Phosphorus Incorporation into Co9S8 Nanocages for Highly Efficient Oxygen Evolution Catalysis

The improvement of activity of electrocatalysts lies in the increment of the density of active sites or the enhancement of intrinsic activity of each active site. A common strategy to realize dual active sites is the use of bimetal compound catalysts, where each metal atom contributes one active site. In this work, a new concept is presented to realize dual active sites with tunable electron densities in monometal compound catalysts. Dual Co²⁺ tetrahedral (Co²⁺(T_d)) and Co³⁺ octahedral (Co³⁺(O_h)) coordination active sites are developed and adjustable electron densities on the Co²⁺(T_d) and Co³⁺(O_h) are further achieved by phosphorus incorporation (P‐Co₉S₈). The experimental results and density functional theory calculations show that the nonmetal P doping can systematically modulate charge density of Co²⁺(T_d) and Co³⁺(O_h) in P‐Co₉S₈ and simultaneously improve the electrical conductivity of Co₉S₈, which substantially enhances oxygen evolution reaction performance of P‐Co₉S₈.     

Unveiling the Origin of Co3O4 Quantum Dots for Photocatalytic Overall Water Splitting

Spinel cobalt oxide displays excellent photocatalytic performance, especially in solar driven water oxidation. However, the process of water reduction to hydrogen is considered as the Achilles’ heel of solar water splitting over Co₃O₄ owing to its low conduction band. Enhancement of the water splitting efficiency using Co₃O₄ requires deeper insights of the carrier dynamics during water splitting process. Herein, the carrier dynamic kinetics of colloidal Co₃O₄ quantum dots-Pt hetero-junctions is studied, which mimics the hydrogen reduction process during water splitting. It is showed that the quantum confinement effect induced by the small QD size raised the conduction band edge position of Co₃O₄ QDs, so that the ligand-to-metal charge transfer from 2p state of oxygen to 3d state of Co²⁺ occurs, which is necessary for overall water splitting and cannot be achieved in Co₃O₄ bulk crystals. The findings in this work provide insights of the photocatalytic mechanism of Co₃O₄ catalysts and benefit rational design of Co₃O₄-based photocatalytic systems.     

Effects of Co content and annealing temperature on the structure and optical properties of CoxMg1−xAl2O4 nanoparticles

Co_xMg_1−xAl₂O₄ (x = 0–0.8) nanoparticles were synthesized by sol–gel method, and characterized by X-ray powder diffraction and transmission electron microscopy. X-ray photoelectron spectroscopy and ²⁷Al solid-state NMR spectroscopy were performed to study the chemical environments of cations in the nanoparticles as a function of cobalt content and annealing temperature. The results show that the crystallite size of the particles is about 20–40 nm. Besides the tetrahedral and octahedral coordinations, the second octahedrally coordinated Al³⁺ ions are observed in the samples. The inversion parameter (two times the fraction of Al³⁺ ions in tetrahedral sites) decreases with the increase of annealing temperature and cobalt content. The fraction of octahedral Mg²⁺ decreases with the increase of Co concentration. The absorption spectra indicate that Co²⁺ ions are located in the tetrahedral sites as well as in the octahedral sites in the nanoparticles. The intensity of the absorption peak corresponding to octahedral Co²⁺ ions (300–500 nm) decreases with increasing annealing temperature.     

Synthesis of nanosized (Zn1−xCox)Al2O4 spinels: New pink ceramic pigments

The Zn_1?xCo_xAl₂O₄ system has been reported to show a blue color with particles sizes that range from 36 nm to 30 μm. However, we have observed a pink color in this system with nanoparticles of an average size of 10 nm.Such differences in the optical properties of this nanosystem are related to the distribution of the cobalt cations in octahedral sites rather than in tetrahedral sites as in standard cobaltite spinels.The solid solution Zn_1?xCo_xAl₂O₄ (x = 0.2, 0.4, 0.6, 0.8 and 1) was synthesized by a co-precipitation reaction from the metallic salts and subsequent calcinations.The color properties and the high thermal stability (1400 °C) of these nanostructures suggest that they have the potential to be applied as satisfactory ceramic pigments.     

