Electrocatalytic Activity of the Pyrochlores Ln2M2O7−δ (Ln = Lanthanoids) for Oxygen Reduction Reaction

Electrocatalytic oxygen reduction reaction (ORR) activities of the pyrochlore oxides Ln₂Zr₂O_7?δ (LnZ) and Ln₂Sn₂O_7?δ (LnS) (Ln = La, Pr, Nd, Sm) were examined in 0.1 M KOH solution at 70 °C. The onset potential (E _on) of the oxygen reduction current and the efficiency (Eff ₄) of 4-electron reduction of oxygen were evaluated by semi-steady state voltammetry with a rotating ring-disk electrode. In both LnZ and LnS series, the E _on values were ~0.85 V versus reversible hydrogen electrode. A relation was found between the E _on values and the lattice parameters; i.e. on the whole, the ORR activity became high with an increase in the lattice parameters. When the Ln ion was the same, the LnZ series exhibited higher ORR activities than the LnS series. The pyrochlore LaZ with the highest ORR activity showed a Eff ₄ value higher than 85%. Moreover, Mn-incorporation to LaZ led to a mixed-oxide (1–xLaZ?xLaM) of LaZ and the perovskite LaMnO₃ (LaM). However, the E _on value apparently sifted to a more positive potential probably due to LaMnO₃, and the magnitude of the cathodic ORR current increased with an increase in the mixing content up to x = 0.3. The mixed-oxide 0.7LaZ–0.3LaM exhibited the highest ORR activity (E _on = ~0.90 V and Eff ₄ > 95%), which was comparable to that of a conventional 20 mass% Pt/C catalyst.     

Catalytic wet peroxide oxidation of phenol solution over Fe–Mn binary oxides diatomite composite

Mn–Fe binary oxides incorporated into diatomite (denoted as FM-diatomite) was prepared by the redox reaction of KMnO₄ and FeSO₄ with pH ranging from 3 to 9. The catalytic activities of FM-diatomite were studied for phenol oxidation and were compared with iron oxide modified diatomite (F-diatomite) and manganese oxide modified diatomite (M-diatomite). The obtained catalysts were characterized by scanning electron microscope, powder X-ray diffraction, energy dispersive spectroscopy, transmission electron microscope, X-ray photoelectron spectroscopy, and nitrogen adsorption/desorption isotherms. The results show that Fe–Mn binary oxides were highly dispersed on the diatomite surface in which manganese oxide and iron oxide displayed multiple oxidation states including Mn⁴⁺, Mn³⁺, Fe²⁺ and Fe³⁺. The phenol oxidation by H₂O₂ through the use of Mn–Fe-diatomite as a catalyst was conducted. FM-diatomite exhibited as an excellent catalyst for the total oxidation of phenol and main intermediates (catechol and hydroquinone). The conversion of phenol and main intermediates by means of FM-diatomite was 100 % under 50 min while that by F-diatomite also was 100 % after 110 min but other intermediates still remained. While phenol conversion by M-diatomite was close to zero due to speedy hydroperoxide decomposition over the manganese oxide catalyst. These results show that there was a synergized effect of iron and manganese oxide present in FM-diatomite.     

Electrocatalytic activity of manganese oxides prepared by thermal decomposition for oxygen reduction

The oxygen reduction reaction (ORR) was studied in KOH electrolyte on manganese oxides supported on Vulcan carbon (Mn_yO_x/C). The oxides were prepared by thermal decomposition of manganese nitrate at different conditions. The oxides were characterized by X-ray diffraction (XRD) and in situ X-ray absorption near edge structure (XANES). The electrochemical studies were conducted using cyclic voltammetry (CV) and steady state polarization measurements carried out with a thin layer rotating ring/disk electrode. XRD results showed that the manganese oxide prepared at 200 °C in air is formed by a major phase of β-MnO₂ and the polarization curves indicated the highest activity for this material. In situ XANES evidenced the occurrence of a redox process involving Mn(II)/Mn(III) and Mn(III)/Mn(IV) in the range of potentials of the CV measurements. The electrocalytic activity was related to the occurrence of a mediation process involving the reduction of Mn(IV) to Mn(III), followed by the electron transfer of Mn(III) to oxygen and by a disproportionation reaction of the HO₂⁻ species in the Mn_yO_x sites. In situ XANES results showed that the Mn(IV) species is MnO₂ and the Mn(III) is most likely MnOOH.     

