As
a continuation of the Direct–Indirect (D-I) model theoretical
approach presented in Part I of this publication, concerning the photocatalytic
oxidation of organic molecules in contact with TiO2 dispersions,
a comparative photooxidation kinetic analysis of three model organic
molecules, benzene (BZ) dissolved in acetonitrile (ACN), phenol (PhOH)
dissolved in either water or acetonitrile, and formic acid (FA) dissolved
in water, is presented to test the applicability of the D-I model
under both equilibrium and nonequilibrium adsorption–desorption
conditions. A previous analysis involving diffuse reflectance ultraviolet–visible
(DRUVS) and Fourier transform infrared (FTIR) spectroscopy, combined
with adsorption isotherm plots, shows that BZ chemisorption on the
TiO2 surface is not allowed, physisorption being in this
case the only possible adsorption mode. In line with D-I model predictions,
BZ photooxidation is observed to take place via an adiabatic indirect
transfer (IT) mechanism, with the participation of photogenerated
terminal −Os
•– radicals as oxidizing agents. In contrast,
because of their strong chemisorption, FA species dissolved in water
are found to be mainly photooxidized via inelastic direct transfer
(DT) trapping of photogenerated valence-band free holes (h
f
+). Finally,
when dissolved in water, PhOH chemisorption is not favored because
of the strong electronic affinity of water molecules with the TiO2 surface, while chemisorption strength considerably increases
when PhOH is dissolved in ACN, as far as the electronic interaction
of solvent molecules with the TiO2 surface is negligible.
Consequently, as predicted by the D-I model, PhOH dissolved in water
is photooxidized via a combination of IT and DT mechanisms, the IT
photooxidation rate (v
ox
IT) being about 1 order of magnitude higher
than DT photooxidation rate (v
ox
DT). In contrast, when ACN is
used as solvent, v
ox
IT remains practically unchanged, while v
ox
DT increases by about 2 orders of magnitude. These photooxidation results
sustain the central D-I model hypothesis that the degree of substrate
species interaction with the TiO2 surface is a decisive
factor determining the kinetics of photocatalytic reactions. The effect
of adsorption–desorption equilibrium rupture on the photooxidation
kinetics of dissolved substrate species, predicted by the D-I model,
is analyzed for the first time from experimental kinetic data concerning
the photooxidation of PhOH dissolved in water under high enough illumination
intensity (ρ ≈ 1017 cm–2 s–1).
