The mechanism of ascorbic acid oxidation by Cu(II)-poly-4-vinylpyridine complexes

The mechanism of ascorbic acid (DH₂) oxidation with molecular oxygen catalysed by the polynuclear complex of Cu²⁺ with poly-4-vinylpyridine (PVP), partially quaternized by dimethylsulphate, has been studied. The half-conversion time of the reaction of DH₂ with Cu(II) PVP under anaerobic conditions is independent of [Cu²⁺]. At pH 3.5, t_0.5 (sec) = 0.8 + 5 × 10^?4 [DH₂]. The formation of an intermediate cupric-ascorbate complex is suggested (K_c ≈ 10⁴ M^?1). Free radicals of ascorbic acid are detected by the ESR-method combined with a flow technique. The small steady-state concentration of radicals indicates that their decay occurs inside the macromolecular complex. The rate constant of the PVP Cu(II) DH^? ternary complex dissociation is ≈0.4 sec^?1 (pH 3.5). The reaction of Cu(I) PVP with O₂ is not accompanied by formation of O₂^? outside the macromolecule bulk. The rate constant of this reaction is 1.3 ± 0.15) × 10² M^?1 sec^?1 (pH 3.5). The cyclic mechanism of the catalytic reaction is suggested to include interchange of the redox state of copper-ions. About $\text{2}\text{3}$ of the total copper ion exists in the form Cu(I) PVP during the reaction at pH 3.5. The rate of DH₂ oxidation under these conditions is limited by the rate of Cu(I) PVP reaction with O₂. At pH 4.5 the overall reaction rate is limited by the rate of interaction of Cu(II) PVP with DH^?.     

Construction of Cu-Ce interface for boosting toluene oxidation: Study of Cu-Ce interaction and intermediates identified by in situ DRIFTS

A facile hydrothermal method was applied to gain stably and highly efficient CuO-CeO₂ (denoted as Cu1Ce2) catalyst for toluene oxidation. The changes of surface and inter properties on Cu1Ce2 were investigated comparing with pure CeO₂ and pure CuO. The formation of Cu-Ce interface promotes the electron transfer between Cu and Ce through Cu²⁺ + Ce³⁺ ↔ Cu⁺ + Ce⁴⁺ and leads to high redox properties and mobility of oxygen species. Thus, the Cu1Ce2 catalyst makes up the shortcoming of CeO₂ and CuO and achieved high catalytic performance with T₅₀ = 234 °C and T₉₉ = 250 °C (the temperature at which 50% and 90% C₇H₈ conversion is obtained, respectively) for toluene oxidation. Different reaction steps and intermediates for toluene oxidation over Cu1Ce2, CeO₂ and CuO were detected by in situ DRIFTS, the fast benzyl species conversion and preferential transformation of benzoates into carbonates through C=C breaking over Cu1Ce2 should accelerate the reaction.     

A kinetic study of the oxidation of ζ phase praseodymium oxide: 109Pr9O16 + 19O2 → Pr10O18

Kinetic and thermodynamic studies of the oxidation of ζ (n = 9 in Pr_nO_2n?2) to epsilon (n = 10) phase have been carried out using a thermogravimetric technique. The experiments covered a temperature range of 464 to 503°C and oxygen pressure varied from 0.01 to 50 Torr. The existence of a reproducible hysteresis loop which depends on the temperature and pressure is shown. Measurements of the oxidation rate under isothermal conditions were fitted to different rate equations and have been found to be well represented by the equation f = 1 ? (1 ? kt)³. This fact indicated that the process of oxidation was predominantly controlled by the chemical reaction at the interface. The activation energy for the reaction has been calculated to be 15.9 kcal/mole.     

