Synthesis and size-dependent exchange bias in inverted core-shell MnO|Mn3O4 nanoparticles

Core-shell nanoparticles of MnO|Mn3O4 with average particle sizes of 5-60 nm, composed of an antiferromagnetic (AFM) core and a ferrimagnetic (FiM) shell, have been synthesized and their magnetic properties investigated. The core-shell structure has been generated by the passivation of the MnO cores, yielding an inverted AFM-core|FiM-shell system, as opposed to the typical FM-core|AFM-shell. The exchange-coupling between AFM and FiM gives rise to an enhanced coercivity of approximately 8 kOe and a loop shift of approximately 2 kOe at 10 K, i.e., exchange bias. The coercivity and loop shift show a non-monotonic variation with the core diameter. The large coercivity and the loop shift are ascribed to the highly anisotropic Mn3O4 and size effects of the AFM (i.e., uncompensated spins, AFM domains, and size-dependent transition temperature).     

Interaction of three pradimicin derivatives with divalent cations in aqueous solution

This study focused on further analysis of the aggregation behavior of pradimicin derivatives and their interaction with cations in aqueous solution. BMY was compared with two other pradimicin antibiotics (T2 and FB) with the same aglycone moiety but consisting of different substitute groups. The surface tension measurement showed a clear critical micelle concentration at 1-2 mM of the BMY aqueous solution. The role of Zn2+ in replacing the Ca2+ was examined using 1H nuclear magnetic resonance (NMR) method. From changes in the NMR spectrum and precipitability, it was concluded that zinc ion has lower affinity and higher precipitating ability to BMY than the divalent cations of alkaline earth metal. The aggregation behavior of T2 and FB in aqueous solution was also studied using NMR method. The results suggest that the supramolecular behavior of T2 is similar to BMY whether or not Ca2+ ions are present in solution and that there are two binding sites for calcium ions in a T2 molecule. Unlike BMY and T2, the NMR spectrum of FB does not show distinct change upon Ca2+ addition. The interaction of pradimicin antibiotics with divalent metal ions was thought to be related to ionic electronegativity and to the amphoteric property of the antibiotics.     

Kinetics of the reduction of MnO4− by aqueous NH3 solutions

Transition Metal Chemistry -     

A radiochemical study of the kinetics of exchange between manganese oxides and some cations in solution

The kinetics of the exchange between⁵⁶Mn-labelled manganese dioxide and cations in aqueous solution was studied by measuring the β⁻ activity acquired by the solution. The results of the exchange between a chemical γ MnO₂ and a divalent M²⁺ ion (M=Mn, Co, Cu or Zn) or a trivalent M³⁺ ion (M=Ga, Fe, In, Rh or Al) indicate a fast initial process followed by a diffusion—controlled exchange. It is assumed that M²⁺ ions exchange with Mn²⁺ ions and M³⁺ ions exchange with Mn³⁺ ions in MnO₂. The process depends on the radii of the host and substituent ions and on consideration of crystal field stabilisation energies. It seems that the γ MnO₂ studied contains more Mn³⁺ than Mn²⁺ ions. The possibility of the exchange between Mn ions and cations of a different charge cannot be ruled out. The exchange between Co²⁺ ions and MnO₂ was enhanced in presence of pyrophosphate, which stabilises Mn(III) as a complex. The fraction of Mn in different samples of MnO₂ exchanged with a given cation depends on the type and not on the surface area of the sample.     

Kinetics and mechanism of the oxidation of hydroxylamine by a {Mn3O4}4+ core in aqueous acidic media

In this work we report the kinetics of oxidation of hydroxylamine by a trinuclear Mn(IV) oxidant, [Mn(3)(μ-O)(4)(phen)(4)(H(2)O)(2)](4+) (1, phen = 1,10-phenanthroline), in aqueous solution over a pH range 2.0-4.0. The trinuclear Mn(IV) species (1) deprotonates in aqueous solution at physiological pH: 1 ? 2 + H(+); pK(1) = 4.00 (± 0.15) at 25.0 °C, I = 1.0 (M) NaNO(3). Both 1 and 2 are reactive oxidants reacting with the conjugate acid of hydroxylamine, viz. NH(3)OH(+) where the deprotonated oxidant 2 reacts faster. This finding is in contrast to a common observation and belief that protonated oxidants react quicker than their deprotonated analogues. Mn(IV)(3) to Mn(II) transition in the present reaction proceeds through the intervention of a spectrally detected mixed-valent Mn(III)Mn(IV) dimer that quickly collapses to Mn(II). The rate of the reaction was found to be lowered in D(2)O-enriched media in comparison to that in pure H(2)O media. An initial one electron one proton transfer to Mn(IV)(3) (electroprotic; 1e, 1H(+)) could be mechanistically conceived as the rate step. We also demonstrate by means of high level DFT studies that, among the two sets of Mn(IV) atoms in the trinuclear oxidant, the unique one that is coordinated with two phen ligands and two oxo-bridges is reduced to Mn(III) at the rate step. This is explained based on energetic and spin density calculations. Moreover, this result agrees with the charge distribution on the Mn atoms of the trinuclear complex.     

