Remarkable N2-selectivity enhancement of practical NH3-SCR over Co0.5Mn1Fe0.25Al0.75Ox-LDO: The role of Co investigated by transient kinetic and DFT mechanistic studies

A Co0.5Mn1Fe0.25Al0.75Ox-LDO catalyst was developed which showed excellent performance for the low-temperature NH3-SCR. NOx conversions 100% were achieved in the whole 100-250 degrees C range, while after 10-h operation at 150 degrees C with 100 ppm SO2/5 vol% H2O in the feed, the NOx conversion was maintained at 80%. This catalyst provided a much better N-2-selectivity than the Mn1Fe0.25Al0.75Ox-LDO and Mn1Al1Ox-LDO, especially at 150-300 degrees C. It was found that Co0.5Mn1Fe0.25Al0.75Ox possessed higher surface acidity and reducibility, while XPS analyses indicated an electron transfer between Co3+/Co2+ and Mn4+/Mn3+ redox cycles, leading to a much lower N2O formation, supported by Density Functional Theory (DFT) calculations. Detailed analysis of gas responses obtained upon various step-gas switches was performed, which allowed to measure the surface concentration and reactivity of preadsorbed NOx-s and NHx-s leading to N-2 and N2O. Transient kinetic and DFT studies strongly suggested likely mechanisms of NH3-SCR and the critical role of Co for N-2-selectivity enhancement.