O4 Spinel Research Articles

CrxN/Mn1.7Cu1.3 coating on ferritic stainless steel for solid oxide fuel cell interconnects

CrxN/Mn1.7Cu1.3 dual layer coating was prepared on SUS 430 steel via multi-arc ion plating and magnetron sputtering in sequence for use of solid oxide fuel cell (SOFC) interconnects. The coated steel was exposed to air at 800 °C corresponding to SOFC cathode environment to evaluate the oxidation behavior and electrical property of the surface scale. Results indicated that the Mn1.7Cu1.3 outer layer was thermally converted into Mn1.7Cu1.3O4 spinel outer layer and the CrxN inner layer was gradually oxidized to form a protective Cr2O3 layer at the interface of CrxN and Mn1.7Cu1.3O4. After the CrxN layer was completely oxidized, the surface scale mainly consisted of an outer Mn1.7Cu1.3O4 spinel layer and an inner Cr2O3 layer. Area specific resistance of the surface scale formed on CrxN/Mn1.7Cu1.3 coated steel after 15 weeks oxidation was 17.28 mΩ·cm2. The superior performance of the coated steel benefited from the combination of CrxN barrier layer and Mn1.7Cu1.3O4 spinel.

Determining the Role of Fe-Doping on Promoting the Thermochemical Energy Storage Performance of (Mn1- x Fex )3 O4 Spinels.

Mn oxides are promising materials for thermochemical heat store, but slow reoxidation of Mn3 O4 to Mn2 O3 limits efficiency. In contrast, (Mn1- x Fex )3 O4 oxides show an enhanced transformation rate, but fundamental understanding of the role played by Fe cations is lacking. Here, nanoscale characterization of Fe-doped Mn oxides is performed to elucidate how Fe incorporation influences solid-state transformations. X-ray diffraction reveals the presence of two distinct spinel phases, cubic jacobsite and tetragonal hausmannite for samples with more than 10% of Fe. Chemical mapping exposes wide variation of Fe content between grains, but an even distribution within crystallites. Due to the similarities of spinels structures, high-resolution scanning transmission electron microscopy cannot discriminate unambiguously between them, but Fe-enriched crystallites likely correspond to jacobsite. In situ X-ray absorption spectroscopy confirms that increasing Fe content up to 20% boosts the reoxidation rate, leading to the transformation of Mn2+ in the spinel phase to Mn3+ in bixbyite. Extended X-ray absorption fine structure shows that FeO length is larger than MnO,but both electron energy loss spectroscopy and X-ray absorption near edge structure indicate that iron is always present as Fe3+ in octahedral sites. These structural modifications mayfacilitate ionic diffusion during bixbyite formation.

Optimization of Polarization Resistance of (Cu,Mn)3O4 as Cathode Current‐Collecting Layer for Solid Oxide Fuel Cells

The polarization resistance optimization of the cathode plays a crucial role in reducing the energy consumption of the solid oxide fuel cell (SOFC) stack. Herein, a (Cu,Mn)3O4 spinel material is used for the development of the cathode current‐collecting layer (CCCL) because it has superior price and good electrical conductivity than conventional current‐collecting materials. Symmetrical cells are fabricated using screen printing techniques. It is optimized by studying the influence of sintering temperature, thickness, and adding amount of Co3O4 on the microstructure and polarization resistance of the CCCL. The research found that 850 °C is the most convenient sintering temperature and 1–2 wt% of the adding amount of Co3O4 can optimize the polarization resistance. In addition, it is found that for the cathode side, the total thickness of the CCCL and the cathode should be controlled between 70 and 80 μm, the performance of the cathode is stable, and the polarization resistance is also rationally optimized.

