Energy-saving seawater electrolysis for hydrogen production

The change in the polarization potentials of anode and cathode due to pH change on electrode surfaces during galvanostatic polarization was examined in 0.5 M NaCl solutions of different pH. On the basis of these results, feeding of the anolyte after oxygen evolution to the cathode compartment for hydrogen production was examined for energy-saving seawater electrolysis. This was assumed to prevent the occurrence of a large pH difference on cathode and anode in electrolysis of neutral solution if sufficient H⁺ is permeated through the membrane. The cell performance was examined using Nafion 115 or Selemion HSF membranes for separation of anode and cathode compartments. The permeation fraction of H⁺ with Nafion 115 was 45–65% in 0.5 M NaCl and was about 90% in 0.25 M Na₂SO₄. These values were smaller than 97% necessary for prevention of the occurrence of pH difference on cathode and anode. The permeation fraction of H⁺ with Selemion HSF became more than 97% during electrolysis of 0.025 M Na₂SO₄, and the cell voltage was kept at low values. These results indicate the effectiveness of our seawater feeding system if the 97% H⁺ permeation fraction through the membrane is attained. Contribution to the Fall Meeting of the European Materials Research Society, Symposium D: 9th International Symposium on Electrochemical/Chemical Reactivity of Metastable, Warsaw, 17th-21st September, 2007.     

Oxygen reduction in acid media by a Ru<Subscript>x</Subscript>Fe<Subscript>y</Subscript>Se<Subscript>z</Subscript>(CO)<Subscript>n</Subscript> cluster catalyst dispersed on a glassy carbon-supported Nafion film

Electrocatalytic oxygen reduction was studied on a Ru_xFe_ySe_z(CO)_n cluster catalyst with Vulcan carbon powder dispersed into a Nafion film coated on a glassy carbon electrode. The synthesis of the electrocatalyst as a mixture of crystallites and amorphous nanoparticles was carried out by refluxing the transition metal carbonyl compounds in an organic solvent. Electrocatalysis by the cluster compound is discussed, based on the results of rotating disc electrode measurements in a 0.5 M H₂SO₄. A Tafel slope of −80.00±4.72 mV dec⁻¹ and an exchange current density of 1.1±0.17×10⁻⁶ mA cm⁻² was calculated from the mass transfer-corrected curve. It was found that the electrochemical reduction reaction follows the kinetics of a multielectronic (n=4e⁻) charge transfer process producing water, i.e. O₂+4H⁺+4e⁻→2H₂O. Electronic Publication     

A novel approach for highly proton conductive electrolyte membranes with improved methanol barrier properties: Layer-by-Layer assembly of salt containing polyelectrolytes

A series of highly proton conductive electrolyte membranes with improved methanol barrier properties are prepared from polyallylamine hydrochloride (PAH) and polystyrene sulfonic acid (PSS) including salt by Layer-by-Layer (LbL) method. The effects of added salt type (NaCl, MgCl₂) and salt concentration (1.0 M, 0.1 M) on proton conductivity (σ) and methanol barrier properties of the LbL self-assembled composite membranes are discussed in terms of controlled layer thickness and charge density. Furthermore, the influences of ion type in the multilayered composite membranes are studied in conjunction with physicochemical and thermal properties.The deposition of the self-assembly of PAH/PSS film on Nafion is followed by UV–Vis spectroscopy and it is observed that the polyelectrolyte layers growth on both sides of Nafion membrane regularly. (PAH/PSS)₅–Na⁺ and (PAH/PSS)₅–H⁺ with 1.0 M NaCl exhibits 49.6 and 27.8% reduction in lower methanol permittivity in comparison with the pristine Nafion^®117, respectively, while the proton conductivities are 12.97 and 74.69 mS cm⁻¹. Promisingly, it is found that the membrane selectivity values (Φ) of all multilayered membranes in H⁺ form are much higher than that of salt form (Na⁺ and Mg²⁺) and perfluorosulfonated ionomers reported in the literature. Also, we find out that the use of polyelectrolytes with high charge density causes a further improvement in proton conductivity and methanol barrier properties simultaneously. These encouraging results indicate that upon a suitable choice of LbL deposition conditions, composite membranes exhibiting both high proton conductivity and improved methanol barrier properties can be tailored for fuel cells.     

