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 ... 

Classification of Reactions by Step Density and Carbonaceous Overlayer Dependence ... 

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

Well-characterized vanadium carbide thin films can be prepared by exposing a clean V(110) surface to ethylene or 1,3-butadiene at 600 K. The formation of vanadium carbide, rather than other forms of carbon-containing species such as graphite or carbonaceous overlayers, was confirmed by the characteristic AES and NEXAFS spectra.4 The stoichiometry and average thickness of the thin carbide films can be estimated by... 

Heterogeneous catalysts for hydrocarbon conversion may require metal sites for hydrogenation-dehydrogenation and acidic sites for isomerisation-cyclisation and these reactions may be more or less susceptible to the effect of carbonaceous overlayers depending on the size of ensembles of surface atoms necessary for the reaction. In reality we must expect species to be transferred and spilled-over between the various types of sites and if this transfer is sufficiently fast then it may affect the overall rate and selectivity observed. If there is spillover of a carbonaceous species [4] then there may be a common coke precursor for the carbonaceous overlayer on the two types of site. Nevertheless, the rate of deactivation of a metal site or an acidic site in isolation may be very different from the situation in which both types of site are present at a microscopic level on the same catalyst surface. The rate at which metal and acid sites deactivate with carbonaceous material may of course not be identical. Indeed metal sites may promote the re-oxidation of a carbonaceous species in TFO at a lower temperature than the acid sites would allow on their own and this may allow differentiation of the carbonaceous species held on the two types of site. 

The structure of the hydrogen-poor carbon deposit has not been clarified by surface science and is referred to as the carbonaceous overlayer. The structural nature of the hydrogen-rich carbon deposit, however, was amenable to a suite of surface science techniques and the unique structure of alkylidines [61] was derived. These adsorbate species, which have molecular coun-... 

The highest barrier among the recombination reactions for C—C bond formation is always found for fhe reaction of two bare carbon atoms. C—C recombination occurs preferentially on a terrace. This elementary reaction may initiate the formation of a carbonaceous overlayer, which will deacfivafe fhe cafalysf. 

Here hydrocarbon conversion reactions occur wholly or at least partly on the carbonaceous overlayer on the metal and oxide surfaces, as reported by others (13,15-20). Poly-condensed EDA complexes may behave as giant alkenes in which by reversible catalytic hydrogenation/dehydrogenation occurs. This mechanism is similar to the intermolecular hydrogen transfer mechanism proposed (IS) for hydrogenation of unsaturated hydrocarbons. 

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. 

There are clear similarities between palladium-catalyzed acetylene and ethylene hydrogenation since both occur in the presence of a relatively unreactive carbonaceous overlayer. In the case of acetylene hydrogenation, this is a combined ethyli-dyne and vinylidene layer while in the case of ethylene hydrogenation this consists of only ethylidyne species. Since ethylidyne species are removed by high pressures of hydrogen (see Fig. 1.7), the general picture can therefore be expected to be similar to ethylene hydrogenation where, in this case, a more open surface is formed by ethylidyne removal. 

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... 

Morkel M, Kaichev VV, Rupprechter G, Freund H-J, Prosvirin IP, Bukhtiyarov VI (2004) Methanol dehydrogenation and formation of carbonaceous overlayers on Pd(lll) studied by high-pressure SFG and XPS spectroscopy. J Phys Chem B 108 12955... 

Deposited Carbon as a Catalyst. - There is a growing body of evidence that, at least in some cases, carbon deposited on a catalyst can be catalytically active in its own right. The most obvious example of this is the catalytic role of carbon in promoting the formation of more carbon.It is very difficult to establish the significance of this, since it is almost impossible to identify a non-catalysed system, but there are several other cases where carbonaceous overlayers have been suggested to be active catalysts. 

The existence of such carbonaceous overlayers has also been explored using radioactive tracer techniques.By use of Pd/SiOa, Rh/Si02, and Pd/Al203, it was shown that adsorption of ethylene and acetylene occurred in two distinct stages at 298 K a non-linear irreversible region and a linear second region. The non-linear region was found to correspond to a mono-layer. 

Carbonaceous overlayers have always been associated with catalytic poisoning, but it does seem, in some cases, that they can also be associated with catalytic activity. In one sense, this is not suprising, since carbonaceous overlayers can have exactly the same structure as carbonaceous catalysts. Nonetheless the association of carbon deposits and catalyst poisoning is too strong to be easily forgotten. 

Such higher residual activity could be in accordance with the model proposed by Sachtler, which assumes that S is preferentially adsorbed on Re sites in bimetallic Pt-Re particles [20]. Indeed, as the Pt-Re interaction is the highest, sulfur ought to divide the Pt-Re surface into very small Pt ensembles. Consequently, the reorganization of the carbonaceous overlayer into pseudo-graphitic entities, which are detrimental to the metallic function, could be impeded. 

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