Time evolution of surface speciation during heterogeneous photocatalysis: Gallic acid on titanium dioxide

Abstract

Gallic acid adsorbs onto TiO2 films deposited onto an ATR crystal in two steps, both described by the kinetic Langmuir equation, as shown by measuring the spectral changes due to adsorbed species. Both pathways lead to species with the same spectral signature. Upon irradiation for preset times, integrated spectral absorbance profiles (log(A/A0) vs time) for the adsorbate peaks coincides with the analogous profile of remnant concentration in solution (log(C/C0) vs time). In our experiments, initially there are similar amounts of adsorbed and dissolved gallic acid. Hence, the rate of destruction of surface complexed gallic acid is approximately twice as large as the net rate of ligand uptake from solution. Upon irradiation, ligand surface concentration falls below equilibrium values, as also shown by plotting spectral signal intensity as a function of solution concentration. Under intermittent irradiation, the original rates were the same, but at larger radiation doses, the overall conversion was higher. This result demonstrates that during the intervening dark periods thermal reactions contribute to the overall rate. These thermal reactions may include desorption of intermediates, dark oxidation of intermediates, and re-equilibration of the surface with increasing gallic acid surface concentration. A description of photocatalysis based in the concept of oxidation length is provided.