Cobalt Spinel Research Articles

Influence of preparation method on dispersion of cobalt spinel over alumina extrudates and the catalyst deN2O activity

A series of supported catalysts of the Co3O4 spinel active phase dispersed over alumina extrudates (9×25mm) was prepared by several methods (incipient wetness impregnation with Co(NO3)2 and CoCl2, with glycerol-assisted impregnation with Co(NO3)2, combustion synthesis, and two variants of spray deposition with Co(NO3)2 on pristine and ammonia soaked extrudates). The catalysts were characterized by XRF, XRD, RS, UV–vis, SEM/TEM/EDX, and their catalytic deN2O activity was investigated in the temperature programmed surface reaction (TPSR) mode. The relation between the spinel active phase particle size and its radial dispersion over the alumina extrudate and the deN2O activity was revealed and quantified. For the assessment of the active phase utilization, the N2O concentration profile across the extrudates was calculated using Thiele modulus and compared with the radial distribution of the spinel. It was shown that the dispersion of spinel active phase exhibits optimal profile when the sample is obtained in the presence of the organic components of the precursor mixture (glycerol or urea). The obtained results were discussed in the context of practical implications for the development of an efficient, low-cost catalyst for the N2O abatement.

Spinel cobalt manganese oxide nano-composites grown hydrothermally on nanosheets for enhanced photocatalytic mineralization of Acid Black 1 textile dye: XRD, FTIR, FESEM, EDS and TOC studies

Spinel cobalt manganese oxide nano-composites were grown on nanosheets using acetate precursors in mono-ethylene glycol. Crystal structures and morphologies of nano-composites were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy and energy dispersive X-ray spectroscopy to characterize the element composition. Fourier transform infrared spectroscopy was used for structural characterization and UV–Vis diffuse reflectance spectra (UV–Vis DRS) for optical properties. XRD results showed tetragonal spinel cobalt manganese oxide (Co,Mn)(Co,Mn)2O4 and cubic spinel cobalt manganese oxide MnCo2O4.5 structural phases. The crystallite size calculated by the Scherrer’s equation was 17 nm. The morphological studies displayed the existence of 40–63 nm nano-powders grown on nanosheets with a good degree of crystallization. Optical properties of cobalt manganese oxide nano-composites exhibit absorbance edge, and the band gap calculated from UV–Vis DRS results was 1.78 eV. FTIR spectra indicated that hydroxyl and oxide groups were major active sites. The absorption bands observed at 656 and 568 cm−1 are related to stretching vibrations of Mn–O and Co–O, respectively. The photocatalytic activities of nano-composites for photocatalytic mineralization of Acid Black 1 textile dye showed an outstanding performance. Photocatalytic process yielded 91% total organic carbon removals within 2.5 h of irradiation. The enhanced photocatalytic activity was attributed to better charge separation of the photo-generated electron–hole pairs in nano-composite.

Thermochemical hydrogen production using manganese cobalt spinels as redox materials

Mn and Co spinels (Mn3-xCoxO4) were thermally reduced and subsequently oxidized showing successful hydrogen production from water splitting by a three-step thermochemical cycle involving sodium hydroxide (NaOH). The spinel materials overcome the main limitation of the Mn2O3/MnO redox cycle, reducing the required temperatures from 1300–1400 °C to 850–1050 °C. Additionally, the reduction process takes place through a single step reaction, avoiding the formation of intermediate species that makes much more complex the chemistry of the Mn2O3 redox cycle. On the other hand, the subsequent reaction with NaOH allows a hydrogen production of 52.5 cm3 STP/gmaterial·cycle, which is comparable to the obtained with other spinel-oxide cycles at similar temperature. The cyclability and stability of the hydrogen production with these materials after operation of several cycles have been assessed in a high temperature tubular furnace.

