Effect of the Cr6+ concentration in Cr-incorporated TiO2-loaded MCM-41 catalysts for visible light photocatalysis

Applied Catalysis B: Environmental 57 (2005) 139–149 www.elsevier.com/locate/apcatb

Effect of the Cr6+ concentration in Cr-incorporated TiO2-loaded MCM-41 catalysts for visible light photocatalysis Bo Sun, Ettireddy P. Reddy, Panagiotis G. Smirniotis* Department of Chemical Engineering and Material Science, University of Cincinnati, Cincinnati, OH 45221-0012, USA Received 28 August 2004; received in revised form 21 October 2004; accepted 28 October 2004 Available online 8 December 2004

Abstract The activity of 25% TiO2-loaded various amounts of Cr-incorporated MCM-41 (25%TiO2/Cr-MCM-41) was studied as a catalyst for photodegradation of 4-chlorophenol under visible light (400–800 nm). Cr-MCM-41 with different ratios of Si to Cr was synthesized by hydrothermal method and loaded with 25 wt.% TiO2 utilizing sol–gel method. The surface areas of Cr-MCM-41 and 25% TiO2/Cr-MCM-41 samples decreased with increasing the amount of Cr incorporated. XRD analysis showed that the MCM-41 structure was formed only in the samples with Si/Cr ratio larger than 20. Temperature program reduction (TPR) and UV–vis spectra results clearly indicated that only Cr6+ was present in the samples with the ratio of Si to Cr more than 20. Whereas the material with the ratio of Si to Cr less than 20 contained chromium with mixed oxidation states. It was found that the photodegradation ability of 25% TiO2/Cr-MCM-41 was highly related to the amount of Cr6+ which strongly interacted with TiO2; the optimum atomic ratio of Si to Cr was 20. This optimum catalyst was compared with two kinds of 25 wt.% TiO2-loaded CrSi20Ox bulk oxides. The overall photodegradation ability of these titania loaded bulk oxide catalysts under visible light was about 45% less active than 25% TiO2/Cr-MCM-41 (Si/Cr = 20). TPR results clearly showed that the interaction of Cr6+ and TiO2 in the bulk oxide catalysts was weaker than that in TiO2/Cr-MCM-41. A good correlation was demonstrated between the Cr6+–TiO2 interaction (Cr–O–Ti) and catalyst’s photoactivity. As a result, the MCM-41 structure can enhance the interaction between TiO2 and Cr6+. All the TiO2/Cr-MCM-41 catalysts deactivate gradually under visible light due to the reduction of Cr6+ in the catalysts. TPR and temperature program oxidation (TPO) results clearly indicated that the average oxidation state of chromium in TiO2/Cr-MCM-41 (Si/Cr = 20) was +2 after 23 h of photoreaction. The reduced chromium in the photocatalyst can be completely reoxidized by heating the catalyst at 450 8C for 3 h. # 2004 Elsevier B.V. All rights reserved. Keywords: TiO2/Cr-MCM-41; UV–vis; TPR; 4-Chlorophenol; Photocatalysis; Deactivation; Reactivation

1. Introduction Environmental pollution has increased public concern nowadays and decontamination of polluted water and air by photocatalysis has been attracting a lot of attention for its efficiency and promising economy. Semiconductor photocatalysis, as one of the advanced physicochemical processes, was extensively studied for solving existing environmental problems [1–6]. Titanium dioxide in the form of anatase is the most studied photocatalyst, but it can only work under ultraviolet light because of its large bandgap [2]. Solar light * Corresponding author. Tel.: +1 513 556 1474; fax: +1 513 556 3473. E-mail address: [email protected] (P.G. Smirniotis). 0926-3373/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2004.10.016

contains only 4% ultraviolet light when it reached to earth surface. In order to utilize the full spectrum of solar light, a number of modifications were tried mainly on TiO2 to develop a catalyst that will work under visible light [7–13]. The use of supported titanium dioxide has allowed to enhance the photodegradation rates in comparison with neat titania [13,14]. Doping TiO2 with metal ions may enhance its photoactivity [15,16] or even may enable its sensitization under visible light [16]. In our earlier studies involving TiO2-loaded transition-metal-incorporated MCM-41 [17], it was shown that only TiO2-loaded Cr-incorporated MCM-41 worked under visible light. In this study, the role of Cr in the catalyst was studied parametrically and the optimum amount of Cr-incorporated in the MCM-41 framework was found. A

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mechanism was proposed on the basis of UV–vis and TPR characterization results. A comparative catalytic study was performed with TiO2-loaded CrSi20Ox bulk oxides.

