Substantial hydrogen absorption in nanometer-sized palladium crystallites on a thin film alumina support is observed at $90–350\mathrm{K}$ under low ${\mathrm{H}}_2$ pressure conditions $(<2\times {10}^{-3}\mathrm{Pa})$ by ${}^1\mathrm{H}({}^{15}\mathrm{N},\alpha \gamma ){}^{12}\mathrm{C}$ nuclear reaction analysis. The enthalpy of H solution varies with the absorbed H content and with $-(0.28\pm 0.02)\mathrm{eV}∕\mathrm{H}$ exceeds that of bulk Pd at $\mathrm{H}∕\mathrm{Pd}$ ratios below 0.20. By resolving absorbed H from surface-adsorbed H, this enhanced H stability is identified as an intrinsic volume property of the small crystallites rather than being associated with particular binding sites on or near their surfaces.

• Received 25 February 2008

DOI:https://doi.org/10.1103/PhysRevB.77.113412