Cobalt Spinel Research Articles

Synthesis of Zn x Co3−x O4 spinels at low temperature and atmospheric pressure

In order to synthesize cobalt-based spinel oxides, air was bubbled through aqueous hydroxides suspensions of cobalt and zinc with atomic ratios of Co:Zn = 100:0, 85:15, 67:33 (precisely 2:1), 50:50, 30:70, 15:85, and 0:100 at 70 °C and atmospheric pressure. When cobalt was absent in the suspensions, hexagonal ZnO nanocrystals with ca. 82.2 nm size were observed, whereas cubic Co3O4 ones with ca. 52.7 nm were seen when zinc was not present. Zinc-contained cobalt spinel oxides, i.e., Zn x Co3−x O4 (x = 0–1), were obtained when both hydroxides were present, e.g., spherical nanoparticles with ca. 107.6 and 85.0 nm diameters were observed for Co:Zn = 50:50 and 67:33, respectively. The lattice constant, a, for the cubic spinel increased with the increase in zinc atomic %, suggesting the increase in zinc concentration in the spinel. The Zn x Co3−x O4 synthesized was normal spinel with a cubic crystal structure, whereas it was also suggested that a very small portion of zinc ion was incorporated into the octahedrally coordinated sites (inversion of spinel). BET surface area of the synthesized catalyst increased with increasing the cobalt atomic % except for Co:Zn = 67:33, for which local minimal surface area was obtained. Oxygen storage capacity of the catalyst was the largest for Co:Zn = 85:15 at 150 and 200 °C, whereas it was for 50:50 at 250 °C. After reducing the synthesized catalysts with hydrogen, metallic cobalt was formed on zinc oxide. CO chemisorption number on the cobalt was the largest for Co:Zn = 67:33, for which the smallest metallic cobalt diameter was also obtained. On the other hand, catalytic CO2 hydrogenation activity and methane selectivity were the highest for Co:Zn = 50:50, suggesting that zinc oxide as a basic support played an important role in the hydrogenation of acidic CO2. It was also shown that the catalytic hydrogenation activity and the methane selectivity were higher for the catalysts prepared by the present liquid-phase approach than for those prepared by conventional coprecipitation and impregnation methods, which were ascribed to larger surface area and number of active sites for the former preparation technique than for the latter two.

Silver‐Doped Cobalt (Magnesium) Aluminum Mixed Metal Oxides as Potential Catalysts for Nitrous Oxide Decomposition

AbstractN2O is a potent greenhouse gas released among others in nitric and adipic acid production as well as from stationary and mobile combustion sources. A catalytic decomposition is highly attractive but requires catalysts active over a broad temperature range and in the presence of inhibiting compounds such as NO and O2. Ag‐doped Co‐(Mg)‐Al‐Ox mixed metal oxides obtained by the coprecipitation of metal nitrate precursors followed by calcination were evaluated as potential catalysts for N2O decomposition (deN2O) also in the presence of NO and/or O2. Ag integration during synthesis proved to be essential for superior catalytic activity. The catalyst with the optimum composition, AgCo3Al‐Ox (Co/Al=3:1 mol %, 1.0 wt % Ag), achieved full N2O conversion at 350 and 450 °C under N2O/N2 and N2O/NO/O2/N2 conditions, respectively. Catalyst characterization, with a focus on H2 temperature‐programmed reduction and X‐ray photoelectron spectroscopy, evidenced that abundant Ag0 on the surface of (1.0–2.8 wt %)AgCo3Al‐Ox enhanced the reduction of cobalt spinels to enable the superior catalytic performance.

