Hydrogen Evolution by Photocatalytic Decomposition of Water under

Photocatalytic decomposition of water to produce hydrogen under UV irradiation was studied over layered perovskite K[Bi3PbTi5O16] photocatalysts prepa...
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Ind. Eng. Chem. Res. 2003, 42, 419-425

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KINETICS, CATALYSIS, AND REACTION ENGINEERING Hydrogen Evolution by Photocatalytic Decomposition of Water under UV Irradiation over K[Bi3PbTi5O16] Perovskite: Effect of Cerium Species Chong-Heng He† and O-Bong Yang* School of Environmental and Chemical Engineering, Center for Advanced Radiation Technology, Chonbuk National University, 664-14 First Street, Dukjin-Dong, Chonju, Chonbuk 561-756, Korea

Photocatalytic decomposition of water to produce hydrogen under UV irradiation was studied over layered perovskite K[Bi3PbTi5O16] photocatalysts prepared by the polymerized complex (PC) method. The photocatalysts were characterized by X-ray diffraction, ultraviolet-visible spectra, Brunauer-Emmett-Teller specific surface area, and FT-IR. The complete phase of K[Bi3PbTi5O16] perovskite was formed at a lower calcination temperature (1073 K, 2 h) by the PC method than a solid-state (SS) reaction method (1273 K, 6 h). The K[Bi3PbTi5O16] photocatalyst prepared by the PC method showed much higher activity than that prepared by SS reaction method. The evolution rate of hydrogen over PC-K[Bi3PbTi5O16] was significantly enhanced and affected by the addition of Ce(SO4)2 in the aqueous solution, which showed a volcano plot as a function of the concentration of Ce(SO4)2 passing through the maximum hydrogen evolution at [Ce(SO4)2] ) 2.4 mM. It was found that the essential promoters for the photocatalytic decomposition of water to produce hydrogen in the Ce(SO4)2 aqueous solution were cerium(IV) cations adsorbed on the perovskite surface (Cead4+) as well as cerium(III) cations in the aqueous phase (Ceaq3+), which were transformed from Ceaq4+ in the aqueous solution of cerium(IV) sulfate during the induction period. Introduction Photocatalytic decomposition of water into H2 and O2 under UV irradiation has been extensively studied in the past decade. It is well-known that hydrogen is an environmentally friendly and high-efficiency fuel; any breakthrough in the application of this field would bring a great benefit to the human beings as an alternate energy source.1-3 Therefore, it is a very interesting and challenging field and has attracted a lot of researchers’ attention since Fujishima and Honda4 discovered that the titania (TiO2) photoelectrode decomposes water into H2 and O2. There are several advanced methods for the preparation of a catalyst with special properties, such as the sol-gel process, gas-phase aerosol routes, etc. The solgel process involves the evolution of inorganic networks through the formation of a colloidal suspension (sol) and gelation of the sol to form a network in a continuous liquid phase (gel). The precursors for synthesizing these colloids consist of a metal or metalloid element surrounded by various reactive ligands. Metal alkoxides are most popular because they react easily with water. Recently, the advances of flame aerosol technology for the inexpensive synthesis of nanoparticle catalysts with precisely controlled characteristics are widely investi* Corresponding author. Tel.: +82-63-270-2313. Fax: +8263-270-2306. E-mail: [email protected]. † Permanent address: East China University of Science and Technology, P.R. China.