Alumina-supported manganese- and manganese–palladium oxide catalysts for VOCs combustion

Alumina-supported manganese- and palladium–manganese oxide catalysts were prepared and tested in the combustion of formaldehyde. Total combustion of formaldehyde/methanol was achieved at 220 °C over a 18.2% Mn/Al₂O₃ catalyst. This temperature decreased either to 90 or 80 upon adding 0.1% or 0.4% Pd, respectively, to the base 18.2% Mn/Al₂O₃ catalyst. The combined use of X-ray diffraction, temperature-programmed reduction and photoelectron spectroscopy (XPS) techniques revealed that a Mn⁴⁺/Mn³⁺ oxide(s) and PdO_x species are present on the surface of the fresh catalysts and remain along the catalytic reaction.     

Hydrogen peroxide sensor based on riboflavin immobilized at the nickel oxide nanoparticle-modified glassy carbon electrode

A simple and sensitive electrochemical sensor based on nickel oxide nanoparticles/riboflavin-modified glassy carbon (NiONPs/RF/GC) electrode was constructed and utilized to determine H₂O₂. By immersing the NiONPs/GC-modified electrode into riboflavin (RF) solution for a short period of time (5–300 s), a thin film of the proposed molecule was immobilized onto the electrode surface. The modified electrode showed stable and a well-defined redox couples at a wide pH range (2–10), with surface-confined characteristics. Experimental results revealed that RF was adsorbed on the surface of NiONPs, and in comparison with usual methods for the immobilization of RF, such as electropolymerization, the electrochemical reversibility and stability of this modified electrode has been improved. The surface coverage and heterogeneous electron transfer rate constants (k _s) of RF immobilized on a NiO _x –GC electrode were approximately 4.83 × 10^?11 mol cm^?2, 54 s^?1, respectively. The sensor exhibits a powerful electrocatalytic activity for the reduction of H₂O₂. The detection limit, sensitivity and catalytic rate constant (k _cat) of the modified electrode toward H₂O₂ were 85 nM, 24 nA μM^?1 and 7.3 (±0.2) × 10³ M^?1 s^?1, respectively, at linear concentration rang up to 3.0 mM. The reproducibility of the sensor was investigated in 10 μM H₂O₂ by amperometry, the value obtained being 2.5 % (n = 10). Furthermore, the fabricated H₂O₂ chemical sensor exhibited an excellent stability, remarkable catalytic activity and reproducibility.     

Electrophoretic deposition of surface-modified titanate nanosheets via layer-by-layer assembly and deposited film properties

The surface of H₂Ti₄O₉·xH₂O titanate nanosheets was modified using the sulfonated tetrafluoroethylene-based polymer Nafion^®, via layer-by-layer assembly. The surface modification allowed the titanate nanosheets to be highly dispersed in hydrophobic organic solvents. Thick films of surface-modified nanosheets were prepared on indium tin oxide (ITO)-coated glass substrates as a negative electrode by electrophoretic deposition. The thickness of the films increased with increasing deposition time and grew to more than 8 μm in 600 s under potentiostatic conditions at 7.5 V. The electrophoretically deposited thick films showed significant hydrophobicity with contact angle for water 95°, and enhanced adsorption and higher photocatalytic activity for hydrophobic dyes such as thionine than those of thick films prepared from unmodified titanate nanosheets.     

Preparation of excessively Ni2+-exchanged zeolite Y (FAU,Si/Al = 1.70) and its single-crystal structure

A single crystal of excessively Ni²⁺-exchanged zeolite Y (FAU, Si/Al = 1.70) was prepared by exchange of |Na₇₁|[Si₁₂₁Al₇₁O₃₈₄]-FAU with an aqueous stream 0.05 M Ni(NO₃)₂ at 293 K and pH 4.9, followed by vacuum evacuation at room temperature and 1.3 × 10^?4 Pa. Its crystal structure was determined by single-crystal synchrotron X-ray diffraction techniques in the cubic space group Fd \(\overline{ 3}\) m and was refined to the final error indices R ₁/wR ₂ = 0.0554/0.1557 for |Ni_24.7(NiOH)_12.1(Ni₂O(OH)₂)_4.8(Ni₄AlO₄)_1.7Na_17.0(H₃O)_6.9|[Si₁₁₇Al₇₅O₃₈₄]-FAU. Crystal has about 53 Ni²⁺ ions per unit cell, indicating the uptake of excess Ni(OH)₂, perhaps as NiOH⁺ ions. Some dealumination of the framework occurred during Ni²⁺ exchange. In this structure, Ni²⁺ ions occupy sites I, I′, II′, II, and III′. The residual Na⁺ ions are found at sites II′ and II. Due to the low pH of the Ni²⁺ exchange solution, some H₃O⁺ ions are observed. Nonframework oxygen atoms as oxide and hydroxide ions and orthoaluminate coordinate to some of Ni²⁺ ions to give NiOH⁺, Ni₂O(OH)₂, and Ni₄AlO₄ ³⁺ groups.     

