**3.1 Hydrogen production using heterogeneous photocatalysis**

In recent decades, research has been conducted on the possibility of using hydrogen as energy vector with low carbon emissions. The policy guidance for reducing the emission of greenhouse gases, and the prospect of decline in oil and other fossil fuels, has brought to light again the discussion about the use of hydrogen and technologies related to it. However, it is clear that large-scale use of hydrogen will only be possible if renewable sources are used in its production (Preguer et al., 2009). Currently, renewables contribute only about 5% of the commercial production of hydrogen, while the remaining 95% are derived from fossil fuels, given the still high cost of production from renewable sources.

The photocatalytic degradation of water to produce hydrogen, under the action of solar energy, offers a promising way to produce hydrogen cleanly, inexpensive and environmentally friendly. While great progress in photocatalysis using radiation in the ultraviolet region has occurred in recent decades, it has been extended with some difficulty,

The International Energy Agency (IEA) estimates that world demand for energy should suffer an increase of 45% by 2030 (Birol, 2008). Based on the projections presented, one can expect a worsening of global warming, if no measures are taken that result in significant reduction of CO2 emissions. In addition, we expect a worrying shortage of fossil fuels, if

Among the alternative energy sources, H2 is a very attractive option, as it concentrates high energy per unit mass – 1.0 kg of hydrogen contains approximately the same energy furnished by 2.7 kg of gasoline, which facilitates the portability of energy (Smith & Shantha,

Experts have pointed out three major obstacles to the expansion of consumption of hydrogen taking into consideration the technology available at the moment: clean production, low cost, storage and transportation. As a result, most efforts to expand the use of hydrogen as a source of cheap energy has been based on the development of new

Among the technologies for hydrogen production, biomass gasification (Albertazzi et al., 2005; Smith & Shantha, 2007), photocatalysis (Ni et al. 2007; Patsoura et al.,2007; Jing et al., 2010), and biological processes (Peixoto, 2008), have been focus of many studies for being routes clean and renewable. The heterogeneous photocatalysis and hydrogen generation by decomposition of water using concentrated solar radiation as primary source energy are

The great expectation of the global market for the use of hydrogen gas as an important input in the production of energy has been driven by the sectors of energy generation and distribution, which moves large numbers of capital around the world, and is in frank expansion, due to the enormous demand for energy by all sectors (Steinfeld, 2005; Preguer et al., 2009; Pagliaro et al., 2010). Most efforts to expand the use of hydrogen as a renewable energy source has been based on the development of fuel cell technology, both for expansion of its service life, by minimizing costs. Volumes of hydrogen gas have already

In recent decades, research has been conducted on the possibility of using hydrogen as energy vector with low carbon emissions. The policy guidance for reducing the emission of greenhouse gases, and the prospect of decline in oil and other fossil fuels, has brought to light again the discussion about the use of hydrogen and technologies related to it. However, it is clear that large-scale use of hydrogen will only be possible if renewable sources are used in its production (Preguer et al., 2009). Currently, renewables contribute only about 5% of the commercial production of hydrogen, while the remaining 95% are derived from fossil fuels, given the still high cost of production from renewable sources.

The photocatalytic degradation of water to produce hydrogen, under the action of solar energy, offers a promising way to produce hydrogen cleanly, inexpensive and environmentally friendly. While great progress in photocatalysis using radiation in the ultraviolet region has occurred in recent decades, it has been extended with some difficulty,

between the most promising having gain attention due to their potential.

been produced, both in EU-funded projects, such as the United States.

**3.1 Hydrogen production using heterogeneous photocatalysis** 

**3. Obtention of gaseous hydrogen for energy production** 

alternative sources of energy are not being widely used.

2007). Besides, its combustion generates no contaminants.

materials and processes of production.

considering the use of visible radiation as a trigger for photocatalytic processes. Particularly, we have achieved some progress in this direction, involving the association between a photosensitizer dye and a semiconductor oxide.

The development of semiconductor oxides capable to be excited by radiation in the visible region became one of the most important topics in photocatalysis research, since the visible light represents a significant fraction of solar energy usable (Hwang et al., 2004). However, finding another photocatalyst than TiO2, which has good chemical stability, corrosion resistance, be able to efficiently absorb radiation in the visible, and is environmentally friendly, has proved an arduous task. However, no semiconductor material capable of catalyzing the overall water splitting under action of visible radiation around 600 nm, with a quantum efficiency high enough to make possible the commercial application (Maeda & Domen, 2007; Jing et al., 2010). Besides, many of the photocatalysts capable to induce hydrogen production with commercially acceptable quantum efficiency, with excitation between 300 and 450 nm, are expensive and inadequate from the environmental point of view (Zeug et al., 1985; Maeda et al., 2006; Bao et al., 2008).

