Colloidal Manganese Dioxide Research Articles

Oxidative degradation of dipeptide (glycyl–glycine) by water-soluble colloidal manganese dioxide in the aqueous and micellar media

The kinetics of the oxidative degradation of dipeptide glycyl–glycine (Gly-Gly) by water-soluble colloidal MnO 2 in acidic medium has been studied by employing visible spectrophotometer in the aqueous and micellar media at 35 °C. To obtain the rate constants as functions of [Gly-Gly], [MnO 2] and [HClO 4], pseudo-first-order conditions were maintained in each kinetic run. The first-order-rate is observed with respect to [MnO 2], whereas fractional-order-rates are determined in both [Gly-Gly] and [HClO 4]. The addition of sodium pyrophosphate and sodium fluoride has composite effects (catalytic and inhibition). The reaction proceeds through the fast adsorption of Gly-Gly on the surface of the colloidal MnO 2. The observed results are discussed in terms of Michaelis–Menten/Langmuir–Hinshelwood model. The Arrhenius and Eyring equations are found valid for the reaction over a range of temperatures and different activation parameters have been evaluated. A probable reaction mechanism, in agreement with the observed kinetic results, has been proposed and discussed. The influence of changes in the surfactant concentrations on the observed rate constant is also investigated and the reaction followed the same type of kinetic behavior in micellar media. The pseudo-first-order rate constant ( k ψ) is found to increase about two-fold with increase in [TX-100]. The catalytic effect of nonionic surfactant TX-100 is explained in terms of the mathematical model proposed by Tuncay et al.

Water-soluble colloidal manganese dioxide as an oxidant for l-tyrosine in the absence and presence of non-ionic surfactant TX-100

Water-soluble colloidal manganese dioxide has been used to oxidize l-tyrosine in aqueous-acidic medium. The kinetics of the reaction was studied in the absence and presence of non-ionic surfactant (TX-100) using a spectrophotometric technique. As the reaction was fast under pseudo-first-order conditions ([ l-tyrosine] ≫ [MnO 2]), the rate constants as a function of [ l-tyrosine], [MnO 2], [HClO 4] and temperature were obtained under second-order conditions. The rate of the reaction increased and decreased with the increase in [ l-tyrosine] and [MnO 2], respectively. Perchloric acid, sodium pyrophosphate and sodium fluoride showed catalytic effect. The effect of externally added manganese(II) sulphate is complex. It is not possible to predict the exact dependence of the rate constants on manganese(II) concentration, which has a series of reactions with other reactants. The reaction is inhibited by the non-ionic surfactant TX-100. Activation parameters have been evaluated using Arrhenius and Eyring equations. Based on observed kinetic results, a probable mechanism for the reaction has been proposed which corresponds to fast adsorption of the reductant and hydrogen ion on the surface of colloidal MnO 2 followed by one-step two-electron transfer rate limiting process.

Formation and Decomposition of Water Soluble Colloidal Manganese Dioxide During the Reduction of by Cysteine. A Kinetic Study

Upon addition of cysteine to a solution of , a transient species appears within the time of mixing, which is stable for several months. In order to conform the nature of the transient species, spectroscopic, kinetic and coagulation experiments were carried out. For the formation of water-soluble colloidal MnO2, the stiochiometry was found to be 1:1 (cysteine:). On adding of HClO4, there is a decrease in the absorbance of colloidal MnO2. At higher [cysteine] (≥2.0 × 10−4 mol dm−3), as the reaction became so fast, the formation and decomposition of colloidal MnO2 are not observed. Thus in presence of [HClO4] and excess of [cysteine], the intermediate colloidal MnO2 is unstable. Kinetic studies of the decomposition of water soluble colloidal MnO2 have been investigated by visible spectrophotometry at 25°C. The observed kinetics was explained by assuming fast adsorption of cysteine on the surfaces of colloidal MnO2. The adsorption formation constants between MnO2-cysteine and MnO2-F− have been determined (Kad = 8.5 × 103 dm3 mol−1 and Kf = 4.9 × 104 dm3 mol−1 for cysteine and F−, respectively). The deposition of solid MnO2 was observed in presence of externally added Mn(II) and Ag+ ions. On the basis of the various observations, a most plausible mechanism has been envisaged for the decomposition of water soluble colloidal MnO2.

