Cyclic voltammetric studies of electroless cobalt in NaOH

The electrochemical behaviour of electroless cobalt in 1N NaOH was studied at 35, 60 and 85°C by cyclic voltammetry. Three anodic peaks corresponding to the formation of Co(OH)₂/CoO, Co₃O₄, Co OOH were observed. The observed cathodic peaks were due to the reduction of the higher cobalt oxides to Co(OH)₂ and also due to the reduction of Co(OH)₂ to cobalt. The variation of peak potential with sweep rate was found to be appreciable only in the case of anodic peak (I). The total charge Q_T, for the passivation to occur was obtained by estimating the area under the first anodic peak at each sweep rate. It was found that the Q_T value decreases with increase of sweep rate. The increase of peak current with increase of sweep rate has been explained as due to the insufficient time available for the film growth at the reversible potential, and that the dissolution of the metal continues until a potential at which the rate of film growth exceeds that of active dissolution is reached. It has been suggested that the passivating film is CoO while the prepassive film is Co(OH)₂. The shift of the anodic peak (I) potentials to more anodic values compared to that for pure cobalt is probably due to the presence of phosphorus in the cobalt deposit.     

Vacuum evaporated thin films of mixed cobalt and nickel oxides as electrocatalyst for oxygen evolution and reduction

Mixed cobalt and nickel oxides, obtained by vacuum coevaporation of Co, Ni and TeO₂ are investigated as electrocatalysts for oxygen reduction and evolution reaction. Gas-diffusion bifunctional oxygen electrodes (GDE) are prepared by direct deposition of catalyst on gas-supplying membrane. Thus obtained GDE with different atomic ratio R_Co/Ni and R_(Co+Ni)/Te of the catalyst are electrochemically tested by means of steady-state voltammetry. It is shown that the films exhibit high catalytic activity toward both oxygen reduction and evolution reactions despite very small catalyst loading of about 0.07 mg cm⁻².     

Comportement electrochimique des oxydes mixtes MexCo1 − x Mn2O4 (Me = Ni,Cu; 0,75 ⩾ x ⩾ 0,25)

It is demonstrated through the electrochemical reduction of cobalt manganese spinels that it is possible to increase the cathodic reactivity by replacement of cobalt ions by nickel and copper cations. The reduction reaction occurs on active sites formed by Mn⁴⁺ ions associated, in octahedral sites, with Mn³⁺ ions, for the Ni_xCo_{1 ? x} Mn₂O₄ compounds. For the copper manganites oxides, Cu_xCo_{1 ? x}Mn₂O₄, the electrochemical reaction is likely to occur by the redox on solid state between Mn³⁺ and Cu²⁺ cations.     

Electrochemical oxygen reduction on oxide catalysts

The process of oxygen electroreduction and adsorption in alkaline medium on nickel and cobalt oxides has been studied by the potentiodynamic and the disc-ring electrode methods. The oxide films were formed on the surface of metal specimens by thermal or electrochemical oxidation.After oxidation, the surface was examined by X-ray diffraction and electron diffraction. It was found that, depending on the oxidation conditions, simple or complex oxides are formed on the surface of metal specimens. Nickel and Cobalt oxide films obtained by thermal oxidation at t° ?300°C are more active than pure metals. The oxides of spinel structure Co₃O₄ and NiCo₂O₄ have an optimum activity. It is shown that oxygen adsorbed during the heat treatment of the specimens accelerates the process of the molecular oxygen reduction. On the oxides of spinel structure the oxygen reduction reaction proceeds to OH^?, whereas on simple nickel oxides the final product of oxygen reduction is HO^?₂.     

Fischer–Tropsch synthesis using Co/SiO2 catalysts prepared from mixed precursors and addition effect of noble metals

