Optical and Chemical Measurements of Solvated Electrons Produced in Plasma Electrolysis with a Water Cathode.

It is known that glow discharges with a water anode inject and form solvated electrons at the plasma-liquid interface, driving a wide variety of reduction reactions. However, in systems with a water cathode, the production and role of solvated electrons are less clear. Here, we present evidence for the direct detection of solvated electrons produced at the interface of an argon plasma and a water cathode via absorption spectroscopy. We further quantify their yield using the dissociative electron attachment of chloroacetate, measuring a yield of 1.04 ± 0.59 electrons per incident ion, corresponding to approximately 100% faradaic efficiency. Additionally, we estimate a yield of 2.09 ± 0.93 hydroxyl radicals per incident ion. Comparison of this yield with other findings in the literature supports that these hydroxyl radicals are likely formed directly in the liquid phase rather than by diffusion from the vapor phase.

Exploring the Unusual Reactivity of the Hydrated Electron with CO2.

Many questions remain about the reactions of the hydrated electron despite decades of study. Of particular note is that they do not appear to follow the Marcus theory of electron transfer reactions, a feature that is yet to be explained. To investigate these issues, we used ab initio molecular dynamics (AIMD) simulations to investigate one of the better studied reactions, the hydrated electron reduction of CO2. The rate constant for the hydrated electron-CO2 reaction complex to react to form CO2- is for the first time estimated from AIMD simulations. Results at 298 and 373 K show the rate constant is insensitive to temperature, consistent with the low measured activation energy for the reaction, and the implications of this behavior are examined. The sampling provided by the simulations yields insight into the reaction mechanism. The reaction is found to involve both solvent reorganization and changes in the carbon dioxide structure. The latter leads to significant vibrational excitation of the bending and symmetric stretch vibrations in the CO2- product, indicating the reaction is vibrationally nonadiabatic. The former is estimated from the calculation of an approximate collective solvent coordinate and the free energy in this coordinate is determined. These results indicate that AIMD simulations can reasonably estimate hydrated electron reaction activation energies and provide new insight into the mechanism that can help illuminate the features of this unusual chemistry.

Kinetics of the Temperature-Dependent eaq - and ⋠OH Radical Reactions with Cr(III) Ions in Aqueous Solutions.

The reactivity of chromium(III) species with the major oxidizing and reducing radiolysis products of water was investigated in aqueous solutions at temperatures up to 150 °C. The reaction between the hydrated electron (eaq - ) and Cr(III) species showed a positive temperature dependence over this temperature range. The reaction was also studied in pH 2.5 and 3.5 solutions for the first time. This work also studied the reaction between acidic Cr(III) species and the hydroxyl radical (⋅OH). It was found that Cr3+ did not react significantly with the ⋅OH radical, but the first hydrolysis species, Cr(OH)2+ , did with a rate coefficient of k= (7.2±0.3)×108  M-1 s-1 at 25 °C. The oxidation of Cr(OH)2+ by the ⋅OH radical formed an absorbing product species that ultimately oxidized to give Cr(VI). These newly measured reaction rates allow for the development of improved models of aqueous chromium speciation for the effective remediation of liquid high-level nuclear waste via vitrification processes.

Empirically Optimized One-Electron Pseudopotential for the Hydrated Electron: A Proof-of-Concept Study.

Mixed quantum-classical molecular dynamics simulations have been important tools for studying the hydrated electron. They generally use a one-electron pseudopotential to describe the interactions of an electron with the water molecules. This approximation shows both the strength and weakness of the approach. On the one hand, it enables extensive statistical sampling and large system sizes that are not possible with more accurate ab initio molecular dynamics methods. On the other hand, there has (justifiably) been much debate about the ability of pseudopotentials to accurately and quantitatively describe the hydrated electron properties. These pseudopotentials have largely been derived by fitting them to ab initio calculations of an electron interacting with a single water molecule. In this paper, we present a proof-of-concept demonstration of an alternative approach in which the pseudopotential parameters are determined by optimizing them to reproduce key experimental properties. Specifically, we develop a new pseudopotential, using the existing TBOpt model as a starting point, which correctly describes the hydrated electron vertical detachment energy and radius of gyration. In addition to these properties, this empirically optimized model displays a significantly modified solvation structure, which improves, for example, the prediction of the partial molar volume.

