Recent Patents on Engineering 2010, 4, 000-000 1

1872-2121/10 $100.00+.00 © 2010 Bentham Science Publishers Ltd.

Photocatalytic Hydrogen Production

Gian Luca Chiarello and Elena Selli*

Dipartimento di Chimica Fisica ed Elettrochimica, Università degli Studi di Milano, Via Golgi 19, I-20133 Milano, Italy

Received: September 6, 2010; Accepted: October 20, 2010; Revised: October 30, 2010

Abstract: The photocatalytic production of hydrogen from aqueous systems is reviewed, stressing the very promising fea-

tures of the process as an environmentally friendly, perfectly renewable way to produce hydrogen, the ideal fuel for the

future. Starting with a brief historical background, the most recent achievements in the field are discussed, both in the de-

velopment of innovative materials able to capture a larger portion of the solar spectrum with respect to traditional photo-

catalytic materials, and on the different setups and devices which have been developed and tested.

Keywords: Photocatalytic hydrogen production; photosplitting of water; photocatalytic steam reforming; solar hydrogen.

1. INTRODUCTION

The increased concern on global warming, emission of pollutants and exhaustion of energy resources consequent to the raise of world energy demand, up to now mainly fulfilled by fossil fuels exploitation, urgently requires the develop-ment of alternative, environmentally friendly and totally re-newable energy sources. In this prospect, sunlight is the most promising renewable energy source of the future. Indeed, the amount of solar energy impinging on the earth’s surface is estimated to be ca. 3 10

24 J per year, that is approximately

104 times greater than the worldwide yearly energy con-

sumption. Moreover, increasing interest is addressed towards hydrogen, which is widely considered the clean energy vec-tor of the future, because the chemical energy stored in the H-H bond formation is released when it reacts with oxygen, yielding only innocuous water as a product, in a highly exo-thermic reaction:

H2 + O2 H2O Gr° = – 237 kJ mol-1

However, nowadays about two-thirds of the world’s hy-drogen production, which is ca. 48 millions metric tons per year, is employed in ammonia synthesis to produce fertiliz-ers; large amounts are also used in refineries, in the synthesis of methanol, and in the catalytic hydrogenation of organic compounds [1].

Although the technologies of energy production from hy-drogen, e.g. fuel cells and internal hydrogen combustion en-gines, are already mature, hydrogen production remains a major problem. In fact, nearly all hydrogen production is still based on fossil raw materials and only 4% is produced via water electrolysis [2]. The most important industrial process of hydrogen production consists in the catalytic steam re-forming of hydrocarbons, implying gaseous or vaporized hy-drocarbons treatment with steam at high pressure (15-40 bar) and high temperature (650° – 950° C) over nickel-based catalysts. In the case of methane the reaction is:

*Address correspondence to this author at the Dipartimento di Chimica

Fisica ed Elettrochimica, Università degli Studi di Milano, Via Golgi 19, I-

20133 Milano, Italy; Tel: ???????????; Fax: ???????????; E-mail: elena.selli@unimi.it

CH4 + H2O CO + 3 H2 Hr° = 205 kJ mol-1

Additional hydrogen can be recovered by a subsequent, lower temperature, “water gas shift” step, in which CO is further oxidized by steam, yielding CO2:

CO + H2O CO2 + H2 Hr° = – 42 kJ mol-1

Because of the endothermic nature of the steam reform-ing reaction, heat must be supplied for the reaction to pro-ceed and this is usually provided by the combustion of part of the feed stock, with a consequent decrease of the net yield of the process.

After the pioneering work of Fujishima and Honda [3], who first demonstrated in the 1972 (just before the 1973 oil crisis) the photocatalytic cleavage of water into H2 and O2 using a photoelectrochemical (PEC) cell, a great interest started in this research topic, which greatly increased espe-cially in the last decade, as testified by the exponential raise of the number of papers published per year up today. The PEC cell consisted in an UV-illuminated n-type TiO2 semi-conductor anode connected through an external circuit to a Pt-black counter electrode (cathode) in the presence of an external electrical bias. Such a great interest is justified by the potential application of this technology to the harvesting, conversion and storage of solar into chemical energy, in the form of H2, as a clean and renewable energy source, even on a large scale [4].

Up today, the photocatalytic production of hydrogen can be obtained mainly by two processes, i.e. either by the direct splitting of water into H2 and O2 (see section 4), or by the photo-reforming of organic compounds (see section 5). Both processes are briefly reviewed here, with the attempt of un-derlining the most salient problems to the large scale devel-opment and application of photocatalysis for hydrogen pro-duction. Indeed, although hundreds of patents were produced in the last decades on this field, the greatest part of them is focused on synthetic methods of novel photoactive semicon-ducting materials and only few of them deal with new photo-catalytic technologies and devices. However, the most im-portant issue to be solved still remains the low quantum effi-ciency of photocatalytic hydrogen production, partly associ-

2 Recent Patents on Engineering 2010, Vol. 4, No. 3 Chiarello and Selli

ated to limitations in band gap excitation [5], leading to low hydrogen production rates, which limit the practical applica-tion of this technology on a large scale.

2. HYDROGEN PRODUCTION THROUGH PHOTO-CATALYSIS

Several excellent reviews were recently published, pro-viding an exhaustive description of the fundamentals of pho-tocatalysis and its application to hydrogen production [5-18]. Furthermore, Ohtani recently published a valuable critical overview providing important advice on illusions, miscon-ceptions and speculations beyond photocatalysis [19].

Photocatalytic reactions on semiconductors are usually initiated by the absorption of a photon with energy equal to, or greater than, the semiconductor band gap. This promotes an electron from the valence band (VB) to the conduction band (CB), with the consequent formation of an electron (eCB

-) - hole (hVB

+) pair (Fig. 1). The so produced charge car-

riers can either recombine, with the consequent loss of en-ergy as heat or emission of photons, or initiate electron trans-fer reactions at the semiconductor surface. These consist in the reduction of electron acceptor species, having a reduction potential lower in energy than the CB level, and the oxida-tion of electron donor species, with a potential higher in en-ergy than the VB, respectively, both adsorbed on the semi-conductor surface. In the case of water cleavage, the electron acceptor species would be the H

+ ion, whereas water, or hy-

droxyl anions, would be the electron donor species, accord-ing to the following reactions:

TiO2 + h eCB- + hVB

+ (1)

2 H+ + 2 eCB

- H2 (2)

H2O + 2 hVB+ O2 + 2 H

+ (3)

Formally, the overall photocatalytic water splitting reac-tion is, therefore:

H2O + 2 h H2 + O2 Gr° = 237 kJ mol-1

(4)

Because reaction (4) is accompanied by a positive Gibbs free energy change (i.e. it is an up-hill reaction), it may be regarded as a sort of artificial photosynthesis, in which the photon energy is converted and stored in the form of chemi-cal energy, i.e. as H2.

