Tuning H-Loading in Pd(111) via Metal Overlayers: Insights from Ab Initio Thermodynamics and Pourbaix Diagrams

Understanding and tuning the surface reactivity of Palladium (Pd) to enhance Hydrogen (H) sorption and catalytic performance has garnered significant research interest. While most studies focus on evaluating surfaces at low adsorbate coverages, understanding surfaces at higher coverages is crucial for practical applications. Inspired by the recent advances in overlayer- and metallene-based nanomaterials, we employed density functional theory to investigate hydrogen sorption behavior on Pd(111) systems with a single metal overlayer, M_MLPd(111) (where M = Ag, Au, Co, Cu, Ir, Ni, Pt, and Rh). Using ab initio-based phase and surface Pourbaix diagrams, we identified the thermodynamically preferred hydrogen-coverage of these surfaces as a function of temperature, H₂ pressure, pH, and applied potential (U_SHE). These diagrams reveal the surface states under realistic cathodic conditions, considering also hydride formation and overlayer segregation. Notably, Cu and Pt overlayers exhibited higher stability than pristine Pd(111), while Ag and Au remained stable but displayed weak hydrogen adsorption. Among the systems studied, Cu_MLPd(111) emerged as a promising candidate due to its lower surface-to-subsurface hydrogen diffusion barriers and optimal hydrogen adsorption energies, suggesting potential applications in selective hydrogenation and reduction reactions. A partial density of states (PDOS) analysis revealed distinct M-H interaction peaks around -6 to -8 eV, and a clear strain → d-band center → ΔG_ads relationship, underscoring how lattice mismatch tunes electronic structure and thus adsorption strength. By using single-metal overlayers as atomic-scale "tuning knobs", we show that H binding can be tuned on the metal overlayer surface, while the underlying Pd substrate serves as a high-capacity hydrogen reservoir for rapid uptake and release. Overall, this work demonstrates how hydrogen coverage and subsurface hydride formation govern the thermodynamic stability and reactivity of M_MLPd(111) surfaces, offering a predictive framework for designing advanced Pd-based electrocatalysts for hydrogen-involving reactions.