Photoreduction Processes over TiO2 Photocatalyst

Abstract

This chapter presents the study of TiO2 photocatalyst for the photoreduction of several reducible chemicals. The photocatalytic reduction of several toxic metal ions, including Ag(I), Cu(II), Cr(VI), Hg(II), and U(VI) in the presence of TiO2, in order to decrease their toxicity, is described. Photodeposition of the noble metals, such as Ag(I), Au(III), Pt(IV), and Pd(II) for doping purposes by photocatalytic reduction over TiO2, is also addressed. Conversion of the greenhouse gas of CO2 into useful hydrocarbons and methanol by photocatalytic reduction using TiO2 photocatalyst is highlighted. Several operating parameters in photoreduction processes that are photocatalyst dose, time of the irradiation, pH of the solution, and the initial concentration of the substrates (the reducible chemicals) are also reviewed.

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

$${\mathrm{TiO}}_2+\mathit{h\nu }\to {\mathrm{TiO}}_2\left({\mathrm{e}}^{-}+h^{+}\right)$$ E1

$$h^{+}+{\mathrm{H}}_2\mathrm{O}\to ·\mathrm{OH}+{\mathrm{H}}^{+}$$ E2

$$h^{+}+\text{TiOH}\to \mathrm{Ti}\mathrm{OH}$$ E3

$${\mathrm{e}}^{-}+{\mathrm{O}}_2\to {{\mathrm{O}}_2}^{-}$$ E4

$${\mathrm{TiO}}_2+\left({\mathrm{e}}^{-}+h^{+}\right)\to {\mathrm{TiO}}_2+\mathrm{\text{heat}}$$ E5

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, 6, 7, 8, 9, 10, 11, 12, 13], Au(III) [13, 14, 15, 16, 17], Cr(VI) [18, 19, 20, 21, 22, 23], Cu (II) [24, 25, 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, 37, 38, 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.

2. Photocatalytic reduction over TiO₂ to detoxify metal ions

The reducible hazardous heavy metal ions that are widely found in the wastewater are silver Ag(I), copper Cu(II), hexavalent chromium Cr(VI), mercury Hg(II), and uranium U(VI). Silver pollutant can be found with high concentration in the wastewater of radiophotography activity that is usually disposed from hospitals [41, 42], silver electroplating, and electronic fabrication [43]. 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 [48, 49].

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

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 effectively, due to its high standard reduction potential (E⁰) = 0.79 V [52]. The reaction is 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:

$${\mathrm{Ag}}^{+}+{\mathrm{e}}^{-}\to {\mathrm{Ag}}^0{\mathrm{E}}^0=0.79\mathrm{V}$$ E6

Hexavalent chromium in the aqueous solution existed as chromate (CrO₄⁼) and bichromat (Cr₂O₇⁼) ions, depending on the pH that has high standard reduction potential (E⁰) = 1.33 V in 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, 19, 20, 21, 22, 23] with satisfaction result. Further, the Cr³⁺ ions can precipitate into undissolved Cr(OH)₃ in basic condition that is remediable by solidification/stabilization technique:

$${\mathrm{Cr}}_2{{\mathrm{O}}_7}^{=}+14{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\to \text{2}{\mathrm{\text{Cr}}}^{3+}+7{\text{H}}_2\mathrm{O}{\mathrm{E}}^0=1.52\mathrm{V}$$ E7

Copper ions in the solution formed as Cu²⁺ can be reduced into dissolved Cu (I) and/or undissolved Cu(0), with respective E⁰ = 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, 25, 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, E⁰ = 0.34 V [52]. To improve the effectiveness, a reducing agent, such as oxalic acid, can be added in the Cu(II) photoreduction [26]:

$${\mathrm{Cu}}^{2+}+\mathrm{e}\to {\mathrm{Cu}}^{+}{\mathrm{E}}^0=0.153\mathrm{V}$$ E8

$${\mathrm{Cu}}^{2+}+2\mathrm{e}\to {\mathrm{Cu}}^0{\mathrm{E}}^0=0.34\mathrm{V}$$ E9

