ZnO–Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>–Au Hybrid Composites for Thioanisole Oxidation Under Visible Light: Experimental and Theoretical Studies

Organic sulfides are present in many materials such as coal, organic polymers, and xenobiotics and cause severe pollution problems during disposal, and their oxidation is essential from the environmental point of view. ZnO and ZnO–Fe₃O₄–Au NPs were prepared, and characterized by analytical methods (XRD, TEM, HRTEM, XPS and Confocal Microscope Analysis) for employment as photo-catalysts for the oxidation of thioanisole under visible light. A considerable visible light photo-oxidation of sulfur compound was observed in the presence of ZnO–Fe₃O₄–Au NPs, where the content Fe₃O₄ facilitates its isolation and recovery from the reaction medium by an external magnet for reuse in the oxidation. The oxidation of thioanisole was monitored spectroscopically, and was found to follow first order kinetics in the substrate, for which a possible mechanism is proposed. Density functional theory was used to analyze the oxidation pathway in order to predict the rate limiting step in the reaction since the sulfoxidation through sulfide radical cation >S^+· → >S=O is important biotechnologically and environmentally.     

Reduction of Oxide Mixtures of (Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> + CuO) and (Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> + Co<Subscript>3</Subscript>O<Subscript>4</Subscript>) by Low-Temperature Hydrogen Plasma

The paper presents experimental results pertaining to the reduction of oxide mixtures namely (Fe₂O₃ + CuO) and (Fe₂O₃ + Co₃O₄), by low-temperature hydrogen plasma in a microwave hydrogen plasma set-up, at microwave power 750 W and hydrogen flow rate 2.5 × 10^?6 m³ s^?1. The objective was to examine the effect of addition of CuO or Co₃O₄, on the reduction of Fe₂O₃. In the case of the Fe₂O₃ and CuO mixture, oxides were reduced to form Fe and Cu metals. Enhancement of reduction of iron oxide was marginal. However, in the case of the Fe₂O₃ and Co₃O₄ mixture, FeCo alloy was formed within compositions of Fe₇₀Co₃₀, to Fe₃₀Co₇₀. Since the temperature was below 841 K, no FeO formed during reduction and the sequence of Fe₂O₃ reduction was found to be Fe₂O₃ → Fe₃O₄ → Fe. Reduction of Co₃O₄ preceded that of Fe₂O₃. In the beginning, the reduction of oxides led to the formation of Fe–Co alloy that was rich in Co. Later Fe continued to enter into the alloy phase through diffusion and homogenization. The lattice strain of the alloy as a function of its composition was measured. In the oxide mixture in which excessive amount of Co₃O₄ was present, all the Co formed after reduction could not form the alloy and part of it appeared as FCC Co metal. The crystallite size of the alloy was in the range of 22–30 nm. The crystal size of the Fe–Co alloy reduced with an increase in Co concentration.     

Preparation and adsorption properties of magnetic Co<Subscript>0.5</Subscript>Ni<Subscript>0.5</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript>–chitosan nanoparticles

Magnetic chitosan microspheres were prepared by the emulsification cross-linking technique in the presence of glutaraldehyde as cross-linking agent, liquid paraffin as dispersant, and Span-80 as emulsifier. The optimal cross-linking time and Co_0.5Ni_0.5Fe₂O₄: chitosan ratio were determined. The morphology of particles was studied by different techniques. The adsorption characteristics were studied and the effect exerted by the initial concentration of methyl orange, the time of cross-linking, and the amount of the adsorbent was determined. It is found that the product obtained at the Co_0.5Ni_0.5Fe₂O₄: chitosan ratio 1: 4 and the crosslinking time 5 h has the uniform morphology. At room temperature, the Co_0.5Ni_0.5Fe₂O₄–chitosan magnetic composite has maximal adsorption for methyl orange at the dosage 20 mg.     

