Electrochemical reduction of CO2 in methanol with aid of CuO and Cu2O
Introduction
Carbon dioxide (CO2) is the ultimate by-product of all processes involving oxidation of carbon compounds and its increasing presence in the atmosphere. In view of the vastness of its supply, CO2 represents a potential source of C1 feedstock for the production of chemicals and fuels. The thermodynamic stability and the relative kinetic inertness of CO2 require its preliminary activation or, alternatively, the activation or modification of the substrates. Therefore, the electrochemical method appears to become one of the very suitable methods for the conversion and reduction of CO2 [1], [2], [3].
A number of researchers have actively investigated the electrochemical reduction of CO2 using various metallic electrodes in organic solvents, because organic nonaqueous solvents dissolve much more CO2 than water does [4], [5], [6], [7]. It has been described that two-electron reduced products were produced by the electrochemical CO2 reduction in dimethyl sulfoxide (DMF), N,N-dimethyl formamide (DMSO), acetonitrile (MeCN), and propylene carbonate (PC). However, even at a copper electrode, few hydrocarbons such as methane and ethylene have been obtained in these organic solvents [7].
Methanol (MeOH) is a better medium of CO2 than water, particularly at low temperature. The solubility of CO2 in MeOH is about four times that in water, at ambient temperature, and more than seven times that in water, at temperatures below 273 K [8], [9], [10]. Therefore, MeOH has been industrially used as a physical absorbent of CO2 in the Rectisol method, at 243–263 K [10]. Currently, over 70 large-scale plants apply the Rectisol process. Therefore, the direct electrochemical reduction of CO2 in MeOH is an advantageous choice, especially when the process is performed under energetically efficient conditions. Many research groups (our group [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], Fujishima and co-workers [25], Koleli et al. [26], Schrebler et al. [27], Eggins et al. [28]) have brought focus into the electrochemical reduction of CO2 in methanol-based electrolyte.
On the other hand, Frese [29] have studied the electrochemical reduction of CO2 at intentionally oxidized Cu electrodes in water. Methanol, CO and methane were detected as the reduction products. Terunum et al. have evaluated the relationship between hydrocarbon production in the electrochemical reduction of CO2 in water and the characterization of Cu electrode [30]. Chang et al. [31] have investigated the electrochemical reduction of CO2 with Cu2O-catalyzed carbon clothes. In the study, only methanol was formed as the product, and its Faradaic efficiency was not clarified. Thus far, in the electrochemical reduction of CO2 in methanol-based catholyte, little information on the electrolysis with the copper oxide electrocatalyst has been reported. This study deals mainly with the electrochemical reduction of CO2 in methanol with aid of CuO and Cu2O.
Section snippets
Fabrication of powder-pressed cathode
The powders of zinc (ϕ ∼ 7 μm) and Cu2O (ϕ ∼ 100 nm) were provided by Nacalai Tesque Inc., Kyoto, Japan. The CuO powders (ϕ ∼ 33 nm) were obtained from Sigma–Aldrich, Japan. First, zinc particles were mixed with CuO or Cu2O powders. A mixture of 500 mg was pressed with an FTIR disk creator at 200 kg/cm2. A diameter and a thickness of the disk were 10 and 1.0 mm, respectively. The fabricated electrode was used for the electrolysis without further purification.
Electrochemical CO2 reduction
The apparatus and experimental conditions for
Results and discussion
Literature data [8], [9] for the solubilities of CO2 in pure methanol and water, at 288 K, were of 4.6 and 1.07 cm3 cm−3, respectively. It has been presented in a previous study [21] that the solubility of CO2 in a solution of 80 mmol dm−3 KOH/methanol, at 243 K, was about 16 cm3 cm−3 (about 710 μmol CO2/cm3 methanol). Although KHCO3 may be formed in the methanol while bubbling CO2 through the solution for several minutes, the amount is assumed to be very small or negligible, since one cannot observe any
Conclusion
The electrochemical reduction of CO2 in methanol-based electrolyte was studied for the evaluation of characteristic of copper oxide catalyst. Zinc particle-pressed electrodes containing CuO and Cu2O were tested as the cathode. Without the present of copper oxide particles, carbon monoxide and formic acid were formed in the electrochemical reduction of CO2, and little formation of hydrocarbon could not be observed. On the contrary, the formation of hydrocarbons was obtained with the zinc
Acknowledgments
The present research was partly supported by Grant-in-Aid for Young Scientists (B) 21710079 from the Ministry of Education, Culture, Sports, Science and Technology of Japan. Any opinions, findings, conclusions, or recommendations expressed in this paper are those of the authors and do not necessarily reflect the view of the supporting organizations.
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