Energy & Fuels 2005, 19, 1143-1147
1143
Photocatalytic Water Splitting over Pt-TiO2 in the Presence of Sacrificial Reagents Anna Galin´ska* and Jerzy Walendziewski Institute of Chemistry and Technology of Petroleum and Coal, Wrocław University of Technology, ul. Gdan´ ska 7/9, 50-310 Wrocław, Poland Received June 29, 2004. Revised Manuscript Received December 22, 2004
Hydrogen production by water splitting over a Pt-TiO2 catalyst has been studied. The main goal of the studies was to examine the influence of sacrificial reagents on the photocatalytic efficiency in water splitting reactions under ultraviolet (UV) irradiation. Various sacrificial reagents, such as methanol, Na2S, and ethylene diamine tetraacetic acid (EDTA), as well as Iand IO3- ions, were used to obtain an effective water splitting to H and O atoms. It was revealed that, in the case of using methanol as the sacrificial reagent, hydrogen evolution is also a result of methanol conversion. Photocatalytic water splitting was obtained when EDTA and Na2S were used as the sacrificial reagents.
1. Introduction Recently, the production of hydrogen has received much attention, because of its potential application as a clean source of energy. Water splitting by a photocatalytic process using oxide semiconductors can be considered to be successful, in regard to it being an economic and clean technology; as a result, it can resolve the problems of energy consumption and environmental degradation, at least partially. Efficient conversion of solar energy has been a challenging problem, and numerous studies have been undertaken in this direction. Biomass conversion, photocatalytic cells, and photocatalytic water splitting are the best-known examples. It has been reported that some metal oxides, such as TiO2, SrTiO2, ZrO2, BaTi4O9, and CeO2,1-10 possess reasonable activities for water splitting into H2 and O2 in the stoichiometric ratio under ultraviolet (UV) and visible-light irradiation. TiO2 has been widely used in photocatalysis, because of its favorable band-gap energy (3.2 eV in anatase) and its high stability in aqueous solution under UV irradiation. Photocatalytic process over TiO2 is initiated by the absorption of a photon with energy equal to or greater * Author to whom correspondence should be addressed. Telephone/ fax: (+048-71) 322-15-80. E-mail:
[email protected]. (1) Sayama, K.; Arakawa, H.; Domen, K. Catal. Today 1996, 28, 175-182. (2) Sayama, K.; Arakawa, H.; Domen, K. J. Photochem., Photobiol. A 1994, 77, 243-245. (3) Takata, T.; Shinohara, K.; Tanaka, A.; Hara, M.; Kondo, J. N.; Domen, K. J. Photochem. Photobiol. A 1997, 106, 45-49. (4) . Moon, S.-Ch.; Mametsuka, H.; Tabata, S.; Suzuki, E. Catal. Today 2000, 58, 125-132. (5) Kato, H.; Kudo, A. Catal. Today 2003, 78, 561-569. (6) Kato, H.; Kudo, A. Chem. Phys. Lett. 1998, 295, 487-492. (7) Hara, K.; Sayama, K.; Arakawa, H. Appl. Catal., A 1999, 189, 127-137. (8) Sayama, K.; Mukasa, K.; Abe, R.; Abe, Y.; Arakawa, H. J. Photochem. Photobiol. A 2002, 148, 71-77. (9) Bamwenda, G. R.; Arakawa, H. J. Mol. Catal. A 2000, 161, 105113. (10) Chung, K.-H.; Park, D.-C. Catal. Today 1996, 30, 157-162.
