5. Variables influencing the photoreduction process

In the study of the photocatalytic reduction method, it is found that some variables including photocatalyst dose, reaction time, UV lamp types, pH, and concentration of the reducible ions considerably play an important role in the photoreduction results.

5.3. The influence of pH

with TiOH (TiO2 hydrated) surface to form Ti OH2

mum results is observed at pH 4 [40].

6. Conclusions

5.4. The influence of the initial concentration

100 mg/L for Hg(II) [28], and the CO2/H2O ratio is 1/2 [66].

The other important variable is pH, since pH determines the species of both TiO2 surface and the reducible metal ions that further affect in the photoreduction efficiency [12]. The general trend of the metal ion photoreduction efficiency with the alteration pH is that at the low pH, the photoreduction is usually low, and then increasing pH in the acid range gives a rise in the photoreduction results, but the further increasing pH leads to the photoreduction decreased. In the aqueous media, the low pH provides more amount of hydrogen ion H<sup>+</sup> that can interact

difficult to release electrons [2–4], although the metal ions mostly existed as the species that are easier to be reduced in large amount; the low photoreduction efficiency is usually observed. It is clear that the number of the electrons plays a prominent role on the photoreduction process. Increasing pH in the acid range, the H<sup>+</sup> concentration declines that make TiOH available increased. This can raise the release of the electrons, and the metal ions are found as reducible species, so the photodegradation can be considerably enhanced. Finally, the number of OH may be in excess at higher pH that can create negative surface of TiO2 to form TiO. This can retard the electron released. On the other side, generally the metal ions are precipitated with the hydroxide anions to form M(OH)n (M = metal, n = valence) that is impossible to be reduced. These two situations stimulate the negative effect on the photoreduction. As the other variables, there are also optimum pH values that are varied depending on the respective metal ions. Optimum pH value for Ag is found at 6–7 [8], at pH = 2 for Cr(VI) [23], and for Cu(II) is 2–3.5 [26], and pH = 4–4.1 is reported for Hg(II) [27]. The photoreduction of CO2 with maxi-

The initial concentration of the reducible metal ions is also important to be paid of attention in the photoreduction. In general, the relationship of the initial concentration is as follows: in the increase of the initial concentration, the metal ions in the photoreduction are enhanced. When the amount of the reduced ions is compared/relative to the initial concentration, it is usually observed that the photoreduction percentage decreases as the initial concentration increases. With the low initial concentration, the small number of the reducible ions is present with abundant electrons that can be reduced completely or 100%. By increasing the initial concentration of the metal ion, the number of the metal ions to be reduced is also increased, but the percentage is usually decreased. The optimum initial concentrations are reported to be <sup>4</sup> <sup>10</sup><sup>3</sup> mole/L for Ag (I) [8], 3 <sup>10</sup><sup>3</sup> mole/L for Cu(II) [24], 10 mg/L for Cr(VI) [23],

Photocatalytic reduction over TiO2 has shown as an attractive method that can be applied for solving the environmental problems due to the toxic metal ion contamination, providing new

+

. Such protonated titanol Ti OH2

Photoreduction Processes over TiO2 Photocatalyst http://dx.doi.org/10.5772/intechopen.80914

<sup>+</sup> is more

139
