Spectroscopic and electrochemical examination of the dark and photo-catalysed redox reactions that occur at the interface between solid α-[Hex4N]4[S2Mo18O62], solid triphenylphosphine and water

Abstract

Solid α-[Hex₄N]₄[S₂Mo₁₈O₆₂] was found to be reduced by solid triphenylphosphine when the two species were ground together as powders and left for periods of several days in the presence of atmospheric water vapour. Electrospray mass spectrometry confirmed that the final product formed by oxidation of the phosphine was PPh₃O, while experiments in the presence of H₂¹⁷O demonstrated that the oxygen source was atmospheric water vapour rather than [S₂Mo₁₈O₆₂]^4–. Voltammetry and UV/visible spectroscopy of samples of reacted solids dissolved in MeCN imply that the two-electron, two-proton reduced species [Hex₄N]₄[H₂S₂Mo₁₈O₆₂] was the major product formed upon reaction of an equimolar solid mixture of PPh₃ and [Hex₄N]₄[S₂Mo₁₈O₆₂]. The rate of this process is accelerated by irradiation with 300–400 nm light, corresponding to the wavelength of an absorption band of [S₂Mo₁₈O₆₂]^4–. When PPh₃ was in excess, three-electron and four-electron reduced forms of [S₂Mo₁₈O₆₂]^4– were detected by EPR spectroscopy and voltammetry, respectively. Direct evidence for the photooxidation of PPh₃ was obtained through solid-state photovoltammetric experiments in which a phototransient response for the oxidation of reduced forms of [S₂Mo₁₈O₆₂]^4– was recorded for solid state mixtures which were mechanically attached to a pyrolytic graphite electrode surface in contact with an aqueous medium.