Photocatalytic Approach for CO₂ Fixation

Abstract

In order to address the depletion of fossil fuels and the serious environmental problems accompanying their combustion and the concomitant CO₂ emission, large-scale chemical conversion of CO₂ into energy-rich materials would be an ultimate solution, and several reactions have been proposed. There have been a lot of challenges that have to be addressed in this field of research, but several breakthroughs have been achieved in recent 10 years. In this chapter, photocatalytic CO₂ reduction systems, which are of particular importance, are reviewed, with a focus on both homogeneous and heterogeneous aspects.

1 Introduction

1.1 Research Background

Photocatalytic CO₂ fixation into energy-rich chemicals such as carbon monoxide and formic acid has attracted attention in order to address the depletion of carbon resources and the suppression of global warming as well as to accomplish the CO₂-reduction half cycle in artificial photosynthesis [1, 2]. Because CO₂ is a very stable molecule, the reduction of CO₂ remains a big challenge; it requires a high potential to electrochemically reduce CO₂ by one electron (–1.9 V vs. NHE at pH 7). On the other hand, the required potential for CO₂ reduction can be reduced if one utilizes multi-electron process. For example, the potentials of two-electron reduction of CO₂ into formic acid and CO are –0.61 and –0.53 V (vs. NHE at pH 7), respectively, approximately 1.3 V more positive than the potential required to drive one-electron reduction of CO₂.

$$\begin{aligned} & \text{CO}_{2} + 2\text{H}^{ + } + 2\text{e}^{ - } \to \text{HCOOH}\quad E^{ \circ } = - 0.61{\text{V}} \\ & \text{CO}_{2} + 2\text{H}^{ + } + 2\text{e}^{ - } \to \text{CO + H}_{2} \text{O}\quad E^{ \circ } = - 0.53{\text{V}} \\ & \text{CO}_{2} + 4\text{H}^{ + } + 4\text{e}^{ - } \to \text{HCHO + H}_{2} \text{O}\quad E^{ \circ } = - 0.48{\text{V}} \\ & \text{CO}_{2} + 6\text{H}^{ + } + 6\text{e}^{ - } \to \text{CH}_{3} \text{OH + H}_{2} \text{O}\quad E^{ \circ } = - 0.38{\text{V}} \\ & \text{CO}_{2} + 8\text{H}^{ + } + 8\text{e}^{ - } \to \text{CH}_{4} \text{ + 2H}_{2} \text{O}\quad E^{ \circ } = - 0.24{\text{V}} \\ \end{aligned}$$

Thus, a catalyst that allows one to promote multi-electron transfer is needed. As described below, certain metal complexes and semiconductors (as well as their composites) work as such (photo)catalysts.

Since the seminal work by Lehn et al. who demonstrated selective CO₂ reduction into CO using Re(I) diimine complexes in 1983 [3], photocatalytic CO₂ reduction has been extensively studied for the purpose of light-to-chemical energy conversion. However, a satisfactory system has yet to be devised to date. As the goal of CO₂ fixation by a photocatalyst is to convert solar energy into chemical energy on a large-scale, a given system has to meet the following requirements.

• (1) A given CO₂ fixation system has to be workable under sunlight having low energy density, and to be stable and efficient during long-term operation.

• (2) CO₂ reduction has to be promoted while suppressing any side reaction that can occur during the reaction (e.g., H₂ reduction via water reduction).

• (3) Water should be used as an electron source.

Unfortunately, no photocatalytic system that satisfies all of the above requirements has been reported to date. Besides, energy conversion scheme via CO₂ reduction where the change in Gibbs energy is positive had not been achieved until very recently. Nevertheless, there were some breakthroughs especially in the last 10 years. This chapter describes recent progress on photocatalytic CO₂ reduction using metal complexes and semiconductors, along with some important early works.

1.2 Some Important Aspects in Photocatalytic CO₂ Fixation Research

One should be careful when reading a literature on photocatalytic CO₂ reduction, as some may be problematic. For example, even though a given CO₂ fixation system is claimed to work in water, quantification of O₂, which is the product of water oxidation, is ambiguously or not described. Besides, it is extremely important in any of photocatalytic CO₂ reduction research to investigate the origin of carbon-containing products by isotope tracer experiments with ¹³CO₂, because contaminated carbon species may become such materials upon photo-irradiation especially in a heterogeneous system. For example, using a commercially available titania loaded with nanoparticulate metal promoters as a heterogeneous photocatalyst for CO₂ reduction, it has been revealed that both CO and CH₄ were obtained, but the contribution of surface contaminants to the reaction products cannot be neglected [4]. High turnover number with respect to the amount of catalytically active sites or the photocatalyst itself is also an important indicator to judge whether or not a given reaction photocatalyzes CO₂ reduction. In this section, the author would like to focus on representative, but “reliable” systems in this regard, which are considered important milestones in photocatalytic CO₂ reduction research.

