Sorption of hydrogen by palladium and palladium/silver alloy wires

Abstract

Heats of adsorption q(θ), sticking probabilities and temperature programmed desorption spectra are reported for the adsorption of H₂ onto clean wire surfaces of Pd, all as functions of coverage θ. Data, principally from temperature programmed desorption, have also been obtained for the inter action of H₂ with two Pd/Ag alloys. The low temperature (104 K) sorption properties of Pd were dominated by the occurrence of absorption which limited the applicability of the technique used to determine q(θ). Certain sites on the Pd surface had to be occupied by strongly bound, β, adsorbed hydrogen before absorption could occur at 104 K, and the probability of their occupation depended on the mode of preadsorption. This is interpreted as evidence for the occurrence of metastable surface precursor states to absorption for which q was 30–60 kJ mol^–1. Sorption by both of the Pd/Ag alloys studied was activated, its rate decreasing as temperature decreased. A model is proposed for the absorbed state of H which rationalises why the forms of the H₂ desorption spectra from Pd and Pd/Ag alloys are dependent on metal sample size and which emphasises that large proportion of H₂ sorbed by these alloys (P_{H2 < 5 mPa) may be absorbed even at room temperature. The possible role of absorbed hydrogen in catalytic reactions using H2 and Pd or Pd/Ag alloys is briefly considered.}