Interaction Mechanisms Between Ar–O<Subscript>2</Subscript> Post-Discharge and Stearic Acid I: Behaviour of Thin Films

We study the interaction of thick films (~4 mm) of stearic acid (SA), a C₁₈ alkane skeleton with an acid function, with late Ar–O₂ post-discharge. Contrary to what is observed with thin films of SA (~2–3 μm) which are efficiently etched (part I), only functionalization is observed over the first 2 h of treatment with a plasma source operated in the continuous mode, whatever the temperature. The heat released by surface reactions affects non-linearly the temperature of the substrate. Pulsing the source at a frequency ranging from 0.1 to 1 kHz slows down the functionalization process but does not allow any etching of the material. On the contrary, the SA can be etched as thick films by pulsing the oxygen flow rate at a frequency below 50 mHz. By pulsing the reactive gas, the time averaged value of the [O]/[O₂] ratio is decreased, limiting the functionalization processes due to oxygen atoms, and the mean temperature is lowered, decreasing the diffusion length of O₂ (and/or possibly O₂*) species in the SA which are responsible for the scission of C–C bonds of radicals.     

Formation and dielectric properties of anodic oxide films on Zr–Al alloys

Zr–Al alloys containing up to 26 at.% aluminum, prepared by magnetron sputtering, have been anodized in 0.1 mol dm⁻³ ammonium pentaborate electrolyte, and the structure and dielectric properties of the resultant anodic oxide films have been examined by grazing incidence X-ray diffraction, transmission electron microscopy, Rutherford backscattering spectroscopy, and AC impedance spectroscopy. The anodic oxide film formed on zirconium consists of monoclinic and tetragonal ZrO₂ with the former being a major phase. Two-layered anodic oxide films, comprising an outer thin amorphous layer and an inner main layer of crystalline tetragonal ZrO₂ phase, are formed on the Zr–Al alloys containing 5 to 16 at.% aluminum. Further increase in the aluminum content to 26 at.% results in the formation of amorphous oxide layer throughout the thickness. The anodic oxide films become thin with increasing aluminum content, while the relative permittivity of anodic oxide shows a maximum at the aluminum content of 11 at.%. Due to major contribution of permittivity enhancement, the maximum capacitance of the anodic oxide films is obtained on the Zr–11 at.% Al alloy, being 1.7 times than on zirconium at the formation voltage of 100 V.     

Structural evolution of the KTa0.6Nb0.4O3 alkoxide-based solutions: probing the transition metals local environment by X-ray absorption spectroscopy

The KTa_0.6Nb_0.4O₃ sols for chemical solution deposition of thin films were prepared from potassium acetate and transition metal ethoxides by the 2-methoxyethanol based route. The local environment of both transition metals after reflux times 1, 4, 24, and 48 h, whereby the crystallization behavior of the films was strongly affected, was monitored by extended X-ray absorption fine structure spectroscopy, at Ta L₃ and Nb K edges. The Ta species existed in the sols as monomers, remaining stable even with prolonged reflux time. The Ta–O–K correlations were confirmed in all cases. In contrast, the Nb-alkoxide formed dimers, with a gradual formation of oligomeric species with prolonged refluxing. The Nb–O–K correlations were present after all reflux times. The number of K neighbours around Nb increased upon refluxing, saturating at 24 h.     

Optical Emission and Raman Spectroscopy Studies of Reactivity of Low-Pressure Glow Discharges in Ar–O<Subscript>2</Subscript> and He–O<Subscript>2</Subscript> Gas Mixtures with Coked Catalysts

We demonstrate that low-pressure glow discharges in He–O₂ gas mixture are effective in removing carbonaceous surface layers from coked catalysts. These discharges contain a number of reactive species including O, O₃, and O₂*, and all these could contribute in the decoking process. However, an evolving understanding is that the O atoms in the discharge have a predominant role in this. A working hypothesis is that the O atoms react with the coke to form CO, CO₂ and other carbon complexes. Online measurements using emission spectra from O, H, and CO in the discharges are compared for the cases of He–O₂ and Ar–O₂ gas mixtures. Under the reported experimental conditions the estimated reactivity of the He–O₂ discharges is considerably higher compared with discharges in the Ar–O₂ gas mixture. Raman spectroscopy is used to confirm the removal of the coke from the surface of Pt/Alumina catalyst.     