2 Metal Complexes

2.1 Re Diimine Tricarbonyl Type Complexes

In 1983, Lehn et al. reported that fac-[Re(N^N)(CO)₃X]⁺ (N^N = diimine ligand; X = Cl^–, Br^–) not only works as an efficient CO₂ reduction photocatalyst but also as a catalyst (Scheme 1). These Re(I) complexes selectively produce CO in a dimethylformamide (DMF) /triethanolamine (TEOA) solution. It is noted that even in the presence of water, certain Re(I) complexes are capable of selectively producing CO without noticeable H₂ formation [3].

Scheme 1

Photocatalytic CO₂ reduction into CO on fac-[Re(bpy)CO₃Cl]⁺ in a mixed solution of DMF and TEOA

Follow-up studies have been made to create more active photocatalysts. The chloro ligand on fac-Re(bpy)(CO)₃Cl (bpy = 2,2′-bipyridine) undergoes substitution with various phosphorus ligands, PR₃ (R = alkyl, alkoxy, or allyl group), giving fac-[Re(bpy)(CO)₃(PR₃)]⁺ [5]. The photocatalytic activity for CO₂ reduction was found to depend strongly on the phosphorus ligand [6]. For example, fac-[Re(bpy)(CO)₃{P(OEt)₃}]⁺ is an efficient photocatalyst for CO formation with a quantum yield (QY) of 38 % at 365 nm [7], which is twice as large compared with that with fac-Re(bpy)(CO)₃Cl.

The reaction mechanism of photocatalytic CO₂ reduction in a homogeneous system was investigated by Ishitani et al. in detail using three different Re(I) diimine complexes, fac-[Re(bpy)(CO)₃L] (L = SCN⁻ (1–NCS), Cl⁻ (1–Cl), and CN⁻ (1–CN)) [8]. The reaction scheme is given in Scheme 2. It has been revealed by means of laser spectroscopy that the initial step of the reaction is the reductive quenching of the triplet metal-to-ligand charge transfer (³MLCT) excited-state of the Re complex by TEOA, generating one-electron reduced (OER) species of the rhenium complex [Re^I(N^N^•−)(CO)₃X]⁻ [9–11]. The corresponding OER species of these complexes play two important roles of capturing CO₂ after loss of the monodentate ligand (L) and of donating the second electron to CO₂ by another OER species without losing L. In the case of 1–NCS, the corresponding OER species play these two roles in the photocatalytic reaction, resulting in more efficient CO evolution (30 % QY) than that of 1–Cl (16 % QY), whose OER species are too short-loved to accumulate during the photocatalytic reaction. On the other hand, 1–CN showed no photocatalytic ability, because the corresponding OER species does not dissociate the CN- ligand. Based on this mechanistic information, the most efficient photocatalytic system was successfully developed using a mixture of fac-[Re(bpy)(CO)₃(CH₃CN)]⁺ and fac-[Re{4,4′-(MeO)₂bpy}(CO)₃{P(OEt)₃}]⁺, which respectively work as a catalyst and a redox sensitizer. The QY of this system was 59 % at the optimal condition.

Scheme 2

Photocatalytic reaction mechanism by 1–NCS. (Reproduced with permission from Ref. [8]. Copyright 2008, American Chemical Society)

Based on the mechanistic study, Ishitani et al. have proposed strategies to develop a highly efficient CO₂ reduction photocatalyst.

• (1) Efficient formation of OER species by quenching of ³MLCT excited-state by an electron donor.

• (2) Effective production of [Re(LL^•−)(CO)₃] by dissociation of the ligand from the OER species.

• (3) Efficient reduction of CO₂ adduct(s) by another OER species.

• (4) Prompt recovery of the starting complex by re-coordination of a ligand after CO formation.

