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 precur sors and the treatment conditions affected the Pt-W interactions and t he catalytic activity for toluene hydrogenation. Samples were prepared from {Pt[W(CO)(3)(C5H5)](2)(PhCN)(2)} (Ph = phenyl) and from [PtCl2(P hCN)(2)] + [W(CO)(6)] and characterized by infrared spectroscopy, exte nded X-ray absorption fine structure (EXAFS) spectroscopy, transmissio n electron microscopy (TEM), and chemisorption of H-2, CO, and O-2. Th e samples were treated in H-2 at 400 degrees C prior to most of the ch aracterizations. Incorporation of W reduced the chemisorption of CO an d of H-2, With the reduction in chemisorption being greater for the ca talyst prepared from {Pt[W(CO)(3)(C5H5)](2)(PhCN)(2)} than for the cat alyst prepared from the combination of monometallic precursors. EXAFS spectra measured at both the Pt L(II) edge and the W L(III) edge showe d 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 inter actions 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 catal yst prepared from the bimetallic cluster was more than an order of mag nitude 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 suppo rt, with its influence on the catalysis maximized by its proximity to the Pt. (C) 1996 Academic Press, Inc.