Photoreduction over TiO₂ has been also used to detoxify mercury (Hg²⁺) ion in the aqueous solution, by converting it to be undissolved Hg⁰ [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 methyl mercury (CH₃Hg), Hg⁽⁰⁾ vapor, Hg²⁺ dissolved ion, and Hg⁽⁰⁾ element:

$${\mathrm{Hg}}^{2+}+{\mathrm{\text{2e}}}^{-}\to {\mathrm{Hg}}^0{\mathrm{E}}^0=0.85\mathrm{V}$$ E10

The photoreduction catalyzed by TiO₂ suspension has also been studied for removal of the radioactive uranium (VI) [48, 49] that exists as UO₂²⁺ anionic in the solution. The anion is the most stable form and so the one that is found in the solution. The photoreduction of the anionic has produced the less radioactive uranium (V), Eq. (11) [52]:

$${{\mathrm{UO}}_2}^{2+}+{\mathrm{e}}^{-}\to {{\mathrm{UO}}_2}^{+}{\mathrm{E}}^0=0.163\mathrm{V}$$ E11

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 requirement method in solving the environmental pollution problem.

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):

$${\mathrm{Au}}^{3+}+\text{3}{\mathrm{\text{e}}}^{-}\to {\mathrm{Au}}^0{\mathrm{E}}^0=1.32\mathrm{V}$$ E12

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, 6, 7, 8, 9, 10, 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.

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, 6, 7, 8, 9, 10, 11, 12, 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 110°C 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, 6, 7, 8, 9, 10, 11, 12, 13].

As its high standard reduction potential (E⁰), 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, 14, 15, 16]. The doping follows reaction (13). The other gold ions may form as AuCl₂⁻, Au⁺, and Au³⁺ that are also reducible by the following reactions (14)–(16) with their own standard reduction potentials [52]:

$${{\text{AuCl}}_4}^{-}+\text{3}{\mathrm{\text{e}}}^{-}\to {\mathrm{Au}}^0+\text{2}{\mathrm{\text{Cl}}}_2{\mathrm{E}}^0=0.93\mathrm{V}$$ E13

$${\mathrm{Au}}^{+}+{\mathrm{e}}^{-}\to {\mathrm{Au}}^0{\mathrm{E}}^0=1.83\mathrm{V}$$ E14

$${\mathrm{Au}}^{3+}+\text{3}{\mathrm{\text{e}}}^{-}\to {\mathrm{Au}}^0{\mathrm{E}}^0=1.32\mathrm{V}$$ E15

$${{\text{AuCl}}_2}^{-}+{\mathrm{e}}^{-}\to {\mathrm{Au}}^0+{\mathrm{Cl}}_2{\mathrm{E}}^0=1.15\mathrm{V}$$ E16

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 PtCl₄⁴⁺ and/or Pt²⁺ ions, and the reactions of the photoreduction are represented by Eqs. (17) and (18) [52]:

$${{\text{PtCl}}_4}^{2-}+\text{2}{\mathrm{\text{e}}}^{-}\to {\mathrm{Pt}}^0+\text{2}{\mathrm{\text{Cl}}}_2{\mathrm{E}}^0=0.77\mathrm{V}$$ E17

$${\mathrm{Pt}}^2++\text{2}\mathrm{\text{e}}\to {\mathrm{Pt}}^0{\mathrm{E}}^0=1.20\mathrm{V}$$ E18

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:

$${\mathrm{Pd}}^2++\text{2}{\mathrm{\text{e}}}^{-}\to {\mathrm{Pd}}^0{\mathrm{E}}^0=0.915\mathrm{V}$$ E19

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].

