Mass transfer limitations on fixed-bed reactor for Fischer-Tropsch synthesis

Mass transfer limitations on fixed-bed for Fischer-Tropsch synthesis were investigated by changing synthesis gas superficial velocity, catalyst pellet size, and catalyst amount. To study external mass transfer limitation, synthesis gas superficial velocity was changed from 8.47 × 10^− 4 m s^− 1 to 3.39 × 10^− 3 m s^− 1. As a result, the synthesis gas superficial velocity of 3.39 × 10^− 3 m s^− 1 was most suitable for hydrocarbon chain growth resulting to liquid hydrocarbon formation. In case of internal mass transfer limitations, the effects of catalyst pellet size and catalyst amount (W_cat/F) were discussed. The large catalyst pellet showed higher C₅₊ selectivity and a lower α value compared to the small pellet because of more severe internal mass transfer limitations of α-olefin and long-chained hydrocarbons in the large pellet, respectively. Catalyst amount (W_cat/F) was inversely proportional to the internal mass transfer limitation because increased catalyst amount gave more time for liquid hydrocarbon products to diffuse from the catalyst pellet and, therefore, the catalyst amount of 4.5 g (W_cat/F = 45 g_cat min L^− 1) was most appropriate for liquid hydrocarbon formation.     

Direct electron transfer of horseradish peroxidase on Nafion-cysteine modified gold electrode

Direct electron transfer of horseradish peroxidase, immobilized on a functional membrane-modified gold electrode, was studied. The electrode showed a quasi-reversible electrochemical redox behavior with a formal potential of 60 mV (versus Ag/AgCl) in 20 mM potassium phosphate buffer solution at pH 7.0 and temperature 25 °C. The cathodic transfer coefficient was 0.42 and electron transfer rate constant was evaluated to be 1.6 s⁻¹. Furthermore, the modified electrode was used as a biosensor and exhibited a satisfactory stability and sensitivity to H₂O₂. The linear range of this biosensor for H₂O₂ determination was from 5.0 × 10⁻⁶ to 1.5 × 10⁻⁴ M while its detection limit, based on a signal-to-noise ratio of 3, was 1.3 × 10⁻⁶ M. The apparent Michaelis-Menten constant () for immobilized HRP was calculated to be 1.6 × 10⁻⁴ M.     

Cyclic voltammetry and scanning electrochemical microscopy studies of methylene blue immobilized on the self-assembled monolayer of n-dodecanethiol

Electron transfer (ET) kinetics through n-dodecanethiol (C₁₂SH) self-assembled monolayer on gold electrode was studied using cyclic voltammetry (CV), scanning electrochemical microscopy (SECM) and electrochemical impedance spectroscopy (EIS). An SECM model for compensating pinhole contribution, was used to measure the ET kinetics of solution-phase probes of ferrocyanide/ferricyanide (Fe(CN)₆^4−/3−) and ferrocenemethanol/ferrociniummethanol (FMC^0/+) through the C₁₂SH monolayer yielding standard tunneling rate constant () of (4 ± 1) × 10⁻¹¹ and (3 ± 1) × 10⁻¹⁰ cm s⁻¹ for Fe(CN)₆^4−/3− and FMC^0/+ respectively. Decay tunneling constants (β) of 0.97 and 0.96 Å⁻¹ for saturated alkane thiol chains were obtained using Fe(CN)₆⁴⁻ and FMC respectively. Also, it was found that methylene blue (MB) molecules are effectively immobilized on the C₁₂SH monolayer and can mediate the ET between the solution-phase probes and underlying gold substrate. SECM-mediated model was used to simultaneously measure the bimolecular ET between the solution-phase probes and the monolayer-immobilized MB molecules, as well as tunneling ET between the monolayer-immobilized MB molecules and the underlying gold electrode, allowing the measurement of k_BI = (5 ± 1) × 10⁶ and (4 ± 2) × 10⁷ cm³ mol⁻¹ s⁻¹ for the bimolecular ET and and (7 ± 3) × 10⁻² s⁻¹ for the standard tunneling rate constant of ET using Fe(CN)₆^4−/3− and FMC^0/+ probes respectively.     

