Characterization and Catalytic‐Hydrogenation Behavior of SiO₂‐Embedded Nanoscopic Pd, Au, and Pd–Au Alloy Colloids

Abstract

Colloids embedded in a silica sol-gel matrix were prepared by using fully alloyed Pd-Au colloids, and pure Pd and Au colloids stabilized with tetraalkylammonium bromide following a modified sol-gel procedure with tetrahydrofuran (THF) as the solvent. Tetraethoxysilicate (TEOS) was used as the precursor for the silica support. The molar composition of the sol was TEOS/THF/H2O/HCl = 1:3.5:4:0.05 for the bimetallic Pd-Au and TEOS/THF/H2O/HCl = 1:4.5:4:0.02 for Pd and Au monometallic systems. After refluxing, the colloid was added as a 4.5 wt % solution in THF for Pd-Au, 10.2 wt % solution in THF for Pd and 8.4 wt % solution in THF for Au at room temperature. The gelation was carried out with vigorous stirring (4 days) under an Ar atmosphere. Following these procedures, bimetallic Pd-Au-SiO2 catalysts with 0.6 and 1 wt % metal, and monometallic Pd- and Au-SiO2 catalysts with 1 wt % metal were prepared. These materials were further treated following four different routes: 1) by simple drying, 2) in which the dried catalysts were calcined in air at 723 K and then reduced at the same temperature, 3) in which they were directly reduced in hydrogen at 723 K, and 4) in which the surfactant was extracted using an ethanol-heptane azeotropic mixture. The catalysts were characterized by nitrogen adsorption-desorption isotherms at 77 K, H2 chemisorption measurements, solid-state 1H, 13C, 29Si-CP/MAS-NMR spectroscopy, powder X-ray diffraction (XRD), small angle X-ray scattering (SAXS), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and 197Au Mössbauer spectroscopy. The physical characterization by a combination of these techniques has shown that the size and the structural characteristics of the Pd-Au colloid precursor are preserved when embedded in an SiO2 matrix. Catalytic tests were carried out in selective hydrogenation of 3-hexyn-1-ol, cinnamaldehyde, and styrene. These data showed evidence that alloying Pd with Au in bimetallic colloids leads to enhanced activity and most importantly to improved selectivity. Also, the combination of the two metals resulted in catalysts that were very stable against poisoning, as was evidenced for the hydrogenation of styrene in the presence of thiophene.

Synthesis of Gold Catalysts Supported on Mesoporous Silica Materials: Recent Developments

“…The monometallic Au catalyst displayed very low activity in the selective semi-hydrogenation of 3-hexyn-1-ol, whereas the bimetallic Au-Pd catalyst exhibited high activity and was remarkably resistant to deactivation. Some years later, Parvulescu et al [171] evaluated the catalytic activity of the before mentioned catalysts in the selective hydrogenation of 3-hexyn-1-ol, cinnamaldehyde, and styrene. Their results showed that alloying Pd with Au in bimetallic colloids led to enhanced activity and improved selectivity.…”

Bimetallic gold/palladium catalysts for the selective liquid phase oxidation of glycerol

“…Gold is very useful as an alloying metal and it has been used with metal such as palladium [1][2][3][6][7][8][9][10][11][12][13] and platinum [7,10,[14][15][16] for various catalytic reactions. The interest for Au-Pd catalysts has been improved in recent years, as they were found to be superior to pure Pd and Au supported catalysts in various reactions as hydrogenation [17], hydrodechlorination [18,19] and hydrodesulphuration [20].…”

Mesoporous Pt–SiO2 and Pt–SiO2–Ta2O5 Catalysts Prepared Using Pt Colloids as Templates

“…Previous attempts to incorporate metal colloids into a solid matrix have been presented in the literature. [12] However, reports about embedding Pt colloids are scarce, and those available primarily refer to polymeric matrices. [13] To verify the effect of the Pt particle size, these catalysts were tested in the reduction of NO with isopentane.…”