Cobalt Oxide Phase Research Articles

Controlling lithium cobalt oxide phase transition using molten fluoride salt for improved lithium-ion batteries

LiCoO2 is a historic lithium-ion battery cathode that continues to be used today because of its high energy density. However, the practical capacity of LiCoO2 is limited owing to the harmful phase transition at high voltages, which prevents the realization of its theoretical capacity. Here, we treat LiCoO2 particles with a molten salt of MgF2–LiF as a reaction accelerator to facilitate the diffusion and doping of magnesium into bulk LiCoO2 and to form a stable coating layer on the particle surface. Ex situ X-ray diffraction analysis confirms the inhibition of the harmful phase transition and the emergence of a different phase as the modified LiCoO2 was charged up to 4.7 V. The modified LiCoO2 shows high electrochemical performance during high-voltage operation. This technology provides a guideline for the suppressing fundamental degradation associated with phase transition and achieving ultra-high energy density LiCoO2 cathodes.

Co and Co3O4 in the Hydrolysis of Boron-Containing Hydrides: H2O Activation on the Metal and Oxide Active Centers.

This work focuses on the comparison of H2 evolution in the hydrolysis of boron-containing hydrides (NaBH4, NH3BH3, and (CH2NH2BH3)2) over the Co metal catalyst and the Co3O4-based catalysts. The Co3O4 catalysts were activated in the reaction medium, and a small amount of CuO was added to activate Co3O4 under the action of weaker reducers (NH3BH3, (CH2NH2BH3)2). The high activity of Co3O4 has been previously associated with its reduced states (nanosized CoBn). The performed DFT modeling shows that activating water on the metal-like surface requires overcoming a higher energy barrier compared to hydride activation. The novelty of this study lies in its focus on understanding the impact of the remaining cobalt oxide phase. The XRD, TPR H2, TEM, Raman, and ATR FTIR confirm the formation of oxygen vacancies in the Co3O4 structure in the reaction medium, which increases the amount of adsorbed water. The kinetic isotopic effect measurements in D2O, as well as DFT modeling, reveal differences in water activation between Co and Co3O4-based catalysts. It can be assumed that the oxide phase serves not only as a precursor and support for the reduced nanosized cobalt active component but also as a key catalyst component that improves water activation.

Atomic-scale understanding of the impact of CoO phase in cobalt-catalyzed Fischer-Tropsch synthesis: A combined computational and experimental study

In cobalt-catalyzed Fischer-Tropsch synthesis (FTS), the active metallic cobalt phase always suffers from oxidation under FTS reaction conditions, leading to the coexistence of metallic cobalt and cobalt oxide (CoO) phases. However, there still exist contradictory views on the impact of CoO phase in FTS. Some researchers proposed that the oxidation of metallic Co to CoO causes the deactivation of cobalt-based FTS catalysts, while some held the view that CoO phase could act as the potential active phase in FTS. In this study, by combining theoretical calculations and experiments, we systematically investigated the fundamental FTS reactions including CO adsorption and activation, C-C coupling, and CxHy hydrogenation on CoO phase, and compared them with those on metallic cobalt phase, aiming at identifying the exact role of CoO, understanding how CoO influences the activity and selectivity, and unraveling the intrinsic mechanism that determines the catalytic behaviors of CoO phase in FTS. The results reveal that the CoO(200) crystal facet is the dominantly exposed surface structure for CoO phase under FTS reaction conditions. The d-band center of CoO(200) surface shifts downward in comparison with that of active metallic Co surface, which causes difficulties in adsorbing and activating CO and gives rise to high energy barriers for the dissociation of C-O bond. This determines the inactive nature of CoO phase in FTS reaction. As for the product selectivity, the CoO(200) surface restricts the C-C chain growth due to its higher barriers for C-C coupling, facile desorption ability for light olefins, and lower energy barrier for CH4 formation compared with metallic cobalt, and consequently leads to the loss of selectivity to C5+ hydrocarbons and enhancement of selectivity to light olefins and undesired methane. Moreover, a strong linear relationship between the experimental observed selectivity change and the DFT-calculated energy difference of CoO and metallic cobalt phases was found, validating the DFT-calculated results. This study provides a comprehensive understanding of the nature and intrinsic catalytic behaviors of CoO phase in cobalt-catalyzed FTS.

