Role of Cationic Gold in Supported CO Oxidation Catalysts

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.