Langmuir 1998, 14, 1899-1904
1899
Urea Photosynthesis from Inorganic Carbon and Nitrogen Compounds Using TiO2 as Photocatalyst Susumu Kuwabata, Hiroshi Yamauchi, and Hiroshi Yoneyama* Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamada-oka 2-1, Suita, Osaka 565, Japan Received May 7, 1997. In Final Form: January 5, 1998 Urea was successfully synthesized by illuminating size-quantized TiO2 nanocrystals (Q-TiO2) immobilized in polyvinylpyrrolidinone gel film in CO2-saturated propylene carbonate containing NO3- and 2-propanol, the latter of which served as a hole scavenger. Commercially available TiO2 (Degussa P-25) also showed activities for the urea photosynthesis, though the production rate of urea was lower. The use of HCOOH or CO in place of CO2 also yielded urea. The same was true for the replacement of the nitrogen source from NO3- to NH2OH or NO. The urea production was accompanied by production of other reduction products such as NH4+ and methanol, but their amounts were largely influenced by the kind of reaction substrates used. The sum of quantum efficiencies obtained for the reduction products showed good accordance with the quantum efficiency obtained for the sole oxidation product of acetone.
Introduction Photoinduced conversion of CO2 to useful chemicals with the use of semiconductor particles as photocatalysts has been intensively studied using bulk and quantized particles. In the case of using bulk semiconductor particles of high stability such as TiO2 and SiC, several kinds of reduction products including formate1-14 and formaldehyde15-18 were obtained in aqueous solutions in the absence of any hole scavenger. However, the amounts of products were very small in many cases, and it is not always easy to reproduce the results. Recent studies have focused on the use of semiconductor nanocrystals having size-quantized effects (Q-Sc) in the presence of hole scavengers. Results obtained with Q-ZnS,19-22 Q-CdS,23,24 (1) Thewissen, D. H. M.; Tinnemans, A. H. A.; Reintew, M. E.; Timmer, K.; Mackor, A. Nouv. J. Chem. 1983, 7, 73. (2) Vijayakumar, K. M.; Lichtin, N. N. J. Catal. 1984, 90, 173. (3) Eggins, B R.; Irvine, J. T. S.; Murphy, E. P.; Grimshaw, J. J. Chem. Soc., Chem. Commun. 1988, 1123. (4) Albers, P.; Kiwi, J. New J. Chem. 1990, 14, 135. (5) Goren, Z.; Willner, I.; Nelson, A. J.; Frank, A. J. J. Phys. Chem. 1990, 94, 3784. (6) Inoue, T.; Fujishima, A.; Konishi, S.; Honda, K. Nature 1979, 277, 637. (7) Tinnemans, A. H. A.; Koster, T. P. M.; Thewissen, D. H. M. W.; Macker, A. Stud. Inorg. Chem. 1983, 3, 239. (8) Halmann, M.; Ulman, M.; Aurian-Blajeni, B. Sol. Energy 1983, 31, 429. (9) Halmann, M.; Katzir, V.; Borgarello, E.; Kiwi, J. Solar Energy Mater. 1984, 10, 85. (10) Yamamura, S.; Kojima, H.; Iyoda, J.; Kawai, W. J. Electroanal. Chem. 1988, 333, 247. (11) Aliwi, S. M.; Aushana, A. M.; Al-Jubori, K. F. J. Sol. Energy Res. 1988, 6, 57. (12) Aliwi, S. M.; Al-Jubori, K. F. Sol. Energy Mater. 1989, 18, 223. (13) Irvine, J. T. S.; Eggins, B. R.; Grimshann, J. Sol. Energy 1990, 45, 27. (14) Khan, M. M. T.; Rao, N. N.; Chatterjee, D. J. Photochem. Photobiol., A 1991, 60, 311. (15) Aurian-Blajeni, B.; Halmann, M.; Manassen, J. Sol. Energy 1980, 25, 165. (16) Tennakone, K. Sol. Energy Mater. 1984, 10, 235. (17) Khalil, L. B.; Youssef, N. S.; Rphael, M. W.; Moawad, M. M. J. Chem. Technol. Biotechnol. 1992, 55, 391. (18) Matsumoto, Y.; Obata, M.; Hombo, J. J. Phys. Chem. 1994, 98, 2950. (19) Henglein, A.; Guttierrez, M. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 852. (20) Henglien, A.; Guttierrez, M. Ber. Bunsen-Ges. Phys. Chem. 1983, 88, 170.
