Characterization of Titania Loaded V-, Fe-, and Cr-Incorporated MCM

Electronic structure and optical properties of anatase doped with bismuth and carbon. V. M. Zainullina , V. P. Zhukov. Physics of the Solid State 2013...
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J. Phys. Chem. B 2002, 106, 3394-3401

Characterization of Titania Loaded V-, Fe-, and Cr-Incorporated MCM-41 by XRD, TPR, UV-vis, Raman, and XPS Techniques Ettireddy P. Reddy, Lev Davydov, and Panagiotis G. Smirniotis* Department of Chemical Engineering, UniVersity of Cincinnati, Cincinnati, Ohio 45221-0171 ReceiVed: October 18, 2001; In Final Form: January 17, 2002

Transition metal (Me ) V, Fe, and Cr) incorporated into MCM-41 mesoporous molecular sieves (Si/Me ) 80) have been synthesized by hydrothermal methods and were loaded with TiO2 utilizing a sol-gel technique. These materials were found (refs 22, 23) to be photoactive for the destruction of organics with visible light. A combination of various physicochemical techniques such as N2 physisorption, O2 chemisorption, diffuse reflectance UV-vis (DR-UV-vis), X-ray diffraction (XRD), Raman, temperature program reduction (TPR), and X-ray photoelectron spectroscopy (XPS) were used to characterize the chemical environment of these transition metals in the prepared photocatalysts. The dispersion of transition metals as determined by O2 chemisorption suggests that they are well dispersed inside the MCM-41 framework, but the dispersion values decreased with the loading of TiO2. This indicates that the loaded titania promotes the transformation of incorporated metal ions into different phases. DR-UV-vis spectra of the Me-Ti-MCM-41 materials exhibit substantial absorption of visible light in the range of 400-600 nm. However, the same materials loaded with titania show higher absorption in the UV range (250-400 nm) because of the presence of titania. XRD patterns of Me-Ti-MCM-41 are similar to that of siliceous MCM-41 and demonstrate that the transition metals are atomically dispersed in the framework. The titania loaded onto the Me-Ti-MCM-41 was of low anatase crystallinity as shown by Raman. The TPR results for Me-Ti-MCM-41 revealed a lower number of reduction transitions than the titania loaded Me-Ti-MCM-41. These reduction transitions depend on the nature of transition metal species in the MCM-41 framework. The Me/Ti and Me/Si surface atomic ratios, which are determined by XPS measurements, reveal that considerable diffusion of transition metal ions to the surface occurs upon loading of titania. The XPS line shapes, binding energies, and surface atomic ratios for Me-Ti-MCM-41 indicated one type of surface electronic level such Me-O-Si, whereas two types of surface electronic levels were found in the case of 25% TiO2/Me-Ti-MCM-41, which corresponds to MeO-Si and Me-O-Ti. The Me/Si and Me/Ti surface atomic ratios show that the incorporated transition metals interact preferable with the loaded titania as in the form of Me-O-Ti heterojunction instead of staying inside the framework as in the form of Me-O-Si electronic level.

Introduction Mesoporous materials are of great interest to catalysis because of their large and uniform pore size (15∼100 Å) and their potential promise as effective absorbents and catalysts in processes which allow stereo-hindered molecules facile diffusion to internal active sites. The discovery of the M41S family of mesoporous molecular sieves was reported by Beck and coworkers.1,2 A great amount of research has been devoted to the well-defined mesoporous molecular sieves that belong to the M41S family.2-5 One form of this series, MCM-41, which possessed a uniform arrangement of hexagonally shaped mesopores of diameter varying from 20 to 100 Å, has received great attention in materials science and catalysis. The objective is to take advantage of the large pore size of these materials, which will facilitate the flow of reactant and product molecules in and out of the pore system. Large pore systems are needed for shapeselective conversions of bulky molecules such as those increasingly encountered in the refining industry during the upgrading of heavy fractions, oxidation of heavy organics from industrial wastewaters, and in the manufacture of fine chemicals and pharmaceuticals. * To whom correspondence should be addressed. Phone: (513) 5561474. Fax: (513) 556-3473. E-mail: [email protected].

Pure silica MCM-41 showed limited catalytic applications. Therefore, incorporation of metal centers in the silicate framework is necessary for their use in catalysis. Therefore, isomorphous substitution of silicon with a transition metal is an excellent strategy in creating catalytically active sites and anchoring sites for reactive molecules in the design of new heterogeneous catalysts. Several successful examples of transition-metal-incorporated mesoporous silicates had been demonstrated with metals such as Al, Ti, Ga, Mn, Cr, and Fe.6-10 Aluminum incorporated MCM-41, which possesses moderate acidity, has been claimed to be an effective hydrothermal and isomerization6,11 and alkylation catalysts.12 Transition-metalincorporated MCM-41 materials were used to study oxidation of organics in liquid or gas-phase reaction with or without oxidants.13-15 Fe-MCM-41 has been used for oxidation of sulfur dioxide in highly concentrated gases.16 Transition-metalincorporated MCM-41 materials fulfill the requirements for catalyzing the reactions of large organic molecules while retaining a uniform pore size. Transition-metal-incorporated MCM-41 has also shown unique reactivities not only for catalytic reactions17,18 but also for photocatalytic reactions.19,20 However, these materials can operate only under UV light irradiation, and they do not exhibit any photocatalytic activity

10.1021/jp0138983 CCC: $22.00 © 2002 American Chemical Society Published on Web 03/08/2002