Synthesis and behaviour of hybrid polymer-silica membranes made by sol gel process with adsorbed carbonic anhydrase enzyme,in the capture of CO<Subscript>2</Subscript>

Thin nylon-SiO₂ membranes made by sol–gel SiO₂ coating of a nylon weaving were impregnated in a second step with an aqueous carbonic anhydrase solution. The biocatalytic hybrid membranes obtained were applied to the capture of CO₂ from a N₂–CO₂ gas mixture containing 10% CO₂, under a total pressure ≈ 1 atm. The CO₂ permeance of these membranes was at least similar to those previously reported for liquid membranes. When impregnated with a 0.2 mg mL⁻¹ enzyme solution in a pH ≈ 8 NaHCO₃ buffer, the permeance of a nylon-SiO₂ membrane was multiplied by a factor ≈ 3 when the buffer molarity was increased from 0.1 to 1 M. By comparison, this permeance only increased by a factor ≈ 1.3 without any enzyme in the same buffers. The permeance was also higher with the enzyme than without it: respectively ≈3.7 10⁻⁸ and ≈4.7 10⁻⁹ mol \textm_{\textmembrane} ^{- 2} {\text{m}}_{\text{membrane}}^{{^{ - 2} }} s⁻¹ Pa⁻¹ with and without enzyme, in a 1 M NaHCO₃ buffer. A maximum permeance was observed for an enzyme concentration of ≈0.2 mg mL⁻¹, possibly due to a competition between the H⁺ ions produced from CO_2,aq by the enzyme and the H⁺ captured by the buffer. Besides, when the SiO₂–CO₂ contact was enhanced by the membrane architecture, SiO₂ improved the CO₂ permeance. The influence of an in situ CaCO₃ deposit was also investigated and it improved the CO₂ permeance when no enzyme was added.     

Preparation of PFSA/PSf hollow fiber composite membranes with recovered PFSA for the pervaporation separation of EtOH/H2O

Perfluorosulfonic acid/Polysulfone(PFSA/PSf) hollow fiber composite membranes have been prepared by dip-coating method using PSf ultrafiltration (UF) membrane as substrate with recovered PFSA. The composite membranes were applied to the pervaporation separation of 95% ethanol (EtOH)/H₂O mixture. SEM images show that the thickness of the PFSA skin layer of the composite membranes is about 2 μm, much thinner than those of other PFSA composite membranes revealed in the literatures. Effects of annealing temperature, coating solution concentration and counter-ions of PFSA on the pervaporation performances of the composite membranes were investigated. The total flux decreases and separation factor increases with the increase of annealing temperature. The highest permeation flux of 3230 g m^?2 h^?1 and a separation factor of 5.4 is obtained for the composite membrane annealed at 80°C. The lowest permeation flux of 396 g m^?2 h^?1 and a separation factor of 27.7 is obtained for the composite membrane annealed at 160°C. The permeation performances of the PFSA/PSf composite membrane are evidently influenced by the counter-ions of PFSA. The flux sequence of the PFSA/PSf composite membranes with different counter-ions is H⁺>Li⁺>Ca²⁺>Mg²⁺>Na⁺>K⁺>Ba²⁺>Fe³⁺>Al³⁺, and the separation factor sequence is H⁺<Li⁺<Al³⁺<Na⁺<Mg²⁺<Ca²⁺<K⁺<Ba²⁺<Fe³⁺. The apparent activation energy ΔE _app values of the composite membranes with different counter-ions were calculated by Arrhenius law. The sequence of ΔE _app values for the membranes with monovalent counter-ions is Li⁺>Na⁺>K⁺. There are very little variations of ΔE _app values between the composite membranes with three divalent counter-ions (Mg²⁺, Ca²⁺ and Ba²⁺), and the ΔE _app values of the composite membranes with two trivalent counter-ions (Fe³⁺ and Al³⁺) are relatively high.     

