Enhanced activity and N2 selectivity for manganese oxides catalysts modified with transition metals: Mechanism and N2O formation pathways

Various transition metals were employed to enhance the catalytic performance and N2 selectivity of Mn-based catalysts. The modification of Fe and Cu promoted the low-temperature activity, while the Al-modified catalyst displayed the highest N2 selectivity close to 100 %. The characterization results proved that the content of Oβ and Mn4+ were increased after adding transition metals. N2 selectivity experiments revealed that the reduction of NO to N2O and the oxidation of NH3 to N2O did not occur. The product of NH3 oxidation was only N2. Combined with the results of the in-situ DRIFTS experiments, it was proposed that N2O was generated from the decomposition of NHNO and NH4NO3 in the SCR reaction. The poor N2 selectivity of the unmodified catalyst was related to the existence of inactive N2O22- species on the surface, while the high content of active NH2- species on the Mn0.1Ho0.005Ce0.015Al0.01/TiO2 catalyst enhanced its catalytic activity and N2 selectivity. In addition, the results also revealed that only the E-R mechanism existed on the unmodified catalyst, while both the E-R and L-H mechanism were activated after the transition metal modification.