Platinum carbonaceous overlayer

VII. The Role of the Carbonaceous Overlayer in Hydrocarbon Reactions on Platinum Surfaces... 

We have been able to identify two types of structural features of platinum surfaces that influence the catalytic surface reactions (a) atomic steps and kinks, i.e., sites of low metal coordination number, and (b) carbonaceous overlayers, ordered or disordered. The surface reaction may be sensitive to both or just one of these structural features or it may be totally insensitive to the surface structure, The dehydrogenation of cyclohexane to cyclohexene appears to be a structure-insensitive reaction. It takes place even on the Pt(l 11) crystal face, which has a very low density of steps, and proceeds even in the presence of a disordered overlayer. The dehydrogenation of cyclohexene to benzene is very structure sensitive. It requires the presence of atomic steps [i.e., does not occur on the Pt(l 11) crystal face] and an ordered overlayer (it is poisoned by disorder). Others have found the dehydrogenation of cyclohexane to benzene to be structure insensitive (42, 43) on dispersed-metal catalysts. On our catalyst, surfaces that contain steps, this is also true, but on the Pt(lll) catalyst surface, benzene formation is much slower. Dispersed particles of any size will always contain many steplike atoms of low coordination, and therefore the reaction will display structure insensitivity. Based on our findings, we may write a mechanism for these reactions by identifying the sequence of reaction steps ... 

Fig. 28. Schematic representation of a platinum catalyst with a monolayer of carbonaceous overlayer showing the exposed platinum clusters.

Studies by Sachtler et al have indicated that the ensemble control may also retard the formation of carbonaceous overlayers. Also, it was suggested that the chemisorption of sulfur on rhenium may enrich the surfaces of the platinum particles with rhenium. 

Catalysis of cyclohexene hydrogenation has been studied extensively both in the vapour and liquid phases on platinum ", palladium and other metallic surfaces. Here the kinetics of the cyclohexene hydrogenation on platinum have been considered lu terms of the specific activities of samples of silica-supported platinum, previously characterised by hydrogen chemisorption. Particular attention has been paid to the structure sensitivity-insensitivity of the reaction and how this varies as carbonaceous overlayers are built up on the catalysts with increasing reaction time. 

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. 

Marsh AL, Somorjai GA (2005) Structure, reactivity, and mobility of carbonaceous overlayers during olefin hydrogenation on platinum and rhodium single crystal surfaces. Top Catal 34 121... 

Carbonaceous Overlayers What is the composition of the working platinum catalyst surface When the surface is examined after carrying out any one of the hydrocarbon conversion reactions, it is always covered by a near-monolayer amount of carbonaceous deposit. 

Somorjai and his colleagues have developed a model for the states of carbon on a platinum surface containing steps and kinks, in which much of the surface was obscured by a carbonaceous overlayer with islands of 3D carbon , leaving only a few single atoms or pairs at steps uncovered. It was felt that the higher activity of sites at steps would cause hydrogen if present to break C—M bonds. If this is so, then very small metal particles that expose only atoms of low coordination number should be more resistant to carbon deposition than larger particles, powders or macroscopic forms. Quantitative evidence on a particle-size effect is... 

More important, the carbonaceous deposit on the platinum catalyst surfaces was often ordered, and ordering imparted to it unique properties. The presence of an ordered overlayer eliminated the poisoning of dehydrogenation reactions (C6H10 to C6H6). The dehydrocyclization of -heptane to... 

It was also observed that while the low-pressure reaction took place on an essentially clean surface, the high-pressure reaction occurred in the presence of a near mono-layer of carbonaceous species. In an eflFort to investigate this overlayer more fully, a C-radioisotope technique was employed. It was observed that when the platinum surface was dosed with C-labeled hydrocarbons, the lability of the deposited radioactive carbonaceous layer varied with deposition temperature. ... 

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