Gold-based catalysts for the water–gas shift reaction: Active sites and reaction mechanism

The water–gas shift (WGS, CO + H2O → H2 + CO2) reaction was studied on a series of gold/oxide catalysts. The results of in situ measurements with X-ray absorption spectroscopy indicate that the active phase of Au-ceria and Au-titania catalysts under the reaction conditions of the water–gas shift consists of metallic nanoparticles of gold on a partially reduced oxide support. In spite of the lack of catalytic activity of Au (1 1 1) and other gold surfaces for the water–gas shift process, gold nanoparticles dispersed on oxide surfaces are excellent catalysts for this reaction. Results of density-functional calculations point to a very high barrier for the dissociation of H2O on Au (1 1 1) or isolated Au nanoparticles, which leads to negligible activity for the WGS process. In the gold-oxide systems, one has a bifunctional catalyst: the adsorption and dissociation of water takes place on the oxide, CO adsorbs on the gold nanoparticles, and all subsequent reaction steps occur at oxide–metal interfaces. The nature of the support plays a key role in the activation of the gold nanoparticles. Although zinc oxide is frequently used in industrial WGS catalysts, the Au/ZnO (0 0 0 1¯) system displays low WGS activity when compared to Au/CeO2 (1 1 1), Au/TiO2 (1 1 0) or Au/CeOx/TiO2 (1 1 0). The ceria and titania supports contain a substantial number of metal cations that are not fully oxidized under WGS reaction conditions and may participate directly in the dissociation of water and other important steps of the catalytic process. The results for Au/CeOx/TiO2 (1 1 0) illustrate the tremendous impact that an optimization of the chemical properties of gold and the oxide phase can have on the activity of a WGS catalyst.