Methane conversion to C2 hydrocarbons in solid electrolyte membrane reactors

Solid electrolyte membrane reactors (SEMRs) have been used to both study and influence catalytic reaction rates. Methane coupling is the reaction most thoroughly and intensively studied in these membrane reactors. In the last 20 years, oxygen ion (O^2?), proton (H⁺) and mixed (O^2?-e^?, H⁺-e^?) conducting membranes have been tested in order to maximize the conversion of methane to C₂ compounds. The present review contains the fundamental operating principles of the various SEMR types and their applications in this reaction. The difficulties that should be overcome in order to promote this SEMR process to an industrial scale are discussed.     

Modelling spur chemistry for alkaline and acidic water at high temperatures

The effect of pH and associated ionic strength on the primary yields in the radiolysis of pressurised water has been assessed by diffusion-kinetic calculations for temperatures in the range 100–300°C. Account has been taken for ionic strength I up to 0.1 mol kg⁻¹, assuming that the counter ions of H⁺ in acid solutions and of OH⁻ in base solutions have unit charge. In acid solutions, the H⁺ ions react with e⁻ _aq. The decrease in G(e⁻ _aq) and the increase in G(H) with decreasing pH becomes substantial for [H⁺] ≥ 1 × 10⁻⁴ m, but the primary yields of oxidising species are almost constant. In alkaline solutions, the OH⁻ anions affect the spur chemistry of radiation-generated protons and hydroxyl radicals for [OH⁻] ≥ 1 × 10⁻⁴ m. The scavenging of H atoms and hydrogen peroxide becomes significant for [OH⁻] ≥ 1 × 10⁻² m. The total yields G(OH) + G(O⁻) and G(H₂O₂) + G(HO₂ ⁻) are independent of base concentration below 0.01 m. In more alkaline solutions, G(OH) + G(O⁻) increases, whereas G(H₂O₂) + G(HO₂ ⁻) decreases with increasing [OH⁻]. Calculations showed the substantial yield of the reaction O⁻ + e⁻ _aq in 0.1 m base solution. Spur chemistry in alkaline hydrogenated water is not affected by the presence of H₂ if less than 0.001 m of hydrogen is added.     

Kinetics of the Formation of the Blue Complex CrO(O2)2 Formed by Dichromate and H2O2 in Acid Solutions. A Stopped-Flow Investigation Using Rapid-Scan UV-VIS Detection

 The kinetics of the CrO(O₂)₂ formation by H₂O₂ and Cr₂O₇ ²⁻ in aqueous acidic media was measured at 293 ± 2 K in a pH range between 2.5 and 3.3. Using the stopped-flow method with rapid scan UV-VIS detection, the rate law of the formation of CrO(O₂)₂ was determined. For the media HClO₄, HNO₃ and CH₃COOH, the reaction order in the Cr₂O₇ ²⁻ concentration was found to be 0.5. For [H₂O₂] as well as for [H⁺], the reaction was first order in all acids used. In HCl and H₂SO₄ media the reaction was first order in Cr₂O₇ ²⁻. At T = 293 ± 2 K the rate constant for the formation of Cr(O)(O₂)₂ was found to be (7.3 ± 1.9) · 10² M^−3/2 s⁻¹ in HClO₄.     

Kinetics of the Formation of the Blue Complex CrO(O2)2 Formed by Dichromate and H2O2 in Acid Solutions. A Stopped-Flow Investigation Using Rapid-Scan UV-VIS Detection

Summary.  The kinetics of the CrO(O₂)₂ formation by H₂O₂ and Cr₂O₇ ²⁻ in aqueous acidic media was measured at 293 ± 2 K in a pH range between 2.5 and 3.3. Using the stopped-flow method with rapid scan UV-VIS detection, the rate law of the formation of CrO(O₂)₂ was determined. For the media HClO₄, HNO₃ and CH₃COOH, the reaction order in the Cr₂O₇ ²⁻ concentration was found to be 0.5. For [H₂O₂] as well as for [H⁺], the reaction was first order in all acids used. In HCl and H₂SO₄ media the reaction was first order in Cr₂O₇ ²⁻. At T = 293 ± 2 K the rate constant for the formation of Cr(O)(O₂)₂ was found to be (7.3 ± 1.9) · 10² M^−3/2 s⁻¹ in HClO₄. Corresponding author. E-mail: grampp@ptc.tu-graz.ac.at Received January 30, 2002; accepted (revised) June 5, 2002     

