Water Photocatalytic

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Energy & Fuels 2009, 23, 2192–2196

Hydrogen Production from Methanol/Water Photocatalytic Decomposition Using Pt/TiO2-xNx Catalyst Wen-Churng Lin,*,† Wen-Duo Yang,‡ I-Lun Huang,‡ Tser-Son Wu,§ and Zen-Ja Chung| Department of EnVironmental Engineering, Department of Chemical and Materials Engineering, and Chemical Engineering DiVision, Kun Shan UniVersity, Tainan 710, Taiwan, National Kaohsiung UniVersity of Applied Sciences, Kaohsiung 807, Taiwan, and Institute of Nuclear Energy Research, Lungtan, Taoyuan 325, Taiwan ReceiVed December 13, 2008. ReVised Manuscript ReceiVed February 10, 2009

Nitrogen-doped titanium oxide powders were synthesized by the two-microemulsion technique and used as a support for Pt toward photocatalytic hydrogen evolution. Two solutions of microemulsion with same water/ oil (w/o) ratio were mixed together to form a slurry of titania precursor, one contains Ti4+ ions chelated with citric acid aqueous droplets and the other has aqueous ammonia droplets. After the consecutive procedures of evaporation, drying, calcination, and grinding, the nanosized TiO2-xNx or Pt/TiO2-xNx photocatalysts were obtained. The synthesized nanosized photocatalysts were then utilized to produce hydrogen by photocatalytic methanol/water splitting in visible light. The effects of Pt loading content, methanol/water ratio, and pH of the methanol/water solution on the performances of the photocatalysts for hydrogen evolution were investigated.

1. Introduction

2hν f 2e- + 2h+

(1)

Conventional energy resources such as coal, petroleum products, etc., which are being used to meet most of the world’s energy requirements, have been depleted to a great extend. It is therefore necessary to produce an alternative fuel, which should in principle be pollution free and storable. Hydrogen is one potential candidate of the best materials to satisfy the requirements, and should be used more widely in the near future because it is environmentally friendly. Since Fujishima and Honda1,2 first reported the photocatalytic ability of TiO2 to generate hydrogen by photosplitting of water in 1972, people realized the potential applications of TiO2 for solar energy conversion. This inexpensive, high-activity, and high-stability metal oxide semiconductor has also been applied in organic and inorganic pollutant degradation to purify air and water. Technologies for generating hydrogen by splitting water using a photocatalyst have attracted much attention. The principle of photocatalytic water decomposition is based on the conversion of light energy into electricity on exposure of a semiconductor to light. Light results in the intrinsic ionization of n-type semiconducting materials over the band gap, leading to the formation of electrons in the conduction band and electron holes in the valence band (eq 1). The light-induced electron holes split water molecules into oxygen gas and hydrogen ions (eq 2). Simultaneously, the electrons generated as a result of eq 1 reduce the hydrogen ions to hydrogen gas (eq 3).

H2O(l) + 2h+ f 1/2O2(g) + 2H+

(2)

2H+ + 2e- f H2(g)

(3)

* Corresponding author. E-mail: [email protected]. † Department of Environmental Engineering, Kun Shan University. ‡ National Kaohsiung University of Applied Sciences. § Department of Mechanical Engineering, Kun Shan University. | Institute of Nuclear Energy Research. (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37–38. (2) Fujishima, A.; Kohayakawa, K. J. Electrochem. Soc. 1975, 122, 1487–1489.

