© Copyright 1996 by the American Chemical Society
VOLUME 100, NUMBER 40, OCTOBER 3, 1996
LETTERS Photocatalytic Decomposition of NO at 275 K on Titanium Oxides Included within Y-Zeolite Cavities: The Structure and Role of the Active Sites Hiromi Yamashita, Yuichi Ichihashi, and Masakazu Anpo* Department of Applied Chemistry, College of Engineering, Osaka Prefecture UniVersity, Gakuen-cho, Sakai, Osaka 593, Japan
Mitsuo Hashimoto Central Technical Research Laboratory, Nippon Oil Company, Ltd., Chidori-cho, Naka-ku, Yokohama 231, Japan
Catherine Louis and Michel Che Laboratoire de Re´ actiVite´ de Surface, UniVersite´ P. et M. Curie, UA 1106-CNRS, 4 Place Jussieu, Tour 54, 75252 Paris Cedex 05, France ReceiVed: May 31, 1996; In Final Form: July 31, 1996X
Titanium oxide species anchored in the Y-zeolite cavities by an ion-exchange method exhibits a high and unique photocatalytic reactivity for the direct decomposition of NO into N2, O2, and N2O at 275 K with a high selectivity for the formation of N2. The in situ photoluminescence and XAFS (XANES and FT-EXAFS) investigations indicate that the titanium oxide species are highly dispersed in the zeolite cavities and exist in a tetrahedral coordination. The charge-transfer excited state of the titanium oxide species plays a significant role in the direct decomposition of NO with a high selectivity for the formation of N2, while the catalysts involving the aggregated octahedrally coordinated titanium oxide species show a high selectivity to produce N2O, being similar to reactions on the powdered TiO2 catalysts.
The design of molecular and/or cluster size catalysts within zeolite cavities and frameworks is of special interest because zeolites offer unique nanoscaled pore reaction fields, an unusual internal surface topology, and ion-exchange capacities. In fact, the advantages of using zeolite catalysts having such void shapeselective cavities and channels have already been established in a number of significant industrial catalytic processes.1 Furthermore, the modification of the zeolite cavity spaces is also vital to the design and application of highly efficient and selective photocatalytic systems for reducing global air pollution. X
Abstract published in AdVance ACS Abstracts, September 15, 1996.
S0022-3654(96)01596-1 CCC: $12.00
Unique photocatalytic properties which cannot be realized in normal catalytic systems can be expected in such modified reaction spaces.2,3 Thus, the well-defined nanopores of zeolites can provide one of the most promising reaction space fields for photocatalytic reactions.4,5 In recent years, we have reported that Cu+/ZSM-5 zeolite and Ag+/ZSM-5 zeolite catalysts work as efficient photocatalysts for the direct decomposition of NO into N2 and O2 at 275 K,3,6-10 in which the highly dispersed Cu+ and Ag+ monomer species in the zeolite cavities play a significant role as active species in the photocatalytic decomposition of NO: the local charge separation on these electronically excited Cu+ and Ag+ © 1996 American Chemical Society
16042 J. Phys. Chem., Vol. 100, No. 40, 1996 results in the selective formation of N2 and O2. On the other hand, Courbon and Pichat have investigated the photocatalytic decomposition of NO on powdered TiO2 catalysts and found that the powdered TiO2 photocatalysts decompose NO into N2O molecules not into N2 and O2.11 However, the photocatalytic decomposition of NO on highly dispersed titanium oxide species prepared in the nanosize pores of zeolites has yet to be investigated. Anpo et al. have reported that the highly dispersed titanium oxide catalyst having a tetrahedral coordination can be prepared in the micropores of Vycor glass and have shown that the catalyst exhibits a high and characteristic photocatalytic reactivity compared to the bulk TiO2 powder.12-14 Highly dispersed titanium oxides included within zeolite cavities (Ti oxide/zeolite) were prepared using an ion-exchange method and used as the photocatalyst for the direct decomposition of NO at 275 K. The photocatalytic properties of this highly dispersed titanium oxide catalyst were then compared