Comparison of iron and copper doped manganese cobalt spinel oxides as protective coatings for solid oxide fuel cell interconnects

MnCo2O4, MnCo1.7Cu0.3O4 and MnCo1.7Fe0.3O4 are investigated as coatings for corrosion protection of metallic interconnects in solid oxide fuel cell stacks. Electrophoretic deposition is used to deposit the coatings on Crofer 22 APU alloy. All three coating materials reduce the parabolic oxidation rate in air at 900 °C and 800 °C. At 700 °C there is no significant difference in oxidation rate between coated samples and uncoated pre-oxidized Crofer 22 APU. The cross-scale area specific resistance (ASR) is measured in air at 800 °C using La0.85Sr0.1Mn1.1O3 (LSM) contact plates to simulate the interaction with the cathode in a SOFC stack. All coated samples have three times lower ASR than uncoated Crofer 22 APU after 4370 h aging. The ASR increase with time is lowest with the MnCo2O4 coating, followed by the MnCo1.7Fe0.3O4 and MnCo1.7Cu0.3O4 coatings. LSM plates contacted to uncoated Crofer 22 APU contain significant amounts of Cr after aging, while all three coatings effectively prevent Cr diffusion into the LSM. A complex Cr-rich reaction layer develops at the coating-alloy interface during oxidation. Cu and Fe doping reduce the extent of this reaction layer at 900 °C, while at 800 °C the effect of doping is insignificant.

Synthesis and electrochemical investigation of spinel cobalt ferrite magnetic nanoparticles for supercapacitor application

Cobalt ferrite magnetic nanoparticles (CoFe2O4-MNPs) were synthesized by hydrothermal and co-precipitation methods using different precursors such as nitrates, chlorides, and acetates, at different concentrations with/without surfactant under different growth conditions. The structural and morphological analyses reveal the formation of a single-phase CoFe2O4 in nanoplatelet-shaped NPs with average particle size between 11 and 26 nm depending on synthesis condition. The specific surface area of these NPs obtained by hydrothermal method was ~ 34 m2 g−1. Electrochemical performances of the obtained nanoparticles in a three-electrode configuration with a 6 M KOH electrolyte revealed a specific capacitance (C s) of 429 F/g at 0.5 A/g, with excellent capacitance retention of 98.8% after 6000 cycles at 10 A/g for the electro-active NPs synthesized by hydrothermal method at 200 °C for 18 h.

Dioxygen Activation Pathways over Cobalt Spinel Nanocubes—From Molecular Mechanism into Ab Initio Thermodynamics and 16O2/18O2 Exchange Microkinetics

A unified molecular, thermodynamic, and kinetic picture of dioxygen activation, surface diffusion, and reactivity over stoichiometric and defected cobalt spinel (100) termination was provided by combination of GGA–DFT+U modeling and experimental isotopic exchange investigations using Co3O4 nanocubes. Various diatomic (CoO5c–O2––CoO5c superoxo, CoO5c–O22––CoT2c peroxo, (O–Osurf)2– peroxo) and monatomic (O–CoT2c and O–CoO5c metal-oxo) reactive oxygen species were described in detail regarding their electronic and magnetic structure. The band alignment diagrams between the pDOS of dioxygen and the exposed cobalt cations were constructed and used to rationalize the revealed pronounced speciation of the surface oxygen, depending on the adsorption geometry (monodentate η1, bidentate η2, bridging μ), varying extent of oxygen reduction (one, two, four electrons), and the entailed complex spin relaxation. It was shown that surface cobalt cations work in tandem constituting dual CoO5c–CoO5c and CoO5c–CoT2c sites fo...

Cobalt Spinel Nanocubes on N-Doped Graphene: A Synergistic Hybrid Electrocatalyst for the Highly Selective Reduction of Carbon Dioxide to Formic Acid

Carbon dioxide reduction into useful chemical products is a key technology to address urgent climate and energy challenges. In this study, a nanohybrid made by Co3O4 and graphene is proposed as an efficient electrocatalyst for the selective reduction of CO2 to formate at low overpotential. A comparison between samples with different metal oxide to carbon ratios and with or without doping of the graphene moiety indicates that the most active catalyst is formed by highly dispersed and crystalline nanocubes exposing {001} oriented surfaces, whereas the nitrogen doping is critical to obtain a controlled morphology and to facilitate a topotactic transformation during electrocatalytic conditions to CoO, which results in the true active phase. The nanohybrid made up by intermediate loading of Co3O4 supported on nitrogen-doped graphene is the most active catalyst, being able to produce 3.14 mmol of formate in 8 h at −0.95 V vs SCE with a Faradaic efficiency of 83%.

