Preparation and formation mechanism of Cr-free spinel-structured high entropy oxide (MnFeCoNiCu)3O4

In this work, two Cr-free high entropy oxides (HEOs), an equimolar (MnFeCoNiCu)₃O₄ and a non-equimolar (Mn_0.272Fe_0.272Co_0.272Ni_0.092Cu_0.092)₃O₄ have been synthesized by a solid-state reaction method. The reaction sequence and electrical conductivity were also studied for these two HEOs. It is demonstrated that a rock-salt phase containing a solid solution of NiO and CuO appears in the synthesizing process of (MnFeCoNiCu)₃O₄, which is ascribed to the incomplete solubilization of rock-salt phase in the spinel phase. For (Mn_0.272Fe_0.272Co_0.272Ni_0.092Cu_0.092)₃O₄, a single spinel phase (Fd-3m) is obtained at 750 °C, which is much lower than that of the (MnFeCoNiCu)₃O₄ sample. Furthermore, Mn, Fe, Co, Ni elements exist in the chemical states of +2 and + 3, and Cu exists in Cu²⁺ state. The electrical conductivity of (Mn_0.272Fe_0.272Co_0.272Ni_0.092Cu_0.092)₃O₄ is approximately 15.77 S cm^-1 at 800 °C, which is nearly three times higher than that of the (MnFeCoNiCu)₃O₄ sample.     

Defect structure and transport properties in (Co,Cu,Mg,Ni,Zn)O high entropy oxide

The defect structure and chemical diffusion in the (Co,Cu,Mg,Ni,Zn)O high entropy oxide were investigated. The material was synthesized in a form of pellets by means of a solid-state reaction, through sintering the mixture of composing oxides at 1273 K for 20 h, followed by the air quenching. The presence of the single-phase, high entropy solid solution structure was confirmed with the use of SEM + EDS and XRD methods and later confirmed after each of the experimental steps. The modification of the standard marker experiment was applied, to determine the predominantly disordered sublattice, which turned out to be the anionic one. The nonstoichiometry and its dependence on the oxygen partial pressure were investigated by means of the thermogravimetric method. The results allowed establishing the (Co,Cu,Mg,Ni,Zn)O_1-y general formula, with values of y reaching values as high as 0.07 under low oxygen partial pressures. The non-logarithmic character of the obtained pressure dependence suggests the presence of a complex structure of interacting point defects. The chemical diffusion coefficient was also determined, based on the measured re-equilibration kinetics. The obtained values of diffusivity are within the range typical for the transition metal oxides, do not confirm the presence of the sluggish diffusion effect in high entropy oxides. The established strong dependence of the chemical diffusion coefficient on the p(O₂), further confirms the presence of the complex defect structure.     

Catalytic performance of LaCo0.5M0.5O3 (M = Mn,Cr, Fe,Ni, Cu) perovskite-type oxides and LaCo0.5Mn0.5O3 supported on cordierite for CO oxidation

Catalytic combustion of CO over perovskite-type oxides LaCo_0.5M_0.5O₃ (M = Mn, Cr, Fe, Ni, Cu) and LaCo_0.5Mn_0.5O₃ supported on cordierite were investigated. The catalysts were synthesized by impregnation method with citrate and characterized by XRD, SEM and TPR. The LaCo_0.5Mn_0.5O₃ catalyst showed much higher activity in CO oxidation compared with LaCo_0.5M_0.5O₃ (M = Cr, Fe, Ni, Cu) due to different kinds of valence state and lattice oxygen content. When LaCo_0.5Mn_0.5O₃ was supported on cordierite, the activity was improved significantly. However, calcining temperature and the presence of water vapor affected the catalytic activity due to sintering and competition of H₂O with CO for adsorption, respectively.     

Formation of sol–gel derived (Cu,Mn,Co)3O4 spinel and its electrical properties

An (Mn,Co)₃O₄ spinel layer effectively suppresses Cr outward diffusion, but the occurrence of spallation between coating and metal interfaces and the increasing oxidation rate over time significantly deteriorate cell performance. A transition metal cation, particularly Cu, is added into the (Mn,Co)₃O₄ to improve the long-term stability of the interconnect materials for low-to-intermediate-temperature solid oxide fuel cells (SOFCs). In this work, fine-crystalline (Cu,Mn,Co)₃O₄ spinel powders with an average crystallite size of 21 nm were successfully synthesized via the citric acid–nitrate method. The potential for use as a coating material for low-to-intermediate-temperature SOFC interconnects was explored. According to the TG curves, IR spectra, and XRD patterns, 800 °C is recommended as the minimum calcination temperature for (Cu,Mn,Co)₃O₄ spinel materials. Compared with (Mn,Co)₃O₄ spinel materials, the addition of Cu does not induce significant changes in the crystal structure but markedly improves the electrical conductivity (116 Scm⁻¹) and the activation energy (0.394 eV) of the (Cu,Mn,Co)₃O₄ spinel. Overall, the sol–gel derived (Cu,Mn,Co)₃O₄ spinel can be used as a coating material for low-to-intermediate-temperature SOFC interconnects and is superior to (Mn,Co)₃O₄ spinel because high electrical conductivity is preferred to minimize ohmic losses between the electrodes of adjacent cells.     