The overall decomposition of water may be expressed as 2hν + H2O(l) f 1/2O2(g) + H2(g)

(4)

To induce this reaction, the energy of the absorbed photon must be at least 1.23 eV.3 According to this equation, the optimum band gap for high hydrogen production is below 2.0 eV. The photocatalytic formation of hydrogen and oxygen on a TiO2 photocatalyst is more ineffective because the amount of hydrogen produced is limited by the rapid recombination of holes and electrons, resulting in water formation.4 Recently, photocatalytic hydrogen production has been extended to the photodecomposition of aqueous solutions containing C1-C3 alcohol sacrificial reagents. These alcohols in water-alcohol mixtures are satisfactory hole scavengers, and in addition, they undergo a relatively rapid and irreversible oxidation.5 Kawai5 and Chen6 proposed that the overall methanol decomposition reaction, which has a lower splitting energy than water, was as follows: (3) Bak, T.; Nowotny, J.; Rekas, M.; Sorrell, C. C. Int. J. Hydrogen Energy 2002, 27, 991–1022. (4) Jeon, M. K.; Park, J. W.; Kang, M. J. Ind. Eng. Chem. 2007, 13, 84–91. (5) Kawai, T.; Sakata, T. J. Chem. Soc. Chem. Commun. 1980, 15, 694– 695. (6) Chen, J.; Ollis, D. F.; Rulkens, W. H.; Bruning, H. Water Res. 1999, 33, 669–676.

10.1021/ef801091p CCC: $40.75  2009 American Chemical Society Published on Web 03/20/2009

Photocatalytic Decomposition Using Pt/TiO2-xNx

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CH3OH(l) T HCHO(g) + H2(g) ∆G1° ) 64.1 kJ/mol (5) HCHO(g) + H2O(l) T HCO2H(l) + H2(g) ∆G2° ) 47.8 kJ/mol (6) HCO2H(l) T CO2(g) + H2(g) ∆G3° ) -95.8 kJ/mol (7) With the overall reaction being CH3OH(l) + H2O(l) T CO2(g) + 3H2(g) ∆G◦ ) 16.1 kJ/mol (8) Consequently, the decomposition energy for methanol is 0.7 eV. Most investigations of hydrogen production via methanol photodecomposition have focused on TiO2,7,8 which has a relatively high activity and chemical stability under UV irradiation. However, TiO2 is active only under ultraviolet (UV) light due to its wide band gap of ca. 3.2 eV (for crystalline anatase phase at room temperature). As the fraction of UV radiation in solar spectrum is less than 5%, TiO2 is not an excellent candidate to efficiently exploit solar radiation (dominant visible light) on Earth. For smarter utilization of the dominant visible light part of the solar spectrum, and even for the indoor applications under weak interior lighting, photocatalyst absorbing visible light photons is a prerequisite. Consequently, the search for visible light active photocatalysts is a subject of intense research today. Recently, N-doped TiO2 has been studied by many researchers9-13 and demonstrated its photoactivity for photocurrent generation and decomposition of many organic compounds under visible light irradiation. If the light energy is equal to or greater than the bandgap of the semiconductor, the electron in the valance band can be excited to the conduction band. This energy change will result in the formation of positive holes in the valance and free electrons in the conduction band. However, the positive hole and electron are easily recombined in a very short time, which will therefore lead to a very low activity for the photocatalyst. The loaded platinum on the surface of TiO2 can play an important role in preventing the rapid recombination between positive hole and electron. It is because the platinum can capture the transferred electrons onto the surface of the TiO2.14,15 In addition, the loaded platinum can also enhance the photoproduction of H2 over semiconductor materials, such as TiO2, SrTiO3, ZnO, WO3, and others.16-18 The importance of nanophase technology has resulted in tremendous research efforts toward the development of new techniques for the synthesis of nanosized particles. One such (7) Kang, M.; Choi, D. H.; Choung, S. J. J. Ind. Eng. Chem. 2005, 11, 240–247. (8) Kim, S. B.; Lee, J. Y.; Jang, H. T.; Cha, W. S.; Hong, S. C. J. Ind. Eng. Chem. 2003, 9, 440–446. (9) Rhee, C. H.; Bae, S. W.; Lee, J. S. Chem. Lett. 2005, 34, 660–661. (10) Irie, H.; Watanabe, S.; Yohino, N.; Hashimoto, K. Chem. Commun. 2003, 11, 1298–1299. (11) Jang, J. S.; Kim, H. G.; Ji, S. M.; Bae, S. W.; Jung, J. H.; Shon, B. H.; Lee, J. S. J. Solid State Chem. 2006, 179, 1067–1075. (12) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269–271. (13) Sakatani, Y.; Koike, H. Japan Patent P2001-72419A, 2001. (14) Linsebigler, A. L.; Lu, G.; Yates, J. T. Chem. ReV. 1995, 95, 735– 758. (15) Fox, M. A.; Dulay, M. T. Chem. ReV. 1993, 93, 341–357. (16) Kiwi, J.; Graetzel, M. J. Phys. Chem. 1984, 88, 1302–1307. (17) Courbon, H.; Herrmann, J. M.; Pichat, P. J. Phys. Chem. 1984, 88, 5210–5214. (18) Renault, N. J.; Pichat, P.; Foissy, A.; Mercier, R. J. Phys. Chem. 1986, 90, 2733–2738.