and Q-TiO225,26 show that the size-quantized semiconductor particles possess much higher activities for CO2 reduction; reduction products were obtained with a quantum efficiency of 0.1 to several percent. As already reported, Q-TiO2 is easily prepared and possesses high stability against corrosion.27-30 In our previous studies,31,32 we successfully immobilized Q-TiO2 particles in polyvinylpyrrolidinone film (Q-TiO2/ PVPD) by means of electrophoretic deposition of Q-TiO2 particles in the presence of PVPD as a stabilizing agent, and the resulting film was found to possess high activities for reduction of CO2 to methanol in propylene carbonate containing 2-propanol as a hole scavenger. Also reported using the Q-TiO2/PVPD film as a photocatalyst was photoinduced reduction of NO3-, where NH4+ was obtained as a major product at a high reaction rate as compared to the rate obtained with bulk TiO2.31 Such findings prompted us to explore simultaneous photoreduction of NO3- and CO2 using the Q-TiO2/PVPD film, and in the course of progress of the studies, NH2OH and NO gas were also used as nitrogen sources instead of NO3-, and HCOOH and CO gas were used instead of CO2 as carbon sources. As will be shown below, all cases tested in the present study gave urea having C-N bonds as a reduction product, but the reaction rate and distribution of products were largely influenced by the combination of carbon and nitrogen sources used. Urea is widely used as an (21) Inoue, H.; Torimoto, T.; Sakata, T.; Mori, H.; Yoneyama, H. Chem. Lett. 1990, 1483. (22) Kanemoto, M.; Shiragami, T.; Pac, C.; Yanagida, S. Chem. Lett. 1990, 931. (23) Inoue, H.; Moriwaki, H.; Maeda, K.; Yoneyama, H. J. Photochem. Photobiol., A 1995, 86, 191. (24) Inoue, H.; Nakamura, R.; Yoneyama, H. Chem. Lett. 1994, 1227. (25) Anpo, M.; Chiba, K. J. Mol. Catal. 1992, 74, 207. (26) Inoue, H.; Matsuyama, T.; Liu, B.-J.; Sakata, T.; Mori, H.; Yoneyama, H. Chem. Lett. 1994, 653. (27) Anpo, M.; Shima, T.; Kodama, S.; Kubokawa, Y. J. Phys. Chem. 1987, 91, 4305. (28) Yoneyama, H.; Haga, S.; Yamanaka, S. J. Phys. Chem. 1989, 93, 4833. (29) Miyoshi, H.; Nippa, S.; Uchida, H.; Mori, H.; Yoneyama, H. Bull. Chem. Soc. Jpn. 1990, 63, 3380. (30) Al-Thabaiti, S.; Kuntz, R. R. Langmuir 1990, 6, 782. (31) Uchida, H.; Hirao, S.; Torimoto, T.; Kuwabata, S.; Sakata, T.; Mori, H.; Yoneyama, H. Langmuir 1995, 11, 3725. (32) Kuwabata, S.; Uchida, H.; Ogawa, A.; Hirao, S.; Yoneyama, H. J. Chem. Soc., Chem. Commun. 1995, 829.
S0743-7463(97)00478-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/18/1998
1900 Langmuir, Vol. 14, No. 7, 1998
Kuwabata et al.
