Characterizations of alumina-supported gold with temperature-programmed reduction

Supported Au/Al2O3 samples of different gold loadings were prepared by the deposition-precipitation of AuCl3 onto alumina. Prepared samples were physically characterized by techniques of atomic emission (ICP), X-ray diffraction (XRD), transmission electron microscope (TEM) and temperature-programmed reduction (TPR). According to TEM and XRD observations, the size of gold crystallites dispersed on samples freshly prepared at 300 K was ca. 8 nm. TPR characterizations of oxidized samples distinguished three kinds of reductive peaks at temperatures ca. 180, 300 and 1000 K. These peaks are attributed to reductions of gold oxides ((AuOx)-O-s) and gold chloride ((AuCly)-Cl-s) on the surface of supported gold crystallites, and of gold ions (Au-1) incorporated into the subsurface of alumina support, respectively. The stoichiometry x of (AuOx)-O-s, increased with the temperature of oxidation treatment (T-0) and attained a peak value of 2.0 ca. T-0=600 K. Using this adsorption stoichiometry, variations in the dispersions of gold crystallites on alumina were determined. Through dispersion measurements, supported nano-crystallites of gold were found to sinter at T>673 K in both oxidative and reductive environments. (C) 1998 Elsevier Science B.V. All rights reserved.