Insight into the NH3-Assisted Selective Catalytic Reduction of NO on β-MnO2(110): Reaction Mechanism, Activity Descriptor, and Evolution from a Pristine State to a Steady State

To understand the molecular-level reaction mechanism and crucial activity-limiting factors of the NH 3 -SCR process catalyzed by MnO 2 -based oxide to eliminate NO (4NH 3 + 4NO + O 2 →4N 2 + 6H 2 O) at middle–low temperature, a systematic computational investigation is performed on β-MnO 2 (110) by first-principles calculations together with microkinetic analysis. Herein, the favored reaction pathways are unveiled. (i) NH 3 tends to adsorb at the unsaturated Lewis acid Mn 5c site on MnO 2 (110) and then partially dissociates into NH 2 * (assisted by the surface lattice O bri ) at the steady state, triggering the subsequent reactions. (ii) Interestingly, NO, either in the gas phase or at the adsorbed state, can readily react with NH 2 * to give the key intermediate NH 2 NO, with the former (i.e., the Eley–Rideal pathway) being slightly more kinetically preferred. (iii) NH 2 NO conversion is identified to proceed easily to N 2 through the dehydrogenation/hydrogenation processes NH 2 NO → NHNO → NHNOH → N 2 + H 2 O. (iv) The removal of the accumulated surface H into H 2 O, assisted by O 2, is relatively difficult, which preferentially occurs via the Mars–van Krevelen mechanism. Quantitatively, a kinetic analysis is conducted to deal with such a complex reaction network, revealing that the rate-limiting steps are NH 2 * + NO(g) → NH 2 NO* and O bri H + O 2 # →OOH# + O bri . Moreover, a sensitivity analysis shows that the adsorption strengths of H on O bri and O 2 in the O bri vacancy (O vac ) are two main activity-determining factors for the overall NH 3 -SCR on MnO 2 (110); notably, it is further found that the O vac formation energy correlates well with both factors and can thus serve as a unified activity descriptor. In addition, the effects of catalyst surface environment under the reaction conditions on the NH 3 -SCR activity and selectivity are discussed. In comparison with the pristine state of MnO 2 (110), both the overall activity and N 2 selectivity (versus N 2 O) would be interestingly enhanced when it arrives at the kinetically steady state that the surface O bri are largely covered by H. These results could provide a consolidated theoretical basis for understanding and optimizing MnO 2 catalysts for the NH 3 -SCR process.