gated. More specifically, by using the mode of reactant gas mixing in diffusion flame reactors, one can widely control the primary particle size and crystallinity of product powders5 and even make unagglomerated or nonaggregated particles.6,7 In the aerosol synthesis route, the reactants should be evaporable, and the reactor system is complex and it is difficult to control the feed flows when the reactants have a larger number. At the initial stages of photocatalysis studies, most of the heterogeneous photocatalysts consisted of oxide powders mainly based on titanates and niotates, with loading of certain a metal or metal oxide such as NiOx, RuO2, Pt, etc., as cocatalysts.8,9 Recently, many kinds of photocatalytic systems for decomposition of water under UV and visible light irradiation have been developed.10-13 This includes the application of several photocatalysts based on ion-exchangeable layered perovskites, such as K4Nb6O17, Rb4Nb6O17, Sr2Ta2O7, RbPb2Nb3O10, K2La2Ti3O10, etc.14-16 Hydrated layered perovskites are more active than unhydrated layered perovskite for photocatalytic water decomposition17 because the hydrated layered structure usually has a high surface area and some metal oxides, such as NiOx and RuO2, can be intercalated into the interlayer space to form suitable active sites for water decomposition. Therefore, these perovskites, especially doped with metal oxides, usually show high activity for water decomposition in comparison with the classical photocatalysts. However, the preparation of those layered perovskites by sol-gel techniques has not yet been reported.

10.1021/ie010978k CCC: $25.00 © 2003 American Chemical Society Published on Web 01/10/2003

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Figure 1. Schematic diagram for the preparation of K[Bi3PbTi5O16] perovskite by the PC method.

A member of the Dion-Jacobson series of layered perovskites (A[A′n-1BnO3n+1], n ) 5), K[Bi3PbTi5O16], prepared by Gopalakrishnan et al.,18 has a hydrated layered structure. These perovskites were prepared by the solid-state (SS) reaction method in which the mixtures of metal oxides or carbonate powders were calcined for a long time at high temperature with several interval grindings during the period of reaction to render interdiffusion of ions.18,19 Drawbacks of the sample prepared by the SS method are a severe loss of alkaline-metal components and large grain growth (surface area shrinkage by heat treatment at high temperature), which may lead to low photcatalytic activities. These drawbacks of the SS method can be overcome by the preparation of perovskite using the polymerized complex (PC) method, which is a low-temperature processing method. To our knowledge, no publication is available on the photocatalytic decomposition of water over the K[Bi3PbTi5O16] perovskite prepared by the PC method. In this work, photocatalytic water decomposition to produce hydrogen under UV irradiation was investigated over K[Bi3PbTi5O16] perovskites prepared by the PC method, which showed unique photocatalytic properties in the aqueous solution containing cerium species. The reaction mechanism is proposed in terms of the role of cerium ions during the photocatalytic water decomposition. Experimental Section Preparation of K[Bi3PbTi5O16] Perovskite. Figure 1 shows the schematic diagram for the preparation of

