Reactions of modulated molecular beams with pyrolytic graphite. III. Hydrogen

Abstract

The reaction of hydrogen and pyrolytic graphite was studied by modulated molecular beam–mass spectrometric methods. Because molecular hydrogen does not react with graphite (within the detection limits of the technique), an atomic hydrogen reactant beam was generated by thermal dissociation of H2 in an effusion oven. For surface temperatures up to 800 °K, methane was the sole product. Acetylene was observed at temperatures above 1000 °K. Between 800 and 1000 °K, no carbon gasification occured; the surface acted only to recombine H atoms to form H2. The data were analyzed in terms of a model in which methane is formed by sequential addition of H atoms to CH n (n=0,1,2,3) and acetylene is formed by surface combination of two CH groups. The agreement between the model and the molecular beam data is very good. The model also predicts previous high pressure (1 atm H2) carbon gasification rates determined in conventional kinetic experiments.