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

Abstract

MnO_X-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 MnO_X-CeO₂ in NO reduction.