Kiyohisa Ohta

Photocatalytic reduction of carbon dioxide to hydrocarbon using copper-loaded titanium dioxide

The photocatalytic reduction of carbon dioxide using copper-loaded titanium dioxide powders at ambient temperature has been reported. The CuTiO2 powders suspended in the solution, which was pressurized with CO2 of 28 kgf/cm2, were illuminated with an Xe lamp. The catalyst, Cu(≤5 wt%)TiO2, is specific for the products (i.e., the main products were methane and ethylene, and not methyl alcohol and formaldehyde). By the photochemical reduction, the yields for methane, ethylene, and ethane were 21.8 μl/g, 26.2 μl/g, and 2.7 μl/g, respectively, under optimum conditions. The carbon dioxide reduction system developed might well become of practical interest for photochemical fuel production, storage of solar energy, and production of raw materials for the photochemical industry.

Photocatalytic reduction of high pressure carbon dioxide using TiO2 powders with a positive hole scavenger

The photocatalytic reduction of high pressure CO2 using Ti02 powders with a positive hole scavenger has been reported. The Ti02 powders suspended in iso-propyl alcohol solution were irradiated with a Xe lamp. The main reduction product from CO2 by photocatalytic reduction was methane. Iso-propyl alcohol acted as the positive hole scavenger. Under the optimum experimental conditions, 1.3 μmol· (g-Ti)−1 of methane was obtained by the photochemical reduction of high pressure CO2, corresponding to 0.43 μmol · (g-Ti)−1 h−1 of the formation rate. The C02-reduction system developed may become of practical interest for the efficient C02-conversion and fixation system, storage of solar energy and the production of raw materials for the photochemical industry.

Electrochemical reduction of carbon dioxide to ethylene with high Faradaic efficiency at a Cu electrode in CsOH/methanol

Abstract The electrochemical reduction of CO2 with a Cu electrode in CsOH/methanol-based electrolyte was investigated. The main products from CO2 were methane, ethylene, ethane, carbon monoxide and formic acid. A maximum Faradaic efficiency of ethylene was 32.3% at −3.5 V vs. Ag/AgCl saturated KCl. The best methane formation efficiency was 8.3% at −4.0 V. The ethylene/methane current efficiency ratio was in the range 2.9–7.9. In the CsOH/methanol, the efficiency of hydrogen formation, being a competitive reaction against CO2 reduction, was depressed to below 23%.

Removal of heavy metals in rinsing wastewater from plating factory by adsorption with economical viable materials

The removal of heavy metals from plating factory wastewater with economical materials was investigated by the column method. Montmorillonite, kaolin, tobermorite, magnetite, silica gel and alumina were used as the economical adsorbents to wastewater containing Cd(II), Cr(VI), Cu(II) and Pb(II). This removal method of heavy metals proved highly effective as removal efficiency tended to increase with increasing pH and decrease with increasing metal concentration. The removal percentages by adsorption onto montmorillonite, tobermorite, magnetite, and silica gel showed high values for all metals. From the results for the heat of adsorption, the adsorption process in the present study might be chemisorption. The proposed method was successfully applied to the removal of Cd(II), Cr(VI) and Cu(II) in rinsing wastewater from plating factory in Nagoya City, Aichi Prefecture, Japan. Since the economical adsorbents used can be obtained commercially because they are easily synthesized, the wastewater treatment system developed is rapid, simple and cheap for the removal of heavy metals.

Photocatalytic reduction of CO2 using TiO2 powders in supercritical fluid CO2

The photocatalytic reduction of CO2 was investigated using TiO2 powders in supercritical fluid CO2. These were irradiated in a stainless steel vessel at 9.0MPa and 35°C. After reducing the CO2 pressure to the ordinary state, pure water was added to the vessel while avoiding air contamination. No gaseous reduction products were observed. Formic acid was obtained only in aqueous solution. The optimal irradiation time for the production of formic acid was 5h. Addition of acidic solutions rather than pure water was preferable for formic acid formation. Formic acid seems to be produced through the protonation of reaction intermediates on TiO2 powders in solutions. The CO2-reduction system described here may be of practical value for efficient CO2-conversion and fixation, storage of solar energy, and production of raw materials for the photochemical industry.

