Nitric Oxide Reaction Pathways on Rutile TiO2(110): The Influence of Surface Defects and Reconstructions

TiO2 exhibits excellent catalytic performance in degrading NO to N2O or N-2. However, up to now, the detailed reaction pathways of NO on TiO2 surfaces are still debatable. In this paper, we studied NO adsorption and reactions on differently treated rutile TiO2(110) surfaces by using polarization/azimuth-resolved infrared reflection absorption spectroscopy (IRRAS). It is found that the surface defects [the oxygen vacancies (Vo)] and reconstructions on TiO2(110) have a strong effect on the reaction pathways of NO -> N2O conversion. The simplest pathway occurs on the defect-free oxidized TiO2(110) surface in which two NO molecules adsorbed on adjacent surface Ti (Ti-5c ) sites first couple to the cis-(NO)(2)/Ti&Ti dimer though a N-N bond, and then convert to N2O species. On the moderately reduced TiO2(110)-(1 x 1) surface, due to the presence of surface Vo and the resulting polaron, two NO molecules adsorbed, respectively, on Vo sites and adjacent Ti-5c sites couple to the trans-(NO)(2)/Ti&Vo dimer, and then convert to N2O before the cis-(NO)(2)/Ti&Ti dimers occur. On the highly reduced quasi-TiO2(110)-(1 X 2) surface, however, the Ti2O3 row fragments hamper the conversion of trans-(NO)(2)/Ti&Vo -> N2O, and thus hamper the subsequent cis-(NO)(2)/Ti&Ti formation without polaron. In this case, the conversion of both the trans-(NO)(2)/Ti&Vo dimer and the isolated NO monomer to N2O is likely to be triggered by the gas NO impingement. The structure-reactivity relationship we proposed is helpful in understanding the catalytic mechanism of NO degradation on TiO2 surfaces.