Relation between the Hydrated Electron Solvation Structure and Its Partial Molar Volume.

It is now generally accepted that the hydrated electron occupies a cavity in water, but the size of the cavity and the arrangements of the solvating water molecules have not been fully characterized. Here, we use the Kirkwood-Buff (KB) approach to examine how the partial molar volume (VM) provides insight into these issues. The KB method relates VM to an integral of the electron-water radial distribution function, a key measure of the hydrated electron structure. We have applied it to three widely used pseudopotentials, and the results show that VM is a sensitive measure of the fidelity of hydrated electron descriptions. Thus, the measured VM places constraints on the hydrated electron structure that are important in developing and evaluating the model descriptions. Importantly, we find that VM does not reflect only the cavity size (and thus should not be used to infer the cavity radius) but is strongly dependent on the extended solvation structure.

High-Temperature Reaction Kinetics of the eaq- and HO2• Radicals with Iron(II) Ions in Aqueous Solutions.

Pulsed electron radiolysis was used to determine the chemical reaction kinetics and Arrhenius parameters for iron(II) reactions in aqueous solutions under irradiation. The second-order Fe2+ reactions with the hydrated electron (eaq-) and the perhydroxyl radical (HO2•), arising from water radiolysis, were measured to high temperatures using custom-built flow-through cells with a multichannel optical detection system. The reaction with the HO2• radical was found to proceed via the formation of a metal-ion adduct species, Fe2+-HO2•. The adduct's molar extinction coefficient and its first-order decay rate coefficients are also reported.

Reactions of Nickel Ions in Water Radiolysis up to 300 °C.

Radiation chemistry of hydrated metal ions plays a significant role in the field of nuclear energy, especially regarding water radiolysis in coolant water in nuclear reactors. This work reports new experimental data on the reactivity of Ni2+/+ species under critical conditions of temperature and pressure. The reaction rates of hydrated nickel ions with water radiolysis products (e-aq, •OH, H, and H2O2) have been investigated for a 25-300 °C temperature range and 200 bar pressure using electron pulse radiolysis/transient absorption. Extensive experiments with the Ni2+/+ species in various salts and pH up to 300 °C were performed. Kinetic decay traces of short-lived monovalent nickel ions were fitted to extract the rate constants versus temperature up to 300 °C. A blue shift of the absorption spectrum of the monovalent nickel ion with increasing temperature was demonstrated, which may indicate a change in the average coordination number. The Arrhenius parameters for the reactions of Ni2+ with e-aq and Ni+ with •OH, H, and H2O2 were obtained. All measured rate constants increased with temperature and followed Arrhenius behavior.

Pulse Radiolysis and Transient Absorption of Aqueous Cr(VI) Solutions up to 325 °C.

Pulse radiolysis with a custom multichannel detection system has been used to measure the kinetics of the radiation chemistry reactions of aqueous solutions of chromium(VI) to 325 °C for the first time. Kinetic traces were measured simultaneously over a range of wavelengths and fit to obtain the associated high-temperature rate coefficients and Arrhenius parameters for the reactions of Cr(VI) + e aq -, Cr(VI) + H•, and Cr(V) + •OH. These kinetic parameters can be used to predict the behavior of toxic Cr(VI) in models of aqueous systems for applications in nuclear technology, industrial wastewater treatment, and chemical dosimetry.

Investigation of the Failure of Marcus Theory for Hydrated Electron Reactions.

Reactions of the hydrated electron with a wide variety of substrates have been found to exhibit unusually similar activation energies in a manner incompatible with Marcus electron transfer theory. Given the fundamental linear response assumption of Marcus theory, one possible explanation for this apparent failure is that the underlying free energy surfaces governing the reactions are not harmonic; i.e., hydrated electron structural fluctuations exhibit non-Gaussian behavior. In this work, we test this hypothesis by using simulations to calculate the hydrated electron vertical detachment energy distribution. We consider both cavity and noncavity models for the hydrated electron, between which the actual hydrated electron behavior is expected to lie. Our results identify a possible origin for non-Gaussian behavior of the hydrated electron but show that it is not of sufficient magnitude to explain the failure of Marcus theory to describe its reactions. Thus, other explanations must be sought.

