Oxidation Of Thiourea Research Articles

Synthesis of Tungsten Oxide Powders Via Thiourea‐Assisted Route: Tailoring from Hierarchical Microspheres to Mesoporously Structured Nanoparticles

Tungsten oxide powders with controlled micro‐to‐nanoscale features, i.e. from microspheres to mesoporous structure, have been synthesized using peroxopolytungstic acid (PTA) and thiourea. Using PTA alone, elliptical tungsten oxide microspindles are hierarchically assembled from rectangular nanoparticles (NPs) after heat treatment at 450°C. With an appropriate addition of thiourea into the PTA precursor, hierarchical microspheres are obtained through the aggregation of isotropic NP unit blocks. When the thiourea content in the precursor increases further, high‐surface‐area mesoporous tungsten oxides composed of well‐dispersed isotropic NPs can be synthesized. We suggest that thiourea acts as a capping agent for forming the isotropic NP unit blocks in the precursors. Moreover, gaseous species released at high temperatures through decomposition and oxidation of nonchelated thiourea in the precursor will suppress the aggregation of NP unit blocks. As a result, tungsten oxide powders in form of microspheres or mesoporous nanostructures can be tailored by the thiourea content in the precursor.

Effect of Interfacial Oxides on the Electrochemical Activity of Lead Dioxide Film Electrodes on a Ti Substrate

The electrochemical activities of PbO(2) thin-film electrodes were systematically studied with an oxygen-transfer reaction, i.e., the oxidation of thiourea. Five different types of PbO(2) film electrodes were prepared using various methods to investigate the influence of the interfacial oxide layer formed between the Ti substrate and the PbO(2) layer contacting the electrolyte. Among the electrodes tested, PbO(2) electrode prepared by minimizing the formation of the interfacial oxide by either the electrochemical or thermal decomposition method generally exhibited better electrochemical activity. From the results, it can be inferred that the enhanced electrochemical activity of the electrode is due to minimization of the interfacial resistance by the introduction of a metallic Pb layer between the Ti substrate and the PbO(2) layer, and to an increase in the roughness on the surface of the PbO(2) layer.

Kinetics of the peroxomonosulfate oxidation of sulfur(-II) compounds : thioacetamide, dithiooxamide and thiourea

Kinetics of relatively faster oxidation of thioacetamide (TA), dithiooxamide (DTO) and thiourea (TA) by peroxomonosul fate (PMS) has been studied spectrophotometrically under pseudo order condition by keeping oxidant in excess. Under these conditions, the sulfur(-II) compounds are oxidized to corresponding S-dioxides. The kinetics rate law for the oxidation of TA and TU both is eq. (A) and for the oxidation of DTO is eq. (B). d(S(- II )) kK (PMS) (S(- II ))

Oxidative conversion of thiourea and N-substituted thioureas into formamidine disulfides with acidified chloramine-T: a kinetic and mechanistic approach

The kinetics of oxidation of thiourea and N-substituted thioureas namely: N-methylthiourea, N-allylthiourea N-phenylthiourea and N-tolylthiourea to the corresponding formamidine disulfides by sodium N-chloro-p-toluenesulfonamide or chloramine-T (CAT) in the presence of HClO4 has been investigated at 278 K. The reactions follow identical kinetics for all thioureas, being first order each with respect to [CAT]o, [Thiourea]o and [H+]. Ionic strength of the medium and addition of p-toluenesulfonamide or halide ions have negligible influence on the rate. The solvent isotope effect has been studied using D2O in the case of the oxidation of thiourea. Decrease in the dielectric constant of the medium by adding methanol decreases the rate. The reactions were studied at different temperatures, and the composite activation parameters have been computed. An isokinetic relationship was observed with β=314 K, indicating that enthalpy factors control the reaction rate. Under comparable experimental conditions, the rate of oxidation of thioureas increases in the order: N-allylthiourea>N-phenylthiourea>N-methylthiourea>thiourea>N-tolylthiourea. A mechanism involving the interaction of conjugate acid (CH3C6H4SO2NHCl) and substrate giving an intermediate complex, in a slow step, has been suggested. The derived rate law is in agreement with the observed kinetics.

Nonlinear Oxidation of Thiourea in an Unbuffered Medium

AbstractThe reaction between periodate and thiourea has been studied in an unbuffered medium. A variety of interesting dynamical behavior including variable stoichiometries, characteristic "clock reaction" and oligooscillations or single peak oscillations in pH, the concentrations of both iodide and iodine were found in a closed reactor. When studied in an open reactor, the above reaction system displays fascinating damped oscillations and bistability in an unbuffered aqueous solution. A 12‐step simplified mechanism has also been proposed here. Our investigation illustrates that it is critical to include a rapid H+‐mediated preequilibrium step in the suggested mechanism. The modified mechanism gives a better agreement between the calculated results and experimental observations.