Interactions with the support in Co/TiO2 and Co3O4/TiO2 systems

TiO₂-supported cobalt systems have been studied both in the oxidized and in the reduced state. After high temperature (770 K) reduction no hydrogen adsorption is observed, indicating the occurrence of strong interactions with the support. Oxidation at room temperature is not observed, but after 770 K oxidation, Co₃O₄ particles are detected, decreasing the dispersion of the supported phase and the extent of their interaction with the support.     

Synthesis and investigation of bimetallic Ni-Co/Al2O3 nanocatalysts using the polyol process

ABSTRACT

Ni-Co/Al₂O₃ catalysts with different Ni:Co ratios by weight were prepared using a simple polyol process. The activities of the catalysts were evaluated for the catalytic partial oxidation of methane (CPOM) in the temperature range of 600–800°C. Numerous techniques such as x-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area analysis, inductively coupled plasma-mass spectroscopy (ICP-MS), thermogravimetric analysis (TGA), high-resolution transmission electron microscopy analysis (HRTEM), scanning electron microscopy analysis (SEM-EDS) and temperature-programmed oxidation (TPO) were applied to characterize fresh and spent catalysts. The XRD analysis confirmed that the loaded particles were metals and showed possible bimetallic nano-alloy Ni-Co formation for Ni- and Co-containing catalysts. The highest metal dispersion was 15.7% for the Ni_2.8Co_2.6/Al₂O₃ catalyst. The catalytic test results showed no correlation between metal dispersion and the metal particle size, and the activity decreased in the order of Ni_7.7/Al₂O₃ > Ni_2.8Co_2.6/Al₂O₃ ≈ Ni_3.8Co_1.5/Al₂O₃ > Ni_2.0Co_3.8/Al₂O₃ >> Co_6.8/Al₂O₃ under a flow rate of 157,500 L kg^?1 h^?1 with CH₄/O₂ = 2 (using air as an oxidant) at 800°C. The obtained results also showed that when the actual atomic Ni/Co ratio was 1.07 in the Al₂O₃-supported catalyst, the dispersion of the active sites appeared to be promoted by Co addition, and the catalytic activity was stable over a reaction time of 10 h. Among all the tested catalysts, the Ni_2.8Co_2.6/Al₂O₃ catalyst exhibited acceptable activity (75%) without coking.     

Annealing Induced Enhancement in Magnetic Properties of Co0.5Zn0.5Fe2O4 Nanoparticles

In this paper, cobalt zinc ferrite (Co_0.5Zn_0.5Fe₂O₄) nanoparticles (NPs) have been prepared using chemical co-precipitation method. In order to investigate the annealing induced effects on their various physical properties, the prepared samples have been annealed at 500 °C, 650 °C and 1000 °C and then compared with as-prepared sample. X-ray diffraction (XRD) patterns of as-prepared and annealed samples at various temperatures exhibit single phase spinel structure. Enhancement in crystallinity and crystallite size is observed with the increase in annealing temperature. The annealing has also greatly influence the morphology and grain size of prepared NPs. The Co_0.5Zn_0.5Fe₂O₄ NPs have shown remarkable enhancement in magnetic moment with increase in annealing temperature. The bandgap energies of Co_0.5Zn_0.5Fe₂O₄ NPs have been measured via UV Spectrometer and observed to decrease with annealing temperature. FTIR spectra of the samples reveal the presence of both high frequency and low-frequency bands due to tetrahedral and octahedral sites, which corroborate well with the XRD results. The observed characteristics of cobalt zinc ferrite NPs as a function of annealing temperature are the rising contender for many data storage and nanodevice applications. Finally, the genotoxicity of prepared nanoferrites has been checked via comet assay.     