Reactivity of Zn+aq in high-temperature water radiolysis.

Reactivity of transients involving Zn+ in high-temperature water radiolysis has been studied in the temperature range of 25-300 °C. The reduced monovalent zinc species were generated from an electron transfer process between the hydrated electron and Zn2+ ions using pulse radiolysis. The Zn+ species can subsequently be oxidized by the radiolytically-produced oxidizing species: ˙OH, H2O2 and ˙H. We find that the absorption of monovalent zinc is very sensitive to the pH of the medium. An absorption maximum at 306-311 nm is most pronounced at pH 7 and the signal then decreases in acidic media where the reducing electrons are competitively captured by protons. At pH values higher than 7, hydroxo-forms of Zn2+ are created and the maximum of the absorption signal begins to shift to the red spectral region. We find that the optical spectrum of Zn+aq cannot be fully explained in terms of a charge-transfer to solvent (CTTS) process, which was previously proposed. Reaction rates of most of the recombination reactions investigated follow the empirical Arrhenius relationship at temperatures up to 200 °C and have been determined at higher temperatures for the first time. A bimolecular disproportionation reaction of Zn+aq is not observed under the conditions investigated.

Failure of molecular dynamics to provide appropriate structures for quantum mechanical description of the aqueous chloride ion charge-transfer-to-solvent ultraviolet spectrum.

The lowest band in the charge-transfer-to-solvent ultraviolet absorption spectrum of aqueous chloride ion is studied by experiment and computation. Interestingly, the experiments indicate that at concentrations up to at least 0.25 M, where calculations indicate ion pairing to be significant, there is no notable effect of ionic strength on the spectrum. The experimental spectra are fitted to aid comparison with computations. Classical molecular dynamic simulations are carried out on dilute aqueous Cl-, Na+, and NaCl, producing radial distribution functions in reasonable agreement with experiment and, for NaCl, clear evidence of ion pairing. Clusters are extracted from the simulations for quantum mechanical excited state calculations. Accurate ab initio coupled-cluster benchmark calculations on a small number of representative clusters are carried out and used to identify and validate an efficient protocol based on time-dependent density functional theory. The latter is used to carry out quantum mechanical calculations on thousands of clusters. The resulting computed spectrum is in excellent agreement with experiment for the peak position, with little influence from ion pairing, but is in qualitative disagreement on the width, being only about half as wide. It is concluded that simulation by classical molecular dynamics fails to provide an adequate variety of structures to explain the experimental CTTS spectrum of aqueous Cl-.

H2 Production in the 10B(n,α)7Li Reaction in Water.

We demonstrate a method for measuring the H2 produced in water from the 10B(n,α)7Li fission reaction. Low energy neutrons from the NIST Center for Neutron Research interact with borate-containing water in a temperature-controlled high pressure cell made from titanium. After exposure for one to several hours, the water is extracted and sparged with argon. H2 entrained in the sparging gas is sampled with a small mass spectrometer. To determine the neutron exposure, a small amount of sodium is included in the borate solution. The water is collected and 24Na activation is measured in a counting apparatus on the following day. The G-value for H2 at room temperature is found to be (1.18 ± 0.10) molecules H2/100eV, in good agreement with previous estimates and recent modeling calculations.

One-electron redox kinetics of aqueous transition metal couples Zn2+/+, Co2+/+, and Ni2+/+ using pulse radiolysis.