On the formation and diffusion of oxygen vacancies in non-stoichiometric mixed Co3-x Mn x O4 spinel structures from first-principles calculations

Using first-principles simulations, we focus on the study of Co3O4–Mn3O4 mixed oxides, which have recently shown alluring features as thermochemical heat storage materials. We provide fundamental atomistic-level insight into the thermodynamics and kinetics of a series of non-stoichiometric Co3-x Mn x O4-y (0 ⩽ x ⩽ 3 and y = 0, 0.125, 0.250) bulk systems, by examining in detail the formation and diffusion processes of oxygen vacancies as a function of Mn content. We find a preference for the formation of vacancies at x = 1.5. And we predict a significant drop of diffusion barriers for x ⩾ 1.5, when Mn atoms start to populate the spinel octahedral sites as Mn3+. Our results pave the way for better understanding the underlying mechanisms that govern oxygen vacancy dynamics in Co3-x Mn x O4 in general, and, in particular, the reversible reduction and re-oxidation reactions of these promising mixed oxides for thermal energy storage. Nevertheless, some discrepancies are found between our calculations on bulk models and recent experimental insights from the literature, which suggests that surface and finite size effects might play an important role in controlling the observed macroscopic behavior of these materials during reversible reduction and re-oxidation cycles.

Michel Boudart, Catalytica and catalytic combustion—A scientific odyssey

Catalytic steam reforming of propylene, benzene and toluene was investigated over Mn-Cr-O spinel oxides with different stoichiometry. The catalysts were structurally stable during reforming conditions except for Mn3O4, which reduced in situ to MnO. The Cr2O3 catalyst showed the highest initial rate for propylene reforming, whereas the Mn-rich Mn1.5Cr1.5O4 spinel was the most active and stable catalyst for aromatic reforming. Reforming rates of olefins and aromatics were comparable under 0.05 bar hydrocarbon and 0.40 bar steam at 873 K. The reforming rate of olefins and aromatics was first order in hydrocarbon and slightly inhibited by H2O, which is consistent with a Mars-van Krevelen type mechanism. Of the hydrocarbons investigated, only the rate of toluene reforming was negative first order in excess H2. Results from diffuse reflectance infrared Fourier transform spectroscopy of adsorbed toluene on a spinel catalyst suggest that H2 inhibits the rapid oxidation of the methyl group of toluene.

Conversion of Co-Mn-Al hydrotalcites in highly active spinel-type catalysts for peroxide decomposition

The fast decomposition of peroxides (M2O2, where M = H+, Li+) at room temperature has gained increased attention given its relevance, from propulsive systems to metal-O2 conversion batteries. This study describes, by a rational design of experiment, the influence of selected parameters such as: pH, initial concentration of K2CO3, precipitate aging, precipitate washing, and thermal treatment for the hydrotalcite co-precipitation, characterizing it for each synthesis variable. The results show that hydrotalcites can efficiently be converted in Co-Mn-Al spinels and as a result, a new highly active Co2Mn0.5Al0.5O4 spinel oxide was synthesized after thermal treatment. Its activity and mechanical resistance were tested in a micro-thruster reactor where it showed total decomposition (> 98%) of H2O2 and in a Li-O2 battery, with increased cyclability, been able to reduce the charging potential in ~200 mV. These encouraging results enhance the importance not only of this class of material, but also, the importance of robust synthesis methodologies for reproducible results.

Study on the Variation in Microstructure of a Ferritic Stainless Steel with Surface Roughness and Thermal Cycling in Air

A ferritic stainless steel, Crofer 22 APU, is one of candidates for metallic interconnects of solid oxide fuel cells. Ferritic stainless steel Crofer 22 APU specimens with different surface roughnesses were prepared by grinding with SiC powder papers of various grits and were then thermally cycled. Polished Crofer 22 APU specimens after one thermal cycle and five thermal cycles had relatively straight oxide layers with similar thicknesses of 30 μm, suggesting that after one cycle (total oxygen exposure time of 100 h at 1073 K), the oxidation does not progress. Micrographs of a trench made by milling with the FIB (focused ion beam) for a Crofer 22 APU specimen rubbed with grit 80 SiC powder paper after 8 thermal cycles (total oxygen exposure time of 200 h at 1073 K), captured by ESB, InLens, and SE2, showed that the surface of the sample was very coarse and its oxide layer was undulated. In the oxide layer, the phase of the sublayer was Cr₂O₃, and that of the top layer was (Cr, Mn)₃O₄ spinel. The sample ground with grit 80 SiC powder paper after 60 thermal cycles (total oxygen exposure time of 1500 h at 1073 K) was very coarse. Some ridges were quite straight and continuous. After 20 and 40 thermal cycles, ASR (area specific resistance) decreased as the number of grit of the SiC powder paper increased, suggesting that the polished Crofer 22 APU is better than those with rougher surfaces for application as an interconnect of SOFC.

