Platinum catalysts ultrathin films

When one considers the various results from the reactions of labeled and unlabeled hexanes over supported catalysts and over thick and ultra-thin films, the conclusion emerges that catalysts with very small platinum particles (ultrathin films or 0.2% platinum/alumina) strongly favor reactions via an adsorbed C5 cyclic intermediate, but at large particle size... 

In all films there is a distribution of crystallite diameters. An example is shown in Fig. 2 for the film with a specific weight of 0.12 fig cm-2. The smallest particles whose diameters can be measured in a micrograph (and then only very approximately) have diameters of about 10 A, and this is the lower size limit used in Fig. 2. However, particles smaller than this can readily be observed in the micrograph, and there is no doubt that this type of film contains some crystallites down to the limit of microscopic resolution (about 8 A in our case), and presumably beyond. However, their number appears to be relatively small. It is interesting to compare the specific film weight of these ultrathin platinum films with the amount of platinum per unit actual surface area of support for typical supported platinum catalysts. A typical supported catalyst would have 1% (w/w) of platinum on a... 

Fig. 12. Variation with average platinum particle diameter of the initial rate of reaction (isomerization plus dehydrocyclization) of n-hexane (- -) and 2-methylpentane (-O-) over ultrathin film catalysts at 275°C. Hydrogen/reactant hydrocarbon, 10/1 total reactant pressure 100 Torr.

Our conclusion regarding the importance of reaction via an adsorbed carbocyclic intermediate, together with the data in Fig. 12, lead to the conclusion that over ultrathin film catalysts, the specific rate of isomerization via the carbocyclic pathway decreases as the average platinum particle... 

Platinum is an important example of a metal where, even on an uncontaminated surface such as is offered by an evaporated film, there is a strong tendency for only one C—C bond to be ruptured in any particular reacting molecule. On this basis, one may express the distribution of reaction products in terms of relative C—C bond rupture probabilities. Some data of this sort are contained in Table XI for thick and ultrathin film catalysts, and for comparison there are included some data for reactions on a silica-supported catalyst containing 0.8% platinum. These data all refer to reactions carried out in the presence of a large excess of hydrogen, although the results of Kikuchi et al. (128) indicate that on platinum catalysts the position of C—C bond rupture (in n-pentane) is very little dependent on hydrogen pressure. The data in Table XI show that, on the whole, the 0.8% platinum/silica catalyst used by Matsumoto et al. (110) was inter-... 

Fig. 1. Electron micrograph of ultrathin platinum film average platinum density 0.12 jug cm-2, deposited on mica in UHV at 275°C. Film catalyst used in reaction of n-hexane at 275°C before microscopic examination. Micrograph obtained by direct transmission through mica sliver. (X400,000).

Work with ultrathin and thick fringe-free platinum films has shown that not only does the product distribution change with catalyst structure, but the specific rate of raction (per unit platinum area) changes also (30). The data in Fig. 12 for the reaction of 2-methylpentane and n-hexane show a decrease in the specific rate with increasing particle size. 

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