A highly efficient CO₂ reduction has thus been established. However, there still remained unclear point(s) on the reaction mechanism, especially a CO₂ addition step to the Re center. So far, 17-electron species, i.e., [Re⁰(N^N)(CO)₃] and/or [Re^I(N^N^•−)(CO)₃] [3, 12–14] and a Re dimer with CO₂ as a bridge ligand [14, 15] have been proposed as possible intermediates derived from reduced Re complexes. However, no clear evidence had been obtained to clarify the mechanism. Another important aspect in photocatalytic CO₂ reduction on Re complexes is that TEOA works as a special electron donor , which enhances catalytic turnover number and selectivity of CO production, compared with other reductants such as triethylamine [16], suggesting a special action of TEOA during the reaction.

Ishitani et al. pointed out that fac-[Re^I(bpy)(CO)₃{R₂N–CH₂CH₂O–COO}] (R = CH₂CH₂OH) could be a predominant complex in various photocatalytic CO₂ reduction reactions using [Re^I(N^N)(CO)₃X]ⁿ⁺ (X = monodentate ligand; n = 0, 1) type complexes in a DMF–TEOA mixed solution (Scheme 3) [17]. A DMF-coordinated complex, fac-[Re^I(bpy)(CO)₃(DMF)]⁺ (1), underwent transformation upon addition of TEOA to generate fac-[Re^I(bpy)(CO)₃(OCH₂CH₂NR₂)] (1′) with an equilibrium constant of 19. Further exposure of 1′ to CO₂ resulted in the generation of 2, whereas a similar CO₂ treatment of 1 did not. The equilibrium constant between 1′ and 2 (K =  [2] /[1′][CO₂]) in a CO₂ atmosphere was estimated to be 1.7 × 10³ M⁻¹, which means the exclusive formation of 2 in the CO₂-saturated mixed solution of DMF and TEOA. This also suggests that even a very low concentration of CO₂ is enough to produce the CO₂−TEOA adduct. Electrochemical analysis indicated the superior CO₂ reduction ability of 2 to 1. On the basis of these observations, it was claimed that 2 should be the catalytically active species in many photocatalytic CO₂ reduction systems that have been reported so far, because TEOA was used as an electron donor in these reported systems.

Scheme 3

Structural change of fac-[Re(bpy)CO₃DMF]⁺ in the presence of TEOA and CO₂. (Reproduced with permission from Ref. [17]. Copyright 2013, American Chemical Society)

2.2 Supramolecular Metal Complexes

As introduced above, Re(I) diimine complexes work as photocatalysts and electrocatalysts for CO₂ reduction. However, there are several problems in these Re-based complexes including (1) insufficient visible-light-absorption, (2) low stability, and (3) reliance on a strong electron donor. A strategy to address the problems (1) and (2) is to couple a catalytic metal complex with a redox photosensitizer so as to improve the electron transfer process from the excited-state sensitizer to the catalytic unit.

Ishitani et al. prepared a series of Ru(II)−Re(I) binuclear complexes (see Fig. 1), and examined their photocatalytic activities [18]. As shown in Fig. 1, improved photocatalytic activity was obtained with the binuclear complex [d ₂ Ru-Re] ²⁺, compared to either the corresponding Ru or Re unit alone and a physical mixture of the Ru and Re unit. This is attributed to improved intramolecular electron transfer from the OER species, which were produced following the selective excitation and subsequent reductive quenching of the Ru ³MLCT excited state, to the catalytic Re moiety. It is also noted that photocatalytic responses were extended further into the visible region by applying a Ru moiety as the sensitizing unit.

Fig. 1

Photocatalytic activities of supramolecular metal complexes (0.05 mM) for CO formation in a CO₂-saturated DMF–TEOA (5:1 v/v) solution containing 0.1 M 1-Benzyl-1,4-dihydronicotinamide (BNAH) as a sacrificial electron donor under selective photoexcitation of the Ru moieties (λ > 500 nm). (Reproduced with permission from Ref. [18]. Copyright 2005, American Chemical Society)