The low efficiency for the hydrogen production by semiconductor photocatalysis already with appropriate band gap should be due to the following reasons: 1) quick electron/hole recombination in the bulk or on the surface of semiconductor particles; 2) quick back reaction of oxygen and hydrogen to form water on the surface of catalyst; and 3) inability to promote efficient use of visible radiation. It is known that photogenerated electrons easily recombine with holes in the semiconductor (Hoffmann et al., 1995; Li et al., 2010; Kumar & Devi, 2011), compromising the quantum efficiency of the photocatalytic process (Kudo, 2006). Noble metal loading can suppress to some extent the charge recombination by forming a Schottky barrier (Chand & Bala, 2007; Fu et al., 2008). Often, sacrificial reagents has been added to the reaction media for the elimination of photo-generated holes, minimizing the electron/hole recombination, improving the quantum efficiency (Liu et al., 2006; Zaleska, 2008a; Jing et al., 2010). Methanol, ethanol and acetic acid have usually been employed as agents of sacrifice. Toxic organic substrates can also be a good option of sacrificial reagent (Jing et al., 2010).

Much progress has been made in photocatalytic water splitting since the Fujishima-Honda effect was reported (Fujishima & Honda, 1971, 1972). Thermodynamically, water splitting into H2 and O2 can be seen as an unfavorable reaction (G = +238 kJ/mol) (Jing et al., 2010; Melo & Silva, 2011). However, the efficiency of water splitting is determined by the band gap, band structure of the semiconductor and the electron transfer process (Linsebigler et al., 1995; Hagfeldt & Grätzel, 1995; Melo & Silva, 2011).

Generally for efficient H2 production using visible light-driven semiconductor the band gap should be less than 3.00 eV (ca. 420 nm) and higher than 1.23 eV (ca. 1000 nm), corresponding to the water splitting potential (Jing et al., 2010; Melo & Silva, 2011). Moreover CB and VB levels should satisfy the energy requirements set by the reduction and oxidation potentials for H2O, respectively: the bottom of the conduction band must be located at a more negative potential than the H+/H2 reduction potential (Eo = 0 V *vs.* NHE at pH 0), while the top of the valence band must be more positively positioned than the H2O/O2 oxidation potential (Eo = 1.23 V *vs.* NHE) (Melo & Silva, 2011). Band engineering is thus necessary for the design of new semiconductors with the combined properties (Jing et al., 2010).

Oxides as HPb2Nb3O10, MgWOx and NiInTaO4 among others, active under the action of ultraviolet radiation, were also active in the visible region after doping using C, N and S ( TiO2Nx, TiO2Cx, TaON and Sm2Ti2O5S) (Hwang et al., 2004), as well as certain perovskite-type photocatalysts, with significant absorption in the visible. Zhang & Zhang (2009) reported the synthesis of a photocatalyst based on BiVO4 which showed high photocatalytic activity in the visible region. However, most of these catalysts are not environmentally friendly as TiO2.

Photocatalytic induced water-splitting technology involving nanosized TiO2, despite the considerable variety of semiconductor photocatalysts capable to split water using solar energy and other photocatalytic processes has great potential to support an economy based on low-cost and environmentally friendly hydrogen production using solar radiation (Ashokkumar, 1998; Ni et al., 2007).

The photocatalytic hydrogen production using TiO2 as photocatalyst can be schematized through **Figs. 1 and 10**.

Fig. 10. Band gap of (a) Rutile sand (b) Anatase compared to the redox potential of water at pH 1.

For an efficient production of H2, the energy level of the CB should be more negative than the energy level of the reduction of hydrogen, while the energy level of VB should be more positive than the energy level of the oxidation of water to formation of O2 (**Fig.10**) (Ashokkumar, 1998; Ni et al., 2007), eqs 5 to 7. As outlined in **Fig. 1**, the vacancies photogenerated in the VB oxidize water into oxygen and hydrogen cations. These cations are reduced to molecular hydrogen in the conduction band. In other words, the separated electrons and holes act as reducer and oxidizer, respectively, in the water splitting reaction to produce hydrogen and oxygen. However, for this to happen effectively, it is necessary to

Oxides as HPb2Nb3O10, MgWOx and NiInTaO4 among others, active under the action of ultraviolet radiation, were also active in the visible region after doping using C, N and S ( TiO2Nx, TiO2Cx, TaON and Sm2Ti2O5S) (Hwang et al., 2004), as well as certain perovskite-type photocatalysts, with significant absorption in the visible. Zhang & Zhang (2009) reported the synthesis of a photocatalyst based on BiVO4 which showed high photocatalytic activity in the visible region. However, most of these catalysts are not environmentally friendly as TiO2.

