ARTICLE pubs.acs.org/JPCC
Novel H2Ti12O25-Confined CeO2 Catalyst with Remarkable Resistance to Alkali Poisoning Based on the “Shell Protection Effect” Xiongbo Chen,† Haiqiang Wang,† Zhongbiao Wu,*,†,‡ Yue Liu,† and Xiaole Weng† † ‡
Department of Environment Engineering, Zhejiang University, Yuhangtang Road 866, Hangzhou 310058, P. R. China Zhejiang Provincial Engineering Research Center of Industrial Boiler & Furnace Flue Gas Pollution Control, Yuhangtang Road 866, Hangzhou 310058, China
bS Supporting Information ABSTRACT: In this paper, a novel catalyst, titanate nanotube (TNT) confined CeO2 was designed and synthesized. Such catalyst showed a remarkable resistance to alkali metal poisoning in deNOx application. The catalyst effectively shielded the main active phase, CeO2, from the poisons with the tubular channel of H2Ti12O25. Furthermore, the poisons (e.g., Na+) could also be stabilized in the interlayer of H2Ti12O25 through ion exchange. This catalyst developed herein gives a new sight for the design of “shell protection” catalysts to improve their tolerance to poisons.
1. INTRODUCTION Nanotubes (e.g., carbon nanotubes, titanate nanotubes) are at the leading edge of the emerging field of nanotechnology. This structure can provide nanosized confined space (inner nanotubes), which could act as a nanoreactor, particularly beneficial to the reaction activity and selectivity in catalysis application.1,2 As for the nanotubes supported catalysts, a unique metalsupport interaction may occur between the introduced metal oxide and support.3 And their redox properties could be tuned by varying the nature of host and the location on the exterior or interior surface of the nanotubes.4,5 Furthermore, this tubular channel structure may shield the metal oxide inner tube from the poisoning components. However, little attention was paid to this shell protection function of nanotubes supported catalysts. The catalyst poisoning or deactivation has always been concerned in catalysis application, such as the poisoning of deNOx catalysts by alkali and alkaline earth metals,68 the deactivation of three way catalysts by lead,9 the suppression of zeolite catalysts and hydrotreating catalysts by coke,10,11 etc. For example, the selective catalytic reduction (SCR) of NOx with ammonia is a widespread process for reducing NOx from stationary sources and the commercial vanadia-based SCR catalysts have shown excellent catalytic activity.1214 But problems associated with catalysts deactivations still remain,68 especially, by the presence of alkali metals and alkaline earth metals.15,16 Till now, the attempt to improve the resistance to the alkali metals/alkaline earth metals of SCR catalysts seems unsuccessful. Recently, the unique tubular structure of Nanotubes gives us some inspirations to solve the problem of catalyst poisoning. The titanate nanotubes (TNTs) as well as some other novel nanostructured titanates have been widely studied for catalytic applications due to their unique electric, mechanical, and structural characteristics.1719 Compared with carbon nanotubes (CNTs), the TNTs were often easy to synthesize and stable in r 2011 American Chemical Society
thermoreaction in the presence of oxygen, which significantly extended the area of possible applications.2022 These properties and their relatively low synthesis cost encouraged us to explore its application in thermo-catalysis. Our previous work23 has found that the TNTs with introduced cerium dioxide nanoparticles showed a superior performance in SCR of NO due to the improved redox potential and special adsorption capacity of NH3. Furthermore, the catalyst was more active than the commercial vanadia catalysts and N2O was not detected at any temperature. As we discussed above, the deactivation of the SCR deNOx catalyst by alkali metal remained an unsolved problem. And the “shell-protection effect” of nanotube supported catalyst may give us some inspirations in the improvement of catalyst poisoning tolerance. In this paper, we designed and synthesized two catalysts with the ceria nanoparticles deposited inside and outside the TNTs (denoted as Ce/TNTs and Ce/TNTs-out, respectively) to ascertain the protection effect of the TNT channels. The comparison in catalytic performances of two catalysts in the presence of alkali metal was carried out. After that, the catalysts were then characterized by X-ray diffraction analysis (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectrum analysis (XPS) to explore the relationship between catalyst structure and resistance ability to alkali metal. We expect that the work conducted herein could give a new insight to solve the big problem of catalyst poisoning.
