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Platinum-Promoted Catalysis by Ceria?

This might be subtitled "Is the Frost hypothesis right after all, if not for methanol synthesis, then for some CO oxidations?".

A recent publication by Lambert's group at Cambridge [C. Hardacre, R.M. Ormerod and R.M. Lambert, J. Phys. Chem., 98 (1994) 10901] presents some strikingly un- expected results on carbon monoxide ox- idation over model platinum/ceria cata- lysts. These are clearly relevant to the mechanisms occurring on auto exhaust catalysts, and, if Lambert has uncovered a general phenomenon, to much other cata- lysis as welt.

The model catalyst was made from a Pt(111) crystal face upon which Ce metal was deposited and then oxidized to ceria. This method produced a disordered CeO2 on the platinum which became ordered upon annealing at 700 K or above. Sub- monolayers of ceria strongly promoted the oxidation of CO - - this was not surprising as similar enhancing effects of partial coverage of metal by a support oxide have been observed before. With increasing coverage of ceria the rate of oxidation fell "to an undetectably low level between 0.8 and 1.3 ML'. However, "Above 1.3 ML, the rate again rises steeply to a value which is much greater than that observed over the clean Pt(111/ surface." This holds for 10 ML of ceria (the highest loading examined) when no bare Pt metal is detectable by CO chemisorption, so the fast oxidation must be occurring on a modified ceria. Anneal- ing the ceria almost eliminated this en- hanced rate of reaction.

Lambert et al. "conclude that the reac- tion is occurring at the surface of the thin oxide film whose properties are radically

altered by the presence of the underlying, fully encapsulated Pt metal". They also suggest that this can be interpreted in terms of the proposal made by Frost [J.C. Frost, Nature, 334 (1988) 577], in which electron transfer from a metal phase to an oxide phase can reduce the enthalpy for oxygen vacancy formation in the oxide, so forming a catalytically active site. This model of the system is the most plausible on offer at present but one wonders if the electron transport is likely to be sufficiently efficient through ten monolayers of ceria. However, as the catalytically active over- layer of ceria is disordered, the active sites may well be rather nearer to the Pt surface than to the top of the ceria. Lambert et al. finish with a note of caution on interpreta- tion: "the techniques at our disposal do not provide absolute and incontrovertible evi- dence of total encapsulation of the Pt crys- tal. It could always be argued that some Pt species, undetectable by our methods, are present at the oxide surface and respon- sible for the observed high catalytic activ- ity: a possibility which would also be of considerable interest.".

MIKE SPENCER

Transient Species in Surface Chemi- cal Reactions

For workers in catalysis, and especially those concerned with reaction mechan- isms, some of the most revealing surface chemistry in recent years has been the discovery of highly reactive, transient sur- face species in surface reactions. For example, Ertl and co-workers [H. Brune, J. Winterlin, R.J. Behm and G. Ertl, Phys. Rev. Lett., 68 (1992) 624] have observed that the

applied catalysis A: General Volume 122 No. 2 - 16 February 1995