Efficient solar hydrogen production by photocatalytic water splitting: From fundamental study to pilot demonstration

international journal of hydrogen energy 35 (2010) 7087–7097

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Efficient solar hydrogen production by photocatalytic water splitting: From fundamental study to pilot demonstration Dengwei Jing, Liejin Guo*, Liang Zhao, Ximin Zhang, Huan Liu, Mingtao Li, Shaohua Shen, Guanjie Liu, Xiaowei Hu, Xianghui Zhang, Kai Zhang, Lijin Ma, Penghui Guo State Key Lab of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, 28 Xianning West Road, Xi’an 710049, PR China

article info

abstract

Article history:

Photocatalytic water splitting with solar light is one of the most promising technologies for

Received 19 November 2009

solar hydrogen production. From a systematic point of view, whether it is photocatalyst

Received in revised form

and reaction system development or the reactor-related design, the essentials could be

6 January 2010

summarized as: photon transfer limitations and mass transfer limitations (in the case of

Accepted 9 January 2010

liquid phase reactions). Optimization of these two issues are therefore given special

Available online 9 February 2010

attention throughout our study. In this review, the state of the art for the research of photocatalytic hydrogen production, both outcomes and challenges in this field, were

Keywords:

briefly reviewed. Research progress of our lab, from fundamental study of photocatalyst

Solar hydrogen

preparation to reactor configuration and pilot level demonstration, were introduced,

Photocatalytic

showing the complete process of our effort for this technology to be economic viable in the

Energy conversion

near future. Our systematic and continuous study in this field lead to the development of

Water splitting

a Compound Parabolic Concentrator (CPC) based photocatalytic hydrogen production solar rector for the first time. We have demonstrated the feasibility for efficient photocatalytic hydrogen production under direct solar light. The exiting challenges and difficulties for this technology to proceed from successful laboratory photocatalysis set-up up to an industrially relevant scale are also proposed. These issues have been the object of our research and would also be the direction of our study in future. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Conventional energy resources, which are being used to meet most of the world’s energy requirements, have been depleted to a great extent. It is therefore necessary to produce an alternative fuel that should in principle be pollution-free, storable, and economical. Hydrogen satisfies the first two conditions, and research has been focused on fulfilling the third requirement in the past decades [1,2]. To be an economical and sustainable pathway, hydrogen should be manufactured from a renewable energy source, i.e., solar energy. Photocatalytic water splitting is the most promising technology for the

purpose, since H2 could be obtained directly from abundant and renewable water and solar light from the process. If successfully developed with economic viability, this could be the ultimate technology that could solve both energy and environmental problems altogether in the future [3–9]. Water splitting using light energy has been studied for a long time using powder systems since the Honda–Fujishima effect was reported [10,11]. Much progress has been made in the past decades. Thermodynamically, water splitting into H2 and O2 is an uphill reaction, accompanied by a large positive change in the Gibbs free energy (DG ¼ 238 kJ/mol). The efficiency of water splitting is determined by the band gap and

* Corresponding author. E-mail address: [email protected] (L. Guo). 0360-3199/$ – see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.01.030

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Fig. 1 – Basic principle of overall water splitting on a cocatalyst-loaded semiconductor.

band structure of the semiconductor and the electron transfer process, as shown in Fig. 1 [12,13]. Generally for efficient H2 production using a visible-light-driven semiconductor, the band gap should be less than 3.0 eV (420 nm), but larger than 1.23 eV, corresponding to the water splitting potential and a wavelength of ca. 1000 nm. Moreover, the conduction band (CB) and valence band (VB) levels should satisfy the energy requirements set by the reduction and oxidation potentials for H2O, respectively. Band engineering is thus necessary for the design of semiconductors with these combined properties. In the past decades, efforts have been made to address the following two important issues. One issue is the development of efficient visible-light-driven photocatalyst which has undergone a rapid progress especially in the past decade. The other 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 resolved by converting solar energy into storable hydrogen energy. For the second, the solution could be the use of solar concentrator. The strategies are schematically illustrated in Fig. 2. As can be found in Fig. 2, whether it is photocatalyst development or the reactor- and system-related design, the essentials could be summarized as: photon transfer limitations and mass transfer limitations (in the case of liquid phase reactions)[14]. For photon transfer optimization, it concerns

