Kinetics and mechanism of peroxymonocarbonate formation

The kinetics and mechanism of peroxymonocarbonate (HCO(4)(-)) formation in the reaction of hydrogen peroxide with bicarbonate have been investigated for the pH 6-9 range. A double pH jump method was used in which (13)C-labeled bicarbonate solutions are first acidified to produce (13)CO(2) and then brought to higher pH values by addition of base in the presence of hydrogen peroxide. The time evolution of the (13)C NMR spectrum was used to establish the competitive formation and subsequent equilibration of bicarbonate and peroxymonocarbonate following the second pH jump. Kinetic simulations are consistent with a mechanism for the bicarbonate reaction with peroxide in which the initial formation of CO(2) via dehydration of bicarbonate is followed by reaction of CO(2) with H(2)O(2) (perhydration) and its conjugate base HOO(-) (base-catalyzed perhydration). The rate of peroxymonocarbonate formation from bicarbonate increases with decreasing pH because of the increased availability of CO(2) as an intermediate. The selectivity for formation of HCO(4)(-) relative to the hydration product HCO(3)(-) increases with increasing pH as a consequence of the HOO(-) pathway and the slower overall equilibration rate, and this pH dependence allows estimation of rate constants for the reaction of CO(2) with H(2)O(2) and HOO(-) at 25 °C (2 × 10(-2) M(-1) s(-1) and 280 M(-1) s(-1), respectively). The contributions of the HOO(-) and H(2)O(2) pathways are comparable at pH 8. In contrast to the perhydration of many other common inorganic and organic acids, the facile nature of the CO(2)/HCO(3)(-) equilibrium and relatively high equilibrium availability of the acid anhydride (CO(2)) at neutral pH allows for rapid formation of the peroxymonocarbonate ion without strong acid catalysis. Formation of peroxymonocarbonate by the reaction of HCO(3)(-) with H(2)O(2) is significantly accelerated by carbonic anhydrase and the model complex [Zn(II)L(H(2)O)](2+) (L = 1,4,7,10-tetraazacyclododecane).     

Preparation and formation mechanism of microporous spheric zinc phosphate

In this paper, we investigate the formation of microporous spheric zinc phosphate by anodization method. Zinc phosphate microporous spheres with diameters of 5–23 μm were prepared in H₃PO₄ and NH₄H₂PO₄ aqueous electrolyte when the applied voltage was 1 V. Microstructure was characterized by scanning electron microscopy. Composition and crystal structure were confirmed through energy dispersive spectrum and X-ray diffraction analysis. The formation mechanism of this special structure was also discussed in this paper.     

Kinetics and mechanism of benzyl chloride reaction with zinc in dimethylacetamide

Oxidative dissolution of zinc in the system of benzyl chloride-dimethylacetamide was investigated. The reaction stereochemistry as well as intermediates and reaction products formed were studied. The kinetic and thermodynamic parameters of the process were measured. The process was shown to follow the Langmuir-Hinshelwood mechanism with the formation of benzyl radicals and mono-solvated organozinc compound on the zinc surface. The components of mixture are adsorbed at various sites of the zinc surface, while recombination and the isomerization of the benzyl radicals occurs in solution.     

Kinetics and mechanism for oxime formation from 2-quinolinecarboxaldehyde

Oxime formation from 2-quinolinecarboxaldehyde occurs with rate-limiting carbinolamine dehydration under both acidic and neutral conditions. Carbinolamine dehydration occurs via a transition state bearing a single positive charge, similar to the corresponding reaction for 2-pyridinecarboxaldehyde and unlike the same reaction for 2-, 3-, and 4-formyl-1-methylpyridinium ions and for the conjugate acids of 3- and 4-pyridinecarboxaldehydes. © 1996 John Wiley & Sons, Inc.     

Kinetics and mechanism of the formation of CO2 hydrate

This article proposes a mechanism of CO₂ hydrate formation taking into account both diffusion and reaction, and gives an analysis of its kinetics. The most important assumptions on the model are that water dissolves into liquid CO₂ and reacts to form CO₂ hydrate, and that the hydrate blocks the dissolution and diffusion of water. Computational simulations were conducted, and the model proposed explains well the many observations on the CO₂ hydrate formation in previous articles. It is concluded that liquid CO₂ disposed in a deep ocean will form a very thin film of CO₂ hydrate, and this will greatly control the CO₂ diffusion in the ocean. © 1993 John Wiley & Sons, Inc.     

