Kinetic characterization of Prussian Blue-modified graphite electrodes for amperometric detection of hydrogen peroxide

Prussian Blue-modified graphite electrodes (G/PB) with electrocatalytic activity toward H₂O₂ reduction were obtained by PB potentiostatic electrodeposition from a mixture containing 2.5 mm FeCl₃ + 2.5 mm K₃[Fe(CN)₆] + 0.1 m KCl + 0.1 m HCl. From cyclic voltammetric measurements, performed in KCl aqueous solutions of different concentrations (5 × 10⁻²–1 m), the rate constant for the heterogeneous electron transfer (k _s) was estimated by using the Laviron treatment. The highest k_s value (10.7 s⁻¹) was found for 1 m KCl solution. The differences between the electrochemical parameters of the voltammetric response, as well as the shift of the formal potential, observed in the presence of Cl⁻ and NO₃⁻ compared to those observed in the presence of SO₄²⁻ ions, points to the involvement of anions in the redox reactions of PB. The G/PB electrodes showed a good electrochemical stability proved by a low deactivation rate constant (0.8 × 10⁻¹² mol cm² s⁻¹). The electrocatalytic efficiency, estimated as the ratio , was found to be 3.6 (at an applied potential of 0 mV vs. SCE; Γ = 5 × 10⁻⁸ mol cm⁻²) for a H₂O₂ concentration of 5 mm, thus indicating G/PB electrodes as possible H₂O₂ sensors.     

Characterization of anodically formed polypyrrole/tungsten trioxide composites

Composite PPy/WO₃ materials were synthesized anodically under various conditions. The cyclovoltammetric switching curves are very close to the blank polypyrrole. The redox behaviour of the filler is only detected through electrochromism, leading to a yellow grey colour at +0.4 V vs SCE and to a green surface at –0.6 V. For overoxidation of the composite, a degree of overoxidation Y = 5 is found, the same as for the matrix alone. Photoelectrochemical response is due to the conducting polymer at negative potentials and even at 0 V vs SCE. A strong effect at positive potentials is due to the WO₃ pigment. From this, a flat band potential of +0.15 V vs SCE is determined for aqueous 0.1 m LiClO₄. Some non-electrochemical properties are also reported. Surface roughness increases with increasing WO₃ concentration, but for thicker layers, from MeCN, it decreases. SEM micrographs reveal homogeneous distribution of WO₃. Large secondary particles are observed in NBu₄BF₄ electrolyte. True and nominal thicknesses differ by up to a factor 1.65 due to the porous structure and this explains differences between densities measured by the flotation and the jump method. X-ray diffraction analysis exhibits unchanged WO₃-lines, but no PPy-lines due to the amorphous character. The conductivity of the composite from MeCN is much lower than expected by the volume filling of the WO₃.     

Electrochemical behaviour of poly(pyrrole) coatings on steel

The electrochemical and corrosion protective properties of electrochemically deposited thin films of polypyrrole (PPy) have been studied. Two systems have been investigated: (1) Oxalate-doped PPy; and (2) a dual layer system consisting of PPy-oxalate and PPy-poly(styrene sulfonate). Large anions like PSS are interesting, because they possibly prevent ingress into the layer by halide ions like chloride. Open circuit potential measurements and impedance spectra of both systems are compared. For intact coating systems the impedance spectra of the four systems investigated can be described by an equivalent circuit model consisting of an R _f Q _f circuit, due to the polypyrrole film itself, and a modified Randles circuit in series, to account for the double layer at the polypyrrole/electrolyte interface and the polypyrrole reaction. In 0.1 m NaCl the time to failure for the dual layer is almost 6-fold that of the single layer. This indicates that PSS is indeed capable of preventing the ingress of chloride ions. In 0.1 m Na₂SO₄ the time to failure for the dual layer is less than double that of the single layer.     

Thermal instability of plane poiseuille flow in the thermal entrance region

The onset of thermal convection in plane Poiseuille flow is investigated theoretically. New stability equations are derived by using the propagation theory considering the variations of disturbance amplitudes in the main flow direction. In the thermal entrance region an analytical procedure to predict the critical conditions for extremely small Prandtl-number fluids is described, based on the local similarity. For x_c≤0.01 the critical Rayleigh numbers are well represented in the whole domain of the Prandtl number by Ra_c = 200(1 + 0.123Pr^-1)Ra _C =200(1+0.123Pr⁻¹)x _C ⁻¹ under the conventional boundary layer theory. It is of much interest that the time-independent, three dimensional disturbances become more stable with a decrease in the Prandil number.     

