Reaction mechanism and kinetics of NOx reduction by methane on In/ZSM-5 under lean conditions

Abstract

Indium supported on ZSM-5 was investigated as a promising catalyst for NOx reduction by methane under lean conditions. Indium on protonated H-ZSM-5 showed a higher activity in dry conditions while indium on unprotonated ZSM-5 with a high loading of 7.3% prepared by ion exchange had a higher activity in the presence of 8% water. NO2 in feed promoted the methane oxidation and NOx reduction. In situ FTIR analysis revealed on In/ZSM-5 the presence of inhibiting compounds at lower temperatures and many carbonaceous surface compounds. In the reaction initiation conditions, the surface coverage of nitrogen containing carbonaceous compounds and any adsorbed species were low. It is proposed that the formation of NO2, partially oxidized methane surface derivatives and actual surface reductants containing NC or NH bondings are crucial in NO reduction. Intrazeolitic InO+ was proposed to be the active catalytic site in NO reduction. Free In2O3, detected by XRD and XPS, has possibly a promoting effect on the reactions. A micro kinetic model, including the surface intermediates, was derived for an In/ZSM-5 catalyst by the NOCH4O2 experiments where the reactant concentrations, space velocity and temperature were varied. The adsorbed H2NCO intermediate, formed in the reaction between NO2 and partially oxidized methane, was proposed to act as an actual NO reductant in the reaction mechanism and kinetic equations. The model was able to follow measured responses and predict the dynamic performance in NOCH4O2 reactions quantitatively in usual steady state lean conditions. Adsorbed oxygen and NO2 were simulated to exist with higher coverage on active sites in reaction conditions. Gas and adsorbed compounds were simulated as a function of reactor length in different unmeasured conditions, which simulations can be used as a tool in catalyst reactor design.