Electroreduction of peroxycitric acid coexisting with hydrogen peroxide in aqueous solution

The electrochemical reduction of peroxycitric acid (PCA) coexisting with citric acid and hydrogen peroxide (H₂O₂) in the equilibrium mixture was extensively studied at a gold electrode in acetate buffer solutions containing 0.1 M Na₂SO₄ (pH 2.0-6.0) using cyclic and hydrodynamic voltammetric, and hydrodynamic chronocoulometric measurements. The reduction of PCA was characterized to be an irreversible, diffusion-controlled process, and the cyclic voltammetric reduction peak potential () was found to be more positive by ca. 1.0 V than that of the coexisting H₂O₂, e.g., the values obtained at 0.1 V s⁻¹ for PCA and H₂O₂ were 0.35 and −0.35 V, respectively, vs. Ag|AgCl|KCl (sat.) at pH 3.3. The of PCA was found to depend on pH, i.e., at pH > 4.5, the plot of vs. pH gave the slope (−64 mV decade⁻¹) which is close to the theoretical value (−59 mV decade⁻¹) for an electrode process involving the equal number of electron and proton in the rate-determining step, while at pH < 4.5, the was almost independent of pH. The relevant electrochemical parameters, Tafel slope, number of electrons, formal potential (E⁰′), cathodic transfer coefficient and standard heterogeneous rate constant (k⁰′) for the reduction of PCA and the diffusion coefficient of PCA were determined to be ca. 100 mV decade⁻¹, 2, 1.53 V (at pH 2.6), 0.29, 1.2 × 10⁻¹² cm s⁻¹ and 0.29 × 10⁻⁵ cm² s⁻¹, respectively, and except for E⁰′, the obtained values were almost independent of the solution pH. The overall mechanism of the reduction of PCA was discussed.     

Investigation on diffusion behavior of Li in LiFePO4 by capacity intermittent titration technique (CITT)

Capacity intermittent titration technique (CITT) was used to investigate the chemical diffusion coefficient () of lithium-ion in LiFePO₄ cathode material. The values of at the galvano-charge current of 0.2 and 0.4 mA were respectively found to range from 8.8 × 10⁻¹⁶ to 8.9 × 10⁻¹⁴ cm² s⁻¹ and from 1.2 × 10⁻¹⁶ to 8.5 × 10⁻¹⁴ cm² s⁻¹ in the voltage range from 3.2 to 4 V (vs. Li⁺/Li). The transfer coefficients of cathode (0.32-0.42) and anodic (0.26-0.3), and the standard rate constant (1.58 × 10⁻⁹ to 1.30 × 10⁻⁸ cm s⁻¹) were measured from the Tafel plots of LiFePO₄ in the equilibrium potential range from 3.06 to 3.45 V. From these kinetic parameters, the finite kinetics at interface was taken into account to revise the above values of . The revised values of at the galvano-charge current of 0.2 and 0.4 mA were respectively found to range from 2.44 × 10⁻¹⁵ to 2.21 × 10⁻¹³ cm² s⁻¹ and from 5.81 × 10⁻¹⁶ to 3.22 × 10⁻¹³ cm² s⁻¹ in the voltage range from 3.2 to 4 V. Results show that the approximation of infinite charge-transfer kinetics leads to a spurious value of which is lower than the revised value, and the spurious extent depends on the galvano-charge current of CITT experiment.     

Electrochemistry of thorium in LiCl-KCl eutectic melts

This work presents a study of the electrochemical properties of Th chloride ions dissolved in a molten LiCl-KCl eutectic, in a temperature range of 693-823 K. Transient electrochemical techniques such as cyclic voltammetry, chronopotentiommetry and chronoamperometry have been used in order to investigate the reduction mechanism on a tungsten electrode and the diffusion coefficient of dissolved Th ions. All techniques showed that only one valence state was stable in the melt. The reduction into Th metal was found to occur according to a one-step mechanism, through a nucleation-controlled process which requires an overpotential of several 100 mV. At 723 K, the diffusion coefficient is D_Th(723 K) = 3.15 ± 0.15 × 10⁻⁵ cm² s⁻¹. EMF measurements indicated that, at 723 K, the standard apparent potential is (723 K) = −2.582 V versus Cl₂/Cl⁻, and the activity coefficient γThCl₄ (723 K) = 4.6 × 10⁻⁴ on the mole fraction scale (based on a pure liquid reference state).     