Improvement of protection performance of polypyrrole by dopant anions

Electropolymerization of pyrrole on mild steel was carried out in 0.01 m sodium molybdate and 0.1 m pyrrole solution at constant current 1.5 mA cm⁻². The film is homogenous and adherent. The corrosion protection of polypyrrole films containing inhibitor anion molybdate as dopant anion was investigated with open circuit potential (OCP) in 0.1 m NaCl. The passivating effect of molybdate was demonstrated. The mobility of anion was investigated by cyclic voltammetry (CV), electrochemical quartz microbalance (EQCM) and electrochemical impedance spectroscopy (EIS) measurements.     

Reduction of dioxygen in neutral solution at polypyrrole electrode

Oxygen electroreduction has been investigated in 0.1 m Na₂SO₄ aqueous solution on an electrode covered by a polypyrrole (PPy) layer. The dependence of the reduction rate on the thickness of the PPy film has been established. The electrocatalytic properties of PPy in the studied electrode process have been confirmed. It was concluded that the nickel electrode covered with a PPy layer can be employed for water deoxygenation.     

Cathodic electrosynthesis of iron oxide films for electrochemical supercapacitors

Cathodic electrosynthesis has been utilized for the fabrication of γ-Fe₂O₃ films, containing chitosan additive as a binder. The films were studied by X-ray diffraction analysis, X-ray photoelectron spectroscopy, scanning electron microscopy, differential thermal analysis, and thermogravimetric analysis. Cyclic voltammetry and chronopotentiometry data showed that the iron oxide films exhibit electrochemical capacitance in the voltage window of −0.9 to −0.1 V vs SCE in 0.25 m Na₂SO₄ and 0.25 m Na₂S₂O₃ aqueous solutions. The highest specific capacitance (SC) of 210 F g⁻¹ was achieved using 0.25 m Na₂S₂O₃ as electrolyte, at a scan rate of 2 mV s⁻¹. The SC decreased with increasing film thickness, scan rate and cycle number. Heat treatment of the films at 140 °C resulted in increasing SC.     

Heterogeneous photocatalytic reduction of Fe(VI) in UV-irradiated titania suspensions: effect of ammonia

Results of the heterogeneous photocatalytic reduction of Fe(VI) in UV-irradiated TiO₂ suspensions in the presence of ammonia are presented. The initial rate of Fe(VI) reduction, R, may be expressed as R = k _Fe(VI)[Fe(VI)]^1.25 where k _Fe(VI) = a[Ammonia]+b), a = 6.0 × 10³ μm ^0.25 s and b = 4.1 × 10⁶ μm ^−1.25s⁻¹. The rate constant, k _Fe(VI), increases with the ammonia concentration. The photocatalytic oxidation of ammonia is enhanced in the presence of Fe(VI). A mechanism involving Fe(V) as a reactive intermediate is presented which explains the faster photocatalytic oxidation of ammonia in the presence of Fe(VI).     

Saturated fatty acid adsorption by acidified rice hull ash

Rice hull ash (RHA) was treated with 1.0 M HNO₃ (RHA-A1) and another batch was treated with 14.0 M HNO₃ (RHA-A14). RHA-A1 and RHA-A14 had a pH of 6.58 and 6.13, respectively. Adsorption of saturated fatty acids (C₈, C₁₀, C₁₂, C₁₄, C₁₆, and C₁₈) was carried out on RHA-A1 and RHA-A14 at 32±1°C. The adsorption data conformed to the Langmuir isotherm. The specific surface area of RHA-A1 was 183.84 m² g⁻¹ while that of RHA-A14 was 174.67 m² g⁻¹. The specific pore volume of RHA-A1 was 0.216 cm³ g⁻¹ while that of RHA-A14 was 0.234 cm³ g⁻¹. The acid-treated ash, RHA-A14 (q _m =0.43±0.03 mmol g⁻¹ where q _m is the amount of adsorbate adsorbed to form a monolayer coverage on the ash particles) showed a twofold increase in the adsorption of fatty acid per gram ash compared to RHA-A1 (q _m =0.25±0.03 mmol g⁻¹). The free energy of adsorption, ΔG° _ads, was determined to be −7.06±0.10 and −6.75±0.11 kcal mol⁻¹ for RHA-A1 and RHA-A14, respectively. The reduced ΔG° _ads values observed for RHA-A14 were attributed to the electrostatic repulsion of the hydrophobic chain of the fatty acid adsorbed on adjacent sites and brought into close proximity of each other. The ΔG° _ads values showed that the process of adsorption took place through physisorption on both RHA.     

