NO oxidation properties of Pt(111) revealed byab initiokinetic simulations

As a well-known oxidation catalyst, platinum is currently being used to convert NO to NO₂ for absorption in so-called NOₓ traps under excess oxygen conditions, where direct conversion of NO to nitrogen gas is prohibitive. By performing kinetic Monte Carlo simulations with parameters derived from density-functional theory, we show that this oxidation process is not an inherent property of the platinum catalyst itself. In fact, the intrinsic NO+O→NO₂ reaction is found to be inhibited (endothermic) rather than promoted on Pt(111), due to strong oxygen--platinum bonds. Only at sufficient oxygen chemical potential does platinum become an efficient oxidation catalyst, as its oxygen bonds are weakened with increasing coverage, and the NO₂ formation reaction becomes exothermic. At that point, Pt catalyzes the reaction by lowering the activation barrier for the kinetic reaction. Congruent with flow-reactor experiments, we note a strong temperature dependence for the turnover frequency, which should encourage further ultra-high vacuum studies.