Aluminum‐Tailored Energy Level and Morphology of Co3−xAlxO4 Porous Nanosheets toward Highly Efficient Electrocatalysts for Water Oxidation

Tuning energy levels plays a crucial role in developing cost‐effective, earth‐abundant, and highly active oxygen evolution catalysts. However, to date, little attention has been paid to the effect of using heteroatom‐occupied lattice sites on the energy level to engineer electrocatalytic activity. In order to explore heteroatom‐engineered energy levels of spinel Co₃O₄ for highly‐effective oxygen electrocatalysts, herein Al atoms are directly introduced into the crystal lattice by occupying the Co²⁺ ions in the tetrahedral sites and Co³⁺ ions in the octahedral sites (denoted as Co²⁺_Td and Co³⁺_Oh, respectively). Experimental and theoretical simulations demonstrate that Al³⁺ ions substituting Co²⁺_Td and Co³⁺_Oh active sites, especially Al³⁺ ions occupying the Co²⁺_Td sites, optimizes the adsorption, activation, and desorption features of intermediate species during oxygen evolution reaction (OER) processes. As a result, the optimized Co_1.75Al_1.25O₄ nanosheet exhibit unprecedented OER activity with an ultralow overpotential of 248 mV to deliver a current of 10 mA cm^–2, among the best Co‐based OER electrocatalysts. This work should not only provide fundamental understanding of the effect of Al‐occupied different Co sites in Co_3–xAl_xO₄ composites on OER performance, but also inspire the design of low‐cost, earth‐abundant, and high‐active electrocatalysts toward water oxidation.     

Understanding the Role of (W,Mo, Sb) Dopants in the Catalyst Evolution and Activity Enhancement of Co3O4 during Water Electrolysis via In Situ Spectroelectrochemical Techniques

Unlocking the potential of the hydrogen economy is dependent on achieving green hydrogen (H₂) production at competitive costs. Engineering highly active and durable catalysts for both oxygen and hydrogen evolution reactions (OER and HER) from earth-abundant elements is key to decreasing costs of electrolysis, a carbon-free route for H₂ production. Here, a scalable strategy to prepare doped cobalt oxide (Co₃O₄) electrocatalysts with ultralow loading, disclosing the role of tungsten (W), molybdenum (Mo), and antimony (Sb) dopants in enhancing OER/HER activity in alkaline conditions, is reported. In situ Raman and X-ray absorption spectroscopies, and electrochemical measurements demonstrate that the dopants do not alter the reaction mechanisms but increase the bulk conductivity and density of redox active sites. As a result, the W-doped Co₃O₄ electrode requires ≈390 and ≈560 mV overpotentials to reach ±10 and ±100 mA cm⁻² for OER and HER, respectively, over long-term electrolysis. Furthermore, optimal Mo-doping leads to the highest OER and HER activities of 8524 and 634 A g⁻¹ at overpotentials of 0.67 and 0.45 V, respectively. These novel insights provide directions for the effective engineering of Co₃O₄ as a low-cost material for green hydrogen electrocatalysis at large scales.     

Sensing of oxidizing and reducing gases by sensors prepared using nanoscale Co3O4 powders: A study through Cu substitution

We demonstrate the sensing-mechanism of both oxidising and reducing gases by nanoscale Co₃O₄ powders through a strategy of Cu-doping in Co₃O₄. The sensitivity towards both types of gases arises due to presence of Co²⁺ and Co³⁺ cations in the (nearly) normal spinel structure of Co₃O₄. Pellets made up of nanopowders were employed for the detection of ～6.5 ppm CO gas present in either pure N₂ or in a mixture of synthetic air as carrier gas (which represents the O₂ sensing-gas too). The high sensitivity of Co₃O₄ nanoparticles to detect ～6 ppm CO (in N₂) arises due to the high surface area of nanopowders exposing a higher number of octahedral Co³⁺ cations as adsorption sites, whereas the sensitivity towards O₂ arises due to partial presence (less number) of octahedral Co²⁺. To support this mechanism, octahedrally Cu²⁺ substituted Co₃O₄ specimens are investigated. The inactive Cu²⁺ at the octahedral site changes the unexposed tetrahedral Co²⁺ into Co³⁺. The presence of inactive Cu²⁺ at the octahedral site and the burial of the Co³⁺ at the tetrahedral sites reduce the adsorption sites for O₂, thereby drastically reducing the overall O₂-gas sensitivity shown by the Cu-substituted sample, although they have higher surface-area (nanoparticles).     

