Drastically enhanced H2flux through asymmetric quantum Pd films

Abstract

It has been established recently that the quantum mechanically confined motion of conduction electrons within ultrathin metal films can strongly modify the various atomic rate processes and chemical reactivity on top of the films with thickness-dependent stability. In this work, we investigate theoretically how such quantum size effects (QSE) can alter the energetics and kinetics as gaseous hydrogen molecules adsorb dissociatively on one side of an ultrathin Pd membrane serving as a model separating system, and desorb associatively from the other side.We first establish that these quantum Pd films possess extra (or magic) stability at certain film thicknesses, and demonstrate that the H 2 flux through such quantum films can be tuned significantly by the QSE-induced variations in the key molecular/atomic rate processes.Furthermore, when the permeate side of the films is coated with a layer of less H-reactive Cu or Ag element, the depth of the H chemisorption well is lowered substantially without introducing other larger barriers, which in turn can enhance the H 2 flux by several orders of magnitude around 500 K. These findings of fundamental nature may prove to be inspirational in future design of ideal membrane-based devices with maximal hydrogen permeability and low cost.