Low temperature synthesis of zinc ferrite nanoparticles

Zinc ferrite (ZnFe₂O₄) nanocrystalline powder materials with various particle sizes were prepared by a unique solid-state combustion method. Phase purity of ZnFe₂O₄ was confirmed by X-ray diffraction studies. High resolution transmission electron microscopic analysis and selected area diffraction pattern also confirmed the correct crystalline phase formation. Particle size was determined from both the transmission electron microscopic images and also from the XRD peak broadening analysis. Oxidation states of different elements present in ZnFe₂O₄ were determined by X-ray photoelectron spectroscopy. Frequency dependent dielectric constant and a.c. conductivity were measured as a function of particle size and both of them were found to decrease with decreasing particle size. These studies indicated that good quality zinc ferrite nanocrystalline powdered materials can be synthesized at low temperature.     

The mechanism of particle formation during homogeneous precipitation of zinc sulfide

Monodisperse zinc sulfide particles were prepared by homogeneous precipitation from zinc sulfate solutions. Particle size could be adjusted in the range of 1 to 5 microns by changing the viscosity of the reaction medium. From electron micrographs of particles prepared in high viscosity media subparticles up to 50 nm in diameter could be observed. From these results it was concluded that particle growth took place predominantly by Brownian coagulation.Furthermore, it was established that stirring the reactor content during the initial phase of particle formation alone did not influence particle size, nor did it promote coagulation of micron-sized spheres. We were able to prepare this material in amounts up to 2 kg of solid material from a reactor with a working volume of 36 l.     

Kinetics and mechanism of formation of adhesive joints based on ethylene-vinyl acetate copolymers

The structure of transient zones in PVC-poly(ethylene-co-vinyl acetate), PET-poly(ethylene-co-vinyl acetate), and steel-poly(ethylene-co-vinyl acetate) adhesion systems is studied. It is shown that PET-poly(ethylene-co-vinyl acetate) and PVC-poly(ethylene-co-vinyl acetate) are related in incompatible and partially compatible systems, respectively. In the temperature range 100?C180°C, diffusion coefficients and the activation energy of diffusion are determined. The depth of penetration of copolymer macromolecules into the PVC phase is calculated. The kinetics of adhesive-joint formation is studied. For all systems, the increase in joint strength has a common character; for each temperature, the steady state is attained. The results are analyzed in terms of wetting and diffusion theories. Both models satisfactorily describe the kinetics of joint formation in compatible and incompatible systems (including steel-poly(ethylene-co-vinyl acetate). The effective activation energy of the kinetics of adhesive-joint formation is determined and compared with the activation energies of diffusion, the viscous flow of copolymers, the ?? transition, and the rate of conformation rearrangements in the surface layers of ethylene-vinyl acetate copolymers. It is suggested that the data obtained can be generalized in terms of the Bikerman theory of a ??weak boundary layer.??     

Electropolymerization : direct film formation on metal substrates-I: Kinetics and mechanism

Attempts have been made to produce, in situ, polymer films on tinplate cathodes by the electrolysis of conducting solutions of vinyl monomers for use in the can-lacquering industry. Study of a range of vinyl monomers revealed that film formation occurs at low monomer conversion only in the electrolysis of acrylonitrile and methacrylonitrile in NN'-dimethyl formamide. The highest rates of film formation were obtained by constant current electrolysis when tetraethyl ammonium p-toluene sulphonate (McKee Salt) was used as electrolyte. The rate of film formation increases with monomer concentration to a maximum and then falls rapidly. Chain propagation occurs by an anionic mechanism with ion pair formation favoured at high monomer concentrations. The physical properties of the coloured films produced rarely approach those required industrially and the method does not represent an alternative approach to the lacquering of food and beverage cans.     

5-Arylidene aminouracils: III. Kinetics and mechanism of sodium salts formation

It extension of the studies on the search for the new biologically active 5-aminouracyl derivatives, we synthesized by the reaction of dialkylphosphites with arylideneuracils respective aminomethylphosphonates. The high level of the antiviral and anti-mycobacterial activity of the target compounds correlates well with the physicochemical parameters characterizing their structure.     

Kinetics and mechanism of formation of isomeric 1-methyl- and 2-methyl-5-vinyltetrazoles

The alkylation of 5-(β-dimethylaminoethyl)tetrazole (1) with dimethyl sulfate afforded 5-(β-dimethylaminoethyl)-1-methyltetrazole (2) and 5-(β-dimethylaminoethyl)-2-methyltetrazole (3). The exhaustive alkylation of compounds 2 and 3 at the terminal dimethylamino group gave 1-methyl-(4) and 2-methyl-5-(β-trimethylammonioethyl)tetrazole (5) methyl sulfates. The proton elimination from the α-methylene (with respect to the tetrazole cycle) groups of the quaternary ammonium cations of salts 4 and 5 by the action of a base leads to the corresponding zwitterions 4 ^± and 5 ^±, which in the rate-determining step undergo the cleavage of the nitrogen—carbon bond with the formation of 1-methyl-5-vinyl- (6) and 2-methyl-5-vinyltetrazole (7). The true constant of the transformation of zwitterion 4 ^± into tetrazole 6 is 21 times higher than that for the transformation of zwitterion 5 ^± into tetrazole 7.     