The role of solvent structure in ligand substitution and solvent exchange at some divalent transition-metal cations

The role of the solvent in reactions involving ions is considered in relation to the structure of liquids. The rate constants and activation parameters for ligand substitutions at divalent transition metal cations in various solvents are compared with those for solvent exchanges. The differences are related to structural properties of the solvents, represented by their heats of evaporation and fluidities, and interpreted with the aid of a model developed from that of Frank and Wen. Water is not a typical solvent.This paper was presented at the symposium, The Physical Chemistry of Aqueous Systems, held at the University of Pittsburgh, Pittsburgh, Pennsylvania, June 12–14, 1972, in honor of the 70th birthday of Professor H. S. Frank.     

Removal and regeneration of aqueous divalent cations by boehmite

Boehmite (Al(OH)O) was employed for the removal of aqueous Mg(2+), Cu(2+), Cd(2+), Pb(2+), and Co(2+) at 298 K. Although boehmite was able to remove these divalent cations, the greater removal rate with boehmite of Pb(2+) (28.7%) than with Mg(2+), Cu(2+), Cd(2+), and Co(2+) (5.6, 25.3, 10.9, and 13.3%, respectively) was observed under acidic conditions. Under stronger alkaline conditions, in which the lead species was completely dissolved, a greater removal rate of Pb(2+) (more than 80%) was observed under the corresponding conditions employed for the acidic conditions. The removed lead species could not be dissolved from boehmite in an acidic solution while an evident dissolution of lead species was detected using an aqueous NaOH solution. The results shown in the present study reveal that boehmite can be employed as a reagent for the removal and regeneration of aqueous metal cations.     

Strong Morphological Effect of Mn3O4 Nanocrystallites on the Catalytic Activity of Mn3O4 and Au/Mn3O4 in Benzene Combustion

Gold nanoparticles (3–4 nm) were deposited on Mn₃O₄ nanocrystallites with three distinct morphologies (cubic, hexagonal, and octahedral). The resulting structures were characterized, and their activities for benzene combustion were evaluated. The dominant exposed facets for the three kinds of Mn₃O₄ polyhedrons show the activity order: (103)≈(200)>(101). A similar activity order was derived for the interfaces between the Au and the Mn₃O₄ facet: Au/(200)≈Au/(103)>Au/(101). The metal–support interactions between the Au nanoclusters and specific facets of the Mn₃O₄ polyhedrons lead to a unique interfacial synergism in which the electronic modification of the Au nanoparticles and the morphology of the Mn₃O₄ substrate have a joint effect that is responsible for a significant enhancement in the catalytic activity of the Au/Mn₃O₄ system.     

Competition between ligands for Al2O3 in aqueous solution

The adsorption of two classes of carboxylic ligands (i.e., aliphatic and aromatic small molecules), onto α-alumina nanoparticles was investigated. A new methodology was used whereby two molecules were simultaneously equilibrated with the inorganic material. A two-dimensional representation of the adsorption of the two complexing molecules enables us to differentiate between pairs of ligands with (i) independent adsorption on different sites of the alumina particles, (ii) competing adsorption on the same sites, or (iii) a mix thereof. Both the highest affinity ligands (tetracarboxylic acid, citric acid, and tiron), and the way they compete with lower affinity ligands have been identified. The combination of carbon skeleton and complexing groups required to produce the ligand of highest affinity at pH 5 has been recognized. In particular, the role of the OH in the α position of a carboxylic group and the role of the distance between two carboxylic groups are emphasized.     

Interactions between cyclic phosphate anions and various cations in aqueous solution

Interactions of tetrameta-, hexameta- and octameta-phosphate ions with proton, alkali metal ions and hexamminecobalt(III) ion were studied by the conductivity and potentiometric method. It was shown that the logarithms of thermodynamic stability constants of 1 : 1 proton associated molecules with these cyclic phosphates increase linearly with the anion charges contrary to those with the linear phosphates. The anomalous order of the stability constants obtained for alkali metal ion-pairs with a cyclic phosphate suggests the possibility of partial dehydration of the alkali metal ions with smaller atomic weights in the ion-pair formation. By the use of ion-selective electrodes, it was shown that in a solution containing an excess of sodium, potassium or rubidium ions compared with cyclic phosphate ions, more than one cation is associated with a cyclic phosphate ion. The apparent association constants of these highly associated species were determined. Interactions of cyclic phosphate ions with hexamminecobalt(III) ion, which was selected as an example of tervalent cations, are discussed in comparison with those for uni- and bi-valent cation complexes.     

Photoacoustic determination of the band gaps of Ca2MnO4 and Ca3Mn2O7

Black powders of the magnetically layered semiconductors Ca₂MnO₄ and Ca₃Mn₂O₇ prepared by a carbonate-precursor technique have been investigated by photoacoustic spectroscopy (PAS) over the energy range 1.25–4 eV. The band-gap energies of these two compounds have been evaluated to be 1.6 and 1.4 (± 0.1) eV, respectively. It is concluded that PAS is a most useful method for determining the variation of the band gap with composition in large band-gap polycrystalline semiconductors.     

