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Supported Au/Al 2 O 3 samples of different gold loadings were prepared by the deposition-precipitation of AuCl 3 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 (Au s O x ) and gold chloride (Au s Cl y ) on the surface of supported gold crystallites, and of gold ions (Au i ) incorporated into the subsurface of alumina support, respectively. The stoichiometry x of Au s O x 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. |
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