Regularities of photostimulated conversions in nanometer aluminum layers

The gravimetric and optical spectroscopic methods reveals that light irradiation with λ = 300–750 nm and intensity I = 6.9 × 10¹⁴–1.1 × 10¹⁶ quanta cm⁻² s⁻¹ for τ = 1–160 min in atmospheric conditions significantly changes the absorption and reflection spectra and mass of aluminum films (d = 2–200 nm). The kinetic curves of the degree of conversion versus aluminum film thickness are satisfactorily described in the inverse logarithmic and parabolic terms. The contact potential difference is measured for Al and Al₂O₃ films along with the photo-EMF of Al-Al₂O₃ systems. The suggested model includes the stages of generation and redistribution of nonequilibrium charge carriers in the contact field of Al-Al₂O₃ systems, oxygen adsorption, Al³⁺ diffusion, and Al₂O₃ formation.     

Reactions of alkyl 4-hydroxybut-2-ynoates with arenes under superelectrophilic activation with triflic acid or HUSY zeolite: Alternative propargylation or allenylation of arenes,and synthesis of furan-2-ones

Reactions of alkyl 4-aryl(or 4,4-diaryl)-4-hydroxybut-2-ynoates [Ar(H or Ar')(OH)C⁴–C³≡C²–CO₂Alk] with arenes under the action of triflic acid TfOH or HUSY zeolite result in the formation of two main compounds, aryl substituted furan-2-ones or products of propargylation of electron rich arenes. Key reactive intermediates in these transformations are the corresponding O,O-diprotonated forms of starting butynoates, Ar(H or Ar')(⁺OH₂)C⁴–C³≡C²– C(=O⁺H)(OAlk), dehydration of which gives rise to mesomeric propargyl-allenyl cations Ar(H or Ar')(OH)⁴C⁺–C³≡C²–C(=O⁺H)(OAlk) ? Ar(H or Ar')(OH)⁴C = C³ = ²C⁺–C(=O⁺H)(OAlk), having two electrophilic centers on the carbons C4 and C2 respectively. Reactions of these species with arenes at C4 lead to products of arene propargylation, alternatively, reactions at C2 result in allenylation of arenes, followed by further transformation into furan-2-ones. Using quantum chemical calculations by the DFT method, it has been shown that the reactivity of such propargyl-allenyl cations is mainly explained by orbital factors. Plausible reaction mechanism is discussed.     

Two-Temperature Two-Dimensional Non Chemical Equilibrium Modeling of Ar–CO<Subscript>2</Subscript>–H<Subscript>2</Subscript> Induction Thermal Plasmas at Atmospheric Pressure

Here the authors developed a two-dimensional two-temperature chemical non-equilibrium (2T-NCE) model of Ar–CO₂–H₂ inductively coupled thermal plasmas (ICTP) around atmospheric pressure (760 torr). Assuming 22 different particles in this model and by solving mass conservation equations for each particle, considering diffusion, convection and net production terms resulting from 198 chemical reactions, chemical non-equilibrium effects were taken into account. Species density of each particle or simply particle composition was also derived from the mass conservation equation of each one taking the non chemical equilibrium effect into account. Transport and thermodynamic properties of Ar–CO₂–H₂ thermal plasmas were self-consistently calculated using the first order approximation of the Chapman–Enskog method at each iteration point implementing the local particle composition and temperature. Calculations at reduced pressure (500 and 300 torr) were also done to investigate the effect of pressure on non-equilibrium condition. Results obtained by the present model were compared with results from one temperature chemical equilibrium (1T-CE) model, one-temperature chemically non equilibrium (1T-NCE) model and finally with 2T-NCE model of Ar–N₂–H₂ plasmas. Investigation shows that consideration of non-chemical equilibrium causes the plasma volume radially wider than CE model due to particle diffusion. At low pressure with same input power, presence of diffusion is relatively stronger than at high pressure. Comparison of present reactive model with non-reactive Ar–N₂–H₂ plasmas shows that maximum temperature reaches higher in reactive C–H–O molecular system than non-reactive plasmas due to extra contribution of reaction heat.     