Structural and complex electromagnetic properties of cobalt ferrite (CoFe2O4) with an addition of niobium pentoxide

Niobium pentoxide (Nb2O5) was added to cobalt spinel ferrite (CoFe2O4) powders for the first time, at varying amounts of 0, 5, 10 and 15wt%. The purpose was to evaluate the effect of niobia on the crystalline phases, microstructure and complex electromagnetic behaviour (complex permittivity and permeability between 300MHz and 10GHz) of CoFe2O4. The samples were prepared by conventional ceramic methods and sintered at 1475°C, as potential applications are as aerospace materials (radomes) which have to survive at such temperatures upon re-entry. The only crystalline phase observed in all samples was CoFe2O4, but microstructural evaluation showed that a non-crystalline, niobium-rich intergranular region was formed between the grains with niobium addition, and this apparent liquid/glassy phase aided sintering as considerable grain growth was also observed. It was shown by Raman spectroscopy that this niobium-rich amorphous intergranular phase was FeNbO4. The electromagnetic measurements of complex permittivity (ε*) and permeability (μ*) measurements indicated a steady decrease in both permittivity and permeability with increasing niobium oxide addition, although the values for each sample were relatively stable between 300MHz and 10GHz. The real permittivity, ε′, decreased from ~ 12 in the pure CoFe2O4 to ~ 5.5 with increasing addition, while its imaginary part assumed values very close to zero. At the same time, the real permeability, μ′, decreased from ~ 1.4 to ~ 1.1, and a similar effect can be observed in the permeability curves. The results of the complex measurements also allowed us to obtain reflectivity graphs representing the energy loss of the incident electromagnetic wave when crossing the layer of the evaluated composition. The graphs are presented in the frequency domain, and indicate that the reflection loss increases with the addition of niobia.

Surface structure of cobalt, palladium, and mixed oxide‐based catalysts and their activity in methane combustion studied by means of micro‐Raman spectroscopy

AbstractThis paper reports the results of the study of surface properties and catalytic activity of cobalt, palladium, and mixed oxide catalysts supported on alumina. Catalytic materials were prepared by the wetness impregnation method directly from solutions of metal salts and using the ultrasound irradiation. The material surface was investigated by micro‐Raman spectroscopy, and the activity in methane combustion was monitored. The results indicated the presence of different metal oxides on the catalysts' surfaces. Cobalt spinel Co3O4 was the main phase of the cobalt catalysts as well as species resulted from interactions with alumina support. In the case of palladium catalysts, the PdO was most prevalent. The article also discusses the observed variation in band position in the Raman spectra and compares the band assignment with literature data. The palladium and mixed oxide catalysts showed average conversion higher than 50% for methane combustion at 673.15 K (400 °C) especially those after sonication.

Shape control of Co3O4 micro-structures for high-performance gas sensor

Recently, spinel cobalt oxide (Co3O4) structure has been widely investigated due to its excellent sensitivity towards various noxious gases and good response/recovery speed at low concentration. In this work, we designed and synthesized two kinds of different Co3O4 micro-structure (cube and octahedron) with a similar size. After fabricating them into gas sensors, we found that the crystal plane structure of Co3O4 has an important effect on its gas sensing performance. Furthermore, the {111} planes of Co3O4may be more sensitive than {100} planes to various testing gases. Co3O4 octahedrons micro-structure exhibits an excellent sensitivity (about 12.6), good response/recovery speed and cycling stability (no decline even after 2 days) under 50ppm ethanol gases at working temperature of 200°C. As such, thisCo3O4 octahedrons micro-structure is a promising candidate for a high-performance gas sensing material.

