Co-based Oxides Research Articles

Co3O4/Co-N-C modified ketjenblack carbon as an advanced electrocatalyst for Al-air batteries

Abstract Nitrogen-doped carbon materials containing non-precious metal (TM-N-C) and Co-based oxides have been extensively investigated as promising catalysts for oxygen reduction reaction (ORR). Herein, we report a novel Co3O4/Co-N-C modified ketjenblack carbon (KB) catalyst via a one-pot and scalable pyrolysis process using cheap melamine, cobalt acetate tetrahydrate and KB as raw materials. Owing to the high specific surface area and good electrical conductivity, this KB-based catalyst exhibits remarkable catalytic activity with a half-wave potential of 0.798 V (vs RHE) and a limiting current density of 5.10 mA cm−2 in alkaline solution, which are comparable with those of the commercial 20 wt% Pt/C. More importantly, it displays superior stability to Pt/C, which makes it one of the most promising non-noble-metal catalysts. Al-air batteries with this catalyst are also tested and generate a maximum power density of 161.1 mW cm−2, which is close to that with 20 wt% Pt/C catalyst (161.9 mW cm−2). After the discharge for 18 h at 50 mA cm−2, the voltage degradation of Al-air battery with Co3O4/Co-N-C modified KB is 7%, while that using Pt/C is increased to 12%. By virtues of its remarkable performance, low cost and simple fabrication method, Co3O4/Co-N-C modified KB here can be used as an efficient ORR cathode catalyst instead of the commercial Pt/C for practical Al-air batteries.

Activity of Water Oxidation on Pure and (Fe, Ni, and Cu)-Substituted Co3O4

Co-based oxides have recently emerged as suitable electrocatalysts for the water oxidation reaction due to their low cost and high activity. Here, we present a comparative study of the electrochemical behavior of pure and metal-ion-substituted cobalt oxides, Co3O4(110) and MxCo3–xO4(110), (M = Fe, Ni, Cu; x = 25% and 50%) in order to elucidate the effect of the cation distribution in the surface of the crystal lattice. We have used density functional theory calculations with on-site Coulomb repulsion of the energetics of the oxygen evolution reaction (OER) on these surfaces, and substantial work has been targeted to understand the relation between structure, mechanism, and activity. The activity of the substituted spinels increases relative to that of pure Co3O4, with a considerable decrease in overpotential values. This agrees well with the experimental findings. We find the activity of the Co-based oxides toward the OER is in the following order where we have considered that O2 evolution occurs at the s...

Iron-Based Spinel Oxide Nanoparticles for Electrocatalysis of Oxygen Evolution Reaction

The design of cost-effective and highly active, durable electrocatalysts for the sluggish oxygen evolution reaction (OER) is critical for promoting various energy conversion process such as water splitting and rechargeable metal-air batteries. Due to a wider selection of earth-abundance and low cost catalyst candidates (e.g., mixed 3d-transition metal oxides), OER in alkaline environment has attracted considerable interests during past years. Besides the recently developed Ni-Fe layered double hydroxides (LDH) catalysts, spinel phase transition metal oxides have also drawn particular interests owing to their well-defined rigid structure and thus potentially high stability during electrocatalysis. While improved OER activities have been demonstrated, currently most spinel transition metal oxides (mainly Co-based spinel oxides) still underperformed the state-of-the-art benchmark OER catalyst IrO2and the Ni-Fe LDH catalysts. Moreover, atomic insights into the surface structure that governs their OER reactivities and stabilities still remain very limited. We report here the exploration of iron-based spinel oxide nanoparticles (MxFe3-xO4, M= Mn, Fe, Co, Ni, Cu) as a new class of alkaline OER electrocatalysts with potentially both high electrocatalytic activity and high stability. Emphasis will be placed on atomistic understanding on the surface cation chemistry and surface atomic structures of the spinel oxide nanoparticles and their correlations with OER activities and stabilities. This will be achieved by comparative microscopic and spectroscopic studies of the nanoparticle surfaces before and after OER electrocatalysis using high-resolution (scanning) transmission electron microscopy, electron energy loss spectroscopy and X-ray photon spectroscopy, which will be further complemented by density functional theory calculations. The results will provide important insights into the structure-activity-stability relations of spinel phase transition metal oxides as well as other mixed metal oxides for OER electrocatalysts.

