The preparation and characterization of highly dispersed supported bimetallic catalysts from inorganic and organometallic molecular precursors

A series of silica supported Pt-Au and Pt-Cu catalysts were prepared using inorganic and organometallic precursors as the sole metal source. These new catalysts were compared to traditionally prepared (wetness impregnated or coimpregnated) catalysts to determine the effects of the coinage metal and precursor type on the resulting catalyst. The molecular precursors (both monometallic precursors and bimetallic cluster molecules) were be deposited onto the silica carrier by spontaneous adsorption from organic solvents. Activation conditions will be chosen to be mild based on temperature programmed oxidation and reduction experiments with the supported precursors. Particular emphasis was placed on developing methods for the preparation of uniform particles (in terms of both size and composition) from the molecular precursors.; The resulting catalysts were characterized by a variety of physical techniques such as carbon monoxide chemisorption, infrared spectroscopy of adsorbed CO, hydrogen-oxygen titration, and transmission electron microscopy. A primary goal was to prepare and characterize highly dispersed and uniform bimetallic particles. Particularly for the Pt-Au system (which has a large bulk miscibility gap), such particles are unattainable via traditional impregnation routes. Characterization data indicated that metal particles on the resulting catalysts were indeed small (<15 Å for the Pt-Cu catalysts) and there was strong evidence for intimate contact between the two metals after thermal removal of the organic ligands.; Hydrocarbon reforming (n-hexane conversion) and hydrogenation/dehydrogenation reactions (e.g. propane dehydrogenation, toluene hydrogenation) were used as the primary tests for catalytic activity. The presence of phosphorus in some of the molecular precursors effectively poisoned skeletal rearrangement reactions during reforming catalysis. Hydrogenation and dehydrogenation processes, however, were relatively unaffected in catalysts derived from phosphine ligated precursors. Catalysts prepared from the organometallic cluster precursor Pt ₂M₄(C≡CtBu)₈ (M = Cu, Ag, Au) were active in hydrocarbon reforming reactions and showed increased selectivity for light hydrocarbon production. The cluster derived Pt-Au catalyst also had greatly enhanced resistance to deactivation processes, whereas similar Pt-Cu catalysts did not. The new catalysts were also used to examine structure sensitivity of hydrogen chemisorption and toluene hydrogenation catalysis. Both processes were significantly different on the cluster derived catalysts than on traditional catalysts.