posted on 2001-09-07, 00:00authored byMauricio E. Calvo, Roberto J. Candal, Sara A. Bilmes
Processes that occur in the TiO2-photocatalysis of binary
aqueous solutions containing model photolytes with
different affinity for the TiO2 surface (methanol and oxalic
and salicylic acids) are analyzed from the photoelectrochemical response of TiO2 films under bias in a time window
of 1−100 s. Long-lived oxidized intermediates produced
upon illumination at 0.6 VSCE are detected by cathodic sweep
run in the dark after irradiation. The main conclusion
derived from this work is that a scheme of competitive
kinetics describes only those cases in which one of the
components is weakly or nonadsorbed on TiO2, whereas for
two photolytes with high affinity for the surface cooperative
effects may occur. The methanol−oxalate system is
quantitatively modeled by considering that oxalate forms
surface complexes with different reactivity and a parallel
pathway for hole transfer to −OH and adsorbed oxalate.
In this case as well as for electrolytes containing methanol
and salicylate photooxidation of methanol (with low
affinity for the surface) via intermediates formed by reaction
with trapped holes (−•OH) is partially or fully suppressed.
For electrolytes containing oxalic and salicylic acids in
which both components chemisorb on TiO2 the photoelectrochemical response depends on preadsorption, the
photooxidation pathways deviates those of single component
systems, and there is remotion of salicylate adsorbed
byproducts assigned to cooperative effects.