Is Transition Metal Oxide a Must? Moisture-Assisted Oxygen Activation in CO Oxidation on Gold/γ-Alumina†

Although a poor catalyst under dry conditions, γ-Al2O3-supported gold (Au/γ-Al2O3) turns out to be a superior CO oxidation catalyst under moisture conditions. In this work, extensive density functional theory calculations have been carried out to investigate the physical origin of the moisture promotion effect. By supporting Au strips on the two most stable γ-Al2O3 surfaces, namely, the (110) and (100) faces, we show that the majority (110) surface is catalytically inert due to the saturation of Al cationic sites by dissociated H2O. On the other hand, the minority (100) surface in combination with Au is responsible for CO oxidation activity, where O2 can adsorb at the Au/γ-Al2O3(100) interface with a tilted Au-O-O-Al5c configuration. In the presence of coadsorbed H2O and CO, the adsorption energy of O2 reaches to 0.7 eV. We find that H2O enables the direct dissociation of the reaction intermediate cis-OCOO produced by the bimolecular coupling between CO and O2, whereas an extra cis-to-trans rotation of OCOO is required in the absence of H2O. In the H2O-assisted pathway, no atomic oxygen is produced, and the overall barrier is only 0.25 eV, which is 0.28 eV lower than that without H2O. By electronic structure analyses, we suggest that a modest acidity of the γ-Al2O3 (100) surface contributes to the O2 adsorption, although Al2O3 lacks the d-states that were shown to be important for O2 activation. 1