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

HYPOTHESIS: Manganese-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.

EXPERIMENTS: In 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.

FINDINGS: As 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.