Ambient Pressure X-ray Photoelectron Spectroscopy Study of Oxidation Phase Transitions on Cu(111) and Cu(110)

The surface structure effect on the oxidation of Cu has been investigated by performing ambient-pressure X-ray photoelectron spectroscopy (APXPS) on Cu(111) and Cu(110) surfaces under oxygen pressures ranging from 10⁻⁸ to 1 mbar and temperatures from 300 to 750 K. The APXPS results show a subsequential phase transition from chemisorbed O/Cu overlayer to Cu₂O and then to CuO on both surfaces. For a given temperature, the oxygen pressure needed to induce initial formation of Cu₂O on Cu(110) is about two orders of magnitude greater than that on Cu(111), which is in contrast with the facile formation of O/Cu overlayer on clean Cu(110). The depth profile measurements during the initial stage of Cu₂O formation indicate the distinct growth modes of Cu₂O on the two surface orientations. We attribute these prominent effects of surface structure to the disparities in the kinetic processes, such as the dissociation and surface/bulk diffusion over O/Cu overlayers. Our findings provide new insights into the kinetics-controlled process of Cu oxidation by oxygen.     

Kinetics and mechanism of oxidation of dimethyl sulfide with molecular oxygen catalysed by K[RuIII(EDTA-H)Cl] complex in water-dioxan medium

Oxidation of dimethyl sulfide (Me₂S) with molecular oxygen catalysed by [Ru^III(EDTA)(H₂O)]⁻1a (EDTA = ethylenediaminetetraacetate anion) was studied spectrophotometrically in water-dioxan medium at constant pH 5.0 (acetic acid-acetate buffer) and ionic strength 0.2 M (KCl). The reaction proceeds through the formation of a [Ru^III(EDTA)(Me₂S)]⁻2 intermediate which undergoes oxidation with molecular oxygen to give dimethyl sulfoxide (Me₂SO) as the oxidation product. The rate of formation of 2 and its decomposition was followed spectrophotometrically by monitoring the reactions at 528 nm the characteristic peak of 2. The rate of formation of 2 was found to be first order in the concentrations of 1a and Me₂S. The rate of decomposition of 2 is independent of the concentration of Me₂S and is half-order with respect to oxygen concentration. Both the formation and decomposition reactions of 2 were studied at different temperatures, and the activation parameters ΔH≠ and ΔS≠ were determined. A suitable mechanism was proposed for the catalytic oxidation of dimethyl sulfide to dimethyl sulfoxide with molecular oxygen.     

Enhanced ammonia oxidation on BDD induced by inhibition of oxygen evolution reaction

Electrochemical oxidation of ammonia (NH₃ and NH₄ ⁺ ) on boron-doped diamond (BDD) electrode was studied using differential electrochemical mass-spectrometry (DEMS) and chronoamperometry. Electro-oxidation of ammonia induces inhibition of the oxygen evolution reaction (OER) due to adsorption of the ammonia oxidation products on the BDD surface. The inhibition of the OER enhances ammonia electro-oxidation, which becomes the main reaction. The amino radicals, formed during ammonia oxidation, trigger a reaction chain in which molecular oxygen dissolved in solution is involved in the ammonia electro-oxidation. Nitrogen, nitrous oxide, and nitrogen dioxide were detected as the ammonia oxidation products, with nitrogen being the main gaseous product of the oxidation.     

Palladium-catalyzed oxidation of lower aliphatic alcohols with oxygen in the presence of aromatic nitriles in aqueous medium

The kinetics of oxidation of lower aliphatic alcohols (C₁–C₄) to the corresponding carbonyl compounds with oxygen in the presence of palladium(II) tetraaqua complex and aromatic nitriles (benzonitrile, phenylacetonitrile, and o-tolunitrile) in aqueous medium (c = 0.01 M) at 65°C under atmospheric pressure were studied. A probable reaction mechanism and kinetic equation were proposed. Aromatic nitriles were found to stabilize decomposition of low-valence palladium species, ensuring activation of molecular oxygen and subsequent oxidation of alcohols.     

Synthesis,magnetochemical studies and X-ray molecular structures of some mononuclear copper(II)–1H-pyrazole adducts and mono(μ-pyrazolate) bridged dicopper(II) complexes derived from N-salicylidenearoylhydrazines