with titanium oxide catalysts prepared by an impregnation method as well as with bulk TiO2 powder catalysts. In the present study, we deal with the characterization of these catalysts by means of in situ photoluminescence and XAFS (XANES and FT-EXAFS) measurements and clarifying the characteristics of the photocatalytic decomposition reaction of NO at 275 K. Special attention has been focused on the relationship between the structure of the titanium oxide species and the reaction selectivity in the photocatalytic decomposition of NO molecules in order to provide the vital information required for the design and application of highly active and selective photocatalytic systems for the efficient decomposition of NOx into N2 and O2. The Ti oxide/Y-zeolite (1.1 wt % as TiO2) catalyst was prepared by ion exchange with an aqueous titanium ammonium oxalate solution using Y-zeolite samples (SiO2/Al2O3 ) 5.5) supplied by the TOSOH corporation (ex-Ti oxide/Y-zeolite). Furthermore, the other two types of Ti oxide/Y-zeolite catalysts having different Ti concentrations (1.0 and 10 wt % as TiO2 , respectively) were prepared by impregnating the Y-zeolite with an aqueous solution of TiCl3 (imp-Ti oxide/Y-zeolite). TiO2 powdered catalysts (JRC-TIO-4: anatase 92%, rutile 8%) were supplied as a standard reference catalyst by the Catalysis Society of Japan. The photocatalytic reactions of NO molecules were carried out with the catalysts (150 mg) in a quartz cell with a flat bottom (60 mL) connected to a conventional vacuum system (10-6 Torr range).3 Prior to the photoreactions and spectroscopic measurements, the catalysts were degassed at 725 K for 2 h, heated in O2 at the same temperature for 2 h, and finally evacuated at 475 K to 10-6 Torr. UV irradiation of the catalysts in the presence of NO (7.8 µmol) was carried out using a 75-W high-pressure Hg lamp (λ > 280 nm) at 275 K. The reaction products were analyzed by gas chromatography. The photoluminescence spectra of the catalysts were measured at 77 K using a Shimadzu RF-5000 spectrophotofluorometer. The UV absorption spectra were recorded with a Shimadzu UV-2200A spectrometer at 295 K. The ESR spectra were recorded at 77 K using a JEOL JES-RE2X spectrometer operating in the X-band mode. The XAFS spectra (XANES and EXAFS) were measured at the BL-7C facility of the Photon Factory at the National Laboratory for High-Energy Physics, Tsukuba. The Ti K-edge absorption spectra were recorded in the transmission mode or fluorescence mode at 295 K. The normalized spectra were obtained by a procedure described in previous literature,15 and Fourier transformation was performed on k3-weighted EXAFS oscillations in the range 3-10 Å-1. The curve-fitting of the EXAFS data was carried out by employing the iterative
Letters TABLE 1: Comparisons of Yields of N2 and N2O and Their Selectivities in the Direct Photocatalytic Decomposition of NO at 275 K on Various Types of the Ti Oxide/Y-Zeolite Catalysts and the Bulk TiO2 Catalyst
catalysts ex-Ti/Y imp-Ti/Y imp-Ti/Y TiO2 powder
Ti content (wt % as TiO2) 1.1 1.0 10
yields (µmol/g of TiO2 h)
selectivity (%)
N2
N2O
total
N2
N2O
14 7 5 2
1 10 22 6
15 17 27 8
91 41 19 25
9 59 81 75
nonlinear least-squares method and the empirical backscattering parameter sets extracted from the shell features of titanium compounds. UV irradiation of the powdered TiO2 and the Ti oxide/Yzeolite catalysts prepared by ion-exchange or impregnation methods in the presence of NO was found to lead to the evolution of N2, O2, and N2O in the gas phase at 275 K with different yields and different product selectivities. The yields of these photoformed N2, O2, and N2O increased linearly to the UV irradiation time and the reaction immediately stopped when irradiation was ceased. The formation of these reaction products were not detected in the dark conditions nor in UV irradiation of the zeolite itself without titanium oxides. After prolonged UV irradiation, the number of the photoformed N2 per total number of Ti ions included within the catalyst exceeded 1.0. These results clearly indicate that the presence of both titanium