Abundant Oxygen Vacancies Research Articles

Key factors for methane combustion over palladium-based catalysts revealed by enhanced and depressed catalytic performance

In this work, metal oxides with different phase structures (stannic oxide, zirconia and tin-zirconium binary oxide) were designed to construct palladium-based catalysts. The factors affecting methane combustion over catalysts were revealed by exploring the reasons for the enhancement and depression of catalytic performance under various reaction conditions. The formation of SnxZr1−xO2 solid solutions not only generated abundant oxygen vacancies to improve the reducibility of PdO and the redox of Pd↔PdO, but also induced the formation of active PdOx, conjointly leading to the excellent catalytic activity and hydrothermal stability. Under SO2-containing atmosphere, the thermodynamically favorable formation of stable Zr(SO4)2 caused the significant diminish of oxygen vacancies and active Pd step sites in catalysts. Consequently, the sulfur-resistance and regeneration performance of Zr-containing catalysts were inferior to those of Pd/SnO2. The above contrast manifested that both oxygen vacancies and active palladium species were responsible for the efficient and stable methane combustion.

The influence of annealing temperature on NH3-SCR performance of CexZr1-x/UiO-66 bimetal MOFs

MOFs possess large surface area and high porosity, but they will convert into derivatives when subjecting to high temperature due to its poor thermal stability. Herein, a series of Ce-Zr bimetal MOFs derivatives were prepared by directly pre-annealing of CexZr1−x/UiO-66 bimetal MOFs at different temperatures under air atmosphere. The as-prepared samples were used as catalysts for selective catalytic reduction of NOx with NH3. The catalytic activity test results indicated that pre-annealing temperature had an obvious promoting effect on NH3-SCR performance. C0.5Z0.5/U-400 catalyst achieved a NOx conversion of 92% at 350 ºC and a complete N2 selectivity in a wide temperature range of 150–500 ºC. Various characterization tools were employed to reveal the effects of pre-annealing temperature on the microstructure and chemical properties of C0.5Z0.5/U bimetal MOFs and its derivatives. The pre-annealing treatment could lead to a distinctive decrease in specific surface area, demonstrating that C0.5Z0.5/U bimetal MOFs gradually converted into derivatives and there was an obvious agglomeration phenomenon between derived metal oxides. But a relatively large specific surface area and fluffy porous microstructure could be obtained for pre-annealing C0.5Z0.5/U at 400 ºC for 2 h. It was conducive to exposure of more active sites, thus enhancing catalytic performance. Moreover, C0.5Z0.5/U-400 catalyst not only possessed a higher Ce3+/(Ce3+ + Ce4+) ratio and a better reduction ability, but also had more abundant oxygen vacancies to adsorb and activate reactant gases. In-situ DRIFTS results indicated that NH3-SCR reactions over C0.5Z0.5/U-400 catalyst followed both E-R and L-H mechanisms.

Single-atom Zr embedded Ti4O7 anode coupling with hierarchical CuFe2O4 particle electrodes toward efficient electrooxidation of actual pharmaceutical wastewater

Electrocatalytic oxidation is commonly restricted by low degradation efficiency, slow mass transfer, and high energy consumption. Herein, a synergetic electrocatalysis system was developed for removal of various drugs, i.e., atenolol, florfenicol, and diclofenac sodium, as well as actual pharmaceutical wastewater, where the newly-designed single-atom Zr embedded Ti4O7 (Zr/Ti4O7) and hierarchical CuFe2O4 (CFO) microspheres were used as anode and microelectrodes, respectively. In the optimal reaction system, the degradation efficiencies of 40 mg L–1 atenolol, florfenicol, and diclofenac sodium could achieve up to 98.8%, 93.4%, and 85.5% in 120 min with 0.1 g L–1 CFO at current density of 25 mA cm–2. More importantly, in the flow-through reactor, the electrooxidation lasting for 150 min could reduce the COD of actual pharmaceutical wastewater from 432 to 88.6 mg L−1, with a lower energy consumption (25.67 kWh/m3). Meanwhile, the electrooxidation system maintained superior stability and environmental adaptability. DFT theory calculations revealed that the excellent performance of this electrooxidation system could be ascribed to the striking features of the reduced reaction energy barrier by single-atom Zr loading and abundant oxygen vacancies on the Zr/Ti4O7 surface. Moreover, the characterization and experimental results demonstrated that the CFO unique hierarchical structure and synergistic effect between electrodes were also the important factors that could improve the system performance. The findings shed light on the single-atom material design for boosting electrochemical oxidation performance.