Cation distribution and microwave absorptive behavior of gadolinium substituted cobalt ferrite ceramics

In the first step, gadolinium substituted cobalt ferrite nanoparticles with composition of CoFe2-xGdxO4 (x = 0–0.05 in a step of 0.01) were synthesized by a hydrothermal technique and without subsequent annealing. X-ray diffraction and field-emission scanning electron microscopy evaluations exhibited that single phase spinel ferrites with narrow size distribution were achieved. The EDS analysis confirmed that the applied process for fabrication of CoFe2-xGdxO4 nanoparticles is a proper process for the synthesis of spinel cobalt ferrites with homogeneity in composition. Mössbauer spectroscopy was utilized to distinguish the site preference of constitutive elements. A well-resolved doublets pattern of ferrite nanoparticles is mainly attributed to the superparamagnetic behavior. The results of magnetic hysteresis revealed that the gadolinium substituted cobalt ferrite nanoparticles exhibit superparamagnetic trend at a room temperature. The zero field cooled magnetization curves of gadolinium substituted cobalt ferrite nanoparticles showed that with an increase in gadolinium content, the blocking temperature increases. Fitting the experimental data of susceptibility with the Neél-Brown model expresses that there are powerful interactions between nanoparticles of gadolinium substituted cobalt ferrite and the Neél-Brown model was not operate for such ferrites. In the second step, the gadolinium substituted cobalt ferrite nanoparticles were annealed and the structural characteristics, magnetic properties and microwave absorptive behavior were evaluated. The Mössbauer spectras show a well-resolved six-line pattern of ferrite nanoparticles which is mostly imputed to the ferrimagnetic treatment. The magnetic hysteresis loops at a room temperature confirmed that the coercive field increased from 802 Oe for x = 0 to 965 Oe for x = 0.02 and then at higher contents, decreased. The magnetic hysteresis curves of CoFe1.98Gd0.02O4 for fields applied parallel and perpendicular to the sample axis, expresses high value of magnetocrystalline anisotropy field of 13 kOe which exhibits that the resonance frequency can be shifted to higher frequency. Consequently the microwave absorber can be covered a wide-band of electromagnetic waves in the X-band.

As-grown enhancement of spinodal decomposition in spinel cobalt ferrite thin films by Dynamic Aurora pulsed laser deposition

Cobalt ferrite CoxFe3−xO4 thin films with composition within the miscibility gap were grown using Dynamic Aurora pulsed laser deposition. X-ray diffraction patterns reveal as-grown phase separation to Fe-rich and Co-rich phases with no post-deposition annealing. The interconnected surface microstructure of thin film shows that this phase separation occurs through spinodal decomposition enhanced by magnetic-field-induced ion-impingement. The lattice parameter variation of the thin films with the magnetic field indicates that the composition fluctuations can be enhanced further by increasing the magnetic field. Results show that spinodal decomposition enhancement by magnetic-field-induced ion-impingement is independent of the lattice-mismatch-induced strain. This approach can promote spinodal decomposition in any thin film with no post-deposition annealing process.

Thermodynamic stability of cobalt oxide's low-index surfaces from density functional theory calculations

Low-index surfaces and morphologies of cobalt spinel (Co3O4) are studied using density functional theory calculations. For each of the surfaces (100), (110), and (111), a set of terminations with different surface compositions and atomic structures are assessed to study their thermodynamic stability under different environmental conditions. Surface energies are calculated as functions of oxygen's chemical potential, which in turn is a function of temperature and partial pressure. We select seven typical shapes of cobalt oxide nanoparticles and compute their free energies of formation using the surface energies of all the surrounding facets. The results show that the shapes of octahedron and cuboctahedron have the lowest Gibbs free energies among the seven shapes for all the environmental conditions being explored.

Hydrogen generation via ethanol steam reforming over Co/HAp catalysts

The catalytic activity of calcium hydroxyapatite (HAp) supported cobalt nanoparticles in ethanol steam reforming (SRE) was investigated. Co was supported on hydrothermally prepared HAp by incipient wetness impregnation method. Co/HAp catalysts were characterized through XRD, FT-IR and Raman spectroscopy, TEM, SEM/EDS, N2 physisorption, TG and TPR-H2. Results showed that spinel cobalt oxide is reduced to CoO and Co and these species are responsible for catalytic activity for hydrogen production via SRE process. The main reactions over Co/HAp are incomplete steam reforming and dehydrogenation of ethanol. Reforming experiment over pre-reduced sample indicated a negative impact of H2 treatment on hydrogen production. The best catalytic properties (YH2 and CEtOH) were obtained over 5%Co/HAp catalyst.