Controllable Fabrication of CoMn2O4 Microflowers and Microspheres As High-Performance Lithium Ion Battery Anode Material

The growing electric vehicle requirements have attracted great interest in development of lithium ion batteries (LIBs) with large gravimetric and volumetric capacities, long cycle lifespan. To meet these applications, high quality electrode materials are needed. However, conventional bulk electrode materials cannot fully reach the increasing demands due to their inherent limits in performance. In recent years, nanostructured electrode materials with higher rate capability have been acknowledged by low lithium-ion diffusion distance and high electrode/electrolyte area. However, electrode made from nanostructured electrode materials meets these challenges. (1) Nanoparticles tend to self-aggregate and the side reaction with the electrolyte due to high specific surface area, which lead to low initial coulombic efficiency and poor cycling life. (2) Nanoparticles have a huge volume changes during Li+ insertion/ extraction that result in fracture of nanostructured particle, delaminating of the conductive coating and low capacity with cycle increasing. (3) Nanostructured particles have low tap density which results in a low volumetric energy density.Therefore, the electrode materials with optimal structure should be developed to deal with above-mentioned problems. Micro-/nanostructures with controlled size and morphology have attracted considerable attention as high-performance anode materials for next-generation LIBs. This micro-/nanostructure should be a porous microstructure composed of primary nanocrystallines tightly compacted to form 3D channels for ion diffusion. Moreover, this structure can exploit many advantages of nanostructure for lithium ion batteries due to the reduction of side reactions with electrolyte, leading to high density and long cycling life with high safety. Some simple micro-/nano-structured metal oxides such as Mn-based oxides, Co-based oxides, Fe-based oxides and Sn-based oxides, have been recently reported for use in LIBs. Mn-based oxide electrodes (MnO, MnO2, Mn2O3, Mn3O4, ZnMn2O4, NiMn2O4 and CoMn2O4) are developed due to low operating voltages (1.3-1.5 V for lithium extraction) and high energy density. Among these Mn-based anodes, CoMn2O4 has been considered for substitution of the conventional graphite anode in LIBs. The shape and micro/nanostructure of CoMn2O4 material play an important role in the electrochemical performances. In this paper, uniform hierarchical porous CoMn2O4 microspheres and microflowers were realized by adjusting the urea concentration through a solvothermal process followed by a post-annealing treatment. In the low urea concentration, the microflower with diameter size of around 10 μm is composed of porous nanosheets with a thickness of about 20 nm. In the high urea concentration, a large amount of uniform CoMn2O4 microspheres is obtained with average diameter of about 8 μm. CoMn2O4 microsphere is highly porous and composed of numerous primary nanoparticles. As expected, the distribution of the O, Mn and Co elements is homogeneously distributed on the surface for CoMn2O4 microflower and microsphere. The actual molar ratios of Mn and Co for CoMn2O4 microflower and microspheres obtained from energy-dispersive X-ray (EDX) analysis are close to 2, which are consistent with the ratio of that in the precursor solution. The chemical compositional and crystalline structural changes of as-prepared samples were characterized using X-ray diffraction (XRD) and thermogravimetric analysis (TGA). These results show the precursor in the low urea concentration is a MnCo-complex structure (including CoMn-layered double hydroxide and CoMn-glycolates or alkoxide derivatives) in an alkaline environment under this solvothermal conditions. Meanwhile, in the high urea concentration, the precursor Co1/3Mn2/3CO3 sample is formed. The possible formation mechanism of as-prepared CoMn2O4 microsphere and microflower precursors is explained due to the presence of CO2 pressure into the reaction solution. When used as anode materials for lithium-ion batteries, the porous CoMn2O4 microflower and microspheres exhibited good long-life cycling performance at high rate density. At a current density of 100 mA g-1, the CoMn2O4 microflower and microspheres exhibit an initial capacity of 1200 and 1225 mA h g-1 and the capacity is maintained at 710 and 220 mAh g-1 after 50 cycles. Excellent rate capabilities (~813 mAh g-1 at 200 mA g-1 and ~612 mAh g-1 at 600 mA g-1) are observed for CoMn2O4 microspheres. Based on these results, the good electrochemical properties can result from the porous CoMn2O4 microspheres. On the one hand, the micro-/nano porous structured CoMn2O4 microspheres could significantly enhance the electrode integrity by buffering the volume changes with repeating Li+ insertion/ extraction, which is contributed to good cycling stability. On the other hand, the micro-/nano porous structured CoMn2O4 microspheres provide extra active position for Li+ storage and short Li+ ion diffusion length, contributing to the high specific capacity and better rate capability. The CoMn2O4 hierarchical porous microspheres, which have good electrochemical performance and ease of fabrication, hold the great promise as the anode materials for high-performance LIBs. Figure 1

