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Journal Cover Kinetics and Catalysis
  [SJR: 0.367]   [H-I: 21]   [3 followers]  Follow
    
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 1608-3210 - ISSN (Online) 0023-1584
   Published by Springer-Verlag Homepage  [2281 journals]
  • Homodesmotic method of determining the O–H bond dissociation
           energies in phenols
    • Abstract: The dissociation energy of the O–H bond has been calculated by the homodesmotic reaction method for phenolic compounds, which are well-known antioxidants, including for natural phenols. Use of moderately complex computational levels, such as B3LYP/6-31G(d), is sufficient for reliably estimating the D(O–H) value for phenols within the homodesmotic approach. The O–H bond dissociation energy for monosubstituted phenols has been calculated, and the additive character of the effect of methyl groups on D(O–H) in methylphenols has been demonstrated: the introduction of a CH3 group into the aromatic ring decreases the D value by 7.8 kJ/mol (ortho position), 1.8 kJ/mol (meta position), and 7.6 kJ/mol (para position). The O–H bond strength has been calculated for a number of ubiquinols, selenophens, flavonoids, and chromanols. The D(O–H) value recommended for α-tocopherol is 328.0 ± 1.3 kJ/mol.
      PubDate: 2016-03-01
       
  • Mechanism of the decomposition of the surface oxide film on
           polycrystalline palladium
    • Abstract: The decomposition of thin surface oxide films on polycrystalline palladium Pd(poly) at 500–1300 K was investigated by mathematical modeling. This process was analyzed in terms of a model including O2 desorption from the chemisorbed oxygen layer (Oads) and the passage of oxygen inserted under the surface layer of the metal (Oabs) and oxygen dissolved in metal subsurface layers (Odis) to the surface. O2 desorption was modeled on a surface with a square lattice of adsorption sites, with account taken of the energy of the lateral repulsive interactions between adjacent Oads atoms (εaa). At εaa = 10 kJ/mol and when the activation energy of O2 desorption for a chemisorbed-oxygen surface coverage of θ ≈ 0 is E des 0 = 230 kJ/mol, the calculated spectra are in agreement with the oxygen temperature-programmed desorption (TPD) spectra obtained for Pd(poly) at θ ≤ 0.5. The passage of Oabs and Odis atoms to the surface was calculated using a first-order equation, with account taken of the activation energy for these atoms coming out to the surface (E 2 and E 3, respectively). As the oxide film is heated, O2 desorption is accompanied by the passage of Oabs and then Odis to the surface, which leads to an increase in the Oads surface coverage and, accordingly, to a buildup of lateral surroundings in the adsorbed layer. Owing to this fact and to the repulsive interactions between Oads atoms, the bonds between Oads and the surface weaken and E des decreases. As a consequence, the O2 desorption rate increases and a low-temperature peak with T max ≈ 710 K, which is due to the passage of Oabs atoms to the surface, and then a high-temperature peak with T max ≈ 770 K, which is due to the passage of Odis atoms to the surface, appear in the TPD spectrum. At εaa = 10 kJ/mol, E des 0 = 230 kJ/mol, E 2 = 145 kJ/mol, and E 3 = 160 kJ/mol and when the number of inserted oxygen monolayers is θabs ≤ 0.3 and the number of oxygen monolayers dissolved in subsurface layers is θdis ≤ 10, the TPD spectra calculated for the given model are in agreement with the O2 TPD spectra that are observed for Pd(poly) and are due to the decomposition of surface oxide films.
      PubDate: 2016-03-01
       
