Platinum-silica catalysts adsorption

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 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. 

Fig. 4 Energy distribution function, (p(e t) (cmol/kJ/mol/), against the dimensionless product of the lateral interaction energy (P) and the local isotherm (0)P0, for carbon monoxide adsorption over a bimetalhc Pto.25-Rho.75 silica supported catalyst, at 698 K. Source From Gas chromatographic kinetic study of carbon monoxide oxidation over platinum-rhodium catalysts, in J. Chromatogr.

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. 

A platinum-iron on silica gel catalyst was prepared by impregnating silica gel (BDH, for chromatographic adsorption) with an aqueous solution of chloroplatinic acid (analytical grade) and sodium hydroxide (analytical grade). The dry product was then impregnated by a ferrous sulfate solution (C.P. grade) and the water was removed in a rotating evaporator. The prepared catalyst contained 1% Pt, 0.7% Fe, and 2% NaOH (by... 

From the study of the influencing of single reactions by products and by other added substances and from the analysis of mutual influencing of reactions in coupled systems, the following conclusions can be drawn concerning adsorption of the reaction components. (1) With the exception of crotyl alcohol on the platinum-iron-silica gel catalyst, all the substances present in the coupled system, i.e. reactants, intermediate products, and final products, always adsorbed on the same sites of the catalytic surface (competitive adsorption). This nonspecificity was established also in our other studies (see Section IV.F.2) and was stated also by, for example, Smith and Prater (32), (2) The adsorption of starting reactants and the desorption of the intermediate and final products appeared in our studies always as faster, relative to the rate of chemical transformations of adsorbed substances on the surface of the catalyst. 

The co-existence of at least two modes of ethylene adsorption has been clearly demonstrated in studies of 14C-ethylene adsorption on nickel films [62] and various alumina- and silica-supported metals [53,63—65] at ambient temperature and above. When 14C-ethylene is adsorbed on to alumina-supported palladium, platinum, ruthenium, rhodium, nickel and iridium catalysts [63], it is observed that only a fraction of the initially adsorbed ethylene can be removed by molecular exchange with non-radioactive ethylene, by evacuation or during the subsequent hydrogenation of ethylene—hydrogen mixtures (Fig. 6). While the adsorptive capacity of the catalysts decreases in the order Ni > Rh > Ru > Ir > Pt > Pd, the percentage of the initially adsorbed ethylene retained by the surface which was the same for each of the processes, decreased in the order... 

The relationship between the two catalytic components is quite complex. Interactions between the support and the hydrogenation component can alter this relationship. For example, Larson et- al. (6) showed that, with platinum on silica-alumina, a selective adsorption of platinum by acid sites causes a reduction in catalyst acidity. Similarly, nickel reacts with the acid sites on silica-alumina forming nickel salts of the silica-alumina acid sites. It has been suggested (J) that one of the effects of sulfiding a nickel on... 

Carbon monoxide undergoes activated adsorption on the surface of palladium oxide. The maximum for this process, at about 350 mm. pressure, is at about 100°C. The gas taken up during activated adsorption can only be recovered as C02 for the most part (57). In a CO-air stream a slight initial reduction of PdO occurs at 23°C., but in the absence of oxygen, there is no reduction below 76°. This process of reduction decreases in rate with time and does not go to completion below 156°. Carbon dioxide, when present in the gas phase, inhibits the reduction of the palladium at 100°C. because it is adsorbed strongly by the PdO (56). Catalysts have been prepared by the deposition of palladium and platinum on asbestos, on silica gel, and on charcoal. 

Type III is relatively rare—a recent example of nitrogen on ice and seems to be characterised by a heat of adsorption equal to or less than heat of liquifaction of the adsorbate. The example of Type II isotherms are furnished by adsorption of nitrogen on an iron or a platinum catalyst at -195 C and those Type III by adsorption of bromine on silica or alumina gas at 80 C. 

Measurements of competitive hydrogenation of butadiene into butenes on palladium and platinum catalysts supported on silica [30] allowed the determination of the relative adsorption coefficients of dienes and olefins. 

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