Hitoshi Ishida

Additional information is available at the end of the chapter Hitoshi IshidaAdditional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.75199

#### **Abstract**

Conversion of CO2 into useful chemicals is attractive as a solution of the fossil fuel shortage and the global warming problems. Reduction of CO2 into carbon monoxide (CO) and formic acid (HCOOH) is also important for obtaining the materials in organic syntheses. There are a lot of studies on the catalysts for electrochemical/photochemical CO2 reduction. Especially, transition metal complexes have actively researched as the molecular catalysts for CO2 reduction. In this chapter, the electrochemical/photochemical CO2 reduction catalyzed by *cis*-[Ru(bpy)2 (CO)2 ]2+ (bpy: 2,2′-bipyridine) and *trans*(Cl)-[Ru(bpy) (CO)2 Cl2 ] is described as a representative example.

**Keywords:** CO2 reduction, artificial photosynthesis, electrochemistry, photochemistry, ruthenium

### **1. Introduction**

Utilization of CO2 becomes more and more important with increasing CO2 emission which causes the global warming and the ocean acidification problems [1, 2]. The huge CO2 emission also relates on depletion of fossil fuels. The conversion of CO2 into useful fuels and chemicals is very urgent to solve the abovementioned problems. The use of biomass instead of fossil fuels is actively researched and partly undertaken [3]. In many chemical laboratories, fixation of CO2 into organic compounds by organometallic catalysts is vigorously studied [4].

Reduction of CO2 with electrons is an attractive chemical conversion to obtain the useful products for fuels and chemical materials. It is so simple that it can be applied to photocatalyses which supply electrons from electron donors such as water. The equilibrium potentials (E0' V vs. SHE

> © 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 reproduction 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.

at pH 7) for CO2 reduction are listed in **Figure 1** [5, 6]; they are thermodynamic values and tend to positively shift with increasing the numbers of electrons participated. One-electron reduction of CO2 requires very high energy. Furthermore, the product, CO2 anion radical (CO2 − ∙), is difficult to give useful organic chemicals because it is a very strong reducing reagent to reduce other molecules and recover CO2 . Thus, the CO2 reductions with multielectrons are desired; however, their reactions are generally difficult even in the electrochemical reduction. A reason is that the intermediates would release from the surface of the electrode as the products before accepting further electrons. To achieve CO2 reduction with more than two electrons, the catalysts which allow to lower the activation energies are required. In other words, the catalysts can undergo the CO2 reduction at the potentials closed to the equilibrium ones. The two-electron reduction of CO2 produces carbon monoxide (CO) and formic acid (HCOOH). The equilibrium potentials are more negative than the proton reduction to afford H<sup>2</sup> . Therefore, the catalysts which can selectively reduce CO2 rather than H+ are also desired. Both CO and HCOOH are useful chemicals: CO can be converted into liquid hydrocarbons by using the Fischer-Tropsch reaction [7], and HCOOH which can be readily converted to H2 is a safe storage material for H2 [8].

A lot of metal complexes have been researched for the CO<sup>2</sup> reduction catalyses [9–16]. Until now, the metal complexes of Mn [17–19], Fe [20, 21], Co [22–24], Ni [24–28], Cu [29], Mo [30], Ru [31–64], Rh [65, 66], Pd [67, 68], W [30], Re [69–76], Os [77, 78], Ir [65, 66, 79, 80] have been reported as the catalysts for CO2 reduction. **Figure 2A** shows the elements of the metal complexes acting as the electrochemical CO2 reduction catalysts. The metal complexes indicated in red include the catalysts for photochemical reduction. **Figure 2B** shows the examples of the metal complexes as the CO2 reduction catalysts. These catalysts based on metal complexes are sometimes called as "molecular catalysts" because they can be designed on the molecular levels by selecting the metal elements and the ligands. The representative and efficient catalysts for CO2 reduction are nickel(II) cyclam (cyclam: 1,4,8,11-tetraazacyclotetradecane),


ruthenium(II) polypyridyl carbonyl complexes and rhenium(I) bipyridyl tricarbonyl complexes. Recently, the complexes with nonprecious metals such as manganese(II) and iron(II) attract much attention. They are abundant and readily available, while they are less durable

the metal complexes for photocatalyses are indicated in red) and (B) the molecular structures.

the examples is described. The reduction products are CO and formic acid, while the nickel and rhenium complexes selectively yield CO. Discussion for the catalytic mechanisms is introduced particularly for the factors determining the product selectivity. In the next section,

procedures, the principles for selecting the photosensitizers and the electron donors, and the photocatalytic mechanisms are summarized. Furthermore, application of the homogeneous catalytic systems to heterogeneous catalyses, which is practically advantageous in the viewpoints of separation of the catalysts from the reactants and the products, is described. In the final section, the artificial photosynthetic systems, which would be realized by utilizing the

reduction catalyzed by the ruthenium complexes as

reduction catalysts: (A) the metal elements in the complexes (the elements in

Electrochemical/Photochemical CO2 Reduction Catalyzed by Transition Metal Complexes

http://dx.doi.org/10.5772/intechopen.75199

19

reduction assisting by the photosensitizers is described. The reaction

and efficient as the disadvantageous points.

In this chapter, the electrochemical CO2

**Figure 2.** Metal complexes reported as CO2

molecular catalysts, are prospected.

the photocatalytic CO2

**Figure 1.** Equilibrium potentials for CO2 reduction (*E0 '* V vs. SHE (pH 7)).

Electrochemical/Photochemical CO2 Reduction Catalyzed by Transition Metal Complexes http://dx.doi.org/10.5772/intechopen.75199 19

at pH 7) for CO2

molecules and recover CO2

selectively reduce CO2

further electrons. To achieve CO2

18 Carbon Dioxide Chemistry, Capture and Oil Recovery

reported as the catalysts for CO2

metal complexes as the CO2

**Figure 1.** Equilibrium potentials for CO2

reduction (*E0*

*'* V vs. SHE (pH 7)).

catalysts for CO2

plexes acting as the electrochemical CO2

of CO2

the CO2

of CO2

reduction are listed in **Figure 1** [5, 6]; they are thermodynamic values and tend

anion radical (CO2

. Therefore, the catalysts which can

reduction catalyses [9–16]. Until

reductions with multielectrons are desired; however,

reduction with more than two electrons, the catalysts which

are also desired. Both CO and HCOOH are useful chemi-

is a safe storage material for H2

reduction catalysts. The metal complexes indicated

reduction. **Figure 2A** shows the elements of the metal com-

reduction catalysts. These catalysts based on metal complexes

reduction are nickel(II) cyclam (cyclam: 1,4,8,11-tetraazacyclotetradecane),

−

[8].

