Kinetics and mechanisms of the carbon-steam reaction on the monolayer and multilayer edges of graphite

Turnover frequency rates, i.e. rates per active edge-carbon site, on both monolayer and multilayer edges were measured for the C + H₂O reaction at 700, 800 and 900°C. The rates followed the rate equation: Rate = k₁P_H2O 1 + k₂P_H2ⁿ + k₃P_H2O where n = 0.5 at 700°C, n = 0.85 at 800°C and n = 1.0 at 900°C for monolayer edges; n = 0.5 for multilayer edges at all three temperatures. Additional experimental results indicated that the 0.5th order for H₂ was due to the dissociative chemisorption of H₂. On monolayer edges, the dissociative chemisorption preferentially took place on the {101̄0}, or zig-zag, face. The turnover rates on the multilayer edges were higher than the monolayer edges by approximately one order of magnitude. Theoretical values of the elementary rate constants, as predicted by transition state theory, compared reasonably well with the experimental values, including the H₂ chemisorption constant. The rates of vacancy formation on the basal plane were also determined. Atomic oxygen formed by dissociation of H₂O was responsible for vacancy formation. H or OH species were responsible when H₂ was present in the gas phase.