Dynamic Complexity in the Electrochemical Oxidation of Thiourea

We explored the temperature-dependent dynamics of the electrochemical oxidation of thiourea under potential-control mode and found complex oscillations with one large peak and one small peak per period. Adjusting the temperature caused the relative amplitudes and positions of the two peaks to vary. Experiments showed that there were two distinct oscillatory regimes as a function of the external current, and at some temperatures, three-frequency quasiperiodic oscillations occurred for current densities between the two oscillatory windows. We examined the species present and the component reactions by employing cyclic voltammetry and electrolysis-HPLC-MS in the two oscillatory regimes. Adsorption and desorption of multiple species, including water and chloride, contribute to the rich dynamical phenomena that were observed.

Simultaneous Tracking of Sulfur Species in the Oxidation of Thiourea by Hydrogen Peroxide

We employ reversed-phase ion-pair high-performance liquid chromatography to quantitatively characterize the oxidation kinetics of thiourea oxidation by hydrogen peroxide. The HPLC technique makes it possible to monitor the concentrations of a variety of sulfur-containing species with different oxidation states and to elucidate the relative phase relations among them. The experimental results are in good agreement with simulations from an 8-step reaction mechanism.

S-Oxygenation of Thiocarbamides. 3. Nonlinear Kinetics in the Oxidation of Trimethylthiourea by Acidic Bromate

The oxidation of trimethylthiourea (TMTU) by acidic bromate has been studied. The reaction mimics the dynamics observed in the oxidation of unsubstituted thiourea by bromate with an induction period before formation of bromine. The stoichiometry of the reaction was determined to be 4:3, thus 4BrO(3)- + 3R(1)R(2)C=S+ 3H(2)O --> 4Br- + 3R(1)R(2)C=O + 3SO(4)(2-) + 6H+. This substituted thiourea is oxidized at a much faster rate than the unsubstituted thiourea. The oxidation mechanism of TMTU involves initial oxidations through sulfenic and sulfinic acids. At the sulfinic acid stage, the major oxidation pathway is through the cleavage of the C-S bond to form a reducing sulfur leaving group, which is easily oxidized to sulfate. The minor pathway through the sulfonic acid produces a very stable intermediate that is oxidized only very slowly to urea and sulfate. The direct reaction of aqueous bromine with TMTU was faster than reactions that form bromine, with a bimolecular rate constant of (1.50 +/- 0.04) x 10(2) M(-1) s(-1). This rapid reaction ensured that no oligooscillatory bromine formation was observed. The oxidation of TMTU was modeled by a simple reaction scheme containing 20 reactions.

Effect of cationic micelles of cetytrimethylammonium bromide on the oxidation of thiourea by permanganate

Abstract Kinetics of MnO 4 − -thiourea redox reaction have been investigated spectrophotometrically in presence of cationic micelles of cetyltrimethylammonium bromide (CTAB). Upon mixing aqueous solutions of permanganate and thiourea, a readily distinguishable brown colour appears and then disappears slowly. Various experiments have been performed to confirm the nature of brown colored solution. The effects of [MnO 4 − ], [thiourea] and [H + ] on the reaction rate were determined in presence of CTAB (=8.0 × 10 −4 mol dm −3 ). Absorbance of the reaction mixture increases with [CTAB] which suggest the incorporation/association of permanganate with the head group of CTAB micelles. Menger–Portnoy model was used to explain the effect of CTAB micelles. The reaction proceeds through the fast formation and slow decomposition of water-soluble colloidal MnO 2 as an intermediate. A suitable mechanism is proposed for the oxidation of thiourea by permanganate.

Formation, characterization and stabilization of water-soluble colloidal MnO2 in the oxidation of methionine, thiourea and thioacetamide by permanganate

Stabilization and characterization of water soluble colloidal MnO2 during the oxidation of sulfur-containing organic reductants "thiourea, thioactamide and methionine" by permanganate in aqueous neutral media are reported for the first time. Upon addition of permanganate to a solution of methionine, a transient species appears within the time of mixing which is stable for several weeks. On the other hand, the transient species is unstable in the presence of thiourea and thioacetamide, respectively. The nature of manganese (IV) species present in solution was characterized by spectrophotometric and coagulation measurements. On addition of HClO4, there is a decrease in the absorbance of the reaction mixture. Under pseudo – first – order conditions ([reductants] > [MnO4-]), the reduction rate was very fast up to the formation of water soluble colloidal MnO2. The effect of various parameters, such as hydrogen ion concentration, amount of MnO4-, concentration of reductants was investigated. Mechanisms consistent with the observed results have been proposed and discussed.

