Chemical discrimination by a kinetic model of organic photooxidation in a heterosupramolecular assembly

The photooxidation of range of common organic pollutants in a dye-sensitised photoelectrochemical cell (DS-PEC) is reported. A photoanode was prepared by the chemisorption of a photosensitiser, cis-bis-(2,2′-bipyridine)-(4,4′-bis-(methyl)phosphonato-2,2′-bipyridine)ruthenium(II) dichloride ([Ru(bpy)2(dmpbpy]2+), to a nanoporous nanocrystalline TiO2 thin film on a conducting glass substrate. The photoanode was coupled to a platinum electroplated fluorine doped tin oxide glass substrate in a two electrode assembly and the cell cavity was filled with an aqueous solution of organic pollutants and irradiated with λ>420nm to give a measurable photocurrent. In addition to the ability of this cell to photooxidise a range of chemically diverse organic pollutants, the application of a kinetic model to observed photocurrent transients allowed the study of interfacial electron transfer processes. Through the mathematical fit of a five-parameter double exponential decay function, evidence to support numerous interfacial reactions for the oxidation process were identified. Rapid oxidation of species in close proximity to the photooxidation centre was proposed as the kinetically fast interfacial process with a first-order rate constant of the order 0.4 s−1. The slower process was attributed to the diffusion of oxidisable species from the solution bulk to the surface prior to oxidation with a first-order rate constant of the order 0.01 s−1. Theoretical profiling of the kinetic events supported the biphasic assignment of interfacial processes and indicated that non-exhaustive oxidation occurs for the solution concentrations examined.