Characterization of MCM-41 under visible light irradiation. Recently, Yamashita et al.21 have reported that Cr-incorporated MCM-41 is effective for the photoreduction of NO in visible light, but no reports have been published on photooxidation reactions with visible light in the open literature. Our latest studies revealed that the titania loaded Cr-incorporated MCM-41 is active for photooxidation of formic acid and other organics.22,23 In the present work, we have undertaken an extensive characterization study of titania loaded Cr-, V-, and Fe-MCM-41 by XRD, XPS, UV-vis, TPR, Raman, oxygen chemisorption, and N2 physisorption techniques in order to understand their surface characteristics. Experimental Section Synthesis. Transition-metal-incorporated MCM-41 supports with Si/Me ) 80 and Si/Ti ) 40 were synthesized as previously reported24,25 using Ludox HS-40 (Dupont) as the source of silica. The precursors used for the incorporation of transition metals in the framework of MCM-41 were vanadium, VO (C3H7O)3 (Alfa); chromium, CrCl3 (Fisher); iron, Fe2(SO4)3 (Fisher); and titanium, titanium isopropoxide (Aldrich). All samples were prepared in the presence of hexadecyltrimethylammonium bromide (Fluka) as a template. The following is the typical preparation procedure: 35 g of Ludox was added to 14.55 mL of water under stirring, and 18.2 mL of 40% tetramethylammonium hydroxide (Fluka) was added. Independently, 18.25 g of the template was dissolved in 33 mL of water, and subsequently, 7 mL of 28% NH4OH was introduced. Finally, the above two solutions containing Ludox and template were mixed together. The corresponding amounts of each transition metal oxide precursor dissolved in either ethanol or water (depending on the nature of the precursor) were added dropwise from a pipet to the resulting mixture. The final mixtures were stirred together for 30 min and then transferred into a polypropylene bottle and treated under autogenous pressure without stirring at 90-100 °C for 3 days. The resulting solids were filtered, washed, dried, and calcined at 550 °C for 10 h under airflow. The temperature profile is very important for the calcination of these materials; it should be a low heating rate otherwise the structure of the MCM-41 collapses. That is the reason we used 2 °C/min while heating and 15 °C/min while cooling. The color of Cr-Ti-MCM-41 changed from pale green to pale yellow, V-Ti-MCM-41 changed from white to orange, and Fe-Ti-MCM-41 changed from dust (nearly white) to pale yellow upon calcination of these materials at 550 °C. The resulting catalyst (typically 1.5 g) was dispersed in ∼100 mL of 2-propanol, and titanium isopropoxide was added to achieve 25% loading. The system was dried while stirring at ambient temperature. It was then placed in the oven to dry at 100 °C for 1 h. They were then transferred into a boatlike crucible and calcined at 450 °C for 3 h with a temperature ramp of 2 °C/min. There is no change in the color of the catalysts upon titania loading. Characterization BET Surface Area and Pore Size Distribution Measurements. The specific surface area (BET) of the transition-metalincorporated MCM-41 and titania loaded transition-metalincorporated MCM-41 materials were measured by nitrogen adsorption at 77 K by the BET method using a Micromeritics Gemini 2360. Horvath-Kawazoe maximum pore volume and adsorption average pore diameter measurements of the supports and the catalysts were performed with a Micromeritics ASAP 2010 using adsorption of N2 at the temperature of liquid N2. All samples were degassed at 250 °C under vacuum before analysis.

J. Phys. Chem. B, Vol. 106, No. 13, 2002 3395 Oxygen Chemisorption. Oxygen chemisorption measurements were used to study the dispersion of transition metals in MCM-41. Oxygen uptake measurements of the transition-metalincorporated MCM-41 and titania loaded transition-metalincorporated MCM-41 were performed with a Micromeritics ASAP 2010 Chemi unit. The known weight of catalysts was placed in a quartz U tube and packed with quartz wool below and above the sample. Then, this U tube was fixed to the analyzer port of the ASAP Chemi system. The catalysts were reduced in a continuous flow of 20 mL/min pure H2 (Matheson, 99.999%) at 370 °C for 2 h. Subsequently, the reduced catalysts were degassed at the same temperature for 1 h. After evacuation, the catalyst was maintained at the same temperature (370 °C), and 20 mL/min of pure O2 (Matheson, 99.999%) gas was passed through the sample to analyze the amount of O2 adsorption. The stoichiometry factor for the metal-to-oxygen ratio equal to one was used to calculate the dispersion of metal on MCM-41. UV-vis. The powders were characterized by UV-vis spectrophotometer (Shimadzu 2501PC) with an integrating sphere attachment ISR1200 for their diffuse reflectance in the range of 200-850 nm. BaSO4 was used as the standard in these measurements. X-ray Diffraction (XRD). XRD was used to identify the crystal phases of titania loaded Cr-, V-, and Fe-incorporated MCM-41 and only Cr-, V-, and Fe-incorporated MCM-41 materials. These studies were performed by using Nicolet powder X-ray diffractometer equipped with a Cu KR radiation source (wavelength 1.5406 Å) to assess the crystallinity of the catalysts. Transition-metal-incorporated MCM-41 powders were run from 2 to 7° (2θ) with a step size of 0.01° and a time step of 1.0 s. Titania loaded transition-metal-incorporated MCM-41 were run from 20 to 50° with step size 0.05° and time step 1.0 s to assess the crystallinity of the TiO2 loading. Raman Spectroscopy. Raman spectra were obtained at room temperature using excitation line from Coherent 90-6 Ar+ (514.5 nm) and K-2 Kr+ (406.7 nm) ion lasers, collecting backscattered photons directly from the surface of spinning (∼2000 rpm) solid samples in 8 mm diameter pressed pellets. Conventional scanning Raman instrumentation equipped with a Spex 1403 double monochromator, with a pair of gratings with 1800 grooves/mm, and a cooled Hamamastsu 928 photomultiplier detector was used to record the spectra under the control of a Spex DM3000 micrometer system. Six scans were performed to improve the signal-to-noise ratio. Typically, each scan was obtained with a 20 mW laser and slit width 6 cm-1 for all samples. H2 Temperature Program Reduction (TPR). TPR experiments were carried out in a gas flow system equipped with a quartz microreactor, using a custom-made setup furnished with TCD detector. Approximately 100 mg of the sample was pretreated in 23 mL/min flow of He at 350 °C for 1 h. After the pretreatment, the catalyst samples were tested by increasing the temperature from 100 to 800 °C at 5 °C/min and keeping the temperature at 800 °C for 2 h under a continuous flow of 6 vol % H2 (Helium as balance) at 25 cm3/min. The amount of hydrogen consumed by the catalyst sample in a given temperature range (in µmol/g) was calculated by integration of the corresponding TCD signal intensities taking into account calibrated values obtained in separate experiments. X-ray Photoelectron Spectroscopy (XPS). XPS was used to analyze the binding energy values and the atomic surface concentration of corresponding elements on Cr-, V-, and Feincorporated MCM-41 and titania loaded Cr-, V-, and Fe-TiMCM-41 materials. The XPS analyses were conducted on a

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TABLE 1: BET Surface Area, Average Pore Diameter, Horvath-Kawazoe Maximum Pore Volume, and Metal Dispersion of Different Catalysts

catalyst MCM-41 25%TiO2/MCM-41 Cr-Ti-MCM-41 25%TiO2/Cr-Ti-MCM-41 Fe-Ti-MCM-41 25%TiO2/Fe-Ti-MCM-41 V-Ti-MCM-41 25%TiO2/V-Ti-MCM-41

average pore pore % of BET SA diameter volume metal (m2/g) (nm) (cm3/g) dispersiona 941 667 702 571 719 536 742 628

4.2 3.4 6.7 5.1 5.7 5.1 7.1 3.9

0.94 0.56 1.1 0.67 0.78 0.68 1.3 0.63

0.0 0.1 51.0 23.4 28.6 37.8 17.7 14.4

a Metal dispersion was determined from oxygen chemisorption measurement by using the stoichiometric factor of metal to oxygen equal to 1.