Fischer–Tropsch synthesis in Co/SiO₂ catalysts, which were prepared by mixed impregnation of cobalt (II) nitrate and cobalt (II) acetate, was studied under mild reaction conditions (Total pressure=1 MPa, H₂/CO=2, T=513 K). X-ray diffraction indicated that highly dispersed cobalt metal was the main active sites on the catalyst prepared by the same method. It was considered that the metallic crystallines, which were readily reduced from cobalt nitrate, promoted the reduction of Co²⁺ to metallic a state in cobalt acetate by H₂ spillover mechanism during the catalyst reduction process. The reduced cobalt, from cobalt acetate, was highly dispersed one and remarkably enhanced the catalytic activity. The addition of a small amount of Ru to this type of catalyst remarkably increased the catalytic activity and the reduction degree. Its turn over frequency (TOF) increased but the selectivity of CH₄ was unchanged. However, when Pt or Pd were added into catalysts, they exhibited a higher selectivity of CH₄. Although Pt and Pd hardly exerted an effect on cobalt reduction degree, they promoted cobalt dispersion and decreased the value of TOF. Characterization of these bimetallic catalysts suggested that a different contact between Co and Ru, Pt or Pd existed. Ru was enriched on the metallic cobalt surface but, Pt or Pd dispersed well in the form of Pt–Co or Pd–Co alloy.     

Effect of sintering temperature on microstructure and magnetic properties of double perovskite Y2CoMnO6

Polycrystalline double perovskite Y₂CoMnO₆ oxides ceramics sintered at four different temperatures from 1000?°C to 1300?°C have been fabricated by conventional sol-gel method. All the Y₂CoMnO₆ compounds are single phase with monoclinic structure (P21/n space group). The mean grain size grows significantly large and the shape becomes regular obviously with increasing sintering temperature. The effect of sintering temperature on magnetic properties of Y₂CoMnO₆ compounds has been studied in detail. We found that the oxygen vacancies are introduced by sintering at high temperature has a certain influence on the magnetic properties. Moreover, the magnetic entropy changes (-?S_M) as well as relative cooling power (RCP) in the double perovskite Y₂CoMnO₆ oxides ceramics around paramagnetic to ferromagnetic transition were also investigated.     

Preparation of metallic cobalt inside NaY zeolite with high catalytic activity in Fischer–Tropsch synthesis

Metallic cobalt clusters are synthesized inside the cages of NaY zeolite by a method including four steps, i.e.: (1) ion exchange, (2) precipitation of exchanged cobalt cations, (3) calcination and (4) reduction of the calcined oxide nano-particles inside the zeolite. As compared with that prepared by the ion exchange followed directly by calcination and reduction and that by the conventional impregnation method, the sample by this four-step method exhibits higher CO conversion and higher selectivity to n-C₁₀–C₂₀ paraffins in Fischer–Tropsch (FT) synthesis. The small metallic cobalt clusters inside the supercages of NaY zeolite probably account for the high catalytic performances.     

Catalytic combustion of methane over mixed oxides derived from Co-Mg/Al ternary hydrotalcites

Co_xMg_3 − x /Al composite oxides (xCoMAO-800) were prepared by calcination of Co_xMg_3 − x/Al hydrotalcites (x = 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, respectively) at 800 °C. The materials were characterized using XRD, TG-DSC, N₂ adsorption-desorption and TPR. The methane catalytic combustion over the xCoMAO-800 was assessed in a fixed bed micro-reactor. The results revealed that cobalt can be homogenously dispersed into the matrices of the hydrotalcites and determines the structure, specific surface areas and porosity of the derived xCoMAO-800 oxide catalysts. The thermal stability and homogeneity of the hydrotalcites markedly depends on the cobalt concentration in the hydrotalcites. The Co-based hydrotalcite-derived oxides exhibit good activity in the catalytic combustion of methane. The catalytic activity over the xCoMAO-800 oxides enhances with increasing x up to 1.5, but subsequently decreases dramatically as cobalt loadings are further increased. The 1.5CoMAO-800 catalyst shows the best methane combustion activity, igniting methane at 450 °C and completing methane combustion around 600 °C. The catalytic combustion activity over the xCoMAO-800 oxides are closely related to the strong Co-Mg/Al interaction within the mixed oxides according to the TG-DSC, TPR and activity characteristics.     