The Kinetics of Oxidation of L-Tryptophan by Water-Soluble Colloidal Manganese Dioxide

The kinetics of the oxidation of L-tryptophan by water-soluble colloidal MnO2 (prepared from potassium permanganate and sodium thiosulfate solutions) has been carried out in aqueous perchloric acid medium at different temperatures. Monitoring the disappearance of the MnO2 spectrophotometrically at 390 nm was used to follow the kinetics. The first-order kinetics with respect to [L-tryptophan] at low concentrations shifted to zero-order at higher concentrations. The reaction followed first-order with respect to [MnO2] but fractional-order with respect to [HClO4]. Adding trapping agents enhanced the rate of the reaction. The Arrhenius and Eyring equations were found valid for the reaction between 35°C and 55°C and different activation parameters (Ea, ΔH#, ΔS#) have been evaluated. On the basis of various observations and product characterization a plausible mechanism has been envisaged for the reaction taking place at the colloid surface. The results suggest formation of an adsorption complex between L-tryptophan and MnO2. The complex decomposes in a rate-determining step, leading to the formation of free radical, which again reacts with the colloidal MnO2 in a subsequent fast step to yield products. Freundlich isotherm is used to explain the adsorption of L-tryptophan on the colloidal MnO2.

Photoinduced redox reactions between colloidal manganese dioxide and some organic compounds of environmental importance

Numerous organic compounds of environmental importance, i.e., phenol, citric, tartaric, and oxalic acids, proved to promote or accelerate reductive dissolution of colloidal manganese dioxide upon irradiation. This is accounted for the formation of surface-located charge-transfer complexes between the MnO2 particulates and the organic electron donors. From the dependences of the rate of the photoassisted and thermal dissolution on the concentration of the organic compounds, the equilibrium constants for the formation of these complexes have been determined in the case of phenol, resorcinol, citrate, and tartaric acid. The quantum yields for these photoinduced reactions (at λir = 365 nm), however, do not show any correlation with the values of the corresponding equilibrium constants, although adsorption is prerequisite for the efficient reductive dissolution of MnO2. The changes in pH markedly affect the rate of this process, indicating that protonation of both the electron donors and the surface of the MnO2 particulates may play significant roles in these systems. The results of experiments carried out in manganese dioxide excess suggest that total mineralization of organic electron donors is strongly hindered by the disadvantageous adsorption properties of the primary redox products.

Colloidal Manganese Oxide Precursor to Octahedral Layered, OL-3 Materials

We report the one-pot synthesis of a hexagonal form of a layered manganese oxide material (OL-3) using mild conditions and low temperature. The oxidation of an aqueous solution of manganese acetate using tetramethylammonium hydroxide and hydrogen peroxide at 4 °C leads to the formation of a colloidal manganese dioxide solution. Colloidal MnO2 was then flocculated using K ions, forming disordered layered manganese oxide nano-flakes having an R $$\bar 3$$ m rhombohedral structure with lattice parameters a = 2.85 A and c = 21.8 A. The potassium manganese oxide nano-flakes were characterized using X-ray diffraction, electron microscopy, chemical analysis, thermal analysis, N2 sorption, and UV/Visible spectroscopy. The results indicate that the colloidal manganese oxide nano-flakes flocculated into ultra-thin, disorderly-stacked hexagonal lamellar sheets composed of a material with the chemical composition of K1.04MnO2.34·0.6H2O.

Reactivity of some sulphur- and non-sulphur-containing amino acids towards water soluble colloidal MnO2. A kinetic study

Kinetic data for the oxidation of glutathione (reduced, GSH), cysteine, glycine and glutamic acid by colloidal manganese dioxide, (MnO2) n are reported. Colloidal MnO2, oxidized glutathione to disulphide (glutathione, oxidized), was reduced to manganese (II). Glycine and glutamic acid (structural units of glutathione) are not oxidized by colloidal MnO2, but the other structural unit, cysteine, is also oxidized by the same oxidant under similar experimental conditions. This is interpreted in terms of the rate-determining colloidal MnO2-S bonded intermediate. The reactivity of GSH towards colloidal MnO2 is very much higher than cysteine. Kinetics of oxidation of GSH and cysteine by colloidal MnO2 were performed spectrophotometrically as a function of [GSH], [cysteine], colloidal [(MnO2) n ], [HClO4], temperature and trapping agents sodium fluoride and manganese (II) (reduction product of colloidal MnO2). The purpose of this work was to study the role of –NH2, –COOH, –SH groups present in the carbon chain of the above amino acids. It was found that the reactivity of –SH group is higher than –NH2 and –COOH groups. The mechanisms, involving a colloidal MnO2 complex with GSH and cysteine, are proposed. The complexes decompose in a rate-determining step, leading to the formation of free radical and manganese (III), which is also an intermediate. The dimerization of radicals takes place in a subsequent fast step to yield the products.

