- Email: [email protected]
Photocatalytic decomposition of nitric oxide on TiO2 modified MCM-41 catalysts
Studies in Surface Science and Catalysis, volume 154 E. van Steen, L.H. Callanan and M. Claeys (Editors) 9 2004 Elsevier B.V. All fights reserved.
2876
PHOTOCATALYTIC DECOMPOSITION OF NITRIC OXIDE ON TiO2_MODIFIED MCM-41 CATALYSTS Chien, S.*, Huang, K. and Kuo, M. Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan, and Department of Chemistry, National Taiwan University, Taipei 10764, Taiwan. E-mail: [email protected]
ABSTRACT Mesoporous siliceous MCM-41 (SiMCM-41) molecular sieves with silicon partially substituted by A1 and Ti were synthesized by direct hydrothermal method. Titania-modified MCM-41 molecular sieves (TiOa/MeMCM-41, where Me = Si, A! or Ti) were then prepared from tetrabutyltitanate and the calcined MeMCM-41. The prepared materials were characterized by ICP-mass, XRD, XANES, nitrogen adsorption isotherm, TEM, SEM, IR, UV-visible, 29Si and 27A1 MAS-NMR, XPS, and EPR spectroscopy. All the spectroscopic studies resulted that AI and Ti located in the framework, and in the titania-modified samples, a well-defined surface layer of TiO2 appeared on the MCM-41 surface. Infrared spectra of pyridine adsorption showed that AIMCM-41 possesses weak Br6nsted and Lewis acidities, which were highly enhanced by the modified surface titania. The EPR spectra of the TiO2/MeMCM-41 after being reduced by evacuating or in hydrogen at 500~ showed strong signals upon NO adsorption, and exhibited highly catalytic activity for the NO decomposition under UV irradiation. Keywords: TiO2-modified MCM-41, photocatalytic decomposition of NO, EPR
INTRODUCTION Since the photocatalytic splitting of water on titania electrode was discovered by Fujishima and Honda in 1972 [1], a new era in heterogeneous photocatalysis was then begun. TiO2 has been shown to be an excellent light mediator in several photocatalytic processes due to its n-type semiconducting property. As a photocatalyst, TiO2 is attractive for its stability, nontoxicity, low cost, and high reactive properties. However, the effective catalytic activity of TiO2 was restricted due to its low surface area. MCM-41 mesoporous molecular sieves were reported by Mobil Oil Corporation researchers in 1992 [2,3], they have attracted much attention in modification of MCM-41 for adsorption, separation and catalysis. MCM-41 possesses uniform channels varying from about 15 to 100 A with a large surface area up to 1000 m2/g. This makes it a suitable support for transition metal oxides. It has been evident that the catalytic activity of MCM-41 can be promoted by either Ti incorporation in the framework [4-10] or the titaniamodified MCM-41 [1 l-15]. The former method was adding appropriate Ti precursors into the MCM-41 synthesized procedure; Ti species could incorporate into the framework of molecular sieves. The latter can be obtained by grafting titanium onto the inner surface wall of MCM-41 by organometallic precursor. Nitrogen oxides (NOx: NO, N20, NO2) are the seriously harmful atmospheric pollutants, which cause acid rain and photochemical smog. The removal of NOx from the atmosphere, e.g., the direct decomposition of NO into N2 and 02, has been a great challenge to many researchers. In the present report, we prepared two types of Ti-containing MCM-41:TiMCM-41 (Ti in framework) and TiO2/MeMCM-41 (TiO2 grafted on surface, where Me = A1 or Ti, and Si for pure silicious MCM-41). NO adsorption and photodecomposition on the catalysts were investigated by in-situ electron paramagnetic resonance (EPR) spectroscopy.
EXPERIMENTAL Synthesis of Ti-containing MCM-41 MeMCM-41 (Me = A1 or Ti) and pure silicious MCM-41 (SiMCM-41) were synthesized by direct hydrothermal method. A given amount of cetyltrimethylammonium bromide (CTABr, TCI) was dissolved in warm deionized water and then adding with sodium silicate (27% SiO2 and 14% NaOH, Aldrich). H202 (30% in water, Showa) was added into the gel mixture followed by the dropwise addition of aluminum nitrate [Al(NOa)3"9H20, Merck] or tetrabutyltitanate [Ti(OC4H9)4, Merck] aqueous solution as the aluminum
2877 or titanium source, respectively. The molar ratio of the resultant gel was x TiO2 : x A102:30 SiO2:15 CTABr : 1600 H20 (x = 0 or 1). After stirring at room temperature for 1 h, a 1 M H2SO4 aqueous solution was added to the gel mixture dropwisely to adjust the pH value to 10. After keeping overnight, the mixture was loaded into a Teflon-lined stainless steel autoclave and statically heated in an oven at 110~ for 6 days. The resultant solid products were retrieved by filtration, washing with deionized water until the pH value of filtrate decreased to about 9, and drying in air at 110~ All the as-synthesized samples were calcined at 540~ with a heating rate of 5~ in a N2 flow for 1 h and subsequently in an air flow for 6 h. TiO2/MeMCM-41 was prepared by modifying the Zheng's procedures [ 14]. 1 g of the calcined MeMCM41 was placed in a 250 ml round-bottom flask and evacuated to a pressure of 10 -3 torr. After introduction of N2 to dispel air, 50 ml dry hexane (Merck) and 5.0 ml tetrabutyltitanate (TBOT, Merck) were injected into the round-bottom flask, and the mixture was stirred and refluxed at 70~ for 20 h under nitrogen. The solid was separated from the mixture using centrifugation and then washed repeatedly with dry ethanol for 4-5 times until free from TBOT. The solid was then hydrolyzed in a beaker with 100 ml deionized water. The mixture was allowed to stir for 2 h, and the product then washed with deionized water, filtered off and airdried at room temperature. After calcination at 540~ for 2 h in a N2 flow, the white TiO2/MeMCM-41 powder was obtained.
