Investigation of intrinsic catalytic mechanism for NO oxidation to NO2 in CeO2 used for NO removal.

[Display omitted] • Resolving the controversy over the NO oxidation mechanism at the CeO 2 surface. • The atomic-scale understanding of NO oxidation by DFT/in-situ experiments. • Interaction between O vac and O 2 , forming a catalytic species O* for NO oxidation. • Optimizing O vac concentration for O 2 -dimer activation to improve the SCR activity. Redox properties of CeO 2 materials have a major impact on the NO oxidation to NO 2 , which is a crucial step for selective-catalytic-reduction (SCR) because NO 2 can trigger the "Fast SCR" reaction. Despite decades of investigations, the mechanism of NO oxidation to NO 2 at the CeO 2 surface is still under debate, the controversy is whether the key oxidants are the CeO 2 catalyst's high-valence metal ions or the O 2 molecules (in the reactant gas) which could be activated by the reducing CeO 2 surface oxygen vacancies (O vac)? We perform density-functional-theory (DFT) simulations and synthesize CeO 2 catalysts for in situ spectroscopy experiments, transient-reaction-analysis (TRA) experiments, and isotopic experiments, to reveal the NO oxidation mechanism and achieve atomic-level understanding. Our results show that O vac at the CeO 2 surface strongly interacts with O 2 gas, leading to the formation of a key oxidizing intermediate O* ("*" means an adsorbed state). TRA experiments, isotopic experiments, and in situ spectroscopy results further provide evidence that O* oxidizes NO to NO 2. Given the theoretical–experimental-joint results, we demonstrate that the effective approach, in terms of utilizing the intrinsic catalytic properties of CeO 2 , should be simultaneously optimizing the surface O vac concentration and the O vac 's activity for O 2 -dimer activation, which provides atomic-scale insights for a rational design of NO-conversion catalysts. [ABSTRACT FROM AUTHOR]