Fig. (1). Schematic representation of the mechanism of photocata-

lytic water splitting over an illuminated TiO2 semiconductor parti-

cle.

The redox potentials of reactions (2) and (3) at pH 7, 25°C and 1 atm are -0.41 V and +0.81 V, respectively (Fig. 2). Overall water splitting thus requires 1.22 V. By applying the Planck’s law, this means that all photons with < 1100 nm, thus the whole visible region, could in principle promote the photocatalytic water cleavage reaction. In particular, the optimal semiconductor acting as photocatalyst in water cleavage should have a band gap close to 2 eV in order to account, in addition to the thermodynamic decomposition potential, for overpotential and Ohmic drop losses [18]. Of course, the right semiconductor would not only have a suit-able band gap, but also VB and CB energy levels matching the potentials of reactions (2) and (3), i.e. the CB edge en-ergy should be more negative than the potential of the H

+/H2

couple and the VB edge energy should be more positive than the O2 evolution potential, on the electrochemical scale.

Fig. (2). Potential energy diagram of H2 production from photocata-

lytic water splitting over an anatase TiO2 semiconductor at pH 7.

Values reported vs. Standard Hydrogen Electrode (SHE).

Figure 3 shows the relative positions of the band struc-tures of some selected semiconductors and the redox poten-tials of water splitting [15]. Almost all most common and widely employed semiconductors have relatively wide band gap ( EB) values. For example, for TiO2 in the anatase form,

EB = 3.2 eV, whereas for TiO2 in the rutile form EB = 3.0 eV, i.e. TiO2 absorbs light only below 400 nm. This limits the photoactivity of such materials only to UV light irradia-tion, so that they are able to use only a small portion (ca. 4 %) of the solar spectrum. This represents one of the major limitations to the practical application of photocatalysis as a tool for solar light harvesting. Moreover, other semiconduc-tors, such as ZnO and CdS, are also possible candidates as photocatalysts for hydrogen production from water, in con-sideration of their narrower band gap compared to TiO2.

Photocatalytic Hydrogen Production Recent Patents on Engineering 2010, Vol. 4, No. 3 3

However, many of these materials have received less atten-tion because, under illumination in contact with water, they can undergo photocorrosion (e.g. Zn

2+ or Cd

2+ dissolution).

This problem could be partly overcome by adopting gas-phase photoreactions, as in the case of the photo-steam re-forming of volatile organic compounds. Alternatively, the system may be stabilized by the addition of redox couple species. On the other hand, more stable semiconductors, such as TiO2, WO3, Fe2O3 and MoS2, have a conduction band po-tential ECB close or even slightly lower than that required to evolve H2. This problem can be partially circumvented by applying a chemical or electrical bias in a two electrodes cell (such as the above mentioned PEC cell), as will be discussed in paragraph 4.2.

3. ENGINEERING EFFICIENT PHOTOCATALYSTS FOR SOLAR H2 PRODUCTION

The main obstacles limiting the practical application of photocatalysis under solar light illumination to up-hill reac-tions, such as hydrogen production from water (solar fuels production), thus are: (i) almost all suitable semiconductor materials absorb light below 400 nm, corresponding to less than 5 % of the solar light spectrum; (ii) photocatalytic proc-esses proceed with low quantum efficiency, due to the high rate of electron-hole pairs recombination. The main strate-gies proposed to shift the adsorption edge towards visible light are doping the semiconducting materials and their sen-sitization by adsorbed species, such as e.g. dyes, able to ab-sorb visible light and activate the semiconductor. Charge car-riers separation can be improved through an appropriate de-sign and control of the surface and bulk properties of semi-conductors, as well as by their surface modification by depo-sition of noble metal nanoparticles and, in the case of hydro-gen production, by using sacrificial agents, able to efficiently combine with photo-produced holes. An interesting review paper [12] recently reported on these strategies to be applied to TiO2, specifically for photocatalytic water splitting. Al-

though the greatest improvements in the development of this research area were achieved in the last decade, strong efforts are still required to achieve the final goal of designing and realizing optimal photocatalysts, able to satisfy the ultimate target, proposed by Bolton in 1996 [5], of at least a 30 % quantum yield in photocatalytic H2 production.

3.1. Doping and Co-Doping Doping was recently shown to be a promising approach to shift the semiconductor absorption threshold into the visi-ble region. In particular, partial substitution of oxygen in semiconducting oxides with non-metal elements, such as ni-trogen [20,21], carbon [22-24], sulfur [25] and boron [26], should produce infra band gap electronic states, thus induc-ing a red-shift of the absorption threshold. However, the pho-tocatalytic performance of doped semiconductors under UV irradiation was usually found to be much poorer than that of the undoped oxides, essentially because doping increases the rate of charge carriers recombination. After the work of Asahi et al. [20], highlighting the absorption and photocata-lytic properties improvements in the vis-region produced by N-doping, an exponentially increasing number of papers on the topic appeared in the literature [27]. An enormous effort is presently devoted to finding the 'best' dopant for photo-catalytic purposes, but any newly published system is only slightly or not even better than the previous one. A more re-cent and effective approach is looking for the ‘best pair' (or group of three) non-metal or metal elements able to improve the absorption in the visible and decrease the rate of charge carriers recombination. Recombination takes place not only at the doping sites, but also at other structural defects in-duced by doping, such as oxygen vacancies. In a well planned co-doped system the different dopants should play electron donor and acceptors roles in a way to avoid the for-mation of point defects through charge compensation mechanisms, thus reducing the recombination probability. Very recently, for instance, Yang et al. [28] showed that F–S

Fig. (3). Relationship between band structure of some selected metal oxide and non-oxide semiconductors and the redox potentials of water

splitting. Reprinted with permission from ref. [4]. Copyright the Royal Society of Chemistry 2009.