important fertilizer and synthesized commercially from NH3 and CO2 under high pressure (>120 atm) and high temperature (>150 °C). In contrast, the photocatalytic process reported in this paper allows urea synthesis under room temperature and pressure. The recent report by Furuya et al. is also worth noting, where urea could be synthesized by electrochemical reduction of CO2 in the presence of NO2- using a Cu-loaded gas diffusion electrode.33 Experimental Section Propylene carbonate used as a solvent was purified by distillation under reduced pressure. 2-Propanol was distilled after storing in the presence of calcium hydride to remove water completely. Water was purified by double distillation of deionized water. Other chemicals of reagent grade were used as received. NO gas was prepared by dropping a 1 mol dm-3 H2SO4 aqueous solution onto solid NaNO2, and the generated NO gas was passed through a glass tube (1 cm diameter × 20 cm) filled with NaOH granules to remove the contained H2O vapor and then stored in a gas bag made of Teflon. The immobilization of TiO2 nanocrystals in polypyrrolidinone film (Q-TiO2/PVPD) was accomplished by electrophoretic deposition of Q-TiO2 particles on a negative electrode. Prior to the electrophoresis, the Q-TiO2 colloid was prepared by adding titanium tetraisopropoxide (Wako Pure Chemicals) to a 2-propanol solution up to a concentration of 67 mmol dm-3. The propanol solution contained 0.6 mol dm-3 water, 0.8 mol dm-3 HCl, and 0.5 wt % polyvinylpyrrolidinone (Nacalai Tesque, K90; average molecular weight ) 360 000), which served as a stabilizing agent for the produced Q-TiO2 particles. The electrophoresis of the resulting Q-TiO2 colloid was carried out using a Pt mesh (10 mm × 25 mm, 80 mesh) as a negative electrode and a Pt foil (10 mm × 25 mm) as a positive electrode. A constant electric field of 30 V cm-1 was applied between the two electrodes for 1 h with the use of a Nichia Type HP51A dc generator, causing deposition of a Q-TiO2/PVPD gel film of approximately 0.2 mm thickness on the negative electrode. The amount of Q-TiO2 in the resulting film was determined to be 7.0 mmol by absorption analysis using disodium catechol-3,5-disulfonates (Tiron) as a complexing agent for tetravalent titanium, which was produced by dissolving Q-TiO2 in fused H2SO4 and (NH4)2SO4 (11:3 in weight ratio). As previously reported,31 the immobilized Q-TiO2 consisted of anatase particles having a size distribution ranging from 1.0 to 7.5 nm. The absorption onset of the Q-TiO2 film was about 340 nm, being blue-shifted from that of bulk anatase TiO2, which shows absorption onset at 380 nm. The film containing bulk TiO2 particles deposited on the Pt mesh was prepared in the same manner as that mentioned above except for the use of bulk TiO2 (Aerosil P-25) in place of Q-TiO2. In this case, electrophoresis for 2.4 h was required to obtain deposition of 7.0 mmol of TiO2. The more detailed procedures of the preparation of the Q-TiO2/PVPD film and its characterization were already described elsewhere.31 Photoinduced reaction experiments were carried out using a quartz cell, the top of which was capped by an airtight glass cap. The Q-TiO2/PVPD film-deposited Pt mesh was fixed through a septum fitted in the glass cap, which contained another septum through which gaseous reaction substrates such as CO2, CO, and NO were introduced into the cell using a stainless steel tube. Propylene carbonate (3 cm3) containing 1 mol dm-3 2-propanol as a hole scavenger was used as a reaction solution. When the gaseous substrates were used, the substrate was bubbled for 1 h at least after dissolving other substrates of solid or liquid state into the reaction solution. If no gaseous substrate was used, N2 gas was bubbled into the solution for 1 h to remove air. When CO2 and NO gases were used together as the reaction substrates, both gases were simultaneously bubbled in the solution. The total flow rate of CO2 and NO was fixed to 5 mL/min in those cases, but the flow rate of each gas was changed to investigate the effect of the relative ratio of the gases on the production behaviors of urea. The amounts of NO and CO2 present in both (33) Shibata, M.; Yoshida, K.; Furuya, N. J. Electroanal. Chem. 1995, 387, 143.