Synthesis of novel direct Z-scheme heterojunction photocatalyst from WO3 nanoplates and SrTiO3 nanoparticles with abundant oxygen vacancies

Excessive use of insecticides in agriculture has raised a global concern owing to their environmental pollution. Various techniques are utilized for removal these compounds which one of the most efficient methods is photocatalytic degradation by heterojunction nanocomposites. Herein, a direct Z-scheme heterojunction nanocomposite with abundant oxygen vacancy defects was fabricated via simple hydrothermal treatment of WO3 nanoplate and SrTiO3 nanoparticle and was utilized as a powerful photocatalyst for decomposition of nitenpyram insecticide under illumination of simulated sunlight. The findings revealed that, the heterojunction nanocomposite with 30wt% of SrTiO3 has the best photocatalytic efficiency. Improvement of the photocatalytic ability may be ascribed to the existence of oxygen vacancy defects in heterojunction interfaces between two semiconductors, facilitating of charge carriers separation, and effective generation of the •O2− and •OH radicals. Furthermore, from the results of the radical capture studies, ESR techniques, and Mott-Schottky analyses, it can be concluded that the hydroxyl radical is the major oxidant for degradation of nitenpyram, and by modulating the charge carrier dynamics and regulation of the charge transfer pathways, a direct Z-scheme heterojunction nanocomposite is formed with improved photocatalytic efficiency. Additionally, the possible decomposition intermediates of nitenpyram were detected from the HPLC-MS analysis.

Cu docking-activated Nb incorporation in multivariate CuO-Nb2O5/CeO2 catalysts for selective reduction of NOx with NH3

Improving the low-temperature selective catalytic reduction of NOx with NH3 (NH3-SCR) meanwhile maintaining N2 selectivity are challenging in the field of catalysis. Herein, we showcase the fabrication of the synergistic catalytic sites in the CuO-Nb2O5/CeO2 catalysts. Impressively, the CuO-Nb2O5/CeO2 is characteristic with abundant oxygen vacancies at the interfacial regions, which facilitate the redox reaction between Ce3+, Nb5+ and Cu2+ with the assistance of surface oxygen, exhibiting high NH3-SCR activity, N2 selectivity and wide temperature window at a gas hourly space velocity of 150,000 h−1. Contrast experiments, EXAFS analysis, in situ infrared and Raman spectroscopy, and theoretical calculations indicate that the introduction of proper copper ions in the CeO2 lattice induces the structure distortion of surface CeO2, improving more niobium ions incorporation into the CeO2. The embedded Nb synergizes Cu and Ce to evoke the generation of newly adsorptive sites for NOx and NH3, thus contributing to the NH3-SCR reaction.

Performance Enhancement and In Situ Observation of Resistive Switching and Magnetic Modulation by a Tunable Two‐Level System of Mn Dopants in a‐Gallium Oxide‐based Memristor

AbstractPurely gallium oxide‐based memristors (GOMRs) show great potentials in resistive random‐access‐memory (RRAM) due to their chemical stability and resistive switching characteristics with Roff/Ron ratios up to 102; indeed, GOMRs with higher Roff/Ron ratios and more functionalities are more expected. In this study, ferromagnetic amorphous gallium oxide (a‐GMO) films with a tunable two‐level system of Mn dopants, i.e., Mn2+ and Mn3+ ions, are prepared by scalable polymer assisted deposition. The Pt/a‐GMO/Pt memristors show a high Roff/Ron ratio of 103, at least one order of magnitude higher than those of previously reported purely GOMRs, thanks to the abundant oxygen vacancies (VOs)‐induced low resistance state and Mn2+‐enhanced high resistance state. Meanwhile, magnetic modulation (MM) is realized electrically in the a‐GOMRs during the RS, through the tuning of bound magnetopolarons (BMPs) by bias voltage‐induced VOs variations, which may be useful for quaternary information coding. Notably, the transition between Mn3+ and Mn2+ions is observed in the GOMRs, which is closely related to the variations of VO concentration and BMP amount, providing an in situ tool to probe the VO‐induced RS and BMP‐dependent MM. The results give insights to Mn‐doped GOMRs and may be useful for design, fabrication, and testing of multifunctional high‐performance RRAMs.