4 Recent Patents on Engineering 2010, Vol. 4, No. 3 Chiarello and Selli

co-doped TiO2 exhibits a photocatalytic activity higher than that of F- or S- singly doped TiO2 under visible light irradia-tion.

A different approach to shift the adsorption threshold to-wards the visible region up to 550 nm consists in metal ion implantation [12,13,29]. With this technique it is possible to modify the electronic properties of TiO2 by bombarding it with high energy metal ions, such as V, Cr, Mn, Fe and Ni, under high voltage acceleration (50-200 keV), thus injecting them into the deep bulk of the oxide, in place of Ti

4+ lattice

ions. For example, H2 evolution from a methanol in water solution was observed on thin films of Pt-loaded TiO2 photo-catalysts implanted with Cr or V (Pt/TiO2–Cr and Pt/TiO2–V) under irradiation with visible light at wavelengths longer than 400 nm, whereas no H2 evolution was observed with the non-implanted Pt-loaded TiO2 thin film [13]. The process for producing visible light responsive ion-implanted titanium oxide photocatalysts was patented by Anpo et al. [30], who reported several examples of different metal ions implanta-tion (Cr, V, Cu, Fe, Mg, Ag, Pd, Ni, Mn and Pt), together with the specific experimental conditions and the characteri-zation of the employed photocatalysts. The photocatalytic test reactions included in the patent are the butane reaction to trans-2-butene and the decomposition of nitrogen monoxide (NO) into N2 and N2O, not the photocatalytic production of hydrogen.

3.2. Dye Sensitization

Visible light excitable molecular dyes, adsorbed on the surface of large band gap n-type semiconductors (e.g. TiO2, ZnO and Fe2O3), are able to inject electrons from their elec-tronically excited state into the semiconductors’ CB, which are suitable for proton reduction to hydrogen. Unfortunately, the photo-oxidized dyes are not able to promote water oxida-tion to molecular oxygen; thus a redox system such as the I3

-

/I- pair [31], or a sacrificial agent such as EDTA [32], are

necessary to regenerate the dye and sustain the reaction cy-cle. Several types of dye were studied for this purpose, in-cluding (i) metal complexes (particularly ruthenium polypyridine complexes); (ii) organic dyes belonging to the class of thiazines, phenazines, xanthenes (e.g. Eosin Y, Rho-damin B and Rose Bengal) and acridines; (iii) natural dyes (pigments) such as cynidin dyes extracted from iris, or tea leaves or flavonoid anthocyanin dyes extracted from Califor-nia blackberry.

3.3. Addition of Sacrificial Agents

Many organic species, including the widely used metha-nol, may act as sacrificial agents in the photocatalytic pro-duction of hydrogen, being able to combine with photogen-erated valence band holes more efficiently than water (see for example ref. [33]). Thus, conduction band electrons be-come more readily available to reduce electron-acceptor spe-cies, e.g. H

+, to produce hydrogen. In this case, the overall

reaction turns into the photo-reforming of the organic com-pounds, ultimately yielding a H2 and CO2 mixture. For ex-ample, in the case of ethanol photo-reforming the reaction is:

CH3CH2OH + 3 H2O TiO2 , h 6 H2 + 2 CO2

Moreover, the co-generation of H2 and CO2, instead of the H2 and O2 mixture, suppresses the highly undesired thermal back reaction between H2 and O2, to give water.

3.4. Photocatalysts Loading with Noble Metals

Noble metal nanoparticles such as Pt, Au, Pd and Ag, de-posited on the semiconductor surface can favor the separa-tion of photoproduced electron-hole pairs [8]. Indeed, the Fermi level of noble metals is usually lower in energy than the CB energy of the TiO2 semiconductor. Thus, photopro-moted electrons can migrate and be captured by the noble metal, whereas photoproduced holes remain in the VB. From this point of view, the semiconductor with noble metal parti-cles deposited on its surface works as a short-circuited mi-cro-photoelectrochemical cell, with the noble metal acting as the photocathode, where H2 evolution occurs, and the semi-conductor surface acting as the photoanode, where the oxida-tion of electron donor species occurs.

The effectiveness in the photocatalytic performance of the noble metal co-catalysts can be related to their work function values , i.e. the energy required to promote an electron from the Fermi energy level into vacuum (the higher is , the lower is the Fermi level energy). In fact, the greater is the difference between the metal work function and that of the TiO2 support, the higher is the Schottky barrier [8,9,34], the electronic potential barrier generated by the band align-ment at the metal–semiconductor hetero-junction, with con-sequent increased efficiency of photogenerated electron transfer and trapping by the metal, ultimately leading to higher H2 production rates.

An example of the effects on the photocatalytic reaction rates of noble metal nanoparticles deposition on the semi-conductor oxide surface is reported in Fig. 4, where the rates of hydrogen, formaldehyde, formic acid and carbon dioxide production obtained from photocatalytic steam reforming of methanol over 1 wt% Ag, Au or Pt deposition on TiO2 Degussa P25 are compared to the rates obtained with bare TiO2 [35]. In this case formaldehyde and formic acid are the intermediate species of methanol oxidation up to CO2. Pt re-sulted the most effective co-catalyst with a more than 50-fold increase in H2 production rate (18.6 vs. 0.36 mmolH2 h

-1 gcat

-

1), followed by gold and silver. For the 111 crystal plane,

= 4.74 eV for Ag, = 5.31 eV for Au and = 5.93 eV for Pt [36], whereas values of 4.6 - 4.7 eV are reported in lit-erature for TiO2 [37]. Consequently, Pt can capture electrons more efficiently than gold. By contrast, the value of Ag, very close to that of TiO2, suggests scarce electron transfer ability, resulting in less efficient charge separation and con-sequent little improvement in the photocatalytic perform-ance.

Furthermore, on the noble metal surface H+ ions reduc-

tion to H2 is easier and more efficient than on the bare semi-conductor surface, because of the lower overpotential re-quired for electron transfer. Thus, although the addition of noble metal nanoparticles to TiO2 has controversial effects on the rate of photocatalytic oxidative reactions under aero-bic conditions [38], it ensures a dramatic increase of the rate of photocatalytic H2 production under anaerobic conditions.

