Manganese Oxide Clusters Research Articles

A cascade approach to hetero-pentanuclear manganese-oxide clusters in polyoxometalates and their single-molecule magnet properties.

Structurally well-defined hetero-pentanuclear manganese-oxide clusters {MMn4} were successfully synthesized in TBA7Hn[MMn4(OH)2(A-α-SiW9O34)2]·2H2O·C2H4Cl2 (, M = Fe(iii), Co(ii), Ni(ii), Cu(ii), Ga(iii)) by sequential introduction of metal cations into the trivacant lacunary polyoxometalates (POMs). The pentanuclear manganese-oxide cluster {Mn5} showed a small spin ground state and a low energy barrier for magnetization relaxation. In contrast, the magnetic interactions in the hetero-pentanuclear clusters could be controlled by the arrangements of metals, and the clusters showed large magnetic anisotropy and single-molecule magnet behavior. In particular, the cluster {FeMn4} in (S = 11/2) showed the slowest relaxation and the highest energy barrier among the previously reported transition metal-containing POMs.

Dimensionality Dependent Water Splitting Mechanisms on Free Manganese Oxide Clusters

The interaction of ligand-free manganese oxide nanoclusters with water is investigated, aiming at uncovering phenomena which could aid the design of artificial water-splitting molecular catalysts. Gas phase measurements in an ion trap in conjunction with first-principles calculations provide new mechanistic insight into the water splitting process mediated by bi- and tetra-nuclear singly charged manganese oxide clusters, Mn2O2(+) and Mn4O4(+). In particular, a water-induced dimensionality change of Mn4O4(+) is predicted, entailing transformation from a two-dimensional ring-like ground state structure of the bare cluster to a cuboidal octa-hydroxy-complex for the hydrated one. It is further predicted that the water splitting process is facilitated by the cluster dimensionality crossover. The vibrational spectra calculated for species occurring along the predicted pathways of the reaction of Mn4O4(+) with water provide the impetus for future explorations, including vibrational spectroscopic experiments.

O-atom transport catalysis by neutral manganese oxide clusters in the gas phase: Reactions with CO, C2H4, NO2, and O2

Reactions of CO, C2H4, NO2, and O2 with neutral Mn(m)O(n) clusters in a fast flow reactor are investigated both experimentally and theoretically. Single photon ionization at 118 nm is used to detect neutral cluster distributions through time of flight mass spectrometry. Mn(m)O(n) clusters are generated through laser ablation of a manganese target in the presence of 5% O2/He carrier gas. A strong size dependent reactivity of Mn(m)O(n) clusters is characterized. Reactions Mn2O5/Mn3O7 + CO → Mn2O4/Mn3O6 + CO2 are found for CO oxidation by Mn(m)O(n) clusters, while only association products Mn2O(3-5)C2H4 and Mn3O(5-7)C2H4 are observed for reactions of C2H4 with small Mn(m)O(n) clusters. Reactions of Mn(m)O(n) clusters with NO2 and O2 are also investigated, and the small Mn2O(n) clusters are easily oxidized by NO2. This activation suggests that a catalytic cycle can be generated for the Mn2O5 cluster: Mn2O5 + CO + NO2 → Mn2O4 + CO2 + NO2 → Mn2O5 + CO2 + NO. Density functional theory (DFT) calculations are performed to explore the potential energy surfaces for the reactions Mn2O(4,5)/Mn3O7 + CO → Mn2O(3,4)/Mn3O6 + CO2, Mn2O5 + C2H4 → Mn2O4 + CH3CHO, and Mn2O4 + NO2 → Mn2O5 + NO. Barrierless and thermodynamically favorable pathways are obtained for Mn2O5∕Mn3O7 + CO and Mn2O4 + NO2 reactions. A catalytic cycle for CO oxidation by NO2 over a manganese oxide surface is proposed based on our experimental and theoretical investigations. The various atom related reaction mechanisms explored by DFT are in good agreement with the experimental results. Condensed phase manganese oxide is suggested to be a good catalyst for low temperature CO oxidation by NO2, especially for an oxygen rich sample.