The Time for Industrialization Has Come: A Pre-Coated Solution for the GW Scale

Green Hydrogen has become present in all mass media thanks to their ubiquitous role to overcome the issues related to climate change. It is estimated that green hydrogen and their associated technologies (i.e. fuel cells and electrolysers) will represent 25 % of the total energy mix by 2050 if a 100 % renewable system is achieved. Among these technologies are Solid Oxide Cells (SOC). This technology has the capability to be the more efficient technologies among fuel cell and electrolyser’s technologies as well as the technology made from cheaper and more abundant materials. In addition to this, it has also the capability to function much more easily in reversible mode that their fuel cell and electrolyser siblings (AEM, PEM or even AEM) which give them a great versatile capacity.In order to make Green Hydrogen a reality and to succeed, an immense work in terms of industrialization needs to be done. In the following years, the industry needs to prove that is ready to deliver the systems in the large volumes that it is going to be demanded if we want to give response to our climate goals (i.e. GW per year). For instance, by 2030 it is estimated that 500 000 micro-CHP (combined heat and power) units will be installed only in the European Union which would require the production of at least as many fuel cell systems for such application. This large number, make a very rapid fuel cell industrialization necessary and a suitable value chain must thus be created.Sandvik Materials Technology (SMT) is a material provider for the more demanding industries and applications. During our more than 160 years of history, the company has delivered large volumes of materials in forms of tubes and strips, for instance. Solid Oxide Cell systems are constituted by many tubes and strips made of special stainless steel alloys in which SMT is specialized of. More recently, SMT has developed a coil-to-coil process to produce pre-coated strip for the fabrication of bipolar plates and interconnects in both PEM fuel cells and electrolysers but as well for SOC. The pre-coated concept allows the industry to have a cost-effective process ready for the rapid ramp-up of both fuel cell and electrolysis technologies.Regarding SOC, Sanergy HT 441 is designed to be used as interconnect material. The product is made at industrial scale by Physical Vapor Deposition in the above-mentioned roll-to-roll coating method. It is based on an ASTM 441 steel grade double coated with thin layers of Ce and Co (< 1 µm). The material can be double side coated with the same coating or with a different coating in the other side. This material has shown to form a protective (Co,Mn)3O4 spinel layer that improves corrosion resistance and heavily reduces Cr evaporation. Recent studies has shown that Cr evaporation is reduces up to 10 times thanks to the Ce and Co coating. On the other hand, very recently it has been put on the table challenges with use of Co due to both societal and economical conflicts. At Surface Research in SMT, different Co-free coated materials has been developed in order to offer a different solution to this. Therefore, this work will show the solutions that SMT put in the market to industrialize fuel cells and electrolysis, and concretely SOCs.

Reevaluating the Cr Evaporation Characteristics of Ce/Co Coatings for Interconnect Applications

