Based on the above results, a photocatalytic mechanism
is proposed and illustrated in Fig. 15 to describe
the reduction of CO2 with H2O. Molecules of CO2 (or
HCO3
−) and H2O interact with trapped electrons on
the Cu cluster. The reduction of CO2 and the decomposition
of H2O proceed competitively, and methanol
is formed. The OH radicals are produced from OH−
by scavenging holes on the TiO2 surface, and these
radicals may react with the carbon species formed
from CO2 to produce methanol. Free oxygen is also
generated from the oxidation of OH− with holes,
then can be partially consumed by the re-oxidation of
methanol.