the choice of photocatalyst, the reaction media and the reactor configuration. Here, reaction media is often the aqueous solution containing various sacrificial agents for the elimination of photo-generated holes and for further improvement of photocatalytic efficiency or simultaneous decomposition of toxic organics. The photocatalytic material should efficiently absorb photos and separate photo-generated charges. Fast transportation of the photo-generated carriers must be guaranteed to avoid bulk electron/hole recombination. The separated electrons and holes act as reducer and oxidizer, respectively, in the water splitting reaction over semiconductors to produce hydrogen and oxygen. In the field of mass transfer optimization, many reactors and reactor configurations have been investigated for their use in photocatalysis. Gas–liquid two phase and gas–liquid–solid three phase flow study in various reactor configurations, especially tubular reactors, are important for mass transfer optimization. Research on photocatalytic hydrogen production in China has been initialed in nineties of the last century and we are among the groups conducting earliest work in the field. In 2003, the project of the Basic Research of Mass Hydrogen Production Using Solar Energy founded by National Basic Research Program of China (973 Plan) was initiated by SKMFPE with the participation of almost all the main teams conducting the related studies at the time. With the support of 973 Project and other financial support from the government, SKMFPE has set their research direction to the development of highly efficient, stable and low-cost visible-light-driven photocatalyst by various modification methods, such as doping, sensitization, supporting and coupling methods to extend the light responsive and performance of the photocatalyst. We have studied the photocatalytic materials as powders for photocatalytic reaction and as solid films for photoelectrochemical hydrogen production as well. Various photocatalytic reactors and relevant instruments have also been developed for photocatalytic hydrogen production, photocatalyst screening and evaluation, which formed a complete platform for further in-depth study. In particular, we have devoted to photocatalytic hydrogen under direct solar light and have also been successful. A series of significant results were obtained in the course of our continuous research. With all these accomplishments, we have been supported by the new 973 project of China which started in 2009. In this review, the state of the art for the research of photocatalytic hydrogen production, both outcomes and challenges in the field, were briefly reviewed. Research progress of our lab, from fundamental study of photocatalyst preparation to the issues related to reactor configuration and pilot level demonstration, were introduced, showing the complete process of our effort for this technology to be economic viable in the near future.

2. Materials for photocatalytic/ photoelectrochemical hydrogen production under visible light 2.1. Fig. 2 – Schematic illustration for the process of photocatalytic water splitting hydrogen production under solar light considered from a systematic point of view.

Materials for photocatalytic hydrogen production

To split water using solar energy, semiconductor photocatalysts, such as TiO2, SrTiO3, Nb2O5, SiC, CdS GaP [2], etc

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have obtained much attention. Various modification methods such as doping [4], sensitization [15], and hybrid composite etc have been attempted. Up to now, over 130 materials and derivatives have been developed to photocatalyze the overall water splitting or produce hydrogen/ oxygen in the presence of external redox agents. Combinatorial method has been developed that has been demonstrated as a convenient way for quick selection of photocatalyst materials [16–18]. We have developed similar instruments that have been successfully applied in our lab. In lab-scaled photocatalytic water splitting and hydrogen production, very high efficiencies were obtained over NiO– La:NaTaO3 (QE ¼ 56%, pure water, 270 nm) [9], Pt–ZnS (QE ¼ 90%, aqueous Na2SO3 solution, 313 nm)[19], Cr/Rh-modified GaN/ZnO (QE ¼ 5.9%, pure water, 420–440 nm) [20], and Ptloaded CdS (QE ¼ 60%, aqueous Na2S/Na2SO3 solution, 420 nm) [21]. However, no semiconducting material has been found to be capable of catalyzing the overall water splitting under visible-light with a QE larger than the commercial application limit 30% at 600 nm [22]. It is considered that the low efficiency for the hydrogen production of semiconductor already with appropriate band gap is 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 efficiently utilize visible-light. It was often observed that photo-generated electrons easily recombine with holes in the semiconductor. This recombination leads to the low quantum efficiency of photocatalysis [23]. Noble metal loading can suppress to some extent the charge recombination by forming a schottky barrier. More often various sacrificial reagents such as inorganic salts and organics were added in the reaction media, effectively restraining the charge recombination process and improve quantum efficiency [24]. Separation of hydrogen gas is also required as oxygen and hydrogen are produced simultaneously. This could be achieved by employing

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a photoelectron-chemical system, in which hydrogen and oxygen are produced at different electrodes.

