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

Morphological effects of nanoparticles are crucial in many solid-catalyzed chemical transformations. We herein prepared two manganese-ceria solid solutions, well-defined MnCeO_x nanorods and MnCeO_x-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. MnCeO_x (111) catalyst outperformed MnCeO_x (100) counterpart due to its higher population density of oxygen vacancy defects. In-situ infrared spectroscopy investigations indicated that the reaction pathway over MnCeO_x 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 MnCeO_x can be traced back to its “Mn–O_L(VÖ)–Mn–O_L(VÖ)–Ce” connectivities. The Mn dopant strongly facilitates the formation of surface oxygen vacancies (VÖ) by liberating surface lattice oxygen (O_L) via CO* + O_L → CO₂* + VÖ and promotes the reduction of NO, according to NO* + VÖ → N* + O_L and 2N* → N₂. 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.