Solid oxide fuel cell (SOFC) is a high-temperature energy conversion device that directly converts the chemical energy of the fuel into electrical energy via electrochemical reactions with a very high efficiency. Corrosion of the interconnects, which leads to high degradation rates, is one of the key issues impeding the commercial success of SOFC. Interconnects are used to provide electrical connection between the individual cells which are stacked together to deliver high power output. Recent developments in materials and manufacturing techniques have significantly reduced the operating temperatures, allowing the use of metallic interconnects. Chromia forming ferritic stainless steels (FSS) are preferred over the other materials owing to a coefficient of thermal expansion (CTE) similar to that of ceramic components, high oxidation resistance, good thermal conductivity, and adequate electrical conductivity of Cr2O3 scale at high temperatures. Nonetheless, the use of FSS as interconnect materials poses some significant challenges.During the operation, the chromia scale grows continuously with time, which in turn increases the resistance across the cell. Moreover, chromia reacts with oxygen in the presence of water vapor to form volatile Cr(VI) species such as CrO2(OH)2. These volatile Cr(VI) species react and get deposited at the triple-phase boundaries hindering the oxygen electrochemical reaction on the cathode, leading to cell degradation, also known as chromium poisoning. Despite the various improvements to the steel composition, chromium evaporation is too high for the long-term operation of SOFC. Applying coatings in a widespread solution to suppress chromium evaporation.Spinel structure-based coatings are considered promising owing to good adhesion and are effective barriers to volatile Cr species and oxygen. (Co,Mn)3O4 spinel oxides deposited using various techniques are widely reported in the literature and are known to reduce Cr evaporation and oxide scale growth. Metallic coatings deposited using physical vapor deposition (PVD) are easier and cheaper in terms of manufacturing and eliminate the need for extra heat treatment step for coating densification used in electrophoretic deposition, dip-coating, and screen printing.PVD CeCo coated steels were reported to reduce the chromium evaporation by about 10 times compared to the uncoated on Sanergy HT, Crofer 22H, AISI 441. Nevertheless, the coated coupons investigated had the coating on the faces, but the edges were uncoated. The uncoated edges contribute to higher chromium evaporation than if the edges were coated. Thus, the reported chromium evaporation of the coated coupons is higher than the real chromium evaporation.To understand the influence of the uncoated edges on the measured chromium evaporation, uncoated coupons and two sets of coated coupons, (1) coated coupons with coated edges and (2) coated coupons with uncoated edges were used in the study. The exposures were conducted in horizontal quartz tube reactors at 800 oC for 500 hours in air humidified with 3 % H2O. The airflow is set at 6 lit/min to achieve a flowrate independent regime in the reactor. To quantify chromium evaporation, Na2CO3 coated denuders were used to capture Cr(VI) species, which were replaced at regular intervals.Although the edges made up to only 4% of the total area, the contribution to chromium evaporation from the edges is significant. The chromium evaporation of the coupons with coated edges is about 50 times lower than the chromium evaporation of the uncoated coupons.

Unveiling the roles of Fe-Co interactions over ternary spinel-type ZnCoxFe2-xO4 catalysts for highly efficient CO2 hydrogenation to produce light olefins

Currently, the CO2 conversion to light olefins is attractive in clean energy research. Despite numerous works on Co-Fe bimetallic catalysts, the lack of well-defined bulk structures with identical crystalline phase and similar textural property hinders the in-depth understanding of the roles of Fe and Co species. Herein, series of uniform K-containing spinel-type ZnCoxFe2-xO4 nanoparticles (x = 0, 0.5, 1.0, 1.5, 2.0) were successfully synthesized via a single source layered double hydroxide precursor route. Compared to ZnFe2O4 and ZnCo2O4, as-fabricated ternary ZnCo0.5Fe1.5O4 spinel nanoparticles as the catalyst exhibited an outstanding performance toward CO2 hydrogenation to produce light olefins, with a 36.1% selectivity toward C2=-C4= products at a 49.6% conversion and an unprecedentedly high iron time yield for CO2 conversion to light olefins (∼29.1 μmolCO2·gFe−1·s−1) at the gaseous hourly space velocity of 24000 mL·gcat−1·h−1. In combination with structural characterizations and reaction results, it was unveiled that during the CO2 hydrogenation over ternary ZnCoxFe2-xO4 catalysts, the formation of electron-rich Fe0 atoms in the CoFe alloy phase significantly promoted in situ the generation of active iron-cobalt carbide, Co2C, and θ-Fe3C phases, thereby improving the reactivity of catalysts for the production of hydrocarbons and simultaneously inhibiting both the CO2 methanation and the secondary hydrogenation of olefins. The present findings provide a clear understanding of the roles of Fe-Co interactions over ternary ZnCoxFe2-xO4 catalysts for the highly efficient hydrogenation of CO2 to produce light olefins.

