Influence of RTIL Nanodomains on the Voltammetry and Spectroelectrochemistry Of Fullerene C\u3csub\u3e60\u3c/sub\u3e in Benzonitrile/Room Temperature Ionic Liquids Mixtures

The cyclic voltammetry of fullerene C60 was examined in mixed benzonitrile/RTIL solvents in order to probe the effect of nanodomains in the mixed RTIL/benzonitrile solutions and their effect upon the voltammetry. In probing the interactions of the fullerides (up to C603−) with RTILs, BMIm+ (1-butyl-3-methylimidazolium, mostly planar) and tetraalkylammonium (more spherical/flexible) salts were used. In order to investigate these shifts in more detail, the ΔE12° (=E°1–E°2) and ΔE23° (=E°2–E°3) values, which were independent of the reference potential, were used. At higher concentrations of the RTILs, greater stabilization of the more highly charged fullerides were observed. These shifts were attributed to the interaction of the fullerides with nanodomains of the RTIL. This was further confirmed by examining the shifts in the E1/2 values of non-RTIL and RTIL salts at constant ionic strength and the changes in diffusion coefficient with %RTIL. The observed shifts in the E1/2 values with increased concentration of the RTIL salts could not be explained by ion pairing equilibria alone. Changes in the visible and near infrared spectra between benzonitrile and mixed benzonitrile/RTIL spectra were most significant for C603−, where voltammetric evidence indicates the strongest interaction between the fullerides and the RTIL. Among the RTILs studied, preliminary DFT calculations showed that the more flexible tetraalkylammonium ion was able to stabilize the C60-anionic species better than the planar BMIm+ species, under similar solution conditions

Electrochemistry and Spectroelectrochemistry of 1,4-Dinitrobenzene in Acetonitrile and Room-Temperature Ionic Liquids: Ion-Pairing Effects in Mixed Solvents

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 (ΔE12° = E1° – E2°) 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 ΔE12° 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

Spectroscopic Evidence of Nanodomains in THF/RTIL Mixtures: Spectroelectrochemical and Voltammetric Study of Nickel Porphyrins

The presence and effect of RTIL nanodomains in molecular solvent/RTIL mixture were investigated by studying the spectroelectrochemistry and voltammetry of nickel octaethylporphyrin (Ni(OEP)) and nickel octaethylporphinone (Ni(OEPone)). Two oxidation and 2–3 reduction redox couples were observed, and the UV–visible spectra of all stable products in THF and RTIL mixtures were obtained. The E° values for the reduction couples that were studied were linearly correlated with the Gutmann acceptor number, as well as the difference in the E° values between the first two waves (ΔE12° = |E1° – E2°|). The ΔE12° for the reduction was much more sensitive to the %RTIL in the mixture than the oxidation, indicating a strong interaction between the RTIL and the anion or dianion. The shifts in the E° values were significantly different between Ni(OEP) and Ni(OEPone). For Ni(OEP), the E1° values were less sensitive to the %RTIL than were observed for Ni(OEPone). Variations in the diffusion coefficients of Ni(OEP) and Ni(OEPone) as a function of %RTIL were also investigated, and the results were interpreted in terms of RTIL nanodomains. To observe the effect of solvation on the metalloporphyrin, Ni(OEPone) was chosen because it contains a carbonyl group that can be easily observed in infrared spectroelectrochemistry. It was found that the νCO band was very sensitive to the solvent environment, and two carbonyl bands were observed for Ni(OEPone)− in mixed THF/RTIL solutions. The higher energy band was attributed to the reduced product in THF, and the lower energy band attributed to the reduced product in the RTIL nanophase. The second band could be observed with as little as 5% of the RTIL. No partitioning of Ni(OEPone)+ into the RTIL nanodomain was observed. DFT calculations were carried out to characterize the product of the first reduction. These results provide strong direct evidence of the presence of nanodomains in molecular solvent/RTIL mixtures

Probing the Effects of Nano-Domains on the Redox Products in Molecular Solvents-RTILs Mixtures

Room temperature ionic liquids (RTIL) have been very attractive as replacements for molecular solvents (MS) in many areas in chemistry. The use of the RTIL in mixture with MS as reaction media will reduce the cost and viscosity of RTILs. The formation of RTIL nano-domains (nano-structures) in the mixture may lead to partitioning of solutes between the MS and the RTIL phase, where the properties and reactivity would be more like those in pure ionic liquids. In the present work, analytical approaches have been employed to probe the presence and effects of RTIL nano-domains on redox processes in the mixture. A focus was on substrates that undergo multi-electron reductions/oxidations, such as: dinitrobenzene, fullerene and metalloporphyrins. Throughout our investigation, there was strong evidence of the presence of RTIL and MS domains in the mixture system. Correlation of the potential shifts with acceptor properties of the mixture enabled to evaluate the extent of ion paring interactions within the molecular and RTIL domains. Influence of RTIL domains on the transport properties of the solution was evaluated versus the change in the viscosity of the mixture. The impact of RTIL domains on the electronic structures of redox products was examined using several spectroscopic methods, including UV-visible, infrared and NMR spectroscopy. Computational tools such as chemometrics, voltammetric simulations and DFT calculations were used to complement the experimental analysis

