固體表面化學反應機制

從 2007 年諾貝爾化學獎談起 ---

蔡茂盛 M. S. ZeiPh.D 1971 Freie University Berlin, Germany, 在哈伯研究所

(Fritz-Haber-Institut) 工作 35 年 (1970 – 2004).

國立中央大學物理系

簡介如何用表面物理科學儀器－光電子顯微鏡 (PEEM) 、低 /高能電子繞射 (LEED,RHEED) 、電子能量損失光譜 (EELS) 、掃瞄穿隧顯微鏡 (STM) 等儀器，來觀察 (測 )一氧化碳 CO在鉑單晶表面上的氧化過程。由 EELS 得知分子態的氧在室溫下因在鉑表面的化學吸附 (chemisorption) ，導致分子鍵 (Molecular bonding) 的減弱，使其解離成原子態氧 (Oad) ，另由 STM 觀察原子態氧(Oad) 及化學吸附態的一氧化碳在鉑 (III) 表面上分別形成 (2×2)-O, 及 c(4×2)-CO 構造的區域 (domains) ，其氧化反應機制是兩吸附態的反應物 (reactants) 在區域交界處 (Domain boundaries) 結合成二氧化碳 CO2 。催化反應過程中，發現反應氣體分子在固體觸媒的化學吸附，導致固體表面原子結構的變化並非如同傳統觀念認為：「催化劑只使化學反應加速，本身不會起任何變化」。 觸媒表面上的原子在反應過程中，是不斷在移動中。觸媒表面原子的遷移性 (mobility) 會因反應分子或離子的強吸附 (Strong bonding) 而導致表面原子遷移性 (mobility) 的提高 ; (mobility-enhancement by chemisorption of atoms/ions) 。

1). Haber-Bosch NH3 Synthesis

2). Fischer-Tropsch synthesis

3). 2007 年諾貝爾化學獎

CO + O2 oxidation on Pt surfaces

4). Methanol fuel cell

5). Structural change of catalyst-surface induced by chemisorption

今天的報告內容( 固體表面化學反應 )

表面原子結構的變化化學反應導致觸媒的

Kaiser Wilhelm Institut fur Physikalische Chemie und Elektrochemie

Fritz-Haber Institut der Max-Planck-Gesellschaft

Ernst Ruska Abteilung Elektron-Mikroskopie 07.2006

每年生產 NH3 10000 萬吨 100 million tons

450o C and 300 bar ?

Haber 哈伯及 Bosch 分別於 1918 及 1931 年獲得諾貝爾化學獎。

天然氣

Ni-catalyst, 700C 20 bar

CO + H2

O2, H2O 排除

450oC

Desorption of the chemisorbed N-species on FeO

為什么在放熱 ( 性 ) 的反应 ΔH = - 46kJ/mol, 合成溫度在 450o C?

WGS = water gas-shift reaction 用於由煤得到的

合成氣体

HistorySince the invention of the original process by the German researchers Franz Fischer and Hans Tropsch, working at the Kaiser Wilhelm Institute in the 1920s, many refinements and adjustments have been made, and the term "Fischer-Tropsch" now applies to a wide variety of similar processes (Fischer-Tropsch synthesis or Fischer-Tropsch chemistry). Fischer and Tropsch filed a number of patents, e.g. US patent no. 1,746,464, applied 1926, published 1930 [3].The process was invented in petroleum-poor but coal-rich Germany in the 1920s, to produce liquid fuels. It was used by Germany and Japan during World War II to produce ersatz fuels. Germany's synthetic fuel production reached more than 124,000 barrels per day (19,700 m³/d) from 25 plants ~ 6.5 million tons in 1944.[4]After the war, captured German scientists recruited 徵募 ( in Operation Paperclip continued to work on synthetic fuels in the United States in a United States Bureau of Mines program initiated by the Synthetic Liquid Fuels Act.In Britain, Alfred August Aicher obtained several patents for improvements to the process in the 1930s and 1940s, e.g. British patent no. 573,982, applied 1941, published 1945 [5]. Aicher's company was named Synthetic Oils Ltd. (There is no connection with the Canadian company of the same name.)

Ni

RdCH4

Another important reaction is the

water gas shift reaction:

H2O + CO → H2 + CO2

Although this reaction results in formation of unwanted CO2, it can be used to shift the H2:CO ratio of the incoming Synthesis gas.