2. EXPERIMENTAL SECTION 2.1. Catalysts Preparation. Ce/TNTs Series. The titanate nanotubes (TNTs) were synthesized by hydrothermal treatment20,21 Received: May 31, 2011 Revised: July 27, 2011 Published: August 03, 2011 17479
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The Journal of Physical Chemistry C of commercial available P25 TiO2 (Degussa, Germany) with 10 N NaOH solution in a Teflonlined autoclave at 150 °C for 24 h. The slurry was washed with 0.1 N HCl solutions for several times until pH value was close to 1.6, and then with distilled water until pH value was close to 7 before being filtrated. After drying at 80 °C for 18 h, the required TNTs were obtained. The introduction of cerium to TNTs or P25 was conducted by wet impregnating the titanium support with the required amount of cerium nitrate solution (Ce/Ti mole ratio at 1:19). The mixture was stirred for 6 h, dried at 80 °C for 18 h and calcined at 450 °C for 3 h to give the final metal oxide loaded catalysts for further analysis and examination. The catalysts were designated as Ce/TNTs for cerium doped inside titanate nanotubes and Ce/ P25 for cerium doped P25, respectively. The wet sodium loaded Ce/titanium catalysts with different molar ratio of Na/Ce at x (denoted as Ce/P25-Na-WI-x or Ce/ TNTs-Na-WI-x) were prepared by impregnating the Ce/Ti catalysts with the required amount of sodium nitrate solution, then stirred for 6 h, dried at 80 °C for 18 h, and calcined at 450 °C for 3 h. For dry solid sodium loaded Ce/TNTs (denoted as Ce/ TNTs-Na-SM-x or Ce/P25-Na-SM-x), the required amount of sodium nitrate solid was added to Ce/TNTs, then mixed for 2 h, dried at 80 °C for 18 h, and calcined at 450 °C for 3 h. Ce/TNTs-Out Series. Many earlier studies19,24 dealt with dispersion of nanoparticles inside TNTs but did not achieve exclusive dispersion of metals on the outside of TNTs. Extensive experiments indicate that TNTs produced by hydrothermal method can be partially filled with the metal precursor solution because of the capillary action. Thus, if we want to obtain the samples with metal oxides doped only outside the surface of TNTs, the TNT channels should be blocked temporarily during the process of preparation. For this purpose, xylene, which is immiscible with water (the solvent of Ce) and has a higher boiling point than water was used to fill the TNTs channel.25 The higher boiling point of xylene allowed the preferential evaporation of water and deposition of Ce on the outer surface while the channels remain occupied. Details of the experiment were presented as follow: The required amount of TNTs were immerged in ethanol for 6 h and dried at 80 °C for 12 h. Then, the TNTs were immerged in superfluous xylene for 12 h to packing the tubular channels, followed by the addition of cerium nitrate solution (Ce/Ti mole ratio at 1:19). However, most TNTs were still suspended in the upper layer of xylene even after 24 h. Thus, a solution of NH4HCO3 dissolved in aqueous ammonia (2628%) was added to facilitate the extraction of TNTs from xylene into the aqueous phase.25 After stirring for 6 h, a standing and layering process for the solutions was followed. Then, the mixture was evaporated within 1 h by heating to 80 °C under stirring. Subsequently, the sample was subjected to the same drying and calcination treatment of Ce/TNTs series. The catalyst was designated as Ce/ TNTs-out for cerium doped outside titanate nanotubes. The wet sodium loaded Ce/TNTs-out and solid sodium loaded Ce/TNTs-out catalysts with different molar ratio of Na/Ce at x (denoted as Ce/TNTs-out-Na-WI-x or Ce/TNTsout-Na-SM-x) were prepared by the same process of Ce/TNTsNa and Ce/P25-Na samples. 2.2. Method of Characterization. The crystal phases of the samples were analyzed by using X-ray diffraction with Cu KR radiation (XRD: model D/max RA, Rigaku Co.). The data were collected for scattering angles (2θ) ranging between 10 and 80° with a step size of 0.02°. X-ray photoelectron spectroscopy with
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Al KR X-ray (hν = 1486.6 eV) radiation operated at 150 W (XPS: Thermo ESCALAB 250) was used to investigate the surface properties and to probe the total density of states (DOS) distribution. The shift of the binding energy due to relative surface charging was corrected using the C 1s level at 284.8 eV as an internal standard. The morphology, structure, and grain size of the samples were examined by transmission electron microscopy (TEM: JEM-2010). An energy dispersive spectrometer (EDS) was used as an assistant to TEM to determine element content. Nitrogen adsorptiondesorption isotherms were obtained using a nitrogen adsorption apparatus (ASAP 2020). All the samples were degassed at 200 °C prior to measurements. The Brunauer EmmettTeller (BET) specific surface area (SBET) was determined by a multipoint BET method using the adsorption data in the relative pressure (P/P0) range from 0.05 to 0.30. 2.3. SCR Activity Evaluation. Selective catalytic reduction of NO with NH3 was carried out in a fixed-bed reactor. The experiments were performed under atmospheric pressure at 200 470 °C. The reactor consisted of a quartz tube of 1 cm i.d. in which 0.5 g of catalyst was filled. A type K thermocouple was placed at the center of the bed for temperature measurements. The typical reactant gas composition was 600 ppm NO, 600 ppm NH3, 3.5% O2, and balance N2. The gas hourly space velocity (GHSV) was about 100 000 h1 for this system. NO, NO2, and O2 concentration were monitored by a flue gas analyzer (KM9106 Quintox Kane International Limited). N2O was detected by a FT-IR gas analyzer (Gasmet DX-4000, Temet Instrument Oy, Finland; Madur Photon Portable IR Gas Analysers, Madur Ltd., Austria).