technique with the ability to precisely control the size and shape of the particles formed was to use water-in-oil microemulsions as a reaction medium. The aqueous cores/water pools of these microemulsions can be used as nanosize reactors for the precipitation of a wide variety of nanoparticles. Compared to other methods, the microemulsion method has several advantages in producing high-purity particles with uniform small size, narrow size distribution, and low aggregation properties.19-23 Accordingly, the TiO2-xNx and Pt/TiO2-xNx photocatalysts used in this study for the photocatalytic hydrogen evolution under visible light irradiation were prepared by using the twomicroemulsion method. Several parameters that affect hydrogen production were also studied, such as Pt content, sacrificial reagent (methanol) concentration, pH, and duration of illumination. 2. Experimental Section 2.1. Materials. n-Hexane (g99% purity, Acros), citric acid monohydrate (g99.5% purity, Riedel-de Haen), titanium(IV) isopropoxide (TIP, 98% purity, Fluka), ammonia solution (15 M, Showa), hydroxypropyl cellulose (HPC, MW ) 100 000, Aldrich), potassium hexachloroplatinate (99% purity, Showa), sorbitan monopalmitate (Span 40, HLB ) 6.7, Sigma), isopropyl alcohol (IPA, g 99.5% purity, ECHO), methanol (g99.9% purity, ECHO), ethanol (g99.5% purity, ECHO), and n-propyl alcohol (g99.5% purity, ECHO) were used for this study. All chemicals were used without further purification. Span 40 and IPA were used as a surfactant and cosurfactant, respectively. HPC behaved as a steric dispersant. Citric acid, serving as a modifying agent, was applied to moderate the hydrolysis and condensation processes of titanium precursor. 2.2. Photocatalysts Preparation. Nanosized nitrogen-doped titanium oxide TiO2-xNx and Pt/TiO2-xNx powders were synthesized by the two-microemulsion method, as shown in Figure 1. The oil phase (continuous phase) of the each microemulsion was composed of n-hexane, Span 40, and IPA. The two microemulsion solutions, one containing 5.0 × 10-5 M hydroxypropyl cellulose, citric acid/ titanium isopropoxide with a molar ratio of 3, and a necessary amount of potassium hexachloroplatinate for the desired Pt loading of 0-0.7 wt % aqueous droplets and the other, with the same water/ oil (w/o) ratio, containing aqueous ammonia droplets, were mixed together and kept continuously stirring at 50 °C for 1 h to form a milky slurry of titania precursor. After the consecutive procedures of evaporation, drying, calcination at 500 °C, and grinding, the desired photocatalyst was obtained. 2.3. Photocatalytic H2 Evolution over TiO2-xNx or Pt/ TiO2-xNx. The photocatalytic H2 evolution reaction, irradiated at room temperature using visible light (λ > 380 nm) from a 400 W halogen lamp, was carried out in a optical quartz cell (ca. 220 mL). The temperature of the reaction system was maintained constant with circulation of water through the internal U-type glass tube from a thermostatic bath. In a typical run, a specified amount of photocatalyst (65 mg) was suspended in an aqueous methanol solution (120 mL of distilled water, 30 mL of methanol) by means of a magnetic stirrer within the cell. The gaseous H2 evolved was periodically collected and analyzed by an online gas chromatograph (Varian CP-3800, molecular sieve 5A, 99.999% N2 carrier), which was connected with a circulation line and equipped with a thermal conductivity detector (TCD) and a stainless steel column (100/120 Carbosieve S-II, 10 ft × 1/8 in.). The injector, column, and detector (19) Tai, C. Y.; Lee, M. H.; Wu, Y. C. Chem. Eng. Sci. 2001, 56, 2389– 2398. (20) Lee, M. H.; Tai, C. Y.; Lu, C. H. J. Eur. Ceram. Soc. 1999, 190, 2593–2603. (21) Lee, J. S.; Lee, J. S.; Choi, S. C. Mater. Lett. 2005, 59, 395–398. (22) Wang, J.; Sun, J.; Bian, X. Mater. Sci. Eng., A 2004, 379, 7–10. (23) Chhabra, V.; Pillai, V.; Mishra, B. K.; Morrone, A.; Shah, D. O. Langmuir 1995, 11, 3307–3311.

