Energy & Fuels 2005, 19, 2209-2213
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New Application of Glycerin from a Photochemical Approach: Dihydrogen Formation from Aqueous Glycerin by Use of Giant Polyoxometalate Photocatalysts Hisao Hori* and Kazuhide Koike National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba West, 16-1 Onogawa, Tsukuba 305-8569, Japan
Yoshitaka Sakai, Hideyuki Murakami, Kunihiko Hayashi, and Kenji Nomiya Department of Materials Science, Faculty of Science, Kanagawa University, Hiratsuka 259-1293, Japan Received May 24, 2005. Revised Manuscript Received June 26, 2005
Photocatalytic H2 formation from aqueous glycerin was achieved over giant polyoxometalate complexes Na21K4[{P2W15Ti3O57.5(OH)3}4Cl]‚104H2O and Na18H15[{P2W15Ti3O59(OH)3}4{µ3-Ti(OH)3}4Cl]‚105H2O to develop a new application for glycerin, an oversupplied compound accompanying biodiesel production. This is the first example of the catalytic formation of H2 by use of a photocatalyst and glycerin and is also the first example of a photochemical application of giant polyoxometalate complexes. The catalytic reactions proceed at room temperature under irradiation with UV-visible light without the use of any cocatalyst. The reaction mechanism can be explained by redox reactions between the complexes, water, and glycerin.
Introduction Recently, there has been a worldwide increase in the production of biodiesel, a fuel that is environmentally benign owing to its low sulfur content and the fact that it does not participate in global warming.1-3 Biodiesel is produced by combining vegetable oils with methanol in a transesterification reaction that yields a fatty acid methyl ester (i.e., biodiesel) and glycerin. Accompanying this increase in biodiesel production, glycerin has become an oversupplied compound, for which new applications are strongly desired.1 Dihydrogen production from inexpensive and readily available sources is attractive because it may contribute to solving both energy and environmental concerns. Several reports have focused on H2 production from biomass-derived hydrocarbons, including glycerin, by use of metal catalysts that work at temperatures near 500 K.4-7 Water-soluble polyoxometalate complexes, such as silicododecatungstate [SiW12O40]4-, have been known for the past 20 years to be efficient homogeneous photocatalysts for the formation of H2 from acidic aqueous solutions containing alcohol (typically methanol),8,9 * Author to whom correspondence should be addressed. Tel: +81298-61-8161. Fax: +81-29-861-8258. E-mail:
[email protected]. (1) McCoy, M. Chem. Eng. News 2005, 83 (8), 19-20. (2) Ma, F.; Hanna, M. A. Bioresour. Technol. 1999, 70, 1-15. (3) Bondioli, P. Top. Catal. 2004, 27, 77-82. (4) Davda, R. R.; Shabaker, J. W.; Huber, G. W.; Cortright, R. D.; Dumesic, J. A. Appl. Catal. B 2005, 56, 171-186. (5) Shabaker, J. W.; Huber, G. W.; Dumesic, J. A. J. Catal. 2004, 222, 180-191. (6) Huber, G. W.; Shabaker, J. W.; Dumesic, J. A. Science 2003, 300, 2075-2077. (7) Czernik, S.; French, R.; Feik, C.; Chornet, E. Ind. Eng. Chem. Res. 2002, 41, 4209-4215.
