Density functional studies of adsorbates in Cu-exchanged zeolites:: model comparisons and SOx binding

The robustness of density functional theory cluster model predictions for a variety of adsorbate complexes in Cu-exchanged zeolites is tested via systematic comparisons of four different models. Results for O, O-2, OH, CO, N-2, NOx, and SOx binding, obtained using an Al(OH)(4) representation of the zeolite support (Z), are in excellent agreement with analogous results for a larger, Z = Al[OSi(OH)(3)](4) model. Smaller models, containing either a bare Cu+ ion or a water-ligand representation of Z, perform well for covalently bonded adsorbates such as CO or NO, but yield pronounced differences for adsorbates whose binding has a large electrostatic component. Only the largest model considered is fully adequate for adsorbed H2O because of its ability to form hydrogen bonds to distant framework oxygen. SO2 and SO3 exhibit a variety of binding modes and, under appropriate conditions, lead to highly stable ZCu-SO4 or ZCu-SO4-CuZ complexes. Such complexes are consistent with X-ray absorption data for Cu-ZSM-5 and are most likely responsible for the reversible inhibition of catalytic activity in this material by SO2.