1H-Pyrazole complexes, [Cu(HL)HPz Cl] nH₂O and [Cu(L)HPz] nH₂O were prepared and characterized, where HL and L, respectively, refer to the mononegative and dinegative N-salicylidenearoylhydrazine anions. The X-ray crystal and molecular structure of [monochloro(N-salicylidenebenzoylhydrazinato)ONO(−1)monopyrazole] copper(II) monohydrate, [Cu(HSBzh)HPz Cl] H₂O, and [(N-salicylidenebenzoylhydrazinato)ONO(−2)monopyrazole] copper(II) hemihydrate, [Cu(SBzh)HPz] 1/2H₂O, were determined. The Cu(II) in [Cu(HSBzh)HPz Cl] H₂O is in a distorted square pyramidal environment and is bound in the equatorial plane with the mononegative tridentate aroylhydrazone anion and pyrazole nitrogen, the axial fifth coordination site is occupied by a chloride ion. On the other hand, the complex [Cu(SBzh)HPz] 1/2H₂O consists of two monomeric crystallographically independent but chemically similar molecules. In each molecule, the Cu(II) is in a distorted square planar geometry and is coordinated to the dinegative tridentate aroylhydrazone via the phenoxy oxygen, azomethine nitrogen and enolimide oxygen, and the fourth coordination site is occupied by the pyrazole nitrogen. The mono(μ-pyrazolate) dicopper(II) complexes, K[Cu₂(L)₂Pz] nH₂O, were also prepared and the X-ray molecular structure of K₂[Cu₄(SBzh)₄(Pz)₂] 2H₂O 1/2CH₃OH was determined. In this complex, two copper(II) atoms are bridged by only one pyrazolate anion forming a dicopper mono(μ-pyrazolate) unit. Each two units are connected together by a five coordinate K⁺ cation forming a tetranuclear assembly. These tetranuclear assemblies are connected together by another K⁺ cation forming a supramolecular structure. Variable temperature magnetic studies on these pyrazolate complexes indicated antiferromagnetic behaviour with −2J values varying from 25 to 36 cm⁻¹.     

Thermal and photochemical production of chromate(VI) ions from some complexes containing the “CrNO”2+ group

Results are reported for the oxidation of complexes of the type [Cr(CN)_5?x(H₂O)_xNO]^x?3 by molecular oxygen in alkaline medium. In the case of the [Cr(CN)₅NO]^3? complex the reaction proceeded photochemically, whereas in othe cases the thermal oxidation was also observed. The influence of pH, CN^? concentration and energy of radiation was investigated.     

Intrinsic kinetic study on the oxidation of 6-pentyl-1,2,3,4-tetrahydroanthacene-9,10-diol

A new working solution consisting of 2-pentylanthraquinone (PAQ) and 6-pentyl-1,2,3,4-tetrahydroantraquinone (4HPAQ) was hydrogenated and then oxidized by O₂ to produce H₂O₂. The oxidation reaction was conducted in a well-stirred batch reactor at 30~50^oC and 0.10~0.20 MPa. By measuring the concentrations of generated H₂O₂ by iodometry, the intrinsic kinetics for the gas-liquid oxidation of 6-pentyl-1,2,3,4-tetrahydroanthacene- 9,10-diol (4HPAQH₂, the only hydrogenated product in the hydrogenated working solution) with molecular oxygen were studied. An exponential model was employed to describe the reaction rate and the kinetic parameters were obtained. The results show that the reaction rate is 0.7 and 1.4 order in the concentration of 4HPAQH₂ and oxygen pressure respectively, and the activation energy for oxidation is 41.3 kJ/mol. This revised version was published online in June 2006 with corrections to the Cover Date.     

Kinetics and mechanism of the oxidation of acrylic acid and methyl methacrylate

The kinetics of the initiated oxidation of acrylic acid and methyl methacrylate in the liquid phase were studied volumetrically by measuring oxygen uptake during the reaction. Both processes proceed via the chain mechanism with quadratic-law chain termination. The oxidation rate is described by the equation w = k ₂/(2k ₆)^1/2[monomer]w _i ^1/2 , where w _i is the initiation rate and k ₂ and k ₆ are the rate constants of chain propagation and termination. The parameter k ₂/(2k ₆)^1/2 is 7.58 × 10^?4 (l mol^?1 s^?1)^1/2 for acrylic acid oxidation and 2.09 × 10^?3 (l mol^?1 s^?1)^1/2 for the oxidation of methyl methacrylate (T = 333 K). For the oxidation of acrylic acid, k ₂ = 2.84 l mol^?1 s^?1 (T = 333 K) and the activation energy is E ₂ = 54.5 kJ/mol; for methyl methacrylate oxidation, k ₂ = 2.96 l mol^?1 s^?1 (T = 333 K) and E ₂ = 54.4 kJ/mol. The enthalpies of the reactions of RO ₂ ^? with acrylic acid and methyl methacrylate were calculated, and their activation energies were determined by the intersecting parabolas method. The contribution from the polar interaction to the activation energy was determined by comparing experimental and calculated E ₂ values: ΔE _μ = 5.7 kJ/mol for the reaction of RO ₂ ^? with acrylic acid and ΔE _μ = 0.9 kJ/mol for the reaction of RO ₂ ^? with methyl methacrylate. Experiments on the spontaneous oxidation of acrylic acid provided an estimate of the rate of chain initiation via the reaction of oxygen with the monomer: w _i,0 = (3.51 ± 0.85) × 10^?11 mol l^?1 s^?1 (T = 333 K).     

