Chemical reactivity of supported gold. A structural study by small-angle X-ray scattering and x-ray absorption spectroscopy

Preparations of Au supported on SiOz and Al2O3 were characterized by small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS), extended X-ray absorption fine structure spectroscopy (EXAFS), and transmission electron microscopy (TEM). The LIII X-ray absorption coefficient was used in the EXAFS analysis and the radial structure function was calculated. The interatomic distances and coordination numbers of Au preparations were obtained. EXAFS data indicated an interaction between Au and A1203, while no evidence of interaction between gold and SiOz could be detected. The average Au-Au coordination number was also determined on the basis of particle diameters measured by SAXS and WAXS. Percentages of total Au present in the metallic phase, directly determined by EXAFS, were compared with the ones computed on the basis of SAXS and WAXS particle size measurements. By SAXS measurements normal-logarithmic particle size distribution curves and metal surface area were obtained. From the particle size and Au concentrations so determined the percentage of Au exposed was calculated. The catalytic activity of the three catalysts examined was studied for the isotopic equilibration reaction H2(g) + D2(g) - BHD(g), and for the reduction of NO by Hz. Moreover, selectivity measurements are reported for the reduction of NO to Nz. An interpretation of possible relationships among catalytic activity, selectivity, degree of dispersion of metal, and interaction between gold and support material is discussed.