No abstract

X-Ray absorption spectroscopies, EXAFS (extended X-ray absorption fine structure) and XAN ES (X-ray absorption near-edge structure), were used for probing the structural and chemical environment of vanadium in two different asphaltenic fractions of a Boscan crude oil. Although the level of porphyrins detectable by u.v.-visible spectrophotometry was substantially reduced (i.8. < 15% from the 4 000 p.p.m. of metal) due to successive solvent extractions, the EXAFS spectra of these fractions were still largely dominated by the typical pattern of vanadyl porphyrins, whereas the intense prepeak observed in the XANES spectra also clearly supported the presence of quite large amounts of porphyrins in these fractions. We thus suggest that the u.v.-visible spectrophotometric analyses tend systematically to underestimate the porphyrinic content in these heavy fractions, probably due to some hardly detectable microheterogeneity of the test solutions. E.s.r. spectra are produced which clearly support these various conclusions. Much more elaborate separation procedures are required in order to isolate and identify any ' non-porphyrinic vanadium fraction ' from Boscan crudes.

No abstract

No abstract

The water electro-osmotic drag coefficient of a cation _ K drag (cation)_ is defined as the number of water molecules which is dragged by a cation when it goes through a membrane at equilibrium state (i.e. in the absence of water concentration gradient within membrane). This coefficient is one of the key elements for modeling the water balance through Nafion membrane in a proton Exchange membrane Fuel cell (PEMFC). In this work, we have applied the method of electrophoresis NMR (ENMR) to determine the coefficient of electro-osmotic drag (K drag ) within Nafion prepared in different cationic forms (H + , Na + and K + ). Applying, in all cases, the same method for determining K drag of Nafion membrane in many cationic forms should be appropriate in order to apprehend the mechanisms of water transport which occur in this membrane. This method consists in measuring the phase of the water NMR signal under the application of an electric current in a diffusion-like experiment, this phase being proportional to the flux of water within Nafion. The results show that the electro-osmotic coefficients of water within Nafion depend on the type of counter-cation and on the water content of Nafion. Due to proton lability, the electro-osmotic drag coefficient of water within Nafion in the acid form (K drag (H + )) is the lowest among the three studied cationic forms. In a dehydrated state of Nafion, the K drag (H + ) is lower than 1, suggesting here that the hopping mechanism is favored. In the case of Nafion in sodium and potassium forms, K drag of these cations is always higher than 1. This can be rationalized by the vehicular mechanism and also by a strong interaction between the water solvation of the cation and the cation itself.