Juan C. Fierro-Gonzalez

Catalysis by gold dispersed on supports: the importance of cationic gold

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).

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.

A highly active catalyst for CO oxidation at 298 K: mononuclear AuIII complexes anchored to La2O3 nanoparticles

Mononuclear La2O3-supported AuIII complexes synthesised from AuIII(CH3)2(C5H7O2) and characterised by X-ray absorption spectroscopy are highly active, stable CO oxidation catalysts at room temperature, demonstrating the importance of the support in stabilizing catalytically active gold species, which need not include zerovalent gold for high activity.

Influence of CO and O2 Treatments on the Structure and Activity of Au/γ-Al2O3 Catalysts for CO Oxidation

Abstract Gold supported on partially dehydroxylated γ-Al2O3 was prepared from Au(CH3)2(C5H7O2). The activities of the materials as CO oxidation catalysts were investigated following pretreatment in flowing O2 and, alternatively, in flowing CO. The catalytic activity data are complemented with extended X-ray absorption fine structure and X-ray absorption near edge structure spectra recorded as the samples in flow reactors were treated in O2, in CO, and in CO + O2 while functioning as CO oxidation catalysts. Mass spectrometry was used to identify the gases evolved during the treatments. The data show that samples pretreated in O2 contained predominantly cationic gold and were more active as catalysts than samples pretreated with CO, which contained gold predominantly as zerovalent clusters. These results reinforce recent reports of the role of cationic gold in the catalytic sites for CO oxidation.

Role of Iron Carbonyls in the Inhibition of Oxygen Activation for the Oxidation of CO Catalyzed by Iron Oxide‐Supported Gold

Iron oxide-supported gold samples were prepared by co-precipitation from HAuCl(4) and Fe(NO(3))(3). The activities of the samples as CO oxidation catalysts were tested without thermal treatment and following treatments in flows of He and O(2) at various temperatures. It was found that the untreated samples and those treated in a flow of He at 150 °C were more active than samples that had been treated at 400 °C in either a flow of O(2) or of He. Infrared spectra recorded during CO oxidation catalysis indicate the presence of bonded CO molecules to cationic gold on all samples, whereas spectra of the least active catalysts indicate a predominant presence of Fe(2+) carbonyls, which were highly stable under the conditions of our experiments. Our results indicate that in the least active samples the Fe(2+)-bound CO blocks sites that would otherwise be available for oxygen activation.