**Visible Light-Driven Water Oxidation Catalyzed by Ruthenium Complexes**

Markus D. Kärkäs, Tanja M. Laine, Eric V. Johnston and Björn Åkermark

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/62272

#### **Abstract**

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A shift in energy dependence from fossil fuels to sustainable and carbon-neutral alternatives is a daunting challenge that faces the human society. Light harvesting for the production of solar fuels has been extensively investigated as an attractive approach to clean and abundant energy. An essential component in solar energy conversion schemes is a catalyst for water oxidation. Ruthenium-based catalysts have received significant attention due to their ability to efficiently mediate the oxidation of water. In this context, the design of robust catalysts capable of driving water oxidation at low overpotential is a key challenge for realizing efficient visible light-driven water splitting. Herein, recent progress in the development within this field is presented with a focus on homogeneous ruthenium-based systems and surface-immobilized ruthenium assemblies for photo-induced oxidation of water.

**Keywords:** water splitting, ruthenium, photochemistry, sustainable chemistry

#### **1. Introduction**

The search for inexpensive and renewable energy is currently one of society's greatest techno‐ logical challenges. As light energy from the sun continuously strikes the earth's surface, harnessing this energy would solve the increasing future energy demand and lead to a more sustainable society. An appealing solution would therefore be to convert light energy to storable fuels, such as hydrogen gas or reduced carbon compounds. For realizing this scenario, novel technologies have to be developed that efficiently utilize solar energy. Furthermore, such systems also need to rely on abundant and inexpensive feedstocks in order to become viable on a large

© 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.

scale.Aswaterisplentiful,itwouldbeattractivetouseitasafeedstockforobtainingthenecessary reducing equivalents—protons and electrons [1–6].

The natural system constitutes an excellent source of inspiration for how to design an artificial system that is capable of harnessing solar energy for fuel production. The concept of artificial photosynthesis emerged in the 1970s and is inspired by nature where light-induced charge separation events sequentially oxidize a Mn4Ca cluster (Figure 1) known as the oxygenevolving complex (OEC) [7, 8]. After four electrons have been abstracted from the OEC, two molecules of water are oxidized to molecular oxygen, thus releasing four electrons and four protons. The natural photosynthetic apparatus subsequently utilizes the generated reducing equivalents to reduce CO2 to carbohydrates [9, 10]. However, instead of using the generated reducing equivalents to reduce CO2 to carbohydrates as in the natural photosynthetic appa‐ ratus, these "artificial leafs" would produce hydrogen gas from the protons and electrons that are liberated when water is oxidized (Eq. 1) [11, 12].

$$2\text{H}\_2\text{O} \rightarrow \text{O}\_2 + 2\text{H}\_2\tag{1}$$

**Figure 1.** Depiction of the Mn4Ca cubane core of the oxygen-evolving complex (OEC).

Water splitting can be divided into two half-reactions; proton reduction and water oxidation. The reductive side of water splitting involves the generation of hydrogen gas from the generated protons and electrons. In contrast to hydrocarbons, hydrogen gas is considered to be environmentally benign, as water is the only combustion product. Although deceptively simple, the other half-reaction, water oxidation (Eq. 2), is a mechanistically complex process and is currently considered as the bottleneck. The oxidation of water thus requires a single catalytic entity capable of accumulating four oxidizing equivalents, breaking several bonds and forming the crucial O–O bond. Splitting of water is an energy demanding process with a Gibbs free energy of 237.18 kJ mol–1 and a minimum electrochemical potential of 1.229 V vs. normal hydrogen electrode (NHE) is required. The basic thermodynamic requirements for splitting water suggest that any light with a wavelength shorter than 1 mm has enough energy to split a molecule of water. Consequently, this allows the use of the entire visible solar spectrum and a majority of the near-infrared spectrum, which collectively constitutes ~80% of the total solar irradiance [13].

$$\text{2H}\_2\text{O} \rightarrow \text{O}\_2 + 4\text{H}^+ + 4\text{e}^- \tag{2}$$

The first example of photoelectrochemical water splitting was reported by Fujishima and Honda in the early 1970s. Their system consisted of a titanium dioxide (TiO2) photoanode which upon irradiation with ultraviolet (UV) light generated oxygen at the anode and hydrogen gas at an unilluminated platinum cathode [14]. Since the seminal work by Fujishima and Honda, several research groups have attempted to improve the system in order to enable the reaction to be driven by visible light instead of UV light [15, 16].

