CoO Clusters Research Articles

Co single atoms and CoO clusters over nitrogen–doped hollow carbon spheres for synergistic oxidation of aromatic alkanes

Single atom catalysts have attracted mountainous attentions in the field of catalysis, owing to their unsaturated coordination environment and maximized utilization of metal atoms. However, single atom catalysts may suffer from low catalytic efficiency toward multistep reactions because of the single active site. Herein, we report a synergistic catalyst (n–Co/CoO@N–HMCS) to solve above problem through supporting Co single atoms (Co–SAs) and CoO clusters (CoO–CTs) on nitrogen–doped hollow carbon spheres for selective oxidation of aromatic alkanes. Among the as–prepared catalysts, 0.03–Co/CoO@N–HMCS exhibits the remarkable catalytic performance, with 96.6% conversion of ethylbenzene and 99.5% selectivity to acetophenone. Density functional theory calculation demonstrates that the synergistic effect between Co single atoms and CoO clusters significantly decreases the energy barrier of cleavage of the C–H bond of ethylbenzene, a rate–determining step for the oxidation of ethylbenzene, so that to accelerate the conversion of ethylbenzene into acetophenone. The strategy is applicable for oxidation of other aromatic alkanes as well.

Large current density for oxygen evolution from pyramidally-coordinated Co oxide

Developing efficient electrocatalyst at large current densities to meet the requirement of industrial water splitting is a big challenge today. Herein, we report a class of catalyst, BiCoO3 with pyramidally coordinated Co3+. BiCoO3 exhibits a large current density of 1000 mA cm−2 at a low overpotential of 402 mV and the overpotential even reduces to only 303 mV to achieve 1000 mA cm−2 by replacing Co with Fe, which belongs to the first-class level among large-current-densities electrocatalysts reported so far. Different from artificially created oxygen vacancies to optimize the bonding strength of the intermediate for enhancement of OER activity, BiCoO3 has the rich active sites and the shortest reaction pathway leading to a large current density, as structurally every Co-O cluster has one oxygen vacancy. This work opens a new avenue for enhanced OER activity at high current densities via optimizing the arrangement of ligand vacancies.

Boosting Propane Dehydrogenation by the Regioselective Distribution of Subnanometric CoO Clusters in MFI Zeolite Nanosheets.

Direct dehydrogenation of propane (PDH) has already been implemented worldwide in industrial processes to produce value-added propylene. The discovery of earth-abundant and environmentally friendly metal with high activity in C-H cleavage is of great importance. Co species encapsulated within zeolite are highly efficient for catalyzing direct dehydrogenation. However, exploring a promising Co catalyst remains a nontrivial target. Direct control of the regioselective distribution of Co species in the zeolite framework through altering their crystal morphology gives opportunities to modify the metallic Lewis acidic features, thus providing an active and appealing catalyst. Herein, we achieved the regioselective localization of highly active subnanometric CoO clusters in straight channels of siliceous MFI zeolite nanosheets with controllable thickness and aspect ratio. The subnanometric CoO species were identified by different types of spectroscopies, probe measurements, and density functional theory calculations, as the coordination site for the electron-donating propane molecules. The catalyst showed promising catalytic activity for the industrially important PDH with propane conversion of 41.8% and propylene selectivity higher than 95% and was durable during 10 successive regeneration cycles. These findings highlight a green and facile method to synthesize metal-containing zeolitic materials with regioselective metal distribution and also to open up a future perspectives for designing advanced catalysts with integrated advantages of the zeolitic matrix and metal structures.

Chemical mass shifts of cluster ions and adduct ions in quadrupolar ion traps revisited and extended.

