Platinum oriented faces

Cyclohexane forms a (9 X 9) surface structure on the Ag(l 11) crystal face and a (j j) surface structure on the Pt(l 11) crystal face at around 200 K. This latter surface structure corresponds to the (001) surface orientation of the monoclinic bulk crystal structure of the molecule. On heating the platinum crystal face to 450 K a ( ) surface structure forms that is identical to the surface structure formed by cyclohexene monolayers at the same temperature. It appears that cyclohexane dehydrogenates at elevated temperatures on platinum to form the same species or that of cyclohexene. 

It has also been shown [173, 233, 234] that the catalytic activity of various crystal faces of platinum for the reduction of N03 ions is very different. This means that similar effects should be expected in the case of platinized systems with various preferred crystallographic orientations [235]. 

A complete model for the stationary CO oxidation on platinum according to Eq. (62) with the use of the QCA and TSM has been discussed in Ref. [136], Its parameters were determined independently from the TDS of CO and 02 on the Pt(lll) face [135]. The experimental evidence [139] for the stationary CO oxidation on a polycrystalline sample with a predominant orientation of the (111) faces (>90%) are described adequately. The numerical analysis of the model indicates that the parameters e — (3) produce a strong effect on the activation barrier height and cannot be... 

Preferred orientation of metal films can be clearly achieved by depositing the metal on the smooth surface of a well-crystallized solid in vacuo. For example, Uyeda (94) and Kainuma (95) obtained (lll)-oriented films of nickel, copper, and platinum when these metals were deposited on a cleavage surface of molybdenite (0001) at temperatures ranging from 20° to 500°C. Miyake and Kubo (96) observed a temperature dependency of the orientation of deposited films of face-centered cubic metals when they were deposited on a cleavage surface of zinc blende (110). 

In this same series of reactions it was also found that C-C bond cleavage took place most readily over the 111 face catalyst (A in Fig. 3.2) and to a considerably less extent on the others. This reaction, which is stmcture sensitive, obviously teikes place over ensembles of face atoms. It was also found > 8 that the dehydrocyclization of hexane to benzene, another structure sensitive reaction, took place four times faster on the 111 face of a platinum crystal than on one cleaved to expose the 100 face. The hydrogenolysis of ethane, on the other hand, was found to be significantly faster on Ni (100) than on Ni (111).22 Thus, it is not sufficient merely to define a site as an ensemble of face atoms the orientation of the atoms must also be specified. 

The existence of these highly mobile quasi-isolated atoms (Fig. 10) could provide new possibilities for catalytic reactions, favoring, for instance, the occurrence of the nonselective cyclic mechanism. Although the potentials used in Hoare and Pal s calculations (Lennard-Jones and Morse-Mye) may be considered as unsuitable for metal clusters, recent calculations (181), made with more realistic potentials, indicate that below 15 A, the clusters with icosahedral symmetry are more stable than the fee cubooctahedra. Unfortunately, the size range in which the polyhedral metal clusters are supposed to be stable does not allow microdiffraction studies. Moreover, when platinum is deposited on an oriented rock-salt face, pseudocrystals with uncommon symmetry are present only in very small amounts, and in a particle size range of 80-120 A, where they obviously result from multiple twinning of fee tetrahedra (182). 

A number of studies has been devoted to these reactions, first considered as taking place on every part of the metal crystallites. Using preferentially oriented platinum films, Anderson and Avery (34) found that the ratio, S, between the rates of isomerization and hydrogenolysis of isobutane was 3.5 times larger on (111) than on (100) faces. They attributed this result to the intervention of an additional symmetrical species triply attached to the metal (Scheme 92). Such a species, which is expected to favor isomerization over hydrogenolysis, would fit the (111) lattice, but not the (100) one. 

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