Modeling H2 transport through a Pd or Pd/Ag membrane, and its inhibition by co-adsorbates, from first principles

We calculate the rate of hydrogen transfer through a Pd and a Pd/Ag selective membrane using transition state theory with adsorption and activation energies calculated by DFT and nudge elastic band methods. The flux through the membrane bulk was evaluated using Fick׳s law with the diffusivity constant calculated from energetic barriers in the diffusing H path while accounting for adsorption and subsurface penetration. A reasonable prediction of expected permeance of a Pd membrane was achieved; the prediction of an ordered Pd3Ag membrane was much lower than expected due to larger energetic barrier, but estimation of diffusivity in a random Pd/Ag membrane, accounting for the probability of encountering the various sites, led to permeance higher than that of Pd. Predicted energies compare well with literature data. The inhibition due to surface adsorption of possible co-adsorbates like propylene, propane, methane, CO and water was also accounted for using DFT-calculated adsorption energies. The inhibitions of propylene and CO are expected to be significant in the corresponding membrane reactors (e.g., for propane dehydrogenation or for methane steam reforming).