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Magnetite
About: Magnetite is a research topic. Over the lifetime, 10277 publications have been published within this topic receiving 278071 citations.
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TL;DR: In this article, the reduction of calcined catalysts was examined in detail using temperature-programmed reduction in H2 and in situ X-ray absorption near-edge structure (XANES) analysis.
Abstract: CO2 hydrogenation to short-chain hydrocarbons was investigated over iron catalysts supported on oxygen- and nitrogen-functionalized multi-walled carbon nanotubes (CNTs) and on silica, which were synthesized by the dry impregnation method using ammonium ferric citrate as precursor. The reduction of the calcined catalysts was examined in detail using temperature-programmed reduction in H2 and in situ X-ray absorption near-edge structure (XANES) analysis. The XANES results revealed that the mixture of hematite and magnetite was gradually transformed into wustite and metallic iron during heating in H2. Iron oxide nanoparticles supported on nitrogen-functionalized CNTs were easier to reduce compared to those on oxygen-functionalized CNTs indicating a promoting effect of the nitrogen functional groups. The interaction between iron oxide and silica was found to be much stronger inhibiting the reduction to metallic iron. As a result, the catalytic activity of iron nanoparticles supported on CNTs in CO2 hydrogenation at 360 °C, 25 bar and a H2:CO2 ratio of 3 was almost twofold higher compared with iron supported on silica. CO2 was converted into C1–C5 hydrocarbons with CO and methane as major products over all catalysts. The Fe/NCNT catalyst achieved the highest olefin selectivity of 11% in the hydrocarbons range of C2–C5. In contrast, mostly paraffins were formed over the Fe/SiO2 catalyst.
82 citations
TL;DR: In this article, the grain-size-dependent properties of the magnetic minerals in Chinese loess/paleosol samples were investigated and the results showed that the oxidation degree of magnetic minerals is strongly affected by both grain size and the paleoclimatic environment in which they were deposited and altered.
Abstract: [1] To investigate the grain-size-dependent properties of the magnetic minerals in Chinese loess/paleosol samples (Touxiangdao, Xining, Qinghai province, China), magnetic extracts were divided into two size fractions by gravitational settling. On the basis of hysteresis measurements, thermal demagnetization of low-temperature saturation isothermal remanent magnetization, and nonmagnetic studies (SEM and XRD) we identified magnetic phases both in the grain size fractions of the magnetic extracts and in the less magnetic residues to provide more accurate and complete descriptions of all the magnetic components in the bulk natural samples. The results show that the oxidation degree (nonstoichiometry) of magnetic minerals is strongly affected by both grain size and the paleoclimatic environment in which they were deposited and altered. In ascending order of the oxidation degree of our samples, we find (1) loess-coarse particles (LC) are multidomain (MD) magnetite with slight oxidation, (2) paleosol-coarse (PC) particles are also MD magnetite but with a higher oxidation degree compared to LC, (3) loess-fine (LF) particles are pseudo-single domain (PSD) magnetite with a high oxidation degree, and (4) paleosol-fine (PF) particles are PSD maghemite. Single domain (SD) and superparamagnetic (SP) maghemite mainly stay in the residues. Further thermomagnetic analysis of PF (PSD maghemite) revealed that this natural maghemite has a Curie temperature identical to that of magnetite and that the conversion efficiency of transformation from maghemite to hematite is only about 50% after a 700°C heating/cooling cycle. These new results identify the sources of multicomponent NRM in Chinese loess sequences as well as clarify the paleoenviromental and paleoclimatic controls on the remanence components.
82 citations
TL;DR: In this paper, the temperature dependence of the exchange constant between 20°C and 573°C using three previously published data sets from inelastic neutron scattering by spin wave excitations in magnetite was calculated.
Abstract: We calculated the temperature dependence of the exchange constant between 20°C and 573°C using three previously published data sets from inelastic neutron scattering by spin wave excitations in magnetite. The exchange constant in magnetite varies as a function of temperature approximately as the saturation magnetization to the power 1.7. Our synthesis of temperature dependent spin wave dispersion data provides an experimental foundation for the temperature variation of the exchange energy which is critical for micromagnetic domain structure calculations and for an understanding of the acquisition of thermoremanent magnetization in rocks.
82 citations
TL;DR: In this paper, the main focus was on the fabrication of deaggregated spherical nanoparticles with a narrow size distribution, and further separation of air-dry EEW nanoparticles was performed in aqueous suspensions.
Abstract: Nanoparticles of iron oxides (MNPs) were prepared using the electric explosion of wire technique (EEW). The main focus was on the fabrication of de-aggregated spherical nanoparticles with a narrow size distribution. According to XRD the major crystalline phase was magnetite with an average diameter of MNPs, depending on the fraction. Further separation of air-dry EEW nanoparticles was performed in aqueous suspensions. In order to provide the stability of magnetite suspension in water, we found the optimum concentration of the electrostatic stabilizer (sodium citrate and optimum pH level) based on zeta-potential measurements. The stable suspensions still contained a substantial fraction of aggregates which were disintegrated by the excessive ultrasound treatment. The separation of the large particles out of the suspension was performed by centrifuging. The structural features, magnetic properties and microwave absorption of MNPs and their aqueous solutions confirm that we were able to obtain an ensemble in...
82 citations
Patent•
30 Sep 1992
TL;DR: In this paper, fine coal is beneficiated in specially designed dense medium cyclones to improve particle acceleration and enhance separation efficiency, and magnetite is recovered from each separated fraction independently, with non-magnetic effluent water from one fraction diluting feed to a smaller-size fraction.
Abstract: Fine particle coal is beneficiated in specially designed dense medium cyclones to improve particle acceleration and enhance separation efficiency. Raw coal feed is first sized to remove fine coal particles. The coarse fraction is then separated into clean coal, middlings, and refuse. Middlings are comminuted for beneficiation with the fine fraction. The fine fraction is deslimed in a countercurrent cyclone circuit and then separated as multiple fractions of different size specifications in dense medium cyclones. The dense medium contains ultra-fine magnetite particles of a narrow size distribution which aid separation and improves magnetite recovery. Magnetite is recovered from each separated fraction independently, with non-magnetic effluent water from one fraction diluting feed to a smaller-size fraction, and improving both overall coal and magnetite recovery. Magnetite recovery is in specially designed recovery units, based on particle size, with final separation in a rougher-cleaner-scavenger circuit of magnetic drum separators incorporating a high strength rare earth magnet.
82 citations