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There are many examples of catalysis in solution by cationic complexes of gold, and recent results, reviewed here in this critical review, demonstrate that cationic gold species on oxide and zeolite supports are also catalytically active, for reactions including ethylene hydrogenation and CO oxidation. The catalytically active gold species on supports are evidently not restricted to isolated mononuclear gold complexes, but include gold clusters, which for at least some reactions are more active than the mononuclear complexes and for some reactions less active. Fundamental questions remain about the nature of cationic gold in supported catalysts, such as the nature of the cationic gold clusters and the nature of gold atoms at metal–support interfaces (88 references).

CO oxidation catalyzed by supported gold has emerged as a vigorous research field in the preceding few years, driven by the recognition that this unreactive metal, when highly dispersed, is remarkably active and selective as a catalyst, offering good prospects for application. It also offers good opportunities for fundamental understanding, because some of the reactants converted by supported gold are small molecules that are informative probes of the catalyst surface. Thus, CO oxidation has been used extensively to characterize supported gold catalysts. Nonetheless, no consensus has yet emerged regarding the catalytically active species, and it seems likely that catalytic activity is not restricted to a single kind of species. In the following review, we focused specifically on one class of catalytically active gold species for CO oxidation, cationic gold. The review includes summaries of (a) evidence of gold in various oxidation states in supported CO oxidation catalysts, including theoretical and experimental results, especially spectroscopic evidence of functioning catalysts; (b) evidence demonstrating that cationic gold plays a role in the catalytic sites in supported catalysts that contain both zerovalent and cationic gold; and (c) evidence that cationic gold plays such a role in supported catalysts that lack detectable zerovalent gold.

This paper is a brief review of suggested structures of catalytic sites and reaction mechanisms in CO oxidation catalyzed by highly dispersed supported gold. The review is focused on evidence of the sites, which has been determined primarily by infrared, X-ray absorption, and other spectroscopies, often combined with transmission electron microscopy; theory has also been applied to characterize supported gold catalysts. The literature gives strong evidence of the reaction of oxygen with CO adsorbed on zerovalent gold at low temperature, consistent with the role of such adsorbed CO in the catalytic reaction. There is also strong evidence of the involvement of cationic gold, even isolated single sites, in the catalysis at higher temperatures (e.g., room temperature). Because CO is only weakly adsorbed on zerovalent gold and more strongly adsorbed on cationic gold, it is suggested that the cationic gold sites may be kinetically more significant than the zerovalent gold sites under potentially practical reaction conditions.