Perkin-Elmer model 5300 X-ray photoelectron spectrometer with Mg KR radiation at 300 W. Typically, 89.45 and 35.75 eV pass energies were used for survey and high-resolution spectra, respectively. The effects of the sample charging were eliminated by correcting the observed spectra for a C 1s binding energy value of 284.5 eV. The powdered catalysts were mounted onto the sample holder and were degassed overnight at room temperature and pressures on the order of 10-7 Torr. The binding energies and atomic concentrations of the catalysts were calculated from the XPS results using the total integrated peak areas of the Me 2p (Me ) Cr, V, and Fe), Ti 2p, Si 2p, and O 1s regions. The spectra were smoothened, and a nonlinear background was subtracted. Results and Discussion The BET surface areas (SA), average pore diameter (APD), and oxygen chemisorption results (% of metal dispersion, MD) are summarized in Table 1. One can observe that the presence of transition metal ions in the gel during synthesis lowers the SA of the resulting MCM-41 material (for example, SA ) 941 m2/g for siliceous MCM-41 and SA ) 703 m2/g for Cr-TiMCM-41). Nonetheless, the BET surface area of the catalysts employed in the present study is much higher than titanias conventionally used for UV-assisted heterogeneous photocatalysis.26 The APD also changes with the introduction of the transition metal. Because the same surfactant template was utilized for the synthesis of both siliceous and transition-metalincorporated MCM-41 materials, one should expect to obtain APD in a close range for both types of catalysts. The above discrepancy is therefore due to the partial breakage of the tubular walls of the MCM-41 structure resulting in the formation of bigger pores as well as lower SA. It should be noted that some loss of SA is observed when titania is deposited on the MCM-41 support, as the pore diameters are expected to decrease. Indeed, the APD of MCM41 also reduces with the loading of titania. This supports the loss of surface area because of partial blockage of the pores. A comparison of the Cr-incorporated MCM-41 samples elucidates the following phenomena. Smaller percentages of titania deposition (∼10%) led to an almost negligible (∼1%) loss of the SA. On the contrary, higher coverages (25%) lead to substantial losses in the SA (∼15%). This clearly indicates that the distribution of titania in the two samples is different. Low loadings lead to a layer-type distribution of titania27 which simply covers the pore walls. Table 1 shows that the higher loadings of titania fill up some of the pores leading to their

Figure 1. UV-vis diffuse reflectance spectra of Me-Ti-MCM-41 materials (Me ) V, Cr, and Fe) and leached Cr-Ti-MCM-41.

partial blockage, such that the removal of isopropyl alcohol upon calcination does not open the pore mouth. The results of oxygen chemisorption (% of metal dispersion, MD) are also presented in Table 1. The siliceous MCM-41 and 25% TiO2/MCM-41 did not show any O2 chemisorption under the experimental conditions employed in this study. Therefore, no MD has been observed for the siliceous MCM-41 sample as well as that containing 25% of TiO2. However, all transitionmetal-incorporated MCM-41 materials exhibited strong oxygen chemisorption. The greatest MD was observed for chromiumincorporated samples, followed by iron- and vanadiumincorporated ones. The MD was determined by using the stoichiometric factor of metal to oxygen equal to one. The oxygen chemisorption results also show a pronounced effect of the TiO2 loading on the MD. Oxygen chemisorption can occur only on the reduced transition metals ions, which contain the coordinatively unsaturated sites. For chromium-incorporated MCM-41, about 50% of the MD is lost upon loading of 25 wt % TiO2; the loss of MD is lower for the V-Ti-MCM-41 sample. This is due to partial blockage of the transition metal sites by the TiO2 loading, making them inaccessible to oxygen. On the contrary, Fe-Ti-MCM-41 exhibits lower MD than that of 25% TiO2/Fe-Ti-MCM-41, as the oxygen chemisorption is higher for the TiO2 loaded sample. Thus, the O2 results are in line with Raman and XPS results discussed in latter paragraphs. The UV-vis spectra of the catalysts in the range of 200850 nm are shown in Figures 1 and 2. As shown in Figure 1, the transition metal incorporated MCM-41 materials exhibit absorption in visible light as well as in the UV range. Figure 2 indicates that significant absorption occurs in visible light even in titania loaded MCM-41 materials. This is consistent with the absorption by mixed oxides of vanadium, chromium, and iron with titania28 drastically differ from neat titanium dioxide (not shown). Their absorption edges are in the vicinity of 500-600 nm, thus shifting the band gap position to ∼2.0 eV. The catalysts absorb light below a threshold wavelength λg, the fundamental absorption edge, which is related to the band gap energy via the relation between wavelength and absorption energy29

λg (nm) ) 1240/Eg (eV)

(1)

This justifies why these photocatalysts can be potentially good candidates for performing photochemical reactions in visible light. The upper branch of the curves (200-350 nm) practically

Characterization of MCM-41

Figure 2. UV-vis diffuse reflectance spectra of titania loaded MeTi-MCM-41 materials (Me ) V, Cr, and Fe) and Degussa P25 TiO2.