Chemical mass shifts in quadrupolar ion traps have been studied previously but only for a limited number of analytes and mass ranges. Here, mass shifts of cluster ions, commonly used as calibrants, and other analytes are qualitatively evaluated on the Bruker amaZon spherical ion trap (QIT) and the Finnigan LXQ linear ion trap (LIT). To extend the mass range from previous experiments m/z up to 4000 are investigated. Chemical mass shifts of CsI, Y(HCOO)3 , and NaCF3 COO cluster ions, CF3 COO- , Na+ , and Cs+ adduct ions, protonated commercial calibration solutions and peptides, and deprotonated peptides were investigated on the Bruker amaZon speed QIT and some of these were also investigated on the Finnigan LXQ LIT. On both instruments, peak distortions and mass shifts toward lower m/z became apparent as m/z approached 1000. To some extent, the issues were more severe at slower scans. Peak distortions included loss of resolution, tailing, or fronting and were different between the amaZon QIT and the LXQ LIT. The noncluster and nonadduct ions analyzed showed no obvious mass shifts or peak distortions under the same analysis conditions. As expected, the ion traps investigated here showed mass shift and peak distortion issues, and such issues persisted at m/z up to 4000 on both instruments. Peak distortions were different between the amaZon QIT and the LXQ LIT, and were not always visible despite mass shifts. Both mass shifts and peak distortions make cluster ions and some adduct ions unsuitable for ion trap calibration.

A dual-functional flexible sensor based on defects-free Co-doped ZnO nanorods decorated with CoO clusters towards pH and glucose monitoring of fruit juices and human fluids

Herein, ZnO nanorods were doped with Co and decorated with CoO clusters through an in situ technique to create a CoO/Co-doped ZnO (CO/CZO) heterostructure at low temperatures (150 °C) on a flexible PET substrate. In the CO/CZO heterostructure, the Co dopant has a low energy barrier to substitute Zn atoms and adsorb over oxygen atoms and their vacancies. Therefore, it decreased the charge density (ND = 2.64 × 1019 cm−3) on non-active sites of ZnO and lowered the charge transfer resistance (317 Ω) at Co-doped-ZnO/electrolyte interface by suppressing the native defects and reducing the Schottky barrier height (− 0.35 eV), respectively. Furthermore, CoO clusters induced a p-n heterostructure with Co-doped ZnO, prevented corrosion, increased the active sites for analyte absorption, and increased the ultimate tensile strength (4.85 N m−2). These characteristics enabled the CO/CZO heterostructure to work as a highly sensitive, chemically stable, and flexible pH and glucose oxidation electrode. Therefore, CO/CZO heterostructure was explored for pH monitoring in human fluids and fruit juices, demonstrating a near-Nernst-limit pH sensitivity (52 mV/pH) and fast response time (19 s) in each human fluid and fruit juice. Also, it demonstrated high sensitivity (4656 µM mM−1 cm−2), low limit of detection (0.15 µM), a broad linear range (0.04 mM to 8.85 mM) and good anti-interference capacity towards glucose-sensing. Moreover, it demonstrated excellent flexibility performances, retained 53% and 69% sensitivity of the initial value for pH and glucose sensors, respectively, after 500 bending, stretching, and warping cycles.Graphical

In situ encapsulated subnanometric CoO clusters within silicalite‐1 zeolite for efficient propane dehydrogenation

AbstractCobalt‐based catalysts are promising alternatives to replace Pt‐ and Cr‐based catalysts for propane dehydrogenation (PDH). However, the sintering and reduction of unstable Co sites cause fast deactivation. Herein, the ultrasmall cobalt oxide clusters encapsulated within silicalite‐1 zeolites (CoO@S‐1) has been obtained via a ligand assistance in situ crystallization method. This CoO@S‐1 catalyst exhibits an attractive propylene formation rate of 13.66 mmolC3H6·gcat−1·h−1 with selectivity of >92% and is durable during 120‐h PDH reaction with five successive regeneration cycles. The high PDH activity of CoO@S‐1 is assigned to the encapsulated CoO clusters are favorable for propane adsorption and can better stabilize the detached H* species from propane, leading to the lower dehydrogenation barriers than framework Co2+ cations and Co3O4 nanoparticles. Additionally, the π‐binding propylene on CoO clusters can prevent the over‐dehydrogenation reaction compared with the di‐σ binding propylene on metallic Co, leading to the superior propylene selectivity and catalytic stability.