Preparation and evaluation of α-Al2O3 supported lithium ion sieve membranes for Li+ extraction

Spinel lithium manganese oxide ion-sieves have been considered the most promising adsorbents to extract Li⁺ from brines and sea water. Here, we report a lithium ion-sieve which was successfully loaded onto tubular α-Al₂O₃ ceramic substrates by dipping crystallization and post-calcination method. The lithium manganese oxide Li₄Mn₅O₁₂ was first synthesized onto tubular α-Al₂O₃ ceramic substrates as the ion-sieve precursor (i.e. L-AA), and the corresponding lithium ion-sieve (i.e. H-AA) was obtained after acid pickling. The chemical and morphological properties of the ion-sieve were confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Both L-AA and H-AA showed characteristic peaks of α-Al₂O₃ and cubic phase Li₄Mn₅O₁₂, and the peaks representing cubic phase could still exist after pickling. The lithium manganese oxide Li₄Mn₅O₁₂ could be uniformly loaded not only on the surface of α-Al₂O₃ substrates but also inside the pores. Moreover, we found that the equilibrium adsorption capacity of H-AA was 22.9 mg·g⁻¹. After 12 h adsorption, the adsorption balance was reached. After 5 cycles of adsorption, the adsorption capacity of H-AA was 60.88% of the initial adsorption capacity. The process of H-AA adsorption for Li⁺ correlated with pseudo-second order kinetic model and Langmuir model. Adsorption thermodynamic parameters regarding enthalpy (∆ H), Gibbs free energy (∆ G) and entropy (∆ S) were calculated. For the dynamic adsorption–desorption process of H-AA, the H-AA exhibited excellent adsorption performance to Li⁺ with the Li⁺ dynamic adsorption capacity of 9.74 mg·g⁻¹ and the Mn²⁺ dissolution loss rate of 0.99%. After 3 dynamic adsorption–desorption cycles, 80% of the initial dynamic adsorption capacity was still kept.     

A novel (μ-OAc)2 bridged unsymmetric coordinated binuclear Mn(II) macrocyclic complex with ligating pendant-arm

A novel binuclear manganese Mn(II) macrocyclic complex with two pyridylmethyl pendant arms, [Mn₂^II(H₂L)(μ-OAc)₂](ClO₄)₂ · H₂O, has been synthesized and characterized crystallographically and magnetically. The crystal structure of the complex shows that two manganese ions locate in the same head of the macrocycle, leaving an uncoordinating cavity to catch protons through oxide of phenolate and the nearby imine groups in another head. The electrochemical study demonstrates that the complex gives two couples of redox peaks with E_1/2 of 0.3775 V and 0.8409 V, respectively. The variable temperature magnetic susceptibility measurement on the sample displays weak antiferromagnetic interaction between two manganese (II) with the J = −3.733(7) cm⁻¹. This complex exhibits a moderate activity for catalyzing disproportionation of H₂O₂ to O₂.     

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 Å and c = 21.8 Å. 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 K_1.04MnO_2.34·0.6H₂O.     