Characterization of the synthesized catalysts The synthesized catalysts were analyzed by inductively coupled plasma-mass spectroscopy (ICP-mass) on a Perkin Elmer Sciex Elan 5000 spectrometer. Powder XRD patterns of the as-synthesized and calcined catalysts were obtained on a Siemens D5000 diffractometer with Cu Ka radiation (40 kV, 30 mA) over the 20 range of 2-10 ~ TiO2/MeMCM-41 powders were also run from 20 to 50 ~ to assess the crystallinity of the TiO2 loading. IR spectra were measured from 0.5 wt% MCM-41 in KBr pellets using a Bomem DA8 spectrometer. The X-ray absorption spectra were performed at the BL17C beamline in the National Syncrotron Radiation Research Center, Taiwan. The Ti K-edge absorption spectra were recorded in the transmission mode at room temperature. Transmission electron microscopy images were obtained on a Hitachi H-7000 microscope. Scanning electron microscopy was observed using a Hitachi S-800 microscope. The specific surface areas of MeMCM-41 and TiOJMeMCM-41 were measured by nitrogen adsorption isotherm at 77 K by the BET method using a Micrometeritics ASAP 2010 surface area analyzer, all samples were degassed at 200~ under vacuum before analysis and the pore size distribution was obtained by the BJH method. The XPS analysis was conducted on an Omicron X-ray photoelectron spectrometer with monochromatic A1 I ~ radiation, and the obtained spectra were correlated to C l s binding energy of 284.5 eV. The diffuse reflectance UV-vis spectra were measured with a Hitachi U3410 spectrophotometer using BaSO4 as a standard. 29Si and 27A1MAS-NMR spectra were performed on a Bruker Avance 300 spectrometer. The nature of acid sites was investigated by in-situ IR experiments using pyridine as the probe molecule. All the samples were prepared as thin pellets of 20 mm in diameter by applying 10 tons pressure. The pellet was placed in an IR cell and evacuated at 500~ to obtain a pressure of 10 .5 torr. 1 tort of pyridine vapor was introduced at 150~ and subsequently evacuated at room temperature. IR spectra were then recorded using a Bomem DA8 FTIR spectrometer.
ln-situ EPR Measurements All samples were dried in a vacuum oven at 120~ for 30 mins before placing in an EPR cell. The EPR cell, as described in our previous paper [16], is designed with a quartz EPR tube on one end and a pyrex reactor on the other end, so that the sample can be treated in-situ for the EPR measurements. The samples were reduced either under directly evacuating or in a hydrogen flow of 25 ml/min followed by evacuating at 500~ to obtain a pressure of 10 .5 tort. After introduction of 1 torr NO, the photoreactions were performed using a Rayonet Photochemical Reactor (model RPR-100) equipped with RPR-2537 A lamps, which gives a 253.7 nm UV source with intensity of 1.65• photons/cm3/sec. All EPR spectra were recorded at 77 K using a Bruker X-band E500CW spectrometer. The g values of EPR signals were measured using a DPPH sample (g = 2.0036) as a reference. RESULTS AND DISCUSSION The ICP results of the calcined MeMCM-41 and TiOjMeMCM-41 (Me = Si, A1 or Ti) samples reveal the atomic ratio of Si/A1 = 20 in A1MCM-41 and Si/Ti = 18 in TiMCM-41, and TiOJSiMCM-41 and TiOz/A1MCM-41 contain about 19.0 and 17.8 wt% of TiO2, respectively.
2878 The powder x-ray diffraction patterns of the as-synthesized SiMCM-41 and the calcined SiMCM-41, A1MCM-41, TiOz/SiMCM-41, and TiOz/A1MCM-41 are shown in Figure 1. All samples present three or four low angle refractions of dl00, d110, d200, and d210, which are characteristics of the hexagonal mesoporous structure. The tubular grains with regular hexagonal array of uniform channels of the calcined catalysts were confirmed by TEM micrographs. After calcination, the intensity of the diffraction peaks of MCM-41 sample increases significantly and slightly shifts to the higher angle-side, as can be seen in Figure 1 (b). The lattice contractions due to the removal of the surfactant template from the channels that caused atomic rearrangement in the calcination process occurred in all samples. The XRD pattems of all the calcined samples shown in Figure 1 (c)-(d) reveal that the regular mesoporous structure was retained either by the introducing A1 or Ti, or by the loading of a TiO2 layer. Apparently, the direct incorporation of Ti or A1 during the hydrothermal synthesis did not cause serious deterioration in the framework order. While the TiO2-modification decreases the crystallinity of MCM-41 that could introduce defects and cause the framework near surface partially collapsed. A weak and broadened XRD signal at around 20 - 25.3 ~ was observed for TiO2/MeMCM-41 samples, that evidenced the grafted titania in a low crystalline anatase form with a particle size of-~13 A as estimated according to the Scherrier equation. IR spectra were performed for structure characterization. All samples exhibit the symmetric stretching vibration band at 795 cm 1 and the asymmetric stretching vibration bands at 1080 cm 1 and 1231 cm 1 of the framework Si-O-Si. Examples as shown in Figure 2 are IR spectra of the calcined TiMCM-41 and TiO2/SiMCM-41. TiMCM-41 presents the Si-O-Ti stretching band at 960 cm 1, which may ascertain the incorporation of Ti into the framework. These results indicate that the incorporated Ti partially substituted Si in the MCM-41 framework during the hydrothermal process. The Si-O-Ti stretching band observed at 950 cm ~ for TiOa/SiMCM-41 is an evidence of TiO2 grafted on the surface [14]. However, absence of the stretching band of Ti-O-Ti at 710 cm ~ in Figure 2 (b) might be due to the low crystalline TiO2 phase [14].
IlOOO -
I0.1 A
. . . .
"7
P !
3
4
5
6
7
8
9 10
20, degree Figure 1. XRD patterns of (a) as-synthesized SiMCM-41 and calcined (b) SiMCM-41, (c) AIMCM-41, (d) TiOz/SiMCM-41, and (e) TiOz/A1MCM-41.
4000
3000
9
i
2000
9
!
'
1000
Wavenumber, cm
-1
Figure 2. IR spectra of calcined (a) TiMCM-41 and (b) TiOz/SiMCM-41.
2879 Table 1. XRD and N2 adsorption data of the calcined SiMCM-41, A1MCM-41, TiMCM-41, TiO2/SiMCM-41, and TiOz/A1MCM-41 catalysts. Sample
SiMCM-41 A1MCM-41 TiMCM-41 TiO2/SiMCM-41 TiO2/A1MCM-41
Before calcinations dloo ao* 39.6 45.7 40.6 46.9 41.3 47.9 -
After calcinations dloo ao* 38.2 44.1 37.4 43.2 35.5 41.1 25.6 36.5 25.2 36.8
Lattic contraction Aao 1.6 3.7 6.7 -
Pore size (A) 27.3 25.5 23.2 25.6 25.2
Wall Pore thickness (A) volume (cm3/g) 16.8 1.194 17.7 1.214 17.9 1.058 !7.4 0.717 18.0 0.888
Surface area (m2/g) 1012 1075 889 902 754
* ao = 2d~oo/~ - the lattice constant The X-ray absorption spectroscopy was carried out to investigate the local structural environment surrounding titanium. Figure 3 shows Ti K-edge XANES spectra of the calcined TiMCM-41, TiO2/SiMCM41, TiO2/A1MCM-41, and Degussa P25 TiO2. The preedge peak was attributed to the transition of the photoelectrons from l s toward t2g states and used as a fingerprint in assessing the local structure around the Ti analogues [18]. The spectrum of TiMCM-41 presents a strong preedge peak at 4.97 keV that was assigned to the tetrahedral coordination for the Ti atoms [18]. It confirms that Ti substitutes Si in the MCM-41 framework in TiMCM-41. Both TiO2-grafted samples, TiO2/SiMCM-41 and TiO2/A1MCM-41, exhibit three preedge peaks similar to those of P25-TiO2, with the greater 4.97 keV peak intensity. It indicates that there are two types of Ti species appeared in the samples, one exhibits symmetric octahedral coordination as TiO2 layer on the surface and the other one located in the tetrahedral sites of the MCM-41 framework that anchored to the surface TiO2. Table 1 summarizes the results of N2 adsorption isotherm at 77 K. The surface areas of SiMCM-41 and A1MCM-41 are over 1000 m2/g. The incorporation of A1 slightly increases surface area and pore volume, but decreases in pore size. The decreases in surface area and pore volume were found in TiMCM-41 that coincides with the findings of Gontier and Tuel [17]. The suppression of pore volumes of TiO2 loaded samples is due to the formation of a TiO2 thin layer on the inner surface wall of MCM-41.
