Role of Pd(0) particles in oxidation of lower olefins in the catalytic system Fe(III)_Pd/ZrO<Subscript>2</Subscript>

Oxidation of metallic Pd(0) particles applied onto an oxide support with Fe(III) ions in a concentration not exceeding 0.06 M at 70°C was studied. In contrast to palladium black, with the supported catalyst Pd/ZrO₂ Pd(II) is formed in the solution in the concentration corresponding to the thermodynamic equilibrium. With an increase in the initial Fe(III) concentration, the equilibrium yield of Pd(II) increases. The initial reaction rate grows with an increase in the weight of the initial Pd-containing catalyst and in the initial Fe(III) concentration. The revealed kinetic relationships of the dissolution of Pd(0) in the reaction with Fe(III) aqua ions allow a conclusion that, in oxidation of lower olefins C₂-C₄ in the catalytic system Fe(III)_Pd/ZrO₂ in aqueous solution, Pd(II) is regenerated in the catalytic cycle by oxidation of Pd(I) species, rather than of Pd(0), with Fe(III) aqua ions.     

Voltammetry and controlled-potential coulometry with boron carbide electrodes

With the boron carbide electrode, E_{p$\text{p}\text{2}$} values were determined for the reduction of the following ions: Cd(II), Co(II), Cu(II), Fe(III), Ni(II), Pb(II), Ru(IV), Sb(V), and U(VI). The linear dependence of peak current on concentration is demonstrated for the U(VI) → U(IV) and Fe(III) → Fe(II) reductions at the boron carbide electrode. The suitability of the electrode for the controlled-potential coulometric ti trations of Fe(II) → Fe(III), Fe(III) → Fe(II), and U(VI) → U(IV) was studied; the results were inconclusive because of the small surface area that could be used conveniently and the possibility of oxygen leaks in the cell.     

Thermochemical properties of trialkylarsenites

The heats of the reaction of sodium with ethyl and methyl alcohol were determined by calorimetry. The difference in the standard heats of the formation of triethylarsenite and arsenic trichloride was obtained by calorimetration of the reaction of arsenic trichloride with sodium ethylate, the value of which was −382.42 ± 3.60 kJ/mol. The standard enthalpies of formation were determined from a critical analysis of all data on thermochemistry of trialkylarsenites for the following compounds: triethylarsenite Δ_f H ₂₉₈^ℴ [(C₂H₅O)₃As(liquid)] = (−704.38 ± 3.85) kJ/mol; trimethylarsenite Δ_f H ₂₉₈^ℴ [(CH₃O)₃As(liquid)] = (−599.36 ± 1.88) kJ/mol. The values of standard enthalpies of formation were not adjusted for the following substances in liquid state: arsenic trichloride (−321.96 ± 3.85 kJ/mol), tris-(diethylamido)arsenic(III) As(NEt₂)₃(liquid) (−129.81 ± 4.41 kJ/mol), tri-n-propylarsenite (−720.61 ± 4.49 kJ/mol), triisopropylarsenite (−756.11 ± 4.65 kJ/mol), tri-n-butylarsenite (−775.11 ± 4.53 kJ/mol), and triisobutylarsenite (−809.71 ± 4.59 kJ/mol). The use of sodium alcoxide solutions for the calorimetration of halogen anhydrides of various acids was demonstrated.     

Fe(III)/Fe(II) separation by solvent extraction for the determination of oxygen stoichiometry in yttrium barium copper oxide superconductor materials

