GROWTH-KINETICS AND STRUCTURE OF FILMS FORMED BY THE THERMAL-DECOMPOSITION OF METHYLENE-CHLORIDE ON IRON

The kinetics of the formation of films by the thermal decomposition of methylene chloride on an iron surface is investigated using a microba lance where the film thickness X is shown to vary with time t as X = X (m)(1 - exp(-Bt)) where X(m) is the maximum film thickness. X-ray phot oelectron, Auger, and Raman spectroscopic analyses show that the film consists of an iron chloride and carbon, where Raman spectroscopy indi cates that the carbon is present as small (similar to 50 A) particles. The observed kinetics are rationalized by assuming that growth is lim ited by thermal decomposition of methylene chloride at the gas-phase/f ilm interface rather than transportation through the film and that the sites for thermal decomposition become poisoned as the reaction proce eds. Surface spectral analysis indicates that both the amount of surfa ce carbon and the proportion of covalently bonded chlorine increase du ring film growth so that either the formation of a carbonaceous layer or the accumulation of an organochlorine species could be responsible for the cessation of growth.