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Drastic Enhancement of TiO2-Photocatalyzed Reduction of Nitrobenzene by Loading Ag Clusters Hiroaki Tada,* Tetsuji Ishida, Ayako Takao, and Seishiro Ito Department of Applied Chemistry, Faculty of Science and Engineering, Kinki University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan Received March 31, 2004. In Final Form: July 8, 2004 Ag clusters (mean diameter ) 1.5 nm, standard deviation ) 0.37 nm) were photodeposited on TiO2 particles in a highly dispersed state. The loading of a small amount of the Ag clusters (0.24 wt %) dramatically enhanced both the activity for the TiO2 photocatalytic reduction of nitrobenzene and the product selectivity of aniline. The essential action mechanism of the Ag clusters is discussed.
Introduction It is an important subject of chemistry to develop environmentally benign chemical conversion processes for synthesizing useful substances. One potential approach is to use sunlight, which is a safe, inexpensive, and sustainable energy source. As such, semiconductor photocatalysts represented by TiO2 are of great interest and importance. Because the holes (or OH radicals) generated by the band-gap excitation of TiO2 completely oxidize almost all the organics, application of the TiO2 photocatalyst to detoxification of harmful pollutants has been studied with avid interest.1-3 Nitroaromatic compounds used widely as raw materials for munitions are notorious pollutants in water, and their detoxification is highly needed. Makarova et al. have reported the oxidation of nitrobenzene (NB) using a TiO2 photocatalyst.4 On the other hand, from the perspective of organic synthesis, selective photocatalytic reductions can be expected because of the mild reducing power of excited electrons. The limited examples include the TiO2 photocatalytic reduction of NB, reported by Brezova et al.5 and Ferry et al.,6 which may also be interesting in connection with the NOx treatment. Both of these studies used P-25 (Degussa) having strong photocatalytic activity in a variety of reactions. We have presented the concept of a “reasonable delivery photocatalytic reaction system” (RDPRS), in which the following three conditions must be satisfied to achieve highly efficient and selective photocatalytic reactions:7 (i) separation of oxidation and reduction sites; (ii) abundant and selective supply of oxidants and reductants to reduction and oxidation sites, respectively; and (iii) restriction of the product readsorption. We report herein a drastic accelerating effect of Ag-cluster loading on the TiO2photocatalyzed reduction of NB to aniline (AN), which is a useful compound as a raw material for medicines, agricultural chemicals, and dyes. TiO2-photocatalyzed reactions may be enhanced with incorporation of metal * To whom correspondence should be addressed. Tel: +81-66721-2332. E-mail:
[email protected]. (1) Reed, B. E.; Matsumoto, M. R.; Jensen, J. N.; Viadero, R.; Lin, W. Treat. Syst. 1998, 70, 449. (2) Cunningham, J.; Al-Sayyed, G.; Sedlak, P.; Caffrey, J. Catal. Today 1999, 53, 145. (3) Tryk, D. A.; Fujishima, A.; Honda, K. Electrochim. Acta 2000, 45, 2363. (4) Makarova, O. V.; Rajh, T.; Thurnauer, M. C. Environ. Sci. Technol. 2000, 34, 4797. (5) Brezova, V.; Blazkova, A.; Surina, I.; Havlinova, B. J. Photochem. Photobiol., A 1997, 107, 233. (6) Ferry, J. L.; Glaze, W. H. Langmuir 1998, 14, 3551. (7) Tada, H.; Soejima, T.; Ito, S. Recent Development of Colloid and Interface; Transworld Research Network: Kerala, India, 2003.