Optimization of Pd catalysts supported on Co3O4 for low-temperature lean combustion of residual methane

A series of Pd/Co3O4 catalysts with increasing palladium loading in the range of 0.5–5wt.% was prepared by incipient wetness impregnation of Co3O4. Solution combustion synthesis with urea as a fuel was used to optimize the Co3O4 reactive support for palladium in the combustion of methane in lean conditions. The obtained catalysts were thoroughly examined by XRD, XPS, XRF, RS, FESEM, H2-TPR, TGA, and N2-BET techniques. The catalytic tests of CH4 combustion were performed for 0.5, 1, and 2vol.% CH4, with constant lambda value. The obtained results revealed that a sub-stoichiometric fuel-to-oxidizer ratio, 0.75, results in the most catalytically active Co3O4 phase. The important differences in the catalysts’ activity were apparent for the highest CH4 concentration, with the 3% Pd/Co3O4 being the most active catalyst. The observed activity was explained considering the physicochemical, spectroscopic, and microscopic characterization of the catalysts with the PdO nanocrystals surface distribution being the determining factor for the catalysts’ reactivity. A simple model accounting for the observed dispersion effect is proposed. The model is based on a two types of the interaction of the surface PdO active phase with the Co3O4 support. Firstly, cobalt oxide helps to remove hydroxyl species from the PdO surface, thus making the active sites more available for methane activation. Secondly, cobalt spinel provides lattice oxygen to the PdO phase, again helping to recreate active sites thereon.

Thermal stability and repartition of potassium promoter between the support and active phase in the K-Co2.6Zn0.4O4|α-Al2O3 catalyst for N2O decomposition: Crucial role of activation temperature on catalytic performance

The effect of calcination temperature of potassium promoted cobalt spinel catalyst supported on α-alumina extrudates was investigated for understanding its diverge role in N2O decomposition. The catalyst were thoroughly characterized by XRD, μRS, IR, XPS, X-ray μ-tomography, SEM, STEM-EDX techniques. Potassium stability and surface location were evaluated by species resolved thermal alkali desorption (SR-TAD). It was found that the potassium promoter is unevenly distributed between the spinel active phase and the support, due to different energetic stability of 2.4-2.6eV and 1.7eV, respectively. Strong beneficial influence of the K-dopant on the catalyst activity, observed for the catalysts calcined at 500°C, substantially attenuated upon increasing the calcination temperature above 600°C. It was accounted for by the migration of potassium from the support to the spinel nanocrystals (below 500°C) and subsequent formation of potassium cobaltate overlayer (above 600°C). Thus while the support is acting as a potassium reservoir, the spinel active phase acts as a potassium sink. The presence of the cobaltate shell around the spinel nanocrystallites was revealed by FTIR and SR-TAD techniques, and corroborated by STEM-EDX elemental mapping. The results point out that in the case of the supported cobalt spinel catalysts promoted by alkali careful selection of the proper activation conditions is crucial for their optimal performance.

Plasma deposited novel nanocatalysts for CO2 hydrogenation to methane

The catalytic hydrogenation of CO2 to methane has been receiving an increasing attention as a method to mitigate the concentration of CO2 in the atmosphere through the production of synthetic natural gas. Finding the effective catalyst is the key factor to make this process feasible. In the present work, hydrogenation of CO2 into CH4 over novel metal oxide nanocatalysts supported on wire gauzes made from kanthal steel (FeCrAl) has been studied. The series of thin films of active phase were prepared by plasma-enhanced metalorganic chemical vapor deposition (PEMOCVD), containing cobalt spinel (Co3O4), ruthenium oxide (RuO2), mixed oxides (RuO2/Co3O4) and iron oxide (Fe2O3). The activity of the prepared nanocatalysts was tested in the gradientless reactor in the temperature range of 150–500°C. The RuO2 catalyst revealed the highest both the yield of methane and the reaction rate, while the Fe2O3 catalyst promoted the CO production through reverse water-gas shift reaction. The results of the present studies are the foundation for further research on a new class of nanomaterials with high catalytic properties for the methanation reaction.