The influence of ionic medium on the kinetics of ligand replacement in the Ptdient H2O2+ complex in an aqueous solution

The influence of the ion background (NaClO₄, LiClO₄, and HClO₄) on the kinetics of the reaction PtdientH₂O²⁺+X⁻→PtdientX⁺+H₂O(X=Cl, Br, I, SCN, and N₃) was studied at 25°C by spectrophotometry. Changes in the rate constant with increase in the ionic strength are described by the Debye-Hückel and Gosh-Bjerrum equations. The reaction PtdienCl⁺+H₂O→PtdientH₂O²⁺+Cl⁻ was studied by potentiometry and its rate constant was established to depend weakly on variations of the medium. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1918–1921, October, 1998.     

Electron transfer at tetrahedral cobalt(II). Part V: kinetics of permanganate ion reduction

Summary The kinetics and mechanism of the reduction of MnO₄ ⁻ by CoW₁₂O₄O₆₋ in aqueous HC1O₄ were studied. The reaction follows the rate law:-d[MnO _inf4 ^sup− ]/dt = 5K _a k[H⁺][MnO _inf4 ^sup− ][CoW₁₂O₄O⁶⁻] with K _a = 2.99 × 10⁻³mol⁻¹ dm³ and k = 2.00 ± 0.02 × 10³dm⁶mol⁻²s⁻¹ at 25°C. Close agreement between k _obs and k _calc on the basis of Marcus theory suggest an outersphere mechanism operates. Alkali metal ions catalyse the reaction in the order K⁺ > Na⁺ > Li⁺ and this result has been rationalized.     

Mechanistic Study of Oxidation of Palladium(II) by Cerium(IV) in Aqueous Acid

The kinetics of oxidation of Pd^II by Ce^IV have been studied spectrophotometrically in HClO₄ media at 40 °C. The reaction is first order each in [Ce^IV] and [Pd^II] at constant [H⁺]. Increasing [H⁺] accelerates the reaction rate with fractional order in [H⁺]. The initially added products, palladium(IV) and cerium(III) do not have any significant effect on the reaction rate. At constant acidity, increasing the added chloride concentration enhances the rate of reaction. H₃Ce(SO₄)₄⁻ and PdCl₄²⁻ are the active species of oxidant and reductant respectively. The possible mechanisms are proposed and the reaction constants involved have been determined.     

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 study of the aerobic reaction between dopamine and Fe(III) in the presence of S<Subscript>2</Subscript>O<Subscript>3</Subscript><Superscript>2−</Superscript>

The reaction between Fe(III) and dopamine in aqueous solution in the presence of Na₂S₂O₃ was followed through UV–Vis spectroscopy, pH and oxy-reduction potential (Eh) measurements. The formation and quick disappearing of the complex [Fe(III)HL¹⁻]²⁺, HL¹⁻ = monoprotonated dopamine was observed with or without S₂O₃ ²⁻ at pH 3. An unexpected reaction occurs in presence of thiosulfate forming the stable anion complex [Fe(III)(L²⁻)₂]¹⁻, L²⁻ = dopacatecholate (λ = 580 nm) and the auto-increasing of the pH, from 3 to 7. It was proposed that H⁺ and molecular oxygen are consumed by free radical thiosulfate formed during the reaction.     

Activation-controlled outer-sphere electron transfer reactions: reduction of heteropoly 11-tungstovanadophosphate and heteropoly 10-tungstodivanado phosphate by thiourea in aqueous acid medium

