5.3. The influence of pH

5. Variables influencing the photoreduction process

considerably play an important role in the photoreduction results.

5.1. The influence of the photocatalyst dose

138 Photocatalysts - Applications and Attributes

that reduces the light contacting with TiO2 surface.

5.2. The influence of the reaction time

(Hg(I)) [27].

(II) [27].

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

In general, the increase of the photocatalyst dose promotes higher photoreduction efficiency, and then it declines when the photocatalyst dose is further increased. Such trend can be explained in terms of the number of active sites available for photocatalytic reactions. The larger number of the active sites is available in as the dose of the photocatalyst increases that would generate more numbers of electrons and so higher photoreduction effectiveness. However, the large amount of catalyst may result in agglomeration form with larger particle size [40] that may provide smaller surface area. The agglomeration can also induce light scattering

The smaller surface area can reduce the number of active sites and so decreases the electron number provided. Consequently lower photoreduction is obtained. Another reason of the decrease in the photoreduction can be attributed to the increase in the turbidity of suspension due to the large amount of photocatalyst. This leads to the inhibition of photon absorption by the photocatalyst. As an effect, the lesser photoinduced electrons can be provided, causing the photoreduction decreased [23]. From several studies, the optimum photocatalyst dose was reported to be 2 g/L (Ag(I)) [8], 1.6 g/L (Cr(VI)) [20], 0.1 g/L (Cu(II)) [25], and 2 g/L

The reaction time determines how long is the contact between light with photocatalyst and that of electrons with the reducible metal ions. The general trend observed is that the photoreduction efficiency improves as the ratio of time is further extended. Longer than the optimum time, the photoreduction is usually independent on the reaction time. In the beginning and further extension time, the contact between light and TiO2 becomes more effective, resulting in more number of electrons. Then, the extension time allows more effective contact between the electrons available with the reducible metal ions. This can enhance significantly the photoreduction. At one time, the photoreduction reaches maximum level, showing the optimum reaction time. For longer time than the optimum one, a very large amount of the products has been resulted that may prevent the contact among the reacting agents. Consequently, TiO2 is hindered to release more electrons that give no increase in the photoreduction. The other reason is the reducible ions in the solution have been completely reduced that no more ions are left in the solution. The optimum reaction time is detected to be varied: 50– 60 min and 5 h for Ag(I) [5, 10], 4 h for Cr(VI) [23], 3 h for Cu(II) [25], and 50–150 min for Hg 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 with TiOH (TiO2 hydrated) surface to form Ti OH2 + . Such protonated titanol Ti OH2 <sup>+</sup> is more 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 maximum results is observed at pH 4 [40].