∙), is diffi-

to positively shift with increasing the numbers of electrons participated. One-electron reduction

cult to give useful organic chemicals because it is a very strong reducing reagent to reduce other

their reactions are generally difficult even in the electrochemical reduction. A reason is that the intermediates would release from the surface of the electrode as the products before accepting

allow to lower the activation energies are required. In other words, the catalysts can undergo

cals: CO can be converted into liquid hydrocarbons by using the Fischer-Tropsch reaction [7],

now, the metal complexes of Mn [17–19], Fe [20, 21], Co [22–24], Ni [24–28], Cu [29], Mo [30], Ru [31–64], Rh [65, 66], Pd [67, 68], W [30], Re [69–76], Os [77, 78], Ir [65, 66, 79, 80] have been

in red include the catalysts for photochemical reduction. **Figure 2B** shows the examples of the

are sometimes called as "molecular catalysts" because they can be designed on the molecular levels by selecting the metal elements and the ligands. The representative and efficient

reduction at the potentials closed to the equilibrium ones. The two-electron reduction

produces carbon monoxide (CO) and formic acid (HCOOH). The equilibrium potentials

requires very high energy. Furthermore, the product, CO2

. Thus, the CO2

are more negative than the proton reduction to afford H<sup>2</sup>

rather than H+

A lot of metal complexes have been researched for the CO<sup>2</sup>

and HCOOH which can be readily converted to H2

**Figure 2.** Metal complexes reported as CO2 reduction catalysts: (A) the metal elements in the complexes (the elements in the metal complexes for photocatalyses are indicated in red) and (B) the molecular structures.

ruthenium(II) polypyridyl carbonyl complexes and rhenium(I) bipyridyl tricarbonyl complexes. Recently, the complexes with nonprecious metals such as manganese(II) and iron(II) attract much attention. They are abundant and readily available, while they are less durable and efficient as the disadvantageous points.

In this chapter, the electrochemical CO2 reduction catalyzed by the ruthenium complexes as the examples is described. The reduction products are CO and formic acid, while the nickel and rhenium complexes selectively yield CO. Discussion for the catalytic mechanisms is introduced particularly for the factors determining the product selectivity. In the next section, the photocatalytic CO2 reduction assisting by the photosensitizers is described. The reaction procedures, the principles for selecting the photosensitizers and the electron donors, and the photocatalytic mechanisms are summarized. Furthermore, application of the homogeneous catalytic systems to heterogeneous catalyses, which is practically advantageous in the viewpoints of separation of the catalysts from the reactants and the products, is described. In the final section, the artificial photosynthetic systems, which would be realized by utilizing the molecular catalysts, are prospected.

#### **2. Electrochemical CO2 reduction**

The representative molecular catalysts based on ruthenium complexes are *cis*-[Ru(bpy)2 (CO)2 ]2+ (bpy: 2,2′-bipyridine), *trans*(Cl)-[Ru(bpy)(CO)2 Cl2 ] and the derivatives (**Figure 3**). They have the bipyridyl ligand which would act as an electron reservoir. The efficient catalysts have the carbonyl ligand, which would draw electrons. It tends to lower the reduction potentials of the metal complexes as well as to lower the overpotentials for the CO2 reduction.

#### **2.1. Electrochemical analysis**

Electrochemical analyses (e.g., cyclic voltammetric measurements) are recommended to know the electrochemical properties of the molecular catalysts. The analyses do not only teach us the reduction potentials of the metal complexes but also show whether the complexes can react with CO2 or not. **Figure 4** shows the cyclic voltammograms (CVs) of *cis*- [Ru(bpy)2 (CO)2 ]2+ in CH3 CN or CH3 CN/H2 O (9:1). The Ag-Ag<sup>+</sup> (CH3 CN) reference electrode (0.10 M Tetrabutylammonium perchlorate (TBAP) /0.01 M AgNO<sup>3</sup> in CH3 CN) is used; the potential (0.00 V vs. Ag/AgNO<sup>3</sup> (CH3 CN)) corresponds to −0.09 V vs. Fc/Fc<sup>+</sup> in CH3 CN. The CV of *cis*-[Ru(bpy)2 (CO)2 ]2+ in CH3 CN under Ar shows an irreversible reduction wave at −1.3 V vs. Ag-Ag<sup>+</sup> (CH3 CN) as shown in **Figure 4** (black line). The irreversible reduction suggests that the one-electron reduction accompanies with a chemical reaction followed by further one-electron reduction. Such a reaction mechanism is called as electrochemical-chemical-electrochemical (ECE) one. The CV under CO2 is a little different from that under Ar, suggesting that the reduced species react with CO2 (**Figure 4**, blue line). In CH3 CN/H2 O (9:1), the CV exhibits a strong cathodic current under CO2 (**Figure 4**, red line), which corresponds to the catalytic reduction of CO2 in the presence of a proton source such as water. The catalytic reduction currents can be analyzed to estimate the efficiency of the catalyst [20, 81]; however, it should be noted that the cathodic currents do not always exhibit the catalytic CO2 reduction [82]. The electrolyses of the metal complexes under CO2 should be carried out to confirm the catalytic efficiency.

**2.2. Electrolysis**

saturated CH3

**Figure 4.** Cyclic voltammograms of *cis*-[Ru(bpy)2

**Figure 5.** Electrolysis cell for electrochemical CO2

O (9:1) (red) containing NBu4

CN/H2

A typical electrolysis cell is shown in **Figure 5**. The cell for reduction (the side of the working electrode) is separated from the cell for oxidation (the counter electrode) with a membrane such as Nafion. A glassy carbon or a Pt plate is used for the electrodes. The metal complex

(0.10 M).

: In Ar (black) or CO<sup>2</sup>

Electrochemical/Photochemical CO2 Reduction Catalyzed by Transition Metal Complexes

http://dx.doi.org/10.5772/intechopen.75199


is bubbled

CN (blue) or in CO2


21

is dissolved in the reaction solution and acts as the homogenous catalyst. CO2

reduction.

(CO)2 ](PF6 )2

ClO4

**Figure 3.** Ruthenium-bipyridyl complexes as electrochemical CO2 reduction catalysts: (A) *cis*-[Ru(bpy)2 (CO)2 ]2+ and (B) *trans*(cl)-[Ru(bpy)(CO)2 Cl2 ].

Electrochemical/Photochemical CO2 Reduction Catalyzed by Transition Metal Complexes http://dx.doi.org/10.5772/intechopen.75199 21

**Figure 4.** Cyclic voltammograms of *cis*-[Ru(bpy)2 (CO)2 ](PF6 ) 2 : In Ar (black) or CO<sup>2</sup> -saturated CH3 CN (blue) or in CO2 saturated CH3 CN/H2 O (9:1) (red) containing NBu4 ClO4 (0.10 M).

#### **2.2. Electrolysis**

**2. Electrochemical CO2**

20 Carbon Dioxide Chemistry, Capture and Oil Recovery

**2.1. Electrochemical analysis**

plexes can react with CO2

potential (0.00 V vs. Ag/AgNO<sup>3</sup>

the catalytic reduction of CO2

]2+ in CH3

(CO)2

ical-electrochemical (ECE) one. The CV under CO2

CV exhibits a strong cathodic current under CO2

[82]. The electrolyses of the metal complexes under CO2

**Figure 3.** Ruthenium-bipyridyl complexes as electrochemical CO2

Cl2 ].

gesting that the reduced species react with CO2

(CH3

(CO)2

CV of *cis*-[Ru(bpy)2

−1.3 V vs. Ag-Ag<sup>+</sup>

catalytic efficiency.