The kinetics of reduction of heteropoly 11-tungstovanadophosphate, [PV^VW₁₁O₄₀]⁴⁻, (HPA1) and heteropoly 10-tungstodivanadophosphate, [PV^VV^VW₁₀O₄₀]⁵⁻, (HPA2) by thiourea has been investigated in HClO₄/phthalate/acetate buffer solutions spectrophotometrically at 25 °C in aqueous medium. The stoichiometry of the reaction is 1:1 in both cases. The HPAs are converted into the corresponding one-electron reduced heteropoly blues, namely, [PV^IVW₁₁O₄₀]⁵⁻ and [PV^IVV^VW₁₀O₄₀]⁶⁻, and thiourea is oxidised to formamidine disulphide. The reaction shows first-order dependence in both [HPA] and [thiourea] at constant pH. The rate–pH profile shows the participation of both the neutral and deprotonated forms of thiourea in the reaction. The reaction proceeds through an outer sphere electron transfer mechanism in which activation-controlled electron transfer is the rate-determining step. Self-exchange rate constants for the couples [PV^VW₁₁O₄₀]⁴⁻/[PV^IVW₁₁O₄₀]⁵⁻, [PV^VV^VW₁₀O₄₀]⁵⁻/[PV^IVV^VW₁₀O₄₀]⁶⁻ and H₂NCSNH₂/H₂NCS^·+NH₂ have been evaluated by Marcus theory.     

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.     

Investigation on the formation and decay of the N,N,N′,N′-tetramethylbenzidine radical cation using various oxidants and reductants by stopped-flow spectrophotometry

N,N,N′,N′-Tetramethylbenzidine (TMB) is an aromatic amine that undergoes oxidation by various oxidizing agents such as Ce⁴⁺, MnO⁻₄, Cr₂O²⁻₇; HSO⁻₅, S₂O⁻₈, H₂O₂, Cl₂, Br₂ and I₂, thereby serving as a reducing substrate. One-electron oxidation of TMB results in a radical cation (TMB^˙+), and on further oxidation leads to the product dication (TMB⁺⁺) were monitored by stopped-flow spectrophotometer at the absorption wavelength of TMB^˙+(λ_max; 460 nm). ESR data was also provided to confirm the formation of radical cation. The rates of both the formation and decay of TMB^˙+ have been followed by a total second-order kinetics, a first-order dependence each on [TMB] (or) [TMB^˙+] and [oxidant]. The kinetic and transition state parameters have been evaluated for the effects of pH and temperature on the formation and decay of TMB^˙+ and discussed with suitable reaction mechanisms. Also, the rate constants for the reactions of the radical cation with various reducing agents such as sulfite (SO²⁻₃), thiosulfate (S₂O²⁻₃), dithionite (S₂O²⁻₄) and disulfite (S₂O²⁻₅) and ascorbic acid (vitamin C, AH₂ were determined. Besides these, this article also explains how TMB acts as a better electron relay than unsubstituted benzidine, even though both of them undergo one-electron oxidation and are used in the chemical routes to solar energy conversions. The observed rate constants for electron transfer were correlated theoretically using Marcus theory. The observed and calculated rate constants have good correlation.     

Composition of the gas phase over Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and the enthalpies of atomization of AlO,Al<Subscript>2</Subscript>O,and Al<Subscript>2</Subscript>O<Subscript>2</Subscript>

The A1, O, AlO, A1₂O, Al₂O₂, WO₂, and WO₃, partial pressures in the vapor over Al₂O₃ in a tungsten Knudsen effusion cell between 2300 and 2600 K were derived from A1⁺, O⁺, AlO⁺, A1₂O⁺, Al₂O₂⁺, WO₂⁺, and WO₃⁺, ion intensities. The mass spectrometer was calibrated against the equilibrium constant of the WO₃(g) = WO₂(g) + O(g) reaction. Refined values of the ionization cross sections of AlO and A1₂O₂ were used in the partial pressure calculations. The enthalpies of atomization of aluminum suboxides were determined to be Δ_at H ^o(AlO, g, 0) = 510.7 ± 3.3 kJ mol⁻¹, Δ_at H ^o(Al₂O, g, 0) = 1067.2 ± 6.9 kJ mol⁻¹, and Δ_at H ^o(Al₂O₂, g, 0) = 1556.7 ± 9.9 kJ mol⁻¹.     

The kinetics of iodide oxidation by hydrogen peroxide in acid solution

The kinetics of the complex reaction between I⁻ and H₂O₂ in acid media was investigated. The particular attention was focused on the determination of the rate constant of the reaction between HIO and H₂O₂ involved in the investigated complex process. The examination of the whole kinetics was performed by simultaneously monitoring the evolution of O₂ pressure, I₃⁻ and I⁻ concentrations. We modeled the behavior of experimentally followed components based on Liebhafsky’s research. Our preliminary results suggest a significantly higher rate constant (3.5 × 10⁷ M⁻¹ s⁻¹) of the reaction between HIO and H₂O₂ as those proposed in the literature.     