Photocatalytic Hydrogen Production Recent Patents on Engineering 2010, Vol. 4, No. 3 5

3.5. Design and Control of Surface and Bulk Semicon-ductor Properties

Both surface and bulk semiconductor characteristics, in-cluding structural defects, crystal phases, particles size, sur-face area and acidity, have great importance in determining their photocatalytic activity [10,14,29]. In general:

(I) structural defects are detrimental because they act as recombination centers for the electron-hole pairs.

(II) the greater is the surface area (the smaller is the parti-cles size), the greater is the number of active and sur-face adsorption sites, with the consequent increase of all photocatalytic reactions rate. However, the surface itself can be envisaged as a crystal defect, which can promote charge carriers recombination. Furthermore, the smaller is the particles size, the shorter is the pho-togenerated charge carriers path to reach the active sites. On the other hand, when the particles size is smaller than the De Broglie wavelength of the elec-trons and holes confined in the semiconductor, the electronic configuration changes as a consequence of the so-called “size quantization effect”. This phe-nomenon is usually observed for particles smaller then 15 nm, where the energy levels available for electrons and holes in the VB and CB become discrete (Fig. 5), leading to a shift of their energy levels towards more negative and positives values, respectively, and thus to an increase of the band gap with respect of that of the bulk crystal. This produces an extra overpotential, which may accelerate the overall interface electron transfer rate, but also a UV-shift of the semiconductor adsorption threshold. From all of these considerations it is clear that an optimal particles size exists for each photocatalyst.

(III) Finally, metal oxides possess different crystal phases. For example, TiO2 can exist in the three main poly-morphs anatase, rutile and brookite, displaying rather different physico-chemical properties and photoactiv-ity. Anatase and rutile are the more frequently investi-gated forms, because pure brookite is quite difficult to synthesize. However, pure brookite in powder form, modified by deposition of Pt nanoparticles, proved to be a very good photocatalyst for hydrogen production from methanol-water vapors [39]. A recent invention was presented on the preparation methods and use of quantum sized anatase, brookite and rutile TiO2 pho-tocatalysts [40].

A powerful tool in characterization studies for the devel-opment of effectively photoactive materials, in particular for understanding a number of fundamental questions related to structure-reactivity relationships, light induced charge carrier separation and intrinsic or deliberately introduced defective states, is Electron Paramagnetic Resonance (EPR) spectros-copy. In fact, this spectroscopic method is very helpful for determining the structure, the dynamics and the spatial dis-tribution of paramagnetic species. Understanding the role played by impurities in N-, S- and F-doped TiO2 [41,42] is an example of the potential of this technique when applied to the study of semiconducting oxides.

Of considerable interest are also the highly ordered, ver-tically oriented TiO2 nanotube arrays fabricated by potenti-ostatic anodization of titanium [43], constituting an architec-ture that offers a large internal surface area without a con-comitant decrease in geometric and structural order. The pre-cisely oriented nature of the crystalline nanotube arrays makes them excellent electron percolation pathways for vec-torial charge transfer between interfaces.

Fig. (4). Effect of 1 wt% Ag, Au or Pt deposition on TiO2 Degussa P25 on the hydrogen, formaldehyde, formic acid and carbon dioxide pro-

duction rates, in the photocatalytic steam reforming of methanol. Specific reaction conditions: 0.014 g of photocatalyst fixed on 3 g of 20-40

mesh (0.85-0.42 mm) quartz beads fed in recirculation mode with 40 mL min-1

of 2% CH3OH / 3% H2O/ N2 (balance) gas mixture.

6 Recent Patents on Engineering 2010, Vol. 4, No. 3 Chiarello and Selli

In conclusion, the synthetic route clearly plays a crucial role in the development of materials photoactive under solar illumination. Among all of the methods of metal oxides preparation and noble metal deposition on solid catalysts we devoted particular attention to the development of Flame Spray Pyrolysis (FP) [44]. This technique is perfectly suit-able for the continuous, single step synthesis (thus poten-tially suitable for large scale productions) of very active tita-nia- and other metal oxide-based photocatalysts, either pure or modified by noble metal nanoparticles, for hydrogen pro-duction via photo steam reforming of methanol [35,45,46].

4. PHOTOCATALYTIC WATER SPLITTING

Fujishima and Honda first achieved the UV-light assisted electrochemical water splitting using a TiO2 photoanode in a PEC cell and published this in 1972 [3]. After their pioneer-ing work, a burst of interest followed in the field of photo-catalysis for water splitting, focused in particular to fine Pt/TiO2 powders dispersed in UV-illuminated water suspen-sions. However, the stoichiometric production of H2 and O2 was always difficult to achieve, because, with the exception of a few reports [47], only H2 evolution was usually ob-served. One of the most important problems in this respect is believed to be the thermal back reaction between H2 and O2 to give water on the Pt surface. The simultaneous evolution of stoichiometric H2 and O2 was reported by Sato et al., who employed NaOH-coated Pt/TiO2 fed with water vapors [48]. The presence of a high concentration (2 M) of Na2CO3 has been more recently reported by Sayama and Arakawa to pre-vent the above-mentioned back reaction on Pt and to aid the desorption of O2 from the TiO2 surface [49]. Tabata et al. [50] studied the influence of the direction of UV irradiation of Pt/TiO2 suspensions in pure water. They found that the stoichiometric evolution of H2 and O2 was achieved by irra-diating the reaction cell from the top, whereas only O2 traces could be detected when the reaction cell was irradiated from

the bottom. This was attributed to mass transfer effects, lim-iting the evolution of gases up to the suspension surface.

The research in the field has been recently focused mainly on the development of new materials able to exploit solar light efficiently and to the development of new devices for hydrogen production.

