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

Abstract

To understand the molecular-level reaction mechanism and crucial activity-limiting factors of the NH3-SCR process catalyzed by MnO2-based oxide to eliminate NO (4NH3 + 4NO + O2 →4N2 + 6H2O) at middle–low temperature, a systematic computational investigation is performed on β-MnO2(110) by first-principles calculations together with microkinetic analysis. Herein, the favored reaction pathways are unveiled. (i) NH3 tends to adsorb at the unsaturated Lewis acid Mn5c site on MnO2(110) and then partially dissociates into NH2* (assisted by the surface lattice Obri) 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 NH2* to give the key intermediate NH2NO, with the former (i.e., the Eley–Rideal pathway) being slightly more kinetically preferred. (iii) NH2NO conversion is identified to proceed easily to N2 through the dehydrogenation/hydrogenation processes NH2NO → NHNO → NHNOH → N2 + H2O. (iv) The removal of the accumulated surface H into H2O, assisted by O2, 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 NH2* + NO­(g) → NH2NO* and ObriH + O2# →OOH# + Obri. Moreover, a sensitivity analysis shows that the adsorption strengths of H on Obri and O2 in the Obri vacancy (Ovac) are two main activity-determining factors for the overall NH3-SCR on MnO2(110); notably, it is further found that the Ovac 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 NH3-SCR activity and selectivity are discussed. In comparison with the pristine state of MnO2(110), both the overall activity and N2 selectivity (versus N2O) would be interestingly enhanced when it arrives at the kinetically steady state that the surface Obri are largely covered by H. These results could provide a consolidated theoretical basis for understanding and optimizing MnO2 catalysts for the NH3-SCR process.

CATKINAS: A large‐scale catalytic microkinetic analysis software for mechanism auto‐analysis and catalyst screening

“…Based on CATKINAS, we have completed a series of work ranging from theoretical methods development to catalyst design of different systems. [36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][53][54] The package aims to construct a systematic microkinetic modeling platform and our group is continuing to perfect it with more functions in the broad field of rational catalyst design, for example, automatic generation of reaction networks and interaction with first-principles calculation packages or external databases.…”

“…The reversibility iteration method (RIM) [36] is firstly developed in an F I G U R E 3 The reaction rate (A), C# coverage distribution (B), coverage summary (C), reversibility of step R1 (D) and the rate (E) and coverage prediction (F) after calculation of 5% of the points . [37][38][39][40][41][42][43][44][45][46][47][48][49][50]…”

“…So far, in the simulation of dozens of different reaction systems and conditions, millions of solutions have been solved without error (under reasonable and meaningful reaction conditions). [ 37–50 ]…”

The Role of Alkali Metal in α‐MnO2 Catalyzed Ammonia‐Selective Catalysis

“…First, K + insertion increased the adsorption energy of NH 3 on vacancy‐free α‐MnO 2 (−2.17 eV vs. −1.35 eV) significantly (Figure e; Supporting Information, Figures S19 and S20). Generally, the NH 3 * (* stands for the active site on the surface) was firstly activated, dissociated and then reacted with the gaseous or adsorbed NO . Thereafter, the N−H bond is broken by O bri species and NH 3 * transformed into NH 2 * and H* through dehydrogenation process (NH 3 *+O bri →NH 2 *+O bri H), the transition states were shown in Figure f,g (see also the Supporting Information, Figures S19 and S20).…”

The Role of Alkali Metal in α‐MnO2 Catalyzed Ammonia‐Selective Catalysis

“…Generally,the NH 3 *(*stands for the active site on the surface) was firstly activated, dissociated and then reacted with the gaseous or adsorbed NO. [16] Thereafter,t he NÀH bond is broken by O bri species and NH 3 *t ransformed into NH 2 *a nd H* through dehydrogenation process (NH 3 * + O bri !NH 2 * + O bri H), the transition states were shown in Figure 5f,g (see also the Supporting Information, Figures S19 and S20). When interfacial oxygen vacancy was formed, it is also found that no matter NH 3 absorbed on the newly formed Mn 5c site or original Mn 5c site,the adsorption energy on K-a-MnO 2Àx (x for the oxygen vacancy)w as significantly higher than that of a-MnO 2Àx ,which is consistent with the results of NH 3 -TPD.F or the K-a-MnO 2Àx ,t he adsorption energy of NH 3 (À1.87 eV) on the original Mn 5c site was higher than the new Mn 5c site (À1.76 eV).…”