Unraveling the contribution of valence state to the NO oxidation activity of manganese oxides

Abstract

Mn-based catalysts are of immense interest in NO oxidation—a crucial subsystem of many deNOx techniques. To meet the industrial requirement and shed light on the effect of Mn valence state on catalytic activity, we synthesized stoichiometric ε-MnO2, which achieved outstanding NO oxidation activity at low temperatures (84% NO conversion at 150 °C), and a series of manganese oxides with different ratios of Mn4+/Mn3+ as comparisons. X-ray absorption spectroscopy and in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) indicated Mn4+ could accelerate the elementary surface reaction. We show that the surface redox efficiency, defined as the average reduction temperature obtained from H2 temperature-programmed reduction (H2-TPR), dictates the rate of surface reaction and further the activity of NO oxidation for various Mn-based catalysts, which can be a reference for advanced catalyst selection in NO catalytic oxidation.