Positron annihilation spectroscopy of sputtered boron carbide films

Positron annihilation spectroscopy (PAS) was carried out on boron carbide films deposited by sputtering and the results correlated to the bombardment conditions during film growth. Films were deposited with substrate bias voltages in the range of 0 to −200 V with a working pressure of 5 mTorr of Ar. Films deposited with bias voltages from −100 to −200 V present the same type of defect and the defect concentration increased linearly with the bias voltage. This defect was ascribed to vacancies in agreement with Monte Carlo simulations of Ar⁺ bombardment of boron carbide. On the other hand, films deposited with 0 V bias presented a higher S parameter values, whose origin was tentatively attributed to a relatively more open nanosized columnar structure, as suggested by the structure zone model. Annealing up to 800 °C for 30 min did not change the defect content.     

Influence of the electrochemical reduction process on the performance of graphene-based capacitors

Electrochemically reduced graphene was used as the key element in the preparation of electric double-layer capacitors where the thickness of the electrode was only a few hundred nano-meters. The resultant electrodes showed different specific capacitances after pre-reduction with scanning potential windows of −1.0 to 1.6 V, −1.5 to 0 V and −1.0 to 1.0 V. Also, a specific capacitance of 246 F/g was obtained as the graphene oxide electrode was reduced with an applied potential of −1.0 to 1.0 V for 4000 s. The influence of the residual oxygen functional groups and sp² domains in electrochemically reduced graphene were investigated for capacitance performance.     

Product distributions and kinetics of cathodic reduction of vanillin in aqueous solution

The cathodic reduction of vanillin in aqueous solution on mercury electrode was studied. The product distributions were mainly dependent on compositions of electrolyte solution. The main products in strongly basic and neutral solutions were hydravanilloin and vanillyl alcohol, respectively. The electrocatalytic effect of sodium ion in the strongly basic solution was also investigated. The results indicated that sodium ion promoted the reduction and the reaction orders were dependent on the cathodic potential. The reaction mechanism was proposed and the reaction kinetics were analyzed. The experimental results and the theoretical analysis revealed that the rate determining step shifted from a chemical reaction step to an electrochemical reaction step when the cathodic potential changed from −1.65 to −1.80 V (vs sce).     

Electrochemical characteristics of boron-doped,undoped and nitrogen-doped diamond films

Boron-doped, undoped and nitrogen-doped diamond films were synthesized by microwave plasma assisted chemical vapor deposition (MP-CVD). Raman spectroscopy, XPS, EPMA and UV–Vis were used to characterize the properties of the synthesized films. Electrochemical characteristics for several redox systems on the three kinds of diamond films were examined. For Li⁺/Li (E⁰=−3.05 V) and H⁺/H₂ (E⁰=0.00 V) redox couples, the marked differences in cyclic voltammetric (CV) behaviors were observed on the nitrogen-doped diamond films, whereas for Fe(CN)³⁻/⁴⁻ (E⁰=0.36), Au/AuCl₄⁻ (E⁰=1.00) and O₂/H₂O (E⁰=1.23 V) couples, the CV behaviors on the nitrogen-doped films were similar to those on the boron-doped or undoped diamond films. The significant differences of CV behaviors could be explained by hypothesizing that the electron transfers of the redox species in the solution on diamond electrodes happened at the top of valence band together with the surface doping model suggested by F. Maier and colleagues [F. Maier, M. Riedel, B. Mantel, J. Ristein, L. Ley, Phys. Rev. Lett. 85 (2000) 3472].     

