Characterization and Cataly tie-Hydrogenation Behavior of SiO_2-Embedded Nanoscopic Pd, Au, and Pd-Au Alloy Colloids

摘要

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 tet-raalkylammonium bromide following a modified sol-gel procedure with tetra-hydrofuran (THF) as the solvent.Tet-raethoxysilicate (TEOS) was used as the precursor for the silica support.The molar composition of the sol was TEOS/THF/H_2O/HCl = 1:3.5:4:0.05 for the bimetallic Pd-Au and TEOS/THF/ H_2O/HC1 = 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-SiO_2 catalysts with 0.6 and 1 wt % metal, and monometallic Pd- and Au-SiO_2 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, H_2 chemisorption measurements,solid-state ~1H,~(13)C,~(29)Si-CP/MAS-NMR spectroscopy,powder X-ray diffraction (XRD),small angle X-ray scattering (SAXS),X-ray photo-electron spectroscopy (XPS),transmission electron microscopy (TEM),and ~(197) Au Mossbauer 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 SiO_2 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