Gold nanoparticles on ceria: Importance of O vacancies in the activation of gold

Abstract

Synchrotron-based techniques (high-resolution photoemission, in-situ X-ray absorption spectroscopy, and time-resolved X-ray diffraction) have been used to study the destruction of SO2 and the water-gas shift (WGS, CO + H2O → H2 + CO2) reaction on a series of gold/ceria systems. The adsorption and chemistry of SO2 was investigated on Au/CeO2(111) and AuOx/CeO2 surfaces. The heat of adsorption of the molecule on Au nanoparticles supported on stoichiometric CeO2(111) was 4-7 kcal/mol larger than on Au(111). However, there was negligible dissociation of SO2 on the Au/CeO2(111) surfaces. The full decomposition of SO2 was observed only after introducing O vacancies in the ceria support. AuOx/CeO2 surfaces were found to be much less chemically active than Au/CeO2(111) or Au/CeO2-x(111) surfaces. In a separate set of experiments, in-situ time-resolved X-ray diffraction and X-ray absorption spectroscopy were used to monitor the behavior of nanostructured {Au + AuOx}-CeO2 catalysts under the WGS reaction. At temperatures above 250 ©C, a complete AuOx → Au transformation was observed with high catalytic activity. Photoemission results for the oxidation and reduction of Au nanoparticles supported on rough ceria films or a CeO2(111) single crystal corroborate that cationic Auδ+ species cannot be the key sites responsible for the WGS activity at high temperatures. The active sites in {Au + AuOx}/ceria catalysts should involve pure gold nanoparticles in contact with O vacancies of the oxide. © 2007 Springer Science+Business Media, LLC.