1. Introduction

Semiconductor TiO2 is regarded as one of the most promising photocatalysts, because it has low cost and high activity, good physical and chemical stability, and nontoxic property [1]. The structure of a semiconductor is characterized by two bands, called as valence band and conduction band, that are separated by a gap named as bandgap energy (Eg). The first band is filled by electrons, while the second is empty or no electron occupying it. The bandgap of TiO2 with anatase typed is 3.2 and 3.0 eV is for the rutile type [2]. Both anatase and rutile are tetragonal in structure, but the anatase has octahedrons that share four edges forming the fourfold axis.

The photocatalyst of TiO2 works by absorption of UV to near visible region, with the energy same as or higher than its bandgap (light with a wavelength of 365 nm is required by rutile and that of 411 nm is for anatase) that generates electron and hole pair. The pair generation is

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

resulted by excitation of some electrons from valence band (evb) to the conduction one (ecb) with the formation of positive holes (hvb+) [3, 4]. The reactions of electron–hole generation from TiO2 are presented as Eqs. (1):

$$\text{TiO}\_2 + h\nu \rightarrow \text{TiO}\_2 \left(\text{e}^- + h^+\right) \tag{1}$$

$$\text{H}^+ + \text{H}\_2\text{O} \rightarrow \cdot \text{OH} + \text{H}^+ \tag{2}$$

$$h^{+} + \text{TiO} \rightarrow \text{TiOH} \tag{3}$$

$$\text{O}^- + \text{O}\_2 \rightarrow \text{O}\_2^- \tag{4}$$

Wastewaters containing copper in high concentration are disposed from electroplating and electrical stuffs [43, 44]. Hexavalent chromium is mostly emitted by metal plating activity and paint industry [45, 46]. Mercury is usually contained in wastewater originated from gold recovery and incinerator in hospitals [46, 47]. The environmental contamination by uranium (VI) ion may originate from uranium purification or extraction from its respected mineral. A leak of nuclear reactor releasing uranium into the water may also contribute to the uranium contamination

Contamination of the hazardous metals can create environmental and human health problems. Silver contamination can induce argyria syndrome [50]. For human, the excessive copper intake can disturb the gastrointestinal (GI) system [44]. Hexavalent chromium is carcinogenic [45, 51], and mercury Hg(II) can cause neurological dysfunction [47]. The uranium ion, as a radioactive element, must be very dangerous, both for environment and human [44]. There-

The products of the reduction of Ag(I), copper Cu(II), hexavalent chromium Cr(VI), mercury Hg(II), and uranium (VI) are Ag(0), Cu(0), Cr(III), Hg(0), and U(IV), respectively, that are less toxic [44]. Hence, these facts motivate many researchers for detoxification of the hazardous heavy metal ions by reduction method, especially by photoreduction catalyzed by TiO2.

Photoreduction of Ag(I) ions in the aqueous solution by electron provided by TiO2 takes place

presented by Eq. (6) that produces undissolved solid silver that is less toxic and easier to be handled. It is clear that by photoreduction process, the silver contaminant is detoxified:

) ions, depending on the pH that has high standard reduction potential (E<sup>0</sup>

acid condition [52]. This allows them to be easily reduced into Cr(III) as Cr3+ ions as presented by reaction (7). The chromate (Cr(VI)) is highly toxic, while Cr3+ is less toxic or is even needed by feeding women [45]. Hence, detoxification of the toxic Cr(VI) can be carried out by reduction method. Photoreduction of Cr(VI) over TiO2 photocatalyst has been frequently reported [18–23] with satisfaction result. Further, the Cr3+ ions can precipitate into undissolved Cr(OH)3

Copper ions in the solution formed as Cu2+ can be reduced into dissolved Cu (I) and/or undissolved Cu(0), with respective E<sup>0</sup> = 0.153 V and 0.34 V [52]. Photoreduction method in the presence of TiO2 has been subjected to detoxify the toxic Cu2+ that can prominently form the undissolved toxic Cu(0), with very small Cu(I) ion [24–26]. The reactions of the Cu(II) reduction are shown by Eqs. (8) and (9). The photoreduction of Cu(II) is found to be less effective compared to Ag(I) photoreduction that may be caused by the low standard reduction potential, E<sup>0</sup> = 0.34 V [52]. To improve the effectiveness, a reducing agent, such as oxalic acid,

Ag<sup>þ</sup> <sup>þ</sup> <sup>e</sup>� ! Ag0 <sup>E</sup><sup>0</sup> <sup>¼</sup> <sup>0</sup>:79 V (6)

<sup>¼</sup> <sup>þ</sup> 14H<sup>þ</sup> <sup>þ</sup> <sup>e</sup>� ! 2Cr<sup>3</sup><sup>þ</sup> <sup>þ</sup> 7H2O E0 <sup>¼</sup> <sup>1</sup>:52 V (7)

) = 0.79 V [52]. The reaction is

Photoreduction Processes over TiO2 Photocatalyst http://dx.doi.org/10.5772/intechopen.80914 131

=

) and bichromat

) = 1.33 V in

fore, a method that can detoxify the hazardous heavy metals is urgently.

Hexavalent chromium in the aqueous solution existed as chromate (CrO4

in basic condition that is remediable by solidification/stabilization technique:

Cr2O7

can be added in the Cu(II) photoreduction [26]:

effectively, due to its high standard reduction potential (E0

[48, 49].

(Cr2O7 =

$$\text{TiO}\_2 + \left(\text{e}^- + h^+\right) \rightarrow \text{TiO}\_2 + \text{heat} \tag{5}$$

Notes: hv represents photon energy of UV light (E = hv), O2 \_ is called as superoxide, and �OH indicates hydroxide radical.

The electrons, in water media and in the presence of dissolve oxygen, can react with the oxygen to form super oxide, as presented by reaction (4) [4]. This is a reduction path. The use of the electrons from TiO2 for some reducible metal ions such as Ag(I) [5–13], Au(III) [13–17], Cr(VI) [18–23], Cu (II) [24–26], Hg(II) [27, 28], and U(VI) [29, 30] has also been developed.

Meanwhile the hole generates the free OH radical after contact with water and TiO2 surface, as illustrated by Eq. (2) and (3). The OH radical acts as a strong oxidizing agent with high oxidation potential (E0 = 2.8 V) that can degrade organic compounds into smaller CO2 and H2O molecules [3, 4]. This is called as oxidation path. Due to strong ability in organic degradation, OH radicals from TiO2 have been intensively applied for cyanide oxidation [31], treating hazardous phenol [32, 33], di-nitrophenol [34, 35], various organic dyes [36–39], and surfactant of detergent [40].

The applications of the photodegradation process catalyzed by TiO2 have been frequently published through journals and books. Meanwhile, less intensive photoreduction methods are explored. In fact, the photoreduction process over TiO2 has some advantages compared to the other reduction reactions, with respect to simplicity, cost-effectiveness, efficiency, less chemical usage, and green chemistry principles.

It is interesting therefore to address the applications of the reduction over TiO2 photocatalyst for various purposes. The applications of the photocatalytic reduction in the presence of TiO2 for detoxifying the toxic metal ions, doping by transition and noble metals, and converting greenhouse gas CO2 into more valuable chemicals are described in more detail.