Kinetics of formation of lead and zinc sulfides in sulfiding roasting in a superheated steam atmosphere

The kinetics of the formation of sulfides in reactions of oxidized lead and zinc compounds with pyrite in a superheated steam atmosphere was studied. The dependences of the rate on the roasting duration at various temperatures were found, the kinetic parameters and modes of sulfide formation were determined, and the mechanism of the process was suggested.     

石墨烯/锌铁氧体复合物的制备及抗菌性能

用氧化还原法和化学共沉淀法分别制备了石墨烯(GE)和石墨烯/锌铁氧体(GE/ZnFe2O4)复合物,通过现代测试技术表征了样品的物相结构、组成和微观形貌.以大肠杆菌、金黄色葡萄球菌和白色念珠菌为测试菌种,分别对样品的抗菌性能进行了研究.结果表明,样品的抗菌活性受GE/ZnFe2O4复合物中GE和ZnFe2O4质量比(mG/Z)以及菌种的影响,其中mG/Z=0.4的复合物对三种菌均有较好的抗菌效果,其最小抑菌浓度分别为25、25和12.5μg/mL;复合物对白色念珠菌的抗菌效果最好,这与菌种的结构有关.此外,对样品的抗菌机理进行了详细研究.     

In-situ high-pressure x-ray diffraction study of zinc ferrite nanoparticles

We have studied the high-pressure structural behavior of zinc ferrite (ZnFe₂O₄) nanoparticles by powder X-ray diffraction measurements up to 47 GPa. We found that the cubic spinel structure of ZnFe₂O₄ remains up to 33 GPa and a phase transition is induced beyond this pressure. The high-pressure phase is indexed to an orthorhombic CaMn₂O₄-type structure. Upon decompression the low- and high-pressure phases coexist. The compressibility of both structures was also investigated. We have observed that the lattice parameters of the high-pressure phase behave anisotropically upon compression. Further, we predict possible phase transition around 55 GPa. For comparison, we also studied the compression behavior of magnetite (Fe₃O₄) nanoparticles by X-ray diffraction up to 23 GPa. Spinel-type ZnFe₂O₄ and Fe₃O₄ nanoparticles have a bulk modulus of 172 (20) GPa and 152 (9) GPa, respectively. This indicates that in both cases the nanoparticles do not undergo a Hall-Petch strengthening.     

Kinetics and mechanism of cyclodextrin inclusion complexation incorporating bidirectional inclusion and formation of orientational isomers

The kinetics of cyclodextrin (CD) inclusion complexation has been usually analyzed in terms of a one-step reaction or a consecutive two-step reaction involving intracomplex structural transformation as a second step. These schemes presume the inclusion of guest molecules through only one side of the CD cavity and the formation of unidirectional CD complexes. However, there has been increasing experimental evidence for the inclusion of guests through both sides of the CD cavity and the formation of orientational isomers for noncentrosymmetric guest molecules. This article presents a novel parallel reaction scheme for CD inclusion complexation, incorporating bidirectional inclusion and the formation of orientational isomers into the scheme. It is shown that the parallel reaction scheme gives the same concentration versus reaction time relationship as the consecutive two-step reaction scheme. The experimental methods for determining the microscopic directional rate constants are presented. The kinetic parameters of the two-step reaction scheme are expressed as functions of the directional rate constants. The ratios of orientational isomers of alpha-CD-based [2]-pseudorotaxanes and the microscopic directional rate constants of the threading and dethreading reactions are estimated from the reported thermodynamic and kinetics data obtained by using either the one-step or two-step reaction scheme. It is shown that the thermodynamic preference of an isomer over the other is mainly due to the slow dethreading rate of the isomer.     

Photocatalysis of cobalt zinc ferrite nanorods under solar light

Co_0.7Zn_0.3Fe₂O₄ nanorods were synthesized from hydrazine precursor by co-precipitation technique. Infrared and thermogravimetric–differential thermogravimetric curves of the precursor indicated the bridging bidentate nature of hydrazine and three-step thermal decomposition. The as-synthesized cobalt zinc ferrite nanorods were characterized by powder X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, transmission electron microscopy, vibrating sample magnetometry and UV–diffuse reflection spectroscopy which proposed the phase structure, morphology, magnetic and optical properties. Co_0.7Zn_0.3Fe₂O₄ nanorods showed a sensible photocatalytic activity on Congo red, Malachite green, Methylene blue, Methyl red, Rhodamine B and Rose bengal under solar light at different time intervals and were magnetically separated.