2. Experimental 2.1. Synthesis of TiO2-loaded Cr-modified MCM-41 specimens The Cr modified MCM-41 with atomic ratios of Si to Cr equal to 320, 120, 80, 40, 30, 20, 10 and 5 along with siliceous MCM-41 were prepared as reported in our earlier work [13]. The precursor of Cr was CrCl3
6H2O (Fisher), and that of silica was 40% Ludox HS-40 colloidal silica (Aldrich). In a typical preparation procedure, 3 ml of water was added to 35 ml of 40% Ludox HS-40 colloidal silica under stirring, then 30.32 ml of 25% tetramethylammonium hydroxide (Aldrich) was added. In a second beaker, 18.25 g of cetyltrimethylammonium bromide (CTABr, Alfa Aesar) was dissolved independently in 33 ml of water, it was heated at 80 8C. Subsequently, 7 ml of ammonium hydroxide (Fisher, 29.6%) was introduced when it became transparent and magnetically stirable after gelation. Finally, the latter solution was transferred to the first one. The corresponding amounts of CrCl3
6H2O were dissolved in 20 ml of water and added drop-wise to the resulting mixture. The final mixture was stirred for 120 min at 80 8C, then transferred into a Teflon bottle and treated under autogenous pressure without stirring at 100 8C for 3 days. The resulting solids were filtered, washed, dried, and calcined at 550 8C for 10 h (temperature increase of 2 8C/min) under a moderate airflow. The resulting catalyst (typically 1.0 g) was dispersed in 100 ml of isopropyl alcohol (Pharmco, 99.8%), and 1.28 ml of titanium(IV) isopropoxide (Aldrich, 97%) was added to achieve 25 % loading. The system was dried while stirring at ambient temperature. Then, the dried materials were calcined at 450 8C for 3 h with a heating rate of 2 8C/min. Two kinds of bulk oxides such as CrSi20Ox-1 and CrSi20Ox-2 with a ratio of Si to Cr equal to 20 were prepared for comparison. The former one was prepared in the same way as the Cr-MCM-41 prepared but without using any template. The latter one was synthesized by a coprecipitation method using urea as hydrolyzing agent. Quantities of CrCl3
6H2O (Fisher) and Na2SiO3
9H2O (Fisher) requisite for yielding the desired 1:20 molar ratio of Cr/Si were dissolved separately in deionized water. Then, the two solutions were mixed together. An excess amount of solid urea was also added. The resulting mixture was heated slowly to 90–95 8C on a hot plate with vigorous stirring. At this temperature the pH of the solution increased slowly at the beginning from 3.5 to 4.5 and then rapidly with increase in time of heating, attaining a final value between 7 and 8. At this point, small amounts of 28% ammonium hydroxide were added to the mixture solution. Then, heating was

continued for another 5–6 h in order to complete precipitation of respective oxides. The resulting precipitate was filtered and washed twice with 0.2 M ammonium nitrate solution to remove sodium ions, then washed with excess distilled water. The obtained solid was dried for 12 h and finally calcined at 550 8C for 10 h with a moderate airflow. The sample obtained was named CrSi20Ox-2. The 25% TiO2-deposited CrSi20Ox-2 was prepared in the same method as above. 2.2. Characterization 2.2.1. BET surface area and pore size BET surface area and pore size distribution studies were conducted by N2 physisorption under 77K using Micromeritics ASAP 2010 apparatus to characterize the synthesized photocatalysts. All samples were degassed at 300 8C under vacuum before analysis. 2.2.2. XRD All samples prepared by using the template were characterized using Siemens powder X-ray diffractometer equipped with a Cu Ka source to assess their regularity. The synthesized Cr-MCM-41 and 25% TiO2/Cr-MCM-41 samples were tested for 2u varying from 28 to 78 to assess the MCM-41 matrix and from 208 to 508 to assess the phase(s) of the TiO2 loaded. XRD results of the samples prepared with Si/Cr = 10 and Si/Cr = 5 did not show any peaks typical for MCM-41. Therefore, these two samples were named CrSi10Ox and CrSi5Ox instead of Cr-MCM-41. 2.2.3. UV–vis The catalyst powders were characterized by UV–vis spectrophotometer (Shimadzu 2501PC) with an ISR1200 integrating sphere attachment for their diffuse reflectance with the wavelength ranging from 200 to 900 nm. BaSO4 was used as the standard for these measurements. 2.2.4. TPR The temperature programmed reduction (TPR) was carried out in a range from 50 to 550 8C on Micromeritics Autochem 2910 instrument with a temperature ramp of 2 8C/ min. The samples were pretreated under 200 8C with ultrahigh-pure O2 (Matheson) for 2 h. 10 ml/min of 10%H2 in argon (Matheson) was passed through the sample tube during the measurement. 2.2.5. O2 pulse chemisorption The Cr dispersion and Cr surface area of the samples were measured using pulse O2 chemisorption with 4% O2 in helium (Matheson) on the Micromeritics Autochem 2910 instrument at the temperature of 550 8C. Prior to the experiment, the catalysts were pretreated with ultra-highpure hydrogen (Matheson) at 550 8C for 180 min. In this study, we assumed that one oxygen atom was adsorbed on each Cr active site.

