Kinetics of hydrogen desorption from palladium and ruthenium-palladium foils
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J. N. Armor Air Products and Chemicals, Inc., 7201 Hamilton Boulevard, Allentown, Pennsylvania 18195 (Received 12 August 1993; accepted 31 October 1994)
The absorption of hydrogen and carbon monoxide at room temperature by palladium and 5% ruthenium-palladium foils was studied using thermal desorption spectroscopy. It was found that hydrogen readily diffused in the palladium and desorbed as one broad peak at about 650 K. Plots of the In (rate) versus inverse absolute temperature indicate that the desorption order is n = 1.25 and the activation energy is about 8.5 Kcal/mol. Carbon monoxide is adsorbed, as two different states, on the surface of the foil and complete coverage is quickly reached below 100 L. Hydrogen also diffuses in 5% ruthenium-palladium foil but to a lesser degree. Two hydrogen desorption peaks are observed in the Ru-Pd alloy. The desorption traces can be fitted with two peaks and the desorption orders are n = 2 for the first peak and n = 1.25 for the second peak. Activation energies of 10.7 and 5.6 Kcal/mol are obtained for the first and second hydrogen peaks, respectively. The first hydrogen desorption peak is regarded as hydrogen desorbing from the surface sites while the second peak is regarded as hydrogen diffusing from below the surface. Activation energies for bulk diffusion were obtained from hydrogen uptake measurements using a sensitive microbalance. These energies corresponded to 4.4 Kcal/mol for Pd foil and 4.9 Kcal/mol for the Ru-Pd alloy. Discussion about the relation between these results with prior studies of hydrogen adsorption on Pd single crystal is included. The appearance of a fractional order for hydrogen desorption is also discussed.
I. INTRODUCTION The interaction of hydrogen with palladium (Pd) is a subject of current interest due to the potentially important technological applications in the area of metallic membranes. Gryaznov1-2 pioneered work on hydrogen diffusion through thin-walled pure metal membranes for catalytic applications. The facile permeation of hydrogen through palladium and palladium alloys suggests a number of applications in some chemical processes. This knowledge can be incorporated into the design of more efficient reactors3 for several hydrogenation or dehydrogenation reactions. Nevertheless, the feasibility of an industrial application (at large scale) relies on reducing the cost of the reactors since a reactor made of Pd would be extremely expensive. Understanding the interaction of hydrogen with Pd may lead to the discovery of other alloys which incorporate inexpensive metals yet provide easy diffusion of hydrogen through the alloy. The absorption of hydrogen by Pd was tentatively explained by Lagos4"6 as a process mediated by suba) Author
to whom correspondence should be addressed. J. Mater. Res., Vol. 10, No. 3, Mar 1995
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surface bonding of hydrogen. According to Lagos' calculations, hydrogen would prefer to bind to subsurface sites than surface sites in Pd based on the energy required for
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