Mechanism of the oxidation of d-glucose onto colloidal MnO2 surface in the absence and presence of TX-100 micelles

Aqueous colloidal manganese dioxide (MnO2) was prepared via titration by using potassium permanganate and sodium thiosulphate in aqueous neutral medium. The kinetics of oxidation of d-glucose onto the surface of colloidal MnO2 have been studied spectrophotometrically. The results show that the rate of initial stage (nonautocatalytic path) increases with increasing the [d-glucose], [H+], and temperature and also upon addition of nonionic surfactant Triton X-100 (TX-100), which indicates that the surfactant enhances the concentration of d-glucose at the surface of the colloidal MnO2. Hydrogen bonding interaction seemingly arises between –OH groups of d-glucose and oxygen of the ether linkages of polyoxyethylene chain of TX-100. A possible mechanism of the oxidative degradation of d-glucose is discussed in terms of d-glucose/TX-100 and colloidal MnO2 interaction.

Reduction of water-soluble colloidal manganese dioxide by thiourea: a kinetic and mechanistic study

Kinetic data for the colloidal MnO2–thiourea redox system are reported for the first time. The reduction of water-soluble colloidal MnO2 by thiourea (sulfur containing reductant) in aqueous perchloric acid medium has shown that it proceeds in two stages, i.e., a fast stage followed by a relatively slow second stage. The log (absorbance) versus time plot deviates from linearity. The kinetics of both the stages was investigated spectrophotometrically. The first-order kinetics with respect to [thiourea] at low concentration shifts to zero-order at higher concentration. The reaction rate increases with [HClO4] and the kinetics reveals complex order dependence in [HClO4]. Addition of P2O 7 4− and F− in the form of Na4P2O7 and NaF, respectively, has inhibitory effect on the reaction rate. The reaction proceeds through the fast adsorption of thiourea on the surface of the colloidal MnO2. A mechanism involving the protonated thiourea as the reactive reductant species is proposed. The observed results are discussed in terms of Michaelis–Menten/Langmuir–Hinshelwood model. From the observed kinetic data, colloidal MnO2–thiourea adsorption constant (K ad1) and rate constant (k 1) were calculated to be 1.25×1010 mol−1 dm3 and 3.1×10−4 s−1, respectively. The variation of temperature does not have any effect on the reaction rate.

Oxidation of gum arabic by soluble colloidal MnO 2

In the present work, the oxidative degradation of gum arabic by colloidal manganese dioxide (MnO 2) was carried out. Monitoring the disappearance of the MnO 2 spectrophotometrically at 375 nm was used to follow the kinetics. The oxidation obeyed fractional-order kinetics with respect to the [gum arabic]. Effect of various experimental parameters such as the initial colloidal [MnO 2], [HClO 4], temperature, and complexing agents (P 2O 7 4−, F −, and Mn 2+) for the oxidation of gum arabic was studied. The reaction was acid catalyzed. Addition of P 2O 7 4−, F −, and Mn 2+ ions enhances the rate of oxidation significantly. Gum arabic adsorbs onto the surface of the colloidal MnO 2 through the equatorial –OH groups of the rhamnose moiety, and the complex breaks down into products. The Arrhenius equation was valid for the oxidation kinetics between 40 and 60 °C. To explain the observed kinetic results, a suitable mechanism and rate law for the reaction taking place at the surface of the colloidal particle has been proposed. The reducing nature of gum arabic is found be due to the presence of –OH group in the skeleton.

Revising the Mechanism of the Permanganate/Oxalate Reaction

The permanganate/oxalate reaction has been known for more than a century; however, its mechanism is still subject to debate. The latest general publication by a French group established a model that involved 14 steps including 8 equilibria. The model was found to be able to simulate experimentally observed phenomena and to account for the bistability in a continually stirred tank reactor (CSTR). However, some earlier reported observations that we found in the literature seem to be inconsistent with this model. We performed electron paramagnetic resonance (EPR) measurements and stopped-flow studies with spectrophotometrical detection to shed light on these contradictions. We found that one of the key steps of the model describing the decomposition of Mn(VII) is not acceptable at least with the indicated rate constant. The only other step in which permanganate is involved is not capable of accounting for the autocatalytic nature. Our striking observation that the reaction still can be autocatalytic when applying a large stoichiometric excess of manganous ions points out that autocatalysis cannot be purely explained by a positive Mn2+ feedback loop. Thus, we propose that the surface-catalyzed formation of colloidal manganese dioxide from Mn(II) and Mn(VII) provides a second positive feedback loop in the reaction.

