Osamu Koga

Electrocatalytic process of CO selectivity in electrochemical reduction of CO2 at metal electrodes in aqueous media

Electrochemical reduction of CO2 at metal electrodes yields CO, HCOO−, CH4, C2H4, and alcohols in aqueous media. Metal electrodes are roughly divided into two groups, CO formation metals (Cu, Au, Ag, Zn, Pd, Ga, Ni, and Pt) and HCOO− ones (Pb, Hg, In, Sn, Cd, Tl). Foreign atom modified electrodes (the coverage is virtually unity) showed variable product selectivity between CO and HCOO−, which depends upon the combination of modifier atom and substrate electrode. Critical investigation of foreign atom modified electrodes derived a series of CO selectivity, as Au > Ag > Cu > Zn > Cd > Sn > In > Pb > Tl > Hg. The electrode potentials of CO2 reduction are well correlated with the heat of fusion of metals and the potential of H2 evolution. The order of CO selectivity agrees roughly with that of the electrode potential of CO2 reduction, and is rationalized in terms of stabilization of intermediate species CO−2 at the electrode surface. CO is produced from stably adsorbed CO−2, and HCOO− is formed from free or weakly adsorbed CO−2.

Electrochemical reduction of CO2 at copper single crystal Cu(S)-[n(111)×(111)] and Cu(S)-[n(110)×(100)] electrodes

Electrochemical reduction of CO2 was studied using two series of single-crystal electrodes, Cu(S)-[n(111)×(111)] and Cu(S)-[n(110)×(100)] at a constant current density of 5 mA cm−2 in 0.1 M KHCO3 aqueous solution. Copper single crystals were grown from 99.999% copper metal in a graphite crucible, and the crystal orientation was determined by the X-ray back reflection method. The surface treatment of the copper single crystal electrodes was studied in detail, and the reproducibility of the CO2 reduction was greatly improved. The product distribution of the CO2 reduction varies greatly with the crystal orientation. CO2 reduction at the Cu(110) (=Cu(S)-[2(111)×(111)]) electrode gives a current yield of 20% of CH3COOH; the formation of CH3COOH in CO2 reduction has not been reported previously. The yield of CH4 was very low (6%) at the Cu(110) electrode. The formation of CH4 and CH3COOH changes significantly with the crystal orientation. A decrease of the step atom density in the Cu(S)-[n(111)×(111)] series reduces the yield of CH3COOH and enhances that of CH4. Introduction of the (100) step to the Cu(110) basal plane, leading to the Cu(S)-[n(110)×(100)] series with kink sites, diminishes the feature of the Cu(110). The Cu(210) (=Cu(S)-[2(110)×(100)]), which has the highest number of dangling bonds of fcc metals, gives a high yield of CH4 with a product distribution similar to that of Cu(111) which has the lowest density of dangling bonds.

Adsorption of CO accompanied with simultaneous charge transfer on copper single crystal electrodes related with electrochemical reduction of CO2 to hydrocarbons

CO2 and CO were electrochemically reduced with Cu(100), Cu(110) and Cu(111) electrodes at a constant current density of 5 mA cm−2 in 0.1M KHCO3 at ambient temperature. C2H4 is favorably produced on Cu(100), and CH4 is predominantly yielded on Cu(111). Cu(110) showed an intermediate product selectivity. Voltammetric measurements in the presence of CO in phosphate buffer solutions gave a pair of sharp charge transfer redox peaks with Cu(100) and Cu(110), not with Cu(111). The charge transfer peaks depend greatly on the anionic species of the electrolyte, and are rationalized by charge displacement due to desorption of specifically adsorbed anions during CO adsorption. Competitive adsorption of anions and CO on Cu single crystal electrode surfaces is discussed on the basis of electrochemical and in situ FTIR spectroscopic measurements.

Infrared spectroscopy of adsorbed CO and intermediate species in electrochemical reduction of CO2 to hydrocarbons on a Cu electrode

Abstract Voltammetric measurements showed in the previous paper that charge transfer is accompanied with CO adsorption on Cu electrode. Adsorbed CO is present at Cu electrode surface below the potential of the cathodic charge transfer, and not present above the potential, as confirmed by infrared spectroscopy. This article reports significant features of in situ infrared spectra of adsorbed CO in detail, obtained with a Cu polycrystal electrode in aqueous electrolytes at 0 to 2 °C with regard to electrochemical reduction of CO 2 to hydrocarbons. Two infrared absorption peaks were assigned to adsorbed CO. A sharp absorption band appeared at ca . 2080 cm − between −0.8 and −1.0 V vs. nhe in 0.2 M KHCO 3 . Another absorption peak was broad at 2040 cm − between −0.7 and −0.8 V in the same solution. The infrared absorption intensity is closely related with the amount of charge accompanied with CO adsorption. The infrared absorption bands were studied using D 2 O electrolyte solutions; the results agreed well with those obtained in H 2 O electrolyte system. Thus the spectra will not be affected by hydrogen. Infrared absorption bands of reduced CO 2 were measured as well, showing identical spectroscopic features with adsorbed CO. The reduced CO 2 is present at −1.0 V at Cu electrode and not present at −0.4 V. The reduced CO 2 at Cu electrode is thus identified as adsorbed CO.