4. Photoreduction method over TiO₂ of the greenhouse gas CO₂ into valuable substances

The level of CO₂ that is a primary greenhouse gas in the atmosphere is continuously rising that creates serious global warming. Conversion of CO₂ into more valuable compounds is believed to be the best way in preventing the excess CO₂ disposal in the air. CO₂ is a thermodynamically stable and chemically inert compound, and it is difficult to oxidize or to reduce it to other useful compounds under normal operating conditions.

Therefore, converting CO₂ into valuable products is possible when catalytic, electrocatalytic, plasmatic, enzymatic, and photocatalytic reduction processes [59] are employed. Among them, photocatalytic reduction seems to be the most intensively developed method.

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

$$2{\mathrm{\text{CO}}}_2+4{\mathrm{\text{H}}}_2\mathrm{O}\to 2{\mathrm{\text{CH}}}_3\mathrm{OH}+3{\mathrm{\text{O}}}_2$$ E20

$$3{\mathrm{\text{CO}}}_2+2{\mathrm{\text{H}}}_2\mathrm{O}\to {\mathrm{CH}}_4+2\mathrm{\text{CO}}+3{\mathrm{\text{O}}}_2$$ E21

$${\mathrm{CO}}_2+2{\mathrm{\text{H}}}_2\mathrm{O}\to 2\mathrm{\text{HCOOH}}+{\mathrm{O}}_2$$ E22

$${\mathrm{CO}}_2+2{\mathrm{\text{H}}}_2\mathrm{O}\to \mathrm{\text{HCOH}}+{\mathrm{O}}_2$$ E23

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 produces different products simultaneously [59].

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

$${\mathrm{H}}_2{\mathrm{CO}}_3+{\mathrm{\text{6H}}}^{+}+{\mathrm{\text{6e}}}^{–}\to {\mathrm{CH}}_3\mathrm{OH}+{\mathrm{\text{2H}}}_2\mathrm{O}{\mathrm{E}}^0=0.044\mathrm{V}$$ E24

$${{\mathrm{CO}}_3}^{2–}+{\mathrm{\text{8H}}}^{+}+{\mathrm{\text{6e}}}^{–}\to {\mathrm{CH}}_3\mathrm{OH}+{\mathrm{\text{2H}}}_2\mathrm{O}{\mathrm{E}}^0=0.209\mathrm{V}$$ E25

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 effective than the photoreduction of the carbonate acid.

Various mechanistic reaction schemes have been proposed for CO₂ reduction with H₂O using TiO₂ photocatalysts. The following are the reaction mechanisms proposed for methane formation [61]:

$${\mathrm{H}}_2\mathrm{O}+{\mathit{\text{2h}}}^{+}\to \mathbf{·}\mathrm{OH}+\mathrm{H}\mathbf{·}$$ E26

$${\mathrm{CO}}_2+{\mathrm{e}}^{-}\to {\mathrm{CO}}_2\mathbf{·}$$ E27

$${\mathrm{\text{2CO}}}_2\mathbf{·}+\mathrm{\text{2e}}\to \mathrm{\text{2CO}}\mathbf{·}+{\mathrm{O}}_2$$ E28

$$\mathrm{\text{2CO}}\mathbf{·}+\mathrm{\text{4H}}\mathbf{·}\to {\mathrm{\text{2·CH}}}_2+{\mathrm{O}}_2$$ E29

$${\mathrm{\text{2·CH}}}_2+\mathrm{\text{2H}}\mathbf{·}\to {\mathrm{\text{2CH}}}_4$$ E30

The above mechanism steps start with the reaction between water molecule with the hole or positive radical from TiO₂ (Eq. (1)) to form radicals of OH and H (Eq. (26)). At the same time, carbon dioxide is reduced by the electron from Eq. (1), to form negative radical of carbon dioxide (Eq. (27)). The further reduction of CO₂ radical is preceded into carbon monoxide radical and oxygen (Eq. (28)). The CO radical then reacts with hydrogen radical from Eq. (26) to produce radical of methylene (Eq. (29)). Finally the methylene radical reacts with hydrogen radical to yield methane (Eq. (30)).

