Small Oxide Particles Research Articles

New generation of monomer-stabilized very small superparamagnetic iron oxide particles (VSOP) as contrast medium for MR angiography: preclinical results in rats and rabbits.

The purpose of this study was to evaluate the signal enhancement characteristics of very small superparamagnetic iron oxide particles (VSOP)-C63, a new monomer-coated, iron oxide-based magnetic resonance (MR) blood pool contrast medium with a very small particle size and optimized physical properties. Equilibrium MR angiography (MRA) of rats (thoracic and abdominal vessels) was performed at 1.5 T with a three-dimensional gradient-recalled echo (3D GRE) technique (TR/TE 6.6/2.3 msec, flip angle 25 degrees ) before and after (every 3-5 minutes up to 50 minutes) i.v. injection of VSOP-C63 [dosages: 15, 30, 45, 60, 75, and 90 micromol Fe/kg; diameter: 8 nm; relaxivities at 0.47 T: R1 = 30 l/(mmol * s); R2 = 39 l/(mmol * s)]. First-pass MRA images (3D-GRE, TR/TE 4.5/1.7 msec, flip angle 25 degrees ) were obtained with 45 micromol Fe/kg VSOP-C63 in comparison with 0.2 mmol Gd/kg of gadolinium diethylene triamine pentaacetic acid (Gd DTPA; before and every 5 seconds p.i.). MRA (3D GRE, TR/TE 4.5/1.7 msec, flip angle 25 degrees) of coronary vessels in rabbits was performed after i.v. injection of 45 micromol Fe/kg of VSOP-C63. In rats maximal S/N ratio in thoracic and abdominal arteries directly after i.v. injection of VSOP-C63 was 25 +/- 1, 43 +/- 2, 49 +/- 4, 57 +/- 3, 64 +/- 3, and 63 +/- 3 for the different dosages. Blood half-life was dose dependent (15 +/- 2, 20 +/- 3, 29 +/- 6, 37 +/- 5, 61 +/- 16, and 86 +/- 21 minutes). At a dose of 30 micromol Fe/kg even small intrarenal arteries were sharply delineated. First-pass MRA showed no significant difference in the S/N ratio between Gd-DTPA (71.5 +/- 11.5) and VSOP-C63 (65.1 +/- 18. 3). The proximal segments of the coronary arteries in rabbits were clearly depicted at a dose of 45 micromol Fe/kg. The monomer-coated, iron oxide-based contrast medium VSOP-C63 exhibits favorable properties as a blood pool agent for both equilibrium and first-pass MRA. J. Magn. Reson. Imaging 2000;12:905-911.

Mössbauer Spectroscopic Study of Iron Containing Hydrotalcite Catalysts for the Reduction of Aromatic Nitro Compounds

Carbonated layered double magnesium hydroxides containing Fe with hydrotalcite structure which are active and selective catalysts for the reduction of aromatic nitro compounds by hydrazine hydrate have been prepared and characterized by X-ray diffraction and Mossbauer spectroscopy. Using these techniques we have shown that when the Fe/(Fe + Mg) ratio was greater than 0.2, which corresponds to the natural phase composition, ferrihydrite Fe5HO8⋅4H2O was formed. This last phase, characterized at room temperature by a superparamagnetic doublet, leads after activation under nitrogen at 723 K to small particles of ferric oxide. An in-situ study of the activated catalysts allowed us to show that only iron cations present in this phase could undergo an oxido-reduction under the conditions of catalysis and that the catalytic properties must be related to this last phase.

