Cobalt Clusters Research Articles

ChemInform Abstract: Cobalt Clusters with Cubane‐Type Topologies Based on Trivacant Polyoxometalate Ligands.

AbstractNa1.5Cs2.5 [Co4(OH)3(H2O)6 (PW9O34)]·9H2O (I) is obtained by reaction of Na(OAc), CoSO4, Na8H[PW9O34] (H2O, 80 °C, 20 min) followed by addition of CsCl (5% yield).

Experimental and Computational Study of Unique Tetranuclear µ3‐Chloride and µ‐Phenoxo/Chloro‐Bridged Defective Dicubane Cobalt(II) Clusters

AbstractTwo tetranuclear CoII clusters [Co4(L)2(µ3‐Cl)2Cl2] have been prepared by using multidentate diaminobisphenolate ligands. The solid‐state structures of the complexes were determined by single‐crystal X‐ray diffraction. The cores of the cluster compounds can be defined as a two‐vertex‐deficient dicubane geometry (pseudo‐dicubane). In the central unit, the cobalt(II) cations are linked through phenoxide oxygen (outer bridges) and chloride anions (inner bridges), previously unprecedented in this type of cobalt cluster. The magnetic properties were studied by both experimental and computational methods. By using a combination of techniques, we were able to determine the nature and strength of the intramolecular magnetic exchange coupling mediated by the different bridging fragments between the crystallographically different cobalt(II) cations.

Catalytic CVD-growth of array of multiwall carbon nanotubes on initially amorphous film Co–Zr–N–O

The possibility of formation of arrays of multiwall carbon nanotubes on catalyst-containing amorphous thin film Co–Zr–N–O with low content of Co (~15at.%) by chemical vapor deposition has been demonstrated. On heating the amorphous alloy crystallizes, whereby the faceted crystal clusters of cobalt are formed on the surface. The rest of the film is cobalt depleted. The growth of CNT occurs on cobalt clusters. When using acetylene at the substrate temperature of 650°C the array of 12μm high CNT is formed after 2min of growing. The diameter of CNT in the array varies in the range 3–11nm. CNTs with the diameter of 5–8nm prevail. CNT growth process on a thin film of Co–Zr–N–O is low sensitive to the thickness of the film, making it technically attractive.

Cobalt electrodeposition in nanoporous anodic aluminium oxide for application as catalyst for methane combustion

The present work is focused on the cobalt electrodeposition in anodic aluminium oxide and the application of the obtained nanocomposite layer as a monolithic catalyst for complete methane oxidation. For the investigation of cobalt nanowires growing in the pores of nanoporous alumina template, cyclic voltametry and galvanostatic method were used. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were applied in order to determine the morphology and the content of the composite material in some stages of its preparation. It was found that the deposition of cobalt from a neutral borate electrolyte is accompanied by release of a small amount of hydrogen, which decreases with the process of nanowires growing.The emergence of the individual flake cobalt clusters on the alumina template is considered as the most suitable structure of the monolithic catalysts. The final resulting laminar composite structure with outer layer of Co- and Pd-oxides exhibits higher active catalytic properties for methane combustion than the catalysts obtained by conventional procedure of preparation by impregnation.

Transition metal pairs on ceria-promoted, ordered mesoporous alumina as catalysts for the CO2reforming reaction of methane

Cobalt and iron clusters dispersed over ordered mesoporous γ-Al2O3enable stable conversion of methane and carbon dioxide to syngas. Tungsten containing catalysts deactivate with TOS. Ceria–zirconia redox promotion is crucial for preventing carbon accumulation.

Two microporous metal–organic frameworks constructed from trinuclear cobalt(ii) and cadmium(ii) cluster subunits

This work presents two novel microporous metal–organic frameworks which are constructed from a tetracarboxylate ligand and trinuclear cobalt(ii) and cadmium(ii) cluster subunits.

Structural and magnetic properties of Co-doped Gd2O3 nanorods

Cobalt-doped Gd2O3 (Gd2−xCoxO3, 0≤x≤0.06) nanorods of about 100nm diameter and 2µm length were synthesized using a simple hydrothermal method. XRD, Raman, XPS, and TEM measurements showed the samples to have a single cubic phase structure of Gd2O3 doped with Co2+ cations, without any cobalt clusters. All the samples showed paramagnetism at room temperature as well as at 5K. The samples’ high magnetization values at 5K were due to reduction of the thermal randomization of the magnetic spins. The Curie–Weiss fitting of the magnetic data reflected antiferromagnetism along with paramagnetism due to the exchange interactions of Gd3+ via O2− ions and coupling between Co2+–Co2+ pairs.