where the alcohol acts as an electron source to the photocatalysts and the reduced photocatalysts cause the reduction of H+ in water to H2. These reported polyoxometalate photocatalysts consist of a single polyanion unit, typically of Keggin-type, and the systems require colloidal platinum as a cocatalyst for the catalytic formation of H2. On the other hand, giant polyoxometalate complexes formed by the combination of several polyanion units are attractive photocatalyst candidates for H2 formation because the multielectron redox capabilities of these giant complexes may facilitate the reduction of H+ in water to H2 without requiring the use of a precious-metal cocatalyst. We report herein that two giant polyoxometalate complexes consisting of four Dawson-type polyanion units, Na21K4[{P2W15Ti3O57.5(OH)3}4Cl]‚104H2O (1) and Na18H15[{P2W15Ti3O59(OH)3}4{µ3-Ti(OH)3}4Cl]‚105H2O (2), can function as photocatalysts to form H2 from aqueous glycerin without requiring any cocatalyst. Experimental Section Materials. Giant polyoxometalate complexes 1 and 2 were prepared as described elsewhere.10-12 Glycerin (>95% purity) was obtained from Wako Pure Chemical Industries Ltd. (8) Akid, R.; Darwent, J. R. J. Chem. Soc., Dalton Trans. 1985, 395399. (9) Yamase, T.; Watanabe, R. J. Chem. Soc., Dalton Trans. 1986, 1669-1675. (10) Sakai, Y.; Yoza, K.; Nozaki Kato, C.; Nomiya, K. J. Chem. Soc., Dalton Trans. 2003, 3581-3586. (11) Sakai, Y.; Yoza, K.; Nozaki Kato, C.; Nomiya, K. Chem. Eur. J. 2003, 9, 4077-4083. (12) Sakai, Y.; Kitakoga, Y.; Hayashi, K.; Yoza, K.; Nomiya, K. Eur. J. Inorg. Chem. 2004, 4646-4652.
10.1021/ef050157i CCC: $30.25 © 2005 American Chemical Society Published on Web 07/30/2005
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(Osaka, Japan). Other reagents and solvents were of high purity and commercially available from Kanto Chemical Co. (Tokyo, Japan). Argon gas (99.99%) for the reaction was purchased from Tomoe Shokai Co. (Tokyo, Japan). Photochemical Procedures. A cylindrical quartz photochemical cell (28.8 mL) fitted with a rubber septum was used. In a typical run, an aqueous H2SO4 solution (0.5 M, 5.0 mL) containing 1 or 2 (3.25 × 10-7 mol, 65.0 µM), glycerin (5.43 mmol, 1.09 M), and a PTFE stirring bar was placed into the cell. The pH of the solution was approximately 0.4. The solution was saturated with argon gas, and the cell was sealed with the rubber septum; the cell was then placed into a water bath and irradiated with UV-visible light from a xenonmercury lamp (200 W; L2001-01L; San-Ei Electric Co., Osaka, Japan) while stirring. For the light irradiation, a cutoff filter (WG305; Schott, Mainz, Germany) and optical quartz fiber were used. The reaction temperature was kept constant at 25 °C. For quantum yield measurements, a quartz cell (9.6 mL) with an aqueous H2SO4 solution (0.5 M, 4.0 mL) containing 1 or 2 (3.25 × 10-7 mol, 81.3 µM) and glycerin (4.34 mmol, 1.09 M) was used, and a band-pass filter (313 nm; Asahi Spectra, Tokyo, Japan) was employed to produce 313-nm monochromatic light. The incident light intensity was determined by a K3[Fe(C2O4)3] actinometer. Measurements. The gases produced were analyzed by a gas chromatograph (GC 323; GL Sciences, Tokyo, Japan) consisting of an injector (150 °C), a column oven (50 °C), and a thermal conductivity detector (130 °C). The column used was an active carbon column (60/80 mesh, 2.17-mm i.d., 2-m length; GL Sciences), and the carrier gas was argon. Emission-lifetime measurements were performed with a frequency-tripled titanium:sapphire laser (Tsunami laser and Spitfire regenerative amplifier; Newport, Irvine, CA) with 266-nm excitation wavelength, 150-fs pulse width, 25-mW pulse energy, and 1-kHz repetition rate. Emission signals were detected with a C4334 streak scope (Hamamatsu Photonics, Hamamatsu, Japan) in photon-counting mode with a time response of 20 ps. Cyclic voltammetry (CV) was performed with an electrochemical analyzer (BAS 100B; BAS, Tokyo, Japan) in an aqueous solution (pH ca. 1.1) containing each complex (0.50 mM) with supporting electrolyte (NaHSO4, 0.1 M) at a scan rate of 100 mV s-1. A glassy carbon working electrode and an Ag/AgCl reference electrode were used. To check the current accompanying the redox processes of the samples, K3[Fe(CN)6] (2.0 mM) as a standard was also subjected to measurement. UV-visible spectra were measured with a Shimadzu UV-2550 spectrophotometer (Shimadzu, Kyoto, Japan). The reaction solutions were transferred into quartz cells (path length 0.5 cm) under an argon atmosphere and were then subjected to measurement.
