3. Photoreduction method over TiO2 for metal recovery and doping purposes

Photoreduction of some reducible metal ions results in the solid phase deposited on the TiO2 structure. This deposition phenomenon has inspired some researchers to use the photoreduction for metal recovery and doping purposes.

Recovery is objected to get pure valuable metals such as silver (Ag) and gold (Au) from the regarding solution, by applying the photoreduction method. The photoreduction of Ag(I) follows Eq. (6). To take the pure Ag particles from Ag deposited on TiO2, one can use ultrasonic shaker [12].

Photodeposition for gold recovery can also be obtained through reduction under UV light in the presence of TiO2 photocatalyst [17]. In this step, the gold is dissolved in the aqueous solution formed as Au3+ ions and then to be reduced as shown by reaction (12):

$$\text{Au}^{3+} + \text{3e}^- \rightarrow \text{Au}^0 \qquad \qquad \text{E}^0 = 1.32 \text{ V} \tag{12}$$

Doping, whether with transition and noble metals on TiO2 recently, has attracted much attention, since it can improve the performance of TiO2 as a photocatalyst under UV light, as well as shift the absorption of TiO2 to visible light region [5–11]. The latter is supposed to give some advantages, as the photocatalytic process under metal-doped TiO2 can take place under sun light that must be low-cost and safer and so greener than that of by UV light irradiation.

Cu<sup>2</sup><sup>þ</sup> <sup>þ</sup> <sup>e</sup> ! Cu<sup>þ</sup> E0 <sup>¼</sup> <sup>0</sup>:153 V (8)

Cu<sup>2</sup><sup>þ</sup> <sup>þ</sup> 2 e ! Cu0 E0 <sup>¼</sup> <sup>0</sup>:34 V (9)

Hg2<sup>þ</sup> <sup>þ</sup> 2e� ! Hg0 <sup>E</sup><sup>0</sup> <sup>¼</sup> <sup>0</sup>:85 V (10)

2+ anionic in the solution. The anion is the

<sup>þ</sup> <sup>E</sup><sup>0</sup> <sup>¼</sup> <sup>0</sup>:163 V (11)

Photoreduction over TiO2 has been also used to detoxify mercury (Hg2+) ion in the aqueous solution, by converting it to be undissolved Hg<sup>0</sup> [27, 28]. Based on the standard reduction potential as seen in the reaction (10) [52], the reduction should proceed effectively. To handle the elemental or solid mercury may be easier than that of the dissolved ions. As presented by previous authors [47], the order of the toxicity level of mercury forms, from the most toxic, is

The photoreduction catalyzed by TiO2 suspension has also been studied for removal of the

most stable form and so the one that is found in the solution. The photoreduction of the anionic

Detoxification of the hazardous (toxic and radioactive) heavy metals by photoreduction pathway offers a simple, practical, economic, and green procedure that meets with the future

Photoreduction of some reducible metal ions results in the solid phase deposited on the TiO2 structure. This deposition phenomenon has inspired some researchers to use the photoreduc-

Recovery is objected to get pure valuable metals such as silver (Ag) and gold (Au) from the regarding solution, by applying the photoreduction method. The photoreduction of Ag(I) follows Eq. (6). To take the pure Ag particles from Ag deposited on TiO2, one can use

Photodeposition for gold recovery can also be obtained through reduction under UV light in the presence of TiO2 photocatalyst [17]. In this step, the gold is dissolved in the aqueous

Doping, whether with transition and noble metals on TiO2 recently, has attracted much attention, since it can improve the performance of TiO2 as a photocatalyst under UV light, as well as

Au<sup>3</sup><sup>þ</sup> <sup>þ</sup> 3e� ! Au<sup>0</sup> E0 <sup>¼</sup> <sup>1</sup>:32 V (12)

solution formed as Au3+ ions and then to be reduced as shown by reaction (12):

3. Photoreduction method over TiO2 for metal recovery and doping

methyl mercury (CH3Hg), Hg(0) vapor, Hg2+ dissolved ion, and Hg(0) element:

<sup>2</sup><sup>þ</sup> <sup>þ</sup> <sup>e</sup>� ! UO2

requirement method in solving the environmental pollution problem.

radioactive uranium (VI) [48, 49] that exists as UO2

132 Photocatalysts - Applications and Attributes

has produced the less radioactive uranium (V), Eq. (11) [52]:

UO2

tion for metal recovery and doping purposes.

purposes

ultrasonic shaker [12].

