Roberto Franco

Nuclear magnetic resonance study of PEO-based composite polymer electrolytes

Abstract The effect of carbon black and ceramic (a-Al 2 O 3 and g-Al 2 O 3 ) particles on the cationic and polymer chain mobilityhave been investigated in poly(ethylene oxide)–lithium salt (LiClO 4 and LiBF 4 ) solid composite polymer electrolytesby using nuclear magnetic resonance (NMR) techniques. Proton ( 1 H) and lithium ( 7 Li) lineshapes and spin–latticerelaxation times were measured as a function of temperature. The 7 Li data indicates that the lineshape and relaxationis affected by the coupling of its quadrupolar moment to the surrounding electric field gradients. Activation energiesextracted from the 7 Li relaxation data are in the range 0.20–0.25 eV. Results show that the addition of a-Al 2 O 3 influenced both, the relative cation mobility and the polymer chain motion. Calculation shows that this result is inquantitative agreement with the conductivity data.

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.

Optical and magnetic characterization of pure and vanadium-doped Bi12TiO20 sillenite crystals

Abstract Visible and Infrared (IR) optical absorption, Raman scattering and Electron Paramagnetic Resonance (EPR) techniques were used for systematic characterization of sillenite type crystals: nominally pure Bi 12 TiO 20 (BTO) and doped with vanadium (BTO:V). IR and Raman scattering results showed that the most conspicuous difference between BTO and BTO:V was the appearance of a band, at 767 cm −1 and at 790 cm −1 , attributed to the (VO 4 ) −3 . EPR, at 20 K, led us to suggest that the valence state of vanadium ions in BTO is 5+. In pure BTO samples, anisotropy of the EPR spectra at 20 K may be related to Fe 3+ impurities coming from starting compounds plus the presence of the (Bi M 3+ + h O + ) intrinsic defect. Results obtained from these different techniques confirm the Oberschmid and Grabmaier model [R. Oberschmid, Phys. Stat. Sol. (a) 89 (1985) 263; B.C. Grabmaier, R. Oberschmid, Phys. Stat. Sol. (a) 96 (1986) 199].

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

NMR and DSC study of polymer electrolyte–Carbon Black composites

Abstract Measurements of 1 H nuclear magnetic resonance (NMR) and differential scanning calorimetry (DSC) are reported for the composite formed by poly(ethylene oxide) (PEO) and Carbon Black (CB) and the composite polymer electrolytes formed by PEO 8 LiClO 4 and CB. The 1 H NMR lineshapes and relaxation times ( T 1 and T 2 ) were investigated as a function of temperature and CB concentration. The results suggest that the dynamic behavior of the polymer chains is affected by the presence of the filler.