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, O2, OH, CO, N2, 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.