Effect of noble metals on activity of MnOx/γ-alumina catalyst in catalytic ozonation of toluene

This work addresses effect of addition of two noble metals, platinum or palladium (1wt.%), on performance of alumina-supported manganese oxides (10wt.%) in catalytic ozonation of toluene in the range of 22–100°C. Catalysts were characterized by ICP-MS, BET, X-ray diffraction (XRD), hydrogen temperature programmed reduction (H2-TPR) and X-ray absorption spectroscopy (XAS) including quantitative X-ray absorption near edge structure (XANES) and qualitative extended X-ray absorption fine structure (EXAFS). It was found that addition of platinum improves activity of manganese oxides leading to complete conversion of toluene at 70°C. Palladium was found to be ineffective in enhancing activity of manganese oxides mainly because of lack of atomic interaction between Pd and Mn. Pt and Mn XANES and EXAFS spectra changed after addition of Pt to manganese oxides implying an atomic interaction between platinum and manganese oxides. It is proposed that the interaction between Pt and Mn occurs via surface oxygen of manganese oxide clusters increasing dispersion of Pt atoms in the presence of Mn. This interaction could increase electron occupancy of Mn 3d orbital which is more favorable in decomposition of ozone and consequently oxidation of toluene.

Experimental and Theoretical Study of Reactions between Manganese Oxide Cluster Cations and Hydrogen Sulfide

Manganese oxide cluster cations Mnm18On+ were prepared by laser ablation and reacted with hydrogen sulfide (H2S) in a fast flow reactor under thermal collision conditions. A time-of-flight mass spectrometer was used to detect the cluster distributions before and after the interactions with H2S. The experiments suggest that oxygen-for-sulfur (O/S) exchange reaction to release water took place in the reactor for most of the manganese oxide cluster cations: Mnm18On++H2S→Mnm18On−1S++H218O. Density functional theory calculations were performed for reaction mechanisms of Mn2O2++H2S, Mn2O3++H2S, and Mn2O4++H2S. The computational results indicate these O/S exchange reactions are both thermodynamically and kinetically favorable, thus in good agreement with the experimental observations. The O/S exchange reactions identified in this gas-phase cluster study parallel similar behavior of related condensed phase reaction systems.

Experimental and Theoretical Study of the Reactions between Manganese Oxide Cluster Anions and Hydrogen Sulfide

Experimental and Theoretical Study of the Reactions between Manganese Oxide Cluster Anions and Hydrogen Sulfide

Magnetic structure variation in manganese-oxide clusters

Negative-ion photoelectron spectroscopy and ab initio simulations are used to study the variation in magnetic structure in Mn(x)O(y) (x = 3, 4[semicolon] y = 1, 2) clusters. The ferrimagnetic and antiferromagnetic ground-state structures of Mn(x)O(y) are 0.16-1.20 eV lower in energy than their ferromagnetic isomers. The presence of oxygen thus stabilizes low-spin isomers relative to the preferred high-spin ordering of bare Mn(3) and Mn(4). Each cluster has a preferred overall magnetic moment, and no evidence is seen of competing states with different spin multiplicities. However, non-degenerate isomags, which possess the same spin multiplicity but different arrangements of local moments, do contribute additional features and peak broadening in the photoelectron spectra. Proper accounting for all possible isomags is shown to be critical for accurate computational prediction of the spectra.

ChemInform Abstract: Nanostructured Cobalt and Manganese Oxide Clusters as Efficient Water Oxidation Catalysts

AbstractReview: 65 refs.

Ab initio study of magnetic interactions of manganese-oxide clusters

The manganese clusters have attracted much attention in relation with the oxygen evolving center (OEC) in photosystem II (PS II) system, which catalyzes the water oxidation reaction. Previously, we examined various spin-structures of Mn(II) 4O 4 model clusters, of which all of magnetic interactions are antiferromagnetic. In this study, we investigated electronic and magnetic structures of simple model clusters, Mn 4O 4(OAc) 6 and Mn 3CaO 4(OAc) 6 using spin unrestricted B3LYP (UB3LYP) method. The UB3LYP method is a standard tool for this study and has been in fact employed by many researchers. However, several peculiar features are observed for these model clusters: for instance the most stable spin state becomes the highest spin state for Mn(IV) 4O 4(OAc) 6 although this model cluster consists of superexchange type of units, Mn(IV) 2O 2 that usually favors antiferromagnetic spin alignments. Implications of the comparative results are discussed in relation to the electrophilic (or radical) mechanism for the O–O bond formation in the OEC.