I0.25 A
~ ~0.2
362 n m
(d) 0
(b),]
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m~ml
~
34Ohm
Z (a) 4.94 4.96 4.98 5.00 5.02 5.04
Photon Energy, keV Figure 3. Ti K-edge XANES spectra of (a) TiMCM-41, (b) TiOz/SiMCM-41, (c) TiOz/A1MCM-41, and (d) Degussa P25 TiO2.
(81.)
_ _
i
o
2;0 "360 "3;0 "460 "4;0 500
Wavelength, nm Figure 4. Diffuse reflectance UV-vis spectra of (a) TiMCM-41, (b) TiOz/SiMCM-41, (c) TiOz/A1MCM-41, (d) Degussa P25 YiO2.
2880 Figure 4 shows the diffuse reflectance UV-Vis spectra of TiMCM-41, TiOJSiMCM-41, TiO2/A1MCM41, and Decussa P25 TiO2. The spectrum of TiMCM-41 exhibits an absorption band below 325 nm due to the presence of the framework Ti. TiO2/SiMCM-41 and TiOjA1MCM-41 reveal the band edge of 380 nm and 362 nm, respectively. The curve shape of TiO2-modified MCM-41 is more similar to that of P25-TiO2, and quite different from that of TiMCM-41. The blue shift from the band edge of 412 nm for P25-TiO2 might be due to the quantum size effect [1,12,14]. 29Si NMR spectra of the calcined SiMCM-41 as shown in Figure 5 (a) exhibits three chemical shifts of Q4 [(SiO)4Si], Q3 [(SiO)3Si(OH)], and Q2 [(SiO)2Si(OH)2] environments at-108.8,-101.5, and-91.6 ppm, respectively. The modification of TiO2 on the surface causes a remarkable enhancement at the chemical shift of Q3 and Q2, as shown in Figure 5 (b), that indicates Ti might be grafted as Q3 [(SiO)3Si(OTi)]and Q2 [(SiO)2Si(OTi)2] environments. Moreover, 27A1 NMR spectra of Al-containing samples confirm that all aluminum substitutes silicon in MCM-41 framework. The enhanced intensity of Ti 2p XPS signal of TiO2/SiMCM-41, as compared to that of TiMCM-41, confirms the successful grafted titania layer on the MCM-41 surface, as shown in Figure 6 (A). In Figure 6 (B), the O ls XPS peak at 533.2 eV is assigned to oxygen in Si-O-Si bonds. The O ls spectrum of TiMCM-41 exhibits oxygen in Si-O-Si bonds overlapped with O species in the interfacial Si-O-Ti cross-linking bonds [12]. A contribution from oxygen in Ti-O-Ti bonds is observed at 530.1 eV [12] for TiO2/SiMCM-41 and is an evidence for the grafted TiO2 layer.
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ppm Figure 5.29Si NMR spectra of the calcined (a) SiMCM-41 and (b) TiOJSiMCM-41.
540 B ind
535 ing
530 E nergy,
525 eV
Figure 6. (A) Ti 2p XPS spectra of (a) TiMCM-41 and (b) TiO2/SiMCM-41. (B) O ls XPS spectra of(a) SiMCM-41, (b) TiMCM-41, and (c) TiOz/SiMCM-41.
The surface acidity of each prepared MCM-41catalyst has been monitored by in-situ IR studies of pyridine adsorption. Figure 7 compares the infrared spectra of ~yridine adsorption on TiMCM-41, TiOJSiMCM-41, and TiOJA1MCM-41 in the range 1400-1700 c m . TiMCM-41 exhibits the absorption bands due to hydrogen bonded pyridine at 1445, 1596, and 1605 cm l and Lewis acid site-bonded pyridine at 1577 and 1490 cm -1, Br6nsted acid site was hardly detected. Thus, the framework Ti reveals no Br6nsted acidity, which is consistent with the Alba's report [6]. TiOJSiMCM-41 exhibits stronger absorption peaks as compared to TiMCM-41, and the additional weak bands due to Lewis acidity at 1620 cm -1 and Br6nsted acidity at 1545 and 1636 cm -1. Apparently, the presence of titania on the surface enhanced the acidity of SiMCM-41. TiO2/A1MCM-41 shows even higher intensities of all absorption bands as can be seen in Figure 7 (c). The incorporation of A1 into the framework of MCM-41 indeed generated Br6nsted catalytic acidity
2881 [20,21]. In our results, both Br6nsted and Lewis acidities of Al-containing MCM-41 are significantly enhanced by the grafted titania. Obviously, the relative acidity is given in the order: TiO2/A1MCM-41 > TiO2/SiMCM-41 > TiMCM-41.
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Wavenumber, cm
l@l = 1.920
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-1
Figure 7. IR spectra of pyridine adsorbed on (a) TiMCM41, (b) TiOz/SiMCM-41, (c) TiO2/A1MCM-41.
Field, G Figure 8. EPR spectra of TiO2/SiMCM-41 at 77K after (a) H2 reduction at 500~ (b) exposure of 1 torr NO, and (c) UV irradiation for 1 h.
