Harry O. Finklea

Preparation and reversible behavior of organized thiol monolayers with attached pentaminepyridineruthenium redox centers

Electroactive monolayers can be formed in which the redox centers are held at a fixed and controllable distance from the electrode. Thiols with attached redox centers (HS(CH2)n CONHCH2pyRu(NH3)52+/3+, n = 10, 11, 15) readily adsorb from acetonitrile solutions onto gold electrodes to form electroactive monolayers. Mixed monolayers are synthesized by co-adsorbing the electroactive thiols with diluent alkanethiols (HS(CH2)nCH3, n = 11, 15) and ω-mercaptoalkanecarboxylic acids (HS(CH2)nCOOH, n = 10, 11, 15). Cyclic voltammetry of the coated electrodes shows a stable redox wave near 0 V vs. SCE in pH 4 aqueous electrolyte. Coverage of the redox centers in mixed monolayers is controllable by adjusting the mole ratio of the electroactive thiol to the diluent thiol in the deposition solution. At sufficiently slow scan rates, cyclic voltammograms of the electroactive monolayers are nearly ideal (ΔEp = 0 mV and ΔEfwhm = 90 to 100 mV) for both diluent thiols and for all coverages of the redox centers. The evidence indicates that the monolayers are well ordered and that the redox centers reside at the interface between the aqueous phase and the hydrocarbon phase. These monolayers are suitable for the study of electron tunneling across the monolayer.

Electrosorption of uranium on carbon fibers as a means of environmental remediation

Abstract Uranium-containing aqueous wastes have been treated by electrosorption on a carbon electrode composed of vapor-grown fibers in a continuous flow-through cell. Effective uranium (VI) removal is accomplished when a negative potential in the range of −0.45 to −0.9 V (vs. Ag/AgCl) is applied to the carbon electrode. For a feed concentration of 100 mg/l, the concentration of U(VI) in the cell effluent is reduced to less than 100 μg/l. The adsorbed uranium is stripped from the carbon fiber by passing a 0.1 M KNO 3 solution through the cell and applying a positive potential on the electrode. Almost all of the stripped uranium is removed as a suspended precipitate and recovered in solid form by filtration. A sorption capacity over 1.20 g uranium /g carbon is reached. The electro-adsorbed uranium is mainly in the form of uranyl hydroxide (UO 3 ·H 2 O), indicating very limited reduction of U(VI) to U(IV) and precipitation of U(IV). It is proposed that ion exchange and double layer charging are the dominant mechanisms for electrosorption of uranium at potentials less negative than −0.3 V, whereas surface-induced precipitation of uranyl hydroxide (UO 3 ·H 2 O) occurs at more negative potentials, thereby greatly enhancing the sorption capacity.

The Preparation of TiO2 Electrodes with Minimum Mott‐Schottky Frequency Dispersion

A preparation procedure for is described which produces electrodes with nearly ideal Mott‐Schottky plots in . The Mott‐Schottky plots are linear and exhibit slopes and intercepts nearly independent of applied frequency from 105 Hz to 50 KHz. Based on Mott‐Schottky intercepts and photocurrent onset potentials, vs. SCE in . Frequency dispersion increases when the electrodes are soaked in , examined in , or used to generate photocurrent in . The origins and possible remedies for frequency dispersion are discussed. The need for statistical analyses of Mott‐Schottky plots over a range of frequencies is emphasized.

Ac voltammetry studies of electron transfer kinetics for a redox couple attached via short alkanethiols to a gold electrode

Abstract Ac voltammetry (ACV) is used to determine the chain length dependence of electron transfer kinetics between gold bead electrodes and ruthenium redox centers attached to the electrode surface via short alkanethiols. The pentaamminepyridine ruthenium redox centers are attached to pre-assembled monolayers of mercaptoalkanecarboxylic acids (HS(CH 2 ) m COOH, m =5, 7, 10). Equations for faradaic admittance of strongly adsorbed, non-interacting electroactive species are fitted to the experimental faradaic admittance data. Kinetic heterogeneity is suggested to be a probable reason for the apparent increase in the standard rate constant ( k s ) and decrease in the total coverage of redox centers ( θ total ) as the perturbation frequency increases. Because of the frequency dependence of k s and because of limitations in the correction for uncompensated resistance, faradaic admittance at a fixed phase angle of 70° is chosen to compare k s for different chain lengths. The plot of ln( k s ) versus m (the number of methylene units) is linear with a slope of −1.2±0.1 per CH 2 .

Consequences of a potential-dependent transfer coefficient in ac voltammetry and in coupled electron-proton transfer for attached redox couples

Abstract The Marcus density-of-states model for simple electron transfer predicts that the transfer coefficient is dependent on overpotential. The nature of the potential dependence is a function of the reorganization energies associated with oxidation and reduction processes. A fifth-order polynomial expression accurately yields the potential dependence of the transfer coefficient and the resulting curved Tafel plots. With this polynomial expression, the effects of the potential-dependent transfer coefficient are examined for two cases, ac voltammetry of an attached redox molecule with simple electron transfer and the kinetic behavior of the 1-electron/1-proton redox system. Simulations of ac voltammograms indicate that the effects are minor and that ac voltammetry is poorly suited for determination of the reorganization energy of the redox molecule. In the coupled electron–proton redox case, the effects are marked. As expected, the apparent standard rate constant decreases dramatically at pH values between the pKa values of the two oxidation states. More surprisingly, the simulated Tafel plots exhibit asymmetry between the anodic and cathodic branches depending on the pH. The path of electron transfer from the oxidized to the reduced species (electron–proton or proton–electron) at a fixed pH depends on the electrode potential.