Characterization of gold supported on Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–CuO–Mn<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts obtained by thermal decomposition of aerosols

The purpose of this work was to employ the differential thermal analysis (DTA) technique to compare variations in the collapse energy of the zeolite Y crystalline structure in a fresh catalyst and in the same catalyst impregnated with nickel and vanadium. A small exothermic signal in the DTA curve at 950–1150 °C indicated the collapse of the crystalline structure. The areas of the exothermic signals in the DTA curves of the two samples indicated a reduction in the curve of the metal impregnated catalyst. These results were compared with X-ray data, leading to the conclusion that metal impregnation affects the zeolite Y crystalline structure and that the DTA technique is a potentially useful tool for measuring the integrity of zeolite Y in catalysts.     

Co<Subscript>3</Subscript>O<Subscript>4</Subscript> nanorods–graphene composites as catalysts for rechargeable zinc-air battery

Composite of Co₃O₄ nanorods with reduced graphene oxide (RG-Co₃O₄) for rechargeable zinc-air battery was prepared by a facile method. The Co₃O₄ nanorods with a length of 1–2 μm were homogeneously distributed on the surface of plicated graphene nanosheets. The samples were characterized by X-ray diffraction, scanning electron microscopy, and Raman spectroscopy. The RG-Co₃O₄ composite showed better electrode potential and higher electrical conductivity compared with pure Co₃O₄. The zinc-air battery can be reversibly charged and discharged for hundred cycles with a good cycle performance. The improved battery performance of RG-Co₃O₄ can be attributed to the synergistic coupling effect between the graphene sheets and Co₃O₄.     

High-Resolution Transmission Electron Microscopy Study of LiNi<Subscript>x</Subscript>Co<Subscript>1−x</Subscript>O<Subscript>2</Subscript> Synthesized by Unconventional Methods

Single phase LiNi_xCo_1–xO₂ (x=0.3) with fine particles were prepared by two low-temperature methods: the modified Pechini sol-gel method and the Self-Propagating Combustion Synthesis (SPCS). It was found that bulk quantities of nano-sized particles of layered LiNi_xCo_1–xO₂ could be obtained at temperatures below 400°C by these solutions technique. The synthesized products were characterized by structural (XRD) and thermal (DTA-TG) analyses. HRTEM was used to evaluate the purity and the phase composition of LiNi_xCo_1–xO₂. Lattice plane images give information about crystal structure and SAED patterns help us to identify the phase. Nano-crystals were obtained with minimum mean diameter about 5nm for both methods.     

Preparation and characterization of Ba<Subscript>2</Subscript>Co<Subscript>2</Subscript>Fe<Subscript>12</Subscript>O<Subscript>22</Subscript> ferrite via glucose sol–gel method

Ba₂Co₂Fe₁₂O₂₂ (Co₂Y) was synthesized by sol–gel method using glucose as chelating agent. X-Ray diffraction studies indicate that sintering temperature as low as 950 °C is sufficient to produce Co₂Y ferrites. Co₂Y ferrites calcined at 1,000 °C exhibit good magnetic prosperities in high frequency, with the resonance frequency up to 11 GHz and intrinsic permeability about 5 even at 6 GHz. The heat-treated temperature dependence of coercivity, initial permeability and resonance frequency is close related to the particle shape and size.     

Effect of Calcination Temperature on the Catalytic Oxidation of Formaldehyde Over Co<Subscript>3</Subscript>O<Subscript>4</Subscript>–CeO<Subscript>2</Subscript> Catalysts

The effect of calcination temperature (350–650?°C) on the structure and catalytic activity of Co₃O₄–CeO₂ mixed oxides prepared by sol–gel method was investigated by XRD, H₂-TPR, O₂-TPD and formaldehyde (HCHO) oxidation. The Co₃O₄–CeO₂ calcined at 450?°C (Co₃O₄–CeO₂-450) exhibited the best performance, showing that the complete oxidation of HCHO was achieved at temperature as low as 80?°C. The results of characterizations revealed that the Co₃O₄–CeO₂-450 had excellent catalytic activity due to the larger specific surface area, the best reducibility and more abundant surface active oxygen species.     

