Solubility of hydrogen in single-sized palladium clusters

Hydrogen concentration-pressure isotherms of surfactant-stabilized palladium clusters and polymer-embedded palladium clusters with diameters of 2, 3, and 5 nm are measured with the gas sorption method at room temperature. The results show that, compared to bulk palladium, the hydrogen solubility in the a phase of the clusters is enhanced fivefold to tenfold, and the miscibility gap is narrowed. Both results can be explained by assuming that hydrogen occupies the subsurface sites of the palladium clusters. The Pd-H isotherms of all clusters show the existence of hysteresis, even though the formation of misfit dislocations is unfavorable in small clusters. Compared to surfactant-stabilized clusters, the polymer-embedded clusters show slow absorption and desorption kinetics. The absorption kinetics can be described by a diffusion model for the composite polymer-cluster system.