Anodic oxidation and electro-Fenton treatment of rotenone

Rotenone, a widely used botanical insecticide submitted to strong restrictions regarding its environmental hazards, was studied as a target compound for electro-Fenton (EF) treatment in aqueous-acetonitrile mixture (70:30) of pH 3.0. In this system, the degradation of organic pollutants occurs by attack of hydroxyl radicals (OH) which are produced from the reaction of added ferrous catalyst (Fe²⁺) and hydrogen peroxide (H₂O₂) electrogenerated by oxygen reduction at carbon felt cathode. The degradative efficiency of EF system was comparatively studied versus anodic oxidation method (AO) in absence and presence of H₂O₂. It was found that only EF is sufficiently powerful to induce fast and efficient mineralization of rotenone and its degradation intermediates.The mineralization of rotenone was found to depend largely on organic solvent type, metal ion catalyst, applied current and initial rotenone concentration. The best operative conditions are achieved using aqueous-acetonitrile mixture of pH 3.0 in the presence of 0.2 mM Fe²⁺ catalyst with a current intensity of 100 mA. Under these optimized conditions, 30 min were sufficient to completely degrade rotenone in 100 mL of a 20 mg L⁻¹ solution. A nearly complete mineralization (∼96% of COD removal) was achieved after 8 h treatment.Rotenone removal kinetic was found to obey the pseudo-first order model and the absolute second order rate constant (k_Rot = 2.49 × 10⁹ M⁻¹ s⁻¹) for the reaction between the substrate and OH was derived.HPLC-MS and HPLC-DAD analysis were applied to identify and follow the evolution of rotenone oxidation products. Three stable aromatic intermediates were observed and two of these were identified as 12aβ-hydroxyrotenone and hydroquinone. Subsequent attack of these intermediates by OH radicals leads to the formation of aliphatic carboxylic acids such as succinic, acetic, oxalic and formic, quantified by ion-exclusion chromatography.     

Optimum operating conditions for a two-stage gasification process fueled by polypropylene by means of continuous reactor over ruthenium catalyst

We studied fuel gas production by means of pyrolysis and steam reforming of waste plastics for applications in solid oxide fuel cells. More specifically, we evaluated the effects of pyrolytic gasification temperature, catalyst content, steam reforming temperature, and weight hourly space velocity for a Ru catalyst used in a 60 g h^− 1-scale continuous experimental apparatus, which consisted of a tank reactor for pyrolysis and a packed-bed catalytic reactor for steam reforming. Polypropylene (PP) pellets were used as a model waste plastic. Ru/γ-Al₂O₃ catalysts with two different Ru contents were investigated. To suppress residue formation, the optimum operating temperature of the pyrolyzer was 673 K. To ensure suppressed coke formation, sufficient carbon conversion to gaseous products, and minimized heat loss from the reactor, the optimum operating conditions for the reformer were determined to be 903 K and 0.11 g-sample g-catalyst^− 1 h^− 1 with a 5 wt.% Ru/γ-Al₂O₃ catalyst. The composition of the gas produced with the 5 wt.% catalyst was almost the same as that predicted by chemical equilibrium laws, and it was applicable for a direct hydrocarbon fuel cell.     

