An official website of the United States government
Tailoring the magnetic properties of iron oxide nanosystems is essential to expanding their biomedical applications. In this study, 34 nm iron oxide nanocubes with two phases consisting of Fe3O4 and α-Fe2O3 were annealed for 2 h in the presence of O2, N2, He, and Ar to tune the respective phase volume fractions and control their magnetic properties. X-ray diffraction and magnetic measurements were carried out post-treatment to evaluate changes in the treated samples compared to the as-prepared samples, showing an enhancement of the α-Fe2O3 phase in the samples annealed with O2 while the others indicated a Fe3O4 enhancement. Furthermore, the latter samples indicated enhancements in crystallinity and saturation magnetization, while coercivity enhancements were the most significant in samples annealed with O2, resulting in the highest specific absorption rates (of up to 1000 W/g) in all the applied fields of 800, 600, and 400 Oe in agar during magnetic hyperthermia measurements. The general enhancement of the specific absorption rate post-annealing underscores the importance of the annealing atmosphere in the enhancement of the magnetic and structural properties of nanostructures.
more »
« less
Chemical State of Potassium on the Surface of Iron Oxides: Effects of Potassium Precursor Concentration and Calcination Temperature
Potassium is used extensively as a promoter with iron catalysts in Fisher–Tropsch synthesis, water–gas shift reactions, steam reforming, and alcohol synthesis. In this paper, the identification of potassium chemical states on the surface of iron catalysts is studied to improve our understanding of the catalytic system. Herein, potassium-doped iron oxide (α-Fe2O3) nanomaterials are synthesized under variable calcination temperatures (400–800 °C) using an incipient wetness impregnation method. The synthesis also varies the content of potassium nitrate deposited on superfine iron oxide with a diameter of 3 nm (Nanocat®) to reach atomic ratios of 100 Fe:x K (x = 0–5). The structure, composition, and properties of the synthesized materials are investigated by X-ray diffraction, differential scanning calorimetry, thermogravimetric analysis, Fourier-transform infrared, Raman spectroscopy, inductively coupled plasma-atomic emission spectroscopy, and X-ray photoelectron spectroscopy, as well as transmission electron microscopy, with energy-dispersive X-ray spectroscopy and selected area electron diffraction. The hematite phase of iron oxide retains its structure up to 700 °C without forming any new mixed phase. For compositions as high as 100 Fe:5 K, potassium nitrate remains stable up to 400 °C, but at 500 °C, it starts to decompose into nitrites and, at only 800 °C, it completely decomposes to potassium oxide (K2O) and a mixed phase, K2Fe22O34. The doping of potassium nitrate on the surface of α-Fe2O3 provides a new material with potential applications in Fisher–Tropsch catalysis, photocatalysis, and photoelectrochemical processes.
more »
« less
- Award ID(s):
- 1903744
- PAR ID:
- 10377339
- Date Published:
- Journal Name:
- Materials
- Volume:
- 15
- Issue:
- 20
- ISSN:
- 1996-1944
- Page Range / eLocation ID:
- 7378
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Iron oxide nanoparticles (IONPs) were synthesized via a block copolymer-assisted hydrothermal method and the phase purity and the crystal structure were investigated by X-ray diffraction. The Rietveld analysis of X-ray diffractometer spectra shows the hexagonal phase symmetry of α-Fe2O3. Further, the vibrational study suggests Raman active modes: 2A1g + 5Eg associated with α-Fe2O3, which corroborates the Rietveld analysis and orbital analysis of 2PFe. The superparamagnetic behavior is confirmed by magnetic measurements performed by the physical properties measurement system. The systematic study of the Congo red (CR) interaction with IONPs using a UV-visible spectrophotometer and a liquid chromatography–tandem mass spectrometry system equipped with a triple quadrupole mass analyzer and an electrospray ionization interface shows effective adsorption. In visible light, the Fe2O3 nanoparticles get easily excited and generate electrons and holes. The photogenerated electrons reduce the Fe3+ ions to Fe2+ ions. The Fe2+/H2O2 oxidizes CR by the Fenton mechanism. The strong adsorption ability of prepared nanoparticles towards dyes attributes the potential candidates for wastewater treatment and other catalytic applications.more » « less
-
Abstract While cobalt-based catalysts have been used in industrial Fischer-Tropsch synthesis for decades, little is known about how the dynamics of the Co-Co2C phase transformation drive their performance. Here we report on the occurrence of hysteresis effects in the Fischer-Tropsch reaction over potassium promoted Co/MnOxcatalyst. Both the reaction rate and the selectivity to chain-lengthened paraffins and terminally functionalized products (aldehydes, alcohols, olefins) show bistability when varying the hydrogen/carbon monoxide partial pressures back and forth from overall reducing to carbidizing conditions. While the carbon monoxide conversion and the selectivity to functionalized products follow clockwise hysteresis, the selectivity to paraffins shows counter-clockwise behavior. In situ X-ray diffraction demonstrates the activity/selectivity bistability to be driven by a Co-Co2C phase transformation. The conclusions are supported by High Resolution Transmission Electron Microscopy which identifies the Co-Co2C transformation, Mn5O8layered topologies at low H2/CO partial pressure ratios, and MnO at high such ratios.more » « less
-
Abstract Fe‐based catalysts are an active, selective, and low‐cost option for tuning Fischer‐Tropsch synthesis (FTS) selectivity toward desirable light olefins. By encapsulating Fe within ZSM‐5, the resultant core‐shell catalysts have the potential to control the product distribution via secondary reactions that occur over the acid sites of the zeolite shell. In this paper, Fe is encapsulated within ZSM‐5 via the seed‐directed growth technique and characterized with a suite of analytical techniques including Mössbauer spectroscopy and X‐ray absorption fine structure (XAFS). Characterization of the core‐shell catalysts indicates that some of the Fe‐based active phase is destabilized during seed‐directed growth, demonstrating the challenges associated with encapsulating an Fe‐based active phase within zeolites. However, comparing FTS performance of the core‐shell catalyst with the Fe‐based control synthesized via incipient wetness impregnation demonstrates improved selectivity toward the desired C2‐C4olefins and C5+hydrocarbons.more » « less
-
The effects of adding Mn and Na promoter metals to graphene oxide (GO)-supported iron-based catalysts for Ficher-Tropsch Synthesis (FTS) reactions to olefins at 20 bars were investigated in a 3D-printed stainless steel (SS) Microreactor. While promoter metals encourage reduction of iron oxide to iron to form iron carbide, the active metal catalysts in GO allow hydrogenation of CO. These catalysts were synthesized by layer deposition method and characterized by different techniques. The TEM images show the integration of graphene oxide into the catalysts. The XRD and XPS studies confirmed the crystal structure and oxidation states of the metals. The catalytic activity and product selectivity were studied in the temperature range of 200–350°Cwith a 2:1 M ratio of H2: CO. Higher CO conversion with greater selectivity for olefins was observed in the presence of the promoters. FeMnNa@GO showed better stability than both Fe@GO and FeMn@GO catalysts in time-on-stream studies.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3390/ma15207378