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Figure 3. Dependence of photocatalytic activity of hydrogen evolution on Pt loading content over Pt/TiO2-xNx [photocatalyst, 65 mg; 150 mL of 20% (v/v) methanol/water].

Figure 1. Preparation of TiO2-xNx or Pt/TiO2-xNx using the twomicroemulsion method.

Figure 2. Time course of photocatalytic hydrogen evolution over Pt/ TiO2-xNx with different Pt loading content [photocatalyst, 65 mg; 150 mL of 20% (v/v) methanol/water].

temperature was maintained at 110, 60, 250 °C, respectively. The flow rate of nitrogen carrier gas was 10 mL/min.

3. Results and Discussion 3.1. Photocatalytic Activity Comparison between TiO2-xNx and Pt/TiO2-xNx. The photocatalytic activity of the TiO2-xNx and Pt/TiO2-xNx was quantitatively examined by photocatalytic H2 evolution from the suspension of the photocatalysts in an aqueous methanol solution for 6 h irradiation using visible light. As can be seen from Figures 2 and 3, the amount of H2 produced increased with increasing the irradiation time. The activity of Pt/TiO2-xNx with Pt loading in the range of 0.1-0.6 wt % is much higher than that of TiO2-xNx in the H2 generation. It is because the conduction band of TiO2-xNx is close to the electric potential of H2 precipitation and the electron-hole couple is easy to recombine.24 The photocatalytic activity will be enhanced if the overpotential of H+/H and the recombination ratio of electron-hole couple can be reduced. (24) Ashokkumar, M. Int. J. Hydrogen Energy 1998, 6, 427–438.

In this regard, the loaded platinum plays a very important role for Pt/TiO2-xNx catalyst, because platinum is able to capture electrons and decrease the overpotential of H+/H, leading to the decrease of the electron-hole recombination.25,26 As shown in Figure 3, relatively small H2 evolution was observed using TiO2-xNx. The deposition of Pt on TiO2-xNx resulted in a substantial improvement in the H2 evolution. In addition, the H2 evolution rate depends on the Pt loading. With Pt loading, TiO2-xNx particles possesses more Pt nanoclusters, which are indispensable for the removal of photogenerated electrons from TiO2-xNx for the reduction reaction and lead to an increase in the photocatalytic activity. After reaching a maximum, the decrease in the activity with further increase in the Pt content beyond a certain optimum value may be, at least partly, due to too many Pt nanoclusters on TiO2-xNx. These clusters would shield the photosensitive TiO2-xNx surface, scatter the visible light to decrease the absorption by TiO2-xNx, and subsequently decrease the surface concentration of the electrons and holes available for further reactions. Another explanation is that, at high metal loadings, the deposited metal particles may act as the recombination centers for the photoinduced species.17 3.2. Effect of Methanol Concentration. The initial steps in the photocatalytic process, prior to the initiation of methanol degradation, are well-known: TiO2 + hν f TiO2(e-, h+)

(9)

