To study the temperature dependence of the CO2
photoreduction, the experiment was repeated at different
temperatures in the range of 43–100 C. As can be seen
from Fig. 5 the methanol yield increased for higher reaction
temperatures. Saladin et al. [10] reported that the
kinetic of CO2 photoreduction can be modeled with
the adsorption/desorption equilibrium of the various
species involved in the reaction. At low temperatures
surface coverage is high and products do not easily desorb.
In this case product desorption is rate limiting and
the apparent activation energy (Ea) becomes positive.
On the contrary, at high temperatures the adsorption
of reactants becomes more important due to the scarcely
covered surface and products readily desorbs and therefore
Ea is negative [10].
Inset of Fig. 5 shows a typical Arrhenius plot for
Degussa P25 and 3%CuO/TiO2 catalysts. A mean value
for Ea of ca. +26 and +12 kJ/mol was calculated for
Degussa P25 and 3%CuO/TiO2, respectively. These positive
values of Ea imply that the desorption of products
is the rate limiting step in the photosynthetic formation
of methanol. In addition, the apparent lower activation
energy of 3%CuO/TiO2 catalyst may indicate a catalytic
role of copper species as an active site to provide methanol,
and hence improving the photoefficiency of TiO2
photocatalyst. The in situ IR experiments are still in
progress to investigate the mechanism aspects of the
catalyst.