Oxygen is expected as a product of the CO2 reduction
(Eq. (1)). Fig. 12 shows that the time dependency
of oxygen generation approximately matches
that of the methanol yield. By and large, the ratio of
methanol to oxygen is near constant, ∼3–1. Free O2
was suggested as being adsorbed on the surface of
TiO2 in the presence of water [14]. Consequently, the
oxygen sensor detected only part of dissolved oxygen
in these experiments. The O2 evolution also plateaued
after 16–18 h of reaction, revealing O2 consumption
by methanol re-oxidation.