than the forbidden band gap of the semiconductor. Following light irradiation, the electron-hole pairs are formed. Photogenerated electrons migrate to the conduction band, where they reduce H+ to H2 and the holes on the surface of semiconductor decompose H2O to O2 and H+.11,12 hυ > Eg
TiO2 w TiO2(e-cb + h+vb) 2e
-
+
cb
•
•
+ 2H w H + H w H2
(1) (2)
h+vb + H2O w H2O+ w OH• + H+
(3)
1 OH• + OH• w H2O + O2 2
(4)
It has been reported4,13-15 that pure TiO2 could not split water into H2 and O2 in the simple aqueous suspension system. The main problem is the fast, undesired electron-hole recombination reaction, which is thermodynamically favored. Therefore, it is important to prevent the electron-hole recombination process. A sacrificial reagent helps to control this process. Photoefficiency of the process can be improved by the addition of so-called sacrificial reagents.4,13,17 Their main task is separation of the photoexcited electrons and holes, which are available for reversible reaction. They play the role of electron donors or electron-acceptor scavengers. (11) Mills, A.; Le Hunte, S. J. Photochem. Photobiol. A 1997, 108, 1-35. (12) Hameed, A.; Gondal, M. A. J. Mol. Catal. A 2004, 219, 109119. (13) Abe, R.; Sayama, K.; Arakowa, H. Chem. Phys. Lett. 2003, 371, 360-364. (14) Sato, S.; White, J. M. Chem. Phys. Lett. 1980, 72, 83. (15) Tabata, S.; Nishida, H.; Masaki, Y.; Tabata, K. Catal. Lett. 1995, 34, 245. (16) Moon, S.-Ch.; Matsumara, Y.; Kitano, M.; Matsuoka, M.; Anpo, M. Res. Chem. Intermed. 2003, 29, 233-256. (17) Lee, S. G.; Lee, S.; Lee, H.-I. Appl. Catal., A 2001, 207, 173181.
10.1021/ef0400619 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/25/2005
1144
Energy & Fuels, Vol. 19, No. 3, 2005
Galin´ ska and Walendziewski the magnetic stirrer for 15 min, and then the UV lamp was turned on. Methanol, EDTA, Na2S, NaI, and NaIO3 were used as sacrificial reagents. No special gases were used to remove oxygen from the reaction mixture. The gases were collected in the glass trap (see Figure 1) and were analyzed by the gas chromatography (GC) method (TCD was used as a detector, 5 Å molecular sieves were used as the column, and N2 was used as the carrier gas).
3. Results and Discussion
Figure 1. Apparatus for photocatalytic water splitting.
Figure 2. Emission spectrum of the mercury lamp.
For photocatalytic hydrogen generation, compounds such as methanol, ethanol, EDTA (an ethylenediaminetetraacetic derivative), Na2S, Na2SO4, or ions such as I-, IO3-, CN-, and Fe3+ can be used as sacrificial reagents.7,8,13,17-19 The same reports indicate that one can also use many other organic pollutants (such as oxalic acid, formic acid, and formaldehyde) as electron donors in wastewater.7,18 In this study, the influence of some selected sacrificial reagents on the photoactivity of Pt-TiO2 in hydrogen production by water splitting is presented. 2. Experimental Section Commercially available TiO2 powder (P25, Degussa) was used without further purification. H2PtCl6 (0.3 wt % platinum) was deposited on the P25 powder, using the dry impregnation method. The obtained catalysts were calcined at temperatures 400, 773, and 1000 °C for 4 h. The photocatalytic reactions were performed using a closed system with an inner-irradiation-type quartz reactor. A mediumpressure mercury lamp (Ace Glass Co., power of 450 W) was the light source. (Figure 1 shows a schematic of the water splitting apparatus, and Figure 2 gives the emission spectrum of the mercury lamp.) The size of band gap (Eg) is 3.05 eV for rutile and 3.20 eV for anatase, corresponding to wavelengths of 420 and 385 nm, respectively. The lamp was plunged in a quartz immersion well with cooling water. The temperature of the reaction cell was controlled in the temperature range of 30-60 °C by cooling water. The photocatalyst, sacrificial reagents, and distilled water (400 mL) were added into the reactor, to the point where the reactor was completely filled. The suspension was mixed with (18) Li, Y.; Lu, G.; Li, S. Chemosphere 2003, 52, 843-850. (19) Abe, R.; Sayama, K.; Domen, K.; Arakawa, H. Chem. Phys. Lett. 2001, 344, 339-344.
3.1. Evolution of H2 over Pt-TiO2 Using Methanol. In the first step, we examined photocatalytic water splitting over a Pt-TiO2 photocatalyst, using methanol as the sacrificial electron donor. Figure 3 presents the H2 yield over 0.3 wt % Pt-TiO2 using methanol, as a function of time. The production of hydrogen started 5 min after the lamp is turned on. It was probably due to the stabilization of lamp radiation and saturation of water with evolved gases. The amount of the evolved H2 increased linearly with UV irradiation time. After 120 min of irradiation, the experiment had to be stopped, because the UV lamp was overheating. It is expected that the photocatalytic activity does not diminish after 120 min of irradiation time. It has been reported that, in the Pt-TiO2 aqueous suspension, alcohol molecules are oxidized to CO2 and transformed to CO and/or CH4.20,21 The presence of oxygen vacancy defects strongly enhances such interactions, because of electron back-donation from surface Ti3+ into the π* orbital of molecular CO.20 According to the literature,22,23 methanol is oxidized to CO2: hυ, cat.
