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.

AuCl4

134 Photocatalysts - Applications and Attributes

AuCl2

PtCl4

degradation and displayed the effective results [14].

useful compounds under normal operating conditions.

valuable substances

reaction (19) that yields small Pd particles on TiO2 structure:

of PtCl4

and (18) [52]:

� <sup>þ</sup> 3e� ! Au0 <sup>þ</sup> 2Cl2 <sup>E</sup><sup>0</sup> <sup>¼</sup> <sup>0</sup>:93 V (13)

Au<sup>þ</sup> <sup>þ</sup> <sup>e</sup>� ! Au0 E0 <sup>¼</sup> <sup>1</sup>:83 V (14)

Au<sup>3</sup><sup>þ</sup> <sup>þ</sup> 3e� ! Au<sup>0</sup> E0 <sup>¼</sup> <sup>1</sup>:32 V (15)

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 deg-

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

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

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

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

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

4+ and/or Pt2+ ions, and the reactions of the photoreduction are represented by Eqs. (17)

radation under UV light and showed more satisfaction result than undoped TiO2 [13].

� <sup>þ</sup> <sup>e</sup>� ! Au0 <sup>þ</sup> Cl2 <sup>E</sup><sup>0</sup> <sup>¼</sup> <sup>1</sup>:15 V (16)

<sup>2</sup>� <sup>þ</sup> 2e� ! Pt<sup>0</sup> <sup>þ</sup> 2Cl2 <sup>E</sup><sup>0</sup> <sup>¼</sup> <sup>0</sup>:77 V (17)

Pt<sup>2</sup> þ þ2e ! Pt<sup>0</sup> <sup>E</sup><sup>0</sup> <sup>¼</sup> <sup>1</sup>:20 V (18)

Pd2 þ þ2e� ! Pd0 <sup>E</sup><sup>0</sup> <sup>¼</sup> <sup>0</sup>:915 V (19)

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].

$$2\text{CO}\_2 + 4\text{H}\_2\text{O} \to 2\text{CH}\_3\text{OH} + 3\text{O}\_2\tag{20}$$

$$2\text{CO}\_2 + 2\text{H}\_2\text{O} \rightarrow \text{CH}\_4 + 2\text{CO} + \text{3O}\_2 \tag{21}$$

$$2\text{ CO}\_2 + 2\text{H}\_2\text{O} \to 2\text{HCOOH} + \text{O}\_2\tag{22}$$

$$\text{CO}\_2 + 2\text{H}\_2\text{O} \rightarrow \text{HCOH} + \text{O}\_2\tag{23}$$

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]:

$$\text{CH}\_2\text{CO}\_3 + 6\text{H}^+ + 6\text{e}^- \rightarrow \text{CH}\_3\text{OH} + 2\text{H}\_2\text{O} \qquad \quad \text{E}^0 = 0.044\text{ V} \tag{24}$$

$$\text{CO}\_3^{2-} + 8\text{H}^+ + 6\text{e}^- \rightarrow \text{CH}\_3\text{OH} + 2\text{H}\_2\text{O} \qquad \quad \text{E}^0 = 0.209\text{ V} \tag{25}$$

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]:

$$\cdot \text{H}\_2\text{O} + 2\text{h}^+ \rightarrow \cdot \text{OH} + \text{H}\cdot \tag{26}$$

$$\text{CO}\_2 + \text{e}^- \rightarrow \text{CO}\_2\text{-}\tag{27}$$

$$2\text{CO}\_2 + 2\text{e} \to 2\text{CO} + \text{O}\_2 \tag{28}$$

$$2\text{CO} \cdot + 4\text{H} \cdot \to 2\cdot \text{CH}\_2 + \text{O}\_2 \tag{29}$$

$$2\cdot \text{CH}\_2 + 2\text{H}\cdot \rightarrow 2\text{CH}\_4\tag{30}$$

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)):

$$\text{CO}\_2 + \text{e}^- \rightarrow \text{CO}\_2^- \tag{31}$$

monoxide radical is further split into carbon radical and oxygen (Eq. (42)). The overall reaction is

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),

The following is the other mechanism in forming methanol, methane, and ethylene that is proposed [64–66]. 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

CO2 þ e� ! CO2

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

CO2

CO2 þ e ! �CO2 (40)

Photoreduction Processes over TiO2 Photocatalyst http://dx.doi.org/10.5772/intechopen.80914 137

�CO2 ! �CO þ 1=2O2 (41)

�CO ! �C þ 1=2O2 (42)

CO2 ! �C þ O2 (43)

4H<sup>þ</sup> þ �C þ 4e ! CH4 (44)

TiO2 þ UV light ! TiO2 þ e� þ h<sup>þ</sup> (46)

2H2O þ 2h<sup>þ</sup> ! �OH þ H� (47)

� þ 2H� ! OC�H þ OH� (49)

OC�H þ OC�H ! HOCCOH (50)

HOCCOH þ 4H� ! CH3COH (51)

