Modeling nitrogen oxide catalysis over cobalt-exchanged zeolites: The role of cobalt-zeolite coordination environment

Nitrogen oxides (NO_x) are by-products of combustion and are precursors of atmospheric smog and acid rain. Exhaust gases must be treated after combustion to prevent NO_x from reaching the atmosphere. The primary treatment technology is catalytic. The catalytic converter has been very successful for removing NO_x from automobile exhaust, but only when the air-fuel mixture is nearly stoichiometric. In order to take advantage of the increased fuel efficiency associated with lean-burn combustion conditions, a new catalyst is needed. Metal-exchanged zeolites have been shown to be promising for treating NO_x from oxygen-rich exhaust streams.; Quantum chemical calculations were performed to investigate the role of the metal-zeolite coordination environment on the catalytic activity for NO_x reduction. The properties of divalent cobalt cations in two ferrierite extraframework sites were calculated and compared to experiment. Specific cobalt-ferrierite environments were proposed based on the relative agreement between theory and experiment. The infrared spectra of ferrierite exchanged with six additional divalent metals were also calculated to resolve the general characteristics of metal-induced infrared bands in ferrierite.; Molecular adsorption is the first step in a catalytic reaction. NO adsorption was examined in detail to elucidate the differences in the cobalt-nitrosyl structures, adsorption thermodynamics, and temperature programmed desorption profiles as a function of the local zeolite environment. Two primary factors were identified that control the preferential adsorption of molecules to cobalt in the ferrierite B site over the G site: the relative coordinative unsaturation of cobalt and the degree of adsorbate-induced strain on the cobalt-zeolite environment. The ability of the zeolite environment to promote secondary hydrogen bonds as well as the electrostatic field of the cobalt-zeolite environment were also identified as factors influencing preferential adsorption. Hence, the local zeolite environment near cobalt has a strong influence on the molecular adsorption properties of cobalt-ferrierite. This suggests that the catalytic activity of cobalt-exchanged zeolites for the selective catalytic reduction of NO_x depends on the same general factors.