Adsoprtion of carbon monoxide at a copper electrode accompanied by electron transfer observed by voltammetry and IR spectroscopy

Abstract Copper is known as a unique metal electrode which effectively reduces Co 2 to CH 4 , C 2 H 4 and alcohols in aqueous electrolytes with intermediate formation of adsorbed CO. Voltammetric measurements revealed that CO is adsorbed on Cu electrodes belwo −7.7V vs. nhe accompanied with quasi-reversible electron transfer. CO is not adsorbed above −0.7V on Cu electrode. Proton is involved with the adsorption process. Other metal electrodes (Au, Ag, Ni, Zn, Cd, Sn and Pb) did not show such activity for CO adsoprtion accompanied with electron transfer. In situ FTIR spectroscopic measurements indicated that adsorbed Co is present with CO stretching adsoprtion band at 2087 cm −1 on Cu electrode below −0.7V. Analysis of experimental results suggest that CO is likely to be adsorbed in a hydridocarbonly complex on Cu electrode without net electron transfer to CO molecule. The surface complex formation may be a preceding step to electrochemical reduction of CO to hydrocarbons and alcohols.

Reduction of adsorbed Co on a Ni electrode in connection with the electrochemical reduction of CO2

Electrochemical reduction of CO2 at Ni electrodes in aqueous media yields CH4, C2H4 and C2H6 with the simultaneous formation of H2 as reported previously. The partial current density of CO2 reduction is ca. 0.2 mA cm−2 when the electrolysis is carried out at a constant current of 5 mA cm−2. During the electroreduction of CO2, CO is formed and strongly adsorbed on the surface of the Ni electrode. The adsorbed CO prevents hydrogen evolution since CO occupies most of the surface sites on which hydrogen may proceed. The extent of prevention of hydrogen evolution by the adsorbed CO may be estimated from the current density on a CO adsorbed electrode with reference to that on a CO free electrode at a constant potential. The present paper presents a new technique for estimating the coverage of adsorbed CO and the reduction rate of adsorbed CO from the extent of prevention of hydrogen evolution and the time course of the cathodic current in constant potential measurements. The rate of reduction of adsorbed CO obtained in this manner agrees quite well with that obtained by coulometric measurements reported previously. This agreement indicates the usefulness of the new estimation technique of surface coverage of adsorbed species, and verifies the hypothesis that the adsorbed CO is an intermediate species formed in the electroreduction of CO2 to hydrocarbons and alcohols at Ni electrodes.

FTIR measurements of charge displacement adsorption of CO on poly- and single crystal (100) of Cu electrodes

Abstract CO is reversibly adsorbed on a Cu electrode with simultaneous charge transfer, as observed by voltammetric measurements. In-situ FTIR measurements were conducted with a Cu(100) electrode in a phosphate buffer solution (pH 6.8) and a polycrystalline Cu electrode in a carbonate buffer solution (pH 10.3). With increase of the negative potential, the infrared absorption band of anions (phosphate anion and CO 3 2− ) diminishes at the potential of the charge transfer, whereas that of the adsorbed CO increases. Specifically adsorbed anions will remain still on the electrode surface below the potential of zero charge ( pzc ) to some extent. The apparent charge transfer is observed at a potential below the pzc , where CO is adsorbed, displacing specifically adsorbed anions.

Configuration of adsorbed CO affected by the terrace width of Pt(S)-[n(111)×(111)] electrodes

Abstract CO adsorbed on Pt(S)- [n(1 1 1)×(1 1 1)] electrodes was studied using infrared reflection absorption spectroscopy (IRAS) in 0.1 M HClO 4 saturated with CO. The configuration of adsorbed CO depends on the terrace width remarkably: on-top and threefold CO are co-adsorbed on the surfaces with terrace atomic rows ( n ) more than nine, whereas only on-top CO appears on the surfaces with n less than eight, on which no multi-bonded CO was observed. Integrated intensity of the IRAS band and peak shift (d ν /d E ) do not depend on the crystal orientation.

Charge displacement adsorption of carbon monoxide on [110] zone copper single crystal electrodes in relation with PZC

Abstract CO is adsorbed on Cu electrode with a simultaneous transfer of charge. The apparent charge transfer is derived from displacement of anions specifically adsorbed on the electrode during the adsorption of CO. This paper presents the results of voltammetric measurements of the adsorption of CO on single crystal electrodes of Cu[110] zone in phosphate buffer solution (pH 6.8) at 0°C. The profile of the charge transfer wave depends significantly on the crystal orientation of Cu single crystals. The potential of the charge transfer is well correlated with the work functions and the potential of zero charge of the Cu single crystals. We propose a model that H 2 PO 4 − is specifically adsorbed on (100) steps of Cu[110] zone crystal electrodes occupying 3 to 4 atomic sites.

Radiation Effect in Hexatriacontane Thin Film

The secondary electron emission spectra of hexatriacontane thin film were measured as a function of irradiation time with an electron beam of about 19 eV. The spectral features which originate in the conduction band features in the crystalline state shifted to the low kinetic energy side linearly with the logarithm of the irradiation time. The shift is explained as an increase in the electron affinity due to the decomposition of hexatriacontane molecules.