Effect of local framework heterogeneity on NO adsorption in cobalt-ferrierite

Quantum chemical calculations have been performed to evaluate the adsorption of NO on cobalt in several extraframework environments within ferrierite zeolites. All possible arrangements of the two nearby framework aluminum atoms have been considered for the B and G ferrierite cobalt sites. The cobalt-nitrosyl orientation, i.e., bent versus linear, depends on the local zeolite environment. The enthalpies and free energies of adsorption are also functions of the local zeolite environment. NO adsorption is favored by about 50 kJ/mol on cobalt in the B site compared to the G site. This large difference is attributable to two factors: differences in NO-induced strain on the cobalt-zeolite environments as well as an intrinsic difference based on the coordinative unsaturation of cobalt. A simulation of the temperature-programmed desorption profile reveals three distinct cobalt environments for adsorbed NO, showing the dependence of a macroscopic property that is related to catalytic activity on the local zeolite environment near a metal cation. This observation suggests that the increase in the turnover of NO to N₂ per cobalt atom with increasing cobalt loading reported in the literature may be related to the same factors. A detailed analysis of the cobalt-nitrosyl bond was also performed.