Preparation and characterisation of carbon-supported palladium nanoparticles for oxygen reduction in low temperature PEM fuel cells

Pd nanoparticles have been synthesised using different reducing agents, including ethylene glycol (EG), formaldehyde and sodium borohydride and their activity for the oxygen reduction reaction (ORR) evaluated. The use of EG led to the best morphology for the ORR and this synthetic method was optimised by adjusting the system pH. Carbon-supported Pd nanoparticles of approximately 7 nm diameter were obtained when reduction took place in the alkaline region. Pd synthesised by EG reduction at pH 11 presented the highest mass activity 20 A g^?2 and active surface area 15 m² g^?1. These synthetic conditions were used in further synthesis. The effect of heat treatment in H₂ atmosphere was also studied; and increased size of the palladium nanoparticles was observed in every case. The Pd/C catalyst synthesised by reduction with EG at pH 11 was tested in a low temperature H₂/O₂ (air) PEMFC with a Nafion^® 112 membrane, at 20 and 40 °C. Current densities at 0.5 V, with O₂ fed to the cathode, at 40 °C were 1.40 A cm^?2 and peak power densities 0.79 W cm^?2, approximately; which compared with 1.74 A cm^?2 and 0.91 W cm^?2, respectively for a commercial Pt/C.     

Influence of Rare-Earth and Bimetallic Promoters on Various VPO Catalysts for Partial Oxidation of n-Butane

Vanadium phosphorous oxide (VPO) catalyst was prepared using dihydrate method and tested for the potential use in selective oxidation of n-butane to maleic anhydride. The catalysts were doped by La, Ce and combined components Ce + Co and Ce + Bi through impregnation. The effect of promoters on catalyst morphology and the development of acid and redox sites were studied through XRD, BET, SEM, H₂-TPR and TPRn reaction of n-butane/He. Addition of rare-earth element to VPO formulation and drying of catalyst precursor by microwave irradiation increased the fall width at half maximum (FWHM) and reduced the crystallite size of the Vanadyl hydrogen phosphate hemihydrate (VOHPO₄ · 1/2 H₂O, VHP) precursor phase and thus led to the production of final catalysts with larger surface area. The Ce doped VPO catalyst which, assisted by the microwave heating method, exhibited the highest surface area. Moreover, the addition of promoters significantly increased catalyst activity and selectivity as compared to undoped VPO catalyst in the oxidation reaction of n-butane. The H₂-TPR and TPRn reaction profiles showed that the highest amount of active oxygen species, i.e., the V⁴⁺–O^? pair, was removed from the bimetallic (Ce + Bi) promoted catalyst. This pair is responsible for n-butane activation. Furthermore, based on catalytic test results, it was demonstrated that the catalyst promoted with Ce and Bi (VPOD1) was the most active and selective catalyst among the produced catalysts with 52% reaction yield. This suggests that the rare earth metal promoted vanadium phosphate catalyst is a promising method to improve the catalytic properties of VPO for the partial oxidation of n-butane to maleic anhydride.     

Synthesis,structure, CO oxidation,and H2 production activities of CaCu3−xMnxTi4−xMnxO12 (x = 0, 0.5, and 1.0)

Synthesis of nanocrystalline pristine and Mn-doped calcium copper titanate quadruple perovskites, CaCu_3?xMn_xTi_4?xMn_xO₁₂ (x = 0, 0.5, and 1.0) by modified citrate solution combustion method has been reported. Powder X-ray diffraction patterns attest the phase purity of the perovskite materials. Average particle sizes of all the materials obtained from the Scherrer's formula are in the range of 55–70 nm. The specific surface areas for all the perovskites obtained from BET isotherms are found to be low as expected for the condensed oxide systems and fall in the range of 13–17 m² g^?1. Transmission electron microscopy studies show a reduction in particle size of CaCu₃Ti₄O₁₂ with increase in Mn doping. Ca and Ti are present in +2 and +4 oxidation states in all the materials as demonstrated by X-ray photoelectron spectroscopy analyses. Cu²⁺ gets reduced in CaCu₃Ti₄O₁₂ with higher Mn content. Mn is observed to be present only in +3 oxidation state. All the materials have been examined to be active in CO oxidation as well as H₂ production from methanol steam reforming. CaCu₃Ti₄O₁₂ with ~14 at.% Mn is found to show best catalytic activities among these materials. A comprehensive analysis of the catalytic activities of these perovskites toward CO oxidation and H₂ production from MSR reveal the cooperative activity of copper-manganese in the doped perovskites and it is more effective at lower manganese content.     