(B) *trans*(cl)-[Ru(bpy)(CO)2

[Ru(bpy)2

(bpy: 2,2′-bipyridine), *trans*(Cl)-[Ru(bpy)(CO)2

 **reduction**

metal complexes as well as to lower the overpotentials for the CO2

CN or CH3

]2+ in CH3

(0.10 M Tetrabutylammonium perchlorate (TBAP) /0.01 M AgNO<sup>3</sup>

(CH3

CN/H2

The representative molecular catalysts based on ruthenium complexes are *cis*-[Ru(bpy)2

Cl2

the bipyridyl ligand which would act as an electron reservoir. The efficient catalysts have the carbonyl ligand, which would draw electrons. It tends to lower the reduction potentials of the

Electrochemical analyses (e.g., cyclic voltammetric measurements) are recommended to know the electrochemical properties of the molecular catalysts. The analyses do not only teach us the reduction potentials of the metal complexes but also show whether the com-

suggests that the one-electron reduction accompanies with a chemical reaction followed by further one-electron reduction. Such a reaction mechanism is called as electrochemical-chem-

reduction currents can be analyzed to estimate the efficiency of the catalyst [20, 81]; however,

it should be noted that the cathodic currents do not always exhibit the catalytic CO2

or not. **Figure 4** shows the cyclic voltammograms (CVs) of *cis*-

CN)) corresponds to −0.09 V vs. Fc/Fc<sup>+</sup>

CN) as shown in **Figure 4** (black line). The irreversible reduction

(**Figure 4**, blue line). In CH3

in the presence of a proton source such as water. The catalytic

(CH3

CN under Ar shows an irreversible reduction wave at

O (9:1). The Ag-Ag<sup>+</sup>

(CO)2 ]2+

CN) reference electrode

in CH3

CN/H2

CN) is used; the

CN. The

O (9:1), the

reduction

(CO)2

]2+ and

] and the derivatives (**Figure 3**). They have

reduction.

in CH3

is a little different from that under Ar, sug-

(**Figure 4**, red line), which corresponds to

reduction catalysts: (A) *cis*-[Ru(bpy)2

should be carried out to confirm the

A typical electrolysis cell is shown in **Figure 5**. The cell for reduction (the side of the working electrode) is separated from the cell for oxidation (the counter electrode) with a membrane such as Nafion. A glassy carbon or a Pt plate is used for the electrodes. The metal complex is dissolved in the reaction solution and acts as the homogenous catalyst. CO2 is bubbled

**Figure 5.** Electrolysis cell for electrochemical CO2 reduction.

with a needle through the septum before electrolysis. Electrochemical CO2 is carried out in batch mode. Reduction of CO2 occurs on the working electrode at the electrochemical cell. Sampling of the gaseous and liquid phases is performed by a syringe through the septum. The gaseous products (CO and H2 ) are analyzed by gas chromatography. The liquid product, HCOOH, is analyzed by electrophoresis, ion chromatography or gas chromatography. The electrolysis is carried out by the controlled potential method, where the potential is determined from the electrochemical analysis (e.g., CVs). The chronopotentiometry, in which the current is constant during the electrolysis, is important for the industrial use. However, the results in the constant potential lead to elucidate the catalyses because the electrolysis potential relates on the catalytic species. Thus, almost all the scientific researches adopt the controlled potential electrolyses.

Thus, the mechanism involving the equilibrium among the carbonyl complex [Ru(bpy)2

(CO)(C(O)OH)]+

plexes were isolated, and the crystal structures were characterized [83]. In the mechanism,

adduct complex is protonated to give the carboxylic acid complex [Ru(bpy)2

acid complex could be reduced to yield HCOOH, and the carbonyl complex to produce CO. The

gives CO and HCOOH under protic and less protic conditions, respectively. This idea is also

depending on the different equilibrium constants [58]. The carbonyl complex reacts with dimeth-

reduction in the presence of dimethylamine produces *N,N*-dimethylformamide (DMF)

reduction catalyzed by *cis*-[Ru(bpy)2

2+ is reduced to yield the coordinated unsaturated species [Ru(bpy)2

and the CO2

Electrochemical/Photochemical CO2 Reduction Catalyzed by Transition Metal Complexes

the carboxylic acid complex [Ru(bpy)2

)] was proposed for the catalytic CO2

evolving CO. The five coordinated complex reacts with CO<sup>2</sup>

)]+

)]0

and further protonated to recover the carbonyl complex [Ru(bpy)2

proposed idea reasonably elucidates the experimental results that the catalytic CO2

supported by the result that the ruthenium complex derivatives give the CO/HCOO<sup>−</sup>

[84]. It is also an evidence which the carbonyl complex would exist in the catalysis.

)], in which CO2

(CO)(COO<sup>−</sup>

ylamine to afford the carbamoyl complex [Ru(bpy)<sup>2</sup>

**Figure 7.** Two proposed mechanisms for CO2

adduct mechanism and hydride mechanism.

Metal-CO2

(CO)(CO2

(CO2

[Ru(bpy)2

[Ru(bpy)2

The CO2

OH)]+

cal CO2

(CO)2 ]

(CO)(CO2

tronic structure of the CO2

is drawn as [Ru(bpy)2

it is drawn as [Ru(bpy)2

(CO)2 ] 2+,

(CO)] with

adduct complex,

ligand. In **Figure 7**,

(CO)(COO<sup>−</sup>

2+. The carboxylic

, and the electrochemi-

(CO)X]n+ (X = CO (n = 2); H (n = 1)):

(CO)(C(O)

reduction

selectivity

(CO)

23

)]+ .

adduct complex [Ru(bpy)2

http://dx.doi.org/10.5772/intechopen.75199


(CO)2 ]

reduction (**Figure 7**, left cycle) [58, 62]. All the com-

to afford the *η*<sup>1</sup>

coordinates to the metal center at the carbon atom. The elec-

bound complex still remains unknown. In the original report [62], it

in which as electron localizes on the CO2

which is the resonance structure of [Ru(bpy)2

(CO)(C(O)N(CH3

) 2 )]+

#### **2.3. Electrocatalytic CO2 reduction by** *cis***-[Ru(bpy)2 (CO)2 ] 2+**

The ruthenium complexes are used as the homogeneous catalysts by dissolving in the reaction solution. The electrolysis of the CO2 -saturated H2 O/DMF (1:1) solution of *cis*-[Ru(bpy)2 (CO)2 ] 2+ was carried out at −1.50 V vs. SCE with an Hg pool as the working electrode (**Figure 6**) [62]. The catalyst could selectively reduce CO2 to afford CO and HCOOH, while H<sup>2</sup> , the reduction product of water, scarcely evolved. As the reaction proceeded, the speed for CO production got slow, but HCOOH production became fast. It was interpreted as the result of the decreasing the proton concentration ([H+ ]) in the reaction solution by consumption of the proton during the reduction. Actually, the reactions in the buffered solution with H<sup>3</sup> PO4 -NaOH exhibited that the production speeds of the CO2 reduction were unchanged during the reactions. It was the decisive result that HCOOH selectively produced when phenol with the high pKa (ca. 9.95) was used as the proton source. These results suggest that there is an acid-base equilibrium between two intermediates in which one is for CO production and another for HCOOH.

**Figure 6.** Plots of the amounts of products vs. the electricity in the electrolysis (−1.50 V vs. SCE) of CO<sup>2</sup> -saturated H2 O/ DMF (1:1 v/v) solution containing *cis*-[Ru(bpy)2 (CO)2 ](PF6 )2 (5.0 × 10−4 M) and LiCl (0.10 M) as the supporting electrolyte at room temperature.