2.1.1.

CdS and CdS-based photocatalyst

The most often studied photocatalysts that have suitable band gaps for photocatalytic hydrogen production are illustrated in Fig. 3. Among these materials, Pt-loaded CdS photocatalyst is the earliest and most studied showing high activity for H2 production from aqueous solutions containing S2 and SO2 3 ions as sacrificial electron donors, under visible-light irradiation. Sacrificial electron donors that irreversibly consume photo-generated holes may promote hydrogen evolution. If the reaction could be turned into a practical application for the production of hydrogen gas from byproducts such as hydrogen sulfide and sulfur dioxide, which are emitted in hydrogenation and flue-gas desulfurization processes at chemical plants, it would be especially interesting in light of current energy and environmental concerns [25,26]. It should be noted that CdS is prone to photocorrosion in the photocatalytic reaction. In order to enhance the activity of CdS, efforts have been made to combine CdS with other semiconductors having different band energies (e.g., TiO2/ CdS, ZnO/CdS, ZnS/CdS [27,28], K4Nb6O17/CdS [29] or K2Ti4O9/ CdS composites [30,31]. An alternative approach to enhance the photoactivity of CdS is to couple CdS with mesoporous materials to form hybrid or composite photocatalysts. In these cases, the photo-generated electrons in CdS are able to move freely into an attached semiconductor or a framework of porous molecules, while the photo-generated holes are trapped in CdS. Therefore, high charge separation and photo utilization would be achieved. Efforts have also to be made to improve the stability of the metal sulfide. We have developed a novel two-step thermal sulfuration method for the preparation of highly stable and active CdS. As shown in Fig. 4, the surface of the CdS photocatalyst was modified with nanostep structures which resulted in much higher hydrogen production rate than CdS prepared by common procedures [32]. The enrichment of Pt nanoparticles at the nanostep region are

Fig. 3 – The band gap positions for various traditional semiconductors relative to the redox potential of water.

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Fig. 4 – TEM image for CdS photocatalyst prepared by two-step surfuration method and its hydrogen production. The preparation of the photocatalyst can be found in literature [32].

considered indispensable for the much enhanced hydrogen production. The activity of the this photocatalyst was further improved by coupled with a zirconium titanium phosphate mesoporous support [33]. It is expected that such composite photocatalyst would reduce the use of noble metal and improve that stability and efficiency of the photocatalyst. It is also worth noting that this strategy has been successfully applied to other sulfide such as WS2 [34].

2.1.2.

Solid solution photocatlyst

In recent years, solid solution photocatalysts with controlled electronic structures has been suggested as a promising direction. Various solid solutions such as (GaN)(ZnO) [35], (AgIn)xZn2(1x)S2 sulfide solution [36] and other oxide solution [37] have been developed for photocatalytic hydrogen production in pure water, sulfide and alcohol. ZnS with 3.6 eVband gap is a well-known photocatalyst for H2 evolution though it responds to only UV. It shows high activity without any assistance of co-catalysts such as Pt. Chen et al. reported a nanoporous ZnS–In2S3–Ag2S solid solution synthesized by a facile template-free method that showed high activities for H2 evolution under visible-light irradiation in the absence of co-catalysts. The initial rate of photocatalytic hydrogen yield reached 3.3 mmol h1 with 0.015 g photocatalyst employed [38]. In view of practical application of photocatalytic hydrogen production technique, cost reduction of the photocatalyst is one of the key issues. Thus, active photocatalysts free of noble metal like Cd1xZnxS is valuable in this consideration. The controllable band structure of this solid solution further adds

