Poisonous effect of carbon bearing species on adsorption of hydrogen on Pd-membrane surfaces

Hydrogen separation through Pd-membrane is one of the most promising technologies to produce pure H₂. In this study, for the first time, the adsorption of hydrogen on covered Pd-membranes with both hydrogen and impurity gaseous species (CO, CO₂ and CH₄) was investigated using density functional theory (DFT). Specifically, the study aims to sample the energy landscape associated with hydrogen-adsorption while varying the structural and compositional degrees of freedom. The relative orientations of adsorbing molecule with respect to the surface of the slab, the crystallographic plane of the slab and various configurations of the adsorbed species have been investigated. In the case of compositional degrees of freedom, adsorption as a function of surface coverage, of hydrogen, is studied. The physisorption of H₂ on Pd-surface at different coverages of H-atoms (and without any impurities) has revealed that among the available adsorption sites, top site with horizontally oriented gaseous molecule is energetically the most favorable (−0.343 eV). Among the impurity gaseous species, when investigated for their binding to the Pd-surface and for their effect on the physisorption energies of the H2, CO is found to alter the energetics of the adsorption process, indicating a higher poisoning effect relative to CO₂ and CH₄. Also, it has the highest adsorption energy (−1.887 eV). Moreover, the study also showed that at higher coverages of H-atoms, the physisorption energies of H₂ decrease (−0.063 eV).