Direct electron transfer of horseradish peroxidase at Co<Subscript>3</Subscript>O<Subscript>4</Subscript>–graphene nanocomposite modified electrode and electrocatalysis

In this paper, the mixture of Co₃O₄–graphene nanocomposite and horseradish peroxidase (HRP) was spread on the surface of carbon ionic liquid electrode (CILE). Then, Nafion film was used for the immobilization. The results of spectroscopy proved that HRP kept up its native structure in the complex material. Direct electrochemistry of HRP resulted in a couple of quasi-reversible redox waves on cyclic voltammograms, reflecting the realization of direct electron transfer of HRP with electrode. The improvement in electrochemical responses was due to the usage of highly conductive Co₃O₄–graphene nanocomposite with biocompatible interface. Electrochemical parameters such as the electron transfer coefficient (α) was estimated as 0.47, and the apparent heterogeneous electron transfer rate constant (k _s) was calculated as 2.90 s^?1. The HRP modified electrode exhibited good electrochemical catalytic ability toward the reduction of trichloroacetic acid and NaNO₂. As a consequence, an updated third-generation electrochemical HRP biosensor with Co₃O₄–GR/CILE was constructed successfully.     

CeO<Subscript>2</Subscript>–ZrO<Subscript>2</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Ternary Oxides Synthesized via Supercritical Anti-Solvent and as a Support for Pd Catalyst for CO Oxidation

CeO₂–ZrO₂–Al₂O₃ ternary oxides as a support for CO oxidation was synthesized successfully via supercritical anti-solvent (SAS) precipitation using CO₂ as the anti-solvent and methanol as the solvent. It was found that the CeO₂–ZrO₂–Al₂O₃ fabricated by SAS precipitation (CZA1) had superior resistance to sintering compared to the traditional co-precipitation method (CZA2). Meanwhile, the oxygen storage/release rate of CAZ1 was almost 1.5 times higher than that of CZA2 and the total oxygen storage capacity (OSC) of CAZ1 was almost twice as high as CZA2. The interactions between the Pd and the CeO₂–ZrO₂–Al₂O₃ support were stronger for the support synthesized by SAS precipitation. The conversion of CO oxidation of Pd/CZA1 was even better than that of Pd/CZA2, especially at high GHSV.     

An enhanced oxime-based biomimetic electrochemical sensor modified with multifunctional AuNPs–Co<Subscript>3</Subscript>O<Subscript>4</Subscript>–NG composites for dimethoate determination

An enhanced oxime-based electrochemical sensor decorated with gold nanoparticles (AuNPs) and Co₃O₄ hexagonal nanosheets coupled with nitrogen-doped graphene has been developed for dimethoate determination dramatically. The introduction of Co₃O₄ hexagonal nanosheets tackles agglomeration of AuNPs and also enhances the sensitivity of electrochemical sensors greatly. The structure and properties of the synthesized composites were characterized by scanning electron microscopy, X-ray diffraction, Raman spectroscopy and Fourier transform infrared spectroscopy, confirming the successful modification of 2-(4-mercaptobutoxy)-1-naphthaldehyde oxime and Co₃O₄ supported AuNPs in a great experiment. In addition, differential pulse voltammetry further revealed that the developed electrochemical sensor exhibited excellent selectivity, sensitivity and stability in real samples analysis. Under optimal conditions, the modified sensor displayed a broad linear range from 1?×?10^?14 M to 1?×?10^?6 M with a fairly low detection limit of 8.4?×?10^?14 M (S/N?=?3) and was expected to act as a superior method for dimethoate determination.     