Improved phenol degradation by catalytic wet air oxidation on the Co3O4 spinel catalysts supported on SBA-15 modified with TiO2

The removal of phenol from aqueous industrial effluents by catalytic wet-air oxidation (CWAO) is a challenging process that requires novel catalysts resistant to leaching of the active phases during residence time in a batch reactor. In this sense, this work demonstrated that this can be archived with the catalyst having the spinel phase Co3O4 supported on mesoporous silica SBA-15 doped with an optimized amount of TiO2 (3 wt%). In the phenol CWAO reaction carried out at 160 °C and 10 bar O2, this best Co3O4/TiO2/SBA-15 catalyst showed an increase of phenol conversion from 49.0 % to 68.3 % and of CO2 selectivity from 87 % to 98 %. The extensive catalyst characterization and overall activity results indicated that this was related to the strength and nature of the acid sites and the oxidation states of the cobalt oxide phases. Combined factors influenced the final behaviour of the catalyst, such as its intrinsic activity, dispersion of the active phase, inhibition of CoSiO4 phase formation, high Lewis acidity and the large amount of active Co3O4 spinel phase. The presence of TiO2 nanoparticles (NPs) decorating the support surface improved the stability of the catalyst by inhibiting the leaching of cobalt species during the reaction. After three reaction cycles, the most efficient Co/3Ti/S15 catalyst showed high CO2 selectivity (∼92.7 %) and low selectivity towards carboxylic acid (∼5.3 %) and polyphenolic compounds (∼2%). It is proposed that surface oxidation is the rate-determining step in the CWAO reaction over TiO2/SBA-15-supported Co3O4 spinel catalysts.

Properties of cobalt-doped SrTiO3 derived from two methods: the modified Pechini method and the citrate combustion method

Two wet-synthesis methods: citrate-combustion and modified (with PVA addition) Pechini synthesis were used to obtain Co-doped strontium titanate with assumed chemical compositions of SrTi1−xCoxO3 (where x = 0.02; 0.06; 0.10). All samples were calcined at 900 °C and sintered at 1200 °C, with PVA series being also calcined at 700 and 800 °C, since TG measurements indicated for this method materials lower minimal calcination temperature than for the citrate-combustion series materials. In general, the modified Pechini approach lowers calcination temperature of materials (up to 200 °C) and produces more structurally homogeneous materials than in the case of the citrate-combustion methods. However, the HT XRD measurements reveal the presence of extra cobalt oxide phases in pva_STO_xCo materials that were not visible using conventional XRD measurements. The presence of cobalt oxide has been confirmed for both series of materials based on TPR tests. Moreover, these measurements indicate the incorporation of some amount of cobalt into the perovskite structure. The fact of substituting titanium with cobalt in SrTiO3 structure seems to be confirmed by the results of XAS studies. Additionally, XAS results allowed us to state that Co2+/Co3+ ratio fell as cobalt loading increased in the materials. The examination of SrTiO3 lattice parameter changes for sintered samples is consistent with the findings of the XAS tests and indicates that cobalt adopts +2 oxidation state in the case of the material with the lowest Co content, and the share of Co3+ increases with the growth of the Co amount introduced into the system.

The High Efficient Investigations of Properties of Co3O4 Nanostructured by Facile Spray Pyrolysis Technique for Future Gas Sensors Application

Herein, the cobalt oxide thin films are elaborated by spray pyrolysis technique. The effect of temperature of substrates on several properties of cobalt oxide is studied. The thin layers are characterized by using X‐ray diffraction (XRD), Raman spectrometry (RS), and UV–vis spectrophotometer. The XRD analysis shows that Co3O4 thin layers have been formed along <111> preferential orientation. The RS displays the characteristic vibration modes of cobalt oxide phase, which confirms the formation of a single phase of spinel Co3O4 thin layers. The UV–vis spectrophotometer allows us to investigate the optical properties such as the absorbance, the transmittance, and the optical bandgap. The variation of electrical properties with the variation of temperature deposition confirms the high impact of the effect that study to make the cobalt oxide an important candidate for gas sensor devices.

