Tm. Salama et al., EXAFS XANES, XRD, AND UV-VIS CHARACTERIZATION OF INTRAZEOLITIC GOLD(I) PREPARED BY MONOLAYER DISPERSION OF AUCL3 INSIDE NA-Y ZEOLITE/, Journal of physical chemistry, 100(9), 1996, pp. 3688-3694
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.