Effect of CO2 pressure on photocatalytic reduction of CO2 using TiO2 in aqueous solutions

Abstract The photocatalytical reduction of CO 2 at high pressure was investigated using TiO 2 suspensions in water and caustic solution. An increase in CO 2 pressure significantly accelerated the CO 2 reduction in both water and the caustic solution. The major reduction products are liquid phase products such as acetic acid and alcohols, and gaseous products such as methane, ethane and ethylene were also observed as a minor portions.

Photocatalytic reduction of CO2 using TiO2 powders in liquid CO2 medium

Abstract The photocatalytic reduction of CO2 was investigated using TiO2 powders in liquid CO2 medium. The TiO2 powders with liquid CO2 medium were illuminated in a stainless steel vessel. After reducing CO2 pressure to the ordinary state, purified water was added to the vessel containing the TiO2 powders without air contamination. No gaseous reduction products were observed and formic acid was exclusively obtained in the aqueous solution. It seems that formic acid is produced through the protonation of the reaction intermediates on the TiO2 powders with purified water.

Electrochemical conversion of carbon dioxide to formic acid on Pb in KOH/methanol electrolyte at ambient temperature and pressure

The electrochemical reduction of CO2 in KOH/methanol-based electrolyte has been investigated on a lead wire electrode at ambient temperature and pressure. The major products of electrochemical reduction of CO2 were formic acid, CO and methane. The formation of formic acid from CO2 predominated in the potential range −1.8 to −2.5V vs Ag/AgCl (saturated KCl). Hydrogen evolution in competition with CO2 reduction was observed at only 3.5% faradaic efficiency. The partial current density for CO2 reduction was more than 22 times larger than that for hydrogen evolution. Study of the Tafel plot showed that the reduction of CO2 to formic acid and CO was not limited by mass transfer in this potential range.

Electrochemical reduction of carbon dioxide in methanol at low temperature

Abstract The electrochemical reduction of carbon dioxide in methanol has been investigated at low temperature. The electrolysis with a copper cathode and a platinum anode was performed in methanol catholyte containing benzalkonium chloride and in 0.1 M hydrogen carbonate or 80 mM benzalkonium chloride anolyte. By the electrochemical reduction in methanol catholyte at −2.2 v vs. sce, carbon monoxide, methane and ethylene with 31.8, 18.9 and 16.9% of Faradaic efficiency, respectively, were produced from carbon dioxide at 0°C. At −15°C, 24.0% carbon monoxide, 39.1% methane and 4.4% ethylene were obtained. Under the optimal experimental conditions, the Faradaic efficiency of hydrocarbons such as methane and ethylene were better than that obtained in aqueous catholyte.

Electrochemical conversion of carbon dioxide to methane in aqueous NaHCO3 solution at less than 273 K

The electrochemical reduction of CO2 on a Cu electrode was investigated in aqueous NaHCO3 solution, at low temperature. A divided H-type cell was employed, the catholyte was 0.65 mol dm−3 NaHCO3 aqueous solution and the anolyte was 1.1 mol dm−3 KHCO3 aqueous solution. The temperature during the electrolysis of CO2 was decreased stepwise to 271 K. Methane and formic acid were obtained as the main products. The maximum Faradaic efficiency of methane was 46% at −2.0 V and 271 K. The efficiency of hydrogen formation, a competing reaction of CO2 reduction, was significantly depressed with decreasing temperature. Based on the results of this work, the proposed electrochemical method appears to be a viable means for removing CO2 from the atmosphere and converting it into more valuable chemicals. The synthesis of methane by the electrochemical method might be of practical interest for fuel production and the storage of solar energy.