Physicochemical properties of Mn1.45Co1.45Cu0.1O4 spinel coating deposited on the Crofer 22 H ferritic steel and exposed to high-temperature oxidation under thermal cycling conditions

The Crofer 22 H ferritic steel substrate was coated with an Mn1.45Co1.45Cu0.1O4 spinel by means of electrophoresis. After high-temperature oxidation under thermal cycling conditions, the physicochemical properties of the obtained system were evaluated. During 48-h cycles that involved heating the samples up to temperatures of either 750 or 800 °C, the oxidation kinetics of both coated and unmodified steel approximately obeyed the parabolic rate law. The unmodified steel was oxidized at a higher rate than the system consisting of the substrate and the coating. In its bulk form, the spinel consisted entirely of the cubic phase and it exhibited high electrical conductivity. The Mn1.45Co1.45Cu0.1O4 coating, on the other hand, was compact and consisted of two phases—the cubic and the tetragonal one—and it was characterized by good adhesion to the metallic substrate. After cyclic oxidation studies conducted for the two investigated temperatures (750 or 800 °C), the coating was determined to provide a considerable improvement in the electrical properties of the Crofer 22 H ferritic steel, as demonstrated by the area-specific resistance values measured for the steel/coating system. The evaporation rate of chromium measured for these samples likewise indicates that the coating is capable of acting as an effective barrier against the formation of volatile compounds of chromium. The Mn1.45Co1.45Cu0.1O4 spinel can therefore be considered a suitable material for a coating on the Crofer 22 H ferritic steel, with intermediate-temperature solid oxide electrolyzer cells as the target application.

Alternating-Current Electrophoretic Deposition of Spinel Coatings on Porous Metallic Substrates for Solid Oxide Fuel Cell Applications

The performance of solid oxide fuel cells (SOFCs) can be degraded by “chromium poisoning” where thermally grown Cr2O3 on metallic surfaces forms volatile Cr-containing species that are redeposited on active regions of the cathode. This phenomenon is further exacerbated for porous metallic interconnects and metal-supported electrodes due to their large surface-to-volume ratios. In this study, electrophoretic deposition (EPD) of CuNi0.2Mn1.8O4 spinel powders on porous SUS430 metallic substrates using alternating current (AC) was explored. Two-step densification heat treatment was used to form a thin, uniform, protective spinel coating. The area-specific resistance (ASR) and weight gain were tracked during 100-h oxidation tests at 700°C in air. The results showed that, despite the considerable complexity of the sample shape, AC EPD was able to form a protective coating layer that significantly limited the growth rate of the thermally grown oxide (TGO) by reducing the kg value by a factor of 25.

Steam reforming kinetics of olefins and aromatics over Mn-Cr-O spinel oxides

Catalytic steam reforming of propylene, benzene and toluene was investigated over Mn-Cr-O spinel oxides with different stoichiometry. The catalysts were structurally stable during reforming conditions except for Mn3O4, which reduced in situ to MnO. The Cr2O3 catalyst showed the highest initial rate for propylene reforming, whereas the Mn-rich Mn1.5Cr1.5O4 spinel was the most active and stable catalyst for aromatic reforming. Reforming rates of olefins and aromatics were comparable under 0.05 bar hydrocarbon and 0.40 bar steam at 873 K. The reforming rate of olefins and aromatics was first order in hydrocarbon and slightly inhibited by H2O, which is consistent with a Mars-van Krevelen type mechanism. Of the hydrocarbons investigated, only the rate of toluene reforming was negative first order in excess H2. Results from diffuse reflectance infrared Fourier transform spectroscopy of adsorbed toluene on a spinel catalyst suggest that H2 inhibits the rapid oxidation of the methyl group of toluene.

Cu1.5Mn1.5O4 spinel type composite oxide modified with CuO for synergistic catalysis of CO oxidation

Composite oxides of copper and manganese are widely used in oxidation reactions, and the main active component is copper-manganese spinel, whereas the copper oxide and manganese oxide show extremely low activity, despite that they are beneficial for CO oxidation. In this paper, it was found that the synergistic effect of Cu1.5Mn1.5O4 and CuO can promote the catalytic oxidation of CO. The catalysts were prepared using the citric acid complexation method and they were characterized through combined the techniques of N2-adsorption desorption, XRD, H2-TPR, TEM, CO-TPD and O2-TPD. The catalytic performance of various catalysts in CO oxidation was then evaluated. The results confirmed that Cu1.5Mn1.5O4 modified with CuO exhibited the best catalytic performance and the highest unit surface activity (defined as the CO conversion rate per unit surface area of catalyst). CuO and Cu1.5Mn1.5O4 had a synergistic effect, where O2 was activated by CuO and it then interacted with CO, activated by Cu1.5Mn1.5O4, to form CO2, thus increasing the catalytic activity.