Thermal behavior of Co<Subscript>x</Subscript>Fe<Subscript>3−x</Subscript>O<Subscript>4</Subscript>/SiO<Subscript>2</Subscript> nanocomposites obtained by a modified sol–gel method

The thermal behavior of Co_xFe_3?xO₄/SiO₂ nanocomposites obtained by direct synthesis starting from nonahydrate ferric nitrate and hexahydrate cobalt nitrate in different ratios with and without the addition of 1,4-butanediol was studied. For the synthesis of Co_xFe_3?xO₄ (x = 0.5–2.5) dispersed in the silica matrix a wide Co/Fe molar ratio was used. The decomposition processes, formation of crystalline phases, gases evolvement and mass changes during gels annealing at different temperatures were assessed by thermal analysis. The absence of succinate precursor and a low mass loss were observed in the case of the gel obtained in the absence of 1,4-butanediol. In case of gels obtained using a stoichiometric ratio of Co/Fe, no clear delimitation between Co and Fe succinates was observed, while for samples with a Fe or Co excess, the formation of the two succinates was observed. The evolution of the crystalline phase after annealing (673, 973 and 1273 K) investigated by X-ray diffraction analysis and Fourier transformed infrared spectrometry revealed that in samples with Fe excess, stoichiometric Fe/Co ratio or low Co excess, the cobalt ferrite (CoFe₂O₄) was obtained as a single phase, while in samples with higher cobalt excess, olivine (Co₂SiO₄) as a main phase, cobalt oxide and CoFe₂O₄ as secondary phases were obtained after annealing at 1273 K. The SEM images confirmed the nanoparticles embedding in the silica matrix, while the TEM and X-ray diffraction data showed that the obtained nanoparticles’ size was below 10 nm in most samples.     

Stabilization of TiO<Subscript>2</Subscript>–Co<Subscript>3</Subscript>O<Subscript>4</Subscript> thin films on a glass fiber material by introduction of silica into the matrix

The TiO₂–Co₃O₄–SiO₂ oxide system supported on glass fiber was synthesized and studied. The oxide layers attached to the glass fiber surface have a porous structure. Characteristics of thin-film coatings on the glass fiber substrate (oxide layer phase composition and adhesion to the glass fiber surface) depend on the silica concentration. The obtained materials are catalytically active towards the exhaustive oxidation of propane.     

Electrochemical characterization of polypyrrole–LiNi<Subscript>1/3</Subscript>Mn<Subscript>1/3</Subscript>Co<Subscript>1/3</Subscript>O<Subscript>2</Subscript> composite cathode material for aqueous rechargeable lithium batteries

A series of polypyrrole (PPy)–LiNi_1/3Mn_1/3Co_1/3O₂ composite electrodes are formed by physical mixing of polypyrrole with LiNi_1/3Mn_1/3Co_1/3O₂ cathode material. LiNi_1/3Mn_1/3Co_1/3O₂ is synthesized by reaction under autogenic pressure at elevated temperature method. Highly resolved splitting of 006/102 and 108/110 peaks in the XRD pattern provide an evidence to well-ordered layered structure of the compound. The ratios of the intensities of 003 and 104 peaks are found to be >1, which indicate no pronounced mixing of the cation. Cyclic voltammetry and AC impedance studies revealed that the addition of polypyrrole significantly decreases the charge-transfer resistance of LiNi_1/3Mn_1/3Co_1/3O₂ electrodes. The electrochemical reactivity of PPy–LiNi_1/3Mn_1/3Co_1/3O₂ composite electrode is examined during lithium ion insertion and de-insertion by galvanostatic charge–discharge testing; 10 wt.% PPy–LiNi_1/3Mn_1/3Co_1/3O₂ composite electrode exhibits better electrochemical performance by increasing the reaction reversibility and capacity compared to that of the pristine LiNi_1/3Mn_1/3Co_1/3O₂ electrode. The cell with 10 wt.% PPy added cathode shows significant improvement in the electrochemical performance compared with that having pristine cathode. The capacity remains about 70% of the initial value after 50 cycles while for cell with pristine cathode only about 28% of initial capacity remains after 40 cycles.     