2,2,6,6-Tetramethyl piperidine-1-oxyl (TEMPO)-mediated catalytic oxidation of benzyl alcohol in acetonitrile and ionic liquid 1-butyl-3-methyl-imidazolium hexafluorophosphate [BMIm][PF6]: Kinetic analysis

TEMPO (2,2,6,6-tetramethyl piperidine-1-oxyl) is electrochemically oxidized to a stable form of the cation (TEMPO⁺) in acetonitrile (CH₃CN) or 1-butyl-3-methyl-imidazolium hexafluorophosphate ([BMIm][PF₆]) media. Cyclic voltammograms were characterized by a well-defined one-electron reversible redox couple in both media at low scan rates. The reduced form of TEMPO⁺ is catalytically regenerated in a follow-up chemical reaction with benzyl alcohol (BA) in the presence of 2,6-lutidine. It was observed that in [BMIm][PF₆], the redox currents are largely suppressed compared to that in CH₃CN. The apparent heterogeneous electron-transfer rate constant () of the quasi-reversible redox reaction of TEMPO was determined at a Pt electrode and found to be 1.9 × 10⁻³ cm s⁻¹ and 4.5 × 10⁻² cm s⁻¹ in [BMIm][PF₆] and CH₃CN, respectively. With the aid of chronoamperometry (CA), the homogeneous rate constant for the catalytic oxidation of benzyl alcohol by TEMPO, in the presence of 2,6-lutidine in CH₃CN was estimated to be 5.53 × 10¹ M⁻¹ s⁻¹ which is approximately double, relative to the value of 2.91 × 10¹ M⁻¹ s⁻¹ determined in [BMIm][PF₆].     

Relative stabilities of Ce(IV) and Ce(III) limiting carbonate complexes at 5-50 °C in Na aqueous solutions, an electrochemical study

The normal potential of the Ce(IV)/Ce(III) redox couple was determined by square wave voltammetry (SWV) at different temperatures in solutions with a constant ratio [CO₃²⁻]/[HCO₃⁻] ≈10 for high ionic strengths (3.29 mol dm⁻³ at 4.39 mol dm⁻³): varies from 259.5 to 198.0 mV/S.H.E. in the 15-50 °C range. Linear variations were found for versus (RT/F)ln(mCO₃²⁻), leading to the stoichiometry, Ce(CO₃)₆⁸⁻ for the Ce(IV) limiting complex. But the slopes of these linear variations were actually found in the range 1.8-1.9, not exactly 2. This was interpreted as dissociation of the Ce(IV) limiting complex following the reaction: Ce(CO₃)₅⁶⁻ + CO₃²⁻ → Ce(CO₃)₆⁸⁻ and as dissociation of the Ce(III) limiting complex following the reaction: Ce(CO₃)₃³⁻ + CO₃²⁻ → Ce(CO₃)₄⁵⁻; for which maximum possible values of log₁₀ K_IV,6 and log₁₀ K_III,4 were estimated via fitting in the 15-50 °C temperature range (log₁₀ K_IV,6 = 0.42 (0.97) and log₁₀ K_III,4 = 0.88 (7.00) at 15 °C (50 °C). The normal potential was found to decrease linearly with T, these variations correspond to , with T⁰ = 298.15 K and . The apparent diffusion coefficient of Ce(IV) was determined by direct current polarography (DCP), cyclic voltammetry (CV) and square wave voltammetry. It was found to depend on the ionic strength and to be proportional to T.     

Estimation of standard reduction potentials of alkyl radicals involved in atom transfer radical polymerization

The redox properties of some alkyl radicals, which are important in atom transfer radical polymerization both as initiators and mimics of the propagating radical chains, have been investigated in CH₃CN by an indirect electrochemical method based on homogeneous redox catalysis involving alkyl halides (RX) and electrogenerated aromatic or heteroaromatic radical anions (D⁻). Dissociative electron transfer between RX and D⁻ yields an intermediate radical (R), which further reacts with D⁻ either by radical coupling or by electron transfer. Examination of the competition between these reactions, which depends on ED/D−°, allows determination of the standard reduction potential of R as well as the self-exchange reorganization energy λR/R⁻. The standard reduction potentials obtained for the radicals CH₂CN, CH₂CO₂Et and CH(CH₃)CO₂Me are −0.72 ± 0.06, −0.63 ± 0.07 and −0.66 ± 0.07 V vs. SCE, respectively. Quite high values of λR/R⁻ (from 122 to 164 kJ mol⁻¹) were found for all radicals, indicating that a significant change of structure accompanies electron transfer to R.     

