Photocurrent kinetics at the electron emission from a metal into electrolyte solution: Part I. Basic relations

A solution of the equation of diffusion describing the photocurrent kinetics created by photoemission of electrons from a metal electrode into the electrolyte solution is presented. The time dependence of the emitted charge involving the rate constant of the solvated electron capture by acceptor, the diffusion coefficient, the life time and the rate constant of electrode reactions of the product and its further transformation, which serve as parameters, has been obtained. Some particular cases are considered, in which these parameters may be obtained experimentally.     

Photocurrent kinetics at the electron emission from metal into electrolyte solution X. Discharge of short-lived intermediate species

Electrode reactions of intermediate species (IS), generated by a short pulse of laser photoemission (LPE), result in the time-dependent change of emitted charge Q(t). Analytical expressions for the kinetic curves Q(t) are derived by solving non-stationary diffusion equations for e⁻_aq and IS. For the IS adsorption Gibbs energy less than −25 kJ mol ⁻¹, kinetic curves are exponential over the very wide range of electrode reaction rate constant W, from 1 up to 10⁷ s⁻¹. The dependence Q(t) ∝ t⁻¹² is typical for the case of activated adsorption of IS or their discharge from the non-adsorbed state. Voltammograms of IS generated by pulse radiolysis, modulated photolysis and pulsed or alternating photoemission current are demonstrated to be described by similar expressions. The difference between half-wave and equilibrium potentials depends on the reactant and product lifetimes and rates of desorption. A characteristic trapezoid of Tafel lines is introduced as a new way to characterize completely the kinetics of two-electron electrode reactions. The relations obtained were applied to the analysis of hydrogen evolution reactions and carbon dioxide and formaldehyde reduction, where hydrogen atoms and organic radicals HCO₂ and CH₂OH adsorbed on a mercury electrode participate as IS.     

Photocurrent kinetics at the electron emission from a metal into electrolyte solution: Part II. Solutions without acceptors and solutions of N2O,NO2−, NO3−

A method of measuring the kinetics of currents arising at the electron photoemission from a metal into electrolyte solution when affected by the u.v. laser pulses for 10^?8 s at the frequency of repetitions 10–25 Hz is described. Measurements have been taken in solutions without acceptors and in those containing N₂O and NO₂^?, NO₃^? ions as electron acceptors. The rate constants of capture of the solvated electrons by N₂O ((6±1)×0⁹ mol^?1 s^?1) and NO₂^? ((4.5±1)×10⁹ mol^?1 s^?1) and the diffusion coefficients of OH-radicals ((1.0±0.3)×10^?5 cm² s^?1) and of NO ((1.2±0.3)×10^?5 cm² s^?1) are found. The oxidation rate of NO₃^2? has been shown to decrease from 40 cm s^?1 in the range of potentials ?0.55 to ?1.0 V. The rate constant of bimolecular recombination of the solvated electrons ((1.3±0.4)×10¹⁰ mol^?1 s^?1) has been found from the dependence of the emitted charge on the light intensity.     

Photocurrent kinetics at electron emission from metal to electrolyte solution: Part V. Ionization rate constants of hydrogen and deuterium atoms adsorbed on mercury

Absolute ionization rate constant values of hydrogen and deuterium atoms adsorbed on mercury were measured using the method of pulse photoelectronic emission from metal into solution. In accordance with the Tafel law, these constants decrease from 2.5×10⁷s^?1 to 9×10⁵s^?1 (Table 1) in a 1 M solution of KCl in the range ?0.25 to ?0.5 V SCE. The transfer coefficient is 0.33±0.03 and the isotope ratio about 2.5. Owing to specific anion adsorption, rate constants increase as their concentration increases and KBr is added to the solution. In 0.05 M solutions of HCl and H₂SO₄, transfer coefficients are 0.30±0.05. From a comparison of measured values, with the hydrogen ion discharge rate constants found by extrapolation of experimental values into the potential range mentioned (taking into account the transfer coefficient change), the change of the Gibbs free energy in the reaction H₃O⁺+e_Me^?→H_ads+H₂O was calculated and found to be 0.87–0.99 eV at the potential of the normal hydrogen electrode. Adsorption energy of the hydrogen atom from the gas phase on a mercury electrode is 1.55±0.10 eV.The volt-ampere dependence of the hydrogen ion discharge current in the range ?0.25 to ?2.25 V corresponding to the current change by 18 orders of magnitude, agrees with the theoretically determined values (maximum deviation in the current is less than a factor of 3) for the medium reorganization energy E_r=1.75 eV. Despite constancy of the transfer coefficients of the elementary stages, in the range ?0.5 V (SCE), the effective transfer coefficient of the total hydrogen evolution processes increases from 0.5 to 1.0, as the ionization rate of the adsorbed hydrogen atoms becomes greater than their electrochemical desorption rate.     