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2.3. Catalytic activity 2.3.1. Visible light photocatalytic performance experiments The catalysts prepared were tested with the photodegradation of 4-chlorophenol, which was performed in a batch round flat-plate reactor described elsewhere [13]. The irradiation source was a 450 W medium pressure mercury lamp (Jelight). A double acrylic OP-2 (Museum quality) sheet was placed between the light source and the reactor for the purpose of excluding ultraviolet radiation when conducting visible-light experiments. The cooling jacket around the reactor allowed to effectively preclude the IR part of the spectrum from penetrating into the reaction solution and cooled the lamp. The temperature for reaction was kept at 25.0  0.5 8C. 400 mg of 25% TiO2-loaded catalyst was mixed with 500 ml of 1 mM 4-chlorophenol. The pH of the reaction suspension was not adjusted. The suspended catalyst in aqueous system was oxygenated (Wright Brothers, 99.9%) at 0.5 L/min to assure the complete saturation. The samples of reaction suspension were collected with a syringe at different intervals and filtered with Cameo 25P polypropylene syringe filters (OSMONICS, Cat# DDP02T2550). The sample solutions were analyzed with a total organic carbon analyzer (TOC-VCSH, Shimadzu). 2.3.2. Regeneration of the deactivated photocatalyst The Cr6+ in all the 25% TiO2/Cr-MCM-41 catalysts was found to be reduced during the photoreactions, which resulted in deactivation of the catalysts. Two methods were attempted to reactivate the reduced catalysts. In our study, 25% TiO2/Cr-MCM-41 (Si/Cr = 20) demonstrated the best performance for degradation of 4-chlorophenol. Therefore, we used this catalyst for the reactivation study in the following way. The suspensions of deactivated 25% TiO2/ Cr-MCM-41 (Si/Cr = 20) catalyst particles after each reaction experiment were filtered with a cellulose nitrate membrane filter (0.2 mm in pore size, MFS, A020A090C). The obtained deactivated catalyst was divided into two parts. One part of the deactivated catalyst was recalcined at 450 8C under airflow for 3 h; this temperature was used in order not to change the TiO2 phase in the 25% TiO2/Cr-MCM-41 (Si/ Cr = 20) catalyst. The other part was irradiated at 80 8C under the 450 W UV lamp used above with oxygen flow for 24 h. Afterwards, the reactivated catalysts were characterized with UV–vis spectrometer. The pore size and pore volume of the reactivated catalyst were measured on the Micromeritics 2010 analyzer. The photoactivity of the reactivated catalyst by calcinations was studied under visible light with the same setup that was used in the previous experiment. The sample solutions filtered in the same way as above were also scanned on the Shimadzu UV–vis spectrophotometer from 210 to 700 nm with a step size of 0.5 nm. The peaks appearing in the spectra were calibrated with CrO3, CrCl3, and 4-chlorophenol water solutions, and these peaks were also referred to for

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identifications of Cr6+ and Cr3+ in the spectra of CrMCM-41 and 25% TiO2/Cr-MCM-41. The deactivated and reactivated catalysts were also characterized with TPR–TPO on the Autochem 2910 apparatus. The temperature ranged from 50 to 550 8C with the rate of 2 8C/min.