layered K[Bi3PbTi5O16] perovskite by the PC method, which is denoted as PC-K[Bi3PbTi5O16]. Titanium isopropoxide (Ti[OCH(CH3)2]4, abbreviated as Ti(O-iPr)4; Aldrich, 99.9%), potassium nitrate (KNO3; Aldrich, >99%), bismuth citrate ([O2CCH2C(OH)(CO2)CH2CO2]Bi; Aldrich, 99.99%), and lead oxide (PbO; Aldrich, >99.9%) were used as precursors. Ethylene glycol (HOCH2CH2OH, EG; Aldrich, >99%) and methanol (CH3OH; Merck, 99.8%) were used as the solvents in the initial stage of preparation, while citric acid monohydrate (HO2CCH2C(OH)(CO2H)CH2CO2H‚H2O, CA; Aldrich, 99%) was used as a complexing agent to stabilize Bi, K, Pb, and Ti ions against water generated during the polymerization between EG and CA at the later stage of preparation. A total of 14.2 g of Ti(O-iPr)4 was dissolved in the mixture of 30 mL of CH3OH and 18.6 g of EG, followed by the addition of 21.5 g of CA under continuous stirring to convert Ti(O-iPr)4 to a stable Ti-CA complex. After complete dissolution of CA, 2.23 g of PbO and 2.02 g of KNO3 were added, and then the solution of 11.94 g of [O2CCH2C(OH)(CO2)CH2CO2]Bi in 20 mL of CH3OH and 10 g of CA was added to the above mixture. The mixture was stirred with refluxing at 343 K for 3 h until a transparent solution containing Bi-, Pb-, K-, and Ti-CA metal complexes was obtained. The clear solution thus prepared was heated to 423 K, while stirring with a Teflon stirrer without refluxing, to promote polymerization reactions between CA and EG and to remove the excess solvents (CH3OH and H2O). The optimal reactant molar ratio for the preparation of well-developed PC-K[Bi3PbTi5O16] perovskite by the PC method was K:Bi:Pb:Ti:EG:CA ) 2:3:1:5:30:15. However, successful PC-K[Bi3PbTi5O16] perovskite was not synthesized by using a stoichiometric ratio of potassium. After continuous heating at 423 K for 3 h, the solution became more viscous, and it finally gelled into a brown resin without any visible precipitation or turbidity. This viscous polymeric gel was then heated at 623 K for 4 h. Then it was lightly ground into a black powder, which is denoted as a “powder precursor”. To make hydrated products, the powder precursor was heated in a furnace under static air at temperatures between 873 and 1173 K, and then it was washed with distilled water. The dehydrated and dihydrated products were obtained by drying the hydrated materials at 673 and 383 K, respectively.18 The perovskite of K[Bi3PbTi5O16] was also synthesized by the conventional SS reaction method according to the method described by Gopalakrishnan et al.,18 which is denoted as SS-K[Bi3PbTi5O16]. KNO3, Bi2O3, PbO, and TiO2 (all Aldrich made) were mixed and ground with the mole ratio of KNO3:Bi2O3:PbO:TiO2 ) 3:1.5:1:5. The ground mixture was calcined in a furnace at 1273 K for 6 h with an interval grinding in the middle of the calcinations. Calcined products were washed with distilled water and dried at 383 K in an oven. In this method, excess alkali is necessary not only to suppress the formation of the competitive Aurivillius20 phases but also to bring down the reaction temperature for the formation of the desired phases. Supported photocatalysts, NiO/K[Bi3PbTi5O16] and RuO2/K[Bi3PbTi5O16], were prepared by impregnation of the aqueous solution of Ni(NO3)2 or RuCl4 on K[Bi3PbTi5O16]. A total of 0.10 g of Ni(NO3)2 or 0.037 g of RuCl4 were dissolved in 3 mL of distilled water, and then 4.0 g each of the perovskite powder prepared by the PC and SS methods was added into the solutions,