Structure, magnetism, and dissociation energy of small bimetallic cobalt-chromium oxide cluster cations: A density-functional-theory study

We study CoxCryOm+ (x+y=2, 3 and 1≤m≤4) clusters by means of density-functional-theory calculations. It is found that the clusters grow preferentially through maximizing the number of metal–oxygen bonds with a favor on Cr sites. The size- and composition-dependent magnetic behavior is discussed in relation with the local atomic magnetic moments. While doped species show an oscillatory magnetic behavior, the total magnetic moment of pure cobalt and chromium oxide clusters tends to enhance or reduce as increasing the oxygen content, respectively. The dissociation energies for different evaporation channels are also calculated to suggest the stable patterns, as fingerprints for future photofragmentation experiments.

An unprecedented twelve-connected 3D metal-organic framework based on heptanuclear cobalt cluster building blocks

Solvothermal reaction of flexible tetrapodal linker tetrakis[4-(carboxyphenyl) oxamethyl]methane acid (H4L) with cobalt salt yields a new metal-organic framework [Co7(L)3(μ3-O)3(DMF)3]∙(DMF)13(H2O)25. X-ray crystallography reveals that the structure exhibits a unique 2-nodal (4,12)-connected topology with the Schläfli symbol of (433.630.83)(46)3, constructed from heptanuclear cobalt clusters as 12-connected nodes and tetrahedral ligands as 4-connected nodes. Temperature-dependent magnetic susceptibility measurements reveal that the heptanuclear cobalt clusters exhibit antiferromagnetic property.

Water Adsorption on Free Cobalt Cluster Cations.

Cationic cobalt clusters complexed with water Con(+)-H2O (n = 6-20) are produced through laser ablation and investigated via infrared multiple photon dissociation (IR-MPD) spectroscopy in the 200-1700 cm(-1) spectral range. All spectra exhibit a resonance close to the 1595 cm(-1) frequency of the free water bending vibration, indicating that the water molecule remains intact upon adsorption. For n = 6, the frequency of this band is blue-shifted, but it gradually converges to the free water value with increasing cluster size. In the lower-frequency range (200-650 cm(-1)) the spectra contain several bands which show a very regular frequency evolution, suggesting that the exact cluster geometry has little effect on the water-surface interaction. Density functional theory (DFT) calculations are carried out at the OPBE/TZP level for three representative sizes (n = 6, 9, 13) and indicate that the vibrations responsible for the resonances correspond to bending and torsional modes between the cluster and water moieties. The potential energy surfaces describing these interactions are very shallow, making the calculated harmonic frequencies and IR intensities very sensitive to small geometrical perturbations. We conclude that harmonic frequency calculations on (local) minima structures provide insufficient information for these types of cluster complexes and need to be complemented with calculations that provide a more extensive sampling of the potential energy surface.

Size effects in the magnetic anisotropy of embedded cobalt nanoparticles: from shape to surface

Strong size-dependent variations of the magnetic anisotropy of embedded cobalt clusters are evidenced quantitatively by combining magnetic experiments and advanced data treatment. The obtained values are discussed in the frame of two theoretical models that demonstrate the decisive role of the shape in larger nanoparticles and the predominant role of the surface anisotropy in clusters below 3 nm diameter.

Nanostructured magnetic metamaterials based on metal-filled carbon nanotubes

Hybrid systems formed by carbon nanotubes filled with metal represent a class of nanomaterials with manifold functionality that can find applications in different fields as, for example, magnetic storage and spintronics. Here a detailed and comprehensive study is reported on the magnetic properties of template grown cobalt cluster-filled multiwall carbon nanotube arrays. The temperature and magnetic field dependence of the samples magnetization are investigated in the range 5–760 K and from −50 kOe to +50 kOe, respectively. Hysteresis cycles are slightly open and transmission electron microscopy reveals that the cobalt clusters inside the nanotubes exhibit a crystalline nature. The zero-field cooling/field-cooling magnetization temperature dependence, analyzed in the framework of a “two-state” progressive crossover model, reveals spin disorder and suggests a spin glass-like scenario. These findings show that cobalt-filled nanotube arrays represent a unique system of magnetically coupled nanostructures, allowing to envision a new class of nanoscale carbon-based magnetic metamaterials, ideal platforms for novel magneto-electronic devices.

Molecular Cobalt Clusters as Precursors of Distinct Active Species in Electrochemical, Photochemical, and Photoelectrochemical Water Oxidation Reactions in Phosphate Electrolytes.