4.1. Composite Photocatalysts

Although TiO2 is still the object of wide research in con-sideration of its high activity, chemical inertness, low cost and non toxicity [6], great attention has been devoted in the last decade to several alternative semiconductor materials, in particular to mixed metal oxides with rather complex struc-ture. These newly developed solid solutions (Fig. 6) can be classified into two groups: the first one comprises elements employed to construct only crystal structures; the second group includes elements suitable to construct both crystal and energy structures. All of the 1

st, 2

nd and 3

rd groups ele-

ments of the periodic table are included in the first class, whereas metal cations with d

0 (both transition metals such as

Ti, Zr, V, Nb, Ta, Mo and W or rare earths such as Ce, Pr, Nb and Sm) or d

10 configuration (such as Cu, Ag, Zn, Cd,

Ga, In, Ge, Sn Pb and Bi) belong to the second class. As shown in Fig. 6, other elements can be introduced both as co-catalysts for hydrogen evolution (i.e. Pt, Au, Rh, NiOx and RuO2) or as dopants, in order to shift the adsorption thresh-old and photoactivity to the visible, as already mentioned in section 3. The application of such materials as photocatalysts for water splitting have been reviewed very recently by both Kudo and Miseki [15] and Kitano and Hara [16].

The mixed metal oxides investigated so far mainly in-clude perovskite-like structures, either pure (general formula ABO3, e.g. SrTiO3 [51] and NaTaO3 [52]), or differently doped (AxA’1-xByB’1-yO3, e.g. Sr or La doped NaTaO3 [53,54]), or layered, such as Ba5Nb4O15 [55], K2La2Ti3O10 [56,57], and also pyrochlore-like structures with general

Fig. (5). Energy levels diagram in semiconductor illustrating the Quantum Size Effect.

Photocatalytic Hydrogen Production Recent Patents on Engineering 2010, Vol. 4, No. 3 7

formula A2B2O6 (e.g. K2Ta2O6, [58]). The photoactivity of these materials is significantly increased by the addition of NiOx or RuO2 as H2 evolution site co-catalyst. However, al-most all of these materials have a wide band gap, between 3.2 and 4.8 eV, which makes them active only under UV-light irradiation. By contrast, exceptionally high apparent quantum yield values, i.e. even up to 56 % at 270 nm, were reported by Kato et al. over lanthanum-doped NiO/NaTaO3.

Some remarkable exceptions of visible light ( > 420 nm) photoactive mixed metal oxides materials are repre-sented by 1.0wt%NiOx/In0.9Ni0.1TaO4, with a wolframite-type structure and a band gap of ca. 2.3 eV, exhibiting a 0.66 % quantum yield under visible light irradiation [59,60], and by 0.5wt%RuO2/BiYWO3 [61]. Moreover, Domen and co-workers proposed mixed metal oxynitrides and oxysulfides, such as Pt–Ru/Y2Ta2O5N2 [62] or Pt/Sm2Ti2O5S2 [63], as new visible light photoactive materials. However, these class of materials displays a quantum efficiency lower then 1 % under visible light irradiation and presently they are not ac-tive in the simultaneous H2 and O2 evolution in the absence of sacrificial reagents.

Some of these composite photocatalytic materials for hy-drogen production have also been patented, together with their method of synthesis and uses [64-67].

4.2. Separate H2 and O2 Evolution from Photocatalytic Water Splitting

The photocatalytic water splitting systems described so far imply the evolution of a mixture of hydrogen and oxy-gen; of course, a separation step would be required prior to any use of hydrogen. Thus, a PEC cell, such as that proposed by Fujishima and Honda [3], would be preferred in order to obtain a pure H2 stream. As already mentioned, the CB flat-band potential of the greatest part of suitable semiconductor materials is very close or even more positive than the H2

evolution potential, resulting in limited spontaneous pho-toinduced H2 evolution. An external voltage (i.e. V) or chemical bias (i.e. pH) is therefore very often required in order to attain H2 evolution. By applying an external poten-tial difference, as in the case of the early Fujishima and Honda cell [3], a useful excess overpotential is created, so that cell reactions can be driven to higher current density. A second important advantage consists in the fact that an elec-tromotive force is provided by this way, so that photopro-moted electrons flow to the counter electrode, thus avoiding charge carriers recombination. The photocurrent increase re-sulting in the presence of an external potential is usually higher than the energy required to generate the voltage bias, with an overall net positive energy gain.

The chemical bias can be achieved by using a basic anolyte (e.g. a NaOH solution) and an acidic catholyte (e.g. a H2SO4 solution). Indeed, the H

+/H2 and H2O/O2 potentials

can be shifted towards more positive or negative values by changing the pH according to the Nernst equation: E = E° – 0.05916 pH, at 278 K. Thus, the largest chemical bias is at-tained when the photoanode is immersed in a basic solution at pH 14 (EO2/H2O = 0.40 V) and the counter Pt-electrode is in contact with an acidic solution at pH = 0 (EH+/H2

= E°H+/H2 =

0 V). In particular, the shift of EO2/H2O from 1.23 V at pH 0 to 0.40 V at pH 14 leads to a 0.83 V increased potential for O2 evolution on the photocatalyst surface. By contrast, if we consider the CB energy, ECB = – 0.65 V, of TiO2 at pH 0, a small excess potential of 0.65 V would be available for H2 evolution, whereas at pH 14 EH+/H2

= – 0.83 V, which is more negative than ECB and consequently no H2 evolution could occur.

In a recent patent [68], the separate production of hydro-gen and oxygen was obtained by dissociating steam, employ-ing a selective ceramic or metal extraction membrane, coated with a photocatalytic material. The setup comprises a para-bolic mirror for solar light focusing on the photocatalyst tar-

Fig. (6). Elements constructing heterogeneous photocatalysts. Reprinted with permission from ref. [4]. Copyright the Royal Society of

Chemistry 2009.

8 Recent Patents on Engineering 2010, Vol. 4, No. 3 Chiarello and Selli

get placed in a treatment chamber. The dissociation reaction is carried out at 3-15 bars pressure and at 300-1000°C tem-perature. Several types of suitable membranes are listed in the patent for both H2 or O2 separation, including perovskite-type ceramic materials and metal dense Pd-based alloys. The separation was achieved under the effect of H2 or O2 partial pressure, or by an electrical gradient. H2 and O2 are produced on the photocatalyst surface by the combination of high tem-perature photocleavage and photocatalytic processes, and readily separated by means of the selective extracting mem-brane, thus avoiding any undesired back reaction.

