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Evaluación de propiedades superficiales y catalíticas del
proceso de adsorción de etileno y oxígeno sobre catalizadores de oro soportados en óxidos mixtos CeZr
Lucía Pérez Ramírez
Universidad Nacional de Colombia
Facultad de Ingeniería, Departamento de Ingeniería Química y Ambiental
Bogotá, Colombia
2017
Evaluación de propiedades superficiales y catalíticas del
proceso de adsorción de etileno y oxígeno sobre catalizadores de oro soportados en óxidos mixtos CeZr
Lucía Pérez Ramírez
Tesis de grado presentada como requisito parcial para optar al título de:
Magister en Ingeniería – Ingeniería Química
Director:
Julio César Vargas Sáenz
Línea de Investigación:
Catálisis Heterogénea
Grupo de Investigación:
Procesos Químicos y Bioquímicos
Universidad Nacional de Colombia
Facultad de Ingeniería, Departamento de Ingeniería Química y Ambiental
Bogotá, Colombia
2017
Evaluation of the surface and catalytic properties of the ethylene and oxygen adsorption process on gold catalysts supported on mixed
oxides CeZr
Lucía Pérez Ramírez
A dissertation submitted in partial fulfilment of the requirements for the degree of:
Master of Engineering – Chemical Engineering
Advisor:
Julio César Vargas Sáenz
Research field:
Heterogeneous Catalysis
Research group:
Chemical and Biochemical Processes
Universidad Nacional de Colombia
School of Engineering, Department of Chemical and Environmental Engineering
Bogotá, Colombia
2017
He aquí que el temor del Señor es la sabiduría,
Y el apartarse del mal, la inteligencia.
Job 28:28
Acknowledgements
First, I praise God for its faithfulness. He filled me with strength and courage during this
challenging journey.
Many thanks to my advisor, Professor Julio César Vargas, for his direction, patience and
persistent help in all circumstances, and to Professor Sébastien Thomas, who provided me
with the opportunity to conduct part of my research at the Institut de Chimie et Procédés
pour l’Energie, l’Environnement et la Santé (ICPEES) facilities at Université de Strasbourg
under a CNRS collaboration partnership. I also acknowledge the economic support
provided by the ICPEES-CNRS during this research stay, as well as the funding received
by the Department of Chemical Engineering and the “División de Investigación y Extensión
Sede Bogotá” through the “Call for International Mobility 2016-2018”.
Thanks to Professor Anne-Cécile Roger for having me at her team at the ICPEES. To
Thierry Dintzer, Thierry Romero, Sécou, Christophe Mélart and Corinne Ulhaq for their
precious help on the characterisation of the catalysts.
I am especially grateful to have been received in the Laboratoire Catalyse et Spectrochimie
(LCS), Caen, and to have learned so much of infrared from Philippe Bazin.
The funding received during my master programme from Universidad Nacional de
Colombia by the scholarship “Asistente Docente”, is gratefully acknowledged, as well as
the funding received by Colciencias through the call for “Jovenes Investigadores”.
Thanks to my friends at Universidad Nacional de Colombia, Université de Strasbourg and
LCS Caen, specially Valentin, Kasia and Ibrahim, for making the path even more enjoyable.
Last but not least, I thank my family for being my unconditional support and motivation.
Resumen y Abstract XI
Abstract
Surface and catalytic properties of mixed oxides CeO2, Au/CeO2, CeZr and Au/CeZr, during
the total oxidation of ethylene is presented in this work. Four types of supports were used:
synthesised and commercial CeO2 and CeZr mixed oxides. Synthesised supports were
prepared by pseudo sol gel method while commercial supports were provided by Solvay-
Rhodia. The mass CeO2/ZrO2 ratio was 0.8/0.2 for the synthesised support and 0.6/0.4 for
the commercial one. Gold was subsequently deposited via direct anion exchange. XRD
and Raman spectroscopy showed the effective formation of the fluorite type structure on
each catalyst. Textural properties were determined via N2 adsorption, demonstrating the
low influence of gold deposited as active phase. Temperature programmed desorption
(TPD) and IR operando spectroscopy were used to stablish the fluid-surface interaction,
elucidate the reaction mechanism during the oxidation reaction and determine the role of
gold as active phase. Results show that there is no adsorption of ethylene or oxygen on
any of the catalysts at room temperature, despite increasing the amount of gold deposited
from 1 to 2%. In fact, only catalysts with gold are active for the oxidation reaction and only
at temperatures above 100 °C. The formation of formate and carbonate species as
intermediary precursors for the subsequent CO2 desorption, are seen as responsible for
the oxidation of ethylene following the Mars and Van Krevelen model.
Keywords: mixed oxides, IR operando spectroscopy, ceria-based catalysts, total
oxidation, ethylene
XII Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
Resumen
Las propiedades superficiales y catalíticas de los óxidos mixtos CeO2, Au/CeO2, CeZr y
Au/CeZr, durante la oxidación total del etileno se presentan en este trabajo. Se utilizaron
cuatro tipos de soportes: óxidos CeO2 y CeZr sintetizados y comerciales. Los soportes
sintetizados se prepararon mediante el método de pseudo sol gel, mientras que los
soportes comerciales fueron proporcionados por Solvay-Rhodia. La proporción másica de
CeO2/ ZrO2 fue de 0.8/0.2 para el soporte sintetizado y de 0.6/0.4 para el soporte comercial.
El oro como fase active se depositó posteriormente a través del intercambio aniónico
directo. La espectroscopia XRD y Raman mostró la formación efectiva de la estructura del
tipo fluorita en cada catalizador mediante difracción de rayos X (XRD) y espectroscopía
Raman. Las propiedades texturales se determinaron por adsorción de N2, demostrando la
baja influencia del oro depositado como fase active sobre estas. Se utilizaron las técnicas
de desorción a temperatura programada (TPD) y espectroscopia infrarrojo (IR) operando
para establecer la interacción fluido-superficie, elucidar el mecanismo de reacción durante
la oxidación total de etileno y determinar el papel de la fase activa. Los resultados muestran
que no hay adsorción de etileno ni de oxígeno en ninguno de los catalizadores a
temperatura ambiente, a pesar de aumentar la carga de oro de 1 a 2%. De hecho, sólo los
catalizadores con oro son activos para la reacción de oxidación y sólo a temperaturas por
encima de 100 ºC. Se observa la formación de especies de formiato y carbonato como
precursores intermedios para la posterior formación y desorción de CO2, y responsables
de la oxidación del etileno siguiendo el modelo propuesto por Mars y Van Krevelen.
Palabras clave: óxidos mixtos, espectroscopía IR operando, catalizadores a base de
ceria, oxidación total, etileno
Contents XIII
Contents
Pág.
Abstract.......................................................................................................................... XI
List of figures................................................................................................................ XV
List of tables .............................................................................................................. XVIII
Introduction ..................................................................................................................... 1
1. Catalytic oxidation of ethylene ................................................................................ 3 1.1 Ethylene........................................................................................................... 3
1.1.1 Basic properties and reactivity ............................................................... 4 1.1.2 Ethylene as aging hormone ................................................................... 5 1.1.3 Technologies for ethylene elimination ................................................... 6
1.2 Catalyst selection ........................................................................................... 10 1.2.1 Cerium oxides ..................................................................................... 14 1.2.2 Cerium-Zirconium oxides .................................................................... 15 1.2.3 Gold as active phase ........................................................................... 17
1.3 Studies on the fluid-surface interaction .......................................................... 18 1.3.1 Temperature programmed desorption (TPD) ....................................... 18 1.3.2 Infrared spectroscopy .......................................................................... 21
1.4 Scope ............................................................................................................ 31
2. Cerium oxides as oxidation catalysts for ethylene elimination .......................... 33 2.1 Catalyst synthesis .......................................................................................... 33 2.2 Characterisation............................................................................................. 34
2.2.1 Surface area – N2 isothermal adsorption-desorption ........................... 34 2.2.2 Crystalline structure – XRD ................................................................. 35 2.2.3 Solid phase formation – Raman spectrocopy ...................................... 38
2.3 Evaluation of the fluid-surface interactions ..................................................... 39 2.3.1 Study of the adsorption-desorption mechanisms of probe molecules C2H4 and O2 via TPD ......................................................................................... 39 2.3.2 Study of the intermediary species during ethylene total oxidation via operando IR spectroscopy ................................................................................. 42
3. Cerium-zirconium oxides as oxidation catalysts for ethylene elimination ........ 55 3.1 Catalyst synthesis .......................................................................................... 55 3.2 Characterisation............................................................................................. 56
3.2.1 Surface area – N2 isothermal adsorption-desorption ........................... 56 3.2.2 Crystalline structure – XRD ................................................................. 57 3.2.3 Solid phase formation – Raman spectroscopy .................................... 59
XIV Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
3.3 Evaluation of the fluid-surface interactions .................................................... 61
3.3.1 Study of the adsorption-desorption mechanisms of probe molecules C2H4 and O2 via TPD ........................................................................................ 61 3.3.2 Study of the intermediary species during ethylene total oxidation via operando IR spectroscopy ................................................................................ 61
4. Adsorbent (zeolite) + catalyst: a promissing alternative ..................................... 65
5. Conclusions and recommendations ..................................................................... 73
References .................................................................................................................... 75
Contents XV
List of figures
Pág.
Figure 1-1: Synthesis of chemical intermediates and products from ethylene ...................4
Figure 1-2: Generalized patterns of growth, respiration and ethylene during development,
maturation and senescence of climacteric and non-climacteric fruits .................................6
Figure 1-3: Schematic representation of the possible reaction paths and products of the
C2H4 + O2 reaction ...........................................................................................................11
Figure 1-4: Mars-van Krevelen mechanism of oxidation reactions on oxide based
catalysts ..........................................................................................................................13
Figure 1-5: CeO2 crystal structure ..................................................................................14
Figure 1-6: Cubic Close Packed (FCC) Anion Arrangement, a) circles labelled T
represent the centres of the tetrahedral interstices, b) circles labelled O represent the
centres of the octahedral interstices ................................................................................15
Figure 1-7: Oxygen (a) storage (b) release mechanism in Ce-Zr oxides .........................16
Figure 1-8: Phase diagram of the CeO2-ZrO2 binary system ...........................................17
Figure 1-9: O2-TPD profiles of (a) Pt(0.2%)/CeO2 submitted to CO2 adsorption, (b)
Pt(0.2%)/CeO2 without CO2 adsorption, and (c) CeO2 samples .......................................20
Figure 1-10: Scheme of a classic dispersive IR spectrometer .........................................22
Figure 1-11: Scheme of a typical FTIR spectrometer ......................................................24
Figure 1-12: Energy profile of a non-catalysed reaction (---) and a catalysed reaction (---)
........................................................................................................................................25
Figure 1-13: A one-dimesional potential energy schematic showing qualitative aspects of
oxygen adsorption ...........................................................................................................27
Figure 1-14: Potential adsorbed-ethylene species ..........................................................28
Figure 1-15: Chatt-Dewar-Duncanson model for ethylene adsorption .............................28
Figure 1-16: Experimental IR frequency bands of different types of formate species over
metal oxides [comb1 stands for νas(COO) + δ(CH) and comb2 stands for νs(COO) + δ(CH)]
........................................................................................................................................30
Figure 1-17: Experimental IR frequency bands of different types of surface carbonate
species over metal oxides ...............................................................................................31
Figure 2-1: XRD patterns of ceria samples with synthesised and commercial (*) supports
........................................................................................................................................36
Figure 2-2: Raman spectra for synthesised and commercial (*) ceria .............................38
Figure 2-3: Scheme of the TPD setup used ....................................................................40
Figure 2-4: Possible configurations of the valve system in the TPD setup ......................40
XVI Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
Figure 2-5: Evolution of mass spectra signal intensities with time on stream during (a)
ethylene desorption (b) oxygen desorption over Au(2%)/CeO2 sample ........................... 41
Figure 2-6: Scheme of the operando system .................................................................. 43
Figure 2-7: General and detailed views of the “sandwich” reactor-cell. 1=Spectrometer
base-plate, 2=IR cell support, 3=adjusting nut for air tightness, 4=gas outlet, 5=air cooling
outlet, 6=gas inlet, 7=air cooling inlet, 8=external shell, 9=thermocouple location, 10=IR
beam, 11=stainless steel ring, 12=Kalrez O-ring, 13=KBr windows, 14=wafer holder,
15=oven location ............................................................................................................. 43
Figure 2-8: Evolution of temperature and mass spectra signal (m/z= 28 for ethylene, 18
for water, 44 for CO2) intensities with time on stream [total flow 25 cc·min-1: 400 ppm C2H4
+ 20% O2 in Ar] when the reaction is performed over CeO2 ............................................ 45
Figure 2-9: IR spectra of CeO2 before (a) and after (b) the reaction ............................... 45
Figure 2-10: Evolution of temperature and mass spectra signal (m/z= 28 for ethylene, 18
for water, 44 for CO2) intensities with time on stream [total flow 25 cc·min-1: 400 ppm C2H4
+ 20% O2 in Ar] when the reaction is performed over Au(1%)/CeO2 ................................ 46
Figure 2-11: IR spectra of Au(1%)/CeO2 before (a) and after (b) the reaction ................ 47
Figure 2-12: IR stacked spectra (800-1150 cm-1) of Au(1%)/CeO2 (a) just before the
reaction; (b) 0 min, 25 °C; (c) 20 min, 35 °C; (d) 40 min, 80 °C; (e) 1 h, 126 °C; (f) 1 h 25
min, 171 °C; (g) 1 h 45 min, 214 °C; (h) 2 h 10 min, 259 °C; (i) 2 h 30 min, 280 °C; [total
flow 25 cc·min-1: 400 ppm C2H4 + 20% O2 in Ar] ............................................................. 48
Figure 2-13: IR stacked spectra (1200-1700 cm-1) of Au(1%)/CeO2 (a) just before the
reaction; (b) 0 min, 25 °C; (c) 20 min, 35 °C; (d) 40 min, 80 °C; (e) 1 h, 126 °C; (f) 1 h 25
min, 171 °C; (g) 1 h 45 min, 214 °C; (h) 2 h 10 min, 259 °C; (i) 2 h 30 min, 280 °C; [total
flow 25 cc·min-1: 400 ppm C2H4 + 20% O2 in Ar] ............................................................. 49
Figure 2-14: IR stacked spectra (2800-3000 cm-1) of Au(1%)/CeO2 (a) just before the
reaction; (e) 1 h, 126 °C; (f) 1 h 25 min, 171 °C; (g) 1 h 45 min, 214 °C; (h) 2 h 10 min,
259 °C; (i) 2 h 30 min, 280 °C; [total flow 25 cc·min-1: 400 ppm C2H4 + 20% O2 in Ar] ..... 49
Figure 2-15: Proposed reaction mechanism for ethylene total oxidation over
Au(1%)/CeO2 (part a) ...................................................................................................... 51
Figure 2-16: Proposed reaction mechanism for ethylene total oxidation over
Au(1%)/CeO2 (part b) ...................................................................................................... 52
Figure 3-1: Global Au/CexZr1-xO2 catalyst synthesis procedure ...................................... 56
Figure 3-2: XRD patterns of ceria-zirconia samples with synthesised and commercial (*)
supports. Composition (wt%) Ce/Zr for CZ=0.8/0.2, for CZ*=0.6/045. ............................. 58
Figure 3-3: Raman spectra for synthesised and commercial (*) ceria-zirconia (CZ) ....... 60
Figure 3-4: Evolution of temperature and mass spectra signal (m/z= 28 for ethylene, 18
for water, 44 for CO2) intensities with time on stream [total flow 25 cc·min-1: 400 ppm C2H4
+ 20% O2 in Ar] when the reaction is performed over Au(2%)/CZ* .................................. 62
Figure 3-5: IR spectrum of Au(2%)/CZ* before (a) and after (b) the reaction .................. 63
Figure 3-6: IR stacked spectra (1200-1700 cm-1) of Au(2%)/CZ* (a) just before the
reaction (b) 0 min, 25°C (c) 20 min, 35 °C (d) 40 min, 80 °C (e) 1 h, 123 °C (f) 1 h 25 min,
167 °C (g) 1 h 45 min, 211 °C (h) 2 h 5 min, 252 °C (i) 2 h 25 min, 280 °C [total flow 25
cc·min-1: 400 ppm C2H4 + 20% O2 in Ar] .......................................................................... 64
Contents XVII
Figure 4-1: Schematic of different zeolite structures .......................................................66
Figure 4-2: Evolution of mass spectra signal (m/z= 27 for ethylene, 44 for CO2) intensities
with time on stream during ethylene desorption under air over Zeolite A .........................67
Figure 4-3: Evolution of mass spectra signal (m/z= 27 for ethylene, 44 for CO2) intensities
with time on stream during ethylene desorption (a) under air (b) under argon over Zeolite
A + Au(1%)/CeO2 system ................................................................................................69
Figure 4-4: Evolution of mass spectra signal (m/z= 27 for ethylene, 44 for CO2) intensities
with time on stream during ethylene desorption (a) under air (b) under argon over Zeolite
A + Au(2%)/CZ* system...................................................................................................70
Contents XVIII
List of tables
Pág.
