ABSTRACT
Particular emphasis in transformation of organics at solid surfaces (e.g., catalysts) has been given to carbonaceous deposits (coke, pyrocarbon) inhibiting reactions on active surface sites due to their blocking (Delmon and Froment 1980, Hughes 1984). On the other hand, pyrocarbon on solid supports can be interesting as a catalyst (Lisovskii and Aharoni 1994) and an adsorbent (Hubbard 2002) per se or as an important component of hybrid adsorbents (Vasilieva et al. 1961, Leboda 1974, 1980a-d, 1981, 1987, 1992, 1993), which can be applied in chromatography (Colin and Guiochon 1976, Bebris and Kiselev 1978, Gierak and Leboda 1989), trace analysis (Rudzinski et al. 1995), catalyst supports (Vissers et al. 1988), etc. Variations in the nature of solid matrices, e.g., from catalysts with metals or metal oxides to practically inert silica gels or nanosilicas, can result in changes in the structure of pyrocarbon synthesized, however, under close conditions. The pyrolysis of organics is a complex multistage process. It includes dehydrogenation of organic substances CnHmXl → CnHm-z + (H2)k + (HX)i where X = OH, O, Cl, etc., n ≫ m-z. The activation energy of these reactions can be mainly between 100 and 300 kJ/mol. They are accompanied by changes in the electronic conguration of carbon atoms from sp3 or sp to sp2 in polyaromatic structures. Carbon transport and chemisorption can occur on a matrix and then on formed pyrocarbon grains. There are many chemical transformations leading to formation of pyrocarbon (graphene) clusters or practically individual carbon particles with certain surface functionalities, especially O-containing ones (Buyanov 1983, Lisovskii and Aharoni 1994, Fenelonov 1995). However, there is an opinion based on analysis of intermediates upon coke formation on solid surfaces that the difference between catalytic and noncatalytic cokes is linked only with a thin structure of the reaction spectra but not overall coke morphology (Buyanov 1983). This can be explained by preferable growth of the carbon phase on carbon nuclei formed at the beginning of the process.
