The Formation, Structure, and Function of Carbonaceous Deposits

It may be helpful at this point to try to draw together some of the many threads concerning carbon deposition that have appeared many times in the previous chapters a comprehensive and unifying model is not yet available, and indeed it is doubtful if it ever will be, so many are the factors that determine the state of carbon on metal surfaces. Nevertheless it is possible to make a few generalisations, and an attempt to do so is opportune now because the last section of this chapter concerns a constructive use of surface carbon to create useful products. The term carbon will be used as an omnium gatherum for what has been variously named coke, acetylenic residue, carbonaceous deposit and probably other things as well. The following short survey may be amplified by reference to review articles. 

The form and quantity of carbon existing on the surface of a metal catalyst depends inter alia upon the following variables (i) the nature of the metal, (ii) its physical form, i.e. single crystal, powder or black , small supported particle etc., (iii) the nature of the support, if any, (iv) the type of hydrocarbon applied, (v) the presence of other molecules, especially hydrogen, and the hydrogen hydrocarbon ratio, (vi) the time and especially the temperature of exposure. We may briefly consider the importance of each of these factors. 

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 

Because of its importance and frequent occurrence, the process of carbon formation and its structure have been widely studied, and many physical techniques (including recently positron-emission tomography ) have been deployed. Of these, temperature-programmed methods (oxidation, TPO reaction with hydrogen, TPRe) are the simplest and most informative. TPO can distinguish between carbon on the metal, which is relatively easily oxidised, from carbon on the support, which is less reactive. Admixture of the sample with a Pd/Si02 catalyst ensures that the effluent contains only carbon dioxide, and no monoxide. The use of the TPRe strictly requires estimation of the methane (and possibly other alkanes) that emerges, since because the H/C ratio in the carbon is unknown, the 

Little attempt seems to have been made to estimate the number of free surface metal atoms in coked catalysts, and hence to find TOFs, assuming these to be the seat of the residual activity. While the use of hydrogen chemisorption might be considered risky, that of carbon monoxide ought to be suitable. Based on the loss of its IR intensity, the active metal area of Pt/Al203 used for n-heptane reforming was only 8% of its initial value, but its extent of adsorption slowly increased as it displaced some of the carbon .  

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