Surface Microstructure of Nanoaluminized CoCrAlY Coating Irradiated by HCPEB

A thermal sprayed CoCrAlY coating was prepared by air plasma spray on the surface of Ni-based superalloy GH4169; then, a nanoscale aluminum film was deposited with electron beam vacuum deposition on it. The coatings irradiated by high-current pulsed electron beam were investigated. After HCPEB treatment, the Al film was remelted into the bond coat. XRD result shows that Al and Al2O3phase were recorded in the irradiated and aluminized coatings, while Co-based oxides which originally existed in the initial samples disappeared. Microstructure observations reveal that the original coating with porosity, cavities, and inclusions was significantly changed after HCPEB treatment as compact appearance of interconnected bulged nodules. Moreover, the grains on the irradiated coating were very refined and homogeneously dispersed on the surface, which could effectively inhibit the corrosive gases and improve the coating oxidation resistance.

Microstructure characterization of Co–20Cr–(5,10)Al oxide dispersion strengthened superalloys

In order to develop Co-based oxide dispersion strengthened (ODS) superalloys with excellent mechanical properties and oxidation resistance at the temperature above 1000°C, Co–20Cr–(5,10)Al ODS superalloys were designed and fabricated by mechanical alloying (MA), spark plasma sintering (SPS), hot rolling and the final annealing at 1200°C. Two different phases were identified; the Co solid solution of a fcc structure, and the B2 phase highly enriched with Aluminum. Composition and volume fraction of both phases are almost consistent with the Co–Cr–Al ternary phase diagram computed with the Pandat software. Another coarse fcc phase with smaller content of oxide particles was also characterized. This phase was probably formed through an incipient melting at the initial stage of SPS process. Fine oxides particles are densely distributed inside both, the fcc solid solution and the B2 phase. Grain boundaries and phase boundaries are decorated with the oxide particles, which are beneficial to grains and phases stability at elevated temperature. The observed unique core–shell structure was characterized as Y2Hf7O17 type in a core and Y2Hf2O7 type at a periphery of the oxide particles.

Facile synthesis of MgCo2O4 nanowires as binder-free flexible anode materials for high-performance Li-ion batteries

MgCo2O4 nanowires are synthesized for the first time through two-step synthesis method, followed by annealing of the MgCo2O4 precursors. The microstructure and morphology of MgCo2O4 nanowires are examined by powder X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy. MgCo2O4 nanowires give rise to a BET surface area of 45.1 m2 g−1 and the adsorption average pore size of 11.5 nm. When tested as an anode for lithium-ion batteries, the MgCo2O4 nanowires electrode exhibit exceptional properties in terms of specific capacity, cycling performance, and rate capacity compared with previously reported Co-based binary metal oxides. For instance, when the current densities increase from 5, 10, 15, and 20 A g−1, the discharge capacities of the MgCo2O4 nanowires electrode are about 649, 348, 188, and 121 mAh g−1, respectively. The enhanced electrochemical performance of the MgCo2O4 nanowires can be mainly attributed to the nanostructures which lead to decreased lithium-ion diffusion distances and increased active sites for Li insertion/extraction reactions.

A Novel Hybrid Chemical-Looping Oxy Combustor Process for the Combustion of Solid and Gaseous Fuels: Thermodynamic Analysis

