J. Phys. Chem. B 2000, 104, 11501-11505
11501
In Situ Investigation of the Photocatalytic Decomposition of NO on the Ti-HMS under Flow and Closed Reaction Systems Jinlong Zhang,† Madoka Minagawa, Terukazu Ayusawa, Srinivasan Natarajan,‡ Hiromi Yamashita, Masaya Matsuoka, and Masakazu Anpo* Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture UniVersity, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan ReceiVed: July 13, 2000; In Final Form: September 20, 2000
Mesoporous materials such as Ti-HMS, prepared at ambient temperature, were studied both under continuous flow and closed reaction systems for the photocatalytic decomposition of NO. The results indicate Ti-HMS to be a good photocatalyst with a continuous reactivity under prolonged irradiation. The direct decomposition of NO into N2, O2, and N2O at 275 K with a high selectivity for N2 and O2 in a closed system is noteworthy. In situ photoluminescence and EXAFS investigations show that the Ti exists in a tetrahedral coordination and is highly dispersed within the mesoporous material. The TPD studies, before and after the photocatalytic investigations, clearly indicate that NO molecules cannot be adsorb strongly on the surface of Ti-HMS photocatalysts, in contrast to the powdered TiO2 photocatalysts, where strong NO adsorption species have been observed. It is likely that the charge transfer excited state of the tetrahedrally coordinated titanium oxide species plays a significant role in the direct decomposition of NO.
1. Introduction NOx is one of the harmful atmospheric pollutants which causes acid rain and photochemical smog. The removal of nitrogen oxides (NOx: NO, N2O, NO2) from the atmosphere, i.e., the direct decomposition of NOx into N2 and O2, has been a great challenge to many researchers.1-5 As a photocatalyst, TiO2 is attractive for its stability, nontoxicity, low cost, and high reactive properties, significantly for chemical waste remediation.6-9 To apply these photocatalysts to real systems, it is necessary to develop a large-scale flow system specifically for the decomposition of NO. Investigations under a continuous flow system, however, invariably lead to a decline in the photocatalytic reactivity on the powdered titanium oxide photocatalysts.10,11 On the other hand, it is to be noted that the physicochemical and the reactivity/selectivity properties of oxide catalysts often strongly depend on their local structural characteristics. Extensive investigations on titanium silicalites and titanium oxide species included within zeolites for the design and development of effective and efficient photocatalysts have been carried out in recent years.12,13 The results of such studies indicate that the local structure of Ti plays a significant role in controlling the reaction rate and selectivity for the direct decomposition of NO. Along these lines, in the present study, highly dispersed tetrahedrally coordinated titanium oxide species have been prepared within a mesoporous material (HMS) and their role in the direct photocatalytic decomposition of NO has been investigated both under closed and flow reaction conditions. Special attention has been focused on the intimate relationship * To whom correspondence should be addressed. E-mail: anpo@ ok.chem.osakafu-u.ac.jp. † Permanent address: Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, P. R. China. ‡ Permanent address: Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560 064, India.