CH3OH w HCHO + H2 hυ, cat.
HCHO + H2O w HCO2H + H2 hυ, cat.
HCO2H w CO2 + H2
(5) (6) (7)
The chromatographic analysis of the exposed mixture revealed that the methanol concentration was decreased by 10%, in comparison to the concentration prior to exposure. This observation suggests that some quantity of methanol was consumed during the reaction. We identified CO2, in an amount of 0.5%, and CH4, in an amount of 4 wt %, in the gas mixture. The detection of CO2 in the mixture of evolved gases directly confirms the aforementioned statement. On the other hand, the presence of CH4 in the reaction mixture confirms not only that methanol is oxidized according to reactions 5-7, but also that, in the next step, CH4 is produced as a result of the reaction of CO2 with water.24 However, the abundance of methane production indicates that methanol is reduced, presumably by evolved (20) Zou, Z.; Ye, J.; Abe, R.; Arakawa, H. Catal. Lett. 2000, 68, 235239. (21) Zou, Z.; Arakowa, H. J. Photochem. Photobiol. A 2003, 158, 145-162. (22) Millard, L.; Browker, M. J. Photochem. Photobiol. A 2002, 148, 91-95. (23) Kawai, T.; Sakata, T. J. C. S. Chem. Commun. 1980, 694-695. (24) Linsebigler, A. L.; Lu, G.; Yateas, J. T., Jr. Chem. Rev. 1995, 735, 735-758.
Photocatalytic Water Splitting over Pt-TiO2
Figure 3. Photocatalytic H2 evolution over Pt-TiO2 using a CH3OH/H2O solution. Temperature ) 60 °C, catalyst weight ) 0.3 g.
Energy & Fuels, Vol. 19, No. 3, 2005 1145
Figure 5. Photocatalytic H2 evolution over 0.3 wt % Pt-TiO2 (0.3 g) using a Na2S/H2O solution.
Figure 6. Photocatalytic O2 evolution over 0.3 wt % Pt-TiO2 (0.3 g) using a Na2S/H2O solution. Figure 4. Photocatalytic H2 evolution over Pt-TiO2 using a CH3OH/H2O solution. Catalyst weight ) 0.3 g, temperature ) 40 °C.
hydrogen or as a result of the reaction of CO2 with water.24
CO2 + H2O
hυ
w
M-TiO2
{
CH4 CH2O CH3COOH (where M ) Pt, Pd, Au, Cu, Ru)
The yield of the production of hydrogen is slightly dependent on the concentration of methanol in the reaction mixture. At the highest amount of methanol (15 mL), we have observed a very low evolution of hydrogen after 80 min of reaction time. After 120 min, we added the next portion of methanol (10 mL) into the solution; however, we did not obtain an increase in the production of hydrogen. In the second variant, we added 0.3 g of catalysts, which resulted in further production of hydrogen (see Figure 3). These results reveal that the decreasing yield in hydrogen productionsand, finally, the stoppage of the reactionscan be the result of deactivation of the catalyst. The relatively high reaction rate was a result of the heightened test temperature (60 °C). For the comparison, we conducted the same experiment at a lower temperature (40 °C). The yield was much lower than when we obtained 2.4 mmol of hydrogen after 120 min from the beginning of this study (Figure 4).
3.2. H2 Evolution over Pt-TiO2 Using Na2S. Figure 5 presents the results of the study of water splitting over 0.3 wt % Pt-TiO2 catalyst using different amounts of Na2S as the sacrificial reagent. The results indicate that the yield of hydrogen is dependent on the concentration of Na2S. H2 evolution under UV irradiation increases rapidly with the increasing amount of this sacrificial reagent in the reaction mixture during the first 20 min. As the reaction proceeds, the rate of hydrogen production decreases. This could be attributed to disintegration of the sacrificial reagent over this period of exposure. We also identified SO42- ions in the solution after reaction. This means that some quantity of oxygen was consumed in the reaction of sulfide ions (oxygenation); therefore, the hydrogen-oxygen recombination process could be avoided. The hydrogen production under UV irradiation over Pt-TiO2, using a low concentration of Na2S (1 and 2 mmol), was very low and appreciably higher at a higher Na2S concentration, whereas the evolution of oxygen with small amounts of Na2S (1 and 2 mmol) was observed (Figure 6). Higher Na2S concentrations seemed to favor oxygen production; therefore, it could not be observed in the reaction product while hydrogen was evolved, only because the recombination reaction was not possible. The O2 and H2 generation was complete after 60 min of exposure. The zero production of these gases indicates
1146
Energy & Fuels, Vol. 19, No. 3, 2005
Figure 7. Evolution of H2 over 0.3 wt % Pt-TiO2 using a Na2S/H2O solution.