CH3COH þ H�!�CH3 þ CO (52)

�CH3 þ H� ! CH4 (53)

�CH3 þ �CH3 ! C2H6 (54)

� (48)

2H<sup>þ</sup> þ 2e ! H2 (45)

written as Eq. (43):

respectively:

(Eq. (49)):

other, ethane is produced (Eq. (54)).

$$\text{2H}^+ + \text{e}^- \rightarrow \text{H}\_2\tag{32}$$

$$\text{CO}\_2^- + 2\text{H}^+ + \text{e}^- \rightarrow \text{CO} + \text{H}\_2\text{O} \tag{33}$$

$$\text{CH}\_2\text{}^- + 2\text{H}^+ + \text{e}^- \rightarrow \text{CH}\_4 + \text{O}\_2 \tag{34}$$

$$\text{CH}\_2\text{}^- + 2\text{H}^+ + \text{e}^- \rightarrow \text{CH}\_3\text{OH} + \text{H}\_2\text{O} \tag{35}$$

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):

$$\text{TiO}\_2 + \text{LU} \text{ light} \rightarrow \text{TiO}\_2 + \text{e}^- + h^+ \tag{36}$$

$$\text{H}\_2\text{O} + \text{h}^+ \rightarrow \cdot \text{OH} + \text{H}^+ \tag{37}$$

$$\text{-OH} + \text{H}\_2\text{O} + 3\text{h}^+ \rightarrow \text{O}\_2 + 3\text{H}^+ \tag{38}$$

$$2\text{H}\_2\text{O} + 4\text{h}^+ \to \text{O}\_2 + 4\text{H}^+ \tag{39}$$

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):

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

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

CO2 þ e� ! CO2

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

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

� (31)

2H<sup>þ</sup> þ e� ! H2 (32)

� þ 2H<sup>þ</sup> þ e� ! CO þ H2O (33)

� þ 2H<sup>þ</sup> þ e� ! CH4 þ O2 (34)

� þ 2H<sup>þ</sup> þ e� ! CH3OH þ H2O (35)

TiO2 þ UV light ! TiO2 þ e� þ h<sup>þ</sup> (36)

�OH þ H2O þ 3h<sup>þ</sup> ! O2 þ 3H<sup>þ</sup> (38)

2H2O þ 4h<sup>þ</sup> ! O2 þ 4H<sup>þ</sup> (39)

H2O þ h<sup>þ</sup> ! �OH þ H<sup>þ</sup> (37)

radical to yield methane (Eq. (30)).

136 Photocatalysts - Applications and Attributes

electron to yield methanol and water (Eq. (35)):

CO2

CO2

CO2

the reactions (37) and (38) is represented by Eq. (39):

$$\cdot \text{CO}\_2 + \text{e} \rightarrow \cdot \text{CO}\_2 \tag{40}$$

$$\cdot \text{CO}\_2 \rightarrow \cdot \text{CO} + 1/2\text{O}\_2 \tag{41}$$

$$\cdot \text{CO} \rightarrow \cdot \text{C} + 1/2\text{O}\_2 \tag{42}$$

$$\cdot \text{CO}\_2 \rightarrow \cdot \text{C} + \text{O}\_2 \tag{43}$$

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:

$$\cdot 4\text{H}^+ + \cdot \text{C} + 4\text{e} \rightarrow \text{CH}\_4\tag{44}$$

$$\text{2H}^+ + \text{2e} \rightarrow \text{H}\_2\tag{45}$$

The following is the other mechanism in forming methanol, methane, and ethylene that is proposed [64–66]. 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)):

$$\text{TiO}\_2 + \text{LIV light} \rightarrow \text{TiO}\_2 + \text{e}^- + h^+ \tag{46}$$

$$\cdot 2\text{H}\_2\text{O} + 2\text{h}^+ \rightarrow \cdot \text{OH} + \text{H}\cdot\tag{47}$$

$$\text{CO}\_2 + \text{e}^- \rightarrow \text{CO}\_2^- \tag{48}$$

$$\text{CO}\_2^- + 2\text{H} \cdot \rightarrow \text{OC} \cdot \text{H} + \text{OH}^- \tag{49}$$

$$\text{OC}\cdot\text{H} + \text{OC}\cdot\text{H} \to \text{HOC}\text{OH} \tag{50}$$

$$\text{HOCCOH} + 4\text{H} \cdot \text{D} \xrightarrow{} \text{CH}\_3\text{COOH} \tag{51}$$

$$\cdot \text{CH}\_3\text{COOH} + \text{H} \cdot \rightarrow \cdot \text{CH}\_3 + \text{CO} \tag{52}$$

$$\cdot \text{CH}\_3 + \text{H} \cdot \rightarrow \text{CH}\_4 \tag{53}$$

$$\cdot \text{CH}\_3 + \cdot \text{CH}\_3 \rightarrow \text{C}\_2\text{H}\_6 \tag{54}$$

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)).