Structural and Electrochemical Investigation of High Energy Spinel LiMn1.5Ni0.5O4 Produced at Low Temperature

Spinel LiMn1.5Ni0.5O4 (LMNO) materials, without the environmentally toxic Co, are one of the most promising potential cathode materials for lithium ion batteries operated at high voltages. According to the distribution of Mn and Ni within the cubic crystal structure, spinel LMNO can be categorized into either ordered or disordered. Normally, disordered LMNO materials are produced at temperatures which are higher than the theoretical oxygen release temperature of spinel LMNO (715 ℃)[1, 2]. This leads to some Mn4+ atoms being reduced to Mn3+ to maintain the electroneutrality of spinel LMNO. The existence of Mn3+ causes Jahn-Teller distortion and affects the structural stability of spinel LMNO. In order to reduce the amount of Mn3+, high temperature produced LMNO materials are post-annealed at lower temperature, i.e. 700 ℃, to introduce O into spinel LMNO again[3]. Both the high calcination temperatures and the long annealing times increase the preparation cost of the spinel LMNO. In other words, low-temperature calcination without post-annealing would be a preferred way to produce spinel LMNO at lower cost. However, LMNO materials calcinated at 700 ℃ in air show ordered distribution of Mn and Ni, which is supposed to be the reason for poor cycling and rate performance[4, 5]. This indicates changing the ordered distribution of Mn and Ni in 700 ℃ produced LMNO may improve the electrochemical performance.In the present work, different co-precipitation processes are attempted to change the distribution of the Mn and Ni precursors used in the LMNO synthesis. Three different spinel LMNO materials, named as N-, CS- and SW-LMNO, are produced by three hydroxide precursors with stoichiometric Li2CO3 at 700 ℃ without post-annealing. Scanning electron microscope (SEM) images, shown in Figure 1(a) – (c), demonstrate different morphologies of N-, CS- and SW-LMNO. This indicates that different precipitation approaches result in different particle sizes and shapes of spinel LMNO. However, the use of different precipitation processes has little effect on the bulk crystal structure of spinel LMNO, reflected by similar X-ray diffraction (XRD) patterns in Figure 1(d). As shown in Figure 1(e), SW-LMNO shows high initial discharge capacity (140 mAhꞏg-1) at room temperature which is the highest amongst all reported LMNO[6]. SW-LMNO also shows good cycling performance in the first 100 cycles at 1 C (1 C = 150 mAꞏg-1), which are not only better than those of other low-temperature produced spinel LMNO but also comparable with those of most reported high-temperature produced spinel LMNO[7]. This means, this work provides a new strategy to produce high-energy spinel LMNO by controlling the microstructure but save energy consumption. Moreover, transmission electron microscope (TEM), X-ray pair distribution function (PDF) and X-ray absorption spectroscopy (XAS) are performed to explain the reason for improvement of SW-LMNO in comparison with N- and CS-LMNO.Reference[1] A. Manthiram et al. A perspective on the high-voltage LiMn1.5Ni0.5O4 spinel cathode for lithium-ion batteries. Energy & Environmental Science[J]. 2014, 7 (4): 1339-1350[2] M. Kunduraci et al. Effect of oxygen non-stoichiometry and temperature on cation ordering in LiMn2-xNixO4 (0.50≥ x≥ 0.36) spinels. Journal of Power Sources[J]. 2007, 165 (1): 359-367[3] D. W. Shin et al. Role of cation ordering and surface segregation in high-voltage spinel LiMn1.5Ni0.5-xMxO4 (M= Cr, Fe, and Ga) cathodes for lithium-ion batteries. Chemistry of Materials[J]. 2012, 24 (19): 3720-3731[4] Z. Moorhead-Rosenberg et al. Electronic and electrochemical properties of Li1-xMn1.5Ni0.5O4 spinel cathodes as a function of lithium content and cation ordering. Chemistry of Materials[J]. 2015, 27 (20): 6934-6945[5] B. Aktekin et al. Cation ordering and oxygen release in LiNi0. 5-xMn1.5+xO4-y (LNMO): In situ neutron diffraction and performance in Li ion full cells. ACS Applied Energy Materials[J]. 2019, 2 (5): 3323-3335[6] O. Tiurin et al. Atomic layer deposition (ALD) of lithium fluoride (LiF) protective film on Li-ion battery LiMn1.5Ni0.5O4 cathode powder material. [J]. 2020, 448: 227373[7] G. Liang et al. Developing high-voltage spinel LiNi0.5Mn1.5O4 cathodes for high-energy-density lithium-ion batteries: current achievements and future prospects. Journal of Materials Chemistry A[J]. 2020, 8, 15373-15398 Figure 1