4.2.1. Devices Employing an External Voltage Bias

A part from TiO2, also Fe2O3 [69-73], WO3 [74-77] and BiVO4 [78,79] have been employed in the form of thin films photoanodes deposited on conductive glass supports (typi-cally tin-doped indium oxide, ITO, or F-doped SnO2 coated pyrex glass), which have been tested for the photoelectro-chemical cleavage of water. The interest on these materials is due to their relatively narrow band gap energy (2.5 eV for WO3, 2.4 eV for BiVO4 and 2.2 eV for Fe2O3), that makes them able to absorb a larger portion of visible light, up to 550-600 nm. In the so obtained devices, the photoanodes are connected through an external circuit to a metal counter elec-trode, typically a Pt electrode, where separate H2 evolution takes place. The two electrodes are usually immersed in a 1 M NaOH electrolyte solution. However, the effective amounts of H2 and O2 evolved under irradiation in this type of systems usually is not reported, the main attention being generally focused on photocurrent density measurements as a function of the applied potential. The amount of photopro-duced H2 and O2 is usually calculated from the photocurrent density, by taking into account that two electrons are re-quired to produce one H2 molecule.

The photocatalytic performance of this kind of films was found to be strongly affected by the preparation conditions and by the nature of the incorporated dopant. For example, an increased photoresponse of a nano-crystalline -Fe2O3 thin film has been reported upon doping it with Ti, Al, Zn [70,71] and Si [72,73]. Furthermore, an increase of Incident-Photon-to-Current Efficiency (IPCE) in the visible, from 25% up to over 40 %, was obtained upon Si-doping of nanostructured Fe2O3 electrodes.

Khan et al. reported a photoconversion efficiency of 8.35 % using a 40 mW cm

-2 Xe lamp illuminating a C-doped TiO2

film [22], prepared by oxidation of a Ti metal foil by means of a premixed CH4/O2 flame. However, a lively discussion [80-82] followed the publication of this paper, particularly concerning photoconversion efficiency calculations.

BiVO4 photoanodes were found to be particularly prom-ising under visible light irradiation, exhibiting a maximum IPCE up to 44 % at 420 nm [78,79]. However, the overall solar energy conversion was estimated to be much lower, i.e. around 0.2 %. More brilliant results were very recently re-ported by Abe et al. [83], who obtained a significantly high quantum efficiency (IPCE = ca. 56 % at 400 nm at 0.6 eV vs. Ag/AgCl) and demonstrated overall water splitting into H2 and O2 under visible light using an IrO2.nH2O-loaded TaON photoanode.

Lin et al. [84] designed a photoelectrochemical cell for water splitting, which combines a highly conductive TiS2 nanonet with a photoactive 27 nm-thick TiO2 coating (TiO2/TiS2 hetero-structures). Two are the advantages of-fered by this system: (i) the TiO2/electrolyte junction area is maximized by the nanonet microstructure and (ii) the trans-port of photopromoted charges is improved by the low thick-ness of the TiO2 layer and they are readily transported away through the highly conductive TiS2 material. Furthermore, visible light absorption of the TiO2 film was ensured by dop-ing it with W, leading to a Ti0.7W0.3O2 composite material with an anatase crystalline structure and an absorption threshold at ca. 600 nm. A peak power conversion efficiency of 0.83 % was so achieved under simulated solar light.

In a recent patent Gan et al. [85] claimed high photocata-lytic activity in hydrogen production by decomposition of water using a double-response (DR) photoanode deposited on a conductive glass. The DR-photoanode consists in a layer of semiconductor photocatalytic material (e.g. titania, tungsten trioxide, tantalum oxynitride, ferric oxide or stron-tium titanate) provided with a protrusion and externally coated with a nickel-iron oxide layer. The photocatalyst layer is manufactured by sputtering it on the conductive glass for 1-4 h. The protrusions are obtained by attaching a wire mesh on this first layer, followed by further sputtering for 1-3 h.

In order to attain pure solar H2 production, a “tandem cell” was developed, by associating a photoelectrocatalytic (PEC) cell, consisting in a semiconductor photoanode and a Pt cathode, with a photovoltaic (PV) cell [76], providing an external voltage bias. Similar systems were patented first by Grätzel and Augustynski [86] and more recently by Zheng et al. [87]. The Grätzel and Augustynski device consists of two superimposed photo systems electrically connected in series: (i) a front mesoporous semiconductor (WO3 or Fe2O3) film photoanode deposited on a transparent, conducting oxide coated, glass sheet in contact with an aqueous solution; (ii) a back dye sensitized TiO2 solar photo-cell, driving bias. The first oxide should absorb the blue and green part of the solar spectrum, while the remaining yellow and red light is trans-mitted through it and then captured by the solar cell mounted behind. The inventors suggest that also saline sea-water could be employed as electrolyte solution.

A similar PEC/PV solid state tandem cell, based on a non-oxide p-type gallium indium phosphide (p-GaInP2) semiconductor electrode, was earlier proposed by Khaselev and Turner [88]. Fig. 7 shows a schematic representation of this monolithic PEC/PV device. The system consists in a GaAs p/n junction (PV) bottom cell connected to the p-GaInP2 layer through a tunnel diode interconnect. The inci-dent light first reached the wider band-gap (1.83 eV) p-GaInP2 layer, absorbing the most energetic fraction of pho-tons, resulting in electron-hole pairs generation. The less en-ergetic photons penetrated through the GaInP2 layer and were absorbed by the bottom GaAs p/n junction (band gap 1.42 eV), generating the photovoltage bias. Contrarily to the systems described so far, in this case H2 evolution occurred at the photoactive semiconductor surface, consisting in a p-type semiconductor, whereas water oxidation and O2 evolu-tion occurred at the Pt-counter electrode (Fig 7). The evolved

Photocatalytic Hydrogen Production Recent Patents on Engineering 2010, Vol. 4, No. 3 9

Fig. (7). Sketch of the monolithic PEC/PV device described in ref.

[88].

gases were collected and analyzed by mass spectrometry, which proved the effective H2 and O2 stoichiometric produc-tion. A photocurrent density of 120 mA cm

-2 at 0.15 V was

reported, corresponding to H2 production efficiency of 12.4 %.

