Investigation of H2 and H2S Adsorption on Niobium- and Copper-Doped Palladium Surfaces

Alloying or doping Pd may be an option for overcoming sulfur poisoning. The current investigation probes the mechanism associated with sulfur binding to determine if Nb and Cu are appropriate doping metals. In this study, the effect of doping Pd with Cu or Nb on the binding strength of H2 and H2S was investigated using plane-wave density functional theory-based electronic structure calculations to determine mechanisms of adsorption. Results of this work indicate that for pure Pd and Pd-doped surfaces, H2 dissociates with the H atoms most stable on the fcc-fcc site. The overall d-band centers calculated for H2 adsorption at the fcc-fcc site for the pure and doped-Pd surfaces indicate that the H2 adsorption strength trend is Pd> Cu> Nb. Regarding H2S adsorption on Pd and Pd-doped surfaces, it was found that Cu has a lower affinity for H2S compared to Pd and Nb. The calculation of the local density of states of the s-, p-, and d-orbitals of the adsorbate-surface complex reveals an increase in the occupation of s-and p-states of the adsorbate and d-states of the dopant metals upon adsorption. In addition, the H2S binding trend is found to be Cu < Pd < Nb, with the doped-Cu surfaces exhibiting the weakest binding and doped-Nb surfaces the strongest binding. Geometry comparisons of each H2S-adsorbed complex shows that the hydrogen atoms are located closest to the surface in the case of Nb, indicating that the strong H-surface interaction leads to the enhanced adsorption behavior, rather than the S-surface interaction; in fact, the sulfur atom is located furthest from the surface doped with Nb