Electrochemical and Structural Characterisation of Dip-Coated Fe/Ti Oxide Films Prepared by the Sol-Gel Route

Thin solid films of mixed Fe/Ti oxide composition (Fe/Ti molar ratios: 0.5∶1, 1∶1, 1.5∶1) have been made from Fe(NO₃)₃ alcoholic solution to which Ti(OiPr)₄ was added. Films have been deposited by the dip-coating technique and heat-treated at 300°C and 500°C. Powders of Fe/Ti oxide heat-treated at 300°C are amorphous, while powders annealed at 500°C for 40 hours transformed to mixed rutile, pseudobrookite and hematite phases. The structure of the XRD amorphous films was identified with the help of near-normal reflection absorption (6°) (IRRA) and near-grazing incidence angle (NGIA) spectroscopy. NGIA FT-IR spectra of films are characterised with a single phonon mode appearing in the spectral range 600–950 cm⁻¹ which shifts with increasing Ti concentration from 675 cm⁻¹ (Fe₂O₃) to 904 cm⁻¹ (TiO₂) thus exhibiting one-mode behavior. Electrochemical investigations made with the help of cyclic voltammetry (CV) and chronocoulometry (CPC) performed in 0.01M LiOH and in 1M LiClO₄/propylene carbonate electrolytes revealed that films are able to uptake reversibly Li⁺ ions with a charge capacity (Q) per film thickness (d) in the range 0.1–0.26 mC/cm²nm and 0.06 mC/cm²nm, respectively. The temperature at which the films were prepared alters the rate of Li⁺ insertion which is faster for less compact films obtained at 300°C. In situ UV-VIS spectroelectrochemical measurements revealed that Fe/Ti oxide films bleached in the UV spectral region (300 nm<λ<450 nm) and colored in the VIS spectral region (450 nm<λ<800 nm), thus exhibiting mixed anodic and cathodic electrochromism.     

IR spectroelectrochemical studies of Fe2V4O13, FeVO4 and InVO4 thin films obtained via sol-gel synthesis

In this paper we generalize the IR spectroscopic properties of M³⁺VO₄ (M=Fe, In) orthovanadate and Fe₂V₄O₁₃ films. The films were prepared using the sol-gel synthesis route from M³⁺ nitrates and vanadium oxoisopropoxide. The vibrational bands in the IR absorbance spectra of the films are classified in terms of terminal V-O stretching (1050–880 cm^–1), bridging V-O^...Fe and V^...O^...Fe stretching (880–550 cm^–1), mixed V-O-V deformations and Fe-O stretching (<550 cm^–1) modes. Ex situ IR spectra of films were measured after consecutive charging/discharging to various intercalation coefficients x and correlated to the current peaks in the cyclic voltammetry curves measured in 1 M LiClO₄/propylene carbonate electrolyte. We classified the ex situ IR spectra of charged/discharged films according to their vibrational band changes. The results reveal that, for small values of the intercalation coefficient, crystalline FeVO₄, InVO₄ and Fe₂V₄O₁₃ films exhibit a simultaneous decrease in the intensity of all IR bands while the band frequencies remain unaffected. For the higher intercalation levels, IR mode frequencies are shifted, signaling the presence of reduced vanadium. Further charging leads to an amorphization of the film structure, which was established from the similarity of the IR spectra of charged films with those of amorphous films prepared at lower annealing temperatures. The results confirm that ex situ IR spectroelectrochemical measurement is an effective way to assess the structural changes in films with different levels of intercalation. Electronic Publication     