Thermodynamics of electrolyte solutions. The system HCl + CaCl2 + H2O at 298.15°K

Activity coefficients of hydrochloric acid have been determined from electromotive-force measurements of cells containing mixtures of hydrochloric acid and calcium chloride at constant total ionic strengthsI=0.1, 0.5, 1.0, 2.0, and 3.0 mole-kg^–1 at 298.15°K. Interpretations based on Scatchard's and Pitzer's equations indicate that Pitzer's equations probably provide a more convenient guide to the thermodynamic properties of the mixed-electrolyte solutions. Activity coefficients for calcium chloride were derived from these equations.     

稀散金属化合物水溶液热力学研究 1: HCL+GaCl3+H2O体系

在HCl+GaCl3+H2O体系中,恒定五个总离子强度I=0.4,0.6,0.8,1.0,1.5mol/kg,控制混合电解质中氯化镓离子强度分数YB=0.0,0.1,0.3,0.5,0.7,并在278.15～318.15K范围内测定了五个温度的无液接电池：Pt|H2(101.325kPa)|HCl(mA),GaCl3(mB),H2O|AgCl|Ag的电动势。根据150个实验点的电动势数据,确定了HCl的活系数及其随氯化镓浓度变化规律,结果发现HCl活度系数遵守Harned规则。同时本文在Pitzer电解质溶液理论基础上提出一个确定氯化镓的pitzer参数和活度系数的方法,指出了氯化镓在这个混合电解质溶液中遵守扩展的Harned规则。     

Vapor pressures of non-aqueous electrolyte solutions. Part 1. Alkali metal salts in methanol

Precise vapor pressure data for solutions of NaCl, NaBr, NaI, NaClO₄, KBr, KI, RbI, and CsI in methanol at 25°C in the concentration range 0.02m (mol-kg^–1)0.7 are communicated and discussed. Measurements were carried out by procedures and equipment known to produce data of high precision. Polynomials of fourth degree in molalities are given for the vapor pressure of methanol solutions which can be used for calculating precise reference values as needed in indirect methods. Osmotic coefficients were calculated by taking into account the second virial coefficient of methanol vapor. Discussion of the data is based on the chemical model of electrolyte solutions taking into account short range interactions. Ion-pair association constants are compared to those of conductance measurements.     

Chemical lasers produced from O(3P) atom reactions. III. 5-μm CO laser emission from the O + CH reaction

Very strong laser emission at 5 μm was detected when SO₂ and CHBr₃ were flash photolyzed in the vacuum ultraviolet (λ ≥ 165 nm) in the presence of a large amount of diluent (SF₆, He, or Ar). About 110 vibration–rotation transitions ranging from Δv = 18 → 17 to 3 → 2, except 16 → 15, were identified. The primary reactions leading to the CO stimulated emission are as follows: The product analysis results and the variation of laser intensity with flash energy and SO concentration indicate that the following side reactions are also occurring. Addition of a small amount of O₂ enhances the laser output by both eliminating these side reactions and simultaneously producing vibrationally excited CO via reaction (8), which has been previously shown to generate CO stimulated emission. The effects of various reactive (NO and H₂) and inert (He, Ar, SF₆, CO, N₂, N₂O, and CO₂) gases have been examined. All additives (P ≤ 20 torr), except NO and H₂, increase the total laser output. N₂O enhances the power most efficiently, whereas CO, N₂, and CO₂ are less effective and have similar efficiencies. The enhancement of the laser intensity by these near-resonant gases is ascribed to the depletion of CO population at lower levels which thus increases the rates cascading from higher levels. NO and H₂ quench the laser output by chemically reducing the concentration of the CH radical.     

