| Recently, electroreduction of CO2to value-added chemicals in aqueous solution are being got more attentions. As is well known, CO2is recognized to be nontoxic, naturally abundant and cheap. In addition, it is also one of the greenhouse gases and brings about many ecological and environmental problems such as global warming. So we should find some methods of conversion and utilization of carbon dioxide, for decreasing the concentration of CO2in the air, such as adsorption by physical method, conversion by biological method and chemical fixation method. Electrochemical reduction of CO2is one of the useful chemical fixation methods, which rises only in recent decades. In aqueous solution, it can get lots of useful chemicals such as CO, HCOOH, HCHO, methanol and hydrocarbons, which are the primary basic materials in the chemical industry. The hydrogen evolution reaction always happens during electrochemical reduction of CO2in aqueous solution, then we can collect the hydrogen to store. Overall electrochemical reduction of CO2in aqueous solution is an environmentally and friendly way. In the long term, electrochemical reduction of CO2in the solution has a significant prospect. But CO2is an inert gas with strong chemical stability, the standard potential is-1.9V vs.NHE in the solution (pH=7). So in the process of electrochemistry, some catalysts need to be added into solution to decease the overpotential of reaction.The details are given as follows:(1) Electrochemical reduction of CO2to methanol using homogeneous catalyst in the aqueous solutionIn the chemical industry, reduction of CO2to useful chemicals is more popular. While the electrochemical reduction of CO2in aqueous solution is still in the infancy, the methods of detection produces are various. In this chapter, we firstly setup the analysis and detection method. Then we make a comparison using three disposal methods after the electrolyte reaction. It was found that micro-distillation method was better than other two methods on the base of our experimental conditions. According to different products, we used different analysis methods:the gaseous products were detected by GC; the liquid products were detected by GC and1HNMR. We study the electroreduction of CO2on pyridine in aqueous solution. Firstly, cyclic voltammetry of pyridine was carried out in the self-design cell at room temperature. There was a pair of reversible redox peaks when the solution is acidic, and the peak current kept a well linear relation with v1/2when increasing the scan rate. The Faradaic efficiency of methanol was mostly around3%, but when the catalyst is0.02M, Faradaic efficiency of methanol is5.3%. At last, we investigated other organic bases to electrocatalytic reduction of CO2in order to compare with the pyridine. A study of the electrochemical behavior of organic bases revealed no redox reaction in the same potential region except4-methylpyridine. The redox reaction on the4-methylpyridine was the same as that on pyridine. Faradaic efficiency of methanol was2.2%on4-methylpyridine, but no methanol was detected when electroreduction of CO2on other organic bases. This is because of the no-shared lone pair of electrons on N atom. In this case, we study the BMIMBF4under the same conditions. There was a pair of reversible redox peak in the same potential region, and the Faradaic efficiency of methanol is2.4%. The results showed that the unshared lone pair of electrons played an important role during the process of electrochemical reduction of CO2-(2) Electroreduction of CO2on platinum complexes in the aqueous solutionIn aqueous solution, electroreduction of CO2to small molecules on platinum complex catalysts, which are PtPz(platinum pyrazole), PtPc(platinum phthalocyanine), PtDAT(the platinum complex of3,5-diamino-l,2,4-triazole), PtTPA(the platinum compounds of tris[(2-pyridyl)methyl]amine), respectively. Cyclic voltammetry of the four catalysts was carried out in the standard three-electrode cell at room temperature. There are two electrolytes:one is1M KCl, and another is1M KHCO3. In the latter electrolyte, the onset potentials of four catalysts are more positive than that in the former one in the presence of CO2, which indicates that the KHCO3solution is more favored for electroreduction of CO2than KCl solution. Under the room temperature and ambient pressure conditions, CO, HCOOH were detected after electroreduction of CO2on platinum complexes in the flow reactor. Based on the experimental results, the Faradaic efficiencies of CO and HCOOH were below10%, which suggest that four platinum complexes are not suitable for electrochemical reduction of CO2in the solution.(3) Investigation of a Cu(core)/CuO(shell) catalyst for electrochemical reduction of CO2In this work, we studied the electrochemical reduction of CO2on a Cu(core)/CuO(shell) catalyst in aqueous electrolytes in a flow reactor. Faradaic efficiencies of CO and HCOOH were around10～20%, respectively. A small amount of methanol was produced. A study of the electrochemical behavior of Cu(core)/CuO(shell) in a standard three-electrode cell revealed transformations between Cu, Cu(Ⅰ), and CuO as a function of applied potential in the presence of Ar and CO2, and the activity of catalyst decreased firstly then remain after continuous scanning. Then catalyst was detected by XPS and XRD. This interplay between different Cu oxidation states and the presence of the oxide surface to promote proton adsorption may benefit the formation of methanol over other products. It may guide the development of catalyst that are more selective for individual product such as the desirable selective production of methanol.(4) Kinetic analysis for electroreduction of carbon dioxide on Cu(core)/CuO(shell) in aqueous solutionThis chapter focused on the experimental and kinetics of electroreduction process of CO2into CO and HCOOH using a Cu(core)/CuO(shell) catalyst. Faradaic efficiencies of products increased with increasing the loading of catalyst on the gas diffusion electrode. We also carried out electroreduction of CO2on catalyst in a long time, which can help us understand the activity of the catalyst during the process of reaction. From the results, Faradaic efficiency of CO and HCOOH are18.1%and26.3%, respectively, when the reaction time is80min and the catalyst is1.5mg/cm2loading on GDE. It was shown that the activity of catalyst was better than other conditions. At last, reactions were modeled by in-series reaction mechanism and it was verified that the models could reasonably describe the products during the electroreduction of CO2. According to the results of kinetic analysis, the rate constant for HCOOH production was higher than that of CO.(5) Investigate of hydrogen evolution during electroreduction of CO2in aqueous solutionHydrogen evolution reaction always competed with electroreduction of CO2in aqueous solution. Faradaic efficiency of H2was50%or more on platinum complexes catalysts. Using the Cu(core)/CuO(shell) catalyst, Faradaic efficiency of H2increased and then decreased when potential shifted negative. Faradaic efficiency of H2always increased with the increment of catalyst loadings. Hydrogen evolution reaction might not affect during the long reaction time. The hydrogen evolution reaction was modeled by differential equations, and it was verified that the models could reasonably describe the reaction during the process of electrochemical reduction. According to the kinetic analysis, the rate constant for H2was not affected when increasing catalyst loadings. |
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