High Catalytic Activity in CO Oxidation over MnO x Nanocrystals

Manganese oxides of various stoichiometry were prepared via Mn-oxalate precipitation followed by thermal decomposition in the presence of oxygen. A non-stoichiometric manganese oxide, MnO _x (x = 1.61…1.67) was obtained by annealing at 633 K and demonstrated superior CO oxidation activity, i.e. full CO conversion at room temperature and below. The activity gradually decreased with time-on-stream of the reactants but could be easily recovered by heating at 633 K in the presence of oxygen. CO oxidation over MnO _x in the absence of oxygen proved to be possible with reduced rates and demonstrated a Mars—van Krevelen—type mechanism to be in operation. A TEM structural analysis showed the MnO _x phase to form microrods with large aspect ratio which broke up into nanocrystalline manganese oxide (MnO _x ) particles with diameters below 3 nm and a BET specific surface area of 525 m²/g. Annealing at 798 K rather than 633 K produced well crystalline Mn₂O₃ which showed lower CO oxidation activity, i.e. 100% CO conversion at 335 K. The catalytic performance in CO oxidation of various Mn-oxides either studied in this work or elsewhere was compared on the basis of specific reaction rates.     

Active Species of Nonheme Iron and Manganese-Catalyzed Oxidations

The intense catalytic and spectroscopic studies of the last decade provided important insights into the mechanisms of some nonheme iron-catalyzed oxidations with hydrogen peroxide. For manganese-based analogs, direct spectroscopic data on the structure of the reactive intermediates are scarce; mechanistic proposals are mainly based on catalytic studies and on analogy with iron systems. Herein, these data are summarized and contemporary mechanistic landscape is presented. We have mainly focused on iron and manganese complexes with N ₄-donor aminopyridine ligands, which are one of the most successful catalysts for chemo-, regio- and enantioselective transformations of organic substrates with H₂O₂ and H₂O₂/CH₃COOH as oxidants. The low-spin Fe^III–OOH, Fe^IV = O and Fe^V = O species can be spectroscopically trapped in the catalyst systems studied, low-spin Fe^V = O intermediate being the most likely key oxidizing agent.     

Electron Microscopy Study of γ-Al2O3 Supported Cobalt Fischer–Tropsch Synthesis Catalysts

Three supported catalysts containing 20 wt% cobalt and 0.5 wt% rhenium were subjected to electron microscopy studies in their calcined state. The catalysts were prepared by incipient wetness impregnation of γ-Al₂O₃ supports of different pore characteristics with aqueous solutions of cobalt nitrate hexahydrate and perrhenic acid. The influence of the support on the Co₃O₄ crystallite size and distribution was studied by X-ray diffraction and electron microscopy. There was a positive correlation between the pore diameter of the support and the post calcination Co₃O₄ crystallite size. On all three γ-Al₂O₃ supports, Co₃O₄ was present as aggregates of many crystallites (20–270 nm in size). Cobalt oxide did not crystallise as independent crystallites, but as an interconnected network, with a roughly common crystallographic orientation, within the matrix pore structure. The internal variations in crystallite size between the catalysts were maintained after reduction. Fischer–Tropsch synthesis was carried out in a fixed-bed reactor at industrial conditions (T = 483 K, P = 20 bar, H₂/CO = 2.1). Although the cobalt-time yields varied significantly (4.6–6.7 × 10^?3 mol CO/mol Co s), the site-time yields were constant (63–68 × 10^?3 s^?1) for the three samples. The C₅₊ selectivity could not be correlated to the cobalt oxide aggregate size and is more likely related to the cobalt particle size and chemical properties of the γ-Al₂O₃ support.     

pH-controlled the formation of 4-sulfocalix[4]arene-based 1D and 2D coordination polymers

Three manganese/4-sulfocalix[4]arene complexes, namely, {H[(C₂₈H₂₀O₁₆S₄)Mn(H₂O)₄Mn_0.5(H₂O)₂]}_n ·6nH₂O (1), {NH₄[(C₂₈H₂₀O₁₆S₄)Mn(H₂O)₄Mn_0.5(H₂O)₂]}_n · 5nH₂O (2), [(C₂₈H₂₀O₁₆S₄)Mn₂(H₂O)₈]_n · 6nH₂O (3), have been synthesized under different pH conditions. Complex 1, which exhibits a one-dimensional (1D) structure, is formed at [H⁺] = 2.0 mol L⁻¹. Reaction at pH 4 leads to another one-dimensional (1D) coordination polymer of 2. At pH 5, a two-dimensional (2D) coordination polymer of complex 3 is formed, showing clearly structural effects on pH response.     

