Reactant-promoted reaction mechanism for water-gas shift reaction on ZnO as the genesis of surface catalysis

Journal Article · Sat Jun 01 04:00:00 EDT 1991 · Journal of Catalysis; (USA)

DOI:https://doi.org/10.1016/0021-9517(91)90040-B· OSTI ID:5719984

Shido, Takafumi; Iwasawa, Yasuhiro ^[1]

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1. Univ. of Tokyo (Japan)

The behavior of reaction intermediates in the catalytic water-gas shift reaction (WGSR) on ZnO surfaces has been studied by means of FT-IR spectroscopy, and a reactant-promoted mechanism including intermediate-reactant interaction is proposed. On-top (terminal) hydroxyl groups on Zn ions which are formed by the first-adsorbed water molecules react with CO to produce bidentate and bridge formates. Seventy percent of them were decomposed to original CO and surface hydroxyls and only 30% of them were converted to H{sub 2} and CO{sub 2} (adsorbed) under vacuum. On the contrary, 100% of the formates were converted to the WGSR products, H{sub 2}, and CO{sub 2} in coexistence with second-adsorbed water molecules. The rate of the formate decomposition was promoted by a factor of more than 10 by the presence of a second water molecule. The activation energy of the decomposition of the surface formates decreases in the presence of water; 155 kJ mol{sup {minus}1} under vacuum and 109 kJ mol{sup {minus}1} with ambient water. The rate-determining step of the decomposition is the scission of the C-H bond of the formates according to isotope effects. In the absence of ambient water vapor, adsorbed CO{sub 2} species exist as unidentate carbonate and carboxylate on ZnO surface. Steady-state rate of catalytic WGSR agree with the decomposition rates of the bidentate formate and the unidentate carbonate; the two decomposition rates are balancing during the steady-state WGSR on ZnO. Water molecules not only act as a reactant to form the formate, but also activate the bidentate formate(ad) to decompose to H{sub 2} and unidentate carbonate(ad) and promote the desorption of the carbonate as CO{sub 2}.

• OSTI ID: 5719984
• Journal Information: Journal of Catalysis; (USA), Journal Name: Journal of Catalysis; (USA) Vol. 129:2; ISSN 0021-9517; ISSN JCTLA
• Country of Publication: United States
• Language: English

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