Methanation of CO2 on H2-reduced Ni(II)-or Co(II)-bearing ferrites at 200 °C

The methanation of CO₂ gas at 200 °C was investigated in Co(II) or Ni(II)-bearing ferrites with substitution of up to 0.262 and 0.143, respectively. The metal substitution in the ferrite facilitated the methanation of the deposited carbon from CO₂ gas in the H₂-reduced M(II)-bearing ferrite in comparison with the reactivity of the H₂-reduced magnetite with the same spinel structure. The effect on the reactivity of methanation was found to be much larger in Ni(II)-substitution. The degree of the effect increased with increases in the Ni(II)-substitution, while it remained about the same with increases in the Co (II)-substitution. The maximum methanation (86.9%) of CO₂ was attained in the Ni(II)-bearing ferrite activated by H₂ gas for 3.0 h.     

CO2 decomposition with oxygen-deficient Mn(II) ferrite

An oxygen-deficient Mn(II) ferrite (Mn_0.97Fe_2.02O_3.92) was synthesized and its reactivity to reduce CO₂ gas into carbon was studied at 300°C. The oxygen-deficient Mn(II) ferrite was obtained by flowing H₂ gas through Mn(II) ferrite with a nearly stoichiometric composition of Mn_0.97Fe_2.02O_4.00 at 300° C. The lattice constant of the oxygen-deficient Mn(II) ferrite (0.8505nm) is larger than that of the Mn(II) ferrite with a nearly stoichiometric composition (0.8498nm). The chemical composition of the Mn(II) ferrite changed from Mn_0.97Fe_2.02O_4.00 to Mn_0.97Fe_2.02O_3.92 during the H₂ reduction process, indicating that the oxygen is deficient in the spinel structure of the Mn(II) ferrite. This was confirmed by Mössbauer spectroscopy and X-ray diffractometry. The efficiency of CO₂ decomposition into carbon at 300°C with the oxygen-deficient Mn(II) ferrite was much lower by about 10⁵ than that of oxygen-deficient magnetite. This is considered to be due to the difference in electron conductivity between Mn(II) ferrite and magnetite, which determines the reductivity for CO₂ into carbon by donation of an electron at the adsorption site.     

Carbon dioxide decomposition into carbon with the rhodium-bearing magnetite activated by H2-reduction

The CO₂ decomposition into carbon with the rhodium-bearing activated magnetite (Rh-AM) was studied in comparison with the activated magnetite (AM). The Rh-AM and the AM were prepared by flowing hydrogen gas through the rhodium-bearing magnetite (Rh-M) and the magnetite (M), respectively. The rate of activation of the Rh-M to the Rh-AM was about three times higher than that of the M to the AM at 300 °C. The reactivity for the CO₂ decomposition into carbon with the Rh-AM (70% CO₂ was decomposed in 40 min) was higher than that with the AM (30% in 40 min) at 300 °C. The Rh-M was activated to the Rh-AM at a lower temperature of 250 °C, and the Rh-AM decomposed CO₂ into carbon at 250 °C. On the other hand, the M was little activated at 250 °C.     

Decomposition of carbon dioxide to carbon by hydrogen-reduced Ni(II)-bearing ferrite

Hydrogen-activated Ni(II)-bearing ferrite, Ni _0.37 ²⁺ Fe _0.49 ²⁺ Fe _2.09 ³⁺ O_4.00, showed a high rate of decomposition of carbon dioxide to carbon at 300°C. This is based on the redox process of the Ni(II)-bearing ferrite with the spinel type of crystal structure. The rates of both activation by hydrogen gas and oxidation in carbon dioxide gas were much improved in the Ni (II)-bearing ferrite. The rate of decomposition was 0.178 mol h^–1 for the activated Ni(II)-bearing ferrite and 0.005 92 mol h^–1 for the activated magnetite in the batch mode, being 30 times larger. The rate of carbon dioxide decomposition was 16 times higher in the flow system in comparison with that of the activated magnetite.     