Hori et al.
Figure 1. Schematic views of the structures (anion parts) of giant polyoxometalate complexes 1 and 2. For details of the structures involving the cation parts, see refs 10-12.
Results and Discussion
Figure 2. Absorption of glycerin (1.09 M in 0.5 M H2SO4), 1 (65.0 µM in 0.5 M H2SO4), and 2 (65.0 µM in 0.5 M H2SO4) and transmittance of the WG305 cutoff filter plotted against wavelength. Concentrations of glycerin, 1, and 2 are the same as those in the catalysis experiment (Figure 3). The path length for measurement of the absorption spectra was 0.5 cm.
Photocatalysis. The structures of both giant polyoxometalate complexes are shown in Figure 1. In both 1 and 2, PO4 tetrahedra and WO6 octahedra combine to form single Dawson-type polyanion units, and four of these polyanion units combine to form a giant structure that uses TiO6 octahedra for 1 and TiO6 octahedra and Ti(OH)3 bridging groups for 2. During the photocatalytic reactions, the acidic aqueous solutions of 1 or 2 with glycerin were irradiated with UV-visible light from the xenon-mercury lamp through the WG305 cutoff filter. With this filter in place, the lamp emits light above a wavelength of 280 nm (Figure 2). Glycerin, however, has no absorption above 260 nm, whereas 1 and 2 have strong absorptions in the deepUV region to approximately 400 nm. Hence, 1 and 2 are the only species that can absorb light from the lamp during the photocatalytic reactions.
As expected, photoreactions using 1 or 2 with light wavelengths >280 nm resulted in formation of H2 in both cases (Figure 3). After 18 h of irradiation, 19.2 µmol of H2 was produced when 1 was used, corresponding to a turnover number [(moles of H2 formed)/(moles of initial 1)] of 59.1, whereas when 2 was used, 7.56 µmol of H2 was produced, with a turnover number [(moles of H2 formed)/(moles of initial 2)] of 23.3. These observations indicate that the formation of H2 proceeded catalytically and that the catalytic ability of 1 was higher than that of 2. As for the quantitative relation between the H2 produced and glycerin, the molar ratio of H2 to initial glycerin was 0.35% for 1 and 0.14% for 2 after 18 h of irradiation. Small amounts of CO were also detected. After 18 h of irradiation, 2.77 µmol of CO was detected for 1 and 2.82 µmol for 2, showing that the molar ratio of H2 to the total gaseous product (H2 +
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Figure 4. Relationship between the catalytic formation of H2 over 1 or 2 and duration of 313-nm light irradiation. An aqueous H2SO4 solution (0.5 M, 4.0 mL) containing the complex (1 or 2; 3.25 × 10-7 mol; 81.3 µM) and glycerin (4.34 mmol, 1.09 M) was irradiated with a xenon-mercury lamp under an argon atmosphere. The temperature of the solution was maintained at 25 °C.
Figure 3. Catalytic formation of H2 and CO over 1 (top) and 2 (bottom) as a function of irradiation time. An aqueous H2SO4 solution (0.5 M, 5.0 mL) containing the complex (1 or 2; 3.25 × 10-7 mol, 65.0 µM) and glycerin (5.43 mmol, 1.09 M) was irradiated (λ > 280 nm) with a xenon-mercury lamp under an argon atmosphere. The temperature of the solution was maintained at 25 °C.
