Intercalation of Methylene Blue into vanadium pentoxide gels

Abstract

The kinetics of the intercalation of Methylene Blue (MB⁺) into vanadium pentoxide gels (V₂O₅·nH₂O) is reported. The intercalation was studied by absorption spectroscopy in the 800–500 nm region. The driving force for intercalation is the reduction of Methylene Blue (formation of a semiquinone) by V⁴⁺ species. This reduction is followed by protonation, reoxidation by oxygen (formation of protonated Methylene Blue) and finally dimerization of the dye. The absorption bands of adsorbed protonated Methylene Blue are at ca. 693 and 770 nm. The absorption coefficient of the 693 nm band is 30 000 ± 2600 M^–1 cm^–1 and that of the 770 nm band is unclear and ranges between 53 000 and 98 000 M^–1 cm^–1. The dimer has an absorption band around 562 nm with an absorption coefficient of 23 000 M^–1 cm^–1. The transformation of protonated Methylene Blue into the dimer follows second-order kinetics with a rate constant of ca. 100 M^–1 s^–1.