Rate determination for F + CH4 by real-time competitive kinetics

The rate constant for the title reaction was found to be (5.72 ± 0 30) × 10^?11 cm³ molecule^?1 s^?1. Fluorine atoms were generated via IR multiphoton dissociation of SF₆ in a mixture of CH₄, D₂, and Ar. The time-dependent concentration of fluorine atoms, and hence the rate of reaction, could be monitored either by HF IR luminescence or by DF luminescence. We have thus established that real-time observation of competitive kinetics can be used to measure a variety of reaction rates.     

Oxygen electroreduction at catalysts PtM (M = Co,Ni, or Cr)

Bimetallic catalysts PtM (M = Co, Ni, or Cr) are synthesized. They exceed purely platinum commercial catalyst E-TEK (20 wt % Pt) in its mass activity (mA/mg_Pt) and specific activity (mA/c_Pt²) in the oxygen reduction reaction. According to XRD data, the high-temperature synthesis involving metal N₄-complexes, chloroplatinic acid, and XC72 carbon black as precursors, yields alloys (or solid solutions) of the metals. The higher activity of the bimetallic catalyst PtCo/C is likely to be caused by the practically entire formation of solid solutions (Pt₃Co and PtCo), unlike PtNi and PtCr where nickel and chromium exist also as oxides that decorate the electrode surface and partly block active centers. It is shown that the mechanism of the oxygen reduction reaction at the synthesized catalysts is similar to that of oxygen reduction at the purely platinum catalyst. The slow stage in the process is transfer of the 1st electron; at potentials more positive than 0.6 V the reaction mainly yields water. The higher electrocatalytic activity of the bimetallic systems is caused by the alloy formation, which leads to changes in the bond length between platinum atoms. The achieving of the optimal bond length, as a result of the alloy formation, provides appropriate conditions for dissociative adsorption of oxygen molecules; the surface coverage with oxygen-containing particles adsorbed from water (which block active centers for O₂ adsorption) decreased. The increase in the activity may also be caused by the formation of the “core-shell” structures whose surface is enriched with platinum whose surface properties are changed under the ligand action of the core formed by the metal alloy     

Structure, magnetic, and electric properties of bismuth niobates doped with d -Elements: VI. Magnetic behavior of Bi3Nb3?3 x Ni3 x O15?δ solid solutions

Magnetic susceptibility of nickel-containing solid solutions with layered perovskite-like structure of bismuth niobate, Bi₅Nb₃O₁₅, was studied. High-spin nickel atoms, Ni(III), were present at low concentrations of the solid solutions. The formation of exchange-bonded aggregates (dimers) of nickel(II) atoms with antiferromagnetic type of exchange was found in the solid solutions. The parameters of exchange interactions in the dimer clusters and the distribution of monomers and dimers of nickel atoms were calculated as a function of the solid solution concentration. Original Russian Text ? N.A. Zhuk, I.V. Piir, N.V. Chezhina, 2008, published in Zhurnal Obshchei Khimii, 2008, Vol. 78, No. 3, pp. 393–399. For communication V, see [1].     

Specific Features of the Structure of Electrical Double Layer on Mechanically Renewed Ag–Sn Electrodes in Aqueous Surface-Inactive Electrolytes

The behavior of an Ag–Sn binary alloy (3–9 at. % Sn; region of solid solutions of Sn in Ag) in aqueous NaF solutions is studied with the aid of an impedance method. After the electrode surface renewal in potentiostatic conditions in the region of ideal polarizability, the EDL differential capacitance rapidly changes with time. These changes testify that the content of tin atoms in a surface layer increases with the duration of contact between a renewed surface and the solution, i.e. tin is a surface-active component of the alloy. On the basis of an analysis of the observed time effects in the framework of available phenomenological models for the electrode/solution interface and the experimental data obtained earlier for an Ag₃Sn electrode in aqueous NaF solutions, a mechanism is suggested, which allows one to qualitatively interpret the formation kinetics of an equilibrium interface between alloys under study and the electrolyte, which occur in the metallic phase. The mechanism includes two stages: (1) a fast escape of atoms of the surface-active component of the alloy (Sn) into a surface layer, and (2) a slower stage of the formation of intermetallic compound Ag₃Sn in a surface layer.     