Green preparation of vanadium carbide through one-step molten salt electrolysis

Vanadium carbide (VC) as excellent ceramic and functional material is usually prepared by carbothermal reduction of V₂O₅ which must be extracted from a typical V slag by complex processes. Pollutants, such as ammonia-nitrogen wastewater, NH₃ and CO₂ are inevitably discharged. A novel and green method for VC preparation was proposed by one-step co-electrolysis of soluble NaVO₃ and CO₂ in molten salt. It was found that VC with high purity was easily obtained by reducing electrolysis temperature and CO₂ flow rate to 600 °C and 10 mL min⁻¹ at 3.0 V. Besides VC with particles and layered stacking structure in products, a small amount of carbon and oxygen elements existed. The atomic percentage contents of C, V, and O elements in VC were about 50.0%, 44.5% and 3.8%, respectively. During electrolysis, CO₃²⁻ and VO₃⁻ was reduced at about −0.55 V (vs. Ag/AgCl) and −1.38 V (vs. Ag/AgCl), respectively. CO₃²⁻ ions were more easily reduced than VO₃⁻, and was firstly reduced to CO₂²⁻ and then converted to C. Then, VC was prepared by two routes from CO₂ and NaVO₃. One route is that VO₃⁻ ions are firstly electroreduced to VO₂⁻ ions and then are further electroreduced to VC with C. Another route is that VO₃⁻ ions are electroreduced to V which in-situ reacted with C to VC. Both VO₃⁻ and CO₃²⁻ ions are electroreduced by two-step process. In final, VC is in-situ deposited on cathode. It provides a novel and green way to prepare VC and also achieves the high value-added utilization of vanadium slag and CO₂.     

The electrical properties of (1−x)(Bi0.5Na0.5TiO3–Bi0.5K0.5TiO3–BaTiO3)–xCaZrO3 lead-free piezoelectric ceramics

Lead-free (1−x)(0.0852Bi_0.5Na_0.5TiO₃–0.12Bi_0.5K_0.5TiO₃–0.028BaTiO₃)–xCaZrO₃ piezoelectric ceramics (BNT−BKT−BT−xCZ, x=0, 0.01, 0.02, 0.03, 0.04 and 0.05) were prepared by using a conventional solid-state reaction method. The effects of CZ-doping on the structural, dielectric, ferroelectric and piezoelectric properties of the BNT−BKT−BT−xCZ system were systematically investigated. The polarization and strain behaviors indicated that the long-range ferroelectric order in the unmodified BNT−BKT−BT ceramics was disrupted by the increase of CZ-doping content, and correspondingly the depolarization temperature (T_d) shifted down from 109 °C to below room temperature. When x>0.03, accompanied with the drastic decrease in the remnant polarization (P_r) and piezoelectric coefficient (d₃₃), the electric-field-induced strain was enhanced significantly. A large unipolar strain of 0.35% under an applied electric field of 70 kV/cm (S_max/E_max=500 pm/V) was obtained in the BNT−BKT−BT−0.04CZ ceramics at room temperature, which was attributed to the reversible electric-field-induced phase transition between the relaxor and ferroelectric phases.     

The Redox Chemistry of Manganese(III) and -(IV) Complexes

The oxidation-reduction thermodynamics for the manganese(III), -(IV), and -(II) ions, and their various complexes, are reviewed for both aqueous and aprotic media. In aqueous solutions the reduction potential for the manganese(III)/(II) couple has values that range from +1.51 V vs. NHE (hydrate at pH 0) to −0.95 V (glucarate complex at pH 13.5). The Mn(IV)/(III) couple has values that range from +1.0 V (solid Mn^IVO₃ at pH 0) to −0.04 V (tris gluconate complex at pH 13.5). With anhydrous media the propensity for the Mn(III) ion to disproportionate to solid Mn^IVO₂ and Mn(II) ion is avoided. For aprotic systems the range of redox potentials for various manganese complexes is from +2.01 V and +1.30 for the Mn(IV)/(III) and Mn(III)/(II) couples (bis terpyridyl tri-N-oxide complex in MeCN), respectively, to −0.96 V for the Mn(IV)/(III) couple (tris 3,5,-di-tert-butylcatecholate complex in Me₂SO). The redox reactions between manganese complexes and dioxygen species (O₂, O₂, and H₂O₂) also are reviewed.     

