Although several authors have found anisotropy of the chemical reactivity of pyrolytic graphite they disagree regarding the existence of a temperature coefficient. New data are presented which together with earlier data show the rate ratio for the two major directions is temperature dependent with an activation energy difference of about 19 kilojoules (4500 thermochemical calories) per gram atom of carbon reacted. This probably arises because the relative number of sites is sufficiently different to cause a shift in which reaction step is rate-controlling.
On the 'faces' chemisorption is slow followed by rapid decomposition of surface oxides. On the 'edges' chemisorption is rapid followed by decomposition. 65. Catalysis of the oxidation of graphite in the temperature range 1000"-fOOOยฐC J. R. Egerton and R. F. Strickland-Constable (Department of Chemical Engineering and Chemical Technology, Imperial College, London, U.
K.).
A study has been made of the catalytic effect of substances applied to the surface of graphite filaments in the temperature range 900"-2000ยฐC on the rate of oxidation with oxygen and also with nitrous oxide. Strong catalysis was observed with some metals, but boron and silicon appear to have little effect. Further studies have been made of the so-called hysteresis effect: the effect is found to change sign above a certain rather low pressure.