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respectively. The slurry was stirred at room temperature overnight, followed by drying at 383 K in an oven. RuO2/K[Bi3PbTi5O16] was obtained by calcination at 773 K for 3 h. For the preparation of the NiO/K[Bi3PbTi5O16] photocatalyst, a dried Ni-loaded sample was reduced by a hydrogen stream (5% H2 in N2, 10 mL/min) at 773 K for 2 h, and then it was reoxidized with air at 473 K for 1 h. Characterization. The catalysts were characterized by X-ray diffraction (XRD) using Cu KR radiation (40 kV, 40 mA; Rigaku, Japan) with a scan speed of 8°/min. The specific surface area of the catalyst was measured by the Brunauer-Emmett-Teller (BET) method with nitrogen gas adsorption at 77 K (Micromerities ASAP 2100). The FT-IR spectra of the catalysts were obtained by a Fourier transform infrared spectroscope of Jasco (FT-IR-300E, Japan). The UV-vis spectra of the cerium ion solutions were recorded by a diode-array spectrophotometer (Hewlett-Packard 8452A). Photocatalytic Decomposition of Water under UV Irradiation. Photocatalytic decomposition of water was carried out in an inner irradiation quartz cell reaction system under UV irradiation. In the outer Pyrex cell, 700 mL of an aqueous slurry containing 1.0 g of a fine powder photocatalyst was well mixed by a magnetic bar during the reaction. The reaction cell was purged by bubbling with argon (99.9999%) until oxygenfree. A high-pressure mercury lamp (450 W, KUV-250D, Kum Kang, Korea) in the inner quartz cell was used as a UV light source. The average reaction temperature was 296 ( 3 K. For the analysis of reaction products after some time under UV irradiation, 500 µL of gas was sampled with a gas-tight syringe and the amounts of H2 and O2 evolved by a gas chromatograph equipped with a thermal conductivity detector and a molecular sieve 5A column (30 m × 0.35 mm × 50.0 µm film thickness; HP) were measured. Results and Discussion Characterization of K[Bi3PbTi5O16] Perovskite. Figure 2 shows the effect of the calcination temperature and preparation method on the structures of PC- and SS-K[Bi3PbTi5O16]. These are two typical XRD patterns of dehydrated and dihydrated forms of perovskite prepared by the SS and PC methods, respectively. It is noted that PC-perovskite is susceptible to easy hydration in moist air. There is a halving of the c periodicity and an expansion of the c axis by ca. 2 Å (per perovskite slab) on the dihydrated PC-perovskite in comparison with the dehydrated SS-perovskite.18 In the PC-perovskite, crystallization was obviously reached at 873 K and the fully developed crystalline phase of K[Bi3PbTi5O16] was observed at 1073 K. The unidentified impurities were not found, and the overall XRD pattern remained the same and without any significant change up to 1173 K (Figure 2b). However, in the SS-perovskite, the K[Bi3PbTi5O16] perovskite started to crystallize at 1173 K and form a fully developed crystalline phase at 1273 K. Few impurity peaks on the fully developed samples are observed in this sample (Figure 2a). Similar results were also reported by Gopalakrishnan et al.18 It is realized in the present study that K[Bi3PbTi5O16] perovskite can be obtained at a lower calcination temperature by the PC method than the conventional SS method. The effect of the calcination temperature of PC-K[Bi3PbTi5O16] perovskite on photocatalytic activities for water decomposition is shown in Figure 3. As the

Figure 2. XRD patterns of the dehydrated samples prepared by the SS method (a) and the dihydrated samples prepared by the PC method (b) as a function of calcination temperature and time: (3) K2O; (2) TiO2; (0) PbO; (b) Bi2O3; (+) unidentified impurity.

Figure 3. Effect of calcination temperature on hydrogen evolution in the aqueous solution of 2.4 mM Ce(SO4)2 over PC-K[Bi3PbTi5O16] perovskite under UV irradiation.

calcination temperature increases from 873 to 1073 K, the photocatalytic activity also increases even though

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Table 1. BET Analysis Data of the Perovskite Prepared by the PC and SS Methods average pore surface pore method preparation calcination calcination area volume diameter method temp (K) time (h) (m2/g) (mL/g) (nm) SS PC

1273 873 1073 1173

6 2 2 2

0.1 12.5 9.7 6.4

0.05 0.04 0.03

15.3 17.1 21.0

Table 2. Results of Photocatalytic Decomposition of Water under UV Irradiation

catalyst

additive in solution

rate of gas evolution (µmol/20 h) H2

O2

K[Bi3PbTi5O16]

none 27.6 7.0 2.4 mM Ce(SO4)2 704.2 26.4 2.4 mM Ce2(SO4)3 87.9 2.1 K[Bi3PbTi5O16] (S-S method)a none 2.4 trace H[Bi3PbTi5O16] (H+ exchange) none 63.0 34.4 NiO(1.0 wt %)/K[Bi3PbTi5O16] none 100.4 0 RuO2(0.5 wt %)/K[Bi3PbTi5O16] none 179.4 83.0

a The host perovskite was prepared by the SS method and calcined at 1273 K for 6 h. The other perovskite used as a support was prepared by the PC method and calcined at 1073 K for 2 h. Reaction condition: catalyst, 1.0 g; solution, 700 mL; high-pressure Hg lamp (450 W); inner irradiation-type quartz reaction cell; mixing with a magnetic stirrer.