In-situ EPR experiments were carried out to study the adsorption and photocatalytic decomposition of NO on all Ti-containing MCM-41 catalysts. Figure 8 (a) shows the EPR spectrum a t 77 K of TiOz/SiMCM-41 after being reduced in hydrogen flow at 500~ for 2 h followed by evacuation for 1 h. A strong EPR signal of an axial g tensor with g• = 1.972 and gll = 1.920 was observed, which is attributed to Ti 3+ in distorted octahedral symmetry. The EPR signals of O2- at gl = 2.022, g2 = 2.012, and g3 = 2.002 and a weak F-center were also observed. After introduction of 1 torr NO to the reduced sample, the EPR spectrum [Figure 6 (b)] showed an intense signal at g• = 2.003 and gll = 1.931 with a hyperfine constant A l = 28 G due to N (I = 1), that were attributed to the NO adsorbed on Ti 3+. After UV irradiation with wavelength of 253.7 nm for 1 h, the EPR signal of NO adsorption completely disappeared, as shown in Figure 8 (c). For TiOJA1MCM-41, with the sample being activated by evacuation at 500~ for 2 h, the EPR spectrum at 77 K exhibited a stronger O2- and overlapped with an F-center as well as a relatively weak Ti 3+ signals as shown in Figure 9 (a). While after being reduced in hydrogen at 500~ the Ti 3+ EPR signal of TiO2/A1MCM-41 was highly enhanced as can be seen in Figure 10 (a). Upon introduction of NO, the EPR spectrum of the high temperature evacuated sample exhibited significant NO adsorption signals, as shown in Figure 9 (b), that are due to NO adsorbed on A13+ and Ti 3+ with a hyperfine parameters A• = 17 and 30 G, respectively. The greater signal intensity upon NO adsorption on TiO2/A1MCM-41 than on TiOJSiMCM-41 indicated that the NO adsorption was highly enhanced by the presence of framework A1. After UV irradiation for 1 h, the EPR spectra indicated some NO still retained adsorbed on the evacuated TiO2/A1MCM-41 sample that disappeared after the further irradiation for 1 h. Apparently, NO adsorption on A13§ species was significant in the high temperature evacuated sample, that was not responsible for NO photodecomposition. The EPR spectrum of NO adsorbed on the H2-reduced TiO2/A1MCM-41 showed that NO adsorbed mostly on Ti 3+ sites, as shown in Figure 10 (b), that was completely suppressed after UV irradiation for 1 h. The gases in the EPR cell were finally analyzed with quadrapole mass residual gas analyzer. The conversion of photocatalytic decomposition of NO was nearly 100% and the product
2882 selectivities of N2 and N20 were 60.7% and 39.3%, respectively. For a TiO2/MeMCM-41 catalyst, the H2reduced sample showed greater photocatalytic activity than the high temperature evacuated one. It has been suggested that NO can be used as a probe molecule for characterization of Lewis acidity [19]. NO is recognized to adsorb on Lewis site and form a complex, such as NO-AI + complex in H-ZSM-5 zeolite. In our experiments, the IR spectra of pyridine adsorption evidenced the Lewis acidity for all MeMCM-41 and TiO2/MeMCM-41 samples. Both AI 3+ and Ti 3+ species should be capable for NO adsorption. However, the EPR spectra of the AIMCM-41, as well as TiMAM-41, only present an F-center signal after NO adsorption, the NO adsorption spectra are hardly detected on these samples.
~20k
IDPPH
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-
IDPPH
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_
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-
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.
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32bo33bo34bo35 bo 36bo
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Field, 13
Field, G
Figure 9. EPR spectra of TiO2/A1MCM-41 at 77K after (a) evacuating at 500~ (b) exposure of I ton" NO, and UV irradiation for l h (c) and 2 h (d).
Figure 10. EPR spectra of TiO2/AIMCM-41 at 77K after (a) H2 reduction at 500~ (b) exposure of 1 torr NO, and (c) UV irradiation 235.7 nm for 1 h.
It is well established that Ti 3+ defect sites can be created on the TiO2(110) surface by thermal annealing [20]. Heating TiO2 to high temperatures can induce desorption of surface oxygen. One missing O atom at the bridging oxygen site leaves a vacancy in TiO2 lattice and then two subsurface Ti 3+ sites are exposed. Therefore, Ti 3+ species on the TiO2 surface can act as Lewis acid sites as similar as AI 3+ species. NO molecules adsorb on both Ti 3+ and Al 3+ sites near the surface of catalyst to form Ti3+-NO and A13+-NO species. TiO2 creates electron-hole pairs during UV irradiation to undergo electron transfer to the antibonding orbital of NO. Subsequently, the dissociative adsorbed NO species lead to the formation of N20 and N2, and leave oxygen retained to lattice vacancies near the surface of catalysts. Since Ti 3+ is considered as the active site for photodecomposition of NO, the higher concentration of Ti 3~ produced by hydrogenreduced sample should lead to a higher photocatalytic activity. Consequently, the photocatalytic activity of the hydrogen-reduced catalysts is greater than that of the high temperature evacuated one, and the photocatalytic activity of Ti-containing MCM-41 is in the order: TiO2/AIMCM-41 > TiO2/SiMCM-41 > P25 TiO2 > TiMCM-41. CONCLUSIONS
All MeMCM-41 and TiO2/MeMCM-41 (Me = Si, A1 or Ti) molecular sieves prepared in this study exhibit well-defined MCM-41 crystalline structures and high surface areas. The spectroscopic studies resulted that AI and Ti located in the framework, and in the titania-modified samples, TiO2 grafted as a thin
2883 layer on the surface of MCM-41. The acidity of MeMCM-41 is remarkably enhanced by the grafted TiO2 layer, and further improved the photocatalytic activity. It elucidated that titania-grafted MCM-41 is a promising catalyst for the photodecomposition of NO. The activity for photocatalytic decomposition of NO was highly correlated with the acidity, the reducibility and the electronic structure of the active state of titanium in the catalyst. ACKNOWLEDGEMENTS
We gratefully acknowledge financial support from the National Science Council of the Republic of China and would like to thank Dr. S.M. Lin for helping in XANES measurements. REFERENCES
1. Linsebigler, A.L., Lu, G., Yates, Jr., J.T., Chem. Rev., 95 (1995), 735-758. 2. Kresge, C.T., Leonowicz, M.E., Roth, W.J., Vartuli, J.C., Beck, J.S., Nature, 359 (1992), 710-712. 3. Beck, J.S., Vartuli, J.C., Roth, W.J., Leonowicz, M.E., Kresge, C.T., Schmitt, K.D., Chu, C.T-W., Olson, D.H., Sheppard, E.W., McCullen, S.B., Higgins, J.B., Schlenker, J.L., J. Am. Chem. Soc., 114 (1992), 10834-10843. 4. Tanev, P.T., Chibwe, M., Pinnavaia, T., Nature, 368 (1994), 321-323. 5. Blasco, T., Corma, A., Navarro, M.T., P6rez Pariente, J., J. Catal., 156 (1995) 65-74. 6. Alba, M.D., Luan, Z., Klinowski, J., J. Phys. Chem., 100 (1996) 2178-2182. 7. Prakash, A.M., Sung-Suh, H.M., Kevan, L., J. Phys. Chem. B, 102 (1998) 857-864. 8. Occelli, M., Biz, S., Auroux, A., Appl. Catal. A, 183 (1999) 231-239. 9. Bal, R., Chaudhari, K., Srinivas, D., Sivasanker, S., Ratnasamy, P., J. Mol. Catal. A-Chem., 162 (2000) 199-207. 10. Laha, S.C., Kumar, R., Micropor. Mesopor. Mat., 53 (2002) 163-177. 11. Maschmeyer, T., Rey, F., Sankar, G., Thomas, J.M., Nature, 378 (1995), 159-162. 12. Aronson, B.J., Blanford, C.F., Stein, A., Chem. Mater., 9 (1997) 1849-1855. 13. Oldroyd, D., Sankar, G., Thomas, J.M., Ozkaya, J. Phys. Chem. B, 102 (1998), 1849-1855. 14. Zheng, S., Gao, L., Zhang, Q., Guo, J., J. Mater. Chem., 10 (2000), 723-727. 15. Zheng, S., Gao, L., Zhang, Q., Zhang, W., Guo, J., J. Mater. Chem., 11 (2001), 578-583. 16. Chien, S.H., Wei, Y.W., Lin, M.C., J. Chin. Chem. Soc., 44 (1997) 195. 17. Gontier, S., Tuel, A., Zeolites, 15 (1995) 601-610. 18. Rhee, C.H., Lee, J.S., Catal. Today, 38 (1997) 2!3-219. 19. Gutsze, A., Plato, M., Karge, H.G., Witzel, F., J. Chem. Soc., Faraday Trans., 92 (1996) 2495-2498. 20. Lu, G., Linsebigler, A., Yates, Jr., J.T., J. Phys. Chem., 98 (1994) 11733-11738.
