Ionic liquids (ILs) have been established as effective promoters for the electrocatalytic upconversion of CO2 to various commodity chemicals. Imidazolium ([Im]+) cathode combinations have been reported to selectively catalyze the 2e−/2H+ reduction of CO2 to CO. Recently our laboratory has reported energy-efficient systems for CO production featuring inexpensive bismuth-based cathode materials and ILs comprised of 1,3-dialkylimidazolium cations. As part of our ongoing efforts to understand the factors that drive CO2 reduction at electrode interfaces, we sought to evaluate the catalytic performance of alternative ILs in combination with previously described Bi cathodes. In this work, we demonstrate that protic ionic liquids (PILs) derived from 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) effectively promote the electrochemical reduction of CO2 to formate (HCOO−) with high selectivity. The use of PILs comprised of the conjugate acid of DBU, [DBU-H]+, efficiently catalyzed the reduction of CO2 to HCOO− (FEFA ≈ 80%) with significant suppression of CO production (FECO ≈ 20%) in either MeCN or MeCN/H2O (95/5) solution. When they were used in combination with [DBU-H]+-based PILs, Bi-based cathodes achieved current densities for CO2 reduction (jtot ≈ 25–45 mA/cm2) that are comparable to or greater than those reported with imidazolium ILs such as [BMIM]PF6. As we demonstrate herein, the selectivity of the 2e− reduction of CO2 toward HCOO− or CO can be dictated through the choice of the IL promoter present in the electrolysis solution, even in cases in which the same electrocatalyst material is studied. These findings highlight the tunability of bismuth/IL systems for the electrochemical reduction of CO2 with high efficiency and rapid kinetics.
Bismuth electrodes undergo distinctive electrochemically induced structural changes in nonaqueous imidazolium ([Im] + )-based ionic liquid solutions under cathodic polarization. In situ X-ray reflectivity (XR) studies have been undertaken to probe well-ordered Bi (001) films which originally contain a native Bi 2 O 3 layer. This oxide layer gets reduced to Bi 0 during the first cyclic voltammetry (CV) scan in acetonitrile solutions containing 1-butyl-3-methylimidazolium ([BMIM] + ) electrolytes. Approximately 60% of the Bi (001) Bragg peak reflectivity is lost during a potential sweep between −1.5 and −1.9 V vs Ag/AgCl due to a ∼ 4−10% thinning and a ∼40% decrease in lateral size of Bi (001) domains, which are mostly reversed during the anodic scan. Repeated potential cycling enhances the thinning and roughening of the films, suggesting that partial dissolution of Bi ensues during negative polarization. The mechanism of this behavior is understood through molecular dynamics simulations using ReaxFF and density functional theory (DFT) calculations. Both approaches indicate that [Im] + cations bind to the metal surface more strongly than tetrabutylammonium (TBA + ) as the potential and the charge on the Bi surface become more negative. ReaxFF simulations predict a higher degree of disorder for a negatively charged Bi (001) slab in the presence of the [Im] + cations and substantial migration of Bi atoms from the surface. DFT simulations show the formation of Bi•••[Im] + complexes that lead to the dissolution of Bi atoms from step edges on the Bi (001) surface at potentials between −1.65 and −1.95 V. Bi desorption from a flat terrace requires a potential of approximately −2.25 V. Together, these results suggest the formation of a Bi•••[Im] + complex through partial cathodic corrosion of the Bi film under conditions (potential and electrolyte composition) that favor the catalytic reduction of CO 2 .
Room-temperature ionic liquids (RTILs) have been shown to have a significant effect on the redox potentials of compounds such as 1,4-dinitrobenzene (DNB), which can be reduced in two one-electron steps. The most noticeable effect is that the two one-electron waves in acetonitrile collapsed to a single two-electron wave in a RTIL such as butylmethyl imidazolium-BF4 (BMImBF4). In order to probe this effect over a wider range of mixed-molecular-solvent/RTIL solutions, the reduction process was studied using UV–vis spectroelectrochemistry. With the use of spectroelectrochemistry, it was possible to calculate readily the difference in E°’s between the first and second electron transfer (ΔE 12° = E 1° – E 2°) even when the two one-electron waves collapsed into a single two-electron wave. The spectra of the radical anion and dianion in BMImPF6 were obtained using evolving factor analysis (EFA). Using these spectra, the concentrations of DNB, DNB–•, and DNB2– were calculated, and from these concentrations, the ΔE 12° values were calculated. Significant differences were observed when the bis(trifluoromethylsulfonyl)imide (NTf2) anion replaced the PF6 – anion, leading to an irreversible reduction of DNB in BMImNTf2. The results were consistent with the protonation of DNB2–, most likely by an ion pair between DNB2– and BMIm+, which has been proposed by Minami and Fry. The differences in reactivity between the PF6 – and NTf2 – ionic liquids were interpreted in terms of the tight versus loose ion pairing in RTILs. The results indicated that nanostructural domains of RTILs were present in a mixed-solvent system.
Via conversion to Katritzky pyridinium salts, alkyl amines can now be used as alkyl radical precursors for a range of deaminative functionalization reactions. The key step of all of these methods is single-electron reduction of the pyridinium ring, which triggers C−N bond cleavage. However, little has been done to understand how the precise nature of the pyridinium influences these events. Using a combination of synthesis, computation, and electrochemistry, this study delineates the steric and electronic effects that substituents have on the canonical steps and the overall process. Depending on the approach taken, consideration of both the reduction and the subsequent radical dissociation may be necessary. Whereas the electronic effects on these steps work in opposition to each other, the steric effects are synergistic, with larger substituents favoring both steps. This understanding provides a framework for future design of pyridinium salts to match the mode of catalysis or activation.
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