Transition metal oxide clusters with character of oxygen-centered radical: a DFT study

Density functional theory (DFT) calculations are applied to study the structure and bonding properties of groups 3–7 transition metal oxide clusters M x=1–3O and Scx=4–6O with 2y − nx + q = 1, in which n is the number of metal valence electrons and q is the charge number. These clusters include MO2, M 2O3 +, M 2O4 −, and M 3O5 (M = Sc, Y, La); MO2 +, MO3 −, M 2O4 +, M 2O5 −, M 3O6 +, and M 3O7 − (M = Ti, Zr, Hf), and so on. The obtained lowest energy structures of most of these clusters are with character of oxygen-centered radical (O·). That is, the clusters contain oxygen atom(s) with the unpaired electron being localized on the 2p orbital(s). Chromium and manganese oxide clusters (except CrO4 −) do not contain O· with the adopted DFT methods. The binding energies of the radical oxygen with the clusters are also calculated. The DFT results are supported by available experimental investigations and predict that a lot of other transition metal oxide clusters including those with mixed-metals (such as TiVO5 and CrVO6) may have high oxidative reactivity that has not been experimentally identified. The chemical structures of radical oxygen over V2O5/SiO2 and MoO3/SiO2 catalysts are suggested and the balance between high reactivity and low concentration of the radical oxygen in condensed phase catalysis is discussed.

Nanostructured cobalt and manganese oxide clusters as efficient water oxidation catalysts

Recent development of new methods of preparing cobalt oxide and manganese oxide clusters has led to oxygen evolving catalysts that operate under mild conditions and modest overpotentials at rates approaching practical utility. Synthesis of nanostructured Co3O4 and Mn oxide clusters in mesoporous silica scaffolds affords catalysts with very high densities of surface metal sites per projected area, with the silica environment providing stability in terms of dispersion of the clusters and prevention of restructuring of catalytic surface sites. Stacking of the nanoclusters of these earth abundant, durable oxide catalysts in the scaffold results in turnover frequencies per projected area that are sufficient for keeping up with the photon flux at high solar intensity. Opportunities for expanding the metal oxide/silica interface approach to heterogeneous water oxidation catalysis to a more general approach for multi-electron catalyst designs based on core/shell constructs are discussed. The results are reviewed in the context of all-inorganic materials for catalytic water oxidation reported recently from other laboratories, in particular electrodeposits generated from Co phosphate solutions, a molecular water oxidation catalyst based on a polyoxotungstate featuring a Co oxide core, and Mn oxide materials with incorporated Ca ions.

Nanostructured manganese oxide clusters supported on mesoporous silica as efficient oxygen-evolving catalysts

Nanostructured Mn oxide clusters supported on mesoporous silica KIT-6 efficiently evolve O(2) in aqueous solution under mild conditions.

Manganese(III,IV) and Manganese(III) Oxide Clusters Trapped by Copper(II) Complexes

Reactions of quinquedentate Schiff base ligands with Mn and Cu ions afforded icosa- and hexadecanuclear mixed-metal clusters in which dinuclear CuII complexes trapped oxo-bridged [MnIII8MnIV4O12] and [MnIII6O6] cores, respectively. Maximum entropy method analysis for synchrotron X-ray diffraction data was used to determine the oxidation states of the Mn ions.

Manganese doped gallium oxynitride prepared by nitridation of citrate precursor

Manganese doping of gallium oxynitride was investigated to obtain a magnetic semiconductor by nitriding a precursor in ammonia flow. The precursor was obtained by prefiring a mixed gel made of Mn 2+ and Ga 3+ nitrates and citric acid in aqueous solution. The products were isostructural with hexagonal GaN. The doping limit was 10 and 5 at% in the samples nitrided at 750 and 850 °C, respectively. Chemical analysis of the products suggested that they were manganese doped gallium oxynitrides. Oxide and nitride ions were randomly distributed in the anion sites in the latter product. Manganese oxide cluster might be formed in the former. The product nitrided at 850 °C showed an antiferromagnetic interaction with a Weiss temperature θ = −38 K, while the product nitrided at 750 °C was paramagnetic.

Energetics of the manganese oxide cluster cations MnNO+ (N=2–5): Role of oxygen in the binding of manganese atoms

The photodissociation of manganese oxide cluster cations Mn(N)O+ (N = 2-5), into Mn(N-1)O+ (one-atom loss) and Mn(N-2)O+ (two-atom), was investigated in the photon-energy range of 1.08-2.76 eV. The bond-dissociation energies D0(Mn(N-1)O+...Mn) for N = 3, 4, and 5 were determined to be 1.84+/-0.03, 0.99+/-0.05, and 1.25+/-0.14 eV, respectively, from the threshold energies for the one- and two-atom losses. As Mn2O+ did not dissociate even at the highest photon energy used, the bond dissociation energy of Mn2O+, D0(Mn+...MnO), was obtained from a density-functional-theory calculation to be 3.04 eV. The present findings imply that the core ion Mn2O+ is bound weakly with the rest of the manganese atoms in Mn(N)O+.