H2O + h+ f •OH + H+

(10)

O2 + e- f O2•-

(11)

The hydroxy radicals (•OH) and superoxide radical anions (O2•-) are the primary oxidizing species in the photocatalytic oxidation processes. After the generation of these radicals, the decomposition of alcohols is known to proceed via radical reactions on the TiO2 surface.25,27-29 The dependency of (25) Cui, W.; Feng, L.; Xu, C.; Lu, S.; Qiu, F. Catal. Commun. 2004, 5, 533–536. (26) Bamwenda, G. R.; Tsubota, S.; Nakamura, T.; Haruta, M. J. Photochem. Photobiol. A: Chem. 1995, 89, 177–189. (27) Ikeda, K.; Sakai, H.; Baba, R.; Hashimoto, K.; Fujishima, A J. Phys. Chem. B 1997, 101, 2617–2620. (28) Ohko, Y.; Tryk, D. A.; Hashimoto, K.; Fujishima, A J. Phys. Chem. B 1998, 102, 2699–2704. (29) Schwitzgebel, J.; Ekerdt, J. G.; Gerischer, H.; Heller, A. J. Phys. Chem. 1995, 99, 5633–5638.

Photocatalytic Decomposition Using Pt/TiO2-xNx

Figure 4. Time course of photocatalytic hydrogen evolution over 0.6 wt % Pt/TiO2-xNx in different concentration of methanol solution (photocatalyst, 65 mg; 150 mL of methanol/water).

Figure 5. Effect of methanol concentration on photocatalytic hydrogen production over 0.6 wt % Pt/TiO2-xNx for 6 h (photocatalyst, 65 mg; 150 mL of methanol/water).

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Figure 6. Time course of photocatalytic hydrogen evolution over 0.2 wt % Pt/TiO2-xNx in methanol solutions of different pH [photocatalyst, 65 mg; 150 mL of 20% (v/v) methanol/water].

Figure 7. Effect of pH on photocatalytic hydrogen production over 0.2 wt % Pt/TiO2-xNx for 6 h [photocatalyst, 65 mg; 150 mL of 20% (v/v) methanol/water].

hydrogen evolution upon methanol concentration on Pt/TiO2-xNx catalyst is shown in Figures 4 and 5. The hydrogen production noticeably increases with the increase of methanol concentration, and maximum activity is exhibited at 80% (v/v). However, the hydrogen production rate decreases significantly when the methanol concentration is over 80% (v/v). This reflects the fact that more and more methanol molecules get adsorbed on the surface of the photocatalyst as the concentration of methanol increases. Therefore, the reactive species (•OH and O2•-) needed to increase for the requirement of decomposition. However, the generations of relative amounts of •OH and O2•- on the surface of the catalyst do not increase as the intensity of light, irradiation time, and catalyst amount remain constant. Consequently, the decomposition efficiency of methanol decreases, resulting in the decrease of the hydrogen production when the methanol concentration is over 80% (v/v). 3.3. Effect of pH. According to previous studies, pH appears to play an important role in the photocatalytic reaction.30-33 In the illuminated titanium oxide system, the effect of pH on the

Figure 8. Comparison of hydrogen production for methanol, ethanol, and propanol for 6 h [0.6 wt % Pt/TiO2-xNx, 65 mg; 150 mL of 20% (v/v) alcohol/water].

(30) Robert, D.; Dongui, B.; Weber, J. V. J. Photochem. Photobiol. A: Chem. 2003, 156, 195–200. (31) Daneshvar, N.; Salari, D.; Khataee, A. R. J. Photochem. Photobiol. A: Chem. 2003, 157, 111–116. (32) Piscopo, A.; Robert, D.; Weber, J. V. Appl. Catal. B: EnViron. 2001, 35, 117–124. (33) Lizama, C.; Freer, J.; Baeza, J.; Mansilla, H. D. Catal. Today 2002, 76, 235–246.