The one-electron redox potentials for aqueous metal couples Co2+/+ and Ni2+/+ have been investigated by using pulse radiolysis using their reactions with a series of reference compounds to establish the most positive upper limits of E0. Experiments with Zn+ were also carried out to confirm the characteristic shape of the expected reduction kinetics. Both formate ions and t-BuOH were employed to scavenge ˙OH radicals and ˙H atoms. Kinetics and fitted first and second order reaction rates have been reported for reactions with methyl viologen, fluorescein, Ru(NH3)63+, Co(en)32+, Co(sepulcrate)3+, Ru(bpy)32+, Cr(bpy)33+, and Ni(Me6[14]4,11-dieneN4)2+. Previous work demonstrated that both Co2+ and Ni2+ can be reduced by CO2˙- radicals, giving a negative E0 limit of -1.9 V vs. SHE. A definite reaction of Ni+ with fluorescein di-anions provides a new upper limit of the Ni2+/+ couple as -0.906 V vs. SHE. The reaction of Co+ with Ru(bpy)32+ has been confirmed, giving E0 = -1.3 V vs. SHE as a rigorous upper limit of the Co2+/+ couple. In the case of Co2+/+, kinetics were complicated by a self-catalyzed metal clustering phenomenon. Initiation rate constants of this process have also been reported.

Chemical Analysis of Secondary Electron Emission from a Water Cathode at the Interface with a Nonthermal Plasma.

When a nonthermal plasma and a liquid form part of the same circuit, the liquid may function as a cathode, in which case electrons are emitted from the liquid into the gas to sustain the plasma. As opposed to solid electrodes, the mechanism of this emission has not been established for a liquid, even though various theories have attempted to explain it via chemical processes in the liquid phase. In this work, we tested the effects of the interfacial chemistry on electron emission from water, including the role of pH as well as the hydroxyl radical, the hydrogen atom, the solvated electron, and the presolvated electron; it was found that none of these species are critical to sustain the plasma. We propose an emission mechanism where electrons, generated from ionized water molecules in the uppermost monolayers of solution, are emitted into the plasma directly from the conduction band of the water.

γ-Radiolysis of Room-Temperature Ionic Liquids: An EPR Spin-Trapping Study.

The radiolytic stability of a series of room-temperature ionic liquids (ILs) composed of bis(trifluoromethylsulfonyl)imide anion (Tf2N-) and triethylammonium, 1-butyl-1-methylpyrrolidinium, trihexyl(tetradecyl)phosphonium, 1-hexyl-3-methylpyridinium, and 1-hexyl-3-methylimidazolium (hmim) cations was studied using spin-trap electron paramagnetic resonance (EPR) spectroscopy with a spin-trap α-(4-pyridyl N-oxide)-N-tert-butylnitrone (POBN). The trapped radical yields were measured as a function of POBN concentration and as a function of radiation dose by double integration of the broad unresolved lines. Well-resolved motionally narrowed EPR spectra for the trapped radicals were obtained by dilution of the ILs with CH2Cl2 after irradiation. The trapped radicals were identified as mainly carbon-centered alkyl and •CF3, and their ratio varies greatly across the series of ILs. Expected nitrogen-centered radicals derived from Tf2N- were not observed. The hmim liquid proved most interesting because a large part of the trapped radical yield (entirely carbon-centered) grew in over several hours after irradiation. We also discovered a complicated narrow-line stable radical signal in this neat IL with no spin trap added, which grows in over several hours after irradiation and decays over several weeks.

Ultraviolet charge-transfer-to-solvent spectroscopy of halide and hydroxide ions in subcritical and supercritical water.

The temperature dependence of the vacuum ultraviolet charge-transfer-to-solvent (CTTS) absorption spectra of aqueous halide and hydroxide ions was measured for the first time up to 380 °C in subcritical and supercritical water. With increasing temperature, absorption spectra are observed to broaden and redshift, much in agreement with previous measurements below 100 °C. These changes are discussed alongside classic cavity models of the solvated species, which tie in the configuration of the adjoining polarized medium and its critical role in light absorption for electronic transitions. The data seemingly confirm the validity of the "diffuse" model pioneered by Platzman and Franck and later revised by Stein and Treinin, which has largely gone untested for nearly 60 years due to lack of experimental data in this extended temperature range. A gradual increase in anion cavity size is inferred as a function of increasing temperature while the enthalpy and entropy of hydration are largely unaffected. The changes in solvation properties are considered in the context of recent studies of the ultraviolet spectroscopy of subcritical and supercritical water and historic studies of the CTTS absorption. The "diffuse" polarizable continuum model succeeds in describing the absorption due to lack of well-defined ion hydration shells for these ions. CTTS spectra for iodide in supercritical water show no energy shift as a function of pressure/density, suggesting dielectric saturation of the I- anion by the adjacent H2O molecules at all experimental pressures/densities.

