Mixed-Phase Oxide Catalyst Based on Mn-Mullite (Sm, Gd)Mn 2 O 5 for NO Oxidation in Diesel Exhaust

Abstract

Oxidation of nitric oxide (NO) for subsequent efficient reduction in selective catalytic reduction or lean NO_x trap devices continues to be a challenge in diesel engines because of the low efficiency and high cost of the currently used platinum (Pt)–based catalysts. We show that mixed-phase oxide materials based on Mn-mullite (Sm, Gd)Mn₂O₅ are an efficient substitute for the current commercial Pt-based catalysts. Under laboratory-simulated diesel exhaust conditions, this mixed-phase oxide material was superior to Pt in terms of cost, thermal durability, and catalytic activity for NO oxidation. This oxide material is active at temperatures as low as 120°C with conversion maxima of ~45% higher than that achieved with Pt. Density functional theory and diffuse reflectance infrared Fourier transform spectroscopy provide insights into the NO-to-NO₂ reaction mechanism on catalytically active Mn-Mn sites via the intermediate nitrate species.