Page 220 - Multifunctional Photocatalytic Materials for Energy

204 Multifunctional Photocatalytic Materials for Energy

Moreover, the charge status of the photocatalyst surface can be changed by vary-
ing the pH of the solution, depending on the semiconductor’s pH at the zero point
of charge (pH zpc ). In particular, if the surface consists of amphoteric groups, such as
TiO 2 , one must take into account the equilibria [134]

pK a1
+
> OH H + Û> OH +
2
a
pK 2
+
> OH OH - Û> O - + H O
2
+
−
where >OH, >OH 2 , and >O indicate the neutral, positive, and negative surface hy-
droxyl groups, respectively. The point of zero charge is estimated as
1
pH zpc = p ( K a1 + pK a )
2
2
where pK a1 and pK a2 are the negative logarithms of equilibrium constants under acidic
and alkaline conditions, respectively. If the pH < pH zpc , the surface charge of the cat-
alyst is mainly positive, when the pH > pH zpc , the negative charge predominates, and
neutral conditions stand when the pH = pH zpc .
The pH zpc values for the most common metal oxides and other semiconductors
have been reported in the scientific literature [135,136].
A charge modification of the solid surface leads to a change in the electrostatic
interaction between the reagents (i.e., sacrificial agents, redox mediators) and the pho-
tocatalyst [137].
The nature of the photocatalyst surface is important in photocatalytic reforming.
For example, surface hydroxyl groups play the role of scavenging centers for photo-
generated holes and adsorption sites for sacrificial agents [138].
Under acidic conditions, photocatalyst particles may agglomerate, in which case,
the surface area available for the adsorption is reduced. In catalytic photoreforming,
hydrogen production increases with increasing concentrations of the sacrificial spe-
cies [139], but at very high organic concentrations, no further beneficial effects on the
rates of hydrogen generation are recorded because of the saturation of the adsorption
sites of the photocatalyst [12,140].
The influence of temperature on the photocatalytic reaction rates is not negligible,
and the activation energy value for each physicochemical step involved in the photo-
catalytic process is different.
It is reported that a moderate increase in temperature enhances the rate of hydrogen
production [121,141,142].
Higher temperatures promote electron transfer from the valence band to the con-
duction band and increase the mobility of charge carriers, thus lowering the likeli-
hood of electron-hole recombinations [10]. A convenient temperature range is about
60−80°C, although the optimal operating temperature varies depending on the photo-
catalytic system. Temperature values higher than 80°C disfavor the adsorption of the
reagents, thus lowering the process’s photocatalytic efficiency.