oxides, metal calcogenides,8 and noble metals including Ag.9-13 In most cases, the effect is due to the improvement of the charge separation efficiency (RDPRS condition i). To our knowledge, the present system is the first example for the RDPRS. The essential action mechanism of the Ag clusters was discussed with particular emphasis placed on RDPRS conditions ii and iii, which are quite important in terms of the application of the photocatalytic reaction to organic synthesis. Experimental Section Ag clusters were incorporated with TiO2 particles by using a photodeposition method (Ag/TiO2).14 Table 1 summarizes the characterization results of TiO2 samples that were used [A-100 (Ishihara) and P-25]. The size distribution of the Ag clusters deposited on TiO2 was observed by a transmission electron microscope (JEOL, JEM-3010) at an acceleration voltage of 300 kV. The adsorption isotherms of NB and AN on TiO2 (or Ag/TiO2) particles were obtained by measuring the concentrations in solutions before and after adsorption. Preliminary experiments confirmed that negligible amounts of NB were adsorbed on the inner walls of the glass vessels used for the adsorption measurements. All suspensions (1 g of TiO2/50 mL of solvent or 0.25 g of Ag/TiO2/50 mL of solvent) were kept at 25 °C with shaking using a water bath incubator (M-305, Masuda Co.) in the dark overnight [solvent, H2O-acetonitrile (99/1 v/v)].15 After equilibration, the solids were separated by centrifugation, and the supernatant was analyzed by UV-vis spectroscopy (U-4000, Hitachi). TiO2 or Ag/TiO2 particles (50 mg) were added to a 1.1 mM solution of NB (50 mL). After the suspension had been bubbled with argon for 20 min in the dark (pH ) 6.8), irradiation (λex > 300 nm) was carried out with a 400-W high-pressure mercury arc; the light intensity integrated from 320 to 400 nm was measured to be 3.8 mW cm-2. The argon bubbling was continued throughout the reaction. After filtration with a syringe filter, the filtrate was subjected to high-performance liquid chromatography (LC-6AD, SPD-6A, C-R6A (Shimadzu)) for product analysis [measurement conditions: column ) Fluofix (8) Rajeshwar, K.; de Tacconi, N. R.; Chenthamarakshan, C. R. Chem. Mater. 2001, 13, 2765 and references therein. (9) Vamathevan, V.; Amal, R.; Beydoun, D.; Low, G.; S. McEvoy, S. J. Photochem. Photobiol., A 2002, 6001, 1. (10) Arabatzis, I. M.; Stergiopoulos, T.; Bernard, M. C.; Labou, D.; Neophytides, S. G.; Falaras, P. Appl. Catal., B 2003, 42, 187. (11) Zhang, L.; Yu, J.-C.; Yip, H.-Y.; Li, Q.; Kwong, K.-W.; Xu, A.-W.; Wong, P. K. Langmuir 2003, 19, 10372. (12) Moonsiri, M.; Chavadej, S.; Rangsunvigit, P.; Gulari, E. Chem. Eng. J. 2004, 97, 241. (13) Tan, T. T. Y.; Yip, C. K.; Beydoun, D.; Amal, R. Chem. Eng. J. 2003, 95, 179. (14) Nishimoto, S.-I.; Ohtani, B.; Kajiwara, H.; Kagiya, T. J. Chem. Soc., Faraday Trans. 1 1983, 79, 2685. (15) Although the adsorption amount of NB was almost constant after 1 h, its adsorption was continued overnight in order to guarantee the adsorption equilibrium.
10.1021/la049167m CCC: $27.50 © 2004 American Chemical Society Published on Web 08/11/2004
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Langmuir, Vol. 20, No. 19, 2004 7899 Table 1. Characterization of Photocatalysts and the TiO2-Photocatalyzed Reduction of NB to AN
catalyst
crystal form
A-100
anatase
P-25
anatase (70%) + rutile (30%)
AG/A-100
anatase
da (TiO2)/ nm
Sb (TiO2)/ m2g-1
150
8.1
30 150
50 8.1
crystallite size (TiO2)c/nm
x (Ag)/ mass %d
da (Ag)/ nm
43 anatase 25 rutile 29 43
0.24
1.5
[NB]0/ mM
reaction time/h
[CH3OH]0/ mM
conversion of NB/%
selectivity of AN/%
5.4 1.1 1.1 5.4 1.1 1.1 5.4 1.1 1.1
2 1 1 2 1 1 2 1 1
0 10 100 0 10 100 0 10 100
9.6 10.7 9.0 46.9 42.0 78.2 51.5 64.8 84.3
8.2 40.9 100 10.8 59.4 100
a Mean diameters determined from TEM observation. b Surface area determined by the Brunauer-Emmett-Teller method. c Crystallite size obtained from X-ray diffraction analysis. d Loading amount determined by inductively coupled plasma spectroscopy.
Figure 2. Dependence of the reaction rate on the methanol concentrations: TiO2 (A-100) system (a); TiO2 (P-25) system (b); Ag/TiO2 (A-100) system (c).
Figure 1. TEM images of A-100 and Ag/A-100 (upper) and the size distribution of Ag clusters (lower). INW425, 4.6 × 250 mm (NEOS); mobile phase, H2O-CH3OH (1/1 v/v); flow rate ) 1.0 mL min-1; λ(NB) ) 270 nm and λ(AN) ) 230 nm].