A combined spectroscopic investigation of cobalt oxide films prepared by DC reactive sputtering

The structure and vibrational characteristics of cobalt monoxide and tetraoxide films, prepared by direct-current DC reactive sputtering, were investigated by different characterization techniques. According to X-ray diffraction analyses, the development of the Co3O4 phase takes place under low discharge power and high substrate temperature (80 W-623 K) conditions, whereas the CoO phase predominates when adopting high discharge power and low substrate temperature (240 W-473 K). Results of infrared absorption and Raman scattering spectroscopies indicate the presence of Co3O4 in all films, that are compatible with the optical transmittance spectra in the UV-Vis-NIR range − showing electronic transitions due to Co2+/3+ cations (related to the Co3O4 spinel structure) in the films deposited at 80 W-623 K and, to a lesser extent, at 240 W-473 K conditions. The investigations include Raman scattering measurements in the 83–623 K range. The results evidence a mixture of phases in the 240 W-473 K film, whose main Raman scattering peaks evolve irreversibly towards Co3O4 upon increasing measurement temperatures. Altogether, the present report provides deeper information about the cobalt oxide phases that shall be useful to improve the control of properties and production of cobalt oxide films by DC reactive sputtering.

Physical properties of Ni: Co3O4 thin films and their electrochemical performance

In this research work, we have deposited cobalt oxide as pure and Ni-doped thin films using spray pyrolysis. The concentration of Ni has been changed from 0 to 6 wt% in all films while other deposition parameters are fixed. The characterization of deposited films has been c using X-ray diffraction (XRD), energy dispersive x-ray spectroscopy (EDX), x-ray photoelectron microscope (XPS), scanning electron microscope (SEM), and optical spectroscopy. The XRD results confirm the formation of Co3O4 as the low-temperature stable phase of cobalt oxide and the successful doping with nickel. The XRD also shows the preferred orientation of growth of film is along the (111) plane and the crystallite size was found to decrease with increasing Ni content. The SEM micrograph of the deposited film surfaces revealed that the samples are porous and in some parts of the sample, the particles are agglomerated due to doping. The Ni doping was confirmed by both EDX and XPS. The Ni concentration was found to increase according to dopant concentration. The XPS data of the 4 wt% film has been recorded which confirms the existence of Ni+2 in the deposited films. Both optical transmission and reflection spectra have been recorded using a spectrophotometer. The band gap values have been found to decrease upon Ni-doping. The electrochemical properties of the pure and nickel-doped cobalt oxide films were measured by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) in 0.3 M KOH electrolyte. The specific capacitance of 4 wt% Ni doped Co3o4 was found to be 804 Fg-1 at a 2 mVs−1 scan rate which is 90% higher than pure films. The important property of this material is that it shows excellent retention of 96% and remains almost constant for 10000 cycles. The impedance analysis reveals that 4 wt% Ni doped cobalt oxide film has the lowest RS value of 0.2 Ω cm−2 and lowest RCT value of 0.05 Ω cm−2 in comparison to other films which have excellent super-capacitive nature. These outstanding electrochemical properties of 4 wt % Ni-doped Co3O4 thin film have made it a potential candidate for anode material in supercapacitor devices.

Speciation of Oxygen Functional Groups on the Carbon Support Controls the Electrocatalytic Activity of Cobalt Oxide Nanoparticles in the Oxygen Evolution Reaction.

The effective use of the active phase is the main goal of the optimization of supported catalysts. However, carbon supports do not interact strongly with metal oxides, thus, oxidative treatment is often used to enhance the number of anchoring sites for deposited particles. In this study, we set out to investigate whether the oxidation pretreatment of mesoporous carbon allows the depositing of a higher loading and a more dispersed cobalt active phase. We used graphitic ordered mesoporous carbon obtained by a hard-template method as active phase support. To obtain different surface concentrations and speciation of oxygen functional groups, we used a low-temperature oxygen plasma. The main methods used to characterize the studied materials were X-ray photoelectron spectroscopy, transmission electron microscopy, and electrocatalytic tests in the oxygen evolution reaction. We have found that the oxidative pretreatment of mesoporous carbon influences the speciation of the deposited cobalt oxide phase. Moreover, the activity of the electrocatalysts in oxygen evolution is positively correlated with the relative content of the COO-type groups and negatively correlated with the C═O-type groups on the carbon support. Furthermore, the high relative content of COO-type groups on the carbon support is correlated with the presence of well-dispersed Co3O4 nanoparticles. The results obtained indicate that to achieve a better dispersed and thus more catalytically active material, it is more important to control the speciation of the oxygen functional groups rather than to maximize their total concentration.