The transition metals that have been examined as dopants on TiO2 are Cu(II) [53, 54], Fe [55, 56], Co [56, 57], Ni [57], Mn [56], and Cr [58]. Moreover, Ag(I) [5–13], Au(III) [13, 14], Pd(II) [14], and Pt(IV) [14] are the noble metals that are frequently doped into TiO2 structure. All the metals doped on TiO2 are reported to improve the photocatalytic activity of TiO2 under UV irradiation as well as to shift the visible absorption with various effects, from very low, shown by Cr(III) up to the very significant effect, observed on Ag(I).

A doping process basically involves the conversion of the metal ions in the solution to be deposited solid metal on TiO2 powder that is frequently carried out by sol–gel method [56]. However, hydrothermal [57] and chemical vapor [58] methods are also introduced.

From the above methods, the regard salt solution is usually used as the dopant source, and high temperature is required that makes the method costly due to high energy consumption. In addition, large metal particle is usually resulted from the process that retards the metal insertion into gap of valence and conduction gaps. As a consequence, the small absorption shift is resulted that yields less significant improvement of the photocatalyst activity under UV light or the slight visible light responsiveness.

In addition to the four doping methods, photoreduction has also been examined. The photoreduction method becomes a great of interest, because the process takes place at room temperature, no need of chemicals, except UV light, and has resulted small cluster of metal dopant particles. The small particles are well inserted into the gap between valence and conduction band of TiO2. Such insertion has considerably shortened the gap that enhances the activity under UV light and pronounces shift of the light absorption into wide visible region. However, the photoreduction method is limited only for dopants that are reducible metal ions, including Cu(II) representing transition metal and Ag(I), Au(III), Pd(II), and Pt(IV) for noble metal ions.

In general, the doping process by photoreduction method is carried out by UV light irradiation toward the regard metal salt solutions in a certain period of time. Then M/TiO2 (M = metal dopant) resulted is dried at about 110C to remove the water.

Photoreduction of Ag (I) in the solution over TiO2 for doping purpose principally follows the same procedure as in the detoxification, as described earlier. The starting salt for Ag doping usually used is AgNO3 [5–13].

As its high standard reduction potential (E<sup>0</sup> ), the photoreduction of Ag+ takes place efficiently, and the small Ag particle resulted can enter into the gap between the conduction and the valence. The present of the small particle dopant in the gap shortens the bandgap. This allows the metal-doped TiO2 to be active under visible light and to work better with UV irradiation, whether for degradation of the organic pollutants or for bacterial combating.

In the doping Au on TiO2 through photoreduction method, the salt frequently introduced as gold source is KAuCl4 that dissolves to form AuCl4 [13–16]. The doping follows reaction (13). The other gold ions may form as AuCl2 , Au<sup>+</sup> , and Au3+ that are also reducible by the following reactions (14)–(16) with their own standard reduction potentials [52]:

$$\text{AuCl}\_4^- + \text{3e}^- \rightarrow \text{Au}^0 + 2\text{Cl}\_2 \qquad \qquad \text{E}^0 = 0.93 \text{ V} \tag{13}$$

$$\text{Au}^+ + \text{e}^- \rightarrow \text{Au}^0 \qquad \qquad \text{E}^0 = 1.83 \text{ V} \tag{14}$$

Therefore, converting CO2 into valuable products is possible when catalytic, electrocatalytic, plasmatic, enzymatic, and photocatalytic reduction processes [59] are employed. Among them,

The photoreduction of CO2 with water vapor catalyzed by titania-based photocatalysts results in methane (CH4), methanol (CH3OH), carbon monoxide (CO), formic acid (HCOOH), and

The conversion reaction pathways are not specific and mainly depend on the reaction conditions. This is therefore a complex mechanism that proceeds through branching pathways and

Reduction of CO2 in the presence of NaOH solution photocatalyzed by TiO2 supported on a polymer has been reported to produce methanol and methane, accompanied with formic acid and formaldehyde. The CO2 is being soluble in NaOH solution and forms carbonate and bicarbonate ions based on the pH measurement. The reductions of carbonate acid and carbon-