(Bi13Co11)Co2O40–Co3O4 composites: Synthesis, structural and magnetic properties

Nanocrystalline of Co sillenite-Co3O4 composites were prepared by combustion and sol-gel methods. The synthesized materials were further investigated by XRD, FT-IR, Raman spectroscopy, DC magnetometry, FESEM and EDX. The XRD characterization revealed that the sillenite phase is nanostructured with a crystallite size in the range from 48.4 to 64.4 nm. Both syntheses methods yielded between 75 and 80 wt% of sillenite. The FESEM micrographs showed that the particles had sphere like and a nanosheets - sphere like shapes when prepared by combustion and sol-gel methods, respectively. Confirmation of the chemical structure of composites was determined by the FT-IR and Raman spectra through the vibrational modes of bismuth-oxygen and cobalt-oxygen bonds. The M-H and M-T magnetic measurements showed ferromagnetic like behavior at 6 K. An antiferromagnetic transition due to Co3O4 was observed at TN = 36 K. Both samples showed a antiferromagnetic Curie-Weiss behavior with a large -θCW, (θCW » TN) their calculated frustration degrees were of f ≈ 7.5 and 15.1 for samples prepared by sol-gel and combustion methods, respectively. The sample prepared by combustion method showed a blocking temperature at Tb = 148 K, Tb decreased when applied a higher field, it was ascribed to CoO clusters like phase. These results show that the combustion method allows to produce a multiphasic sample with a high degree of geometric magnetic frustration, while, the sol-gel method allows to synthesize a sample with a high crystallinity and purity.

Re-Oxidation of ZnO Clusters Grown on HOPG

This article studies the chemical interaction between ZnO and highly oriented pyrolytic graphite for as grown and thermally treated samples. In-situ X-ray photoelectron spectroscopy and ex-situ Raman spectroscopy confirm that graphite is affected by these processes, becoming oxidized and defective only in the presence of ZnO clusters that become recrystallized upon thermal re-oxidation processes performed at 400 °C. By comparing these results with other identical experiments performed with ZnO clusters grown on graphene and even with CoO clusters grown on graphite, the present results show how the interaction of the ZnO clusters with graphitic substrates depend on two factors—firstly, the mode of growth and corresponding morphology, and secondly, the reactivity of the graphitic substrates, either graphene or graphite. The results presented here will help us understand the fundamental interactions in ZnO/graphitic heterostructures and to define their operating limits.

A novel Co-O cluster based coordination polymer for efficient hydrogen production photocatalysis

The development of efficient and economical catalysts for photocatalytic water splitting and hydrogen production is one of the promising methods to alleviate the current energy crisis. In this work, we synthesized a novel Co-O cluster based coordination polymer, [Co2(HL)2·(DMF)2·H2O]·solvents (Co-CP) by a facile solvothermal method from amino-functionalized ligand and non-precious cobalt salt. The Co-CP possesses a bandgap of 2.65 eV, whose conduction and valence band are -0.8 and 1.85 eV, respectively. Efficiently, the visible-light driven photocatalytic hydrogen evolution rate of Co-CP reaches 1778 μmol g−1 h−1, due to the rapid charge transfer and efficient suppression of electron-hole recombination as determined by photocurrent response and photoluminescence measurement. Possible photocatalytic mechanisms are proposed, for which the Co-O clusters are predicted to be the active sites.

