Cristiane B. Rodella

Structural characterization of the V2O5/TiO2 system obtained by the sol–gel method

The influence of the vanadium load and calcination temperature on the structural characteristics of the V2O5/TiO2 system was studied by X-ray diffraction and X-ray absorption spectroscopy (XAS) techniques. Samples of the V2O5/TiO2 system were prepared by the sol – gel method under acid conditions and calcined at different temperatures. The rutile phase was found to predominate in pure TiO2 calcined at 450 8C as a result of the reduction of phase transition temperature promoted by the sol– gel method under acid conditions. The anatase phase became predominant at 450 8C as the amount of vanadium increased from 6 to 9 wt%. A structural change in the TiO2 phase from predominantly anatase to totally rutile with increased calcination temperature was observed in 6 wt% samples. An analysis of the vanadium X-ray Absorption Near Edge Structure (XANES) spectra showed that the oxidation state of vanadium atoms in the samples containing 6 and 9 wt% of vanadium and calcined at 450 8C was predominantly V 4þ . However, the presence of V 5þ atoms cannot be ruled out. A qualitative analysis of extended Xray absorption fine structure (EXAFS) spectra of the samples containing 6 and 9 wt% of vanadium calcined at 450 8C showed that the local structure around vanadium atoms is comparable to that of VO2 crystalline phase, in which vanadium atoms are fourfold coordinated in a distorted structure. For the sample after calcination at 600 8C, the EXAFS and XANES results showed that a significant portion of vanadium atoms were incorporated in the rutile lattice with a VxTi(12x )O2 solid solution formation. The conditions of sample preparation used here to prepare V2O5/TiO2 samples associated with different amounts of vanadium and calcination temperatures proved to be useful to modifying the structure of the V2O5/TiO2 system. q 2002 Elsevier Science Ltd. All rights reserved.

Surface Characterisation of V2O5/TiO2 Catalytic System

Samples of the V 2 O 5 /TiO 2 system were prepared by the sol-gel method and calcined at different temperatures. Surface species of vanadium, their dispersion, as well as the structural evolution of the system were analysed by XRD, Raman, EPR. and XPS techniques. The results of XRD showed the evolution of TiO 2 from anatase phase to rutile phase. The Raman spectra for calcination temperatures up to 500 °C showed a good dispersion of vanadium over titania in the form of monomeric vanadyl groups (V 4+ ) and polymeric vanadates (V 5+ ). At least three families of V 4+ ions were identified by EPR investigations. Two kinds of isolated V 4+ species are placed in sites of octahedral symmetry, substituting Ti 4+ in the rutile phase. The third is formed by pairs of V 4+ species on the surface of titania. Above 500 °C part of superficial V 4+ is inserted into the matrix of titania and part is oxidized to V 5+ . The XPS results showed that the V/Ti ratio rises with increasing calcination temperature, indicating a smaller dispersion of vanadium.

V2O5/TiO2 catalyst xerogels: Method of preparation and characterization

This work describes a modified sol-gel method for the preparation of V2O5/TiO2 catalysts. The samples have been characterized by N2 adsorption at 77 K, X-ray Diffractometry (XRD), Scanning Electronic Microscopy (SEM/EDX) and Fourier Transform Infrared Spectroscopy (FT-IR). The surface area increases with the vanadia loading from 24 m2 g−1 for pure TiO2 to 87 m2 g−1 for 9 wt% of V2O5. The rutile form is predominant for pure TiO2 but becomes enriched with anatase phase when vanadia loading is increased. No crystalline V2O5 phase was observed in the diffractograms of the catalysts. Analysis by SEM showed heterogeneous granulation of particles with high vanadium dispersion. Two species of surface vanadium were observed by FT-IR spectroscopy: a monomeric vanadyl and polymeric vanadates. The vanadyl/vanadate ratio remains practically constant. Ethanol oxidation was used as a catalytic test in a temperature range from 350 to 560 K. The catalytic activity starts around 380 K. For the sample with 9 wt% of vanadia, the conversion of ethanol into acetaldehyde as the main product was approximately 90% at 473 K.