its value for industrial application. A series of Cd1xZnxS (x ¼ 0– 0.92) photocatalysts were prepared by co-precipitation method and were calcined at 723 K under N2 atmosphere [39]. The band gap of the photocatalyst can be continuously adjusted by changing the composition of the solid solution (see Fig. 5). At the optimal composition, the solid solution showed high activity toward hydrogen production even in the absence of noble metal loading. However, Cd1xZnxS prepared by conventional co-precipitation method often shows poor crystallinity. The activity and stability of the prepared material is far from being satisfactory for its commercial utilization. Recently, in our group a series of Cd1xZnxS solid solution photocatalysts was prepared by thermal sulfuration of corresponding oxide precursors [40]. The band gap control of solid solution photocatalyst can also be achieved by varying its composition. The final composition for all the samples prepared by thermal sulfuration of corresponding mixed precursors is close to their stoichiometric composition. It is found that Cd0.8Zn0.2S solid solution with nominal x value of 0.2 showed the highest activity toward hydrogen production as shown in Fig. 6, the quantum efficiency achieves 9.6% at 420 nm. For pure Cd1xZnxS, it is assumed that the band gap of Cd1xZnxS would be quite large when the conduction band of the solid solution is high enough for efficient hydrogen production. Doping Ni2þ into Cd1xZnxS solid solution can tune its band structures by both solid solution and metal ion doping. In this case, Ni2þ is expected to form a donor lever above the valence band of Cd1xZnxS to reduce its band gap and increase its visible-light absorption, while still maintaining its high

Fig. 5 – Conduction and valence band potentials of the Cd1–xZnxS photocatalysts with various Cd/Zn ratios.

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Fig. 6 – Hydrogen production for Cd1LxZnxS solid solution prepared by various methods. a) Cd0.8Zn0.2S–S, prepared by two-step thermal sulfuration (b) Cd0.8Zn0.2S–C, prepared by co-precipitation (c) Cd0.8Zn0.2S–N, prepared by two thermal sulfuration under N2 atmosphere.

conduction band. In our study it was found that 0.1 wt% Ni2þ doped Cd0.1Zn0.9S photocatalyst showed the highest activity with the apparent quantum yield of 15.9% at 420 nm [41]. Band structure of the Cd1xZnxS solid solution was further modified with Cu doping and high efficiency is also obtained [42].

2.1.3.

Formation of hybrid or composite photocatalyst

Coupling of two photocatalyst has been considered effective for improvement of photocatalytic efficiency. To extend the light absorption of such wide band gap semiconductors as TiO2 and Ta2O5, it is doped with cationic and anionic ions [43– 45]. In our study, nitrogen doped TiO2 was coupled with WO3 and after loaded with noble metal, high efficiency was obtained [43]. CdS nanocrystallites have been successfully incorporated into the mesopores of Ti-MCM-41 by a two-step method involving ion-exchange and sulfuration forming a CdS@Ti-MCM-41 composite photocatalyst. Owning to the quantum confinement effect and efficient charge separation, the activity of CdS photocatalyst has been greatly improved [46,47]. The activity of CdS@Ti-MCM-41 was much improved by loading Pt co-catalyst. This demonstrates that Ti-MCM-41 serves as a stable host to protect the loaded CdS particles from photocorrosion. In our another study, CdS nanoparticles were decorated on Na2Ti2O4(OH)2 nanotubes through partial ionexchange method [48]. The results showed that The high activity of the prepared composite photocatalyst can be attributed to the enhanced charge separation due to the onedimensional nanotube structure of the Na2Ti2O4(OH)2. This further suggest that materials with special morphology or structure are favorable for enhanced photo utilization.

2.1.4.

Other novel photocatalyst developed

Recently, mesoporous silicate materials involving transitionmetal ions within the mesoporous framework have opened new possibilities in many research areas not only for catalysis

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but also for various photochemical processes, such as photocatalytic degradation of organic pollutant. In our study, molecular zeolite such as MCM-41 and SBA-15 was decorated with transitional metals such as Cr,V,Ti etc to extend their absorption [49,50]. Multi-component sulfide photocatalyst ZnIn2S4 is the only member of the AB2X4 family semiconductor with a layered structure. In addition, the structure and morphology of ZnIn2S4 could be controlled by the various surfactants and solvents added in the hydrothermal condition. Therefore, the authors’ group conducted a systematic investigation on the effects of solvents, surfactants and hydrothermal time [51–53] on the photocatalytic activity of ZnIn2S4 for hydrogen evolution under visible-light irradiation. It was found that the assistance of CTAB not only greatly enhanced the photocatalytic activity but also strongly affected the crystal structure of ZnIn2S4 compared to the other surfactant-assisted ZnIn2S4 photocatalysts. Such correlation between the photocatalytic activity and the structure distortion has so far been reported for some metal oxide photocatalysts. In our further research on ZnIn2S4, a series of Cudoped ZnIn2S4 photocatalysts was synthesized by a facile hydrothermal method, with the copper concentration up to 2.0 wt% [54]. Most recently, Domen et al. showed [55] that an abundant material, polymeric carbon nitride, can produce hydrogen from water under visible-light irradiation in the presence of a sacrificial donor. Contrary to other conducting polymer semiconductors, carbon nitride is chemically and thermally stable and does not rely on complicated manufacturing device. The results represent an important step towards photosynthesis in general where artificial conjugated polymer semiconductors can be used as energy transducers. In another interesting study, Demuth et al. found TiSi2 can be used for overall water splitting with simultaneous hydrogen and oxygen production. It is also worth noting that this material has the ability for hydrogen storage [56]. All the above finding indicates that both traditional semiconductor material with improved properties and semiconductor of new compositions are promising candidate materials for future application. Nevertheless, stability and cost is still the priority for the choice and design of the new photocatalyst from a practical point of view.