Modeling aqueous electrolyte solutions: Part 1. Fully dissociated electrolytes

Liquid densities (pvT), vapor pressures (VLE), and mean ionic activity coefficients (MIAC) at 25 °C of 115 single-salt electrolyte solutions containing univalent up to trivalent ions are modeled with the ePC-SAFT equation of state proposed by Cameretti et al. [L.F. Cameretti, G. Sadowski, J.M. Mollerup, Ind. Eng. Chem. Res. 44 (2005) 3355–3362; ibid., 8944]. For each ion, only two model parameters were adjusted to experimental density and MIAC data. Without using any additional binary parameters, ePC-SAFT is able to reproduce experimental data of the respective salt solutions up to high electrolyte molalities. Moreover, it is even able to describe the reversed MIAC series for alkali hydroxides and fluorides.     

Solid electrolyte interphase (SEI) electrode: Part III. Deposition-dissolution process of lithium in thionyl-chloride solution

The deposition-dissolution mechanism of lithium on stainless steel and calcium electrodes in 1 M LiAlCl₄ -thionyl-chloride solution is studied by pulse galvanostatic and ac techniques. The metal -solution interfacial capacitance of the stainless steel electrode is about 30 μF cm^?2 which is higher by an order of magnitude than the capacitance of lithium-coated stainless steel and either pure or lithium-coated calcium. The lower capacitance is attributed to the existence of a solid electrolyte interphase (SEI) on the coated stainless steel or the calcium electrode.Significantly different is observed upon deposition of lithium on stainless steel or calcium. Deposition on stainless steel takes place only after prior formation of a SEI on the electrode (by passage of about 20 mC cm^?2), while deposition on calcium starts immediately after the electrode capacitance has been charged (by about 5 μC cm^?2). Furthermore, deposition of about 3% of a monolayer of lithium on calcium is enough to stabilize its potential at 0.0 V vs. LiRE.On the lithium-coated stainless steel electrode, a linear relationship between the current and over-potential is observed for up to 700 mV. This indicates a Tafel slope > V. During lithium deposition on stainless steel, the SEI resistivity is about 1.5 × 10⁷ Ω-cm and its thickness is about 10 nm.Under open circuit potential, the deposited lithium corrodes at an apparent rate of 100 μA cm^?2. Rapid fluctuations of the electrode potential during the corrosion or dissolution process are accounted for by a break and repair mechanism of metallic contact between lithium deposited within the SEI and the current collector.     

Freezing points and related properties of electrolyte solutions. III. The systems NaCl−CaCl2−H2O and NaCl−BaCl2−H2O

The freezing temperatures of aqueous calcium chloride and barium chloride and their mixtures with sodium chloride were measured at equivalent molalities of 0.1 to 1.5 mole-kg^?1. Osmotic and activity coefficients of the mixtures were calculated at the freezing points of the mixtures. From the freezing points and calorimetric enthalpies of mixing of sodium chloride-magnesium chloride solutions, osmotic and activity coefficients were calculated at 298°K. Agreement of the calculated properties with isopiestic and electrochemical measurements at 298°K is excellent.     

Jet cooled spectroscopy of H2DO+: Barrier heights and isotope-dependent tunneling dynamics from H3O+ to D3O+