Reactivity of oxygen-deficient Mn(II)-bearing ferrites (Mn x Fe3-x O4-δ, O⩽x⩽1, δ>0) toward CO2 decomposition to carbon

The reduction of CO₂ to carbon was studied in oxygen-deficient Mn(II)-bearing ferrites (Mn _x Fe_3-x O_4-, Ox1, >0) at 300 °C. They were prepared by reducing Mn(II)-bearing ferrites with H₂ gas at 300°C. The oxygen-deficient Mn(II)-bearing ferrites showed a single phase with a spinel structure having an oxygen deficiency. The decomposition reaction of CO₂ to carbon was accompanied by oxidation of the oxygen-deficient Mn(II)-bearing ferrites. The decomposition rate slowed when the Mn(II) content in the Mn(II)-bearing ferrites increased. A Mössbauer study of the phase changes of the solid samples during the H₂ reduction and CO₂ decomposition indicated the following. Increases in the Mn(II) content lowered the electron conductivity of the Mn(II)-bearing ferrites. Increases in the oxygen deficiency, , contributed to an increase in electron conductivity and suggested that electron conduction due to the electron hopping determines the reductivity of CO₂ to carbon by the donation of an electron at adsorption sites.     

Methanation of CO2 with H2-reduced magnetite

The methanation reaction of CO₂ was studied with H₂-reduced magnetite. A high conversion ratio of about 0.9 (in 30 min of the reaction time) with a selectivity of nearly 100% was obtained at 300°C and at 0.1 MPa for H₂-reduced magnetite which had been prepared by passing H₂ gas for 1–5 h at 300°C. From the results of X-ray diffractometry and Mössbauer spectroscopy, and from chemical analysis of the deposited carbon, H₂-reduced magnetite is considered to decompose adsorbed CO₂ into carbon, and to incorporate the oxygen of the CO₂ into the spinel-type structure of the magnetite, associated with oxidation of the Fe²⁺ ion into Fe³⁺ ion in the magnetite. The high conversion ratio in the methanation reaction is considered to come from a higher reactivity of the elementary carbon deposited on the surface of the H₂-reduced magnetite.     

Formation and characterization of bi-layer oxide coating on carbon-steel for improving corrosion resistance

Attempts were made to improve the corrosion resistance of carbon steel by developing an additional barrier layer of magnesium ferrite (MgFe₂O₄) on magnetite (Fe₃O₄) film formed at high temperature in aqueous medium. The magnetite film was developed by exposing the carbon steel specimen in LiOH solution at 265°C for 10days. Subsequently, the magnesium ferrite (MgFe₂O₄) film was deposited by pulsed laser deposition (PLD) technique. X-ray diffraction analysis of the film revealed the formation of spinel phase of MgFe₂O₄. Relative atomic ratio of Mg and Fe estimated from X-ray photoelectron spectroscopy further confirmed the spinel phase of MgFe₂O₄. Scanning electron microscopy and atomic force microscopy techniques were used to analyze the film surface morphology. The corrosion behavior of the coated specimens was studied electrochemically. Impedance measurements showed an increase in impedance by more than two times in PLD coated samples compared to the Fe₃O₄ coated carbon-steel.     

The preparation of zinc ferrite nanorods by using single ferrocenyl complex as precursor

Zinc ferrite nanorods were prepared by the decomposition of ferrocenyl complex [Zn(fca)₂] 1 (Hfca, ferroceneylacetone) as single source precursor. In this work, we presented a new and simple route to synthesize nanostructured ferrites by decomposition of ferrocene-based complexes without the assistance of catalysts or template, and successfully obtained zinc ferrite nanorods with perfect morphology. The synthesis of ZnFe₂O₄ nanorods was carried out via decomposition of zinc ferrocenyl complexes at the presence of sodium hypochlorite under hydrothermal condition without the attendance of catalysts or templates. The results show that ferrocenyl complexes play an important role in the control synthesis of good morphology ferrites. To our knowledge, this is the first report to describe the synthesis of nanostructured zinc ferrites based on one-step hydrothermal decomposition of zinc ferrocenyl complexes. This exploration provides a useful method to seek new nanomaterials with perfect morphology in the shape-controlled synthesis of complicated inorganic composition oxides.     

Characterization of wet processed (Ni, Zn)-ferrites for CO2 decomposition

Ultrafine (Ni, Zn)-ferrites were prepared by two different methods of coprecipitation and hydrothermal synthesis, and their oxygen-deficient ferrites (ODF) produced by hydrogen reduction were investigated on the efficiency of CO₂ decomposition. The crystalline sizes of (Ni, Zn)-ferrites were less than 30 nm with high Brunauer–Emmett–Teller (BET) surface areas, ranging from 77 to 172 m² g^–1. The (Ni, Zn)-ferrites by hydrothermal synthesis resulted in smaller crystalline sizes, higher BET surface areas and better efficiencies of CO₂ decomposition than by coprecipitation. Compared with the binary NiFe₂O_4– ferrite, the ternary (Ni _x , Zn_1–x ) Fe₂O_4– ferrites showed higher efficiency for CO₂ decomposition, indicating a potential catalyst for the reduction of CO₂ emission in the environmental atmosphere.     

