ABSTRACT
8.1 INTRODUCTION
8.1.1 Experimental Results of CO2 Reduction
The basis and starting point for modeling are experiments. Some typical experimental results will be briefly introduced and the measured properties are shown in Table 8.1. Recently, Bocarlsy and coworkers proposed a promising way of reducing CO2 into methanol in the presence of pyridine with low overpotential achieving nearly 100% faradaic efficiency [1–4]. The role of pyridine and detailed mechanism are still on debate [3]. However, the formation of a complex between CO2 and pyridine derivatives, as the key step for reduction of CO2, was considered by all the researchers [3–8]. Rosen et al. [9] reported a highly efficient method of reduction of CO2 selectively into CO at overpotentials below 0.2 V catalyzed by an ionic liquid called “1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF4).” The faradaic efficiencies of producing CO greater than 96% were obtained [9]. Other benefits of this technology are that ionic liquid itself is an electrolyte and CO2 is remarkably soluble in the imidazolium-based ionic liquids. For instance, the solubility of CO2 in the ionic liquid increases dramatically with increasing pressure, reaching 0.72 mole fraction CO2 in BMIM-PF6 at 93 bar [10]. An in situ spectroscopic observation of the reaction by sum frequency generation (SFG) showed that the adsorbed EMIM layer can react with CO2 to form a complex such as CO2–EMIM, which was then converted to CO [11]. It was found that the reaction rate varied remarkably with the metal electrode employed [11]. The addition of water to EMIM-BF4 can surprisingly increase the efficiency of CO2 conversion to CO, which was possibly due to the increased proton caused by the hydrolysis of tetrafluoroborate [12]. Other ionic liquids such as EMIM-Tf2N [13] and BMIM-BF4 [14] also showed selectivity to produce CO with high faradic efficiency.