Electrochemical characterization of PANI-Nafion membranes and their electrocatalytic activity

PANI-Nafion^® membranes were prepared by a chemical method in which the faces of a commercial Nafion membrane were contacted with two acid solutions containing an oxidant (Fe³⁺) and aniline respectively. They were then characterized by a variety of electrochemical techniques (cyclic voltammetry, chronoamperometry, ac impedance). PANI was stored mostly in the vicinity of the Nafion face exposed to the oxidant. When this face was contacted with Hg and the other with electrolyte solutions, typical electrochemical responses of PANI were detected. Electrocatalytic reduction of O₂ and oxidation of N₂H₄ were achieved on PANI-Nafion electrodes and found to be only slightly slowed down with respect to ordinary PANI film electrodes, at variance with a fast process such as Fe³⁺ reduction, the rate of which was severely limited by diffusion through the membrane.     

Effect of thermal and acid treatment on the structure of NH4Na—Y zeolite by IR data

The structural transformations of ammonium-exchanged forms of Na-Y zeolite during thermal evacuation and acid treatment were studied using IR spectroscopy. The formation of the zeolite H-forms by shallow-bed evacuation of NH₄N-Y at 570 K is accompanied by a high-frequency shift of the bands in the IR lattice vibration spectra. In the formation of the H⁺-forms resulting from the decationization of zeolite by treatment with an aqueous HCl solution, no shifts of the bands are observed. During deep-bed calcination in air the H⁺-form, is transformed into the H-form completed by the formation of a highly-crystalline stabilized zeolite at 623 K. A rapid increase in the shallow-bed calcination temperature results in a collapse of the structure of the H- and H⁺-forms followed by the formation of amorphous SiO₂.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 236–240, February, 1995.     

Mixed-conducting dense ceramic membranes for air separation and natural gas conversion

Non-perovskite SrFeCo_0.5O _x (SFC2) was found to have high electronic and ionic conductivities as well as structural stability. At 800°C in air, total and ionic conductivities of 17 and 7 S·cm⁻¹ were measured, respectively; the ionic transference number was calculated to be ≈0.4. This material is unique because of its high electronic conductivity and comparable electronic and ionic transference numbers. X-ray diffraction analysis showed that air-sintered SFC2 consists of three phase components, ≈75 wt% , ≈20 wt% perovskite , and ≈5 wt% rock salt CoO. Argon-annealed SFC2 contains brownmillerite Sr₂(Fe_1−x Co _x )₂O₅ and rock salt CoO. Dense SFC2 membranes were able to withstand large pO₂ gradients and retain mechanical strength. A 2.9-mm-thick disk membrane was tested in a gas-tight electrochemical cell at 900°C; an oxygen permeation flux rate ≈2.5 cm³(STP)·cm⁻²·min⁻¹ was measured. A dense thin-wall tubular membrane of 0.75-mm thickness was tested in a methane conversion reactor for over 1,000 h. At 950°C, the oxygen permeation flux rate was ≈10 cm³(STP)·cm⁻²·min⁻¹ when the SFC2 thin-wall membrane was exposed with one side to air and the other side to 80% methane balanced with inert gas. Results from these two independent experiments agreed well. The SFC2 material is a good candidate as dense ceramic membranes for oxygen separation from air or for use in methane conversion reactors.     

Study of equilibria and kinetics of the acid catalysed interaction of iron(III) with salicylaldehyde ando-hydroxyacetophenone

The equilibria and kinetics of the reaction of Fe^III with salicylaldehyde ando-hydroxyacetophenone, leading to 1∶1 chelate formation, have been studied at different temperatures (25–35°C) and ionic strength, I = 1.0 mol dm⁻³ (NaClO₄+HClO₄). A dual path mechanism involving both Fe _aq ³⁺ and Fe(OH) _aq ²⁺ species and undissociated free ligand (LH) is consistent with the experimental observations where [H⁺]≫[Fe]_T≫[L]_T (where [Fe]_T and [L]_T stand for total concentrations of iron and ligand respectively). The results conform to k_obs/B = k₁[H⁺]+k₂K_h where B = [Fe]_T/(K_h+[H⁺])+1/Q; K_h = hydrolysis constant of Fe _aq ³⁺ ; k₁, k₂ are the forward second order rate constants of Fe _aq ³⁺ and Fe(OH) _aq ²⁺ , respectively, and Q is the equilibrium constant of the reaction, Fe³⁺+LH⇋FeL²⁺+H⁺. Thermodynamic parameters for each of the steps have been determined. Fe(OH) _aq ²⁺ appears to react in a dissociative fashion (Eigen-Tamm mechanism), whilst Fe _aq ³⁺ appears to react through the associative inter-change (I_a) mechanism. The equilibrium constants (Q) obtained spectrophotometrically are compared with those obtained from kinetic studies. TMC 2638     

