NH3-SCR reaction mechanisms of NbOx/Ce0.75Zr0.25O2 catalyst: DRIFTS and kinetics studies

Journal article, 2016

A NbOx/Ce0.75Zr0.25O2 (NbCZ) catalyst was synthesized by a citric acid-aided sol-gel method. It shows that above 80% NOx conversion and above 95% N-2 selectivity for the selective catalytic reduction of NOx by ammonia over this catalyst are achieved in the temperature range 200-450 degrees C. Based on the DRIFTS and kinetic studies over NbCZ and Ce0.75Zr0.25O2, the promotion mechanism by niobia loading was elucidated with an overall reaction pathway. Two different reaction routes, "L-H" mechanism via "NH4NO3 + NO" at low temperatures (<200 degrees C) and "E-R" mechanism via "NH2 + NO" at high temperatures (>350 degrees C), are presented. The niobia addition increases the surface acidity and promotes the formation of nitrates species at low temperatures. In this way, the reaction between the ads-NH3 and nitrates species is accelerated to form NH4NO3 intermediates, which then decompose to N-2 and H2O. The reaction of the ads-NH3 species with gaseous NOx at high temperatures is also promoted due to the enhanced acidity and weakened thermal stability of nitrates after niobia loading.