Platinum crystal surfaces

Gdowski GE, Farr JA, Madix RJ. 1983. Reactive scattering of small molecules from platinum crystal surfaces D2CO, CH3OH, HCOOH and the nonanomalous kinetics of hydrogen atom recombination. Surf Sci 127 541. 

D. Hydrocarbon Reactions on Platinum Crystal Surfaces at High Pressures... 

Platinum crystal surfaces that were prepared in the zones indicated by the arrows at the sides of the triangle are thermally unstable. These surfaces, on heating, will rearrange to yield the two surfaces that appear at the end of the arrows. There is reason to believe that the thermal stability exhibited by various low and high Miller index platinum surfaces is the same for other fee metals. There are, of course, differences expected for surfaces of bcc solids or for surfaces of solids with other crystal structures. 

This and similar instruments (3,4) that allow one to study reaction rates and product distributions on small-area crystal and catalyst surfaces have been used in our studies of the mechanism of heterogeneous catalysis and the nature of active sites. These studies, which concentrated primarily on hydrocarbon reaction as catalyzed by platinum crystal surfaces, will be reviewed in the next section. 

B. Dehydrogenation and Hydrogenolysis of Cyclohexane on Platinum Crystal Surfaces at Low Pressures (< 10-4 Torr)... 

Initial Turnover Numbers on Platinum Crystal Surfaces for Various Hytlrocarbon Reactions at Hit/h and Low Pressures ... 

VI. Active Sites for C-H, H-H, and C-C Bond Breaking on Platinum Crystal Surfaces... 

Studies to correlate the reactivity and the surface structure and composition of platinum surfaces indicate that the active platinum crystal surface must be heterogeneous. The heterogeneity involves the presence of various atomic sites that are distinguishable by their number of nearest neighbors (atoms in terraces, steps, and kinks), and also variation in surface chemical composition. A model that depicts the active platinum surface is shown schematically in Fig. 28. Part of the surface is covered with a partially de-... 

The Initial turnover frequencies observed are comparable with those reported by Somorjai et al on unsupported stepped platinum crystal surfaces and by Boudart et al on supported platinum and decrease with reaction time and reaction temperature. 

Primary structure sensitivity resulting from the effect of changing particle size on step and kink density appears therefore to be present here at short reaction times. Secondary structure sensitivity (including the effect of carbonaceous poisoning on the reaction rate) appears not to be present here. Thus Somorjai has reported that the dehydrogenation reaction of cyclohexane to cyclohexane is insensitive to both structural featureSt whereas the dehydrogenation of cyclohexene to benzene la very sensitive to the densities of atomic steps and kinks and the order of the carbonaceous overlayer on the platinum crystal surface. 

The reactivity pattern displayed by platinum crystal surfaces for alkane isomerization reactions is completely different from that for aromatization. Studies revealed that maximum rates and selectivity (rate of desired reaction /total rate) for butane isomerization reactions are obtained on the flat crystal face with the square unit cell. Isomerization rates for this surface are four to seven times higher than those for the hexagonal surface. Isomerization rates are increased to only a small extent by surface irregularities (steps and kinks) on the platinum surfaces (Figure 7.39). 

S. M. Davis, G. A. Somorjai, Correlation of cyclehexene reactions on platinum crystal surfaces over a ten-order-of-magnitude pressure range variation of stmcture sensitivities, rates, reaction probabilities, J. Catal. 1980, 65, 78-83. 

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