2.2. Materials for photoelectrochemical hydrogen production For photoelectrochemical decomposition of water which takes place in photoelectrochemical cells (PECs), hydrogen and oxygen are separately generated on the surface of photo cathode and photoanode. During the past decades many oxide photoelectrodes such as TiO2, WO3, and SrTiO3 have been extensively studied for hydrogen production [57–59]. However, due to their wide band gaps, these oxides can respond only to ultraviolet (UV) light. Materials that have visible-light response and can be readily prepared as film form are considered. This field is also the interest of our team. We build up an automated photocurrent spectroscopy system to evaluate the response or the material to different wavelength of light. The wavelength region of solar spectrum contributing to the water splitting reaction can be determined. Systematic

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3. Instruments and reactors for photocatalytic hydrogen production 3.1.

Fig. 7 – Instruments for ultrasonic spray pyrolysis (USP) film preparation developed in our Lab.

study of the intensity modulated photocurrent spectroscopy system (IMPS) and the intensity modulated photovoltage spectroscopy system (IMVS) have also been carried out. Various semiconductor photoelectrodes, especially in thin film form, have been investigated. Traditional TiO2 photoelectrode only respond to UV light which occupies only 4% of the whole solar energy. Development of new PEC materials or the new preparation method for high quality film is thus of importance. We have designed an ultrasonic spray pyrolysis (USP) instrument for film preparation, as shown in Fig. 7. ZnIn2S4, BiVO4 and WO3 films with different crystal structure were deposited by USP. For BiVO4 film, the material with different Bi/V ratio in each deposition precursor has been studied to explore their potential application in hydrogen production by photoelectrochemical water splitting. The effect of Bi/V ratio on structure and photoelectrochemical properties were also studied which showed that USP is a reliable instrument for the preparation of high quality film. ZnIn2S4 thin films can be prepared on ITO conductive glass by spray pyrolysis from a mixed aqueous solution [60]. The deposited film is endowed with a cubic spinel structure with strong absorption in visible-light. As a photoanode, the deposited film also exhibits a good photo response. In 0.1 mol/L Na2SO3 þ 0.1 mol/L Na2S mixed solution, the IPCE amounts to over 30% at a potential of 0.3 V vs. SCE in 400 nm irradiation wavelength. WO3 films were also prepared by us with ultrasonic spray pyrolysis using precursor obtained by dissolving tungsten acid in hydrogen peroxide aqueous solution in 363K water bath. The effect on the structure and photoelectrochemical properties of WO3 films by varying amount of hydrogen peroxide added and concentration of precursor was investigated. As mentioned above, semiconductor with small band gap can absorb more light and excite more electron-hole pairs, but if these excited electron-hole pairs are not separated and transport to anode or cathode in time, recombination will occur subsequently. Along with reducing the band gap, electron separation and transport improvements are also important ways to improve conversion efficiency. One dimensional Nano-structure such as nanotube, nano-wire and nanocolumn arrays are excellent electron percolation pathways for charge transfer and offer a large internal surface area. We proposed that low band gap semiconductor films with nanostructure can be a promising approach to efficient photoelectrochemical water splitting.

