A First-Principle Study of Synergized O2 Activation and CO Oxidation by Ag Nanoparticles on TiO2(101) Support

We performed density functional theory (DFT) calculations to investigate the synergized O-2 activation and CO oxidation by Ag-8 cluster on TiO2(101) support. The excellent catalytic activity of the interfacial Ag atoms in O-2 dissociation is ascribed to the positive polarized charges, upshift of Ag d-band center, and assistance of surface Ti-5c atoms. CO oxidation then takes place via a two-step mechanism coupled with O-2 dissociation: (i) CO + O-2 -> CO2 + O and (ii) CO + O -> CO2. The synergistic effect of CO and O-2 activations reduces the oxidation energy barrier (E-a) of reaction (i), especially for the up-layered Ag atoms not in contact with support. It is found that the coadsorbed CO and O-2 on the up layered Ag, atoms form a metal-stable four-center O-O-CO structure motif substantially promoting, CO oxidation. On the oxygen defective Ag-8/TiO2(101) surface, because of the decreased positive charges and the down-shift of d-band centers in Ag, the metal cluster exhibits low O-2 adsorption and activation abilities. Although the dissociation of O-2 is facilitated by the TiO2(101) defect sites, the dissociated 0 atoms would cover the defects so strongly that further CO oxidation would be prohibited unless much extra energy is introduced to recreate oxygen defects.