Characterization of Gold(I) in NaY Zeolite and Acidity Generation

Abstract

Au(I) ions were incorporated into NaY zeolite via solid-vapor reaction between Au 2Cl 6 vapor and partially dehydrated zeolite in high vacuum. The process involved the evolution of HCl, as monitored by temperature-programmed desorption-mass, resulting in the reduction of Au(III) into Au(I). X-ray photoelectron spectroscopy characterization of Au n+ /NaY revealed the Au 4 f 7/2 peak exhibited a chemical shift by +1.95 eV relative to that of Au 0, implying the formation of surface AuCl or like species. Upon the introduction of Au 2Cl 6 into NaY zeolite, the IR OH band at 3690 cm −1 due to cation-water interaction in NaY zeolite declined. On the other hand, a strong hydroxyl band at 3640 cm −1 was delivered due to polarization of H 2O in zeolite by the electrostatic potential associated with the Au n+ ( n > 1) ions. Adsorption of D 2O onto Au 2Cl 6/NaY at room temperature produced a peak at 2656 cm −1 due to OD species associated with Au n+ ( n > 1). Degassing of D 2O at 333 K reduced this peak in favor of a new band at 2690 cm −1 which is assigned to O-D stretching mode in the Au (OD)Cl moiety. The CO chemisorption onto Au(I)/NaY at 77 K followed by evacuation to reduced pressures showed a unique carbonyl band at 2188 cm −1, which is characteristic of Au(I) ions. The evolution of HCl during the solid-vapor reaction produced protonic sites, which were detected by Fourier transform infrared of pyridine adsorption. The concentration of these sites increased up to 353 K, where the solid-vapor reaction leveled off. The subsequent pyridine-Cl 2 adsorption did not lead to changes in the oxidation state of Au(I) ions, indicating that NaY zeolite stabilized their structure.