Three cobalt model molecular compounds, Co-cubane ([Co4 (µ3 -O)4 (µ-OAc)4 py4 ]), Co-trimer ([Co3 (μ3 -O)(µ-OAc)6 py3 ]PF6 ), and Co-dimer ([Co2 (μ-OH)2 (µ-OAc)(OAc)2 py4 ]PF6 ), are investigated as water oxidation reaction (WOR) catalysts, using electrochemical, photochemical, and photoelectrochemical methodologies in phosphate electrolyte. The actual species contributing to the catalytic activity observed in the WOR are derived from the transformation of these cobalt compounds. The catalytic activity observed is highly dependent on the initial compound structure and on the particular WOR methodology used. Co-cubane shows no activity in the electrochemical WOR and negligible activity in the photochemical WOR, but is active in the photoelectrochemical WOR, in which it behaves as a precursor to catalytically active species. Co-dimer also shows no activity in the electrochemical WOR, but behaves as a precursor to catalytically active species in both the photochemical and photoelectrochemical WOR experiments. Co-trimer behaves as a precursor to catalytically active species in all three of the WOR methodologies.

Morphological impact on the reaction kinetics of size-selected cobalt oxide nanoparticles.

Apart from large surface areas, low activation energies are essential for efficient reactions, particularly in heterogeneous catalysis. Here, we show that not only the size of nanoparticles but also their detailed morphology can crucially affect reaction kinetics, as demonstrated for mass-selected, soft-landed, and oxidized cobalt clusters in a 6 nm to 18 nm size range. The method of reflection high-energy electron diffraction is extended to the quantitative determination of particle activation energies which is applied for repeated oxidation and reduction cycles at the same particles. We find unexpectedly small activation barriers for the reduction reaction of the largest particles studied, despite generally increasing barriers for growing sizes. We attribute these observations to the interplay of reaction-specific material transport with a size-dependent inner particle morphology.

Thermal Desorption Spectroscopy Study of the Adsorption and Reduction of NO by Cobalt Cluster Ions under Thermal Equilibrium Conditions at 300 K.

Adsorption of NO molecules on gas phase cobalt cluster ions, Con(+) (n = 4-9), was investigated in thermal equilibrium with He gas at 300 K. The Con(+) clusters, contrary to the isolated clusters in a vacuum, adsorbed NO without undergoing significant dissociation. Thermal desorption spectroscopy of Con(+)(NO)m indicated that Con(+) clusters with n = 4-6 and n = 7-9 can have four and six adatoms chemisorbed, respectively. Reduction of NO occurred, releasing N2 molecules, to form Con(+)Ok(NO)m-k (k = 2, 4, ...). The reaction mechanism involved the exchange of chemisorbed N atoms with the O atom in NO bound to the clusters. The reactivity of Con(+) (n = 4-9) exhibited periodic n dependence, and Co6(+) and Co9(+) was similar to the case of the isolated Co16(+) clusters holding up to eight adatoms reported by Anderson et al. ( J. Chem. Phys . 2009 , 130 , 10992 - 11000 ).

An unusual 2D nanoscaled quadruple-layer metal–organic framework based on octanuclear cobalt clusters

Self-assembly of a flexible tricarboxylate ligand (H3L) and cobalt salt under hydrothermal conditions yields an unprecedented two-dimensional (2D) nanoscaled quadruple-layer networks [Co4(HL)3(μ3-OH)2(H2O)2]·5H2O (1) (H3L=(3,5-dicarboxyl-phenyl)-(4-(2′-carboxyl-phenyl)-benzyl)ether). The structure of compound 1 was determined by single-crystal X-ray diffraction analysis and further characterized by elemental analyses, IR spectrum, and thermogravimetric analysis. Compound 1 represents the first 2D nanoscaled multiple-layer architecture constructed from octanuclear cobalt cluster building blocks. Furthermore, the adjacent sidearm-containing 2D quadruple-layer motifs are interdigitated in a mutual relationship to form a 3D supramolecular framework. In addition, the magnetic properties of compound 1 have been studied by measuring its magnetic susceptibility in the temperature range of 2–300K, indicating the existence of antiferromagnetic coupling within the octanuclear cobalt cluster.