Competition Between O2 and H2O2 in the Oxidation of Fe(II) in Natural Waters

The oxidation rates of nanomolar levels of Fe(II) in seawater (salinity S = 36.2) by mixtures of O₂ and H₂O₂ has been measured as a function of pH (5.8–8.4) and temperature (3–35∘C). A competition exists for the oxidation of Fe(II) in the presence of both O₂ (μ mol⋅L⁻¹ levels) and H₂O₂ (nmol⋅L⁻¹ levels). A kinetic model has been applied to explain the experimental results that considers the interactions of Fe(II) with the major ions in seawater. In the presence of both oxidants, the hydrolyzed Fe(II) species dominate the Fe(II) oxidation process between pH 6 and 8.5. Over pH range 6.2–7.9, the FeOH⁺ species are the most active, whereas above pH 7.9, the Fe(OH)⁰₂ species are the most active at the levels of CO²⁻₃ concentration present in seawater. The predicted Fe(II) oxidation rate at [Fe(II)]₀ = 30nmol⋅L⁻¹ and pH = 8.17 in the oxygen-saturated seawater with [H₂O₂]₀ = 50nmol⋅L⁻¹ (log ₁₀ k = −2.24s⁻¹) is in excellent agreement with the experimental value of log ₁₀ k = −2.29s⁻¹ ([H₂O₂]₀ = 55nmol⋅L⁻¹, pH = 8).     

Effect of Ionic Strength and Temperature on the Protonation of Oxidized Glutathione

The protonation constants for oxidized glutathione, H _i−1L^(4−i+1)−, K _i ^H=[H _i L⁽⁴⁻ⁱ⁾⁻]/[H _i−1L^(4−i+1)−][H⁺] i=1,2,…,6 have been measured at 5, 25 and 45 °C as a function of the ionic strength (0.1 to 5.4 mol⋅[kg(H₂O)]⁻¹) in NaCl solutions. The effect of ionic strength on the measured protonation constants has been used to determine the thermodynamic values (K _i ^H0) and the enthalpy (ΔH _i ) for the dissociation reaction using the SIT model and Pitzer equations. The SIT (ε) and Pitzer parameters (β ⁽⁰⁾, β ⁽¹⁾ and C) for the dissociation products (L⁴⁻, HL³⁻, H₂L²⁻, H₃L⁻, H₄L, H₅L⁺, H₆L²⁺) have been determined as a function of temperature. These results can be used to examine the effect of ionic strength and temperature on glutathione in aqueous solutions with NaCl as the major component (body fluids, seawater and brines).     

Performance of a photocatalytic hybrid system coupled with a stainless steel membrane for removal of humic acid

The objective of this study was to evaluate the performance of a photocatalysis/H₂O₂/metal membrane hybrid system in the degradation of humic acid. A metal membrane of nominal pore size 0.5 μm was used in the experiment for separation of TiO₂ particles. Hydrogen peroxide was tested as an oxidant. The efficiency of removal of COD_Cr and color increased rapidly for initial hydrogen peroxide concentrations up to 50 mg L⁻¹. The efficiency of removal of COD_Cr and color by 50 mg L⁻¹ initial hydrogen peroxide concentration was approximately 95 and 98%, respectively. However, addition of hydrogen peroxide over 50 mg L⁻¹ inhibited the efficiency of the system. Addition of hydrogen peroxide to a UV/TiO₂ system enhanced efficiency of removal of COD_Cr and color compared with no addition of hydrogen peroxide. This may be ascribed to capture electrons ejected from TiO₂ and to the production of OH radicals. Application of the metal membrane in the UV/TiO₂/H₂O₂ system enhanced the efficiency of removal of COD_Cr and color because of adsorption by the metal membrane surface and the production of OH radicals. By application of a metal membrane with a nominal pore size of 0.5 μm, TiO₂ particles were effectively separated from the treated water by metal membrane rejection. The photocatalytic metal membrane had much less resistance than the humic acid, TiO₂, and humic acid/TiO₂ because of the degradation of humic acid by the photocatalytic reaction.     