Thus, the mechanism involving the equilibrium among the carbonyl complex [Ru(bpy)2 (CO)2 ] 2+, the carboxylic acid complex [Ru(bpy)2 (CO)(C(O)OH)]+ and the CO2 adduct complex [Ru(bpy)2 (CO) (CO2 )] was proposed for the catalytic CO2 reduction (**Figure 7**, left cycle) [58, 62]. All the complexes were isolated, and the crystal structures were characterized [83]. In the mechanism, [Ru(bpy)2 (CO)2 ] 2+ is reduced to yield the coordinated unsaturated species [Ru(bpy)2 (CO)] with evolving CO. The five coordinated complex reacts with CO<sup>2</sup> to afford the *η*<sup>1</sup> -CO2 adduct complex, [Ru(bpy)2 (CO)(CO2 )], in which CO2 coordinates to the metal center at the carbon atom. The electronic structure of the CO2 bound complex still remains unknown. In the original report [62], it is drawn as [Ru(bpy)2 (CO)(COO<sup>−</sup> )]+ in which as electron localizes on the CO2 ligand. In **Figure 7**, it is drawn as [Ru(bpy)2 (CO)(CO2 )]0 which is the resonance structure of [Ru(bpy)2 (CO)(COO<sup>−</sup> )]+ . The CO2 adduct complex is protonated to give the carboxylic acid complex [Ru(bpy)2 (CO)(C(O) OH)]+ and further protonated to recover the carbonyl complex [Ru(bpy)2 (CO)2 ] 2+. The carboxylic acid complex could be reduced to yield HCOOH, and the carbonyl complex to produce CO. The proposed idea reasonably elucidates the experimental results that the catalytic CO2 reduction gives CO and HCOOH under protic and less protic conditions, respectively. This idea is also supported by the result that the ruthenium complex derivatives give the CO/HCOO<sup>−</sup> selectivity depending on the different equilibrium constants [58]. The carbonyl complex reacts with dimethylamine to afford the carbamoyl complex [Ru(bpy)<sup>2</sup> (CO)(C(O)N(CH3 ) 2 )]+ , and the electrochemical CO2 reduction in the presence of dimethylamine produces *N,N*-dimethylformamide (DMF) [84]. It is also an evidence which the carbonyl complex would exist in the catalysis.

with a needle through the septum before electrolysis. Electrochemical CO2

 **reduction by** *cis***-[Ru(bpy)2**

ing the reduction. Actually, the reactions in the buffered solution with H<sup>3</sup>

**Figure 6.** Plots of the amounts of products vs. the electricity in the electrolysis (−1.50 V vs. SCE) of CO<sup>2</sup>

(CO)2 ](PF6 )2


Sampling of the gaseous and liquid phases is performed by a syringe through the septum.

uct, HCOOH, is analyzed by electrophoresis, ion chromatography or gas chromatography. The electrolysis is carried out by the controlled potential method, where the potential is determined from the electrochemical analysis (e.g., CVs). The chronopotentiometry, in which the current is constant during the electrolysis, is important for the industrial use. However, the results in the constant potential lead to elucidate the catalyses because the electrolysis potential relates on the catalytic species. Thus, almost all the scientific researches adopt the

The ruthenium complexes are used as the homogeneous catalysts by dissolving in the reaction

was carried out at −1.50 V vs. SCE with an Hg pool as the working electrode (**Figure 6**) [62].

product of water, scarcely evolved. As the reaction proceeded, the speed for CO production got slow, but HCOOH production became fast. It was interpreted as the result of the decreas-

the decisive result that HCOOH selectively produced when phenol with the high pKa (ca. 9.95) was used as the proton source. These results suggest that there is an acid-base equilibrium between two intermediates in which one is for CO production and another for HCOOH.

**(CO)2 ] 2+**

to afford CO and HCOOH, while H<sup>2</sup>

]) in the reaction solution by consumption of the proton dur-

reduction were unchanged during the reactions. It was

batch mode. Reduction of CO2

The gaseous products (CO and H2

22 Carbon Dioxide Chemistry, Capture and Oil Recovery

controlled potential electrolyses.

solution. The electrolysis of the CO2

ing the proton concentration ([H+

that the production speeds of the CO2

DMF (1:1 v/v) solution containing *cis*-[Ru(bpy)2

at room temperature.

The catalyst could selectively reduce CO2

**2.3. Electrocatalytic CO2**

is carried out in

(CO)2 ] 2+

, the reduction



(5.0 × 10−4 M) and LiCl (0.10 M) as the supporting electrolyte

O/

occurs on the working electrode at the electrochemical cell.

) are analyzed by gas chromatography. The liquid prod-

O/DMF (1:1) solution of *cis*-[Ru(bpy)2

PO4

**Figure 7.** Two proposed mechanisms for CO2 reduction catalyzed by *cis*-[Ru(bpy)2 (CO)X]n+ (X = CO (n = 2); H (n = 1)): Metal-CO2 adduct mechanism and hydride mechanism.

On the other hand, the ruthenium hydride complex [Ru(bpy)2 (CO)H]+ is known to react with CO2 to yield the formate complex [Ru(bpy)2 (CO)(OC(O)H)]+ [85]. In the conversion, CO2 is inserted into the Ru-H bond. The formate complex can release formate ion (HCOO<sup>−</sup> ) and is considered to be an intermediate for HCOO<sup>−</sup> production. Based on the results, the hydride mechanism is proposed (**Figure 7**, right cycle). In the mechanism, the coordinated unsaturated species [Ru(bpy)2 (CO)] does not react with CO2 but a proton to yield the hydride complex. The hydride mechanism reasonably explains the CO2 reduction to produce HCOO<sup>−</sup> . However, it has a couple of problems [16]. One is that the mechanism is difficult to elucidate the CO production. Production of HCOO<sup>−</sup> may occur through the hydride mechanism, while CO may produce through the M-CO2 adduct mechanism. In this case, the product selectivity (CO/HCOO<sup>−</sup> ) should be controlled by the reactivity difference between CO and H<sup>+</sup> with the coordinated unsaturated complex. Under the protic conditions, the selectivity of HCOO<sup>−</sup> production should be enhanced; however, the selectivity of the catalyses gives the opposite tendency. Thus, the pH in the solution or the pKa value of the proton source dependence on the electrochemical CO2 reduction cannot be explained. Another is that the ruthenium catalyst does not evolve H2 so much in the CO2 reduction. It suggests that the catalyst intermediate strongly binds with CO2 rather than H+ .

Nevertheless, the hydride mechanism is supported by many researchers. It is because there are many research works on the CO2 insertion into Metal-H bonds to afford the corresponding metal formate complexes. On the other hand, the research works on the carboxylic acid complex are fewer, and no mechanical pathways of HCOO<sup>−</sup> production from the carboxylic acid complex are not understood on the molecular levels.

the modified electrode stable have been actively done: introduction of pyrrole groups to the

The catalytic reaction mechanisms are also unknown but are considered similar as these of

dine [35]. The complex does not polymerize because of the steric hindrance. They discussed the reaction mechanisms based on the hydride mechanism (**Figure 9**). The precursor complex is at

mechanism, the formate complex converts to the carboxylic acid complex and then the carbonyl complex by dehydration. The conversion of the formate complex to the carbonyl complex via the carboxylic acid complex is not known, and therefore further researches are expected.