the complete disintegration of the sacrificial reagent in the reaction mixture and a lack of oxygen as a trapped reagent. It is well-known that the catalyst concentration greatly influences the reaction rate. Figure 7 shows that the photocatalytic production of hydrogen under UV irradiation is dependent on the amount of Na2S catalyst in reaction solutions. An increase of more than a factor of 3 in the catalyst concentration in the reaction suspension resulted in a 30%-40% increase in hydrogen production. In the first 20 min of exposure, the production of hydrogen is similar, despite different amounts of the catalyst (∼0.9 mmol). After this time, the rate of the gases’ evolution rapidly decreased, which could be caused by the consumption of Na2S, the oxygenation of sulfide ions, and the reduction of their concentration in the reaction mixture. It was revealed earlier that sulfate ions were found there. 3.3. H2 Evolution over Pt-TiO2 Using NaI and NaIO3 as the Sacrificial Reagents. The next series of studies was conducted using I- and IO3- ions as the sacrificial reagents. After 30 min of exposure of the IO3- ions catalyst in the aqueous suspension, we have observed a very low production of hydrogen (3.1 µmol). We have also identified the evolution of oxygen; however, the rate of evolution decreased with the time of exposure. According to some works,13 the rate of hydrogen evolution was very low, because the reaction of IO3- reduction runs competitively with H+ reduction. The time dependence of O2 evolution is shown in Figure 8. The highest activity is observed in the solution using NaIO3; it is 450 µmol after 45 min. However, the photocatalytic activity ended after 60 min. In the case of the solution using NaI and NaI + NaIO3, the photoactivity was similar. In all experiments, the rate of oxygen and hydrogen evolution decreased with the reaction time. This observation could be ascribed to the consumption of the sacrificial reagent ions or catalyst deactivation. 3.4. H2 Evolution over Pt-TiO2 Using EDTA. As mentioned previously, EDTA is also treated as a hole scavenger. The results of the photoactivity studies of catalyst in aqueous EDTA solution are shown in Figure 9. We obtained high yield in the production of hydrogen in the reaction with the addition of 3 and 4 mmol EDTA
Galin´ ska and Walendziewski
Figure 8. Evolution of O2 over 0.3 wt % Pt-TiO2 (0.3 g).
Figure 9. Evolution of H2 over 0.3 wt % Pt-TiO2 using an EDTA/H2O solution.
(∼1.8 mmol after 65 min of reaction). Although there was no oxygen production, the hydrogen production yields for EDTA concentrations of 3 and 4 mmol were similar. This suggests that EDTA trapped all the produced oxygen; therefore, a high yield evolution of hydrogen was possible. After addition of only a small amount of EDTA (2 mmol) into the aqueous suspension of the catalyst, both H2 and O2 were evolved. The H2/O2 ratio was not stoichiometric (2.75) during exposure. After 90 min from the beginning of the reaction, we obtained 0.735 mmol of hydrogen and 0.268 mmol of oxygen. The evolution of both gases indicates that the OH radicals were only partly trapped by the small amount of EDTA, which allows the formation of O2 (eq 4). The reaction stopped when EDTA was utilized. 3.5. Influence of Calcination Temperature of the TiO2 Catalyst on H2 Evolving under UV Irradiation. Finally, we have investigated the dependence of the photoactivity of the Pt-TiO2 catalyst in the water splitting under UV irradiation at the calcination temperature of the TiO2 support (Figure 10). The high-temperature calcined TiO2 catalysts present a very low photoactivity and the gas evolving yield was strongly decreasing with the calcination temperature of the catalyst. This is a result of the transformation of the active anatase structure to the low catalytically active rutile structure. The anatase structure shows
Photocatalytic Water Splitting over Pt-TiO2
Energy & Fuels, Vol. 19, No. 3, 2005 1147
splitting products, hydrogen and oxygen, was observed; however, because sulfide ions (S2-) were oxidized to SO42-, this reagent was also consumed during this period of exposure. This means that sulfide ions should also be treated as one of the raw materials. However, the real reaction mechanism is not currently known.