Water splitting in low-temperature ac plasmas at atmospheric pressure

Plasma-induced water splitting at atmospheric pressure has been studied with a novel fan-type Pt reactor and several tubular-type reactors: an all-quartz reactor, a glass reactor, and three metal reactors with Pt. Ni, and Fe as electrodes. Reaction products have been analyzed by using GC (gas chromatography) and Q-MS (quadrupole mass spectrometry). Optical emission spectroscopic studies of the process have been carried out by employing a CCD (charge-coupled device) detector. Water splitting with tubular quartz and glass reactors is probably non-catalytic. However, a heterogeneous catalytic function of surface of metal electrodes has been observed. The variation of hydrogen yield (Y_H) and energy efficiency (E_e) with operational parameters such as input voltages (U_in), flow rates of carrier gas (F_He), and concentrations of water (C_W) has been examined. Plasma-induced water splitting can be described with a kinetic equation of-dC_w/dt = kC_W ^0.2. The rate constants at 3.25 kV are 2.8 × 10⁻⁴, 3.5 × 10⁻³, and 3.4 × 10⁻² mol^0.8L^−0.8 min⁻¹ for tubular glass reactor, a tubular Pt reactor, and a fan-type Pt reactor, respectively. A CSTR (continuous-stirred tank reactor) and PFR (piston-flow reactor) model have been applied to a fan-type reactor and tubular reactor, respectively. A mechanism on the basis of optical emission spectroscopic data has been obtained comprising the energy transfer from excited carrier gas species to water molecules, which split via radicals of HO^·, O^·, and H^· to form H₂ and O₂. The fan-type Pt reactors exhibit highest activity and energy efficiency among the reactors tested. Higher yields of hydrogen are achieved at higher input voltages, low flow rates, and low concentrations of water (Y_H = 78 % at U_in of 3.75 kV, F_He of 20 mL/min, and C_W of 0.86 %). The energy efficiency exhibits an opposite trend (E_e = 6.1 % at U_in of 1.25 kV, F_He of 60 mL/min and C_W of 3.1 %).     

Amperometric determination of xanthine in tea,coffee, and fish meat with graphite rod bound xanthine oxidase

A method is described for construction of an amperometric xanthine biosensor based on graphite rod modified through adsorption of xanthine oxidase. Enzymatically produced H₂O₂ from xanthine was split into 2H⁺ + O₂ + 2^e− at 0.6 V and the current was measured, which was directly proportional to xanthine concentration ranging from 1 ° 10⁻⁷ to 6 ° 10⁻⁷ M with a detection limit of 1 ° 10⁻⁷ M. The biosensor exhibited optimum response within 35 sec at pH 7.0 and 35°C. It was employed for determination of xanthine in tea leaves (0.9 ° 10⁻⁵−2.5 ° 10⁻⁵ mmol/g), coffee powder (3.2 μmol/g) and fish meat (90 mmol/g). The content of xanthine in fish meat increased 6.5 times with its storage at room temperature during 15 days. The enzyme electrode could be reused 200 times during the span of 30 days, when stored in reaction buffer at 4°C.     

Kinetics and mechanism of the ligand substitution reaction between aquapentacyanoruthenate(II) and 4-cyanopyridine in the presence of anionic surfactant micelles

The kinetics of ligand substitution between aquapentacyanoruthenate(II) ion, [Ru(CN)₅H₂O]³⁻ and 4-cyanopyridine (4-CNpy) has been investigated spectrophotometrically in the presence of anionic surfactant micelle, namely sodium dodecylsulphate (SDS) at 400 nm (λ_max of the intense yellow product [Ru(CN)₅4-CNpy]³⁻) under pseudo-first-order conditions using at least 10% excess of 4-CNpy over [Ru(CN)₅H₂O]³⁻. The reaction was studied as a function of [Ru(CN)₅H₂O³⁻], [4-CNpy], [SDS], pH, ionic strength and temperature, by varying each of these variables one at a time. The reaction exhibited overall second-order kinetics, being first order each in [4-CNpy] and [Ru(CN)₅H₂O³⁻] over a wide concentration range. Variation of ionic strength of the medium had a significant negative effect on the rate. The SDS micelle, being negatively charged, does not reveal any regular effect except at or near its critical micelle concentration (c.m.c). The rate of reaction was measured at different temperatures, and the activation parameters were computed using Arrhenius and Eyring plots. A plausible mechanism consistent with the experimental results has been proposed.