Another important information obtained from this work is that the photocatalytic abilities of the binuclear complexes with a 4-methyl-4′- [1, 10] phenanthroline-[5,6-d]imidazol-2-yl)bipyridine (abbreviated as BL) bridging ligand, i.e., [Ru-BL-Re] ²⁺ and [Re-BL-Ru] ²⁺, were much lower than that containing a bpyC₃bpy bridging ligand ([d ₂ Ru-Re] ²⁺). When (CF₃)₂bpy or bpy were used as peripheral ligands, the binuclear complexes also had poor photocatalytic ability, giving a turnover numbers (TNs) for CO formation of 3 and 50, respectively, for [tfbRu-Re] ²⁺ and [b ₂ Ru-Re] ²⁺ (data not plotted in Fig. 1). In these metal complexes that showed poorer photocatalytic activity, the intramolecular electron transfer was found to be endothermic, hindering the forward electron transfer and leading to inferior performance. In addition to the endothermic character in the intramolecular electron transfer event, it appears that electron localization on the bridging ligand appears to be another decisive factor. One can assume that, electrons in [Ru-BL-Re] ²⁺ are mainly localized on the Ru end of the bridging ligand, because the energy level of the π* orbital on the phenanthroline-imidazolyl motif of BL is lower than that on the bpy one coordinating to the Re unit. The low electron density on the catalytic Re site in the OER species could account for the low photocatalytic activity of [Ru-BL-Re] ²⁺.

In the case of [Re-BL-Ru] ²⁺, in contrast, electron localization must occur on the Re site, which is supposed to contribute to higher photocatalytic activity. It has been reported that photocatalytic CO₂ reduction activities of mononuclear complexes of fac-[Re(N^N)(CO)₃(PR₃)]⁺ (R = an alkyl group) depend strongly on their reduction potentials, E ^red_1/2 (LL/LL^•–), and efficient photocatalytic reduction of CO₂ requires E ^red_1/2 (LL/LL^•–) < –1.41 V vs. Ag/AgNO₃ [6]. In view of the wide conjugation of BL and the strong electronic interaction across the bridging ligand, the reduction ability of the OER species (E ^red_1/2  = –1.1 V vs. Ag/AgNO₃) should be insufficient for prompt reduction of CO₂ under these conditions.

According to the same strategy, they also recently developed photocatalytic CO₂ reduction systems using various multinuclear Ru complexes, which selectively produce HCOOH with high turnover numbers (~671) under visible light (λ = 480 nm) [19]. This kind of supramolecular metal complexes has been shown to be applicable to a Z-scheme CO₂ reduction system, in combination with a suitable semiconductor material, as will be discussed later.

2.3 Metal Complex Catalysts Based on Earth-Abundant Elements

Most of the metal complexes that have been developed to date for CO₂ reduction are comprised of Re- and Ru-based ones. In order to replace such precious metals from the sustainable chemistry point of view, an earth-abundant material that is sufficiently active, and alternative to precious metals, is highly desirable. Bourrez et al. reported that a manganese -based complex exhibited electrocatalytic activity for CO₂ reduction to CO, although the TON was moderate (TON = 13 for 4 h) [20]. Ishitani et al. used the same Mn complex as a catalyst, in combination with a ruthenium (II) tris-diimine complex as a redox sensitizer, to achieve CO₂ reduction into HCOOH in the presence of BNAH as an electron donor [21]. The QY of HCOOH formation by this Mn complex was 5.9 %, which is comparable to that achieved by a similar Re complex (6.9 %).

2.4 Enhanced Photocatalytic Activity of Rhenium(I) Complex by Light-Harvesting Periodic Mesoporous Organosilica

As briefly mentioned above, sunlight is a very “dilute” energy source (ca. 10 photons nm⁻² s⁻¹) especially for small molecules such as metal complexes. Therefore, light-harvesting would be highly desirable for efficient photocatalysis by a homogeneous metal complex. In nature, a wheel-like array of chlorophylls in LH1 and LH2 of purple photosynthetic bacteria efficiently absorbs sunlight, funneling the captured energy to a reaction center by resonance energy transfer (RET) with a QY of almost unity [22]. For the construction of an artificial photosynthetic system, the three-dimensional organization of molecular parts, that is, light absorbers and multi-electron catalysts, at appropriate positions is of particular importance, because the RET efficiency is strongly dependent on the distance between the energy donor and acceptor molecules and their orientation [23].

Inagaki et al. employed a mesoporous biphenyl-silica (Bp-PMO) anchoring fac-[Re^I(bpy)(CO)₃(PPh₃)]⁺(OTf)^– (OTf = CF₃SO₃) in the mesochannels for CO₂ reduction [24]. The structure of this hybrid material is depicted in Fig. 2. The incident photons (λ = 280 nm) were effectively absorbed by the biphenyl groups in Bp-PMO, and the excited energy was funneled into the Re complex by RET. As the result, photocatalytic CO evolution from CO₂ was enhanced by a factor of 4.4, compared with direct excitation of the Re complex. In addition, Bp-PMO helped to protect the Re complex against photo-decomposition. These results demonstrate the potential of PMOs as a light-harvesting antenna for designing various photoreaction systems, mimicking the natural photosynthesis.