Theoretical mechanism for the oxidation of thiourea by hydrogen peroxide in gas state

This article presents a theoretical reaction mechanism for the oxidation of thiourea by hydrogen peroxide using density functional theory and ab initio methods. This work also focuses on the analysis of the potential energy surface (PES), thermodynamic and kinetic properties for the designed oxidation mechanism of thiourea using BH&HLYP and B3LYP methods. Both DFT calculations lead to similar reaction paths. The results of the thermodynamic analysis for the designed oxidation mechanism of thiourea are reasonable and feasible. Based on the research on the activity energies, activity enthalpies and activity Gibbs free energies obtained from the determined reaction pathways, we conclude that the oxidation of thiourea by hydrogen peroxide in vacuum was well completed. Because the active relative energies, the relative enthalpies and the relative Gibbs free energies of Reaction 4 predicted by Path II are higher than those predicted by Path I. Kinetics calculations suggest that the designed Path I is more favorable than Path II. The basis sets and calculation methods have strong effects on the thermodynamics and kinetics calculations.

A kinetic study of water-soluble colloidal MnO 2 formed by the reduction of permanganate by thiourea

A conventional spectrophotometric technique was used to study the oxidation of thiourea by permanganate in a dilute perchloric acid solution. Preliminary observations indicate that the reduction of permanganate by thiourea proceeds through the fast formation of colloidal MnO 2 as an intermediate. Various experiments were performed to confirm the nature of intermediate(s) formed during the reduction of permanganate by thiourea. The kinetic and spectroscopic data are consistent with the formation of water-soluble colloidal MnO 2. The stoichiometry was found to be 1:2 (MnO 2:thiourea). The stability of water-soluble colloidal MnO 2 depends upon the [H +]. The effect of total [MnO 4 −], [thiourea] and [H +] on the reaction rate was determined. On the basis of various observations, a mechanism is proposed for the oxidation of thiourea by permanganate.

Oxidation and Decomposition Kinetics of Thiourea Oxides

In this report the decomposition and oxidation kinetics of thiourea dioxide and thiourea trioxide are investigated with a reversed-phase ion-pair high-performance liquid chromatography (HPLC) method. The HPLC method allows us to simultaneously determine and quantify several sulfur-containing reagents such as (NH2)2CSO2, (NH2)2CSO3, (NH2)2CO, and SO3(2-) (HSO3-). Experiments illustrate that the decomposition of thiourea oxides is a first-order reaction, in which the rate constants increase with the pH of the solution. Oxidations of thiourea oxides by hydrogen peroxide are first order with respect to both reagents within the studied pH range between 4.0 and 8.0. Oxidation rate constants are measured under different pH conditions, which show that increasing the pH of the reaction solution significantly accelerates the oxidation process.

Cooperativity effect of mercaptosuccinic acid on the oxidation of thiourea by methylene blue in acidic medium

The principal substrate, thiourea (TU) interacts with methylene blue (MB) in a molar ratio of 2: 1 and the rate of oxidation is considerably enhanced in presence of mercaptosuccinic acid (TMA). The order of reaction is zero in MB and unity with respect to TU. The rate initially increases on increasing [TMA] but it is not affected at higher concentrations of TMA ( ca ≥ 2.0 × 10−4 M). The order in MB changes from zero to –½ at lower concentrations of MB ( ca ≤ 1.0 × 10−5 M). The reaction is characterised by a high negative entropy of activation. The kinetic features have been explained in terms of a competitive reaction between the half-reduced MB radical (HM·) and the TU molecule. This postulate has been verified by making the runs in presence of EDTA. The possibility of the participation of metal ion(s) as an impurity was ruled out on the grounds that the reactions of TU and TMA with MB individually in acidic medium do not show a negative order.

