Mechanism of the Selective Reduction of NOx over Co/MFI: Comparison with Fe/MFI

Abstract

Co/MFI and Fe/MFI have been compared in catalytic tests of NOx reduction with isobutane and methane. FTIR, TPD-MS, and MS data of catalytic results with labeled molecules reveal a similar multistep mechanism over both catalysts for reduction with isobutane, but NOx reduction with methane over Co/MFI is qualitatively different. Reactive NOy (nitro and nitrate) groups are assumed to be formed from NO and an adsorbed superoxide ion, which was identified by its EPR signal at 77 K. Formation of nitrate ions and NO2 ligands is easily explained with this model. With isobutane as the reductant, Fe/MFI is more active, but Co/MFI is more selective, leading to 100% N2 yield. The primary intermediate is a deposit, exposing nitrile and isocyanate groups; the latter react with NOx to form N2 with one N atom from the deposit and the other from NO. An important difference between Co/MFI and Fe/MFI is the formation of zerovalent metal clusters in the former case and its absence in the latter. Clusters of Co0n have been detected by EPR in catalysts with high Co loading after reduction at 300°C. They catalyze the H/D exchange of CH4 with D2. The possibility is discussed that under certain conditions such clusters could contribute to the activation of methane.