General introduction

Our group has also developed series instruments for photocatalytic hydrogen production, photocatalyst screening and activity evaluation. We have developed small closed circulation reactor, photocatalytic hydrogen production reactor with simulated solar light and direct solar photocatalytic hydrogen production reactor with compound parabolic collector (CPC). These reactors ensure the evaluation of the developed photocatalyst from lab scale to out-door demonstration scale. As for material development, a system for quick preparation and selection of photocatalyst has been designed in our group, based on the development of a novel hydrogen gas sensor that can quantitatively determines the hydrogen concentration in a precise way. On the other hand, a novel multi-channel photocatalyst evaluation system has also been developed that can simultaneously evaluate six groups of photocatalysts in a precise, convenient and in-situ manner. To the best of our knowledge, no similar system has been reported or patented in other groups. The set-up of all these photocatalyst evaluation system provided a powerful support for the relative research in SKMFPE and lead to the finding of many active photocatalysts. As for the new reaction system, most traditional rector can only operate in a batch mode. And the sacrificial agent is often irreversibly consumed. We have designed a double bed photocatalytic system, with photocatalytic hydrogen production occurs on one bed and the sacrificial agent re-generated on another bed. This design leads to the formation of a continuous reaction system with very stable hydrogen production rate. For the realization of mass production of hydrogen, development of efficient solar light concentrator is one of the key issues in regard that solar light is diffuse and has low energy intensity. We have selected (CPC) to construct sunlight-driven hydrogen production system (Fig. 8). Sustainable solar light water splitting hydrogen production on a relatively lager scale has been realized by coupling CPC with an inner-circulated reactor. In the following part, our consideration for the design of solar photocatalytic hydrogen production reactor and the preliminary results will be briefly introduced [61].

3.2. Design of solar photocatalytic hydrogen production reactor Although various solar reactors have been developed, it is surprising that most of them aim only at photocatalytic detoxification [62–64]. To the best of our knowledge no solar reactor designed for photocatalytic hydrogen production have been reported in literatures. The object of our work is to explore the possibility of mass solar hydrogen production by coupling photocatalytic reactors with solar light concentrators by fully considering the similarity and dissimilarity between photocatalytic hydrogen production and photocatalytic detoxification process, both based on the existing technologies, literatures and on our theoretical and

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Fig. 8 – Photocatalytic hydrogen production reactors developed in our Lab. a) small closed circulation reactor; b) photocatalytic hydrogen production reactor with simulated solar light; c) direct solar photocatalytic hydrogen production reactor.

experimental study. Following this consideration, special requirement for photocatalytic hydrogen production process must be addressed for the design of an efficient solar photocatalytic hydrogen production reactor (SPHR). An East–West alignment with small half acceptance angle has been chosen. For the maximum acceptance of solar radiation, the aperture of the CPC should be perpendicular to the incident light as far as possible. Thus the CPC aperture should be tilted at the angle close to the latitude of the local site facing south, to maximize the available solar irradiation. In our case tilt of 35 degrees was chosen which is the latitude of Xi’an city. It is necessary that uniform flow be maintained at all times in the tubular reactor, since non-uniform flows causes nonuniform residence times that can lower efficiency compared to the ideal conditions [65]. In the case of the heterogeneous process with photocatalyst powder in suspension, sedimentation and depositing of the catalyst along the hydraulic circuit should be avoided and turbulent flow in the reactor must be guaranteed. As has been demonstrated [66] Reynolds’s number varying between 10 000–50 000 ensures fully turbulent flow and avoids the settlement of TiO2 particles in the tubes. This choice of Reynolds’s number can thus be extended to our design. Furthermore, every photoreactor design must guarantee that all the useful incoming photons are used and do not escape without having intercepted a particle in the reactor. For these reasons, and also from a practical point of view, diameters of less than 12.5 mm are not feasible.

3.3.

Preliminary results for the designed SPHR

The prototype SPHR includes the following components: solar collector, Pyrex photoreactor tubes, reflective surface, flow