oxides included within the zeolites as well as UV irradiation light are indispensable for the photocatalytic reaction to take place while the direct decomposition of NO to produce N2, O2, and N2O occurs photocatalytically on the surface of the titanium oxide catalysts. The photocatalytic reactivities of various titanium oxide catalysts for the direct decomposition of NO are shown in Table 1. Of special interest is the comparison of the photocatalytic activities of the Ti oxide/Y-zeolite catalysts with that of the widely used bulk TiO2 powdered catalyst. It can be seen that the specific photocatalytic reactivities of the Ti oxide/Y-zeolite catalysts which have been normalized for the unit amount of TiO2 in the catalysts, are much higher than the bulk TiO2 catalysts. Table 1 also shows the yields of the photoformed N2 and N2O (efficiency) and their distribution (selectivity) in the photocatalytic decomposition of NO on various types of titanium oxide catalysts. From Table 1, it is clear that the efficiency and selectivity for the formation of N2 strongly depend on the type of catalyst. The ex-Ti oxide/Y-zeolite catalyst exhibits a high reactivity and a high selectivity for the formation of N2 while the formation of N2O is found to be the major reaction on the bulk TiO2 catalyst as well as on the imp-Ti oxide/Yzeolite catalyst. Thus, the results obtained with the ex-Ti oxide/ Y-zeolites clearly show the large difference in selectivity as well as efficiency with the imp-Ti oxide/Y-zeolite and the bulk TiO2 catalyst. The absorption spectra of the Ti oxide/Y-zeolite catalysts and bulk TiO2 catalysts were measured by the UV diffuse reflectance method. The Ti oxide/Y-zeolite catalysts exhibit absorption bands in the wavelength regions of 280-330 nm, shifting into the shorter wavelength regions as compared to the bulk TiO2 catalyst. Such a blue-shift in the absorption band of titanium oxides can be attributed to the size quantization effect due to the presence of extremely small Ti oxide particles and/or the presence of highly unsaturated Ti oxide species having a tetrahedral coordination.12,14,16 A significant blue-shift in the absorption band was observed with the ex-Ti oxide/Y-zeolite
Letters
J. Phys. Chem., Vol. 100, No. 40, 1996 16043
Figure 1. XANES (a-c) and FT-EXAFS (A-C) spectra of the ex-Ti oxide/Y-zeolite catalyst (a, A), and the imp-Ti oxide/Y-zeolite catalysts with Ti content of 1.0 wt % (b, B) and 10 wt % as TiO2 (c, C).
Figure 2. ESR spectrum of the Ti3+ ions generated by the photoreduction of the ex-Ti oxide/Y-zeolite catalyst with H2 at 77 K. (ESR signal was also recorded at 77 K.)
TABLE 2: Results of the Curve-Fitting of Ti K-Edge EXAFS Data for the Various Types of the Ti Oxide/ Y-Zeolite Catalysts and the Bulk TiO2 Catalyst
(Ti-O). The ex-Ti oxide/Y-zeolite catalyst exhibits only Ti-O peaks, indicating the presence of the isolated titanium oxide species on the catalyst. From the results shown in Table 2, this ex-Ti oxide/Y-zeolite catalyst can be seen to consist of the 4-coordinate titanium ions. In Figure 1B and C, the imp-Ti oxide/Y-zeolite catalysts exhibit an intense peak at around 2.7 Å. This peak can be assigned to the neighboring titanium atoms (Ti-O-Ti) as well as to the Ti-O peak, indicating the aggregation of the titanium oxide species in these catalysts. Taking the coordination numbers shown in Table 2 in consideration, the presence of the aggregated octahedral titanium oxide species can be suggested with the impregnated catalysts. These XANES and FT-EXAFS investigations indicate that the ex-Ti oxide/Y-zeolite catalyst involves only the well-isolated tetrahedral Ti oxide species, while the imp-Ti oxide/Y-zeolite catalyst involves the aggregated octahedral Ti oxide species. To investigate the local structure of the titanium oxide species of the catalysts prepared within zeolite cavities, an ESR technique was incorporated to by monitor the Ti3+ ions which were formed by the photoreduction of the Ti oxide with H2 at 77 K. In Figure 2, the ESR spectrum of the Ti3+ ions formed in this way on the ex-Ti oxide/Y-zeolite catalyst shows