Phase transformation of FeCr2O4 to (Fe,Cr)2O3 solid solution pigment powders: Effect of post-heating temperature

Iron chromite powders were synthesized via solution combustion route using iron(III) nitrate nonahydrate and chromium(III) nitrate nonahydrate as starting materials, as well as glycine–urea, glycine–citric acid, and glycine–ethylene glycol mixtures as fuels. The effect of postheating at different temperatures on the structure, molecular, microstructure, and chromatic properties of powders and tiles colored by in-glaze powders was studied. The X-ray diffraction patterns showed that as-synthesized powders were obtained in crystalline FeCr₂O₄ phases moreover, postheating of the powders led to d-space shift and oxidation and formation of (Fe,Cr)₂O₃ solid solution phase regardless of fuel type. Phase transformation of FeCr₂O₄ to (Fe,Cr)₂O₃ solid solution was observed at 500/750°C depending on the dominant phase of as-synthesized particles. Fourier transform infrared analysis illustrated that the band positions of octahedral M–O and tetrahedral M–O bonds were shifted due to Fe cations movement from their position and lattice shrinkage by increasing of post-heating temperature. Moreover, scanning electron micrographs showed that Fe_0.7Cr_1.3O₃ semispherical fine particles were formed from porous spongy FeCr₂O₄ particles due to oxidation and phase transformation during the postheating. Furthermore, chromatic properties of the samples were represented. The color properties of the pigments showed that the formation of brown pigments is provided with the phase transformation from FeCr₂O₄ to (Fe,Cr)₂O₃ at a temperature of up to 750°C. Moreover, increasing the color purity to this temperature is related to the removal of residual carbonaceous matters. The chromatic properties of the glazed tiles colored using the pigments showed that postheating between 250 and 500°C led to more brown appearance.     

Insight into the strengthening mechanism of α-Al2O3/γ-Fe ceramic-metal interface doped with Cr,Ni, Mg,and Ti

Corrosion prevention is an important issue in thermal energy storage. The poor bonding strength between the ceramic coatings and substrates in these energy storage devices arises because of the high corrosivity of the energy storage materials at high temperatures, which reduces the service life of the devices. In this study, an alloy element x (Cr, Ni, Mg, or Ti) was innovatively doped into the α-Al₂O₃ (0001)/γ-Fe (111) interface to enhance the interfacial bonding strength, and the strengthening mechanism of the doped interface was quantitatively analysed by first-principles calculations at the electronic level. The results show that Ti and Mg doping can clearly increase the bonding strength of the Al₂O₃/Fe interface, whereas doping with Ni and Cr is ineffective. More importantly, the micro-mechanism was revealed by discussing the segregation behaviour, where the Ti and Mg dopants readily segregated to the interface and then combined with Al or O atoms to form new compounds (Ti₂O₃, TiAl₂, MgO, and MgAl₃), whereas the Cr and Ni dopants were preferentially and stably localized in the Fe sites. Therefore, Mg and Ti atoms appear to play the enhancement effect as interfacial binders. Furthermore, evaluation of the electronic structure confirmed that all doped interfaces exhibited metallic and covalent characteristics at the interfaces, whereas greater hybridisation of the electron orbitals was found at the doped Ti and Mg interfaces than at the other interfaces, such as between Al-s and Mg-p, between Al-p and Mg-p, between Ti-p and Al-p, between Ti-s and Al-s, and between Ti-p, d and O-s. The more electron orbitals hybridizing, the larger the bonding strength is.     