the surface area decreases. The highest activity is achieved in the case of the sample calcined at 1073 K, which was the highest crystallinity as shown in Figure 2b. A slight decrease in photocatalytic activity is observed in the sample calcined at 1173 K, which is due to the decrease in the surface area as the calcination temperature was increased. The specific surface area measured by the BET method shows that the specific surface area as well as pore volume is reduced as the calcination temperature increases. It is found that PC-perovskite showed a significantly larger surface area and pore volume than SS-perovskite. This, therefore, supports the higher photocatalytic activity by PC-perovskite than SS-perovskite. The corresponding values listed in Table 1 are very similar to those of layered perovskite of K2La2Ti3O10 prepared by the PC and SS methods.16 The PC method used in this work involves the formation of organic networks through the polymerized reaction between EG and CA. The stable metal-CA complexes are fixed on the chain of polymer by chemical reaction or inside the polymer network. Even though they are similar to the sol-gel method in principle, the network ligands in the sol-gel system are not strong, and they are easily destroyed during dry calcination in comparison with those in the PC method. Therefore, the PC method is better and makes it relatively convenient to prepare the stable K[Bi3PbTi5O16] perovskite. However, the preparation of layered perovskites by sol-gel techniques has not yet been reported as mentioned above. Photocatalytic Decomposition of Water. The results of the photocatalytic decomposition of water over various catalysts are presented in Table 2. Native SSK[Bi3PbTi5O16] calcined at 1273 K for 6 h produces only 2.4 µmol of H2/20 h and a trace amount of O2 under UV irradiation. However, the rate of hydrogen evolution of

Figure 4. Time courses of hydrogen evolution over PC-K[Bi3PbTi5O16] perovskite in a 2.4 mM cerium aqueous solution under UV irradiation. The other conditions are the same as those described in Table 2: (b) hydrogen and (O) oxygen in a Ce(SO4)2 aqueous solution; (2) hydrogen and (4) oxygen in a Ce2(SO4)3 aqueous solution.

native PC-K[Bi3PbTi5O16] calcined at 1073 K for 2 h was significantly increased by more than 1 order in comparison with SS-K[Bi3PbTi5O16]. The photocatalytic activity (63 µmol of H2/20 h) of the PC sample exchanged by a proton, H[Bi3PbTi5O16], is 2 times higher than that of its original potassium form, which is comparable with the evolution rate of 5.9 µmol of H2/h over Sr2Nb2O7 with a layered perovskite structure under UV irradiation.14 When NiOx and RuO2 were loaded on PC-K[Bi3PbTi5O16], the rate of hydrogen evolution was increased as much as 3.6 and 6.5 times, respectively. The rate of photocatalytic water decomposition depended strongly on the electronic structure of the substrate as well as the loaded metal and pretreatment conditions. Of several metals loaded on the perovskite materials, nickel was found to be the most effective for water decomposition, in which nickel might act as a charge separator intercalated into the interlayer space21 or located on the exterior surface of the substrate.14,15 Also, the amount of metal loading should be optimized for efficient water decompositions.15 Effect of Ce(SO4)2 on H2 Evolution. It is wellknown that the cerium shows +1, +3, or +4 valence states according to the pretreatment conditions. Various oxidation states make them attractive cations as sacrificial agents in aqueous photoelectrochemical reactions because they can oxidize and reduce reversibly. Even so, the role of cerium cations in photocatalytic water decomposition has not been well explored. In our blank runs, a trace amount of O2 was detected from the solution containing 2.4 mM Ce(SO4)2 without K[Bi3PbTi5O16] and only 27.6 µmol of H2 was evolved in an aqueous suspension containing 1.0 g of PC-K[Bi3PbTi5O16] under UV irradiation for 20 h. H2 and O2 were not detected in both of the reaction systems containing 2.4 mM Ce(SO4)2 and 2.4 mM Ce2(SO4)3 under dark conditions for 20 h, respectively. The production rate of hydrogen was greatly increased when a certain amount of Ce(SO4)2 was added in an aqueous suspension of PCK[Bi3PbTi5O16] as shown in Table 2 and Figure 4. Figure 4 shows the time course of H2 evolution from the aqueous solution containing 2.4 mM of Ce(SO4)2 or Ce2(SO4)3. The time course of H2 evolution in the solution containing 2.4 mM of Ce(SO4)2 showed an S-shaped