Boiling Temperature and Reversed Deliquescence Relative Humidity Measurements for Mineral Assemblages in the NaCl + NaNO3 + KNO3 + Ca(NO3)2 + H2O System

Boiling temperature measurements have been made at ambient pressure for saturated ternary solutions of NaCl + KNO₃ + H₂O, NaNO₃ + KNO₃ + H₂O, and NaCl + Ca(NO₃)₂ + H₂O over the full composition range, along with those of the single salt systems. Boiling temperatures were also measured for the four component NaCl + NaNO₃ + KNO₃ + H₂O and five component NaCl + NaNO₃ + KNO₃ + Ca(NO₃)₂ + H₂O mixtures, where the solute mole fraction of Ca(NO₃)₂, x{Ca(NO₃)₂}, was varied between 0 and 0.25. The maximum boiling temperature found for the NaCl + KNO₃ + H₂O system is ≈134.9 ^∘C; for the NaNO₃ + KNO₃ + H₂O system is ≈165.1 ^∘C at x(NaNO₃) ≈ 0.46 and x(KNO₃) ≈ 0.54; and for the NaCl + Ca(NO₃)₂ + H₂O system is 164.7 ± 0.6 ^∘C at x{NaCl} ≈ 0.25 and x{Ca(NO₃)₂} ≈ 0.75. The NaCl + NaNO₃ + KNO₃ + Ca(NO₃)₂ + H₂O system forms molten salts below their maximum boiling temperatures and the temperatures corresponding to the cessation of boiling (dry-out temperatures) of these liquid mixtures were determined. These dry-out temperatures range from ≈300 ^∘C when x{Ca(NO₃)₂} = 0 to ≥ 400 ^∘C when x{Ca(NO₃)₂} = 0.20 and 0.25. Mutual deliquescence/efflorescence relative humidity (MDRH/MERH) measurements were also made for the NaNO₃ + KNO₃ and NaCl + NaNO₃ + KNO₃ salt mixture from 120 to 180 ^∘C at ambient pressure. The NaNO₃ + KNO₃ salt mixture has a MDRH of 26.4% at 120 ^∘C and 20.0% at 150 ^∘C. This salt mixture also absorbs water at 180 ^∘C, which is higher than expected from the boiling temperature experiments. The NaCl + NaNO₃ + KNO₃ salt mixture was found to have a MDRH of 25.9% at 120 ^∘C and 10.5% at 180 ^∘C. The investigated mixture compositions correspond to some of the major mineral assemblages that are predicted to control brine composition due to the deliquescence of salts formed in dust deposited on waste canisters in the proposed nuclear repository at Yucca Mountain, Nevada.     

Vibrational spectroscopy of intermediates in benzene-to-pheno conversion by FeO<Superscript>+</Superscript>

Gas-phase FeO⁺ can convert benzene to phenol under thermal conditions. Two key intermediates of this reaction are the [HO-Fe-C₆H₅]⁺ insertion intermediate and Fe⁺(C₆H₅OH) exit channel complex. These intermediates are selectively formed by reaction of laser ablated Fe⁺ with specific organic precursors and are cooled in a supersonic expansion. Vibrational spectra of the sextet and quartet states of the intermediates in the O-H stretching region are measured by infrared multiphoton dissociation (IRMPD). For Fe⁺(C₆H₅OH), the O-H stretch is observed at 3598 cm⁻¹. Photodissociation primarily produces Fe⁺+C₆H₅OH; Fe⁺(C₆H₄)+H₂O is also observed. IRMPD of [HO-Fe-C₆H₅]⁺ mainly produces FeOH⁺+C₆H₅ and the O-H stretch spectrum consists of a peak at ∼3700 cm⁻¹ with a shoulder at ∼3670 cm⁻¹. Analysis of the experimental results is aided by comparison with hybrid density functional theory computed frequencies. Also, an improved potential energy surface for the FeO⁺+C₆H₆ reaction is developed based on CBS-QB3 calculations for the reactants, intermediates, transition states, and products.     