Structural Characterization of Doped Calcium Tartrate Tetrahydrate

Crystals of calcium tartrate tetrahydrate were grown in silica gel medium in pure form and with barium, strontium, cobalt, nickel, manganese, zinc, and cadmium as dopants. The crystals, with formula D_xCa_1-x(C₄H₄O₆).4H₂O (where D=doping atom), were characterized by X-ray diffraction, photoelectron spectroscopy, and Raman spectroscopy. Some of the doping atoms are located in the host lattice and others are distributed at random positions, especially on the surface of the crystals. In the host lattice the doping atom replaces Ca when an earth alkaline atoms is used but occupies an interstitial site when a transition metal atom is used. The dielectric permittivity is higher in doped compounds.     

Kinetics of mechanochemical dissolution of chromium in iron

The kinetics of solid Fe–Cr solution formation has been studied during mechanical alloying of Fe and Cr powders taken in an atomic ratio of 80: 20. X-ray diffraction and Mössbauer spectroscopy data have been analyzed within the framework of the energetic approach. It has been established that, in this system, reaction product yield N is related to mechanical energy dose D and specific surface area S of Fe nanograin boundaries in an ideal manner: N ~ D² at S ~ D and N ~ D at S = const.     

The kinetics of reactions of nickel clusters with hydrogen and deuterium

The kinetics of reactions of nickel clusters with hydrogen and deuterium are studied in a laser-vaporization cluster source coupled to a continuous-flow reactor. The abslute rate constants for the addition of the first H₂ (D₂) molecule to nickel clusters Ni _n (n=7→36 for H₂ andn=7→60 for D₂) have been measured. Rate constants are found to be only weakly dependent onn forn≧14, showing a gradual increase with size that scales approximately withn ^(2/3), i.e., the cluster geometrical cross section. Reaction probabilities for clusters in this size range are approximately 0.6 for H₂ and 0.3 for D₂. Belown=14, there is a stronger dependence of reactivity on size, with Ni₉ being far less reactive than any other cluster studied. These results are compared to bulk nickel studies, and a discussion of possible correlation of reactivity to cluster structure is presented.     

Evolution of chemical bonding and electron density rearrangements during D3h → D3d reaction in monolayered TiS2: A QTAIM and ELF study

Monolayered titanium disulfide TiS₂, a prospective nanoelectronic material, was previously shown to be subject to an exothermic solid‐state D_3h–D_3d reaction that proceeds via a newly discovered transition state. Here, we study the reaction in detail using topological methods of quantum chemistry (quantum theory of atoms in molecules and electron localization function analysis) and show how electron density and chemical bonding between the atoms change in the course of the reaction. The reaction is shown to undergo a series of topological catastrophes, associated with elementary chemical events such as break and formation of bonds (including the unexpected formation of S? S bonding between sulfur layers), and rearrangement of electron density of outer valence and core shells. © 2014 Wiley Periodicals, Inc.     

Kinetics of complex formation of poly-n-vinyl carbazole and n-ethyl carbazole with electron acceptors at different molar ratios

The rates of complex formation for poly-N-vinyl carbazole and its saturated low molecular analogue, N-ethyl carbazole, with tetracyanoethylene, chloranil, trinitrobenzene and picric acid in chloroform solution have been investigated at molar ratios of donor to acceptor, D/A = 1, 2, 4, 8 and 10. The reaction rate was studied by the “stopped flow” method. At mole ratio D/A > 1 the complex forming reactions follow kinetics of first order. The apparent rate constants were calculated by the least squares method. The complex formation rate constants for the monomer analogue are higher than those for the polymer. They also depend on the electron affinity of the acceptor. The reaction rate rises with increase of the electron affinity of the acceptor.     