Fig. 2

Schematic representation of light-harvesting by PMO and enhancing of photocatalysis of Re complex. (Reprinted with permission from Ref. [24]. Copyright 2010, American Chemical Society)

3 Semiconductors

As described above, certain metal complexes catalyze CO₂ reduction to CO or HCOOH photocatalytically or electrochemically with high selectivity and QYs in a homogeneous system. From the viewpoint of large-scale application and efficient solar energy utilization, however, semiconductor-based heterogeneous photocatalysts would be more advantageous over molecular-based homogeneous catalysts, considering their superior oxidation ability to utilize a mild reductant (ideally, water) and potential recyclability [25]. In this section, several kinds of heterogeneous photocatalysts based on inorganic semiconductors for CO₂ reduction are described.

3.1 CO₂ Reduction on Wide Gap Semiconductors in the Presence of CH₄ or H₂

Tanaka et al. have developed several wide-gap semiconductors (some of them may be mentioned as insulators) such as ZrO₂ and MgO for CO₂ reduction into CO in the presence of CH₄ or H₂ gas as a reductant [26–28]. Their results indicated that CO₂ adsorbed on ZrO₂ and MgO, which are solid base oxides, is reduced to formate species by gas phase H₂. Importantly, the stable linear form of CO₂ transforms into a reactive species upon adsorption, which are more susceptible to reduction than the linear form.

Their group also reported that β-Ga₂O₃ having a band gap of ca. 4.5 eV showed higher activity for photocatalytic CO₂ reduction into CO in the presence of H₂ than other solid bases such as MgO [29]. This reaction is subject to a Langmuir-Hinshelwood type mechanism, where the monodentate bicarbonate species was reduced by the dissociatively adsorbed hydrogen, thereby generating bidentate formate on β-Ga₂O₃ that was finally decomposed into CO under photo-irradiation (Scheme 4). Despite the inferior affinity of β-Ga₂O₃ with CO₂, the dissociatively adsorbed H₂ on β-Ga₂O₃ was responsible for the higher photocatalytic activity.

Scheme 4

Mechanism of photocatalytic reduction of CO₂ over β-Ga₂O₃ in the presence of H₂. (Reprinted with permission from Ref. [29]. Copyright 2010, American Chemical Society)

3.2 Semiconductor Photocatalysts Workable in Water

As described above, some of wide-gap semiconductor oxides (or insulators) have been shown to exhibit activity for CO₂ reduction into CO in the presence of CH₄ or H₂ as an electron donor. However, a semiconductor photocatalyst that is capable of reducing CO₂ using water as an electron source and of producing stoichiometric amount of O₂ had not been reported until recently.

In 2011, Kudo et al. reported that ALa₄Ti₄O₁₅ (A = Ca, Sr, and Ba) layered perovskites having 3.79–3.85 eV band gaps showed photocatalytic activity for CO₂ reduction to form CO and water oxidation into O₂ [30]. ALa₄Ti₄O₁₅ (A = Ca, Sr, and Ba) has been originally developed as a photocatalyst for overall water splitting into H₂ and O₂ under UV irradiation [31]. Table 1 summarizes the photocatalytic activities of ALa₄Ti₄O₁₅ (A = Ca, Sr, and Ba) for CO₂ reduction in water. While the CO₂ reduction activity was negligible in the absence of a cocatalyst, the activity could be enhanced by modification with Ag cocatalysts. Among three semiconductors tested, the Ba derivative was found to exhibit the highest performance. Under Ar bubbling condition, water splitting reaction proceeds on Ag/BaLa₄Ti₄O₁₅, but continuous CO₂ bubbling of the reactant solution allowed one to proceed CO₂ reduction into CO and HCOOH as the major and minor product, respectively. Although H₂ evolution via overall water splitting could not be suppressed completely, modification of BaLa₄Ti₄O₁₅ with ~10 nm Ag nanoparticles, which was achieved by employing the liquid-phase reduction method, improved the selectivity for CO₂ reduction while suppressing the undesirable H₂ evolution.