Magneto-transport and magneto-dielectric anomalies in Co doped iron oxide thin films – Microwave assisted sol-gel route

Use of microwave (MW) energy as an alternate to high temperature treatment during material synthesis is gaining much attention as it offers the advantages of being faster and economical than other heating methods. A comprehensive study of structure, magnetization, and polarization behaviour of Cobalt (Co) doped Fe3O4 thin films, prepared by means of microwave assisted sol–gel method is performed. Co doped (0-10 wt%) sols are prepared after irradiating them at MW power of 1000 W. Phase analysis is confirmed using XRD where Fe3O4 phase persisted with Co concentrations of 0-6 wt%. Higher concentration results in the appearance of cobalt oxide phase. Magnetic and transport properties are strongly affected by the presence of Co ions. Co doped Fe3O4 films show anisotropic behaviour with high saturation magnetization obtained for 6 wt% Co doped Fe3O4 thin films, i.e. ∼ 125 emu/g. Verwey transition is observed for thin films with low dopant concentration while no sharp transition in magnetization behaviour is observed for higher dopant concentration. High frequency anomaly in dielectric analysis is observed at high frequencies for Co doped Fe3O4 thin films. Negative values of magneto-dielectric coefficient show coupling of magnetic and dielectric behaviour at room temperature.

Overcharge‐Induced Phase Heterogeneity and Resultant Twin‐Like Layer Deformation in Lithium Cobalt Oxide Cathode for Lithium‐Ion Batteries

Overcharging is expected to be one of the solutions to overcome the current energy density limitation of lithium-ion battery cathodes, which will support the rapid growth of the battery market. However, high-voltage charging often poses a major safety threat including fatal incendiary incidents, limiting further application. Numerous researches are dedicated to the disadvantages of the overcharging process; nonetheless, the urgent demand for addressing failure mechanisms is still unfulfilled. Herein, it is revealed that overcharging induces phase heterogeneity into layered and cobalt oxide phases, and consequent "twin-like deformation" in lithium cobalt oxide. The interplay between the uncommon cobalt(III) oxide and the deformation is investigated by revealing the atomistic formation mechanism. Most importantly, abnormal cracking is discovered in the vicinity of the cobalt oxide where structural instability induces substantial contraction. In addition, surface degradation is widely observed in the crack boundary inside the particle. As unintentional overcharging can occur due to local imbalance in state-of-charge in severe operating conditions such as fast charging, the issues on overcharging should be emphasized to large extent and this study provides fundamental knowledge of overcharge by elucidating the crack development mechanism of layered cathodes, which is expected to broaden the horizon into high voltage operation.

Gold-Based Catalysts for Complete Formaldehyde Oxidation: Insights into the Role of Support Composition

Formaldehyde (HCHO) is recognized as one of the most emitted indoor air pollutants with high detrimental effect on human health. Significant research efforts are focused on HCHO removal to meet emission regulations in an effective and economically profitable way. For over three decades, the unique electronic properties and catalytic abilities of nano-gold catalysts continue to be an attractive research area for the catalytic community. Recently, we reported that mechanochemical mixing is a relevant approach to the preparation of Co-Ce mixed oxides with high activity in complete benzene oxidation. A trend of higher surface defectiveness, in particular, oxygen vacancies, caused by close interaction between cobalt oxide and cerium oxide phases, was observed for a mixed oxide composition of 70 wt.% Co3O4 and 30 wt.% CeO2. These results directed further improvement by promotion with gold and optimization of mixed oxide composition, aiming for the development of an efficient catalyst for room temperature HCHO abatement. Support modification with potassium was studied; however, the K addition caused less enhancement of HCHO oxidation activity than expected. This motivated the preparation of new carrier material. In addition to Co3O4-CeO2 mixed metal oxides with preset ratio, γ-Al2O3 intentionally containing 33% boehmite and shortly named Al2O3-b was used for synthesis. Analysis of the role of support composition in HCHO oxidation was based on the characterization of nano-gold catalysts by textural measurements, XRD, HRTEM, XPS, and TPR techniques. Gold supported on mechanochemically treated Co3O4-CeO2-Al2O3-b (50 wt.% Al2O3-b) exhibited superior activity owing to high Ce3+ and Co3+ surface amounts and the most abundant oxygen containing species with enhanced mobility. This catalyst achieved oxidation to CO2 and H2O by 95% HCHO conversion at room temperature and 100% at 40 °C, thus implying the potential of this composition in developing efficient catalytic materials for indoor air purification.

