Synthesis of γ-(Al1-xFex)2O3 solid solutions from oxinate precursors and formation of carbon nanotubes from the solid solutions using methane or ethylene as carbon source

This work reports for the first time the synthesis of ?-(Al1-xFex)2O3 solid solutions with a high specific surface area (200-230 m2/g) by the decomposition of metal oxinate [(Al1-xFex)(C9H6ON)3] and investigated the potential of these materials as catalysts for the synthesis of carbon nanotubes by catalytic chemical vapor deposition using methane or ethylene as carbon the source. The nanocomposite powders prepared by reduction in H2-CH4 contain carbon nanotubes (CNTs), which are mostly double-walled but also contain a fair amount of undesirable carbon nanofibers, hollow carbon particles, and metal particles covered by carbon layers. Moreover, abundant metallic particles are observed to cover the surfaces of the matrix grains. By contrast, the nanocomposite powders prepared by reduction in N2-C2H4 are not fully reduced, and the CNTs are much more abundant and homogeneous. However, they are multiwalled CNTs with a significant proportion of defects. The powders were studied by several techniques including Mössbauer spectroscopy and electron microscopy

Surface Composition of Carbon Nanotubes-Fe-Alumina Nanocomposite Powders: An Integral Low-Energy Electron Mo1ssbauer Spectroscopic Study

The surface state of carbon nanotubes-Fe-alumina nanocomposite powders was studied by transmission and integral low-energy electron Mo¨ssbauer spectroscopy. Several samples, prepared under reduction of the R-Al1.8-Fe0.2O3 precursor in a H2-CH4 atmosphere applying the same heating and cooling rate and changing only the maximum temperature (800-1070 °C) were investigated, demonstrating that integral low-energy electron Mo¨ssbauer spectroscopy is a promising tool complementing transmission Mössbauer spectroscopy for the investigation of the location of the metal Fe and iron-carbide particles in the different carbon nanotubenanocomposite systems containing iron. The nature of the iron species (Fe3+, Fe3C, R-Fe, ç-Fe-C) is correlated to their location in the material. In particular, much information was derived for the powders prepared by using a moderate reduction temperature (800, 850, and 910 °C), for which the transmission and integral low-energy electron Mössbauer spectra are markedly different. Indeed, R-Fe and Fe3C were not observed as surface species, while ç-Fe-C is present at the surface and in the bulk in the same proportion independent of the temperature of preparation. This could show that most of the nanoparticles (detected as Fe3C and/or ç-Fe-C) that contribute to the formation of carbon nanotubes are located in the outer porosity of the material, as opposed to the topmost (ca. 5 nm) surface. For the higher reduction temperatures Tr of 990 °C and 1070 °C, all Fe and Fe-carbide particles formed during the reduction are distributed evenly in the bulk and the surface of the matrix grains. The integral low-energy electron Mo¨ssbauer spectroscopic study of a powder oxidized in air at 600 °C suggests that all Fe3C particles oxidize to R-Fe2O3, while the R-Fe and/or ç-Fe-C are partly transformed to Fe1-xO and R-Fe2O3, the latter phase forming a protecting layer that prevents total oxidation

Mössbauer spectroscopy on Mars: goethite in the Columbia Hills at Gusev crater

In January 2004 the USA space agency NASA landed two rovers on the surface of Mars, both carrying the Mainz Mössbauer spectrometer MIMOS II. The instrument on the Mars-Exploration-Rover (MER) Spirit analyzed soils and rocks on the plains and in the Columbia Hills of Gusev crater landing site on Mars. The surface material in the plains have an olivine basaltic signature suggesting physical rather than chemical weathering processes present in the plains. The Mössbauer signature for the Columbia Hills surface material is very different ranging from nearly unaltered material to highly altered material. Some of the rocks, in particular a rock named Clovis, contain a significant amount of the Fe oxyhydroxide goethite, α-FeOOH, which is mineralogical evidence for aqueous processes because it is formed only under aqueous conditions. In this paper we describe the analysis of these data using hyperfine field distributions (HFD) and discuss the results in comparison to terrestrial analogues