Mechanism of formation and electronic structure of semiconducting ZnSb nanoclusters electrodeposited from an ionic liquid

Electrocrystallization of Sb and the compound semiconductor ZnSb has been investigated by in situ SPM methods at the electrified ionic liquid/Au(1 1 1) interface at an elevated temperature of 50 °C for the first time employing the ionic liquid ZnCl₂-[C₄mim]⁺Cl⁻ (45:55). Prior to the underpotential deposition (UPD) process of Sb, ZnCl₃⁻ anions adsorb on the gold surface at the open-circuit potential (OCP). An ordered region - showing the characteristic of a Moiré-like pattern - coexists with a disordered region indicative of an interfacial phase transition. When the potential is reduced to −0.40 V versus Pt/Pt(II), 2D electrocrystallization of Sb starts showing a typical structure of the first monolayer. Further decreasing the potential to −0.5 V a second layer of Sb islands occurs. Stepping the potential from the UPD region to −0.60 V, the OPD of Sb sets in showing randomly dispersed clusters of homogeneous size. Near the ZnSb deposition potential, at ∼−0.95 V, a nearly homogeneous distribution of clusters of spherical shape with diameters up to 15 nm is found. Their corresponding STS curves exhibit an obvious semiconducting behaviour with a gap-energy of ∼0.6 ± 0.2 eV. Experiments at deposition conditions on the Sb-rich or Zn-rich side relative to the ZnSb deposition potential show an obvious doping effect - in the case of Zn excess - which is revealed by the corresponding normalized conductance (NC) spectra.     

Selective oxidation of toluene over the new catalyst cobalt tetra (4-hydroxyl) phenylporphyrin supported on zinc oxide

Cobalt tetra(4-hydroxyl)phenylporphyrin/ZnO was prepared and used for the catalytic oxidation of toluene with O₂. Its activity has been increased by 70% over that of the cobalt tetra(4-hydroxyl)phenylporphyrin and the effective reuse of the supported catalyst for toluene oxidation is seven times per 1 mg (1.35 × 10⁻⁶ mol) cobalt tetra(4-hydroxyl)phenylporphyrin, demonstrating the promotion by zinc oxide.     

Characterization of mass transfer in supercritical-phase Fischer-Tropsch synthesis reaction

Mass transfer, which includes the diffusion of either reactant or hydrocarbon products in the supercritical-phase Fischer-Tropsch synthesis reaction, was studied experimentally as well as by numerical simulations. On the diffusion of the reactant gas, the relationship between catalyst effectiveness factor and the catalyst particle size or reaction temperature was studied in supercritical phase, gas phase and liquid phase, respectively. The lowest apparent Arrhenius activation energy appearing in the liquid-phase reaction could be attributed to the lowest catalyst effectiveness factor shown in this reaction phase, which was caused by the slowest diffusion of reactant in the liquid-filled catalyst pores. The higher carbon-chain growth probability achieved on the catalyst calcined at high temperature is attributed partly to the quick diffusion of CO inside the catalyst pellets as well as to the quick transportation of the primary -olefin products. Secondary reaction and diffusion behavior of the primary -olefins were studied in the various reaction phases. The effect of catalyst pellet size or contact time is also discussed.     

Ionic conductivity, infrared and Raman spectroscopic studies of 1-methyl-3-propylimidazolium iodide ionic liquid with added iodine

Polyiodides (I_x⁻, x = 3 and 5) and 2I⁻…I₂ adducts were established from the Raman spectra study of 1-methyl-3-propylimidazolium iodide (MPIm⁺I_x⁻; 1 ≤ x ≤ 5) ionic liquids containing various amounts of iodine (0 mol ≤ I₂ ≤ 2 mol). The existence of I₃⁻ and 2I⁻…I₂ was established for 1 ≤ x ≤ 2.5, symmetric I₃⁻ ions for x = 3, while linear and discrete I₅⁻ was substantiated for 3 ≤ x ≤ 5. The presence of polyiodide species in MPIm⁺I_x⁻ (1 ≤ x ≤ 5) was correlated with an enhanced ionic conductivity, attributed to the established relay-type Grotthus mechanism. Two-step conductivity increase was also reflected in decrease of the hydrogen bond interactions between the CH ring groups and polyiodides. While in the concentration range 1 ≤ x ≤ 3 (triiodides and tetraiodides) IR bands changed only slightly in intensity, in the concentration range x > 3 the CH stretching bands (3040-3170 cm⁻¹) split and the new band at 1585 cm⁻¹ appeared in the IR spectra beside the already existing Im⁺ ring stretching mode at 1566 cm⁻¹.     