Combustion, performance and emission characteristics of a DI diesel engine fueled with ethanol-biodiesel blends

In this study, Euro V diesel fuel, biodiesel, and ethanol-biodiesel blends (BE) were tested in a 4-cylinder direct-injection diesel engine to investigate the combustion, performance and emission characteristics of the engine under five engine loads at the maximum torque engine speed of 1800 rpm. The results indicate that when compared with biodiesel, the combustion characteristics of ethanol-biodiesel blends changed; the engine performance has improved slightly with 5% ethanol in biodiesel (BE5). In comparison with Euro V diesel fuel, the biodiesel and BE blends have higher brake thermal efficiency. On the whole, compared with Euro V diesel fuel, the BE blends could lead to reduction of both NO_x and particulate emissions of the diesel engine. The effectiveness of NO_x and particulate reductions increases with increasing ethanol in the blends. With high percentage of ethanol in the BE blends, the HC, CO emissions could increase. But the use of BE5 could reduce the HC and CO emissions as well.     

A viscosity function for thermoplastics blended with a thermotropic liquid crystalline polymer

The capillary flow behavior of thermoplastic blends containing thermotropic liquid crystalline polymers (TLCP) was studied both theoretically and experimentally. Significant viscosity reductions were observed for a polyethersulfone (PES) blended with a TLCP at 10 and 30 wt %, respectively. A viscosity function was developed and tested to evaluate the relationship between the blend viscosity reduction and weight fraction including some property-related parameters: η = η₀[1 − φ + φ/δ + K(1 − φ)(1 − λ)φ^ϵ]⁻¹, where η and η₀ stand for blend and matrix viscosity, respectively. ϕ is the weight fraction; δ, the viscosity ratio of the two parent components; λ, the critical stress ratio; and ϵ, an exponent. Interfacial slippage in the blend has been suggested as the flow mechanism by which a substantial reduction in the melt viscosity takes place upon the addition of a small amount of TLCP. The fact that experimental data could be fitted to the viscosity function with satisfactory accuracy should make the function an acceptable way of modeling, analyzing, and parameterizing the experimental rheological data of these blends. © 1996 John Wiley & Sons, Inc.     

Different views regarding the kinetics and mechanisms of oxygen reduction at Pt and Pd electrodes

Two diametrically different pH dependences have been reported for oxygen reduction at prereduced Pt electrodes. In the current density (cd) region with the Tafel slope of −2.3RT/F, the pH dependence expressed as dE/dpH is, according to one group of workers, −90 mV, ie −3 × 2.3RT/(2F) and − 30 mV, ie −2.3RT/(2F) in acid and alkaline solutions, respectively. The same dependences, according to another group of workers are −60 mV, ie −2.3RT/F in both acid and alkaline solutions. In the cd region with the Tafel slope of −2 × 2.3 RT/F, dE/dpH according to the first group of workers is − 120 mV and 0 mV in acid and alkaline solutions, respectively. However, according to the second group, this dependence is −60 mV in both acid and alkaline solutions. The reasons for the variation in pH dependences is discussed. A previous analysis for the pH dependence of − 2.3 RT/F at all pHs in both cd regions is in error. An account of the pH dependences of −3 × 2.3 RT/(2F) and −2.3 RT/(2F) in terms of Temkin conditions of adsorption is satisfactory and allows for the change of Tafel slopes and pH dependences with increasing cd.     

Spectroscopic study of the electrochemical behaviour of tantalum(V) chloride and oxochloride species in 1-butyl-1-methylpyrrolidinium chloride

