Morphology effects in MnCeOx solid solution-catalyzed NO reduction with CO: Active sites, water tolerance, and reaction pathway

Abstract

Morphological effects of nanoparticles are crucial in many solid-catalyzed chemical transformations. We herein prepared two manganese-ceria solid solutions, well-defined MnCeOx nanorods and MnCeOx-nanocubes, exposing preferentially (111) and (100) facets of ceria, respectively. The incorporation of Mn dopant into ceria lattice strongly enhanced the catalytic performance in the NO reduction with CO. MnCeOx (111) catalyst outperformed MnCeOx (100) counterpart due to its higher population density of oxygen vacancy defects. In-situ infrared spectroscopy investigations indicated that the reaction pathway over MnCeOx and pristine CeO2 is similar and that besides the direct pathway, an indirect pathway via adsorbed hyponitrite as an intermediate cannot be ruled out. X-ray photoelectron and Raman spectroscopies as well as first-principles density functional theory (DFT) calculations indicate that the enhanced catalytic performance of MnCeOx can be traced back to its "Mn-O-L(Vo)-Mn-O-L(Vo)-Ce " connectivities. The Mn dopant strongly facilitates the formation of surface oxygen vacancies (Vo) by liberating surface lattice oxygen (O-L) via CO* + O-L -> CO2* + Vo and promotes the reduction of NO, according to NO* + Vo -> N* + O-L and 2N* -> N-2. The Mn dopant impact on both the adsorption of CO and activation of O-L reveals that a balance between these two effects is critical for facilitating all reaction steps.