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

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 AuO _x /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. AuO _x /CeO₂ surfaces were found to be much less chemically active than Au/CeO₂(111) or Au/CeO_2−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 + AuO _x }–CeO₂ catalysts under the WGS reaction. At temperatures above 250 °C, a complete AuO _x → 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 Au^δ+ species cannot be the key sites responsible for the WGS activity at high temperatures. The active sites in {Au + AuO _x }/ceria catalysts should involve pure gold nanoparticles in contact with O vacancies of the oxide.