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

MnO X -CeO 2 mixed oxide is a highly effective catalyst for NO reduction with NH 3 in exhaust gases, demonstrating excellent NO conversion and N 2 selectivity at low temperatures. However, at high temperatures, both conversion and selectivity decline, leading to increased formation of N 2 O and NO 2 . 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 N 2 and H 2 O. At high temperatures, the Langmuir-Hinshelwood mechanism becomes predominant, involving the adsorption of both NH 3 and NO, with NO undergoing oxidation to nitrate species before reacting with NH 3 -derived intermediates. The excessive deprotonation of adsorbed NH 3 under these conditions leads to the formation of N 2 O as a by-product. These insights provide a deeper understanding of the temperature-dependent catalytic performance of MnO X -CeO 2 in NO reduction. • FTIR of NO SCR with NH 3 on MnO X -CeO 2 reveals two distinct reaction pathways. • Low temperature, Eley-Rideal (ER); high temperature, Langmuir-Hinshelwood (LH). • In ER mechanism, coordinated ammonia adsorbed on a Lewis site reacts with NO (g). • In LH mechanism, adsorbed NO is oxidised to nitrate species. • In LH mechanism, N 2 O is formed by the excessive deprotonation of adsorbed ammonia.