Boosting borohydride hydrolysis for H2 generation by MOF-templated void-engineered shaggy cobalt oxide: Abundant oxygen vacancy-mediated enhancement

Designing an advantageous catalyst for enhancing the release of hydrogen (H2) from NaBH4 hydrolysis is still desirable. Herein, a void-engineered shaggy cobalt oxide (VSCO) is constructed via facile carving and calcination using the cuboid cobalt-based metal organic framework (Co-MOF) as a template. The as-prepared VSCO shows unique structural properties, such as large internal void, high specific surface area, and abundant oxygen vacancy, enabling VSCO to boost H2 production from NaBH4 hydrolysis. VSCO also exhibits an activation energy (Ea) of 28.9 kJ mol−1, which is much lower than that of commercial Co3O4 NP (62.9 kJ mol−1) and most of recent reported noble metals. VSCO also retains its outstanding catalytic activity over multiple cycles. This work sheds a light into the new approach of constructing metal oxide material with the void structure and abundant oxygen vacancies for catalyzing the hydrolysis of NaBH4.

Fe-doping and oxygen vacancy achieved by electrochemical activation and precipitation/dissolution equilibrium in NiOOH for oxygen evolution reaction

The poor conductivities and instabilities of accessible nickel oxyhydroxides hinder their use as oxygen evolution reaction (OER) electrocatalysts. Herein, we constructed Fe-NiOOH-OV-600, an Fe-doped nickel oxide hydroxide with abundant oxygen vacancies supported on nickel foam (NF), using a hydrothermal method and an electrochemical activation strategy involving 600 cycles of cyclic voltammetry, assisted by the precipitation/dissolution equilibrium of ferrous sulfide (FeS) in the electrolyte. This two-step method endows the catalyst with abundant Fe-containing active sites while maintaining the ordered structure of nickel oxide hydroxide (NiOOH). Characterization and density functional theory (DFT) calculations revealed that synergy between trace amounts of the Fe dopant and the oxygen vacancies not only promotes the generation of reconstructed active layers but also optimizes the electronic structure and adsorption capacity of the active sites. Consequently, the as-prepared Fe-NiOOH-OV-600 delivered large current densities of 100 and 1000 mA cm−2 for the OER at overpotentials of only 253 and 333 mV in 1 mol/L KOH. Moreover, the catalyst is stable for at least 100 h at 500 mA cm−2. This work provides insight into the design of efficient transition-metal-based electrocatalysts for the OER.

Scalable Synthesis of Oxygen Vacancy-Rich Unsupported Iron Oxide for Efficient Thermocatalytic Conversion of Methane to Hydrogen and Carbon Nanomaterials.

Thermocatalytic methane decomposition (TCMD) involving metal oxides is a more environmentally friendly and cost-effective strategy for scalable hydrogen fuel production compared to traditional methane steam reforming (MSR), as it requires less energy and produces fewer CO/CO2 emissions. However, the unsupported metal oxide catalysts (such as α-Fe2O3) that would be suited for this purpose exhibit poor performance in TCMD. To overcome this issue, a novel strategy was developed as a part of this work, whereby oxygen vacancies (OVs) were introduced into unsupported α-Fe2O3 nanoparticles (NPs). Systematic characterization of the obtained materials through analytical techniques demonstrated that mesoporous nanostructured unsupported α-Fe2O3 with abundant oxygen vacancies (OV-rich α-Fe2O3 NPs) could be obtained by direct thermal decomposition of ferric nitrate at different calcination temperatures (500, 700, 900, and 1100 °C) under ambient conditions. The thermocatalytic activity of the resulting OV-rich α-Fe2O3 NPs was assessed by evaluating the methane conversion, hydrogen formation rate, and amount of carbon deposited. The TCMD results revealed that 900 °C was the most optimal calcination temperature, as it led to the highest methane conversion (22.5%) and hydrogen formation rate (47.0 × 10-5 mol H2 g-1 min-1) after 480 min. This outstanding thermocatalytic performance of OV-rich α-Fe2O3 NPs is attributed to the presence of abundant OVs on their surfaces, thus providing effective active sites for methane decomposition. Moreover, the proposed strategy can be cost-effectively scaled up for industrial applications, whereby unsupported metal oxide NPs can be employed for energy-efficient thermocatalytic CH4 decomposition into hydrogen fuel and carbon nanomaterials.

