Adsorption and Absorption Energies of Hydrogen with Palladium

Thermal recombinative desorption rates of HD on Pd(111) and Pd(332) are reported from transient kinetic experiments performed between 523 and 1023 K. A detailed kinetic model accurately describes the competition between recombination of surface-adsorbed hydrogen and deuterium atoms and their diffusion into the bulk. By fitting the model to observed rates, we derive the dissociative adsorption energies (E-0, ads H-2 = 0.98 eV; E-0, ads D-2 = 1.00 eV; E-0,E-ads (HD)= 0.99 eV) as well as the classical dissociative binding energy epsilon(ads) = 1.02 +/- 0.03 eV, which provides a benchmark for electronic structure theory. In a similar way, we obtain the classical energy required to move an H or D atom from the surface to the bulk (epsilon sb = 0.46 +/- 0.01 eV) and the isotope specific energies, E-0,sb( H) = 0.41 eV and E-0, sb (D) = 0.43 eV. Detailed insights into the process of transient bulk diffusion are obtained from kinetic Monte Carlo simulations.