Crystal Facet Induced Single‐Atom Pd/CoxOy on a Tunable Metal–Support Interface for Low Temperature Catalytic Oxidation

Atomic dispersed metal sites in single‐atom catalysts are highly mobile and easily sintered to form large particles, which deteriorates the catalytic performance severely. Moreover, lack of criterion concerning the role of the metal–support interface prevents more efficient and wide application. Here, a general strategy is reported to synthesize stable single atom catalysts by crafting on a variety of cobalt‐based nanoarrays with precisely controlled architectures and compositions. The highly uniform, well‐aligned, and densely packed nanoarrays provide abundant oxygen vacancies (17.48%) for trapping Pd single atoms and lead to the creation of 3D configured catalysts, which exhibit very competitive activity toward low temperature CO oxidation (100% conversion at 90 °C) and prominent long‐term stability (continuous conversion at 60 °C for 118 h). Theoretical calculations show that O vacancies at high‐index {112} facet of Co_xO_y nanocrystallite are preferential sites for trapping single atoms, which guarantee strong interface adhesion of Pd species to cobalt‐based support and play a pivotal role in preventing the decrement of activity, even under moisture‐rich conditions (≈2% water vapor). The progress presents a promising opportunity for tailoring catalytic properties consistent with the specific demand on target process, beyond a facile design with a tunable metal–support interface.     

Effect of silver oxide doping on surface and catalytic properties of Co3O4/Al2O3 system

Cobaltic oxide catalyst supported on gamma-alumina having the formula 0.1 Co₃O₄/Al₂O₃ was prepared by wet impregnation method using finely powdered Al(OH)₃ and cobalt nitrate dissolved in the least amount of distilled water. Four silver oxide-doped samples were prepared by impregnating Al(OH)₃ solid with calculated amounts of silver nitrate dissolved also in the least amount of distilled water prior to impregnation with cobalt nitrate solution. The amounts of Ag₂O were 0.2, 0.4, 0.8 and 1.6 wt.%. Pure and doped solids were subjected to heat treatment at 400, 600 and 800 °C. Diffraction lines of Co₃O₄ phase were detected in the XRD patterns of pure and doped solids precalcined at different temperatures. However, the doping process conducted at different temperatures brought about a progressive significant increase in the particle size and degree of ordering of Co₃O₄ phase. Ag₂O-doping of the investigated system affected a measurable decrease in its specific surface areas. The catalytic activities of different solids, in CO-oxidation by O₂ conducted at 125–225 °C, were found to increase significantly by increasing the amount of dopant added reaching to a maximum limit at 0.8 wt.% Ag₂O. The maximum increase in the catalytic activity expressed as reaction rate constant k measured at 150 °C over the solids precalcined at 400 °C and at 175 °C for the solids precalcined at 600 and 800 °C attained 337%, 118% and 219%, respectively. The doping process did not modify the activation energy (E_a) of the catalyzed reaction, and the observed apparent changes in E_a values resulted from a compensation effect as evidenced from almost same changes in the values of pre-exponential factor of Arrhenius equation. Therefore, the observed increase in the catalytic activity of the investigated system due to Ag₂O treatment, which did not change the mechanism of the catalyzed reaction, could be attributed to an effective increase in the concentration of active sites contributing in chemisorption and catalysis of CO oxidation by O₂.     

Dielectric properties of cobalt substituted M-type barium hexaferrite prepared by co-precipitation

Barium hexaferrite was synthesised via the co-precipitation method using high purity nitrates, oxides and carbonates of iron (III), barium (II) and ammonium hydroxide. Once a phase pure sample of barium hexaferrite was obtained, it was doped, by weight, with 1, 2, 3, 5, 10, 15, 20 and 30% cobalt oxide (Co₃O₄). The addition of cobalt to the BaM had the effect of reducing the permittivity and loss tangent until a doping of 10% whereupon it remained constant at around 9. Thermogravimetric (TG) studies of green bodies showed decarboxilation to occur at around 825°C and the transformation of residual Co₃O₄ to Co₂O₃ at around 900°C. The X-ray diffraction (XRD) studies confirmed the Co ions substituting in the iron sites until a doping level, of 10–15% where the structure underwent a transition to one more closely resembling the W-type hexaferrite. The measured densities were found to vary with doping levels, with a maximum of 4.45 g/cm³ at 1% Co doping.     