only one type of Ti3+ signal with a g value of g⊥ ) 1.9781. From the g value and the characteristic shape of the spectrum it can be concluded that the Ti3+ species are present in a tetrahedral coordination.21 The addition of NO onto the ex-Ti oxide/Y-zeolite catalyst leads to the appearance of a new ESR signal with a g value of g⊥ ) 2.0015 at 77 K. This new signal can be attributed to the NO species adsorbed on the Ti ions.6,22 UV irradiation of the catalyst having the adsorbed NO species was found to lead to a gradual decrease in the intensity of the ESR signal linearly with the UV irradiation time. After UV irradiation was discontinued, the intensity of the ESR signal returned to its original level. These changes in the ESR signal of the NO species clearly indicate not only that the adsorbed NO species act as reaction precursors but also that the decomposition of NO proceeds photocatalytically. Figure 3 shows that the ex-Ti oxide/Y-zeolite catalyst exhibits a photoluminescence spectrum at around 490 nm by excitation at around 290 nm at 77 K. The observed photoluminescence and absorption bands are in good agreement with those of the highly dispersed tetrahedrally coordinated titanium oxides anchored onto Vycor glass where the absorption of UV light at around 280 nm brings about an electron transfer from the lattice oxygen (Ol2-) to the titanium ion (Ti4+) to form a charge-transfer
catalysts ex-Ti/Y imp-Ti/Y imp-Ti/Y Ti(OPri)4 TiO2 powder a
Ti content (wt% as TiO2) 1.1 1.0 10
shell
Ra (Å)
CNb
Ti-O Ti-O Ti-O Ti-O Ti-O
1.78 1.88 1.93 1.76 1.96
3.7 5.1 5.8 4 6
Bond distances. b Coordination number.
catalyst clearly suggesting that the dispersion of the Ti oxide species on this catalyst is higher than on the catalysts prepared by impregnation methods. Thus, a clear relationship can be seen between the reactivity for the photocatalytic decomposition of NO and the magnitude in the blue shift of these catalysts. The XANES spectra of the Ti oxide catalyst at the Ti K-edge show several well-defined preedge peaks which are related to the local structures surrounding the Ti atom. Also, the relative intensities of the preedge peaks provide useful information on the coordination number surrounding the Ti atom.17-20 The bulk powdered anatase and rutile TiO2 catalysts were found to exhibit three characteristic small preedge peaks which are attributed to the transitions from the 1s core level of Ti to three different kinds of molecular orbitals (1t1g, 2t2g, and 3eg). On the other hand, tetrahedrally coordinated Ti such as Ti(OPri)4 was found to exhibit an intense single preedge peak since a regular tetrahedron structure lacks an inversion center. Figure 1 shows the XANES spectra of the Ti oxide/Y-zeolite catalysts. The ex-Ti oxide/Y-zeolite catalyst exhibits an intense single preedge peak, indicating that the titanium oxide species in the zeolite has a tetrahedral coordination. As can be seen in Figure 1b, for the imp-Ti oxide/Y-zeolite catalyst having a small Ti content, the single characteristic preedge peak is rather weak, indicating that the catalyst consists of a mixture of tetrahedrally and octahedrally coordinated titanium oxide species. On the other hand, the imp-Ti oxide/Y-zeolite catalyst having a high Ti content exhibits three characteristic weak preedge peaks due to the presence of the crystalline anatase TiO2. Figure 1 also shows the FT-EXAFS spectra of the catalysts, and all data are given without corrections for phase shifts. Table 2 shows the results obtained by the curve-fitting analysis of the EXAFS spectra. As shown in Figure 1, all of the catalysts investigated in the present study exhibit a strong peak at around 1.6 Å which can be assigned to the neighboring oxygen atoms
16044 J. Phys. Chem., Vol. 100, No. 40, 1996
Figure 3. Photoluminescence spectrum of the ex-Ti oxide/Y-zeolite catalyst (a), its excitation spectrum (EX), and the effect of the addition of NO on the photoluminescence spectrum (b-e). Measured at 77 K, excitation beam: 290 nm, emission monitored at 490 nm; amounts of added NO: (a) 0.0, (b) 0.2, (c) 0.8, (d) 7.6, (e) 21.3 µmol g-1.