The larger reactor volume, additional oxygen polishing unit, and carbon stripper for the separation of oxygen carriers and ash in the chemical looping combustion (CLC) and/or chemical looping oxygen uncoupling (CLOU) processes for solid fuels are anticipated not only to incur operational complexity but also to increase the capital and operating costs. As an alternative, this paper proposes a novel hybrid process, called “Chemical Looping Oxy Combustor (CLOC)”. This novel process provides an integration of chemical looping air separation (CLAS) with fluidized bed oxy-fuel combustion and is expected to eliminate the need for an additional oxygen polishing unit and carbon stripper. It can be retrofitted to any existing coal circulating fluidized bed (CFB) at low cost. The other advantages of CLOC includes less solid handling issues, flexibility in handling low-grade coal with high moisture, no/less contamination of oxygen carriers, no/less slip of CO2/SOx in an air reactor, low energy penalty, etc. Also, in the CLOC process, coal combustion will occur in a separate fluidized bed combustor with relatively faster kinetics, because of the availability of high oxygen concentration (i.e., ∼25–28 vol %), which eliminates the need for a larger fuel reactor volume. In the current paper, thermodynamic simulations of CLOC process using Cu-, Mn-, and Co-based metal oxide oxygen carriers were performed. Their performances were also compared against the conventional air-firing and oxy-firing technologies, e.g., oxy-fuel combustion integrated with cryogenic air separation unit (CASU) and CLOU. It was identified that the CLOC process needs external heat for reduction reactor provided by either direct or indirect methane combustion. Moreover, a maximum plant thermal efficiency was achieved for CLOC using Cu-based oxygen carrier. The energy penalty of the CLOC process, compared with the air-firing base case, was found to be ∼2%–3%, which is ∼4–5 times smaller than those of the CASU cases and only half of that of the CLOU process, indicating that CLOC offers a promising option for the combustion of solid fuels.

Solvent-free oxidation of ethylbenzene over hierarchical flower-like core–shell structured Co-based mixed metal oxides with significantly enhanced catalytic performance

Hierarchical flower-like core–shell structured Co-based mixed metal oxides exhibited excellent catalytic performance in solvent-free oxidation of ethylbenzene.

Surface modification of CoCrAlY coating by high-current pulsed electron beam treatment under the “evaporation” mode

High current pulsed electron beam (HCPEB) was conducted on a CoCrAlY coating. The microstructure evolution and surface morphology of the coatings were characterized by means of X-ray diffraction (XRD), scanning electron microscope (SEM), and three-dimensional laser scanning microscope (LSM), respectively. XRD result shows that Al2O3 and YAlO3 phases were recorded in the irradiated coatings. Co-based oxides which originally existed in the initial samples disappeared and the content of CoAl peaks significantly reduced as the pulses increased. Microstructure observations reveal that the coarse surface was melted and the structural defects inherent in the initial coatings were sealed after HCPEB irradiation. Surface features such as wavy aspects with abundant particulates as well as nano-scale bubbles and chemistry modifications were discussed as the consequence of the evaporation–condensation process.

Surface microstructure and high temperature oxidation resistance of thermal sprayed CoCrAlY coating irradiated by high current pulsed electron beam

A thermal sprayed CoCrAlY coating prepared by high-current pulsed electron beam (HCPEB) was investigated. Oxidation test of the coated specimens was conducted at 1050°C in air for 100hrs. The microstructure evolution and surface morphology of the coatings were analyzed by means of X-ray diffraction (XRD), scanning electron microscope (SEM), and 3D laser scanning microscope, respectively. XRD result shows that Al2O3 and YAlO3 phases were recorded in the irradiated coatings, but the Co-based oxides originally which existed in the initial samples disappeared. Microstructure observations reveal that the original coarse surface of the air plasma sprayed (APS) CoCrAlY coating was significantly changed after HCPEB irradiation, with a compact appearance of interconnected bulged nodules. Moreover, the grains on the irradiated coating were very fine and homogeneously dispersed on the surface. After high temperature oxidation test, a uniform and dense α-Al2O3 layer on "discontinuously" modified surface was obtained, and a quite negligible amount of Co and Cr was inflicted away from the layer. For the HCPEB irradiation of 30 pulses, the high temperature oxidation resistance of CoCrAlY coating was improved most dramatically.

Nanoscale Spin-State Ordering in LaCoO3 Epitaxial Thin Films

The nature of magnetic ordering in LaCoO3 epitaxial thin films has been the subject of considerable debate. We present direct observations of the spin-state modulation of Co ions in LaCoO3 epitaxial thin films on an atomic scale using aberration-corrected scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS), and ab initio calculations based on density functional theory (DFT) calculations. The results of an atomic-resolution STEM/EELS study indicate that the superstructure is not associated with oxygen vacancies; rather, it is associated with a higher spin state of Co3+ ions and their ordering. DFT calculations successfully reproduced the modulation of lattice spacing with the introduction of spin ordering. This result identifies the origins of intrinsic phenomena in strained LaCoO3 and provides fundamental clues for understanding ferromagnetism in Co-based oxides.