Characterization of Fe-promoted sulfated zirconia catalysts used for the n-butane isomerization by XPS and Mössbauer spectroscopies

Iron‐promoted sulfated zirconia catalysts have been characterized by Mossbauer spectroscopy and XPS before and after catalytic test for the isomerization of n‐butane. The data obtained by Mossbauer spectroscopy show that iron is present in the fresh catalysts at the surface of zirconia both as isolated cations and small ferric oxide particles. The diameters of these particles do not exceed 4 nm. The characterization of the catalysts by Mossbauer spectroscopy after 20 and 120 min of reaction shows no apparent reduction of the iron cations. However, the analysis of the peak of Fe 3d3/2 in the XPS spectra shows that iron has been partially reduced in the used catalysts. These apparently contradictory results could be explained by the study of a pre‐reduced catalyst by both techniques. This study shows first that the Fe3+ cations in the particles can be reduced into Fe2+ and reoxidized at room temperature, and second that the reduction observed by XPS corresponds to the departure of the O2 re‐adsorbed at room temperature under air, occurring when the catalysts are placed under vacuum. All the data obtained seemed to confirm that the loss of the promoting effect of iron during the catalytic reaction may partially be due to the irreversible reduction of the iron species susceptible to undergo such reduction.

Synthesis and Characterization of CoO, Co3O4, and Mixed Co/CoO Nanoparticules

Very small nanometric oxide particles (CoO and Co3O4, around 2 nm in diameter) well dispersed inside a polymer matrix were prepared by solid-state oxidation of a metallic colloid precursor (1.6 nm ...

Characterization of high surface area smectite supported cobalt oxides catalysts for hydrodesulfurization by means of TPR, TPS and ESR

Catalysts of cobalt oxide loaded on smectic clays having high surface areas were prepared. Smectic clays used were montmorillonite, saponite, porous saponite, hectorite and stevensite. The catalysts were tested for hydrodesulfurization (HDS) activity with thiophene in a pulse flow reactor at different temperatures. Co–porous saponite catalyst prepared by an ion-exchange method in the series including CoMo–Al 2O 3 shows the highest thiophene HDS activity so far studied. The catalytic properties of the selected catalysts were characterized by temperature-programed-reduction (TPR) and -sulfiding (TPS) together with electron spin resonance (ESR). The HDS performance with thiophene is closely dependent upon the reduction temperature of cobalt oxide obtained by TPR for Co–smectite catalysts. The TPS measurements indicate that small particles of cobalt oxide are highly dispersed in the smectic layer by means of ion-exchange method. It is suggested that the properties such as sensitivity toward H 2S and catalyst color of cobalt oxide on Co–porous saponite by ion-exchange method are different from those of Co–porous saponite by impregnation method. Reactivity of loaded cobalt oxide with H 2S is strongly dependent upon the sulfiding paths.

Acidic and basic sites in NaX and NaY faujasites investigated by NH3, CO2 and CO molecular probes

The acid-base properties of samples of NaY and NaX faujasites have been investigated by adsorbing different probe molecules and measuring IR spectra. In the case of the NaY sample, only Na+ ions were found to be involved in the adsorption of CO2 and CO, confirming the overwhelming Lewis acid character of this material. In contrast, carbonate-like species were formed by adsorbing carbon dioxide on the NaX sample, due to the reaction of basic framework oxygen atoms with CO2 molecules polarised on neighbour Na+ ions. The spectroscopic analysis of NaX also showed evidence of Bronsted acid hydroxyls and OH groups bonded to extra-framework Al atoms. Ammonia adsorption revealed that the amount of Bronsted acid hydroxyls is significantly lower than the Lewis acid Na+ countercations. Moreover, small oxide particles, carrying carbonate-like species on their surface, are present in the zeolitic cavities. These particles could be responsible for the basic reactivity towards CO observed after outgassing the NaX sample at high temperature.

EXAFS Study of Ti, Fe and Ga Substituted Silicalites

The local environment of isomprphous substituted Ti, Fe and Ga metal atoms in silicalite (hereafter indicated as TS-1, Fe-S and Ga-S respectively) has been studied by means of EXAFS spectroscopy. Measurements have been carried out on the samples in presence of template and after template removal at increasing temperatures in order to follow the evolution of the local environment of the metal heteroatoms. Ti(IV) atoms in (TS-1) show a great stability, while Ga(III) and particularly Fe(III) atoms have propensity to migrate from framework into extraframework positions, forming small oxide particles trapped inside the MFI channels upon increasing the template burning temperature. The established stability scale is Ti(IV) > Ga(III) > Fe(III). These conclusions are qualitatively supported by UV-Vis, IR and Raman data.

