Diamonds in the Rough: Direct Surface Enhanced Infrared Spectroscopic Evidence of Nitrogen Reduction on Boron-Doped Diamond Supported Metal Catalysts.

In situ investigations of electrocatalytic processes of increasing societal interest such as the nitrogen reduction reaction (NRR) require aggressive experimental conditions that are not readily compatible with surface sensitive techniques such as attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS). A method for performing ATR-SEIRAS studies at very negative potentials where conventional IR-active films delaminate and fail is reported. The method relies on a thin film of very robust boron-doped diamond deposited on a micromachined Si wafer, which provides extended mid-IR transparency at long wavelengths. SEIRAS activity is achieved by electrodepositing gold nanoparticles onto the conductive BDD layer. The Au@BDD layers are shown to sustain prolonged periods of electrolysis at negative potentials, with no degradation of the modifying layer. The efficacy of these substrates for electrocatalysis is demonstrated by studying the reduction of N2 at -1.5 V vs Ag/AgCl in an aqueous-based electrolyte. Under these conditions, direct spectroscopic evidence of both NH3 and hydrazine formed from the nitrogen reduction reaction (NRR) is provided.

Reversible Proton-Coupled Reduction of an Iron Nitrosyl Porphyrin within [DBU-H]+-Based Protic Ionic Liquid Nanodomains.

The reduction of [Fe(OEP)(NO)] has been studied in the presence of aprotic room-temperature ionic liquids (RTIL) and protic (PIL) ionic liquids dissolved within a molecular solvent (MS). The cyclic voltammetric results showed the formation of RTIL nanodomains at low concentrations of the RTIL/PIL solutions. The pKa values of the two PILs studied (i.e., trialkylammonium and [DBU-H]+-based ionic liquids) differed by four units in THF. While voltammetry in solutions containing all three RTILs showed similar potential shifts of the first reduction of [Fe(OEP)(NO)] to [Fe(OEP)(NO)]- at low concentrations, significant differences were observed at higher concentrations for the ammonium PIL. The trialkylammonium cation had previously been shown to protonate the {FeNO}8 species at room temperature. Visible and infrared spectroelectrochemistry revealed that the [DBU-H]+-based PIL formed hydrogen bonds with [Fe(OEP)(NO)]- rather than formally protonating it. Despite these differences, both PILs were able to efficiently reduce the nitrosyl species to the hydroxylamine complex, which could be further reduced to ammonia. On the voltammetric time scale and when the switching potential was positive of the Fe(II)/Fe(I) potential, the hydroxylamine complex was re-oxidized back to the NO complex via direct oxidation of the coordinated hydroxylamine at low scan rates or initial oxidation of the ferrous porphyrin at high scan rates. The results of this work show that, while [DBU-H]+ does not protonate electrochemically generated [Fe(OEP)(NO)]-, it still plays an important role in efficiently reducing the nitroxyl ligand via a series of proton-coupled electron transfer steps to generate hydroxylamine and eventually ammonia. The overall reaction rates were independent of the PIL concentration, consistent with the nanodomain formation being important to the reduction process.

Ion Pairing versus Solvation of Dinitrobenzene Anions in Room-Temperature Ionic Liquids (RTILs): Vibrational Signatures of RTIL-Substrate Interactions.

The mechanism of solvation of ions by ionic liquids is more complex than solvation in most molecular solvents as the ionic liquid itself provides the counter ion. Solvation and ion pairing of anionic substrates in room-temperature ionic liquids (RTILs) were investigated using resonance Raman spectroscopy and DFT calculations. The purpose of this study was to differentiate between the formation of discrete cation/anion structures and a double-layer cloud of counter ions without specific atomic interactions between the ionic species. In acetonitrile/RTIL mixtures, the radical anion and dianion of dinitrobenzene (DNB) are stabilized by RTILs through solvation and ion pairing. The formation of the lowest-energy ion pair led to the largest shifts in the Raman band in DNB-·, while significantly smaller shifts were predicted for general solvation. The effect of general solvation and ion pair formation was studied using DFT with the implicit solvation model. Identification of the bands most sensitive to tight ion pairing allowed for the interpretation of the observed vibrational changes. The formation of tight ion pairs between the anionic solutes depends on both cation-solute and RTIL cation-anion interactions. Tight ion pairs were observed in RTILs, but general solvation was also important. This work establishes the advantageous use of vibrational spectroscopy to provide detailed structural information not accessible from voltammetry alone.

