H₂ saturation on palladium clusters

The interaction of Pd_N clusters (N = 2, 3, 4, 7, and 13) with multiple H₂ adsorbate molecules is investigated using density functional theory with the hybrid PBE0 functional. The optimal structure for each Pd_NH_2(L) complex is determined systematically via a sequential addition of H₂ units. The adsorption energy for each successive H₂ addition is computed to determine the maximum number of molecules that can be stably added to a Pd_N at T = 0 K. The Gibbs free energy is then used to determine the saturation coverage at finite temperature. For N = 2, 3, and 4, a single H₂ is found to dissociate, and up to two additional molecular H₂ units per Pd atom can bind stably to the clusters at 0 K. At 300 K, one H₂ unit dissociates, and only one additional H₂ molecular unit per Pd atom is stably bound. For N = 7 and T = 0 K, two H₂ units dissociate, and 11 additional H₂ units bind molecularly. At 300 K, two units dissociate, and eight are bound molecularly. For N = 3, 4, and 7, we find that an additional H₂ unit may dissociate if the underlying cluster structure rearranges. Eight H₂ units dissociate on Pd₁₃ at 0 K. At least one additional H₂ binds molecularly at 0 K, but none bind at 300 K. This suggests that only dissociated H₂ units will stably bind to larger Pd particles at room temperature. The influence of molecularly adsorbed H₂ units on the migration of dissociated H atoms is investigated in a preliminary way. Both barrier heights and the relative stability of local minima of Pd₄H_2(L) are found to be affected by the degree of molecular H₂ coverage.