3. RESULTS AND DISCUSSION 3.1. Crystal Structure and Phase. To get a better understanding of the structure of catalysts, X-ray powder diffraction was conducted on the samples. As shown in Figure 1, the Ce/P25 sample showed a typical composite structure of TiO2. The XRD peaks belonging to the anatase (PDF-#21-1272) phase (at 2θ = 25.28°, 37.80°, 48.05°, 53.89°, 55.06°, and 62.69°) and rutile (PDF-#21-1276) phase (at 2θ = 27.45°, 36.09°, and 54.32°) were clearly observed. For pure TNTs, the XRD peaks at 24.7°, 28.02°, and 48.22° were observed, attributed to (110), (211), and (020) faces of H2Ti3O7, respectively.26,27 However, the XRD patterns of Ce/TNTs and Ce/TNTs-out samples only showed two evident peaks (at 2θ = 25.10°, 48.20°) and one weak peak (at 2θ = 28.50°), indicating that the structure of titanium compounds in Ce/TNTs and Ce/TNTs-out catalysts was an intermediate condensed layered titanate (e.g., H2Ti6O13 and H2Ti12O25). It has been reported that the structure of TNTs generally followed the sequence H2Ti3O7 f H2Ti6O13 f H2Ti12O25 f monoclinic TiO2(B) f anatase during the calcination process (T = 120400 °C, through topotactic mechanisms).27,28 The intermediate tunnel structure of H2Ti12O25 was normally formed when the calcination temperature was at 400 °C. Furthermore, previous research has reported that the doping of ceria could induce an inhibition in the transformation to anatase.29 Thus one could conclude that the crystal phase of titanium compounds in the Ce/TNTs or Ce/TNTs-out samples was H2Ti12O25, although the Ce/TNTs or Ce/TNTs-out samples had been pretreated at 450 °C. As a protonated titanate, H2Ti12O25 had the effective ion-exchange property like H2Ti3O7.22,3032 This property of H2Ti12O25 gave us a surprising result in the following study. In addition, the Ce/TNTs and 17480
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Figure 1. Powder XRD patterns for Ce/titanium catalysts.
Figure 3. TEM and HRTEM images: (a) general view of Ce/TNTsout; (b) Ce particles on the exterior surface of Ce/TNTs-out; (c) general view of Ce/TNTs-Na-WI-1; (d) Ce particles in the tube for Ce/TNTs-Na-WI-1.
Figure 2. TEM and HR-TEM images of Ce/TNTs: (a) and (b) general view; (c) the wall of TNTs; (d) isolated particles; (e) and (f) CeO2 particles in the tube.