The relation between structural features and electrochemical activity of MnO2 nanoparticles synthesized from a polyol-made Mn3O4 precursor

A simple hybrid synthesis processing method was developed to synthesize γ-MnO₂ nanocrystalline particles. The polyol method was modified by the addition of nitric acid in order to allow the synthesizing of single-phase Mn₃O₄ in a large scale. In the sequence, the acid digestion technique was used to transform Mn₃O₄ into γ-MnO₂. Structural and morphological characterization was carried out by X-ray diffractometry, Infrared and Raman spectroscopy, thermogravimetric analysis, nitrogen adsorption isotherm, scanning electron microscopy, and transmission electron microscopy. The electrochemical properties were investigated by cyclic voltammetry and galvanostatic charge–discharge measurements. The synthesized material exhibits a specific capacitance of 125.1 F?g^?1 at a mass loading of 0.98 mg?cm^?2. The relation between structural features and electrochemical activity is discussed by comparing the synthesized material with commercial electrolytic manganese dioxide.     

MnO2 electrodeposition at the positive electrode of zinc-ion aqueous battery containing Zn2+ and Mn2+ cations

Journal of Solid State Electrochemistry - Effects of MnO2 electrodeposition on α, β, γ, and δ-MnO2 polymorphs from aqueous zinc sulfate solution with manganese sulfate additive...     

Possibility of the existence of divalent actinium in aqueous solution

Radiopolarographic reduction of Ac(III) was investigated in aqueous solutions. It was found that a very large shift of the half-wave potential was induced by the addition of 1, 4, 7, 10, 13, 16-hexaoxacyclooctadecane (18-CROWN-6) to the aqueous solution. The shift was explained by complex formation of Ac(II) with 18-CROWN-6. The ionic radius and the electronic configuration of Ac²⁺ were determined to be 1.25 Å and [Rn] 6d¹.     

Removal of divalent nickel cations from aqueous solution by multi-walled carbon nano tubes: equilibrium and kinetic processes

Release of heavy metals into water as a result of industrial activity may pose a serious threat to the environment. In this study, the potential of multi-walled carbon nano tubes (MWCNT) to remove Ni²⁺ cations from aqueous solutions was investigated in a batch reactor under different experimental conditions. The effects on the removal process of conditions such as initial concentration of Ni²⁺ ions, temperature, and adsorbent mass were investigated. Nickel uptake was quantitatively evaluated by use of the Langmuir, Freundlich, and Dubinin?CKaganer?CRadushkevich isotherm models. For 20?mg/L initial Ni²⁺ cation concentration, adsorption capacity increased from 8.12 to 11.75?mg/g when the temperature was increased from 25 to 65?°C, an indication of the endothermic nature of adsorption process. In addition, the adsorption equilibrium was well described by the Langmuir isotherm model; maximum adsorption capacity was 17.86?mg/g Ni²⁺ cations on HNO₃-treated MWCNT (t-MWCNT). The results obtained in this study show that adsorption of Ni²⁺ on t-MWCNT is a spontaneous and endothermic process. By use of second-order kinetic constants and the Arrhenius equation, the activation energy of adsorption (E _a) was determined as 5.56?kJ?mol^?1.     

Ion exchange of some transition metal cations on hydrated titanium dioxide in aqueous ammonia solutions

The adsorption of transition metal cations on hydrated titanium dioxide in complexing ammonia and amine solutions has been studied as a function of ammonia (amine) concentration. The relationships between the distribution coefficients and ammonia concentration as well as the effects of various amines on sorption of transition metals indicate that a coordinate bond is formed between the metal ions and the hydroxy groups of the sorbent. The distribution coefficients of silver(I) and cobalt(III), which form strong ammonia complexes in aqueous solutions, decrease with increasing concentration of ammonia already at concentrations exceeding 10^–3. mol·dm^–3. Cations of zinc, manganese and mercury which form much weakerammonia complexes do not exhibit any effect of ammonia concentration in the whole range investigated. In the case of sorption of macroamounts of ammonia or amine complexes of silver, the molecular sieve effect plays an important role. The differences in the affinity of hydrated titanium dioxide for ammonia solvates of various transition metal ions can serve as a tool for effective separation of these ions in ammonia solutions.     

Kinetics of the N-chlorination of 2-aminobutyric, 3-aminobutyric, 3-aminoisobutyric and 4-aminobutyric acids in aqueous solution

We studied the kinetics of the N-chlorination of 2-aminobutyric, 3-aminoisobutyric, and 4-aminobutyric acids by sodium hypoclorite in strongly alkaline aqueous solution. As in the case of other amines, the rate of formation of the four N-chloroamino acids was proportional to the concentrations of hypochlorite and amino acid, and inversely proportional to the concentration of hydroxyl ions. A reaction mechanism compatible with these results is proposed and discussed. © John Wiley & Sons, Inc.