Summary Oxygen stoichiometry is a critical parameter defining the T_c of cuprate superconductors (e.g. YBa₂Cu₃O₇). On dissolution excess or deficiency of oxygen can be converted into shifts of the Fe(II)/Fe(III) concentration ratio of an aqueous solution. For small samples a solvent extraction technique for the separation of Fe(III) from Fe(II) was developed, to make use of the superior sensitivity of atomic spectrometry (AAS, ICP-AES). The system n-benzoylphenylhydroxylamine (BPHA)/CHCl₃ was chosen because it is relatively inactive as a redox partner. Despite the catalytic effect of Cu(II) on the oxidation of Fe(II), oxidation blanks can be kept down at negligible levels. Less than 0.55% of residual Fe(II) is converted to Fe(III) during the extraction procedure (argon atmosphere). In the presence of air, oxidation levels are still practical (3%). Extraction is from 0.3 mol/l HBr providing excellent recovery of Fe(III) (e.g. 98.8%). All Fe(II), Y, Ba (including BaSO₄ precipitate) and 99.4% of the Cu remain in the aqueous phase. Fe(III) is rapidly back-extracted into an aqueous phase by 6 mol/l HCl for dilution and aspiration into the flame or ICP. Recovery of Fe(III) after the two extraction steps is still 98.3%.     

A re-evaluation of the enthalpy of sublimation of some metal acetylacetonate complexes

Sublimation enthalpies for Al(III), Fe(III) and Zn(II) acetylacetonates derived using isoteniscopic, sublimation bulb and spoon gauge techniques are reported. Selected sublimation enthalpies are Al(C₅H₇O₂)₃, 118.6 ± 7.8; Fe(C₅H₇O₂, 113.6 ± 3.8; and Zn(C₅H₇O₂)₂, 117 ± 3 kJ mole^?1, respectively.     

Studies on synthesis,determination of CMC values,kinetics and the mechanism of iron(II) reduction of surfactant–Co(III)–ethylenediamine complexes in aqueous acid medium

The surfactant–Co(III) complexes of the type cis-[Co(en)₂AX]²⁺ (A?=?Tetradecylamine, X?=?Cl^?,?Br^?) were synthesised from corresponding dihalogeno complexes by the ligand substitution method. The critical micelle concentration (CMC) values of these surfactant complexes in aqueous solution were obtained from conductance measurements. The kinetics and mechanism of iron(II) reduction of surfactant–Co(III) complexes, cis-[Co(en)₂(C₁₄H₂₉NH₂)Cl](ClO₄)₂ and cis-[Co(en)₂(C₁₄H₂₉NH₂)Br] (ClO₄)₂ ions were studied spectrophotometrically in an aqueous acid medium by following the disappearance of Co(III) using an excess of the reductant under pseudo-first-order conditions: [Fe(II)]?=?0.25?mol?dm^?3, [H⁺]?=?0.1?mol?dm^?3, [μ]?=?1.0?mol?dm^?3 ionic strength in a nitrogen atmosphere at 303, 308 and 313?K. The reaction was found to be of second order and showed acid independence in the range [H⁺]?=?0.05–0.25?mol?dm^?3. The second-order rate constant increased with surfactant–Co(III) concentration and the presence of aggregation of the complex itself altered the reaction rate. The effects of [Fe(II)], [H⁺] and [μ] on the rate were determined. Activation and thermodynamic parameters were computed. It is suggested that the reaction of [Fe(II)] with Co(III) complex proceeds by an inner-sphere mechanism.     

Reaction of hot H atoms with CD3Br

Energetic H atoms produced by photolysis of gaseous HI react with CD₃Br by D abstraction (1) and Br abstraction (2) and possibly also by the substitution reactions (4) and (5). Yields of HD and CD₃H have been determined for several defined initial translational energies of H^*. The phenomenological threshold energy of reaction (1) is 53 ± 5 kJ/mol. Over the range of initial energies of 76–109 kJ mol the integral probability of reaction (1) increases substantially, but the sum of the integral probabilities of reactions (2) and (5) shows little change. The ratio of the sum of the integral yields of reactions (2) and (5) to the integral yield of reaction (1), when normalized to equal numbers of Br and D atoms, is 69 ± 33 at an initial energy of 76 kJ/mol and 31 ± 6 at 109 kJ/mol.     