Voltammetric detection of the interactions between RNO2 and electron acceptors in aqueous medium at highly boron doped diamond electrode (HBDDE)

This paper describes the electrochemical behavior of the nitrofurazone (NFZ), in predominantly aqueous medium, in the absence and presence of glutathione (reduced form) (GSH), l-cysteine (Cys) and O₂ using a highly boron doped diamond electrode (HBDDE). In presence of [Thiol] ≥ 3.7 × 10⁻² mol L⁻¹ NFZ is directly reduced to RNO-Thiol adducts in an electrochemical process involving two electrons and two protons. On the other side, O₂ acts as a RNO₂^•− scavenger and the velocity constant for the reaction, kO₂, is 60 L mol⁻¹ s⁻¹. The process is catalytic and can be used to the analytical determination of NFZ in the range of 9.9 × 10⁻⁷ ≤ [NFZ] ≤ 1.1 × 10⁻⁵ mol L⁻¹ at pH 8.0, with sensitivity of 2.2 × 10⁶ μA mol⁻¹ cm⁻² and detection limit of 3.4 × 10⁻⁷ mol L⁻¹. The analytical parameters were similar to those obtained at pH 4.0 using the direct reduction of NFZ to the respective amine derivative in a process involving six electrons and six protons. The characterization of NFZ global reduction process in aqueous medium and at relative low scan rate, 100 mV s⁻¹, was only possible due the intrinsic superficial characteristics of the HBDDE, which stabilize the RNO₂⁻ free radical, allowing to work in a large potential window, without losing the RNO₂⁻ oxidation signal.     

Grain boundaries and the influence of the ionophilic-ionophobic balance on Li and F NMR and conductivity in low-dimensional polymer electrolytes with lithium tetrafluoroborate

⁷Li and ¹⁹F NMR linewidths and impedance spectra are reported for low-dimensional CmOn (I):LiBF₄ mixtures. Data for the ionophilic polymer C18O5 is compared with that for the ionophobic C18O1 and the block copolymer C16O1O5(21%) (21 mol% of C16O5). In C18O5:LiBF₄ (1:1) narrow ⁷Li linewidths, which were observed in the liquid crystal phase above the side chain melting temperature (∼50 °C), persist in the crystal down to ca. 0 °C and broaden below −20 °C. However, in C18O1:LiBF₄ (1:0.6) narrow ⁷Li linewidths were also observed down to −20 °C suggesting highly mobile neutral aggregates of salt since this system is non-conductive. In the copolymer C16O1O5(21%):LiBF₄ (1:0.7) the linewidths were even narrower down to −70 °C with weak temperature dependence. In all systems ¹⁹F linewidths were significantly broader than ⁷Li linewidths. The complex plane plots obtained by impedance spectroscopy exhibit characteristic minima identified with ‘grain boundary’ resistance and, following heat treatment, minima with weak temperature dependence identified with ‘internal crystal’ resistance, R_i, and conductivities, σ_i ≥ 10⁻⁴ S cm⁻¹. Four-component mixtures of copolymers CmO1O5 and CmO1O4 with LiBF₄ and ‘salt-bridge’ poly(tetramethylene oxide)-dodecamethylene copolymers gave conductivities of ca. 4 × 10⁻⁴ S cm⁻¹ at 20 °C with weak temperature dependence. A novel carrier-hopping mechanism of lithium transport decoupled from side chain melting in the crystalline state is postulated.      