Efficient and selective oxidation of olefins and alcohols using nanoparticles of WO3-supported manganese oxides (W1−x Mn x O3)

Nanoparticles of manganese oxide supported on tungsten oxide (WO₃) were synthesized by an impregnation method using Mn(NO₃)₂ and Na₂WO₄ as a source of manganese and tungsten. Atomic absorption spectroscopy (AAS), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the physicochemical properties of compounds. Due to a highly dispersed state of manganese or insertion of manganese ions into the WO₃ lattice, no manganese oxide peak was observed in the XRD patterns of the W_1?x Mn _x O₃ nanoparticles. Investigation of W_1?x Mn _x O₃ by AAS and EDX showed that the relative atomic abundance of Mn present in the bulk and on the surface of WO₃ was 3.68% and 4.8% respectively. For the first time, the catalytic oxidation of olefins and alcohols, in the presence of these materials and hydrogen peroxide (H₂O₂) as a green oxidant at room temperature was studied. The recoverability and catalyst leaching of the W_1?x Mn _x O₃ nanoparticles in epoxidation of styrene as a model reaction were also investigated.     

Synthesis of Pure VSH-2 in Large Quantities and Its Characterization

A method to produce pure VSH-2 in large quantities (~20 g) was developed. The reagents were silica sol, V₂O₅, H₂SO₄, CsOH, and ethanol. Despite the fact that V₂O₅ was used as the vanadium source the oxidation states of most of the vanadium atoms in the produced VSH-2 were 4+, indicating that ethanol acts as a reducing agent. The crystals adopted individual octahedral shapes or aggregated states depending on the gel composition. The total surface area of the pristine VSH-2 with the general formula of Cs₂(VO)(Si₆O₁₄)·3H₂O was only 40 m²/g, indicating that the pores are blocked by the large Cs⁺ ions. The surface area increased to 149 m²/g upon exchanging Cs⁺ with Na⁺. Analyses of the diffuse reflectance UV–Vis spectra of Mⁿ⁺–VSH-2 (Mⁿ⁺ = Cs⁺, Na⁺, Ca²⁺, and Pb²⁺) revealed that the 215, 250, and 313 nm bands arise due to the V⁴⁺ to O^2? metal-to-ligand charge transfer (MLCT) and the 437, 590, and 914 nm bands arise due to the d–d transition of V⁴⁺. This reveals an unprecedented interesting situation that in dehydrated VSH-2 the framework oxide plays the role of both acceptor to V⁴⁺ and donor to Mⁿ⁺. The measured atomic magnetic moment (μ) was 1.64 BM, indicating that most of the V atoms exist in V⁴⁺. The ESR spectrum of VSH-2 showed a strong signal due to V⁴⁺ with the g value of 1.959 with ΔH_pp value of 168 G. The Raman spectra of Mⁿ⁺–VSH-2 revealed the existence of strong V=O stretching at 960 cm^?1, and other weak peaks. The V=O stretching band shifted to a higher energy region upon increasing the Sanderson’s electronegativity of Mⁿ⁺. The thermogravimetric (TGA) analysis showed that VSH-2 is thermally stable up to 550 °C and above which the oxidation of V⁴⁺ occurs.     

The Redox Chemistry of Manganese(III) and -(IV) Complexes

The oxidation-reduction thermodynamics for the manganese(III), -(IV), and -(II) ions, and their various complexes, are reviewed for both aqueous and aprotic media. In aqueous solutions the reduction potential for the manganese(III)/(II) couple has values that range from +1.51 V vs. NHE (hydrate at pH 0) to −0.95 V (glucarate complex at pH 13.5). The Mn(IV)/(III) couple has values that range from +1.0 V (solid Mn^IVO₃ at pH 0) to −0.04 V (tris gluconate complex at pH 13.5). With anhydrous media the propensity for the Mn(III) ion to disproportionate to solid Mn^IVO₂ and Mn(II) ion is avoided. For aprotic systems the range of redox potentials for various manganese complexes is from +2.01 V and +1.30 for the Mn(IV)/(III) and Mn(III)/(II) couples (bis terpyridyl tri-N-oxide complex in MeCN), respectively, to −0.96 V for the Mn(IV)/(III) couple (tris 3,5,-di-tert-butylcatecholate complex in Me₂SO). The redox reactions between manganese complexes and dioxygen species (O₂, O₂, and H₂O₂) also are reviewed.