Table 1-1: Observed wavenumbers (cm-1) and assignments for ethylene adsorbed on
metal oxides .................................................................................................................... 29
Table 2-1: Specifications of the precursor used for the Ce-based catalyst preparation ... 33
Table 2-2: Textural characteristics of samples with synthesised and commercial (*) ceria
supports .......................................................................................................................... 34
Table 2-3: Structural characteristics of CeO2, Au(1%)/CeO2, CeO2* and Au(2%)/CeO2*
oxides. ............................................................................................................................ 37
Table 3-1: Specifications of the precursors used for the Ce-Zr-based catalyst preparation
....................................................................................................................................... 56
Table 3-2: Textural characteristics of samples with synthesised and commercial (*) ceria-
zirconia supports ............................................................................................................. 57
Table 3-3: Structural characteristics of CZ, Au(1%)/CZ, CZ* and Au(2%)/CZ mixed
oxides. ............................................................................................................................ 59
Table 4-1: Summary of the results of the TPD experiences for zeolite A under air, zeolite
A +Au/CeO2 (under air and argon), and zeolite A +Au/CZ* (under air and argon) ........... 68
Introduction
Ethylene is a plant hormone that regulates the processes of maturation and senescence of
various horticultural products and is one of the compounds responsible for post-harvest
losses, which can be as high as 40% [1]. This problem is particularly important in tropical
regions like Colombia, since most horticultural products are climacteric and, therefore, emit
ethylene and are sensitive to concentrations above 0.1 μL L-1 [2].
Many technologies have been developed to control ethylene emissions. The most widely
used are cold storage and hormone control, which have presented drawbacks due to their
extensive energy consumption, to the toxicity of the compounds used and to the rapidity
with which some products regenerate their ethylene receptors [3]. Taking this into account,
the use of non-invasive and efficient technologies for the removal of ethylene is a must in
order to develop industrial solutions.
The actual trend in research for ethylene elimination has been the hormonal control of
ethylene by the use of 1-methylcyclopropene (1-MCP). Regarding physicochemical
methods, some authors have reported the use of substances to absorb ethylene such as
silica, zeolites or activated carbon, having the problem of rapidly saturating with water due
to the high relative humidity (80% - 100%) in the environment where the horticultural
products are stored. It has also been reported the use of strongly oxidizing agents such as
potassium permanganate, although it is a toxic substance. As for the catalytic oxidation,
metals such as palladium, platinum and gold have shown an important activity for oxidation
reactions at relatively low temperatures (0°C - 200°C). Advanced oxidation through the use
of titanium dioxide and ultraviolet light, has also been reported. It is noteworthy that these
researches have shown successful results in horticultural products in temperate latitudes,
finding problems in tropical products such as bananas, mango and avocado [4].
The present work aims to evaluate the surface and catalytic properties of mixed oxides of
cerium-zirconium for the elimination of ethylene by catalytic oxidation as a strategy to
2 Introduction
increase the post-harvest useful life of fruits and vegetables. This thesis complements the
doctoral work of Rolando Mendoza [5], in which it is proposed to develop a catalyst for the
total oxidation of ethylene based on gold nanoparticles supported on mixed oxides of Ce-
Zr-Co. The influence of the presence of gold and the nature of the support on the adsorption
mechanism of ethylene and oxygen, are studied through the use of spectroscopic
techniques.
A catalyst composed of a mixed oxide of cerium/zirconium as the support/carrier and gold
nanoparticles active site, is used. The selection of gold as active principle is due to its high
catalytic activity at low temperatures, when it has a particle size smaller than 4 nm. The
selection of a mixed oxide cerium-zirconium is due to the high dispersion of gold in ceria
(cerium oxide) and the possibility of having a good redox cycle due to the oxygen vacancies
that can be generated by zirconium and cerium oxide.
This document has been divided into five main chapters. The first one is a brief introduction
to the state of the art and some generalities concerning current ethylene elimination
methods for horticultural applications, including total catalytic oxidation as an alternative. It
describes the catalyst used, some structural and physicochemical properties, and the
reason for selecting a ceria-based oxide for this specific purpose. It also introduces the
main techniques used to elucidate the fluid-surface interactions that take place during the
oxidation reaction.
Chapter two and three are dedicated to the study of ceria and ceria-zirconia based
catalysts, respectively, on the total oxidation of ethylene. The influence of gold as active
phase is elucidated. Synthesis, characterisation and evaluation techniques used are
described in detail among with the results obtained.
Chapter four explores a novel strategy for ethylene elimination through a system composed
of an adsorbent and a catalyst. This, in view of the fact that the catalyst is only active at
temperatures above 100 °C. The system allows to use a regenerable adsorbent (in this
case, a zeolite) to trap a certain quantity of ethylene at room temperature. Ethylene is
subsequently desorbed and reacts with the catalyst in the presence of hot air.
Chapter five summarises some conclusions and recommendations for future work.
1. Catalytic oxidation of ethylene
In this chapter, basic properties of ethylene are described, as well as its role as ripening
hormone in plants and fruits. Current technologies for ethylene elimination are presented,
introducing total catalytic oxidation as an alternative to what is conventionally used. The
use of a proper catalyst is key in the development of this strategy at the industrial scale,
thus, the selection of ceria-zirconia based catalyst is explained in detail. Finally, the main
techniques used to study the fluid-surface interaction are introduced.
1.1 Ethylene
Ethylene (C2H4), also known as ethene, is a colourless, gaseous organic compound. It is
the simplest of the alkenes (structures that contain a carbon-carbon double bond) and is
the most important organic compound manufactured nowadays, with a worldwide
production of over 150 million tons in 2016, exceeding that of any other organic compound
[6]. It is the building block for a vast range of chemicals from polymers to solvents.
It is mostly used to produce ethylene oxide, ethylene dichloride, ethylbenzene and
polyethylene, which are precursors to many other compounds (Figure 1-1) [7]. Ethylene is
usually obtained from petroleum and natural gas, by steam cracking, which is the thermal
breakdown of long chain saturated hydrocarbons into smaller, often unsaturated ones. In
the US, Canada and the Middle East, it is primarily obtained by cracking of ethane and
propane. In Europe and Asia, it is mainly obtained from cracking naphtha, gasoil and
condensates with the coproduction of propylene, C4 olefins and aromatics [8]. It can also
be obtained by advanced catalytic cracking of olefins.
4 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
Figure 1-1: Synthesis of chemical intermediates and products from ethylene [7]
1.1.1 Basic properties and reactivity
Ethylene has a poor solubility in water 3.5 mg·100 mL-1 but a good solubility in diethyl ether
due to its nonpolar nature. Its gas density is 1.178 kg·m-3 at 15 °C, its boiling point −103.7
°C (1 atm) and its melting point −169.2 °C (1 atm) [9].
A closer look into the molecular structure of ethylene allows us to understand the reasons
for its multiple applications, functionality and reactivity. In order to have a deeper
understanding, we must remember that a double carbon-carbon bond is composed by σ-
bond and a π-bond. A σ-bond is a covalent bond resulting from the formation of a molecular
orbital by the axial or end-to-end overlap of atomic orbitals, and a π-bond, results from the
formation of a molecular orbital by side-to-side overlap of atomic orbitals along a plane
perpendicular to a line connecting the nuclei of the atoms. π-bonding engages overlapping
of p (in the ethylene case) or d atomic orbitals, which result in the formation of π (bonding)
and π* (antibonding) molecular orbitals. Bonding orbitals stabilise the molecu le and have
lower energies because they are closer to the nuclei of the atoms. Antibonding orbitals have
higher energy levels and less electron density between the nuclei [10].
Usually the π-bond is weaker than the σ-bond, thus, it is not necessary a large amount of
energy to break it. It dominates the chemistry of ethene and is responsible for its high
reactivity. Being a region of high electron density, the π-bond is very vulnerable to attack
by electrophiles. Many reactions of ethylene are catalysed by transition metals, which bind
transiently to the ethylene using both the π and π* orbitals.
Catalytic oxidation of ethylene 5
1.1.2 Ethylene as aging hormone
In addition to having a wide range of practical and commercial applications, ethylene is also
known as the aging hormone in plants. It is synthesised in most tissues of plants and has
several effects on their physiology. It plays a key role in the processes of growth,
development, maturation, and senescence of all climacteric plants or fruits. Ethylene is
biologically active at very low concentrations in the ppm and ppb range, in fact, it can induce
quality loss of a wide variety of fruits and vegetables when present at concentrations above
0.1 μL·L-1 [11].
Fruits and vegetables may be classified depending on whether or not they exhibit a peak
in respiration and ethylene production during ripening (Figure 1-2). Climacteric species are
characterised by an increase in respiration ethylene production as they ripen [12], thus,
they are subject to massive deterioration during the post-harvest period, characterised by
excessive softening and changes in taste, aroma and suface colour [13]. Non-climacteric
species, such as grape, orange, pineapple and leafy vegetables, are characterised by the
lack of ethylene-associated respiratory peak, thus, they do not continue to ripen after
harvest. They may eventually soften and rot, but this is due to moisture loss, decay and
tissue degeneration.
Deterioration of climacteric fruits is accentuated during the commercialisation stage since
these fruits are transported for great distances and stored during long periods of time.
Taking into account the importance of the product perception by the final consumer, and
that this can be associated to quality parameters such as fruit firmness, percentage of
acidity and soluble solids which at the same time are indirectly related to sensory quality
characteristics (such as colour, taste, texture and aroma) and nutritional characteristics
(content of vitamins, minerals, antioxidants, etc.), a deterioration in these properties results
in large economic losses [14]. In fact, according to the Food and Agriculture Organization,
post-harvest losses are estimated to be as high as 40% [1]. This is why there has been a
broad interest in the search for alternatives and strategies to prolong the conditions of the
fruit during the post-harvest, consumption and commercialisation stage, by removing
ethylene and, therefore, suppressing its effects [15].
6 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
Figure 1-2: Generalized patterns of growth, respiration and ethylene during development,
maturation and senescence of climacteric and non-climacteric fruits [16]
1.1.3 Technologies for ethylene elimination
Techniques to limit biosynthesis of ethylene mostly focus on increasing or decreasing
temperature and modifying the atmosphere in which fruits are preserved, which implies
economic investments in transport and cold storage [17]. One of the disadvantages of using
low temperatures is cold damage, when temperatures below the critical temperature of
each product are used [3], which in tropical fruits can be as high as 10-12 °C. As for the
use of controlled and modified atmospheres, it is sought to delay the maturation of the
product by varying the contents of O2 and CO2 in the surrounding environment, affecting
the processes of respiration and gas emission. However, industrial application of this
alternative has not been very successful due to the lack of economic incentives to extend
its use. In addition, the product is generally exposed to varying temperatures during
Catalytic oxidation of ethylene 7
handling, transportation, storage and marketing. This creates a problem in the design of
modified atmospheres because it is difficult to maintain optimum gas concentrations when
the surrounding temperature is not constant [18].
The complete elimination of ethylene can be achieved through various chemical or physical
methods, which exploit the ability of certain substances to oxidise, decompose or adsorb
ethylene through different mechanisms (degradation/oxidation, adsorption, inhibition of the
receptor and the synthesis, catalysis and photocatalysis).
1.1.3.1 Chemical/physical methods
Ethylene can undergo various reactions for degradation. In fact, its double bond makes it
highly reactive and prone to different reaction mechanisms. Ethylene can adsorb UV
radiation at 161, 166 and 175 nm [19]. When it reacts with ozone, it produces water, carbon
dioxide (CO2), carbon monoxide (CO), and formaldehyde [20]. Even in low ozone
concentrations, effective removal of ethylene can also be achieved as ultraviolet light
interacts with oxygen (O2) in the air to form ozone, which breaks the ethylene molecule,
and at the same time, UV light interacts directly with ethylene to degrade it [21]. However,
this reaction is inefficient in atmospheres of very low ethylene concentration, such as those
found in the storage and transport processes of fresh fruit, so the commercial potential of
ozone as an ethylene scrubber is limited [22].
One of the first chemical agents used for the oxidation of ethylene was potassium
permanganate (KMnO4), with the ability to decompose ethylene to acetaldehyde and,
subsequently, acetic acid, which in turn can be oxidised to CO2 and H2O [23]. KMnO4 is
often supported on celite, vermiculite, silica gel, zeolite or alumina pellets in order to
increase the surface area [24]. Currently, commercial formulas containing KMnO4 are the
most commonly used in the industry to reduce post-harvest losses. However, direct contact
of potassium permanganate with fruits is not allowed due to its high toxicity, so films or
filters are used in combination with inert substrates such as silica gel, activated carbon,
zeolite or alumina in order to avoid this [25].
