The effects of Cu loading and dispersion on methanol
yields are shown in Fig. 10. The methanol yields
increase with Cu loading, but then decrease when
the Cu loading exceeds 2 wt.%. The Cu dispersions
of Cu/TiO2 increased slightly to 40% for Cu loading
below 2 wt.%, then decreased with further Cu
loading. The turnover frequency (TOF) of Cu/TiO2
catalyst can be estimated if we assume that entire
photoreduction occurs on Cu active sites. Table 3
lists the TOFs of Cu/TiO2 catalysts for known Cu
dispersion. The TOF of 0.6 wt.% Cu/TiO2 catalysts
gave the highest value near 58, and TOFs decrease
with increasing Cu loadings. Obviously, more Cu
loading can increase methanol yield because of the
amount of active sites. However, the photoactivity
(TOF) of each Cu site declines with more Cu loading
on TiO2 surface. Therefore, catalysts with more
than 2 wt.% Cu loading cannot further increase the
methanol yield due to the decreasing TOF. Furthermore,
excess Cu loading can mask the TiO2 surface,
reducing the photoexciting capacity of TiO2. There
exists an optimum amount of copper loading, around
2 wt.% under the experimental conditions of this
work.