Reaction pathway of NOX reduction on a MnOX-CeO2 catalyst: An in-situ FTIR study

MnOX-CeO2 mixed oxide is a highly effective catalyst for NO reduction with NH3 in exhaust gases, demonstrating excellent NO conversion and N2 selectivity at low temperatures. However, at high temperatures, both conversion and selectivity decline, leading to increased formation of N2O and NO2. To understand this behavior, the reaction mechanism was investigated using in situ FTIR spectroscopy, which revealed two distinct pathways. At low temperatures, an Eley-Rideal mechanism dominates, where ammonia adsorbed on Lewis acid sites reacts with gas-phase NO, forming nitrosamine intermediates that rapidly decompose into N2 and H2O. At high temperatures, the Langmuir-Hinshelwood mechanism becomes predominant, involving the adsorption of both NH3 and NO, with NO undergoing oxidation to nitrate species before reacting with NH3-derived intermediates. The excessive deprotonation of adsorbed NH3 under these conditions leads to the formation of N2O as a by-product. These insights provide a deeper understanding of the temperature-dependent catalytic performance of MnOX-CeO2 in NO reduction.