Kinetics and mechanism of the reduction of colloidal manganese dioxide by d-fructose

Kinetics of the reduction of water-soluble colloidal manganese dioxide by d-fructose was investigated in acidic perchlorate media at different temperatures. The observed kinetics is explained by assuming fast adsorption of d-fructose on the surface of colloidal MnO 2. The reaction is fractional-order each in [ d-fructose] and [H +] with a unit order in [MnO 2] 0. The results suggest formation of an adsorption complex between d-fructose and MnO 2. The complex decomposes in a rate-determining step, leading in the formation of a free radical, which again reacts with the colloidal MnO 2 in a subsequent fast step to yield the products. The reaction rate decreased by addition of F −, P 2O 7 4− and Mn 2+ ions. The rates of Mn 2+ and P 2O 7 4− inhibited reactions further increase with increasing the concentrations of those ions. The Arrhenius equation was valid for the reaction between 30 and 60 °C. On the basis of various observations and product characterization a plausible mechanism has been envisaged for the reaction taking place at the colloid surface. The colloidal MnO 2 has been found to be reduced by gum arabic (a protective colloid). The adsorption hypothesis viz. Freundlich isotherm for the mechanism of the reaction was tested and the Freundlich exponents ( n = 0.57 and n′ = 0.69) have been calculated.

Oxidation of lactic acid by water soluble (colloidal) manganese dioxide

AbstractSpectrophotometric method has been used to characterize water‐soluble colloidal manganese dioxide obtained by the redox reaction between sodium thiosulphate and potassium permanganate in neutral aqueous medium which shows a single peak in the visible region with λmax = 425 nm. The kinetics of the oxidation of lactic acid by colloidal manganese dioxide (oxidant) has been investigated spectrophotometrically under pseudo‐first‐order conditions of excess lactic acid. The rate of the noncatalytic reaction pathway was slow which increased with increasing lactic acid concentration. The reaction was first‐order with respect to [oxidant] as well as [lactic acid]. In presence of manganase(II) and fluoride ions, the noncatalytic path disappeared completely while the oxidation rate of autocatalytic path increased and decreased, respectively with increasing [Mn(II)] and [F−]. A mechanistic scheme in conformity with the observed kinetics has been proposed with the rate‐law: equation image © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 359–366 2004

Coagulation of colloidal manganese dioxide by divalent cations

Abstract The coagulation of colloidal MnO 2 by several divalent cations, both in the absence and presence of a complexing ligand ( l -histidine), has been studied. The efficiency as coagulating agents increases in the order Mg 2+ 2+ 2+ 2+ 2+ 2+ . For a given cation, the coagulating efficiency increases with increasing pH and with increasing temperature, and decreases with increasing concentration of l -histidine. Apparent values for the equilibrium constants of complexation of divalent cations by l -histidine have been obtained. The apparent equilibrium constant of formation of the complex increases in the order Mg 2+ 2+ 2+ 2+ 2+ . The value corresponding to Co 2+ could not be determined because l -histidine acted as a catalyst for the reduction of colloidal MnO 2 by that cation.

Reduction of Colloidal Manganese Dioxide by Manganese(II)

The reduction of colloidal MnO2 by Mn2+ in aqueous HClO4 has been studied by a spectrophotometric method. The reaction product is Mn(III). The reaction is of first order in both colloidal MnO2 and H+, whereas it presents a fractional order (0.58±0.02) in Mn2+. The reaction is retarded by addition of NaClO4, but is not affected by addition of tert-butanol. The corresponding activation energy is 29.5±1.3 kJ mol−1. The reaction is catalyzed by Na4P2O7, and the pyrophosphate-catalyzed reaction is of first order in both colloidal MnO2 and pyrophosphate and of fractional order (0.64±0.01) in Mn2+, whereas its rate presents a complex dependence on the concentration of H+. The pyrophosphate-catalyzed reaction is accelerated by addition of both NaClO4 and tert-butanol. The corresponding activation energy is 49.7±3.0 kJ mol−1. Mechanisms in agreement with the experimental data are proposed for both the parent and the pyrophosphate-catalyzed reactions.

