Catalysis by Gold: Isolated Surface Au3+ Ions are Active Sites for Selective Hydrogenation of 1,3‐Butadiene over Au/ZrO2 Catalysts

Abstract

In recent years, there has been an increasing interest in catalysis by nanosized gold catalysts, which are active in reactions such as CO oxidation, propylene epoxidation, hydrogenation of unsaturated compounds, vinyl chloride synthesis, and others. 5,11] Despite such a diverse spectrum of reactions, efforts have been mainly devoted to elucidating the catalytically active sites in the oxidation reactions. Little attention has been paid to the nature of active gold, especially cationic gold species, in the other reactions such as hydrogenation of unsaturated compounds. This is surprising, since catalytic hydrogenation is one category of the best studied and industrially most important reactions on conventional transition metal catalysts, 19] and Au ions in solution can activate carbon– carbon double and triple bonds and are finding applications as efficient homogeneous catalysts in many organic reactions. 21] Hydrogenation of 1,3-butadiene has long been a model reaction for the study of active sites on conventional metal catalysts. 19] A few preliminary investigations on the hydrogenation of 1,3-butadiene with supported gold catalysts showed that small gold nanoparticles were active in the reaction, although theoretical calculations seem to suggest that the surface of metallic gold is chemically inert for hydrogen activation. By using Au/ZrO2 catalysts with very low loadings of Au (< 0.1%), we have now discovered that isolated surface-type Au ions are actually the active sites for the selective hydrogenation of 1,3-butadiene. The activity and selectivity of these Au ions are comparable to those of conventional precious transition metals such as Pd and Pt. This finding that low loadings of Au ions are more active for the hydrogenation reaction than high loadings of Au nanoparticles could raise intensive interest in exploring the potential of surface-type Au ions in heterogeneous catalytic reactions. The use of isolated and heterogenized Au ions in catalysis and related materials may be the key to using the noble gold in areas usually reserved for conventional precious metal catalysts, for example, to significantly improve the economics and reduce the use of other precious metals. Figure 1 shows the effect of gold loading on 1,3-butadiene conversion over Au/ZrO2 catalysts calcined at 200 8C. No butane (i.e., the product of complete hydrogenation) but only butenes were detected as the hydrogenation products over all