Methanation of carbon monoxide over promoted flame-synthesized cobalt clusters stabilized in zirconia matrix

Methanation catalysts based on 20% Co–ZrO2 were synthesized in a one-step flame spray pyrolysis that at the same time allows the doping of noble metal promoters, hereby demonstrated with 0.4wt.% Pd, Rh, Pt, Ru, and Ag. Unlike conventional catalysts, the cobalt content is internally dispersed and stabilized within the zirconia matrix, allowing the formation of very fine surface Co0 clusters (1–2nm) upon reduction as unraveled from its dispersion. In situ X-ray absorption spectroscopy (XAS) showed that doping of noble metal promoters significantly improved the reducibility of cobalt in the order of undoped<Ag<Ru<Pt<Rh<Pd, as assisted by the H-spillover. The number of active Co0 sites on the surface was enhanced by almost 2-fold upon addition of promoters, and so are the corresponding rates of CO conversion. Detailed characterization by XAS and X-ray photoelectron spectroscopy (XPS) unraveled that homogeneously dispersed promoters (Rh, Pt, and Ru) merely enhanced the reducibility of cobalt and did not affect the turnover frequencies (TOFs, based on CO conversion) relative to that of the bare Co–ZrO2 catalyst. On the contrary, surface-enriched promoters (Pd and Ag) lowered the apparent TOFs due to the mixed influence of the noble metals and Co0. We further found in the representative case of Rh-promoted Co–ZrO2 that the promoter ameliorated hydrogenation and hence resulted in higher CH4 selectivity with increasing H2 partial pressure (i.e., higher reaction order), compared to the unpromoted catalyst.

The electronic structure of nanoparticle: theoretical study of small Cobalt clusters (Co n , n = 2–5) (part A)

Electronic and geometrical structures of small Cobalt cluster (Co n , n = 2–5), which were used as building block in Cobalt cluster compounds, were fully investigated in this article. Since small Cobalt clusters as a nanoparticle have many applications in scientific works and technology, it is essential for understanding their relatively accurate electronic structure. The most stable symmetrically and electronically structure confirmed by frequency test for studied clusters is as follows: 5-et (quintet) Co2 is of course linear, and its HOMOs described by degenerate d xy orbitals in β-spin part located on both atoms; 8-et (octet) Co3 is almost scalene triangle and its HOMO in s-orbital of α-spin part located on Co(1); 11-et Co4 is rhombus, d xz orbital of β-spin part located on Co(2) is its HOMO part. 12-et Co5 is pyramidal with HOMO in p x orbital located on Co(5) in β-spin part, which show very low (0.06) occupation number. This large depletion of electron in HOMO occupation of Co5 explains high chemical reactivity of this species. Density functional theory, DFT, was used through software Gaussian 09. Xc-correlation functional was chosen among the many recommended xc-functionals for transition metal in the literature by calibration procedure in such a way that the results can match with experimental values. So suitable xc-functional B3P86 and 6-311++G* basis set were found. Natural bond orbital, NBO, has also been used for analysis.

Cobalt supported on Graphene – A promising novel Fischer–Tropsch synthesis catalyst

Graphene and CNTs-supported cobalt catalyst, is introduced for Fischer–Tropsch synthesis (FTS). The catalysts were prepared using wet impregnation method with cobalt loading of 15.0wt%. The catalysts were characterized by TGA, BET, XRD, XPS, H2 chemisorption, TPR, and TEM techniques. The activity, selectivity and stability of the catalysts in FTS were assessed in a fixed-bed micro-reactor, and the results were compared with each other. Comparing TEM images showed dispersion of cobalt clusters in graphene-supported catalyst is higher and the average clusters size decreased from 7.5 to about 6.0nm. Graphene as the support shifted the reduction temperature of cobalt oxide species to lower temperatures. Degree of CO conversion increased from 61 to 74%, FT synthesis rate increased by about 22%; the product distribution shifted to the higher molecular weight hydrocarbons, compared to CNTs-supported catalyst. On the graphene-supported cobalt catalyst, 480h continuous FT synthesis decreased 22% of the initial CO conversion. However, at the same time for CNTs-supported catalyst the CO conversion decreased 34%.

Electronic and magnetic properties ofConMomnanoclusters from density functional calculations (n+m=xand 2≤x≤6atoms)

We present the results of the density functional theory study of Co_n Mo_m nanoclusters with n+m=x and 2<=x<=6 atoms on the all-electron level using the generalized gradient approximation. The discussion of the properties of the pure cobalt and molybdenium cluster is followed by an analysis of the respective mixed clusters of each cluster size x. We found that the magnetic moment of a given cluster is mainly due to the Co content and increases with increasing n. The magnetic anisotropy on the other hand becomes smaller for larger magnetic moments S. We observe an increase in the binding energy, electron affinity, and average bond length with increasing cluster size as well as a decrease in the ionization potential, chemical potential, molecular hardness and the HOMO-LUMO gap.