The first high resolution spectroscopic data for jet cooled H2DO+ are reported, specifically via infrared laser direct absorption in the OH stretching region with a slit supersonic jet discharge source. Transitions sampling upper (0-) and lower (0+) tunneling states for both symmetric (nu1+ <-- 0+, nu1- <-- 0-, and nu1- <-- 0+) and antisymmetric (nu3+ <-- 0+ and nu3- <-- 0-) OH stretching bands are observed, where +/- refers to wave function reflection symmetry with respect to the planar umbrella mode transition state. The spectra can be well fitted to a Watson asymmetric top Hamiltonian, revealing band origins and rotational constants for benchmark comparison with high-level ab initio theory. Of particular importance are detection and assignment of the relatively weak band (nu1- <-- 0+) that crosses the inversion tunneling gap, which is optically forbidden in H3O+ or D3O+, but weakly allowed in H2DO+ by lowering of the tunneling transition state symmetry from D(3h) to C(2v). In conjunction with other H2DO+ bands, this permits determination of the tunneling splittings to within spectroscopic precision for each of the ground [40.518(10) cm(-1)], nu1 = 1 [32.666(6) cm(-1)], and nu3 = 1 [25.399(11) cm(-1)] states. A one-dimensional zero-point energy corrected potential along the tunneling coordinate is constructed from high-level ab initio CCSD(T) calculations (AVnZ, n = 3,4,5) and extrapolated to the complete basis set limit to extract tunneling splittings via a vibrationally adiabatic treatment. Perturbative scaling of the potential to match splittings for all four isotopomers permits an experimental estimate of DeltaV0 = 652.9(6) cm(-1) for the tunneling barrier, in good agreement with full six-dimensional ab initio results of Rajamaki, Miani, and Halonen (RMH) [J. Chem. Phys. 118, 10929 (2003)]. (DeltaV0 (RMH) = 650 cm(-1)). The 30%-50% decrease in tunneling splitting observed upon nu1 and nu3 vibrational excitations arises from an increase in OH stretch frequencies at the planar transition state, highlighting the transition between sp2 and sp3 hybridizations of the OHD bonds as a function of inversion bending angle.     

Kinetic studies of atmospherically relevant silicon chemistry. Part III: Reactions of Si+ and SiO+ with O3, and Si+ with O2

Silicon ions are generated in the Earth's upper atmosphere by hyperthermal collisions of material ablated from incoming meteoroids with atmospheric molecules, and from charge transfer of silicon-bearing neutral species with major atmospheric ions. Reported Si(+) number density vs. height profiles show a sharp decrease below 95 km, which has been commonly attributed to the fast reaction with H(2)O. Here we report rate coefficients and branching ratios of the reactions of Si(+) and SiO(+) with O(3), measured using a flow tube with a laser ablation source and detection of ions by quadrupole mass spectrometry. The results obtained are (2σ uncertainty): k(Si(+) + O(3), 298 K) = (6.5 ± 2.1) × 10(-10) cm(3) molecule(-1) s(-1), with three product channels (branching ratios): SiO(+) + O(2) (0.52 ± 0.24), SiO + O(2)(+) (0.48 ± 0.24), and SiO(2)(+) + O (<0.1); k(SiO(+) + O(3), 298 K) = (6 ± 4) × 10(-10) cm(3) molecule(-1) s(-1), where the major products (branching ratio ≥ 0.95) are SiO(2) + O(2)(+). Reactions (1) and (2) therefore have the unusual ability to neutralise silicon directly, as well as forming molecular ions which can undergo dissociative recombination with electrons. These reactions, along with the recently reported reaction between Si(+) and O(2)((1)Δ(g)), largely explain the disappearance of Si(+) below 95 km in the atmosphere, relative to other major meteoric ions such as Fe(+) and Mg(+). The rate coefficient of the Si(+) + O(2) + He reaction was measured to be k(298 K) = (9.0±1.3) × 10(-30) cm(6) molecule(-2) s(-1), in agreement with previous measurements. The SiO(2)(+) species produced from this reaction, which could be vibrationally excited, is observed to charge transfer at a relatively slow rate with O(2), with a rate constant of k(298 K) = (1.5 ± 1.0) × 10(-13) cm(3) molecule(-1) s(-1).     

Experimental determination of the ψ′-potential by means of electron photoemission from metals into electrolyte solutions

The measurements of the photoelectron emission current from a metal electrode into electrolyte solution can be used for direct determination of the ψ′-potential. The limits of applicability of this method are discussed. The values of the ψ′-potential measured at mercury and lead electrodes in HCl solutions are in good agreement with those calculated by means of the Gouy-Chapman theory.     