Confinement of Ultrasmall Cobalt Oxide Clusters within Silicalite-1 Crystals for Efficient Conversion of Fructose into Methyl Lactate

Chemocatalysis of sugars to methyl lactate (MLA) exhibits great advantages over the conventional fermentation approach because of its higher productivity and cost-effective separation process. However, widely used supported metal oxide catalysts suffer from deactivation resulting from sintering during the reaction and removal of coke at high temperatures. Herein, we report ultrasmall cobalt oxide clusters (∼1.7 nm) stabilized within silicalite-1 crystals catalyst (CoO@silicalite-1), exhibiting superior catalytic activity and resistance to sintering for the conversion of fructose into methyl lactate. HAADF-STEM, EDS-mapping, and XRD experiments identify the existence of confined CoO clusters. XANES and Raman spectra demonstrated the covalent interaction between CoO and silicalite-1. Thanks to the ultrasmall CoO particle size (∼1.7 nm), the CoO@silicalite-1 affords nearly 100-fold higher Co-mass-based activity (mg MLA/mg Co) compared with CoO or Co3O4 particles outside the silicalite-1 framework. More impor...

Semiconductive Amine-Functionalized Co(II)-MOF for Visible-Light-Driven Hydrogen Evolution and CO2 Reduction.

A Co-MOF, [Co3(HL)2·4DMF·4H2O] was simply synthesized through a one-pot solvothermal method. With the semiconductor nature, its band gap was determined to be 2.95 eV by the Kubelka-Munk method. It is the first trinuclear Co-MOF employed for photocatalytic hydrogen evolution and CO2 reduction with cobalt-oxygen clusters as catalytic nodes. Hydrogen evolution experiments indicated the activity was related to the photosensitizer, TEOA, solvents, and size of catalyst. After optimization, the best activity of H2 production was 1102 μmol/(g h) when catalyst was ground and then soaked in photosensitizer solution before photoreaction. To display the integrated design of Co-MOF, we used no additional photosensitizer and cocatalyst in the CO2 reduction system. When -NH2 was used for light absorption and a Co-O cluster was used as catalyst, Co-MOF exhibited an activity of 456.0 μmol/(g h). The photocatalytic mechanisms for hydrogen evolution and CO2 reduction were also proposed.

X-ray absorption spectroscopy analysis and photocatalytic behavior of ZnTiO3 nanoparticles doped with Co and Mn synthesized by sonochemical method

ZnTiO3 (ZTO) as a perovskite structure doped with Co and Mn were fabricated by sonochemical process. Structural phase, chemical and local atomic structure and photocatalytic activity of the synthesized samples were characterized, by X-ray diffraction, by X-ray absorption spectroscopy and Rhodamine B (RhB) dye degradation. The XRD results revealed that the mixing phases of cubic and hexagonal zinc titanate phases were found in all samples. The X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure technique (EXAFS) have been used to identify the crystal and atomic local structure of prepared samples. The Fourier transform EXAFS results of these prepared samples agreed well with the model of Co and Mn substituting at the B-site of ABO3 structure together with small cluster of CoO and Mn2O3 cluster. The photocatalytic activity for rhodamine b dye degradation under visible light irradiation exhibited that the doped samples had the superior degradation compared to bare-ZTO sample. The results suggest that the incorporation of transition metal ions of Co and Mn dopants is an effective way to improve the catalytic efficiency of perovskite ZTO structure. The effect of different dopant ions and dopant concentration on the structural, chemical, optical and photocatalytic activity are discussed.

Synthesis of zeolite Beta containing ultra-small CoO particles for ethylbenzene oxidation

Zeolite Beta containing ultra-small CoO particles was synthesized from a one-step hydrothermal process. The synthesis route involves the pre-mixture of hydrofluoric acid with tetraethylammonium hydroxide (in a molar ratio of 1.3–1.5:1) before the addition of a silicon and cobalt source. Investigations by scanning electron microscopy, X-ray diffraction, UV-Vis spectroscopy, X-ray photoelectron spectroscopy, H 2 -temperature-programmed reduction and transmission electron microscopy confirm the presence of ultra-small CoO particles in the zeolite Beta structure. The ultra-small CoO particles in zeolite Beta present high stability against both oxidation and reduction atmospheres at high temperatures. The catalytic performance of the CoO-containing zeolite Beta catalysts was compared with other Co-containing zeolites by evaluating ethylbenzene oxidation reactivity. The CoO-containing zeolite Beta exhibits high ethylbenzene conversion and high selectivity to acetophenone and 1-phenylethanol. The high activity of this catalyst system can be attributed to the high dispersion of the ultra-small CoO particles in the Beta structure. Beta zeolite containing ultra-small CoO clusters was synthesized from a one-step hydrothermal process. The CoO-containing Beta zeolite exhibits high activity and selectivity in ethylbenzene oxidation.

