Ronald J. Willey

Magnetic properties of partially-inverted zinc ferrite aerogel powders

Fine powders of ZnFe2O4 with an average particle size of 10 nm and inversion parameter of 0.21 were synthesized by the aerogel procedure. Portions of the powders were calcined in air at 500 and 800 °C and other portions were ball-milled for 10 h. The materials were characterized by x-ray diffractometry, vibrating sample, and SQUID magnetometry, Mossbauer spectrometry, and low temperature calorimetry. Upon calcination the powders underwent significant changes in grain size, inversion parameter, and hence magnetic properties. The magnetic state of the as-produced and calcined samples is best described as disordered and highly dependent on temperature. Upon ball-milling the grain size varied widely and the inversion parameter attained a value of 0.55. The magnetic properties of the ball-milled sample are similar to those of ferrimagnetic MgFe2O4 powders having comparable grain size and inversion parameters.

Transition metal mixed oxides as combustion catalysts : preparation, characterization and activity mechanisms

Abstract A number of transition metal mixed oxides with spinel-type, corundum-type and perovskite-type structures have been prepared and characterized. Attempts have been made to improve their morphological properties and their stability. Some of them have been tested in the catalytic combustion of methane, CO and H 2 (perovskites), of propane and of phenantrene. FT-IR experiments allowed to obtain a quite complete picture of the mechanism of catalytic combustion of C3 organic compounds on spinel-type oxides MgCr 2 O 4 and Co 3 O 4 . Nucleophilic oxygen species (lattice oxygen) is thought to be involved in both partial and total oxidation.

Preparation and characterization of magnesium chromite and magnesium ferrite aerogels

A supercritical drying process has been used to prepare MgO-Cr2O3, and MgO-Fe2O3 powders from a mixture of magnesium acetate and the metallic acetylacetonates hydrolyzed in a methanolic solvent. Th...

Investigation of iron oxide-chromia-alumina aerogels for the selective catalytic reduction of nitric oxide by ammonia

The selective catalytic reduction of nitric oxide by ammonia over iron oxide-chromia-alumina aerogels was studied over the temperature range of 423 to 750 K. The highest activities were observed for Fe/Cr/Al materials with a 60% conversion at 720 K for a reactor flow velocity of 1008 m3/h-kg. Oxygen significantly enhanced the reaction rate as explained by a redox mechanism. Infrared characterization showed weak adsorption of nitric oxide on the preoxidized materials and quite strong adsorption on the reduced material; ammonia adsorption was strong on both oxidized and reduced materials. Observation of interaction between adsorbed nitric oxide and ammonia (when ammonia was added to reduced catalysts which held preadsorbed nitric oxide) provided insight into the reaction mechanism.

A spectroscopic study of amorphous and crystalline Ti-containing silicas and their surface acidity

Different titanium silicalite (TS-1) samples and TiO2/SiO2 aerogels samples have been prepared and characterized structurally by XRD, skeletal FT-IR and FT-Raman spectroscopies and diffuse reflectance UV spectroscopy. The surface acidity of all materials has been investigated by FT-IR spectroscopy of the surface hydroxy groups and of adsorbed acetonitrile. The TS-1 samples are found to be totally free from extra-framework Ti oxide species, while the characteristic features of framework tetrahedrally coordinated Ti cations are well evident. Framework Ti cations substituting for Si into the silicalite MFI structure act as quite strong Lewis acid sites towards acetonitrile. The presence of Ti neither changes significantly the Brønsted acidity of the silicalite silanols, nor results in the formation of new hydroxy groups. TS-1 samples present lower defectivity (smaller amount of clustered hydrolyzed Si–OH OH–Si bonds) than pure silicalite prepared in the same way. Extra-framework TiO2, instead, is well detectable into aerogel samples, although framework Ti cations are likely also present. The surface of Ti-containing aerogels is covered by large amounts of silanol groups, although these sites do not present enhanced Brønsted acidity. Additionally, Lewis sites are detected whose strength is similar to that observed on TS samples, and is superior to that of the predominant sites on TiO2. It is suggested that silica tends to cover the TiO2 core in the case of aerogels. The surface is consequently dominated by silica with Ti sites in substitutional position.