FTIR spectroscopy was used to identify the oxochloride species of tantalum(V) in ionic liquids and to confirm the correlations between their presence in electrolytes and the changes in the route of electrochemical reduction of tantalum(V). Electrochemical behaviour of the mixtures (x)1-butyl-1-methyl-pyrrolidinium chloride-(1 − x)TaCl₅ at x = 0.80, 0.65, and 0.40 was investigated over the temperature range 90-160 °C with respect to the electrochemical deposition of tantalum and was discussed in terms of spectroscopic data. The mechanism of electrochemical reduction of tantalum(V) in the basic and acidic electrolytes depends strongly on the structure and composition of the electro active species of tantalum(V) defined by the molar composition of ionic liquids and on the competition between tantalum(V) chloride and oxochloride species. In the basic mixture at x = 0.80, with octahedral [TaCl₆]⁻ ions as the electrochemically active species only the first reduction step Ta⁵⁺ → Ta⁴⁺ at −0.31 V was observed. The competitive reduction of tantalum(V) oxochloride species occurs at more anodic potential (−0.01 V) than the reduction of the chloride complexes and can restrict the further reduction of tantalum(IV). In the basic ionic liquid at x = 0.65, the cyclic voltammograms exhibit reduction peaks at −0.31 V and −0.51 V attributed to the diffusion controlled process as [TaCl₆]⁻ + e⁻ → [TaCl₆]²⁻ and [TaCl₆]²⁻ + e⁻ → [TaCl₆]³⁻. The further irreversible reduction of tantalum(III) to metallic state may occur at −2.1 V. In the acidic ionic liquids, at x = 0.40 the electrochemical reduction of two species occurs, TaCl₆⁻ and Ta₂Cl₁₁⁻ and it is limited by two electron transfer for both of them at −0.3 V and −1.5 V, respectively.     

Efficient electrochemical reduction of nitrate to nitrogen on tin cathode at very high cathodic potentials

The electrochemical reduction of nitrate on tin cathode at very high cathodic potentials was studied in 0.1 M K₂SO₄, 0.05 M KNO₃ electrolyte. A high rate of nitrate reduction (0.206 mmol min⁻¹ cm⁻²) and a high selectivity (%S) of nitrogen (92%) was obtained at −2.9 V versus Ag/AgCl. The main by-products were ammonia (8%) and nitrite (<0.02%). Small amounts of N₂O and traces of NO were also detected.As the cathodic potential increases, the %S of nitrogen increases, while that of ammonia displays a maximum at −2.2 V. The %S of nitrite decreases from 65% at −1.8 V to <0.02% at −2.4 V. The kinetic analysis indicated that the formation of nitrogen and ammonia proceeds through the intermediate nitrite.The reduction follows first order kinetics for both nitrate and nitrite at more cathodic potentials than −2.4 V, while at less negative potentials the kinetics is more complicated.The %Faradaic efficiency (%FE) of the reduction at −2.9 V was about 60% initially and decreased to 22% at 40 min.A cathodic corrosion of tin was observed, which was more intensive in the absence of nitrate. At potentials more negative than −2.4 V, small amounts of tin hydride were detected.     

Investigations about the Reducing Properties of Dimeric Dihydropyridines

The reaction of a series of bis-[1,4-dihydropyridinyl-(4)] derivatives P₂ with some benzyl halides, trans-1,2-dibromoethane and several heteroaromatic cations Q^⊕ was investigated in acetonitrile/toluene with electrochemical and kinetic methods. For substrates with a reduction potential between −0.9 and −1.5 V the reaction occurs via the ratedetermining dissociation of P₂ into free radicals P^. as the real reducing agents, which transform the benzyl halides into the corresponding 1,2-diarylethanes and Q^⊕ into the corresponding dimeric product Q₂. For the reduction of trans-1,2-dibromocyclohexane to cyclohexene a simultaneous transfer of two electrons to the substrate combined with the dissociation of P₂ into two pyridinium ions P^⊕ is proposed. In the case of a reduction potential more positive than −0.9 V (e.g. nitro substituted benzyl halides) P₂ itself can act as the reducing agent with an increased reaction rate. With pyrylium ions Q^⊕ a product P-Q was detected by voltammetry and mass spectroscopy. In this case an electrophilic attack onto P₂ instead of the electron transfer from P₂ to Q^⊕ cannot be excluded.     

