Origin of Ammonia Selective Oxidation Activity of SmMn₂O₅ Mullite: A First-Principles-Based Microkinetic Study

Abstract

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.

Nanoporous CeO2-Decorated Cu-SmMn2O5 Composite Mullite Oxide Catalyst for Toluene Oxidation

“…The results of O 2 -TPD are shown in Figure b. According to the previous studies, the peaks below 400 °C are ascribed to surface-adsorbed oxygen species, the peaks between 400 and 600 °C are regarded as surface lattice oxygen species, and the peaks over 600 °C represent the bulk lattice oxygen species. , It is widely accepted that surface-adsorbed oxygen species are active in oxidizing small molecules. , There are nearly no peaks observed in the SMO sample due to the poor mobility of oxygen species even though the temperature reaches 800 °C, which is reported by the previous work of our group . Four main peaks representing the desorption of different oxygen species are observed in the SCMO-Cu-IE sample within the temperature range.…”

“…17,47 It is widely accepted that surface-adsorbed oxygen species are active in oxidizing small molecules. 20,47 There are nearly no peaks observed in the SMO sample due to the poor mobility of oxygen species even though the temperature reaches 800 °C, which is reported by the previous work of our group. 10 Four main peaks representing the desorption of different oxygen species are observed in the SCMO-Cu-IE sample within the temperature range.…”

“…At the current stage, introducing oxygen vacancies into the crystal structure of mullite oxide serves as an efficient way to solve the above-mentioned problem. − Researchers have developed various strategies to create oxygen vacancies including elemental doping − and interfacial decoration. ,, For example, Shen et al reported that Cu ions were doped into the crystal lattice of SmMn 2 O 5 replacing Mn 3+ ions to form the Cu 2+ –O–Mn 4+ structure . This Cu doping with inequivalence successfully created a large number of oxygen vacancies and weakened the strength of the Mn–O bond, thus enhancing the redox ability to convert lattice oxygen into active oxygen species.…”

Unraveling the Lattice O Assisted Internal Selective Catalytic Reduction Mechanism on High N2 Selectivity of CuOx/PtCu Catalysts in NH3-SCO

“…, Fe-ZSM-5 and Cu-SSZ-13), − non-noble metal oxides ( e.g. , Cu-Mg-Al hydrotalcite-derived mixed metal oxides, CuO, and Fe 2 O 3 ), ,− and bifunctional catalyst ( e.g. , Pt/Al 2 O 3 + Cu-SSZ-13 and Pt/Al 2 O 3 + Fe-ZSM-5). − Comparably speaking, noble metal catalysts are studied extensively for their superior oxidation property, among which Pt/Al 2 O 3 is the most reliable in practical application due to its excellent NH 3 -SCO activity below 250 °C.…”