EXAFS/XANES, XRD, and UV-Vis characterization of intrazeolitic gold(I) prepared by monolayer dispersion of AuCl3 inside Na-Y zeolite

Preparations of AuCln in Na-Y zeolite were characterized by means of X-ray diffraction (XRD), ultraviolet diffuse reflectance (UV-vis), Au L(III)-edge X-ray absorption near-edge structure (XANES), and extended X-ray absorption fine structure (EXAFS). Au2Cl6 was first homogeneously dispersed on the zeolite surface by mechanical mixing at room temperature under a nitrogen atmosphere. The thermal evacuation of the mixed sample at 323 K for 72 h led to the eruption of a sole Au2Cl6 crystalline line at 2 theta = 14.70 degrees due to the condensation of the gold species on the zeolite surface. The detected Au-O bond by the EXAFS technique has a coordination distance of 2.16 Angstrom, which is about the same as that registered for the tetrameric molecule of dimethylgold(III) hydroxyl. Ramping the Au2Cl6-Na-Y reaction temperature to 338 K yielded appreciable amounts of Au(I) ions which were embedded with the Cl- ions in a (AuCl), zig-zag-like structure. The Au-O interactions were not apparent due to the removal of OH attached to Au(III) at 338 K, thereby causing the partial reduction of Au(III) to Au(I). In agreement with the EXAFS data, the intensity of threshold resonance due to the unoccupied d-states in the XANES spectrum of the preparation at 323 K declined following an increase of the temperature to 338 K, to resemble that recorded for the AuCl reference compound. This observation was associated with the UV-vis band broadening and shifting at 225 nm to the lower energy side because of the increase in population of the bridged Au-Cl linkages over the terminal ones.