Subsurface Hydride Formation Leads to Slow Surface Adsorption Processes on a Pd(111) Single-Crystal Electrode in Acidic Electrolytes

Palladium is one of the most important catalysts due to its widespread use in heterogeneous catalysis and electrochemistry. However, an understanding of the electrochemical processes and interfacial phenomena at Pd single-crystal electrodes/electrolytes is still scarce. In this work, the electrochemical behavior of the Pd(111) electrode was studied by the combination of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in different acidic electrolytes, namely, sulfuric acid, perchlorate acid, methane sulfonic acid, and hydrofluoric acid. An analysis of CV profiles shows the strong adsorption of all anions at low electrode potential, partially overlapping with underpotential deposited hydrogen (UPD-H), leading to the appearance of a pair of sharp peaks in what would be considered the “hydrogen region”. All anions studied (HSO₄^-, ClO₄^-, CH₃SO₃^-, and F^-) adsorb specifically and interact with (or effectively block) the surface-adsorbed hydroxyl phase formed on the Pd(111) terrace at higher potentials. Strikingly, the scan rate-dependent results show that the process of anion adsorption and desorption is a kinetically rather slow step. EIS measurements show that the exact mechanism of this slow anion ad/desorption process actually stems from (sub)surface phenomena: the direct hydrogen insertion into Pd lattice (hydrogen subsurface absorption) commences from ca. 0.40 V and leads to the formation of (subsurface) Pd hydrides (PdH_x). We argue that the subsurface hydrogen phase significantly alters the work function and thereby the kinetics of the anion adsorption and desorption processes, leading to irreversible peaks in the voltammetry. This precise understanding is important in guiding further fundamental work on Pd single crystals and will be crucial to advancing the eventual design of optimized Pd electrocatalysts.