Revealing the crystal facet effect on N2O formation during the NH3-SCR over alpha-MnO2 catalysts

The detailed atomic-level mechanism of the effect induced by engineering the crystal facet of alpha-MnO2 catalysts on N2O formation during ammonia-selective catalytic reduction (NH3-SCR) was ascertained by combining density functional theory (DFT) calculations and thermodynamics/kinetic analysis. The surface energies of alpha-MnO2 with specific (100), (110), and (310) exposed planes were calculated, and the adsorptions of NH3, NO, and O-2 on three surfaces were analyzed. The adsorption energies showed that NH3 and NO molecules could be strongly adsorbed on the surface of the alpha-MnO2 catalyst, while the adsorption of O-2 was weak. Moreover, the key steps in the oxidative dehydrogenation of NH3 and the formation of NH2NO as well as dissociation of NH2 were studied to evaluate the catalytic ability of NH3-SCR reaction and N-2 selectivity. The results revealed that the alpha-MnO2 catalyst exposed with the (310) plane exhibited the best NH3-SCR catalytic performance and highest N-2 selectivity, mainly due to its low energy barriers in NH3 dehydrogenation and NH2NO generation, and difficulty in NH2 dissociation. This study deepens the comprehension of the facet-engineering of alpha-MnO2 on inhibiting N2O formation during the NH3-SCR, and points out a strategy to improve their catalytic ability and N-2 selectivity for the low-temperature NH3-SCR process.