Porosity of porous Al alloys

Two porosity models of porous Al alloys with different pore types (ball and polygon shape) were established. The experimental results coincide well with theoretical computations. The porosity of Al alloys (Pr _c) consists of three parts, porosity caused by preform particles (Pr _p), additional porosity (Pr _a), and porosity caused by solidification shrinkage (Pr _s).Pr _pis the main part ofPr _c whilePr _a is the key for fabricating porous Al alloys successfully in spite of its little contribution toPr _c.     

Stereoselective synthesis of malyngic acid and fulgidic acid

A new stereoselective total synthesis of malyngic acid has been achieved from a known oxazolidinone derivative via eight steps involving the Evans asymmetric alkylation as the chirality-inducing step and chelation-controlled Zn(BH₄)₂ reduction of an α-hydroxy ketone intermediate for the installation of the 12,13-anti stereochemistry. Fulgidic acid, the C12-epimer of malyngic acid, has also been synthesized in eight steps from the same starting material by using syn-selective K-Selectride reduction of an α-alkoxy ketone intermediate.     

In-situ changes of thermo-mechanical properties of poly(lactic acid) film immersed in alcohol solutions

Poly (lactic acid) (PLA) has properties suitable for many applications. However, PLA's properties are affected by environmental conditions. In this study, the glass-rubber transition temperatures (T_g) of PLA films were measured during immersion (i.e., in-situ) in pure alcohols and alcohol aqueous solutions using a dynamic mechanical analysis technique. The T_g of PLA decreased when immersed in alcohols. For pure aliphatic alcohols, the T_g reduction became smaller as the number of carbons (C1–C10) in the alcohol main chains increased. The Fox equation and the Hansen solubility parameters (HSP)/Flory-Huggins (FH) model were used to explain the T_g reduction. The relationships explained the interactions between PLA and pure alcohols with small molecules (C1–C8), but bigger pure alcohols (C9–C10) did not fit the prediction. The chemical isomerism in pure propanol (i.e., propan-1-ol and propan-2-ol) did not affect the T_g reduction. The T_g reduction in propan-2-ol aqueous solutions was concentration dependent although the partition coefficients based on the HSP and the FH parameters did not fit this relationship. The in-situ immersion of PLA in alcohol solutions could be used to evaluate the change in T_g from the T_g of dry PLA.     

Effect of graphite pore former on oxygen electrodes prepared with La0.6Sr0.4CoO3−δ nanoparticles

This study aimed at fabricating porous crack-free and delamination-free La_0.6Sr_0.4CoO_3?δ electrodes using nanopowders and investigating oxygen reduction (occurring at solid oxide fuel cell cathodes) and oxygen evolution (occurring at solid oxide electrolysis cell anodes) at 600 °C in air. The electrodes were deposited by screen-printing on Ce_0.8Gd_0.2O_1.9 substrates. The pastes were prepared with nanoparticles synthesised by flame spray synthesis and graphite pore former. Without graphite, the electrodes sintered at 1000 °C exhibit relatively low porosity and significant densification which led to partial delamination and large overpotentials. The addition of graphite, which was removed by combustion at ca. 650 °C during sintering, markedly improves electrode performance by increasing porosity and reducing densification. A minimal overpotential for both the oxygen reduction and oxygen evolution was reached for a layer porosity of ca. 50–60 vol.%.     

