The thermal decomposition of carbon tetrachloride on clean iron was studied
in ultrahigh vacuum using molecular beam strategies, where it is found tha
t carbon tetrachloride thermally decomposes on the surface to deposit iron
and carbon with exactly identical kinetics as found at high pressures. No g
as-phase products are detected and the activation energy for the reaction (
14.2 +/- 0.5 kcal/mol) is in good agreement with the value measured at high
pressures. Little carbon is detected on the surface using Auger spectrosco
py following reaction and it is found that this diffuses into the surface m
uch faster when formed from CCl4 than from CH2Cl2. This effect is ascribed
to the effect of co-adsorbed chlorine on the adsorbed carbon, which is prop
osed to decrease the activation energy for diffusion into the bulk of the s
ample. This effect explains the increased tendency for carbon tetrachloride
to form carbides under extreme-pressure tribological conditions.