Kinetics and mechanism of oxidation of ferrocyanide by N-bromosuccinimide in aqueous acidic solution

The kinetics of oxidation of ferrocyanide by N-bromosuccinimide (NBS) has been studied spectrophotometrically in aqueous acidic medium over temperature range 20–35 °C, pH = 2.8–4.3, and ionic strength = 0.10–0.50 mol dm⁻³ over a range of [Fe²⁺] and [NBS]. The reaction exhibited first order dependence on both reactants and increased with increasing pH, [NBS], and [Fe²⁺]. The rate of oxidation obeys the rate law: d[Fe³⁺]/dt = [Fe(CN)₆]^4–[HNBS⁺]/(k ₂ + k ₃/[H⁺]). An outer-sphere mechanism has been proposed for the oxidation pathway of both protonated and deprotonated ferrocyanide species. Addition of both succinimide and mercuric acetate to the reaction mixture has no effect on the reaction rate under the experimental conditions. Mercuric acetate was added to the reaction mixture to act as scavenger for any bromide formed to ensure that the oxidation is entirely due to NBS oxidation.     

Poly(3,4-ethylenedioxythiophene)-modified nafion membrane for direct methanol fuel cells

Membranes Nafion 117 are modified with poly(3,4-ethylenedioxythiophene) (PEDT) by chemical polymerization of EDT with H₂O₂ or FeCl₃ as the oxidants in a two-compartment cell. Depending on the oxidant and polymerization conditions, PEDT is deposited either as a thin film on the membrane surface or inside the Nafion membrane depending on whether FeCl₃ or H₂O₂ is used as the oxidant. The decrease in the ionic conductivity and methanol permeability is studied as a function of the polymerization time. A linear dependence is found with H₂O₂ and a t ^−1/2 dependence, with FeCl₃. The contributions of PEDT and Nafion to the overall conductivity of the composite membranes are separated by impedance measurements. The modified membranes (FeCl₃) are also tested in direct methanol fuel cells (DMFC). The methanol permeation through the membranes is measured by operating the fuel cell in an electrolysis mode. The influence of the modified membranes on the DMFC current-voltage characteristics is studied with 2 M CH₃OH and O₂ at 1.2 bar_abs and 80°C. Membrane electrode assemblies (MEAs) are prepared by hot pressing the modified membrane between two commercial gas diffusion electrodes with 1 mg cm⁻² of Pt loading. A decrease of the methanol permeation of 25% is observed at MEA with the modified membrane (1 h polymerization time), compared with that of MEA with a Nafion membrane. However, the overall DMFC performance decreases in the same relation: a maximal power density of 36 W cm⁻² is measured at MEA with a PEDT-modified membrane compared with 45 W cm⁻² for MEA with a Nafion membrane. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 11, pp. 1330–1339. Based on the report delivered at the 8th International Frumkin Symposium “Kinetics of the Electrode Processes,” October 18–22, 2005, Moscow. The text was submitted by the authors in English.     

Theoretical study of the gas-phase Fe<Superscript>+</Superscript>-mediated oxidation of ethane by N<Subscript>2</Subscript>O

We report herein a comprehensive study of the gas-phase Fe⁺-mediated oxidation of ethane by N₂O on both the sextet and quartet potential energy surfaces (PESs) using density functional theory. The geometries and energies of all the relevant stationary points are located. Initial oxygen-atom transfer from N₂O to iron yields FeO⁺. Then, ethane oxidation by the nascent oxide involves C–H activation forming the key intermediate of (C₂H₅)Fe⁺(OH), which can either undergo C–O coupling to Fe⁺ + ethanol or experience β-H shift giving the energetically favorable product of FeC₂H₄ ⁺ + H₂O. Reaction of FeC₂H₄ ⁺ with another N₂O constitutes the third step of the oxidation. N₂O coordinates to FeC₂H₄ ⁺ and gets activated by the metal ion to yield (C₂H₄)Fe⁺O(N₂). After releasing N₂ through the direct H abstraction and/or cyclization pathways, the system would be oxidized to ethenol, acetaldehyde, and oxirane, regenerating Fe⁺. Oxidation to acetaldehyde along the cyclization –C–to–C hydrogen shift pathway is the most energetically favored channel.     