The effect of ruthenium promotion of the Co/δ-Al2O3 catalyst on the hydrogen reduction kinetics of cobalt

The effect of ruthenium content on the reductive activation of the Co/δ-Al2O3 catalyst was investigated using thermal analysis and in situ synchrotron radiation X-ray diffraction. Data of thermal analysis and phase transformations can be described by a kinetic scheme consisting of three sequential steps: Co3+ → Co2+ → (Co0Co2+) → Co0. The first step is the generation of several CoO clusters within one Co3O4 crystallite followed by their further growth obeying the Avrami–Erofeev kinetic equation (An1) with dimensional parameter n1 < 1, which may indicate the diffusion control of the growth. The second step is the kinetically controlled sequential process of the metallic cobalt phase nucleation (An2), which is followed by the third step of slow particle growth limited by mass transport according to the Jander model (D). Ruthenium promotion of Co/δ-Al2O3 catalysts significantly accelerates the reduction of cobalt. As the ruthenium content is raised to 1 wt%, the characteristic temperature of metal phase formation decreases by more than 200 °C and Ea for An2 step decreases by 25%. For step D, a joint decrease in activation energy and pre-exponential factor in case of ruthenium promotion corresponds to a weaker diffusion impediment at the final step of cobalt reduction. In the case of unmodified Co/δ-Al2O3, the characteristic temperature of the metal phase formation reaches very high values, the metallic nuclei rapidly coalesce into larger ones, and the further process is inhibited by diffusion of the reactants through the product layer. For ruthenium promoted catalysts, each CoO crystallite generates one metal crystallite; thus, ruthenium enhances the dispersion of the active component.

Epitaxial Growth of Co and Ru on Pt(111)

The epitaxial growth of Co/Ru on a well-defined Pt(111) electrode surface has been studied as a function of deposition potentials by the reflection high-energy electron diffraction (RHEED). For ∼1 to 2 ML coverage, Co growth on Pt(111) proceeds initially via nucleation and formation of patchy islands with various levels, and the grain boundaries likely enhance the CoO and Co3O4 cluster formation after emersion. At ∼3 to 4 ML coverage, 2D islands covered by CoO surface layer, in incoherent epitaxy on Pt(111), are observed, and the twinned Co clusters are for the first time detected at ∼5 ML thick islands, showing a Stranski–Krastanov growth mode. Greater than 5 to 6 ML coverage leads to 3D cluster formation growth in the (111) orientation aligned along the Co [110] to the Pt [110] direction. For Ru on Pt(111), at ∼0.2 ML coverage Ru islands are mainly monolayer high with a pseudomorphic growth; beyond this coverage (∼0.6 ML), concurrent growth of higher layers leads to predominantly bilayer island formation, showing an incommensurate growth. Greater than 1 ML coverage develops multilayer islands growing in the (0001) orientation and aligned the [11–20] direction to the [110] Pt direction. The acquired reciprocal lattice net of Co(110)*fcc differs considerably from that of an Ru(110)hcp*, which allows a reliable phase determination of Co/Ru deposits on Pt(111).

Catalytic Oxygen Evolution by Cobalt Oxido Thin Films.