Insights into perovskite-catalyzed peroxymonosulfate activation: Maneuverable cobalt sites for promoted evolution of sulfate radicals

Metal-based catalysis has significantly contributed to the chemical community especially in environmental science. However, the knowledge of cobalt-based perovskite for aqueous phase oxidation still remains equivocal and insufficient. In this study, we discovered that Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) perovskite was exclusively effective for peroxymonosulfate (PMS) activation to produce free radicals, whereas the BSCF was inert to activate peroxydisulfate (PDS) and hydrogen peroxide. The BSCF/PMS exhibited superior performance to the benchmark Co3O4 nanocrystals and other classical PMS activators such as α-MnO2 and spinel CoFe2O4, meanwhile achieving an impressive stability with manipulated cobalt leaching in neutral and basic environment. In situ electron paramagnetic resonance (EPR) revealed the evolution of massive sulfate radicals (SO4−) and hydroxyl radicals (OH) during the oxidation. A comprehensively comparative study of BSCF and Co3O4 nanocrystals was performed, including electrochemical impedance spectroscopy (EIS) and cyclic voltammograms (CV) in PMS solution as well as hydrogen temperature-programmed reduction (H2-TPR) and oxygen temperature-programmed desorption (O2-TPD) tests. The results unveil that the cobalt-based perovskite, BSCF, exhibited a better electrical conductivity and redox potential than the spinel cobalt oxide to interact with PMS. More importantly, the oxygen vacancies and less-electronegativity A-site metals may secure cobalt sites with a lower valence state for donating electrons to PMS simultaneously for radical generation. This study advances the mechanism of cobalt-based heterogeneous catalysis in environmental remediation.

Impact of grain size and structural changes on magnetic, dielectric, electrical, impedance and modulus spectroscopic characteristics of CoFe2O4 nanoparticles synthesized by honey mediated sol-gel combustion method

In this work CoFe2O4 spinel ferrite nanoparticles were synthesized by honey mediated sol-gel combustion method and further annealed at higher temperature 500 °C, 700 °C, 900 °C and 1100 °C. The synthesized spinel ferrite nanoparticles is investigated by x-ray diffraction, Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis/differential scanning calorimetry (TGA/DSC), field emission scanning electron microscopy, x-ray photoelectron spectroscopy and vibrating sample magnetometer. The x-ray diffraction study reveals face-centered cubic spinel cobalt ferrite crystal phase formation. The crystallite size and lattice parameter are increased with annealing temperature. Raman and Fourier transform infrared spectra also confirm spinel ferrite crystal structure of synthesized nanoparticles. The existence of cation at octahedral and tetrahedral site in cobalt ferrite nanoparticles is confirmed by x-ray photoelectron spectroscopy. Magnetic measurement shows increased saturation magnetization 74.4 emu g−1 at higher annealing temperature 1100 °C, high coercivity 1347.3 Oe at lower annealing temperature 500 °C, and high remanent magnetization 32.3 emu g−1 at 900 °C annealing temperature. The magnetic properties of synthesized ferrite nanoparticles can be tuned by adjusting sizes through annealing temperature. Furthermore, the dielectric constant and ac conductivity shows variation with frequency (1–107 Hz), grain size and cation redistribution. The modulus spectroscopy study reveals the role of bulk grain and grain boundary towards the resistance and capacitance. The cole-cole plots in modulus formalism also well support the electrical response of nanoparticles originated from both grain and grain boundaries. The dielectric, electrical, magnetic, impedance and modulus spectroscopic characteristics of synthesized CoFe2O4 spinel ferrite nanoparticles demonstrate the applicability of these nanoparticles for magnetic recording, memory devices and for microwave applications.