Ethylene may be subjected to hydrogenation in the presence of metal catalysts. It can also
react with halogens (mainly chlorine and bromine), in halogenation and hydrohalogenation
8 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
reactions to form dihaloalkanes. In this way, ethylene can be removed by passing it over
brominated activated carbon to form dibromoethane [26]. These types of filters are relatively
efficient in the removal of ethylene, however, the bromine can also react with water to form
HBr and Br2. These compounds are harmful to plant tissues and corrosive to stainless steel.
In addition, the brominated activated carbon is hygroscopic, so it is possible that the filter
adsorbs a lot of moisture, which is inconvenient [22]. Alternatively, the ethylene may react
with hydrogen halides to form ethyl halides. It can also react with concentrated sulfuric acid
to form ethyl hydrogen sulphate or with water in acid medium to form ethanol [27].
It is clear that ethylene is a highly reactive compound and can be degraded by different
reaction routes. However, typical reactions generally require high ethylene concentrations
and/or high pressure and temperature conditions.
In addition to the chemical degradation methods, ethylene can be adsorbed by different
substances including activated carbon, crystalline aluminosilicate molecular sieves,
Kieselguhr, bentonite, Fuller's earth, silica gel, aluminium oxide and zeolites [28]. The
activated carbon may be granular, powder or fibre. According to Martínez-Romero et al.
[29], the best results in terms of ethylene adsorption rates were obtained with granular
carbon (80%), followed by powder (70%) and fibre (40%).
The use of some regenerable adsorbents, such as propylene glycol, hexylene glycol,
squalene, phenylmethylsilicone, polyethylene and polystyrene has also been investigated
[30]. These have the advantage of being reusable after being purged. However, the main
problem is that adsorption techniques only transfer ethylene to the adsorbent solid matrix,
rather than destroying it.
At the commercial scale, only some of the technologies described above are used in
product packaging. Substances designed to remove ethylene during storage are generally
designed to go into the package (in the form of sachets) or integrated with the packaging
material. Adsorbents based on KMnO4, for example, come in the form of sachets, blankets,
or tubes. The sachets are used in individual fruit and vegetable boxes while the others are
used in vehicles where they are transported [31]. KMnO4 is not impregnated directly into
the package due to its toxicity. Typically, these products contain about 4-6% KMnO4
Catalytic oxidation of ethylene 9
supported on an inert substrate [32]. It has been proposed to use other materials such as
activated carbon, bentonite, Kieselguhr, zeolites and aluminosilicates. Metal catalysts on
activated carbon also effectively remove ethylene. Commercially, sachets containing
activated carbon impregnated with palladium are used [33]. A variety of packages
impregnated with clays or 'activated earth' have also been commercialised. Usually these
are bags of polyethylene impregnated with zeolites. Although these minerals have some
capacity for adsorption of ethylene, there is no information to prove its effectiveness. In fact,
in plastic films, the ethylene would have to diffuse through the polymeric matrix of the
package before coming into contact with the dispersed mineral, making this an excessively
slow and inefficient process [31].
1.1.3.2 Biological methods
Alternatively, ethylene synthesis inhibitors such as aminoethoxyvinylglycine (AVG) and
aminooxyacetic acid (AOA) can also be employed, which effectively retard the senescence
of climacteric products by inhibiting the action of 1-aminocyclopropane-1-carboxylate
synthase (ACCS), the enzyme that catalyses the synthesis of 1-aminocyclopropane-1-
carboxylic acid (ACC), a precursor of ethylene [34]. Ethylene synthesis inhibitors are
applied pre-harvest. Another strategy commonly used involves the use of the ethylene
action inhibitor, 1-methylcyclopropene (MCP-1), which block ethylene by binding to its
receptors. It is applied post-harvest [35], [36].
1.1.3.3 Catalytic oxidation of ethylene
During the last years, a recent development has been seen in the removal of biogenic
ethylene by total catalytic oxidation. Among the drawbacks of the catalysts which have been
reported above is that they are limited by their low efficiency and high reaction temperature.
Considering that the storage or transport temperature of the fruit should be lower than 10°C
[37], the presence of an active catalyst at relatively low temperatures is essential for the
development and implementation of this process at the commercial level. Other desirable
characteristics for this catalyst are: a high ethylene adsorption capacity and the capacity to
promote the breakdown of the carbon-carbon bond (C=C).
10 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
Several catalysts with transition metals as active phase (Pd, Pt, Rh) [38] supported on metal
oxides (TiO2, Al2O3, zeolites) [39], [40] or black carbon have been evaluated in the catalytic
oxidation of ethylene. But metals like palladium and platinum are only active at
temperatures above ambient. The use of Cu-Mn porous oxides has also been studied,
however, these materials only exhibit high conversion in the ethylene oxidation reaction at
temperatures around 200 °C [41]. The use of photocatalytic materials that are irradiated
with ultraviolet light to remove ethylene through a light-promoted oxidation mechanism on
TiO2 nanoparticles has also been investigated [42] and also using TiO2/SiO2 mixed phase
nanocomposites [23]. However, the main limitation is the low efficiency of photocatalysis in
TiO2.
A very attractive option is the use of Au/Co3O4 and Au-Ag/Al2O3 catalysts [43]. Ahn et al.
[44] observed that at 0 °C, the total oxidation reaction of ethylene can achieve a conversion
of up to 93.7% on an Au/Co3O4 catalyst. It is interesting that although the use of the Co3O4
support exhibits no activity, the addition of gold as the active phase greatly increases the
activity of the catalyst. In fact gold has a great catalytic activity when it has a crystal size
smaller than 4 nm. But for the oxidation reaction it is necessary, at the same time, a
promoter or support that allows the formation of oxygen vacancies, for which mixed oxides
are generally used to generate a good redox cycle.
1.2 Catalyst selection
Depending on the type of the catalyst and reaction conditions, one could either favour the
selective or partial oxidation of ethylene leading to the formation of ethylene oxide
(epoxide), or the total oxidation or combustion reaction leading to the formation of CO2 and
water, as seen in Figure 1-3. Usually, total oxidation reactions are thermodynamically much
more favourable, although it is common that products of partial oxidation are intermediates
in the combustion process that forms carbon oxides.
Catalytic oxidation of ethylene 11
Figure 1-3: Schematic representation of the possible reaction paths and products of the
C2H4 + O2 reaction [38]
In presence of water, ethylene oxidation may even lead to the formation of acetic acid via
acetaldehyde. It can also just hydrate and form ethanol following another catalytic path [45].
Overall in a catalytic reaction, activity and selectivity depend pretty much on the chemical
nature of the catalyst. This includes the composition and distribution of the components,
the textural properties (surface area, pore volume, pore size), and other process variables
[46]. Among some of the desirable characteristics of the catalyst in order to favour the total
oxidation of ethylene to CO2, the following phenomena is expected:
(i) High ethylene adsorption capacity
(ii) The preferential cleavage of C-C, O-H, and C-H bonds
(iii) No cleavage of C-O bonds
(iv) No hydrogenation of CO or CO2 to form alkanes
(v) High oxygen mobility
(vi) High activity at low temperatures
The C-H bond breaking of alkenes is enhanced by strongly basic surface lattice oxygen
and cations that are soft acid and undergo redox readily. Due to their good redox
(oxidoreductive) properties, multicomponent oxides are often used for oxidation reactions.
These are based on a few number of transition-metal oxides as basic components. For
12 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
example, for selective oxidation reactions, these basic components include oxides of
molybdenum, vanadium, copper, iron, antimony, tin and uranium. For total oxidation
reaction the basic components include oxides of cerium, cobalt, iridium, and zirconium [47],
[48].
Often in these cases, a second element is also added, acting as a promoter which main
purpose is to enhance catalyst activity and selectivity, by enhancing the rate of the rate-
determining steps. Improved catalyst activity can be due to many reasons: the formation of
new compounds, increase of acidity or basicity, or generation of defect sites for
hydrocarbon or oxygen activation. Promoters can be transition or post-transition metals.
Generally, presence of small quantities of promoter can optimise the performance of the
principal active elements [47].
Even though a mixed oxide support favours oxidation reactions due to their high oxygen
exchange, it is still necessary an active principle which acts as a strong dehydrogenating
agent. The most used materials as active phase are noble metals like palladium or platinum,
due to their high dehydrogenating capacity. In addition, since the early studies by Haruta
and co-workers, there has been a growth of papers and reports about the unusual activity
of nanosised gold-based materials in oxidation reactions [49].
Within a mixed oxide based catalyst, total combustion reactions follow a Mars-Van Krevelen
mechanism, where the catalyst surface oxidises the hydrocarbon and then is re-oxidised
by molecular oxygen from the gas phase, following a redox cycle. This is shown in a more
detailed way in Figure 1-4. The reactant molecule interacts strongly with the catalyst at site
containing M1n+. Lattice oxygen is consumed to form products, and the M1 cation is reduced.
The active site and cation M1 are re-oxidised by the migration of lattice oxygen from M2,
which in turn is re-oxidised by the gas phase oxygen. M2 may or may not be distinct from
M1 [50]. Although Figure 1-4 shows a Mars-Van Krevelen cycle for a bimetallic oxide, a
catalyst composed of a transition metal and a metal oxide might follow the same type of
mechanism as well.
Catalytic oxidation of ethylene 13
Figure 1-4: Mars-van Krevelen mechanism of oxidation reactions on oxide based catalysts
[50]
There are some additional features to optimise total combustion reactions. This can be
achieved by increasing the residence time of the surface intermediates, strengthening the
adsorption of the desired products and optimising the amount of weakly adsorbed oxygen
or the density of combustion sites. The support also plays an important role on this, since
reaction products also depend on its surface area. A not so large surface area limits the
rate of total combustion products, since it does not favour the subsequent reactions to CO2
of partially oxidised products. While a large surface area enhances the dispersion of active
elements and favours the formation of total combustion products, which are dependent on
their ability to diffuse from the active centre into the gas phase [51].
Fluorite-type mixed oxides are characterised by a high oxygen mobility. Cerium based
catalysts are widely known for their redox properties and high performance due to the
generation of oxygen vacancies, which make them ideal in oxidation reactions. It has also
been reported that the introduction of zirconium improves the redox properties of cerium
and thermal stability, enhancing the oxygen storage capacity and, therefore, its catalytic
properties [52]. In addition, gold as active phase has shown a great catalytic activity when
it has a particle size smaller than 4 nm. Gold has a strong interaction with ceria-based
14 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
supports due to their defect-type structure, it shows a good dispersion and it helps
enhancing the reducibility of the material [53], [54].
1.2.1 Cerium oxides
Cerium is the second element in the lanthanide series. It has a ground state electron
configuration [Xe] 4f15d16s2 and it can exhibit both +3 and +4 oxidation states, although it
acquires a more stable structure on the tretavalent form Ce4+. Cerium oxide crystallises in
a fluorite-type structure. It has a face-centred cubic unit cell (fcc) with a space group Fm3m.
Each cerium cation (Ce4+) is coordinated by eight equivalent nearest-neighbour oxygen
anions (O2-) at the corner of a cube, each anion being tetrahedrally coordinated by four
cations [55], as seen in Figure 1-5.
Figure 1-5: CeO2 crystal structure [56]
If we take a closer look to the common ccp (cubic close) anion packing, it can have
tetrahedral as well as octahedral interstices. Figure 1-6 helps to illustrate both the centres
of the tetrahedral (8 sites) and octahedral (4 sites) holes in the ccp arrangement of an fcc
unit cell. Comparing to the ceria structure, it is easy to see that all the tetrahedral sites in
ceria are occupied by the oxide ions whereas the octahedral sites remain empty. This is
why oxygen mobility is favoured, since the oxygen anions tend to move into the octahedral
holes [57].
Catalytic oxidation of ethylene 15
Figure 1-6: Cubic Close Packed (FCC) Anion Arrangement, a) circles labelled T represent
the centres of the tetrahedral interstices, b) circles labelled O represent the centres of the octahedral interstices [58]
The outstanding role of ceria has been recognised in three-way catalysis, water-gas shift
(WGS) reactions, wet oxidation, combustion reactions and solid oxide fuel cells [59]-[61].
This is due to its high OSC (oxygen storage capacity) attributed to the reversible
transformation of oxidation states between Ce4+ and Ce3+ ions [62].
1.2.2 Cerium-Zirconium oxides
One well-known drawback in ceria catalysts is their low stability in high temperature ranges.
Textural properties of ceria can be improved by the introduction of doping agents in the
fluorite lattice. One of the most efficient dopants studied has been zirconium [60], [63]. The
incorporation of elements such as Zr4+ in the ceria network enhances the catalyst redox
properties by increasing its oxygen storage capacity, favours the reduction of Ce4+ to Ce3+,
preserves the oxygen vacancies and prevents catalyst sintering by enhancing the thermal
resistance [52].
The catalytic effects of zirconium introduction into the ceria structure can be mainly
explained by the distortion that it causes. This distortion is introduced primarily by the
difference in the ionic sizes between Ce4+ (0.87 Å) and Zr4+ (0.72 Å) when in a hexahedral
position. Taking into account that during the oxygen storage and release process, the
volume of the cerium cell increases due to the change in the cerium oxidation state, from
Ce4+ (0.87 Å) to Ce3+ (1.01 Å), the stress energy generated from this volume increase can
be diminished by Zr introduction, facilitating the valence change process [64], [65]. The
16 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
oxygen storage-release mechanism of a Ce-Zr mixed oxide can be well appreciated in
Figure 1-7.
Figure 1-7: Oxygen (a) storage (b) release mechanism in Ce-Zr oxides [66]
The Ce-Zr solid solution can adopt a different crystal structure depending on the Ce/Zr ratio
and temperature. According to the CeO2-ZrO2 phase diagram (Figure 1-8), below 1000 °C
(1273 K), ceria-zirconia exhibits a cubic (c) structure when Zr content is less than 20%. This
structure is common to both pure ceria and cubic zirconia. However, at high Zr contents
(>80%), the compound exhibits a monoclinic (m) symmetry. In the intermediate region, the
true nature of the Ce-Zr structure is still matter of debate due to the presence of a number
of stable and metastable phases of tetragonal symmetry. In fact, three different tetragonal
phases can be distinguished t, t’ and t’’. The t phase has been reported to be the most
stable tetragonal phases. The t’’ phase is an intermediate between t’ and c; in fact, it shows
no tetragonality and is generally referred to as cubic phase, since its XRD pattern is indexed
in the cubic Fm3m space group [67], [68].
Although according to the phase diagram the limit between the tetragonal and the cubic
phase may be found at CeO2 molar compositions of around 65%, previous reports have
found that a Ce0.5Zr0.5O2 is still compatible with the cubic symmetry, being on the limit of the
phase transition. This composition can either take the tetragonal or cubic form by simply
changing the preparation conditions [69].