4.2.2. Devices Employing a Chemical Bias

The need of an external voltage bias in photocatalytic H2 production remains as a drawback, which inexorably de-creases the overall photoconversion efficiency. Anpo and co-workers [13] designed a H-shaped two compartments glass photocatalytic cell, which allows the separate evolution of hydrogen and oxygen without any external voltage bias. The TiO2-based photocatalytic layer contained in the system has been fully characterized and optimized in subsequent studies, with an improved performance of the whole device [89-95].

A similar photocatalytic plexyglass cell was set-up by our research group as well [96]; a schematic representation of the whole system in Fig. 8. The two cell compartments were separated by a photoactive electrode surmounting a proton exchange Nafion membrane. As better detailed in Fig. 9, the Pt/Ti/TiO2 photoelectrode consisted in a thin titanium dioxide layer, acting as photoanode, deposited on one side of a 10 cm

2 titanium disk and a platinum layer, acting as cath-

ode, deposited on the opposite side. Both layers were pre-pared by radio frequency magnetron sputtering (RF-MS).

One of the two compartments of the cell was filled with 1 M NaOH (side A in contact with the TiO2 film, see Fig. 8), the other with a 0.5 M H2SO4 water solution (side B in con-tact with the Pt film), in order to attain a chemical bias. When side A of the cell was illuminated through a pyrex glass window or different cut off filters, water splitting into molecular oxygen (from side A) and hydrogen (from side B) occurred at constant rate (Fig. 9). The evolved gases were

Fig. (8). Sketch of the cell for photocatalytic water splitting, with

separate H2 and O2 evolution: 1 titanium disk; 2 cation exchange

membrane; 3 glass filter; 4 burette; 5 stopcock; 6 rubber septum; 7

reservoir.

Fig. (9). Mechanism of photocatalytic water splitting in a two com-

partments cell separated by a Pt/Ti/TiO2 photoelectrode disk sur-

mounting a proton exchange membrane.

collected in the two upside down graduated burettes sur-mounting the two cell compartments (Fig. 8) and their vol-

10 Recent Patents on Engineering 2010, Vol. 4, No. 3 Chiarello and Selli

ume was determined from the shift of the liquid solution in the burettes. The composition of the evolved gases was checked, after sampling them with a syringe, by both mass spectrometry and gas chromatographic analysis [96].

The rutile phase, predominant when TiO2 was deposited on the titanium disk kept at 600°C, appeared to be more ac-tive in the photocatalytic production of hydrogen with re-spect to the anatase phase, which was present in higher amount when the TiO2 layer was deposited on the titanium disk kept at 450°C. This is compatible with the better capa-bility of the rutile phase to absorb light at longer wave-lengths, because of its narrower band gap. The 600°C pho-toelectrode showed active also under irradiation at wave-lengths above 400 nm. In particular, a hydrogen production rate up to 226 mol h

-1 (corresponding to 5.6 NL of hydro-

gen per hour per square meter of irradiated electrode surface) was attained with our best performing photoelectrode, with a TiO2 film thickness of 720 nm deposited at 600°C, when il-luminated in the 350-450 nm range using an iron halogenide mercury arc lamp as light source. The electrodes are very stable: their photoactivity performance remained unchanged even after over 300 h of irradiation.

Kitano et al. [92] found that the photoactivity of magne-tron sputtered TiO2 films can be improved by chemical etch-ing after treatment with a HF solution [93] and attributed this to an increased surface area and donor density of the TiO2 film, with a consequent shortening in the holes diffusion path to reach the solid-liquid interface, as well as to a higher con-ductivity. Also the deposition of a double TiO2 layer at two different temperatures has been recently found to have bene-ficial effects [95], leading to an almost doubled hydrogen production rate with respect to that attained with a single layer electrode. The enhanced photoactivity was attributed to the capability of the inner block TiO2 layer to increase elec-tron transfer.

Anpo and co-invertors patented the RF-MS preparation method of these TiO2 films either for manufacturing photoe-lectrical conversion substrates mainly for dye sensitized solar cells [97,98] or more specifically for water splitting [99]. They claimed that the photoactivity in the visible region is enhanced upon N-doping of the TiO2 film, which was achieved by introducing a small percent amount of N2 into the inert gas contained in the MS deposition chamber at a temperature of 400°C or higher.

5. PHOTOCATALYTIC REFORMING OF ORGANICS

Many organic compounds are efficient hole scavengers and can thus be envisaged as sacrificial reagents in photo-catalytic hydrogen production. This point of view, however, is not appropriate, because in the presence of organic com-pounds a photocatalytic reaction other than water splitting occurs, proceeding through a different mechanism and yield-ing different products, i.e. H2 + CO2 (and other oxidation in-termediates), instead of a H2 + O2 mixture. On the other hand, also the use of the term “photocatalytic reforming” could appear inappropriate to indicate the photocatalytic oxi-dation process of organics occurring under anaerobic condi-tions in parallel to the photocatalytic evolution of hydrogen. In fact, catalytic reforming is an industrial chemical process used to convert low octane rating petroleum refinery naphtha

into high-octane gasoline, containing so-called reformates. The reactions involved in the process lead to hydrocarbons’ structural rearrangement or chain breaking, with hydrogen sometimes being a by-product or even a reactant. By con-trast, the catalytic steam reforming consists in the high pres-sure and high temperature reaction of vaporized organics with steam yielding syngas (i.e. a H2 + CO gas mixture).

However, in photocatalysis the term “photocatalytic re-forming” is often used to indicate the liquid phase photocata-lytic reaction of organic compounds yielding H2 + CO2. If the reaction proceeds in the gas phase, then it is called “pho-tocatalytic steam-reforming”. This reaction, at difference with respect to the above mentioned corresponding catalytic (i.e. “thermal”) reaction, proceeds at ambient pressure and temperature, and is of high environmental and energetic in-terest, when organics-containing waste water, bio-ethanol or any other suitable bio-mass are employed as hole scavengers sources. It may also be envisaged as an effective way to combine the abatement of organic pollutants with solar hy-drogen production [100]. Bio-mass is renewable and plants consume atmospheric CO2 during their growth, so that a small net CO2 environmental impact can be achieved com-pared to that involved in the use of fossil fuels.