The contemporary demand to generate fuels from solar energy has stimulated intense effort to develop water splitting catalysts that can be coupled to light-absorbing materials. Cobalt oxido catalyst (Co-OECs) films deposited from buffered Co(II) solutions have emerged as arguably the most studied class of heterogeneous oxygen evolution catalysts. The interest in these materials stems from their formation by self-assembly, their self-healing properties, and their promising catalytic activity under a variety of conditions. The structure and function of these catalysts are reviewed here together with studies of molecular Co-O cluster compounds, which have proven invaluable in elucidating the chemistry of the Co-OECs.

Cobalt-containing layered or zeolitic silicates as photocatalysts for hydrogen generation.

Layered magadiite and zeolites Y containing framework Co or small CoO clusters in the pores have been synthesized and tested as photocatalysts for water splitting, in the absence and presence of methanol, upon UV or simulated sunlight irradiation; the best performing material was Co-magadiite.

Shape, Morphology, and Phase Transitions during Co Oxide Growth on Au(111)

The surface structure and morphology of cobalt oxide thin films grown on Au(111) were studied using scanning tunneling microscopy (STM) and ultraviolet and X-ray photoelectron spectroscopies. Initial growth in O2 led to 2-D CoO clusters exhibiting a superstructure in STM images characteristic of the lattice mismatch between the rocksalt (111) surface and the substrate. The superstructure governed the CoO cluster shape; as a result, the shapes of clusters containing up to thousands of Co atoms were modulated by the most efficient packing of spheres. Continued growth led to a transition to 3-D, fully oxidized, spinel phase Co3O4 clusters. These Co3O4 clusters were embedded with characteristic “Y” shaped grain boundaries that result from high reactivity toward oxygen at specific edges of the CoO clusters, and the doubling of the periodicity when CoO is oxidized to Co3O4. Exposure of the Co3O4 films to O atoms induced oxidation toward Co2O3, which roughened the surface. Reducing these overoxidized films produ...

Local structural properties of Co-ion-implanted ZnO nanorods

We examined the local structural properties around Co and Zn ions in Co-ion-implanted ZnO nanorods by using an X-ray absorption fine structure (XAFS) analysis. Vertically-aligned ZnO nanorods were synthesized on Al2O3 substrates by using a catalyst-free metal-organic chemicalvapor deposition. Co ions (Co+ and Co2+) with energies of 50 and 100 keV and fluxes of 1013 and 1015 particles/cm2 were implanted in the ZnO nanorods, and the ion-implanted ZnO nanorods were annealed at 400–650°C. X-ray absorption near edge structure (XANES) analyses demonstrated that the chemical valence state of the Co ions were mostly 2+. An extended XAFS (EXAFS) analysis revealed that the Co ions were mostly substituted at the Zn sites of ZnO nanorods at a Coion flux of 1015 particles/cm2. However, at a flux of 1013 particles/cm2, Co ions formed Co-O and Co-Co clusters. These results were in contrast to the Co distribution in Co-added ZnO predicted by using a Monte Carlo method.

Cluster-size dependent phase transition of Co oxides on Au(111)

The surface structure of Co oxide nanoparticles supported on Au(111) was studied using scanning tunneling microscopy (STM) and ultra-violet photoelectron spectroscopy (UPS). Initial Co oxide growth at oxygen pressures orders of magnitude above those required to form bulk Co3O4 resulted solely in small 2-D CoO islands, exhibiting a moiré surface characteristic of the rocksalt (111) surface; continued growth led to a transition to fully oxidized 3-D Co3O4 spinel clusters. The CoO–Co3O4 transition is reversible by reduction in vacuum. It is shown that the stabilization of the reduced phase at low coverages can be explained by the higher surface energy of the support and the Co3O4 spinel structure; since a bilayer is the smallest repeat unit of this structure, oxidation increases the exposed area of the high surface energy substrate. In addition, the 2D to 3D transition required to form Co3O4 increases the occupancy of high energy edge sites, thus further stabilizing small CoO clusters against oxidation. The results show how support interactions and particle size can be used to tune the stability of oxide phases.