between the photocatalytic performance reactivity and the local structure of the titanium oxide species in Ti-HMS photocatalysts. Investigations on the different adsorption species of NO were also carried out by temperature-programmed desorption (TPD) methods. 2. Experimental Section Ti-HMS was prepared by following method proposed by Zhang et al.14 A typical procedure is as follows: Tetraethyl orthosilicate (TEOS), tetraisopropyl orthotitanate (TIPOT), and a long-chain alkylamine surfactant (dodecylamine, DDA) were used as the source for the silica, titanium, and template, respectively. The molar composition of the reaction mixture was (0.01-0.10 Ti):(1.0 Si):(0.20 DDA):(9.0 EtOH):(160 H2O). The final products were denoted as Ti-HMS(x), where x is the Ti content in the starting synthesis mixture (wt %). The actual composition of Ti in the Ti-HMS photocatalysts was determined by employing atomic absorption methods. The TiO2 powered catalyst (P-25: anatase 92%, rutile 8%) was supplied as a standard reference catalyst by the Catalysis Society of Japan. For photocatalytic reactions of NO molecules under a closed system, typically 100 mg of the catalyst was taken in a quartz cell with a flat bottom (60 mL) that is connected to a conventional vacuum system (10-6 Torr range). Prior to the photocatalytic reactions 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. Only NO as a reactant was introduced into the cell having a flat bottom. UV irradiation of the catalysts in the presence of NO (7.8 µmol) was carried out using a 75-W highpressure Hg lamp (λ > 280 nm) at 275 K, and an ice bath was used to keep the flat bottom at a constant temperature. The reaction products were analyzed by gas chromatography (Shimadzu, Japan). The photocatalytic reactions under a flow system were carried out by following method described before.10 The reaction cell
10.1021/jp002513y CCC: $19.00 © 2000 American Chemical Society Published on Web 11/11/2000
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Figure 1. Diffuse reflectance UV-vis spectra of (a) Ti-HMS(1), (b) Ti-HMS(2), (c) Ti-HMS(10), and (d) TiO2 (P-25).
was made of quartz (length, 190 mm; inner diameter, 10 mm), and 1.0 g of the Ti-HMS(10) photocatalyst was employed in each experiment. The pretreatment and other reaction conditions of the photocatalysts were similar to that described recently.10 To prevent any movement of the catalyst within the cell, glass wool was placed at both the bottom and the top sides of the reaction cell. The reaction temperature was kept at 295 K, and the reaction gases (NO and He) were introduced at a rate of 25 mL/min as a single pass. A NOx meter (New Cosmos Electric Motors Ltd., Japan) in combination with a gas chromatograph (Shimadzu, Japan) was employed to measure the concentrations of NO and the products of decomposition, viz., N2, O2, and N2O. The NOx meter and gas chromatograph were both connected to a recorder, which continuously monitored the changes in the concentrations of NO and the product yields of N2, O2, and N2O. 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-2200 spectrometer at 295 K. The X-ray absorption spectra (XAFS) were measured at the BL-7C facility of the Photon Factory at the National Laboratory for High-Energy Physics, Tsukuba, Japan. The Ti K-edge absorption spectra were recorded both in the transmission and the fluorescence modes at 295 K. The normalized spectra were obtained by the procedure described previously in the literature.15 The temperature-programmed desorption (TPD) investigations were carried out on the spent catalyst obtained from the closed system.11 A 10-mg sample of the photocatalyst was placed in the TPD cell. Before each run, the photocatalyst was heated to 573 K at 5 K/min in 3 kPa (23 Torr) of O2 to regenerate the original oxide surface, and then the system was evacuated for 2 h. The TiO2 sample was cooled to room temperature after degassing at 573 K for 1 h. NO (1 kPa, 7 Torr) was introduced into the TPD cell and the sample was irradiated for 2 h using UV light. The photocatalyst was then degassed in the system for 15 min prior to TPD measurements. A constant heating rate of 5 K/min was employed using a digital programmable temperature controller, and the TPD profile was measured employing a quadrupole mass analyzer (model no. M-QA100TS) interfaced with a PC-9821 computer. 3. Results and Discussion The initial characterizations of the Ti-HMS photocatalyst were carried out by employing standard techniques such as powder XRD and N2 desorption, which indicated clearly that the desired products have been formed. The diffuse reflectance UV-visible spectra of Ti-HMS is presented in Figure 1. Compared to the bulk anatase (standard reference catalyst P-25),