Electrocatalytic activity of manganese oxides prepared by thermal decomposition for oxygen reduction

The oxygen reduction reaction (ORR) was studied in KOH electrolyte on manganese oxides supported on Vulcan carbon (Mn_yO_x/C). The oxides were prepared by thermal decomposition of manganese nitrate at different conditions. The oxides were characterized by X-ray diffraction (XRD) and in situ X-ray absorption near edge structure (XANES). The electrochemical studies were conducted using cyclic voltammetry (CV) and steady state polarization measurements carried out with a thin layer rotating ring/disk electrode. XRD results showed that the manganese oxide prepared at 200 °C in air is formed by a major phase of β-MnO₂ and the polarization curves indicated the highest activity for this material. In situ XANES evidenced the occurrence of a redox process involving Mn(II)/Mn(III) and Mn(III)/Mn(IV) in the range of potentials of the CV measurements. The electrocalytic activity was related to the occurrence of a mediation process involving the reduction of Mn(IV) to Mn(III), followed by the electron transfer of Mn(III) to oxygen and by a disproportionation reaction of the HO₂⁻ species in the Mn_yO_x sites. In situ XANES results showed that the Mn(IV) species is MnO₂ and the Mn(III) is most likely MnOOH.     

Designing a structured catalyst for selective reduction of O<Subscript>2</Subscript> by hydrogen in the presence of NO

The influence of the promoter (Pd) modifying additives of oxides of rare-earth (La₂O₃, CeO₂) and transition (NiO, CuO) metal oxides on the catalytic activity of Co₃O₄/cordierite in reactions of O₂ and NO reduction by hydrogen was studied. Introducing Pd and rare-earth metal oxides into the composition of cobalt oxide catalyst results in an increase in its activity in H₂ + 1/2O₂ → H₂O, H₂ + NO → 1/2N₂ + H₂O reactions and an increase in selectivity upon oxygen reduction by hydrogen in the presence of nitric oxide, due possibly to a decrease in the strength of oxygen bounds with the surface and the formation of low-temperature forms of oxygen, which is not typical of unpromoted cobalt oxide catalyst. A structured Pd-Co₃O₄-La₂O₃/cordierite catalyst was developed that surpasses the commercial granulated silver-manganese catalyst used in industry to purify the technological gases used in the production of hydroxylamine sulfate of oxygen impurities with reference to activity and selectivity (in the process of oxygen reduction in the presence of nitric oxide), and to thermal stability.     

Cobalt and Copper Composite Oxides as Efficient Catalysts for Preferential Oxidation of CO in H2-Rich Stream

A series of Co–Cu composite oxides with different Co/Cu atomic ratios were prepared by a co-precipitation method. XRD, N₂ sorption, TEM, XPS, H₂-TPR, CO-TPR, CO-TPD and O₂-TPD were used to characterize the structure and redox properties of the composite oxides. Only spinel structure of Co₃O₄ phase was confirmed for the Co–Cu composite oxides with Co/Cu ratios of 4/1 and 2/1, but the particle sizes of these composite oxides decreased evidently compared with Co₃O₄. These composite oxides could be reduced at lower temperatures than Co₃O₄ by either H₂ or CO. CO and O₂ adsorption amounts over the composite oxides were significantly higher than those over Co₃O₄. These results indicated a strong interaction between cobalt and copper species in the composite samples, possibly suggesting the formation of Cu _x Co_3?x O₄ solid solution. For the preferential oxidation of CO in a H₂-rich stream, the Co–Cu composite oxides (Co/Cu = 4/1–1/1) showed distinctly higher catalytic activities than both Co₃O₄ and CuO, and the formation of Cu _x Co_3?x O₄ solid solution was proposed to contribute to the high catalytic activity of the composite catalysts. The Co–Cu composite oxide was found to exhibit higher catalytic activity than several other Co₃O₄-based binary oxides including Co–Ce, Co–Ni, Co–Fe and Co–Zn oxides.     

Promising high temperature thermoelectric properties of dense Ba2Co9O14 ceramics

Layered cobalt oxides have shown high thermoelectric properties. The n = 1 member of the Ba_n+1Co_nO_3n+3(Co₈O₈) family, Ba₂Co₉O₁₄, a new layered cobalt oxides family with Co(II) and Co(III) in the CdI₂ layers, has been synthesized by solid state reaction and sintered as dense ceramics (relative density ∼ 93%) by Spark Plasma Sintering. It presents promising p-type thermoelectric properties at high temperature. The dimensionless figure of merit ZT is 0.032 at 660 K and 0.04 at 1000 K, which is about one quarter to one third of the ZT value of Ca₃Co₄O₉ ceramics.     

