Carbon deposition from propylene on polycrystalline and single crystal iron

Abstract

The kinetics and morphology of the deposition of carbon from propylene on polycrystalline and single crystal iron have been studied over the temperature range 400–700 °C. The dependence of the rate of carbon formation on temperature shows a pronounced maximum at ca. 600 °C. Below this temperature, single crystals and polycrystalline iron foils behave similarly. On fresh samples, carbon deposition appears to be rate controlled by the diffusion of carbon in iron. After the deposition of some carbon it is suggested that the rate is controlled by the carbide-catalyzed production of carbon from propylene. Above ca. 600 °C, carbon formation is considered to be rate controlled by the surface decomposition of propylene. On fresh samples, the rate was affected by the geometry of the surface with carbon formation decreasing in the order (100) ∼ foils > (110). On used catalysts, where the active surface is probably iron carbide, geometric effects were not observed. Encapsulation of the surface by carbon was important at these higher temperatures, and this reaction was independent of the surface geometry.