Table 1 Photocatalytic activities for CO₂ reduction over ALa₄Ti₄O₁₅ (A = Ca, Sr, and Ba) modified with Ag cocatalyst under UV irradiation (λ > 200 nm)^a

Even under CO₂ bubbling, cocatalysts of NiO _x , Ru, Cu, and Au did not achieve any appreciable CO₂ conversion, but promoted overall water splitting. Ag is known to be an efficient electrocatalyst for CO₂ reduction, but has relatively large overpotential for H₂ evolution [32]. This is a preferable feature for use as a cocatalyst for CO₂ reduction on a semiconductor photocatalyst, and appears to contribute to the superior performance of Ag-loaded material for CO₂ fixation. Kudo et al. also pointed out that continuous bubbling of the reactant suspension with CO₂ was important to get more CO, suggesting the occurrence of some backward reactions. It should be noted that in the optimal condition, the ratio of reduction/oxidation products {(CO + HCOOH + H₂)/O₂} was almost equal to 2/1, consistent with the reaction stoichiometry. The stoichiometric evolution of O₂ clearly indicated that water was consumed as a reducing reagent (an electron donor) for the CO₂ reduction. Thus, an uphill reaction of CO₂ reduction accompanied with water oxidation was achieved using the Ag/BaLa₄Ti₄O₁₅ photocatalyst.

Teramura et al. have developed layered double hydroxides (LDHs; [M ²⁺_1–x M ³⁺ _x (OH)₂]^x+(A^n–) _x/n mH₂O) as new heterogeneous photocatalysts for CO₂ reduction workable in water under UV irradiation (λ > 200 nm) [33]. LDHs are natural or synthetic clays that consist of brucite (Mg(OH)₂)-like positively charged two-dimensional sheets interleaved with anionic species (A^n–) such as CO₃ ^2– for charge compensation, in which some divalent cations such as Mg²⁺ are substituted by trivalent cations. Although the reaction products did not meet the stoichiometry (in most cases, excess O₂ evolution was observed) and the reason still remains unclear, several kinds of LDHs showed activity for CO and O₂ evolution in water, as shown in Fig. 3. As exemplified by Mg-In LDH, interestingly, neither Mg- nor In-hydroxide gave CO or O₂ from water in the presence of CO₂, but the combination of the two metals to make a Mg-In LDH resulted in clearly observable CO and O₂ evolution. It indicates the importance of the formation of LDH structure for driving CO₂ reduction in water. They also conducted isotope tracer experiments with ¹³CO₂, which indicated that the main source of CO generated was CO₂ molecules in gas phase, but some residual CO₃ ^2– ions in the interlayer became the source of CO.

Fig. 3

Yields of O₂ and CO for the photocatalytic conversion of CO₂ in water, over various LDHs (M²⁺/M³⁺ = 3) after 10 h of photo-irradiation (λ > 200 nm). (Reprinted with permission from Ref. [33]. Copyright 2011, Wiley–VCH Verlag GmbH and Co. KGaA)

4 Metal-Complex/Semiconductor Hybrid Photocatalysts

4.1 The Proof-of-Concept

As introduced above, certain metal complexes based on rhenium or ruthenium catalyze CO₂ reduction to CO or HCOOH (photo)catalytically with high selectivity and QYs. However, the oxidation ability of these metal complexes is in general too low to oxidize water. By contrast, while the selectivity for CO₂ reduction is not very high, the stability of semiconductors for oxidation reactions is attractive.

On the basis of these backgrounds, one can simply draw a composite material, as shown in Scheme 5, consisting of a light-absorbing semiconductor and a catalytic metal complex, which shows high performance both for water oxidation and CO₂ reduction. In 2010, Morikawa et al. reported a proof-of-concept of this kind of a metal-complex/semiconductor hybrid photocatalyst to reduce CO₂ into formic acid [34]. They used a p-type semiconductor, N-doped Ta₂O₅ (energy gap = ca. 2.6 eV), and ruthenium(II) complexes [Ru(bpy)₂(CO)₂]²⁺, [Ru(dcbpy)(bpy)(CO)₂]²⁺ or [Ru(dcbpy)₂(CO)₂]²⁺ (dcbpy: 4,4′-dicarboxy-2,2′-bipyridine; bpy: 2,2′-bipyridine). [Ru(dcbpy)₂(CO)₂]²⁺ is an electrocatalyst for CO₂ reduction into HCOOH [35]. Under irradiation of the composite with visible light in a mixed solution of acetonitrile (MeCN) and TEOA, electrons and holes are generated in the conduction and valence band of N-Ta₂O₅, respectively. Here, the adsorbed metal complexes do not essentially harvest the incident photons. The conduction band electrons move to the adsorbed Ru complex, thereby reducing CO₂ into HCOOH. On the other hand, holes left behind the valence band are consumed by oxidation of TEOA. The selectivity of HCOOH production was more than 75 % before the turnover number underwent saturation. Unfortunately, however, N-Ta₂O₅ is unable to oxidize water into molecular O₂ because the valence band potential is more negative than the water oxidation potential. Therefore, this system is not applicable to an artificial photosynthetic assembly using water as an electron source.