Study on the Performance of Copper‐Manganese Composite Oxide Catalysts for Toluene

AbstractCopper‐manganese (CuMn) composite oxide catalysts were prepared for the catalytic combustion of a common volatile organic compound, namely toluene. The structural properties of the catalysts were analysed by X‐ray diffraction (XRD), Brunauer‐Emmett‐Teller (BET) theory, X‐ray photoelectron spectroscopy (XPS) and hydrogen temperature‐programmed reduction (H2‐TPR). The catalytic stability was examined after 100 h. It was found that Cu1Mn1 exhibited the highest catalytic ability when the molar ratio of Cu : Mn was 1 : 1. The removal of toluene reached 90 % at a temperature below 229 °C. It was speculated that the Cu1.5Mn1.5O4 spinel structure and CuO were formed in the catalyst. The catalyst surface area was not the main factor affecting the catalytic activity. The catalysts displayed good stability and maintained high catalytic activity for a long time. Notably, fresh catalysts contained high amounts of high‐valent Mn4+ and Cu2+ cations, which played important roles in the catalytic combustion of toluene. The cations initiated the redox process and led to the conversion of toluene to CO2 and H2O. The reduced lattice oxygen supplied from air was transformed into surface oxygen on the catalyst. The catalytic combustion of toluene in the presence of CuMn composite oxide catalysts followed the Mars‐van Krevelen mechanism.

Electrical properties and stability of low temperature annealed (Zn,Cu) co-doped (Ni,Mn)3O4 spinel thin films

ABSTRACT Toward the development of infrared (IR) detectors, nickel–manganite-based thin films were initially prepared from (Ni0.2Mn2.8–x Cu x )Cl2 (0.010 ≤ x ≤ 0.040) solutions using the liquid flow deposition (LFD) method. The influence of Cu on the negative temperature coefficient of resistance (NTCR) characteristic of the films annealed at 400°C was investigated. It was found that the incorporation of Cu can effectively enhance electrical conductivity; however, it degrades both the thermal sensitivity and stability of the nickel–manganite films. The investigation was extended by further modifying the composition with Zn. The results revealed that by co-doping Cu with a proper amount of Zn the temperature coefficient of resistance (TCR) could be tailored, while a relatively low resistivity (ρ) of the final products was retained. Specially, when 0.01 mol Zn was added to a precursor solution containing 0.025 mol Cu, the resulting specimen possessed a TCR = 2.82% K–1 and a ρ = 820 Ω (measured at RT). More importantly, compared to Zn-free films, the (Zn,Cu) co-doped compositions showed much improved electrical stability, with an aging coefficient (ΔR/R) as low as 4.6%, after aging at 150°C in air for 500 h. The results suggest that the (Zn,Cu) co–doped (Ni,Mn)3O4 thin films have a promising application in IR detectors.

