Factors influencing efficiencies of TiO2-sensitised photodegradation. Part 1.—Substituted benzoic acids: discrepancies with dark-adsorption parameters

Abstract

Adsorption isotherm data have been determined at 295 K for salicylic (SA), 4-aminobenzoic (ABZA) and 3-chloro-4-hydroxybenzoic (CHBZA) acids onto TiO₂ particles suspended in dilute aqueous solution. Good fit is achieved between these dark-adsorption data and linearised isotherm expressions based on competitive adsorption–desorption equilibria involving competition between solute and solvent. Such linear analyses yield values for K, the equilibrium constants, and also for (n^s₂)_max, the limiting extents of solute adsorption, attainable in competition with the solvent. The latter values do not correspond to occupancy of each surface O—Ti—O site by a single adsorbed solute, but rather indicate that, on average, each adsorbed acid molecule coexists with ca. 40 adsorbed H₂O molecules. Concentration dependences predicted for rates of photodegradation of SA, ABZA and CHBZA on the basis of the experimentally determined adsorption parameters, plus an assumption that a pseudo-first-order Langmuir–Hinshelwood-type rate expression applies to TiO₂-sensitised photodegradations, differ markedly from initial rates observed for the latter at 297.5 ± 2.5 K. Consideration is given to various mechanistic sources of such discrepancy.