Homogeneous catalysts are advantageous from the viewpoints of elucidating the catalytic reaction mechanisms compared to heterogeneous ones because the homogenous catalysts can be examined by using many spectroscopic techniques. Nevertheless, the mechanisms of the

There may be potentially many intermediates and pathways in the catalyses, and they depend on the reaction conditions and the subtle difference among the catalyst structures [16, 37].

In the preceding section, the electrocatalytic activities of the ruthenium complexes are introduced. The electrocatalyst can be utilized in photocatalytic systems by combining with a photosensitizer (PS). **Figure 10** shows a schematic drawing of the photocatalytic system, in which the excited PS (PS\*) receives an electron from an electron donor to afford the one-electron

to yield the formate complex, which is reduced to produce HCOO<sup>−</sup>

reduction catalyzed by the ruthenium complexes still remain unknown.

], which has two bulky groups at 6,6′-positions in 2,2′-bipyri-

reduction catalyzed by *trans*(Cl)-[Ru(6Mesbpy)(CO)2

Electrochemical/Photochemical CO2 Reduction Catalyzed by Transition Metal Complexes

reduction catalyzed by

Cl2 ]. 25

with

. In the

ion to yield the coordinated unsaturated

http://dx.doi.org/10.5772/intechopen.75199

to afford the hydride complex. The hydride com-

2+. Machan et al. reported the electrochemical CO2

bipyridyl ligand also yields pyrrole polymers to stabilize the ruthenium polymer.

but H+

recovering the original complex. However, the catalyst mainly produces CO not HCOO<sup>−</sup>

Cl2

 **reduction**

the center of the scheme. It is reduced with releasing Cl<sup>−</sup>

*cis*-[Ru(bpy)2

(CO)2 ]

species, which does not bind with CO2

**Figure 9.** A proposed mechanism of electrochemical CO<sup>2</sup>

*trans*(Cl)-[Ru(6Mesbpy)(CO)2

plex reacts with CO2

electrochemical CO2

**3. Photochemical CO2**

#### **2.4. Electrocatalytic CO2 reduction by** *trans***(cl)-[Ru(bpy)(CO)2 Cl2 ]**

*Trans*(Cl)-[Ru(bpy)(CO)2 Cl2 ] is known to be an efficient catalyst for electrochemical CO<sup>2</sup> reduction [58]. The catalytic activity and the product selectivity are similar as these of *cis*- [Ru(bpy)2 (CO)2 ] 2+. Reduction of *trans*(Cl)-[Ru(bpy)(CO)2 Cl2 ] induces to release Cl<sup>−</sup> ion to afford the coordinated unsaturated complex. This complex is considered to an intermediate which can bind with CO2 ; however, it induces polymerization in the absence of CO2 as shown in **Figure 8** [86, 87]. The polymer with Ru(0)-Ru(0) bonds is also an efficient electrocatalyst for CO<sup>2</sup> reduction [50, 54]. The complex is electrochemically reduced to polymerize on the cathode electrode. The electrode modified with the polymer is moved to another electric cell, and it works in the presence of CO2 as the active electrode for electrochemical CO2 reduction. Researches to make

**Figure 8.** Electroreductive polymerization of *trans*(cl)-[Ru(bpy)(CO)2 Cl2 ].

Electrochemical/Photochemical CO2 Reduction Catalyzed by Transition Metal Complexes http://dx.doi.org/10.5772/intechopen.75199 25

**Figure 9.** A proposed mechanism of electrochemical CO<sup>2</sup> reduction catalyzed by *trans*(Cl)-[Ru(6Mesbpy)(CO)2 Cl2 ].

the modified electrode stable have been actively done: introduction of pyrrole groups to the bipyridyl ligand also yields pyrrole polymers to stabilize the ruthenium polymer.

The catalytic reaction mechanisms are also unknown but are considered similar as these of *cis*-[Ru(bpy)2 (CO)2 ] 2+. Machan et al. reported the electrochemical CO2 reduction catalyzed by *trans*(Cl)-[Ru(6Mesbpy)(CO)2 Cl2 ], which has two bulky groups at 6,6′-positions in 2,2′-bipyridine [35]. The complex does not polymerize because of the steric hindrance. They discussed the reaction mechanisms based on the hydride mechanism (**Figure 9**). The precursor complex is at the center of the scheme. It is reduced with releasing Cl<sup>−</sup> ion to yield the coordinated unsaturated species, which does not bind with CO2 but H+ to afford the hydride complex. The hydride complex reacts with CO2 to yield the formate complex, which is reduced to produce HCOO<sup>−</sup> with recovering the original complex. However, the catalyst mainly produces CO not HCOO<sup>−</sup> . In the mechanism, the formate complex converts to the carboxylic acid complex and then the carbonyl complex by dehydration. The conversion of the formate complex to the carbonyl complex via the carboxylic acid complex is not known, and therefore further researches are expected.

Homogeneous catalysts are advantageous from the viewpoints of elucidating the catalytic reaction mechanisms compared to heterogeneous ones because the homogenous catalysts can be examined by using many spectroscopic techniques. Nevertheless, the mechanisms of the electrochemical CO2 reduction catalyzed by the ruthenium complexes still remain unknown. There may be potentially many intermediates and pathways in the catalyses, and they depend on the reaction conditions and the subtle difference among the catalyst structures [16, 37].

#### **3. Photochemical CO2 reduction**

**Figure 8.** Electroreductive polymerization of *trans*(cl)-[Ru(bpy)(CO)2

On the other hand, the ruthenium hydride complex [Ru(bpy)2

plex. The hydride mechanism reasonably explains the CO2

so much in the CO2

complex are fewer, and no mechanical pathways of HCOO<sup>−</sup>

2+. Reduction of *trans*(Cl)-[Ru(bpy)(CO)2

acid complex are not understood on the molecular levels.

Cl2

rather than H+

.

to yield the formate complex [Ru(bpy)2

considered to be an intermediate for HCOO<sup>−</sup>

the CO production. Production of HCOO<sup>−</sup>

CO may produce through the M-CO2

24 Carbon Dioxide Chemistry, Capture and Oil Recovery

are many research works on the CO2

CO2

rated species [Ru(bpy)2

the electrochemical CO2

strongly binds with CO2

**2.4. Electrocatalytic CO2**

*Trans*(Cl)-[Ru(bpy)(CO)2

(CO)2 ]

[Ru(bpy)2

bind with CO2

presence of CO2

does not evolve H2

ity (CO/HCOO<sup>−</sup>

(CO)H]+

production. Based on the results, the hydride

may occur through the hydride mechanism, while

adduct mechanism. In this case, the product selectiv-

reduction. It suggests that the catalyst intermediate

insertion into Metal-H bonds to afford the correspond-

**Cl2 ]**

] induces to release Cl<sup>−</sup>

] is known to be an efficient catalyst for electrochemical CO<sup>2</sup>

Cl2

but a proton to yield the hydride com-

(CO)(OC(O)H)]+

inserted into the Ru-H bond. The formate complex can release formate ion (HCOO<sup>−</sup>

(CO)] does not react with CO2

mechanism is proposed (**Figure 7**, right cycle). In the mechanism, the coordinated unsatu-

However, it has a couple of problems [16]. One is that the mechanism is difficult to elucidate

the coordinated unsaturated complex. Under the protic conditions, the selectivity of HCOO<sup>−</sup> production should be enhanced; however, the selectivity of the catalyses gives the opposite tendency. Thus, the pH in the solution or the pKa value of the proton source dependence on