A study of the reactions of fluorine with hydrogen and methane in the initiation phase using a miniature tubular reactor

A small tubular reactor having an inner diameter of 1–2 mm andused as the source in a molecular beam apparatus is described in detail. This arrangement allows the study of fast reactions with reaction times smaller than 1 msec. The preexplosive reaction phase between F₂ and H₂ and CH₄, respectively, is investigated to find out the initiation reactions. In the F₂/H₂ reaction, initiation is brought about by heterogeneous generation of F atoms or some other surface reaction. Evidence is also obtained for chain branching reactions. In the F₂/CH₄ case the dominant initiation reaction is the homogeneous reaction CH₄ + F₂ → CH₃ + HF + F. The rate constant for the reaction between 300 and 400 K is 10^12.3±0.3 exp[?47 ± 8 kJ/mol/RT] cm³/mol sec. The analysis of the experimental data also yields the rate constant for the propagation reaction CH₃ + F₂ → CH₃ F + F, which is 10^12.3±0.3 exp[?4.6 ±2.1 kJ/mol/RT] cm³/mol sec.     

Catalytic spectrophotometric determination of trace aluminium with indigo carmine

A new catalytic spectrophotometric method is described for the determination of trace amounts of Al(III). The methods based on catalytic action of Al(III) on the oxidation of indigo carmine (IC) by ammonium persulfate in hexamethylene tetramine-hydrochloric acid ((CH2)6N4-HCl) buffer medium (pH 5.4) and in the presence of surfactant-TritonX-100. The effects of some factors on the reaction speed were investigated. Aluminium concentration is linear for 0-1.2x10(-7) g/ml in this method. The detection limit of the proposed method is 1.96x10(-8) g/ml. Most of the foreign ions except for Cu(II), Fe(III) do not interfere with the determination, and the interference of Cu(II) and Fe(III) in this method can be removed by extraction with sodium diethyldithiocarbamate-carbon tetrachloride (DDTC-CCl4). This system is a quasi-zero-order reaction for Al(III), but it is a quasi-first-order reaction for IC. The apparent rate constant is 2.62x10(-5) s-1 and the apparent activation energy is 6.60 kJ/mol in the system. The proposed method was applied to the determination of trace aluminium(III) in real samples with satisfactory results.     

Pd(OAc)2/M(NO3)n (M = Cu(II), Fe(III); n = 2, 3): Kinetic investigations of an alternative Wacker system for the oxidation of natural olefins

Pd-catalyzed oxidative coupling of camphene by dioxygen afforded mainly a diene, which subsequently underwent oxidation to a ring-expanded β,γ-unsaturated ketone with LiNO₃ as reoxidant. However, the instability of LiNO₃ results to the decomposition of NO₃⁻ ions which subsequently deactivates the catalyst. The present investigation describes the oxidation of terpenes catalyzed by Pd(OAc)₂/M(NO₃)_n (M = Cu(II), Fe(III); n = 2 or 3), using dioxygen as final oxidant. Fe(III) and Cu(II) effectively stabilize the nitrate reoxidant as determined by the significant increase of both catalytic activity and stability of the system. Turnover frequency suggests that Fe(III) is the most efficient co-catalyst. Moreover, it is established that the co-catalysts NO₃⁻, Cu(II) and especially Fe(III) ions, change the product distribution (diene/ketone) remarkably. Their involvement in the rate-determining step was investigated and the results of the kinetic investigations clarified important aspects of Pd(II)-catalyzed oxidation reactions. The described protocol offers an alternative to the traditional Wacker system which uses CuCl₂ as co-catalyst and is not effective in promoting the oxidation of bicycle olefins.     