Ce/TNTs-out samples also revealed some slight characteristic peaks (at 2θ = 28.5o, 33.1°, 47.5°, and 56.3°) of cerianite. There was little change after Na adding, indicating that the addition of Na did not change the structure of the Ce/TNTs or Ce/TNTsout samples. 3.2. Morphology and Structure Investigation. The morphology and structure of the Ce/TNTs sample were then investigated by using TEM. As shown in Figure 2a,b, a few isolated particles and plenty of intact nanotubes (outer diameter of 810 nm, inner diameter of ca. 3 nm, and length of several hundred nanometers) were observed in the sample. The inner diameter of nanotubes fitted well with their BET-BJH pore diameter (Supporting Information, Figure S1a). Furthermore, HR-TEM revealed that the nanotubes were multilayered and their interlayer spacing was observed at about 0.71 nm (Figure 2c), which was in agreement with that as reported previous.27 The d-spacing of the isolated particles was measured at ca. 0.35 nm, which was in proximity to the (101) plane of anatase (Figure 2d).33,34 This was further confirmed by the EDS analysis, which revealed only the existence of Ti and O in these particles (Supporting Information, Figure S1b). However, the Ce was found in the
areas containing many nanotubes, which revealed the presence of Ce at ca. 8.82% (Supporting Information, Figure S1c). Because there were little CeO2 particles observed outside the nanotubes, we could assume that the CeO2 particles mainly exist inside the tubular channels of TNTs. Indeed, HR-TEM analysis (see Figure 2e,f) confirmed this assumption, which showed the d-spacing of inside particles at ca. 0.32 nm, belonging to CeO2.35 Note that we rotated the specimen under the microscope in two directions to confirm the locating of these particles inside the TNT channels. The HR-TEM also revealed that the inner wall of nanotubes became uneven after cerium doping while the outer wall remained smooth. Furthermore, significant amounts of TNTs clogged up by CeO2 and large numbers of TNTs with CeO2 attached to the inner wall could also be observed. This could also suggest that the vast majority of CeO2 were located inside the channels rather than on the exterior surface for the Ce/TNTs sample. For the Ce/TNTs-out samples, as shown in Figure 3a,b, the CeO2 particles located inside the channels could not be observed. Instead, a few particles situated on the exterior surface of TNTs were observed. This implied that large quantities, if not all, of cerium species were doped on the outer surface. The tubular structure of the TNT sample retained well after Na was added by the wet impregnation method (sample: Ce/TNTs-Na-WI-1, Figure 3c,d). 3.3. XPS Analysis. To further evaluate the position of the Ce in the samples, XPS was then applied. Because the thickness of the TNT wall is 23 nm (Figure 2), which is close to the penetration depth of XPS, only outside Ce and part of the Ce inside TNTs could be detected. As such, we etched the samples via iron sputtering to cut the wall of the TNTs, exposing the inside Ce. Generally, for the sample of ceria located only on the surface of the supports (e.g., Ce/P25), etching treatment would lower their cerium content, while for the sample of ceria located inside the TNTs, the etching would lead to more exposure of ceria, resulting in an increase of Ce content. Indeed, the XPS results verified this conjecture (Table 1). In addition, cerium content for the 17481
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Table 1. Atomic Concentration with XPS sample Ce/TNTs
Ce/TNTs-out
Ce/TNTs-Na-WI Ce/P25 Ce/P25-Na-WI
treatment
O (at. %)
Ti (at. %)
Ce (at. %)
before etching
78.43
19.53
2.04
50 s etching
72.12
24.64
3.23
100 s etching
69.61
26.63
3.76
150 s etching
69.02
26.95
4.04
before etching
72.17
24.20
3.63
50 s etching
69.83
25.43
4.74
100 s etching
69.66
25.59
4.76
150 s etching before etching
69.42 68.29
25.86 23.91
4.65 1.86
50 s etching
68.62
24.86
2.63
before etching
73.65
22.46
3.61
50 s etching
71.08
25.18
3.49
before etching
72.75
23.86
3.17
50 s etching
71.17
25.87
2.65
Ce/TNTs-out sample increased with the first etching and then was stable with the following etching treatments, which means the cerium species were well dispersed in the Ce/TNTs-out sample. 3.4. Potential Deactivation Routes of Na for Ce/Titanium Catalysts. In the real application circumstance, the contaminants were mainly from the raw gases. For example, in the SCR process, the inorganic components such as alkali metal normally existed in the exhaust gases. The potential deactivation routes of the alkali metals (e.g Na) for the SCR catalysts could be proposed as follows: (1) The solid sodium compounds directly deposited on the surface of catalysts, which was called as “solid mixing model” (SM model). (2) With the contribution of water vapor, the sodium compounds might impregnate into the internal partitions of catalysts, which was called as “wet impregnation model” (WI model). 