Figure 1. Time course of acetone (a), urea (b), NH4+ (c), and methanol (d) by illuminating a Q-TiO2/PVPD film immersed in CO2-saturated (0.16 mol dm-3) propylene carbonate containing 20 mmol dm-3 LiNO3 and 1 mol dm-3 2-propanol. gas and liquid phases were determined by gas chromatography using Yanaco G-1800T equipped with a TCD and a Porapack T column at 100 °C. The photoinduced reaction using the Q-TiO2/ PVPD film as a photocatalyst was initiated by illuminating the film with a 500-W high-pressure mercury arc lamp as a light source. The light of wavelengths shorter than 300 nm was cut off using a glass filter, and the light intensity obtained was 0.36 W cm-2. The determination of urea was made by using a JASCO high-performance liquid chromatography system composed of a PU-980 pump, an MD-910 multichannel detector, and a Toso ODS-80TM column. The eluent used was a mixture of acetonitrile and water (55/45 by volume), and its flow rate was 0.5 mL min-1. Methanol, NH4+, and H2 were determined by gas chromatography (Yanaco G-180) with the use of columns of Gaschropack 54 (GL Sciences), Amipack 141 (GL Sciences), and molecular sieves 5A, respectively. The amount of Ti3+ generated in the illuminated Q-TiO2/PVPD film was determined by a photodiode array spectrophotometer (Hewlett-Packard, 8452A) using the absorption coefficient of 1480 mol-1 dm3 cm-1 at 700 nm, determined in our previous study.31
Results and Discussion Photoinduced Reduction of NO3- with CO2 to Urea. Figure 1 shows the time course of production of several substances obtained by illuminating the Q-TiO2/ PVPD film in CO2-saturated propylene carbonate solution containing 20 mmol dm-3 LiNO3 and 1 mol dm-3 2-propanol. Urea, methanol, and acetone were produced from the beginning of the illumination, whereas production of NH4+ began to occur after illumination for approximately 3 h. Acetone was produced from oxidation of 2-propanol as a hole scavenger. Since methanol and NH4+ were produced by photoinduced reduction of CO2 and NO3-, respectively, as reported in our previous papers,31,32 the reactions giving these products are given by
CH3CH(OH)CH3 f CH3COCH3 + 2H+ + 2e- (1) CO2 + 6H+ + 6e- f CH3OH + H2O
(2)
NO3- + 10H+ + 8e- f NH4+ + 3H2O
(3)
If urea was produced by simultaneous reduction of CO2 and NO3-, 16 Faradays should be required to obtain 1 mol of the product, as given by eq 4.
Urea Photosynthesis Using TiO2 as Photocatalyst
Langmuir, Vol. 14, No. 7, 1998 1901
Table 1. Products Obtained by Photoinduced Reactions of NO3- and CO2 Using Different Kinds of TiO2 Photocatalysts in Propylene Carbonate and Watera amount of products (µmol) [quantum efficienty (%)]d catalystb
solventc
Q-TiO2/PVPD
PC
Q-TiO2 colloid P-25 TiO2/PVPD
PC PC
P-25 colloid P-25 TiO2/PVPD
PC H2O
urea
methanol
NH4+
H2
Ti3+
acetone
5.6 [11.8] 0.22 2.3 [5.2] 0.1 0
1.2 [1.4] 0.12 0.04 [0.03] 0.05 0
2.7 [2.1] 0 1.2 [1.7] 0 0
0.18 [0.1] 0 0.08 [0.02] 0.06 0.01
0.14 [0.02] 0
61.0 [15.9] 2.5 30.1 [7.1] 1.1 0.07
a The photoreaction time was 5 h. b The amount of TiO presented in the reaction cell was fixed at 7.0 µmol. c PC: propylene carbonate. 2 All solutions were saturated with CO2 (0.16 mol dm-3), and they contained 20 mmol dm-3 LiNO3 and 1 mol dm-3 2-propanol. d The quantum efficiency was obtained by illuminating a TiO2 film with a monochromatic light of 300 nm.
2NO3- + CO2 + 18H+ + 16e- f (NH2)2CdO + 7H2O (4) In addition to the products given in Figure 1, small amounts of H2 and Ti3+ were produced in the gas phase and in the Q-TiO2/PVPD film, respectively.