SYNTHESIS OF NANOSIZED ZIRCONIUM DIOXIDE, COBALT OXIDE AND RELATED PHASES IN SUPERCRITICAL CO2 FLUID

This study is devoted to obtaining nanoscale zirconium dioxide, cobalt oxide and related phases by SAS method in supercritical carbon dioxide. The synthesized compounds were characterized by a complex of physico-chemical analytical methods: infrared spectroscopy, differential scanning calorimetry, X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy. The experimental parameters for obtaining the nanoparticles were: pressure 10 MPa, temperature 40 °C, carbon dioxide supply rate 35 g/min, the initial solution supply rate 0.5 ml/min. Individual phases containing zirconium and cobalt, and also samples with zirconium to cobalt molar ratios 3:1, 2:1, 1:1, 2:1 and 1:3 were obtained. The use of zirconium and cobalt acetylacetonates as initial components leads to formation of stable products – nanoparticles of acetates of the corresponding metals in the X-ray amorphous state. When heated to 340-350 °C, the destruction of organometallic complexes to oxides occurs with formation of a continuous series of X-ray amorphous solid solutions in the ZrO2-CoO system. At temperatures above 600 °C, the phases crystallize with the decomposition of solid solutions into ZrO2 and Co3O4. When temperature is above 900 °C, further oxidation of cobalt occurs. Thus, cobalt oxide oxidation into Co3O4 proceeds in two steps, at 600 and 900 °C. For samples of zirconium dioxide with cobalt oxide admixture at a temperature of 700 °C stabilization of the cubic modification is observed which is probably due to the entry of cobalt into the cubic structure of zirconium oxide, which prevents transition to tetragonal and monoclinic modifications.

The role of sulfur sinks and micro-structured supports on the performance of sulfur-sensitive non-PGM catalysts

The presence of traces of sulfur dioxide (i.e. 1–10 ppmV) alongside residual methane emissions induces the rapid deactivation of typical methane abatement catalysts. As a result, sulfur-induced poisoning is the main challenge faced by after-treatment technologies for residual methane abatement. The aim of this study was to synthesise a series of cobalt and molybdenum catalysts and assess their performance under realistic after-treatment conditions. The series of catalysts was characterised by BET, XRD, XPS, TPR, TGA under air flow, SEM, EDX and in situ DRIFTS. In addition, the performance of Co100 supported on an alumina hollow fibre was assessed under similar reaction conditions and characterised by SEM and EDX. It was found that the catalytic activity of the series of cobalt and molybdenum catalysts decreased in the following order: Co100 > Co80Mo20 > Co60Mo40. In addition, it was discovered that the other two catalysts (i.e. Co25Mo75 and Mo100) were inactive at 450 °C due to the lack of highly active Co3+ species. Among the active catalysts, Co100 and Co80Mo20 experienced an initial activation followed by a gradual deactivation whereas Co60Mo40 experienced a 1 h interval with stable methane conversion levels in between the activation and deactivation. The delay in poisoning of the active cobalt (II,III) oxide phase of Co60Mo40 was attributed to the sulfur sink-properties of the cobalt molybdenum oxide present in this catalyst (i.e. 44 wt%). Finally, Co100 supported on an alumina hollow fibre achieved the highest methane conversion per unit mass of catalyst and its performance was stable for a duration of 2.5 h. The high activity was attributed to the large surface area provided by the geometry of the support while the higher resistance to sulfur poisoning was credited to the sulfur sink-properties of the alumina, which delayed the poisoning of the active phase.

