NaOH solution was crucial in the photoreduction
of CO2 [19]. The methanol yield substantially
increased with the addition of NaOH in our
experiments, perhaps due to the following two reasons.
First, the high concentration of OH−; ions in
aqueous solution could act as strong hole-scavengers
and form OH radicals, thereby reducing the recombination
of hole–electron pairs. The longer decay
time of surface electrons would certainly facilitate
the reduction of CO2. Second, caustic NaOH solution
dissolves more CO2 than does pure water. The
initial pH value of 0.2N NaOH solution was approximately
13.3, and decreased to nearly 7.3 after
bubbling CO2 in the reactor. Notably, photoreduction
may have been accelerated by the high concentration
of HCO3.