2H+ + 2e- f H2
(5)
Ti4+(TiO2) + e- f Ti3+(TiO2)
(6)
The amounts of all products obtained after illuminating the Q-TiO2/PVPD film for 5 h are given in Table 1. The apparent quantum efficiency of each product was determined for the solution conditions shown in Table 1 with illumination of monochromatic light of 300 nm obtained by passing light from the Hg arc lamp through an interference filter. The amounts of urea, methanol, NH4+, H2, Ti3+, and acetone obtained by the illumination with 6.94 × 1019 photons were 0.85, 0.27, 0.31, 0.06, 0.02, and 9.2 mmol, respectively, and the quantum efficiency of each product as given in Table 1 was evaluated on the basis of the reaction schemes given by eqs 1-6. The sum of the quantum efficiencies obtained for the reduction products was 15.4, which was fairly consistent with the quantum efficiency obtained for the oxidation product of acetone, suggesting that the reactions given by eqs 1-6 explain well the photoinduced reactions at the Q-TiO2/PVPD film and that other reactions such as oxidative decomposition of PVPD and solvent were negligible, if any. The stability of PVPD against both reduction and oxidation was also confirmed by electrochemical measurement; cyclic voltammetry of a glassy carbon electrode taken in propylene carbonate containing 1 mol dm-3 LiClO4 in the absence and presence of 0.1 g dm-3 PVPD gave the same voltammogram at potentials between -1.0 and +3.3 V versus Ag/AgCl. The obtained voltammogram showed a gradual increase in oxidation currents due probably to decomposition of PC when the electrode potential was shifted to more positive than 2.3 V versus Ag/AgCl. If 1 mol dm-3 2-propanol was dissolved in the electrolyte solution, large oxidation currents appeared at potentials positive of 0.8 V versus Ag/AgCl. Then it is supposed from these results that photogenerated positive holes in the Q-TiO2 film were mostly involved in oxidation of 2-propanol. Figure 2 shows the effect of the concentration of NO3on the production of urea, NH4+, and methanol obtained by illumination for 5 h. Apparently, the production of urea and NH4+ increased with increasing concentration of LiNO3, indicating that NO3- was the indispensable nitrogen source for the production of urea. As shown in this figure, a little decrease in methanol production was observed as the concentration of LiNO3 increased, but the
Figure 2. Effects of the concentration of LiNO3 on the amount of urea (a), NH4+ (b), and methanol (c) production obtained by illuminating a Q-TiO2/PVPD film for 5 h in CO2-saturated (0.16 mol dm-3) propylene carbonate containing LiNO3 and 1 mol dm-3 2-propanol.
rate of decrease was much lower than that of the increase in the amount of urea and NH4+, suggesting that the photoinduced reduction of CO2 to methanol was not greatly influenced by the production of urea. The photoinduced conversion of CO2 and NO3- to urea was attempted using several kinds of photocatalysts and solvents. The results obtained by illumination for 5 h are also summarized in Table 1. The amount of TiO2 used was the same for all cases. The quantum efficiencies of products obtained by the photoreactions in propylene carbonate using the films of Q-TiO2 and commercially available bulk TiO2 (P-25) were evaluated with the same method as that mentioned above, but the quantum efficiencies of other reactions could not be determined because the amounts of products obtained by illumination of the monochromatic light were too small to be precisely analyzed. The following is noted from the results shown in the table. (1) The rate of urea production obtained using the Q-TiO2/PVPD film and the P-25 film was much greater than that obtained using colloidal Q-TiO2 and colloidal P-25, respectively. (2) The Q-TiO2 had a higher activity than P-25, as recognized from the results obtained for both film and colloidal photocatalysts. (3) The use of water instead of propylene carbonate as the solvent did not yield any reduction products of CO2 and NO3-. Experiments using Q-TiO2 in aqueous solution were unsuccessful due to coagulation of Q-TiO2 into big particles.