Synthesis,Characterization and Application of Co–MgO Mixed Oxides in Oxidation of Carbon Monoxide

The present work deals with the synthesis of nanostructured Co–MgO mixed oxides with different weight ratios of cobalt by a facile co-precipitation method as a catalyst for low-temperature CO oxidation. The prepared samples were characterized by X-ray diffraction (XRD), N₂ adsorption/desorption (BET), Fourier transform infrared spectroscopy (FTIR), and transmission and scanning electron microscopies (TEM and SEM) techniques. The results revealed that inexpensive cobalt–magnesium mixed metal oxide nanoparticles have a high potential as catalyst in low-temperature CO oxidation. The Co–MgO mixed oxide with 30 wt.% cobalt had the highest activity. The results showed that the catalysts pretreated under O₂-containing atmosphere possessed higher activity compared to the catalyst pretreated under H₂ atmosphere. Co–MgO catalyst showed a good repeatability in reaction condition. The stability test exhibited that the Co–MgO mixed oxides were highly stable for CO oxidation over a 30 h time on stream in the feed gas containing a high amount of moisture and CO₂.     

Mesoporous hard-templated Me–Co [Me = Cu,Fe] spinel oxides for water gas shift reaction

Copper–cobalt and iron–cobalt oxides, as well as a cobalt oxide sample, were synthesized through the hard template (HT) route by using SBA-15 silica as the HT. Copper- and iron-containing materials with a Me/(Co + Me) atomic ratio of 9 and 17 mol% were obtained and characterized as to their structure, morphology, texture and redox properties by X-ray diffraction, FTIR spectroscopy, transmission electron microscopy, N₂-physisorption and H₂-temperature programmed reduction, respectively. All the oxides were tested in a fixed-bed reactor for the water gas shift reaction in the 200–350 °C temperature range. All the catalysts showed a spinel structure, with the copper ions occupying exclusively the tetrahedral positions in the Cu–Co spinels and the iron ions being present in both tetrahedral and octahedral positions in the Fe–Co spinels. Segregation (to a minor extent) of maghemite phase was detected only for the high-concentration iron–cobalt oxide. The materials were replicas of the topological structure of the template, the channels being void replicas of the former walls of the SBA-15 host and the oxide appearing as nanorods, arranged in a highly ordered way in the case of Cu–Co oxides. Compared to Co₃O₄, the copper-containing and the iron-containing spinels were easier and harder to reduce, respectively. While the catalytic activity of cobalt and iron–cobalt spinels was rather poor, a remarkable water gas shift activity, accompanied (to a minor extent) by methanation, was observed over Cu–Co spinels. The influence of the reduction features on the catalytic performance is discussed.     

Preparation of hierarchical leaf-like cobalt and enhanced magnetic properties by a new low-temperature synthesis method

Hierarchical leaf-like cobalt materials have been synthesized by a simple method at relatively low temperature. The product was characterized by means of XRD, SEM, EDS, and VSM techniques. The effects of temperature and cobalt acetate amount on the final Co were investigated by a series of experiments. It was found that the temperature played an important role in the formation of such novel leaf-like cobalt. When the reaction temperature of the mixture was as low as 40–65 °C, the morphology of final products can be changed from fluffy like to leaf like hierarchical structures. The leaf-like cobalt possessed hexagonal close-packed (HCP) phase structure. The hierarchical leaf-like cobalt exhibited high saturation magnetization (M_s) of 151.6 emu/g and coercivity (H_c) of 158.5 Oe. The low temperature chemical reduction method is quite simple, it will provide possibility for large scale preparation of such leaf-like cobalt. Due to the specific structure and magnetic properties, these cobalt leafs are expected to have potential applications as candidates for microwave absorption and sensors.     

In situ FTIR Study of Cobalt Oxides for the Oxidation of Carbon Monoxide

A high-valance cobalt oxide, CoO _x , was prepared from cobalt nitrate aqueous solution through precipitation with sodium hydroxide and oxidation by hydrogen peroxide. Further, other pure cobalt oxide species were refined from the CoO _x by temperature-programmed reduction (TPR) to 170, 230 and 300 °C. They were characterized by TPR and X-ray diffraction (XRD). Adsorption of CO and the co-adsorption of CO/O₂ over the cobalt oxides were further tested by in situ FTIR. It was shown that Co₃O₄ is quite active for the oxidation of CO at room temperature in the presence of oxygen, leading to the formation of CO₂. The variation in the oxidation of CO was interpreted with a mechanism involving two kinds of oxygen species, i.e., *-O₂ on the CoO _x surface and *-O_L on the surface of Co₃O₄ spinel structure.     

