Mechanism of the selective reduction of NO_x over Co/MFI: Comparison with Fe/MFI

Co/MFI and Fe/MFI have been compared in catalytic tests of NO_x 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 NO_x reduction with methane over Co/MFI is qualitatively different. Reactive NO_y (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 NO_x 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 Co_n⁰ 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.