Photocatalytic induced water-splitting technology involving nanosized TiO2, despite the considerable variety of semiconductor photocatalysts capable to split water using solar energy and other photocatalytic processes has great potential to support an economy based on low-cost and environmentally friendly hydrogen production using solar radiation

The photocatalytic hydrogen production using TiO2 as photocatalyst can be schematized

Fig. 10. Band gap of (a) Rutile sand (b) Anatase compared to the redox potential of water at pH 1.

For an efficient production of H2, the energy level of the CB should be more negative than the energy level of the reduction of hydrogen, while the energy level of VB should be more positive than the energy level of the oxidation of water to formation of O2 (**Fig.10**) (Ashokkumar, 1998; Ni et al., 2007), eqs 5 to 7. As outlined in **Fig. 1**, the vacancies photogenerated in the VB oxidize water into oxygen and hydrogen cations. These cations are reduced to molecular hydrogen in the conduction band. In other words, the separated electrons and holes act as reducer and oxidizer, respectively, in the water splitting reaction to produce hydrogen and oxygen. However, for this to happen effectively, it is necessary to

(Ashokkumar, 1998; Ni et al., 2007).

through **Figs. 1 and 10**.

ensure the fast transportation of the photogenerated carriers, avoiding bulk electron/hole recombination. Separation of hydrogen gas is also required as oxygen and hydrogen are produced simultaneously.

$$H\_2O \xrightarrow{hv > E\_g} H\_2 + \frac{1}{2}O\_2 \tag{5}$$

$$2e^- + 2H^+ \xrightarrow{E\_{R^-} \times E\_{H^+/H\_2O}} H\_2 \tag{6}$$

$$2\text{h}^+ + H\_2O \xrightarrow{E\_{\text{B}V} \ge E\_{O\_2/H\_2O}} \frac{1}{2}O\_2 + 2H^+ \tag{7}$$

Having the adequate semiconductor, capable to induce water splitting when photoexcited by solar radiation, a key issue concerns the efficient utilization of the solar energy itself. Two major drawbacks of solar energy must be considered: (1) the intermittent and variable manner in which it arrives at the earth's surface (2) efficient collection of solar light on a useful scale. The first drawback can be solved by converting solar energy into storable hydrogen energy. For the second, the solution could be the use of solar concentrators (Jing et al., 2010).

For photocatalytic hydrogen production, it is imperative the use of visible radiation, especially if the goal is the storage of the energy supplied by the sun. Thus, photocatalysts able to mediate reactions through the use of visible radiation are more than desirable. Amplify the sensitivity of photocatalysts through the introduction of dopants, impurities and / or association between semiconductor and photosensitizers capable of shifting the absorption of the resulting composite to visible, are alternatives to a more efficient water photolysis (Hwang et al*.*, 2004; Machado et al., 2008; Zaleska, 2008a, 2008b; Zhang & Zhang, 2009).

When a metal (eg platinum) is deposited on a semiconductor, the excited electrons migrate from the semiconductor to the metal until the Fermi levels of both species are aligned. The Schottky's barrier (Chand & Bala, 2007; Fu et al., 2008) formed at the metal/semiconductor interface can serve as a trap for electrons, efficient enough to minimize electron-hole recombination, increasing the efficiency of the photocatalytic process. At the same time, the metal is important for its own catalytic activity. Metals deposited on a semiconductor serve as active sites for the production of H2, in which the trapped electrons are transferred to photogenerated protons to produce H2 (**Fig. 11**) (Melo & Silva, 2011).

Research on photocatalytic hydrogen production in our laboratory is very recent. Our primary aim is the development of highly efficient, stable and low-cost visible-light-driven photocatalyst using different modification methods, such as doping, sensitization, supporting and coupling methods to extend the light response and performance of the photocatalyst aiming its application in environmental photocatalysis and photocatalytic hydrogen production. Despite a considerable variety of semiconductor photocatalysts capable to split water using solar energy and mediate other photocatalytic processes (Ashokkumar, 1998; Kim et al., 2010; Jing et al., 2010; Kumar & Devi, 2011), our studies have focused on improving the photocatalytic activity of TiO2 through its synthesis by different procedures, their use and of hybrid variants, doped or not, and composites involving TiO2 and photosensitizing dyes, especially considering issues related to the environment. In particular, we have studied photocatalytic reactions using solar radiation, with the photocatalyst in aqueous suspensions, with methodologies based on CPC reactor.

Fig. 11. Schematic representation of the photocatalytic water splitting on a platinized semiconductor powder particle.

We have developed small closed circulation reactor for bench-scale tests. These reactors ensure the evaluation of the developed photocatalyst from lab scale to out-door scale, in a batch mode.

The object of these studies is to improve hydrogen production and its storage under low pressure.