X-Ray absorption spectroscopy: sensitive characterization of (model-) catalysts with the electron yield technique

Small chromium oxide particles (Cr2O3, CrO2) supported on titanium dioxide and oxidized Ag(111) single crystals were investigated by X-ray absorption spectroscopy at the oxygen K-edge. The spectra were collected in the electron yield mode in order to increase the surface sensitivity. The shape of the sharp split White line (WL) in the O K-edges spectra depended strongly on the oxidation state of the chromium ions in the probed samples suggesting that the WL can be used as an indicator of different environments in the supported chromium oxide films. On the other hand, the O K-edges of the oxidized Ag(111) crystal indicated that the formation of the distinct oxygen species at the surface and in the near-surface region was accompanied by a different silver-to-oxygen covalent interaction.

The Experimental Accuracy of Lattice Spacing Determination on Small Metal Particles in Commercial Catalysts

Abstract A description of the microstructure of heterogeneous catalysts is important in understanding the mechanisms for catalysis and may result in improvements in catalyst performance. Many commercial catalysts consist of small metal particles and/or metal oxide particles dispersed on a porous support. For bimetallic catalysts, the metals may be in the form of alloys, metal mixtures or completely phase separated. HREM has been demonstrated to be a powerful technique for determining the lattice spacing from small particles[l]. In many studies of catalysts it is necessary to prepare TEM samples by microtomy so that the particle location relative to the porous support is preserved. However, such samples show a reduction in particle visibility and signal-to-noise ratio of lattice fringes. In spite of these difficulties, we have been able to routinely use HREM to rapidly determine the relative distributions of metal and metal oxides in many systems because of the large difference in spacing between strong reflections from the metal and metal oxide (typically 20 %).

Mechanisms of accumulation of small particles of iron oxide in experimentally induced osteosarcomas of rats: a correlation of magnetic resonance imaging and histology. Preliminary results.

This study was conducted to investigate the distribution and kinetics of small particles of iron oxide in osteosarcoma-like tumors. Magnetic resonance (MR) imaging was performed in eight athymic nude rats with an experimentally induced osteosarcoma of the right hind leg immediately after intravenous injection of a superparamagnetic iron oxide preparation. Five animals received 150 mumol iron oxide/ kg and three received 50 mumol iron oxide/kg. The iron oxide preparation consisted of polythylenglycol-coated particles with a core diameter of 6 to 8 nm. The MR images were correlated with histologic slices of the tumors. The tumors accumulated iron oxide rapidly. A marked decrease in signal intensity, preferentially along the periphery of the tumor, was followed by a partial return of the signal intensity within the first minute. The maximum signal decrement throughout the entire tumor exceeded 41% and 21% with one dose each of 150 mumol iron/kg and 50 mumol iron/kg, respectively. The rate of return depended on the injected dose and tumor area, with the signal intensity approaching the initial value before the injection of iron oxide after 45 minutes. Histologic correlation only showed deposition of contrast medium in the proliferative areas of the tumors, mainly confined to the tumor margin. In addition to a predominantly extracellular deposition, intracellular storage could be detected. The findings help to advance the understanding of the distribution and kinetics of intravenous-injected small particles of iron oxide in osteosarcoma-like tumors. A first-pass accumulation of iron oxide could be documented by MR imaging in the periphery of osteosarcomas. Due to sieving of iron oxide particles by liver, spleen, and bone marrow, the signal intensity at 45 minutes after the injection of iron oxide returned to 89% (150 mumol iron oxide/kg) and 95% (50 mumol iron oxide/kg) of the preinjection intensity.