Catalytic oxidation of ethylene 17
Figure 1-8: Phase diagram of the CeO2-ZrO2 binary system [70]
1.2.3 Gold as active phase
Among with group VIII metals, Cu and Ag from group IB have been the most widely used
in catalytic reactions due to their activity. The catalytic excellence of group VIII metals can
be attributed to the presence of unpaired electrons in the d-band, which is why they are
classified as paramagnetic. While, in the case of Cu, Ag and Au from group IB, they have
fully occupied d-electron bands (diamagnetic), however, they have a relatively low
ionization potential, being able to readily lose electrons to yield d-band vacancies. The
presence of unpaired electrons in the d-band is essential to hydrogen chemisorption, as
they stabilise an intermediate weakly-held form without which dissociation cannot occur
[71].
Gold, despite belonging to the group IB of the periodic table, has often been regarded to
have a poor catalytic activity. In bulk, it is chemically inert. However, recent discoveries
have shown that when it is small enough (particle size < 4nm) it turns out to be surprisingly
active for many reactions. In fact, there has been a significant growth on papers about the
high activity of nanosised gold since the early studies by Haruta and co-workers, who
realised its unusual catalytic activity at sub-ambient temperature [72]. It is thought that the
18 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
exceptional ability of gold is ascribed to the presence of Au atoms in low coordination states
on the surface of the nanoparticles [73], [74].
As just mentioned, gold’s particle size is critical when in catalytic reactions. The particle
size of an active metal depends highly on the preparation method used [75]. However, the
general catalytic performance is not defined only by Au particle diameter but also by other
major factors like support selection and contact structure. In fact, in this case, gold’s
remarkable ability also depends on the mutual interaction of the nanoparticles and the
support, which not only depends on the synthesis method, but also the support used [74].
Taking this into account, a ceria-based support is also an excellent alternative because of
gold high dispersion on it. Gold also promotes the reducibility of ceria at low temperatures,
(although this degree of reduction depends on the percentage of incorporated zirconia) [76],
[53]. In some situations, the active site of the correspondent reaction is the interface gold-
support, as it is the case of CO oxidation [77]. But for WGS, for example, active sites are
small gold particles in a close contact with oxygen vacancies of ceria [78].
1.3 Studies on the fluid-surface interaction
Since the presence of an active catalyst at relatively low temperatures is a key for the
development and application of the catalytic oxidation of ethylene at the commercial scale,
the identification and understanding of the adsorption and reaction mechanisms that take
place during this process are fundamental for the proper design of a ceria-based material.
Adsorption-desorption of probe molecules can be studied via temperature programmed
desorption (TPD). This technique helps to quantify the amount of gas that adsorbs on the
catalyst surface at room temperature. Identification of molecular intermediary species that
adsorb on the surface during the reaction can be performed by infrared (IR) operando
spectroscopy.
1.3.1 Temperature programmed desorption (TPD)
The quantification of chemical species that dissociate or desorb the surface can be
performed by temperature programmed desorption (TPD), also referred as thermal
desorption spectroscopy (TDS). Through TPD techniques it is also possible to determine
Catalytic oxidation of ethylene 19
the number, type and strength of available active sites on the catalyst surface from
measurements of the amounts of desorbed gas at several temperatures [79].
Adsorption takes place when an attractive interaction between a particle and a surface is
strong enough to overcome the disordering effect of thermal motion. This can occur by
mechanisms of physisorption or chemisorption. Physisorptive bonds occur when the
interaction is essentially the result of van-der-Waals forces, they are characterised by
dissociation energies of approximately 50 kJ·mol-1. Chemisorption, as stated by its name,
involves the formation of a chemical bond which occurs by the overlap between the
molecular orbitals of the adsorbed molecule and the surface atoms. Chemisorptive bonds
often require that an activation barrier is overcome, this is why chemisorption usually leads
to dissociation of the molecule. These bonds are characterised by dissociation energies
exceeding 50 kJ·mol-1 [80].
A typical TPD system is mainly composed of a gas flow set-up, reactor or sample tube that
goes into a furnace, and detector. The procedure is simple, once the sample has been
degasified, reduced or pre-treated in some way, a constant stream of the analyte gas is
flowed through the sample (a fixed bed of catalyst) so that it adsorbs onto the active sites
of the catalyst. The programmed desorption begins by increasing the temperature linearly
with time while a continuous stream of inert carrier gas flows through the sample. At a
certain temperature, the thermal energy will exceed the heat of adsorption of the molecule,
thus, the bond between the adsorbate and the adsorbent will break, and the adsorbed
species will be released from the surface and swept along with the carrier gas. If there is a
presence of different active metals, the chemical bond between the adsorbed molecule and
each type of metal will probably be of different energy. Consequently, molecules adsorbed
on each active metal will require a different level of thermal energy to break the bond and
desorb, resulting in different peaks in the final TPD spectrum.
The detector is generally a Quadrupole Mass Spectrometer (QMS), although it is also usual
to attach a differential thermal conductivity (TCD) detector. Either the mass over charge
(m/Z) of TCD measured by the detector at any time is proportional to the instantaneous
molecular concentration of desorbed molecules. The volume of desorbed species obtained
from the integration of the peak, combined with a factor of stoichiometry, allows to obtain
20 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
the number of active sites. As said before, presence of multiple peaks indicate the energy
differences in the active sites [81].
One of the important considerations in order to propose the reaction mechanism of the total
oxidation of ethylene on ceria-based samples is to study the adsorption-desorption
mechanisms of the reactants, in this case, C2H4 and O2, via TPD. Literature concerning
C2H4 or O2 desorption profiles on ceria or ceria-zirconia samples is very scarce. In fact,
there are no reported C2H4- or O2-TPD profiles of gold-containing ceria-based samples.
However, there are some reported O2-TPD profiles of other transition metals supported on
ceria. Figure 1-9 shows O2-TPD profiles of high surface area platinum-containing ceria
sample [82]. In this case, Gaillard wanted to stablish whether there was a competitive
adsorption between CO2 and O2, and the role of platinum as active phase during the
activation and adsorption of oxygen. First, it is noticeable that no oxygen desorption is
observed arising directly from the ceria support alone (c). This is probably due to the lack
of Pt atoms which may act as portholes for oxygen adsorption, as generally described in
the literature [83]. This means, the activation of molecular oxygen is done on the transition
metal atoms. Also, the MS signal (m/e = 32) exhibits two desorption peaks at about 450
and 620 °C, consistent with two kinds of adsorbed oxygen species. The fact that the shape
and area of the peaks are not affected by CO2 adsorption indicates the lack of competition
between the CO2 probe molecule and adsorbed oxygen.
Figure 1-9: O2-TPD profiles of (a) Pt(0.2%)/CeO2 submitted to CO2 adsorption, (b)
Pt(0.2%)/CeO2 without CO2 adsorption, and (c) CeO2 samples [82]
Catalytic oxidation of ethylene 21
1.3.2 Infrared spectroscopy
Spectroscopy techniques take advantage of the interaction between light and matter. The
interaction between radiation, which is a flow of energy photons, and a molecule in a given
state can result in different phenomena such as absorption, diffusion and emission. The
exact nature of this interaction depends on the energy of the radiation [84].
In the case of the infrared radiation, it happens to correspond to most of the molecular
vibration frequencies. Thus, if a molecule is irradiated by an electromagnetic wave in the
infrared domain, the incident wave will be absorbed whenever the frequency of the incident
wave is equal to one of the vibration frequencies νvib of the molecule. This will only happen
if the transition is allowed, that is, if the dipole moment of the molecule changes as a result
of the excitation in the vibrational states [85], [86].
The typical units in infrared spectroscopy are wavenumbers, �̃�, which is just the reciprocal
of the wavelength, λ, according to equation (1.1). The energy of the irradiated photon can
be calculated as the product of the Planck’s constant, h, and the frequency, 𝜈, according to
equation (1.2).
�̃� =1
𝜆=𝜈
𝑐 (1.1)
𝐸 = ℎ ∙ 𝜈 = ℎ ∙ �̃� ∙ 𝑐 (1.2)
The total portion of the infrared radiation in the electromagnetic spectrum extends from
14000 to 20 cm-1, although the mid-infrared range, which approximately goes from 4000-
400 cm-1, is the most useful. It gives information about the strength of the bonds between
atoms and the nature of molecular vibrations. The contributions of different groups within a
specific molecule appear in different frequencies. Thus, IR spectroscopy is used for
qualitative analysis to determine specific functional groups and molecular structure. Since
the intensity of the bands observed may indicate the concentration of the species, it is
possible to make a quantitative analysis in some cases [87].
IR spectroscopy is capable to analyse gas, liquid and solid samples. In heterogeneous
catalysis, it is often used to observe adsorbed species on a solid surface. The adsorption
22 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
of a molecule onto a surface often comes with a loss of symmetry, activating vibrational
modes that were not permitted before, when in the gas phase. In conclusion, interaction
between adsorbed species and their environment (the surface or other molecules) influence
the observed bands on the spectrum.
1.3.2.1 Dispersive IR spectrometers
Dispersive IR spectrometers are often referred to as scanning spectrometers. Their basic
components are a radiation source, grating-slit system (monochromator), and detector, as
shown in Figure 1-10. They generally have a double-beam design with two equivalent
independent beams from the same source which pass through both the sample and a
reference species for analytical comparison. These reference and sample beams are
alternately focused on the detector by making use of an optical chopper, such as, a sector
mirror. After the source energy is directed along both the sample and the reference, the
emitted wavefront of radiation is dispersed by a monochromator into its component
frequencies. The monochromator is composed by a grating, which disperses the light into
its component frequencies (spatially separated wavelengths), and a narrow slit, which
chooses which frequencies go onto the detector. Finally, the detector converts the analogue
spectral output into an electrical signal, which is further processed by the computer using
mathematical algorithm to arrive at the final spectrum [88].
Figure 1-10: Scheme of a classic dispersive IR spectrometer [89]
The common IR radiation sources are inert solids that are electrically heated to a
temperature up to 1300-1700 °C to promote thermal emission of radiation in the infrared
Catalytic oxidation of ethylene 23
region of the electromagnetic spectrum. The most used ones are the Nernst glower, which
is a cylinder of rare earth oxides with platinum wires sealed to the ends, the globar source,
which is a silicon carbide rod, the incandescent wire source, which is a tightly wound coil of
nichrome wire, and the carbon dioxide laser [90].
As for the detectors, they can be classified in three categories: thermal, pyroelectric and
photoconducting. Examples of thermal detectors are a thermocouple or a bolometer. They
can be used over a wide range of wavelengths and they operate at room temperature. Their
main disadvantages are slow response time and lower sensitivity relative to other types of
detectors. Pyroelectric detectors consist of a pyroelectric material, triglycerine sulphate is
the most commonly used. They have a fast response time which make them ideal in most
Fourier transform IR instruments. Photoconducting detectors rely on the interaction
between photons and a semiconductor. They consist of a semiconductor materials, mercury
cadmium telluride (MCT) is widely used for mid-IR region. It is the most sensitive detector
with better response characteristics than the others [91].
Dispersive IR instruments operate in the frequency domain. The wavelengths are measured
one at a time, with the gratingslit combination selecting the wavelengths being measured.
The slit controls the spectral bandwidth, which is how wide a range of frequencies are
permitted to strike the detector. Narrower slits give better resolution by distinguishing more
closely spaced frequencies of radiation. Wider slits provide better sensitivity by allowing
more light to reach the detector [86].
1.3.2.2 Fourier transform infrared (FTIR) spectrometers
FTIR spectrometer belongs to the third generation of IR spectrometer, and it is the most
widely used since the mid-1980s. Unlike the dispersive IR spectrometer, it operates in the
time domain and collects all wavelengths simultaneously. It is able to do that by first
collecting an interferogram of a sample signal using an interferometer, and then performing
a Fourier Transform (FT) on the interferogram to obtain the spectrum. Thus, the FT is simply
a mathematical function to convert data from the time domain to the frequency domain [92].
As show in Figure 1-11, an FTIR is typically based on The Michelson Interferometer
Experimental Setup. It consists of a source, interferometer and detector. The interferometer
24 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
consists of a beam splitter and two mirrors, a fixed one and a movable one that translates
back and forth, very precisely thanks to a laser frequency mechanism. Radiation from the
source strikes the beam splitter and separates into two beams roughly of the same intensity.
One beam is reflected off the beam splitter to the fixed mirror; the second is transmitted
through the beam splitter to the movable mirror, thus, the path length of this beam is
variable. The fixed and moving mirrors reflect the radiation back to the beam splitter, where
it is recombined forming an interference pattern. Interference will be constructive if the
difference in path lengths is an exact multiple of the wavelength. If the difference in path
lengths is half the wavelength then destructive interference will result [93], [94].
Figure 1-11: Scheme of a typical FTIR spectrometer [95]
Then, the IR beam passes through the sample, which can absorb some portion of the
energy. The transmitted portion reaches the detector, which records the total intensity and
yields an interferogram. The signal goes then through an amplifier, and is subsequently
converted by an analogue-to-digital converter. Finally, it is transferred to a computer in
which Fourier transform is carried out.
The final spectrum can be presented as transmittance (%T) or absorbance. It is important
to collect a reference or “background” single beam which is then automatically subtracted
from the sample spectrum.
Catalytic oxidation of ethylene 25
FTIR has three major advantages over dispersive IR spectroscopy. Since all wavelengths
being transmitted at once, many scans can be completed and averaged on an FTIR in a
shorter time than one scan on most dispersive instruments. In addition, it does not limit the
amount of light reaching the detector using a slit and it uses fewer mirrors than the
dispersive IR instrument, reducing the occurrence of reflecting losses. This means more
energy reaches the sample and hence the detector, which results in an improved signal-to-
noise ratio. Moreover, it does not require external calibration standards. The laser that
controls the velocity of the movable mirror provides reference for both precision and
accuracy of the spectrometer. Therefore, spectra can be compared with confidence, leading
to a higher precision [96], [97].
1.3.2.3 Operando Infrared Spectroscopy
Characterisation of the active sites within a catalyst is a key to its optimisation and rational
design. It is on these sites where reaction takes place going through the stages of reactant
adsorption, intermediate species formation and generated products desorption. In fact, the
formation of intermediate species is a key when crossing the energy barrier compared to a
non-catalysed reaction, as represented in Figure 1-12. Active centres are linked to
intermediates but can also be poisoned by other species, which affect the overall catalytic
performance. It is important to consider that they are not static entities, but they undergo
modifications depending on the reaction conditions and surface restructuration
phenomena. This is why, the study and characterization of these active centres should be,
as well, under actual reaction conditions.