Cobalt ferrite nanoparticles with controlled composition-peroxymonosulfate mediated degradation of 2-phenylbenzimidazole-5-sulfonic acid

Magnetic spinel cobalt ferrite nanoparticles with variable composition (CoxFe3-xO4; x=0.1, 0.5, 0.7 and 1.0) were synthesized. The nanoparticles were characterized by various surface techniques. Average sizes and surface areas of ferrites were determined in the ranges of 11–34nm and 18.5–49.1m2/g, respectively. Surface analysis of the nanoparticles confirmed the spinel type structures in which Co(II) incorporated into the crystal lattice. The synthesized catalysts were used to dissociate peroxomonosulphate (PMS) into reactive sulfate radicals (SO4•−) and further into hydroxyl radicals (HO•) to degrade a target pollutant, 2-phenylbenzimidazole-5-sulfonic acid (PBSA) in absence of heat and light. As the molar ratio of cobalt (i.e., x) in the ferrite catalyst increased from 0.1 to 1.0, PBSA degradation enhanced from 24 to 75% in 240min. The removal of PBSA increased significantly with the increase in PMS concentration up to 0.1mM, followed by a decrease at PMS levels of >0.1mM. Nitrogen content in PBSA was mineralized by the cobalt ferrite-PMS system mostly into NO3− and NH4+ ions with minor formation of NO2−. Only 32% TOC removal was observed over a 240min reaction time, indicating carbon content in PBSA was not completely mineralized. A chemical probe method, based on free radical scavenging, revealed the contribution of both SO4•− and HO• species in PBSA degradation. Fifteen reaction intermediates were identified using LC/Q-TOF-ESI–MS analysis. Hydroxylation, elimination of sulfonate moiety, and ring cleavage processes were involved in the major degradation pathways. Catalyst reuse experiments demonstrated PBSA degradation efficiency either retained or increased with each subsequent reuse. The magnetic spinel Co-ferrite nanoparticles can be applied effectively to activate PMS without energy aiding for degrading harmful emerging organic contaminants in water.

Low-Temperature Atomic Layer Deposition of Cobalt Oxide as an Effective Catalyst for Photoelectrochemical Water-Splitting Devices

We have developed a low-temperature atomic layer deposition (ALD) process for depositing crystalline and phase pure spinel cobalt oxide (Co3O4) films at 120 °C using [Co(tBu2DAD)2] and ozone as coreagent. X-ray diffraction, UV–vis spectroscopy, atomic force microscopy, field emission scanning electron microscopy, X-ray photoelectron spectroscopy, and time-of-flight elastic recoil detection analysis were performed to characterize the structure and properties of the films. The as-deposited Co3O4 films are crystalline with a low amount of impurities (<2% C and <5% H) despite low deposition temperatures. Deposition of Co3O4 onto thin TiO2 photoanodes (100 nm) for water oxidation resulted in 30% improvement of photocurrent (after 10 ALD cycles yielding small Co3O4 particles) as compared to pristine TiO2 films), and exhibited no detrimental effects on photocurrent response up to 300 deposition cycles (approximately 35 nm thick films), demonstrating the applicability of the developed ALD process for deposition o...

CVD synthesis of cobalt spinel for bio-butanol combustion

As potential substitutes for precious metals, cobalt spinel has been considered as a promising alternative catalyst for the low-temperature catalytic oxidation of biofuels. This work reports the one-step synthesis of cobalt spinel by a more elaborated approach named pulsed-spray evaporation chemical vapor deposition with a new feedstock of Co(acac)3 and toluene. The obtained samples were comprehensively characterized in terms of structure and morphology (XRD, FTIR, Raman and SEM), chemical composition (XPS and EDS) and the redox properties (TPR/TPO). The structural and morphological analysis indicates that the synthesized thin films are pure cubic Co3O4 with homogeneous matrix with well-defined facets and straight edges. Compared to the commonly used feedstock of Co(acac)2/ethanol, Co(acac)3/toluene could result in a much higher growth rate, more lattice oxygen on the surface and better re-oxidation property. The Co3O4 film exhibits attractive performance towards the deep oxidation of n-butanol by preventing the formation of CO and acetaldehyde. The relationship between the peculiar structure and properties of Co3O4 is highlighted. The surface basic sites, specific morphology and the redox properties are proposed to play synergistic roles in the reaction sequence.