Zhang et al. the absorption edges of the Ti-HMS samples exhibit a blue shift of about 100 nm. For Ti-HMS(1) and Ti-HMS(2), the appearance of a peak near 210 nm can be assigned to the legendto-metal charge-transfer transition (LMCT) involving electron transfer from oxygen ligands to the tetracoordinated Ti(IV) ions of the titanium oxide species.16 This shows that the Ti ions are coordinated within the framework of HMS. A small change in the absorption spectrum between Ti-HMS(1) and Ti-HMS(2) (∼20 nm) indicates that the local coordination geometry rearranges with the changes in Ti concentration within the framework. The spectrum for Ti-HMS(10), on the other hand, shows an absorption band in the region of 250 and 300 nm, attributable, partly, due to the presence of some of the Ti in octahedral coordination. Similar absorption peak positions have been observed for titanium oxides having penta- or hexacoordinated Ti species.17,18 It is to be noted that no significant absorption bands around 300-350 nm have been observed, suggesting the absence of segregated crystalline TiO2 anatase phase. The above results clearly show that most of the Ti atoms in the Ti-HMS samples occupy site-isolated positions within the silica framework, except for the high Ti content sample, Ti-HMS(10), where some of the Ti species may be present in an octahedrally coordinated environment. The local structural information around Ti can be determined with the aid of both X-ray absroption near-edge structure (XANES) and extended X-ray absroption fine structure (EXAFS). The preedge peak (due to the 1s-3d transition) was used as a fingerprint in assessing the local structure around the Ti analogues for procedures employed in identifying metal ion position in other similar compounds. In Figure 2, we provide the XANES and Fourier transformed EXAFS spectra of the three different samples studied in this work. The XANES spectra show a single intense peak which can be assigned to the allowed 1s f t2g transition, consistent with the tetrahedral environment for the Ti atoms.19 The intensity of the preedge peak, however, decreases with increasing Ti concentration (Figure 2). This observation suggests that in the sample containing higher Ti concentration, Ti-HMS(10), the Ti atoms possibly exist in higher coordination. It has been shown recently that the presence of octahedral Ti, as extraframework species in anatase or in compounds having significant disorder in Ti-O distances and/ or Ti-O-Si angles, generally leads to changes in preedge features such as loss of intensity and a shift to higher energy.20 Thus, the present observation is consistent with this and also correlates well with the UV-visible spectroscopy studies. The Fourier transformed EXAFS data for all the samples, presented in Figure 2, show a strong peak at around 1.6 Å (without phase shift correction), which can be assigned to the neighboring oxygen atoms (Ti-O). While Ti-HMS(1) and TiHMS(2) exhibit only a single Ti-O peak, Ti-HMS(10) exhibits an additional strong peak at around 2.7 Å. This peak may be due to the presence of Ti-O-Ti environment. This observation is suggestive of the fact that in sample with lower Ti concentrations the Ti probably exists as isolated species while the higher Ti concentration leads to agglomeration. The effect of NO addition on the photoluminescence properties of TI-HMS(1) is presented in Figure 3. Typically, the evacuated Ti-HMS(1) exhibits a strong peak in the emission spectrum at around 450-550 nm, and when excited its LMCT band appears at around 260-290 nm at 77 K. The observed photoluminescence spectrum is in good agreement with those previously observed for highly disperpared tetrahedrally coordinated Ti in silica matrixes.21,22 The absorption of UV light at around 290 nm brings about an electron transfer from the lattice
Photocatalytic Decomposition of NO
J. Phys. Chem. B, Vol. 104, No. 48, 2000 11503
Figure 2. XANES (a-c) and FT-EXAFS (A-C) spectra of the Ti-HMS(1) (a, A), Ti-HMS(2) (b, B), and Ti-HMS(10) (c, C) photocatalysts.
Figure 3. Effect of the addition of NO upon the photoluminescence spectra of the Ti-HMS(1) photocatalyst: (a) 0 Torr, (b) 0.1 Torr, (c) 3.2 Torr, (d) 6 Torr, and (e) 8 Torr.
oxygen (O2-) to the titanium ion (Ti4+) of the tetrahedral coordinated titanium oxide species to form its charge transfer excited state. The observed photoluminescence spectrum can then be attributed to the radiative decay process from the charge transfer excited state to the ground state of the highly dispersed titanium oxide species having a tetrahedral coordination as shown by the following scheme: hν
(Ti4+-O2-) {\ }(Ti3+-O-)* hν′ It can clearly be seen from Figure 3 that the addition of NO leads to an efficient quenching of the photoluminescence of the Ti-HMS(1), indicating that the added NO molecules interact with the charge transfer excited state. The intensity of the emission peak is directly proportional to the amount of added NO. These results clearly demonstrate the accessibility of the tetrahedral Ti for NO in Ti-HMS(1).