The advantage of the photocatalytic reforming process with respect to the corresponding traditional thermal process consists in the fact that, because of the endothermic nature of the steam reforming reaction, heat must be supplied for the reaction to proceed, which is usually provided by the com-bustion of part of the feed stock, with a consequent decrease of the net yield of the process. By contrast, the photo-assisted process occurs at room temperature and atmospheric pressure and no feed stock needs to be burnt, because the re-quired energy is totally supplied by the conversion of the en-ergy of photons. Hence, the overall chemical energy content of the products is effectively higher than that of the reactants, leading to a net photoenergy conversion and storage in the form of chemicals, in particular in the form of hydrogen, the fuel of the future.

A large variety of organic compounds have been em-ployed in recent years in the photocatalytic production of hydrogen. These include methane [101,102], methanol [45,103-110], ethanol [108,110-113], isopropanol [108], n-butanol [108,110], polyalcohols [114], acetaldehyde [115], carboxylic acids [110,115,116], glycerin [117,118], sugars [114,119,120], azo-dyes [110], EDTA [107], and several ali-phatic and aromatic compounds [121]. In general, the H2 production rate was found to significantly increase upon or-ganics’ addition to the photocatalyst water suspension. This modus operandi, however, evidenced mass transfer limita-tions to H2 production and short-time stability of the suspen-sions [45]. An alternative setup has been designed and adopted in our research group to test the photocatalytic activ-ity in the steam photoreforming of volatile organic com-pounds, implying the immobilization of photocatalysts on a bed of quartz beads, which are continuously fed with metha-nol-water vapors [35,46]. This system allowed a much more accurate control of the reaction conditions and led to an in-crease of both hydrogen production rate and overall photon efficiency.

Photocatalytic Hydrogen Production Recent Patents on Engineering 2010, Vol. 4, No. 3 11

Some recent patents claiming the photocatalytic degrada-tion of organic water pollutants with the simultaneous evolu-tion of hydrogen recently appeared [122,123]. A PEC reac-tor, containing a two electrodes configuration with two com-partments for separate evolution of gases, was recently de-signed [124], containing a photoanode based on an ordered array of titania nanotubes (Fig. 10), suitable for H2 produc-tion by water splitting or photoreforming of ethanol (in liq-uid or gas phase). Alternatively, carbon-nanotube based elec-trodes can be employed for the gas phase reduction of CO2 to liquid fuels (mainly isopropanol).

A composite photocatalytic hydrogen production system (Fig. 11), its preparation method and its uses was also pat-ented [125], which are quite different from those described so far. In this case the decomposition of the organic com-pounds occurs at the irradiated photocatalyst surface, acting as an electron donor, whereas hydrogen is evolved at the ac-tive site of an artificial enzyme. The electron transfer from the photocatalyst to the enzyme occurs by means of a re-

versible redox mediator, such as methyl viologen. The artifi-cial enzyme is a hydrogenase mimic enzyme in the form of a 24-subunit, small heat shock protein (HSp) with the active H2 evolution site, typically a Pt nanoparticle, placed into the core of the protein cage. The photocatalyst can be either a semiconductor metal oxide or a metallorganic compound, and can be encapsulated in the protein cage architecture, which, in turn, is immobilized in a polymer gel. The protein cage architecture is intended to mimic the controlled molecu-lar access to the active site of hydrogenase as well as to be used as a multivalent template for the attachment of light harvesting molecules at specific sites. Moreover, the Pt nanoparticles are size and shape constrained by the protein cage, giving rise to very small Pt colloids possessing high surface area. These composite materials showed to be active in the photocatalytic production of hydrogen under simulated solar light irradiation in the presence of an electron donor such as EDTA. However, the photoactivity was reported to slightly decrease after three weeks of irradiation.

Fig. (10). (a) View of the lab-scale PEC device. (b) Image of the photo/electro-catalytic disc. (c) Scheme of the PEC device for CO2 reduc-

tion to fuels and H2 production. Reprinted with permission from ref. [124]. Copyright the Royal Society of Chemistry 2010.

12 Recent Patents on Engineering 2010, Vol. 4, No. 3 Chiarello and Selli

6. CURRENT & FUTURE DEVELOPMENTS

The conversion of solar into chemical energy for the pro-duction solar fuels is surely one of the most important issues in the future worldwide energy scenario. In spite of the enormous scientific efforts carried out in the last decade, the photocatalytic production of hydrogen seems to be still quite far from real large-scale production applications, because its quantum efficiency is still too low. The main achievements attained up today consist in the understanding of the phe-nomena involved in photocatalysis and in the development of new, innovative and always more efficient photocatalytic semiconductor materials and technologies. Future large scale application of photocatalysis in solar hydrogen production may thus be envisaged.

The greatest current efforts are presently addressed to solve the two main general challenges in photocatalysis, i.e. to increase the separation of photopromoted charge carriers and to shift the semiconductors’ adsorption threshold into the visible region, in order to exploit a larger part of the solar spectrum. The first issue requires the engineering of novel composite photocatalytic materials, containing efficient elec-tron or holes trapping co-catalysts. On the other hand, doping and especially co-doping of semiconductor materials, to-gether with the development of chemically different, low band gap new materials, are presently regarded as promising ways to achieve full solar light exploitation.

Of vital importance is of course the development of even more sophisticated preparation techniques, able to control and tune the physico-chemical properties of photoactive ma-terials. Gas phase synthetic methods, such as Radio Fre-quency Magnetron Sputtering (RF-MS) [96], Flame Spray Pyrolysis (FSP) [44] and Pulsed Microplasma Cluster Source (PMCS), coupled with Supersonic Cluster Beam Deposition (SCBD) [126], at the moment appear to be the most promis-ing preparation techniques. Visible-light active semiconduc-tor thin-layers should be the photoactive part of solar light driven devices ensuring separate hydrogen production, ob-tained from further development and optimization of the tan-dem cells described in section 4.2.1 or the two compartment cells presented in section 4.2.2. These presently appear as the

most promising photocatalytic systems for a future scaling up of solar hydrogen, also suitable for the simultaneous pho-tocatalytic abatement of organic water pollutants.

ACKNOWLEDGEMENTS

Financial support from CARIPLO Foundation through the Projects entitled Development of nanostructured photo-catalytic films for energy conversion on microplatforms and Visible Light Sensitive Photocatalytic Materials for Separate Hydrogen Production Devices is gratefully acknowledged.

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