Cyanogel-Based Preparation of Amorphous NiFe Nanoaggregates with Enhanced Activity and Stability for OER

The development of cost-efficient electrocatalysts for oxygen evolution reaction (OER) with high efficiency is crucial to widespread applications of water splitting for hydrogen production. In this work, porous three-dimensional (3D) amorphous NiFe nanoaggregates composed of interconnected nanograins were synthesized by a cyanogel-based wet chemical reduction method using the NiCl2/Na4Fe(CN)6 cyanogel as the precursor and NaBH4 as the reducing agent. The influence of the incorporated Fe amount was carefully studied by slightly changing the feeding molar ratios of the Ni/Fe atoms in the precursors. The intrinsic 3D backbone structure of the cyanogel resulted in crystal nuclei tending to generate along with the backbones, which is key to the formation of NiFe nanoaggregates with a porous 3D interconnected structure. The synthesized NiFe nanoaggregates with a 3D interconnected structure and high porosity, as well as the incorporation of Fe, are in favor of high surface area, more active sites, and abundant oxygen vacancies, leading to superior activity and stability of OER in alkaline electrolytes with a low overpotential of 0.35 V at 10 mA cm−2, a high current density of 24.8 mA cm−2 at 1.65 V, a small Tafel slope of 76.9 mV dec−1, and attractive durability in 1 M KOH solution.

Oxygen vacancy for boosted alkaline oxygen evolution under AC magnetic field

Reducing the overpotential of oxygen evolution reaction (OER) with slow kinetics is important to realize commercial hydrogen production from water electrolysis. Here, ZnCo2O4 with abundant surface oxygen vacancies is employed to clarify the influence of oxygen vacancies on OER performance under AC magnetic field. The additional active sites as well as enhanced reaction activity of each active sites resulting from oxygen vacancies enable N–ZnCo2O4-δ-350 to exhibit outstanding OER performance, where the overpotential is 406 mV at 10 mA cm−2. Furthermore, double exchange interaction between Co2+-O2--Co3+ ignited by oxygen vacancies produces ferromagnetism in N–ZnCo2O4-δ, which can directly promote OER under ACMF. And the mass transport at additional active sites active sites provided by oxygen vacancies is also accelerated under ACMF due to magnetohydrodynamic convection. As a result, non-iR-corrected overpotential at 10 mA cm−2 of N–ZnCo2O4-δ-350 is reduced to 243 mV when the ACMF intensity is 4.752 mT, which is a 44.5% reduction compared to no ACMF. These findings provide a novel and efficient perspective for realizing efficient electrocatalysis.

In-situ construction of Schottky junctions by highly dispersed Ni co-catalysts supported on mesoporous defective t-TiO2 for boosting photocatalytic H2 evolution

Designing photocatalysts with interfacial design and defect engineering are crucial to improved H2-evolution efficiency of photocatalytic water splitting. Herein, mesoporous tablet-like x-wt% Ni/t-TiO2 catalysts with Schottky junction and oxygen vacancies (Oν) were constructed using metal–organic framework (namely MIL-125(Ti)) as template. It is demonstrated that the optimal composition of 1 wt% Ni/t-TiO2 achieves a H2 production rate of 4276.13 μmol/g/h without using noble metal Pt. It was ∼129 times higher than MIL-125(Ti) derived pure TiO2 (t-TiO2) and showed fair durability after least four consecutive cycles test under full spectrum light irradiation. Particularly, its apparent quantum efficiency (AQE) at 350 nm reaches 12.09 %. Our results show that the synergetic effect of Schottky junction and oxygen vacancies (Oν) contribute significantly to the high catalytic performance. Meanwhile, the mesoporous structure provides a large specific surface area which is crucial for further enhancement of the reaction rate. This work opens an effective and low-cost strategy for preparing novel heterojunction photocatalysts with abundant oxygen vacancies and improving the hydrogen generation rate of MOF-derived photocatalysts without using noble metal.