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Figure 5. Effect of the initial concentration of Ce(SO4)2 on the yield of H2 over PC-K[Bi3PbTi5O16] perovskite under UV irradiation for 20 h.

pattern because only a little H2 evolution was observed during the induction period (ca. 6 h), and then the rate of H2 evolution increased sharply and increased slowly after 16 h. H2 evolution is strongly dependent on the initial concentration of Ce(SO4)2 in the reaction solution as shown in Figure 5. The yields of H2 evolution for 20 h showed a volcano plot with a maximum yield of H2 at [Ce(SO4)2] ) 2.4 mM. In dilute aqueous solutions of [Ce(SO4)2] < 2.4 mM, cerium species were effective for the promotion of H2 evolution as adsorbed species on K[Bi3PbTi5O16] or ions in the solution. However, the excess cerium cations in the solution act as deteriorating species on water decomposition. The intensity of UV irradiation can be reduced by excess cerium cations that strongly absorb the UV light as shown in Figure 6. The other possibility is that superfluous cerium cations can act as an electron acceptor to inhibit the formation of H2. However, in the solution containing Ce2(SO4)3, a very low evolution rate of H2 linearly increased with irradiation time without any induction period, which indicates that different electrochemical reactions take place according to the cerium species. The state change of the cerium species undergoing photocatalytic reaction was measured by a UV-vis spectrophotometer as shown in Figure 6. The sampled solution was filtered by a 0.2 µm filter before spectrophotometric measurements. Parts a and b of Figure 6 show the UV-vis spectra of the aqueous solutions of Ce(SO4)2 and Ce2(SO4)3, respectively. Figure 6c shows the UV-vis spectra change of the solution containing Ce(SO4)2 during photocatalytic water decomposition with PC-K[Bi3PbTi5O16] perovskite under UV irradiation. When the PC-perovskite was added and stirred for 30 min in the Ce(SO4)2 aqueous solution, the concentration of Ceaq4+ in the solution decreased slightly as shown in curve 2 of Figure 6c, which suggested that some of Ceaq4+ species might be adsorbed on the layered perovskite as Cead4+. It is notable that most of Ceaq4+ in the solution was transformed to the Ceaq3+ species after 4 h of UV irradiation and only a small amount remained as the Ceaq4+ state as shown in curve 4 of Figure 6c. After 24 h of photocatalytic reaction, only the Ceaq3+ species was left in the solution (curve 5) and the

Figure 6. UV-vis spectra of the aqueous solutions of Ce4+ (a) and Ce3+ (b) and the aqueous solutions containing Ce(SO4)2 during the photocatalytic water decomposition over K[Bi3PbTi5O16] perovskite (c).

concentration of Ceaq3+ was lower than 2.4 mM, indicating that some of the aqueous cerium species were still adsorbed on the surface of PC-K[Bi3PbTi5O16] perovskite as mentioned above. During the 6 h induction period, a negligible amount of hydrogen evolved even in the Ce(SO4)2 solution, which strongly suggested that the enhancement of hydrogen evolution should be ascribed to the cerium(III) species which was transformed from the cerium(IV) in the Ce(SO4)2 solution, and the 6 h induction period might correspond to the time for the transformation of Ceaq4+ to Ceaq3+ species. Predominant oxygen production on the induction period could be explained by the fact that some of the Ceaq4+ species seemed to act as the electron acceptor to produce oxygen by oxidation of water9 and a little Ceaq4+ species might

