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

Abstract

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 H 2 by recombination rather than oxidation to H 3 O + . Charge imbalance between the reactions accounts for the asymmetric voltammetry in SO 4 2− , ClO 4 − , PO 4 3− , and Cl − electrolytes with pH 0.68−4.5. Two-dimensional hydride formation is evidenced by the reduction wave prior to H 2 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 CO 2 reduction to CH 4 and should be broadly useful in electro-organic synthesis.

Quantification of Hydride Coverage on Cu(111) by Electrochemical Mass Spectrometry

“…Unexpectedly, decomposition of the hydride phase by H recombination occurs at potentials positive of the reversible potential. 15 Similar observations of H recombination were reported for polycrystalline Cu electrodes. 16 The minimal charge transfer associated with the recombination reaction accounts for the previously unexplained voltammetric charge asymmetry associated with Cu(111) in acid media.…”

“…Similar observations of H recombination were reported for polycrystalline Cu electrodes . The minimal charge transfer associated with the recombination reaction accounts for the previously unexplained voltammetric charge asymmetry associated with Cu(111) in acid media . Based on H 2 evolved by recombination at positive potentials, the fractional H coverage, θ H , relative to the metal lattice, was estimated from voltammetry to be ≈0.52 and ≈0.94 in sulfuric and perchloric acid, respectively .…”

“…The minimal charge transfer associated with the recombination reaction accounts for the previously unexplained voltammetric charge asymmetry associated with Cu(111) in acid media . Based on H 2 evolved by recombination at positive potentials, the fractional H coverage, θ H , relative to the metal lattice, was estimated from voltammetry to be ≈0.52 and ≈0.94 in sulfuric and perchloric acid, respectively . Scanning tunneling microscopy studies of Cu(111) in sulfuric acid reveal an ordered (√3 × √7) sulfate adlayer at more positive potentials. − Upon stepping to more negative potentials, this gives way to (4 × 4) and (5 × 5) commensurate structures on Cu(111). , In early work, these structures were ascribed to the hydration layer but subsequently were reassigned to adsorbed H .…”

“…15 Based on H 2 evolved by recombination at positive potentials, the fractional H coverage, θ H , relative to the metal lattice, was estimated from voltammetry to be ≈0.52 and ≈0.94 in sulfuric and perchloric acid, respectively. 15 Scanning tunneling microscopy studies of Cu(111) in sulfuric acid reveal an ordered (√3 × √7) sulfate adlayer at more positive potentials. 17−20 Upon stepping to more negative potentials, this gives way to (4 × 4) and (5 × 5) commensurate structures on Cu(111).…”

2022 roadmap on low temperature electrochemical CO2 reduction

“…What drives the reconstruction on Cu is still under discussion and requires future analysis. Not only the surface-electrolyte interactions and the adsorption of the intermediates of the CO 2 R will influence the surface reconstruction mechanism, but also the subsurface hydrogen at potentials close to the hydrogen evolution [167][168][169]. These studies will be also key to investigate the potential-dependence of the degradation mechanism on shape-controlled nanoparticles (figure 22(B)) under reaction conditions.…”