Figure 1-12: Energy profile of a non-catalysed reaction (---) and a catalysed reaction (---)
[98]
26 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
The term operando is borrowed from Latin and means "working" or "operating". Operando
IR spectroscopy allows studying a catalyst under flow of reactants using real operating
conditions very close to the application (industrial conditions). Therefore, this method is
able to provide significant insights on the relevant catalytic steps, active sites and reaction
intermediates during a given process, in order to elucidate the reaction mechanism on a
given catalyst. It can even serve to differentiate an active site from a bare adsorption centre.
Note that IR is a very sensitive technique. Thus, as it serves to identify important reaction
surface species it can also detect surface impurities such as water, carbonates and other
residual species after synthesis. Presence of these species may inhibit probe adsorption,
or simply disturb the correct spectral interpretation by band overlapping. Therefore, the
necessity of their elimination (catalyst pre-treatment). Thermal stability of these species
strongly depends on acid–base properties of the material: on relatively acidic compounds,
like ceria or zirconia, they can be removed at mild temperatures. As a matter of fact, it is
important that the pre-treatment temperature never exceeds the calcination temperature of
the catalyst. If that would be the case, it would lead to surface reconstruction and sintering
phenomena [99], [100].
1.3.2.4 Oxygen activation
The interaction of molecular oxygen with transition-metal surfaces has been largely studied
due to its importance as a fundamental step in many catalytic reactions. Despite extensive
research, the elementary steps of oxygen chemisorption on transition metals remain difficult
to characterize accurately, although many authors agree on a chemisorbed-precursor
mechanism that involves an initial physically and/or chemically intact adsorbed species
acting as a precursor to dissociative chemisorption (Figure 1-13) [101]-[104].
Catalytic oxidation of ethylene 27
Figure 1-13: A one-dimesional potential energy schematic showing qualitative aspects of
oxygen adsorption [101]
On many transition metal surfaces, including Pt, Pd, Ni, Ag and Cu, two chemisorbed
molecular states of oxygen are frequently observed in vibrational spectroscopy: a peroxo-
like form (O22-), with an O-O stretching frequency of 610-650 cm-1, and a superoxo-like form
(O2-), with a stretching frequency of 810-870 cm-1 [105]. The O-O bond would then stretch
until it breaks, to form atomic adsorbed oxygen species.
1.3.2.5 Ethylene activation and oxidation
The interaction of olefins, mainly ethylene, with catalyst surfaces has been the object of
many investigations by vibrational techniques. Although there is no specific literature on
ethylene adsorption onto ceria-based catalysts, there have been studies concerning the
adsorption of ethylene onto some noble and transition metals, metal oxides and some types
of zeolites.
Ethylene has different adsorption mechanisms depending on the noble or transition metal
used (Figure 1-14), which include the (a) π-bonded, (b) di-σ bonded, (c) ethylidyne, (d)
28 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
ethylidene, (e) vinylidene and (f) vinyl [106]. However, in the majority of cases it adsorbs
following the two first cases. The di-σ mode, features a strong interaction where two of the
ethylene carbon atoms attach to two metal surface atoms (the atomic orbitals around the C
atom hybridise as in ethane). The π-bonded mode easily explains the weakening of the
ethylene C=C bond following the Chatt-Dewar-Duncanson model (Figure 1-15). In this
model, π-electrons are donated to an empty orbital of a metal atom on the catalyst surface.
This is accompanied by back bonding from the filled d orbitals on the catalyst to the π*
orbitals of the alkene [107].
Figure 1-14: Potential adsorbed-ethylene species [106]
Figure 1-15: Chatt-Dewar-Duncanson model for ethylene adsorption [108]
Depending on the ethylene adsorption mechanism, it is common to find frequency bans in
the C=C or C-C bond stretching region on the corresponding IR spectra. The weaker
interaction of π -bonded ethylene compared with the di-σ mode implies that the π -bonded
case has a smaller energy cost. [109] For example, in the case of small gold clusters, the
Catalytic oxidation of ethylene 29
π-bonded mode dominates over the di-σ mode. [110] However, its detection is not easy
since adsorbed ethylene is very unstable on metal surfaces. In fact, for its detection to be
possible, it is necessary a low-temperature measure (below room temperature) [111], [112].
Ethylene characteristic vibrational frequencies can be shifted when adsorbed onto a
specific material. The shift with respect to the original position of the bands is due to the
metal- π -bond and highly depends on its strength. In fact, these bands can be even masked
by the strong skeletal vibrations of the catalysts [113].
For example, two main characteristic frequencies of free gaseous ethylene are 1623 and
1342 cm-1, which correspond to C=C stretching and CH2 in-plane bending (scissoring),
respectively. One of the early studies of π -adsorbed ethylene species on Pt, Rh and Pd
based-catalysts states that these bands can be shifted down to 1495 and 1199 cm-1,
respectively, because of the gas-metal interaction [114]-[116]. As for the out-of-plane
deformation modes, such as CH2 wagging, these are generally shifted upwards in the
majority metal oxides. Some of the observed wavenumbers (cm-1) and assignments for
ethylene adsorbed on metal and metal oxides are shown in Table 1-1.
Table 1-1: Observed wavenumbers (cm-1) and assignments for ethylene adsorbed on
metal oxides [113]-[116]
ν1 (sC=C) ν2 (δCH2) ν7+ ν8 (2ωCH2)
Free 1625 1343 1888
Pt 1495 1199 ---
Rh 1499 1214 ---
Pd 1510 1241 ---
SiO2 1618 1340 1922
HY 1612 1338 1940
MgO --- 1338 1898
CaO --- 1342 1898
NiO --- 1338 1908
Fe2O3 1618 1340 1960
ZnO 1599 1340 1960
TiO2 1612 1341 1960
ZrO2 1610 1341 1975
In addition to the strong shift upwards of the out-of-plane deformation modes and the shift
downwards of the C=C stretching frequencies as well as of all CH stretching, the
30 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
perturbation caused by the surface-gas interaction can cause the activation on IR of the
modes that are Raman active in the CH stretching region.
Nevertheless, as said before, ethylene is very unstable when on a catalyst surface. On the
pathway in oxidation reactions, it is not easy to distinguish adsorbed ethylene as
intermediary species. The observation of carboxylate species that end up in the formation
of carbon dioxide, is more common. These species are mainly formates, acetates and
carbonates.
There are three main types of conformations of carboxylate groups that can be adsorbed
on the catalyst surface. Single-bonded species are usually denominated as ‘monodentate’
while double-bonded are called ‘bidentate’. Bidentate species can either be chelating, when
both oxygen atoms bind to one surface site, or bridged if the oxygen atoms bind to different
sites. When all three oxygen atoms are bound to the surface, the conformation is called
polydentate, or bulk, as it can be the case for carbonate species. The main carboxylate
species formed during the production of CO2 as well as their assigned IR bands are shown
in Figure 1-16 and Figure 1-17. Usually, bridged bidentate species are the most stable
conformations.
Figure 1-16: Experimental IR frequency bands of different types of formate species over metal oxides [comb1 stands for νas(COO) + δ(CH) and comb2 stands for νs(COO) + δ(CH)]
[117]
Catalytic oxidation of ethylene 31
Figure 1-17: Experimental IR frequency bands of different types of surface carbonate
species over metal oxides [118]
1.4 Scope
Total catalytic oxidation of ethylene is proposed as an alternative for its elimination within
the storage systems of horticultural products. However, the presence of an active catalyst
at low temperatures is a must for the development of the process at industrial scale. For
this purpose, ceria-based mixed oxides with gold as active phase are proposed as active
materials.
This work aims to contribute with the characterisation of ceria and ceria-zirconia-based
catalysts as active materials for the total oxidation of ethylene, and the elucidation of the
fluid-surface interactions that take place during the adsorption of the reactants in order to
propose a possible reaction pathway.
2. Cerium oxides as oxidation catalysts for ethylene elimination
Ceria-based catalysts were tested on the reaction of total oxidation of ethylene. Two kinds
of ceria supports were used, one was synthesised at the laboratory following a pseudo sol-
gel method, the other one was provided by Solvay-Rhodia. Gold was then deposited in both
supports at different compositions: 1% for synthesised support and 2% for the commercial
one. The catalysts composed with commercial supports will be referred in the text with this
symbol: *.
2.1 Catalyst synthesis
The different catalysts were synthesised according to the pseudo sol-gel method described
by Vargas et al. [119]. The method is based on the thermal decomposition of propionate
precursors. For the case of cerium oxide, the precursor salt was cerium (III) acetate hydrate.
The specifications of the precursor used are shown in Table 2-1. The necessary amount of
the organometallic salt was dissolved in propionic acid, which was used as modifier agent
with a cation concentration of 0.12 mol·L-1. The solution was kept under total reflux at 100
°C for 1 h. This procedure ensured the effective formation of the polymeric precursor in
solution. Then, the solvent was removed by controlled evaporation at atmospheric pressure
and an approximate temperature of 116-120 °C to obtain a yellow resin. The solvent
evaporation took around 3 h. Finally, the resin was calcined in air with a temperature ramp
of 2 °C·min-1 until reaching 500 °C and maintained 6 h at this temperature.
Table 2-1: Specifications of the precursor used for the Ce-based catalyst preparation
Reactive Supplier CAS Formula LOT
Cerium(III) acetate
sesquihydrate
Alfa Aesar 537-00-8 Ce(C2H3O2)3•1.5H2O E03W017
34 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
After obtaining the support, gold was subsequently deposited on the oxide by the method
of direct ion exchange (DAE) between the support and a solution of chloroauric acid
(HAuCl4) with a concentration of 10-4 mol·L-1. The quantity of the solution depended on the
concentration in weight of gold to be deposited. The solution was kept at 70 °C for 1.5 h.
Then, the solid was separated by filtration and washed several times with ammonia and de-
ionised water to remove chlorine, which improves acidity and prevents sintering, as
reported by Ivanova et al. [120]. Finally, the catalyst was calcined in air with a temperature
ramp of 5 °C·min-1 until reaching 300 °C and maintained 4 h at this temperature.
2.2 Characterisation
The ceria-based oxides were characterised by several techniques. The textural
characteristics were determined by N2 adsorption at 760 mmHg and -196 °C. The structural
properties were characterised using X-ray Diffraction (XRD) and Raman spectroscopy,
which were also useful to verify the effective formation of the fluorite-type structure.
2.2.1 Surface area – N2 isothermal adsorption-desorption
The surface area and the pore volume of the mixed oxides catalysts were determined by
N2 adsorption at -196 °C using a Micromeritics ASAP 2420 SurfaceArea and Porosity
Analyser. The samples were previously degasified during 1 night by heating up to 200 °C
in order to eliminate humidity. Total surface area was derived from the BET model, the total
pore volume (Vpore) was measured at P/P0 = 0.95 as a single point and the pore diameter
(Dpore) was calculated according to the formula: Dpore = 4Vpore/SBET, where SBET is the
BET surface area. The textural properties of CeO2, Au(1%)/CeO2, CeO2* and
Au(2%)/CeO2* are shown in Table 2-2.
Table 2-2: Textural characteristics of samples with synthesised and commercial (*) ceria
supports
SBET (m²·g-1) Vpore at P/P° = 0.95 (cm3·g-1) Dpore (nm)
Ceria 79.2 0.19 9.5
Au(1%)/Ceria 78.2 0.19 9.9
Ceria* 199.8 0.14 2.8
Au(2%)/Ceria* 203.1 0.15 2.9
Cerium oxides as oxidation catalysts for ethylene elimination 35
The introduction of gold as active phase does not have a significant effect on the textural
properties of the catalysts, this can be remarked on the fact that the variations on the BET
surface area when comparing the sample with or without gold, fall within the range of the
experimental error of the equipment used. However, it is clear that the ceria commercial
support has a smaller pore diameter than the synthesised one and, consequently, a higher
surface area. The differences in this property can be ascribed to the fact that the synthesis
method for both supports was different, taking into account that the so-called synthesised
supports were obtained via pseudo sol-gel method and the commercial ones were provided
by Rhodia. More specifically, it is very likely that calcination temperatures for both kind of
supports were different. It has been reported that, in general, an increase in the calcination
temperature of a catalyst leads to a decrease in the surface area [121].
2.2.2 Crystalline structure – XRD
The crystalline structure of the mixed oxides catalysts was determined by X-ray diffraction
(XRD) in a Brücker AXS-D8 Advanced equipment, furnished with a LYNXEYE detector with
a nickel filter in order to filtrate Cu-Kα radiation (λ= 1.5404 Å). The 2θ range scatter used
was 10º to 80º with a 0.05º step size at a scan rate of 5º·min-1. The crystallite size was
determined using the Debye-Scherrer equation (eq. 2.1).
𝐷 =𝐾∙𝜆
𝛽 ∙cos𝜃 (2.1)
Where K corresponds to the Scherrer constant (K= 0.9); λ corresponds to the wavelength
of the X-ray used (λ= 1.5404 Å); β is the line broadening at half the maximum intensity
(FWHM), after subtracting the instrumental line broadening, in radians, and θ is the Bragg
reflection angle in radians.
The cubic lattice parameter of the cubic phase-centred fluorite was calculated from the four
most intense diffraction peaks (111), (200), (220) and (311) according to equation 2.2,
considering a FCC system where h, k and l are the Miller indexes and d is the interplanar
distance.
𝑎 = 𝑑ℎ𝑘𝑙√ℎ2 + 𝑘2 + 𝑙2 (2.2)
36 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
The effective formation of the fluorite structure was verified. The XRD patterns for the ceria-
based catalysts are presented in Figure 2-1.
Figure 2-1: XRD patterns of ceria samples with synthesised and commercial (*) supports
For all the samples, diffraction peaks at 28.6°, 33.1°, 47.5°, 56.5°, 59.2° and 69.5° were
observed, which relate to the planes (111), (200), (220), (311), (222) and (400),
respectively. These peaks correspond to a fluorite cubic structure CeO2 (JCPDS 03-065-
5923). No diffraction peaks associated to gold as the active phase were detectable. They
would have corresponded to reflections at 38.5 and 44.8°, relating to (111) and (200) planes
of metallic gold (JCPDS 04-0784). The fact that Au0 is not detected in the samples suggests
that the metallic particle size obtained with the deposition technique used may be smaller
than the dimension required for detection (ca. 5 nm) [53]. The peaks of the commercial
ceria-based supports were broader than the synthesised ones, indicating a lower crystallite
size. Lower crystallite size tends to have a higher active metal dispersion which may lead
to better catalyst activity [122]-[124].
Cerium oxides as oxidation catalysts for ethylene elimination 37
Results for the cubic lattice parameter of the cubic phase-centred fluorite and the average
crystallite size are reported in Table 2-3. Since no significant differences on XRD patterns
from samples with and without gold were observed, it was suggested that the presence of
gold does not induce any modification of the support phase. However, the values for the
lattice parameter of samples with gold are somewhat higher than the ones observed for
ceria bare support. Although the content of gold is very low, a plausible explanation to this
could be due to the fact that deposited metallic gold (Au0) forms an interface with the ceria
support. In this thin interface, the existence of Au3+ is likely, due to the presence of oxygen
atoms on the ceria lattice. These Au3+ ions may partially attract oxygen atoms to the
surface, leaving some vacancies in the oxide bulk and resulting in a reduction of cerium
atoms [125]. Remember that the reduction process results in an increase on the ionic size
of cerium from Ce4+ (0.87 Å) to Ce3+ (1.01 Å). This explains the fact that patterns of the
samples with gold are slightly shifted to lower Bragg angles. It is also noticeable that the
crystallite size of commercial samples is smaller than for synthesised ones.
