On the mechanism of methanol synthesis and the water-gas shift reaction on ZnO
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Catalysis Letters Vol. 110, Nos. 1–2, August 2006 ( 2006) DOI: 10.1007/s10562-006-0088-9
On the mechanism of methanol synthesis and the water-gas shift reaction on ZnO J. Tabatabaei,a,b,* B.H. Sakakini,a and K.C. Waugha a
School of Chemistry, University of Manchester, Faraday Building, Sackville Street, Manchester M60 1QD b Davy Process Technology, Technology Centre, Princeton Drive, Stockton-on-Tees TS17 6PY
Received 7 April 2006; accepted 11 April 2006
Zinc oxide catalyses both methanol synthesis and the forward and ’everse water-gas shift reaction (f- and r- WGSR). Copper also catalyses both reactions, but at lower temperatures than ZnO. Presently the combination of Cu and ZnO stabilized by Al2O3 is the preferred catalyst for methanol synthesis and for the f- and r- WGSR. On Cu, the mechanism of methanol synthesis is by hydrogenation of an adsorbed bidentate formate [1] (the most stable adsorbed species in methanol synthesis), while the f- and rWGSR proceeds by a redox mechanism. The f-WGSR proceeds by H2O oxidizing the Cu and CO, reducing the adsorbed oxide and the r-WGSR proceeds by CO2 oxidising the Cu and H2, reducing it [2–5]. Here we show that the mechanisms of both reactions are subtly different on ZnO. While methanol is shown to be formed on ZnO through a formate intermediate, it is a monodentate formate species which is the intermediate; the f- and r-WGS reactions also proceed through a formate – a bidentate formate - in sharp contrast to the mechanism on Cu.
1. Introduction Following their success in discovering a commercially viable ammonia synthesis catalyst, researchers at Badische Anilin Soda and Fabrik (BASF) turned their attentions to discovering a catalyst which would convert CO/H2 mixtures to hydrocarbons. In 1923 they discovered that ZnO would convert CO/H2 mixtures not to hydrocarbons but to CH3OH [6 and references therein]. Consequently much of the research that has been conducted on ZnO has focused on trying to elucidate the reaction pathway by which CO and H2 are adsorbed on ZnO and transformed to CH3OH [7–14]. A useful means of studying the mechanism of the synthesis of CH3OH on ZnO is to look at its decomposition on that surface. Several authors have done this [7–14]. Having first characterized the vibrational spectrum of a formate species on ZnO by adsorbing HCO2H on to the ZnO at 473 K and having shown that it was formed by CO2/H2 co-adsorption at 473 K and not by CO/H2O co-adsorption at 473 K [7], Tamaru and coworkers showed that the same formate species was formed by adsorbing CH3OH on the ZnO at 473 K [8]. The decomposition products of the adsorbed formate were shown to be H2, CO2 and CO. Using CD3OD Tamaru and co-workers claimed that the CO2 was produced by the reaction
*To whom correspondence should be addressed. E-mail: [email protected]
CD3 OD þ DCOOðaÞ ! CD3 OðaÞ þ CO2 þ D2 : Trapping out the CO2 in the gas phase was found to have no effect on the rate of CO evolution and so Tamaru and co-workers concluded that the decomposition of CO2 to CO on the ZnO surfa
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