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strongly restrain hydrogen evolution. After the induction period, the amount of O2 slightly decreased. However, the addition of Ce2(SO4)3 did not promote H2 evolution as effectively as Ce(SO4)2. It was apparent that the state of the cerium species was changed after participating in the reaction. On the other hand, the amount of H2 from water deconposition is increased with the irradiation time as shown in Figure 4. Accordingly, the cerium species acted as promoters in the photocatalytic decomposition of water. It can be summarized that the essential promoters for H2 evolution are not only the Ceaq3+ species but also some adsorbed cerium(IV) species, which results in the synergistic effect on water decomposition for hydrogen production. Although the rates of H2 evolution are higher over some perovskites, such as Sr2Nb2O7, K2La2Ti3O10, etc.,11,13 than that of our perovskite, the characteristic of photocatalytic decomposition over PC-perovskite in the Ce(SO4)2 aqueous solution is very interesting. The reaction mechanism could be separated into two parts, during and after the induction period. During the induction period, the reaction steps could be proposed as follows:

perovskite + hv f e- + h+

(1)

Ceaq4+ f Cead4+

(2)

Cead4+ + e- f Cead3+ f Ceaq3+

(3)

2H2O + 4h+ f O2(g) + 4Haq+

(4)

First, the perovskite was photoexcited by UV irradiation to form an electron (e) and hole (h+) pair in reaction (1). Some of Ceaq4+ in an aqueous phase will be adsorbed on the perovskite surface as Cead4+ in reaction (2). Then the Cead4+ trap the electrons and form its reduced state Cead3+ according to reaction (3). Simultaneously, the holes get electrons from adsorbed water molecules and produce the oxygen gas9 in reaction (4). In this process Ceaq4+ acts as an electron acceptor to promote oxygen formation and transferred itself into a Ceaq3+ species. In this reaction mechanism, subscripts aq and ad indicate that the species exist in the aqueous phase and the adsorbed state, respectively. After the induction period, most of Ceaq4+ were transferred to Ceaq3+, and the following reaction mechanism could be proposed:

perovskite + hv f e- + h+

(5)

Cead3+ + h+ f Cead4+

(6)

H2O + e- f H2 + OH-

(7)

Cead4+ + OH- f Cead3+ + OH•

(8)

Cead3+ a Ceaq3+

(9)

2OH• f H2O2

(10)

H+ + OH- f H2O

(11)

Under UV irradiation, Cead3+ as an electron donor traps the hole on the perovskite surface and forms its oxide state Cead4+ according to reaction (6). Meanwhile, the photoexcited electrons reduce the water to produce hydrogen gas as described in reaction (7). The recycles of cerium species might be achieved by reactions (8)(11).

Figure 7. FT-IR spectra of PC-K[Bi3PbTi5O16] perovskites: (a) dihydrated perovskite after photocatalytic reaction for 24 h in a Ce(SO4)2 aqueous solution; (b) dihydrated perovskite before photocatalytic reaction; (c) anhydrated perovskite before photocatalytic reaction.

