At high temperatures (IOOO-2OOOT
) and low pressures (10-5-10-2 Torr) ethylene, acetylene and benzene decompose heterogeneously on pyrolytic carbon giving mainly hydrogen and deposited carbon, with collision yields of the order of 10e4. The kinetics of these carbon deposition reactions show some striking similarities with carbon removal reactions by oxygen or oxygenated compounds.
The true reaction order of these decomposition reactions is one above 14OO"C, but becomes smaller at lower temperatures. This behaviour, common in gas-solid reactions, is generally interpreted as an inhibition due to chemisorption of some intermediate or reaction product. Evidence is also obtained that decomposition of the hydrocarbon molecules only occurs on peculiar sites of the carbon surface, i.e. the decomposition is not a purely thermal process. but involves a specific chemical interaction with the surface.
Moreover, the behaviour of the pyrocarbon surface in carbon deposition reactions is similar to that observed in gasification reactions, i.e. the reactivity of the surface accommodates itself to the temperature and pressure conditions. as revealed by the observation of "transitory" and "stationqry rates". Transitory rates show that the surface deactivates with increasing temperatures (Figs. 4 and5) [from which a maximum in the stationary rate results (Figs. l-3)] and decreasing pressures (Figs. 7 and8). The interpretation assumes that reaction sites are continuously created as an effect of carbon atoms deposition, but also deactivated by a thermal healing process.
A main Merence between carbon deposition reactions from hydrocarbons and carbon gasification reactions concerns the temperature range where reactivity is temperature dependent: in carbon deposition reactions, deactivation of the pyrocarbon surface is still effective up to much higher temperatures (Fig. 12).