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Based on first-principles calculations and microkinetic analysis, the reaction routes and origin of the activity of SmMn 2 O 5 mullite for the selective catalytic oxidation of ammonia (NH 3 -SCO) are systematically investigated on three low-index surfaces under experimentally operating conditions. Key influencing factors and contributions of different iconic intermediate species (NH*, N 2 H 4 *, and HNO*) to the overall reaction process have been identified. In detail, Mn 4+ serves as the primary active site for NH 3 adsorption, while lattice oxygen participates in the dehydrogenation of NH 3 on (010) 4+ and (001) 4+ surfaces. Furthermore, the (010) 4+ surface shows both the best activity and the highest N 2 selectivity at low temperatures via the synergy effect of exposed Mn−Mn dimers and the most labile O 2 atoms. We further evaluate the potential catalytic performances of six A-site doped (010) 4+ facets, among which La, Pr, and Nd dopings are predicted to possess better catalytic performances. Our study provides deep insights into the microscope reaction mechanisms and provides the specific optimization strategy for NH 3 -SCO on mullite oxides.

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