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Raman spectra of pure synthetic indigo and of Maya blue of a Mexican clay sculpture are compared. The Raman spectrum of Maya blue shows extra bands assigned to vibrational modes of Bu symmetry as well as an increasing intensity of some other bands. The partial removal of the mutual exclusion rule for the centrosymmetric indigo molecule is supposed to indicate a loss of its planarity due to strong adsorption at the palygorscite matrix.
Redox behaviour of La-Cr compounds formed in CrOX/La2O3 mixed oxides and CrOX/La2O3/ZrO2 catalysts
(2003)
The redox behaviour of chromium oxide in CrOx/La2O3, CrOx/La2O3/ZrO2 and CrOx/ZrO2 mixed oxide systems has been investigated by temperature-programmed reduction (TPR), photoelectron spectroscopy (XPS) and Raman spectroscopy (RS). The formation of La-Cr compounds such as La2CrO6, LaCrO4 was shown to occur by calcination of CrOx/La2O3 and CrOx/La2O3/ZrO2 at 600 °C. With a Cr content of ca. 4 wt.%, it was revealed that, having been reduced to Cr3+-containing compounds, La2CrO6 and/or LaCrO4 in both systems may be restored by reoxidation at 600 °C. In the case of CrOx/La2O3/ZrO2 catalysts, it is suggested that these compounds were real surface species anchored as an overlayer on zirconia. Such a structure may stabilise, on the one hand, the tetragonal modification of zirconia and, on the other hand, highly dispersed state of Cr species in calcined as well as in reduced catalysts.
Structural characterization of thin films formed or changed on materials by micro Raman spectroscopy
(1998)
CrOx/La2O3 mixed oxides, prepared by impregnating La2O3 with appropriate aqueous solutions of (NH4)2CrO4 and calcining at 600 °C for 4 h, have been investigated by means of XRD, TPR, XPS, DRIFTS, and Raman spectroscopy (RS). The formation of the compounds La2CrO6, La(OH)CrO4 and LaCrO4 under these conditions was evidenced. Strong peaks at 864, 884, 913, and 921 cm-1, as well as weak peaks at 136, 180, 354, 370, and 388 cm-1 in the RS spectrum of CrOx/La2O3 have been assigned to La2CrO6.
Maya Blue, a pigment composed of very low concentrations of natural indigo and the clay mineral palygorskite, is one of the most brilliant blue dyes, intensively used for more than 2000 years in Mesoamerica. It is extremely stable against environmental attacks and was applied by the Indians for inside and outside mural paintings, ceramics, textiles and for colouring their famous codices. In the present paper it was studied as a powder (compared with modern synthetic indigo) and as colour on tissues, a Maya clay head, and fragment IV of the famous Codex Huamantla. Investigations using Raman spectroscopy in the visible and near-infrared range showed a high degree of correspondence among all Maya Blue-carrying samples and a good agreement with synthetic indigo. Additional spectral lines may be explained by a transformation of the planar indigo molecule when binding to the palygorskite lattice. Thermal investigations of the original amatl paper of the codex and of recent paper from fig-trees showed a high similarity and thus proved that this tree was chosen for paper making by Mayas, Aztecs and other Indian tribes. This was also true for the codex.
Raman spectroscopic and 35Cl-NMR scans of Hydro-Xan®, a preparation containing the biocatalytically effective Tetrachlorinedecaoxide anion (TCDO), are described. The spectroscopic analyses showed that HydroXan® does not contain chlorite initially used in the preparation anymore. Furthermore, a structure of four equal chlorate-like ClO3-groups was indicated for TCDO by the scans. Such a structure is supported by the long-term stability of HydroXan® found at tests over 267 days.
Permineralized samples of the tree fern Psaronius sp. belong to the spectacular discoveries of the Lower Permian Rotliegend Petrified forest of Chemnitz, one of the most famous palaeontological sites in the world. The cellular tissues of the 280 million years old fossil fern show a delicate three-dimensional preservation by permineralization. Moreover, anatomical detail can be distinguished very well because of the black coloured former cell walls. By means of Raman spectroscopy this fossilised tissue of the adventitious roots is revealed to be carbon which structure seems to correspond to anthracite. The aim of this work is the experimental confirmation of the carbon structure by means of thermogravimetry and Raman spectroscopy on coal samples of different rank, especially anthracite samples from different times and localities.
Coal maturation involves the sequence peatlignitecoalanthracite-microcrystalline graphite by incresasing the density and specific energy. The loss of in situ moisture (typically up to 200 °C) and volatile organic matter (up to 600 °C) has been acquired by thermogravimetry. The increasing carbon content, density resp. structure results in Raman spectra with typical band shape of the D- and G-peaks. We used these methods to characterise and compare the separated coal samples to the dispersed carbon inclusions in permineralized Psaronius.