Investigation on sonocatalytic damage of BSA under ultrasonic irradiation by Fe<Superscript>III</Superscript> complexes with some aminocarboxylic acid

The interaction of BSA and Fe^III complexes ([Fe^III(gly)(H₂O)₄]²⁺, [Fe^III(ida)(H₂O)₃]⁺, and [Fe^III(nta)(H₂O)₂], gly—glyane, ida—iminodiacetic acid, nta—triglycolamic acid) as well as the sonocatalytic damage to BSA was studied by UV-vis and fluorescence spectra. In addition, the influences of ultrasonic irradiation time and Fe^III complex concentration were also examined on the sonocatalytic damage to BSA. The results showed that the fluorescence quenching of BSA solution caused by the Fe^III complexes belonged to the static quenching process. The BSA and Fe^III complexes interacted with each other mainly through weak interaction and coordinate actions. The binding association constants (K) and binding site numbers (n) were calculated. The results were as follows: K ₁ = 0.5353 × 10⁴ l mol⁻¹ and n ₁ = 0.9812 for [Fe^III(gly)(H₂O)₄]²⁺, K ₂ = 1.4285 × 10⁴ l mol⁻¹ and n ₂ = 1.0899 for [Fe^III(ida)(H₂O)₃, and K ₃ = 0.4411 × 10⁴ l mol⁻¹ and n ₃ = 0.9471 for [Fe^III(nta)(H₂O)₂]. Otherwise, under ultrasonic irradiation the BSA were obviously damaged by the Fe^III complexes. The damage degree rose up with the increase of ultrasonic irradiation time and Fe^III complex concentration. And that, [Fe^III(nta)(H₂O)₂] exhibited in a way higher sonocatalytic activity than [Fe^III(gly)(H₂O)₄]²⁺ and [Fe^III(ida)(H₂O)₃]⁺.     

Mechanistic Studies on the Oxidation of Hydroquinone by an Oxo-bridged Diiron(III,III) Complex in Weakly Acidic Aqueous Media

The kinetics of oxidation of hydroquinone (H₂Q) by a μ-oxo-bridged diiron(III,III) complex, Fe₂(μ-O)(phen)₄(H₂O)₂]⁴⁺ (1) has been investigated in aqueous media at 25.0 °C in presence of an excess of 1,10-phenanthroline (phen). The overall redox rate increases with increase in [H⁺]. The title complex (1) and its conjugate bases, [Fe₂(μ-O)(phen)₄(OH)₂]³⁺(2) and [Fe₂(μ-O)(phen)₄(OH)₂]²⁺ (3), participate in the reaction with H₂Q as the only kinetically reactive reducing species. Rate constants (in dm³ mol⁻¹ s⁻¹) for the parallel reactions (1) + H₂Q → Products, (2) + H₂Q → Products and that for (3) + H₂Q → Products are, respectively, 500 ± 40, 100 ± 6 and 30 ± 2. Substantial rate retardation in D₂O media in comparison to that in H₂O media suggests that electron transfer is coupled with proton movements in the rate-determining step.     

Transport properties of vanadium ions in cation exchange membranes:: Determination of diffusion coefficients using a dialysis cell

Diffusion coefficients of vanadium ions in cation exchange membranes are of interest because they allow to calculate the ion exchange across the membrane in an all vanadium redox flow battery which leads to undesired cross contamination and energy losses in the battery system. Diffusion coefficients of V²⁺, V³⁺, VO²⁺ and VO⁺₂ ions in CMS, CMV and CMX cation exchange membranes have been determined by measuring the ion exchange fluxes of these ions with H₃O⁺ ions using a dialysis cell. The experimental data are evaluated on the basis of integrated flux equations which require also ion exchange sorption equilibria obtained already in previous work. The lowest diffusion coefficients are observed in the CMS membrane for all vanadium ions. This membrane turns out to be the most suitable one for being applied in a vanadium battery since it is expected to prevent most effectively cross contamination of vanadium ions.     