Figure 10. Evolution of H2 over 0.3 wt % Pt-TiO2 using an EDTA/H2O solution. The calcination temperatures of Pt-TiO2 were 400, 773, and 1000 °C. Table 1. Effect of Electron Donor on Hydrogen Production (Pt-TiO2, 0.3 g) sacrificial reagent
amount (mmol)
H2 (mmol/90 min)
without sacrificial reagent IIO3Na2S EDTA methanol
1 1 3 3 250
0 0 0.0051 1.232 2.209 11.50
Table 2. Effect of Electron Donor on Oxygen Evolution (Pt-TiO2, 0.3 g)) sacrificial reagent
amount (mmol)
O2 (mmol/30 min)
NaIO3 NaIO3 + NaI NaI EDTA Na2S
1 1 + 40a 1 2 2
0.410 0.138 0.121 0.118 0.060
a
Represents 1 mmol of NaIO3 + 40 mmol NaI.
higher activity because, unlike the rutile structure, it has a longer life in an excited state.25 3.6. Comparative Study of Sacrificial Reagents. According to the Bard’s concept,4 the Pt-TiO2 system could be illustrated as a “short-circuited” photoelectrochemical cell, where a TiO2 semiconductor electrode and a platinum-metal counterelectrode have been brought into contact. Well-dispersed metal particles act as miniphotocathode trapping electrons, which reduces water to hydrogen. Tables 1 and 2 show the results of the application of the different sacrificial reagents. It is understandable that the efficiency of the evolution of hydrogen and oxygen is dependent on the type of sacrificial reagent. Comparing the applied sacrificial reagents, we have found that the highest yields are attained by the use of methanol. The obtained results confirm the partial production of hydrogen through the methanol conversion. The reduced reaction efficiency was caused by the consumption of this agent. The production of oxygen was observed only by using IO3- and I- ions and a small amount (1 and 2 mmol) of Na2S and EDTA. The typical photocatalytic water splitting into H2 and O2 was seemingly observed in the case of using of Na2S as the sacrificial reagent. The production of both water(25) Titanium Dioxide P25 as Photocatalyst, Technical Information, Degussa Aerosil & Silanes.
A similar result was observed while using EDTA as the sacrificial reagent. The studies showed that this reagent traps some of the OH radicals with a small amount of EDTA (2 mmol), whereas the higher EDTA amount (4 mmol) allows for trapping all of the OH radicals to be trapped. We obtained such a high evolution of hydrogen, because of the impossibility of the recombination of pairs of electron-holes and the impossibility of the reaction of oxygen with hydrogen. These reactions suggest that EDTA is the effective scavenger. EDTA has two roles: it traps OH radicals and acts as a hole scavenger, which is confirmed by the obtained results and is consistent with the literature data.26-29 The perfect combined reaction pair seems to occur in the presence of the mixing of NaI and NaIO3, and the overall reaction of water splitting should proceed by the redox cycle between IO3- and I-.13,19 We did not observe this effect. In fact, we observed only the evolution of oxygen. It is speculated that the reduction reaction of IO3proceeds more preferentially than the reduction of water. All studies reveal that the reaction temperature has a large influence on the production of hydrogen. The increase in reaction temperature is accompanied by the increase in the yield of the production of hydrogen. All the studied reactions were photocatalyzed, because no reactions without the catalyst and UV irradiation were observed. We also did not observe any production of hydrogen without the addition of a sacrificial reagent. Therefore, one can say that the sacrificial reagents have a key role in hydrogen production via the photocatalyzed water splitting reaction.
4. Conclusions (1) The highest yield in hydrogen production is obtained using methanol as a sacrificial reagent. Hydrogen is at least partially produced by the conversion of methanol. The relatively high yield was a result of the heightened reaction temperature (60 °C). (2) Photocatalytic water splitting is observed at the presence of applying EDTA and Na2S as the sacrificial reagents. They act as effective hole scavengers; however, they are oxidized due to OH radicals, preventing oxygen formation and the recombination of the reaction of oxygen with hydrogen. EF0400619
(26) Mika, A. M. Fotochemiczna konwersja energii 1994. (27) Darvent, J. R. J. Chem. Soc. Faraday Trans. 2 1981, 77, 1703. (28) Harbour, J. R.; Wolkow, R.; Hair, M. L. J. Phys. Chem. 1981, 85, 4026. (29) Mills, A.; Porter, G. J. Chem. Soc., Faraday Trans. 1 1982, 78, 3659.