Different mechanisms for hydrogen, carbon monoxide, methane, and methanol productions are also proposed as seen in Eqs. (31)–(33) [62]. In this mechanism, only electron plays a role in the photoreduction, while no involvement of the hole is illustrated. Firstly, carbon dioxide gas reacts with the electron released from TiO₂ (Eq. (1)), to form anion of carbon dioxide (Eq. (31)). At same time, the hydrogen ion from water is also reduced by the electron to form hydrogen gas (Eq. (32)). Secondly, the anion of carbon dioxide reacts with hydrogen ion in the presence of electron to result carbon monoxide and water (Eq. (33)). In addition, the reaction of carbon dioxide anion with hydrogen ion and electron also occurred to produce methane and oxygen (Eq. (34)). Simultaneously, the carbon dioxide anion also reacts with the hydrogen ion and electron to yield methanol and water (Eq. (35)):

$${\mathrm{CO}}_2+{\mathrm{e}}^{-}\to {{\mathrm{CO}}_2}^{-}$$ E31

$${\mathrm{\text{2H}}}^{+}+{\mathrm{e}}^{-}\to {\mathrm{H}}_2$$ E32

$${{\mathrm{CO}}_2}^{-}+{\mathrm{\text{2H}}}^{+}+{\mathrm{e}}^{-}\to \mathrm{CO}+{\mathrm{H}}_2\mathrm{O}$$ E33

$${{\mathrm{CO}}_2}^{-}+{\mathrm{\text{2H}}}^{+}+{\mathrm{e}}^{-}\to {\mathrm{CH}}_4+{\mathrm{O}}_2$$ E34

$${{\mathrm{CO}}_2}^{-}+{\mathrm{\text{2H}}}^{+}+{\mathrm{e}}^{-}\to {\mathrm{CH}}_3\mathrm{OH}+{\mathrm{H}}_2\mathrm{O}$$ E35

The methane and hydrogen gas resulted from photoreduction of CO₂ with H₂O under UV light and over TiO₂ is also formulated by mechanisms as follows [63]. The reactions between water molecule with a hole or positive radical photogenerated by TiO₂ under UV light irradiation (Eq. (36)) yield OH radicals and H ion (Eq. (37)). Then the OH radical further reacts with water and the positive radical, to release oxygen gas and hydrogen ion (Eq. (38)). The overall of the reactions (37) and (38) is represented by Eq. (39):

$${\mathrm{TiO}}_2+\mathit{UV}\mathit{\text{light}}\to {\mathrm{TiO}}_2+{\mathrm{e}}^{-}+{\mathit{h}}^{+}$$ E36

$${\mathrm{H}}_2\mathrm{O}+h^{+}\to \mathbf{·}\mathrm{OH}+{\mathrm{H}}^{\mathbf{+}}$$ E37

$$\mathbf{·}\mathrm{OH}+{\mathrm{H}}_2\mathrm{O}+\mathrm{3}{h}^{+}\to {\mathrm{O}}_2+{\text{3H}}^{+}$$ E38

$${\mathrm{\text{2H}}}_2\mathrm{O}+4h^{+}\to {\mathrm{O}}_2+{\mathrm{\text{4H}}}^{\mathbf{+}}$$ E39

On the other side, CO₂ meets with the electron to form a corresponding radical (Eq. (40)). Then the radical is dissociated into carbon monoxide radical along with oxygen (Eq. (41)). The carbon monoxide radical is further split into carbon radical and oxygen (Eq. (42)). The overall reaction is written as Eq. (43):

$${\mathrm{CO}}_2+e\to \mathbf{·}{\mathrm{CO}}_2$$ E40

$$\mathbf{·}{\mathrm{CO}}_2\to \mathbf{·}\mathrm{CO}+1/{\mathrm{\text{2O}}}_2$$ E41

