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 homogeneousl y dispersed on the zeolite surface by mechanical mixing at room temper ature 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 crystal line line at 2 theta = 14.70 degrees due to the condensation of the go ld species on the zeolite surface. The detected Au-O bond by the EXAFS technique has a coordination distance of 2.16 Angstrom, which is abou t the same as that registered for the tetrameric molecule of dimethylg old(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 interaction s were not apparent due to the removal of OH attached to Au(III) at 33 8 K, thereby causing the partial reduction of Au(III) to Au(I). In agr eement with the EXAFS data, the intensity of threshold resonance due t o 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 r esemble that recorded for the AuCl reference compound. This observatio n was associated with the UV-vis band broadening and shifting at 225 n m to the lower energy side because of the increase in population of th e bridged Au-Cl linkages over the terminal ones.