Reactivity of Mixed Iron–Cobalt Spinels in the Lean Methane Combustion

Recent Advances in Co3O4-Based Composites: Synthesis and Application in Combustion of Methane

In recent years, it has been found that adjusting the organizational structure of Co₃O₄ through solid solution and other methods can effectively improve its catalytic performance for the oxidation of low concentration methane. Its catalytic activity is close to that of metal Pd, which is expected to replace costly noble metal catalysts. Therefore, the in-depth research on the mechanism and methods of Co₃O₄ microstructure regulation has very important academic value and economic benefits. In this paper, we reviewed the catalytic oxidation mechanism, microstructure regulation mechanism, and methods of nano-Co₃O₄ on methane gas, which provides reference for the development of high-activity Co₃O₄-based methane combustion catalysts. Through literature investigation, it is found that the surface energy state of nano-Co₃O₄ can be adjusted by loading of noble metals, resulting in the reduction of Co-O bond strength, thus accelerating the formation of reactive oxygen species chemical bonds, and improving its catalytic effect. Secondly, the use of metal oxides and non-metallic oxide carriers helps to disperse and stabilize cobalt ions, improve the structural elasticity of Co₃O₄, and ultimately improve its catalytic performance. In addition, the performance of the catalyst can be improved by adjusting the microstructure of the composite catalyst and optimizing the preparation process. In this review, we summarize the catalytic mechanism and microstructure regulation of nano-Co₃O₄ and its composite catalysts (embedded with noble metals or combined with metallic and nonmetallic oxides) for methane combustion. Notably, this review delves into the substance of measures that can be used to improve the catalytic performance of Co₃O₄, highlighting the constructive role of components in composite catalysts that can improve the catalytic capacity of Co₃O₄. Firstly, the research status of Co₃O₄ composite catalyst is reviewed in this paper. It is hoped that relevant researchers can get inspiration from this paper and develop high-activity Co₃O₄-based methane combustion catalyst.

Facile synthesis of multiphase cobalt-iron spinel with enriched oxygen vacancies as a bifunctional oxygen electrocatalyst

The multiphase cobalt-iron spinel was firstly synthesized via a facile cold plasma method and applied as a bifunctional electrocatalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Compared with the single-phase obtained by the traditional calcination method, the CoFe₂O₄ and Co₃O₄ phase were obtained by the plasma method. The multivalence states of cobalt and iron facilitated electron transport in electrochemical reactions. The plasma sample had a small particle size (5 nm) due to the low operation temperature. Notably, electron impact produced more oxygen vacancies and a larger surface area on Co_xFe_yO₄, which increased the active sites and electronic conductivity. Electrochemical investigations indicated that the multiphase spinel obtained with a quasi-four-electron transfer process showed an onset potential of 0.76 V versus the RHE for the oxygen reduction reaction. In the oxygen evolution reaction, the potential of current density at 10 mA cm^-2 was 1.53 V versus RHE. As for the overall electrocatalytic activity, the multiphase spinel had a ΔE (the difference between E₁₀(OER) and E_1/2(ORR)) of 0.89 V, exhibiting greater bifunctional activity than the other prepared catalysts.

Synergy effect of CuO on CuCo2O4 for methane catalytic combustion

Spinel oxides (AB₂O₄) have been widely studied as catalysts for methane combustion. Increasing attention was focused on the catalysis properties of the [B₂O₃] octahedron; however, the role of the [AO] tetrahedron in the catalytic activity was seldom discussed. Herein, a series of (CuO)_x–CuCo₂O₄ (x = 0, 0.1, 0.2) composite oxides were synthesized by a solvothermal method. The structure, morphology, and physicochemical properties of the as-synthesized samples were characterized by the XRD, SEM, BET, and XPS techniques. The results of the catalytic activity tests showed that the coexistence of CuO with CuCo₂O₄ can improve the catalytic activity. The XPS results demonstrated that there were remarkable Cu⁺ ions present in the composite oxides, which can cause increases in the number of oxygen vacancies on the surface of the catalysts. In addition, the redox of Cu⁺ and Cu²⁺ may improve the oxygen exchange capacity for methane oxidation.

CuO and CuCo₂O₄ exhibit a synergistic effect in catalyzing methane combustion, which increases the oxidation rate of methane on the surface of (CuO)_0.2–CuCo₂O₄ composite oxide and decreasing the methane combustion temperature.

Selectivity of Mixed Iron-Cobalt Spinels Deposited on a N,S-Doped Mesoporous Carbon Support in the Oxygen Reduction Reaction in Alkaline Media

One of the practical efforts in the development of oxygen reduction reaction (ORR) catalysts applicable to fuel cells and metal-air batteries is focused on reducing the cost of the catalysts production. Herein, we have examined the ORR performance of cheap, non-noble metal based catalysts comprised of nanosized mixed Fe-Co spinels deposited on N,S-doped mesoporous carbon support (N,S-MPC). The effect of the chemical and phase composition of the active phase on the selectivity of catalysts in the ORR process in alkaline media was elucidated by changing the iron content. The synthesized materials were thoroughly characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman spectroscopy (RS). Detailed S/TEM/EDX and Raman analysis of the phase composition of the synthesized ORR catalysts revealed that the dominant mixed iron-cobalt spinel is accompanied by minor fractions of bare cobalt and highly dispersed spurious iron oxides (Fe2O3 and Fe3O4). The contribution of individual phases and their degree of agglomeration on the carbon support directly influence the selectivity of the obtained catalysts. It was found that the mixed iron-cobalt spinel single phase gives rise to significant improvement of the catalyst selectivity towards the desired 4e- reaction pathway, in comparison to the reference bare cobalt spinel, whereas spurious iron oxides play a negative role for the catalyst selectivity.