Bi-metallic catalysts of mesoporous Al2O3 supported on Fe,Ni and Mn for methane decomposition: Effect of activation temperature

Methane decomposition reaction has been studied at three different activation temperatures (500 °C, 800 °C and 950 °C) over mesoporous alumina supported Ni–Fe and Mn–Fe based bimetallic catalysts. On co-impregnation of Ni on Fe/Al₂O₃ the activity of the catalyst was retained even at the high activation temperature at 950 °C and up to 180 min. The Ni promotion enhanced the reducibility of Fe/Al₂O₃ oxides showing higher catalytic activity with a hydrogen yield of 69%. The reactivity of bimetallic Mn and Fe over Al₂O₃ catalyst decreased at 800 °C and 950 °C activation temperatures. Regeneration studies revealed that the catalyst could be effectively recycled up to 9 times. The addition of O₂ (1 ml, 2 ml, 4 ml) in the feed enhanced substantially CH₄ conversion, the yield of hydrogen and the stability of the catalyst.     

High entropy oxides (FeNiCrMnX)3O4 (X=Zn,Mg) as anode materials for lithium ion batteries

High entropy oxides (HEOs) have attracted the attention of researchers due to their high theoretical specific capacity and structural stability. However, the effect of elements in HEOs on material properties is unknown. In this work, HEOs (FeNiCrMnZn)₃O₄ and (FeNiCrMnMg)₃O₄ were successfully synthesized and used as anode materials for lithium ion batteries. The effects of Zn and Mg on the properties of HEOs were studied from the aspects of crystal structure, micro morphology, and surface valence. Results show that Zn enhances the lithium storage performance of HEOs theoretically, which is verified in the later electrochemical performance tests. (FeNiCrMnZn)₃O₄ is better than (FeNiCrMnMg)₃O₄ in all aspects of rate properties, cycle performance, and electrochemical impedance spectroscopy. Therefore, the introduction of electrochemically active metals, such as Zn, can improve the performance of HEOs, thereby providing ideas for the subsequent design and application of HEOs.     

Anodic characteristics of SrFe0.9M0.1O3(M: Ni,Co, Ti,Mn) electrodes

Anodic characteristics of the substituted oxides, SrFe_0.9M_0.1O₃(M: Ni, Co, Ti, Mn), were studied. It was found that the catalytic activities of the oxides substituted with Ni and Co ions for the oxygen evolution reaction are high in alkaline solution. The reaction mechanisms for the oxygen evolution reaction are proposed for all the substituted oxides under the assumption of Langmuirian adsorption condition. The current efficiencies of the anodic dissolution of the oxides were much higher in acidic solution than those in alkaline solution. The anodic dissolution is much suppressed by the substitution with Ni or Co ion in alkaline solution. Therefore, the oxides substituted with Ni and Co ions are most suitable as anode materials in alkaline solution. The anodic dissolution is based on the oxygen vacancy formed in the oxygen evolution process. The mechanism of the anodic dissolution of the oxides is proposed.     

NO reduction over nanostructure M-Cu/ZSM-5 (M: Cr,Mn, Co and Fe) bimetallic catalysts and optimization of catalyst preparation by RSM

The selective catalytic reduction (SCR) of NO with NH₃ in the presence of oxygen over a series of H-ZSM-5 supported transition metal oxides (Co, Mn, Cr, Cu and Fe) was investigated. Among them, Cu/ZSM-5 nanocatalyst was found to be the most promising catalyst based on activity. The modification of Cu/ZSM-5 by adding different transition metals (Co, Mn, Cr and Fe) to improve the efficiency of NO conversion was studied. The results indicated that the Fe–Cu/ZSM-5 bimetallic nanocatalyst was the highest active catalyst for NO conversion (67% at 250 °C and 93% at 300 °C). Response surface methodology (RSM) involving central composite design (CCD) was employed to evaluate and optimize Fe–Cu/ZSM-5 preparation parameters (Fe loading, calcinations temperature, and impregnation temperature) in SCR of NO at 250 °C. The optimum condition for maximum NO conversion was estimated at 4.2 wt.% Fe loading, calcinations temperature of 577 °C and impregnation temperature of 43.5 °C. Under these condition, experimental NO conversion efficiency was 78.8%, which was close with the predicted value (79.4%).     