Voltammetric and spectroscopic analysis of interactions of pyridine mono-carboxylic acid isomers with cysteine at physiological pH

The interactions of pyridine mono-carboxylic acid isomers (PCAIs) (nicotinic acid (NA), isonicotinic acid (INA) and picolinic acid (PA)) with cysteine (RSH) at physiological pH (7.40) have been investigated by square-wave and cyclic voltammetry (SWV and CV), and UV-Vis and infrared spectroscopy. By the addition of isomeric pyridine mono-carboxylic acids, the reduction peak current of mercurous cysteine thiolate could be decreased and also its peak potential E _p was shifted to more positive values. Also, the significant changes in formal potential E ⁰′, electron transfer coefficient α and electrode reaction standard rate constant k _s of mercurous cysteine thiolate (Hg₂(SR)₂) in the presence and absence of PCAIs were observed. The results of voltammetric measurements indicated that binding reactions were occurred between PCAIs and RSH and new electroactive molecular complexes were formed, which resulted in the decrease of free cysteine concentration and the decrease of the reduction peak current of mercurous cysteine thiolate. The logarithmic values of binding constants of NA, INA and PA were calculated as 13.4, 17.7 and 18.9, respectively. The binding ratios for NA-RSH, INA-RSH and PA-RSH complexes were determined as 1: 3, 1: 4 and 1: 4, respectively. Both UV-Vis and FTIR studies also confirmed these interaction reactions.     

Formic acid reduction—XVII : Kinetic studies on the formic acid reduction of N-benzylideneaniline

The formic acid reduction of azomethine which has been reported² to proceed nearly quantitatively by the use of the formate reagent, 5HCO₂H·2NEt₃, was kinetically investigated by the carbon dioxide trapping and UV spectroscopic methods, using N-benzylideneaniline as a representative. Rate data gave the rate equation, v = (k + k′ [NEt₃])[N-benzylideneaniline][HCO₂H], indicating two-path mechanism. By the technique of using deuterated formic acids, it was found that the hydrogen bound to the carbon of formic acid is transferred as hydride to the carbon of the CN double bond in the rate determining step. The reaction is facilitated by the electron-releasing substituents of the two benzene rings of N-benzylideneaniline. On the bases of these results a possible mechanism is proposed.     

Development of negligible depletion hollow fiber membrane-protected liquid-phase microextraction for simultaneous determination of partitioning coefficients and acid dissociation constants

A new method based on negligible depletion hollow fiber-protected liquid-phase microextraction coupled with high-performance liquid chromatography (HPLC) was developed for the simultaneous determination of partitioning coefficients (K_OW) and acid dissociation constants (pK_a), by using phenol, 4-chlorophenol and 2,4-dichlorophenol as model compounds. A 37-mm length polypropylene hollow fiber membranes (600 μm inner diameter, 200 μm wall-thickness, 0.2 μm pore size, ∼70% porosity) with two-end sealed were filled with 1-octanol by ultrasonic agitation to prepare the extraction device. The extraction device was deployed in sample solutions, prepared by spiking target analytes in 1-octanol saturated aqueous solutions (500 mL), for negligible depletion extraction. After equilibrium was reached (∼5 h), the 1-octanol in the lumen of the hollow fiber membrane was collected for HPLC determination of the target analytes. As the depletion of the analytes in aqueous samples was negligible, the distribution coefficient (D_OW) could be calculated based on the measured equilibrium concentration in 1-octanol (C_O) and the initial concentration (C_W) in the aqueous sample of the target analyte (D_OW = C_O/C_W). The D_OW values measured at various pH values were nonlinearly regressed with pH to obtain the K_OW and pK_a values of a compound. Results showed that the measured values of the K_OW and pK_a of these model compounds agreed well with literature data.     

Kinetics of oxygen reduction on porous mixed conducting (La0.85Sr0.15)0.9MnO3 electrode by ac-impedance analysis

