Structure and Growth Kinetics of Films Formed by the Thermal Decomposition of CCl4 on Iron Surfaces

Carbon tetrachloride thermally decomposes on iron in the temperature range 500-700 K to form films that consist predominantly of iron chloride. Two growth regimes are found: (1) parabolic, where the film thickness (X) varies as a function of time as X2 ∼ t and where the growth rate is independent of pressure, and (2) a linear growth region (X ∼ t), where the growth kinetics are first order in CCl4 pressure. The kinetics are analyzed by a model which assumes that growth is controlled either by CCl4 thermally decomposingat thegrowinggas-film interfaceorbydiffusion through the film. Analysis of theexperimental data gives an activation energy for diffusion of 21.5 ( 0.3 kcal/mol and an activation energy for CCl4 decompositionof18.3(0.5kcal/mol. In contrast to thebehavior found formethylenechlorideandchloroform, where small particles of carbon were identified in the film using Raman spectroscopy, no carbon particles are found in films formed from carbon tetrachloride decomposition below∼700 K although carbon is found in the filmusingX-ray photoelectron spectroscopy. XPS results also suggest that, in addition to completely thermally decomposing to yield carbon and chlorine, a small portion of the CCl4 can react to form adsorbed CCl2 species.