The Surface Configurations and Their Impact on Pd-Based Alloy Membranes for Hydrogen Separation—An Application of a Universal Neural Network Potential

Pd-based alloy membranes for hydrogen separation are highly efficient and have several important practical advantages over traditional methods like pressure swing adsorption or cryogenic distillation. However, the use of Pd, a rare and expensive metal, and its reactivity with certain gas mixture components pose challenges. To address these issues, Pd alloys have been widely explored in the literature.<br/>In this work, we report our computational study on the alloy configurations and their impact on the adsorption and diffusion of hydrogen at the surface. Our unique universal neural network potential, called Preferred Potential (PFP), was applied to perform necessary atomistic simulations. One of the advantages of using PFP is the capability of simulating chemical reactions at near-DFT accuracy on a much larger system with a much higher speed.<br/>We examined the surface segregation behavior of the fcc Pd-based alloys as a function of temperature and composition using Monte Carlo simulations. Pronounced segregation of the additive species was found in Pd₃Ag and Pd₃Au, but Pd segregation was found in Pd₃Cu, Pd₃Ni, and Pd₃Pt at the fcc(111) surface. These findings are consistent with the previously known facts in the literature.<br/>Beyond the binary phases, selected ternary phases were also explored. The investigation of the combinations of Au-Ag, Au-Cu, and Cu-Ag in host Pd revealed the segregation of Ag and Au at the surfaces in these alloys, similar to the case of the binary alloys. However, interesting subsurface structures were observed and characterized for each composition. The surface segregation of the additional elements significantly affects the surface binding of the hydrogen. The binding energies of the hydrogen atoms on a pure Pd surface are well-defined and distinct: 2.47 and 2.97 eV at top and fcc hollow sites, respectively, in reference to a hydrogen atom. However, the binding energies on the ternary surfaces exhibit very different behavior. There is much more variety in the geometric configurations of the surface adsorption sites. The observed hydrogen binding energies are much higher in general and distributed over a wide range of energy depending on the surface composition. This difference in the surface binding energies has a significant impact on the surface migration as well as the out-of-plane migration energy barriers.<br/>This work will provide the basis for understanding the behavior of the Pd-based alloy membranes and their interaction with hydrogen. The insights and techniques developed in this study can be useful in the design beyond the commonly used Pd-based membranes.