Removal of organic dyes using Cr-containing activated carbon prepared from leather waste

In this work, hydrogen peroxide decomposition and oxidation of organics in aqueous medium were studied in the presence of activated carbon prepared from wet blue leather waste. The wet blue leather waste, after controlled pyrolysis under CO₂ flow, was transformed into chromium-containing activated carbons. The carbon with Cr showed high microporous surface area (up to 889 m² g⁻¹). Moreover, the obtained carbon was impregnated with nanoparticles of chromium oxide from the wet blue leather. The chromium oxide was nanodispersed on the activated carbon, and the particle size increased with the activation time. It is proposed that these chromium species on the carbon can activate H₂O₂ to generate HO radicals, which can lead to two competitive reactions, i.e. the hydrogen peroxide decomposition or the oxidation of organics in water. In fact, in this work we observed that activated carbon obtained from leather waste presented high removal of methylene blue dye combining the adsorption and oxidation processes.     

Decomposition of CO2 and CO into carbon with active wüstite prepared from Zn(II)-bearing ferrite

CO₂ decomposition reaction into carbon was studied at 300 °C using the H₂-reduced Zn(II)-bearing ferrite which consisted of the Zn(II) oxide and the active wüstite. The H₂-reduced Zn(II)-bearing ferrite was prepared from Zn(II)-bearing ferrite by the reduction with H₂ gas at 300 °C. The wüstite (FeO) in the H₂-reduced Zn(II)-bearing ferrite had a higher value (=0.97, active wüstite) than those of the normal wüstites (0.90<<0.95) prepared at high temperatures (>570 °C). The decomposition reaction of CO₂ proceeds in two steps: (1) CO₂ reduction to CO, and (2) CO decomposition into carbon. In the initial stage, the reduction of CO₂ into CO takes place, accompanying both the oxidation of the active wüstite to the slightly oxidized wüstite, and the transformation of active wüstite and Zn(II) oxide into the Zn(II)-bearing ferrite. After the reaction of the initial stage attains equilibrium of an apparent state of rest, the adsorbed CO is decomposed into carbon, associated with the transformation of the slightly oxidized wüstite and the Zn(II) oxide into the Zn(II)-bearing ferrite.     

Experimental and modelling studies of carbon dioxide adsorption by porous biomass derived activated carbon

The goal of the study was to produce a low-cost activated carbon from agricultural residues via single stage carbon dioxide (CO₂) activation and to investigate its applicability in capturing CO₂ flue gas. The performance of the activated carbon was characterized in terms of the chemical composition, surface morphology as well as textural characteristics. The adsorption capacity was investigated at three temperatures of 25, 50 and 100 °C for different types of adsorbate, such as purified carbon dioxide and binary mixture of carbon dioxide and nitrogen. The purified CO₂ adsorption study showed that the greatest adsorption capacity of the optimized activated carbon of 1.79 mmol g^?1 was obtained at the lowest operating temperature. In addition, the adsorption study proved that the adsorption capacity for binary mixtures was lower due to the reduction in partial pressure. The experimental values of the purified CO₂ adsorption were modelled by the Lagergren pseudo-first-order model, pseudo-second-order model, and intra-particle diffusion model. Based on the analysis, it inferred that the adsorption of CO₂ followed the pseudo-second-order model with regression coefficient value higher than 0.995. In addition, the adsorption study was governed by both film diffusion and intra-particle diffusion. The activation energy that was lesser than 25 kJ mol^?1 implied that physical adsorption (physisorption) occurred.     