Amperometric hydrogen peroxide biosensor based on the immobilization of horseradish peroxidase on core-shell organosilica@chitosan nanospheres and multiwall carbon nanotubes composite

The application of the composites of multiwall carbon nanotubes (MWNTs) and core-shell organosilica@chitosan crosslinked nanospheres as an immobilization matrix for the construction of an amperometric hydrogen peroxide (H₂O₂) biosensor was described. MWNTs and positively charged organosilica@chitosan nanospheres were dispersed in acetic acid solution (0.6 wt%) to achieve organosilica@chitosan/MWNTs composites, which were cast onto a glass carbon electrode (GCE) surface directly. And then, horseradish peroxidase (HRP), as a model enzyme, was immobilized onto it through electrostatic interaction between oppositely charged organosilica@chitosan nanospheres and HRP. The direct electron transfer of HRP was achieved at HRP/organosilica@chitosan/MWNTs/GCE, which exhibited excellent electrocatalytic activity for the reduction of H₂O₂. The catalysis currents increased linearly to H₂O₂ concentration in a wide range of 7.0 × 10⁻⁷ to 2.8 × 10⁻³ M, with a sensitivity of 49.8 μA mM⁻¹ cm⁻² and with a detection limit of 2.5 × 10⁻⁷ M at 3σ. A Michaelies-Menten constant value was estimated to be 0.32 mM, indicating a high-catalytic activity of HRP. Moreover, the proposed biosensor displayed a rapid response to H₂O₂ and possessed good stability and reproducibility. When used to detect H₂O₂ concentration in disinfector samples and sterilized milks, respectively, it showed satisfactory results.     

Kinetics and selectivity of asphaltene hydrocracking

Thermal hydrocracking and catalytic hydrocracking over NiMo/γ-Al₂O₃ of a pentane-insoluble asphaltene were conducted in a microbatch reactor at 430 °C. The experimental data of asphaltene conversion fit second-order kinetics adequately, to give the apparent rate constants of 2.435 × 10⁻² and 9.360 × 10⁻² wt frac⁻¹ min⁻¹ for the two processes respectively. A three-lump kinetic model is proposed to evaluate rate constants of parallel reactions from asphaltenes to liquid oil (k₁) and to gas + coke (k₃), and consecutive reaction from liquid to gas + coke (k₂). The evaluated k₁ is 2.430 × 10⁻² and 9.355 × 10⁻² wt frac⁻¹ min⁻¹, k₂ is 2.426 × 10⁻² and 6.347 × 10⁻³ min⁻¹, and k₃ is 5.416 × 10⁻⁵ and 4.803 × 10⁻⁵ wt frac⁻¹ min⁻¹ for asphaltenes hydrocracking in the presence or absence of the catalyst, respectively. Analysis of selectivity shows that the catalytic hydrocracking process promotes liquid production and inhibits coke formation effectively.     

A third-generation hydrogen peroxide biosensor based on Horseradish Peroxidase (HRP) enzyme immobilized in a Nafion-Sonogel-Carbon composite

A third-generation biosensor based on HRP and a Sonogel-Carbon electrode has been fabricated with the aim of monitoring hydrogen peroxide in aqueous media via a direct electron transfer process. The redox activity of native HRP, typical of thin-layer electrochemistry, was observed. The charge coefficient transfer, α, and the heterogeneous electron transfer rate constant, k_s, were calculated to be 0.51 ± 0.04 and 1.29 ± 0.04 s⁻¹, respectively. Topographic study by atomic force microscopy (AFM) shows that the enzyme may have been introduced inside the ionic cluster of the Nafion. The immobilized HRP exhibited excellent electrocatalytical response to the reduction of H₂O₂ and preserved its native state after the immobilization stage. Several important experimental variables were optimized. The resulting biosensor showed a linear response to H₂O₂ over a concentration range from 4 to 100 μM, with a sensitivity of 12.8 nA/μM cm⁻² and a detection limit of 1.6 μM, calculated as (3 S.D./sensitivity). The apparent Michaelis-Menten constant was calculated to be 0.295 ± 0.020 mM. The biosensor showed high sensitivity as well as good stability and reproducibility. The performance of the biosensor was evaluated with respect to four possible interferences.