A Cu-Zn Intermetallic Catalyst Utilized for Electrochemical, Photoelectrochemical and Photocatalytic CO2 Reduction

Abstract

Introduction To reduce the carbon dioxide concentration in the atmosphere and to solve the energy shortage issue, CO2 converting technologies, such as electrochemical, photoelectrochemical, and photocatalytic CO2 reduction, have been paid big attention in the last several decades. Researches in this field aim to convert CO2 into useful fuels such as formic acid, carbon monoxide, methane and methanol using electric or solar energy by means of CO2 reduction catalysis, in which the products can be directly used or easily stored. To achieve high conversion efficiency, the development of CO2reduction catalyst is essential. In the present research, a Cu-Zn intermetallic catalyst was developed on the basis of the understanding of the reaction mechanism of CO2 reduction on the metal surface in an aqueous media. It was reported that the adsorption or desorption strength of CO2 and CO anion radicals on the surface of the metal is the essential factor on the reaction pathway of CO2 reduction. 1,2 Among numerous candidates for catalyst materials, the Cu-Zn intermetallic catalyst is supposed to have a very appropriate adsorption-desorption strengths to achieve high efficiency and selectivity for converting CO2into formic acid. Besides, the Cu-Zn intermetallic catalyst is consisted of abundant and eco-friendly metals, also the synthesis method is very simple and low-cost. Experimental and Evaluation A Cu-Zn intermetallic electrode was prepared by sputtering and vacuum sealing methods. Its electrocatalytic properties were evaluated in an electrochemical cell and the composition was optimized according to the electrochemical performance. The optimized Cu-Zn electrode was then used to construct a photoelectrochemical (PEC) cell with a mesoporous SrTiO3photoanode prepared by screen printing method. According to the performance in the PEC cell, Cu-Zn intermetallic nanoparticles loaded SrTiO3powder photocatalyst was also synthesized by chemical reduction and vacuum sealing methods and its photocatalytic property was evaluated. Results and Discussion According to the electrochemical evaluation, the Cu-Zn intermetallic electrode behaved lower onset potential than pure copper and pure zinc. After the optimization of the Zn concentration, we measured the lowest onset potential (-0.65V vs. Ag/AgCl) when the Zn mass ratio was 53%. In the PEC cell, the optimized Cu-Zn electrode was used as the cathode in a CO2 purged KHCO3 electrolyte. When the SrTiO3 photoanode was irradiated by UV light, CO2 reduction was catalyzed on the surface of Cu-Zn electrode. Figure 1 shows the products detected at the rest potential, and HCOOH, CO, CH4 and H2 were produced. When we purged Ar gas in the electrolyte solution at cathodic side, the products amount was much lower than that of CO2 purged condition, indicating that most of the products were originated from catalytic CO2reduction. Further, products amounts from Cu-Zn intermetallic were two times higher than those from pure Cu. It is noteworthy that the faradic efficiency for HCOOH was as high as 79.72%. The turnover number (TON) of this electrode reached 1458.9, which proved that the Cu-Zn electrode performed high stability. In the photocatalytic evaluation, the Cu-Zn intermetallic nanoparticles loaded SrTiO3 powder was dispersed into CO2 purged KHCO3 solution and irradiated under UV light. Significant amount of HCOOH was also detected, while H2 production was almost negligible, indicating that the Cu-Zn also behaves high selectivity as co-catalyst in a CO2photocatalytic reduction system. The carbon source of the products was also confirmed by our isotope tracing experiment. Conclusion The Cu-Zn intermetallic electrode can catalyze CO2 reduction under low bias-potential in electrochemical and photoelectrochemical CO2 reduction systems with high conversion efficiency and selectivity for HCOOH. Our Cu-Zn electrode was very stable under bias application as well as photon irradiation conditions. The Cu-Zn intermetallic nanoparticles loaded STO powder can also convert CO2into HCOOH and other products under UV irradiation. Acknowledgement This work has been supported by a grant from Advanced Catalytic Transformation program for Carbon utilization (ACT-C), Japan Science and Technology Agency (JST). References Kuhl, Kendra P., et al. J. Am. Chem. Soc., 136,14107-14113 (2014)Hori, Y. Modern aspects of electrochemistry. Springer New York, 2008. 89-189. Figure 1