3. Results and discussion The color of the final calcined Cr-MCM-41 samples changes from light yellow to dark yellow with Si/Cr ratio increasing from 320 to 20, then it becomes jungle green at the Si/Cr ratios of 10 and 5. This indicates that the ability of Cr3+ impregnated in the catalyst to be oxidized after calcination is strongly related to the total amount of Cr incorporated inside MCM-41 framework. Cr3+ cannot be fully oxidized to Cr6+ in samples with Si/Cr ratio less than 20 because of the disruption of the MCM-41 pores at a higher Cr concentration and thus the formation of bulk chromium oxide. 3.1. BET surface area and pore size and pore volume The BET surface areas of Cr-MCM-41 and 25% TiO2/CrMCM-41 (Table 1) decrease from 1117 to 664 m2/g with Si/ Cr changing from 320 to 20. The MCM-41 pore structure tends to collapse at a higher amount of Cr loading because the diameters of Cr and Si ions are different and the ability of MCM-41 structure for dispersing Cr is limited. The surface area of samples with Si/Cr = 10 and 5 is decreased drastically further due to the collapsing of MCM-41 structure, which is shown below by XRD characterization. Correspondingly, the pore size increases unanimously with the Cr concentration in the samples. The surface area decreases by about 200 m2/g with Cr-MCM-41 (Si/Cr  20) after TiO2 loading. The pore volume of Cr-MCM-41 (Table 1) decreased by about 0.25 cm3/g after the TiO2 loading and the pore size also decreased, implying that the majority of TiO2 stayed inside the MCM-41 pores. 3.2. XRD characterization One can observe (Fig. 1A and Table 1) that the unit cell parameters of Cr-MCM-41 samples with the Si/Cr ratio larger than 20 are approximately the same. The peak intensity of Cr-MCM-41 decreases with increasing the Cr concentration. The same trend is observed for the BET surface area of Cr-MCM-41. No XRD peak appears for the samples with Si/Cr ratio equal to 10 or 5, which proves that the MCM-41 structure cannot be formed at high Cr concentrations. For this reason, we use different names for these two samples, CrSi10Ox and CrSi5Ox. The unit cell parameter changes only to a very small extent after the TiO2 loading, thus indicating that the MCM-41 structure still exists. As shown Fig. 1B, the height of XRD peak of the CrMCM-41 samples decreases after TiO2 loading because a

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Table 1 Physical characterization results for Cr-MCM-41 and 25% TiO2/Cr-MCM-41 materials utilized in the present study Si/Cr

320 120 80 40 30 20 10c 5d 20e 20f 20g a b c d e f g

25% TiO2/Cr-MCM-41

Cr-MCM-41 BET (m2/g)a

Pore volume (cm3/g)a

Pore size ˚ )a (A

Unit cell ˚ )b parameter (A

BET (m2/g)a

Pore volume (cm3/g)a

Pore size ˚ )a (A

Unit cell ˚ )b parameter (A

1117 1074 999 912 848 664 239 204 – – –

1.01 1.05 0.99 0.95 1.21 1.14 0.89 0.93 – – –

35 38 40 40 57 69 144 176 – – –

43 42 42 43 42 43 – – – – –

883 851 777 712 657 547 213 198 548 564 583

0.74 0.77 0.65 0.60 0.80 0.81 0.59 0.63 0.83 0.85 0.88

32 35 32 37 48 57 108 124 59 58 58

41 41 41 43 40 42 – – 43 41 42

Measured by physisorption with pffiffiffi liquid nitrogen. Unit cell parameter = 2d100 = 3. CrSi10Ox (this sample did not have the MCM-41 structure). CrSi5Ox (this sample also did not have the MCM-41 structure). Reactivated 25% TiO2/Cr-MCM-41 (Si/Cr = 20) by calcination for the first time. Reactivated 25% TiO2/Cr-MCM-41 (Si/Cr = 20) by calcination for the second time. Reactivated 25% TiO2/Cr-MCM-41 (Si/Cr = 20) by calcination for the third time.

part of the pore structure is blocked with TiO2, which can also be seen from the decrease of the BET surface area and the pore size. The unit cell parameters of the 25% TiO2/CrMCM-41 samples with different Si/Cr ratios are also close to each other (Fig. 1B and Table 1). The peaks of titania in either rutile or anatase form are not detected by XRD in the 2u region from 208 to 508 (figure not shown) indicate that the deposited titania was well dispersed as an amorphous phase in Cr-MCM-41 matrix.