In operando XAS investigation of reduction and oxidation processes in cobalt and iron mixed spinels during the chemical loop reforming of ethanol

FeCo2O4and CoFe2O4nanoparticles have been studied as oxygen carriers for the chemical loop reforming of ethanol by usingin operandoX-ray absorption spectroscopy.

Structural, morphological and magnetic properties of Eu-doped CoFe2O4 nano-ferrites

Europium (Eu) doped spinel cobalt ferrites having composition CoEuxFe2−xO4 where x=0.00, 0.03, 0.06, 0.09, 0.12 were fabricated by co-precipitation route. In order to observe the phase development of the ferrite samples, thermo-gravimetric analysis was carried out. The synthesized samples were subjected to X-ray diffraction analysis for structural investigation. All the samples were found to constitute face centered cubic (FCC) spinel structure belonging to Fd3m space group. Scanning electron microscopy revealed the formation of nanocrystalline grains with spherical shape. Energy dispersive X-ray spectra confirmed the presence of Co, Eu, Fe and O elements with no existence of any impurity. The magnetic hysteresis curves measured at room temperature exhibited ferrimagnetic behavior with maximum saturation magnetization (Ms) of 65emu/g and coercivity (Hc) of 966Oe. The origin of ferrimagnetism in Eu doped cobalt ferrites was discussed in detail with reverence to the allocation of Co2+ and Fe3+ ions within the spinel lattice. The overall coercivity was increased (944–966Oe) and magnetization was decreased (65–46emu/g) with the substitution of Eu3+. The enhancement of former is ascribed to the transition from multi domain to single domain state and reduction in lateral is attributed to the incorporation of nonmagnetic Eu ions for Fe, resulting in weak superexchange interactions.

Spinel cobalt–manganese oxide supported on non-oxidized carbon nanotubes as a highly efficient oxygen reduction/evolution electrocatalyst

Ultrasmall cobalt–manganese spinel oxide nanoparticles supported on non-oxidized carbon nanotubes were prepared as a highly efficient bifunctional electrocatalyst for oxygen reduction and evolution reactions.

Pd catalysts supported on Co3O4 with the specified morphologies in CO and CH4 oxidation

Firstly, the spinel cobalt oxide (Co3O4) was controllably synthesized with different morphologies (cubical, flower-like, plate-like and rectangular-like) via a hydrothermal method. Then Pd was loaded on them by incipient wetness impregnation. The properties of the samples prepared were characterized by XRD, H2-TPR, CO-TPR, SEM, TEM, HRTEM and XPS. The impacts of Pd addition on CH4 and CO oxidation were investigated under lean methane and CO atmosphere, respectively. The effectiveness of Pd was different for CH4 and CO oxidation. For methane oxidation, the activities of Co3O4 were enhanced due to Pd loading. And the orders of activity primarily kept consistent before and after impregnation, while the extent of improvement on activity from Co3O4 to Pd/Co3O4 was consistent with the order of surface Oads/Olat of Pd/Co3O4, indicating that different interaction occurred between Pd species unit cell and the Co3O4 unit cell with different crystal planes.In the case of CO oxidation, a significant decline on activity was observed over the Pd/Co3O4-R (rectangular-like Co3O4), Pd/Co3O4-F (flower-like Co3O4), Pd/Co3O4-P (plate-like Co3O4), while a distinct promotion on activity was observed over the Pd/Co3O4-C (cubical Co3O4). Similarly the variations in the activity of Pd/Co3O4 were ascribed to the shifts on surface Oads/Olat molar ratio and the specific area reaction rate.