Table 2-3: Structural characteristics of CeO2, Au(1%)/CeO2, CeO2* and Au(2%)/CeO2*
oxides.
Catalyst Cubic lattice “a” (Å) Crystallite size (nm)
CeO2 5.39 9.0
Au(1%)/CeO2 5.40 9.0
CeO2* 5.42 4.7
Au(2%)/CeO2* 5.43 4.7
The above behaviour was already seen by Fu et al. [126], although they prepared gold-
ceria samples by a different synthesis method (deposition precipitation and coprecipitation),
they also noticed and increase in the lattice constant when gold was present. Also, they did
not observe any peaks corresponding to metallic gold for samples with gold content less
than 3%.
Results from XRD analysis allows to corroborate the effectiveness of the synthesis method
in the formation of the solid cubic phase of ceria.
38 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
2.2.3 Solid phase formation – Raman spectrocopy
The solid phase formation was studied by Raman spectroscopy. The Raman spectra were
recorded in a dispersive Horiva Jobin Yvon LabRam HR800 Microscope, with a He-Ne
green laser (532.14 nm) working at 5 mW, and with grating of 600 g.mm-1. The microscope
used a 50x objective and a confocal pinhole of 1000 μm.
The Raman spectra of CeO2 and CeO2* are presented in Figure 2-2. Raman spectra from
gold-ceria samples are not shown a very poor signal to noise (S/N) ratio. This could be
possible due to a high fluorescence interference from gold. Even weak fluorescence from
the sample is often much stronger than the Raman scattering, resulting in a large
background. Fluorescence can be reduced by selecting lasers in the UV or NIR region
[127], [128].
Figure 2-2: Raman spectra for synthesised and commercial (*) ceria
Bands at 260, 470 and around 600 cm-1 were noticed. The main band at 470 cm-1
correspond to a first order F2g Raman active mode of fluorite type lattice, which originates
from Ce–8O stretching vibrations. This mode in CeO2 shifts and broadens with decreased
particle size, as seen for the commercial support. The band at 260 cm-1 is present only in
ceria lattice with high defect concentration and it usually indicates the presence oxygen
Cerium oxides as oxidation catalysts for ethylene elimination 39
containing species on the surface. A weak and broad peak at about 600 cm-1 can be
assigned to the intrinsic oxygen vacancies [122], [129], [130].
The results obtained from the Raman spectra are found to be consistent with the results
obtained from the XRD patterns of CeO2 samples. The characterisation analysis allowed to
verify the effective formation of the fluorite-type structure of the ceria samples, leading to
the conclusion that the pseudo sol-gel synthesis method was effective.
2.3 Evaluation of the fluid-surface interactions
Evaluation of the catalyst was divided in two stages. First stage comprehended the study
of the adsorption-desorption mechanisms of C2H4 and O2 separately on the catalyst
surface, at room temperature. For this purpose, a TPD system was proposed, in order to
quantify the amount of gas adsorbed. Second stage was the identification of molecular
species that adsorbed on the surface during the reaction. This was done via operando IR
spectroscopy.
2.3.1 Study of the adsorption-desorption mechanisms of probe molecules C2H4 and O2 via TPD
The TPD system was composed of the gas flow set-up, where activation and reaction flows
could be prepared independently, reactor, furnace, and detector (Figure 2-3). In this case,
the detector was a Hiden Analytical QGA (quantitative gas analysis) mass spectrometer
system configured for continuous analysis of gases and vapours at pressures near
atmosphere in standard form.
Note that instead of having a 6-way valve joining the reactor, as in the majority of setups,
two 4-way valves were installed. This allowed to independently investigate either the carrier
flow of the analyte flow via MS, via bypass or reactor. Four configurations were possible,
as shown in Figure 2-4, (a) analysis of analyte gas when flowing via bypass, (b) analysis of
carrier gas when flowing through the reactor, (c) analysis of analyte gas when flowing
through the reactor, (d) analysis of carrier gas when flowing via bypass. This configuration
helped to create baselines in an easier way and to detect possible leakages in the system
during the reaction.
40 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
Figure 2-3: Scheme of the TPD setup used
Figure 2-4: Possible configurations of the valve system in the TPD setup
The procedure used was: during the activation, the sample was heated up to 300 °C with a
flow of 50 cc·min-1 of dry air and a ramp of 5 °C·min-1. Sample was kept at 300 °C for 1 h,
30 min under dry air and 30 min under argon. Then, the sample was cooled down under
argon, while the analyte gas (either pure O2 or ethylene with a concentration of 1000 ppm
in N2) was being sent via bypass. Once the reactor temperature reached 25 °C, the valve
was switched to send the analyte gas through the reactor with a flow of 25 cc·min-1 to
Cerium oxides as oxidation catalysts for ethylene elimination 41
perform the adsorption at room temperature. Once the analyte gas m/Z signal was stable,
meaning that adsorption sites were full or that adsorption did not take place at all, the carrier
gas was sent through the sample (either dry air or argon). After stabilisation of the signal,
the temperature was raised progressively up to 300 °C (8 °C·min-1) for analyte desorption.
The quantity of catalyst loaded was 50 mg.
Contrary to what was expected, according to the results by Ahn et. al. using a ceria-based
catalyst [44], no adsorption of ethylene or oxygen took place on ceria-based catalysts (with
or without gold) at room temperature (Figure 2-5).
Figure 2-5: Evolution of mass spectra signal intensities with time on stream during (a) ethylene desorption (b) oxygen desorption over Au(2%)/CeO2 sample
42 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
This could either mean that ethylene does not effectively adsorb on this kind of surfaces at
room temperature, or that its partial pressure was not high enough for the adsorption to
take place, taking into account that the concentration used was 1000 ppm. Remember that
according to Langmuir model [131], the dynamic process of adsorption can be considered
as (eq. 2.3):
𝐴(𝑔) + 𝑙 𝑘𝑎𝑑𝑠↔ 𝐴 − 𝑙 (2.3)
Where A(g) is the adsorbent, 𝑙 is a free surface site, kads is the velocity constant for the
adsorption process and kdes is the constant for the desorption process. According to this,
the global constant would be K= kads/kdes. If A(g) has a low partial pressure, then there is a
very low chance that the surface is covered by adsorbed species, since their permanence
highly depend on the reactant pressure.
2.3.2 Study of the intermediary species during ethylene total oxidation via operando IR spectroscopy
The operando system used for this study is provided in Figure 2-6; it is composed of four
main parts: the infrared spectrometer, the IR reactor-cell, the gas flow set-up and the
exhaust gas analysers. The reactor-cell is connected to the gas-system including mass flow
controllers for the introduction of gases into the lines. The two gas mixtures, so called the
activation and the reaction flow, can be prepared and sent independently to the reactor cell.
The system allows investigating the exhaust gases (reactive and/or reaction products) by
a Quadrupole Mass Spectrometer (Pfeiffer Omnistar GSD 301), while complementary
information on the gas phase can be gained by FTIR spectroscopy through a gas micro-
cell of 0.088 cm3 volume. The IR operando system is equipped with one single IR source,
thus, if complementary information on the gas phase is required, the IR beam is
alternatively sent to the catalyst or to the gas phase analysis through a mobile mirror [132].
Regarding the IR data, 32 scans were collected per spectrum at a time resolution of 1
spectrum each two minutes with a Thermo Scientific Nicolet 6700 spectrometer, equipped
with a MCT detector. Spectra were processed by the Nicolet OMNIC software.
𝑘𝑑𝑒𝑠
Cerium oxides as oxidation catalysts for ethylene elimination 43
Figure 2-6: Scheme of the operando system [132]
Figure 2-7: General and detailed views of the “sandwich” reactor-cell. 1=Spectrometer
base-plate, 2=IR cell support, 3=adjusting nut for air tightness, 4=gas outlet, 5=air cooling outlet, 6=gas inlet, 7=air cooling inlet, 8=external shell, 9=thermocouple location, 10=IR beam, 11=stainless steel ring, 12=Kalrez O-ring, 13=KBr windows, 14=wafer holder, 15=oven location [133]
44 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
For each experiment, the catalyst was pressed into a self-supported wafer with a diameter
of 16 mm and a mass of 20 mg approximately. The sample was first activated through an
oxidative procedure, rising the temperature to 287 °C with a ramp of 5 °C·min-1, and
maintaining at this temperature for 1 h. Then, the sample was cooled down to 25 °C in 40
min. During the cooling, the reaction mixture was prepared independently and sent via
bypass. Once the sample arrived 25 °C, the 6-way valve was switched to direct the reaction
flow through the cell. Stabilisation was reached after 15 min. The sample was afterwards
heated up to 287 °C with a ramp of 2 °C.min-1, to observe the reaction temperature. The
flow conditions were as follows: with a total flow rate of 25 cc·min-1, the activation flow was
composed of 20% of oxygen diluted in Argon and the reaction flow was composed by a
mixture 400 ppm C2H4 and of 20% oxygen diluted in Argon. The reaction was studied within
the temperature range from 25 to 287 °C.
Two ceria-based samples were tested. The first one, was a provided ceria support. The
second one, was the same support with 1% gold deposited as active phase. The exhaust
gas analysis showed that there is no production of CO2 when testing the support alone
(Figure 2-8). Common products from partial oxidation reactions were not detected either,
thus, oxidation does not take place. As for the IR data, the spectra before through the
reaction remained almost invariable, thus, no intermediary species were detected (Figure
2-9). A decrease in some peaks around 1000, 1380 and 1500-1600 cm-1 region is observed
but this could be due to a continuous subsequent desorption of trapped bulk carbonate
species. This kind of species could have been ‘trapped’ during the catalyst synthesis, more
specifically, during the evaporation of propionic acid producing bulk CO2 species inside the
catalyst matrix.
Cerium oxides as oxidation catalysts for ethylene elimination 45
Figure 2-8: Evolution of temperature and mass spectra signal (m/z= 28 for ethylene, 18 for water, 44 for CO2) intensities with time on stream [total flow 25 cc·min-1: 400 ppm C2H4
+ 20% O2 in Ar] when the reaction is performed over CeO2
Figure 2-9: IR spectra of CeO2 before (a) and after (b) the reaction
On the contrary, the catalyst with gold exhibited activity on the total oxidation of ethylene,
but only at high temperatures, starting to convert ethylene to CO2 at around 136 °C, as
shown in Figure 2-10. Results for operando IR spectroscopy during the oxidation reaction
on Au(1%)/CeO2 catalyst, are reported in Figures 2-11 to 2-14.
800130018002300280033003800
Ab
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(a.
u.)
Wavenumber (cm-1)
46 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
Figure 2-10: Evolution of temperature and mass spectra signal (m/z= 28 for ethylene, 18 for water, 44 for CO2) intensities with time on stream [total flow 25 cc·min-1: 400 ppm C2H4
+ 20% O2 in Ar] when the reaction is performed over Au(1%)/CeO2
The first remark that can be made from Figure 2-10 is that periodic phenomena were
observed during the reaction (small oscillations on the m/Z signals). This situation was
already observed by Bazin et al. [132], who stated that, since the oscillation period
correlated with the infrared beam scanning duration, they were clearly due to the periodic
infrared beam heating of catalyst surface, equivalent to a thermal effect of +3.5 °C. This is
due to gold nanoparticles sensitivity; as a result, a slight change in the temperature may
influence the reactivity.
The complete IR spectrum (800-4000 cm-1) of the catalyst surface before and after the
reaction is shown in Figure 2-11. Initial bands around 850, 1400-1350 and 1480-1420 cm-1
are detected. These bands can be attributed to the presence of bulk polydentate carbonate
species present within the catalyst framework. The existence of carbonate species before
the reaction can be due to the fact that CO2 could be trapped in the catalyst framework
during the synthesis or calcination procedures. The broad band around 3600 cm-1 is due to
the presence of surface hydroxyl species, which is common in the most of oxide catalysts
[135].
Cerium oxides as oxidation catalysts for ethylene elimination 47
Figure 2-11: IR spectra of Au(1%)/CeO2 before (a) and after (b) the reaction
Figures 2-12 to 2-14 show the stacked IR spectra of the catalyst surface during the
operando experience. Looking into close detail to the region between 800-1150 cm-1
(Figure 2-12), the formation of weak bands around 1105 and 890 cm-1 can be observed.
These bands can be related to the adsorption of dioxygen species onto the surface. They
are associated to the formation of superoxide and peroxide species, respectively. These
bands only appeared at temperatures above 80 °C, which indicates the absence of surface
oxygen species at room temperature. It is clear how these bands attenuated and
disappeared as the sample was progressively heated. This could be due to dissociation of
oxygen molecular species into atomic species (O, O-) and later migration into the ceria
lattice [136]. This confirms what was already seen in Section 2.3.1, in which there was no
evidence of oxygen adsorption at room temperature. It is common within reactions
performed with mixed oxides catalyst that lattice oxygen is consumed for the formation of
products while this is constantly regenerated by molecular oxygen coming from the
reactants. The rise of a strong band at 1070 cm-1 which can be associated to the formation
of surface carbonate species, is also observed.
Higher bands are seen at 2933, 2842, 1545, 1379, 1356 cm-1 (Figure 2-13 and Figure 2-
14). These can be readily attributed to surface formate species [137]. These bands grew
significantly when the sample was heated on the presence of C2H4 and O2, reaching their
maximum at around 194 °C, and then followed by a steeped decrement. It is possible that
these surface formate species are partly oxidised to surface carbonate species at
temperatures above 200 °C.
800130018002300280033003800
Ab
sorb
ance
(a.
u.)
Wavenumber (cm-1)
48 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
Figure 2-12: IR stacked spectra (800-1150 cm-1) of Au(1%)/CeO2 (a) just before the
reaction; (b) 0 min, 25 °C; (c) 20 min, 35 °C; (d) 40 min, 80 °C; (e) 1 h, 126 °C; (f) 1 h 25 min, 171 °C; (g) 1 h 45 min, 214 °C; (h) 2 h 10 min, 259 °C; (i) 2 h 30 min, 280 °C; [total flow 25 cc·min-1: 400 ppm C2H4 + 20% O2 in Ar]
Finally, the bands at 1430 and 1570 cm-1 correspond to the symmetric and asymmetric
stretching of the CO2- bridging mode, respectively, which can be associated to the formation
of bidentate carbonate species. The ‘negative’ peaks around 1140 and 1180 cm-1 are
related to the consumption of surface OH groups [138].
