Nitrate reduction on Cu (100) and Cu (111) surfaces in alkaline and acidic solutions was studied by electrochemical methods (cyclic voltammetry, rotating disc electrode) coupled with online and in situ characterization techniques (mass spectrometry, ion chromatography and Fourier transformed infra-red spectroscopy) to evaluate the reaction mechanism and products on the different surfaces. Electrochemical results show that reduction of nitrate in alkaline media on Cu is structure sensitive. The onset potential on Cu (100) is +0.1 V vs. RHE, ca. 50 mV earlier than on Cu (111). The onset potentials for nitrate reduction on Cu (100) and Cu (111) in acidic media are rather similar. Analytical techniques show a diverse product distribution for both surfaces and for both electrolytes. Whereas in acidic media both Cu electrodes show the formation of NO and ammonia, in alkaline media Cu reduces nitrate to nitrite and further to hydroxylamine. In alkaline media, Cu (100) is a more active surface for the formation of hydroxylamine than Cu (111)

Understanding the competition between hydrogen evolution and CO₂ reduction is of fundamental importance to increase the faradaic efficiency for electrocatalytic CO₂ reduction in aqueous electrolytes. Here, by using a copper rotating disc electrode, we find that the major hydrogen evolution pathway competing with CO₂ reduction is water reduction, even in a relatively acidic electrolyte (pH 2.5). The mass-transport-limited reduction of protons takes place at potentials for which there is no significant competition with CO₂ reduction. This selective inhibitory effect of CO₂ on water reduction, as well as the difference in onset potential even after correction for local pH changes, highlights the importance of differentiating between water reduction and proton reduction pathways for hydrogen evolution. In-situ FTIR spectroscopy indicates that the adsorbed CO formed during CO₂ reduction is the primary intermediate responsible for inhibiting the water reduction process, which may be one of the main mechanisms by which copper maintains a high faradaic efficiency for CO₂ reduction in neutral media.

The complexity of the electrocatalytic reduction of CO to CH₄ and C₂H₄ on copper electrodes prevents a straightforward elucidation of the reaction mechanism and the design of new and better catalysts. Although structural and electrolyte effects have been separately studied, there are no reports on structure-sensitive cation effects on the catalyst’s selectivity over a wide potential range. Therefore, we investigated CO reduction on Cu(100), Cu(111), and Cu(polycrystalline) electrodes in 0.1 M alkaline hydroxide electrolytes (LiOH, NaOH, KOH, RbOH, CsOH) between 0 and −1.5 V vs RHE. We used online electrochemical mass spectrometry and high-performance liquid chromatography to determine the product distribution as a function of electrode structure, cation size, and applied potential. First, cation effects are potential dependent, as larger cations increase the selectivity of all electrodes toward ethylene at E > −0.45 V vs RHE, but methane is favored at more negative potentials. Second, cation effects are structure-sensitive, as the onset potential for C₂H₄ formation depends on the electrode structure and cation size, whereas that for CH₄ does not. Fourier Transform infrared spectroscopy (FTIR) and density functional theory help to understand how cations favor ethylene over methane at low overpotentials on Cu(100). The rate-determining step to methane and ethylene formation is CO hydrogenation, which is considerably easier in the presence of alkaline cations for a CO dimer compared to a CO monomer. For Li⁺ and Na⁺, the stabilization is such that hydrogenated dimers are observable with FTIR at low overpotentials. Thus, potential-dependent, structure-sensitive cation effects help steer the selectivity toward specific products.