1902 Langmuir, Vol. 14, No. 7, 1998
Among these findings, the second was also observed for photoreduction of NO3- to NH4+ 31 and of CO2 to methanol on TiO232 and of CO2 to formate on ZnS.34 The last finding seems to be consistent with the reported results that the photoreduction of CO2 on bulk TiO2 occurred with negligibly low quantum efficiencies when water was used as the hole scavenger. Then, the question arises, why did the film photocatalysts give a higher amount of urea for photoreduction of CO2 and NO3- than the colloidal catalysts? If the photoreducing power of TiO2 particles were enhanced by immobilizing them in the polymer film, the production rate of urea might be increased. However, such an effect cannot be expected especially for bulk TiO2 particles of P-25, because, in general, assembling photocatalyst particles in the polymer film should decrease the surface areas exposed to solution as compared with that for dispersed catalysts in solution, and then the reaction rate must be decreased. Then, we need to pay attention to the fate of urea after its production. Photooxidation of urea was investigated using O2-saturated propylene carbonate containing 1 mmol dm-3 urea and 1 mol dm-3 2-propanol. The dissolved oxygen in that case served as a scavenger for photogenerated electrons in TiO2, and competing oxidation of urea and 2-propanol by the valence band holes of TiO2 was investigated. Illumination for 5 h caused a decrease in the concentration of urea to 0.87 for the Q-TiO2/PVPD, while a decrease to 0.11 mmol dm-3 was observed for the suspended Q-TiO2 particles, indicating that the photoinduced oxidative decomposition of urea took place even in the presence of 2-propanol at a concentration one thousand times larger than that of urea as a hole scavenger. Furthermore, the results show that oxidation of urea took place more easily in the Q-TiO2 colloid than the Q-TiO2/PVPD film. The difference in the rate of the competing oxidation of urea between the two kinds of photocatalysts seems to result from the difference in the collision frequency of urea with the Q-TiO2 particles. It is thought that when the Q-TiO2/PVPD film was used, most of the produced urea diffused away from the TiO2 particle surfaces into the solution bulk and its oxidation was prevented. In the case of the Q-TiO2 colloidal photocatalyst, however, the urea in the solution bulk often struck the TiO2 particle surface because of agitation of the TiO2 suspension, so that it was more easily oxidized by the colloidal photocatalyst than by the film photocatalyst. Such a difference in the probability of oxidation of the photoproduced urea seems responsible for the apparent activity difference between the Q-TiO2/PVPD film and Q-TiO2 colloidal photocatalysts. Use of Other Carbon Sources in Place of CO2. CO gas and formic acid were tested in place of CO2 as carbon sources for the photoinduced reactions using the Q-TiO2/ PVPD film in propylene carbonate solution containing 20 mmol dm-3 LiNO3. The products and their amounts obtained by illumination for 5 h are given in Table 2. Since it was determined by gas chromatography that 0.16 mol dm-3 CO2 was dissolved in CO2-saturated propylene carbonate solution, the same concentration was chosen when HCOOH was used as the carbon source, but the concentration of CO dissolved in the CO-saturated solution was 0.10 mol dm-3. In both cases, urea was produced from the beginning of illumination. Also obtained as the reduction products were NH4+, H2, and methanol when CO was used, whereas methanol production did not occur with the use of formic acid, though the other reduction products were obtained. NH4+ was obtained in both cases (34) Inoue, H.; Torimoto, T.; Sakata, T.; Mori, H.; Yoneyama, H. Chem. Lett. 1990, 1483.
Kuwabata et al. Table 2. Products Obtained by Photoinduced Reduction of NO3- in the Presence of CO or HCOOH as a Carbon Sourcea carbon source COc HCOOHd
urea 2.3 [4.4] 5.7 [10.2]
amount of product (µmol) [relative quantum efficiency (%)]b methanol NH4+ H2 Ti3+ acetone 2.9 [1.5] 0 [0]
4.1 [4.1] 2.7 [3.7]
0.11 [0.03] 0.21 [0.06]
0.13 [0.02] 0.15 [0.01]
44.1 [10.5] 57.1 [14.2]
a The photoreaction time was 5 h. b The quantum efficiency was obtained by illuminating a TiO2 film with a monochromatic light of 300 nm. c The solution used was CO-saturated (0.10 mol dm-3) propylene carbonate containing 20 mmol dm-3 LiNO3 and 1 mol dm-3 2-propanol. d The solution used was propylene carbonate containing 0.16 mol dm-3 HCOOH, 20 mmol dm-3 LiNO3, and 1 mol dm-3 2-propanol.
in an amount comparable to that of urea, but its production was not linearly dependent on the irradiation time; a rapid increase in its production was observed between 1 and 2 h of illumination for the use of CO, followed by stagnation of the production, while the NH4+ production was completed within 1 h of illumination for formic acid. The urea production with the use of these substrates is given by eqs 7 and 8.
