INFLUENCE OF ADSORBED HYDROGEN MOLECULES ON THE HYDROGEN PERMEATION AND RECYCLING
Hydrogen permeation through a thin iron membrane has been measured using a molecular hydrogen gas and/or a weakly ionized rf-discharge as particle sources. Transient and steady state values of the permeation flux phi"rm p" were obtained in the regime where the surface penetration of H and the recombinative release of H"2" are rate-determining. phi"rm p" is then /Waelbroeck 84a/ proportional to the flux density psi of the particles which impinge on the membrane upstream. The proportionality factors depend on the nature (atomic or molecular) of the incident particles, on temperature (in particular for a molecular driving gas), and on the ratio of the surface recombinative release rate constants (2sigmak"rm r")"rm d" up- and down-stream, respectively. The pressure P of the molecular gas, the power coupled to the rf-discharge, the temperature of the membrane, and its electrical potential with respect to the plasma have been varied during the rf-plasma-driven permeation measurements. The results are analyzed on the basis of the model presented in reference /Waelbroeck 84a/, using the equations in reference /Waelbroeck 85a/. According to the model there should be adsorbed hydrogen molecules on the surface when it is exposed to molecular hydrogen gas, and these admolecules should exert a catalytic influence on the mechanism of the surface recombinative release of hydrogen and enhance the value of the recombinative release rate constant. An excellent agreement is achieved assuming that most of the chemisorbed hydrogen is molecular. The heat of adsorption is found to be 21.5pm0.5 kcal/mol. An activation energy for chemisorption is evaluated to be 8pm kcal/mol. The gain in the activation energy for the recombinative release by admolecules of 2pm0.5 kdcal/mol and a cross section of 5times10"18" cm"2" for the disappearance of the chemisorbed molecules by electrons (E ge 100eV) are evaluated.
Bibliographic Reference: REPORT JUL-2114, 1987, AVAILABILITY: KFA, JUELICH, D
Record Number: 1989126073300 / Last updated on: 1989-05-01
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