Interface reactions between glass melts containing transition metal oxides and ferrites

The interface reactions between SiO₂-PbO-MO melts (M = Fe, Mn, Zn and Ni) and Mn-Zn and Ni-Zn ferrites were studied using electron probe microanalysis and X-ray diffraction. Ni ions in the glass melt containing NiO were localized at the interface between the glass and Mn-Zn ferrite. The Ni-rich layer was also detected at the interface between NiO containing glass melt and Ni-Zn ferrite; the composition of this layer was thought to be close to NiFe₂O₄. Pb₈NiSi₆O₂₁ crystal was deposited as the product of the reaction with the glass melt and Mn-Zn ferrite at 700 °C. As compared with Mn-Zn ferrite, no reaction products were formed in Ni-Zn ferrite at various temperatures. The dissolution length of Mn-Zn ferrite in SPN5 glass melt was found to be smaller than for other melts, and it is concluded that the NiO-rich layer at the surface of the ferrite is chemically very durable to the glass melts.     

Cation valencies and surface segregation in copper ferrites under X-ray photoelectron spectroscopy conditions

Cu2p XPS spectra of copper ferrites and copper magnesium manganite are reported. It turns out that the surface layer of the ferrites are enriched with copper and that for the so-called cuprous ferrite Cu_0.5Fe_2.5O₄, a decomposition in the surface layer occurs into αFe₂O₃ and CuFeO₂.Evidence is found that in the ferrites only monovalent copper in A sites is present at room temperature and bivalent copper in A and B sites. The tetrahedral Cu⁺ ions show a remarkable negative shift of the core levels in comparison with metallic copper (1.4 – 1.7 eV).     

CO2 decomposition with mangano-wüstite

Mn(II)-ferrite (Mn_0.97Fe_2.02O_4.00) prepared by the wet method was reduced in a hydrogen at 300°C to form highly reactive mangano-wüstite ((Fe_0.67, Mn_0.32)O) for CO₂ decomposition. Approximately 23% CO₂ injected (3.40 mmol) was decomposed to CO by the mangano-wüstite (3.22 g) in the initial stage of the reaction in a batch system at 400°C. 88% CO was further decomposed to carbon. Approximately 58% CO₂ injected was reversibly adsorbed on the surface and the remaining 12% was unchanged after 200 h reaction. The mangano-wüstite was concurrently transformed to Mn(II)-bearing ferrite (Mn_0.23Fe_2.77O_4.00) and manganeserich mangano-wüstite ((Fe_0.60, Mn_0.40)O). The higher CO₂ decomposition capacity for this mangano-wüstite than that for oxygen-deficient Mn(II)-ferrite is discussed in detail, based on electron hopping and movement of ions in the bulk.     

Methanation of CO2 with the oxygen-deficient Ni(II)-ferrite under dynamic conditions

The continuous methanation of CO₂ has been accomplished over hydrogen-reduced Ni(II)-bearing ferrite (Ni_xFe_3–xO_4–; x=0.39, > 0) in a mixed gas flow of CO₂ and H₂ at 250–375 °C. The yield and the selectivity for the methanation were larger than 50% and 95%, respectively, at the initial stage of the process. They decreased to 31% and 89%, respectively, after 6 h methanation. The innovative results can be ascribed to the use of the new material; hydrogen-reduced Ni(II)-bearing ferrite. Its formation was evinced by chemical analyses and the increase in the lattice constant; the lattice constant of the Ni(II)-bearing ferrite (a₀ 0.8375 nm) was enlarged to 0.8379 nm by hydrogen reduction. The enlarged lattice constant was not changed during the methanation. These findings suggest that the methanation occurs at the oxygen-deficient site of the hydrogen-reduced Ni(II)-bearing ferrite, as well as the formation of water by combination of the incorporated oxygens with hydrogen. The methanation consists of three steps of the elementary reaction. First, the oxygen-deficient sites are formed by hydrogen reduction; second, CO₂ is reduced to elementary carbon and two oxygen ions which are incorporated into the oxygen-deficient sites; and third, the carbon deposited on the surface of the reduced ferrite is selectively hydrogenated to CH₄.     

