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Article

Theoretical Study on Silver- and Gold-Loaded Zeolite Catalysts: Thermodynamics and IR Spectroscopy

Anibal Sierraalta,*^† Rafael Hernandez-Andara,^† and Elena Ehrmann^‡

Laboratorio de Qumica Computacional, Centro de Qumica, Instituto Venezolano de Investigaciones Cientficas, Apartado 21827, Caracas 1020-A, Venezuela, and Departamento de Qumica, Universidad Simn Bolvar, Apartado 89000, Valle de Sartenejas, Baruta-Edo Miranda, Venezuela

J. Phys. Chem. B, 2006, 110 (36), pp 17912–17917

DOI: 10.1021/jp063911p

Publication Date (Web): August 23, 2006

Corresponding author. E-mail: asierral@ivic.ve.

†

Laboratorio de Química Computacional, Centro de Química, Instituto Venezolano de Investigaciones Científicas.

‡

Departamento de Química, Universidad Simón Bolívar.

Abstract

Density functional calculations have been carried out to determine geometries, adsorption energies and vibrational frequencies of NO, N₂O, CO, O₂, and H₂O, on a model for Ag(I) and Au(I) ion-exchanged ZSM-5 catalysts. Using statistical mechanics, the ΔH and ΔG 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 °C the adsorption of two NO molecules and the direct N₂O decomposition on AgZSM-5 are thermodynamically unfavorable. The adsorption of one NO molecule presents a small positive ΔG value. On the contrary, in the case of AuZSM-5, the adsorption of one NO molecule and the direct N₂O decomposition to produce N₂ are thermodynamically favorable. For both models, the N₂O decomposition by AgO and AuO species is thermodynamically very favorable. The analysis of the interaction with H₂O 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 H₂O than the AgZSM-5 catalyst.