Nevertheless, the hydride mechanism is supported by many researchers. It is because there

ing metal formate complexes. On the other hand, the research works on the carboxylic acid

 **reduction by** *trans***(cl)-[Ru(bpy)(CO)2**

; however, it induces polymerization in the absence of CO2

[86, 87]. The polymer with Ru(0)-Ru(0) bonds is also an efficient electrocatalyst for CO<sup>2</sup>

as the active electrode for electrochemical CO2

reduction [58]. The catalytic activity and the product selectivity are similar as these of *cis*-

the coordinated unsaturated complex. This complex is considered to an intermediate which can

tion [50, 54]. The complex is electrochemically reduced to polymerize on the cathode electrode. The electrode modified with the polymer is moved to another electric cell, and it works in the

) should be controlled by the reactivity difference between CO and H<sup>+</sup>

reduction cannot be explained. Another is that the ruthenium catalyst

is known to react with

is

.

with

) and is

[85]. In the conversion, CO2

reduction to produce HCOO<sup>−</sup>

production from the carboxylic

ion to afford

reduc-

as shown in **Figure 8**

reduction. Researches to make

Cl2 ]. In the preceding section, the electrocatalytic activities of the ruthenium complexes are introduced. The electrocatalyst can be utilized in photocatalytic systems by combining with a photosensitizer (PS). **Figure 10** shows a schematic drawing of the photocatalytic system, in which the excited PS (PS\*) receives an electron from an electron donor to afford the one-electron

CO2

than H2

[Ru(bpy)3

of [Ru(bpy)3

CO2

SCE, it requires the reducing ability of PS<sup>−</sup>

production by reduction of H2

To generate the one-electron reduced species PS<sup>−</sup>

]2+ is +0.77 V vs. SCE (CH3

are actually used in photocatalytic CO2

is reduced are desired.

**3.2. Photocatalytic CO2**

of *trans*(Cl)-[Ru(bpy)(CO)2

not utilize the excited state but the one-electron reduced species.

reduction catalyzed by the ruthenium complexes proceeds under electrolysis at −1.30 V vs.

the excited state of the photosensitizer. As the reduction potential of the excited state of

positive potentials than +0.77 V. **Figure 12** shows the examples of the electron donors which

aqueous solution, but amines (triethylamine (TEA) and triethanolamine (TEOA)) cannot work in the presence of water because they are protonated to afford the ammonium ions which cannot give an electron. 1-Benzyl-1,4-dihydronicotineamide (BNAH) is a model compound

the model compound BNAH cannot give two electrons in the oxidation by the excited state

2,3-dihydro-1*H*-benzo[d]imidazole (BIH) and the derivatives (e.g., BI(OH)H), which have much stronger reducing power than BNAH, have been recently utilized in photocatalytic CO<sup>2</sup>

These electron donors are called the sacrificial reagents because the one-electron oxidized species occur chemical changes or decompose so as to prevent back electron transfer. They are useful in order to investigate the reductive half reaction. However, from the viewpoint of the energy balance, the reduction-oxidation (redox) systems in which water is oxidized and

and the electron donor, respectively (**Figure 13**). The catalysis had been carried out in *N,N*dimethylformamide (DMF)/water [57, 59]. However, it was indicated that hydration of DMF affording formate became a serious problem in quantifying formate [93]. We proposed the use of *N,N*-dimethylacetamide (DMA), of which the dehydration does not produce formate but acetate, instead of DMF [39]. Although the photocatalysis strongly depends on the solvent

of NADH in nature. NADH is a two-electron donor and is oxidized to yield NAD<sup>+</sup>

]2+ but provides one electron to afford the dimer BNA<sup>2</sup>

reduction. BIH provides two electrons to yield the oxidation product BI+

], [Ru(bpy)3

system, the reaction proceeds in DMA/water similarly as in DMF/water.

 **reduction**

Our group have investigated the photochemical CO2

Cl2

**Figure 12.** Examples of the electron donors (D) used in photochemical CO2

. In general, the CO2

O, and therefore, the photocatalytic CO2

Electrochemical/Photochemical CO2 Reduction Catalyzed by Transition Metal Complexes

reduction [16, 92]. Ascorbate ion (AscH−

reduction requires higher energy

http://dx.doi.org/10.5772/intechopen.75199

, the electron donors can reductively quench

.

reduction by the system consisting

]2+ and BNAH as the catalyst, the photosensitizer,

reduction.

CN), the electron donors which can be oxidized at less

reduction does

27

) can be used in

. However,

. 1,3-Dimethyl-2-phenyl-

**Figure 10.** A schematic drawing of photocatalytic reduction by combining a photosensitizer (PS) with an electrocatalyst (Cat.).

reduced PS (PS<sup>−</sup> ). The PS<sup>−</sup> is the more powerful reagent than PS\*, and it can inject an electron to the electrocatalyst. The catalyst can work similarly as the electroreduction occurs. In this section, the photocatalytic CO2 reduction by the ruthenium complexes is expounded.

#### **3.1. Photosensitizer and sacrificial electron donors**

The most common photosensitizer used in photocatalytic CO2 reduction is [Ru(bpy)3 ] 2+ and the derivatives. **Figure 11** shows the absorption and emission spectra of [Ru(bpy)3 ] 2+ in acetonitrile. The complex exhibits an absorption band at 400–500 nm, which is assignable to metal-to-ligand charge transfer (MLCT). When excited at the band, the emission at the longer wavelengths is observed. The emission is not fluorescence but room-temperature phosphorescence, which is sensitive to O2 . Therefore, the emission spectrum should be carefully measured under deaerated conditions [88]. The lifetime of the excited state of [Ru(bpy)3 ] 2+ is 1.10 μs in acetonitrile [89, 90]. The quantum yield has been recently reevaluated as 0.095 in acetonitrile [91]. The oxidation potential (corresponding to the reducing ability) of the excited state (PS\*) is −0.81 V vs. SCE (CH3 CN), while this of the one-electron reduced species (PS<sup>−</sup> ) is −1.33 V. As the electrochemical

**Figure 11.** Absorption and emission (phosphorescence) spectra of [Ru(bpy)<sup>3</sup> ]2+ in deaerated CH3 CN at room temperature.

CO2 reduction catalyzed by the ruthenium complexes proceeds under electrolysis at −1.30 V vs. SCE, it requires the reducing ability of PS<sup>−</sup> . In general, the CO2 reduction requires higher energy than H2 production by reduction of H2 O, and therefore, the photocatalytic CO2 reduction does not utilize the excited state but the one-electron reduced species.

To generate the one-electron reduced species PS<sup>−</sup> , the electron donors can reductively quench the excited state of the photosensitizer. As the reduction potential of the excited state of [Ru(bpy)3 ]2+ is +0.77 V vs. SCE (CH3 CN), the electron donors which can be oxidized at less positive potentials than +0.77 V. **Figure 12** shows the examples of the electron donors which are actually used in photocatalytic CO2 reduction [16, 92]. Ascorbate ion (AscH− ) can be used in aqueous solution, but amines (triethylamine (TEA) and triethanolamine (TEOA)) cannot work in the presence of water because they are protonated to afford the ammonium ions which cannot give an electron. 1-Benzyl-1,4-dihydronicotineamide (BNAH) is a model compound of NADH in nature. NADH is a two-electron donor and is oxidized to yield NAD<sup>+</sup> . However, the model compound BNAH cannot give two electrons in the oxidation by the excited state of [Ru(bpy)3 ]2+ but provides one electron to afford the dimer BNA<sup>2</sup> . 1,3-Dimethyl-2-phenyl-2,3-dihydro-1*H*-benzo[d]imidazole (BIH) and the derivatives (e.g., BI(OH)H), which have much stronger reducing power than BNAH, have been recently utilized in photocatalytic CO<sup>2</sup> reduction. BIH provides two electrons to yield the oxidation product BI+ .

