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

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.