coincides with that of titania. Then, the absorption by titania itself sharply decreases, and the absorption of light in the range of 350-450 nm is exhibited by the heterojunction of titania with the corresponding transition metal oxide. The absorption after 450 nm is probably due to the transition metal oxide itself. It should be noted that the flat line of the absorption spectra observed after about 450-500 nm does not mean the absence of absorption. As shown in Figure 2, vanadium-incorporated catalysts allow the expanding of the absorption spectrum even further toward the infrared part, especially in the reduced form. It should be noted that we have chosen to analyze 25 wt % loaded composites to ensure that the presence of a large quantity of the second oxide does not overshadow the true optical properties. One can observe that the loading of titania significantly alters the absorption spectrum of the composite. As shown in Figure 1, the neat UV-vis spectrum of V-Ti-MCM-41 indicates the presence of two bands at 258 and 374 nm as the main feature of this catalyst, which are associated to the appearance of V+5 species in 4- and 6-fold coordination state, respectively.30 However, some of these V species could be formed by the interaction of atmospheric moisture.31 Therefore, the presence of V+5 species at the onset of its absorption spectrum (∼625 nm) coincides with that of pure V2O5. One can conclude that isolated V+5 species, tetrahedrally coordinated, are present in V-Ti-MCM-41. They can interact with moisture, which can partially modify the nature of V species. The presence of 25 wt % of TiO2, however, shifts the absorption edge from about 625 to 450 nm, and the presence of vanadium is observable in the tail of the spectrum. This clearly signals the unavailability of the V+5 species inside the framework of MCM-41, signifying that all of these species are fully covered. The reduced TiO2/ V-Ti-MCM-41 samples exhibited significant absorption in the visible range. Two shoulders are observable on this spectrum: in the range of 350-400 nm and 550+ nm. This also coincides with the behavior of the reduced pure vanadium oxide prepared independently. Figure 2 shows that iron-incorporated MCM-41 samples behave similarly to vanadium-incorporated ones. The loading of titania simply shifts the absorption edge by about 100 nm toward UV. The spectra of both samples are rather monotonical, which do not allow us to make qualitative conclusions about the nature and placement of iron species inside MCM-41. One can symbolically divide the curve into three parts: 200-400, 400-500, and 500-600 nm. The first part will correspond to the absorption by pure titania, the second will correspond to

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Figure 3. XRD patterns of transition-metal-incorporated MCM-41’s: (a) Cr-Ti-MCM-41, (b) V-Ti-MCM-41, and (c) Fe-Ti-MCM41.

the absorption by iron doped titania, and the third will correspond to the absorption by iron oxide itself. The Cr-Ti-MCM-41 and 25% TiO2/Cr-Ti-MCM-41 situation is somewhat similar to the prior work (see Figures 1 and 2). It was found previously32 that there are three species inside Cr-incorporated MCM-41: framework Cr3+, extraframework Cr3+, and extraframework Cr6+. Neat Cr-Ti-MCM-41 exhibits an absorption maximum at ∼390 nm which corresponds to the Cr6+ species. When loaded with titania, this maximum (corresponding to Cr6+) is overshadowed by the absorption of TiO2, leaving only an absorption shoulder. Finally, the reduced sample of titania-loaded Cr-Ti-MCM-41 does contain a small remainder of the above maximum. This can be attributed to the incomplete reduction of Cr6+ species. It was found33 that some of the Cr6+ species inside Cr-Ti-MCM-41 are nonreducible. Furthermore, the deposition of titania further deprives the reduction process as some of the Cr6+ becomes unavailable. On the other hand, the presence of an absorption minimum in the vicinity of 550 nm (green) clearly indicates the presence of Cr3+ species in abundance. A comparison of the UV-vis spectra of various neat oxide standards with chromium-containing materials was made. There are absorption peaks at ∼275 and ∼370 nm and shoulders at ∼470 and ∼600 nm on the spectra of Cr-Ti-MCM-41 (Figure 1). The same materials, but loaded with 25% TiO2 (Figure 2), exhibit higher absorption in the UV range because of the presence of titania. All the materials still retain high absorption in visible light (up to 600 nm) and have a distinct shoulder at ∼370 nm. As shown in Figure 1, a leaching experiment was performed to determine the ratio of Cr6+ to Cr3+ in the calcined Cr-Ti-MCM-41 sample. The presence of chromium ions in the powders and in solution was monitored spectrophotometrically. The disappearance of absorption peaks at 275 and 380 nm indicates that the Cr6+ species was leached out upon stirring in water for 12 h, whereas the absorption peak at ∼600 nm was still shown, which indicates Cr3+ species were retained in the material. The amount of Cr6+ leached into the two independent experiments constituted about 80% of the chromium introduced during the synthesis. More interestingly, no significant leaching of Cr6+ was detected in the case of the most active 25% TiO2/Cr-Ti-MCM-41 catalyst. In conclusion, the UV-vis study points out that there are two species of chromium (Cr3+ and Cr6+) species present inside Cr-TiMCM-41. The XRD analysis was employed to characterize the crystallinity of the catalysts and showed a number of trends. First, the diffractograms recorded from 2 to 7° (Figure 3) exhibited the same location of peaks as siliceous MCM-41.24 However, the

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Figure 4. Raman spectra of titania loaded on transition-metalincorporated MCM-41’s.

intensities of these peaks were lower than those found for siliceous MCM-41. This can be attributed to the presence of foreign ions in the gel during synthesis, which can hinder the structure-directing action of the template by changing the ionic strength of the medium. A similar trend (the lowering of peak intensities) was observed by Kevan et al.32 The second part of the XRD analysis was performed in the range of 20-50° in order to assess the crystallinity of TiO2 loaded onto the transition-metal-incorporated MCM-41 support. It showed that our samples exhibit low crystallinity of titania. Xu and Langford27 observed a different trend when loading titania onto siliceous MCM-41. However, they utilized a different support (unmodified MCM-41) and different loading method (sol-gel precipitation with acid peptization). There are steric hindrances associated with the formation of titania clusters inside zeolites and mesoporous molecular sieves27 because of the relatively low pore size of these materials. This may not allow for efficient crystal growth under the calcination conditions employed, and the overall crystallinity of TiO2 in our samples remains low. It is also worth noting that no peaks corresponding to the transition metal oxides (those of V, Cr, and Fe) were observed on X-ray diffractograms. This indicates that metal ions were either atomically dispersed in the framework of MCM-41 or attained an amorphous form outside the framework. Transition metal ions are expected to be in intimate contact with the loaded titania, provided that uniform distribution of titania on the pore walls of the molecular sieve has been achieved. With this structure, it will be possible to combine the effect of TiO2 and the atomic dispersion of the transition metal inside MCM-41 achieved as a result of the work of a structure-directing agent. This may not be possible for mixed oxides. Raman spectra of Cr-Ti-MCM-41, V-Ti-MCM-41, and Fe-Ti-MCM-41 did not show any peaks corresponding to CrO, V-O, or Fe-O bending or stretching modes, indicating that the transition metals are absolutely well dispersed inside the framework of MCM-41 (figure not shown). Raman spectra of titania loaded transition metals (Cr, V, and Fe) incorporated MCM-41 are shown in Figure 4. In this figure, four bands at 144, 397, 518, and 641 cm-1 indicate the existence of titania (anatase) particles.34 The intensity of these four peaks is very high in the case of Ti-MCM-41 spectra, whereas the peaks are of much less intensity in the case of the other three catalysts such as 25% TiO2/Cr-Ti-MCM-41, 25% TiO2/V-Ti-MCM41, and 25% TiO2/Fe-Ti-MCM-41. Therefore, one can easily understand that the loaded titania is directly interacting with the transition metals incorporated inside the MCM-41 framework. Moreover, the peaks that appear at 144, 397, 518 and 641 cm-1 are not due to the Ti incorporated inside the MCM41 framework; as such we could not see any peaks. Therefore,

