Reveal of free radicals in manganese-based catalysts and their roles during selective catalytic reduction of nitrogen oxide

Abstract

HypothesisManganese-based catalysts attract extensive attentions in low-temperature selective catalytic reduction (SCR) of nitrogen oxide (NOx). However, seldom work focuses on the existence of free radicals and their roles in SCR. ExperimentsIn this work, electron paramagnetic resonance spectrometer and density functional theory are combined to reveal surface characterizations of manganese oxide and Ce/La-doped manganese oxides. FindingsAs a result, superoxide radical (O2*) exists on manganese-oxide surface, produces nitrogen-containing free radical, and functions as electron transfer between NOx and NH3 at 100 °C, resulting in good NOx conversion as well as N2 selectivity at the same time. The O2* is born of accepting electrons on adsorbed oxygen through overlapped orbits of Mn-d and O-p. Additional metal doping increases the percentage of O2* among all oxygen species from 2.9 % to 6.9 % (Ce doping) and 5.1 % (La doping). The order of O2* percentage is consistent with the NOx-conversion order at 100 °C, i.e., Ce doping (91.6 %) > La doping (55.7 %) > catalyst without doping (27.5 %). Above result helps to understand interface behaviors of manganese-based catalyst in SCR. This work is also in favor of developing more effective low-temperature catalysts.