Figure 4. Products distribution of the photocatalytic decomposition of NO on the different photocatalysts: (a) Ti-HMS(1), (b) TiHMS(2), (c) Ti-HMS(10), and (d) TiO2 (P-25).
The results of the photocatalytic decomposition of NO, carried out in a closed system at 275 K under UV irradiation, on the various photocatalysts are presented in Figure 4. For comparison the photocatalytic decomposition of NO has also been performed on standard reference powdered TiO2 (P-25). In all the cases, the decomposition of NO leads to the formation of N2, O2, and N2O, differing in yields and product selectivities and also exhibiting a good linearity against the irradiation time. The results suggest that the photocatalytic decomposition of NO may be proceeding at the Ti sites in all these photocatalysts. It can be seen from Figure 4, that the specific photocatalytic reactivities of the Ti-HMS catalysts, which have been normalized for the unit amount of TiO2, are much higher than the bulk TiO2 catalyst. It appears that the efficiency and selectivity of the formation of N2 and O2 strongly depends on the type of titanium oxide photocatalyst employed, Ti-HMS(1) and Ti-HMS(2) exhibiting a higher reactivity and selectivity for the formation of N2 and O2, while N2O is favored on bulk TiO2.
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Figure 5. NO conversion rates of the photocatalytic decomposition reaction of NO on the Ti-HMS and P-25 photocatalysts: (a) Ti-HMS(1) (TiO2: 1 wt %; 1.00 g), (b) Ti-HMS(10) (TiO2: 10%, 1.00 g), and (c) P-25 (0.1 g); pretreatment temperature of 573 K; flow rate of NO (10 ppm) was 25 mL/min.
From the results, it is clear that the photocatalytic performance of titanium oxide species appears to be modified by their incorporation into the framework of HMS, which may arguably be due to the changes in the coordination and reaction environment. While the pure bulk TiO2 does not appear to be a high selectivity for formation of N2, the highly dispersed titanium oxides in Ti-HMS show good photocatalytic selectivity and activity for the formation of N2. In this connection it is worth noting that the catalytic activity and selectivity of the Ti-HMS photocatalysts also changes with the Ti content, which may be associated with the changes in the coordination geometry and the aggregation of the titanium oxide species. The photocatalytic performances of the standard reference TiO2 (P-25) and Ti-HMS were investigated under a flow system with prolonged UV irradiation. The results are summarized in Figure 5. During the reaction, P-25 photocatalyst appears to lose its photocatalytic activity, and after about 2 h, the conversion of NO levels off and becomes one-quarter to one-fifth of the activity observed at the beginning of the reaction. However, Ti-HMS(10) appears to show constant photocatalytic activity (about 20%) under identical conditions. The reaction products of the decomposition of NO under the flow system were N2, O2, and N2O, just as in the closed reaction system. To understand the differences in the photocatalytic reactivity between P-25-TiO2 and Ti-HMS in the closed reaction system, temperature-programmed desorption (TPD) investigations have been carried out. The results of the TPD investigations after the photocatalytic reactions of NO, under closed reactions, on Ti-HMS(10) are presented in Figure 6. It can be seen very clearly that only the desorption peak corresponding to NO could be observed. Though a negligible amount of N2 is present, the desorption peaks corresponding to O2 and N2O are absent, indicating that only NO binds to the active sites of titanium oxide species. In this connection it is worth noting that three types of TPD peaks due to the desorption of N2O, N2, and O2 could be observed for P-25 photocatalyst in addition to three strong peaks for NO under identical conditions.11 In Figure 7, we compare the results of TPD studies of NO on P-25 and Ti-HMS(10) photocatalysts. It can be seen that there are three clear NO adsorbed species present in P-25 catalyst compared to only one for Ti-HMS(10), and the desorption from the Ti-HMS(10) is very much weaker than that of P-25. Moreover, the peak at 524 K, related to the strong adsorption species of NO, could be observed for P-25 but not
Zhang et al.
