MgO-supported platinum-tungsten catalysts prepared from organometallic precursors: Platinum clusters isolated on dispersed tungsten

MgO-supported Pt and Pt-W catalysts were prepared from organometallic precursors and characterized to determine how the nature of the precursors and the treatment conditions affected the Pt-W interactions and the catalytic activity for toluene hydrogenation. Samples were prepared from {Pt[W(CO)(3)(C5H5)](2)(PhCN)(2)} (Ph = phenyl) and from [PtCl2(PhCN)(2)] + [W(CO)(6)] and characterized by infrared spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy, transmission electron microscopy (TEM), and chemisorption of H-2, CO, and O-2. The samples were treated in H-2 at 400 degrees C prior to most of the characterizations. Incorporation of W reduced the chemisorption of CO and of H-2, With the reduction in chemisorption being greater for the catalyst prepared from {Pt[W(CO)(3)(C5H5)](2)(PhCN)(2)} than for the catalyst prepared from the combination of monometallic precursors. EXAFS spectra measured at both the Pt L(II) edge and the W L(III) edge showed substantial Pt-W contributions for the former catalyst (but not the latter), with a Pt-W coordination number of about 2 and an average Pt-W distance of 2.71 Angstrom. TEM and EXAFS results indicate that interactions between Pt and W cations in the sample made from {Pt[W(CO)(3)(C5H5)](2)(PhCN)(2)} maintained the Pt in a highly dispersed form, with supported Pt clusters being smaller than about 10 Angstrom. The catalyst prepared from the bimetallic cluster was more than an order of magnitude less active for toluene hydrogenation at 1 atm and 60 degrees C than the catalyst prepared from the two monometallic precursors; the comparison indicates that the W acted like an isolated dispersed support, with its influence on the catalysis maximized by its proximity to the Pt. (C) 1996 Academic Press, Inc.