Chemiluminescence in O2 oxidation of organosodium compounds of polycyclic aromatic hydrocarbons and benzophenone

Chemiluminescence (CL) in oxidation of organosodium compounds by O₂ in THF was studied. Emitters of CL are excited complexes of polycyclic aromatic hydrocarbons, excimers¹(R·R)^*. The mechanism of their formation was proposed. The Na⁺, R^.−+O₂ CL system is a unique source for the selective generation of excimers of aromatic hydrocarbons. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 284–288, February, 1997.     

Synthesis of ZrO2–SiO2 mixed oxide by alcohol-aqueous heating method

The preparation of ZrO₂–SiO₂ mixed oxide has been carried out simply by heating an alcohol-aqueous mixture. Preparation conditions such as volume ratio of alcohol to water and prehydrolysis time of tetraethoxysilane had more important effect on the homogeneity of mixed oxide. A self-catalytic effect on the resultant Zr–O–Si formation is observed. By employing FT-IR, XRD, NH₃-TPD and pyridine adsorption FT-IR techniques, the mixing homogeneity of the mixed oxides in terms of the amount of Zr–O–Si hetero-linkages has been investigated in detail. The results indicate that the amount of Zr–O–Si hetero-linkages increases with the increase of ZrO₂ content in mixed oxides. Moreover, the phase segregation and acidity generation of mixed oxides are studied and factors responsible for good mixing are also discussed. Using this approach, a series of highly homogeneous mixed oxides with ZrO₂ content of 20–70 mol% are obtained at the optimized preparation condition.     

Mixed amphoteric oxide ZrO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> as catalyst for the conversion of 2-methyl-3-butyn-2-ol

The TPR spectra of the conversion of 2-methyl-3-butyn-2-ol (MBOH) on mixed ZrO₂-Al₂O₃ oxides with different ZrO₂ contents were obtained by desorption mass spectrometry. It was shown that MBOH is dehydrated to 3-methyl-3-buten-1-yne on the acid centers (H₀ ≥ –3.3) and is decomposed to acetylene and acetone at the basic centers (H_– ≤ +6.8) of this oxide. The formation of acid centers in the amorphous structure of ZrO₂-Al₂O₃ at 1.5 < Al/Zr < 3is explained by the Tanabe rule.     

Growth and field crystallization of anodic films on Ta–Nb alloys

The growth behavior of amorphous anodic films on Ta–Nb solid solution alloys has been investigated over a wide composition range at a constant current density of 50 A m⁻² in 0.1 mol dm⁻³ ammonium pentaborate electrolyte. The anodic films consist of two layers, comprising a thin outer Nb₂O₅ layer and an inner layer consisting of units of Ta₂O₅ and Nb₂O₅. The outer Nb₂O₅ layer is formed as a consequence of the faster outward migration of Nb⁵⁺ ions, compared with Ta⁵⁺ ions, during film growth under the high electric field. Their relative migration rates are independent of the alloy composition. The formation ratio, density, and capacitance of the films show a linear relation to the alloy composition. The susceptibility of the anodic films to field crystallization during anodizing at constant voltage increases with increasing niobium content of the alloy.     

Protonation sites in methyl nitrate and the formation of transient CH4NO3 radicals. A neutralization—reionization mass spectrometric and computational study