Figure 6. TPD spectra of the Ti-HMS(10) photocatalyst after the photocatalytic decomposition of NO: (a) NO, (b) N2, (c) N2O, (d) O2. Ti-HMS(10) sample size was 100 mg. Reaction time of the decomposition reaction was 2 h. The rate of temperature increase was 5 K/min.
Figure 7. TPD spectra of NO on P-25 and Ti-HMS(10) after the photocatalytic decomposition of NO: (a) Ti-HMS(10) (TiO2: 10 wt %; 100 mg); (b) P-25 (10 mg). Photocatalytic reaction time was 2 h, and the rate of temperature increase was 5 K/min.
for Ti-HMS(10). It can, therefore, be deduced that the amount of the irreversible adsorption of NO is very low for Ti-HMS(10). The decline phenomenon of the photocatalytic reactivity of P-25 for the decomposition of NO at 295 K may then be associated with the formation of strongly adsorbed species of NO on the TiO2 surface. In fact, it has been proposed that the photocatalytic deactivation of TiO2 (P-25) for NO decomposition was due to the adsorbed species on the surface.15 The P-25 catalyst can be completely regenerated by heating under O2 and/ or Ar flow (rate ) 25 mL/min) at around 573 K for 1 h,11 confirming that the deactivation of the catalyst is indeed caused by the formation of strong adsorption species on the surface. 4. Conclusions The photocatalytic decomposition of NO on various Ti-HMS photocatalysts was carried out both under closed and flow conditions. The results obtained by the diffuse reflectance, EXAFS, and in situ photoluminescence studies of Ti-HMS photocatalysts clearly showed that the environment of Ti in these photocatalysts is predominantly tetrahedral and these tetrahedrally coordinated titanium oxide species are highly dispersed within the mesoporous structure of HMS. It was found that TiHMS photocatalysts were effective and efficient for the decomposition of NO into N2 and O2 under a flow reaction system, even for prolonged irradiation periods, and the charge transfer excited state of the highly dispersed tetrahedrally coordinated titanium oxide species played a significant role in these photocatalytic reactions. The efficiency and selectivity of
Photocatalytic Decomposition of NO the formation of N2 and O2 were found to strongly depend on the local structure of titanium oxide photocatalysts. The analyses of the TPD studies of NO on various photocatalysts showed the presence of a strongly adsorbed NO species for P-25 photocatalyst, while no such species were observed for TiHMS photocatalysts. These results clearly suggested that the performance of the titanium oxide species was greatly modified by their incorporation into the framework of HMS, being attributed to the changes in the coordination and reaction environment. Acknowledgment. The present work has been supported in part by the Ministry of Education of Japan, Grant-in-Aid for JSPS Fellows. References and Notes (1) Anpo, M. Catal. SurV. Jpn. 1997, 1, 169, and references therein. (2) Ollis, D. F.; Al-Ekabi, H. Eds. Photocatalytic Purification and Treatment of Water and Air; Elsevier: Amsterdam, 1993. (3) Anpo, M.; Zhang, S. G.; Higashimoto, S.; Matsuoka, M.; Yamashita, H. J. Phys. Chem. 1999, 103, 9295. (4) Anpo, M.; Che, M. AdV. Catal. 1999, 44, 119, and references therein. (5) Serpone, N.; Pelizzetti, E. Photocatalysis; Wiley: New York, 1989. (6) Anpo, M.; Yamashita, H.; Matsuoka, M.; Park, D. R.; Shul, Y. G.; Park, S. E. J. Ind. Eng. Chem. 2000, 6, 59, and references therein. (7) Alekabi, H.; Serpone, N. J. Phys. Chem. 1988, 92, 5726.
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