Investigation of the selective catalytic reduction of nitric oxide with ammonia over Mn/TiO2 catalysts through transient isotopic labeling and in situ FT-IR studies

Abstract

The transient isotopic labeling studies were performed under steady state conditions by using 15N (15NO and 15NH3) and 18O (18O2) containing species to investigate the reaction mechanism of the low-temperature SCR of NO over Mn/TiO2. Our investigation has suggested that the nitric oxide forms neither nitrous oxide nor it is oxidized to nitrogen dioxide with the gas phase oxygen. The results acquired with the time resolution illustrated that the reaction of ammonia with lattice oxygen was practically instantaneous. The formation of labeled N218O, N18O, and H218O species is evident through the NH3 contact with catalyst surface lattice oxygen but not direct contact with the gas phase oxygen. This is consistent with the view that the lattice oxygen of Mn/TiO2 catalyst has direct effect on the reaction mechanism rather than gas phase oxygen. Our labeling experiments suggest that the SCR reaction rate is very small in the absence of gas phase oxygen and enhanced distinctly by the addition of excess oxygen. The cross-labeled 15N14N is the dominant form of nitrogen during the NH3 labeling studies. The increase in the SCR reaction temperature monotonically enhanced the ammonia oxidation. The evolution of 15N14N, 15N14NO, and 15NO species from the labeled ammonia (15NH3) and unlabeled nitric oxide (14NO) revealed the occurrence of ammonia oxidation. The results obtained in these experiments were combined with oxygen exchange experiments to arrive at conclusions regarding the plausible mechanism of the SCR reaction. The oxygen exchange between nitric oxide and the catalyst lattice oxygen established in the catalytic reduction of NO reaction. The effect of oxygen on activity and selectivity is noteworthy in both SCR and ammonia oxidation reactions over Mn/TiO2. The role of ammonia oxidation has been quantified for the overall SCR reaction network at low-temperatures. The role of ammonia oxidation and surface oxygen species has been addressed to substantiate the conclusions drawn.