Hideyuki Katsumata"'*, Kouichirou Matsushitaa, Satoshi Kaneco3, Tohru Suzukib and Kiyohisa Ohtaa a Department of Chemistry for Materials, Faculty of Engineering, Mie University, Tsu, Mie 514-8507, Japan b Environmental Preservation Center, Mie University, Tsu, Mie 514-8507, Japan * Corresponding author. E-mail address: [email protected]
Reduction of CO2 was performed in the presence of metal powders such as iron, zinc and magnesium under ambient temperature and pressure. When iron powder was used, methane, ethylene and ethane were obtained as reduced products. On the other hand, carbon monoxide and methane were only produced in the presence of zinc and magnesium powders, respectively. In the case of iron and magnesium powders, amounts of methane obtained were 2.5 and 4 |j.mol after 72 and 48 h, respectively. In the presence of zinc powder, CO was produced about 200 (imol corresponding to 20% of CO2 amounts dissolved in the solution. Furthermore, the reduction mechanism of CO2 was discussed on the basis of characteristics of metal powders, such as bond radius and redox potential. This method can be considered to be simple and useful for the reduction of CO2.
In the past 60 years, the amount of anthropogenic carbon dioxide (C02) emitted to the atmosphere, primarily because of expanding use of fossil fuels for energy, has risen from preindustrial levels of 280 patrs per million (ppm) to present levels of over 365 ppm . Consequently, the dire warning of severe weather perturbations and globally rising temperatures has been given. Therefore, it has been desired that reduction methods of CO2 should be developed for the conversion and removal of C02. From the viewpoint, the research in the photochemical, electrochemical, and photoelectrochemical CO2 reduction has strongly increased in recent years .
Recently, many researchers have actively studied the electrochemical reduction of C02 using various metal electrodes in organic solvents [3-5]. It has been reported that low levels of reduced products containing carbon monoxide, oxalic acid and formic acid were produced by the electroreduction of C02 in dimethyl sulfoxide, /V.A'-dimethyl formamide, propylene carbonate and acetonitrile , Previously, we have investigated the electrochemical reduction of CO2 on copper electrodes by using methanol as a solvent at 243 K [6-12]. In only methanol-based catholyte, the formations of methane and ethylene were observed.
Although the electrochemical reduction of CO2 using metal electrodes is useful, it is not suited for long term reduction. Therefore, it is required to establish more simple method for the reduction of CO2. Recently, it has been reported that iron powder was applied to the reduction of halogenated compounds [13,14], Therefore, in the present study, we performed the reduction of CO2 by using metal powder such as iron, zinc and magnesium, at ambient temperature and pressure. Furthermore, the mechanism for the reduction of CO2 was discussed on the basis from the characteristics of metal powders.
Methanol (99%, Nacalai Tesque, Inc., Japan) was used as received. The purity of carbon dioxide gas used was 99.9999%. Iron powder (purity 99.9%, average 45 |im) was obtained from Wako Pure Chemicals Co., Japan, and magnesium (purity 98%, 210-710 jam) and zinc powders (purity 90%, average 75 ¡im) from Nacalai Tesque, Inc., Japan. These metal powders were used without further purification. Demineralized water as medium for the reduction of C02 was purified by an ultra pure water system (Advantec MFS Inc., Japan).
A gas chromatography (GC) combined with a TCD (GC-320, GL Science, Japan) or a FID (GC-14B, Shimadzu, Japan) was used for separation and detection of the reduced products of CO2. The GC-TCD was equipped with a Molecular Sieve 5A column for analysis of H2 and/or a Molecular Sieve 13X-S one for CO. Ar or He was used as a carrier gas. The GC-FID was installed with a Porapak Q column for analysis of hydrocarbons. N2 was used as a carrier gas. A high performance liquid chromatography (HPLC) was also used for detection of liquid products. A solution of 0.1% of H3PO4 as a mobile phase was pumped by a Model 576 (GL Science, Japan). The separation column was KC-811 (Shodex, Japan). The absorbance was measured at 220 nm with a UV spectrophotometer (L-4000, Hitachi, Japan).
The reduction of C02 was carried out in a Pyrex glass cell, which was 115 mL of a cylindrical reactor. The reduction procedure was as follows. CO2 gas was bubbled into 30 mL of solvent (water or 33% methanol) for 1 h at a flow rate of 30 mL min '. The pH of the sample solution was 4.0. Then, the suitable weight of metal powder was placed into the C02-saturated solution. The metal powders tested were iron, zinc and magnesium. The reaction cell was then closed using PTFE covered septum and the solution was magnetically stirred in the presence of the metal powder at room temperature. Gaseous products formed during reduction were sampled from the septum and were analyzed by GC with TCD and/or FID. Products soluble in the sample solution were analyzed by HPLC with the UV detector.
Methanol is a much better solvent for C02 than water because the solubility of CO2 in methanol is approximately five times that in water at ambient temperature and pressure [15-17]. Therefore, the reduction of CO2 was performed in methanol with metal powders. When methanol was used as medium, amount of reduced products increased in the presence of iron powder. On the other hand, in other metal powders this phenomenon was not observed. Methane, ethylene and ethane were obtained as reduced products of C02 in the presence of iron powder. The amount of the products showed a maximum at 33% of methanol (Fig. 1).
When the reduction was conducted under a nitrogen atmosphere, these hydrocarbons were not obtained. Therefore, the hydrocarbons were reduced products from CO2 and were not originated from methanol.
Effect of iron powder amount on the reduction of CO2 was investigated over the range of 0
- 1 g for 5 h. The reduced amounts of product increased with increasing iron amount up to 0.8 g. At 0.8 g of iron powder, amounts of methane, ethylene and ethane produced were 0.5, 0.1 and 0.1 (J.mol, respectively. Under this condition, formation amount of hydrogen from H2O was about 600 jimol.
Effect of reaction time on the reduction of CO2 was examined over the range of 0 - 96 h. The reduced product amounts increased with increasing reaction time up to 72 h. For the reaction time of 72 h, amounts of methane, ethylene and ethane produced were 2.5, 1 and 1 prnol, respectively. In addition, amount of hydrogen was about 2000 (imol after 72 h.
3.2. Zinc powder
Effect of zinc powder amount on the reduction of CO2 was investigated over the range of 0
- 24 g for 5 h. The results are shown in Fig. 2. The reduced product was only obtained carbon monoxide. CO amount increased with increasing zinc amount. At 20 g of zinc powder, amount of CO produced was 30 |_imol. On the other hand, amount of hydrogen produced from H20 was ca. 100 |-imol in the presence of 20 g zinc powder.
Effect of reaction time on the reduction of C02 was examined over the range of 0 - 96 h. The reduced product amount (CO) increased with increasing reaction time up to 24 h. For the reaction time of 24 h, amount of carbon monoxide produced was 180 (xmol. However, formation of hydrogen increased with increasing the reaction time and hydrogen amount was about 2700 nmol after 96 h.
Previously, electrochemical reduction of C02 in a KOH/methanol-based electrolyte was investigated using a zinc wire electrode at ambient temperature and pressure . Carbon
Fig. on using C2H4 medii time,
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