Co3O4 doped over SBA 15: excellent adsorbent materials for the removal of methyleneblue dye Pollutant

Nanosized cobalt oxide particles are incorporated into SBA 15 mesoporous silica materials and are effectively used for the first time as adsorbent materials for aquatic dye pollutant removal. Cobalt is found to exist in its Co₃O₄ spinel structure as evident from FTIR and X-ray diffraction studies. The best weight ratio of metal loading to show excellent adsorption of methyleneblue is found to be 10 wt% Co over the support. There, Co₃O₄ spinel nanoparticles lie inside the pores of mesoporous silica. Further increase in the percentage of metal loading decreases the adsorption capacity which may be due to the agglomeration of nanoparticles over the silica support as evident from TEM photographs. Cobalt-doped systems of the present study, having good adsorption capacity of methyleneblue, are prepared via impregnation of cobalt nitrate over SBA 15 in aqueous medium. Here, we introduce a new SBA 15-based system for the fast removal of aquatic dye pollutants which is highly economical for industrial applications.     

Some physical properties of V2O5-Fe2O3 and V2O5-Fe2O3-Li2O systems

Various methods have been used to study the physical properties of the V₂O₅-Fe₂O₃ and V₂O₅-Fe₂O₃-Li₂O systems, including X-ray, electron microscope, Mössbauer effect, NMR and thermogravimetric measurements. The iron ions are approximately equally distributed in substitutional and interstitial sites in the V₂O₅ lattice. The maximum number of iron ions dissolved in the V₂O₅ matrix corresponds to 4 mol % Fe₂O₃. In all the samples a quantity of Fe₂O₃ which has not been included in lattice is observed. The V₂O₅-Fe₂O₃ and V₂O₅-Fe₂O₃-Li₂O systems are formed from solid solutions mixed with very small Fe₂O₃ particles. The analysis of the charge compensation of iron ions suggests that V₂O₅ is a quasi-amorphous semiconductor. Irradiation of V₂O₅-based samples with an electron beam induces the V₂O₅ platelets to convert to the VO _x phase.     

Defects‐Induced In‐Plane Heterophase in Cobalt Oxide Nanosheets for Oxygen Evolution Reaction

Cobalt oxides as efficient oxygen evolution reaction (OER) electrocatalysts have received much attention because of their rich reserves and cheap cost. There are two common cobalt oxides, Co₃O₄ (spinel phase, stable but poor intrinsic activity) and CoO (rocksalt phase, active but easily be oxidatized). Constructing Co₃O₄/CoO heterophase can inherit both characteristic features of each component and form a heterophase interface facilitating charge transfer, which is believed to be an effective strategy in designing excellent electrocatalysts. Herein, an atomic arrangement engineering strategy is applied to improve electrocatalytic activity of Co₃O₄ for the OER. With the presence of oxygen vacancies, cobalt atoms at tetrahedral sites in Co₃O₄ can more easily diffuse into interstitial octahedral sites to form CoO phase structure as revealed by periodic density functional theory computations. The Co₃O₄/CoO spinel/rocksalt heterophase can be in situ fabricated at the atomic scale in plane. The overpotential to reach 10 mA cm^?2 of Co₃O₄/CoO is 1.532 V, which is 92 mV smaller than that of Co₃O₄. Theoretical calculations confirm that the excellent electrochemical activity is corresponding to a decline in average p‐state energy of adsorbed‐O on the Co₃O₄/CoO heterophase interface. The reaction Gibbs energy barrier has been significantly decreased with the construction of the heterophase interface.     