Tungsten oxide decorating-regulated iron-nickel oxide heterostructure as electrocatalyst for oxygen evolution reaction

The development of efficient and long-term stable electrocatalysts for oxygen evolution reaction (OER) is the key to harness hydrogen energy by water electrolysis. In this work, tungsten oxide decorated iron-nickel oxide heterostructure was grown on nickel skeleton by hydrothermal synthesis through simultaneously chemical etching reaction and decomposition of transition metal oxalate, forming porous WO3/Fe2O3-NiO/NF composite. The decoration of WO3 creates WO3/Fe2O3-NiO heterostructure interface with high-density heterometal-oxygen bridge sites, which can effectively modulate the intrinsic electronic structure of the catalyst and optimize the adsorption energy of the reaction intermediates, thus accelerate the OER through the adsorbate evolution mechanism. In addition, abundant oxygen vacancy (VO) sites induced by high valence W6+ further boost OER activity through the lattice oxygen-mediated mechanism. With the unique porous interfacial structure, the as-synthesized WO3/Fe2O3-NiO/NF exhibits remarkable OER electrocatalytic activity with low overpotential of 211 mV at OER current density of 100 mA cm-2, and high stability at 100 mA cm-2 with overpotential fluctuates within ± 2.8% range during the 100 h test. The as-proposed method is effective in lowering the overpotential of OER, and may be applicable for other electrochemical energy conversion applications.

Elucidating the effect of Ce with abundant surface oxygen vacancies on MgAl2O4-supported Ru-based catalysts for ammonia decomposition

Ammonia is a promising COx-free hydrogen (H2) carrier because of its high volumetric H2 density. However, developing highly active/stable ammonia decomposition catalysts for H2 production remains challenging. In this study, the role of ceria (CeO2) as a promoter for Ru-based catalysts was investigated. Ru/Cex/MgAl(y00) catalysts were prepared by the incipient wetness impregnation (IWP) and deposition–precipitation (DP) methods. The surface oxygen vacancy (OV) concentration was controlled by the Ce loading and calcination temperature. By evaluating the surface reaction behavior of the ammonia decomposition catalysts, this study proposes that CeO2 effectively improves the recombination/desorption of N and H atoms. The optimal catalyst Ru/Ce5/MgAl(600) achieved a high H2 formation rate (27.4 mmolH2·gcat−1∙min−1 at 30,000 mL·gcat−1·h−1, 450 °C) and stable performance after exposure at 650 °C for 20 h. This study provides insight into the effect of abundant surface OVs in CeO2 on the physicochemical properties and ammonia decomposition performance of the catalysts.

High-Entropy Oxides as Nanozymes for Non-Immunological Electrochemical Monitoring of Cortisol