  • Photophysics and photochemistry of uranyl ions in aqueous solutions:
           Refining of quantitative characteristics
    • Abstract: The photochemistry and photophysics of aqueous solutions of uranyl nitrate have been investigated by nanosecond laser photolysis with excitation at 266 and 355 nm and by time-resolved fluorescence spectroscopy. The quantum yield has been determined for (UO 2 2+ )* formation under excitation with λ = 266 and 355 nm light (φ = 0.35). The quantum yield of uranyl luminescence under the same conditions is 1 × 10–2 and 1.2 × 10–3, respectively, while the quantum yield of luminescence in the solid state is unity, irrespective of the excitation wavelength. The decay of (UO 2 2+ )* in the presence of ethanol is biexponential. The rate constants of this process at pH 3.4 are k 1 = (2.7 ± 0.2) × 107 L mol–1 s–1 and k 2 = (5.4 ± 0.2) × 106 L mol–1 s–1. This biexponential behavior is explained by the existence of different complex uranyl ion species in the solution. The addition of colloidal TiO2 to the solution exerts no effect on the quantum yield of (UO 2 2+ )* formation or on the rate of the reaction between (UO 2 2+ )* and ethanol. The results of this study have been compared with data available from the literature.
      PubDate: 2016-03-01
       
  • Synthesis, characterization, and photocatalytic properties of La 3+ -doped
           BiPO 4 photocatalysts
    • Abstract: La3+-doped BiPO4 photocatalysts were prepared via a hydrothermal process. Their morphologies, structures, and light absorption properties were evaluated. A small amount of La-doping (optimized at 2 mol %) could significantly improve the activity of BiPO4 in the degradation of methylene blue under ultraviolet irradiation. After five recycles, the La3+-doped BiPO4 did not exhibit any apparent loss in activity, confirming its stability despite recycling.
      PubDate: 2016-03-01
       
  • Effect of the oxidation state of manganese in the manganese oxides used in
           the synthesis of Mn-substituted cordierite on the properties of the
           product
    • Abstract: Catalysts based on Mn-substituted cordierite 2MnO · 2Al2O3 · 5SiO2 have been synthesized using different manganese oxides (MnO, Mn2O3, and MnO2) at a calcination temperature of 1100°C. The catalysts differ in their physicochemical properties, namely, phase composition (cordierite content and crystallinity), manganese oxide distribution and dispersion, texture, and activity in high-temperature ammonia oxidation. The synthesis involving MnO yields Mn-substituted cordierite with a defective structure, because greater part of the manganese cations is not incorporated in this structure and is encapsulated and the surface contains a small amount of manganese oxides. This catalyst shows the lowest ammonia oxidation activity. The catalysts prepared using Mn2O3 or MnO2 are well-crystallized Mn-substituted cordierite whose surface contains different amounts of manganese oxides differing in their particle size. They ensure a high nitrogen oxides yield in a wide temperature range. The product yield increases with an increasing surface concentration of Mn3+ cations. The highest NO x yield (about 76% at 800–850°C) is observed for the MnO2-based catalyst, whose surface contains the largest amount of manganese oxides.
      PubDate: 2016-03-01
       
  • Thermal stability of surface nitrogen–oxygen complexes and phase
           transitions in ZrO 2
    • Abstract: The IR spectra of surface compounds observed in the course of the temperature-programmed desorption (TPD) of NO x and the TPD spectra are compared. The high-temperature peaks of desorption are related to the decomposition of surface nitrites and nitrates. The low-temperature peaks of NO x desorption with maximums below 140°C are caused by the decomposition of surface nitrosyls. On the heating of surface nitrosyls, the following two reaction paths are possible: desorption at low temperatures and conversion into nitrates. The shape of the TPD spectra of NO depends on the phase composition of test samples. The transition of a tetragonal phase into a monoclinic one occurred upon the surface dehydroxylation of polycrystalline particles with the formation of particles with a tetragonal nucleus and a monoclinic crust. This transition is reversible. The cooling of a sample in a moist atmosphere leads to the transition of the monoclinic crust to the tetragonal phase.
      PubDate: 2016-03-01
       
