Reactions of modulated molecular beams with pyrolytic graphite IV. Water vapor

Publisher Website
Google Scholar
Abstract
The reaction of water vapor with the prism plane face of anneal pyrolytic graphite was investigated by modulated molecular beam–mass spectrometry methods. The equivalent water vapor pressure of the beam was ∼2×10−5 Torr and the graphite temperature was varied from 300 to 2500°K. The mechanism was deduced from three types of experiments: isotope exchange utilizing modulated H2O and steady D2O beams; measurements of the phase difference between H2O and neon reflected from the surface from a mixed primary beam of these species; and reaction of a modulated H2O beam to produce CO and H2. Based upon the isotope exchange experiments chemisorption of water on graphite was found to be dissociative and reversible. Incident water molecules chemisorbed with a sticking probability of 0.15±0.02 to form the complexes C–OH and C–H. Recombination of the surface complexes reverses the adsorption step and is responsible for the isotope exchange properties of the graphitesurface. This process is unactivated. Reaction to produce CO and H2 also results from collisions of the primary surface complexes, but this step has an activation energy of 170 kJ/mole. This reaction yields bound complexes tentatively identified as C–O and H–C–H, which then decompose to produce the stable reaction products. All of the above steps exhibit characteristic times on the order of milliseconds, and are therefore detectable by the modulated beam method. All surface intermediates are strongly affected by solution and diffusion in the bulk of the solid.