Influence of modifying additives on the catalytic activity and stability of Au/Fe2O3–MO x catalysts for the WGS reaction

The activity and stability of Au/Fe₂O₃–MO_x catalysts (M = Zr, Mg, Ca, Ni, La, Cu, Zn, Al, Ba, Cr, Co, Ce, Mo, Bi, Ti, Mn) in water-gas shift reaction were investigated extensively. The WGS activity and stability of Au/Fe₂O₃ is improved significantly upon addition of ZrO₂ and to a lesser extend MgO, CaO, NiO, La₂O₃, Cr₂O₃, CuO. In contrast, Bi, Ti and Mn oxides seriously decrease the catalytic activity while additions of Zn, Al, Ba, Co, Ce and Mo oxides do not influence evidently the catalytic activity and its stability. Based on the characterization using the methods of BET-surface area and pore structure XRF, XRD, and H₂–TPR for some of as-prepared and spent samples, it could be concluded that the catalytic activity of gold catalysts supported on composite oxide of Fe₂O₃–MO_x depends not only on the dispersion of the gold particles but also on the reduction property of composite oxide supports, regardless of the fluctuation of gold loading and some change of specific surface area and pore structure due to introduction of the modifying metal oxides. The improvement of catalytic stability may be attributed to the comparative stabilization of high dispersion of gold particles and uneasily sintering of Fe₃O₄ crystallites during the catalytic operation. However, the chemical (electronic) effects exerted by the modifying addition of metal oxides on the catalytic performance of gold catalyst may not be ruled out.     

Microwave dielectric properties of ATiO3 (A = Ni,Mg, Co,Mn) ceramics

Relationships between structural characteristics and microwave dielectric properties of ATiO₃ (A = Ni, Mg, Co, Mn) with ilmenite structure were investigated. The oxygen octahedral distortion was dependent on the type of A-site ions which affected to the temperature coefficient of the resonant frequency (TCF) of ATiO₃ ceramics. The quality factor (Qf) of ATiO₃ (A = Mn, Ni, Co) specimens was appreciably lowered than that of MgTiO₃ specimens due to the degree of covalency of cation–oxygen ion bond and the ordering of A-site ions. Also, the dielectric constant (K) was dependent on the electronic oxide polarizabilities of ATiO₃ ceramics.     

原子吸收光谱法测定四氧化三锰产品中多种杂质元素

用盐酸溶解样品 ,火焰原子吸收光谱法测定四氧化三锰中铁、钙、镁、钾、钠 ,石墨炉原子吸收光谱法测定其中的铬。对测定条件进行了试验 ,找出了最佳溶样方法和酸度 ,并对共存元素的干扰及排除进行试验 ,获得了满意的效果。该法简便、快速、准确 ,样加标回收率为 98.6%～ 1 0 9.6%之间 ,相对标准偏差为 1 .61 %～ 4.96%之间 ,倍比实验、标准曲线法和标准加入法结果相吻合 ,适用于四氧化三锰样品中铁、钙、镁、钾、钠、铬元素的分析。     

High-temperature stability of Mg(Al,Cr)2O4 spinel co-existing with calcium aluminates in air

Cr₂O₃ is a well-known corrosion resistant oxide used in refractory applications. However, it can oxidize into toxic and water-soluble Cr(VI) compounds upon reaction with calcium aluminate cement phases in the presence of oxygen, which subsequently causes disposal problems after use. This study describes the extent to which chromium in the spinel Mg(Al,Cr)₂O₄ phase can be oxidized to Cr(VI) when it reacts with the calcium aluminate cement phases C₁₂A₇, CA, CA₂ and free CaO at 1300 °C in air, using XRD, XPS and leaching tests (TRGS 613 standard) as analytical tools. On reaction with CaO, the Mg(Al,Cr^III)₂O₄ spinel mainly transformed into hauyne (Ca₄Al₆Cr^VIO₁₆) and Ca₅Cr₃O₁₂ which contains both Cr(IV) and Cr(VI). The reaction of C₁₂A₇ and CA with the spinel phase also resulted in the formation of Ca₄Al₆CrO₁₆. Conversely, the reaction of Mg(Al,Cr^III)₂O₄ spinel with CA₂ resulted in the formation of only a trace amount of Cr(VI). Water-soluble Cr(VI) leached in large quantities (>100 mg/L) from samples where the Mg(Al,Cr^III)₂O₄ reacted with either C₁₂A₇ or CA. Almost no Cr(VI) leached from the sample when Mg(Al,Cr^III)₂O₄ reacted with CaO, using the standard TRGS 613 leach test, but a significant amount of Cr(VI) was released into solution when leached with a HCl solution for 12 h. Both Cr(IV) and Cr(VI) present in the Ca₅Cr₃O₁₂ dissolved into acidic solution. Only a small amount of Cr(VI) leached from the sample that resulted when spinel was reacted with CA₂, even after a prolonged HCl leach. Cr(III) in spinel Mg(Al,Cr)₂O₄ is very stable and does not leach in either distilled water or acidic solution.     