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.     

Oxygen dependent oxidation of trimethoprim by sulfate radical: Kinetic and mechanistic investigations

Trimethoprim (TMP) is a typical antibiotic to treat infectious disease, which is among the most commonly detected antibacterial agents in natural waters and municipal wastewaters. In the present study, the impacts of dissolved oxygen (DO) on the oxidation efficiency and pathways of TMP by reaction with sulfate radicals (SO₄⁻) were investigated. Our results revealed that the presence of DO was favourable for TMP degradation. Specifically, TMP would react initially with SO₄⁻ via electron-transfer process to form a carbon-centered radical. In the absence of oxygen, the carbon-centered radical could undergo hydrolysis to produce α-hydroxytrimethoprim (TMP−OH), followed by the further oxidation which generated α-ketotrimethoprim (TMP=O). However, in the presence of oxygen, the carbon-centered radical would alternatively combine with oxygen, leading to a sequential reaction in which peroxyl radical and a tetroxide were formed, and finally generated TMP−OH and TMP=O simultaneously. The proposed pathways were further confirmed by density functional theory (DFT) calculations. The results obtained in this study would emphasize the significance of DO on the oxidation of organic micro-pollutants by SO₄⁻.     

Spin transition in the molecular heterospin complex of Cu(hfac)2 with 4,4,5,5-tetramethyl-2-(1-methylpyrazol-5-yl)-4,5-dihydroimidazole-1-oxyl 3-oxide

The synthesis, structure, and magnetic properties of the products of the reaction for Cu(hfac)₂ (hfac is hexafluoroacetylacetonate) with spin-labeled nitronyl nitroxides 4,4,5,5-tetramethyl-2-(1-R-1H-pyrazol-5-yl)-3-imidazoline-1-oxyl 3-oxides L^5/R (R = Me, Et, Pr, Bu), viz., binuclear complex [Cu(hfac)₂L^5/Me]₂ and chain polymer complexes [Cu(hfac)₂L^5/R]_n, are described. The polymer heterospin chains are built according to “head-to-head” (R = Me, Et, Pr, Bu) and “head-to-tail” (R = Pr, Bu) motifs. Compound [Cu(hfac)₂L^5/Me]₂ is characterized by the ability to reveal the reversible effect of thermally induced spin transition at a temperature about 75 K (without hysteresis). In the set of heterospin Cu^II compounds with spin-labeled pyrazoles, this is the earlier unknown example of a molecular complex exhibiting a similar magnetic anomaly.     

Catalytic efficacy of Schiff-base copper(II) complexes: Synthesis,X-ray structure and olefin oxidation

Three copper(II) Schiff-base complexes, [Cu(L¹)(H₂O)](ClO₄) (1), [Cu(L²)] (2) and [Cu(L³)] (3) have been synthesized and characterized [where HL¹ = 1-(N-ortho-hydroxy-acetophenimine)-2-methyl-pyridine], H₂L² = N,N′-(2-hydroxy-propane-1,3-diyl)-bis-salicylideneimine and H₂L³ = N,N′-(2,2-dimethyl-propane-1,3-diyl)-bis-salicylideneimine]. The structure of complex 1 has been determined by single crystal X-ray diffraction analysis. In complex 1, the copper(II) ion is coordinated to one oxygen atom and two nitrogen atoms of the tridentate Schiff-base ligand, HL¹. The fourth coordination site of the central metal ion is occupied by the oxygen atom from a water molecule. All the complexes exhibit high catalytic activity in the oxidation reactions of a variety of olefins with tert-butyl-hydroperoxide in acetonitrile. The catalytic efficacy of the copper(II) complexes towards olefin oxidation reactions has been studied in different solvent media.     