Fabrication and electroanalytical characterization of label-free DNA sensor based on direct electropolymerization of pyrrole on p-type porous silicon substrates

Label-free DNA sensors based on porous silicon (PS) substrate were fabricated and electrochemically characterized. p-type silicon wafer was electrochemically anodized in an ethanolic hydrofluoric (HF) solution to construct a PS layer on which polypyrrole (PPy) film was directly electropolymerized. To achieve direct electropolymerization of PPy on PS substrate without pre-deposition of any metallic thin-film underlayer, a low resistivity wafer (0.01–0.02 Ω cm) was used. The rough surface of the PS layer allowed for a strong adsorption of the PPy film. Intrinsic negative charge of the DNA backbone was exploited to electrostatically adsorb 26 base pairs of probe DNA (pDNA) into the PPy film by applying positive bias. The pDNA was designed to hybridize with the target DNA (tDNA) which is the insertion element (Iel) gene of Salmonella enterica serovar Enteritidis. Dependence of peak current (i _p ) around 0.2 V vs Ag/AgCl on tDNA concentration and incubation time were shown from the cyclic voltammograms of PS/PPy + pDNA + tDNA substrates in a 0.01 M potassium perchlorate solution. Plot of i _p vs incubation time showed a reduction in current density (J) by ca. 29 μA cm⁻² every hour. Sensitivity obtained from a plot of i _p vs tDNA concentration was −166.6 μA cm⁻² μM⁻¹. Scanning electron microscopy (SEM) image of the cross-section of a PS/PPy + pDNA + tDNA multilayered film showed successful direct electropolymerization of PPy for a nano-PS DNA biosensor.     

Electrochemical behaviour of zirconium metal and its oxide film in LiOH and HF solutions

The electroformation andin situ characterization of ZrO₂ in 0.1m LiOH at 25°C reveal an insulating behaviour equivalent to that observed in other alkaline and acid media. Etched zirconium is found to be covered by a 2.0nm oxide film, while the growth rate of the anodic film in 0.1m LiOH is 2.1nm V^–1 and the dielectric constant is 30/r (r being the roughness factor). Chemical dissolution of the anodic film after times of up to 2.5×10⁴s is very slow in 0.1m LiOH and 1m Hcl, whereas ZrO₂ dissolves rapidly in 0.05m HF, as shown by the reciprocal capacitanceC ^–1, which is found to follow the empirical relationC ^–1=A–Bt^1/2 whereB=2nm s^–1/2 in 0.05m HF.     

Multiwalled carbon nanotubes/polypyrrole/graphene/nonwoven fabric composites used as electrodes of electrochemical capacitor

The reduced graphene oxide/nonwoven fabric (rGO/NWF) composites have been fabricated through heating the NWF coated with the mixture of GO and HONH₂·HCl at 130°C, during which the GO is chemically reduced to rGO. Then the composites of polypyrrole (PPy)/rGO/NWF have been prepared through chemically polymerizing pyrrole vapor by using the FeCl₃·6H₂O adsorbed on rGO/NWF substrate as oxidant. Finally, multiwalled carbon nanotubes (MWCNTs) are used as conductive enhancer to modify PPy/rGO/NWF through dip‐dry process to obtain MWCNTs/PPy/rGO/NWF. The prepared composites have been characterized and their capacitive properties have been evaluated in 1.0M KCl electrolyte by using two‐electrode symmetric capacitor test. The results reveal that MWCNTs/PPy/rGO/NWF possesses a maximum specific capacitance (C_sc) of about 319 F g^?1 while PPy/rGO/NWF has a C_sc of about 277.8 F g^?1 at the scan rate of 1 mV s^?1 and that optimum MWCNTs/PPy/rGO/NWF retains 94.5% of initial C_sc after 1000 cycles at scan rate of 80 mV s^?1 which is higher than PPy/rGO/NWF (83.4%). Further analysis reveals that the addition of MWCNTs can increase the charger accumulation at the outer and inner of the composites, which is favorable to improve the stability and the rapid charge‐discharge capacity. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41023.     

