Selective Reduction of NO x by Methane on Co-Ferrierites : II. Catalyst Characterization

Co-ferrierite, active for the selective NO reduction by CH 4, was characterized by X-ray photon spectroscopy and magnetic susceptibility measurements. The adsorption of NO and NO 2 was studied by diffuse-reflectance FTIR spectroscopy to discover which intermediates participate in the NO x reduction. The valence state of cobalt in Co-ferrierite and other Co-zeolite catalysts was found to be 2+. The dominant NO species adsorbed on Co-Y, Co-ZSM-5, and Co-ferrierite is in a dinitrosyl form appearing at 1810 and 1897 cm −1 for Co-Y and 1810 and 1890 cm −1 for Co-ZSM-5 and Co-ferrierite. The mononitrosyl form of the adsorbed NO is a minor species appearing at 1930-1935 cm −1 on all three samples. The mononitrosyl species on all samples is extremely weakly adsorbed. The dinitrosyl species adsorbed on Co-ferrierite is strongly adsorbed and needs to be heated above 300°C to desorb. Interestingly, the weakly adsorbed mononitrosyl species is enhanced in an O 2 environment (100 Torr of O 2) and is now stable to 200°C. However, in an oxygen environment, the dinitrosyl species is less stable, desorbing at ∼200°C. All adsorbed NO species disappear at >200°C in 100 Torr O 2, and adsorbed NO 2 species were observed. NO 2 adsorbed on Co-ferrierite shows a weakly adsorbed, covalent N 2O 5 in addition to stable species, such as nitro, nitrito, and nitrato species. Together with earlier kinetic and reaction studies, we suggest a mechanism for the selective reduction of NO x by CH 4.