Cerium oxides as oxidation catalysts for ethylene elimination 49
Figure 2-13: IR stacked spectra (1200-1700 cm-1) of Au(1%)/CeO2 (a) just before the
reaction; (b) 0 min, 25 °C; (c) 20 min, 35 °C; (d) 40 min, 80 °C; (e) 1 h, 126 °C; (f) 1 h 25 min, 171 °C; (g) 1 h 45 min, 214 °C; (h) 2 h 10 min, 259 °C; (i) 2 h 30 min, 280 °C; [total flow 25 cc·min-1: 400 ppm C2H4 + 20% O2 in Ar]
Figure 2-14: IR stacked spectra (2800-3000 cm-1) of Au(1%)/CeO2 (a) just before the
reaction; (e) 1 h, 126 °C; (f) 1 h 25 min, 171 °C; (g) 1 h 45 min, 214 °C; (h) 2 h 10 min, 259 °C; (i) 2 h 30 min, 280 °C; [total flow 25 cc·min-1: 400 ppm C2H4 + 20% O2 in Ar]
50 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
Taking into account the above considerations, a basic reaction pathway according to the
IR results can be proposed. Although it is not clear how or where ethylene adsorption takes
place, it is clear that it breaks to form two kinds of surface species: formates and
carbonates. These two intermediary species are the precursor for the subsequent formation
of CO2. Lattice oxygen is being constantly regenerated by molecular oxygen that comes
from the gas stream. Water is easily formed due to the initial presence of hydroxyl species
on the catalyst surface. These hydroxyl species are constantly being regenerated, as well,
by the dehydrogenation of ethylene.
If a detailed description with elementary steps is desired, two types of mechanisms are
possible. The first one is a non-competitive mechanism where ethylene and oxygen adsorb
on different active sites. The active sites are represented by 𝑙1 and 𝑙2, where 𝑙1=ceria and
𝑙1=Au, or vice versa.
1) 3[𝑂2 + 2𝑙1 → 2𝑂 − 𝑙1]
2) 𝐶2𝐻4 + 6𝑙2 → 2𝐶 − 𝑙2 + 4𝐻 − 𝑙2
3) 2𝐶 − 𝑙2 + 4𝑂 − 𝑙1 → 2𝐶𝑂2 + 4𝑙1 + 2𝑙2
4) 4𝐻 − 𝑙2 + 2𝑂 − 𝑙1 → 2𝐻2𝑂 + 4𝑙2 + 2𝑙1
The second one, is a competitive mechanism where both ethylene and oxygen adsorb onto
the same active site.
1) 3[𝑂2 + 2𝑙1 → 2𝑂 − 𝑙1]
2) 𝐶2𝐻4 + 6𝑙1 → 2𝐶 − 𝑙1 + 4𝐻 − 𝑙1
3) 2𝐶 − 𝑙1 + 4𝑂 − 𝑙1 → 2𝐶𝑂2 + 6𝑙1
4) 4𝐻 − 𝑙1 + 2𝑂 − 𝑙1 + 6𝑙2 → 4𝐻 − 𝑙2 + 2𝑂 − 𝑙2 + 6𝑙1
5) 4𝐻 − 𝑙2 + 2𝑂 − 𝑙2 → 2𝐻2𝑂 + 6𝑙2
In both competitive and non-competitive cases, it is possible that the adsorbed species
migrate to the ceria surface by spillover phenomena.
The most feasible detailed reaction pathway is represented in Figures 2-15 and 2-16.
Ethylene is adsorbed and activated on the metallic gold surface. Dehydrogenation takes
place and the given species migrate to the ceria surface by spillover mechanisms. Oxygen
adsorption takes place directly on the ceria surface.
Cerium oxides as oxidation catalysts for ethylene elimination 51
Figure 2-15: Proposed reaction mechanism for ethylene total oxidation over
Au(1%)/CeO2 (part a)
52 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
Figure 2-16: Proposed reaction mechanism for ethylene total oxidation over
Au(1%)/CeO2 (part b)
The proposed reaction mechanism can be described in detail by the following steps:
1) Ethylene adsorption on gold (π-bonded)
2) Ethylene dehydrogenation and π-bond weakening
3) Hydrogen migration to the ceria surface by spillover mechanisms to form water by
associating with surface Ce-OH species (consumption of OH groups), and ethylene
C=C bond cleavage to form two C-H adsorbed species
4) Oxygen rearrangement on the ceria lattice
5) C-H group migration to the ceria surface by spillover mechanisms and formation of
formate species
6) Partial oxidation of formates to produce adsorbed carbonate species
7) CO2 formation and desorption
Cerium oxides as oxidation catalysts for ethylene elimination 53
8) Regeneration of ceria lattice oxygen by adsorption and dissociation of molecular
oxygen coming from the reactive atmosphere, following a Mars van Krevelen
mechanism
In conclusion, gold has a crucial role on the performance of the catalyst, since the support
alone does not show any activity on the reaction of ethylene oxidation. Moreover, it is likely
that the dissociative adsorption of oxygen takes place on the gold metallic sites and
regeneration of ceria lattice oxygens takes place by migration of adsorbed oxygens by
spillover mechanisms. Besides gold, three main active sites on the ceria surface get
involved during the oxidation reaction. These include surface hydroxyl species, surface
oxygen species and generated vacancies. The presence of these species is also vital for
the reaction to take place since hydroxyl species favour the later formation of water and
vacancies are necessary in order for the Mars van Krevelen oxidation-reduction cycle to
take place.
3. Cerium-zirconium oxides as oxidation catalysts for ethylene elimination
Ceria-zirconia mixed oxides were tested on the reaction of total oxidation of ethylene. Two
kinds of ceria-zirconia supports were used, one was synthesised at the laboratory following
a pseudo sol-gel method, the other one was provided by Solvay-Rhodia. The mass
CeO2/ZrO2 ratio was 0.8/0.2 for the synthesised support and 0.6/0.4 for the commercial
one. Gold was then deposited in both supports at different compositions: 1% for
synthesised support and 2% for the commercial one. The catalysts composed with
commercial supports will be referred in the text with this symbol: *.
3.1 Catalyst synthesis
Cerium-zirconium mixed oxide catalysts were synthesised following the same procedure
mentioned in section 2.1, according to the pseudo sol-gel like method described by Vargas
et al. [119]. In this case, the salts used were cerium (III) acetate hydrate and zirconium (IV)
acetylacetonate. The specifications of the precursors used are shown in Table 3-1. Note
that the amount of the organometallic salt of zirconium depends on the final molar
composition desired. The salts were dissolved separately in propionic acid, kept under total
reflux at 100 °C for 1 h, and subsequently mixed. The solvent was removed by controlled
evaporation at atmospheric pressure and an approximate temperature of 116-120 °C to
obtain a yellow resin, which was finally calcined at 500 °C. Procedure for gold deposition
was the same as described in section 2.1. The global procedure is represented in Figure
3-1.
56 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
Table 3-1: Specifications of the precursors used for the Ce-Zr-based catalyst preparation
Reactive Supplier CAS Formula LOT
Cerium(III) acetate
sesquihydrate
Alfa Aesar 537-00-8 Ce(C2H3O2)3•1.5H2O E03W017
Zirconium(IV) 2,4-
pentanedionate
Alfa Aesar 17501-44-9 C20H28O8Zr G31Y032
Figure 3-1: Global Au/CexZr1-xO2 catalyst synthesis procedure
3.2 Characterisation
The ceria-zirconia mixed oxides were characterised by several techniques. The textural
characteristics were determined by N2 adsorption at 760 mmHg and -196 °C. The structural
properties were characterised using X-ray Diffraction (XRD) and Raman spectroscopy,
which were also useful to verify the effective formation of the fluorite-type structure.
For details concerning the characterisation analysis and calculations performed, please
refer to Section 2.2.
3.2.1 Surface area – N2 isothermal adsorption-desorption
The textural properties of CZ, Au(1%)/CZ, CZ* and Au(2%)/CZ* mixed oxides are shown in
Table 3-2.
Cerium-zirconium oxides as oxidation catalysts for ethylene elimination 57
Table 3-2: Textural characteristics of samples with synthesised and commercial (*) ceria-
zirconia supports
SBET (m²/g) Vpore at P/P° = 0.95 (cm3/g) Dpore (nm)
CZ 67.1 0.07 4.1
Au(1%)/CZ 62 0.06 4.1
CZ* 52.8 0.15 11.1
Au(2%)/CZ* 59.7 0.15 10.4
Contrary to what was seen for the ceria-based catalysts, the introduction of gold has a
remarkable effect on the BET surface area of the CZ catalysts, more specifically, a
reduction of the surface area for synthesised samples, and an increase for commercial
ones. A decrease on this property can be explained by the fact that the material was
subjected again to heat treatment after gold deposition. It is not clear why the opposite
happens with the commercial support, although it may be related to the fact that both
supports have different contents of gold. It seems then that for the latter case, the
introduction of gold has a favourable effect on the textural properties of the catalyst, even
after the heat treatment (calcination after gold deposition).
For the four samples it is clear that the surface area is low when compared to ceria-based
catalysts from Table 2-3. This behaviour was already observed by Gómez-García [139]
when synthesising the samples via co-precipitation method. There is no evident explanation
for this phenomena and it would be necessary to prepare more sample varying the Ce/Zr
ratio in order to make more general conclusions.
In this case, it seems to be that the introduction of zirconium into the fluorite matrix as
“doping agent” does not have a favourable effect on the textural properties of the catalyst,
since it induces a loss in the surface area.
3.2.2 Crystalline structure – XRD
The formation of the fluorite structure was verified by XRD. The XRD patterns for the ceria-
zirconia mixed oxides catalysts are presented in Figure 3-2.
58 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
Figure 3-2: XRD patterns of ceria-zirconia samples with synthesised and commercial (*)
supports. Composition (wt%) Ce/Zr for CZ=0.8/0.2, for CZ*=0.6/045.
For the CZ and Au(1%)/CZ samples, diffraction peaks at 28.8°, 33.5°, 48.0°, 57.0°, 59.9°
and 69.5° were observed, which relate to the planes (111), (200), (220), (311), (222) and
(400) of the FCC structure, respectively. Although in the case of CZ* and Au(2%)/CZ*
samples these peaks are shifted to higher Bragg angles, all samples exhibit XRD patterns
similar to cubic CeO2 (JCPDS 034-0394), indicating the integration of zirconium into the
CeO2 cubic lattice forming a solid solution. Also, no diffraction peaks associated to the
presence of tetragonal or monoclinic zirconia were observed.
No diffraction peaks associated to gold as the active phase were detectable. They would
have corresponded to reflections at 38.5 and 44.8°, relating to (111) and (200) planes of
metallic gold (JCPDS 04-0784). The fact that Au0 is not detected in the samples suggests
that the metallic particle size obtained with the deposition technique used may be smaller
than the dimension required for detection (ca. 5 nm) [53]. The peaks of the synthesised
ceria-zirconia based supports were broader than the commercial ones, indicating a lower
crystallite size.
Cerium-zirconium oxides as oxidation catalysts for ethylene elimination 59
Results for the cubic lattice parameter of the cubic phase-centred fluorite and the average
crystallite size are reported in Table 3-3. For comparison, the structure of all the catalysts
was assimilated to a cubic structure and the pseudo-cubic lattice parameter has been
calculated. No significant differences on XRD patterns and on lattice parameters from the
samples with and without gold were observed, which suggested that the introduction of gold
does not induce any modification of the support phase. From Figure 3-2 it can be seen that
the diffraction peaks were shifted systematically to higher Bragg angles with increasing
ZrO2 content. This observation was attributed to the shrinkage of the lattice due to the
replacement of Ce4+ (0.87 Å) with smaller cation radius Zr4+ (0.72 Å), and can be confirmed
when comparing the lattice parameters of both supports with different Zr content.
Table 3-3: Structural characteristics of CZ, Au(1%)/CZ, CZ* and Au(2%)/CZ mixed oxides.
Catalyst Cubic lattice “a” (Å) Crystallite size (nm)
Ce0.8Zr0.2O2 5.36 5.4
Au(1%)/Ce0.8Zr0.2O2 5.36 5.1
Ce0.5Zr0.5O2* 5.29 8.0
Au(2%)/ Ce0.5Zr0.5O2* 5.29 8.1
The above behaviour was already remarked by Araque [52] and Gómez-García [139] who
observed that the cubic lattice parameter diminished with an increase of the the zirconium
content in the sample.
Differences on the crystallite size of synthesised and commercial samples might be due to
the difference on the synthesis methods.
3.2.3 Solid phase formation – Raman spectroscopy
The presence of a tetragonal phase in the mixed oxides synthesised was studied by Raman
spectroscopy. The Raman Spectra of CZ and CZ* are presented in Figure 3-3.
60 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
Figure 3-3: Raman spectra for synthesised and commercial (*) ceria-zirconia (CZ)
Bands at 185, 303, 470 and 629 cm-1 were noticed. The main band at 470 cm-1 correspond
to a first order F2g Raman active mode of fluorite type lattice, which originates from Ce–8O
stretching vibrations. This mode in CeO2 shifts and broadens with decreased particle size,
as seen for the commercial support. The band at 303 cm-1 has been attributed to the
tetragonal substitution of oxygen atoms from the ideal fluorite lattice, while the band at 630
cm-1 has been attributed to the presence of oxygen vacancies. The high incorporation of Zr
atoms in the ceria network generated a tetragonal phase-like distortion, which is observed
in the intensity and location of these bands, although this distortion does not mean a
modification of the cubic solid phase.
The results obtained from the Raman spectra can be compared with those obtained by
Araque [52], who observed the same peaks for CZ samples with different mass ratios. They
are also found to be consistent with the results obtained from the XRD patterns of CZ
samples. The characterisation analysis allowed to verify the effective formation of the
fluorite-type structure of the ceria-zirconia samples, leading to the conclusion that the
pseudo sol-gel synthesis method was effective.
Cerium-zirconium oxides as oxidation catalysts for ethylene elimination 61
3.3 Evaluation of the fluid-surface interactions
The study of the adsorption-desorption mechanisms of C2H4 and O2 separately on the
catalyst surface was made via TPD. The identification of molecular species that adsorbed
on the surface during the reaction was done via operando IR spectroscopy.
For details concerning the TPD and operando IR systems, please refer to Section 2.3.
3.3.1 Study of the adsorption-desorption mechanisms of probe molecules C2H4 and O2 via TPD
Similar to what was seen for ceria-based catalyst, no adsorption of ethylene or oxygen took
place on ceria-zirconia based catalysts (with or without gold) at room temperature. As for
the ceria case, this could either mean that ethylene does not effectively adsorb on this kind
of surfaces at room temperature, or that its partial pressure was not high enough for the
adsorption to take place, taking into account that the concentration used is 1000 ppm.
Further tests can be made by increasing the partial pressure of ethylene using a higher
concentration in order to increase the adsorption constant.