The oxygen reduction reaction on mixed conducting (La_0.85Sr_0.15)_0.9MnO₃ electrodes with various porosities was investigated by analysis of the ac-impedance spectra. To attain a mixed electronic/ionic conducting state of (La_0.85Sr_0.15)_0.9MnO₃ with high oxygen vacancy concentration, the electrode specimen was purposely subjected to cathodic polarisation. The ac-impedance spectrum clearly showed a straight line inclined at a constant angle of 45° to the real axis in the high-frequency range, followed by an arc in the low-frequency range, i.e. it exhibited the Gerischer behaviour. This strongly indicates that oxygen reduction on the mixed conducting electrode involves diffusion of oxygen vacancy through the electrode coupled with the electron exchange reaction between oxygen vacancies and gaseous oxygen (charge transfer reaction) at the electrode/gas interface. It was further recognised that the two-dimensional electrochemical active region for oxygen reduction extends from the origin of the three-phase boundaries (TPBs) among electrode, electrolyte and gas into the electrode/gas interface segments, which is on average approximately 0.7 to 1.1 μm in length below the electrode porosity 0.12. Based from the fact that the ac-impedance spectrum deviated more significantly from the Gerischer behaviour with increasing electrode porosity above 0.22, it is proposed that due to the increased length of TPBs, the rate of the overall oxygen reduction on the highly porous electrode is mainly determined by the charge transfer reaction at the TPBs, and the subsequent diffusion of oxygen vacancy occurs facilely through the electrode.     

Dimerization of electrogenerated Mo(V) in sulphuric acid solutions

The electrochemical behaviour of molybdenum(VI) in sulphuric acid solutions was investigated by cyclic voltammetry. In the reduction of Mo(VI) to Mo(III) a dimerization reaction of Mo(V) is involved; the rate constant for the reaction was estimated to be 2.79×10² M⁻¹ s⁻¹ and the activation energy was ca. 35 kJ mol⁻¹ in 0.1 M H₂O₄. Oxidation of the monomer and dimer Mo(V) species take place at −0.31 and +0.18 V (vs. SCE), respectively.     

Oxidation with alkaline permanganate using formic acid for the back-titration : Potentiometric determination of chromium

Determination of chromium by oxidation of chromite with permanganate does not give accurate results. KmnO₄ is reduced to MnO₂. Titration of KmnO₄ with Cr⁺³ solution in the presence of 0.8–1.5N NaOH and Ba⁺² ions yields manganate and gives good results. In the absence of Ba⁺² ions and in the presence of 0.5–2N NaOH reduction of KmnO₄ passes quantitatively to MnO₂.Cr⁺³ can be determined by adding the chromic solution to KmnO₄ while stirring in presence of 1N NaOH and Ba⁺² ions, or a. 2.5N NaOH in the absence ofBa⁺² ions. The excess KmnO₄ is then back-titrated with formic acid.     

Electrochemical reduction of mucochloric acid and its 5-alkoxy derivatives

5-Alkoxy-3-chloro-2(5H)-furanones were synthesized by the electrochemical reduction of 5-alkoxy derivatives of mucochloric acid in acetonitrile on the lead electrode in the presence of acetic acid as a proton donor. A combined analysis of the experimental data and quantum chemical calculation of intermediates indicates the EE _D C mechanism of reduction, including the tandem transfer of two electrons with chloride ion elimination at the stage of transfer of the second electron and protonation.     

The role of surface oxides in formic acid oxidation on Au

Compared to Pt or Pd electrodes, Au is a poor catalyst for the direct anodic oxidation of HCOOH, but the formation of Au surface oxides in acidic solutions is accompanied by a fast oxidation of HCOOH. This fast reaction is not simply a secondary reaction of Au surface oxides since those oxides are kinetically stable in HCOOH solutions. They do oxidize HCOOH only via a slow and purely electrochemical process which occurs on free Au sites and is “driven” by oxide reduction. The fast HCOOH oxidation is due to a highly reactive intermediate which is able either to form stable Au oxides Au_nO_m or to react with HCOOH. Our results are consistent with the model that by the charge transfer step a reactive non-equilibrium {Au…O> species is formed which converts to stable equilibrium oxides Au_nO_m after migration and rearrangement steps. Pre-equilibrium <Au…O> oxidizes HCOOH and this oxidation is of lower order with respect to <Au…O> compared with the formation of Au_nO_m.     