V2O5/TiO2 Catalytic Xerogels Raman and EPR Studies

Raman spectroscopy and Electron Paramagnetic Resonance (EPR) studies were performed on a series of V2O5/TiO2 catalysts prepared by a modified sol-gel method in order to identify the vanadium species. Two species of surface vanadium were identified by Raman measurements, monomeric vanadyls and polymeric vanadates. Monomeric vanadyls are characterized by a narrow Raman band at 1030 cm−1 and polymeric vanadates by two broad bands in the region from 900 to 960 cm−1 and 770 to 850 cm−1. The Raman spectra do not exhibit characteristic peaks of crystalline V2O5. These results are in agreement with those of X-ray Diffractometry (XRD) and Fourier Transform Infrared (FT-IR) previously reported (C.B. Rodella et al., J. Sol-Gel Sci. Techn., submitted). At least three families of V4+ ions were identified by EPR investigations. The analysis of the EPR spectra suggests that isolated V4+ ions are located in sites with octahedral symmetry substituting for Ti4+ ions in the rutile structure. Magnetically interacting V4+ ions are also present as pairs or clusters giving rise to a broad and structureless EPR line. At higher concentration of V2O5, a partial oxidation of V4+ to V5+ is apparent from the EPR results.

Electric and magnetic properties of polymer electrolyte/carbon black composites

Abstract Measurements of 1H Nuclear Magnetic Resonance (NMR) relaxation times, Electron Paramagnetic Resonance (EPR) and AC Impedance Spectroscopy (IS) are reported for composites based on PEO8:LiClO4 and carbon black (CB), prepared by two methods: solvent and fusion processing. Three nuclear relaxation processes were identified for 1H nuclei: (i) belonging to the polymer chains in the amorphous phase, loosely bound to the CB particles, whose dynamics is almost the same as for unfilled polymer, (ii) belonging to the polymer chains which are tightly attached to the CB particles, and (iii) belonging to the crystalline phase in the loose polymer chain. The paramagnetic electronic susceptibility of the composite samples, measured by EPR, was interpreted by assuming a contribution of localized spin states that follow a Curie law, and a Pauli-like contribution of delocalized spins. A significant change of the EPR linewidth was observed at 40 K, which is the temperature where the Curie and Pauli susceptibilities equally contribute to the paramagnetic electronic susceptibility. The electrical properties are very sensitive to the preparation methods of the composites, which conditions the interaction between carbon particle–carbon particle and carbon particle–polymer chain. Classical statistic models to describe the conductivity in these media were not satisfactory.

Chemical and structural characterization of V2O5/TiO2 catalysts

Univ Sao Paulo, Inst Fis Sao Carlos, Dept Fis & Ciencia Mat, BR-13560970 Sao Carlos, SP, Brazil

CARACTERIZAÇÃO TEXTURAL E ESTRUTURAL DE V 2 O 5 /TiO 2 OBTIDOS VIA SOL-GEL: COMPARAÇÃO ENTRE SECAGEM CONVENCIONAL E SUPERCRÍTICA

This work describes a modified sol-gel method for the preparation of V2O5/TiO2 catalysts. The samples have been characterized by N2 adsorption at 77K, x-ray diffractometry (XRD) and Fourier Transform Infrared (FT-IR). The surface area increases with the vanadia loading from 24 m2 g-1, for pure TiO2, to 87 m2 g-1 for 9wt.% of V2O5. The rutile form is predominant for pure TiO2 but became enriched with anatase phase when vanadia loading is increased. No crystalline V2O5 phase was observed in the catalysts diffractograms. Two species of surface vanadium observed by FT-IR spectroscopy a monomeric vanadyl and polymeric vanadates, the vanadyl/vanadate ratio remains practically constant.

Surface characterisation of V(2)O(5)/TiO(2) catalytic system

Univ Fed Sao Carlos, Dept Mat Engn, Ctr Caracterizacao & Desenvolvimento Mat, BR-13565905 Sao Carlos, SP, Brazil