$$\mathbf{·}\mathrm{CO}\to \mathbf{·}\mathrm{C}+1/{\mathrm{\text{2O}}}_2$$ E42

$${\mathrm{CO}}_2\to \mathbf{·}\mathrm{C}+{\mathrm{O}}_2$$ E43

The hydrogen ion obtained from the reaction (39) then reacts with the carbon radical from reaction (43) to yield methane and hydrogen gas, as represented by Eqs. (44) and (45), respectively:

$${\mathrm{\text{4H}}}^{+}+\mathbf{·}\mathrm{C}+\mathrm{\text{4e}}\to {\mathrm{CH}}_4$$ E44

$${\mathrm{\text{2H}}}^{+}+\mathrm{\text{2e}}\to {\mathrm{H}}_2$$ E45

The following is the other mechanism in forming methanol, methane, and ethylene that is proposed [64, 65, 66]. After being released from TiO₂ (Eq. (46)), the hole reacts with water molecule to form radicals of hydroxyl and hydrogen (Eq. (47)), while the electron reduces the carbon dioxide to yield its anionic form (Eq. (48)). Then the anion of carbon dioxide reacts with the hydrogen radical from Eq. (47), to result an intermediate radical and hydroxide anion (Eq. (49)):

$${\mathrm{TiO}}_2+\mathit{UV}\text{light}\to {\mathrm{TiO}}_2+{\mathrm{e}}^{-}+h^{+}$$ E46

$${\mathrm{\text{2H}}}_2\mathrm{O}+{\mathit{\text{2h}}}^{+}\to \mathbf{·}\mathrm{OH}+\mathrm{H}\mathbf{·}$$ E47

$${\mathrm{CO}}_2+{\mathrm{e}}^{-}\to {{\mathrm{CO}}_2}^{-}$$ E48

$${{\mathrm{CO}}_2}^{-}+\mathrm{\text{2H}}\mathbf{·}\to \mathrm{OC·H}+{\mathrm{OH}}^{\mathbf{-}}$$ E49

$$\mathrm{OC·H}+\mathrm{OC·H}\to \text{HOCCOH}$$ E50

$$\mathrm{\text{HOCCOH}}+\mathrm{\text{4H}}\mathbf{·}\to {\mathrm{CH}}_3\mathrm{COH}$$ E51

$${\mathrm{CH}}_3\mathrm{COH}+\mathrm{H}\mathbf{·}\to \mathbf{·}{\mathrm{CH}}_3+\mathrm{CO}$$ E52

$$\mathbf{·}{\mathrm{CH}}_3+\mathrm{H}\mathbf{·}\to {\mathrm{CH}}_4$$ E53

$$\mathbf{·}{\mathrm{CH}}_3+\mathbf{·}{\mathrm{CH}}_3\to {\mathrm{C}}_2{\mathrm{H}}_6$$ E54

The product of the radical (Eq. (50)) reacts with hydrogen radical to produce acetic acid (Eq. (51)). The acid can also further react with hydrogen radical to form methyl radical (Eq. (52)). Next, the methyl radical reacts with hydrogen to produce methane. When two methyl radicals react to each other, ethane is produced (Eq. (54)).

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.

5.1. The influence of the photocatalyst dose

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 that reduces the light contacting with TiO₂ surface.

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 (Hg(I)) [27].

5.2. The influence of the reaction time

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(II) [27].

5.3. The influence of pH

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⁺ that can interact with TiOH (TiO₂ hydrated) surface to form Ti ·OH₂⁺. Such protonated titanol Ti ·OH₂⁺ is more difficult to release electrons [2, 3, 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⁺ 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].

5.4. The influence of the initial concentration

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 4 × 10⁻³ mole/L for Ag (I) [8], 3 × 10⁻³ mole/L for Cu(II) [24], 10 mg/L for Cr(VI) [23], 100 mg/L for Hg(II) [28], and the CO₂/H₂O ratio is 1/2 [66].

6. Conclusions

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 and renewable energy by conversion of CO₂ into syngas, improvement TiO₂ photocatalyst activity under UV and solar light by doping process, and recovery of valuable economic metals based on the metal deposited.