A study of the solid salt film on nickel and stainless steel

During steady state diffusion controlled dissolution of one-dimensional pit electrodes of stainless steel SS 302 and nickel, impedance and transient measurements were carried out to determine the ohmic potential drop across the salt film Δε_ΩF, the electric conductivity of the film σ_F and the film thickness d_F as a function of potential and solution composition. It was found that Δε_ΩF increases with decreasing diffusion limited current density, i_p, and with potential. For i_p → 0 it approaches a value which is independent on the bulk concentration. The critical potential for salt film formation is −0.9 and −0.08 V vs sce for SS 302 and nickel, respectively. The specific electric conductivity of the salt films is approximately 1.4 × 10⁻⁵ and 4.4 × 10⁻⁵ S cm⁻¹ for SS 302 and nickel, respectively. The salt film thickness linearly increases with potential and bulk concentration and is very thin compared to the pit depth (less than 1.5% at potentials < 1.2 V vs sce and chloride concentrations < 6 moll⁻¹).     

Electrocatalytic investigations of a tri-iron cluster towards hydrogen evolution and relevance to [FeFe]-hydrogenase

Electrochemical investigations of a tri-iron cluster, [Fe₃(μ₃-S)₂(CO)₉] (designated as Fe₃CO thereafter), which possesses a core of [Fe^IIFe^IFe^I], is described. In 0.5 M [NBu₄]BF₄/dichloromethane, the cluster exhibits a fairly reversible redox process at −1.03 V and irreversible reduction wave at ca. −1.75 V. The latter is attributed to the reductions of both the decomposed product of the monoanion, [Fe₃CO]⁻, and the isomer of this anion. This cluster catalyses proton reduction with the presence of acid HBF₄·Et₂O in dichloromethane. Possible mechanisms were proposed to elucidate its electrochemistry and electrocatalytic behaviours on proton reduction. Digital simulations were performed to verify the proposed mechanisms and kinetic parameters were generated at our best estimation in the simulations. The simulated cyclic voltammograms fit well with the experimentally observed ones, which largely supports the proposed mechanisms.     

Synthesis, electropolymerization and oxidation kinetics of an anthraquinone-functionalized poly(3,4-ethylenedioxythiophene)

The chemical synthesis of an EDOT derivative endowed with an electron acceptor anthraquinone moiety (AQ-EDOT) is described. The electrochemical polymerization of the monomer has been studied by cyclic voltammetry, chronoamperometry and chronopotentiometry. The monomer oxidation-polymerization takes places on platinum at potentials more positive than 1.3 V vs. Ag/AgCl. The polymer film presents a stable redox process with E⁰ = 0.22 V, that can be assigned to the characteristic exchange process of the parent unsubstituted PEDOT polymer. An unstable redox process at E⁰ = −1.00 V, present decreasing charges on the consecutive cycles despite that the lost reduction charge is recovered by two irreversible oxidation processes taking place at high anodic potentials 0.00 and 0.16 V. A structural charge trapping effects occurring by reduction at −1.1 V and re-oxidation at 0.16 V of the anthraquinone moiety is suggested. The stable redox process is not affected by cycling allowing the obtention of the oxidation empirical kinetics, kinetic coefficients and reaction orders. Different initial states attained by reduction at different cathodic potentials for a constant time were explored for the kinetic study.     

Electrochemical investigation of thin films of cobalt phthalocyanine and cobalt-4,4′,4″,4′″-tetracarboxyphthalocyanine and the reduction of carbon monoxide,formic acid and formaldehyde mediated by the Co(I) complexes

Cyclic voltammetric investigations of thin films of cobalt(II) phthalocyanine(1) and cobalt(II)-4,4′,4″,4′″-tetracarboxyphthalocyanine(2) in N₂-saturated 1 M sulfuric acid solution revealed that both compounds gave irreversibly semi-stable reduction products, R-1 and R-2, at around −0.70 V vs sce. Reductions of CO, formaldehyde and formic acid by R-1 or R-2 were conducted with vibrating carbon fiber electrode (cfe) modified with 1 or 2. Products of the reduction of CO were mainly formaldehyde and a small amount of methanol. In the reduction of CO, R-2/cfe was ca 17 times more efficient than R-1/cfe, which was however more efficient than the former in the reduction of formaldehyde. Reduction of formic acid by R-2 and R-1 seemed to be difficult under the conditions. Reduction of CO on the 2/cfe polarized with rectangular potential pulse (−0.659 V for 5s and 0.20 V vs Ag/AgCl for 85s) produced nearly quantitative yield of formaldehyde and small amount of methanol.     