Figure 7 shows the FT-IR spectra of PC-K[Bi3PbTi5O16] perovskites before and after photocatalytic reaction. The changes are not much even after 20 h of UV irradiation in an aqueous suspension containing Ce(SO4)2 except two new small peaks at around 1057 and 1107 cm-1 on the sample used in the reaction, corresponding to the vibration of ν(C-O) bands adsorbed on cerium oxide.22 The formation of the two new peaks indicates that some cerium species are adsorbed on the perovskite surface, which seems to act as crucial promoters for the production of hydrogen. Cerium species intercalated in the layered structure of K[Bi3PbTi5O16] and forming the surface-adsorbed species may improve the charge separation of the photoexcited electron and hole as mentioned in the previous mechanism. The peak of -OH around 3440 cm-1 on the perovskite used in the reaction is bigger than that of a fresh sample, indicating that the sample used in the cerium aqueous solution has a higher concentration of hydroxyl species on the surface than that of the fresh sample. It is generally suggested that surface hydroxyl species play an important role in determining the photocatalytic activity because these species act as traps for photoexcited holes to form hydroxyl radicals and effectively slow the rate of electron-hole recombination.23 It may also be a reason for improving the photocatalytic activity of K[Bi3PbTi5O16] for H2 evolution. Conclusions The complete phase of K[Bi3PbTi5O16] perovskite was formed by the PC method at a lower calcination temperature than by the SS method. The perovskite prepared by the PC method showed higher photocatalytic activity than that prepared by a conventional hightemperature SS method in photocatalytic water decomposition to hydrogen under UV irradiation. The rate of hydrogen evolution was greatly improved and affected by the addition of Ce(SO4)2 in an aqueous suspension, which showed a volcano plot as a function of the concentration of Ce(SO4)2 passing through the maximum hydrogen evolution at [Ce(SO4)2] ) 2.4 mM. The

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essential promoters for hydrogen production in the cerium-containing aqueous solution were the cerium(IV) cations adsorbed on the perovskite surface as well as Ceaq3+ in the aqueous phase, which were transformed from the Ceaq4+ species in the aqueous solution of cerium(IV) sulfate during the induction period. Those cerium cation promoters seem to be effective recombination inhibitors of the photoexcited electron and hole as discussed in the proposed reaction mechanism. Acknowledgment This research was supported by the project (No. 19991-307-007-3) granted from the Korea Science and Engineering Foundation (KOSEF). The financial support from Chonbuk National University for Dr. He’s postdoc is gratefully acknowledged. Literature Cited (1) Takata, T.; Tanaka, A.; Hara, M.; Kondo, J. N.; Domen, K. Recent Progress of Photocatalysts for Overall Water Splitting. Catal. Today 1998, 44, 17-26. (2) Armor, J. N. The Multiple Roles for Catalysis in the Production of H2. Appl. Catal. A 1999, 176, 159-176. (3) Momirlan, M.; Veziroglu, T. Recent Directions of World Hydrogen Production. Renewable Sustainable Energy Rev. 1999, 3, 219-231. (4) Fujisima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37-38. (5) Pratsinis, S. E.; Zhu, W.; Vemury, S. The Role of Gas Mixing in Flame Synthesis of Titania Powders. Powder Technol. 1996, 86, 87-93. (6) Pratsinis, S. E. Flame Aerosol Synthesis of Ceramic Powders. Prog. Energy Combust. Sci. 1998, 24, 197-219. (7) Biswas, P.; Yang, G.; Zachariah, M. R. In Situ Processing of Ferroelectric Materials from Lead Waste Streams by Injection of Gas-Phase Titanium Precursors: Laser Induced Fluorescence and X-ray Diffraction Measurements. Combust. Sci. Technol. 1998, 134, 183-200. (8) Ohno, T.; Tanigawa, F.; Fujihara, K.; Izumi, S.; Matsumura, M. Photocatalytic Oxidation of Water on TiO2-coated WO3 Particles by Visible Light Using Iron(III) Ions as Electron Acceptor. J. Photochem. Photobiol., A 1998, 118, 41-44. (9) Towata, A.; Uwamino, Y.; Sando, M.; Iseda, K.; Taoda, H. Synthesis of Titania Photocatalysts Dispersed with Nickel Nanosized Particles. Nanostruct. Mater. 1998, 10, 1033-1042. (10) Takata, T.; Shinohara, K.; Tanaka, A.; Hara, M.; Kondo, J. N.; Domen, K. A Highly Active Photocatalyst for Overall Water Splitting with a Hydrated Layered Perovskite Structure. J. Photochem. Photobiol., A 1997, 106, 45-49. (11) Kato, H.; Kudo, A. New Tantalate Photocatalysts for Water

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Received for review December 5, 2001 Revised manuscript received October 23, 2002 Accepted November 8, 2002 IE010978K