Hierarchical MnCo2O4 nanosheet arrays/carbon cloths as integrated anodes for lithium-ion batteries with improved performance.

To solve the reduced output voltage caused by the high lithium redox potential of Co3O4 when applied as an anode material in full cells, an effective strategy is to partially replace Co by Mn to form MnCo2O4 without changing the original crystal structure. Herein, 3D hierarchical MnCo2O4 nanosheets arrays grown via a hydrothermal method on carbon cloths, as binder-free anodes for lithium-ion batteries, exhibit a high areal capacity of 3.0 mA h cm(-2) at a current density of 800 μA cm(-2), excellent cycling stability, good rate performances and a discharge voltage plateau of 0.25 V which is lower than that of their Co3O4 nanosheet counterparts. Due to the increased output voltage of the full cell induced by the introduction of Mn species with a lower lithium extraction potential, MnCo2O4 based full cells display higher or comparative capacity in a certain voltage range compared with Co3O4, while still retaining the excellent conductivity of Co3O4 electrodes. Our work here paves the way for the design of high performance full cells with Co-based oxide electrodes.

Structural features and electrochemical properties of nanostructured ZnCo2O4 synthesized by an oxalate precursor method

As a Li-ion battery anode, the active substance with a porous nanostructure can be endowed with a high electrochemical performance because of its porosity and remarkable surface area. In this paper, the thermal decomposition of zinc–cobalt binary oxalate precursors, precipitated from a solvothermal medium of ethanol and water (75/25, v/v) at 100 °C, has been performed to synthesize phase-pure ZnCo2O4 spinels, thoroughly giving porous and rod-like configurations with an average length of a few micrometers. Interestingly, each of the as-obtained porous microrods has been well characterized to consist of ~35.2-nm single-crystalline nanoparticles with polydisperse interspaces. More interestingly, porous ZnCo2O4 microrods can deliver an initial specific discharge capacity of 1,293.7 mAh g−1 with the coulombic efficiency of 76.8 % at 0.2 A g−1, reaching a value of 937.3 mAh g−1 over 100 discharge–charge cycles. Even at a high current density of 2.0 A g−1, the porous ZnCo2O4 nanostructures can still possess a reversible discharge capacity of ~925.0 mAh g−1, further assigned to the synergistic effect of Zn- and Co-based oxide components. Anyway, the facile oxalate precursor method can realize the controlling synthesis of porous and rod-like ZnCo2O4 nanostructures with a high electrochemical performance.

Importance of Correlation in Determining Electrocatalytic Oxygen Evolution Activity on Cobalt Oxides

Co-based oxides are suitable electrode materials for the electrocatalytic oxygen evolution reaction (OER) with promising activity and stability, in addition to being widely available and relatively cheap. We investigate OER on Co3O4(001) and β-CoOOH (011̅2) surfaces using density functional theory calculations (DFT). We construct surface Pourbaix diagrams and investigate the theoretical overpotential for the elementary steps involved in OER on these surfaces. We show that inclusion of the Hubbard-U correction to DFT (DFT+U) is necessary to recover experimentally observed trends in the activity for the strongly correlated cobalt oxides. We find that the inclusion of the Hubbard-U correction lowers the activity of both Co3O4(001) and β-CoOOH(011̅2) when compared to results from pure DFT. In addition, the Hubbard-U correction shifts the location of Co3O4 and β-CoOOH from the strong binding leg to the weak binding leg of the OER volcano plot. The calculations also suggest that the theoretical overpotentials for Co3O4 and β-CoOOH are very nearly the same. We ascribe this to a similar local coordination environment of the active Co site in Co3O4 and CoOOH under OER conditions.