meter; fitting, pipes and tanks; pump and sensors (pH, temperature, oxygen and pyranometer). The adopted CPC parameters in our case are: the maximum half incident angle for CPC is 14 , for cost reduction, the half acceptance after truncation 30 . Three consecutive clear days form June 16–18 2006 in Xi’an city was chosen for the test of the optical properties of the designed CPC. For the case of our local site it is found that the solar radiation shows an initial increase from 12:00 to 13:00 where it reached a maximum of 527 W/m2. For our CPC based solar reactor, the concentration factor at this point was determined to be 4.76. Thereafter, the solar radiation undergoes a rapid decrease to 369 W/m2 at 15:30, where the concentration is also the smallest. Our results indicated that for a CPC based solar reactor to function efficiently, such an operation site is necessary where strong solar radiation are available. For photocatalytic hydrogen production, total volume of water in our SPHR is 11.4 L. To optimize the design of the SPHR, such parameters as tube radius, flow velocity, photocatalyst and sacrificial agent concentrations, were investigated. In all these tests, water used is distilled water avoiding the possible influence from the inorganic salts on the photocatalytic reaction. As shown in Table 1, the maximum hydrogen production rate amounted to 1.88 L/h for our designed SPHR at optimum conditions of case 7, corresponding to a hydrogen production rate of 0.164 L/h per unit volume of reaction solution. While for lab scale photocatalytic hydrogen production under visible-light irradiation (lS430), it is 0.126 L/h. The higher hydrogen production rate per unit volume may be attributed to the design of tubular reactor well illuminated by CPC on one hand. This combination enables solar rays to illuminate the complete perimeter of the round receiver, rather than just the ‘‘front’’ of it, which may greatly

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Table 1 – Investigation of various parameters for SPHR. No.

1 2 3 4 5 6 7 8 9

Tube radius (mm)

Flow velocity (cm/s)

Catalyst Concentration (g/L)

Sacrificial Agent (mol/L)

Hydrogen production Rate (L/h)

Solar intensity W/m2

h%

11 11 11 11 11 11 11 15 15

11.7 15.4 20.3 25.6 25.6 25.6 25.6 15.4 25.6

0.5 0.5 0.5 0.5 0.8 1.0 1.0 0.5 1.0

0.05,0.06 0.05,0.06 0.05,0.06 0.05,0.06 0.05,0.06 0.05,0.06 0.1,0.1 0.05,0.06 0.05,0.06

0.45 0.71 0.94 1.50 1.71 1.49 1.88 0.29 0.86

453 473 419 518 491 410 490 447 491

0.12 0.18 0.27 0.35 0.42 0.44 0.47 0.08 0.21

reduce the available solar rays. On the other hand, efficient utilization of UV part of solar light by CPC may be another important reason. As is discussed previously, though having limited number, photos in UV range are much more efficient in driving the photocatalytic hydrogen production.

4. Numerical investigations of catalyst– liquid slurry flow in the photocatalytic reactor To investigate the operation of a photocatalytic reactor with high photocatalytic efficiency, research on photocatalyst, ray transfer, local absorption of photons has been carried out frequently [67,68] However, up to now almost all the relevant investigations have ignored the influence of different flow regions or catalyst distributions on the photocatalysis in the reactor. In fact, the distribution characteristics of catalyst particle determine the radiation distribution and photon absorption in the reactor. So the investigations on the

catalyst-liquid two-phase flow in the reactor play an important part in the reactor design and its operation. The research on the solid–liquid two-phase flow has been carried out widely including pressure drop, flow region, particle deposition velocity, which have been obtained by experiments and theoretical analysis [69,70]. With the development of numerical simulation, relevant prediction models have also been established. For example, the particle trajectory and collisions in two-phase flow has been simulated by Lagrangian method. And the Eulerian method based on kinetic theory has been widely used in the conditions of dense particles. In our study, we focused on the effects of solid– liquid two-phase flow characteristics on the photocatalytic process during hydrogen production by numerical simulation, especially the effects of catalyst distribution coupling with ray transfer. Considering the characteristics of catalyst–liquid slurry flow, an algebraic slip mixture (ASM) model was selected in our study. Slurry pressure gradient is an important parameter in practice, reflecting the operation conditions of the reactor.

Fig. 9 – Catalyst volume fraction distribution contours and profiles under different mean slurry velocities with catalyst volume fraction.

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5.

Fig. 10 – Slip velocity distributions along the vertical center line of the cross section under different catalyst volume fractions with mean slurry velocity 1.5 m/s.