  • Effect of the n -butanol addition on propargyl radical issued from the
           combustion of benzene
    • Abstract: A global kinetic model was constructed from the combination of two existing and broadly validated kinetic schemes for benzene and butanol. Then the combined model, which was able to predict the combustion behavior of benzene-butanol fuel mixtures, was implemented in the PREMIX code in conjunction with CHEMKIN II in order to investigate the effect of n-butanol addition on the formation-depletion of propargil radical C3H3, which is known to be soot precursor, in fuel-rich n-butanol-benzene flames. It was treated the dependence of the soot precursor amounts on n-butanol percentage in the fuel mixture. Then was considered the causes of the variation of this species mole fractions with benzene replacement percentage by oxygenate additive. The principal objective of the current study was to obtain fundamental understanding of the mechanisms through which the oxygenate compound affects the soot precursor amounts. It was found that n-butanol-benzene fuel mixture displayed lower propargyl radical concentrations as compared to the pure benzene flame.
      PubDate: 2016-03-01
       
  • Catalytic properties of copper chromite ferrites in water gas shift
           reaction and hydrogen oxidation
    • Abstract: The catalytic properties of a series of copper chromite ferrite samples with the composition CuCr2–x Fe x O4 (where x = 0–2) and a spinel-type structure in reactions with reducing (water gas shift reaction, WGSR) and oxidizing (the oxidation of hydrogen) reaction atmospheres were studied. The samples were obtained by the thermal decomposition of mixed hydroxo compounds. The distribution of Cu2+ ions in the tetrahedral and octahedral crystallographic positions of spinel, which depends on the Cr3+/Fe3+ ratio, affects the apparent activation energy (E a) in both of the reactions. In WGSR, E a is ∼33 kJ/mol for CuCr2O4, in which Cu2+ ions mainly occupy tetrahedral positions, whereas E a ≈ 100 kJ/mol for CuFe2O4, in which Cu2+ ions mainly occupy octahedral positions. In the reaction of hydrogen oxidation, E a is ∼71 kJ/mol for CuCr2O4 or ∼42 kJ/mol for CuFe2O4. The value of E a for the mixed chromite ferrites changes with the replacement of chromium ions by iron ions and, hence, with a ratio between the amounts of copper ions in the tetrahedral and octahedral oxygen positions of spinel.
      PubDate: 2016-03-01
       
  • Quantum chemical simulation of propylene oxidation on Ag 20
    • Abstract: The adsorption of atomic oxygen and the mechanism of propylene (C3H6) oxidation to oxide (C3H6O) on an Ag20 tetrahedral cluster were studied using density functional theory. The effects of cluster structure and active site structure on the mechanism of this reaction were considered. The oxidation of C3H6 can occur both on an edge and at the apex of the silver cluster. The C3H6O formation steps on the cluster edge are characterized by lower activation energies.
      PubDate: 2016-03-01
       
  • The role of para – meta isomerization in the selective synthesis of
           para - tert -butylphenol in the presence of modern macroporous sulfonic
           cation-exchange resins
    • Abstract: The formation of meta-tert-butylphenol (m-TBP), which decreasing the quality of the commercial product, is shown to be possible in the target synthesis of para-tert-butylphenol (p-TBP) in the presence of sulfonic cation-exchange resins using macroporous Amberlyst 36 Dry as an example. The equilibrium concentration of m-TBP in the liquid phase (the maximum value at 409 K is 47 wt %) is calculated from experimental and published data. Up to 10 wt % m-TBP (on the reaction mixture basis) is formed at the maximum values of temperature (409 K) and contact time (450 min). Kinetic characteristics are determined for the \(p - TBP + phenol\underset{{{k_{ - 6}}}}{\overset{{{k_6}}}{\longleftrightarrow}}m - TBP\) + phenol reaction, which supplements the kinetic model for the production of p-TBP. The rate constants of this reaction are several orders of magnitude smaller than those for the main (ortho–para) transformations. The temperature range (353–383 K) and contact time at which the formation of m-TBP is minimum and exerts no effect on the qualitative characteristics of the target p-isomer are determined.
      PubDate: 2016-03-01
       