Scheme 5

Photocatalytic CO₂ reduction and water oxidation on a metal complex/semiconductor hybrid. V.B. and C.B. indicate the valence and conduction band of a semiconductor, respectively

4.2 Metal-Complex/Polymeric Semiconductor Hybrid

As an alternative semiconductor material that has stronger oxidation ability, Maeda et al. focused on carbon nitride polymers [36, 37]. Carbon nitride is an earth-abundant polymer semiconductor photocatalyst, which has recently been developed for water splitting with visible light by the same group [38, 39]. It has several proposed allotropes with diverse properties, but the graphitic phase is regarded as the most stable under ambient conditions. Graphitic carbon nitride is yellow powder with high chemical stability both in acid and base, exhibiting a steep absorption edge at around 450 nm and a tail extending to 600 nm. Importantly, the material shows photocatalytic activity for water oxidation, in contrast to nitrogen doped Ta₂O₅. However, there had been no reliable report on photocatalytic CO₂ reduction using C₃N₄ as a photocatalyst until very recently.

Mesoporous graphitic carbon nitride (mpg-C₃N₄) polymers with a ruthenium complex, cis, trans-[Ru{4,4′-(CH₂PO₃H₂)₂-2,2′-bipyridine}(CO)₂Cl₂] (abbreviated Ru for simplicity), that works a catalyst for CO₂ reduction were combined together to create a new CO₂ reduction photo-assembly [36]. Table 2 summarizes CO₂ reduction activities, which were tested in a MeCN–TEOA mixture (4:1 v/v) under >400 nm irradiation. Mpg-C₃N₄ alone did not show any activity for CO₂ reduction. However, combining mpg-C₃N₄ with Ru resulted in the production of HCOOH and CO as CO₂ reduction products, with H₂ as a byproduct. Under optimal condition, turnover number with respect to the adsorbed Ru exceeded 200 after 20 h of visible light irradiation, with selectivity of formic acid production of higher than 80 %. These numbers are higher than those recorded by the previous report using nitrogen doped Ta₂O₅. Control experiments showed that using an insulator, alumina, instead of carbon nitride, did not give any products. Nothing happened without catalyst sample as well. When the reaction was conducted under argon atmosphere, no carbon-containing product was obtained, evolving H₂ alternatively. Without TEOA, the amounts of produced HCOOH and CO became very low.

Table 2 Photocatalytic activities for CO₂ reduction over Ru/mpg-C₃N₄ in a CO₂-saturated MeCN⁻–TEOA mixed solution under visible light (λ > 400 nm)^a

Figure 4 shows an action spectrum of formic acid production on Ru/mpg-C₃N₄. The apparent quantum yield (AQY) decreased with increasing the wavelength of incident light, and reached zero at 550 nm. This change in AQY corresponds to the light-absorption profile of carbon nitride, which is shown by red curve. It clearly means that the formic acid production originates from light absorption by carbon nitride. Because Ru/mpg-C₃N₄ photocatalyst consists of large amount of carbon, isotope tracer experiments were conducted using ¹³CO₂ as the reactant. Interestingly, the main product of HCOOH was found to originate solemnly from CO₂, not the decomposition of C₃N₄ component. Based on these results, it was concluded that Ru and carbon nitride work as CO₂ reduction catalyst and light-absorber, respectively. Also, TEOA is an electron donor to scavenge holes in the valence band of carbon nitride, and a proton source.

