Theoretical Study of NO Conversion on Ag/TiO2 Systems. I. Anatase (100) Surface

A theoretical study of nitric oxide (NO) conversion on the anatase (100) surface covered with silver clusters has been performed. Two complementary approaches based on density functional theory (DFT) have been applied, in which the electron density was expanded in plane waves and in atom-centered Gaussian-type orbitals, respectively. It was observed that the NO interaction with the surface occurs mainly via the N atom. Adsorption of NO on silver clusters or at the border between silver and the TiO2 surface is more exothermic than at the uncovered anatase surface. Therefore, all stages of NO degradation proceed mainly on these active sites. Further adsorption of NO molecules leads to the formation of dimer species with previously adsorbed ones. Only acyclic cis-isomers of ONNO are formed according to the calculated energies. An analysis of electron density shows that the LUMO of adsorbed (NO)(2) becomes partially occupied so that the adsorbed nitric oxide dimers are negatively charged. As a result of this charge transfer, the (NO)(2) species are decomposed by breaking one or two N-O bonds, followed by the formation and desorption of N-2 or N2O. In the case of decomposition at silver-surface boundaries, the main gas-phase product is N2O, whereas on the silver cluster both N-2 and N2O are formed. After the (NO)(2) decomposition, oxygen atoms remain on the surface and can further react with NO molecules from the gas phase, leading to the formation of rather tightly bound nitrogen dioxide molecules.