The results of quantum efficiency calculation are displayed
in Table 2. The formation of methanol was found
to be much more effective on Cu2+ loaded TiO2 catalyst.
The highest methanol yield as well as quantum efficiency
was achieved by 3%CuO/TiO2 indicating that this catalyst
had the highest reactivity among all the prepared
catalysts. Besides describing the catalyst reactivity,
quantum efficiency value also figured the effectiveness
of the reaction system.
The effect of copper loading on methanol yields is
shown in Fig. 4. The methanol yields increased with
Cu loading, but then decreased when the Cu loading exceeded
3 wt%. Evidently, more Cu loading can increase
methanol yield because of the amount of active sites.
Copper can serve as an electron trapper and prohibits
the recombination of electron and hole, significantly
increasing photoefficiency [1]. However, catalysts with
more than 3 wt% Cu loading cannot further increase
the methanol yield due to its shading effects which are
much higher, consequently reducing the photo exciting
capacity of TiO2. Thus it is estimated that an optimum
amount of copper loading, is approximately 3 wt% under
the experimental conditions of this work.
The dispersion capacity of 2.2 wt% Cu, as determined
by XRD analysis, indicated that the increasing of CuO
above 2.2 wt% could increase the shading effects. However,
below 3 wt% loading, we found that its shading effects
could still be covered by its high ability to trap
electrons. The absorption spectra pattern of low copper-
loaded catalysts (