Figure 5. (A) TPR profiles of tranisiton-metal-incorporated MCM41’s: (a) Cr-Ti-MCM-41, (b) V-Ti-MCM-41, and (c) Fe-TiMCM-41. (B) TPR profiles of titania loaded on transition-metalincorporated MCM-41’s: (a) 25% TiO2/Cr-Ti-MCM-41, (b) 25% TiO2/V-Ti-MCM-41, and (c) 25% TiO2/Fe-Ti-MCM-41.

the results obtained from these spectra agree well with the XPS results explained in the latter paragraphs. Temperature programmed reduction (TPR) was used in the present study to investigate different oxidation states of the transition-metal-incoporated MCM-41 materials. Figure 5A compares the TPR profiles for Cr-Ti-MCM-41, V-Ti-MCM41, and Fe-Ti-MCM-41. One can observe that these catalysts differ in their reduction behavior. As shown in Figure 5A, two major peaks at 440 and ∼800 °C were found for Cr-Ti-MCM41. As reported by others35 the first peak at 440 °C corresponds to the reduction of Cr6+ f Cr3+, which validates our earlier observations by UV-vis about the presence of Cr (VI) in the MCM-41 based materials made with Cr (III) precursor.22 The second peak at 800 °C was observed for all catalysts (even siliceous MCM-41), so it can be attributed to the hydroxyl groups leaving the surface of amorphous silica. In the case of V-Ti-MCM-41 (see Figure 5A), one can observe that there are two intense peaks at 547 and 800 °C along with one shoulder peak at 480 °C. The shoulder peak at 480 °C corresponds to the reduction temperature of V+5 (V+5 to V+4), and the peak at 547 °C corresponds to the reduction of V+4 to V+3. The TPR profile of Fe-Ti-MCM-41 indicates that there are three reduction peaks at 493 and 581 °C which correspond to Fe3+ and Fe2+, respectively, and a peak at 654 °C corresponding to the reduction behavior of Ti which is incorporated inside the framework of MCM-41 along with Fe. This reduction behavior of Ti is due to the surface interaction between the Fe and Ti. When TiO2 is loaded onto our Cr-containing materials, the TPR profiles show marked difference (Figure 5B). We still observe the two major peaks at 440 and 800 °C, but a shoulder appears before the first peak and another peak appears between them. The shoulder in the range of 250-350 °C corresponds to the dehydroxylation of the TiO2 surface and also the reduction of titanium from +4 to +3.36 The position of the new peak at 570 °C is apparently due to the transition of Cr3+ f Cr2+.36 The lower temperature of the reduction of titania is due to the high degree of interaction with the framework chromium. Furthermore, when incorporated into MCM-41 during synthesis, chromium is expected to attain the tetrahedral coordination.32 This may also contribute to the peculiar interaction of chromium and titanium oxides in our active catalysts for the photodegradation of aqueous organic pollutants in visible light. The TPR profile of 25% TiO2/Fe-Ti-MCM-41 show two different peaks at 467 and 540 °C, which correspond to the reduction temperatures of the Fe3+ and Fe2+ states of iron (see Figure 5B). Very

Characterization of MCM-41

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TABLE 2: XPS Binding Energy Values and Surface Atomic Concentration of Transition-Metal-Incorporated MCM-41 and Titania Loaded Transition-Metal-Incorporated MCM-41s binding energy (eV)a

surface atomic ratiosa

catalysts

Ti 2p3/2

Me 2p3/2

Si 2p

O 1s

Me/Tib

Me/Sib

V-Ti-MCM-41 25%TiO2/V-Ti-MCM-41 Cr-Ti-MCM-41 25%TiO2/Cr-Ti-MCM-41 Fe-Ti-MCM-41 25%TiO2/Fe-Ti-MCM-41

N.D. 458.5 N.D. 458.5 N.D. 458.5

516.6 517.2 576.9 579.3 710.7 710.9

103.1 103.2 103.2 103.2 103.1 103.1

532.7 530.2 532.8 530.1 532.6 530.2

N.D. 0.1615 N.D. 0.136 N.D. 0.039

0.017 0.202 0.0014 0.258 0.009 0.024

a N.D. ) Ti 2p was not detected by XPS. b Me ) V, Cr, and Fe. These are the transition metals, which are incorporated inside the MCM-41 framework during the synthesis.