The Microstructure and Conductivity Evolution of Plasma-Sprayed (Mn, Co)3O4 Spinel Coatings during Conductivity Measurements at Elevated Temperature

(Mn, Co)3O4 spinel is widely used to protect the metallic interconnect of solid oxide fuel cells while it suffers deoxidization during the preparation by plasma spray. This work was proposed to study the effect of spray parameters on the microstructure and conductivity of spinel coatings. In this work, spinel coatings were prepared by the atmospheric plasma spray. The prepared coatings were heated up to 700 °C and held on for 15 h to allow the conductivity evolution. The microstructure and composition of coatings were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectrum (XPS). The results show that all coatings were evidently densified in two hours of heating while the measured conductivities were continuously evolved. The phase composition was found contributed more to the conductivity evolutions than the densification. The conversion of CoO to MnCo2O4 was observed and thus endowed the coatings a conductivity of 40 S/cm. A high fraction of Co3+ diffraction peaks, a high amount of Mn2+ and a low content of Co2+ jointly showed that more Co3+ occupied the B site of AB2O4 phase and more Mn2+ stood at the A site, indicating a stoichiometric composition of MnCo2O4. Annealing twins were detected by TEM and EBSD for the heated coatings but only a limited contribution to the evolution of the conductivity was considered. Finally, we suggest a high flowrate of plasma gas and a high input energy to prepare spinel coatings with designed conductivity.

A study on microstructure and magnetic properties of nanostructured CoxNi1-xMn0.5Fe1.5O4(x=0,0.25,0.5,0.75,1) spinel ferrites

A low-temperature synthesis of novel nanostructured CoxNi1-xMn0.5Fe1.5O4(x=0,0.25,0.5,0.75,1) ferrites was carried out by sol-gel auto-combustion technique. The obtained nanostructured ferrites were investigated by employing the techniques of powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX) and vibrating sample magnetometry (VSM). The XRD diffractograms of the prepared ferrites revealed the formation of a spinel phase with face centered cubic (fcc) structure belonging to Fd- m space group. The average lattice parameter ‘a’ of ferrites exhibited a rise versus a rise in Co2+ concentration in accordance with the Vegard’s law. The SEM investigation of NiMn0.5Fe1.5O4 powder revealed an existence of octahedral-shaped morphology of ferrite grains. The TEM investigation of NiMn0.5Fe1.5O4 powder showed nanostructures of ferrite particles with sizes consistent with the crystallite sizes as estimated by Debye-Scherer’s formula. An EDX spectrum of NiMn0.5Fe1.5O4 powder confirmed its elemental composition. The M-H hysteresis loops recorded by VSM at room temperature revealed a dependence of coercivity (Hc), maximum magnetization (Mmax) and retentivity (Mr) on Co2+concentration. Due to the shape dependence of M-H loops on Co2+ concentration in compounds enabled their candidature for applications in memory devices and magnetic sensors.

Influence of Dy3+ Ions on the Microstructures and Magnetic, Electrical, and Microwave Properties of [Ni0.4Cu0.2Zn0.4](Fe2-x Dy x )O4 (0.00 ≤ x ≤ 0.04) Spinel Ferrites.

[Ni0.4Cu0.2Zn0.4](Fe2–xDyx)O4 spinel ferrite nanoparticles with different Dy3+ concentrations (0.00 ≤ x ≤ 0.04) were prepared by a citrate sol–gel auto-combustion technique. A strong correlation among Dy concentration, structural parameters, and magnetic, electrical, and microwave properties was established. An increase in the Dy3+ concentration is the reason for a rise in the crystal structure parameters (due to different ionic radii of Fe and Dy ions) and a slight increase in the average particle size with a minor reduction in the specific surface area. It was observed that Dy3+ ions prefer to occupy the octahedral B site due to their large ionic radius (0.91 Å). The explanation of the electrical and magnetic properties was given in terms of the features of Dy3+–O2––Fe3+ dysprosium–oxygen–iron indirect exchange. The occurrence of the intensive changes in amplitude–frequency characteristics was observed from 1.6 to 2.7 GHz. The explanation of electromagnetic absorption was given in terms of the peculiarities of the microstructure (resonance of domain boundaries). The results open perspectives in the utilization of [Ni0.4Cu0.2Zn0.4](Fe2–xDyx)O4 spinel ferrite nanoparticles as functional materials for targeted drug delivery and hyperthermia applications.