On-site formation of hypochlorite for indigo oxidation – Scale-up and full scale operation of an electrolyser for denim bleach processes

On-site production of hypochlorite for denim bleach was investigated using undivided unipolar electrolysers. The current efficiency of the electrolysis process was studied at laboratory scale with up to 20 A cell current and at the technical scale using a 1200 A electrolyser. NaCl solutions in the concentration range 0.236–0.944 M NaCl (13.8–55.2 g l⁻¹) were used. Anodic current densities up to 400 A m⁻² were used. Hypochlorite solutions with active chlorine concentration c(Cl)=0.10–0.12 M were obtained with current efficiency of 58–70%. For denim bleach processes a concentration of active chlorine up to c(Cl)=0.12 M is required. This corresponds to approximately 30 ml l⁻¹ of commercial hypochlorite solution. Electrical energy consumption calculated per kg of active chlorine formed ranges from 5.7 to 8.6 kWh kg⁻¹. By coupling the electrolyser to a commercial 30 kg capacity drum washing machine the electrochemically prepared oxidant was successfully used in bleach processes. The technique overcomes problems of insufficient reproducibility of bleach, chemical costs and released wasted water due to the use of commercial NaOCl solution.     

Usefulness of a composite electrode with a carbon surface modified by electrosynthesized polypyrrole for supercapacitor applications

Thin polypyrrole (PPy) films (thickness = 5–10 nm) were electrochemically deposited in situ on a carbon paste (97% of graphite plus 3% of Teflon) by means of cyclic voltammetry (CV), from an acetonitrile solution of pyrrole (0.2 M) and NaClO₄ (0.1 M). The obtained PPy/graphite composite electrode was investigated by CV and chronopotentiometry in 0.3 M NaClO₄ aqueous electrolytic solution. The capacitance of a composite electrode, calculated by CV, was about 10 F g⁻¹. The capacitance value of the composite electrode was approximately nine times larger than that of pure graphite. The massic charge and discharge capacity (Q) values, calculated by chronopotentiometry, were considerably higher for the composite electrode—by more than 60 times—than for the pure graphite electrode. Electrochemical impedance spectroscopy (EIS) measurements, performed under stationary conditions, led to an interfacial capacitance value intermediate between that of pure graphite and that of the composite electrode.     

Stable Carbon Isotope Composition of c9,t11-Conjugated Linoleic Acid in Cow’s Milk as Related to Dietary Fatty Acids

This study explores the potential use of stable carbon isotope ratios (δ¹³C) of single fatty acids (FA) as tracers for the transformation of FA from diet to milk, with focus on the metabolic origin of c9,t11-18:2. For this purpose, dairy cows were fed diets based exclusively on C₃ and C₄ plants. The FA in milk and feed were fractionated by silver-ion thin-layer chromatography and analyzed for their δ¹³C values. Mean δ¹³C values of FA from C₃ milk were lower compared to those from C₄ milk (−30.1‰ vs. −24.9‰, respectively). In both groups the most negative δ¹³C values of all FA analyzed were measured for c9,t11-18:2 (C₃ milk = −37.0 ± 2.7‰; C₄ milk −31.4 ± 1.4‰). Compared to the dietary precursors 18:2n-6 and 18:3n-3, no significant ¹³C-depletion was measured in t11-18:1. This suggests that the δ¹³C-change in c9,t11-18:2 did not originate from the microbial biohydrogenation in the rumen, but most probably from endogenous desaturation of t11-18:1. It appears that the natural δ¹³C differences in some dietary FA are at least partly preserved in milk FA. Therefore, carbon isotope analyses of individual FA could be useful for studying metabolic transformation processes in ruminants.     

Synthesis and characterization of secondary doped polypyrrole/organic modified attapulgite conductive composites

Secondary doping method was introduced into fabricating polypyrrole/oganic modified attapulgite conductive composites. The preparation conditions, such as amount of hexadecylpyridinium chloride (CPC, modifying agent), organic modified attapulgite (OATP), and HCl (secondary dopant) have been optimized to get the composites with the highest conductivity. When m_CPC/m_ATP, m_OATP/m_Py, and n_HCl/n_SA (SA is sulfamic acid) reaches 0.03, 0.6, and 0.5, respectively, the PPy/OATP composites possess the highest conductivity of 87.59 S cm^?1 as well as the highest thermal degradation temperature of 249.29°C. Scanning electron microscopy, transmission electron microscopy, X‐ray diffraction, Fourier transform infrared spectroscopy, UV‐Visible diffuse reflectance study, and X‐ray photoelectron Spectroscopy results showed that PPy chains form the core‐shell structure and may combine with OATP via π–π stacking interaction. Thermogravimetric analysis showed that the thermal stability of PPy/OATP‐SH composites was enhanced and these could be attributed to the retardation effect of OATP as barriers for the degradation of PPy. This method may open a new door for PPy‐based composites with special structures, higher performance, and thus broader application ranges. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41407.     