2876
PHOTOCATALYTIC DECOMPOSITION OF NITRIC OXIDE ON TiO2_MODIFIED MCM-41 CATALYSTS Chien, S.*, Huang, K. and Kuo, M. Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan, and Department of Chemistry, National Taiwan University, Taipei 10764, Taiwan. E-mail: [email protected]
ABSTRACT Mesoporous siliceous MCM-41 (SiMCM-41) molecular sieves with silicon partially substituted by A1 and Ti were synthesized by direct hydrothermal method. Titania-modified MCM-41 molecular sieves (TiOa/MeMCM-41, where Me = Si, A! or Ti) were then prepared from tetrabutyltitanate and the calcined MeMCM-41. The prepared materials were characterized by ICP-mass, XRD, XANES, nitrogen adsorption isotherm, TEM, SEM, IR, UV-visible, 29Si and 27A1 MAS-NMR, XPS, and EPR spectroscopy. All the spectroscopic studies resulted that AI and Ti located in the framework, and in the titania-modified samples, a well-defined surface layer of TiO2 appeared on the MCM-41 surface. Infrared spectra of pyridine adsorption showed that AIMCM-41 possesses weak Br6nsted and Lewis acidities, which were highly enhanced by the modified surface titania. The EPR spectra of the TiO2/MeMCM-41 after being reduced by evacuating or in hydrogen at 500~ showed strong signals upon NO adsorption, and exhibited highly catalytic activity for the NO decomposition under UV irradiation. Keywords: TiO2-modified MCM-41, photocatalytic decomposition of NO, EPR
INTRODUCTION Since the photocatalytic splitting of water on titania electrode was discovered by Fujishima and Honda in 1972 [1], a new era in heterogeneous photocatalysis was then begun. TiO2 has been shown to be an excellent light mediator in several photocatalytic processes due to its n-type semiconducting property. As a photocatalyst, TiO2 is attractive for its stability, nontoxicity, low cost, and high reactive properties. However, the effective catalytic activity of TiO2 was restricted due to its low surface area. MCM-41 mesoporous molecular sieves were reported by Mobil Oil Corporation researchers in 1992 [2,3], they have attracted much attention in modification of MCM-41 for adsorption, separation and catalysis. MCM-41 possesses uniform channels varying from about 15 to 100 A with a large surface area up to 1000 m2/g. This makes it a suitable support for transition metal oxides. It has been evident that the catalytic activity of MCM-41 can be promoted by either Ti incorporation in the framework [4-10] or the titaniamodified MCM-41 [1 l-15]. The former method was adding appropriate Ti precursors into the MCM-41 synthesized procedure; Ti species could incorporate into the framework of molecular sieves. The latter can be obtained by grafting titanium onto the inner surface wall of MCM-41 by organometallic precursor. Nitrogen oxides (NOx: NO, N20, NO2) are the seriously harmful atmospheric pollutants, which cause acid rain and photochemical smog. The removal of NOx from the atmosphere, e.g., the direct decomposition of NO into N2 and 02, has been a great challenge to many researchers. In the present report, we prepared two types of Ti-containing MCM-41:TiMCM-41 (Ti in framework) and TiO2/MeMCM-41 (TiO2 grafted on surface, where Me = A1 or Ti, and Si for pure silicious MCM-41). NO adsorption and photodecomposition on the catalysts were investigated by in-situ electron paramagnetic resonance (EPR) spectroscopy.
EXPERIMENTAL Synthesis of Ti-containing MCM-41 MeMCM-41 (Me = A1 or Ti) and pure silicious MCM-41 (SiMCM-41) were synthesized by direct hydrothermal method. A given amount of cetyltrimethylammonium bromide (CTABr, TCI) was dissolved in warm deionized water and then adding with sodium silicate (27% SiO2 and 14% NaOH, Aldrich). H202 (30% in water, Showa) was added into the gel mixture followed by the dropwise addition of aluminum nitrate [Al(NOa)3"9H20, Merck] or tetrabutyltitanate [Ti(OC4H9)4, Merck] aqueous solution as the aluminum
2877 or titanium source, respectively. The molar ratio of the resultant gel was x TiO2 : x A102:30 SiO2:15 CTABr : 1600 H20 (x = 0 or 1). After stirring at room temperature for 1 h, a 1 M H2SO4 aqueous solution was added to the gel mixture dropwisely to adjust the pH value to 10. After keeping overnight, the mixture was loaded into a Teflon-lined stainless steel autoclave and statically heated in an oven at 110~ for 6 days. The resultant solid products were retrieved by filtration, washing with deionized water until the pH value of filtrate decreased to about 9, and drying in air at 110~ All the as-synthesized samples were calcined at 540~ with a heating rate of 5~ in a N2 flow for 1 h and subsequently in an air flow for 6 h. TiO2/MeMCM-41 was prepared by modifying the Zheng's procedures [ 14]. 1 g of the calcined MeMCM41 was placed in a 250 ml round-bottom flask and evacuated to a pressure of 10 -3 torr. After introduction of N2 to dispel air, 50 ml dry hexane (Merck) and 5.0 ml tetrabutyltitanate (TBOT, Merck) were injected into the round-bottom flask, and the mixture was stirred and refluxed at 70~ for 20 h under nitrogen. The solid was separated from the mixture using centrifugation and then washed repeatedly with dry ethanol for 4-5 times until free from TBOT. The solid was then hydrolyzed in a beaker with 100 ml deionized water. The mixture was allowed to stir for 2 h, and the product then washed with deionized water, filtered off and airdried at room temperature. After calcination at 540~ for 2 h in a N2 flow, the white TiO2/MeMCM-41 powder was obtained.