Fig. 4

Schematic illustration of photocatalytic CO₂ reduction into HCOOH on Ru/mpg-C₃N₄ under visible light, along with the action spectrum. (Reproduced with permission from Ref. [36]. Copyright 2013, The Royal Society of Chemistry)

In contrast to inorganic semiconductors, it is possible to control both bulk and surface properties of C₃N₄ based on an organic chemistry protocol [40], thereby modulating the band-gap structure and introducing a desired organic moiety that anchors a metal-complex catalyst. Besides, C₃N₄ is structurally flexible, exhibiting various shapes with the aid of a hard template such as silica during the synthesis [41]. Maeda et al. have also reported that the activity of Ru/C₃N₄ for the CO₂ reduction reaction is sensitive to specific surface area and crystallinity of carbon nitride, but is largely insensitive to the pore size and the volume [37].

4.3 Artificial Z-Scheme

Very recently, Ishitani et al. developed a new type of photocatalytic CO₂ fixation system using Ag-loaded TaON semiconductor and a Ru(II) binuclear complex, which works according to Z-scheme principle somewhat similar to natural photosynthesis in green plants [42]. As illustrated in Scheme 6, two components of TaON and light-harvesting Ru unit both undergo photoexcitation upon visible light in the initial step. The photogenerated hole in the TaON valence band oxidizes methanol, and the conduction band electrons move to the excited or oxidized photosensitizer unit, but cannot be transferred to the ground state. An OER species generated as a result of the interfacial electron transfer is consumed by intramolecular electron transfer, which is thermodynamically down-hill, finally reducing CO₂ into HCOOH on the catalytic Ru unit. Because formic acid production from CO₂ involves a two-electron reduction, the stepwise two-photon absorption and subsequent electron transfer processes would occur twice during the reduction of CO₂ to give one HCOOH molecule. It should be noted that the whole reaction is energetically up-hill, involving a positive change in the Gibbs energy of 83.0 kJ mol⁻¹. Isotope tracer experiments indicated that this hybrid material photocatalytically produced formic acid as the major reduction product and formaldehyde as the oxidation product from CO₂ and methanol, respectively. Under visible light (λ > 400 nm), both Ag/TaON and the sensitizer unit in the supramolecular complex undergo photoexcitation. The conduction band electrons in Ag/TaON migrate to the excited state or oxidized sensitizer unit, producing a one-electron reduced species. Subsequent intermolecular electron transfer occurs from the one-electron- reduced species in the photosensitizer unit to the catalyst unit, as it is a thermodynamically downhill process. Finally, holes left in the valence band of Ag/TaON oxidize methanol to give formaldehyde, whereas electrons transferred to the catalyst unit reduce CO₂ into formic acid. Since the CO₂ reduction to give formic acid is a two-electron process, the stepwise two-photon absorption and the subsequent electron transfer events are likely to occur twice during the reaction to give one HCOOH molecule.

Scheme 6

Artificial Z-scheme for photocatalytic CO₂ reduction using Ag-loaded TaON semiconductor and a Ru(II) binuclear complex (RuBLRu′). (Reproduced with permission from Ref. [42]. Copyright 2013, American Chemical Society)

Currently, the main problem of this Z-scheme system includes competitive H₂ evolution that lowers the selectivity of CO₂ reduction, significant electron–hole recombination in the TaON component [43], and possible back electron transfer from the excited-state photosensitizing unit to Ag/TaON, which is thermodynamically a down-hill process. To address these problems, refinement of preparation condition of TaON as well as proper design of the metal complex component to maximize the forward electron transfer rate is required. In addition, role(s) of Ag deposits on TaON need to be clarified, as the efficiency of this system becomes very low in the absence of the Ag modification.

5 Summary and Future Outlook

In this chapter, photocatalytic CO₂ reduction both in homogeneous and heterogeneous systems are reviewed. Certain metal complexes consisting rhenium, ruthenium, or manganese are shown to work as efficient (photo)catalysts for CO₂ reduction into CO or HCOOH with high quantum yields and selectivity. Unfortunately, however, no metal complex that is capable of oxidizing water to drive CO₂ reduction has been reported so far. On the other hand, semiconductor photocatalysts having high photooxidation ability could be applicable to a CO₂ reduction system that should utilize water as an electron source. For example, ALa₄Ti₄O₁₅ (A = Ca, Sr, and Ba) modified with Ag nanoparticles exhibit activity for CO₂ reduction and water oxidation under band-gap irradiation. A suitable combination between a metal complex and a semiconductor led to the development of new visible-light CO₂ reduction systems. Importantly, some of them could work non-sacrificially, converting visible-light energy into chemical energy.