2NO3- + CO + 16H+ + 14e- f (NH2)2CdO + 6H2O (7) 2NO3- + HCOOH + 16H+ + 14e- f (NH2)2CdO + 7H2O (8) In both cases, the oxidation product was only acetone and no CO2 was obtained. The quantum efficiency obtained for the production of acetone and the sum of those obtained for the production of all the reduction products given in Table 2 were in good agreement with each other. Use of Other Nitrogen Sources in Place of NO3-. The effects of the nitrogen source on the urea production were also investigated using NH2OH or NO gas in place of NO3-. The CO2-saturated propylene carbonate containing 1 mol dm-3 2-propanol was used in those cases. As is well-known, NH2OH and NO are intermediate species in the assimilatory and dissimilatory reduction of NO3- to give NH4+ and N2, respectively, in metabolism.35 The time courses of the photoinduced reaction products are shown in Figures 3 and 4 for the use of NH2OH and NO, respectively. The urea production in these cases is given by eqs 9 and 10.
2NH2OH + CO2 + 4H+ + 4e- f (NH2)2CdO + 3H2O (9) 2NO + CO2 + 10H+ + 10e- f (NH2)2CdO + 3H2O (10) It is recognized by comparing the results shown in Figures 3 and 4 with those shown in Figure 1 that the use of NH2OH and NO as nitrogen sources gave higher production rates of urea than the use of NO3-, resulting in higher quantum efficiency of the urea production. When NH2OH was used, the quantum efficiency of the urea production was 44.3% and those of NH4+, CH3OH, and acetone were 14.3, 7.4, and 66.5%, respectively, and when NO was used, 46.2% was obtained for the urea production and 5.0, 1.8, (35) Payne, W. J. Bacteriol. Rev. 1973, 409, 1973.
Urea Photosynthesis Using TiO2 as Photocatalyst
Figure 3. Time course of acetone (a), urea (b), NH4+ (c), and methanol (d) production obtained by illuminating a Q-TiO2/ PVPD film immersed in CO2-saturated (0.16 mol dm-3) propylene carbonate containing 20 mmol dm-3 NH2OH and 1 mol dm-3 2-propanol.
Figure 4. Time course of acetone (a), urea (b), NH4+(c), and methanol (d) production obtained by illuminating a Q-TiO2/ PVPD film immersed in propylene carbonate containing 1 mol dm-3 2-propanol. The reaction cell was bubbled by CO2 and NO gases with the same flow rate of 5 mL min-1 prior to the reaction, giving concentrations of 44 mmol dm-3 CO2 and 1.2 mmol dm-3 NO.
and 53.4% were obtained for the production of NH4+, CH3OH, and acetone, respectively. It is then suggested from these results that the reduction of NO3- was the rate-determining step in the photoinduced production of urea from CO2 and NO3-. The higher production rate of NH4+ obtained with the use of NH2OH also suggests the validity of this view. In contrast, the amount of NH4+ was very small as compared to that of urea when NO was used as the nitrogen source, as recognized from the results shown in Figure 4, and the highest selectivity and quantum efficiency were obtained for the urea production in that case among the reactions investigated in this study. When the experiments shown in Figure 4 were carried out, CO2 and NO gases were bubbled in the reaction solution for 30 min at the same flow rate of 2.5 mL min-1 prior to illumination. It was found that the resulting solution contained 3.6 µmol of NO and 132 µmol of CO2
Langmuir, Vol. 14, No. 7, 1998 1903
Figure 5. Effects of the molar ratio of NO to CO2 in the reaction cell on the amount of urea (a), NH4+ (b), and methanol (c) obtained by illuminating a Q-TiO2/PVPD film for 3 h in propylene carbonate containing 1 mol dm-3 2-propanol.