excited state.12-14 Therefore, it can easily be concluded that the observed photoluminescence spectrum is attributed to the radiative decay process from the thus-formed charge-transfer excited state to the ground state of the highly dispersed titanium oxide species having a tetrahedral coordination.12-14 The lifetime of the charge-transfer excited state was determined to be 54 µs, being much longer than that of the TiO2 powders (nanosecond order). Such a long lifetime of the charge-transfer excited state of the ex-Ti oxide/Y-zeolite catalyst is well associated with the presence of highly dispersed homogeneous tetrahedral titanium oxide species. On the other hand, the impTi oxide/Y-zeolite catalysts did not exhibit any photoluminescence spectrum. Thus, these results clearly indicate that the ex-Ti oxide/Y-zeolite catalyst consists of the highly dispersed isolated tetrahedral titanium oxide species, while the imp-Ti oxide/Y-zeolite catalysts involve the aggregated octahedral titanium oxide species which do not exhibit any photoluminescence spectrum, being in agreement with results obtained by XAFS investigations. As shown in Figure 3, the addition of NO onto the ex-Ti oxide/Y-zeolite catalyst leads to an efficient quenching of the photoluminescence spectrum of the catalyst. The lifetime of the charge-transfer excited state was also found to be shortened by the addition of NO, its extent depending on the amount of NO added. These results indicate not only that the tetrahedrally coordinated titanium oxide species locate at positions accessible to the added NO but also that the added NO easily interacts with the charge-transfer excited state of the species. From these results, it can be emphasized that a high photocatalytic efficiency and selectivity for the formation of N2 in the photocatalytic decomposition of NO was achieved with the ex-Ti oxide/Y-zeolite catalyst having highly dispersed isolated tetrahedral titanium oxide species while the formation of N2O in the photocatalytic decomposition of NO was found to proceed on the bulk TiO2 catalysts and on the imp-Ti oxide/ Y-zeolite catalysts involving aggregated octahedrally coordinated titanium oxide species. On the isolated tetrahedral titanium oxide species, NO species are able to adsorb onto these oxide species as weak ligands to form the reaction precursors. Under UV irradiation the charge-transfer excited complexes of the oxides, (Ti3+-O-)*, are formed. Within their lifetimes the electron transfer from the electron trapped center, Ti3+, into the π-antibonding orbital of NO takes place, and simultaneously, the electron transfer from the π-bonding orbital of another NO
Letters into the trapped hole center, O-, occurs. These electron transfers lead to the direct decomposition of two sets of NO on (Ti3+O-)* into N2 and O2 under UV irradiation in the presence of NO even at 275 K. On the other hand, with the aggregated or bulk TiO2 catalysts, the photoformed holes and electrons rapidly separated from each other with large space distances between the holes and electrons, thus preventing the simultaneous activation of two NO on the same active sites and resulting in the formation of N2O and NO2 in place of N2 and O2. The decomposed N and O species react with NO on different sites to form