Theoretical study on silver- and gold-loaded zeolite catalysts: thermodynamics and IR spectroscopy

Density functional calculations have been carried out to determine geometries, adsorption energies and vibrational frequencies of NO, N(2)O, CO, O(2), and H(2)O, on a model for Ag(I) and Au(I) ion-exchanged ZSM-5 catalysts. Using statistical mechanics, the DeltaH and DeltaG values were calculated in order to evaluate the stability of the adsorbates on Ag(I) and Au(I) sites. The calculated vibrational frequencies are in reasonable agreement with the reported experimental values. The analysis of the results shows that at 475 degrees C the adsorption of two NO molecules and the direct N(2)O decomposition on AgZSM-5 are thermodynamically unfavorable. The adsorption of one NO molecule presents a small positive DeltaG value. On the contrary, in the case of AuZSM-5, the adsorption of one NO molecule and the direct N(2)O decomposition to produce N(2) are thermodynamically favorable. For both models, the N(2)O decomposition by AgO and AuO species is thermodynamically very favorable. The analysis of the interaction with H(2)O shows that water displaces the adsorbed NO on AgZSM-5 but not on AuZSM-5 which indicates that the AuZSM-5 catalyst is less sensitive to deactivation by H(2)O than the AgZSM-5 catalyst.