Experimental evidence of consecutive electron transfers for the Sb(III)−Sb(Hg) couple in 4 M H(Cl,ClO4) at low chloride concentrations

A dc polarographic and cyclic voltammetric study has been made of the reduction of Sb(III) ions from 0.01 M HCl+3.99 M HClO₄ and 0.001 M HCl+3.999 M HClO₄ supporting electrolytes in which a quasi-reversible, respectively irreversible behaviour is observed. It is shown that the Sb(III) reduction can be explained on the assumption of a reaction mechanism that consists of three successive one-electron transfers. Along the reduction wave the Sb(III)→Sb(II) and Sb(II)→Sb(I) step are rate determining, respectively at more negative and more positive potentials. Kinetic parameters were determined and the rate constants are shown to increase with chloride ion concentration.     

Reduction of nitroblue tetrazolium to formazan by folic acid

The reduction of nitroblue tetrazolium (NBT) to formazan by folic acid, N-(4-aminobenzoyl) glutamic acid, and other amino acids was studied in this paper. The reduction involves only one of the two tetrazolium rings of NBT. The reaction is considerably more rapid with folic acid and N-(4-aminobenzoyl) glutamic acid than with the other amino acids under study. The electron donor moiety appears to be the carboxylic acid in the alpha position. N-ethyl-N′(3-dimethylaminopropyl) carbodiimide notably increases the rate of the reaction and promotes the reduction of both tetrazolium rings.     

Characterization of nanoporous β-SiC fiber complex prepared by electrospinning and carbothermal reduction

The nanoporous β-silicon carbide (SiC) nano-sized fiber complex were made from precursors of various ratios with (SiO₂) as silicon source and polyacrylonitrile as carbon source by simple electrospinning method and economical carbothermal reduction. The prepared samples were characterized by SEM for surface shape, XRD for crystalline properties, TGA in air for oxygen resistance, and BET for porosity according to the precursor components (C/Si mol ratio). The samples with carbon ratio to silicon (C/Si) of five or more in the precursor showed the long fiber shape. Increasing the C/Si in the precursor solution tended to lead higher β-SiC crystalline and smaller crystallite size than low C/Si in the precursor solution because excess carbon could act as a dispersant and barrier to prevent neck growth or adhesion of SiC. The prepared samples have superior oxidation resistance as a whole. The increment of the C/Si leads to the increment of porosity. Also, it seems that the gaseous pore development of carbon during carbothermal reduction leads to mainly small pores (micro- or mesopores).     

Enantioselective reduction of ketophosphonates using chiral acid adducts with sodium borohydride

A method for asymmetric reduction of α-and β-ketophosphonates using a chiral complex prepared from sodium borohydride and D-or L-tartaric acid is developed. Reduction of α-or β-ketophosphonates by these reagents led to formation of corresponding (S)-or (R)-hydroxyphosphonates. Reduction of chiral di(1R,2S,5R)-menthylketophosphonates by the chiral complex NaBH₄/(R,R)-tartaric acid due to the dual compliant asymmetric induction resulted in increased stereoselectivity of the reaction and led to formation of the hydroxyphosphonates with ee 90% or higher. On the other hand, reduction of di(1R,2S,5R)-methylketophosphonates by the chiral complex NaBH₄/(S,S)-tartaric acid proceeded as non-compliant dual asymmetric induction and resulted in decreased reaction stereoselectivity leading to formation of hydroxyphosphonates with ～45–60% ee. The developed methodology was applied to the synthesis of (R)-phosphocarnitine in multigram amounts.     

