A kinetic investigation on the palladium hydrogen system

We investigated room temperature kinetics of hydrogen diffusion in 200 mesh palladium powder using a unique radio-frequency eddy current apparatus. This apparatus is sensitive to resistivity changes in palladium caused by hydrogen absorption or desorption. We developed a theoretical model to calculate the diffusivity constants for hydrogen absorption and desorption in palladium powder. These isothermal diffusivity values were used in an Arrenhius plot to determine the activation energy values of hydrogen absorption and desorption in either argon, nitrogen or dry air. We observed differences in the absorption and desorption cycles for hydrogen in argon and nitrogen. In addition, larger signals in all temperature ranges were recorded in nitrogen atmospheres. The activation energy values for hydrogen desorption, Edes, obtained from this experiment show little deviation from an average value of 5.2 kcal. Edes values for the individual carrier gases are (-5.2 +/- 0.1) kcal for H2 in argon; (-5.3 +/- 0.3) kcal in nitrogen and (-5.2 +/- 0.5) kcal for H 2 in mixtures of nitrogen and argon. The activation energy for hydrogen desorption in the presence of air was significantly higher than the previous two carrier gases. We calculated this Edes value to be (-7.3 +/- 0.4) kcal and attribute this high heat of desorption to hydroxyl ion formation on the palladium surface. Enthalpies of adsorption for hydrogen and oxygen were calculated using the Langmuir-Hinshelwood mechanism for heterogeneous catalysts. The enthalpy values, DeltaHad, compare favorably with reported values of DeltaHad for palladium metal films, namely, DeltaH ad = -27 kcal (Bond, 1962). The average values for the enthalpies of adsorption for hydrogen are; (-27 +/- 3) kcal in argon; (-27 +/- 5) kcal in nitrogen and (-26 +/- 1) kcal in mixtures of both nitrogen and argon. We investigated the 'mass-transfer' phenomenon of hydrogen by varying the flow rates of the carrier gases. Our results indicate that 'mass transfer' effects were absent in all our experiment data.