Reduced Graphene Oxide Supported Nickel–Manganese–Cobalt Spinel Ternary Oxide Nanocomposites and Their Chemically Converted Sulfide Nanocomposites as Efficient Electrocatalysts for Alkaline Water Splitting

The electrolysis of water provides a powerful pathway for the storage and conversion of clean and renewable energy. Therefore, the development of earth-abundant, inexpensive, highly efficient electrocatalysts contributes a great deal to the overall efficiency of a water electrolytic system. Here, inspired by the low charge transfer resistance of mixed-valence cations, the favorable H atom binding energy of cobalt, and high electrical conductivity of graphene, we report a facile synthesis strategy to synthesize a spinel ternary oxide material consisting of nickel, manganese, and cobalt supported on reduced graphene oxide (rGO/NMC) with further conversion into a spinel ternary sulfide via a gaseous sulfurization protocol. The rGO/NMC-312 oxide material is found to be an efficient OER electrocatalyst with an overpotential as low as 320 mV for a current density of 10 mA cm–2, which is comparable to that of the state of the art OER catalysts. In addition, when used as HER electrocatalysts, the as-converted rGO...

Insights into electrode/electrolyte interfacial processes and the effect of nanostructured cobalt oxides loading on graphene-based hybrids by scanning electrochemical microscopy

Nanostructured cobalt oxide polymorphs (CoO and Co3O4) deposited via electrodeposition allowed optimal loading on supercapacitive graphene nanosheets producing a set of graphene-based hybrids namely, CoO/GO, CoO/ErGO, Co3O4/GO, Co3O4/rGO, and Co3O4/ErGO, as pseudocapacitive electrochemical electrodes. We gained fundamental insights into the complex physicochemical interfacial processes at electrode surfaces and electrode/electrolyte (or solid/liquid) interfaces by scanning electrochemical microscopy operating in the feedback probe approach and imaging modes while monitoring and mapping the redox probe (re)activity behavior. We determined the various experimental descriptors including diffusion coefficient, electron transfer rate, and electroactive site distribution on electrodes. We emphasize the interplay of (1) heterogeneous basal and edge plane active sites, (2) graphene surface functional moieties (conducting/semiconducting), and (3) crystalline spinel cobalt oxides (semiconducting/insulating) coated graphene, reinforcing the available electron density of states in the vicinity of the Fermi level contributing to higher electroactivity, faster interfacial diffusion, and shorter distances for electron transfer, facilitated through molecular and chemical bridges obtained by electrodeposition as compared with the physical deposition.

Sonochemical oxidation of vanillyl alcohol to vanillin in the presence of a cobalt oxide catalyst under mild conditions

The heterogeneous oxidation of vanillyl alcohol to vanillin was investigated on new grounds under eco-friendly conditions in the presence of hydrogen peroxide as an oxidant and water as solvent, coupled with low frequency ultrasonic irradiation. The sono-Fenton-like-assisted vanillyl alcohol oxidation was performed with a high-surface area nanostructured spinel cobalt oxide catalyst exhibiting small crystallites size. The catalytic reaction was also carried out under conventional heating conditions for comparison purposes. The influence of the reaction parameters, namely catalyst loading and hydrogen peroxide concentration was studied with the aim of determining the optimum yield and selectivity to the desired vanillin product. The chemical effects of ultrasound (ability to generate hydroxyl radicals) along with increased mass transfer appeared to be key prerequisites for enhancing the efficiency of the process, while decreasing the overall energy consumption.

Cobalt and cobalt-iron spinel oxides as bulk and silica supported catalysts in the ethanol combustion reaction

Cobalt and mixed iron-cobalt spinel oxides CoxFe(3-x)O4 supported on SiO2 ‘IMP-Cox-500′ and their bulk analogues ‘CP-Cox-500′ were prepared via impregnation and co-precipitation methods respectively and calcined at 500°C. Thermogravimetric analysis ‘ATG/DTG’, X-ray Diffraction method ‘XRD’, Texture measurements ‘BET/BJH’ and Temperature Programmed Reduction technique ‘TPR’ were used for their characterization. Their catalytic behavior in ethanol combustion was investigated. Results showed that nanostructured oxide materials with spinel structure were obtained with a good dispersion of the active phase at the surface of silica in the case of IMP-Cox-500 compounds. The reduction behavior of the prepared samples is highly dependent on the oxide composition and interaction with the support; which is directly related to the catalytic behavior. The use of supported catalysts improves the catalytic performance and leads the reaction towards a total oxidation by decreasing greatly the formation of partial oxidation products. The obtained values of activation energies are relatively low in comparison with the literature data. A compensation effect was found by carrying out ethanol combustion through the spinel systems considered in this work.