Formation and evolution of a subsurface layer in a metalworking process

The effect of metalworking on the structure and composition of the near-surface workpiece region was studied using, as an example, the hot rolling of an aluminum alloy. In this work, it was found that hot rolling resulted in the formation of a subsurface layer 1.5–8 μm thick with structure and properties different from those of the underlying bulk metal. Electron microprobe analysis, transmission electron microscopy, and electron diffraction studies showed this layer to be composed of magnesium and aluminum oxides mixed with the metal, and to be non-uniform in thickness and composition. The thickness of the subsurface layer gradually decreased in the sequential stages of hot rolling. The metal grain size ranged from 0.04 to 0.2 μm, which was at least 25 times smaller than the grain size of the underlying bulk material. The oxides in the subsurface layer were found to contain, in addition to amorphous Al 2O 3, a mixture of crystalline oxides. The particle size of the crystalline oxides in the subsurface layer was found to be in the range from 28 to 300 Å, with almost half of the particles in the range 28–33 Å. After the first passes the oxide mixture was composed of MgO, γ-Al 2O 3, and MgAl 2O 4 spinel, later of MgO and γ-Al 2O 3, and after the last pass only of MgO. The subsurface mixed layer was covered with a continuous surface oxide layer 250–1600 Å thick containing MgO and Al 2O 3. A hypothesis that the subsurface grain growth was retarded by Zener pinning by the small oxide particles was suggested and tested. Image analysis of transmission electron micrographs was used to obtain the size distribution and volume fraction of small oxide particles used to test this hypothesis. The experimental metal grain size was found to be in good agreement with the results of three-dimensional computer simulation studies of grain growth inhibition reported by Anderson et al., in literature which supported the Zener pinning hypothesis.

97/01759 Preparation of naphthalene-based mixtures from coal liquids

Alumina-supported cobalt catalysts have been prepared from different cobalt precursors to study the influence of the precursor on the ultimate metal particle size. Furthermore, the effect of the particle size on the catalytic performance (activity and selectivity) during Fischer-Tropsch synthesis has been investigated. The preparation of low-loaded cobalt catalysts (2.5 wt%) by incipient wetness impregnation using cobalt EDTA and ammonium cobalt citrate precursors resulted initially in very small cobalt oxide particles, as determined by XPS. The small oxide particles reacted during the thermal treatment in a reducing gas flow with the alumina support to cobalt aluminate, which was neither active nor selective during Fischer-Tropsch synthesis. The catalysts prepared with cobalt nitrate had larger particles that could be easily reduced to metallic cobalt. These catalysts were active under reaction conditions. High-loaded cobalt catalysts (5.0 wt%) prepared using ammonium cobalt citrate showed a larger particle size than the low-loaded catalyst prepared from the citrate precursor. The extent of reduction to metallic cobalt that could be achieved with the high-loaded catalyst was significantly higher than that with the low-loaded catalyst, as shown by magnetic measurements. Accordingly, the high-loaded catalyst exhibited a reasonable activity and, in addition, an interesting and remarkably high selectivity toward higher hydrocarbons, and also a very high Schultz-Flory parameter.

Preparation and properties of supported cobalt catalysts for Fischer-Tropsch synthesis

Alumina-supported cobalt catalysts have been prepared from different cobalt precursors to study the influence of the precursor on the ultimate metal particle size. Furthermore, the effect of the particle size on the catalytic performance (activity and selectivity) during Fischer-Tropsch synthesis has been investigated. The preparation of low-loaded cobalt catalysts (2.5 wt%) by incipient wetness impregnation using cobalt EDTA and ammonium cobalt citrate precursors resulted initially in very small cobalt oxide particles, as determined by XPS. The small oxide particles reacted during the thermal treatment in a reducing gas flow with the alumina support to cobalt aluminate, which was neither active nor selective during Fischer-Tropsch synthesis. The catalysts prepared with cobalt nitrate had larger particles that could be easily reduced to metallic cobalt. These catalysts were active under reaction conditions. High-loaded cobalt catalysts (5.0 wt%) prepared using ammonium cobalt citrate showed a larger particle size than the low-loaded catalyst prepared from the citrate precursor. The extent of reduction to metallic cobalt that could be achieved with the high-loaded catalyst was significantly higher than that with the low-loaded catalyst, as shown by magnetic measurements. Accordingly, the high-loaded catalyst exhibited a reasonable activity and, in addition, an interesting and remarkably high selectivity toward higher hydrocarbons, and also a very high Schultz-Flory parameter.