Pre-treatment and oxidation behavior of sol–gel Co coating on 430 steel in 750 °C air with thermal cycling

Research and development efforts continue to improve oxidation resistance and electrical conductivity of solid oxide fuel cell (SOFC) interconnects. This research evaluates the performance of a ferritic stainless steel (AISI 430 — a candidate SOFC interconnect material) with and without a Co coating (via sol–gel dip coating technique) with various pre-treatments, during cyclic oxidation exposures up to 750°C in laboratory air. A pre-treatment exposure of Co coated samples to a 750°C reducing atmosphere (prior to oxidation exposures) led to the formation of an effective Co-based spinel oxide coating that lowers oxidation rates by more than 40 times, and substantially lowers area specific resistance (ASR) in comparison to uncoated specimens. The development of effective sol–gel Co coatings with reducing pre-treatments, and their evolution within simulated SOFC interconnect environments is presented and discussed.

Microstructure and Hot Corrosion Properties of Laser Cladding of a Co-Base Alloy on the Leaf Surface of Gas Turbine

The temperatures inside the gas turbine reach up to 1000°C, alloys used for gas turbine components must be oxidation, and corrosion resistant, and stable in structure under high temperature circumstances. A Co-based alloy was cladded on the 1Cr18Ni9Ti stainless steel surface using a high power carbon dioxide laser. The microstructure evolution and hot corrosion properties of samples in 75%Na2SO4+25%NaCl saline were investigated. The results show that the microstructures of the cladded layer is fine, and the hot corrosion resistance of the cladded layer was significantly improved because of the formation of a protective oxide film of CoO and CoO•Cr2O3. Under high temperature corrosive atmosphere, the high content of Co promoted the formation of the protective oxide film. The refinement of dendritic structures and the formation of Co-based alloy oxides lower the penetration rate of the sulphur ions that induce the intergranular corrosion.

Catalysis of Cu-Doped Co-Based Perovskite-Type Oxide in Wet Oxidation of Lignin To Produce Aromatic Aldehydes

The aim of the present work was to investigate the influence of Cu substitution on the physicochemical properties of LaCo1−xCuxO3 (x = 0, 0.1, and 0.2) prepared by the sol−gel method, to clarify the relation between perovskite properties and their catalytic behaviors in the catalytic wet aerobic oxidation (CWAO) of lignin. X-ray diffraction (XRD), temperature-programmed reduction of H2 (H2-TPR), and X-ray photoelectron spectroscopy (XPS) indicated that Cu and Co were finely dispersed on the perovskite framework of the samples and an interaction between the Cu2+ and Co3+ sites in the perovskite lattice occurred. The percentages of surface chemisorbed oxygen species were 64.2, 75.2, and 81.1%, when x = 0, 0.1, and 0.2, respectively, obtained from O 1s XPS, indicating that the content of chemisorbed oxygen was improved with the increased content of Cu2+ in the mixed oxides. The samples were applied to the CWAO of lignin, and the experiments illustrated that the catalyst activity improved with an increase in the Cu content. A mechanism was proposed that anion vacancies were generated after Cu incorporation into the LaCoO3 catalyst, which promoted the amount of adsorbed oxygen surface active site [Co(surf)3+O2−] species and improved the yield of activated species [Cu(surf)2+O2−]. Under the combined effect of these factors, the catalytic activity of the Cu-substituted catalyst improved in comparison to the LaCoO3 catalyst as the Cu content was increased.

Neutron diffraction studies and the magnetism of an ordered perovskite: Ba2CoTeO6

The complex perovskite Ba(2)CoTeO(6) (BCTO) has been synthesised, and the crystal structure and magnetic properties have been investigated using a combination of X-ray and neutron powder diffraction, electron microscopy and dielectric, calorimetric and magnetic measurements. It is shown that at room temperature this compound adopts the 6L-trigonal perovskite structure, space group P3[combining macron]m (s.g. 164) (a = 5.7996(1) A, c = 14.2658(3) A). The structure comprises dimers of face-sharing octahedra as well as octahedra which share only vertices with their neighbours. Dielectric measurements indicate a diffuse transition of antiferroelectric nature near 280 K. A long-range antiferromagnetically ordered state has been identified from neutron diffraction and magnetic studies. The magnetic diffraction peaks were registered below the magnetic transition at about 15 K and a possible model for the magnetic structure is proposed. The structural and magnetic features of this compound are discussed and compared with those of other Co-based quaternary oxides adopting the perovskite structure.

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

Co x Mg 3 − x /Al composite oxides ( xCoMAO-800) were prepared by calcination of Co x Mg 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, N 2 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.