Strain induced magnetic anisotropy and 3d7 ions effect in CoFe2O4 nanoplatelets

Cobalt ferrite (CoFe2O4) magnetic nanoplatelets were synthesized by hydrothermal method at 120 °C (H120) and 180 °C (H180) respectively. The formation of inverse spinel cobalt ferrite was confirmed by X- ray diffraction pattern (XRD) and Transmission electron microscopy (TEM). The X-ray diffraction studies shows the linear variation of microstrain with inverse crystallite size. The compressive microstrain of 0.024 and 0.016 was estimated for CoFe2O4 samples H120 and H180 respectively using Williamson-Hall (W-H) plot analysis assuming uniform deformation model. The valence state of metal ions and single phase formation single domain CoFe2O4 was confirmed by X-ray photoemission spectroscopy (XPS) and Raman spectroscopy. X-ray photoemission spectra (XPS) exhibit Fe 2p3/2 peak and Co 2p3/2 peaks in both samples composed of two peaks corresponding to octahedral sites and tetrahedral sites. The strain induced magnetic anisotropy is estimated on basis of strain measured by W-H plot at 300 K. The contribution of the Co2+ ions on octahedral sites of both samples of CoFe2O4 is assigned to the magnetostriction ions in cubic structure of cobalt ferrite by assuming ground state. The magnetic ions with 3d7 transition in CoFe2O4 lattice introduced the local magnetostriction through spin-orbit-lattice interaction with distorted cubic crystal field.

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

In this work a series of mixed iron-cobalt spinel catalysts was synthesized by solution combustion synthesis and doped with 3 wt% of palladium via incipient wetness impregnation. The catalysts were synthesized with different Co:Fe ratios, characterized from the structural and surface point of view (XRD, XRF, μRS, BET, FESEM), and tested toward the oxidation of methane in lean conditions, in gas mixtures containing from 0.5 to 2 vol% of CH4. The temperature window for the catalytic activity of the undoped mixed iron–cobalt spinels spread from 350 to 650 °C. While lower iron substitution levels lead to a decrease in the spinel’s activity, the catalyst with 2/3 of the cobalt in the Co3O4 substituted with iron shows similar activity to the parent Co3O4. Although the materials were proved to be heterogeneous from the crystal phase point of view (a mixture of cobalt–iron spinels), the observed reactivity still depends on the amount of iron introduced during the synthesis. The addition of Pd lowered the temperature window for the catalytic activity to the range of 250–550 °C. The promoting effect of Pd is most evident for the pure Co3O4 catalyst. In contrast to the Pd-free catalysts, the best activity of Pd-promoted samples was observed for the catalysts with the highest iron content.

Fabrication of Yolk/Shell Partially Inverse Spinel Cobalt Ferrite/Mesoporous Silica Nanostructured Catalysts for Organic Pollutants Degradation by Peroxymonosulfate Activation