Photocatalytic and photoelectrocatalytic degradation of metoprolol tartrate in aqueous media by recyclable Co doping Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>/TiO<Subscript>2</Subscript> magnetic core–shell nanocomposites

Magnetically recoverable cobalt doping Fe₃O₄/TiO₂ magnetic nanocomposites with an acceptable core–shell structure were prepared via a sol-gel process at low calcination temperature. The crystalline size and structure, morphology, and magnetic properties of resulting particles have been characterized by X-ray diffraction (XRD), fourier transform infrared (FT-IR), FT-Raman, high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and vibrating sample magnetometry (VSM). Metoprolol tartrate (MET) as a pharmaceutical pollutant was used to observe the photocatalytic degradation ability of the magnetically recoverable particles. The process of degradation under UV irradiation at controlled temperature was studied and the remaining concentrations of MET as a contaminant were measured by UV-Vis spectrometer at λ = 229 nm. This ability remained 95.76% after three times of repetitive use at the same conditions. Various parameters such as reaction temperature, pH, and speed of stirring of the aqueous solution had an effect on the rate of degradation. The amount of cobalt dopant and nanocomposites are also effective on the rate of degradation. Coupling of electrical current with photocatalytic process has proven to be effective in the degradation of MET aqueous solution clearly.     

Simple synthesis of Co<Subscript>3</Subscript>O<Subscript>4</Subscript> nanoflakes using a low temperature sol–gel method suitable for photodegradation of dyes

In this work we describe and discuss the synthesis, and characterization of Co₃O₄ nanoflakes prepared by citric acid based sol–gel method using cobalt nitrate precursor. The average crystalline size of Co₃O₄ was estimated from the X-ray diffraction peaks of powders using Scherrer’s formula. The size of the particles deduced from TEM agrees with XRD based values. The FTIR confirms formation of Co₃O₄. The SEM micrographs show the nanoflake morphology. Here we report the lowest temperature sol–gel method of preparation of Co₃O₄ nanoflakes and its photocatalytic study for the degradation of dye pollutant methylorange.     

Steam reforming of dimethoxymethane,methanol and dimethyl ether on CuO–ZnO/γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalyst

The performance of a СuO–ZnO/γ-Al₂O₃ catalyst for the reactions of methanol, dimethyl ether (DME) and dimethoxymethane (DMM) steam reforming (SR) to hydrogen-rich gas was studied. The catalyst was found to be active and selective for methanol and DMM SR producing hydrogen-rich gas with low content of CO (<1 vol %). It provided complete conversion of methanol and DMM at 300°C, and hydrogen productivity of, respectively, 15 and 16.5 L_H2g _cat ^-1 h^-1. With the use of physicochemical methods and catalytic experiments, it was shown that the catalyst surface contained the acid sites typical for γ-Al₂O₃, and CuO–ZnO agglomerates, responsible, respectively, for DMM hydration to methanol and formaldehyde, and SR of these compounds to hydrogen-rich gas.     

Structural and low temperature Raman scattering studies in SrTi<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3</Subscript> nanoparticles synthesized by sol–gel method

We investigate effects of Co dopant concentration on the structure and low temperature Raman scattering properties in SrTi_1−x Co _x O₃ (x = 0.00, 0.10, 0.20, 0.30) nanoparticles prepared by sol–gel method. The dopant induced changes are studied by XRD, and Raman scattering measurements. The results show an average particle size of about 20 nm depending on the Co content and the lattice parameters decrease as increasing the Co content. In the Raman spectra, a broad structure in the region 100–500 cm⁻¹ is almost absent and the peaks in the region 600–800 cm⁻¹ show different weights with respect to SrTiO₃, relating to structural changes. The anomalous change in the area ratio of Raman peaks as function of temperature suggests a phase transition in our samples in the range of 110–130 K. These results indicate that the Co ion has replaced the site of Ti in unit cell. This novel route also demonstrates the advantage of synthesizing the compound with low annealing temperature.