KINETICS OF HYDROGEN DESORPTION FROM PALLADIUM AND RUTHENIUM-PALLADIUM FOILS

The absorption of hydrogen and carbon monoxide at room temperature by palladium and 5% ruthenium-palladium foils was studied using thermal d esorption 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 d esorption order is n = 1.25 and the activation energy is about 8.5 Kca l/mol. Carbon monoxide is adsorbed, as two different states, on the su rface of the foil and complete coverage is quickly reached below 100 L , Hydrogen also diffuses in 5% ruthenium-palladium foil but to a lesse r degree. Two hydrogen desorption peaks are observed in the Ru-Pd allo y. The desorption traces can be fitted with two peaks and the desorpti on orders are n = 2 for the first peak and n = 1.25 for the second pea k. Activation energies of 10.7 and 5.6 Kca/mol are obtained for the fi rst and second hydrogen peaks, respectively. The first hydrogen desorp tion peak is regarded as hydrogen desorbing from the surface sites whi le the second peak is regarded as hydrogen diffusing from below the su rface, Activation energies for bulk diffusion were obtained from hydro gen 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 pri or studies of hydrogen adsorption on Pd single crystal is included, Th e appearance of a fractional order for hydrogen desorption is also dis cussed.