Characterization of the synthesized catalysts The synthesized catalysts were analyzed by inductively coupled plasma-mass spectroscopy (ICP-mass) on a Perkin Elmer Sciex Elan 5000 spectrometer. Powder XRD patterns of the as-synthesized and calcined catalysts were obtained on a Siemens D5000 diffractometer with Cu Ka radiation (40 kV, 30 mA) over the 20 range of 2-10 ~ TiO2/MeMCM-41 powders were also run from 20 to 50 ~ to assess the crystallinity of the TiO2 loading. IR spectra were measured from 0.5 wt% MCM-41 in KBr pellets using a Bomem DA8 spectrometer. The X-ray absorption spectra were performed at the BL17C beamline in the National Syncrotron Radiation Research Center, Taiwan. The Ti K-edge absorption spectra were recorded in the transmission mode at room temperature. Transmission electron microscopy images were obtained on a Hitachi H-7000 microscope. Scanning electron microscopy was observed using a Hitachi S-800 microscope. The specific surface areas of MeMCM-41 and TiOJMeMCM-41 were measured by nitrogen adsorption isotherm at 77 K by the BET method using a Micrometeritics ASAP 2010 surface area analyzer, all samples were degassed at 200~ under vacuum before analysis and the pore size distribution was obtained by the BJH method. The XPS analysis was conducted on an Omicron X-ray photoelectron spectrometer with monochromatic A1 I ~ radiation, and the obtained spectra were correlated to C l s binding energy of 284.5 eV. The diffuse reflectance UV-vis spectra were measured with a Hitachi U3410 spectrophotometer using BaSO4 as a standard. 29Si and 27A1MAS-NMR spectra were performed on a Bruker Avance 300 spectrometer. The nature of acid sites was investigated by in-situ IR experiments using pyridine as the probe molecule. All the samples were prepared as thin pellets of 20 mm in diameter by applying 10 tons pressure. The pellet was placed in an IR cell and evacuated at 500~ to obtain a pressure of 10 .5 torr. 1 tort of pyridine vapor was introduced at 150~ and subsequently evacuated at room temperature. IR spectra were then recorded using a Bomem DA8 FTIR spectrometer.
ln-situ EPR Measurements All samples were dried in a vacuum oven at 120~ for 30 mins before placing in an EPR cell. The EPR cell, as described in our previous paper [16], is designed with a quartz EPR tube on one end and a pyrex reactor on the other end, so that the sample can be treated in-situ for the EPR measurements. The samples were reduced either under directly evacuating or in a hydrogen flow of 25 ml/min followed by evacuating at 500~ to obtain a pressure of 10 .5 tort. After introduction of 1 torr NO, the photoreactions were performed using a Rayonet Photochemical Reactor (model RPR-100) equipped with RPR-2537 A lamps, which gives a 253.7 nm UV source with intensity of 1.65• photons/cm3/sec. All EPR spectra were recorded at 77 K using a Bruker X-band E500CW spectrometer. The g values of EPR signals were measured using a DPPH sample (g = 2.0036) as a reference. RESULTS AND DISCUSSION The ICP results of the calcined MeMCM-41 and TiOjMeMCM-41 (Me = Si, A1 or Ti) samples reveal the atomic ratio of Si/A1 = 20 in A1MCM-41 and Si/Ti = 18 in TiMCM-41, and TiOJSiMCM-41 and TiOz/A1MCM-41 contain about 19.0 and 17.8 wt% of TiO2, respectively.
2878 The powder x-ray diffraction patterns of the as-synthesized SiMCM-41 and the calcined SiMCM-41, A1MCM-41, TiOz/SiMCM-41, and TiOz/A1MCM-41 are shown in Figure 1. All samples present three or four low angle refractions of dl00, d110, d200, and d210, which are characteristics of the hexagonal mesoporous structure. The tubular grains with regular hexagonal array of uniform channels of the calcined catalysts were confirmed by TEM micrographs. After calcination, the intensity of the diffraction peaks of MCM-41 sample increases significantly and slightly shifts to the higher angle-side, as can be seen in Figure 1 (b). The lattice contractions due to the removal of the surfactant template from the channels that caused atomic rearrangement in the calcination process occurred in all samples. The XRD pattems of all the calcined samples shown in Figure 1 (c)-(d) reveal that the regular mesoporous structure was retained either by the introducing A1 or Ti, or by the loading of a TiO2 layer. Apparently, the direct incorporation of Ti or A1 during the hydrothermal synthesis did not cause serious deterioration in the framework order. While the TiO2-modification decreases the crystallinity of MCM-41 that could introduce defects and cause the framework near surface partially collapsed. A weak and broadened XRD signal at around 20 - 25.3 ~ was observed for TiO2/MeMCM-41 samples, that evidenced the grafted titania in a low crystalline anatase form with a particle size of-~13 A as estimated according to the Scherrier equation. IR spectra were performed for structure characterization. All samples exhibit the symmetric stretching vibration band at 795 cm 1 and the asymmetric stretching vibration bands at 1080 cm 1 and 1231 cm 1 of the framework Si-O-Si. Examples as shown in Figure 2 are IR spectra of the calcined TiMCM-41 and TiO2/SiMCM-41. TiMCM-41 presents the Si-O-Ti stretching band at 960 cm 1, which may ascertain the incorporation of Ti into the framework. These results indicate that the incorporated Ti partially substituted Si in the MCM-41 framework during the hydrothermal process. The Si-O-Ti stretching band observed at 950 cm ~ for TiOa/SiMCM-41 is an evidence of TiO2 grafted on the surface [14]. However, absence of the stretching band of Ti-O-Ti at 710 cm ~ in Figure 2 (b) might be due to the low crystalline TiO2 phase [14].
IlOOO -
I0.1 A
. . . .
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3
4
5
6
7
8
9 10
20, degree Figure 1. XRD patterns of (a) as-synthesized SiMCM-41 and calcined (b) SiMCM-41, (c) AIMCM-41, (d) TiOz/SiMCM-41, and (e) TiOz/A1MCM-41.
4000
3000
9
i
2000
9
!
