Site Switching from Di-σ Ethylene to π-Bonded Ethylene in the Presence of CoadsorbedNitrogen on Pt(111)

Jun Yin,† Michael Trenary,‡ and Randall Meyer*,†

Departments of Chemical Engineering and Chemistry, UniVersity of Illinois at Chicago, Chicago, Illinois 60607

ReceiVed: April 7, 2010; ReVised Manuscript ReceiVed: May 30, 2010

The formation of π-bonded ethylene on a nitrogen-covered Pt(111) surface was studied by reflection absorptioninfrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD). The observation with RAIRSof a peak at 975 cm-1 due to the dCH2 wagging (out of plane) mode is a clear indication of the presence ofπ-bonded ethylene on the surface. This form of ethylene desorbs from the surface at 216 K, a temperaturemuch lower than ethylene desorption from the clean surface. Density functional theory calculations confirmthat π-bonded C2H4 becomes thermoneutral compared with di-σ C2H4 when 0.25 ML of N atoms are coadsorbedon the Pt(111) surface.

Introduction

The adsorption and hydrogenation of ethylene on Pt surfaceshave been extensively studied as model systems for alkenehydrogenation.1-5 Under ultrahigh vacuum (UHV) conditionsbelow 240 K, ethylene is known to adsorb on Pt(111) via twoσ bonds to the metal surface. The di-σ bonded ethylene thendehydrogenates to the ethylidyne (CCH3) species upon anneal-ing. Although ethylidene (CHCH3) and surface ethyl specieshave been argued as relevant hydrogenation reaction intermedi-ates in RAIRS and kinetic studies, it is now generally appreci-ated that π-bonded ethylene is the critical species for theethylene hydrogenation reaction.4-7 Cremer and coworkersstudied ethylene hydrogenation on the Pt(111) surface using sumfrequency generation (SFG) under reaction conditions anddetermined that π-bonded ethylene is the dominant reactionintermediate that leads to the formation of ethane.7 Whereasvarying the concentration of di-σ bonded ethylene did not affectthe hydrogenation rate at 295 K, the coverage of ethylidynewas directly proportional to the presence of di-σ bonded ethylene,indicating that both ethylidyne and di-σ bonded ethylene arespectator species.8-10 In contrast, the π-bonded ethylene peak isunchanged by the coverage of ethylidyne. However, in Cremer’sexperiment, the features ascribed to π-bonded ethylene areweak, indicating that it is present as a minority species on thesurface.

Further study has revealed that two types of π-bondedethylene may potentially exist on the Pt(111) surface: phys-isorbed and chemisorbed. Physisorbed π-bonded ethylene canbe observed on the clean Pt(111) surface at low temperature,but above 52 K, it begins to convert to di-σ bonded ethylene.11

Chemisorbed π-bonded ethylene has been reported to form onPt(111) surfaces modified with such coadsorbates as oxygen12

and potassium or cesium.11,13,14 These studies have typicallyrelied on indirect methods for determining the presence ofπ-bonded ethylene. In addition, Kubota et al. have usedreflection absorption infrared spectroscopy (RAIRS) to detectπ-bonded ethylene on Pt(111) at 112 K in the presence of 10-3

Torr of ethylene.15 The π-bonded ethylene disappeared as soon

as the gas-phase ethylene was pumped away and UHV wasrestored. So far, there is no RAIRS evidence of chemisorbedπ-bonded ethylene on Pt(111) under UHV conditions. Here wedemonstrate that N atoms on the Pt(111) surface can stabilizeπ-bonded ethylene relative to di-σ bonded ethylene, a resultsupported by density functional theory (DFT) calculations. Inpromoting the stability of the form of ethylene that is reactivetoward hydrogenation on Pt(111), nitrogen is revealed to havethe desirable ability to alter the chemical properties of a catalystsurface subtly while not being a reactant itself.

Experimental Section

The experiments were performed in two separate UHVchambers using two different Pt(111) crystals. The TPD resultswere obtained in a chamber (chamber 1) with a base pressureof ∼1 × 10-10 Torr. Detailed description of the system canbeen found elsewhere.16 In brief, the system is equipped withlow-energy electron diffraction (LEED), an XPS system, and aquadrupole mass spectrometer (QMS) (UTI 100C) for TPD. Theheating rate in the TPD experiment was 2 K/s. The data weresmoothed using the Savitzky-Golay method. The RAIRSexperiments were performed in a second chamber (chamber 2)with a base pressure of ∼2 × 10-10 Torr. A detailed descriptionof this system can be found elsewhere.17 In brief, this UHVchamber is equipped for AES, LEED, and RAIRS using acommercial Fourier transform infrared (FTIR) spectrometer(Bruker IFS 66v/S). The IR beam enters and exits the UHVchamber through differentially pumped O-ring-sealed KBrwindows and passes through a polarizer before reaching theinfrared detector. An MCT (HgCdTe) detector and a SiC IRsource were used. In cases where the sample was annealed toa temperature > 90 K, the sample was cooled back to 90 Kbefore a spectrum was acquired. The background referencespectrum was also taken at 90 K. The resolution for the spectrais 4 cm-1. The Pt(111) surfaces were cleaned and judged freeof impurities by a standard procedure previously described.18

Ammonia (99.9992%), oxygen (99.998%), and ethylene (99.8%)were purchased from Matheson Trigas and used without furtherpurification.