单磺化酞菁镓在水、微乳液中的解聚反应动力学研究

本文研究了单磺化酞菁镓（ＳＰｃＧａ）在水、微乳液（ＴｒｉｔｏｎＸ－１００－壬烷－正戊醇－水）中的二聚现象和解聚反应动力学，测定了二聚平衡常数ＫＤ和解聚速率常数ｋ。结果表明：ＳＰｃＧａ的解聚反应速率与ＳＰｃＧａ的单体浓度ＣＭ和双体浓度ＣＤ的关系为：Ｖ＝ｋ１ＣＤ－ｋ２ＣＭ２     

“Through-space” P spin-spin couplings in ferrocenyl tetraphosphine coordination complexes: Improvement in the determination of the distance dependence of JPP constants

From the analysis of several nickel and palladium halide complexes of a constrained ferrocenyl tetraphosphine, the existence in solution phase of unique ³¹P-³¹P “through-space” nuclear spin-spin coupling constants (J_PP) had been previously evidenced. Due to the blocked conformation of the species in solution, and based on the NMR spectra obtained for the complexes and their corresponding solid state X-ray structures, these J_PP constants had been shown to clearly depend on the mutual spatial position of the corresponding phosphorus atoms. Herein, the quantitative correlation disclosed at that time (P?P distance dependence of coupling constants) is remarkably confirmed, and mathematically refined owing to the study of a new palladium dibromide tetraphosphine complex, for which the synthesis and the solution NMR and solid state X-ray characterizations are reported.     

Kinetics and mechanism of the reaction between dimethylformamide and chromium(VI)

The kinetics of oxidation of N,N‐dimethylformamide by chromium(VI) has been studied spectrophotometrically in aqueous perchloric acid media at 20°C. The rate showed a first‐order dependence on both [Cr(VI)] and [DMF], and increased markedly with increasing [H⁺]. The order with respect to [HClO₄] was found to lie between 1 and 2. The rate was found to be independent of ionic strength as well as of any inhibition effect of Mn(II). The formation of superoxochromium(III) ion was detected in an aerated solution of chromium(VI), DMF and HClO₄. The proposed mechanism, involving two reaction pathways, leads to the rate law, rate = K_a1 [HCrO₄⁻] [DMF] (k_I K_a2 [H⁺]²+k_II[H⁺]). The first pathway, with rate constant k_I, involves the formation of chromium(V) and a free radical. The second pathway, with rate constant k_II, involves the formation of Cr(IV), CO₂ and dimethylamine. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 409–415, 1999     

The chemisorption of carbon monoxide on Ni(110) studied by electron energy loss spectroscopy

The vibrational spectrum of carbon monoxide chemisorbed on Ni(110) at 300K has been recorded as a function of surface coverage. At low and intermediate coverage the adsorbate is bonded either to single nickel atoms (linear site) or to two nickel atoms in contact (B₂ site). As the coverage approaches unity the spectrum changes rapidly until at saturation virtually all adsorbed molecules are of B₂ type.     

The formation and molecular structure of acetylcyclopentadienylsodium-tetrahydrofuranate

Acetylcyclopentadienylsodium has been isolated in crystalline form as a THF adduct from a reaction between cyclopentadienylsodium and methyl acetate in THF solution. The product has been characterized by means of a single-crystal X-ray diffraction study. {[C₅H₄CMeO]Na·THF}_n crystallizes in the monoclinic space group P2₁/c with unit cell parameters a 6.698(3), b 16.095(4), c 10.661(3) Å, β 92.93(3)° and D_c 1.17 g cm^?3 for Z = 4. Least-squares refinement led to a final R value of 0.080 based on 661 independent observed reflections. The coordination sphere around each sodium atom consists of the oxygen atoms from two C₅H₄CMeO ligands, the oxygen atom of the THF molecule, and an ion contact pair between the sodium and the five ring carbon atoms of the C₅H₄CMeO ligand.     

Ni(II)-baicalein complex modified multi-wall carbon nanotube paste electrode toward electrocatalytic oxidation of hydrazine

A modified electrode Ni(II)-BA-MWCNT-PE has been fabricated by electrodepositing nickel(II)-baicalein [Ni(II)-BA] complex on the surface of multi-wall carbon nanotube paste electrode (MWCNT-PE) in alkaline solution. The Ni(II)-BA-MWCNT-PE exhibits the characteristic of improved reversibility and enhanced current responses of the Ni(III)/Ni(II) couple compared with Ni(II)-BA-CPE. It also shows good electrocatalytic activity toward the oxidation of hydrazine. Kinetic parameters such as the electron transfer coefficient α, rate constant k_s of the electrode reaction, the diffusion coefficient D of hydrazine and the catalytic rate constant k_cat of the catalytic reaction are determined. Moreover, the catalytic currents present linear dependence on the concentration of hydrazine from 2.5 μM to 0.2 mM by amperometry. The detection limit and sensitivity are 0.8 μM and 69.9 μA mM⁻¹, respectively. The modified electrode for hydrazine determination is of the property of simple preparation, good stability, fast response and high sensitivity.     

