Understanding Strain and Ligand Effects in Hydrogen Evolution over Pd(111) Surfaces

Pseudomorphic catalytic systems can exhibit enhanced or inhibited activity relative to the pure surface parent metal, based on a combination of strain and ligand effects. In contrast, mechanically strained and dealloyed systems can exhibit pure strain effects. Density functional calculations for hydrogen adsorption at different coverages between 0.25 and 1 monolayer on biaxially strained Pd(111) are carried out to illustrate its differing catalytic behavior for the hydrogen evolution reaction (HER) in comparison to selected pseudomorphic Pd overlayers (Pd/M). The separation of the ligand and strain effects present in Pd/M pseudomorphs and the consequent modification of the binding strengths caused by them individually are estimated. The strain exhibits a systematic contribution to binding energy changes while the ligand effect can act to either intensify or weaken the strain effect. In certain systems (e.g., Pd/Ir) the ligand effect is more pronounced than the strain effect while in others (e.g., Pd/Au) the strain effect is larger. The individual contributions of strain and ligand effects to shifts in the d-band center are also calculated and found to correlate well with the observed binding energy changes. We suggest that in the absence of a ligand effect—as would be expected in mechanically strained Pd (111)—H binding is tunable, and a differential free energy of hydrogen adsorption of ∼0 eV (at 0 V vs RHE) is achieved at various combinations of strain and coverage. For pure Pd under compressive strain, this leads to a prediction of a broad region of enhanced activity for the HER which may compare favorably to Pd overlayers supported on more expensive metals such as Pt and PtRu.