Density functional theory investigation of cobalt siting in ferrierite

Cobalt-exchanged ferrierite is a selective catalyst for the reduction of nitrogen oxides in the presence of oxygen. The activity of each isolated cobalt cation has been suggested to depend on the local zeolite environment. Two of the three cobalt extraframework sites proposed in the literature were investigated with density functional theory calculations. The electronic structure, geometry, and infrared spectra of eight clusters were calculated and compared to experiment. Exchanged cobalt cations were most stable in the high-spin electronic configuration. The stability of cobalt in each site was evaluated by calculating the cobalt-zeolite binding energy. The calculations also show the important role of zeolite aluminum location in extraframework cobalt siting. Only one of the eight clusters was able to reproduce all of the experimental characteristics of the predominant cobalt species. A secondary cobalt species was predicted to most likely correspond to one of four possible candidate clusters.