Co1−xZnxFe2O4 (0 ≤ x ≤ 1) nanocrystalline solid solution prepared by the polyol method: Characterization and magnetic properties

Highly crystalline stoichiometric Co_1?xZn_xFe₂O₄ (0 ≤ x ≤ 1) nanoparticles were successfully synthesized by the polyol process. X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), transmission electron microscopy (TEM), infrared spectroscopy (IR), zero-field ⁵⁷Fe Mössbauer spectrometry and magnetic measurements using a SQUID magnetometer were employed to investigate the effect of the substitution of Zn²⁺ ions for Co²⁺ ones on the structure, and the magnetic properties of the cobalt ferrite, CoFe₂O₄. The unit cell parameter almost increases linearly with increasing Zn concentration, x, following Vegard's law. The red and blue shifts observed for the metal-oxygen ν₁ and ν₂ IR vibration bands, respectively, were consistent with the preferential entrance of Zn²⁺ ions in tetrahedral sites. Besides, detailed magnetic investigation in correlation with the cation distribution has been reported. All the particles exhibit superparamagnetic behaviour at room temperature. In addition, the magnetic characteristics (blocking temperature, saturation magnetization, coercivity, Curie temperature) clearly depend on the chemical composition and cation distribution. Both the blocking temperature and Curie temperature decrease drastically with Zn composition, x, increase. Further, the saturation magnetization follows an almost bulk-like behaviour with values notably larger than that of the bulk, mainly attributed to cation distribution deviation.     

Synthesis,structural and electrical properties of triethylene glycol (TREG) stabilized Mn0.2Co0.8Fe2O4 NPs

Triethylene glycol (TREG) stabilized Mn_0.2Co_0.8Fe₂O₄ NPs was synthesized by a glycothermal reaction. XRD analysis identified the product as Mn_0.2Co_0.8Fe₂O₄ with a high phase purity. Nano-sized particles with an average size of about 6–8 nm were obtained with nearly single crystalline nature with TEM analysis. Superparamagnetic-like behavior of TREG stabilized Mn_0.2Co_0.8Fe₂O₄ NPs was observed by VSM. The binding between TREG and Mn_0.2Co_0.8Fe₂O₄ NPs was investigated with FT-IR and found to be via O on the TREG and NP surface. TG analysis indicated that the Mn_0.2Co_0.8Fe₂O₄ NP content was about 40%, with a TREG-shell content to be around 60%. Overall conductivity of the nanocomposite is in the range of 10^?10 to 10^?7 S cm^?1 with a strong dependence on temperature and frequency, indicating ionic conductivity. The nanocomposite exhibited lower ?’ and ?″ compared to TREG stabilized Mn_0.2Co_0.8Fe₂O₄ NPs due to the doping of co-doping of manganese and cobalt.     

Construction of Complex Co3O4@Co3V2O8 Hollow Structures from Metal–Organic Frameworks with Enhanced Lithium Storage Properties

A novel metal–organic‐framework‐engaged strategy is demonstrated for the preparation of multishelled Co₃O₄@Co₃V₂O₈ hybrid nanoboxes. This strategy relies on the unique reaction of zeolitic imidazolate framework‐67 with the vanadium source of vanadium oxytriisopropoxide. Benefitting from the synthetic versatility, a series of nanostructures can be realized including triple‐shelled and double‐shelled Co₃O₄@Co₃V₂O₈ nanoboxes and single‐shelled Co₃V₂O₈ nanoboxes. When evaluated as electrode materials for lithium‐ion batteries, these unique hollow structures demonstrate remarkable lithium storage properties. For example, the triple‐shelled Co₃O₄@Co₃V₂O₈ nanoboxes retain a high capacity of 948 mAh g^?1 after 100 cycles at 100 mA g^?1.     

Interaction of V2O5-NaY zeolite with H2S and SO2

V₂O₅-NaY zeolite catalyst was prepared by vacuum impregnation of ammonium metavanadate solution in water on NaY-zeolite (zeolite). The slurry was filtered, washed, dried, and calcined at 600°C. On heating the catalyst at 450°C in vacuum, VO²⁺ species were detected by e.s.r. It was observed that the electron-accepting and -donating sites increased when V₂O₅ or vanadium species were loaded on it. Vanadium species poisoned the active sites of the zeolite that were responsible for the formation of SO₂⁻ ions. Pretreated V₂O₅-NaY zeolite with SO₂ plays an important role in the formation of VO²⁺ species when it is treated with H₂S at room temperature. V₂O₅-NaY zeolite can be used for the reduction of SO₂ by H₂S.