Protonation sites in methyl nitrate (1) were evaluated computationally at the Gaussian 2(MP2) level of ab initio theory. The methoxy oxygen was the most basic site that had a calculated proton affinity of PA = 728–738 kJ mol⁻¹ depending on the optimization method used to calculate the equilibrium geometry of the CH₃O(H)-NO₂⁺ ion (2⁺). Protonation at the terminal oxygen atoms in methyl nitrate was less exothermic; the calculated proton affinities were 725, 722, and 712 kJ mol⁻¹ for the formation of the syn-syn, anti-syn, and syn-anti ion rotamers 3a⁺, 3b⁺, and 3c⁺, respectively. Ion 2⁺ was prepared by an ion-molecule reaction of NO₂⁺ with methanol and used to generate the transient CH₃O(H)-NO₂^. radical (2) by femtosecond collisional electron transfer. Exothermic protonation of 1 produced a mixture of 3a⁺–3c⁺ with 2⁺ that was used to generate transient radicals 3a–3c. Radical 2 was found to be unbound and dissociated without barrier to methanol and NO₂. Radicals 3a–3c were calculated to be weakly bound. When formed by vertical neutralization, 3a–3c dissociated completely on the 4.2 μs time scale of the experiment. The main dissociations of 3a–3c were formations of CH₃O^. + HONO and CH₃ONO + OH^.. The gas-phase chemistry of radicals 3a–3c and their dissociation products, as studied by neutralization—reionization mass spectrometry, was dominated by Franck—Condon effects on collisional neutralization and reionization. The adiabatic ionization energies of 3a–3c were calculated as 7.54, 7.57, and 7.66 eV, respectively.     

Iron(III)–salen–H<Subscript>2</Subscript>O<Subscript>2</Subscript> as a peroxidase model: electron transfer reactions with anilines

Abstract  

Iron(III)–salen complexes catalyze the H₂O₂ oxidation of various ring-substituted anilines in MeCN have been studied, and [O=Fe^IV(salen)]^+· is proposed as the active species. Study of the kinetics of the reaction by spectrophotometry shows the emergence of a new peak at 445 nm in the spectrum which corresponds to azobenzene. Further oxidation of azobenzene by H₂O₂ leads to the formation of azoxybenzene. ESI–MS studies also support the formation of these products. The rate constants for the oxidation of meta- and para-substituted anilines were determined from the rate of decay of oxidant as well as the rate of formation of azobenzene, and the reaction follows Michaelis–Menten kinetics. The rate data show a linear relationship with the Hammett σ constants and yield a ρ value of −1.1 to −2.4 for substituent variation in the anilines. A reaction mechanism involving electron transfer from aniline to [O=Fe(salen)]^+· is proposed. The presence of axial ligands modulates the activity of the complex.     

Preparation of Co–Al and Ni–Al layered double hydroxide thin films by a sol–gel process with hot water treatment

Using hot water treatment of sol–gel derived precursor gel films, Co–Al and Ni–Al layered double hydroxide (LDH) thin films were prepared. The precursor gel films of Al₂O₃–CoO or Al₂O₃–NiO were prepared from cobalt or nickel nitrates and aluminum tri-sec-butoxide using the sol–gel method. Then, the precursor gel films were immersed in a NaOH aqueous solution of 100 °C. Nanocrystallites of Co–Al and Ni–Al LDH were precipitated with the hot water treatment with NaOH solution. The largest amounts of nanocrystals were obtained with a solution of pH = 10 for Co–Al LDH, and with that of pH = 9 for Ni–Al LDH. X-ray diffraction measurements confirmed that this process formed CO₃ ²⁻ intercalated LDHs. Both Co–Al and Ni–Al LDH thin films were confirmed to work as electrodes for electrochemical devices by cyclic voltammogram measurements.     

Electrochemical behavior of nanocrystalline diamond thin films grown in electrical arc plasma

Nitrogenated nanocrystalline diamond films with controlled electrical conductivity are grown in electrical arc plasma in CH₄/H₂/Ar/N₂ gas mixtures and characterized by scanning electron microscopy and spectroscopic measurements. Their electrochemical properties are studied by electrochemical impedance spectroscopy. Transfer coefficients of reactions in the [Fe(CN)₆]^3−/4− redox system are determined. The electrochemical behavior of the material is controlled by its nitrogenation (3–20% N₂ in the reaction gas mixture). The nitrogenated nanocrystalline diamond has higher differential capacitance in indifferent electrolyte (1 M KCl) solution than not nitrogenated one; the nitrogenation also increases the reversibility of reactions in the [Fe(CN)₆]^3−/4− redox system. By and large, with nitrogenation of diamond, its electrochemical behavior changes from the one characteristic of a “poor conductor” to that characteristic of metallike conductor. In this respect the nanocrystalline diamond electrodes grown in the electrical arc plasma are similar to those grown in microwave plasma.     

