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

Abstract

Redox properties of CeO2 materials have a major impact on the NO oxidation to NO2, which is a crucial step for selective-catalytic-reduction (SCR) because NO2 can trigger the “Fast SCR” reaction. Despite decades of investigations, the mechanism of NO oxidation to NO2 at the CeO2 surface is still under debate, the controversy is whether the key oxidants are the CeO2 catalyst’s high-valence metal ions or the O2 molecules (in the reactant gas) which could be activated by the reducing CeO2 surface oxygen vacancies (Ovac)? We perform density-functional-theory (DFT) simulations and synthesize CeO2 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 Ovac at the CeO2 surface strongly interacts with O2 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 NO2. Given the theoretical–experimental-joint results, we demonstrate that the effective approach, in terms of utilizing the intrinsic catalytic properties of CeO2, should be simultaneously optimizing the surface Ovac concentration and the Ovac’s activity for O2-dimer activation, which provides atomic-scale insights for a rational design of NO-conversion catalysts.