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

Abstract

Preparations of Au supported on Si02 and A1203 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 Lm 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 Si02 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) -»• 2HD(g), and for the reduction of NO by H2. Moreover, selectivity measurements are reported for the reduction of NO to N2. An interpretation of possible relationships among catalytic activity, selectivity, degree of dispersion of metal, and interaction between gold and support material is discussed.

In situ ESR study of gold supported on NaY zeolite

“…ESR is also a power tool to study the metal complexes and the nature of the ligands; there is extensive literature on the ESR spectroscopic properties of complexes of copper(II) with 4N or other ligands. [14] Gold(II) compounds would be paramagnetic with electron configuration 5d 9 as copper(II) compounds, it is possible to infer the existence of this oxidation state from a study of the ESR absorption spectra. Although the divalent state of gold is one of the most interesting oxidation states in transition metal chemistry, only a few paramagnetic Au 2+ complexes have been reported.…”

CO oxidation over Au/TiO2 prepared from metal-organic gold complexes

“…Although the early publications by Bond et al [1,2] and Parravano et al [3,4] reported that small gold particles exhibited catalytic activity for some reactions, such as hydrogenation of olefins and oxygen transfer reactions, gold was generally considered to be catalytically inactive [5]. Over the past 20 years, the volume of literature on the application of gold catalysts has been steadily increasing since Haruta reported that highly dispersed gold particles supported on metal oxides are extremely active for CO oxidation [6,7].…”

“…Gates and coworkers reported that a supported mononuclear gold complex was active for ethylene hydrogenation at 353 K [11]. Recently, Iwasawa and coworkers [12][13][14][15][16] prepared highly active CO oxidation catalysts by grafting Au-phosphine complexes onto as-precipitated Ti(OH) 4 . They found that the as-precipitated Ti(OH) 4 was essential for obtaining highly dispersed gold catalysts and the use of a conventional TiO 2 support resulted in a much less active catalyst with a particle size greater than 15 nm.…”

Small-Angle Scattering of Heterogeneous Catalysts

“…The first one was aimed at reducing the magnitude of the void scattering through some experimental procedures (Gunn, 1958). This procedure, known as the pore-maskant method, has been used in some SAXS laboratories (White et al, 1972;Renouprez & Imelik, 1973;Renouprez et al, 1974;Cocco et al, 1979;Lopez et al, 1992), but it is limited by the fact that the imbibition liquids may affect the catalyst in unknown ways, and it is difficult to ensure reproducibility in filling the pores. The intensities due to the metal phase can be used to determine the average particle sizes and/or the particle-size distribution N(d) (d = characteristic dimension of the particle).…”