Predominant formation of CH3OH (119 ppm) occurred after
120 min of photoelectrochemical CO2 reduction at pH 9 and 10,
but maximum CH3OH production took place at pH 8.0, clearly indicating
its stabilization in this condition. C2H5OH (23.8 ppm) and
CH2O (2.4 ppm) also emerged, but at much lower concentrations.
The results obtained at pH 8 are consistent with the proposed
mechanism (Scheme 1). The generation of hydrogen radical is necessary
to the CH3OH formation, which is suppressed at more alkaline
condition. At this condition, the hydroxyl radical in the
reaction is preponderant and the C2H4O and CH3COCH3 formation
are preferential.