Magnetic cobalt ferrite composite as an efficient catalyst for photocatalytic oxidation of carbamazepine.

A magnetic spinel cobalt ferrite nanoparticle composite (CFO) was prepared via an ultrasonication-assisted co-precipitation method. The morphological structure and surface composition of CFO before and after reaction were investigated by using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray, and Fourier transform infrared spectroscopy, indicating the consumption of iron oxide during photodegradation. X-ray photoelectron spectroscopy and vibrating sample magnetometry confirm the preparation of the ferrite nanoparticle composite and its magnetic properties. The prepared CFO was then used for the photocatalytic degradation of carbamazepine (CBZ) as an example of pharmaceuticals and personal care products (PPCPs) from aqueous solution. The effects of the nanocomposite dosage, contact time, and solution pH on the photodegradation process were investigated. More than 96% of the CBZ was degraded within 100min at 0.2g·L-1 CFO in the presence of UV light. The reactive species for CBZ degradation in the CFO/UV system was identified as hydroxyl radicals by the methanol scavenging method. Combined with the detection of leached iron ions during the process, the CBZ degradation mechanism can be presumed to be heterogeneous and homogeneous photocatalytic degradation in the CFO/UV system. Furthermore, iminostilbene and acridine were detected as intermediate products by GC-MS.

Surface oxygen dynamics and H2 oxidation on cobalt spinel surface probed by 18O/16O isotopic exchange and accounted for by DFT molecular modeling: facile interfacial oxygen atoms flipping through transient peroxy intermediate

Surface dynamics of reactive oxygen species (ROS) and hydrogen oxidation on cobalt spinel nanocatalyst, faceted predominantly with (100), were investigated by means of pulsed 16O2/18O2 isotopic exchange and thermoprogrammed surface reaction investigations, corroborated by periodic spin-unrestricted DFT-PW91+U modelling of the principal surface reaction molecular events. Three temperature windows [T < 350 °C (i), 350 °C < T < 700 °C (ii), and T > 700 °C (iii)] where identified and associated with diatomic oxygen species of superoxo (CoO–(O2)−–CoO, peroxo (CoT–(O2)2−–CoO) (i), and monoatomic metaloxo (CoT–O, CoO–O) nature (ii), and with oxygen vacancies VO (iii). A new oxygen isotopic exchange pathway was proposed that involves peroxy transient intermediates produced during ROS oxygen surface diffusion. Flipping of the supra- (RSO) and intra-facial (lattice) oxygen moieties within the [18Osup–16Oint]2− peroxy unit requires 0.39 eV only, opening an easy pathway for rapid isotopic exchange without explicit formation of energetically more costly oxygen vacancies. The latter may occur effectively at T > 700 °C. The catalytic activity of ROS species was probed by H2 oxidation reaction. The diatomic ROS reactivity (below 160 °C) is characterized by Ea = 16 kcal/mol, and for monoatomic species (between 160 °C and 300 °C) it falls to Ea = 9.2 kcal/mol. It was shown that suprafacial dehydroxylation of ROS generated water is energetically less costly (Ea = 1.15 eV) than intrafacial dehydroxylation (Ea = 1.71 eV) entailing removal of water associated with the lattice oxygen. Thus, the former may operate even at relatively low temperatures (below 300–350 °C). The appearance of significant amount of H216O in the reaction products is related to easy isotopic 18O/16O scrambling via transient peroxo intermediates, and is not diagnostic of direct involvement of the Mars van Krevelen mechanism.