for the solution volume of 3 mL used in the experiments. The amount of NO present in the solution is judged to have been too small to produce 17 µmol of urea, as shown in Figure 4. Therefore, the NO present in the gas phase in the cell must have been involved in the urea production by being successively dissolved in the solution during the course of the photoreaction. As the solubilities of NO and CO2 in the propylene carbonate solution are greatly different, the photoinduced experiments were carried out by changing the ratio of flow rates of NO and CO2 bubbled into the solution to achieve different molar ratios of NO to CO2 in the cell. Figure 5 shows the amount of urea, methanol, and NH4+ produced by illumination for 3 h as a function of the ratio of NO to CO2 introduced into the cell. As shown in the figure, the highest urea production was achieved at the NO/CO2 ratio of approximately 2, which is in coincidence with the ratio of N to C contained in a urea molecule, while the amounts of methanol and NH4+ produced were not much influenced by the NO/CO2 ratio. Conclusion It was shown in this study that production of urea was achieved by photoinduced reduction of inorganic carbon and nitrogen compounds with the use of Q-TiO2 and bulk TiO2 catalyst films. The combinations of carbon and nitrogen compounds used here were CO2-NO3-, CONO3-, HCOOH-NO3-, CO2-NH2OH, and CO2-NO. In all cases, urea was produced with multielectron reduction of the substrates, as given by eqs 4 and 7-10. It is of current interest to synthesize valuable products using semiconductor photocatalysts in which multielectron transfers are involved. So far several photoreactions were attempted, such as conversion of alkene and alkyne to alkane,36-39 conversion of CO2 to CH3OH and CH4,25,40,41 (36) Boonstra, A. H.; Mustsaera, C. A. H. A. J. Phys. Chem. 1975, 79, 1940. (37) Boonstra, A. H.; Mustsaera, C. A. H. A. J. Phys. Chem. 1975, 79, 2025. (38) Anpo, M.; Aikawa, N.; Kodama, S.; Kubokawa, Y. J. Phys. Chem. 1984, 88, 2569.
1904 Langmuir, Vol. 14, No. 7, 1998
and conversion of N2 to NH2NH2 and NH3.42,43 It is suggested from these studies that photoreactions performed in a gas phase are more favorable than those performed with the use of dispersed catalysts in solutions for inducing multielectron photoreactions, especially for the photoreduction of CO2.40,41 Those findings as well as the results obtained in this work suggest strongly that it is important to design a reaction system in such a way as to enable reaction products to leave easily the photocatalysts for obtaining their high production rates, since oxidative decomposition of the products causes an appreciable decrease in the apparent production rate. In addition, it was reported in some papers32,34,37-40 that semiconductor nanocrystals exhibiting size-quantized effects possess higher photocatalytic activities for multielectron reactions, and the results obtained in this study (i.e. Table 1) also supported the advantage of the use of the size-quantized photocatalyst. The increase in the (39) Anpo, M.; Shima, T.; Kodama, S.; Maruyama, K.; Onishi, T. J. Phys. Chem. 1987, 91, 4305. (40) Yamashita, H.; Nishiguchi, H.; Kamada, N.; Anpo, M.; Teraoka, Y.; Hatano, H.; Ehara, S.; Kikui, K.; Palmisano, L.; Sclafani, A.; Schiavello, M.; Fox, M. A. Res. Chem. Intermed. 1994, 20, 815. (41) Hemminger, J. C.; Carr, R.; Somorjai, G. A. Chem. Phys. Lett. 1978, 57, 100. (42) Schrauzer, G. N.; Guth, T. D. J. Am. Chem. Soc. 1977, 99, 7189. (43) Endon, E.; Leland, J. K.; Bard, A. J. J. Phys. Chem. 1986, 90, 6223.
Kuwabata et al.
surface areas of the catalysts with decreasing their size and the size-quantized effect itself seem to play important roles in the enhancement of the multielectron reactions, although their concrete correlation has not yet been established. The reductive synthesis of urea from CO2 and NO3with the use of the oxidation of 2-propanol to acetone is a spontaneous reaction because the ∆G of the reaction is estimated to be -1059 kJ mol-1. The photosynthesis of urea using other combinations of carbon and nitrogen compounds also gives a negative ∆G as long as 2-propanol is used as a hole scavenger. However, imposition of high energy is sometimes needed to overcome an energy barrier existing in one of the elementary steps which comprise a multielectron reduction, as shown, for example, in the hydration of N2, giving NH3 (∆G ) -159 kJ mol-1). Thus, the photoassisted production of urea with the use of the present reaction system has scientific significance though a sacrificial reagent of 2-propanol was used. Acknowledgment. This research was supported by Grant-in-Aid for General Scientific Research No. 07455340 and by Grant-in-Aid for Priority Areas No. 06239110 from Ministry of Education, Science, Sports, and Culture. LA970478P
