An activation-deactivation model for catalytic deposition of carbon

Abstract

A sequence of reaction steps that corresponded to the fundamental phenomena of adsorption/desorption of a hydrocarbon and hydrogen, hydrogenation/dehydrogenation surface reactions, carbon diffusion and precipitation, catalytic site activation, and deactivation involved in catalytic deposition of carbon from hydrocarbons is proposed. Rate expressions derived from the reaction sequence yielded a kinetic model for simultaneous carbon deposition, catalytic site activation, and catalyst deactivation, which exhibited trends qualitatively similar to experimental results for Ni-catalyzed catbon deposition from propylene/hydrogen mixtures. It was found that increasing the H 2 concentration decreased the rates of deposition and poisoning but did not affect the rate of site activation. Thus, higher H 2 concentrations lead to lower initial deposition rates, but the total level of deposition increases because of reduced deactivation rates. The hydrocarbon concentration affected the rates of activation, poisoning, and deposition similarly so that the ultimate level of deposition was independent of the hydrocarbon concentration. However, as the hydrocarbon concentration increased, the time over which deposition occurred decreased due to an increased rate of deactivation. Optimized fits of the model to experimental data at various temperatures showed that the model agreed well with data at H 2 concentrations from 0.0 to 45%, propylene concentrations up to 3%, and temperatures up to 550°C.