interestingly, the reduction behavior of 25% TiO2/Fe-TiMCM-41 is similar, with a little variation of reduction temperatures, to that of Fe-Ti-MCM-41. This suggests that there is negligible interaction between titania and iron in this particular catalyst. As shown in Figure 5B, three different TPR peaks were observed in the case of 25% TiO2/V-Ti-MCM-41. This was due to vanadium reducing at three different temperatures. The peak at 505 °C is due to the reduction of V+5 to V+4, whereas the peak at 564 °C was due to the reduction of V+4 to V+3, and the third peak at 736 °C was due to the reduction of V+3 oxidation state. The TPR results of these three catalysts indicate that the dispersion of titania is different in each sample. Kevan et al.36 reported that the reduction temperature is also dependent on the reduction conditions, such as the H2 partial pressure and the heating rate. It is therefore difficult to discuss the reduction temperature differences from the data obtained under different reduction conditions reported in the literature. The reported reduction temperatures are quite different, even for the same system in the literature. The samples of Cr-, V-, and Fe-incorporated MCM-41 and 25% TiO2 loaded transition-metal-incorporated MCM-41 were investigated by XPS. The XPS bands of O 1s, Si 2p, and Ti 2p core levels are shown in Figures 6A, 6B and 7, respectively. The binding energy values of O 1s, Si 2p, Ti 2p, Cr 2p, V 2p, and Fe 2p photoelectron peaks and Me/Si and Me/Ti surface atomic concentration ratios as determined by XPS of the above catalysts are summarized in Table 2. All of these figures clearly indicate that the XPS photoelectron peaks depend on the surface concentration of transition metals in the Me-Ti-MCM-41 and titania loaded Me-Ti-MCM-41 (Me ) transition metals Cr, V, and Fe). The O 1s profile, as shown in Figure 6A, is due to the overlapping contribution of oxygen from silica and the transition metal in the case of Me-Ti-MCM-41 and silica, the transition metal and titania in the case of 25% TiO2/Me-Ti-MCM-41, respectively. As shown in Figure 6A from the XPS spectra of O 1s corresponding to 25% TiO2/Me-Ti-MCM-41, one can clearly detect that there are three types of O 1s peaks with binding energy values of 529-530, 530.2, and 532.7 eV (Table 2) corresponding to the oxygen atoms that are more bound to Me (MeOx), Ti (TiO2),37 and Si (SiO2),38 respectively. The binding energy values of three different types of O 1s peaks can be judged from the difference in the electronegativity of the elements.39 It should be noted that there was only one oxygen photoelectron peak at 532.7 eV belonging to SiO2 in the case of transition-metal-incorporated MCM-41. The peak intensity of the O 1s XPS band corresponding to SiO2 reduced drastically when titania was loaded on Cr-, V-, and Fe-incorporated MCM41. This clearly indicates that loading of titania on Cr-, V-, and Fe-incorporated MCM-41 changes its structural behavior and also points out the possible migration of oxygen atoms is taking place.

Figure 6. (A) O 1s XPS core level spectra of transition-metalincorporated MCM-41’s and titania loaded on transition-metalincorporated MCM-41’s: (a) Cr-Ti-MCM-41, (b) V-Ti-MCM-41, (c) Fe-Ti-MCM-41, (d) 25% TiO2/Cr-Ti-MCM-41, (e) 25% TiO2/ V-Ti-MCM-41, and (f) 25% TiO2/Fe-Ti-MCM-41. (B) Si 2p XPS core level spectra of transition-metal-incorporated MCM-41’s and titania loaded on transition-metals-incorporated MCM-41’s: (a) Cr-TiMCM-41, (b) V-Ti-MCM-41, (c) Fe-Ti-MCM-41, (d) 25% TiO2/ Cr-Ti-MCM-41, (e) 25% TiO2/V-Ti-MCM-41, and (f) 25% TiO2/ Fe-Ti-MCM-41.

Figure 6B shows the binding energy of the Si 2p core levels, found around 103.2 eV, which agrees well with the values reported in the literature.38,40 As shown in Figure 6B, the intensity of Si 2p is very strong in the case of transition-metalincorporated MCM-41 as compared to the titania loaded transition-metal-incorporated MCM-41. Si 2p peak intensity for Cr-Ti-MCM-41 is higher than those of V- and Fe-incorporated MCM-41, though equal amounts of transition metals are incorporated inside the MCM-41 framework, indicating that Cr is well incorporated inside the framework. However, the intensity of the Si 2p peak decreased drastically with loading of titania on to the Me-Ti-MCM-41, suggesting that titania is covering the surface of transition-metal-incorporated MCM41. At the same time, the covered titania is also interacting with transition metals, which are incorporated inside the MCM-41 framework. Figure 7 shows the binding energies of the Ti 2p core levels, found at 458.5 and 464.5 eV for Ti 2p3/2 and Ti2p1/2 lines, respectively, which agrees well with the values reported in the literature.41 Very interestingly, the peak intensity of the Ti 2p core level is very high in the case of 25%TiO2/Cr-TiMCM-41 as compared to that of 25% TiO2/V-Ti-MCM-41 and 25% TiO2/Fe-Ti-MCM-41. However, the appearance of the Ti 2p peak is mainly due to the loading of TiO2 but not due to the incorporated titanium inside the Me-Ti-MCM-41 framework. This was clarified from the XPS spectra of transition-metal-incorporated MCM-41s such as Cr-Ti-MCM41, V-Ti-MCM-41, and Fe-Ti-MCM-41. Furthermore, XPS detected very small peaks of Cr 2p, V 2p, and Fe 2p, though the concentration of these metal ions is 50% less than titanium during the synthesis of Me-Ti-MCM-41 gel. It is evident that the incorporated titanium is well dispersed inside the MCM-41

3400 J. Phys. Chem. B, Vol. 106, No. 13, 2002

Figure 7. Ti 2p XPS core level spectra of titania loaded on transitionmetal-incorporated MCM-41’s: (a) 25% TiO2/Cr-Ti-MCM-41, (b) 25% TiO2/V-Ti-MCM-41, and (c) 25%TiO2/Fe-Ti-MCM-41.

framework, since it is not detectable on the surface by XPS technique. The XPS results of titania loaded Me-Ti-MCM41 catalysts are in perfect agreement with the results of Raman and XRD results that we discussed in the previous paragraphs. The binding energies and corresponding relative dispersions of Me/Si and Me/Ti were depicted in Table 2. The binding energy of Cr 2p3/2 in Cr-Ti-MCM-41 is 576.9 eV; it probably corresponds to the Cr3+ state. The binding energy values increased from 576.9 to 579.3 eV upon titania loading (Table 2). The increase in binding energy with loading of titania indicates that a part of chromium changes its oxidation state from +3 to +6. This may be due to either the interaction of incorporated chromium and the loaded titania or the breakage of the bond between Si-O-Cr and diffusion of chromium on to the surface of MCM-41. Thus, diffused chromium may be stabilized in the hexavalent state by interacting with the loaded titania. It appears that some amount of chromium in the trivalent state stabilized in the MCM-41 framework. As can be noted from Table 2, the binding energy of V 2p3/2 increased from 516.6 to 517.2 eV with titania loading on V-Ti-MCM-41. A careful examination of the literature reveals that the V 2p3/2 binding energy reported for the V+5 oxidation state ranges between 517.4 and 516.4 eV; the next oxidation, V+4, shows values in the range of 515.7-515.4 eV.42,43 The increase in binding energy value indicates that vanadium oxide is stabilized in a higher oxidation (pentavalent) state after the loading of titania. This may be due to the same reason that was proposed for Cr-Ti-MCM-41. The binding energy of Fe 2p3/2 is 710.7 and 710.9 eV for Fe-Ti-MCM-41 and 25% TiO2/Fe-TiMCM-41, respectively. The increasing trend of Fe 2p3/2 core level binding energy is similar to that of Cr 2p3/2 and V 2p3/2 core levels, because of the interaction of titania. However, the increase in the binding energy value is much less in this case. The binding energy of Fe 2p3/2 is 110.7 eV for Fe-Ti-MCM41, indicating that some amount of Fe is in the +2 oxidation state. However, after loading titania, the binding energy value increased to 110.9 eV, which agrees well with the Fe3+ oxidation state binding energy value reported in the literature.44 On the basis of the binding energy values of Cr 2p3/2, V 2p3/2, and Fe 2p3/2 in the corresponding titania loaded MCM-41, one can conclude that the interparticle distance between incorporated transition metal and the silica in MCM-41 framework increases, thereby leading to the diffusion of transition metals onto the surface of MCM-41. Consequently, diffused transition metal ions may be stabilized in the higher oxidation states such as

Reddy et al.