Partial Molar Volume of the Hydrated Electron.

The partial molar volume of the hydrated electron was investigated with pulse radiolysis and transient absorption by measuring the pressure dependence of the equilibrium constant for e-aq + NH4+ ⇔ H + NH3. At 2 kbar pressure, the equilibrium constant decreases relative to 1 bar by only 6%. Using tabulated molar volumes for ammonia and ammonium, we have the result V̅(e-aq) - V̅(H) = 11.3 cm3/mol at 25 °C, confirming that V̅(e-aq) is positive and even larger than the hydrophobic H atom. Assuming on the basis of recent molecular dynamics simulations that the molar volume of the H atom is somewhat less than that of H2, we estimate V̅(e-aq) = 26 ± 6 cm3/mol. The positive molar volume is consistent with an electron that exists largely in a small solvent void (cavity), ruling out a recent model ( Larsen , R. E. ; Glover , W. J. ; Schwartz , B. J. Science 2010 , 329 , 65 - 69 ) that suggests a noncavity structure with negative molar volume.

Total Internal Reflection Absorption Spectroscopy (TIRAS) for the Detection of Solvated Electrons at a Plasma-liquid Interface.

The total internal reflection absorption spectroscopy (TIRAS) method presented in this article uses an inexpensive diode laser to detect solvated electrons produced by a low-temperature plasma in contact with an aqueous solution. Solvated electrons are powerful reducing agents, and it has been postulated that they play an important role in the interfacial chemistry between a gaseous plasma or discharge and a conductive liquid. However, due to the high local concentrations of reactive species at the interface, they have a short average lifetime (~1 µs), which makes them extremely difficult to detect. The TIRAS technique uses a unique total internal reflection geometry combined with amplitude-modulated lock-in amplification to distinguish solvated electrons' absorbance signal from other spurious noise sources. This enables the in situ detection of short-lived intermediates in the interfacial region, as opposed to the bulk measurement of stable products in the solution. This approach is especially attractive for the field of plasma electrochemistry, where much of the important chemistry is driven by short-lived free radicals. This experimental method has been used to analyze the reduction of nitrite (NO2-(aq)), nitrate (NO3-(aq)), hydrogen peroxide (H2O2(aq)), and dissolved carbon dioxide (CO2(aq)) by plasma-solvated electrons and deduce effective rate constants. Limitations of the method may arise in the presence of unintended parallel reactions, such as air contamination in the plasma, and absorbance measurements may also be hindered by the precipitation of reduced electrochemical products. Overall, the TIRAS method can be a powerful tool for studying the plasma-liquid interface, but its effectiveness depends on the particular system and reaction chemistry under study.

Vacuum ultraviolet spectroscopy of the lowest-lying electronic state in subcritical and supercritical water.

The nature and extent of hydrogen bonding in water has been scrutinized for decades, including how it manifests in optical properties. Here we report vacuum ultraviolet absorption spectra for the lowest-lying electronic state of subcritical and supercritical water. For subcritical water, the spectrum redshifts considerably with increasing temperature, demonstrating the gradual breakdown of the hydrogen-bond network. Tuning the density at 381 °C gives insight into the extent of hydrogen bonding in supercritical water. The known gas-phase spectrum, including its vibronic structure, is duplicated in the low-density limit. With increasing density, the spectrum blueshifts and the vibronic structure is quenched as the water monomer becomes electronically perturbed. Fits to the supercritical water spectra demonstrate consistency with dimer/trimer fractions calculated from the water virial equation of state and equilibrium constants. Using the known water dimer interaction potential, we estimate the critical distance between molecules (ca. 4.5 Å) needed to explain the vibronic structure quenching.