The preparation of a well-ordered p(2 × 2)-N/Pt(111) surfacehas been discussed previously in detail.19 In brief, the 0.25monolayer (ML) N-Pt(111) surface was created by the oxida-

* Corresponding author. E-mail: rjm@uic.edu.† Department of Chemical Engineering.‡ Department of Chemistry.

J. Phys. Chem. C 2010, 114, 12230–1223312230

10.1021/jp103116m 2010 American Chemical SocietyPublished on Web 06/24/2010

tive dehydrogenation of ammonia. Ammonia and oxygen arecoadsorbed at 85 K, followed by annealing the sample to 400K, generating water (which desorbs) and leaving behind a p(2× 2)-N/Pt(111) surface with a nitrogen coverage of 0.25 ML.

Computational Methodology

The DFT calculations in this work were performed using theVienna ab initio simulation package (VASP).20,21 A plane-wavebasis set with a cutoff energy of 400 eV and ultrasoft Vanderbiltpseudopotentials (US-PP) were employed.22 Calculations wereperformed using the Perdew-Wang (PW-91) form of theexchange-correlation functional23 for thermodynamics of C2H4

adsorption. The (2 × 2) unit cell representing the system is fourlayers thick, in which the two uppermost metal layers wereallowed to relax and approximately five layers of vacuum wereused to separate the slabs. The Brillouin zone is sampled witha uniform 7 × 7 × 1 k-point grid (Monkhorst-Pack), asdetermined by convergence tests.24 In general, the structures’geometries were optimized within a convergence tolerance of10-3 eV.

Results and Discussion

Figure 1 shows the TPD results for m/e ) 28 and 27 afterexposing 0.8 L of ethylene to both clean and nitrogen-coveredPt(111). On clean Pt(111), the saturation coverage of ethylenegives only a single peak around 300 K in agreement withprevious work. Simultaneous to ethylene desorption, some di-σbonded ethylene is converted to ethylidyne. When C2H4 isexposed to a surface precovered by 0.25 ML of nitrogen atoms,two peaks are seen at 216 and 300 K. Additional features form/e ) 28 are observed above 400 K (but not for m/e ) 27),which are ascribed to N2 desorption (to be further discussed ina forthcoming publication).

Figure 2 shows RAIRS spectra following exposure of 0.8 Lof C2H4 to the p(2 × 2)-N/Pt(111) surface at 90 K. The mostcharacteristic and intense peak due to π-bonded ethylene15,25,26

is the dCH2 wagging (out-of-plane) mode, which appears at975 cm-1 in Figure 2. The peak at 2920 cm-1 is assigned to theCH2 symmetric stretching mode of di-σ bonded ethylene. Thepeak at 2964 cm-1 follows the same changes with coverageand temperature as the 975 cm-1 peak and is therefore assignedto a CH stretching mode of π-bonded ethylene. The highfrequency of the CH stretch peak at 3083 cm-1 indicates that itis also due to a form of π-bonded ethylene. The fact that thispeak disappears with annealing to only 130 K, whereas the 975and 2964 cm-1 peaks persists to 210 K, indicates that the 3083cm-1 is due to a form of π-bonded ethylene that is less stabilizedby the presence of N atoms on the surface. The π-bondedethylene species disappears upon annealing to 210 K, whereasdi-σ bonded ethylene remains on the surface before ultimatelyconverting to ethylidyne at 310 K. Therefore, we assign the eth-ylene desorption peak at 216 K in TPD to the π-bonded ethylenespecies because the dCH2 wagging mode of π-bonded ethyleneis diminished at the same temperature.

Previous DFT calculations suggest that π-bonded ethyleneoccupies an atop site on Pt(111),27 in analogous fashion to itsbonding in organometallic clusters.28 Therefore, one possibleexplanation for the observation of π-bonded ethylene at lowethylene coverages on the p(2 × 2)-N/Pt(111) surface is that Natoms block the preferred adsorption sites for di-σ bondedethylene. However, our DFT calculations indicate that nitrogenatoms occupy fcc three-fold hollow sites29 whereas di-σ bondedethylene bonds atop/atop to the Pt(111) surface in agreementwith earlier experiments,30,31 allowing coadsorption of 0.25 MLof C2H4 on the N-Pt(111) surface. This implies that a simplesite blocking mechanism is not likely to control the adsorptionof C2H4. The calculations also confirm that whereas adsorptionof di-σ C2H4 on clean Pt(111) in an atop/atop arrangement isfavored by 0.44 eV over π-C2H4 at a C2H4 coverage of 0.25ML, the situation changes dramatically in the presence ofcoadsorbed nitrogen atoms. On the N-Pt(111) surface, thefavored site for di-σ bonded ethylene switches from atop/atopto atop/bridge where the molecule is centered over the fcc site.

