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

A NbO x /Ce 0.75 Zr 0.25 O 2 (NbCZ) catalyst was synthesized by a citric acid-aided sol-gel method. It shows that above 80% NO x conversion and above 95% N 2 selectivity for the selective catalytic reduction of NO x by ammonia over this catalyst are achieved in the temperature range 200–450 °C. Based on the DRIFTS and kinetic studies over NbCZ and Ce 0.75 Zr 0.25 O 2 , the promotion mechanism by niobia loading was elucidated with an overall reaction pathway. Two different reaction routes, “L-H” mechanism via “NH 4 NO 3 + NO” at low temperatures (<200 °C) and “E-R” mechanism via “NH 2 + NO” at high temperatures (>350 °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-NH 3 and nitrates species is accelerated to form NH 4 NO 3 intermediates, which then decompose to N 2 and H 2 O. The reaction of the ads-NH 3 species with gaseous NO x at high temperatures is also promoted due to the enhanced acidity and weakened thermal stability of nitrates after niobia loading. [ABSTRACT FROM AUTHOR]