Carbon Carbonaceous overlayer

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... 

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. 

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. 

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. 

Augustine et al. working at atmospheric pressure observed that presulfiding has a profound effect on both the quantity and the quality of the carbonaceous overlayer formed during n-hexane conversion reactions, and the effects are greater for bimetallic samples than for Pt monometallic catalysts. Carbon is decreased in the presence of sulfur. The carbonaceous layer is more rich in hydrogen over the sulfided catalysts. They assumed that sulfur can interact with Lewis acid sites and thus deactivate sites for coking. 

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 conventional, chemically stable glasses also have modified surfaces relative to their bulk as a result of interactions with the ambient atmosphere. In a review of glass and ceramic surface analyses, Pantano has shown that commercial sodalime silicate glasses exposed to the ambient have carbonaceous overlayers (consisting of both hydrocarbon and carbonate phases) on surfaces that are alkali-depleted as a result of interactions with atmospheric water and carbon dioxide. Pantano uses techniques including XPS, AES, ion scattering spectrometry (ISS), SIMS, and sputtered-induced photon spectrometry (SIPS) to qualitatively and quantitatively characterize these surface layers. 

It appears that the stronger metal-carbon interaction on iridium surfaces imposes the periodicity on the carbon atoms in the overlayer, while the structure of the graphite overlayer on the Pt( III) face is independent of the substrate periodicity and rotational symmetry. Ordering of the dehydrogenated carbonaceous residue on the stepped iridium surface is absent when the surface is heated to above 1100 K. Atomic steps of (100) orientation appear to prevent the formation of ordered domains that are predominant on the Ir(lll) crystal face. The reasons for this are not clear. Perhaps the rate of C-C bond breaking on account of the steps is too rapid to allow nucleation and growth of the ordered overlayer. On the (111) face, the slower dehydro-... 

Exposure of solid surfaces to ambient air invariably results in a modest overlayer of carbonaceous byproducts collectively labeled adventitious carbon (AC). The employment of the adventitious carbon to establish binding energy scales has been the standard in more ESCA analysis than all other referencing methods combined. In order to define the use of adventitious carbon (AC) [30-34], we must first try to describe what it is and how it behaves. Although each of these studies exhibits slight differences, the striking feature of the many thousands of XPS observations of adventitious carbon is their apparent chemical and physical similarity. Any attempt at analyzing the key details of AC requires a short review of the properties of the possible candidates. 

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