The effects of substitution on the electrochemical reduction of naphthacenetetrone in N,N-Dimethylformamide

The polarographic reduction of 5,6,11,12-naphthacenetetrone (NT) and several of its chloromethyl, and nitro derivatives were investigated. The electrochemical behavior of NT was compared with 6, 11-dihydroxy-5, 12-naphthacencequinone (DHNQ) previously studied. Like DHNQ, NT exhibits two reduction waves in N,N-dimethylformamide. However, two additional post adsorption waves which were not present in DHNQ reduction were also observed. The first step reduction of NT, representing the formation of the radical anion NT^?, has a potential (0.01 V vs sce) much more anodic than that of its DHNQ counterpart ( ?0.75 V vs sce). The substituent effects of NTs were noted. A linear Hammett relationship similar to that of the DHNQ series was also found for the NTs. In both cases, the averaged values of σ_meta and σ_para were used as the substituent constants     

Reduction potentials of H3+xPMo12–xVxO40 and H6+xP2Mo18−xVxO62 heteropolyacid (HPA) catalysts and their catalytic activity for the vapor-phase oxidative dehydrogenation of isobutyric acid

Vanadium-containing H_6+xP₂Mo_18−xV_xO₆₂ (x = 0, 1, 2, and 3) Wells-Dawson heteropolyacid (HPA) catalysts were prepared for use in the oxidative dehydrogenation of isobutyric acid (IBA) to methacrylic acid (MAA). Vanadium-containing H_3+xPMo_12−xV_xO₄₀ (x = 0, 1, 2, and 3) Keggin HPA catalysts were also investigated for the purpose of comparison. The reduction potentials of H_3+xPMo_12−xV_xO₄₀ and H_6+xP₂Mo_18−xV_xO₆₂ catalysts were determined by temperature-programmed reduction (TPR) measurements. The reduction potentials of H_3+xPMo_12−xV_xO₄₀ and H_6+xP₂Mo_18−xV_xO₆₂ catalysts showed volcano-shaped curves and exhibited the same trend with respect to vanadium substitution. The conversions of IBA over H_3+xPMo_12−xV_xO₄₀ and H_6+xP₂Mo_18−xV_xO₆₂ catalysts also showed volcano-shaped curves and exhibited the same trend with respect to vanadium substitution. However, the H_6+xP₂Mo_18−xV_xO₆₂ catalysts showed a higher reduction potential and a higher conversion of IBA than the H_3+xPMo_12−xV_xO₄₀ catalysts at the same level of vanadium substitution. A correlation between reduction potential and catalytic activity of the HPA catalysts clearly demonstrated that the conversion of IBA increased monotonically with increasing reduction potential of the HPA catalysts, across both families of vanadium-containing HPA catalysts examined in this work.     

Apparent enthalpies of activation of electrodic oxygen reduction at platinum in different current density regions—II. Alkaline solution

Oxygen reduction (OR) at Pt electrodes in aqueous LiOH solutions containing LiClO₄ (pH = 11.6) is examined at temperatures ranging from 5 to 45°C. At all temperatures, two distinct linear E-logi regions are observed. At low current densities the Tafel slopes are close to −2.3RT/F, while at high cds they are close to −2 × 2.3 RT/F. Though the apparent enthalpy of activation is the low cd region (48 kJ mole⁻¹ at 1.0 V vs rhe) is higher than the apparent enthalpy of activation in the high cd region (27 kJ mole⁻¹ at 0.8 V vsrhe), it is concluded that the enthalpies of activation at the zero Galvani potential difference are the same in both cd regions. Consequently, the rate-determining step and the reactants and reaction products in the step are the same in both cd regions. The observed change in the Tafel slopes and the difference in the apparent enthalpies of activation arise solely from different conditions of adsorption of reacton intermediates and oxygen containing species in these two cd regions.