A simple depiction of an artificial photosynthetic system is shown in Figure 2 and consists of three components: a chromophore (photosensitizer) for light-absorption, a water oxidation catalyst (WOC), and a reduction catalyst for proton reduction. The light-absorbing component, the molecular chromophore, is in general coordinated to the surface of a semiconductor, such as TiO2. The initial step in such a system involves light absorption by the photosensitizer, generating a long-lived charge-separated state by transferring an electron to the conduction band of the semiconductor. The oxidized photosensitizer subsequently recovers an electron from the covalently bound oxidation catalyst (the WOC) or from the functionalized semicon‐ ductor surface to regenerate the ground state photosensitizer. After four successive electron transfers, the highly oxidized WOC is reduced by oxidizing two molecules of water, thus releasing molecular oxygen. Although the events seem trivial, the overall process of lightdriven water splitting requires interfacing of several nontrivial chemical steps such as accumulation and abstraction of several electrons at the reduction and oxidation catalyst, respectively. This requires the integration of efficient light absorption, generation of long-lived charge separation, organized proton reduction at the cathode, and fast oxidation of water at the anode [17–19].

**Figure 2.** Three-component molecular assembly for water splitting consisting of a photosensitizer (PS), a water oxida‐ tion catalyst (WOC), and a hydrogen-evolving catalyst (HEC). Reprinted with permission from ref. 29. Copyright 2014 American Chemical Society.

#### **2. Ruthenium-Based Photosensitizers**

scale.Aswaterisplentiful,itwouldbeattractivetouseitasafeedstockforobtainingthenecessary

The natural system constitutes an excellent source of inspiration for how to design an artificial system that is capable of harnessing solar energy for fuel production. The concept of artificial photosynthesis emerged in the 1970s and is inspired by nature where light-induced charge separation events sequentially oxidize a Mn4Ca cluster (Figure 1) known as the oxygenevolving complex (OEC) [7, 8]. After four electrons have been abstracted from the OEC, two molecules of water are oxidized to molecular oxygen, thus releasing four electrons and four protons. The natural photosynthetic apparatus subsequently utilizes the generated reducing equivalents to reduce CO2 to carbohydrates [9, 10]. However, instead of using the generated reducing equivalents to reduce CO2 to carbohydrates as in the natural photosynthetic appa‐ ratus, these "artificial leafs" would produce hydrogen gas from the protons and electrons that

2 22 2H O O 2H ® + (1)

reducing equivalents—protons and electrons [1–6].

190 Applied Photosynthesis - New Progress

are liberated when water is oxidized (Eq. 1) [11, 12].

the total solar irradiance [13].

**Figure 1.** Depiction of the Mn4Ca cubane core of the oxygen-evolving complex (OEC).

Water splitting can be divided into two half-reactions; proton reduction and water oxidation. The reductive side of water splitting involves the generation of hydrogen gas from the generated protons and electrons. In contrast to hydrocarbons, hydrogen gas is considered to be environmentally benign, as water is the only combustion product. Although deceptively simple, the other half-reaction, water oxidation (Eq. 2), is a mechanistically complex process and is currently considered as the bottleneck. The oxidation of water thus requires a single catalytic entity capable of accumulating four oxidizing equivalents, breaking several bonds and forming the crucial O–O bond. Splitting of water is an energy demanding process with a Gibbs free energy of 237.18 kJ mol–1 and a minimum electrochemical potential of 1.229 V vs. normal hydrogen electrode (NHE) is required. The basic thermodynamic requirements for splitting water suggest that any light with a wavelength shorter than 1 mm has enough energy to split a molecule of water. Consequently, this allows the use of the entire visible solar spectrum and a majority of the near-infrared spectrum, which collectively constitutes ~80% of