Electrocatalytic oxidation of 2-chlorophenol on a composite PbO2/polypyrrole electrode in aqueous solution

The electrocatalytic oxidation of 2-chlorophenol on a composite PbO₂/polypyrrole (PPy) electrode was carried out in 0.1 m H₂SO₄ solution. The composite PbO₂/PPy electrode was developed by the codeposition of polypyrrole and PbO₂ microparticles on the PbO₂/SnO₂/Ti substrate. The PbO₂ microparticles and polypyrrole in the composite electrode were observed to be hydrophilic active-sites and hydrophobic inactive-sites, respectively. The results indicated that the conversion of 2-chlorophenol and the efficiency of electrooxidation were improved on the hydrophobic-modified PbO₂/PPy electrode. The performance for electrooxidation of 2-chlorophenol on the composite PbO₂/PPy electrode was better than that on Pt or PbO₂/SnO₂/Ti electrodes. The thicker the composite (PbO₂/PPy) layer, the more active-sites in the composite electrode and the more 2-chlorophenol could be oxidized.     

C2-ceramide attenuates phenylephrine-induced vasoconstriction and elevation in [Ca2+] i in rat aortic smooth muscle

In the present study, we examined the effects of cell-permeable C₂-ceramide on contraction of aortic smooth muscle and intracellular free Ca²⁺ ([Ca²⁺] _i ). C₂-ceramide (10⁻⁷ to 10⁻⁴M) alone did not elicit any significant changes in either basal tension or resting levels of [Ca²⁺] _i in rat aortic smooth muscle. However, C₂-ceramide (10⁻⁷ to 10⁻⁴ M) attenuated phenylephrine-induced contractions in isolated rat aortic rings in a concentration-related manner, and inhibited elevations in [Ca²⁺] _i in cultured rat aortic smooth muscle cells induced by phenylephrine. C₂-ceramide-induced relaxation was found to be only slightly endothelium-dependent. However, nitric oxide inhibitors (l-NNA, l-NMMA), an inhibitor of prostanoid synthesis (indomethacin), an inhibitor of opiate actions, and several inhibitors of the pharmacologic actions of various vasoactive amines all failed to interfere with the vasorelaxant responses of C₂-ceramide. Three different inhibitors of protein kinase C, when used in a wide concentration range, also failed to interfere with the ceramide-induced relaxations. Our results suggest that the sphingomyelin-signaling pathway may play an important regulatory role in arterial wall tone.     

Electrodeposition of polypyrrole layers on aluminium from aqueous electrolytes

A screening of twenty aqueous electrolytes for a film-forming electropolymerization of pyrrole with Al was performed. Electrodeposition of well adhering homogeneous polypyrrole layers on aluminium is possible from aqueous electrolytes containing 0.1–0.8M oxalic acid. Pretreatment of the metal by polishing (PD) or by anodic (galvanostatic) activation (GA) is an essential step. In all cases, the Al₂O₃ surface layer with pores, usually filled with electrolyte, is transformed to a Al₂O₃ layer with polypyrrole filled pores. The PPy-layers on Al allow an easy cathodic coating with metals like copper. The hydrogenoxalate doped layers exhibit the ususal redox capacity,y=0.3. The sandwich structure Al/Al₂O₃/PPy represents a condenser with an unusual composite dielectric with the electronically conducting plates Al and PPy. According to our impedance measurements, the PPyin the pores is a high resistivity material due to overoxidation in the course of electropolymerization at high local current densities. The composite, Al₂O₃/overoxidized PPy, exhibits an unusually high permittivity, in the order of 10³.     

Polypyrrole electropolymerized on aluminum alloy 1100 doped with oxalate and tungstate anions

Polypyrrole films doped with oxalic acid and tungstate were potentiostatically electropolymerized on aluminum alloy 1100. Two statistical factorial designs (fractional and complete) were used to study the influence of the synthesis variables on the film performance against corrosion. Corrosion protection of the polypyrrole films doped with oxalate and tungstate anions (PPy/OXA/W) on the aluminum alloy was evaluated by potentiometric and electrochemical impedance spectroscopy (EIS) measurements in a 0.05 mol L⁻¹ NaCl solution. The results obtained showed that the best performance against corrosion was detected with the PPy/OXA/W film synthesized at 1.0 V, 1.5 C in 0.2 mol L⁻¹ pyrrole, 0.1 mol L⁻¹ oxalic acid and 0.05 mol L⁻¹ sodium tungstate solutions provide a protective effect against corrosion.