'
1000
Wavenumber, cm
-1
Figure 2. IR spectra of calcined (a) TiMCM-41 and (b) TiOz/SiMCM-41.
2879 Table 1. XRD and N2 adsorption data of the calcined SiMCM-41, A1MCM-41, TiMCM-41, TiO2/SiMCM-41, and TiOz/A1MCM-41 catalysts. Sample
SiMCM-41 A1MCM-41 TiMCM-41 TiO2/SiMCM-41 TiO2/A1MCM-41
Before calcinations dloo ao* 39.6 45.7 40.6 46.9 41.3 47.9 -
After calcinations dloo ao* 38.2 44.1 37.4 43.2 35.5 41.1 25.6 36.5 25.2 36.8
Lattic contraction Aao 1.6 3.7 6.7 -
Pore size (A) 27.3 25.5 23.2 25.6 25.2
Wall Pore thickness (A) volume (cm3/g) 16.8 1.194 17.7 1.214 17.9 1.058 !7.4 0.717 18.0 0.888
Surface area (m2/g) 1012 1075 889 902 754
* ao = 2d~oo/~ - the lattice constant The X-ray absorption spectroscopy was carried out to investigate the local structural environment surrounding titanium. Figure 3 shows Ti K-edge XANES spectra of the calcined TiMCM-41, TiO2/SiMCM41, TiO2/A1MCM-41, and Degussa P25 TiO2. The preedge peak was attributed to the transition of the photoelectrons from l s toward t2g states and used as a fingerprint in assessing the local structure around the Ti analogues [18]. The spectrum of TiMCM-41 presents a strong preedge peak at 4.97 keV that was assigned to the tetrahedral coordination for the Ti atoms [18]. It confirms that Ti substitutes Si in the MCM-41 framework in TiMCM-41. Both TiO2-grafted samples, TiO2/SiMCM-41 and TiO2/A1MCM-41, exhibit three preedge peaks similar to those of P25-TiO2, with the greater 4.97 keV peak intensity. It indicates that there are two types of Ti species appeared in the samples, one exhibits symmetric octahedral coordination as TiO2 layer on the surface and the other one located in the tetrahedral sites of the MCM-41 framework that anchored to the surface TiO2. Table 1 summarizes the results of N2 adsorption isotherm at 77 K. The surface areas of SiMCM-41 and A1MCM-41 are over 1000 m2/g. The incorporation of A1 slightly increases surface area and pore volume, but decreases in pore size. The decreases in surface area and pore volume were found in TiMCM-41 that coincides with the findings of Gontier and Tuel [17]. The suppression of pore volumes of TiO2 loaded samples is due to the formation of a TiO2 thin layer on the inner surface wall of MCM-41.
I0.25 A
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Z (a) 4.94 4.96 4.98 5.00 5.02 5.04
Photon Energy, keV Figure 3. Ti K-edge XANES spectra of (a) TiMCM-41, (b) TiOz/SiMCM-41, (c) TiOz/A1MCM-41, and (d) Degussa P25 TiO2.
(81.)
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i
o
2;0 "360 "3;0 "460 "4;0 500
Wavelength, nm Figure 4. Diffuse reflectance UV-vis spectra of (a) TiMCM-41, (b) TiOz/SiMCM-41, (c) TiOz/A1MCM-41, (d) Degussa P25 YiO2.
2880 Figure 4 shows the diffuse reflectance UV-Vis spectra of TiMCM-41, TiOJSiMCM-41, TiO2/A1MCM41, and Decussa P25 TiO2. The spectrum of TiMCM-41 exhibits an absorption band below 325 nm due to the presence of the framework Ti. TiO2/SiMCM-41 and TiOjA1MCM-41 reveal the band edge of 380 nm and 362 nm, respectively. The curve shape of TiO2-modified MCM-41 is more similar to that of P25-TiO2, and quite different from that of TiMCM-41. The blue shift from the band edge of 412 nm for P25-TiO2 might be due to the quantum size effect [1,12,14]. 29Si NMR spectra of the calcined SiMCM-41 as shown in Figure 5 (a) exhibits three chemical shifts of Q4 [(SiO)4Si], Q3 [(SiO)3Si(OH)], and Q2 [(SiO)2Si(OH)2] environments at-108.8,-101.5, and-91.6 ppm, respectively. The modification of TiO2 on the surface causes a remarkable enhancement at the chemical shift of Q3 and Q2, as shown in Figure 5 (b), that indicates Ti might be grafted as Q3 [(SiO)3Si(OTi)]and Q2 [(SiO)2Si(OTi)2] environments. Moreover, 27A1 NMR spectra of Al-containing samples confirm that all aluminum substitutes silicon in MCM-41 framework. The enhanced intensity of Ti 2p XPS signal of TiO2/SiMCM-41, as compared to that of TiMCM-41, confirms the successful grafted titania layer on the MCM-41 surface, as shown in Figure 6 (A). In Figure 6 (B), the O ls XPS peak at 533.2 eV is assigned to oxygen in Si-O-Si bonds. The O ls spectrum of TiMCM-41 exhibits oxygen in Si-O-Si bonds overlapped with O species in the interfacial Si-O-Ti cross-linking bonds [12]. A contribution from oxygen in Ti-O-Ti bonds is observed at 530.1 eV [12] for TiO2/SiMCM-41 and is an evidence for the grafted TiO2 layer.
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535 ing
530 E nergy,
525 eV
Figure 6. (A) Ti 2p XPS spectra of (a) TiMCM-41 and (b) TiO2/SiMCM-41. (B) O ls XPS spectra of(a) SiMCM-41, (b) TiMCM-41, and (c) TiOz/SiMCM-41.
The surface acidity of each prepared MCM-41catalyst has been monitored by in-situ IR studies of pyridine adsorption. Figure 7 compares the infrared spectra of ~yridine adsorption on TiMCM-41, TiOJSiMCM-41, and TiOJA1MCM-41 in the range 1400-1700 c m . TiMCM-41 exhibits the absorption bands due to hydrogen bonded pyridine at 1445, 1596, and 1605 cm l and Lewis acid site-bonded pyridine at 1577 and 1490 cm -1, Br6nsted acid site was hardly detected. Thus, the framework Ti reveals no Br6nsted acidity, which is consistent with the Alba's report [6]. TiOJSiMCM-41 exhibits stronger absorption peaks as compared to TiMCM-41, and the additional weak bands due to Lewis acidity at 1620 cm -1 and Br6nsted acidity at 1545 and 1636 cm -1. Apparently, the presence of titania on the surface enhanced the acidity of SiMCM-41. TiO2/A1MCM-41 shows even higher intensities of all absorption bands as can be seen in Figure 7 (c). The incorporation of A1 into the framework of MCM-41 indeed generated Br6nsted catalytic acidity
2881 [20,21]. In our results, both Br6nsted and Lewis acidities of Al-containing MCM-41 are significantly enhanced by the grafted titania. Obviously, the relative acidity is given in the order: TiO2/A1MCM-41 > TiO2/SiMCM-41 > TiMCM-41.