  • How does the extent of substitution of methane with chlorine influence the
           mechanism and kinetics of the reactions between chloromethanes and atomic
           chlorine
    • Abstract: A theoretical investigation of the reaction mechanism and kinetics of the reaction between chloromethanes CH4–x Cl x (x = 1–3) and chlorine atoms was performed. The height of the reaction barrier was found to decrease with the degree of substitution of chloromethanes with atomic chlorine. A direct dynamics method was employed to study the kinetic nature of these hydrogen-abstraction reactions. The sequence of calculated reaction rate coefficients is: k(CH3Cl + Cl) < k(CH2Cl2 + Cl) < k(CHCl3 + Cl).
      PubDate: 2016-03-01
       
  • Kinetic study of spiramycin removal from aqueous solution using
           heterogeneous photocatalysis
    • Abstract: Spiramycin macrolide antibiotic (SPM) can be photocatalytically degraded on TiO2 (anatase variety). The experiments are done in a batch reactor and the effect of some key parameters is investigated under low energy of artificial UV light. The reaction rate is affected by varying TiO2 dose, pH and SPM concentration. Under optimized conditions, a photodegradation efficiency of 98% is achieved and the SPM photodegradation follows pseudo-first order kinetics. The Langmuir–Hinshelwood (L–H) model is successfully used to fit the experimental data, indicating the dependence of the reaction rate on the chemical reaction step. The L–H model led to the determination of both reaction kinetic and adsorption/desorption equilibrium constants. In order to give an overall estimate of the by-products, chemical oxygen demand, total organic carbon, and calculated average oxidation state monitor the photodegradation process.
      PubDate: 2016-03-01
       
  • Macrokinetics of the cationic copolymerization of isobutylene with
           isoprene
    • Abstract: A theoretical model has been developed for the macrokinetics of cationic isobutylene–isoprene copolymerization in a stirred reactor and in a tubular turbulent reactor. The model provides means to calculate the reaction mass temperature, velocity, and turbulence kinetic energy fields. The adequacy of the model has been demonstrated using the Fisher criterion. Computational experiments have been carried out to estimate the effects of the catalyst concentration and the rotational speed of the stirrer (in the case of the synthesis conducted in a stirred reactor) and the effects of reaction mixture velocity and apparatus diameter (in the case of the synthesis conducted in a tubular turbulent reactor) on molecular weight characteristics of the resulting copolymer (butyl rubber).
      PubDate: 2016-03-01
       
  • Determination of kinetic constants of the cationic copolymerization of
           isobutylene with isoprene
    • Abstract: A theoretical kinetic model has been developed for cationic isobutylene–isoprene copolymerization in methyl chloride with an AlCl3 catalyst. Kinetic constants of this process have been derived from experimental data available on copolymerization kinetics (isobutylene conversion curve) and on molecular weight characteristics of the isobutylene–isoprene copolymer (butyl rubber). The adequacy of the theoretical kinetic model of the isobutylene–isoprene copolymerization process has been demonstrated by comparing the calculated molecular weight characteristics and degree of unsaturation of butyl rubber to the corresponding independent experimental data.
      PubDate: 2016-03-01
       
  • Kinetic description of the hydrogenation of nitrobenzene and
           nitrosobenzene on skeletal nickel in aqueous solutions of propan-2-ol of
           different compositions
    • Abstract: The kinetics of the hydrogenation of nitrosobenzene and nitrobenzene is simulated taking into account accompanying processes and specific features of the conversion of reactive groups. The constants of the hypothetical reaction steps are calculated. The influence of sodium hydroxide and acetic acid on the kinetic parameters of the reactions is discussed.
      PubDate: 2016-03-01
       