Scanning electrochemical microscopy investigation of the rate of formation of a passivating TiO2 layer on a Ti G4 dental implant

Titanium and alloys with titanium as the major component are widely used for making biomedical implants, such as artificial dental roots. In our laboratory, we have studied the kinetics of the self-healing reaction of the TiO₂ film that forms on the surface of such an implant. Amperometric SECM approach curves were recorded over the surface of a grade 4 titanium (Ti G4) dental implant sample at specific times after the metal surface had been exposed to an air-saturated buffer solution. A ferrocene methanol redox mediator and a platinum microelectrode tip (r = 12.5 μm) were used in the experiments. The effective rate coefficient (k_eff) values for the mediator regenerating surface reaction were estimated using Wittstock's method from the approach curves recorded at different time points. Decreasing values of k_eff over time indicated an increasing rate of formation of the passivating TiO₂ film.     

Iron(II) sulfate oxidation with oxygen on a Pt/C catalyst: A kinetic study

The kinetics of iron(II) sulfate oxidation with molecular oxygen on the 2% Pt/Sibunit catalyst was studied by a volumetric method at atmospheric pressure, T = 303 K, pH 0.33–2.4, [FeSO₄] = 0.06?0.48 mol/l, and [Fe₂(SO₄)₃] = 0?0.36 mol/l in the absence of diffusion limitations. Relationships were established between the reaction rate and the concentrations of Fe²⁺, Fe³⁺, H⁺, and Cl^? ions in the reaction solution. The kinetic isotope effect caused by the replacement of H₂O with D₂O and of H⁺ with D⁺ was measured. The dependence of Fe²⁺ and Fe³⁺ adsorption on the catalyst pretreatment conditions was studied. A reaction scheme is suggested, which includes oxygen adsorption, the formation of a Fe(II) complex with surface oxygen, and the one-electron reduction of oxygen. The last step can proceed via two pathways, namely, electron transfer with H⁺ addition and hydrogen atom transfer from the coordination sphere of the iron(II) aqua complex. A kinetic equation providing a satisfactory fit to experimental data is set up. Numerical values are determined for the rate constants of the individual steps of the scheme suggested.     

Mechanism of the reaction of 2-haloketones with 2-aminopyridine

Time-dependent ¹H NMR spectra of DMSO-d₆ solutions of p-substituted phenacyl bromides and 2-aminopyridine indicate that the formation of imidazo[1,2-a]pyridines occurs via two relatively long-lived intermediates, C and D, which are in equilibrium with each other. The assigned structures are in accord with chemical shifts, pK_a estimates, and substituent effects (-OMeCH₃, -H, and -NO₂) on both the equilibrium constant (C?D) and rates of reaction. The slowest step in the reaction with phenacyl bromide is conversion of the intermediate D to product E. With phenacyl chloride no intermediates are observed and initial formation of C determines the overall rate. Even through the intermediate D is already protonated, its conversion to E is subject to acid catalysis. Compared to the p-OMe substituent, the p-NO₂ group enhances the rate of formation of C and D by a factor of only 2.6. The same rate enhancement is observed in the reaction of pyridine with phenacyl bromides. Rates of reaction of a given phenacyl halide with pyridine and 2-aminopyridine are similar. It is concluded that the initial reaction is alkylation of the pyridine nitrogen atom to give C and that the other possible initial condensation product, the carbinolamine F, cannot be a kinetically significant intermediate. Reasons for preferred N-alkylation are presented. Recommendations for improved syntheses of imidazo[1,2-a]pyridines are included.     

黏土负载镍的纳米片高效芳烃加氢催化剂

以天然层状黏土蒙脱石（MMT）为前体,通过液相沉积-沉淀将镍物种引入水溶液中剥离为MMT纳米片表面的简易方法制得Ni/MMT纳米片。该Ni/MMT纳米片由于是二维（2D）结构,利于芳烃及其加氢产物的传质扩散,相比Ni/SBA-15和Ni/γ-Al₂O₃催化剂,具有更为高效的芳烃加氢性能,且在镍负载量高达18.5%时,其四氢萘加氢的转化频率（TOF）达到最高值。