Oxidation of manganese(II) by ozone and reduction of manganese(III) by hydrogen peroxide in acidic solution

Manganese(II) is oxidized by ozone in acid solution, k=(1.5±0.2)×10³ M⁻¹ s⁻¹ in HClO₄ and k=(1.8±0.2)×10³M⁻¹ s⁻¹ in H₂SO₄. The plausible mechanism is an oxygen atom transfer from O₃ to Mn²⁺ producing the manganyl ion MnO²⁺, which subsequently reacts rapidly with Mn²⁺ to form Mn(III). No free OH radicals are involved in the mechanism. The spectrum of Mn(III) was obtained in the wave length range 200–310 nm. The activation energy for the initial reaction is 39.5 kJ/mol. Manganese(III) is reduced by hydrogen peroxide to Mn(II) with k(Mn(III)+H₂O₂)=2.8×10³M⁻¹ s⁻¹ at pH 0–2. The mechanism of the reaction involving formation of the manganese(II)-superoxide complex and reaction of H₂O₂ with Mn(IV) species formed due to reversible disproportionation of Mn(III), is suggested. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 207–214, 1998.     

Theoretical investigation of water gas shift reaction catalyzed by iron group carbonyl complexes M(CO)5 (M = Fe, Ru, Os)

We have investigated the mechanism of M(CO)(5) (M = Fe, Ru, Os) catalyzed water gas shift reaction (WGSR) by using density functional theory and ab initio calculations. Our calculation results indicate that the whole reaction cycle consists of six steps: 1 → 2 → 3 → 4 → 5 → 6 → 2. In this stepwise mechanism the metals Fe, Ru, and Os behave generally in a similar way. However, crucial differences appear in steps 3 → 4 → 5 which involve dihydride M(H)(2)(CO)(3)COOH(-) (4') and/or dihydrogen complex MH(2)(CO)(3)COOH(-) (4). The stability of the dihydrogen complexes becomes weaker down the iron group. The dihydrogen complex 4_Fe is only 11.1 kJ/mol less stable than its dihydride 4'_Fe at the B3LYP/II(f)++//B3LYP/II(f) level. Due to very low energy barrier it is very easy to realize the transform from 4_Fe to 4'_Fe and vice versa, and thus for Fe there is no substantial difference to differentiate 4 and 4' for the reaction cycle. The most possible key intermediate 4'_Ru is 38.2 kJ/mol more stable than 4_Ru. However, the barrier for the conversion 3_Ru → 4'_Ru is 23.8 kJ/mol higher than that for 3_Ru → 4_Ru. Additionally, 4'_Ru has to go through 4_Ru to complete dehydrogenation 4'_Ru → 5_Ru. The concerted mechanism 4'_Ru → 6_Ru, in which the CO group attacks ruthenium while H(2) dissociates, can be excluded. In contrast to Fe and Ru, the dihydrogen complex of Os is too unstable to exist at the level of theory. Moreover, we predict Fe and Ru species are more favorable than Os species for the WGSR, because the energy barriers for the 4 → 5 processes of Fe and Ru are only 38.9 and 16.2 kJ/mol, respectively, whereas 140.5 kJ/mol is calculated for the conversion 4' → 5 of Os, which is significantly higher. In general, the calculations are in good agreement with available experimental data. We hope that our work will be beneficial to the development and design of the WGSR catalyst with high performance.     

Mixed Valence Chemistry of Pyrimidine Bridged Binuclear Complex of Pentacyanoferrate and Pentaammineruthenium

The pyrimidine bridged binuclear complex (CN)₅FepymRu(NH₃)₅- (I) was prepared in aqueous solution by mixing cquimolar of Fe(CN)₅OH₂^3? and Ru(NH₃)₅pym²⁺. Its mixed valence state molecule (CN)₅FepymRu(NH₃)₅(II) was obtained upon oxidation of I by one equivalent of peroxydisulfate ion. Both binuclear complexes and corresponding Fe(II) and Ru(II) mononuclear complexes displayed a metal-to-ligand charge transfer absorption in 400–450 nm region. Rate constants of formation and dissociation of I and II were measured, and the values of k_f (?10³M^?1s^?1) and k_d (?10^?3-10^?4 s^?1) were consistent with kinetic results expected for the substitution of Fe(CN)₅OH₂^3? with di- and trivalent ligands. Cyclic voltammetry of I exhibited two one-electron steps of oxidation corresponding to [III, L, II] + e → [II, L, II] and [III, L, III] + e → [III, L, II], respectively. The mixed valence binuclear complex II showed an intervalence band at 955 nm with a molar extinction coefficient 5.80 × 10² M^?1cm^?1 and a half-width 5100 cm^?l. The properties of the IT band conform to Hush's theory. Spectroscopic, electrochemical and kinetic results of II suggest that the mixed valence complex features a trapped - valence formulation with localized oxidation states of Fe(II) and Ru(III).     