Coupled electron/proton transfer of galvinol attached to SAMs on gold electrodes

The theory for a 1-electron, 1-proton redox couple with potential-dependent transfer coefficients is tested using a self-assembled monolayer with attached galvinol/galvinoxyl redox couples on a gold electrode. The potential dependence of the transfer coefficients is derived from the Marcus density-of-states model of heterogeneous electron transfer. The pKa of surface-attached galvinol is 11.3. The predicted asymmetry in kinetically controlled cyclic voltammograms is observed at pHs lower than 11. Apparent standard rate constants between pH 10 and 13 are fitted to the theory to yield a standard rate constant of 4.5 × 10 s−1 for the deprotonated galvinol/galvinoxyl radical redox couple; this standard rate constant is comparable to the standard rate constant for ferrocene/ferricenium with the same number of atoms linking the redox center to the electrode. Anomalous behavior is seen for the apparent standard rate constants at pHs between 7 and 9.

Proton coupled electron transfer of galvinol in self-assembled monolayers

Abstract Proton coupled electron transfer (PCET) half-reactions play a vital role in a variety of biological and chemical processes. A previous kinetic analysis of proton coupled redox couples on electrodes assumes that electron and proton transfers proceed stepwise and that the transfer coefficient for electron transfer steps remains constant at all overpotentials. This work explores the consequences of the stepwise model with a potential-dependent transfer coefficient using a surface-attached 1-electron/1-proton-redox couple. The potential dependence of the transfer coefficient is based on the Marcus density-of-states model for heterogeneous electron transfer. Galvinol is attached to a self-assembled monolayer on a gold electrode and its voltammetry investigated from pH 2 to 14. The formal potential, standard rate constant and p K a for galvinoxyl/deprotonated galvinol/protonated galvinol are obtained from the pH dependence of the apparent formal potential and apparent standard rate constant. The standard rate constant for galvinoxyl/deprotonated galvinol (5600 s −1 ) suggests that the reorganization energy of the galvinol system is comparable to or higher than the reorganization energy of Ru(NH 3 ) 5 (pyridine) complex (0.8 eV). Deviations from the predicted pH dependence of the apparent standard constant are seen at pH less than 9. The deviations suggest that the assumption of stepwise electron and proton transfer should be questioned.

Electrochemical and Electrochemically Modulated Reflectance AC Voltammetry Studies of Electron Transfer Kinetics Between Attached Redox Centers and a Mirror Gold Electrode

Small amplitude electrochemical ac voltammetry (ACV) is employed to investigate kinetics of electron transfer between Ru redox centers attached to the electrode surface via C 10 alkanethiols and gold mirror electrodes. The equations for faradaic admittance of strongly adsorbed electroactive species in the case of a Langmuir isotherm are applied to determine the total coverage of redox centers (θ total ) and kinetic parameters: the standard rate constant (k s ) and transfer coefficient, k s appears to increase and θ total appears to decrease as the perturbation frequency increases. In a separate experiment, large amplitude ACV is performed simultaneously with electrochemically modulated reflectance ac voltammetry (EMR ACV) on the same electroactive monolayer/electrode system. The electromodulation reflectance coefficient (X) is defined in the frequency domain as a ratio of ac electroreflectance to both dc electroreflectance and the interfacial ac potential (E inter ). X is shown to be a more useful quantity than the ratio of ac electroreflectance to de electroreflectance for representation of the electroreflectance data. X is found to be exactly out-of-phase with the faradaic admittance at the wavelength region (410-440 nm) corresponding to the absorption band of the reduced form of the Ru complex. Therefore, the ac electroreflectance signal is dominated by the modulation of the electrode coverage of a given redox state with E inter . Electrochromic effects are negligible. Accurate calculation of k s from EMR ACV data is complicated because of the nonlinear relationship between the faradaic ac current and large amplitude E inter . Thus, under our experimental conditions, ACV is the preferred method to determine k s .

Synthesis and characterization of N-2-aryl-1,2,3-triazole based iridium complexes as photocatalysts with tunable photoredox potential

N-2-Aryl chelated 1,2,3-triazole-Ir(III) complexes with various substituents were prepared for the first time. These photoactive Ir(III) complexes were characterized by X-ray crystallography and their redox potentials were evaluated. This study revealed a new class of photocatalysts with tunable photoredox potentials.

Overview of SOFC Anode Interactions with Coal Gas Impurities

An overview of the results of SOFC anode interactions with phosphorus, arsenic, selenium, sulfur, antimony, and hydrogen chloride as single contaminants or in combinations is discussed. Tests were performed using both anode- and electrolyte-supported cells in synthetic and actual coal gas for periods greater than 1000 hours. Post-test analyses were performed to identify reaction products formed and their distribution, and compared to phases expected from thermochemical modeling. The ultimate purpose of this work is to establish maximum permissible concentrations for impurities in coal gas, to aid in the selection of appropriate coal gas clean-up technologies.