Platinum- silica catalyst surface area

Fig. 4. Platinum crystallite size distribution for 2.5% (w/w) platinum/silica catalyst (Fig. 3). Full line, number distribution broken line, surface area distribution. After T. A. Dorling, and R. L. Moss, J. Catal. 7, 378 (1967) and R. L. Moss, Platinum Metals Rev. 11 (4), 1 (1967).

A platinum on silica gel catalyst was prepared by impregnation of silica gel (BDH, for chromatographic adsorption) by a solution containing 0.5% (wt.) of sodium hydroxide and 0.5% (wt.) of chloroplatinic acid (both of analytical grade). The dried catalyst contained 1% (wt.) of platinum and a corresponding amount of the alkaline component. The BET surface area of the catalyst was 40 m2/g, the mean pore radius 150 A. The catalyst was always reduced directly in the reactor in a stream of hydrogen at 200°C for 2 hr. 

In order to increase the contact of a catalyst with hydrogen and the compounds to be hydrogenated platinum (or other metals) is (are) precipitated on materials having large surface areas such as activated charcoal, silica gel, alumina, calcium carbonate, barium sulfate and others. Such supported catalysts are prepared by hydrogenation of solutions of the metal salts, e.g. chloroplatinic acid, in aqueous suspensions of activated charcoal or other solid substrates [28. Supported catalysts which usually contain 5, 10 or 30 weight percent of platinum are very active, and frequently pyrophoric. 

A large number of heterogeneous catalysts have been tested under screening conditions (reaction parameters 60 °C, linoleic acid ethyl ester at an LHSV of 30 L/h, and a fixed carbon dioxide and hydrogen flow) to identify a suitable fixed-bed catalyst. We investigated a number of catalyst parameters such as palladium and platinum as precious metal (both in the form of supported metal and as immobilized metal complex catalysts), precious-metal content, precious-metal distribution (egg shell vs. uniform distribution), catalyst particle size, and different supports (activated carbon, alumina, Deloxan , silica, and titania). We found that Deloxan-supported precious-metal catalysts are at least two times more active than traditional supported precious-metal fixed-bed catalysts at a comparable particle size and precious-metal content. Experimental results are shown in Table 14.1 for supported palladium catalysts. The Deloxan-supported catalysts also led to superior linoleate selectivity and a lower cis/trans isomerization rate was found. The explanation for the superior behavior of Deloxan-supported precious-metal catalysts can be found in their unique chemical and physical properties—for example, high pore volume and specific surface area in combination with a meso- and macro-pore-size distribution, which is especially attractive for catalytic reactions (Wieland and Panster, 1995). The majority of our work has therefore focused on Deloxan-supported precious-metal catalysts. 

Figure 11 shows conversion to iso-heptanes to be negligible for (0.5 wt. %) platinum supported on activated carbon (Pt/C) as the only catalyst, and also for (0.4 wt. %) platinum on silica-gel (Pt/Si02). No detectable conversion was obtained with silica-alumina. A mechanical mixture of either of the Pt-bearing particles with silica-alumina of about 150 m.Vg-surface area, both in millimeter diameter particle size (1000m), immediately resulted in appreciable isomerization ( SiAl with Pt/C SiAl with Pt/Si02). Isomerization increases rapidly for smaller component particle sizes, of 70/i and S i diameters. It approaches the performance of a silica-alumina that has been directly impregnated with platinum, and which has... 

The preparation and characterisation of silica-supported platinum catalysts Indicated in Table 1 have been discussed The extent of H2 chemisorption was estimated by extrapolation of volumetric data at 295K to the gero-pressure intercept and vas converted to a Pt surface area assuming H/Pt 1 and that there were 1.2 x 10 Pt atoms per Table I shows... 

Rioux RM, Song H, Hoefelmeyer JD, Yang P, Somorjai GA (2005) High-surface-area catalyst design synthesis, characterization, and reaction studies of platinum nanoparticles in mesoporous SBA-15 silica. J Phys Chem B 109 2192... 

Platinum and chlorine (samples made with chloride precursors) contents of the catalyst samples were determined with X-ray fluorescence spectroscopy (XRF) (Phillips PW 1480 spectrometer). BET surface areas of catalysts were within 5% that of the silica support material. Platinum dispersion was measured with hydrogen chemisorption in a volumetric set-up, using a procedure described elsewhere [3]. Stoichiometry of H/Pt = 1 was assumed for calculating the platinum dispersion [4]. Transmission electron microscopy (TEM) (Phillips CM 30, 300kV) was used to check the platinum particle size in some of the catalysts. Average platinum particle size was determined based on analysis of about 100 platinum crystallites. 

The BET surface area measurements and pore volimie measurements were performed on a Micromeritics ASAP 2000 by nitrogen adsorption using a volumetric method. The BET siuface areas for the fresh alumina, ceria, silica and titania catalysts were 124, 91, 64 and 87 m /g washcoat, respectively. Spent catalysts from the ethanol oxidation experiments have been characterized. The results from these measurements show that the platinum on ceria catalyst has lost more than half of its original surface area, while Pd/Ce02 exhibits stability towards sintering (see Table 1). The titania catalysts have lost ca 30% of their surface areas, while the surface areas of the alumina and silica catalysts have remained practically unchanged. 

These catalytic converters contain a high surface area, a honeycombed ceramic or stainless steel core that is coated with silica and alumina, called a washcoat. Precious metal catalysts, such as platinum, palladium, and rhodium, are added as a suspension to the washcoat. As the hot gases pass through the catalytic converter, they are converted by the catalysts to the reduced or oxidized products. 

Although the most widely used supporting material for firel cell catalysts is high-surface-area carbon (such as Vulcan XC72), at elevated temperatures the carbon-to-catalyst contact bums. It has been found that the platinum particles are able to catalyze the combustion of the carbon support even at moderate temperatures (125-195 °C) [83]. In order to avoid these problems, some have tried to synthesize Pt catalysts supported on silica. Peled et al. have synthesized and characterized carbon- and silica-supported PtNi and PtCo catalysts via the same... 

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