97/00819 Carbidization of iron supported on SiO 2 and Al 2O 3 in Fischer-Tropsch synthesis

Alumina-supported cobalt catalysts have been prepared from different cobalt precursors to study the influence of the precursor on the ultimate metal particle size. Furthermore, the effect of the particle size on the catalytic performance (activity and selectivity) during Fischer-Tropsch synthesis has been investigated. The preparation of low-loaded cobalt catalysts (2.5 wt%) by incipient wetness impregnation using cobalt EDTA and ammonium cobalt citrate precursors resulted initially in very small cobalt oxide particles, as determined by XPS. The small oxide particles reacted during the thermal treatment in a reducing gas flow with the alumina support to cobalt aluminate, which was neither active nor selective during Fischer-Tropsch synthesis. The catalysts prepared with cobalt nitrate had larger particles that could be easily reduced to metallic cobalt. These catalysts were active under reaction conditions. High-loaded cobalt catalysts (5.0 wt%) prepared using ammonium cobalt citrate showed a larger particle size than the low-loaded catalyst prepared from the citrate precursor. The extent of reduction to metallic cobalt that could be achieved with the high-loaded catalyst was significantly higher than that with the low-loaded catalyst, as shown by magnetic measurements. Accordingly, the high-loaded catalyst exhibited a reasonable activity and, in addition, an interesting and remarkably high selectivity toward higher hydrocarbons, and also a very high Schultz-Flory parameter.

Manufacturing of Optical Phantoms and Measurement of Their Optical Properties Using an Integrating Sphere.

Noninvasive diagnosis by near-infrared light is widely used because it can provide physiological information about biological tissues. For quantitative measurement, however, estimation of the optical properties of biological tissues is necessary with respect to understanding of photon migration in media which scatter light strongly. The purpose of this study is to fabricate stable, solid phantoms which optically simulate biological tissues, and to measure their optical properties using an integrating sphere and a Monte Carlo simulation. The solid phantoms were fabricated using either polyester or epoxy resin as the base material, and small titanium oxide particles were mixed to provide appropriate scattering of light. Their optical properties were measured by means of spectroscopy using an integrating sphere and numerical simulation using a Monte Carlo method. The reduced scattering coefficients were measured within an uncertainty of approximately 4% and ranged from 0.3 to 1.5 mm-1, which are typical values of biological tissues in the near-infrared wavelength range. The measured absorption coefficients of the phantoms were below 0.0034 mm-1, which are satisfactorily small values.

Iron oxide-enhanced MR lymphography: the evaluation of cervical lymph node metastases in head and neck cancer.

Accurate diagnosis of cervical lymph node metastasis is challenging, even with the latest computed tomography or MR equipment and technique. The lack of definitive criteria for distinguishing metastatic from benign nodes is a serious shortcoming of current imaging options. Dextran-coated, ultrasmall superparamagnetic iron oxide is a new MR contrast agent, which accumulates in the reticuloendothelial system of lymph nodes. Small iron oxide particles are taken up by macrophages within normal functioning nodes, reducing their signal on postcontrast MR because of the magnetic susceptibility effects of iron oxide. Metastatic nodes, on the other hand, remain high in signal on postcontrast T2*-weighted gradient echo images. Early clinical experience in cancer patients suggests that iron oxide-enhanced MR lymphography is a valuable imaging technique that may improve diagnostic accuracy for nodal metastases. This article reviews development of superparamagnetic iron oxide compounds, their imaging characteristics, and clinical experience for evaluating head and neck cancer metastases.