Intermediates in the intramolecular ligand transfer reactions of h5-C5H5Fe(CO)2R complexes

Dissolution of h⁵-C₅H₅Fe(CO)₂R (I) (R = cyclohexyl or cyclohexylmethyl) in DMSO leads to the formation of a solvent coordinated acyl complex, h⁵-C₅H₅Fe(CO)(COR)(DMSO) (II). Treatment of this complex with triphenylphosphine leads to its conversion to h⁵-C₅H₅Fe(COR)(PPh₃) (III). Rates for the reaction I ? and II → III have been determined. A comparison of the rates of the reaction I → III in eight solvents shows no specific rate acceleration in DMSO and no correlation with solvent donicity. The results are in accord with a two step mechanism in which the first intermediate is the coordiantively-unsaturated species h⁵-C₅H₅Fe(COR)(CO). The small spread in rates for solvents of widely different dielectric constants suggests little charge separation in the transition state for this step.     

Chlorine dioxide reduction by aqueous iron(II) through outer-sphere and inner-sphere electron-transfer pathways

The reduction of ClO(2) to ClO(2)(-) by aqueous iron(II) in 0.5 M HClO(4) proceeds by both outer-sphere (86%) and inner-sphere (14%) electron-transfer pathways. The second-order rate constant for the outer-sphere reaction is 1.3 x 10(6) M(-1) s(-1). The inner-sphere electron-transfer reaction takes place via the formation of FeClO(2)(2+) that is observed as an intermediate. The rate constant for the inner-sphere path (2.0 x 10(5) M(-1) s(-1)) is controlled by ClO(2) substitution of a coordinated water to give an inner-sphere complex between ClO(2) and Fe(II) that very rapidly transfers an electron to give (Fe(III)(ClO(2)(-))(H(2)O)(5)(2+))(IS). The composite activation parameters for the ClO(2)/Fe(aq)(2+) reaction (inner-sphere + outer-sphere) are the following: DeltaH(r)++ = 40 kJ mol(-1); DeltaS(r)++ = 1.7 J mol(-1) K(-1). The Fe(III)ClO(2)(2+) inner-sphere complex dissociates to give Fe(aq)(3+) and ClO(2)(-) (39.3 s(-1)). The activation parameters for the dissociation of this complex are the following: DeltaH(d)++= 76 kJ mol(-1); DeltaS(d)++= 32 J K(-1) mol(-1). The reaction of Fe(aq)(2+) with ClO(2)(-) is first order in each species with a second-order rate constant of k(ClO2)- = 2.0 x 10(3) M(-1) s(-1) that is five times larger than the rate constant for the Fe(aq)(2+) reaction with HClO(2) in H(2)SO(4) medium ([H(+)] = 0.01-0.13 M). The composite activation parameters for the Fe(aq)(2+)/Cl(III) reaction in H(2)SO(4) are DeltaH(Cl(III))++ = 41 kJ mol(-1) and DeltaS(Cl(III))++ = 48 J mol(-1) K(-1).     

Electrometric titrations of Cr(VI), Mn(IV), V(V), Co(III) and U(VI) with a standard iron(II) solution in alkaline media containing hexitols

Direct and indirect potentiometric, bipotentiometric and biamperometric titrations with a standard iron(II) solution are described for some inorganic compounds in alkaline media containing hexitols (mannitol, dulcitol and sorbitol). The optimal conditions for titrations based on the Cr(VI) → Cr(III), Mn(IV) → Mn(III) → Mn(II), V(V) → V(IV), Co(III) → Co(II) and U(VI) → U(IV) systems are discussed. Of the hexitols studied, sorbitol has the greatest effect on the value of the redox potential of the Fe(III)/Fe(II) system; the E_f° value is about —1.10 V vs. SCE.     

