Surface Reconstruction of a Mullite-Type Catalyst via Selective Dissolution for NO Oxidation

Abstract

We present a facile selective dissolution method for the surface modification of SmMn2O5 mullite (SMO) to increase the exposed sites for NO oxidation on the premise of excellent thermal stability. Surface Sm cations are partially removed during the treatment (SMO-H), leading to the exposure of B-site cations with higher electronegativity and oxygen vacancies. Under laboratory-simulated diesel combustion conditions, the SMO-H exhibits higher NO oxidation activity than the SMO and is comparable to that of the Pt/Al catalyst under a gas hourly space velocity (GHSV) of 120 000 mL g–1 h–1. In addition, the SMO-H possesses good thermal and steam stability during a 50 h test at 300 °C. X-ray absorption spectroscopy (XAS) and electron paramagnetic resonance (EPR) spectroscopy results reveal that the SMO-H presents more Mn4+O6 octahedral groups and oxygen vacancies than the SMO. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), and density functional theory (DFT) calculations show that the valence of surface Mn decreased after NO capture up to 300 °C and fulfilled the adsorption procedure as nitrite and/or nitrate species when O2 is involved in the flue gas. The results indicate that Mn-terminated surface rather than Sm-terminated one is the major adsorption site for NO.

Constructing a Pt/YMn2O5 Interface to Form Multiple Active Centers to Improve the Hydrothermal Stability of NO Oxidation

“…O v binds very readily to oxygen molecules, which in turn generates more reactive oxygen species and facilitates the inhibition of competitive adsorption of NO and oxygen molecules in the NO oxidation reaction. 1,49 And the lattice oxygen (O latt ) is present at low binding energies. 50−52 In particular, O lab was attributed to labile oxygen.…”

“…Therefore, diesel oxidation catalysts (DOCs) are installed in the tail gas post-treatment line of diesel vehicle exhausts to treat the generated NO to meet the strict emission standards. 1 Given that the downstream diesel particle filter (DPF) requires NO 2 as an oxidant to participate in the reaction and that the rapid selective catalytic reduction (SCR) reaction requires a 1:1 molar ratio of NO to NO 2 , DOCs play a pivotal role in the conversion of NO to NO 2 in DOC + DPF + SCR + ASC, a diesel vehicle reprocessing system in use. Moreover, with increasingly stringent regulations and the increase of catalyst life requirements, as well as when the back-end DPF particulate matter accumulates to a certain extent, oil injection is required to remove soot particles by oxidation, resulting in a sharp increase in the tail gas temperature.…”

“…However, the difficulty of removing NO from their exhaust gases has limited their development. Therefore, diesel oxidation catalysts (DOCs) are installed in the tail gas post-treatment line of diesel vehicle exhausts to treat the generated NO to meet the strict emission standards . Given that the downstream diesel particle filter (DPF) requires NO 2 as an oxidant to participate in the reaction and that the rapid selective catalytic reduction (SCR) reaction requires a 1:1 molar ratio of NO to NO 2 , DOCs play a pivotal role in the conversion of NO to NO 2 in DOC + DPF + SCR + ASC, a diesel vehicle reprocessing system in use.…”

Ultralow-Temperature NOx Reduction over SmMn2O5 Mullite Catalysts Via Modulating the Superficial Dual-Functional Active Sites

“…Wang et al proved that Mn–Mn dimers in Mn-mullite catalysts own the special activation behaviors of NO and can further promote the conversion of NO into NO 2 . Surface modification of SmMn 2 O 5 mullite catalysts by a method of selective dissolution using nitric acid can increase Mn 4+ O 6 octahedral groups and improve NO or propane oxidation performance. , However, the synergistic effects and the potential of multiple Mn sites over SmMn 2 O 5 -based catalysts in NO x reduction have not even been explored.…”

“…22 Surface modification of SmMn 2 O 5 mullite catalysts by a method of selective dissolution using nitric acid can increase Mn 4+ O 6 octahedral groups and improve NO or propane oxidation performance. 23,24 However, the synergistic effects and the potential of multiple Mn sites over SmMn 2 O 5based catalysts in NO x reduction have not even been explored. Herein, ultralow-temperature NO x reduction over acidetched SmMn 2 O 5 mullite catalysts has been originally demonstrated.…”