High entropy oxides (HEOs) are a class of materials that have garnered significant attention in recent years due to their potential use as catalysts in energy-related applications[1]. These materials are derived from high entropy compounds, which consist of equimolar or near equimolar ratios of multiple primary elements and have configurational entropy in the formation of a single-phase structure. The cocktail effect of HEOs gives rise to various catalytically active sites due to the synergistic interaction of the different elements, and lattice distortion during the formation of a single-phase solid solution results in the generation of abundant oxygen vacancies, which enhances the adsorption activity of HEOs and improves their catalytic performance[2]. HEOs can also be cost-efficient by utilizing low-cost transition metal elements as their active centers. Despite their widespread use as catalysts, the application of HEOs in electrocatalysis for molecule monitoring and early diagnosis remains largely unexplored.Cortisol, a glucocorticoid hormone that regulates many aspects of metabolism, stress, and immune response, is closely related to various diseases and disorders[3]. Current methods for noninvasive monitoring of cortisol relies upon immunological assays and typically limited to labor-intensive and time-consuming laboratory techniques. As a consequence, detecting cortisol in a label-free manner that is low cost and offers significant sensor stability and high sensitivity is desirable for many emerging applications, particularly for on-body monitoring.Here, we present work that examines the catalytic activities of HEOs as nanozymes for the electrocatalysis of cortisol. The electrochemical cortisol sensor is based on a novel one-pot microwave-assisted reaction synthesized (Fe, Co, Mn, Ni, Cu) Ox. The catalytic mechanism of electrochemical sensors depends on the formation of various valence metal redox species pairs, where the higher valence state oxidizes cortisol species present on the electrode surface, accompanied by a reduced lower valence state. The one-pot microwave-assisted synthesized HEOs exhibited ultra-high catalytic activity towards cortisol species as measured by cyclic voltammetry and chronoamperometry. Experimental results show a superior sensing performance including a detection limit as low as 0.5 µM (s/N=3), a good linear detection range between 0.01 ~ 4.01 µM, and a sensitivity of 46.5 µA µM-1 cm-2.

BiOBrxI(1-x) solid solution with oxygen vacancies boosting the photocatalytic degradation of multiple pollutants

In this work, we prepared BiOBrxI(1-x) solid-solution with abundant oxygen vacancies by a facile hydrothermal method. The efficiency in the photocatalytic degradation of rhodamine B with BiOBr0.5I0.5 as photocatalyst was determined to be more than 99.9% in 12 min under visible light irradiation, which was 2 and 5 times higher than pure BiOBr and BiOI, respectively. Further tests revealed that BiOBr0.5I0.5 sample was able to efficiently decompose other contaminants including CrVI, tetracycline hydrochloride, methyl orange and methylene blue. In addition, the resultant solution after the degradation process has considerable biocompatibility through the growth of wheat seeds. Various spectroscopic characterizations and electrochemical studies were performed including PL, EIS etc. The excellent photocatalytic performance can be ascribed to the effective separation of photogenerated carriers, where the synergistic impact of solid solution and oxygen vacancies played an essential role. The band structure of the sample was established and the reaction mechanism was discussed. The atomically mixed oxyhalide is proved to be a promising photocatalyst for practical environmental applications.

Melamine intercalation approach for the synthesis of C, N codoped net-like NiCo oxides composites and Its Application towards oxygen evolution reaction in alkaline solution

Electrolysis of water is a leading hydrogen production pathway with the advantages of simple technology, mature development and small environmental impact. However, the energy conversion efficiency seriously decreased due to the high overpotential produced by oxygen evolution reaction (OER). To decrease the overpotential of OER, NiCoOx particles with small sizes embedded in layered structure constituted with carbon and nitrogen composites (NiCoOx-CN) were prepared through melamine intercalation method with acetylacetonate as cobalt and nickel source. Finally, NiCoOx-CN showed excellent OER performance with an overpotential of only 326 mV to obtain current density of 10 mA cm−2 and Tafel slope of 70.8 mV·dec−1. Besides, NiCoOx-CN possessed excellent long-term durability in alkaline environments. Scanning electron microscopy (SEM), electron paramagnetic resonance (EPR) and other techniques were implemented to study the mechanism for the increased OER performance of NiCoOx-CN. The microstructure of NiCoOx-CN that the nanoparticles embedded into the interlinked layered nanosheets was helpful to expose more active sites. Meanwhile, the doping of C and N heteroatom and abundant oxygen vacancies also led to the enhanced conductivity of NiCoOx-CN and the optimized the electron transfer. Thus, the OER performance of NiCoOx-CN significantly boosted. As a result, this strategy holds great promise in preparing other high performance OER electrocatalysts with melamine.