Influence of Different Birnessite Interlayer Alkali Cations on Catalytic Oxidation of Soot and Light Hydrocarbons

A series of layered birnessite (AMn4O8) catalysts containing different alkali cations (A = H+, Li+, Na+, K+, Rb+, or Cs+) was synthesized. The materials were thoroughly characterized using X-ray diffraction, X-ray fluorescence, X-ray photoelectron spectroscopy, Raman spectroscopy, specific surface area analysis, work function, thermogravimetry/differential scanning calorimetry, and transmission electron microscopy. The catalytic activity in soot combustion in different reaction modes was investigated (tight contact, loose contact, loose contact with NO addition). The activity in the oxidation of light hydrocarbons was evaluated by tests with methane and propane. The obtained results revealed that alkali-promoted manganese oxides are highly catalytically active in oxidative reactions. In soot combustion, the reaction temperature window was shifted by 195 °C, 205 °C, and 90 °C in tight, loose + NO, and loose contact conditions against uncatalyzed oxidation, respectively. The catalysts were similarly active in hydrocarbon combustion, achieving a 40% methane conversion at 600 °C and a total propane conversion at ~450 °C. It was illustrated that the difference in activity between tight and loose contacts can be successfully bridged in the presence of NO due to its facile transformation into NO2 over birnessite. The particular activity of birnessite with H+ cations paves the road for the further development of the active phase, aiming at alternative catalytic systems for efficient soot, light hydrocarbons, and volatile organic compounds removal in the conditions present in combustion engine exhaust gases.

Catalytic Performance of Palladium Supported on Sheaf-Like Ceria in the Lean Methane Combustion

Sheaf-like CeO₂ (CeO₂-S) in microscale was prepared by the hydrothermal method, and then etched with KOH aiming to obtain an imperfect fluorite structure (CeO₂-SK) with high content of oxygen vacancies and oxygen mobility. With CeO₂-S and CeO₂-SK as supports respectively, a modified colloidal deposition method was employed to obtain Pd/CeO₂ catalysts for being used in lean methane combustion. According to the inductively coupled plasma (ICP), N₂ physisorption and scanning electron microscopy (SEM) results, the Pd supported catalysts are very similar in their Pd loading, surface area and morphologies. SEM and transmission electron microscopy (TEM) results revealed various nanorods exposed CeO₂ (110) and (100) facets on Pd/CeO₂-SK surface after KOH etching. Raman spectra and H₂-temperature programmed reduction (H₂-TPR) results indicated that Pd/CeO₂-SK catalyst has a much higher content of catalytic active PdO species than Pd/CeO₂-S catalyst. It was also found that the catalytic performance of Pd/CeO₂ in lean methane combustion depends greatly upon the exposing crystal planes and oxygen vacancies content of sheaf-like CeO₂, and Pd/CeO₂-SK exhibits higher activity than Pd/CeO₂-S. The larger amount of CeO₂ (110) and (100) planes on Pd/CeO₂-SK surface can enhance the formation of oxygen vacancies, active Pd species and migration of lattice oxygen, which all evidently improve the redox ability and catalytic activity of the Pd/CeO₂-SK catalysts in lean methane combustion.

Synthesis, Characterization and Kinetic Behavior of Supported Cobalt Catalysts for Oxidative after-Treatment of Methane Lean Mixtures

The present work addresses the influence of the support on the catalytic behavior of Co₃O₄-based catalysts in the combustion of lean methane present in the exhaust gases from natural gas vehicular engines. Three different supports were selected, namely γ-alumina, magnesia and ceria and the corresponding catalysts were loaded with a nominal cobalt content of 30 wt. %. The samples were characterized by N₂ physisorption, wavelength dispersive X-ray fluorescence (WDXRF), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction with hydrogen and methane. The performance was negatively influenced by a strong cobalt-support interaction, which in turn reduced the amount of active cobalt species as Co₃O₄. Hence, when alumina or magnesia supports were employed, the formation of CoAl₂O₄ or Co-Mg mixed oxides, respectively, with a low reducibility was evident, while ceria showed a lower affinity for deposited cobalt and this remained essentially as Co₃O₄. Furthermore, the observed partial insertion of Ce into the Co₃O₄ lattice played a beneficial role in promoting the oxygen mobility at low temperatures and consequently the catalytic activity. This catalyst also exhibited a good thermal stability while the presence of water vapor in the feedstream induced a partial inhibition, which was found to be completely reversible.