photocatalytic reaction is generally attributed to the surface charge of titanium oxide. The point of zero charge (pzc) of TiO2 is at a pH value of 6.3; the catalyst’s surface is positively charged at pH values lower than 6.3 and negatively charged at higher pH values.34 Electrostatic attraction or repulsion between the catalyst’s surface and the reactant molecule is taking place, depending on the ionic form of the compound (anionic or

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Figure 9. Time course of photocatalytic hydrogen production over 0.6 wt % Pt/TiO2-xNx [photocatalyst, 65 mg; 150 mL of 60% (v/v) methanol/ water].

cationic) and consequently enhances or inhibits, respectively, the photocatalytic reaction rate. The role of pH on the photocatalytic H2 evolution was studied in the pH range of 1-13. HCl and NaOH were used to adjust the pH in this work. The influence of the initial pH value on the H2 evolution for the Pt/TiO2-xNx suspension is demonstrated in Figures 6 and 7. As can be seen, the higher H2 evolution rate occurs at pH ) 6.5, which is the point of zero charge of the catalyst detected in this work. At the point of zero charge, the reactant (methanol) molecules are allowed to reach easily the catalyst’s surface and achieve higher reaction rates. 3.4. Effect of Different Alcohol on Hydrogen Evolution. As can be seen from Figure 5, the hydrogen production noticeably increases with the addition of methanol to the water solution. A comparison of hydrogen evolution for methanol, ethanol, and propanol solutions is presented in Figure 8. The hydrogen yield decreases with the increase of the carbon number of alcohol. This effect is mainly due to the decrease in the formation of formaldehyde for higher alcohols because it involves carbon-to-carbon bond breaking. The extent of carbonto-carbon bond breaking decreases with the increase in chain elongation and complexity, as these factors contribute to enhancing the steric hindrance in the molecule.35 3.5. Reaction Time. The variation of hydrogen production from methanol/water photocatalytic decomposition over 0.6 wt % Pt/TiO2-xNx was examined at 30 °C and a methanol/water concentration of 60% (v/v). Figure 9 shows the variation of hydrogen production against reaction time. The H2 production rate increases significantly after 12 h of reaction. After 18 h, hydrogen production rate increases to a high level and then is stable for a long time. This is because the hydrogen production reaction is an activated process, and the photons absorbed on (34) Evgenidou, E.; Fytianos, K.; Poulios, I. J. Photochem. Photobiol. A: Chem. 2005, 175, 29–38. (35) Hameed, A.; Gondal, M. A. J. Mol. Catal. A: Chem. 2005, 233, 35–41.

the surface of catalyst increase with an increase in the irradiation time, which in turn helps in the photodecomposition process. The results indicate that the Pt/TiO2-xNx will be a stable photocatalyst for a long period for the methanol/water photocatalytic decomposition. 4. Conclusions This present work uses the two-microemulsion method to synthesize TiO2-xNx and Pt/TiO2-xNx photocatalyst powders for hydrogen production by photocatalytic methanol/water splitting in visible light. From the experimental results of tests, the following conclusions can be drawn: (1) The amount of hydrogen production was found to be dependent on the Pt content on the photocatalyst, initial pH, sacrificial reagents, and the concentration of the sacrificial reagent. (2) The addition of Pt on TiO2-xNx surface can increase the photocatalytic reaction and enhance hydrogen evolution. The optimum loading of Pt on TiO2-xNx is 0.2 wt %. (3) Among the sacrificial reagentssmethanol, ethanol, and propanolsmethanol was found to be the most effective and strongest sacrificial reagent to yield the highest photocatalytic H2 evolution activity. The hydrogen production first increases with the increase of methanol concentration, but the maximum activity is exhibited at the 80% (v/v) methanol/water concentration. (4) The photocatalytic hydrogen evolution is favorable in the neutral pH range. (5) Using 65 mg of Pt/TiO2-xNx powder in 150 mL of 60% methanol solution at pH values of 5-7 produces about 475 µmol of hydrogen in 6 h. Acknowledgment. The authors thank the National Science Council (NSC 94-2211-E-168-001) for the financial support of this work. EF801091P