Hydrogen-Driven Surface Segregation in Pd Alloys from Atomic-Scale Simulations

The safe widespread application of hydrogen-based fuels requires sensors that are long-term stable, inexpensive, hydrogen-specific, and have a short response time. In this regard, optical sensing based on nanostructured Pd alloys has shown great potential, but challenges remain, including understanding controlling the surface composition under operation conditions. While latter is crucial for functionality stability, it experimentally very challenging to obtain accurate atomic-scale information. Here, we therefore scrutinize behavior two particularly relevant alloys, {111} AuPd CuPd, in H environments ranging from vacuum fully covered surfaces. To end, employ combination alloy cluster expansions trained using density functional theory calculations, Monte Carlo simulations, thermodynamic analysis coverage as well layer-by-layer near-surface region function partial pressure, temperature, bulk composition. overcome symmetry reduction implicit systems, exploit local symmetries, which enables us achieve reliable models at low computational cost. case AuPd, Au segregates while 100% coverage, transition between these regimes occurs over narrow pressure interval. other hand, varies much more gradually with coupled nonmonotonic variation Cu concentration topmost layer. there pronounced depletion both 0 enrichment observed 50% concentrations up least 10%, providing nontrivial explanation an apparent discrepancy experiment calculations was previously. At same time, layer 2 continuously shifts increasing coverage. results demonstrate rich can result coupling metal–metal metal–hydrogen interactions surfaces, even apparently simple concentrated systems. Moreover, they underline advantages simulations account temperature effects accurately capture wide range.