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 N2. 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 (Ti5c) 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 × 1) surface, due to the presence of surface Vo and the resulting polaron, two NO molecules adsorbed, respectively, on Vo sites and adjacent Ti5c 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 × 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