3.5. Test of Resistance to Alkali Metal by SM Model: A Good Shield. The SCR performance of Ce/titanium catalysts before or after solid Na mixing is shown in Figure 4. Similar to other SCR catalysts, such as V2O5/TiO2, V2O5WO3/TiO2, and LaFe-ZSM-5,3639 the Ce/P25 sample was deactivated seriously after solid mixing with Na (Na/Ce molar ratio at 1). For the Ce/TNTs-out catalyst, a serious sodium deactivation in the Ce/TNTs-out-Na-SM-1 sample was also observed. However, for the Ce/TNTs samples, after adding Na (Na/Ce molar ratio at 1), the Ce/TNTs-Na-SM-1 sample still showed a NO conversion of 90% at 350 °C. This result is quite promising, as such high resistance to alkali poisoning has never been reported before. The different performance of Ce/P25-SM-1, Ce/TNTs-Na-SM1, and Ce/TNTs-out-Na-SM-1 catalysts is in accord well with the potential deactivation routes of Na. It clearly indicates that the good shield provided by the tubular structure of TNTs indeed improves the sodium resistance ability of the Ce/TNTs sample. 3.6. Test of Resistance to Alkali Metal by WI Model: A Strong Scavenger. Thereafter, another deactivation model of more serious poisoning circumstance by the wet impregnation of Na was evaluated. Figure 5 shows the SCR performance of the Ce/titanium samples after the wet impregnation of Na. As expected, the Ce/P25 sample was significantly deactivated after the wet impregnation of Na (Na/Ce molar ratio at 0.5, 1, or 2). The catalytic activities of Ce/P25-Na-WI samples all became negligibly low. In contrast, for the Ce/TNTs samples, after adding Na
Figure 4. Variation of NO conversion with reaction temperature over catalysts before or after Na adding. Reaction conditions: [NO] = [NH3] = 600 ppm, [O2] = 3.5%, balance N2, catalyst 0.5 g, and GHSV about 100 000 h1.
Figure 5. Variation of NO conversion with reaction temperature over different catalysts. Reaction conditions: [NO] = [NH3] = 600 ppm, [O2] = 3.5%, balance N2, catalyst 0.5 g, and GHSV about 100 000 h1.
(Na/Ce molar ratio at 1), the Ce/TNTs-Na-WI-1 sample still showed a NO conversion of 85% at 350 °C while the Ce/TNTsout-Na-WI-1 sample also showed a NO conversion similar to that for the Ce/TNTs-Na-WI-1 sample. This means that the Ce/ TNTs and Ce/TNTs-out samples all possess a good alkali resistance under the wet impregnation model. This result is very surprising as usually, the wet-impregnated Na+ would enter into the nanotubes due to the effect of capillary, leading to the deactivation of interior CeO2. The reason will be explained in the following section. In addition, the Ce/TNTs-Na-WI-1 sample was subjected to a durability test, which was conducted at the reaction temperature of 350 °C. No decrease of NO conversion was observed after the 48 h test (Supporting Information, Figure S2). This indicates that the sample has very good stability. 3.7. Proposed Mechanism of Duplicate Resistance to Alkali Poisoning for Ce/TNTs. On the basis of the activity experimental 17482
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serve as inspiration for developing more catalysts with improved poisoning tolerance.
’ ASSOCIATED CONTENT
bS
Supporting Information. BJH desorption and EDS results are shown in Figure S1. The results of durability test are shown in Figure S2. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author Figure 6. Schematic of protection mechanism for the Ce/TNTs-Na samples.
results, we could get an intuitive estimation that the unique tubular structure of TNTs most likely act as a good shield and protect active species by introducing them into the tubular channels, which shields the contact between CeO2 and alkali metal compounds, hence preventing the alkali poisoning. On the contrary, once CeO2 was doped on the outer surface of the supports, the poisons could easily contact with CeO2 and deactivate them. This estimation has been supported by the different performance of Ce/P25-SM-1, Ce/TNTs-Na-SM-1, and Ce/ TNTs-out-Na-SM-1 catalysts. The next immediate question was “how to explain the good sodium resistance ability of Ce/TNTs and Ce/TNTs-out samples with wet impregnation model?” From the result of XRD analysis, we could confirm that the structure of titanium compounds in the Ce/TNTs and Ce/TNTs-out samples was H2Ti12O25, which was one kind of condensed layered titanate. According to the crystal structure of titanate nanotubes, protons could occupy the cavities between the layers of TiO2 octahedra.22 The open morphology of the nanotubes results in effective ionexchange properties. And all protons in the titanate nanotubes are exchangeable to alkaline ion in the reaction as22,3032 xMenþ þ H2 Ti12 O25 ---Mex H2x Ti12 O25 xðn 1Þþ þ xHþ Consequently, it could be deduced that the TNTs provided dual protections for the interior CeO2 from alkali poisoning. The catalysts developed in this paper could be considered as “shell protection catalyst” and their schematic of protection mechanism is described in Figure 6. First, solid Na could not enter the channels of TNTs. In addition, the residual H+ of H2Ti12O25 in the interlayer could easily exchange with Na+ ions, locking them as the formation of NaxH2xTi12O25 after calcination.