Creep behaviour of ODS steels

The creep properties of oxide dispersion strengthened (ODS) steels, DT and DY alloys, were investigated in the temperature range 500–700°C. The evaluation of the creep threshold stress was performed by creep dip tests. The Norton's creep law, corrected with this experimental threshold stress, gives a good description of the creep behaviour of DT and DY alloys. Transmission electron microscopy (TEM) observations showed the dislocations to be pinned to the small oxide particles based on thermally activated dislocation detachment from oxide particles. The results are discussed in the light of Arzt's model.

Structural and magnetic properties of metastable fcc Cu-Fe alloys.

The structural properties of the metastable fcc Cu-Fe alloys obtained by high-energy ball milling of elemental powders have been studied by transmission electron microscopy. The same technique has been used to study the effect of thermal annealing on the alloyed samples. Experimental observations show that in some Cu-rich grains, even for short processing times, there is an Fe concentration up to about 30 at. % with an essentially flat concentration profile. In the same grains one observes also a relatively low density of dislocations, several small aggregates of point defects and small particles of oxide precipitates all having the same orientation with respect to the matrix. In a few cases it is also possible to observe a lamellar structure with a periodicity of a few nm which has been interpreted as arising from spinodal decomposition. The same origin has been attributed to structures modulated on a similar length scale, observed quite frequently in the annealed samples. All these observations suggest that during plastic deformation there is a transient supersaturation of point defects which increases considerably the atomic mobility and the kinetics of the solid-state reactions in the deformed grains, and that the driving force for alloying arises from the mechanical energy supplied to the sample by the ball impact. It is also shown that the observation of (partial) spinodal decomposition, prior to and after annealing, is compatible with the magnetic properties of these samples if one accepts that Fe atoms in fcc Fe-Cu alloys are present in two electronic states, as observed in other fcc iron-late transition metal alloys. \textcopyright{} 1996 The American Physical Society.

The internal oxidation of two-phase binary alloys under low oxidant pressures

The main features of the internal oxidation in two-phase binary alloys are examined for insignificant and important diffusion of the most-reactive component and are compared with the behavior of corresponding single-phase systems. It is shown that two-phase alloys may have two different types of internal oxidation, one of which is similar to that of the single-phase alloys (classical type), producing a uniform distribution of small oxide particles in the zone of internal oxidation, while another is typical of two-phase systems and involves the in situ conversion of the most-reactive component into its oxide. It is also shown that, under the same values of all the relevant parameters, the classical internal oxidation of two-phase alloys involves faster kinetics and smaller degrees of enrichment of the most-reactive component in the zone of internal oxidation than for single-phase alloys. As a consequence of this, the transition to the external oxidation of the most-reactive component in these systems involves higher overall concentrations of the most-reactive component than in corresponding single-phase alloys.

Effects of ethylene glycol addition on the properties of Ru/Al2O3 catalyst prepared by sol-gel method

Ru/Al2O3 catalysts were prepared by sol-gel method with an organic additive (ethylene glycol). The effect of the addition of ethylene glycol on the properties of Ru/Al2O3 was characterized by BET, XRD, EXAFS, and TGA/DTA. Ethylene glycol was effective to promote the phase transition of α-Al2O3 even at 800°C calcination with high surface area. This finding is ascribed to the modified structure of aluminum alkoxide by ethylene glycol addition in the solution state. Ethylene glycol is also effective to get small particles of ruthenium after the reduction at 500°C. The EXAFS and UV-Vis spectra of Ru complex revealed that the coordination structure of Ru depended on the additive used. The ethylene glycol sol prefers to form octahedral Ru complex. This Ru complex in alumina matrix is stable up to 200°C and forms small Ru oxide particles even at 300°C calcination. This suggests that ethylene glycol coordinates to the Ru complex as well as to aluminum ion in the initial state, which is important to control the final properties of the Ru/Al2O3 catalyst.