Electronic structure and magnetic properties of small manganese oxide clusters

To investigate the electronic structure and magnetic properties of manganese oxide clusters, we carried out first-principles electronic structure calculations for small MnO clusters. Among various structural and magnetic configurations of the clusters, the bulklike [111]-antiferromagnetic ordering is found to be favored energetically, while the surface atoms of the clusters exhibit interesting electronic and magnetic characteristics which are different from their bulk ones. The distinct features of the surface atoms are mainly attributed to the reduction of Mn coordination numbers and the bond-length contractions in the clusters, which may serve as a key factor for the understanding of physical and chemical properties of magnetic oxide nanoparticles.

Structure and Dynamics of Manganese Oxide Clusters and Related Systems

AbstractFor Abstract see ChemInform Abstract in Full Text.

Experimental and theoretical study of the photoelectron spectra of MnOx−(x=1–3) clusters

We report a combined experimental and theoretical investigation of MnOx− and MnOx(x=1–3) clusters. Theoretically, geometrical configurations of various isomers of the clusters were optimized and vertical detachment energies for the anions were evaluated. The ground state of MnO− was predicted to be Σ+5, followed by an excited state (7Σ+) 0.14 eV higher in energy. The ground state of MnO2− is B25, with a B13 isomer 0.15 eV higher. MnO3− is predicted to be a singlet D3h cluster. Vibrationally resolved photoelectron spectra of MnOx− were measured at several photon energies and under various experimental conditions, and were interpreted based on the theoretical results. The electron affinities of MnO, MnO2, and MnO3 were determined to be 1.375 (0.010), 2.06 (0.03), and 3.335 (0.010), respectively. Five excited states of MnO were observed and assigned using the theoretical results. The Σ+7 excited state of MnO− was found to be significantly populated and was distinguished from the ground state of the anion by temperature dependent studies. We observed two isomers for MnO2− and the detachment features from both isomers were assigned. Only one vibrationally resolved band was observed for MnO3−, which corresponds to transitions from the ground state of MnO3− to that of MnO3. The combined experimental and theoretical studies allow us to elucidate the complicated electronic and geometric structures of the various manganese oxide clusters and their anions.

Colloidal Synthesis and Electroluminescence Properties of Nanoporous MnIIZnS Films

An organometallic colloidal route was employed to prepare orange fluorescing Mn2+ functionalized ZnS and CdS particles. For this purpose, tributylphosphine-capped manganese oxide clusters were first synthesized, which served as heterogeneous nucleation centers in the ZnS condensation process. An exposure of the resulting Mn:ZnS particles to Cd2+ initiates a metal replacement yielding Mn:CdS. This process broadens the characteristic internal Mn2+-emission band peaking at 600 nm and enhances the fluorescence quantum yield from 3% to 6%. The size of the metal sulfide particles was determined to be 3−4 nm using HRTEM and XRD measurements. The colloids were further used to prepare thin, crack-free layers placed between ITO and Al contact electrodes, which allowed us to investigate their electroluminescence properties. We have found that an infiltration of ZnI2 or other molecular cluster species dramatically improves the air stability and efficiency of the EL device. The most efficient samples have shown a lumi...

Symmetry and broken symmetries in molecular orbital descriptions of unstable molecules II. Alignment, flustration and tunneling of spins in mesoscopic molecular magnets

As an extention of previous work [Yamaguchi K (1983) J Mol Structure (Theochem) 103:101], our theoretical efforts toward molecular materials are briefly reviewed. Axial, helical and general (cone, tetrahedron, etc) spin structures of carbon clusters, manganese oxide clusters and iron-sulfur clusters were investigated using the classical Heisenberg model and generalized Hartree-Fock theory. The spin flustrations in the clusters were also studied in relation to theoretical descriptions of the magnetism and the chemical bonds in active sites of several enzymes. The macroscopic quantum tunneling and coherence of spins in the manganese oxide clusters were analyzed using the instanton model, and the tunneling rate of spins was calculated by the coherent state path integral method. The spin transitions induced by an external magnetic field were studied by the path integral Monte Carlo method. The theoretical results were explained in terms of the symmetry and broken symmetry of the wavefunctions in the mesoscopic molecular systems, which are intermediate in the scale factor. The results were also applied to molecular design of mesoscopic clusters of clusters in the intermediate and strong correlation regime. The active control of spins is finally discussed from the viewpoint of functionalities in molecular and biological materials, and technological applications of mesoscopic molecular magnets are discussed with regard to quantum computing.