4. CONCLUSION In summary, we have synthesized TNTs-confined CeO2 catalysts, which introduced the vast majority of ceria into the channels of TNTs. With the doping of ceria, the structure of TNTs maintained the structure as condensed layered titananteH2Ti12O25. The tubular structure of H2Ti12O25 shielded CeO2 from solidphase alkali metal compounds and also locked alkali metal ions in the interlayer through ion exchange. Thus, the dual protection for Ce/TNTs was achieved, resulting in a significantly improved resistance to alkali poisoning for the catalysts. This study gives a new insight into the design of “shell protection catalysts” for practical catalysis applications. This study will also
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’ ACKNOWLEDGMENT This research was financially supported by Changjiang Scholar Incentive Program (Ministry of Education, China, 2009), National Natural Science Foundation of China (NSFC-50878190), Science Foundation of Chinese University, and Natural Science Foundation of Zhejiang Province (Y5090053 and Y5090163). ’ REFERENCES (1) Pan, X. L.; Fan, Z. L.; Chen, W.; Ding, Y. J.; Luo, H. Y.; Bao, X. H. Nat. Mater. 2007, 6, 507. (2) Chen, W.; Pan, X. L.; Willinger, M. G.; Su, D. S.; Bao, X. H. J. Am. Chem. Soc. 2006, 128, 3136. (3) Pan, X. L.; Bao, X. H. Chem. Commun. 2008, 6271. (4) Chen, W.; Pan, X. L.; Bao, X. H. J. Am. Chem. Soc. 2007, 129, 7421. (5) Chen, W.; Fan, Z. L.; Pan, X. L.; Bao, X. H. J. Am. Chem. Soc. 2008, 130, 9414. (6) Kling, A.; Andersson, C.; Myringer, A.; Eskilsson, D.; Jaras, S. G. Appl. Catal. B:Environ. 2007, 69, 240. (7) Kamata, H.; Takahashi, K.; Odenbrand, C. U. I. J. Mol. Catal A: Chem. 1999, 139, 189. (8) Zheng, Y. J.; Jensen, A. D.; Johnsson, J. E.; Thogersen, J. R. Appl. Catal. B: Environ. 2008, 83, 186. (9) Larese, C.; Granados, M. L.; Galisteo, F. C.; Mariscal, R.; Fierro, J. L. G. Appl. Catal. B: Environ. 2006, 62, 132. (10) Mihindou-Koumba, P. C.; Cerqueira, H. S.; Magnoux, P.; Guisnet, M. Ind. Eng. Chem. Res. 2001, 40, 1042. (11) Pacheco, M. E.; Martins Salim, V. M.; Pinto, J. C. Ind. Eng. Chem. Res. 2011, 50, 5975. (12) Alemany, L. J.; Berti, F.; Busca, G.; Ramis, G.; Robba, D.; Toledo, G. P.; Trombetta, M. Appl. Catal. B: Environ. 1996, 10, 299. (13) Busca, G.; Lietti, L.; Ramis, G.; Berti, F. Appl. Catal. B: Environ. 1998, 18, 1. (14) Forzatti, P. Appl. Catal. A: General 2001, 222, 221. (15) Nicosia, D.; Czekaj, I.; Krocher, O. Appl. Catal. B: Environ. 2008, 77, 228. (16) Tang, F.; Xu, B.; Shi, H.; Qiu, J.; Fan, Y. Appl. Catal. B: Environ. 2010, 94, 71. (17) Liu, G.; Wang, L.; Sun, C.; Yan, X.; Wang, X.; Chen, Z.; Smith, S. C.; Cheng, H.-M.; Lu, G. Q. Chem. Mater. 2009, 21, 1266. (18) Huang, Y.; Ho, W.; Lee, S.; Zhang, L.; Li, G.; Yu, J. C. Langmuir 2008, 24, 3510. (19) Ratanatawanate, C.; Xiong, C. R.; Balkus, K. J. Acs Nano 2008, 2, 1682. (20) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. Langmuir 1998, 14, 3160. (21) Yao, B. D.; Chan, Y. F.; Zhang, X. Y.; Zhang, W. F.; Yang, Z. Y.; Wang, N. Appl. Phys. Lett. 2003, 82, 281. 17483
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