Oxidation of alkylaromatics to benzylic ketones using TBHP as an oxidant over LaMO3 (M = Cr,Co, Fe,Mn, Ni) perovskites

A series of LaMO₃ (M = Cr, Co, Fe, Mn, Ni) perovskites were synthesized by citrate precursor decomposition method and characterized by XRD technique. The catalytic activity of the synthesized perovskites was investigated for the oxidative functionalization of alkylaromatics to benzylic ketones using TBHP as an oxidant. Of various perovskites screened, LaCrO₃ exhibited remarkable catalytic activity providing the oxidation of alkylarenes selectively at the benzylic position. The LaCrO₃/TBHP catalytic system provides excellent yields of the desired ketones under solvent-free conditions and the catalyst can be successfully used up to six consecutive cycles with no significant loss in activity.     

Role of Mg–O on phase stabilization in solution combustion processed rocksalt structured high entropy oxide (CoCuMgZnNi)O with high dielectric performance

High entropy oxide (CoCuMgZnNi)O with a phase pure rocksalt was synthesized using low-temperature solution combustion. The precursors were found to combust at 270 °C and 400 °C was considered to be the formation temperature. The high entropy rocksalt oxide (HERO) fully stabilized at 1000 °C shows a single-phase, fcc rocksalt structure with an Fm-3m space group. HERO displays one of its parent oxide Mg–O structural properties as both belong to the cubic family and had lattice parameters very close to each other. The lower cation systems exhibited a transition from spinel to rocksalt structure with the addition of Mg–O. Raman of HERO affirmed a completely disordered occupancy of various metal cations, the formation of HERO at 400 °C, and phase stabilization at 1000 °C. Dielectric measurements at room temperature showed high permittivity (κ) with magnitudes ∼1.9 × 10³, 4.7× 10¹, and 0.9 × 10¹ at 100, 1k, and 100k Hz.     

One-pot hydrothermal synthesis of Mn3O4 nanorods grown on Ni foam for high performance supercapacitor applications

Mn₃O₄/Ni foam composites were synthesized by a one-step hydrothermal method in an aqueous solution containing only Mn(NO₃)₂ and C₆H₁₂N₄. It was found that Mn₃O₄ nanorods with lengths of 2 to 3 μm and diameters of 100 nm distributed on Ni foam homogeneously. Detailed reaction time-dependent morphological and component evolution was studied to understand the growth process of Mn₃O₄ nanorods. As cathode material for supercapacitors, Mn₃O₄ nanorods/composite exhibited superior supercapacitor performances with high specific capacitance (263 F · g^-1 at 1A · g^-1), which was more than 10 times higher than that of the Mn₃O₄/Ni plate. The enhanced supercapacitor performance was due to the porous architecture of the Ni foam which provides fast ion and electron transfer, large reaction surface area, and good conductivity.     

Oxidation of alkylaromatics to benzylic ketones using TBHP as an oxidant over LaMO3 (M = Cr, Co, Fe, Mn, Ni) perovskites

A series of LaMO₃ (M = Cr, Co, Fe, Mn, Ni) perovskites were synthesized by citrate precursor decomposition method and characterized by XRD technique. The catalytic activity of the synthesized perovskites was investigated for the oxidative functionalization of alkylaromatics to benzylic ketones using TBHP as an oxidant. Of various perovskites screened, LaCrO₃ exhibited remarkable catalytic activity providing the oxidation of alkylarenes selectively at the benzylic position. The LaCrO₃/TBHP catalytic system provides excellent yields of the desired ketones under solvent-free conditions and the catalyst can be successfully used up to six consecutive cycles with no significant loss in activity.     

Interaction between metallic interconnect and constituent oxides of (La,Sr)MnO3 coating of solid oxide fuel cells

The chemical interaction between Fe–Cr alloy interconnect and constituent oxides of Sr-doped LaMnO₃ (LSM) coating, La₂O₃, SrO and Mn₂O₃, is investigated at 800–900 °C in air. The Cr deposition reaction between the Fe–Cr alloy metallic interconnect and oxides varies significantly with the nature of the oxides. The interaction between the Fe–Cr alloy and La₂O₃ and Mn₂O₃ oxide coatings primarily results in the formation of LaCrO₃ and (Cr, Mn)₃O₄ while in the case of SrO oxide coating, Cr₂O₃ is the main product. In the case of LSM coating, the formation of (Cr, Mn)₃O₄ and Cr₂O₃ is identified. The results indicate that the chemical interaction between the Fe–Cr alloy interconnect and LSM coating is most likely related to the surface oxide species such as SrO and MnO_x initially enriched or segregated on the surface of LSM particularly in the early stages of the reaction.