2H O O 4H 4e 2 2

+ -

®+ + (2)

The first step in solar energy conversion schemes involves light absorption by a chromophore. In the natural photosynthetic system, a set of specialized chlorophyll-based pigments is responsible for the absorption of visible light, and subsequently transfers the excitation energy to the reaction centers of photosystem II and I. Mimicking these events for constructing artificial photosynthetic devices is a crucial objective and requires tailored photosensitizers that are photostable and efficiently absorb photons across a wide range of wavelengths in the visible spectral region. Furthermore, they should also be easy to modify to allow for straight‐ forward tuning of the photophysical features. The main requirement is that the reduction potential of the oxidized photosensitizer is more positive than that of the WOC and the onset potential for water oxidation (and any overpotential that is produced in the designed system) [20, 21].

#### **2.1. Photophysical Description of Ruthenium-Type Photosensitizers**

Perhaps the most extensively studied metal-based photosensitizers are the [Ru(bpy)3] 2+-type complexes (Figure 3; bpy = 2,2'-bipyridine). Shortly after the seminal report on UV-lightmediated water splitting at TiO2 photoanodes by Honda and Fujishima [14], the basis for artificial photosynthesis appeared when it was realized that metal complexes, such as [Ru(bpy)3] 2+ (**1**), are efficiently quenched by organic compounds [22, 23]. Flash photolysis experiments have revealed that light absorption (*λ* max ≈ 450 nm) by the [Ru(bpy)3] 2+-type photosensitizers triggers excitation of an electron in a metal-centered orbital to a *π*\* orbital located on the ancillary polypyridyl ligand. This metal-to-ligand charge transfer (MLCT) results in a singlet excited state, 1 [Ru(bpy)3] 2+\*, that undergoes rapid intersystem crossing (ISC), affording a triplet state, 3 [Ru(bpy)3] 2+\*. This excited state is relatively long lived and has a dual nature, being that it can participate in either a single-electron oxidation or a single-electron reduction in the presence of an acceptor or a donor, respectively (Figure 4). The [Ru(bpy)3] 2+ type photosensitizers possess several desirable features: 1) photostability, 2) the produced excited state has a sufficient lifetime for it to participate in chemical reactions, 3) they exhibit compatibility with a wide pH range, 4) they display broad absorption of visible light, and 5) the relative ease by which the photophysical properties of the ruthenium photosensitizers can be tuned, allowing e.g. that the absorption of light can be extended from the near infrared to the UV region by simply modifying the ancillary ligands (see Figure 3) [24–26].

**Figure 3.** Examples of [Ru(bpy3)]2+-type photosensitizers.

Visible Light-Driven Water Oxidation Catalyzed by Ruthenium Complexes http://dx.doi.org/10.5772/62272 193

**Figure 4.** Photophysical properties of the [Ru(bpy)3] 2+ complex (**1**).

to the reaction centers of photosystem II and I. Mimicking these events for constructing artificial photosynthetic devices is a crucial objective and requires tailored photosensitizers that are photostable and efficiently absorb photons across a wide range of wavelengths in the visible spectral region. Furthermore, they should also be easy to modify to allow for straight‐ forward tuning of the photophysical features. The main requirement is that the reduction potential of the oxidized photosensitizer is more positive than that of the WOC and the onset potential for water oxidation (and any overpotential that is produced in the designed system)

**2.1. Photophysical Description of Ruthenium-Type Photosensitizers**

Perhaps the most extensively studied metal-based photosensitizers are the [Ru(bpy)3]

experiments have revealed that light absorption (*λ* max ≈ 450 nm) by the [Ru(bpy)3]

[Ru(bpy)3]

the UV region by simply modifying the ancillary ligands (see Figure 3) [24–26].