CO) was 87.4% for 1 and 72.8% for 2. Glycerin was the sole carbon-containing species among the initial species before irradiation. Hence, the carbon in CO was from glycerin. In the absence of either light irradiation or glycerin, no reaction occurred. In the absence of the complex (1 or 2), no H2 was formed. Therefore, it is clear that the combination of the complex, glycerin, and light irradiation is required for the formation of H2 and that the complexes act as photocatalysts with the glycerin effectively used as an electron source. Catalytic formation of H2 over 1 or 2 was also observed when 313-nm monochromatic light was used (Figure 4). When the light intensity was 3.10 × 10-4 einstein h-1 (1 einstein ≈ 6.022 × 1023 photons), the apparent quantum yields for the formation of H2 [(H2 formation rate)/(light intensity)], where the H2 formation rate was taken from the period of irradiation when the H2 amount increased linearly with respect to time, were 7.96 × 10-4 for 1 and 1.72 × 10-4 for 2, showing that 1 is more efficient than 2, a result consistent with photoreactions using light wavelengths >280 nm. Reaction Mechanism. Formation of H2 by use of conventional single-polyanion-unit polyoxometalate photocatalysts such as [SiW12O40]4- in aqueous alcoholic solutions has been explained as follows.8,9 The polyoxometalate complex (P) is raised to the ligand-to-metal charge-transfer excited state by photoirradiation (eq 1), and the excited species (P*) goes back to the initial state
with emission (eq 2). When the alcohol acts as an electron source to the excited-state species P*, P* is quenched to form the one-electron-reduced species P(eq 3). When the platinum cocatalyst is present, the additional electron of P- moves onto the platinum cocatalyst, and the electrons gathered on the cocatalyst reduce H+ in water to H2 (eq 4).8 On the other hand, when the cocatalyst is absent, hardly any H2 is formed; however, in some cases, P- is further reduced by a disproportionation reaction with another P- (eq 5), and the two-electron-reduced species thus formed (P2-) reduces H+ to give H2 (eq 6).9
P + hν f P*
(1)
P* f P + hν
(2)
P* + alcohol f P- + alcohol+
(3)
2P- + 2H+ + Pt f 2P + H2 + Pt
(4)
2P- f P2- + P
(5)
P2- + 2H+ f P + H2
(6)
Complex 1 showed maximum absorption at 274 nm with a molar extinction coefficient of 3.36 × 105 M-1 cm-1 in 0.5 M H2SO4. Complex 2 also showed maximum absorption at 290 nm with a molar extinction coefficient of 4.28 × 105 M-1 cm-1. In addition, emission-lifetime measurements showed that, in 0.5 M H2SO4, 1 has a maximum emission at 356 nm with a lifetime of 31.1 ns and 2 has a maximum emission at 354 nm with a lifetime of 30.6 ns. We observed the behavior of the quenching of excited species 1* and 2* by glycerin. From the relation between glycerin concentration and the rate constant of emission decay from 1* or 2*, we measured the quenching rate constants for 1* and 2* by glycerin to be 4.11 × 108 M-1 s-1 and 5.33 × 108 M-1 s-1, respectively. These facts indicate that both excited species 1* and 2* were fully quenched with the 1.09 M
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Figure 6. Cyclic voltammogram of 1 (0.50 mM) in an argonsaturated aqueous solution of NaHSO4 (0.1 M) with a scan rate of 100 mV s-1. Table 1. Electrochemical Data for the Polyoxometalate Complexesa first redox process
second redox process
complex
E1/2 (mV)
∆E (mV)
∆i (µA)
E1/2 (mV)
∆E (mV)
∆i (µA)
1 2 [SiW12O40]4- b
-253 -236 -144
59 68
31.2 31.2
-404 -408 -402
55 59
27.6 30.0
a Cyclic voltammetry (CV) was performed in an aqueous solution containing the complex (0.50 mM) and a supporting electrolyte NaHSO4 (0.1 M). A glassy carbon working electrode and an Ag/ AgCl reference electrode were used. The pH was approximately 1.1. b Taken from ref 14, where the measurement was carried out in a sulfate solution of pH 1.0.
Figure 5. UV-visible spectra of the sample solutions at various irradiation times. After irradiation for the specified time, the reaction solutions were transferred into quartz cells (path length 0.5 cm) under an argon atmosphere and then subjected to measurement.