Interface-engineering in MOF-derived In2O3 for highly sensitive and dual-functional gas sensor towards NO2 and triethylamine

Nitrogen dioxide (NO2) and triethylamine (TEA) vapor are two typical reactive nitrogen species harmful to personal health and the environment. Thus, multi-functional and reliable sensors were momentous. Herein, the interface engineering of MOF-derived In2O3 with various contents of oxygen vacancy (OV) and different crystalline phases was achieved via pyrolysis of NH2-MIL-68(In) at different temperatures. High-proportional mixed hexagonal and cubic crystalline phases were presented in the MOF-derived In2O3 obtained by pyrolysis of NH2-MIL-68(In) at 400 °C (MOF-In2O3-400). Benefiting from the larger surface and interface area, MOF-In2O3-400 exhibited abundant OV and improved gas-sensing performance compared with the samples obtained at 500 °C and 600 °C. Interestingly, the MOF-derived In2O3 sensors exhibited dual-selective and ppb-level detection of NO2 and TEA vapor at distinct optimum operating temperatures. MOF-In2O3-400 exhibited ultra-high response of 1210 to 200 ppb NO2, and can detect as low as 1 ppb NO2 at 30 °C and TEA vapor as low as 500 ppb at 200 °C. It was also assessed for excellent long-term stability up to 120 days. As a practical demonstration, a prototype device was fabricated on an integrated circuit platform to issue a warning when NO2 or TEA concentrations exceeded the safety thresholds. The results indicate that the MOF-In2O3-400 is an excellent candidate in temperature-dependent dual-functional gas sensors towards NO2 and TEA vapor.

Synthesis of NiCo2O4/CF composites with heterogeneous interfaces as excellent microwave absorbers

The absorbers with strong absorption capacity, wide effective absorption bandwidth (EAB), low filling, lightweight and thin thickness are the future development direction of electromagnetic wave (EMW) absorption field. NiCo2O4/CF composites were synthesized by simple hydrothermal method and calcination method in this study. X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and vector network analyzer (VNA) were used to study the crystal structure, microstructure, surface chemical states and microwave absorption properties of the composites, respectively. Carbon fiber (CF) was used as nucleation carrier, and NiCo2O4 formed a dense conductive network on its surface in the shape of nanoneedles. In addition, there are abundant oxygen vacancies and heterogeneous interfaces on the surface of composite materials, which can cause dipole polarization and interface polarization and lead to more dissipation of electromagnetic wave energy. The minimum reflection loss (RLmin) of the NiCo2O4/CF composite is − 59.75 dB at the thickness of 2.01 mm. The maximum effective absorption bandwidth (RL < −10 dB) of the composite is 4.12 GHz (8.84–12.96 GHz) at the thickness of 2.18 mm. In the thickness range of 1.2–5 mm, the effective absorption frequency range of the composite material accounted for 80.78% of the measurement range (3.46–18 GHz), which covered the whole C-band, X-band and Ku-band, as well as a portion of the S-band. NiCo2O4/CF composites have the characteristics of strong absorption and wide absorption bandwidth due to the synergistic effect of dielectric loss and magnetic loss, good impedance matching characteristics and more Debye relaxation processes, which provide new inspiration for the structure and component design of lightweight microwave absorbing materials (MAMs).

Elemental Fe conditioning for the synthesis of highly selective and stable high entropy catalysts for CO2 methanation

High entropy oxides (HEOs) with high stability and designability have demonstrated excellent catalytic performance in CO2 hydrogenation. However, it is challenging to combine high methane selectivity and high stability of HEOs using an equimolar design strategy. In this study, a non-equimolar design strategy was used to fix the Cr, Co, Mn and Ni elements and adjust different Fe molar amounts to prepare a highly crystalline HEO at 900 °C. After H2 reduction, the catalyst (HEO-4-900/H2) achieved 72.9% CO2 conversion and 98.8% methane selectivity at atmospheric pressure and 400 °C, and maintained a high methane selectivity of over 97% after 100 h stability test. The catalytic activity was influenced by the amount of Fe. Increasing the amount of Fe could precisely regulate the crystallinity and oxygen defects of the crystal, improve the catalyst stability, reduce the interaction between the Co-Ni alloys and HEOs, and allow the catalyst to contain more Co-Ni active sites and more abundant oxygen vacancies. This study demonstrates the considerable potential of HEOs for CO2 methanation, and provides a new design idea for further stimulating highly active CO2 methanation HEOs catalysts.