Visible and Infrared Spectroelectrochemistry of Zinc and Manganese Porphinones: Metal vs. Porphyrin Reduction

The visible and infrared spectroelectrochemistry of zinc and manganese porphinones and porphinediones was carried out in THF solutions. The aim of this work was to use FTIR spectroelectrochemistry and DFT calculation to determine whether the reduction was centered predominantly on the metal or the macrocycle. For zinc(II), the first one-electron reduction must occur on the macrocyclic ring because the metal’s d-orbitals are filled (d10). The carbonyl bands on the macrocyclic ring were used to probe the electronic structure because they can be readily observed in the infrared spectra. The results of this study are complementary to previous spectroelectrochemical studies that have been reported for the iron and cobalt complexes of the same macrocycles. As expected for the formation of a π-radical anion species, significant downshifts in the carbonyl bands were observed. DFT calculations showed that the behavior of the porphinedione complexes were most sensitive to the electronic structure of the M(OEPdione)− species. If a MI species is formed, the two carbonyl groups will be downshifted by similar energies. For MII-radical anions, one carbonyl will be downshifted significantly, and the second one will be downshifted by a small amount. On the basis of this criterion, it was determined that cobalt(I) and iron(I) complexes were formed, while zinc and manganese formed π-radical anion species. The visible spectroelectrochemistry was also consistent with these electronic structures

In Situ Study of the Photodegradation of Carbofuran Deposited on TiO\u3csub\u3e2\u3c/sub\u3e Film under UV Light, Using ATR-FTIR Coupled to HS-MCR-ALS

The in situ study of the photodegradation of carbofuran deposited on a TiO2 catalyst film under UV light was carried out using the ATR-FTIR technique. The data were analyzed using a Hard–Soft Multivariate Curve Resolution-Alternating Least Squares (HS-MCR-ALS) methodology. Using S-MCR-ALS, four factors were deduced from the evolving factor analysis of the data, and their concentrations and spectra were determined. These results were used to draw qualitative and quantitative analyses of the major products of carbofuran photodegradation. The results of this analysis were in good agreement with GC-MS results and with reported mechanisms. Hard-MCR-ALS was then used to refine the spectra and concentrations, using a multistep kinetic model. The rate constant for the first step in the photodegradation of carbofuran was found to be 2.9 × 10–3 min–1. The higher magnitude of the correlation (96.87%), the explained variance (99.87%) and LOF (3.01), are good indicators of the reliability of the outcome of this approach. This method has been shown to be an efficient approach to study in situ photodegradation of pesticides on a solid surface

Proton-Coupled Reduction of An Iron Nitrosyl Porphyrin in The Protic Ionic Liquid Nanodomain

The one-electron reduction of many molecules becomes much more favorable if combined with proton transfers or strong hydrogen bonding. Protic room temperature ionic liquids (RTILs), which can form nanodomains in solutions with molecular solvents (MS), can provide an efficient avenue for this process. In this work, we report on the voltammetry, UV/visible and resonance Raman spectroelectrochemistryof Fe(TPP)(NO) in the presence of aprotic/protic ammonium-based ionic liquids. While aprotic RTILs did shift the reduction to more positive potentials, similar shifts could be observed at much lower concentrations of diethylmethylammonium triflate (HAmOTf, a protic ionic liquid). Deconvolution of the rotating ring-disk electrode (RRDE) voltammetry revealed the partitioning of the reduced species into the ionic liquid nanodomains at low concentrations. The potential shift was substantially in excess of the value expected based on the pKa of the weak acid. Upon the addition of small amounts of the protic RTIL, the electrochemically or chemically generated anion, Fe(TPP)(NO)-, reacted rapidly with the HAm+ acid, forming a Fe(TPP)(HNO) complex. Further reduction to Fe(TPP)(NH2OH) could be observed on the spectroelectrochemical time scale. The outcome of this work revealed the advantageous role of protic RTIL nanodomains in accelerating the proton-coupled reductions to form more energetically favorable product

Direct Observation of Photoinduced Charge Separation in Ruthenium Complex/Ni(OH)\u3csub\u3e2\u3c/sub\u3e Nanoparticle Hybrid