From the equations, based on the standard reduction potential reduction, the photoreduction of carbonate ions (mostly existing in higher pH) to form methanol takes place faster or is more

Various mechanistic reaction schemes have been proposed for CO2 reduction with H2O using TiO2 photocatalysts. The following are the reaction mechanisms proposed for methane forma-

H2CO3 <sup>þ</sup> 6H<sup>þ</sup> <sup>þ</sup> 6e– ! CH3OH <sup>þ</sup> 2H2O E<sup>0</sup> <sup>¼</sup> <sup>0</sup>:044 V (24)

<sup>2</sup>– <sup>þ</sup> 8H<sup>þ</sup> <sup>þ</sup> 6e– ! CH3OH <sup>þ</sup> 2H2O E0 <sup>¼</sup> <sup>0</sup>:209 V (25)

H2O þ 2h<sup>þ</sup> ! �OH þ H� (26)

2CO2� þ 2e ! 2CO� þ O2 (28)

2CO� þ 4H� ! 2�CH2 þ O2 (29)

2�CH2 þ 2H� ! 2CH4 (30)

CO2 þ e� ! CO2� (27)

ate ions with their standard reduction potential are shown as Eqs. (24) and (25) [60]:

2CO2 þ 4H2O ! 2CH3OH þ 3O2 (20)

Photoreduction Processes over TiO2 Photocatalyst http://dx.doi.org/10.5772/intechopen.80914 135

3CO2 þ 2H2O ! CH4 þ 2CO þ 3O2 (21)

CO2 þ 2H2O ! 2HCOOH þ O2 (22)

CO2 þ 2H2O ! HCOH þ O2 (23)

photocatalytic reduction seems to be the most intensively developed method.

formaldehyde (HCHO) and follows the simplified reactions (20)–(23) [59].

produces different products simultaneously [59].

CO3

tion [61]:

effective than the photoreduction of the carbonate acid.

$$\text{Au}^{3+} + 3\text{e}^- \rightarrow \text{Au}^0 \qquad \qquad \text{E}^0 = 1.32 \text{ V} \tag{15}$$

$$\text{AuCl}\_2^- + \text{e}^- \rightarrow \text{Au}^0 + \text{Cl}\_2 \qquad \qquad \text{E}^0 = 1.15 \text{ V} \tag{16}$$

The gold atom resulted from the photoreduction is doped on TiO2 structure or TiO2/Au through insertion or impregnation. The doped photocatalyst has been tested for phenol degradation under UV light and showed more satisfaction result than undoped TiO2 [13].

Platinum (Pt) doped TiO2 or TiO2/Pt can be resulted by irradiating H2PtCl6 salt in the aqueous solution in the presence of TiO2 suspension [14]. The platinum salt is dissolved to form an ion of PtCl4 4+ and/or Pt2+ ions, and the reactions of the photoreduction are represented by Eqs. (17) and (18) [52]:

$$\text{PtCl}\_4^{2-} + 2\text{e}^- \rightarrow \text{Pt}^0 + 2\text{Cl}\_2 \qquad \qquad \text{E}^0 = 0.77\text{ V} \tag{17}$$

$$\text{Pt}^2 + +2\text{e} \rightarrow \text{Pt}^0 \qquad \qquad \text{E}^0 = 1.20 \text{ V} \tag{18}$$

Photoreduction of Pd(II) over TiO2 for the doping purpose is performed by the irradiation of PdCl2 in the aqueous solution with UV light [14]. The reduction of Pd(II) ion is written as reaction (19) that yields small Pd particles on TiO2 structure:

$$\text{Pd}^{2} + + 2\text{e}^{-} \rightarrow \text{Pd}^{0} \qquad \qquad \text{E}^{0} = 0.915 \text{ V} \tag{19}$$

With the same conditions of the photoreduction, it is found that the order of the photodeposition efficiency from the highest is shown by Pt (100%) that is followed by Au (80%) and then by Pd (50%). This sequence photoreduction result is consistent with their standard reduction potentials that are 1.2, 0.93, and 0.915 V, as well as their empirical radii that are 135, 135, and 140, respectively. The higher standard reduction potentials promote more reduction, and smaller size facilitates the effective insertion. They have also been examined for phenol degradation and displayed the effective results [14].
