(594d) Ab Initio Studies of PdNb Alloys for Hydrogen Membranes
Palladium-based membranes have been studied extensively for their application in H2 production due to their high permeability and catalytic activity to H2, their impermeability to other gases, and their stability at relatively high temperatures. Recently there has been a high demand for the development of new alloy materials to reduce the cost of the membranes and to improve the permeability. Alloying materials can allow one to tune the surface and bulk so that it exhibits the desired electronic properties necessary for a given process to take place. In this work, alloys of PdNb are investigated to understand the mechanisms involved in surface site stabilities, reactivity, and subsurface hydrogen diffusion. Niobium is an optimal material to use because it has the one of the highest hydrogen permeability of any other material and under specific conditions can be an order higher than pure Pd and has shown to be resistant to sulfur poisoning. Unfortunately, Nb is notorious for undergoing hydrogen embrittlement and can be a major structural issue that will also be examined through alloying. By combining the two materials a membrane that combines the advantages of each can be developed. Density functional theory (DFT) is employed using the software package Vienna ab initio Simulation Package (VASP) to calculated binding energies of H in alloys of Pd and Nb. Bader charge analysis and a density of state analysis will be used to understand the role that the charge of the H plays in the diffusion process in alloys with different compositions and H concentrations. Another significant technical barrier impeding hydrogen separation membrane development is the susceptibility to Pd membrane deactivation or degradation via sulfur poisoning. Although desulfurization technologies can decrease the level of H2S present in an effluent stream down to the ppm range, these resulting trace concentrations still cannot be tolerated by Pd-based membrane materials. The impact that the Nb surface atoms have on the sulfur tolerance will also be investigated.
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