Figure 1. Temperature-programmed desorption of 0.8 L of ethylenefrom clean Pt(111) and from 0.25 ML of nitrogen on Pt(111). Both N2

and C2H4 contribute to the m/z ) 28 signal, but only C2H4 contributesto the m/z ) 27 signal, which is due to the C2H3 fragment of ethylene. Figure 2. RAIRS spectra following exposure of 0.8 L of ethylene to

0.25 ML of nitrogen on Pt(111).

Site Switching from Di-σ C2H4 to π-Bonded C2H4 J. Phys. Chem. C, Vol. 114, No. 28, 2010 12231

In addition, π-C2H4 and di-σ C2H4 are essentially thermoneutral(di-σ C2H4 is favored by only 0.01 eV), indicating that bothspecies should exist on the surface depending upon variationsin local coverages. As depicted in Figure 3, density of statescalculations of the nitrogen-covered surface reveal a shift inthe d-band center of the metal away from the Fermi edge (from-1.94 to -2.52 eV on the 0.25 ML of N surface). Followingthe d-band model of Norskov and Hammer,32 this shift in the

d-band center readily explains a reduction in the reactivity ofthe surface toward adsorbates when N atoms are coadsorbed.Indeed, the adsorption energy of di-σ C2H4 reduces from -1.13eV on Pt(111) to -0.48 eV on the nitrogen-covered surface.As indicated above, the extent to which the bonding of π-C2H4

is weakened is much less than that of di-σ C2H4. This can beexplained by the lower coordination of π-bonded ethylene tothe surface.

In an effort to better understand the results, further experi-ments varying the coverage of C2H4 were performed. As shownin Figure 4, the presence of π-bonded C2H4 is maximized at anexposure of 0.8 L of C2H4 to the nitrogen-precovered surface.TPD data mirror that of the IR data, although it should be notedthat the peak ascribed to π-bonded ethylene shrinks and narrowsas the coverage of C2H4 increases. At higher exposure, moredi-σ bonded ethylene is formed, which subsequently convertsto ethylidyne when the surface is heated to higher temperatures.The reduced formation of π-bonded ethylene at high ethylenecoverages suggests that the favored bonding mode for C2H4 onthe N-Pt(111) surface may switch again back to di-σ atop/atop in an analogous way to which the favored adsorption siteswitches on the Pt(111) surface from di-σ to π-bonded ascoverage increases. DFT calculations could not, however, locatefavorable adsorption configurations for higher C2H4 coverages.

It is well known that halides and alkali metals may serve aspromoters in metallic heterogeneous catalysts for many impor-

Figure 4. RAIRS spectra of different ethylene exposures to the p(2 × 2)-N/Pt(111) surface at 90 K, followed by annealing to different temperatures.(a) 0, (b) 0.4, (c) 0.8, and (d) 1.6 L.

Figure 3. PDOS for clean Pt(111) and for Pt(111) covered with 0.25ML of N.

12232 J. Phys. Chem. C, Vol. 114, No. 28, 2010 Yin et al.

tant reactions such as ethylene epoxidation33 and Fischer-Tropschsynthesis.34 Our work suggests that nitrogen atoms may be aneffective promoter for the selective hydrogenation of alkenesas the bonding of alkenes to metal surfaces shifts from aspectator state to one that can easily be hydrogenated. Byaltering the nitrogen coverage, one may tune the electronicstructure of the metal and potentially shift the selectivitydramatically. Furthermore, nitrogen atoms do not react with themolecular species and therefore present an intriguing option forpromotion and selectivity control in other systems.

Conclusions

The formation of π-bonded ethylene on p(2 × 2)-N/Pt(111)has been observed by a combination of RAIRS and TPD. DFTcalculations find that π-bonded C2H4 is thermoneutral comparedwith di-σ C2H4 when 0.25 ML of C2H4 is adsorbed on p(2 ×2)-N/Pt(111). However, changing the coverage of ethylenechanges the relative portions of π-bonded C2H4 and di-σ C2H4

because of the repulsive interactions between ethylene and Natoms. Our results suggest that N may serve as a promoter forhydrogenation of alkenes.

Acknowledgment. This work was supported by a grant fromthe U.S. National Science Foundation, CHE-0714562.

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