3.3.2 Study of the intermediary species during ethylene total oxidation via operando IR spectroscopy
Two ceria-zirconia based samples were tested. The first one, was the commercial CZ*
support. The second one, was the same support with 2% gold deposited as active phase.
The exhaust gas analysis showed that there is no production of CO2 when testing the
support alone, thus, oxidation does not take place. On the contrary, the catalyst with gold
exhibited a good performance on the total oxidation of ethylene, but only at high
temperatures, starting to convert to CO2 at around 106 °C, as shown in Figure 3-4. Results
for operando IR spectroscopy during the reaction on Au(2%)/CZ* catalyst, are reported.
62 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
Figure 3-4: Evolution of temperature and mass spectra signal (m/z= 28 for ethylene, 18 for water, 44 for CO2) intensities with time on stream [total flow 25 cc·min-1: 400 ppm C2H4
+ 20% O2 in Ar] when the reaction is performed over Au(2%)/CZ*
The complete IR spectrum (800-4000 cm-1) of the catalyst surface before the reaction is
shown in Figure 3-5. The initial spectrum has carbonate characteristic IR signatures: with
doublet bands at about 1540 and 1450 cm-1 (derived from the asymmetric stretching
vibration, ν3, of the unconstrained CO32– ion) and a singlet band at 880 cm–1 (the out-of-
plane bending vibration, ν2) [140]. The existence of carbonate species before the reaction
can be due to the fact that CO2 could be trapped in the catalyst framework during the
synthesis or calcination procedures. The band centred at 1720 cm-1 can be ascribed to a
bridged CO site, in which the carbonyl oxygen atom interacts with a cerium cation of the
support [141]. This can be due to adsorption of CO present in the air [142]. Two bands at
around 2920 and 2850 cm-1 are assigned to the asymmetrical and symmetrical stretching
vibrations of CH2 groups The broad band around 3600 cm-1 is due to the presence of
surface hydroxyl species [135].
Cerium-zirconium oxides as oxidation catalysts for ethylene elimination 63
Figure 3-5: IR spectrum of Au(2%)/CZ* before (a) and after (b) the reaction
Figure 3-6 show the stacked IR spectra of the catalyst surface during the operando
experience on the region 1200-1700 cm-1, where changes on the IR bands are more
evident. First thing to remark is the disappearing of the band around 1525 cm-1, which can
be associated to the desorption of carbonate species in the catalyst present due to the
synthesis method. Meanwhile, the formation of weak bands around 1546, 1370, 1358 cm-1
can be observed. These can be related to the formation of formate species and correspond
to the vibrational modes of νas(CO2-), δ(CH) and νsy(CO2
-), respectively [137]. Nevertheless,
the band at 1358 cm-1 can also be assigned to νsy(CO32-) of carbonate species, along with
the band near 1430 cm-1 and 1070 cm-1.
800130018002300280033003800
Ab
sorb
an
ce (
a.u
.)
Wavenumber (cm-1)
64 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
Figure 3-6: IR stacked spectra (1200-1700 cm-1) of Au(2%)/CZ* (a) just before the
reaction (b) 0 min, 25°C (c) 20 min, 35 °C (d) 40 min, 80 °C (e) 1 h, 123 °C (f) 1 h 25 min, 167 °C (g) 1 h 45 min, 211 °C (h) 2 h 5 min, 252 °C (i) 2 h 25 min, 280 °C [total flow 25 cc·min-1: 400 ppm C2H4 + 20% O2 in Ar]
Although formate and carbonate bands are difficult to distinguish individually due to
possible overlapping, one can conclude that the ceria-zirconia based catalyst has a similar
reaction mechanism to the ceria-based one, making a special emphasis on the formation
of intermediary formates and carbonates for the later transformation and desorption into
CO2, as previously shown in Section 2.3.2, Figures 2 15 and 2-16.
In conclusion, as seen for the ceria-based samples, gold has a crucial role on the
performance of the ceria-zirconia based catalysts, since the CZ support alone does not
show any activity on the reaction of ethylene oxidation. Ceria-zirconia based oxides also
follow a Mars van Krevelen oxidation-reduction mechanism during ethylene total oxidation
reaction.
4. Adsorbent (zeolite) + catalyst: a promissing alternative
Zeolites are nanoporous crystalline aluminosilicates that present a specific framework
structure with channels on its inside. These channels allow a variety of applications in many
fields such as catalysis and ion exchange. There are many types of zeolites, around 40 on
its mineral form and more than 150 synthetic ones, which vary depending on its framework
structure. The most important mineral type commercially are chabazite, faujasite and
mordenite, and the most popular synthetic ones are types A and X, synthetic mordenite and
their ion-exchanged varieties.
The structure of a zeolite is basically composed of silicon and aluminum atoms bonded via
oxygen. Given that an O atom has a charge of -2, while a Si atom has a charge of +4, a
pure free-defect SiO2 structure would have a neutral charge. However, an atom of Al has a
charge of +3, thus, aluminosilicate structures like zeolites have an unbalanced charge
distribution. This is compensated by positive ions, generally alkali metals Na+, K+, alkaline
metals, Ca2+, Ba2+, and protons H+, which adsorb to the surface. Zeolites are highly
hygroscopic, so besides metal cations they might also contain hydroxyl groups in their
pores or surface [143], [144].
The ‘sodalite cage’ is a common framework structure constructed of silicon, aluminium and
oxygen atoms. Figure 4-1 shows how zeolites are comprised from this sodalite cages to
form different structures of pores and rings. Each line represents a bridging oxygen while
the intersections locate the tetrahedral atoms [145].
66 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
Figure 4-1: Schematic of different zeolite structures [143]
Zeolites have been widely used as adsorbents in separation and purification processes in
the past decades due to their high cation-exchange ability as well as to the molecular sieve
properties [146], [147]. This is because their pores are uniform in size and they are in the
same size range as small molecules, so they can discriminate molecules on the basis of
size [144]. In addition to changes to the cationic structure, the Si/Al ratio can be varied.
Thus, zeolites may be tailored with widely different adsorptive properties, by the appropriate
choice of framework structure, cationic form, and silica-to-alumina ratio in order to achieve
the selectivity required for a given separation. As said before, they have a high affinity by
polar molecules, like water, but they can be hydrophobic when the Si/Al ratio is increased.
Hydrophobic zeolites are generally used in the removal of volatile organic compounds from
air [148].
As mentioned in chapter 1, a lot of C2H4-adsorbing substances such as potassium
permanganate (KMnO4), activated carbon and minerals (zeolites and clays) have been
characterised and used, though the search has been focused considerable attention on
zeolites [149].
Adsorbent (zeolite) + catalyst: a promissing alternative 67
Taking into account the fact that the catalyst proposed in this work is not active at room
temperature, it is proposed an enhanced strategy in which a regenerable adsorbent (a
zeolite) is used before the catalyst to ‘trap’ or adsorb ethylene at room temperature. Once
the zeolite is saturated with ethylene, it is heated under flow of hot gas for the ethylene to
desorb and the reaction to take place at the adequate temperature on the catalyst surface.
The zeolite used on the experiments was a Zeolite type A synthesised from a mining
residue, which characterisation is reported by Medina [150].
The procedure was the same as described at Section 2.3.1 with the only difference that 50
mg of zeolite were loaded before the catalyst bed.
All catalysts (ceria-based and ceria-zirconia based) were tested, but only the two with the
highest conversion upon the use of this system are shown, which are Au(1%)/CeO2 and
Au(2%)/CZ*. Zeolite A was also separately tested in order to quantify the amount of
ethylene that it can trap. TPD profile for zeolite A is reported in Figure 4-2. TPD profiles for
both catalysts are reported in Figure 4-3 and Figure 4-4. Two tests were performed for each
catalyst, the difference was the carrier gas under which desorption took place (air or argon).
Figure 4-2: Evolution of mass spectra signal (m/z= 27 for ethylene, 44 for CO2) intensities
with time on stream during ethylene desorption under air over Zeolite A
68 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
The TPD profile for zeolite A shows that the MS signal intensity for CO2 remains constant
during the whole process, meaning there is no formation of this production during the
heating and desorption process. This indicates that zeolite is not active for ethylene
oxidation, it only serves as ‘trap’ to adsorb it, as mentioned before.
On the opposite, the TPD profiles for both catalysts show that there is effective formation
of CO2 no matter the carrier gas utilised. Moreover, although the form of the peaks are
different when the carrier gas is different (and this is due to slight differences on the flow
conditions), the area is the same, thus, the quantity of desorbed unreacted ethylene and
desorbed CO2 is the same. This proves that the oxygen consumed during the oxidation
reaction is not the one that comes from the air but the one coming from the catalyst lattice.
These lattice oxygens have to be subsequently regenerated by the molecular oxygen that
comes from the air, either way, the reaction won’t keep taking place.
It can be seen from Figures 4-3 and 4-4 that, for both catalysts, CO2 signal is composed of
two peaks. This can be explained by the fact that CO2 formation can be either via two
intermediary species: formate or carbonate.
Table 4-1 shows a summary of the results of all TPD experiences done. It can be seen from
the results that conversions up to 58% can be achieved with this enhanced strategy.
Table 4-1: Summary of the results of the TPD experiences for zeolite A under air, zeolite
A +Au/CeO2 (under air and argon), and zeolite A +Au/CZ* (under air and argon)
mL adsorbed C2H4 /mg zeolite (x10-4)
mL desorbed C2H4 /mg zeolite (x10-4)
mL produced CO2 /mg cat (x10-3)
Ethylene conversion (%)
ZA (air) 11.5 ± 1.7 11.5 ± 1.7 ---- ----
ZA +Au/CeO2 (air) 9.03 ± 0.14 3.90 ± 0.58 1.03 ± 0.15 57%
ZA +Au/CeO2 (argon) 8.82 ± 0.13 2.87 ± 0.26 1.05 ± 0.10 57%
ZA +Au/CZ* (air) 12.3 ± 0.18 5.17 ± 0.47 1.43 ± 0.21 58%
ZA +Au/CZ* (argon) 12.4 ± 0.12 5.30 ± 0.50 1.37 ± 0.13 56%
Adsorbent (zeolite) + catalyst: a promissing alternative 69
Figure 4-3: Evolution of mass spectra signal (m/z= 27 for ethylene, 44 for CO2) intensities
with time on stream during ethylene desorption (a) under air (b) under argon over Zeolite
A + Au(1%)/CeO2 system
70 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
Figure 4-4: Evolution of mass spectra signal (m/z= 27 for ethylene, 44 for CO2) intensities
with time on stream during ethylene desorption (a) under air (b) under argon over Zeolite A + Au(2%)/CZ* system
Adsorbent (zeolite) + catalyst: a promissing alternative 71
Ethylene conversion (X) was calculated by assuming that the initial moles of ethylene
(adsorbed) were equal to the sum of the desorbed moles of ethylene and the ones
consumed during the reaction. This was done due to the fact that it was not possible to
correctly calculate the peak of adsorbed ethylene with the technique, it was only possible
to quantify the desorption peak. Consumed moles of ethylene were calculated based on
stoichiometric calculations with respect to desorbed CO2. It is interesting to see that
although the quantity of adsorbed ethylene should be the same for all the systems (taking
into account that any of the catalyst adsorbs ethylene at room temperature), it appears to
be higher for systems with a zirconia. There is no evident explanation for this phenomena,
although it could be due to slight differences on the time during which the carrier gas was
flown through the system before heating.
According to the results, it appears to be that the enhanced adsorbent+catalyst strategy
proved to be effective for ethylene removal and oxidation in environments of low ethylene
concentration, although it does no achieve total conversion. Further recommendations to
enhance the strategy would include changing the type of zeolite used, to increase the
quantity of trapped ethylene, or increasing the carrier gas temperature very fast so that the
desorption of ethylene from the zeolite takes place at the same time as the catalyst
becomes active. This could help to avoid losses from ethylene that desorbs but does not
react, since the temperature is still not high enough on the catalyst surface. Adsorption
enhancement can be also achieved by the co-incorporation of Au and metal oxide into the
zeolite matrix [151].
5. Conclusions and recommendations
The so-called enhanced adsorbent+catalyst strategy in order to remove ethylene from
environments at low concentrations, such as the ones found in climacteric fruits storage,
proved to be effective for the purpose described. Depending on the catalyst used, the
system can achieve conversions of up to 58%. Other types of zeolites could be used to
enhance the quantity of ethylene that can be adsorbed. Further work should be made in
order to improve the technique and make it possible to quantify the adsorption peak of
ethylene.
The enhanced strategy was proposed due to the low interaction of the catalyst with ethylene
at room temperature. No effective adsorption of ethylene took place on the catalyst surface.
This could be due to the partial pressure of the ethylene in the flow stream, taking into
account the low concentration used (1000 ppm), which could not be enough for the
adsorption to take place, according to Langmuir mechanism.
Gold as active phase is fundamental for the activation of the ethylene and further oxidation
reaction. According to the IR spectra of the catalyst surface, formate and carbonate species
are primarily formed during the reaction. Both species are responsible for subsequent CO2
formation and desorption. No species associated to ethylene adsorption were detected.
This might be due to the low stability of adsorbed ethylene species, fast cleavage of the
double bond and transformation into formate and carbonate.
Ethylene oxidation follows a Mars van Krevelen mechanism, were oxygen in the products
(CO2 and H2O) comes from ceria structural oxygen. This is confirmed by the fact that the
reaction also takes place in the absence of atmospheric oxygen.
74 Evaluation of the surface and catalytic properties of the ethylene and oxygen
adsorption process on gold catalysts supported on mixed oxides CeZr
The presence of polydentate carbonate species in the catalyst before the reaction might be
due to CO2 trapped during the synthesis process. To avoid their presence, a heating
treatment is necessary before the use of the catalyst. It is possible that CO2 species get
constantly ‘trapped’ within the bulk of the catalyst when the ethylene oxidation takes places.
Therefore, a regeneration system (heating treatment after a specific time of use of the
catalyst) might be needed in order to avoid its accumulation.
The formation of the fluorite type structure was effectively verified for all types of oxides by
XRD and Raman spectroscopy. The insertion of gold as active phase does not have an
effect on the structural properties of the catalysts. The insertion of zirconia does not change
the original fluorite cubic-type structure of ceria although it has a clear effect on the lattice
parameter. The shrinkage of the lattice parameter is due to the differences in the Ce4+ and
Zr4+ ionic sizes. Tetragonal phase-like distortions may be seen for high contents of zirconia
(Ce/Zr>0.5).
Recommendations to improve ethylene adsorption on the ceria-based catalysts include the
increase of the ethylene concentration used. As for the adsorbent+catalyst strategy
proposed changing the type of zeolite used in order to increase the quantity of trapped
ethylene, would be an option. Adsorption enhancement can be also achieved by the co-
incorporation of Au and metal oxide into the zeolite matrix.
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