Thermodynamic quantities of surface formation of aqueous electrolyte solutions VII. Aqueous solution of alkali metal nitrates LiNO3, NaNO3, and KNO3

To compare the effect of nitrate anions on the surface tension increments of aqueous solutions with that of halide anions, the surface tension of aqueous solutions of lithium nitrate, sodium nitrate, and potassium nitrate was measured as a function of temperature and concentration. It is shown that the surface tension of aqueous alkali metal nitrate solutions is determined primarily by the kinds of anions, since the surface tension increments of these nitrates were of the same magnitude. The importance of the electrical double layer at the surface is discussed in relation to these surface tension increments.     

Comparison of the kinetics of cathodic H2 evolution from the unhydrated H3O+ ion and its hydrated form H9O: Frequency factor effects and modes of activation

Experiments are described in which the kinetics of cathodic hydrogen evolution from the unhydrated H₃O⁺ ion in pure CF₃SO H₃O⁺ are compared with those from an aqueous solution of CF₃SO₃H where the proton is mainly in a fully hydrated state as H₉O. From the acid hydrate, which exists mainly as the ionic compound CF₃SOH₃O⁺, rates of H₂ evolution at Ni, Pt, and Hg electrodes, measured at a given overpotential or expressed as exchange current densities, are between about 3.5 and 20 times slower than those from the same electrolyte in dilute (1.0M) aqueous solution. Allowing for the concentration differences in these two types of system and double-layer effects, the rate constants are between about 9.4 and 216 times smaller for the reaction from H₃O⁺ than from H₉O at the above electrodes. The evaluation of apparent heats of activation for H₂ evolution from the two types of proton sources allows ratios of real frequency factors to be calculated for discharge from H₃O⁺ and H₉O. These data have a bearing on the theoretical conclusions regarding proton discharge mechanisms and show that frequency factor effects can be as important as activation energy differences in determining the rates of proton discharge from different proton sources. The results are discussed in terms of current ideas about electron and proton transfer in electrochemical reactions, the state of hydration of H⁺, and the role of discharge from paired CF₃SO and H₃O⁺ ions. In particular, the molecular mechanics of discharge of the proton from the molecular ion H₃O⁺ can be different from that from the fully hydrated H⁺ ion where many more HO- vibrational and librational modes can be involved in the process of activation of the H₉O entity.     

Total cross sections for electron scattering from molecules: NH3 and H2O

By developing the semi-empirical formula recently obtained for total cross sections of electron scattering from diatomic molecules in the intermediate- and highenergy range, we calculate the total cross sections for electron scattering from molecules (NH₃ and H₂O) over an incident energy range of 10–1000 eV. The total cross sections have also been calculated by using the complex optical potential and the additivity rule. Compared with other available experimental and calculating data, excellent agreements have been achieved. The developed semi-empirical formula reflects that total cross sections for electron scattering from NH₃ and H₂O in the intermediate- and high-energy range quantitatively depend on the bond length.     

Electrophoresis of a highly charged fluid droplet in dilute electrolyte solutions: Analytical Hückel-type solution

An analytical formula is presented here for the electrophoresis of a dielectric or perfectly conducting fluid droplet with arbitrary surface potentials suspended in a very dilute electrolyte solution. In other words, when the Debye length (κ⁻¹) is very large, or κa$\ll$1, where κ is the electrolyte strength and a stands for the droplet radius. This formula can be regarded as an extension of the famous Hückel solution valid for weakly charged rigid particles to arbitrarily charged fluid droplets. The formula reduces successfully to the ones obtained by Booth for a dielectric droplet, and Ohshima for a perfectly conducting droplet, both under Debye–Hückel approximation valid for weakly charged droplets. Moreover, the formula is valid for a gas bubble and a rigid solid particle as well. Classic results obtained by Hückel for a rigid particle are reproduced as well. We found that for a dielectric droplet, the more viscous the droplet is, the faster it moves regardless of its surface potential, contrary to the intuition based on the purely hydrodynamic consideration. For a perfectly conducting liquid droplet, on the other hand, the situation is reversed: The less viscous the droplet is, the faster it moves. The presence or absence of the spinning electric driving force tangent to the droplet surface is found to be responsible for it. As a result, an axisymmetric exterior vortex flow surrounding the droplet is always present for a dielectric liquid droplet, and never there for a conducting liquid droplet.