Ni(OH)2 have emerged as important functional materials for solar fuel conversion because of their potential as cost-effective bifunctional catalysts for both hydrogen and oxygen evolution reactions. However, their roles as photocatalysts in the photoinduced charge separation (CS) reactions remain unexplored. In this paper, we investigate the CS dynamics of a newly designed hybrid catalyst by integrating a Ru complex with Ni(OH)2 nanoparticles (NPs). Using time resolved X-ray absorption spectroscopy (XTA), we directly observed the formation of the reduced Ni metal site (~60 ps), unambiguously demonstrating CS process in the hybrid through ultrafast electron transfer from Ru complex to Ni(OH)2 NPs. Compared to the ultrafast CS process, the charge recombination in the hybrid is ultraslow (≫50 ns). These results not only suggest the possibility of developing Ni(OH)2 as solar fuel catalysts, but also represent the first time direct observation of efficient CS in a hybrid catalyst using XTA

Voltammetry and Spectroelectrochemistry of TCNQ in Acetonitrile/RTIL Mixtures

Understanding the solvation and ion-pairing interactions of anionic substrates in room-temperature ionic liquids (RTIL) is key for the electrochemical applications of these new classes of solvents. In this work, cyclic voltammetry and visible and infrared spectroelectrochemistry of tetracyanoquinodimethane (TCNQ) was examined in molecular (acetonitrile) and RTIL solvents, as well as mixtures of these solvents. The overall results were consistent with the formation of RTIL/acetonitrile nanodomains. The voltammetry indicated that the first electrogenerated product, TCNQ−, was not incorporated into the RTIL nanodomain, while the second electrogenerated product, TCNQ2−, was strongly attracted to the RTIL nanodomain. The visible spectroelectrochemistry was also consistent with these observations. Infrared spectroelectrochemistry showed no discrete ion pairing between the cation and TCNQ− in either the acetonitrile or RTIL solutions. Discrete ion pairing was, however, observed in the acetonitrile domain between the tetrabutylammonium ion and TCNQ2−. On the other hand, no discrete ion pairing was observed in BMImPF6 or BMImBF4 solutions with TCNQ2−. In BMImNTf2, however, discrete ion pairs were formed with BMIm+ and TCNQ2−. Density function theory (DFT) calculations showed that the cations paired above and below the aromatic ring. The results of this work support the understanding of the redox chemistry in RTIL solutions

Interrogation of the Selectivity and Electrokinetics of CO2 Reduction by AgSn Films in the Presence of Protic Organic [DBU–H]+ Cations as judged by Impedance Spectroscopy and Distribution of Relaxation Times Analysis

The use of renewable electricity to synthesize high energy and high value chemicals via reduction of CO2 is an attractive strategy for renewable energy storage. Improving our understanding of how heterogeneous CO2 reduction electrocatalysts function is important to designing efficient systems for conversion of CO2 into commodity chemicals such as CO and HCO2H. Both Ag- and Sn-based materials have been previously considered as CO2 reduction catalysts and offer distinct CO2RR selectivities. In this work, we have considered electrodeposited composite film electrodes prepared from electroplating baths with varying ratios of Ag+ and Sn2+ triflates to understand how the performance of such composite materials varies as a function of composition. XPS analysis confirms that for each composite film electrodes, Ag existed in the metallic (Ag0) state, while the Sn was mainly oxidized (Sn2+/4+). The AgSn composite film electrodes studied herein are therefore best considered as AgSnOx cathodes with varying ratios of Ag0:Sn2+/4+. These systems were assessed as CO2RR electrocatalysts and were found to promote the 2e–/2H+ reductions to deliver CO and HCOOH with fast kinetics and high efficiencies from electrolyte solutions containing the protic organic cation [DBU–H]+. While Sn-rich composite films showed poor selectivities for CO versus HCO2H, a significant increase in CO versus HCO2H selectivity (up to 99%) is achieved for composite film electrodes in which the Ag content ranged from 25 - 75%. By tuning the ratio of Ag0 to SnOx we prepared composite film cathode materials that support quantitative current efficiencies for generation of CO with geometric current densities approaching 30 mA/cm2 at applied overpotentials that are less than 750 mV were realized. Additionally, electrochemical impedance spectroscopy (EIS) coupled with analysis of the distribution of relaxation times (DRT) was used to better understand factors important to the composites’ activity under CO2RR conditions. Probing the dynamics with DRT analysis revealed that multiple processes relating to both adsorption and diffusion-controlled events are important to the activity of the electrocatalysts considered in this work. The collection of electroanalytical investigations suggest that synergistic interactions between Ag and SnOx give rise to porous films that support enhanced CO2RR kinetics and that mixing of Ag with SnOx enhances the efficacy of adsorption and stabilization of reduced CO2 intermediates and [DBU–H]+ cations to facilitate CO evolution at the cathode/electrolyte interface