Sulfate radical based advanced oxidation process has received popular attention in organic pollutants degradation recently, including which, cobalt ferrite heterogeneous catalyst was reported to be an effective activator. However, issues of particle aggregation and metal leaching still made the activity and stability not so satisfactory. In this study, well-defined partially inverse spinel cobalt ferrite/mesoporous silica ((Co x Fe1−x )[Co1−x Fe1+x ]O4@mSiO2) nanostructured catalysts with uniform yolk/shell architecture were synthesized successfully through a hard-templating method for the first time. The prepared catalysts were characterized by various techniques including X-ray diffraction, transmission electron microscopy, N2 physical adsorption/desorption. And moreover, Mossbauer spectroscopy was employed to confirm the detailed structure of the partially inverse spinel cobalt ferrite. It was shown that the synthesized catalysts consisted of (Co0.21Fe0.79)[Co0.79Fe1.21]O4 nanoparticles encapsulated in mSiO2 hollow spheres and the content of cobalt ferrite can be controlled by varying experimental conditions. The catalysts were utilized as peroxymonosulfate catalytic activator for organic pollutant degradation. Both sulfate radical and hydroxyl radical were certified as intermediate active species during the dye pollutants degradation process. Compared to the conventional mSiO2 supported and naked cobalt ferrites, the yolk/shell nanostructured catalysts exhibited higher activity, excellent reaction stability and sintering resistance. It is clear that the design of such yolk/shell architecture can promote the development of highly efficient catalytic nanomaterials applied to degradation of organic pollutants. Well-defined yolk/shell (Co0.21Fe0.79)[Co0.79Fe1.21]O4@mSiO2 nanostructured catalysts were successfully synthesized through hard-templating method and exhibited excellent activity and stability in dyes degradation.

Mesoporous cobalt and manganese spinels: Synthesis and physicochemical properties

Binary spinels of Со and Мn are obtained by means of hard-template synthesis. Their structural and chemical characteristics are determined via N2-BET, XRD, and TEM. Their catalytic activity is studied using methanol oxidation as the model reaction.

Effect of coating density on oxidation resistance and Cr vaporization from solid oxide fuel cell interconnects

Manganese cobalt spinel oxides are promising materials for protective coatings for solid oxide fuel cell (SOFC) interconnects. To achieve high density such coatings are often sintered in a two-step procedure, involving heat treatment first in reducing and then in oxidizing atmospheres. Sintering the coating inside the SOFC stack during heating would reduce production costs, but may result in a lower coating density. The importance of coating density is here assessed by characterization of the oxidation kinetics and Cr evaporation of Crofer 22 APU with MnCo1.7Fe0.3O4 spinel coatings of different density. The coating density is shown to have minor influence on the long-term oxidation behavior in air at 800 °C, evaluated over 5000 h. Sintering the spinel coating in air at 900 °C, equivalent to an in-situ heat treatment, leads to an 88% reduction of the Cr evaporation rate of Crofer 22 APU in air-3% H2O at 800 °C. The air sintered spinel coating is initially highly porous, however, densifies with time in interaction with the alloy. A two-step reduction and re-oxidation heat treatment results in a denser coating, which reduces Cr evaporation by 97%.

Total Oxidation of Lean Methane over Cobalt Spinel Nanocubes Controlled by the Self-Adjusted Redox State of the Catalyst: Experimental and Theoretical Account for Interplay between the Langmuir–Hinshelwood and Mars–Van Krevelen Mechanisms

Involvement of suprafacial and intrafacial oxygen species in catalytic combustion of methane over the (100) faceted cobalt spinel was systematically examined as a function of temperature and CH4 conversion (XCH4). The clear-cut Co3O4 nanocubes of uniform size were synthesized using a hydrothermal method and characterized with XRD, RS, HR-TEM, XRF, TPSR (CH4/16/18O2), and SSITKA (CH4/16/18O2) techniques. The experimental results were corroborated by first-principles thermodynamic and DFT+U molecular modeling, providing a rational framework for a detailed understanding of the origin of a different redox comportment of the catalyst with the varying temperature and its mechanistic implications. Three temperature/conversion stages of the methane oxidation reaction were distinguished, depending on involvement of the adsorbed or lattice oxygen and the redox state of the catalyst. A stoichiometric (100) surface region (300 °C < T < 450 °C, XCH4 < 25%) is featured by the dominant suprafacial (Langmuir–Hinshelwood)...