N2O and NO2, respectively. Although the study of the detailed mechanism involving such a local charge separation observed on the highly dispersed titanium oxide catalysts will be the subject of our future work, the present study clearly demonstrates that the titanium oxide species anchored within zeolite cavities are promising candidates for new and applicable photocatalysts for the reduction of toxic NOx elements. Acknowledgment. The present work has been supported in part by the Grant-in-Aid on Priority-Area-Research on “Photoreaction Dynamics” (06239110), “Catalytic Chemistry of Unique Reaction Fields” (07242264), and International Joint Project Research (07044162) of the Ministry of Education, Science, Sports, and Culture of Japan. M.A. is much indebted to Osaka Prefecture for the Special Project Research and Tokyo Ohka Foundation for their financial support, as well as to the Tosoh Corp. for kindly providing the Y-zeolite samples. References and Notes (1) Zeolites and Microporous Crystals; Hattori, T., Yashima, T., Eds.; Elsevier: Amsterdam, 1994. (2) Domen, K.; Yoshimura, J.; Sekine, T.; Tanaka, A.; Onishi, T. Catal. Lett. 1990, 4, 339. (3) Anpo, M.; Matsuoka, M.; Shioya, Y.; Yamashita, H.; Giamello, E.; Morterra, C.; Che, M.; Patterson, H. H.; Webber, S.; Ouellette, S.; Fox, M. A. J. Phys. Chem. 1994, 98, 5744. (4) Fox. M. A. Res. Chem. Intermed. 1991, 15, 153. (5) Anpo, M.; Yamashita, H. In Surface Photochemistry; Anpo, M., Ed.; Wiley; West Sussex, 1996; p 117. (6) Giamello, E.; Murphy, D.; Magnacca, G.; Morterra, C.; Shioya, Y.; Nomura, T.; Anpo, M. J. Catal. 1992, 136, 510. (7) Anpo. M.; Nomura, T.; Shioya, Y.; Che, M.; Murphy, D.; Giamello, E. Proc. 10th Int. Cong. Catal. Guczi, L., Solymosi, F., Tetenyi, P., Eds.; Akademiai Kiado: Budapest, 1993; p 2155. (8) Negishi, N.; Matsuoka, M.; Yamashita, H.; Anpo, M. J. Phys. Chem. 1993, 97, 5211. (9) Yamashita, H.; Matsuoka, M.; Tsuji, K.; Shioya, Y.; Anpo M.; Che, M. J. Phys. Chem. 1996, 100, 397. (10) Matsuoka, M.; Matsuda, E.; Tsuji, K.; Yamashita H.; Anpo, M. Chem. Lett. 1995, 375. (11) Courbon, H.; Pichat, P. J. Chem. Soc., Faraday Trans. 1 1984, 80, 3175. (12) Anpo, M.; Nakaya, H.; Kodama. S.; Kubokawa, Y.; Domen, K.; Onishi, T. J. Phys. Chem. 1985, 90, 1633. (13) Anpo, M.; Chiba, K. J. Mol. Catal. 1992, 74, 207. (14) Yamashita, H.; Ichihashi, Y.; Harada, M.; Stewart, G.; Fox M. A.; Anpo, M. J. Catal. 1996, 158, 97. (15) Tanaka, T.; Yamashita, H.; Tsuchitani, R.; Funabiki, T.; Yoshida, S. J. Chem. Soc., Faraday Trans. 1 1988, 84, 2987. (16) Liu, X.; Iu K.; Thomas, J. K. J. Chem. Soc., Faraday Trans. 1993, 89, 1861. (17) Liu, Z.; Davis, R. J. J. Phys. Chem. 1994, 98, 1253. (18) Bordiga, S.; Coluccia, S.; Lamberti, C.; Marchese, L.; Zecchina, A.; Boscherini, F.; Buffa, F.; Genoni, F.; Leofanti, G.; Petrini, G.; Vlaic, G. J. Phys. Chem. 1994, 98, 1253. (19) Bonneviot, L.; On, D. T.; Lopez, A. J. Chem. Soc., Chem. Commun. 1993, 685. (20) Behrens, P.; Felsche, J.; Vetter, S.; Schult-Ekloff, G.; Jaeger, N. I.; Niemann, W. J. Chem. Soc., Chem. Commun. 1991, 678. (21) Anpo, M.; Shima, T.; Fujii, T.; Suzuki, S.; Che, M. Chem. Lett. 1987, 1997. (22) Anpo, M.; Yabuta, M.; Kodama S.; Kubokawa, Y. Bull. Chem. Soc. Jpn. 1986, 59, 259.
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