These electron donors are called the sacrificial reagents because the one-electron oxidized species occur chemical changes or decompose so as to prevent back electron transfer. They are useful in order to investigate the reductive half reaction. However, from the viewpoint of the energy balance, the reduction-oxidation (redox) systems in which water is oxidized and CO2 is reduced are desired.

#### **3.2. Photocatalytic CO2 reduction**

**Figure 11.** Absorption and emission (phosphorescence) spectra of [Ru(bpy)<sup>3</sup>

reduced PS (PS<sup>−</sup>

(Cat.).

sensitive to O2

(CH3

). The PS<sup>−</sup>

26 Carbon Dioxide Chemistry, Capture and Oil Recovery

**3.1. Photosensitizer and sacrificial electron donors**

The most common photosensitizer used in photocatalytic CO2

ated conditions [88]. The lifetime of the excited state of [Ru(bpy)3

CN), while this of the one-electron reduced species (PS<sup>−</sup>

derivatives. **Figure 11** shows the absorption and emission spectra of [Ru(bpy)3

section, the photocatalytic CO2

]2+ in deaerated CH3

is the more powerful reagent than PS\*, and it can inject an electron

reduction is [Ru(bpy)3

]

]

) is −1.33 V. As the electrochemical

2+ is 1.10 μs in acetonitrile

]

2+ in acetonitrile.

2+ and the

reduction by the ruthenium complexes is expounded.

to the electrocatalyst. The catalyst can work similarly as the electroreduction occurs. In this

**Figure 10.** A schematic drawing of photocatalytic reduction by combining a photosensitizer (PS) with an electrocatalyst

The complex exhibits an absorption band at 400–500 nm, which is assignable to metal-to-ligand charge transfer (MLCT). When excited at the band, the emission at the longer wavelengths is observed. The emission is not fluorescence but room-temperature phosphorescence, which is

[89, 90]. The quantum yield has been recently reevaluated as 0.095 in acetonitrile [91]. The oxidation potential (corresponding to the reducing ability) of the excited state (PS\*) is −0.81 V vs. SCE

. Therefore, the emission spectrum should be carefully measured under deaer-

CN at room temperature.

Our group have investigated the photochemical CO2 reduction by the system consisting of *trans*(Cl)-[Ru(bpy)(CO)2 Cl2 ], [Ru(bpy)3 ]2+ and BNAH as the catalyst, the photosensitizer, and the electron donor, respectively (**Figure 13**). The catalysis had been carried out in *N,N*dimethylformamide (DMF)/water [57, 59]. However, it was indicated that hydration of DMF affording formate became a serious problem in quantifying formate [93]. We proposed the use of *N,N*-dimethylacetamide (DMA), of which the dehydration does not produce formate but acetate, instead of DMF [39]. Although the photocatalysis strongly depends on the solvent system, the reaction proceeds in DMA/water similarly as in DMF/water.

**Figure 12.** Examples of the electron donors (D) used in photochemical CO2 reduction.

**Figure 13.** Photochemical CO2 reduction catalyzed by *trans*(Cl)-[Ru(bpy)(CO)2 Cl2 ] with [Ru(bpy)3 ]2+ (a photosensitizer (PS)) and BNAH (an electron donor).

The photochemical CO2

of CO2

5,5′-dimethyl complex than in the 4,4′-complex.

**Figure 14.** A proposed reaction mechanism for photocatalytic CO<sup>2</sup>

because that the homogenous photocatalytic CO2

**3.3. Application to heterogeneous catalysts**

These phenomena have not been observed in electrochemical CO2

ply also sometimes affects the reaction mechanisms [16, 37].

reduction catalyzed by *trans*(Cl)-[Ru(2,2′-bipyridine)(CO)<sup>2</sup>

Electrochemical/Photochemical CO2 Reduction Catalyzed by Transition Metal Complexes

http://dx.doi.org/10.5772/intechopen.75199

29

reduction by *trans*(Cl)-[Ru(bpy)(CO)2

ing two methyl groups at 4,4′- or 5,5′-positions in the ligand has been recently reported [64]. As the catalytic activities of these complexes at low catalyst concentrations are almost the same, the intrinsic activities are considered to be identical. However, the catalytic activities of these complexes are different at high catalyst concentration, where the rate-determining step is not in the catalytic cycle but in the electron relay cycle: the ruthenium complex with dimethyl groups at 5,5′-positions in the 2,2′-bipyridyl ligand is higher than that at 4,4′-positions. The efficiency of the back-electron transfer from the reduced catalyst to the photosensitizer is lower, or the cage escape yield for the sensitizer-catalyst complex is higher in the

the electron relay between the photosensitizer and the catalyst. The speed of the electron sup-

Heterogeneous catalysts are industrially important because they are useful for separating the starting materials and the products from the catalyst and can be recovered and reused. The molecular catalysts can be utilized to develop the heterogeneous catalysts. For photocatalysts

 reduction, combining the molecular catalysts with semiconductor [32, 94, 95], metalorganic frameworks (MOFs) [96, 97] or periodic mesoporous organosilicas (PMOs) [98–101]

Cl2

Cl2 ].

reduction. It is probably

reduction contains the diffusion process of

] bear-

The catalytic reaction proceeds by receiving electrons from the photochemically driven electron relay system. For two-electron reduction of CO2 to CO or HCOOH, the electron relay cycle has to go round two times when the catalytic cycle turns one time. The electron source is not an electrode, but the reaction had been supposed to proceed according to the same mechanism as in electrochemical reduction. However, it has been recently indicated that in some cases, the reaction mechanisms of the photochemical CO2 reduction are likely different from the electrochemical one [16]. For example, unusual catalyst concentration dependence on the product selectivity (CO/HCOO<sup>−</sup> ) in the photocatalysis has been observed: at high catalyst concentration the selectivity of HCOO<sup>−</sup> increases [37]. To elucidate the peculiar catalyst concentration effect, the mechanisms as shown in the right cycle in **Figure 14** are proposed. At the high concentration of the catalyst, the reduced catalyst forms a dimer, which is proposed to selectively afford HCOO− . The dimer of the complex is similar as the intermediate of polymerization, but it is not detected in the photocatalytic system because the absorption spectrum cannot be conformed due to the overlapped absorption of [Ru(bpy)3 ]2+. Alternatively, the photocatalytic CO2 reduction by *trans*(Cl)-[Ru(6Mesbpy)(CO)2 Cl2 ] which does not dimerize because of the steric hindrance of the ligand has been examined. The ruthenium complex selectively produces CO in the photochemical CO2 reduction, and it demonstrates that the dimerization of the catalyst relates on the HCOO<sup>−</sup> production. It is suggested that the catalyst concentration dependence is not observed in a DMA/ethanol solution. It indicates that HCOO<sup>−</sup> also produces in the cycle consisting of mono-nuclear ruthenium complexes as proposed for the electrocatalytic CO2 reduction (**Figure 14**, left cycle) [31].

