Experimental and Theoretical Insight into the Facet-Dependent Mechanisms of NO Oxidation Catalyzed by Structurally Diverse Mn₂O₃ Nanocrystals

Crystalline structure of metal oxides is based on a close-packed array of oxygen anions with metal cations occupying interstitial sites, where the exposed facet determines the surface arrangement and coordination of oxygen and metal ions. Owing to the different physicochemical properties of various ions, the exposed crystal facets have a critical influence on properties of metal oxides, including catalytic behavior. Understanding the facet-dependent mechanisms of catalytic reactions is important for improving the performance of metal oxide catalysts; however, this understanding is currently lacking because research performed to date has been limited by the unilateral experimental or theoretical evidence. Herein, we aim to elucidate the effect of different exposed crystal facets in Mn₂O₃ by constructing rod- and particle-like Mn₂O₃ nanocrystals and investigating the effects on the catalytic performance of these structures for NO oxidation. Distinct catalytic behaviors of these different nanocrystals are investigated by a combination of systematic structural and property characterizations and density functional theory (DFT) calculations. Our results suggest that NO oxidation on the surface of rod-like Mn₂O₃ with exposed (220) and (400) facets follows the Langmuir–Hinshelwood mechanism, whereas the Mars–van-Krevelen mechanism is favored for Mn₂O₃ nanoparticles with exposed (222) facets. These different mechanisms lead to diverse catalytic performances of Mn₂O₃, especially the poisoning resistance. Specifically, rod-like Mn₂O₃ is shown to be deactivated by H₂O because hydroxylated oxygen inhibits the adsorption of NO and adsorbed OH* hinders the chemisorption of O₂ on the (220) and (400) facets, while H₂O only slightly influences the activity of Mn₂O₃ nanoparticles with (222) facets. This work shows that regulating which crystal facets of Mn₂O₃ are exposed allows tuning the catalytic performance of this material and the reaction pathways for NO oxidation. Furthermore, this work provides clear insight that may increase the understanding of facet-dependent reaction mechanisms of other metal oxide catalysts and establishes valuable design principles for future studies to improve heterogeneous catalysts.