Results and Discussion The electronic absorption spectrum of the Ag/TiO2 sample had an absorption band centered at 490 nm due to the Ag surface plasmon, indicating that most Ag photodeposited is in the metallic state. Figure 1 shows transmission electron micrograph (TEM) images of A-100 and Ag/A-100 (loading amount ) 0.24 wt %) and the Ag size distribution. This indicates that Ag clusters with a mean diameter of 1.5 nm and a sharp distribution (σ ) 0.37 nm) are formed on the surfaces of TiO2 in a highly dispersed state. The loading effect of the Ag nanoclusters on the photoinduced reaction of NB was examined. Figure 2 shows the rate (v) of decrease in the NB concentration (CN) with irradiation as a function of the concentration of CH3OH (CM) added as a reducing agent: v ) [CN(0) CN(30 min)]/30 min. The value of v for A-100 (a) is negligibly small even in the presence of CH3OH, whereas P-25 (b) is clearly active. Very interestingly, Ag/A-100 (c) gives a large v value exceeding that of P-25 at CM ) 0, and the value increases significantly with increasing CM. When H2O (+CH3OH) and D2O (+CD3OD) were used as the
solvents, the main product was identified by GC-MS [GC17A, GCMS-QP5050 (Shimadzu); column ) DB-1, 0.25 mm × 30 m (J&W)] as φ-NH2 [MS: m/z ) 93 (M+)] and φ-ND2 [MS: m/z ) 95 (M+)], respectively. The NB conversion and the AN selectivity are also listed in Table 1. In P-25 and Ag/A-100 systems, the conversion and selectivity increase with increases in CM, and the additive effect is more significant in the latter system. Both P-25 (or Ag/A-100) and irradiation were necessary for this reaction to take place. Also, excitation of only the Ag surface plasmon band (λex > 400 nm) led to no reaction. A feature of the TiO2 photocatalytic reaction is that reduction and oxidation simultaneously proceed at adjacent surface sites; however, in Ag/TiO2, efficient charge separation is achieved via the photoinduced electron transfer from the conduction band of TiO2 to Ag, as shown in a previous study [condition i of RDPRS].16 The turnover number was estimated from the amount of NB consumed after 2 h of irradiation to be 1230 by assuming that the reduction sites are the surface Ag atoms of the clusters.17 On the other hand, the holes in the valence band (h+ vb) would oxidize water and/or part of NB in the water system, being consumed by the preferential oxidation of CH3OH to yield the •CH2OH radical having a strong reducing power (half wave oxidation potential Eox 1/2 ) -0.74 V vs standard hydrogen electrode (SHE))18 in the CH3OH-added system. (16) Tada, H.; Teranishi, K.; Ito, S.; Kobayashi, H.; Kitagawa, S. Langmuir 2000, 16, 6077. (17) The number of surface Ag atoms was calculated using a value of 0.5 as the dispersion for the Ag-cluster mean diameter (1.5 nm): Somorjai, G. Introduction to Surface Chemistry and Catalysis; Wiley: New York, 1994. (18) Lilie, V. J.; Beck, G.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1971, 75, 458.
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Alcohols such as CH3OH and C2H5OH are well-known to act as hole-capturing agents to accelerate various TiO2 photocatalytic reactions.19,20 Bard showed using the pulse radiolysis technique that methoxyl radicals react with NB much faster than isopropoxyl radicals in solution (with no TiO2 present).21 Based on these radiochemical data, Ferry and Glaze suggested that the active reducing species in the TiO2 (P-25) photocatalyzed reduction of NB with CH3OH are not •CH2OH radicals but the excited electrons 6 in the conduction band (ecb). The ecb (flat band potential 22 Efb ) -0.52 V vs SHE at pH ) 6.8) has enough potential to reduced NB (half-peak potential Ered 1/2 ) +0.16 V vs SHE).23 Choi and Hoffmann proposed a mechanism for the TiO2-photocatalyzed CCl4 reduction involving both direct reduction via the ecb transfer and indirect reduction by the R-hydroxyl radicals;24 however, the latter contribution would be small in the present system (vide infra). In the Ag/A-100 system, no dark reaction occurred and Ag+ ions were not detected from the solution after irradiation. These facts exclude the possibility that the Ag clusters serve as electron donors in this system (Ag0 f Ag+ + e-). Further, Ohko et al. have recently reported that Ag nanoparticles loaded on TiO2 are oxidized under visible light irradiation in the presence of O2.25 However, the stability is guaranteed under the present conditions, that is, UV-light irradiation without O2. The loading amount of the Ag clusters was confirmed to be almost invariant before and after reaction. Evidently, this is a TiO2-photocatalyzed 6-electron reduction (eq 1), and the photocatalytic activity strongly depends on the kind of TiO2, increasing dramatically with the loading of a small amount of Ag nanoclusters. Ag/TiO2, hν (λex > 300 nm)