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.

Altering the Coordination of Iron Porphyrins by Ionic Liquid Nanodomains in Mixed Solvent Systems.

The solvent environment around iron porphyrin complexes was examined using mixed molecular/RTIL (room temperature ionic liquid) solutions. The formation of nanodomains in these solutions provides different solvation environments for substrates that could have significant impact on their chemical reactivity. Iron porphyrins (Fe(P)), whose properties are sensitive to solvent and ligation changes, were used to probe the molecular/RTIL environment. The addition of RTILs to molecular solvents shifted the redox potentials to more positive values. When there was no ligation change upon reduction, the shift in the E° values were correlated to the Gutmann acceptor number, as was observed for other porphyrins with similar charge changes. As %RTIL approached 100 %, there was insufficient THF to maintain coordination and the E° values were much more dependent upon the %RTIL. In the case of FeIII (P)(Cl), the shifts in the E° values were driven by the release of the chloride ion and its strong attraction to the ionic liquid environment. The spectroscopic properties and distribution of the FeII and FeI species into the RTIL nanodomains were monitored with visible spectroelectrochemistry, 19 F NMR and EPR spectroscopy. This investigation shows that coordination and charge delocalization (metal versus ligand) in the metalloporphyrins redox products can be altered by the RTIL fraction in the solvent system, allowing an easy tuning of their chemical reactivity.

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.

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 DNB(2-) 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 DNB(2-), most likely by an ion pair between DNB(2-) 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.

Nano-Plasticity of an Electrified Ionic Liquid/Electrode Interface: Uncovering Hard-Soft Structuring via Controlled Metal Fill Factor.

Ionic liquids (ILs) nanostructuring at electrified interfaces is of both fundamental and practical interest as these materials are increasingly gaining prominence in energy storage and conversion processes. However, much remains unresolved about IL potential-controlled (re)organization under highly polarized interfaces, mostly due to the difficulty of selectively probing both the distal and proximal surface layers of adsorbed ions. In this work, the structural dynamics of the innermost layer (<10 nm from the surface) were independently interrogated from that of the ionic layers in the sub-surface region (>100 nm from the surface), using an infrared (IR) spectroscopy approach. By tuning the metal fill factor of gold films deposited on conductive metal oxide-modified IR internal reflection elements, the charge-driven (re)structuring of the inner and distal layers of 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate is unveiled. Within a relatively wide potential region (∼±1 V) bounding the potential of zero charges, the ionic liquid is shown to undergo a reversible (i.e., soft) reorganization whereby the innermost layer of anions (cations) is exchanged by a layer of cations (anions). Kinetically unhindered changes in the number density of constituent cations and anions largely follow electrostatic expectations in the subsurface region, whereas the innermost layer exhibits a pronounced hysteresis and very slow relaxation. Under larger negative potential bias, IL restructuring is characterized by a highly irreversible (i.e., hard) and intense interfacial densification of the BMPy+ cations, consistent with the formation of nanoscale segregated liquids. The outcomes of this work reveal a plastic IL nanostructuring under a strong electric field.

In situ study of the photodegradation of carbofuran deposited on TiO2 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.

Dissection of Alkylpyridinium Structures to Understand Deamination Reactions.

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 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.

Directing the Outcome of CO2 Reduction at Bismuth Cathodes Using Varied Ionic Liquid Promoters.

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 (j tot ≈ 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.

Direct Observation of Photoinduced Charge Separation in Ruthenium Complex/Ni(OH)2 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.