This is especially important for synthesis gas derived from coal, which tends to have a ratio of ~0.7 compared to the ideal ratio of ~2.

Fe

↑

COad →

CO2↑

Structural transformation of the reconstructed Pt(100) surface

(100)-hex (100)-(1x1)

COad + O2 → CO2

Pt(100)

Ads. CO

Phys. Rev.Lett. 49(1982) 177

PCO ↓, ψ ↑

Oscillation of CO oxidation on Pt-surface

CO- 氧化反應呈現震盪現象的解釋

EELS Spectrum of ads. Oxygen on Pt(111) at 300 K

只出顯在 300 oK

Oscillation of CO oxidation on Pt surface

(Sub-Micron) 結構成像

一氧化碳及氧在鉑金薄膜表面上氧化過程所形成的光電子顯微鏡 (PEEM) 圖像，其中暗亮區域分別對應於氧分子及一氧化碳在鉑表面上的吸附區域。

STM images for CO + O2 oxidation on Pt(111)

Science 12 (1997) 1931

STM images

C(4x2)-CO Orientation change of the c(4x2)-damain

→

(2x2)-O

via Langmuir-Hinschelwood mechanism

CO oxidation

其氧化反應機制是兩吸附態的反應物 (reactants) 在區域交界處 (Domain boundaries) 結合成二氧化碳 CO2

H+

H+

catalyst

Methanol (CH3OH) fuel cell

Cathode: 1/5 O2 + 6 H+ + 6 e- → 3 H2O

H+

Anode: CH3OH + H2O +Pt → CO2 + 6 H+ + 6 e-

Pt-O + H+ + e-

→Pt-OH

Pt-OH + H+ + e- → Pt-H2O

Pt-(CH3OH)ads → Pt-COads + 4 H+

+ 4 e- Nafion

(+)

(-)

(-) 氧分子在鉑上降低活化能成原子態氧

Anode:

Pt + CH3OH → Pt-CO + 4 H+ + Pt + 4 e-

Pt-CO + Pt- H2O → CO2 + 2 e + 2 H+ 2Pt

CH3OH Pt → CO2 + 6 H+ + 6 e-

Surface reactions on catalyst surface

重奌在說明觸媒表面的功能

Catalyst decreasing the activation energy, facilitating chem. reaction

Pt(111) substrate surface

Ru-deposited

Ru-deposited layers

After Ru/Pt(111) towards CO electro-oxidation in HClO4 , disappearing the long range-order

as demonstrated by RHEED

Ru-islands with regular lateral periodicity of around 20 A grown on Pt(111) surface

証明 Ru- 原子的移動

釕在鉑 (111) 表面上所形成的超結構在電解液中因CO 的氧化過程，導致釕在鉑 (111) 表面上所形成的超結構明顯消失 (loss of the long-range order)[6] 。

RHEED patterns for Pt(111) covered by Ru (0.64 ML) deposit, showing satellite reflections indicated by arrows

Satellite reflections show that the Ru adlayer with additional lateral periodicity of around 20 A is formed

↑

COad →

CO2↑

Structural transformation of the reconstructed Pt(100) surface

(100)-hex (100)-(1x1)

COad + O2 → CO

2

Pt(100)

証明 : 觸媒表面上的原子在反應過程中，是不斷在移動中。

並非如同傳統觀念認為：「催化劑只使化學反應加速，本身不會起任何變化」。

鉑單 晶面 (100) 在 CO- oxidation ; 由六面单胞 (hex-unit cell) 轉化成四面单胞 (square unit cell, 1x1) 來

囘轉 ( 變 ) 化 , 來解釋 陪同 CO- 氧化反應呈現的震盪現象以及鉑 / 釕因 CO- 在氧化反應所引起的結構變化

Conclusion

1). Strong adsorption of the reactants on the catalyst surface leads to decrease the activation energy of molecular dissociation.

2). CO-oxidation on Pt surfaces and NH3 synthesis on FeO proceed via Langmuir-Hinschelwood mechanism, i.e., reaction by the adsorbed atomic species on catalyst surface.

3). Structural change of catalyst surface is accompanied by the catalytic reactions. The mobility of the surface atoms is enhanced by the strong adsorption of the reactants.

實例說 ( 証 ) 明 , 固體表面化學反應的重要

柏林圍牆 1989 前 West Berlin

East Berlin

Berlin, 1989

05.2003, Fritz-Haber Institute der Max-Planck-Gesell. Berlin, Germany