Kinetics and mechanism of gas-carbon reactions: Conformation of etch pits, hydrogen inhibition and anisotropy in reactivity

An etch decoration/transmission electron microscopy study was performed on consecutive reactions of graphite with CO₂ and H₂O and on the inhibition of H₂ in separate reactions with these gases. The results indicated that H₂ chemisorption, at near 700°C, was preferred on the zig-zag {101̄0} face. It was also determined that the chemisorption was responsible for the anisotropy of reactivity on the two principal edge planes when H₂ is present. This anisotropy was successfully used to explain the conformation of etch pits in all reactions observed to date. From the rate data on separate reactions of graphite with CO₂ and H₂O, both with added H₂, the adsorption constant for H₂ on the monolayer zig-zag plane at 700°C was calculated to be 96 and 71 atm^-0.5, respectively. The agreement was satisfactory.