Figure 8. XPS surface atomic percentage ratios of Me/Si and Me/Ti for transition-metal-incorporated MCM-41’s and titania loaded on transition-metal-incorporated MCM-41’s: (a) Cr-Ti-MCM-41, (b) V-Ti-MCM-41, (c) Fe-Ti-MCM-41, (d) 25% TiO2/Cr-Ti-MCM41, (e) 25% TiO2/V-Ti-MCM-41, and (f) 25% TiO2/Fe-Ti-MCM41.

Fe3+, V+5, and Cr6+ by interacting with loaded titania. From the UV-vis studies, the 25% TiO2/Cr-Ti-MCM-41 material does absorb visible light in the region of 400-500 nm, whereas in the 25% TiO2/V-Ti-MCM-41 and 25% TiO2/Fe-TiMCM-41 cases, a flat line in the above region can be observed (see Figure 2). Therefore, 25% TiO2/Cr-Ti-MCM-41 is expected to be more active than 25% TiO2/Fe-Ti-MCM-41 and 25% TiO2/V-Ti-MCM-41 for the photodegradation of organics under visible light.22 This is due to the formation of heterojunctions between higher oxidation state transition metal ions and loaded titania (Me-O-Ti). The relative dispersion of transition metals and titania on the surface of MCM-41 was also estimated from XPS measurements of the Me-Ti-MCM-41, and 25% TiO2/Me-Ti-MCM-41 is illustrated in Table 2 and Figure 8. The Me 2p/Si 2p and Me 2p/Ti 2p atomic ratios can be considered as the relative dispersion of transition metal ions on the MCM-41 framework. In general, the Me/Si ratios are found to decrease upon loading of titania in all three catalysts. The Cr/Si ratio is very low, followed by Fe/Si, followed by V/Si, even though the same amount of transition metal precursors were added during the synthesis of these materials. This indicates that chromium ions are better incorporated inside the MCM-41 framework than Fe and V ions. However, the transition metal to silicon atomic ratio is as follows Cr/Si > V/Si > Fe/Si in the case of titania loaded Me-Ti-MCM-41s. This indicates that the loaded titania is diffusing out more chromium ions than iron and vanadium ions in the form of heterojunction (Ti-O-Cr) onto the surface of the MCM-41. As shown in Table 2 and Figure 8, the difference between the Me/Si and Me/Ti ratios is higher in the case of 25% TiO2/Cr-Ti-MCM-41 than in the case of 25% TiO2/VTi-MCM-41, but the Me/Ti ratio is higher than the Me/Si ratio in the case of 25% TiO2/Fe-Ti-MCM-41. This suggests that iron ions hardly diffused to the surface of MCM-41 because of the loaded titania. One can clearly say from the XPS results that the formation of a heterojunction between titania and transition metal ions is only due to the loading of titania but not due to the already incorporated titanium ions, because XPS could not detect any Ti 2p core level peaks from the surface of V-Ti-MCM-41, Fe-Ti-MCM-41, and Cr-Ti-MCM-41. Moreover, the photocatalytic activity of 25% TiO2/Cr-TiMCM-41 is the same as the activity of 25% TiO2/Cr-MCM41. This indicates that the incorporation of titanium metal ions does not play any role in the photocatalytic activity as well as the formation of heterojunction between transition metal and titania (Ti-O-Cr).22

Characterization of MCM-41 Conclusions Different transition metals (V, Cr, and Fe) containing mesoporous Me-Ti-MCM-41 molecular sieves (Si/Me ) 80) that contain atomically dispersed transition metal centers had been hydrothermally synthesized. These materials possesses a lower BET surface area and higher average pore diameter as compared to the siliceous MCM-41 because of the breakage of pores upon incorporation of transition metals. The surface area and average pore diameter values decreased with the loading of titania, because the loaded titania obstruct the pores of MeTi-MCM-41. The characterization results of Me-Ti-MCM41 and 25% TiO2/Me-Ti-MCM-41 materials by O2 chemisorption, DR-UV-vis, XRD, Raman, TPR, and XPS techniques reveal that the transition metal ions are well incorporated inside the framework of MCM-41 before the loading of titania. However, some amount of transition metal ions is redistributed onto the surface upon the loading of titania. This redistribution phenomenon is strongly promoted by the loaded titania and accompanied by a loss of specific surface area, average pore diameter, and dispersion of transition metal ions inside the framework of MCM-41. DR-UV-vis showed a substantial absorption of visible light in the range of 400-600 nm by all three transition-metal-incorporated MCM-41 molecular sieves. The same materials loaded with titania exhibit higher absorption in the UV range because of the presence of titania. DR-UVvis spectra of leached Cr-Ti-MCM-41 showed the disappearance of the absorption peak at 380 nm because of the leaching of Cr6+ from the extraframework Cr-Ti-MCM-41; however, the Cr6+ leaching was prevented with the loading of TiO2. TPR results for Me-Ti-MCM-41 revealed a lower number of reduction transitions than the titania loaded Me-Ti-MCM41, which indicates that there is a strong interaction between transition metal ions and loaded titania center that immobilizes the Me-O-Ti species and prevents the formation of bulk metal oxides. The Me/Ti and Me/Si surface atomic ratios are determined by XPS measurements and reveal that considerable diffusion of transition metal ions to the surface of MCM-41 occurs upon loading of titania. The XPS spectra, binding energies, and surface atomic ratios for Me-Ti-MCM-41 suggest one type of surface electronic level, such as Me-OSi. However, two types of surface electronic levels were found in the case of 25% TiO2/Me-Ti-MCM-41, namely, Me-OSi and Me-O-Ti. The Me/Si and Me/Ti surface atomic ratios determined by XPS also indicated that there is a significant interaction between the incorporated transition metal ions and the loaded titania as in the form of a Me-O-Ti heterojunction. Only this type of heterojunction located on the MCM-41 wall surface can lead to the photocatalytic activity. Acknowledgment. This work was supported by the United States Department of Army (DOA) through Young Investigator Program (Grant No. DAAD 19-00-1-0399) and NATO (Grant No. SfP-974209). We also acknowledge funding from the Ohio Board of Regents (OBR) that provided matching funds for equipment to the NSF CTS-9619392 Grant through the OBR Action Fund #333. References and Notes (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) 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. 1992, 114, 10834.