of glycerin which we used in the photocatalytic reactions. Consistently, after irradiation started, a new broad absorption band appeared with maxima around 690-710 nm in the UV-visible spectra (Figure 5). The new absorption band reflects the appearance of the reduced species, which was identified by comparison with the reported spectra of the conventional singlepolyanion-unit polyoxometalate complexes,13 indicating that the quenching of 1* and 2* by glycerin occurs effectively during the catalytic reactions. From the discussion above, it is clear that complexes 1 and 2 both satisfy the prerequisites for the H2 formation expressed by eqs 1-3, where 1 and 2 showed similar photoabsorption, emission, and quenching properties. Nevertheless, photocatalytic formation of H2 was seen to be higher over complex 1 than over complex 2. This observation suggests that the processes occurring after quenching by glycerin were different in each complex. To clarify this point, we measured the electrochemical behavior of each complex. In the CV measurements, we observed two reversible waves in the reductive region of complex 1 (Figure 6)s the first with a half-wave potential (E1/2) of -253 mV (vs Ag/AgCl) and the second with an E1/2 value of -404 (13) Papaconstantinou, E. Chem. Soc. Rev. 1989, 18, 1-31.
mV. Complex 2 showed a similar CV pattern. The electrochemical data for 1 and 2 are summarized in Table 1. In both the first and second reduction processes, each complex showed a potential difference (∆E) of approximately 60 mV between the cathodic peak and the corresponding anodic peak, indicating that each wave is a single-electron process. We compared the peak current (∆i) ascribed to these reductions with that ascribed to the reduction of a standard complex [Fe(CN)6]3-. The ∆i values of 1 and 2 (0.50 mM) in the first and second reductions were between 27.6 and 31.2 µA, whereas the ∆i value accompanying the reduction of [Fe(CN)6]3- (2.0 mM) to [Fe(CN)6]4- was 29.3 µA. Therefore, when compared at the same concentration, each peak current for 1 and 2 was approximately 4 times that of the single-electron reduction of the standard complex. This fact indicates that, in both the first and the second reduction processes of 1 and 2, each of the four polyanion units receives one electron. The reduction potential of water (E0, corresponding to the process 2H+ + 2e- T H2) is dependent on the pH of the solution in accordance with the Nernst equation E0 ) -(199 + 59 × pH) mV (vs Ag/AgCl). Under our reaction conditions, the pH of the reaction mixture was approximately 0.4; therefore, the E0 was -223 mV. On the other hand, the E1/2 values for the first reduction were -253 and -236 mV for 1 and 2, respectively, and are thus energetically plausible to reduce H+ to H2. The E1/2 values in the first reduction process of these giant polyoxometalate complexes are much more negative than that of [SiW12O40]4- (-144 mV)14 (see Table 1). This difference may explain why 1 and 2 can cause the
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catalytic formation of H2 without the use of any cocatalyst, while [SiW12O40]4- requires a platinum cocatalyst. For the two giant polyoxometalate complexes, the E1/2 value in the first reduction is more negative for 1 than for 2, indicating that 1 is more able to cause the reduction of H+ to H2. This fact is consistent with the higher catalytic H2 formation ability of 1 than 2. In our reaction system, the quenching of the excited photocatalysts by glycerin was sufficiently fast (in the order of 108 M-1 s-1), although the apparent quantum yields for the formation of H2 were low (in the order of 10-4). To enhance the H2 formation rate, an improvement in the rate of the reduction process from H+ in water to H2 is desired.
H2 formation from aqueous glycerin without the use of any cocatalyst. The glycerin was used effectively as an electron source to the photochemically excited complexes, and the resultant reduced complexes caused H2 formation. After 18 h of irradiation, the turnover number for the formation of H2 reached 59.1 and 23.3 by use of 1 and 2, respectively. The higher catalytic activity of 1 compared to that of 2 can be explained by the more negative reduction potential of 1. Photocatalytic H2 formation systems using glycerin and polyoxometalates other than 1 and 2, including catalytic systems consisting of polyoxometalate-loaded heterogeneous photocatalysts, are being investigated in our laboratory to enhance the H2 formation rate.
Conclusions We have shown that the giant polyoxometalate complexes 1 and 2 can function as photocatalysts to cause
Acknowledgment. We thank Dr. Yoshiyuki Sasaki (AIST) for his valuable discussions. This work was supported in part by a Grant-in-Aid for Scientific Research (No. 17310055) from the Japan Society for the Promotion of Science (JSPS).
(14) Pope, M. T.; Varga, G. M., Jr. Inorg. Chem. 1966, 5, 12491254.
EF050157I
