gamma-Al2O3-supported Pt catalysts with extremely high dispersions resulting from Pt-W interactions

gamma-Al2O3-supported catalysts were prepared from bimetallic precursors, { Pt[W(CO)(3)(C5H5)](2)(PhCN)(2)} and {Pt2W2(CO)(6)(C5H5)(2)(PPh3)(2)}, as we ll as from a mixture of [PtCl2(PhCN)(2)] and [W(CO)(6)], and characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy and chemis orption of H-2, CO, and O-2; the samples were reduced with H-2 at 400 degre es C for 2 h-prior to most characterizations. EXAFS data show that tungsten in either of the bimetallic precursors helped to maintain the platinum in a highly dispersed form during treatment in H-2, leading to supported plati num clusters of only about 4 to 6 atoms each, on average. EXAFS spectra mea sured at the Pt L-II edge indicate substantial Pt-W contributions in sample s prepared from the bimetallic precursors, but not in samples prepared from the two monometallic precursors; the Pt-W coordination numbers in the form er samples were about 2 and 1, respectively, at an average Pt-W distance of 2.71 Angstrom. W L-III edge EXAFS data indicate substantial W-Pt interacti ons in the samples prepared from the bimetallic precursors, with an average W-Pt coordination number of about 1.0 at a distance of 2.71 Angstrom. The results are consistent with the inference that W-Pt-W (or Pt-(W)(2)-Pt) uni ts in the precursor were largely retained following removal of the ligands by H-2 treatment. EXAFS data also show that the Pt-W interactions in sample s prepared from the bimetallic precursors were strong enough to be largely maintained even under oxidation/reduction conditions at temperatures as hig h as 400 degrees C. In addition to the metal-metal contributions, the EXAFS data show substantial interactions of both tungsten and platinum with oxyg en atoms of the gamma-Al2O3 support. The platinum clusters are inferred to be stabilized in a highly dispersed state by their interactions with tungst en cations (identified by oxygen uptake data), which are held in place by i nteractions with surface oxygen atoms of gamma-Al2O3 (indicated by EXAFS da ta). In contrast, the sample prepared from the two monometallic precursors is characterized by a lack of Pt-W interactions las indicated by the EXAFS data) and by relatively large platinum particles. The supported platinum cl usters made from bimetallic precursors are characterized by lower chemisorp tion of CO or of hydrogen and by low catalytic activities for toluene hydro genation at 1 atm and 60 degrees C, relative to the values characterizing t he samples made from the monometallic precursors, which incorporated larger platinum particles. On the other hand, the catalysts containing tungsten w ere an order of magnitude more active than Pt/gamma-Al2O3 for hydrogenation of crotonaldehyde to give crotyl alcohol, although these catalysts were st ill nonselective for this reaction. The data indicate that tungsten formed isolated microscopic islands on gamma-Al2O3, influencing the adsorption and catalysis by platinum by its proximity to the latter. Literature data indi cate that other combinations of oxophilic and noble metals on oxide support s behave similarly. (C) 2000 Academic Press.