Microwave Activation of Processes in Mesopores: The Thiourea Electrooxidation at Mesoporous Platinum

AbstractIn situ microwave activation is applied to the electrochemical oxidation of thiourea at low surface area (polished polycrystalline) and at high surface area (electrodeposited mesoporous platinum coated) platinum microelectrodes. The one electron oxidation of thiourea to formamidine disulfide is monitored as a function of the different activation parameters. In the absence of microwaves (ambient temperature, low mass transport) increasing the surface area (roughness) increases the thiourea oxidation response predominantly due to adsorption effects. In the presence of high microwave intensities, high mass transport and thermal effects further increase the oxidation current at mesoporous platinum. The most effective thickness of the mesoporous platinum film for the thiourea oxidation process is estimated as 3 μm independent of electrode diameter, temperature, or mass transport effects.

Anodic behavior of gold in acid thiourea solutions: A cyclic voltammetry and quartz microgravimetry study

It is shown that at potentials E 100 μA cm−2) starts at E ≥ 0.65 V, with the primary process being the oxidation of thiourea, which gives rise a current peak at E ≃ 1.0 V. The thiourea oxidation at E ≥ 1.1 produces the appearance of catalytically active species, which drastically accelerate the gold dissolution process in the potential region extending from a steady-state value to 0.6 V, where the current efficiency for gold approaches 100% and a peak emerges at E ≃ 0.55 V. The peak’s height is commensurate with the value of the limiting diffusion current associated with the ligand supply. The species in question make no discernible impact on the thiourea oxidation process. Formamidine disulfide, which forms during the anodic oxidation of thiourea or which is added in solution on purpose, exerts no noticeable catalytic influence on the anodic gold dissolution. The catalytically active species is presumably the S2− ion, product of decomposition and deep oxidation of thiourea and formamidine disulfide. Indeed, adding sulfide ions in solution has a strong catalytic effect on the gold dissolution, whose character is identical to that of the effect exerted by products of anodic oxidation of thiourea at E ≥ 1.1 V μA cm−2.

Adsorption and oxidation pathways of thiourea at polycrystalline platinum electrodes

Abstract The present paper is related to study the adsorption and oxidation of thiourea (TU) on platinum electrodes in 0.1 M perchloric acid. A systematic electrochemical investigation was performed combining cyclic voltammetry with in situ Fourier transform infrared spectroscopy (FTIRS) and differential electrochemical mass spectrometry (DEMS) techniques to establish the nature of the species formed during these processes. It was concluded that TU adsorbs parallel on the platinum surface in the 0.20–0.55 V RHE potential range without faradic reaction. A dual-path reaction mechanism was evidenced in the oxidation process when TU was present in solution. The first reaction takes place from dissolved TU at E > 0.55 V RHE to form formamidine disulphide ( TU ) 2 2 + which is oxidized to soluble NH 2 CN at E > 1.30 V RHE . The second parallel reaction occurs from adsorbed TU at E > 0.60 V RHE and implies the formation of adsorbed species (NH 2 CN, SCN and S). In a following oxidation step at E > 1.10 V RHE , N 2 , CO 2 , and HSO 4 - / SO 4 2 - were produced. All these species were characterized from the spectroscopic experiments.

Selective Oxidation of Thioureas in an Ionic Liquid by Employing an Ion‐Supported Hypervalent Iodine(III) Reagent.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Selective Oxidation of Thioureas in an Ionic Liquid by Employing an Ion-supported Hypervalent Iodine(III) Reagent

Several 2,4-dialkyl-3,5-bis(arylimino)-1,2,4-thiadiazolidines were synthesised under mild conditions in good yield by the selective oxidation of N-alkyl-N'-arylthioureas by using 1-(4-diacetoxyiodobenzyl)-3-methyl imidazolium tetrafluoroborate [dibmim]+[BF4]- in ionic liquids.

Thermodynamic Analysis of Decomposition of Thiourea and Thiourea Oxides

Thiourea has exhibited extremely rich dynamical behavior when being oxidized either through a chemical approach or via an electrochemical method. In this study, thermodynamic properties of thiourea and its oxides are investigated by measuring their thermogravimetry (TG), differential thermogravimetry (DTG), and differential scanning calorimetry (DSC) simultaneously. Online FT-IR measurements show that products of the thermal decomposition vary significantly with the reaction temperature. In addition to the determination of their apparent activation energy (E), preexponential factor (A), and entropy (DeltaS++), enthalpy (DeltaH++), and Gibbs energy (DeltaG++) of thermal decomposition, our investigation further illustrates that the decomposition kinetics of thiourea and thiourea oxides follows the Johnson-Mehl-Avrami Equation, f (alpha) = n(1 - alpha)[-ln(1 - alpha)](1-1/n) and G(alpha) = [-ln(1 - alpha)](1/n) with n equal to 2, 3.43, and 3, respectively.