Producing superhard coatings by decomposition of complex salts in shaped-charge explosion

The behavior of two new complex salts containing cobalt and tungsten atoms was studied under different thermal conditions. It was found that when heated in air, the salt [Co(NH₃)₆](WO₄)NO₃ exploded at 260°C, and further heating to 800°C led to the formation of a mixture of oxide phases. Heating of [CoEn₃]₂(W₇O₂₄) · nH₂O was not accompanied by explosion and also led to the formation of cobalt and tungsten oxides. The complex salts were used to produce superhard (Vickers microhardness up to 35.8 GPa) coatings on titanium disks by means of shaped-charge explosion. High microhardness is associated with the formation of carbonitride crystalline phases on the target surfaces.     

Disproportionation of CO on iron-cobalt alloys—I: Thermodynamic study

Catalyst poisoning observed during carbon formation by CO disproportionation can be related to competing carbiding and oxidizing reactions. Hence the thermodynamic stability of the catalytic metal (Fe, Co or Ni) under the conditions of the disproportionation is essential to prevent the formation of oxides or carbides catalytically inactive. We show that alloying the metal, which decreases its thermodynamic activity, increases the (T, p_co, p_co2) domain of its stability. A thermodynamic approach of the carbon-oxygen-iron cobalt alloy system predicts that a maximum expansion of the domain of stability of the metallic phase would be obtained with an alloy containing 50% cobalt, 50% iron. These results are discussed by comparison with some previous experimental results.     

Effect of support morphology and Pd promoter on Co/SBA-15 for Fischer–Tropsch Synthesis

An influence of support morphology and Pd promoter on physicochemical properties and catalytic performance of Co/SBA-15 in Fischer–Tropsch Synthesis (FTS) was investigated. SBA-15(M) from a hydrothermal synthesis with decane addition had smaller particle size, larger pore size and shorter cavity length which enhanced the dispersion of cobalt oxides, eased diffusion of reactants and improved the FTS performance. 10Co/SBA-15(M) provided the highest and most steady conversions of CO and H₂ with the highest yield of C₅–C₉ products. The addition of Pd enhanced the reduction of cobalt oxides but produced more methane and light paraffins.     

Combined X-ray study of lithium (tin) cobalt oxide matrix negative electrodes for Li-ion batteries

The lithium insertion behaviours of the oxides Co₃O₄ and Co₂SnO₄ were studied using a range of electrochemical, spectroscopic and diffraction techniques. Co K-edge EXAFS studies on the Co₃O₄ oxide showed that the reversible lithium insertion is coupled with changes in cobalt oxidation state. On lithium insertion, Co₃O₄ is reduced to yield Co(II) and yields only metallic cobalt species on complete reduction. On lithium removal an oxide of Co is formed, which from coulometry should be CoO, however, EXAFS indicates the short range structure is quite different to that of the rocksalt CoO. The long range structure of the matrix is amorphous according to XRD. The EXAFS and XRD data also revealed that both the metallic and oxide phases were disordered, having low co-ordination numbers and large shell spacings, and that there was an initial reduction to CoO before full reduction to metallic Co. The electrochemical behaviour of Co₂SnO₄ cells was more reminiscent of that of SnO₂ than that of Co₃O₄, but did exhibit significant differences due to the presence of cobalt. EXAFS on Co₂SnO₄ cells revealed that Co is reduced to metallic cobalt on the initial discharge, but that it does not convert back to an oxide on cycling even though the electrochemical treatment was the same as for Co₃O₄. Together the EXAFS and Mössbauer data show that the Co and Sn are reduced concurrently, and that some of the Sn remains in the oxidised form. In summary, we have a surprising result in that the presence of the tin dramatically changes the redox behaviour of the cobalt. In a matrix derived from a cobalt oxide spinel, cobalt undergoes redox cycling, whereas in a matrix derived from a cobalt tin oxide spinel, the cobalt does not cycle whilst the tin does.