The Poisonous Effect of Carbon-Bearing Species on Adsorption of Hydrogen on Pd Alloys Membrane Surfaces

Hydrogen separation through the Pd membrane is one of the most promising technologies for producing pure H₂. This work investigates the effect of gaseous impurities, CO, CO₂ and CH₄ on the adsorption process of hydrogen, i.e. on the physisorption, and the chemisorption of hydrogen on Pd- x (x- Au, Pt) surface using density functional theory (DFT). Specifically, the study aims to sample the energy landscape associated with the hydrogen-adsorption while varying the structural and compositional degrees of freedom. The relative orientations of adsorbing molecules 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 mainly hydrogen, is studied. The physisorption of H2 on the Pd alloys-surface at different coverages of H-atoms (and without any impurities) has revealed that among the available adsorption sites, the top site with the horizontally oriented gaseous molecule is energetically the most favorable. 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. Moreover, the study also showed that at higher coverages of H-atoms, the physisorption energies of H2 decrease.