Electrochemical/Photochemical CO2 Reduction Catalyzed by Transition Metal Complexes http://dx.doi.org/10.5772/intechopen.75199 29

**Figure 14.** A proposed reaction mechanism for photocatalytic CO<sup>2</sup> reduction by *trans*(Cl)-[Ru(bpy)(CO)2 Cl2 ].

The photochemical CO2 reduction catalyzed by *trans*(Cl)-[Ru(2,2′-bipyridine)(CO)<sup>2</sup> Cl2 ] bearing two methyl groups at 4,4′- or 5,5′-positions in the ligand has been recently reported [64]. As the catalytic activities of these complexes at low catalyst concentrations are almost the same, the intrinsic activities are considered to be identical. However, the catalytic activities of these complexes are different at high catalyst concentration, where the rate-determining step is not in the catalytic cycle but in the electron relay cycle: the ruthenium complex with dimethyl groups at 5,5′-positions in the 2,2′-bipyridyl ligand is higher than that at 4,4′-positions. The efficiency of the back-electron transfer from the reduced catalyst to the photosensitizer is lower, or the cage escape yield for the sensitizer-catalyst complex is higher in the 5,5′-dimethyl complex than in the 4,4′-complex.

These phenomena have not been observed in electrochemical CO2 reduction. It is probably because that the homogenous photocatalytic CO2 reduction contains the diffusion process of the electron relay between the photosensitizer and the catalyst. The speed of the electron supply also sometimes affects the reaction mechanisms [16, 37].

#### **3.3. Application to heterogeneous catalysts**

The catalytic reaction proceeds by receiving electrons from the photochemically driven elec-

reduction catalyzed by *trans*(Cl)-[Ru(bpy)(CO)2

cycle has to go round two times when the catalytic cycle turns one time. The electron source is not an electrode, but the reaction had been supposed to proceed according to the same mechanism as in electrochemical reduction. However, it has been recently indicated that in some

the electrochemical one [16]. For example, unusual catalyst concentration dependence on the

centration effect, the mechanisms as shown in the right cycle in **Figure 14** are proposed. At the high concentration of the catalyst, the reduced catalyst forms a dimer, which is proposed to

erization, but it is not detected in the photocatalytic system because the absorption spectrum

because of the steric hindrance of the ligand has been examined. The ruthenium complex

lyst concentration dependence is not observed in a DMA/ethanol solution. It indicates that

also produces in the cycle consisting of mono-nuclear ruthenium complexes as pro-

reduction (**Figure 14**, left cycle) [31].

cannot be conformed due to the overlapped absorption of [Ru(bpy)3

reduction by *trans*(Cl)-[Ru(6Mesbpy)(CO)2

to CO or HCOOH, the electron relay

] with [Ru(bpy)3

reduction are likely different from

]2+. Alternatively, the

]2+ (a photosensitizer

] which does not dimerize

reduction, and it demonstrates that the

production. It is suggested that the cata-

) in the photocatalysis has been observed: at high catalyst

Cl2

. The dimer of the complex is similar as the intermediate of polym-

increases [37]. To elucidate the peculiar catalyst con-

Cl2

tron relay system. For two-electron reduction of CO2

cases, the reaction mechanisms of the photochemical CO2

selectively produces CO in the photochemical CO2

dimerization of the catalyst relates on the HCOO<sup>−</sup>

product selectivity (CO/HCOO<sup>−</sup>

posed for the electrocatalytic CO2

selectively afford HCOO−

**Figure 13.** Photochemical CO2

(PS)) and BNAH (an electron donor).

28 Carbon Dioxide Chemistry, Capture and Oil Recovery

photocatalytic CO2

HCOO<sup>−</sup>

concentration the selectivity of HCOO<sup>−</sup>

Heterogeneous catalysts are industrially important because they are useful for separating the starting materials and the products from the catalyst and can be recovered and reused. The molecular catalysts can be utilized to develop the heterogeneous catalysts. For photocatalysts of CO2 reduction, combining the molecular catalysts with semiconductor [32, 94, 95], metalorganic frameworks (MOFs) [96, 97] or periodic mesoporous organosilicas (PMOs) [98–101]

There would be many other problems to construct the artificial photosynthesis. However, the real system which can efficiently work has already existed in nature. We will realize it with a

Electrochemical/Photochemical CO2 Reduction Catalyzed by Transition Metal Complexes

http://dx.doi.org/10.5772/intechopen.75199

31

This work was supported by a Grant-in-Aid for Scientific Research (C) from the Ministry of Education, Culture, Sports, Science and Technology (17K05815). This work was also supported by the PRESTO Program of JST, and a Grant-in-Aid for Scientific Research on Innovative Areas, "Artificial Photosynthesis (AnApple)" (No. 15H00882), from the Japan Society for the

lot of ideas to overcome many problems one by one.

Address all correspondence to: ishida@sci.kitasato-u.ac.jp

Department of Chemistry, Graduate School of Science, Kitasato University, Japan

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[3] Mikkelsen M, Jorgensen M, Krebs FC. The teraton challenge. A review of fixation and transformation of carbon dioxide. Energy & Environmental Science. 2010;**3**:43-81. DOI:

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–

and the alcohol radicals. The

**Acknowledgements**

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[5] Schwarz HA, Dodson RW. Reduction potentials of CO<sup>2</sup>

**Author details**

Hitoshi Ishida

**References**

**Figure 15.** Photocatalytic CO2 reduction by periodic mesoporous organosilica (PMO) containing two different ruthenium complexes as photosensitizing and catalytic sites.

are actively researched. We have also developed a novel PMO consisting of 2,2′-bipyridyl framework by introducing two different ruthenium complexes as a photosensitizing site (Ru(PS)) and a catalytic site (Ru(Cat)) as shown in **Figure 15** [99]. Photochemical CO2 reduction by the PMO catalyst has catalytically produced CO and formate. The product selectivity (CO/formate) becomes large with increasing the ratio of Ru(PS) to Ru(Cat) (x/y). The photocatalysts can be recycled at least three times without losing the catalytic activity, demonstrating that the Ru(PS) and Ru(Cat) units are strongly immobilized on the BPy-PMO framework.

### **4. Future prospects**

The molecular catalysts are applicable to various photocatalytic systems. Ultimately, our goal is to construct an artificial photosynthetic system. An example is shown in **Figure 16**. In the system, the electrons are not supplied from the sacrificial electron donor but from water which is the same as in natural photosynthetic system. As the CO<sup>2</sup> reduction requires a high potential, two photosensitizing systems would be combined as the Z-scheme mechanism in the natural photosynthesis. In order to realize the artificial photosynthesis, we have to overcome some problems. One is to perform these reactions (water oxidation, photo-induced electron transfer and CO2 reduction, etc.) under the similar conditions or in the separated circumstances. Another is to match the velocities among the reactions; even if the efficient catalyst for CO2 reduction was obtained, the speeds for the water oxidation and the electron supply have to match with that of CO2 reduction.

**Figure 16.** A schematic drawing for an artificial photosynthetic system.

There would be many other problems to construct the artificial photosynthesis. However, the real system which can efficiently work has already existed in nature. We will realize it with a lot of ideas to overcome many problems one by one.
