NO adsorption, desorption, and reduction by CH4 over Mn-ZSM-5

An investigation of the interaction of NO with Mn-ZSM-5 as well as the reduction of NO by methane has been conducted using in situ infrared spectroscopy and mass spectrometry. Adsorption of NO at room temperature on Mn2+ results in the slow oxidation to Mn3+, with the simultaneous formation of N2O. During NO exposure the intensity of the band for Mn2+(NO) (1894 cm(-1)) decreases while that for Mn3+(O-)(NO) (1966 cm(-1)) increases, Elevating the temperature, or introducing O-2, converts the Mn3+(O-(NO) to NO2/NO3 species. Adsorbed NO2/NO3 species are more stable to high temperature than NO. During the reduction of NO by CH4 in the presence of O-2, NO2 is formed via the oxidation of NO. Adsorbed NO2 then reacts with CH4. Cyanide species are observed and found to react very rapidly with NO2, leading to the formation of N-2 and CO2. A series of elementary steps are proposed to account for the reduction of NO by CH4 in the presence of O-2. AS a part of this mechanism, it is hypothesized that the formation of N-2 and N2O occurs via the processes Mn3+(O-)(CN) + NO2 --> Mn3+(O-) + CO2 + N-2, and Mn3+(O-)(CN) + NO2 --> Mn2+ + CO2 + N2O, respectively. (C) 1997 Academic Press.