Binding and transport of MoV in perfluorinated ionomers by EPR and EPR imaging

Abstract

EPR spectra of Mo^V(as MoCl₅) in ethanol solutions and in perfluorinated ionomers (PFI) with long pendant chains (Nafion) have been measured as a function of MoCl₅ concentration in the solutions (in the range 3.60 × 10^–6 to 1.15 × 10^–4M) and degree of neutralization of the sulfonic groups in the ionomers. Two Mo^V sites were characterized in the ethanol solutions, and their relative intensity was found to depend on MoCl₅ concentration. Both sites, however, show evidence for chlorine ligands around the paramagnetic cation. The Mo^V site in PFI swollen by ethanol is identical to the site detected in neat ethanol solutions at higher MoCl₅ content, and was assigned to a molybdenum site with five chlorine ligands and an oxygen ligand from the solvent. The diffusion coefficients of this species were determined by 2D spatial–spectral electron paramagnetic resonance imaging (2D EPRI) in Nafion–ethanol, by simulating the concentration profiles along the spatial coordinate measured during the diffusion process. The diffusion coefficients in Nafion–ethanol are 5.0 × 10^–7, 7.1 × 10^–7, 10.4 × 10^–7 and 13.3 × 10^–7 cm² s^–1 at 280, 300, 315 and 330 K, respectively. The activation energy for diffusion is 15.4 kJ mol^–1.