Platinum-silica, catalysts, structure

Subsequent to the discovery of skeletal rearrangement reactions on plati-num/charcoal catalysts, the reality of platinum-only catalysis for reactions of this sort was reinforced with the observation of the isomerization of C4 and C5 aliphatic hydrocarbons over thick continuous evaporated platinum films (68,108, 24). As we have seen from the discussion of film structure in previous sections, films of this sort offer negligible access of gas to the substrate beneath. Furthermore, these reactions were often carried out under conditions where no glass, other than that covered by platinum film, was heated to reaction temperature that is, there was essentially no surface other than platinum available at reaction temperature. Studies have also been carried out (109, 110) using platinum/silica catalysts in which the silica is catalytically inert, and the reaction is undoubted confined to the platinum surface. 

With higher hydrocarbons, the spectra depend upon the structure of the olefin [70], With platinum—silica catalysts, linear chain olefins tend to form dehydrogenated surface residues more readily than branched chain olefins, which give predominantly saturated adsorbed alkyl species. 

O2 adsorption, 28 38 surface modility, 28 34 surface structure, 28 30, 31 oxidation of CO on, 28 65 oxide-supported metal catalysts, 41 10, 11 -phosphine catalysts achiral, 25 83-85 recovery, 32 354-356, 367-369 selectivity, 30 348 platinum, 30 355 -silica catalysts... 

The platinum particles in this Pt/silica are structurally similar to, but smaller than, those in the standard reference catalyst EUROPT-1. 

Usually they are employed as porous pellets in a packed bed. Some exceptions are platinum for the oxidation of ammonia, which is in the form of several layers of fine-mesh wire gauze, and catalysts deposited on membranes. Pore surfaces can be several hundred mVg and pore diameters of the order of 100 A. The entire structure may be or catalytic material (silica or alumina, for instance, sometimes exert catalytic properties) or an active ingredient may be deposited on a porous refractory carrier as a thin film. In such cases the mass of expensive catalytic material, such as Pt or Pd, may be only a fraction of 1 percent. 

The structure of the metal particles dispersed on a silica powder support ( Aerosil 380, 70 A average silica particle diameter) has been studied by Avery and Sanders (47) using electron microscopy in both bright and dark field, to determine the extent to which the metal particles were multiply twinned or of ideal structure. Platinum, palladium, and gold were examined. These catalysts were prepared by impregnation using an aqueous solution of metal halide derivatives, were dried at 100°-150°C, and were hydrogen... 

Catalysis of cyclohexene hydrogenation has been studied extensively both in the vapour and liquid phases on platinum ", palladium and other metallic surfaces. Here the kinetics of the cyclohexene hydrogenation on platinum have been considered lu terms of the specific activities of samples of silica-supported platinum, previously characterised by hydrogen chemisorption. Particular attention has been paid to the structure sensitivity-insensitivity of the reaction and how this varies as carbonaceous overlayers are built up on the catalysts with increasing reaction time. 

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