Synthesis of Cobalt Oxide on FTO by Hydrothermal Method for Photoelectrochemical Water Splitting Application

Cobalt oxide thin films were successfully grown directly on fluorine-doped tin oxide glass substrates through a simple, green, and low-cost hydrothermal method. An investigation into the physicochemical characteristics and photoelectrochemical (PEC) properties of the developed cobalt oxide thin film was comprehensively performed. At various annealing temperatures, different morphologies and crystal phases of cobalt oxide were observed. Microflowers (Co3O4) and microflowers with nanowire petals (Co3O4/CoO) were produced at 450 °C and 550 °C, respectively. Evaluation of the PEC performance of the samples in KOH (pH 13), Na2SO4 (pH 6.7), and H2SO4 (pH 1) revealed that the highest photocurrent −2.3 mA cm−2 generated at −0.5 V vs. reversible hydrogen electrode (RHE) was produced by Co3O4 (450 °C) in H2SO4 (pH 1). This photocurrent corresponded to an 8-fold enhancement compared with that achieved in neutral and basic electrolytes and was higher than the results reported by other studies. This promising photocurrent generation was due to the abundant source of protons, which was favorable for the hydrogen evolution reaction (HER) in H2SO4 (pH 1). The present study showed that Co3O4 is photoactive under acidic conditions, which is encouraging for HER compared with the mixed-phase Co3O4/CoO.

Cobalt Based Catalysts on Alkali-Activated Zeolite Foams for N2O Decomposition

In this work, we studied the effect of alkali-activated zeolite foams modifications on properties and catalytic activity of cobalt phases in the process of catalytic decomposition of N2O. The zeolite foam supports were prepared by alkali activation of natural zeolite followed by acid leaching and ion exchange. The cobalt catalysts were synthesised by a different deposition technique (direct ion exchange (DIE) and incipient wetness impregnation (IWI) method of cobalt on zeolite foams. For comparison, catalysts on selected supports were prepared and the properties of all were compared in catalytic tests in the pellet form and as crushed catalysts to determine the effect of internal diffusion. The catalysts and supports were in detail characterized by a variety of techniques. The catalyst activity strongly depended on the structure of support and synthesis procedure of a cobalt catalyst. Ion exchange method provided active phase with higher surface areas and sites with better reducibility, both of these factors contributed to higher N2O conversions of more than 80% at 450 °C. A large influence can also be attributed to the presence of alkali metals, in particular, potassium, which resulted in a modification of electronic and acid base properties of the cobalt oxide phase on the catalyst surface. The promotional effect of potassium is better reducibility of cobalt species.

The Initial Surface Structure of Co3O4 as a Factor of Influence on Kinetic Features of the Dissolution of the Solid Phase

Due to the depletion of natural reserves of cobalt-containing ores, it is urgent to optimize the processes of cobalt replenishment, based on the leaching of its compounds, processing of industrial waste, regeneration of cobalt compounds from catalysts. The kinetic characteristics of the dissolution of metal oxides are influenced by the initial state of their surface structure, which requires the accumulation of experimental material and individual targeted studies. The paper presents the results of an experimental study of the surface structure of industrial Co3O4 cobalt oxide samples. The studies were carried out by electron microscopy and X-ray phase analysis and revealed the size and shape of the Co3O4 particles and the nature of their surface distribution. The correspondence of experimental and theoretical data has been established, Co3O4 cobalt oxide has been identified as an α-form of the cubic crystal system. The data obtained are of interest in the study of the effect of the surface characteristics of the solid phase of Co3O4 cobalt oxide on the dissolution kinetics under the chemical and electrochemical influence

Cobalt oxide micro flowers derived from hydrothermal synthesised cobalt sulphide pre-catalyst for enhanced water oxidation

The rational design and development of the low-cost and efficient electrocatalysts towards oxygen evolution reaction (OER) are greatly influenced by the exposed surface area and structure of the electrocatalyst. Herein, we show a flower-like cobalt oxide/cobalt sulphide nanosheet electrocatalyst via a simple and cheap one-step hydrothermal synthesis for electrochemical water oxidation. Tuning the sulphur concentration influences the crystal formation and structural direction to achieve flower-like morphologies that greatly improves the surface area to benefit the OER. The best OER electrocatalysts were achieved on the CoS (1:0.5) electrode, which can deliver a current density of 10 mA cm−2 at a small overpotential of 383 mV, a low Tafel slope (38 mV dec−1) and remains constant for over 24 h. Mechanism study shows that during OER, the cobalt sulphide acts as a pre-catalyst and produces more active cobalt oxide phases with higher Co valence state when undergoing oxidation. This innovative design demonstrates the catalytic activity and overall efficiency which can be applied as an efficient non-noble metal electrocatalyst for OER.

