Lattice model simulation of hydrogen effect on palladium gold alloys used as purification metal membranes
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Hydrogen fuel is seen as energy of the future. Hydrogen molecules (H₂) are mostly used in the petrochemical industry and ammonia production. Hydrogen molecules are produced in large majority by steam reforming of hydrocarbons (methane). However, the purification of hydrogen is still a major factor in the cost of producing hydrogen. The range of membranes is large with advantages and drawbacks of each type of membrane. Among those membranes, metal alloy membranes are widely used because of their selectivity, durability, and resistance to poisoning. In the past, it was assumed an alloy once formed would remain in its initial structure while hydrogen gas was permeating through. It has been shown experimentally that the presence of hydrogen in palladium gold metal alloys will change the structure of the alloy from a disordered to an ordered phase. Hydrogen isotherms at different temperatures were used to demonstrate the change in structure. This structural change resulted in an increase in solubility of hydrogen in the membrane. In this work, using NVT-Monte Carlo we calculated the effect of hydrogen on the structure and on the solubility of Pd₉₆Au₄ and Pd₈₅Au₁₅ alloys. The palladium gold alloy was used because it demonstrated high resistance to sulfur poisoning and similar or higher permeability seen in to pure Pd. The methods used do not require any experimental input except for the structure of the bulk crystal. The interstitial binding energies were calculated using a Cluster Expansion model derived previously by Semidey and Kang from plane wave Density Functional Theory. The metal atoms enthalpies of formation were calculated from a truncated version of the Cluster Expansion derived by Sluiter for fcc metal structures. We conclude that hydrogen presence in the metal membrane will change the membrane from a disordered to a Short Range Ordered structure.