NO Adsorption, Desorption, and Reduction by CH 4 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 Mn 2+ results in the slow oxidation to Mn 3+, with the simultaneous formation of N2O. During NO exposure the intensity of the band for Mn 2+(NO) (1894 cm −) decreases while that for Mn 3+(O −)(NO) (1966 cm −1) increases. Elevating the temperature, or introducing O 2, converts the Mn 3+(O −)(NO) to NO 2/NO 3 species. Adsorbed NO 2/NO 3 species are more stable to high temperature than NO. During the reduction of NO by CH 4 in the presence of O 2, NO 2 is formed via the oxidation of NO. Adsorbed NO 2 then reacts with CH4. Cyanide species are observed and found to react very rapidly with NO 2, leading to the formation of N 2 and CO 2. A series of elementary steps are proposed to account for the reduction of NO by CH 4 in the presence of O 2. As a part of this mechanism, it is hypothesized that the formation of N2 and N2O occurs via the processes Mn 3+(O −)(CN) + NO 2 → Mn 3+(O −) + CO 2 + N 2, and Mn 3+(O −)(CN) + NO 2 → Mn 2+ + CO 2 + N 2O, respectively.