Kinetics and Mechanism of Chromium(III)-oxalates-2-hydroxynicotinate and Chromium(III)-oxalates-3-hydroxypicolinate Aquation in HClO4 Solutions

New chromium(III) complexes, [Cr(C₂O₄)₂(2-hnic)]²⁻ and [Cr(C₂O₄)₂(3-hpic)]²⁻ (where 2-hnic = O,O′-bonded 2-hydroxynicotinic acid and 3-hpic = N,O-bonded 3-hydroxypicolinic acid), were obtained and characterized in solution. The acid-catalyzed aquation of the both complexes leads to liberation of the appropriate pyridinecarboxylic acid and formation of cis-[Cr(C₂O₄)₂(H₂O)₂]⁻. Kinetics of these reactions were studied spectrophotometrically in the 0.1–1.0 M HClO₄ range, at I = 1.0 M. In the case of [Cr(C₂O₄)₂(2-hnic)]²⁻, a slow chelate-ring opening at the Cr–O (phenolate) bond is followed by a fast Cr–O (carboxylate) bond breaking. The rate law: k_obs = k_HQ_H[H⁺] was established, where k_H is the acid-catalyzed rate constant and Q_H is the protonation constant of the coordinated phenolate oxygen atom. In the case of [Cr(C₂O₄)₂(3-hpic)]²⁻, the reversible chelate-ring opening at Cr–N bond is followed by the rate determining step – the one-end bonded ligand liberation. The rate law for the first step was determined: k_obs = k₁+k₋₁/Q₁[H⁺], where k₁ and k₋₁ are the rate constants of the chelate-ring opening and closure and Q₁ is the protonation constant of the pyridine nitrogen atom. The aquation mechanisms are proposed and the effect of ligand coordination mode on complex reactivity is discussed.     

On the nature of the processes occurring in silver doped SiO<Subscript>2</Subscript> films under heat treatment

The processes taking place on air-heating of SiO₂−Ag⁺ films and xerogels produced from the SiO₂ sols of different pH (3.7 or 9.5) were investigated. Silver nanoparticles 10–40 nm in size tolerant to oxidation at temperatures above 600 °C were found to be formed in the systems whatever the pH value of the starting sol. SiO₂ crystallization giving the cristobalite phase in the temperature range from 500 to 800 °C was shown to proceed only in the films produced from the acidic sol, while in those formed from the alkali one SiO₂ remained amorphous. A mechanism by which the formation of Ag nanoparticles and the cristobalite phase occurs in the films at the oxidative conditions is suggested.     

Interaction of O+ ion with water molecules: A quantum-chemical study

The interaction of O⁺ ion with several (from one to four) water molecules was studied by theab initio (UMP4/4-31G^*) and semiempirical (AM1) quantum-chemical methods. It was found that the energy of binding the O⁺ ion to the first water molecule is appreciably higher than those of binding to the subsequent water molecules. In the complex with a water molecule, whose structure corresponds to that of water oxide, the O⁺ ion retains high reactivity. The barrier to the transfer of O⁺ ion to another water molecule is much lower than the barrier to analogous transfer of O atom from the molecule of water oxide, despite the lower dissociation energy of the H₂O−O bond. Consideration of subsequent interactions with water molecules leads to an increase in the barrier to the transfer of O⁺ ion. The doublet and quadruplet excited states of the O⁺+2 H₂O system were also studied. In these cases, the formation energies are well described by the ion-dipole model. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 981–988, June, 2000.