Cyanogen pyrolysis and the CN + NO reaction behind incident shock waves

The reaction chemistry of C₂N₂? Ar and C₂N₂? NO? Ar mixtures has been investigated behind incident shock waves. Progress of the reaction was monitored by observing the cyano radical (CN) in absorption at 388.3 nm. A quantitative spectroscopic model was used to determine concentration histories of CN. From initial slopes of CN concentration during cyanogen pyrolysis, the rate constant for C₂N₂ + M → 2CN + M (1) was determined to be k₁ = (4.11 ± 1.8) × 10¹⁶ exp(?47,070 ± 1400/T) cm³/mol · s. A reaction sequence for the C₂N₂? NO system was developed, and CN profiles were computed. By comparison with experimental CN profiles the rate constant for the reaction CN + NO → NCO + N (3) was determined to be k₃ = 10^{(14.0 ± 0.3)} exp(?21,190 ± 1500/T) cm³/mol · s. In addition, the rate of the four-centered reaction CN + NO → N₂ + CO (2) was estimated to be approximately three orders of magnitude below collision frequency.     

Kinetics and equilibrium constants of some pentacyanoferrate(II) complexes of nitrogen and sulfur containing heterocycles

The kinetics of the substitution reactions of Fe(CN)₅H₂O³⁻ ion with a series of nitrogen and sulfur containing heterocycles were studied in aqueous media. In the presence of excess ligand, varied over a large range of concentrations, second-order rate constants were calculated at μ = 0.100 M NaClO₄. Activation parameters for the formation reactions were found, ΔH*ast; and ΔS*, 28 ± 6 kJ/mol and 135±20 J/mol, respectively. The results are interpreted as being consistent with dissociative, SN₁ mechanism. The kinetics of formation and dissociation were studied by stopped-flow technique at several temperatures. An investigation of the kinetics of exchange of coordinated heterocycles for 1,3,5-triazine, yielded rate saturation that is typical of a limiting SN₁ mechanism. Activation parameters of the limiting first-order specific rate of dissociations were found with ΔH* and ΔS* 53±2 kJ/mol and 105±5 J/mol, respectively. From the specific rates of formation and dissociation reactions the equilibrium constants were calculated. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet: 30: 415–418, 1998     

Electrocatalytic effect of iron sulfates on oxygen reduction. Influence of the solvation sphere of the mediator

The redox catalysis of oxygen reduction was performed on a platinum rotating disk electrode. The Fe(III)/Fe(II)/H₂SO₄ system at different pH's was used as a MEDIATOR. The catalytic effect of mediator was directly related to the solvation sphere of Fe(III) and Fe(II). Only the redox couple FeHSO ₄ ²⁺ /FeHSO ₄ ⁺ (pH<0) showed a catalytic effect on oxygen reduction.     

Selective water sorbents for multiple applications, 8. sorption properties of CaCl2?SiO2 sol-gel composites

This paper presents sorption properties of CaCl₂/SiO₂ composites synthesized by a sol-gel approach. Desorption isobars measured at T=30–140°C and vapor pressure 12.8–81.0 mbar clearly show a correlation between the sorbents pore structure and their sorption properties. The sample adsorptivities are found to exceed 1.2 kg H₂O/kg adsorbent (or 20–25 mol H₂O/mol of the salt). That is markedly higher than any reported before for silica-based materials. This results in a high energy storage capacity reaching 3,400 kJ/kg of dry sorbent, as confirmed by direct calorimetric measurements. The isosteric desorption heat is found to decrease from 67±5 kJ/mol to 46±5 kJ/mol with increase in the surface coverage.