Surface Hydride Formation on Cu(111) and Its Decomposition to Form H₂ in Acid Electrolytes

Mass spectrometry and Raman vibrational spectroscopy were used to follow competitive dynamics between adsorption and desorption of H and anions during potential cycling of three low-index Cu surfaces in acid electrolytes. Unique to Cu(111) is a redox wave for surface hydride formation coincident with anion desorption, while the reverse reaction of hydride decomposition with anion adsorption yields H₂ by recombination rather than oxidation to H₃O⁺. Charge imbalance between the reactions accounts for the asymmetric voltammetry in SO₄^2-, ClO₄^-, PO₄^3-, and Cl^- electrolytes with pH 0.68-4.5. Two-dimensional hydride formation is evidenced by the reduction wave prior to H₂ evolution and vibrational bands between 995 and 1130 cm^-1. In contrast to Cu(111), no distinct voltammetric signature of surface hydride formation is observed on Cu(110) and Cu(100). The Cu(111) hydride surface phase may serve to catalyze hydrofunctionalization reactions such as CO₂ reduction to CH₄ and should be broadly useful in electro-organic synthesis.