The Peltier heat and the standard electrode potential of ferro-ferricyanide couple at 298.15 K determined by electrochemical–calorimetry

An experiment was done on electrochemical–calorimetry to identify the Peltier heats of the ferro-ferricyanide reversible electrode reaction over the concentration range of 0.075–0.3 mol dm⁻³ at 298.15 K. A new approach has been developed to obtain the standard potential of this electrode, which was identified as (+0.3580 ± 0.0030) volt at 298.15 K and compared with previously reported values. An equation derived from the approach is also applied to several standard couples, such as Fe(CN)₆⁻³/Fe(CN)₆⁻⁴, H⁺/H₂, Cu²⁺/Cu, Cl⁻/Hg₂Cl₂,Hg, Fe³⁺/Fe²⁺, and Cl⁻/Cl₂ to determine their respective reaction heats with satisfying results.     

Photo-Fenton and photo-Fenton-like processes for the degradation of methyl orange in aqueous medium: Influence of oxidation states of iron

Degradation of methyl orange (MO) was carried out by the photo-Fenton process (Fe²⁺/H₂O₂/UV) and photo-Fenton-like processes (Fe³⁺/H₂O₂/UV, Fe²⁺/S₂O₈²⁻/UV, and Fe³⁺/S₂O₈²⁻/UV) at the acidic pH of 3 using hydrogen peroxide and ammonium persulfate (APS) as oxidants. Oxidation state of iron had a significant influence on the efficiency of photo-Fenton/photo-Fenton-like processes. It was found that a process with a source of Fe³⁺ ions as the catalyst showed higher efficiency compared to a process with the Fe²⁺ ion as the catalyst. H₂O₂ served as a better oxidant for both oxidation states of iron compared to APS. The lower efficiency of APS is attributed to the generation of excess protons which scavenges the hydroxyl radicals necessary for degradation. Further, the sulfate ions produced from S₂O₈²⁻ form a complex with Fe²⁺/Fe³⁺ ions thereby reducing the concentration of free iron ions in the solution. This process can also reduce the concentration of hydroxyl radicals in the solution. Efficiency of the various MO degradation processes follows the order: Fe³⁺/H₂O₂/UV, Fe³⁺/APS/UV, Fe²⁺/H₂O₂/UV, Fe²⁺/APS/UV.     

Using ESR spectroscopy to study radical intermediates in proton-exchange membranes exposed to oxygen radicals

Direct ESR and spin-trapping experiments were used to study the behavior of Nafion, a perfluorinated ionomer membrane used in fuel cells, when exposed in the laboratory to oxygen radicals produced by Fenton and photo-Fenton reactions. DMPO (5,5-dimethyl-1-pyroline) was used as the spin trap. The results suggest that the two ESR methods provide complementary information on Nafion fragmentation. The presence of membrane-derived fragments was suggested indirectly by the presence of a broad signal (line width ≈ 84 G) after prolonged exposure of the membrane to the Fenton reagent based on Ti(III), and by the DMPO adduct of a carbon-centered radical in the spin-trapping experiments. The most convincing proof for the presence of perfluorinated radicals was obtained in Nafion membranes partially neutralized by Cu(II), Fe(II) and Fe(III) upon exposure to UV-irradiation in the presence or absence of H₂O₂ (photo-Fenton treatment). Identification of the chain-end radical RCF₂CF ₂ ^• with magnetic parameters different to those determined for the chain-end detected in γ-irradiated Teflon, was taken as evidence for the attack of reactive oxygen radicals on the side-chain of the membrane. Additional support for this suggestion was the detection of the “quartet” ESR signal assigned to the CF₃C^•O radical, and of the “quintet” ESR signal assigned to the radical centered at the intersection of the main and side chains. The limitations and advantages of each approach are discussed.