(397f) Permeability of H in Pd-Based Alloys Using Density Functional Theory

The challenge of producing pure hydrogen at lower cost requires the fabrication of high-permeability membranes that are stable at high temperature and pressure for the long operation times typically employed in petrochemical industries. For pure palladium membranes, the absorption and desorption cycles during the operation of membrane reactors causes expansion and contraction of the lattice due to alpha °ê beta phase transformations, making the membrane vulnerable to embrittlement. Alloying palladium with other metals helps by reducing the embrittlement, offering resistance to poisoning, and providing a longer operational lifetime. In addition, experiments in the past have shown that 23-32% silver in Pd-Ag alloy has significantly higher permeability as compared to pure palladium. To design a membrane having a specific permeability absent of experimental input, it is desirable to understand the mechanisms associated with both hydrogen diffusion and hydrogen solubility.

Binding energies of hydrogen at interstitial octahedral (O-) and tetrahedral (T-) sites were estimated using DFT. A plot of binding energy as a function of nearest (NN) and next nearest neighbors (NNN) indicate that binding energy strongly depends upon NN while it is weakly dependent on NNN. For both 15 and 25 mole %Ag in Pd-Ag alloy, the minimum binding energy was found to be at the site where the nearest and next nearest Ag atoms are zero. Also, with Ag addition, the binding energy of the O-site decreases. It was found that binding energy decreases with increasing alloy composition for 0-37 mole %Ag and increases thereafter. The solubility of hydrogen in pure palladium and Pd-Ag alloy was calculated using the Sieverts constant which was estimated with the help of DFT-based binding energy predictions. It was found that at a given temperature, the hydrogen solubility of Pd-Ag alloy increases with increasing Ag concentration until the mole %Ag reaches a maximum at 37.5 mole %Ag and it reduces thereafter matching closely with experimental results. The solubility of hydrogen in Pd-Ag alloy over a 0-50 mole %Ag range was found to be several times higher than that of pure palladium.

Permeability of hydrogen was obtained using the solubility and diffusion coefficient estimates. It was found that permeability increases with increasing mole %Ag, has a maximum value around 25% and decreases thereafter. The permeability of Pd-Ag alloy is significantly higher than that of pure palladium. These results and further analysis of permeability as a function of temperature and pressure are expected to help in the novel design and development of Pd-Ag alloy membranes for improved hydrogen separation.