Kinetics of CH4 formation from the reaction of H with graphite

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Abstract
Small ribbon-shaped samples of spectroscopic grade graphite heated to between 500 and 1250 K have been exposed to atomic hydrogen produced by thermal dissociation of H2 on hot W and the rate of CH4 evolution has been measured mass spectrometrically. H2 pressures were between 1×10−6 and 5×10−5 Torr and arrival rates of H at the graphite surface ranged from 1×1014 to 5×1015 atoms cm−2 sec−1. When a well outgassed graphite surface is initially exposed to H atoms, the rate of CH4 evolution, NM, peaks and then decays to a steady-state rate. A mechanism is proposed and examined which adequately explains the results observed. The mechanism involves the formation of a CH4-producing surface complex by the reaction of an incident H atom with chemisorbed H atoms on the graphite surface. Additional chemisorbed H rapidly reacts with this complex and CH4 desorbs. The decay in NM to a steady-state evolution rate is apparently caused by a surface reaction in which pairs of the surface complexes responsible for CH4 evolution form a stable site-blocking species.