Abstract:
Hydrogen sorption was studied on PdAu wires carefully cleaned in ultra high vacuum, at a hydrogen pressure of 6.25 µPa and wire temperatures of 100, 150, 200 and 304 K. For Pd at 100 K the initial sticking coefficient was 0.16, falling to 0.006 at a fractional monolayer coverage θ= 0.8. Sorption against time curves at 100 K for hydrogen continued smoothly to coverages θ > 3.0. Deuterium for the same collision number (pD2=√2pH2) gave a similar rate up to θ∼ 1.0 when the uptake became slower. Temperature programmed desorption gave one peak and desorption activation energies, which extrapolated to a θ= 0 value of 100 kJ mol–1 for Pd, 90Pd10Au and 70Pd30Au and 45 kJ mol–1 for 55.2Pd44.8Au. These activation energies decreased sharply with coverage, to a value for the three Pd rich alloys of ∼20 kJ mol–1, comparable with the heat of solution. Hydrogen sorptions for a given exposure increased slightly from Pd to 90Pd10Au, decreasing sharply towards zero at 40Pd60Au, where the holes in the d band have disappeared. The results are discussed critically in relation to earlier data, and a tentative model advanced involving (mainly)β chemisorbed H (on Pd atoms, immobile at 100 K), α chemisorbed H2(weakly held on octahedral holes in 100 planes) and dissolved H atoms under the surface. Although it is concluded that holes in the d band are necessary for strongly chemisorbed β-H, a role for PdnAum surface ensembles cannot be ruled out.