Immobilization and activation of cobalt-amine catalyst on NH4OH-treated activated carbon for ethylene dimerization

Catalytic dimerization of ethylene (C2H4) to butene (C4H8) utilizes homogenous catalysts composed of transition metal molecular complexes in the presence of a co-catalyst or Ni, Rh or Cr supported heterogeneous catalysts. Recently, a heterogeneous cobalt catalyst supported on NH4OH-treated carbon (Co/NC) has gained attention. However, the structure of the active site, the identity of the active phase, the role of NH4OH treatment and the effect of thermal activation have remained elusive. Herein, in situ Raman spectroscopy showed the absence of crystalline cobalt oxide phases, such as CoO or Co3O4, in the working catalyst. An intense Raman band at 810 cm−1 was detected in the as-synthesized catalyst, which corresponds to μCo-peroxo species formed via auto-oxidation of the [Co(NH3)x]2+ sites during the drying step. UV–vis DRS absorbance bands at ∼286, 340 and 560 nm indicated that a mixture of octahedral Co3+ and Co2+ is present in the as-synthesized catalyst. During thermal treatment in inert atmosphere, the 265 and 810 cm−1 Raman bands diminished signifying the reduction of μCo-peroxo sites, with the oxidation of some NH3 ligands to N2O(g), indicated by an IR doublet at ∼2225 cm−1. UV–vis DRS during thermal treatment corroborated the reduction of Co3+ to Co2+, with Co2+ sites coordinated in a tetrahedral fashion, as indicated by a new absorbance band at ∼600−700 nm. The presence of the immobilized pseudo-tetrahedral [Co(NH3)x]2+ molecular complexes active for dimerization is proposed as an alternative to previous reports that attribute olefin dimerization activity to crystalline cobalt oxide phase in this catalyst. Steady-state catalytic test results at 1 bar, 80 °C, 5.0 % C2H4 at 10 sccm, 0.3 g catalyst show that model catalyst containing crystalline cobalt oxides (Co/C) exhibited similar initial butene productivity to the [Co(NH3)x]2+ containing Co/NC catalyst, but suffered sharper deactivation due to the irreversible olefin deposition on Co3+ sites.

Mixed metal oxides in synergy at nanoscale: Electrospray induced porosity of in situ grown film electrode for use in electrochemical capacitor

A heterogeneous film electrode with mixed metal oxides of nickel and cobalt constituting the active layer is developed by first electrospraying metal acetate precursors on nickel foam, followed by calcination. The formation of unique cubic phases of nickel oxide (NiO) and cobalt oxide (Co3O4) are confirmed through X-ray diffraction. The growth of two oxides in nanoscale is confirmed from high-resolution transmission electron microscopy, whereas the images from scanning electron microscope show synergistic growth leading to three dimensional porosity. The synergy is further demonstrated from the areal capacity of 267 mC cm−2 at 1 mA cm−2 for mixed oxide electrode, which is much higher than the respective values for pure NiO film (124 mC cm−2), and Co3O4 film (174 mC cm−2) electrodes. The faradaic behaviour of Co3O4 film electrode in cyclic voltammetry and chronopotentiometry scans is significantly modified to a more capacitive charge transfer due to the presence of NiO, resulting in better rate capability. The electrochemical performance of film electrodes is augmented utilizing hybrid mode, where in situ grown carbon film based on electrospray coating of resorcinol formaldehyde on nickel foam serves as counter electrode. The hybrid cell delivers specific energy and specific power to the extent of 22.7 Wh kg−1, and 2.8 kW kg−1 respectively, with the capacitance retention of 89% after 2000 cycles at 10 mA cm−2.