  • Free-radical chain oxidation of 1,4-dioxane inhibited by
           2-thio-6-aminouracil
    • Abstract: The effect of 2-thio-6-aminouracil on the oxygen uptake kinetics has been studied in 1,4-dioxane free-radical chain oxidation as a model system. The presence of a thiocarbonyl group in the 2-position of the uracil ring makes 6-aminouracil highly reactive towards 1,4-dioxane peroxy radicals. The rate constant of the 1,4-dioxane peroxy radical interaction with 2-thio-6-aminouracil has been measured to be k 7 = (3.0 ± 0.5) × 105 L mol–1 s–1 (333 K). The stoichiometric inhibition factor f = 1.1 ± 0.1 has been determined.
      PubDate: 2016-03-01
       
  • Effect of the structure of the ortho , meta , and para isomers of
           perhydroterphenyl on their reactivity in heterogeneous catalytic
           dehydrogenation
    • Abstract: The kinetics of the dehydrogenation of the individual ortho, meta, and para isomers of perhydroterphenyl and their mixtures over a (3 wt % Pt)/C catalyst has been investigated in a flow reactor at 280–340°C. The rate of the isomerization of the stereoisomers of the initial substrate (perhydroterphenyl) and terphenyl dehydrogenation products has an effect on the hydrogen release kinetics. The highest reactivity in isomerization is shown by the ortho isomer. The largest amount of hydrogen (7.0 wt %) is released in the dehy-drogenation of perhydro-meta-terphenyl and perhydro-para-terphenyl, whose conversion at 320°C is 96%.
      PubDate: 2016-03-01
       
  • Activation of nickel–chromium hydrogenation catalysts with hydrogen
    • Abstract: The kinetics of the reduction of nickel cations in nickel oxide and nickel–chromium catalysts whose oxide precursors have different structures has been investigated by thermal analysis. The reduction of nickel oxide with a hydrogen-containing gas takes place at 250–330°C. The apparent activation energy of this reaction is about 88 kJ/mol. The introduction of up to 30 at % chromium cations into the nickel oxide structure shifts the reduction temperature of nickel in the oxide phase to 300–450°C and increases the apparent activation energy of the reduction of nickel cations to ∼108 kJ/mol. The introduction of 67 at % chromium into nickel oxide results in the formation of an oxide precursor with a spinel structure. The apparent activation energy of the reduction of nickel cations in this spinel is about 163 kJ/mol. The results of this study can be used in optimizing the composition of Ni-containing hydrogenation catalysts and their activation and operation conditions.
      PubDate: 2016-03-01
       
  • Palladium hydrogenation catalyst based on a porous carbon material
           obtained upon the dehydrochlorination of a chloro polymer
    • Abstract: The applicability of a porous carbon material obtained as a result of the “chemical” dehydrochlorination of chlorinated polyvinyl chloride as a support for palladium hydrogenation catalysts was demonstrated. The efficiency of the catalyst was evaluated in the liquid-phase reactions of nitrobenzene hydrogenation and chlorobenzene hydrodechlorination. Although the specific activity of the catalyst was lower by a factor of 3–4 than that of the samples based on Sibunit and carbon nanotubes, the complete conversion of the initial compounds with the selective formation of end products under mild conditions was achieved at a relatively low palladium content (1.5%).
      PubDate: 2016-03-01
       
  • Measuring the kinetic isotope effect at natural isotopic abundances for
           discriminating between the homogeneous and heterogeneous catalytic
           mechanisms in the Heck and Suzuki reactions
    • Abstract: The kinetic isotope effect (KIE) at the natural abundances of bromine and carbon isotopes in the substrates of the Heck and Suzuki reactions have been investigated to determine the true nature of catalyst in these reactions. Data processing has demonstrated that statistically significant differences between KIE values for the Suzuki reaction of nonactivated bromobenzene are observed upon the replacement of the soluble catalyst precursor with the insoluble one. This finding unambiguously indicates that the reaction takes place on heterogeneous palladium species. Similar experiments on the Heck reaction have demonstrated that the KIE values are insensitive to the nature of the catalyst precursor, which is consistent with the true homogeneous mechanism of catalysis.
      PubDate: 2016-01-01
       
 
 
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