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Figure 7. IR spectra of pyridine adsorbed on (a) TiMCM41, (b) TiOz/SiMCM-41, (c) TiO2/A1MCM-41.
Field, G Figure 8. EPR spectra of TiO2/SiMCM-41 at 77K after (a) H2 reduction at 500~ (b) exposure of 1 torr NO, and (c) UV irradiation for 1 h.
In-situ EPR experiments were carried out to study the adsorption and photocatalytic decomposition of NO on all Ti-containing MCM-41 catalysts. Figure 8 (a) shows the EPR spectrum a t 77 K of TiOz/SiMCM-41 after being reduced in hydrogen flow at 500~ for 2 h followed by evacuation for 1 h. A strong EPR signal of an axial g tensor with g• = 1.972 and gll = 1.920 was observed, which is attributed to Ti 3+ in distorted octahedral symmetry. The EPR signals of O2- at gl = 2.022, g2 = 2.012, and g3 = 2.002 and a weak F-center were also observed. After introduction of 1 torr NO to the reduced sample, the EPR spectrum [Figure 6 (b)] showed an intense signal at g• = 2.003 and gll = 1.931 with a hyperfine constant A l = 28 G due to N (I = 1), that were attributed to the NO adsorbed on Ti 3+. After UV irradiation with wavelength of 253.7 nm for 1 h, the EPR signal of NO adsorption completely disappeared, as shown in Figure 8 (c). For TiOJA1MCM-41, with the sample being activated by evacuation at 500~ for 2 h, the EPR spectrum at 77 K exhibited a stronger O2- and overlapped with an F-center as well as a relatively weak Ti 3+ signals as shown in Figure 9 (a). While after being reduced in hydrogen at 500~ the Ti 3+ EPR signal of TiO2/A1MCM-41 was highly enhanced as can be seen in Figure 10 (a). Upon introduction of NO, the EPR spectrum of the high temperature evacuated sample exhibited significant NO adsorption signals, as shown in Figure 9 (b), that are due to NO adsorbed on A13+ and Ti 3+ with a hyperfine parameters A• = 17 and 30 G, respectively. The greater signal intensity upon NO adsorption on TiO2/A1MCM-41 than on TiOJSiMCM-41 indicated that the NO adsorption was highly enhanced by the presence of framework A1. After UV irradiation for 1 h, the EPR spectra indicated some NO still retained adsorbed on the evacuated TiO2/A1MCM-41 sample that disappeared after the further irradiation for 1 h. Apparently, NO adsorption on A13§ species was significant in the high temperature evacuated sample, that was not responsible for NO photodecomposition. The EPR spectrum of NO adsorbed on the H2-reduced TiO2/A1MCM-41 showed that NO adsorbed mostly on Ti 3+ sites, as shown in Figure 10 (b), that was completely suppressed after UV irradiation for 1 h. The gases in the EPR cell were finally analyzed with quadrapole mass residual gas analyzer. The conversion of photocatalytic decomposition of NO was nearly 100% and the product
2882 selectivities of N2 and N20 were 60.7% and 39.3%, respectively. For a TiO2/MeMCM-41 catalyst, the H2reduced sample showed greater photocatalytic activity than the high temperature evacuated one. It has been suggested that NO can be used as a probe molecule for characterization of Lewis acidity [19]. NO is recognized to adsorb on Lewis site and form a complex, such as NO-AI + complex in H-ZSM-5 zeolite. In our experiments, the IR spectra of pyridine adsorption evidenced the Lewis acidity for all MeMCM-41 and TiO2/MeMCM-41 samples. Both AI 3+ and Ti 3+ species should be capable for NO adsorption. However, the EPR spectra of the AIMCM-41, as well as TiMAM-41, only present an F-center signal after NO adsorption, the NO adsorption spectra are hardly detected on these samples.
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IDPPH
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-
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.
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Field, G
Figure 9. EPR spectra of TiO2/A1MCM-41 at 77K after (a) evacuating at 500~ (b) exposure of I ton" NO, and UV irradiation for l h (c) and 2 h (d).
Figure 10. EPR spectra of TiO2/AIMCM-41 at 77K after (a) H2 reduction at 500~ (b) exposure of 1 torr NO, and (c) UV irradiation 235.7 nm for 1 h.
It is well established that Ti 3+ defect sites can be created on the TiO2(110) surface by thermal annealing [20]. Heating TiO2 to high temperatures can induce desorption of surface oxygen. One missing O atom at the bridging oxygen site leaves a vacancy in TiO2 lattice and then two subsurface Ti 3+ sites are exposed. Therefore, Ti 3+ species on the TiO2 surface can act as Lewis acid sites as similar as AI 3+ species. NO molecules adsorb on both Ti 3+ and Al 3+ sites near the surface of catalyst to form Ti3+-NO and A13+-NO species. TiO2 creates electron-hole pairs during UV irradiation to undergo electron transfer to the antibonding orbital of NO. Subsequently, the dissociative adsorbed NO species lead to the formation of N20 and N2, and leave oxygen retained to lattice vacancies near the surface of catalysts. Since Ti 3+ is considered as the active site for photodecomposition of NO, the higher concentration of Ti 3~ produced by hydrogenreduced sample should lead to a higher photocatalytic activity. Consequently, the photocatalytic activity of the hydrogen-reduced catalysts is greater than that of the high temperature evacuated one, and the photocatalytic activity of Ti-containing MCM-41 is in the order: TiO2/AIMCM-41 > TiO2/SiMCM-41 > P25 TiO2 > TiMCM-41. CONCLUSIONS
All MeMCM-41 and TiO2/MeMCM-41 (Me = Si, A1 or Ti) molecular sieves prepared in this study exhibit well-defined MCM-41 crystalline structures and high surface areas. The spectroscopic studies resulted that AI and Ti located in the framework, and in the titania-modified samples, TiO2 grafted as a thin
2883 layer on the surface of MCM-41. The acidity of MeMCM-41 is remarkably enhanced by the grafted TiO2 layer, and further improved the photocatalytic activity. It elucidated that titania-grafted MCM-41 is a promising catalyst for the photodecomposition of NO. The activity for photocatalytic decomposition of NO was highly correlated with the acidity, the reducibility and the electronic structure of the active state of titanium in the catalyst. ACKNOWLEDGEMENTS
We gratefully acknowledge financial support from the National Science Council of the Republic of China and would like to thank Dr. S.M. Lin for helping in XANES measurements. REFERENCES
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