The search for renewable energy sources is becoming day by day more urgent for mankind. The global energy demand is indeed predicted to rise considerably within the next decade, and fossil fuels alone cannot satisfy this energy thirst, due to their limited availability and the Earth pollution caused by their combustion. Among renewable energy sources, solar light is the most abundant, cheap, and equally distributed. In order to be used, it has to be transformed into other forms that can be subsequently stored and transported. Artificial photosynthesis converts solar light into chemical energy, by performing the light-driven water splitting into its components: hydrogen (H2) and oxygen (O2) (see Scheme 6.1) (Armaroli and Balzani 2007; Balzani et al. 2008). H2 and O2 can then be recombined to release their chemical energy (the amount of free energy ΔG associated with the reaction of 2mol of H2 with 1mol of O2 is 113.38kcal). The light-driven splitting of water is actually a complex process, where components dealing with light absorption, energy and electron transfers, and redox catalysis are assembled in a modular approach (Figure 6.1). Light is first harvested by an antenna, and the energy is transferred to a photosensitizer (P); the latter, in its excited state, promotes an electron transfer from a suitable donor (D) to a suitable acceptor (A); such charge separation is the crucial step of the overall process. Indeed, electrons in A are converged to a hydrogen-evolving catalyst (HEC), able to reduce protons to H2; at the same time, in the other half-cell, holes in D are used for water oxidation through the intervention of an oxygen-evolving catalyst (OEC). In order to efficiently exploit solar radiation and to avoid unproductive charge recombination, all these events need to be concerted, and therefore, fast catalysis is needed. In particular, the oxidative half-reaction leading to oxygen is by far more difficult to achieve than the reduction to hydrogen, since it is a 4e−/4H+ process, which also requires the formation of an oxygen–oxygen bond. The OEC is usually constituted by one or several redox active metals, since it should act as a charge pool, by reacting four times with the photogenerated holes in D, prior to transforming water into oxygen in a single step, as described in Scheme 6.2. Besides the chemical difficulties of these steps, the reactions shown earlier occur in a very oxidizing environment, since the standard redox potential for the O2/H2O couple is 1.23 V vs. normal hydrogen electrode (NHE). These conditions may lead to oxidative damage of the catalyst, with consequent loss of its activity. Therefore, strategies to preserve catalyst activity under turnover conditions must be considered and adopted. In this chapter, we will focus mainly on two approaches: 1. Oxygen-evolving catalysts able to self repair 2. Oxygen-evolving catalysts that are robust and resist under oxidizing conditions
Pursuit of Long-Lasting Oxygen-Evolving Catalysts for Artificial Photosynthesis Self-Healing Materials and Molecular-Reinforced Structures
BERARDI SPrimo
;
2011
Abstract
The search for renewable energy sources is becoming day by day more urgent for mankind. The global energy demand is indeed predicted to rise considerably within the next decade, and fossil fuels alone cannot satisfy this energy thirst, due to their limited availability and the Earth pollution caused by their combustion. Among renewable energy sources, solar light is the most abundant, cheap, and equally distributed. In order to be used, it has to be transformed into other forms that can be subsequently stored and transported. Artificial photosynthesis converts solar light into chemical energy, by performing the light-driven water splitting into its components: hydrogen (H2) and oxygen (O2) (see Scheme 6.1) (Armaroli and Balzani 2007; Balzani et al. 2008). H2 and O2 can then be recombined to release their chemical energy (the amount of free energy ΔG associated with the reaction of 2mol of H2 with 1mol of O2 is 113.38kcal). The light-driven splitting of water is actually a complex process, where components dealing with light absorption, energy and electron transfers, and redox catalysis are assembled in a modular approach (Figure 6.1). Light is first harvested by an antenna, and the energy is transferred to a photosensitizer (P); the latter, in its excited state, promotes an electron transfer from a suitable donor (D) to a suitable acceptor (A); such charge separation is the crucial step of the overall process. Indeed, electrons in A are converged to a hydrogen-evolving catalyst (HEC), able to reduce protons to H2; at the same time, in the other half-cell, holes in D are used for water oxidation through the intervention of an oxygen-evolving catalyst (OEC). In order to efficiently exploit solar radiation and to avoid unproductive charge recombination, all these events need to be concerted, and therefore, fast catalysis is needed. In particular, the oxidative half-reaction leading to oxygen is by far more difficult to achieve than the reduction to hydrogen, since it is a 4e−/4H+ process, which also requires the formation of an oxygen–oxygen bond. The OEC is usually constituted by one or several redox active metals, since it should act as a charge pool, by reacting four times with the photogenerated holes in D, prior to transforming water into oxygen in a single step, as described in Scheme 6.2. Besides the chemical difficulties of these steps, the reactions shown earlier occur in a very oxidizing environment, since the standard redox potential for the O2/H2O couple is 1.23 V vs. normal hydrogen electrode (NHE). These conditions may lead to oxidative damage of the catalyst, with consequent loss of its activity. Therefore, strategies to preserve catalyst activity under turnover conditions must be considered and adopted. In this chapter, we will focus mainly on two approaches: 1. Oxygen-evolving catalysts able to self repair 2. Oxygen-evolving catalysts that are robust and resist under oxidizing conditionsI documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