The pyrophosphate-assisted reduction of chromium(vi) by manganese(ii) and its reverse reactionElectronic supplementary information (ESI) available: derivation of the theoretical rate laws, apparent kinetic orders of Mn(ii) at various initial concentrations of Cr(vi) and at various temperatures, initial rates at various concentrations of sodium perchlorate, parameters a and b at various temperatures, and initial rates at various temperatures. See http://www.rsc.org/suppdata/nj/b1/b104573n/

We report the discovery of a novel reaction that behaves in an unexpected way. Although Cr(VI) and Mn(II) do not react in the absence of stabilizing ligands, they react in aqueous perchloric acid provided that pyrophosphate [a stabilizing ligand for Mn(III)] is present in the medium. The reaction [whose stoichiometry is Cr(VI) + 3 Mn(II) → Cr(III) + 3 Mn(III)] takes place in a reversible manner, but the kinetic plots are not as those expected for conventional reversible reactions. On the contrary, a period of decay of the Cr(VI) concentration [and formation of Mn(III)] is followed by a period in which that reactant is actually regenerated (and the product consumed). During the course of the reaction, pyrophosphate is slowly hydrolyzed to phosphate. This hydrolysis constitutes the driving force for the backward reaction. A mechanism involving three consecutive 1-electron redox steps is proposed for the reduction of Cr(VI) to Cr(III), in each step a Mn(II) being oxidized to Mn(III), whereas colloidal manganese dioxide might be a key intermediate for the reoxidation of Cr(III) to Cr(VI).

A kinetic study of the reaction between colloidal manganese dioxide and formic acid in aqueous perchloric acid solution in the presence of surface active agents

The kinetics of the reaction between colloidal manganese dioxide (obtained by reduction of potassium permanganate with sodium thiosulfate) and formic acid at 25°C and constant ionic strength in aqueous perchloric acid solution were studied in the presence of surface active agents, such as cationic cetyltrimethylammoniumbromide (CTAB), anionic sodium lauryl sulfate (SDS), non-ionic Triton X-100. The reaction rate was followed spectrophotometrically by measuring the changes in absorbance at 500 nm. The results were analyzed by means of the fixed time method. Cationic surfactants cause flocculation of the oppositely charged colloidal MnO 2 and could not be used, whereas anionic micellar systems were found to be ineffective on the reaction rate. No noticeable effect of SDS on the reaction rate was observed, even at high concentrations. This reaction was found to be accelerated by non-ionic micelles of Triton X-100. The reaction was first-order with and without surfactants, and the rate constant, k, increased as Triton X-100 concentration increased and reached a plateau at higher concentrations.

Photoassisted dissolution of colloidal manganese dioxide in the presence of phenol

The UV-vis absorption spectrum of colloidal manganese dioxide in aqueous solution is considerably modified by addition of phenol. This is accounted for the formation of a charge-transfer complex on the surface of the MnO2 particulates. Irradiation of this system in the range 310–590 nm leads to an enhanced reductive dissolution of the Mn(IV) species. The dependences of both the spectral change and the rate of the photoassisted dissolution on the phenol concentration show Langmuir-type functions, the evaluation of which gives the formation constant of the surface-located complex, K= 1.3 ±0.3 × 104 M−1. The crucial role of this compound in the photoinduced reduction of manganese dioxide is also confirmed by the wavelength dependence of the quantum yield.

A Kinetic Study of the Reduction of Colloidal Manganese Dioxide by Oxalic Acid

A kinetic study of the reaction between colloidal manganese dioxide and oxalic acid in aqueous acetate media (pH 4.3–5.1) is reported. The reaction is autocatalytic and, in order to determine the rate constantsk1andk2corresponding, respectively, to the noncatalytic and autocatalytic reaction pathways, the Mn(III) formed as an intermediate was stabilized by addition of sodium pyrophosphate. Joint iodimetric and spectrophotometric studies indicated that the reduction of colloidal manganese dioxide follows the sequence[formula]where R stands for the reductant. Both reaction pathways exhibit acid catalysis, and the activation energies associated tok1andk2are 74.7 ± 0.9 and 44.6 ± 0.6 kJ mol−1, respectively. The reaction is accelerated by addition of manganese(II) sulfate, andk2increases with rising oxalate concentration, suggesting that MnC2O4is the active autocatalyst. The possibility that Mn(III) might collaborate in the autocatalysis through a free-radical chain mechanism is also pointed out.

Oxidation of higher alcanols by tetra-1-butylammonium permanganate

Oxidations of hexan-1-ol, hexan-2-ol, hexan-3-ol, heptan-1-ol, heptan-2-ol, octan-1-ol, and octan-2-ol with tetra-1-butylammonium permanganate, dissolved in the same alcohols, proceed partly autocatalytically. The rate constants of both catalytic and non-catalytic reactions have been evaluated. Colloidal manganese dioxide, one of the reaction products, has been identified as the catalyst.