According to Wasp et al. [71], the pressure drop in solid–liquid two-phase flow included two parts: pressure drop due to vehicle (homogeneous distribution) and excess pressure drop due to bed formation (heterogeneous distribution). Our results indicate that the mean slurry pressure gradient increases with the mean slurry velocity, and the mean slurry pressure gradient also increases with the catalyst volume fraction [72]. Fig. 9 shows catalyst volume fraction distributions on the outlet of reactor pipe and its vertical center line respectively, under different mean slurry velocities with catalyst volume fraction 0.10. Due to the density difference, some catalyst particles begin to deposit, causing a higher volume fraction of catalyst at the bottom. And when the mean slurry velocity increases, the variation gradient of catalyst volume fraction on the outlet cross section and its center line will be smaller, which means the catalyst distribution is more homogeneous. It can be found that the catalyst distribution is asymmetric due to the deposit along the vertical direction. The volume fraction near the bottom is larger than that of the top. Due to higher concentration at the bottom, the catalyst particles may contact or assemble, causing less effective surface area where photocatalysis reaction occurs. That is not favorable for the photocatalysis in the reactor. Slip velocity is the velocity difference between solid phase and liquid phase, which is determined by catalyst volume fraction and particle diameter. The slip velocity between phases along the vertical center line of the cross section under different catalyst volume fractions is shown in Fig. 10. The slip velocity is much smaller than mean slurry velocity. And the slip velocity near the bottom is smaller than that in the other area relatively. And with the increase of catalyst volume fraction, the slip velocities become smaller. The effects of slurry velocity on slip velocity have also investigated. Theoretical analysis also indicates the slip velocity along the vertical center line of the cross section under different mean slurry velocities. The slip velocities under different mean slurry velocities are almost in the same magnitude. The slip velocity with higher mean slurry velocity is a litter larger.

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Conclusion and remarks

The state of the art for the research of photocatalytic hydrogen production, both outcomes and challenges in this field, were briefly reviewed. Research progress in our lab, from fundamental study of photocatalyst preparation to the issues in reactor configuration and pilot level demonstration, were introduced, showing the complete process of our effort for this technology to be economic viable in the near future. As the quantum efficiency obtained over the present photocatalysts is not yet satisfactory for practical application, more efficient visible-light-driven photocatalysts should be developed. Thus, it is necessary to narrow the band gap of photocatalysts to harvest visible-light in the longer wavelength region and enhance the photo-generated charge separation in photocatalysis. High-efficiency and cost-effective water splitting systems based on these photocatalyst should also be constructed. The factors such as electronic properties, chemical composition, structure and crystallinity, surface states and morphology, determining the photocatalytic activity of materials have to be further elucidated in sufficient detail. Lower cost alternative co-catalysts, such as non-noble metals and derived metal compounds, should be tested for possible substitution for the most frequently used noble metals such as Pt, which is very efficient but expensive. Photocatalyst free of of noble metal is also highly preferred considering that such photocatalyst can be readily used in a more economic way. Additionally, new insights into the water splitting mechanism are needed, particularly with regards to identification of thermodynamic and kinetic bottlenecks, in order to facilitate design of the most effective photocatalytic water splitting systems. On the other hand, in order to achieve enhanced and sustainable hydrogen production, continual addition of electron donors is required to make up half of the water splitting reaction to reduce H2O to H2, as sacrificial electron donors can irreversibly consume photo-generated holes to prohibit charge recombination. Taking into account the lowering cost for solar-to-H2 energy conversion, pollutant byproducts from industries and low-cost renewable biomass from animals or plants are preferred to be used as sacrificial electron donors in water splitting systems. Our systematic study and accomplishments lead to the development of a Compound Parabolic Concentrator (CPC) based photocatalytic hydrogen production solar rector for the first time. We have demonstrated the feasibility for efficient photocatalytic hydrogen production under direct solar light. It is anticipated that this demonstration of concentrator-based solar photocatalytic hydrogen production would draw attention for further studies in this promising direction. It is also considered that the current lack of industrial applications of this technology is mainly due to two reasons: the low photocatalytic efficiency, and related to that the lack of agreement on how to quantify this efficiency, in particular with respect to the photocatalyst preparation and reactor configuration; Nevertheless, both for material and reactor design, reduction of cost have to be given special priority. The other challenge is the lack of examples where the successful laboratory photocatalysis set-up has been scaled up to an industrially relevant scale. These two issues have been the object of our research and would also be the direction of our study in future.

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Acknowledgement [19]

The work was financially supported by the National Natural Science Foundation of China (Contracted No. 90610022, 50821064) and the National Basic Research Program of China (Contracted No. 2009CB220000).

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