Structure and magnetic properties of magnesium ferrite fine powders

Abstract Fine powders of magnesium ferrite, MgFe2O4, were produced through the sol-gel supercritical drying method, with two portions then being calcined at 773 K and 1073 K. The powder structural and magnetic properties were determined from transmission electron microscope micrographs, x-ray diffraction, Mossbauer effect spectroscopy and magnetometry measurements. The powder structure matched the MgFe2O4 spinel phase, with small amounts of α-Fe2O3 being observed in heated samples. As-produced powders were superparamagnetic at room temperature, with single magnetic domain particle behavior being observed at low temperatures, and for the 1073 K heated sample. The particle size distribution for the as-produced powder was evaluated separately from the micrographs, by fitting the magnetization data to a weighted Langevin function, and by fitting Mossbauer spectra taken at temperatures from 25 K to 298 K. Very similar particle size distributions were found from all three methods. The average particle diameter was 11 nm for the as-produced powder, and increased for heated samples. The saturation magnetization and magnetocrystalline anisotropy energy density values were both consistent with bulk values, in contrast to the large differences between particle and bulk values described for other fine particle systems.

A study of the external and internal sites of MFI-type zeolitic materials through the FT-IR investigation of the adsorption of nitriles

Abstract The adsorption of acetonitrile (AN) and pivalonitrile (2,2-dimethylpropionitrile, PN) has been investigated on pure silicalite S1, on a ZSM5 zeolite apparently free from extraframework material and another ZSM5 zeolite apparently rich in extraframework material, on a titanium silicalite TS1 and on a boralite sample. AN enters the cavities of MFI type structures and interacts with both internal and external acid sites while PN does not enter such cavities and only interacts with the external sites. Terminal silanols, whose acidity can vary from weak to medium, are present in all cases mostly at the external surface, while very acidic bridging Si–OH–Al sites of ZSM5 zeolite are apparently exclusively located at the internal channel surface. Lewis acidic Ti cations are evident both in the internal and on the external surface of TS1. Weakly acidic BOH sites of boralite are distributed among internal and external surface. Extraframework alumina-like species of ZSM5 zeolite are apparently located in the internal cavities. Terminal silanols and hydrolized defects are located mostly at the external surface of silicalite. The mechanism of incorporation and charge balance of boron atoms as well as the structure of the hydroxy groups of boralite are still not well established.

FTIR studies on the selective oxidation and combustion oflight hydrocarbons at metal oxide surfaces Part3.—Comparison of the oxidation ofC3 organic compounds over Co3O4,MgCr2O4 and CuO

Oxidation of the C 3 organic compounds propane, propene, acrolein, propan-2-ol and acetone has been investigated over three transition-metal oxide catalysts, Co 3 O 4 , MgCr 2 O 4 and CuO, in a flow reactor and using FTIR spectroscopy to study the adsorbed species. Co 3 O 4 and MgCr 2 O 4 are very active in propane and propene catalytic combustion. FTIR studies suggest that adsorbed isopropoxide species and adsorbed acetone and acetates are intermediates in propane oxidation while adsorbed acrolein and acrylates are intermediates in propene oxidation. Flow reactor studies support these hypotheses. It is suggested that the reaction rates in propane and propene total oxidation can be influenced, at low temperature, by the rate of oxidation of adsorbed acetate and acrylate intermediates, respectively. Co 3 O 4 and MgCr 2 O 4 are also active and quite selective catalysts for the oxy-dehydrogenation of propan-2-ol to acetone at low conversion, suggesting that the same oxygen species are involved in total and partial oxidation of organic compounds. CuO, as such, is not active in the adsorption and oxidation of C 3 hydrocarbons and oxygenates, at low temperature. At higher temperatures the reactants reduce the catalyst and catalytic activity starts. The oxidation state of the CuO x catalyst can be evaluated by IR studying the transmittance of the radiation upon different treatments.

Titania nanotubes prepared by anodization in fluorine-free acids

Recently, we reported the discovery of new high-aspect ratio titania nanotubes. These nanotubes were synthesized by means of anodization in an oxalic acid electrolyte containing chlorine ions and were found to have significant carbon content. In this article, the synthesis of similar titania nanotubes in oxalic, formic, trichloroacetic, gluconic, hydrochloric, and sulfuric acid is reported. Differences in carbon content and morphology are analyzed, which in turn provides information on the chemistry of the formation of these nanotubes. Our results suggest that the carbon content in the nanotubes can be controlled by the use of an appropriate organic acid.

Oxidation of methanol over iron oxide based aerogels in supercritical CO2

Abstract The oxidation of methanol over iron oxide on silica or molybdenum aerogels using supercritical CO 2 as the carrier was investigated for iron oxide content from 1 to 20 wt%. A pure iron oxide aerogel was also prepared. Partial oxidation products of dimethyl ether, methyl formate, or formaldehyde were found between 200 to 300°C. Selectivity was influenced by the catalyst used. Pure iron oxide favored dimethyl ether, low levels of iron oxide on silica favored methyl formate, and iron oxide on molybdenum favored formaldehyde. Results demonstrate the feasibility of aerogels as model catalyst systems for reactions in supercritical fluids.