φ-NO2 + 6H+ + 6ecb98 φ-NH2 + 2H2O (1) It has recently been shown that many TiO2-photocatalyzed oxidations of organic compounds including sucrose,9 methylorange,10 acetone,11 and 4-chlorophenol12 and reduction of selenate ions13 are also enhanced by loading Ag particles. In all these studies, the acceleration effect was attributed to the increase in charge separation efficiency followed by the promotion of the charge transfer to the reaction substrates.26 The relatively high activity of P-25 (Table 1) is also ascribable to the efficient charge separation via the interfacial electron transfer from anatase to rutile27 in addition to its large surface area and high crystallinity.28 Thus, the efficient charge separation is the primary factor affecting increases in the photocatalytic activity for multielectron reactions such as those in this system. (19) Morrison, S. R.; Freund, T. Electrochim. Acta 1968, 13, 1968. (20) Schwitzgebel, J.; Ekerdt, J. G.; Sunada, F.; Lindquist, S. E.; Heller, A. J. Phys. Chem. B 1997, 101, 2621. (21) Bard, A. J. Science 1980, 207, 139. (22) Graetzel, M. In Energy Resources through Photochemistry and Catalysis; Graetzel, M., Ed.; Academic Press: New York, 1983. (23) The Ep/2 was measured on Au single crystals in 0.1 M HClO4. Fan, L.-J.; Wang, C. W.; Chang, S.-C.; Yang, Y.-w. J. Electroanal. Chem. 1999, 477, 111. (24) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (25) Ohko, Y.; Tatsuma, T.; Fujii, T.; Naoi, K.; Niwa, C.; Kubota, Y.; Fujishima, A. Nat. Mater. 2003, 2, 29. (26) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729. (27) Kawahara, T.; Konishi, Y.; Tada, H.; Tohge, N.; Nishii, J.; S. Ito, S. Angew. Chem., Int. Ed. 2002, 41, 2881. (28) The crystallite size is comparable with the mean diameter in P-25, while in A-100, the latter is significantly larger than the former.
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Figure 3. Adsorption isotherms of NB on TiO2 (A-100) (a) and Ag/TiO2 (A-100) (b). Adsorption selectivity of NB on Ag (c).
The adsorptivity of Ag/A-100 for NB and the product was studied and compared with that of A-100. Figure 3 shows the adsorption isotherms of NB, where Ceq is the equilibrium concentration ((a) A-100; (b) Ag/A-100). The adsorption is significantly enhanced with the loading of Ag clusters. The adsorption selectivity defined as [{n(Agnm-2)/{n(Ag-nm-2) + n(TiO2-nm-2)}] × 100% is more than 94%, indicating that NB (oxidant) is adsorbed almost selectively on the Ag surfaces (reduction sites) of Ag/TiO2; n(Ag-nm-2) and n(TiO2-nm-2) are the amounts of NB adsorbed per unit surface areas of Ag and bare TiO2, respectively. This means that the TiO2 surfaces (oxidation sites) are in direct contact with H2O and/or CH3OH (reductant) [condition ii of RDPRS]. Wang et al. have recently reported that the spontaneous adsorption of CH3OH on the surface of TiO2 with a crystal structure of anatase occurs in the gas phase (adsorption free energy ∆G ) -19 ( 1.3 kJ mol-1).29 The main product, AN, was adsorbed on the surfaces of neither TiO2 nor Ag/TiO2 [condition iii of RDPRS]. The difference in the adsorptivity between NB and AN can be explained qualitatively in terms of their solubility to water (0.21 mass % for NB and 3.38 mass % for AN at 298 K),30 whereas the nature of the NB adsorption on the Ag clusters is unclear at present. Noticeably in Table 1, the Ag/A-100 system containing 100 mM CH3OH selectively produces AN. This high selectivity is thought to be achieved because the Ag/A100 system satisfies both conditions ii and iii. In connection with the reaction mechanism, the satisfaction of condition ii enables us to infer that the direct ecb reduction is more advantageous than the indirect reduction by •CH2OH radicals because the former proceeds on the reduction sites (Ag cluster surfaces) with a high NB concentration and the opposite is valid for the latter. In conclusion, in the TiO2-photocatalyzed reduction of NB to AN, the activity and product selectivity have been found to increase dramatically with the loading of a small amount of Ag clusters. This study has also demonstrated that the present reaction system is a good example of RDPRS. The detailed mechanism of the reaction including the adsorption of NB to the Ag cluster is currently being studied. Acknowledgment. The authors acknowledge Ishihara Tecno Co. Ltd. for the gift of a TiO2 (A-100) sample. LA049167M (29) Wang, C.-y.; Groenzin, H.; Shultz, J. M. J. Phys. Chem. B 2004, 108, 265. (30) Handbook of Chemistry and Physics, 82nd ed.; CRC Press: New York, 2001; pp 8-99.
