Cu-Sn alloy electrodes were prepared by simple electrodeposition method for the electrochemical reduction of CO 2 into CO and HCOO − . The alloy electrode surfaces provided good selectivity and efficiency in electrochemical CO 2 conversion because they provided appropriate binding energies between the metal and the reactive species obtained through CO 2 reduction. Therefore, product selectivity can be modulated by altering the Cu-Sn crystal structure of the electrode. Using the Cu-Sn alloy electrodes, electrochemical reduction was performed at applied potentials ranging from − 0.69 to − 1.09 V vs. reversible hydrogen electrode (RHE). During electrochemical CO 2 reduction, all the prepared Cu-Sn alloy electrodes showed prominent suppression of hydrogen evolution. In contrast, Cu 87 Sn 13 has high selectivity for CO formation at all the applied potentials, with maximum faradaic efficiency (FE) of 60% for CO at − 0.99 V vs. RHE. On the other hand, Cu 55 Sn 45 obtained a similar selectivity for electrodeposition of Sn, with FE of 90% at − 1.09 V vs. RHE. Surface characterization results showed that the crystal structure of Cu 87 Sn 13 comprised solid solutions that play an important role in increasing the selectivity for CO formation. Additionally, it suggests that the selectivity for HCOO − formation is affected by the surface oxidation state of Sn rather than by crystal structures like intermetallic compounds.

An interfacial reaction between the UV irradiated water phase (H˙ provides) and the electron-discharged nitrogen-gas phase (N atom provides) produces ammonia efficiently.

Electric‐discharge nitrogen comprises three main types of excited nitrogen species‐atomic nitrogen (Natom), excited nitrogen molecules (N2*), and nitrogen ions (N2+) – which have different lifetimes and reactivities. In particular, the interfacial reaction locus between the discharged nitrogen and the water phase produces nitrogen compounds such as ammonia and nitrate ions (denoted as N‐compounds generically); this is referred to as the plasma/liquid interfacial (P/L) reaction. The Natom amount was analyzed quantitatively to clarify the contribution of Natom to the P/L reaction. We focused on the quantitative relationship between Natom and the produced N‐compounds, and found that both N2* and N2+, which are active species other than Natom, contributed to P/L reaction. The production of N‐compounds from N2* and N2+ was enhanced upon UV irradiation of the water phase, but the production of N‐compounds from Natom did not increase by UV irradiation. These results revealed that the P/L reactions starting from Natom and those starting from N2* and N2+ follow different mechanisms.

There are multiple active species at the interface between the discharged gas phase and the water phase. Activated nitrogen species are generated in the nitrogen plasma gas by dielectric barrier discharge (DBD). The reaction at the interface between the gas phase containing the activated nitrogen species and water phase (P/L reaction) can produce nitrogen-derived compounds (N-compounds) in water. We have already clarified the mechanism of the P/L reaction focusing on highly active atomic nitrogen in a previous study. In this study, we report the mechanism of the P/L reaction by excited but metastable nitrogen molecules. Involving excited nitrogen molecules [N2(A3Σu +)] with a long lifetime, the reaction selectivity of N-compounds was clarified by quantitative analysis. Moreover, the active species in the water phase (H·, HO·, H2O2) of the P/L reaction were measured, and the involvement of the reaction in the generation of N-compounds was confirmed. The findings clarify that each activated nitrogen species reacts differently with H2O. These results suggest that the occurrence of either an oxidation reaction or a reduction reaction can be regulated by controlling the activated nitrogen species.

In the plasma/liquid (P/L) interfacial reaction, nitrogen fixation is performed on a water phase surface. In the P/L reaction, discharged nitrogen gas reacts with water molecules at the interface between the plasma gas phase and the water phase, followed by either a reduction reaction, ammonia production or oxidation reaction, nitric acid production. The production of nitric acid in the P/L reaction is influenced by the concentration of oxygen present in each gas phase and water phase, and the atomic nitrogen contained in the nitrogen plasma. For the reduction reaction at the P/L reaction locus, the water phase was modulated in order to make ammonia production dominant in nitrogen fixation. Ammonia is released into the gas phase under conditions of high water temperature and high pH. To obtain only ammonia using this reaction, it is necessary to incorporate a process for raising the temperature of the water. In the P/L reaction, only the ammonia gas can be obtained in one-step by using the rise in water temperature due to the discharged heat plasma gas. A reaction system was developed to control the water and the gas phase to enable high purity ammonia trapping as released by the gas phase.