Stress sensitivity of inductance in NiMgCuZn ferrites and development of a stress insensitive ferrite composition for microinductors

Three series of NiMgCuZn ferrites were prepared by conventional sintering process. The formation of single phase in these ferrites was confirmed by x-ray diffraction. Initial permeability measurements on these samples were carried out in the temperature range of 30–400°C. The effect of the external applied stress on the open magnetic circuit type coil with these ferrites was studied by applying uniaxial compressive stress parallel to magnetizing direction and the change in the inductance was measured. The variation of ratio of inductance (ΔL/L)% increases upto certain applied compressive stress and there after it decreases, showing different stress sensitivities for different compositions of ferrites studied in the present work. With a view to develop stress insensitive NiMgCuZn ferrite, a low stress sensitivity composition among all the ferrites studied was chosen and different amounts of SiO₂ were added to it and a series of ferrite compositions were prepared. The variation of ratio of inductance (ΔL/L)% with external applied compressive stress was examined. These results show that, 0.05 wt% SiO₂ added Ni_0.3Mg_0.3Cu_0.1Zn_0.3Fe₂O₄ ferrite exhibited stress insensitivity. It was noticed that addition of SiO₂ was found to be effective in reducing the stress sensitivity. This was confirmed from the elastic behaviour studies at room temperature on these ferrite samples. These studies were carried out to develop a ferrite composition for its use as core material for microinductor applications.     

Catalytic Activity of the Spinel Ferrite Nanocrystals on the Growth of Carbon Nanotubes

We prepared three ferrite nanocatalysts: (i) copper ferrite (CuFe₂O₄) (ii) ferrite where cobalt was substituted by nickel (Ni _x Co_1?x Fe₂O₄, with x=0, 0.2, 0.4, 0.6), and (iii) ferrite where nickel was substituted by zinc (Zn _y Ni_1?y Fe₂O₄ with y=1, 0.7, 0.5, 0.3), by the sol-gel method. The X-ray diffraction patterns show that the ferrite samples have been crystallized in the cubic spinel structural phase. We obtained the size of grains by field emission scanning electron microscopy images and their magnetic properties by vibrating sample magnetometer. Next, carbon nanotubes were grown on these nanocatalysts by the catalytic chemical vapor deposition method. We show that the catalytic activity of these nanocrystals on the carbon nanotube growth depend on cation distributions in the octahedral and tetrahedral sites, structural isotropy, and catalytic activity due to cations. Our study may have applications in finding a suitable candidate of doped ferrite nanocrystals as catalysts for carbon nanotube growth. More interestingly, the yield of fabrication of carbon nanotubes can be considered as an indirect tool to study catalytic activity of ferrites.     

Photocatalytic degradation of methyl orange dye by NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanoparticles under visible irradiation: effect of varying the synthesis temperature

Nanoparticles of nickel ferrites (NiFe₂O₄) were synthesized at different temperature of synthesis (25, 50 and 80 °C) through the chemical co-precipitation method. The synthesized powders were characterized using X-ray diffraction for crystallite size and lattice parameter calculation. It reveals the presence of cubic spinel structure of ferrites with crystallite size between 29 and 41 nm. Transmission electron microscopy and scanning electron microscopy showed uniform distribution of ferrite particles with some agglomeration. The Fourier-transform infrared spectroscopy showed absorption bonds, which were assigned to the vibration of tetrahedral and octahedral complexes. Raman spectroscopy is used to verify that we have synthesized ferrite spinels and determines their phonon modes. The thermal decomposition of the NiFe₂O₄ was investigated by TGA/DTA. The optical study UV–visible is used to calculate the band gap energy. Magnetic measurements of the samples were carried out by means of vibrating sample magnetometer and these studies reveal that the formed nickel ferrite exhibits ferromagnetic behavior. Photoluminescence showed three bands of luminescence located at 420, 440 and 535 nm. The photocatalytic properties of nickel ferrite (NiFe₂O₄) nanoparticles were evaluated by studying the photodecomposition of methyl orange as organic pollutant models and showed a good photocatalytic activity.     

Novel Sr-Zn-Co hexagonal ferrite nano-rods by wood-template chemical solution synthesis

Novel one-dimensional structured SrZnCoFe₁₆O₂₇ hexagonal ferrites were successfully prepared by using the nature pine-wood template assisted chemical solution method. The diameters of the w-type hexagonal ferrite nano-rods obtained were in the range of 500 ± 50 nm and the length attained was 5-6 μm. The one-dimensional structured SrZnCoFe₁₆O₂₇ hexagonal ferrite exhibited magnetop lumbite structure and presented hard magnetic properties. The magnetization value of the rod-like ferrites was much higher than that of the bulk hexagonal ferrites. The reasonable explanation of the improvement in the magnetization value was the shape effect. To our knowledge, only a few papers have reported the successful preparation of 1D structured Ba/Sr hexagonal ferrite and this novel SrZnCoFe₁₆O₂₇ hexagonal ferrite can be potentially applied in magnetic material industries.