Keisuke Natsui

Anodic Oxidation on a Boron‐Doped Diamond Electrode Mediated by Methoxy Radicals

The boron-doped diamond (BDD) electrode is known to possess a wide range of applications for the direct generation of hydroxyl radicals and resulting inorganic peroxides such as persulfate, perphosphate, and hypochlorite. In contrast to the extensive use of the BDD electrode in biological and inorganic chemistry, only a few examples exist in which electrochemically generated hydroxyl or alkoxy radicals have been employed to promote reactions of highly functionalized organic substances. The known applications include reactions of relatively simple substrates, such as acetal production by oxidative C C bond cleavage, and BDD-mediated oxidative couplings to construct diaryl linkages. In general, alkoxy radicals, formed by anodic oxidation of the corresponding alcohols, function as mediators in reaction pathways that are different from those occurring under standard electrolysis conditions using carbon or platinum electrodes. Dolson and Swenton have termed reactions that proceed by the generation of methoxy radicals, formed by anodic oxidation of MeOH followed by chemical reaction of the derived radical and ionic species, as EECrCp processes. In a previous effort focusing on the synthesis of ( )-parasitenone (4) (Table 1), we have used an oxidative quantitative dimethoxy-acetalization reaction (1!2), occurring in a basic MeOH solution at a Pt electrode, as the key step (entry 1). A similarly efficient reaction takes place when the BDD electrode is employed (entry 3), while the corresponding aldehyde 3 is produced from 1 in the oxidative reaction occurring on a glassy carbon (GC) electrode (entry 2). The process taking place in a basic MeOH solution (entry 1) might involve the intermediacy of methoxy radicals in the manner described by Swenton. Similarly, reaction at the BDD electrode (entry 3) is anticipated to produce the same radical species. These observations indicated that the acetalization reaction (1!2) at Pt or BDD electrodes proceeds through the EECrCp pathway in which the initially formed arene cation radical intermediate A is converted to a cation B that undergoes addition of methoxide, while the ECEC mechanism proceeds through radical C (Scheme 1). Also, the lack of production of methoxy radicals at the GC electrode enables the initially formed cation radical A to undergo proton loss (to D) in the route for formation of aldehyde 3. Despite the plausibility of the mechanistic pathways suggested for the conversion of 1 to either 2 or 3, no studies exist providing direct proof for the involvement of radical species in processes of this type. In the current study, we have carried out ESR experiments to determine if radical intermediates are generated in electrochemical oxidation reactions of methanol using BDD, Pt, and GC electrodes. In addition, we have probed electrochemical oxidation reactions of isoeugenol (5) as part of the biomimetic preparation of the neolignan, licarin A (6). To gain information about whether or not radical species are involved in electrochemical oxidation reactions conducted in MeOH, we have carried out an ESR study using the radical trapping agent 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) (Scheme 2). Anodic oxidations of methanolic solutions of DMPO were performed employing constant Table 1: Synthesis of ( )-parasitenone (4) using anodic oxidation and electrochemical synthesis of bisketal 2.

Stable and Highly Efficient Electrochemical Production of Formic Acid from Carbon Dioxide Using Diamond Electrodes

High faradaic efficiencies can be achieved in the production of formic acid (HCOOH) by metal electrodes, such as Sn or Pb, in the electrochemical reduction of carbon dioxide (CO2 ). However, the stability and environmental load in using them are problematic. The electrochemical reduction of CO2 to HCOOH was investigated in a flow cell using boron-doped diamond (BDD) electrodes. BDD electrodes have superior electrochemical properties to metal electrodes, and, moreover, are highly durable. The faradaic efficiency for the production of HCOOH was as high as 94.7 %. Furthermore, the selectivity for the production of HCOOH was more than 99 %. The rate of the production was increased to 473 μmol m-2  s-1 at a current density of 15 mA cm-2 with a faradaic efficiency of 61 %. The faradaic efficiency and the production rate are almost the same as or larger than those achieved using Sn and Pb electrodes. Furthermore, the stability of the BDD electrodes was confirmed by 24 h operation.

Selective production of methanol by the electrochemical reduction of CO2 on boron-doped diamond electrodes in aqueous ammonia solution

The electrochemical reduction of CO2 was investigated in an aqueous ammonia solution using boron-doped diamond electrodes. Methanol was mainly produced by reduction at a potential of −1.3 V (vs. Ag/AgCl) with a faradaic efficiency as high as 24.3%. Also, even in an aqueous ammonium bicarbonate solution (pH 7.9) without CO2 bubbling, methanol was produced, while no methanol production was observed at higher (10.6) and lower pH (3.38). These observations suggest that the selectivity for methanol production in aqueous ammonia solutions is due to the electrochemical reduction of bicarbonate ions which are formed by the reaction between ammonia and CO2. Moreover, we present the important role of ammonia as an electrolyte for the selective production of methanol by electrochemical reduction of CO2.

Effect of alkali-metal cations on the electrochemical reduction of carbon dioxide to formic acid using boron-doped diamond electrodes

The electrochemical reduction of carbon dioxide in aqueous solutions using boron-doped diamond (BDD) electrodes was investigated at ambient pressure and temperature. We discuss the effects of the alkali-metal (AM) cations, K+, Na+, Rb+ and Cs+, on the faradaic efficiency (FE) for the formation of formic acid. An FE of 71% was achieved in the case of a 0.075 M Rb+ solution neutralized to pH 6.2 by the addition of HCl. In the case of a Cs+ solution neutralized to pH 6.2, the highest FE was obtained with the more dilute concentration of 0.02 M. Of the four different solutions examined, the lowest FE was observed for the Na+ solution. Moreover, we found that the productivity for the production of formic acid is higher at higher current densities.

Photo-modulation of superconducting and magnetic properties

In this chapter, photo-modulation of superconducting and magnetic properties is described. First, the superconducting property of a boron-doped diamond is investigated, in which the critical current density varies dramatically between the hydrogen- and oxygen-terminated diamond. Second, the superconducting diamond is modified with an azobenzene molecular layer to modulate the superconductivity upon photoirradiation. In this composite superconductor, the critical current density is reversibly amplified by 55% upon photoisomerization of the azobenzene layer. Third, Prussian Blue, a coordination polymer, is integrated with the semiconducting titanium oxide nanosheets to modulate the magnetic property upon photoirradiation. In this magnetic heterostructure, photoinduced magnetic phase transition of Prussian Blue is present by injecting photoexcited electrons from titanium oxide nanosheets.

Surface Hydrogenation of Boron-Doped Diamond Electrodes by Cathodic Reduction

Boron-doped diamond (BDD) has attracted much attention as a promising electrode material especially for electrochemical sensing systems, because it has excellent properties such as a wide potential window and low background current. It is known that the electrochemical properties of BDD electrodes are very sensitive to the surface termination such as to whether it is hydrogen- or oxygen-terminated. Pretreating BDD electrodes by cathodic reduction (CR) to hydrogenate the surface has been widely used to achieve high sensitivity. However, little is known about the effects of the CR treatment conditions on surface hydrogenation. In this Article, we report on a systematic study of CR treatments that can achieve effective surface hydrogenation. As a result, we found that the surface hydrogenation could be improved by applying a more negative potential in a lower pH solution. This is because hydrogen atoms generated from protons in the CR treatment contribute to the surface hydrogenation. After CR treatments, BDD surface could be hydrogenated not completely but sufficiently to achieve high sensitivity for electrochemical sensing. In addition, we confirmed that hydrogenation with high repeatability could be achieved.