[Ru(bpy)3]

complexes (Figure 3; bpy = 2,2'-bipyridine). Shortly after the seminal report on UV-lightmediated water splitting at TiO2 photoanodes by Honda and Fujishima [14], the basis for artificial photosynthesis appeared when it was realized that metal complexes, such as

photosensitizers triggers excitation of an electron in a metal-centered orbital to a *π*\* orbital located on the ancillary polypyridyl ligand. This metal-to-ligand charge transfer (MLCT)

nature, being that it can participate in either a single-electron oxidation or a single-electron reduction in the presence of an acceptor or a donor, respectively (Figure 4). The [Ru(bpy)3]

type photosensitizers possess several desirable features: 1) photostability, 2) the produced excited state has a sufficient lifetime for it to participate in chemical reactions, 3) they exhibit compatibility with a wide pH range, 4) they display broad absorption of visible light, and 5) the relative ease by which the photophysical properties of the ruthenium photosensitizers can be tuned, allowing e.g. that the absorption of light can be extended from the near infrared to

2+ (**1**), are efficiently quenched by organic compounds [22, 23]. Flash photolysis

2+\*, that undergoes rapid intersystem crossing (ISC),

2+\*. This excited state is relatively long lived and has a dual

2+-type

2+-type

2+-

[20, 21].

[Ru(bpy)3]

results in a singlet excited state, 1

**Figure 3.** Examples of [Ru(bpy3)]2+-type photosensitizers.

affording a triplet state, 3

192 Applied Photosynthesis - New Progress

#### **2.2. Evaluating Light-Driven Water Oxidation with Ruthenium Complexes**

A three-component system is typically employed for evaluating light-driven water oxidation and consists of a photosensitizer, a water oxidation catalyst, and a sacrificial electron acceptor (Figure 5). As the sacrificial electron acceptor, sodium persulfate is usually employed since the recombination or reversed electron transfer can be ruled out, thereby simplifying the kinetic analysis of subsequent steps in the catalytic process. The three-component light-driven system using the persulfate anion (S2O8 2–) and a metal-based photosensitizer has been well docu‐ mented and is believed to commence with oxidative quenching of the excited state of the photosensitizer, such as [Ru(bpy)3]2+\*. This results in the generation of [Ru(bpy)3] 3+, sulfate, and a sulfate radical (SO4 −•), which is a strong oxidant (E° > 2.40 V vs. NHE[27]) and has the ability to directly oxidize a second equivalent of [Ru(bpy)3] 2+ [28]. The reduction of two equivalents affords the four equivalents of [Ru(bpy)3] 3+ that are needed to oxidize the WOC, which in turn oxidizes water to molecular oxygen. The processes involved in the light-driven persulfate system are summarized by Eqs. 3–6.

**Figure 5.** Three-component system for light-driven water oxidation consisting of a water oxidation catalyst, a [Ru(bpy)3] 2+-type photosensitizer and persulfate as the sacrificial electron acceptor.

$$2\left[\text{Ru(bpy)}\_{3}\right]^{2\*} + 2h\nu \to 2\left[\text{Ru(bpy)}\_{3}\right]^{2\*} \tag{3}$$

$$2\left[\text{Ru}\left(\text{bpy}\right)\_{3}\right]^{2+\*} + 2\text{S}\_{2}\text{O}\_{8}^{2-} \rightarrow 2\left[\text{Ru}\left(\text{bpy}\right)\_{3}\right]^{3+} + 2\text{SO}\_{4}^{2-} + 2\text{SO}\_{4}^{-}\tag{4}$$

$$2\left[\text{Ru}\left(\text{bpy}\right)\_{3}\right]^{2+} + 2\text{SO}\_{4}^{-} \rightarrow 2\left[\text{Ru}\left(\text{bpy}\right)\_{3}\right]^{3+} + 2\text{SO}\_{4}^{2-} \tag{5}$$

$$4\left[\text{Ru}\left(\text{bpy}\right)\_{3}\right]^{2+} + 2\text{S}\_{2}\text{O}\_{8}^{2-} + 2h\nu \rightarrow 4\left[\text{Ru}\left(\text{bpy}\right)\_{3}\right]^{3+} + 4\text{SO}\_{4}^{2-} \tag{6}$$