Polarographic reduction of pyridinium ion in pyridine tetraethylammonium perchlorate as background electrolyte

Tetraethylammonium perchlorate, compared to lithium perchlorate as background electrolyte for the reduction of pyridinium ion in pyridine, is effective over a wider potential range, but is more difficult to obtain in a pure state; slight amounts of impurities do not, however, affect the pyridinium wave. The pyridinium wave produced in 0.1 M Et₄NClO₄ may occur at a more negative potential than the main pyridinium wave in 0.1 M LiClO₄, depending on the source of the pyridinium ion, but still appears to be due to a diffusion-controlled reduction, whose limiting current is linearly proportional to concentration; the prewave observed in LiCl0₄ background generally does not appear in Et₄NClO₄ background. Specific differences in the effect of Li(I), Na(I) and Et₄N(I) background cation appear to be due to electrocapillary phenomena and perhaps to the extent of solvation of the ions. The constancy of current for solutions containing acetic acid with added acetate, pyridinium nitrate with added nitrate, and benzoic acid with added benzoate indicate that the pyridinium reduction is independent of anion concentration.     

Preparation of hollow nanospheres g-C3N4 loaded by Keggin type Cu mono-substituted heteropoly acid with enhanced visible-light harvesting and electron transfer properties for high-efficiency photocatalysis

Abstract

A series of heterojunction catalysts (CuCN-X) were successfully fabricated by loading the different amounts of Keggin type Cu mono-substituted heteropoly acid (HPW₁₁Cu) on the surface of hollow nanospheres g-C₃N₄ (HSCN). The HSCNs were prepared by using SiO₂ as a hard template. The chemical structure, porosity, morphology and electronic property of the prepared catalysts were investigated using XRD, SEM, N₂-absorption isotherm and XPS. The results indicated that the modified HSCN show prominent absorption in the visible light range and decrease the band gap. The greatly enhanced photocatalytic activity of obtained CuCN-X have been shown by the degradation of Rhodamine B (RhB), reduction of CO₂ and production of photocatalytic hydrogen under visible light irradiation. More significantly, CuCN-15 has shown significantly improved photocatalytic performance at 4.5, 3.5 and 3.3 times higher that of than HSCN for the degradation of RhB, reduction of CO₂ and production of photocatalytic hydrogen, respectively. Furthermore, the mechanism for the enhanced photocatalytic activity of CuCN-X is proposed to be due to the formation of heterojunction. The electrons can be rapidly transferred from HSCN to HPW₁₁Cu, in which facilitate charge separation and charge transfer.     

乙酸自热重整制氢用类水滑石衍生Zn-Ni-Al-Fe-O催化剂研究

采用共沉淀法制备了Zn_(2.4)Ni_(0.6)Al_xFe_(1-x)O_(4.5±δ)(x=1/0.5/0)系列类水滑石型镍基催化剂,用于乙酸自热重整制氢,并利用XRD、H_2-TPR、BET、XPS等表征手段对催化剂进行了表征。结果表明,Zn_(2.4)Ni_(0.6)Al_(0.5)Fe_(0.5)O_(4.5±δ)催化剂在乙酸自热重整中乙酸转化率维持在100%,氢气产率为2.39 mol-H_2/mol-HAc。Zn-Al水滑石前驱体经焙烧后形成了ZnO为骨架的复合氧化物,铁的适量添加增大了催化剂的比表面积,经还原后形成Fe NiZn合金,Fe以及Zn的给电子作用提高了Ni的抗氧化能力,催化剂的抗氧化烧结和抗积炭能力得到提高。     

The effect of Ag addition on electrical properties of the thermoelectric compound Ca3Co4O9

Ca₃Co₄O₉/Ag composites incorporating different amounts of Ag were synthesized by solid-state reaction. Scanning electron microscopy revealed Ag particles dispersed among and combined with Ca₃Co₄O₉ grains several times larger in size. The electrical resistivity (ρ) of the composites is favorably lower than that of Ca₃Co₄O₉ alone and decreases with increasing Ag content. It can thus be inferred that the highly conductive Ag particles between the oxide grains contribute to the reduction of ρ. Although minimal in smaller amounts, the addition of Ag also seems to have a negative impact on the Seebeck coefficient (S) of the composites due to its poor S. Since the reduction of ρ is more significant than the degradation of S, the power factor is found to be improved by the addition of 10 wt% Ag.