Mechanistic study of NO reduction with methane over Co²⁺ modified ZSM-5 catalysts

NO reduction with methane over Co-ZSM-5 has been studied in an oxidizing atmosphere. Although the activity was reduced due to the poisoning of oxygen, NO decomposed over Co-ZSM-5 to nitrogen and oxygen in two different temperature ranges, 100-300°C and > 400°C. This suggests the presence of two types of Co²⁺ cations in ZSM-5. The adsorption of NO, NO₂ and CH₄, as well as the reduction of NO with methane in O₂ were studied with in-situ diffuse reflectance Fourier-transform infrared spectroscopy (DRIFT). NO and methane molecules were only weakly adsorbed on Co-ZSM-5, and would desorb completely at the reaction temperature. The presence of oxygen was found to enhance the NO interaction with Co-ZSM-5, leading to the formation of adsorbed NO₂ intermediates. The adsorbed NO₂ would activate methane molecules and yield hydrogen and methyl radicals, which could in turn react with NO₂ to generate nitromethane intermediates. A tentative reaction mechanism has been proposed to elucidate the production of N₂, N₂O, H₂O, formaldehyde and CO₂.