J. Phys. Chem. B, Vol. 106, No. 13, 2002 3401 (3) Antonelli, D. M.; Ying, J. Y. Curr. Opin. Colloid Interface Sci. 1996, 1, 523. (4) Chen, C. Y.; Li, H. X.; Davis, M. E. Micro. Mater. 1993, 2, 17. (5) Sayari, A.; Danumah, C.; Moudrakouski, I. L. Mater. Res. Soc. Symp. Proc. 1995, 371, 81. (6) Le, O. N.; Thomson, R. T. U.S. Patent 5,232,580, 1993. (7) Corma, A.; Navarro, M. T.; Perez-Pariente, J. J. Chem. Soc. Chem. Commun. 1994, 147. (8) Kosslick, H.; Lischke, G.; Walther, G.; Storek, W.; Martin, A.; Fricke, R. Microporous Mater. 1997, 9, 13. (9) Rey, F.; Shankere, G.; Mashmeyer, T.; Thomas, J. M.; Bell, R. G. Topic Catal. 1996, 3, 121. (10) Thomas, J. M. Faraday Discuss. 1995, 100, C9. (11) Corma, A.; Martinez, A.; Martinez-Soria, V.; Monton, J. B. J. Catal. 1995, 153, 25. (12) Armengol, E.; Cano, M. L.; Corma, A.; Garcia, H.; Navarro, M. T. J. Chem. Soc. Chem. Commun. 1995, 519. (13) Blasco, T.; Corma, A.; Navarro, M. T.; Perez-Pariente, J. J. Catal. 1995, 156, 65. (14) Reddy, K. M.; Moudrakovski, I.; Sayari, A. J. Chem. Soc. Chem. Commun. 1994, 1059. (15) Lim, S.; Haller, G. L. Appl. Catal. A 1999, 188, 277. (16) Wingen, A.; Anastasievic, N.; Hollnagel, A.; Werner, D.; Schuth, F. J. Catal. 2000, 193, 248. (17) Corma, A. Chem. ReV. 1997, 97, 2373. (18) Notari, B. AdV. Catal. 1996, 41, 253. (19) Anpo, M.; Che, M. AdV. Catal. 1999, 44, 119. (20) Yamashita, H.; Zhang, J.; Matsuoka, M.; Anpo, M. In Photofunctional Zeolites; Anpo, M., Eds.; Nova Science Publishers, Inc.: New York, 2000; p 129. (21) Yamashita, H.; Yoshizawa, K.; Ariyuki, M.; Higashimoto, S.; Che, M.; Anpo, M. Chem. Commun. 2001, 435. (22) Davydov, L.; Reddy, E. P.; France, P.; Smirniotis, P. G. J. Catal. 2001, 203, 157. (23) Davydov, L.; Reddy, E. P.; Smirniotis, P. G. Angew. Chem. 2001, submitted for publication. (24) Sayari, A.; Liu, P.; Kruk, M.; Jaroniec, M. Chem. Mater. 1997, 9, 2499. (25) Corma, A.; Fornes, V.; Navarro, M. T.; Perez-Pariente, J. J. Catal. 1994, 148, 569. (26) Davydov, L.; Smirniotis, P. G. J. Catal. 2000, 191, 105. (27) Xu, Y.; Langford, C. H. J. Phys. Chem. B 1997, 101, 3115. (28) Serpone, N.; Lawless, D.; Disdier, J.; Herrmann, J. M. Langmuir 1994, 10, 643. (29) Gratzel, M. Heterogeneous Photochemical electron Transfer; CRC Press: Baton Rouge, FL, 1988. (30) Pena, M. L.; Dejoz, A.; Fornes, V.; Rey, F.; Vazquez, M. I.; Lopez Nieto, J. M. Appl. Catal. A 2001, 209, 155. (31) Gao, X.; Bare, S. R.; Weckhysen, B. M.; Wachs, I. E. J. Phys. Chem. B 1998, 102, 10842. (32) Zhu, Z. D.; Chang, Z. X.; Kevan, L. J. Phys. Chem. B 1999, 103, 2680. (33) Ulagappan, N.; Rao, C. N. R. Chem. Commun. 1996, 1047. (34) Went, G. T.; Oyama, S. T.; Bell, A. T. J. Phys. Chem. 1990, 94, 4240. (35) Uhm, J. H.; Shin, M. Y.; Zhidong, Z.; Chung, J. S. Appl. Catal. B 1999, 22, 293. (36) Zhu, Z.; Hartmann, M.; Maes, E. M.; Czernuszewicz, R. S.; Kevan, L. J. Phys. Chem. B 2000, 104, 4690. (37) Reddy, B. M.; Chowdhury, B.; Reddy, E. P.; Fernandez, A. J. Mol. Catal. A 2000, 162, 431. (38) Reddy, B. M.; Ganesh, I.; Reddy, E. P. J. Phys. Chem. B 1997, 101, 1769. (39) Imamura, I.; Ishida, S.; Taramoto, H.; Saito, Y. J. Chem. Soc., Faraday Trans. 1993, 89, 27. (40) Odenbrand, C. U. I.; Anderson, S. L. T.; Anderson, L. A. H.; Brandin, J. M.g.; Busca, G. J. Catal. 1990, 125, 451. (41) Sawatzky, G. A.; Post, D. Phys. ReV. B 1979, 20, 1546. (42) Nogier, J. Ph.; Delmer, M. Catal. Today 1994, 20, 109. (43) Bukhtiyarov, V. I. Catal. Today 2000, 56, 403. (44) Briggs, D.; Sea, M. P., Eds.; Auger and X-ray Photoelectron Spectroscopy. Practical Surface Analysis, 2nd ed.; Wiley: New York, 1990; Vol. 1.