Experimental study of intermediates and wave phenomena in co oxidation on platinum metal (Pt,Pd) surfaces

The mechanism of catalytic CO oxidation on Pt(100) and Pd(110) single-crystal surfaces and on Pt and Pd sharp tip (～10³ Å) surfaces has been studied experimentally by temperature-programmed reaction, temperature desorption spectroscopy, field electron microscopy, and molecular beam techniques. Using the density functional theory the equilibrium states and stretching vibrations of oxygen atoms adsorbed on the Pt(100) surface have been calculated. The character of the mixed adsorption layer was established by high resolution electron energy loss spectroscopy—molecular adsorption (O_2ads, CO_ads) on Pt(100)-hex and dissociative adsorption (O_ads, CO_ads) on Pt(100)-(1×1). The origin of kinetic self-oscillations for the isothermal oxidation of CO in situ was studied in detail on the Pt and Pd tips by field electron microscopy. The initiating role of the reversible phase transition (hex) ? (1 × 1) of the Pt(100) nanoplane in the generation of regular chemical waves was established. The origination of self-oscillations and waves on the Pt(100) nanoplane was shown to be caused by the spontaneous periodical transition of the metal from the low-active state (hex) to the highly active catalytic state (1 × 1). A relationship between the reactivity of oxygen atoms (O_ads) and the concentration of CO_ads molecules was revealed for the Pd(110) surface. Studies using the isotope label ¹⁸O_ads demonstrated that the low-temperature formation of CO₂ at 150 K is a result of the reaction of CO with the highly reactive state of atomic oxygen (O_ads). The possibility of the low-temperature oxidation of CO via interaction with the so-called “hot” oxygen atoms (O_hot) appearing on the surface at the instant of dissociation of O_2ads molecules was studied by the molecular beam techniques.     

CO在Ag掺杂的氧化锰八面体分子筛上吸附的TAP研究

利用产物瞬时分析反应器中进行的单脉冲实验, 考察了393～493 K温度范围内CO在Ag掺杂的氧化锰八面体分子筛上的吸附行为. 实验表明: CO在催化剂表面发生化学吸附, 并与晶格氧反应生成CO₂. 通过对该过程反应物及产物脉冲响应曲线的模拟, 得到了各基元反应的动力学参数. CO和CO₂在该催化剂表面的脱附活化能分别为83和31 kJ/mol, CO与晶格氧的反应活化能为116 kJ/mol.     

Homogeneous Catalytic Oxidation of Cyclohexene by Nitro Complexes of Copper(II)

This work describes the preliminary study of the homogeneous oxidation reaction of cyclohexene by nitro complexes of copper(II), Cu(CH₃CN)₂(NO₂)Cl (1) and Cu(CH₃CN)₂(NO₂)₂ (2) in acetonitrile, under oxygen pressure (20 atm) at 70 and 100°C. The results show the formation of 2-cyclohexen-1-one and 1,2-epoxyhexane-3-one as main reaction products. Also, it was observed that the oxidation reaction at 100°C with complexes 1 and 2 becomes more selective to the formation of 2-cyclohexene-1-one although the activity of both catalyst decreases.     

Noble‐Metal‐Free Single‐Atom Catalysts CuAl4O7–9− for CO Oxidation by O2

The single copper atom doped clusters CuAl₄O_7–9^? can catalyze CO oxidation by O₂. The CuAl₄O_7–9^? clusters are the first group of experimentally identified noble‐metal free single atom catalysts for such a prototypical reaction. The reactions were characterized by mass spectrometry and density functional theory calculations. The CuAl₄O₉CO^? is much more reactive than CuAl₄O₉^? in the reaction with CO to generate CO₂. One adsorbed CO is crucial to stabilize Cu of CuAl₄O₉^? around +I oxidation state and promote the oxidation of another CO. The widely emphasized correlation between the catalytic reactivity of CO oxidation and Cu oxidation state can be understood at the strictly molecular level. The remarkable difference between Cu catalysis and noble‐metal catalysis was discussed.