As shown in Fig. 2A, UV and visible light with a wavelength less than 550 nm can be absorbed by Cr-MCM41 materials. The light absorbance increases with the Cr concentration in the sample. Two broad light absorption peaks, corresponding to Cr6+, appear in UV range at 274 and 360 nm for all samples with different ratios of Si to Cr. The

intensities of these two peaks increase as the Si/Cr ratio of the Cr-MCM-41 decreases from 320 to 20 and the intensities decrease with further Cr incorporation. One can also observe a shoulder peak at about 430 nm for Cr-MCM-41 samples with Si/Cr = 320, 120, 80, 40, which also corresponds to the existence of Cr6+. This shoulder is less apparent in the case of Cr-MCM-41 (Si/Cr = 30 and 20) probably due to the color of these two samples becoming dark yellow. The UV–vis spectra of the CrSi10Ox and CrSi5Ox samples show two new intense peaks at 465 and 610 nm, which correspond to Cr3+. The Cr3+ peaks do not appear for any other Cr-MCM-41 samples prepared with Si/Cr  20. The loading of 25 wt.% TiO2 in the pores of Cr-MCM-41 increases the light absorbance ability of the samples. One can observe from the comparison of the UV–vis spectra (Fig. 2A and B) that the ability of the final TiO2-loaded Cr-MCM-41 to absorb light in the visible range of the spectrum is highly related with that of Cr-MCM-41. The peaks of Cr6+ at 274 and 360 nm simply

Fig. 1. XRD of the samples prepared from Si/Cr = 320 to 5: (A) Cr-MCM41 (Si/Cr = 320–20), CrSi10Ox (Si/Cr = 10) and CrSi5Ox (Si/Cr = 5) samples. (B) TiO2/Cr-MCM-41 (Si/Cr = 320–20), TiO2/CrSi10Ox (Si/Cr = 10) and TiO2/CrSi5Ox (Si/Cr = 5) samples.

Fig. 2. UV–vis spectra of the samples prepared from Si/Cr = 320–5: (A) CrMCM-41 (Si/Cr = 320–20), CrSi10Ox (Si/Cr = 10) and CrSi5Ox (Si/Cr = 5) samples. (B) TiO2/Cr-MCM-41 (Si/Cr = 320–20), TiO2/CrSi10Ox (Si/ Cr = 10) and TiO2/CrSi5Ox (Si/Cr = 5) samples.

3.3. UV–vis characterization

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become shoulders after the TiO2 loading. And the widths of the two shoulders increase with the Si/Cr ratio increasing from 320 to 20. The shoulder at 430 nm in Fig. 2A disappears or is hidden in the spectra of TiO2/Cr-MCM-41 (Si/Cr = 20–320) (Fig. 2B). The shoulder and peak at 465 nm are still shown as a shoulder for TiO2/CrSi10Ox and as a strong peak for TiO2/CrSi5Ox. The peak at 610 nm remains the same for TiO2/CrSi10Ox and TiO2/CrSi5Ox. The visible light (400–500 nm) absorbance of the final catalysts increases with the amount of Cr incorporated. 3.4. TPR characterization The TPR profiles of Cr-MCM-41 and TiO2/Cr-MCM-41 samples are shown in Fig. 3A and B. As shown in Fig. 3A, only one reduction peak appears at about 340 8C and this reduction peak height and area increase with Si/Cr decreasing from 320 to 20. This indicates that only one oxidation of Cr present in these samples. There are two reduction peaks appearing at about 250 and 370 8C in the samples with Si/Cr ratios of 10 and 5, which proves that different oxidation states of Cr co-exist. This result is consistent with that of UV–vis spectra and the visual observation of the samples’ colors. The TPR profiles of TiO2-loaded MCM-41, Cr-MCM-41 (Si/Cr = 320–20), CrSi10Ox and CrSi5Ox (Fig. 3B) show different reduction peaks from those of the original samples without TiO2 loading (Fig. 3A). The interaction between Cr and TiO2 lowers the beginning temperature of the reduction process by about 70 8C and widens the temperature window for reduction. The appearance of the number of reduction transition peaks of these materials depends on the Cr concentration and the interaction between Cr and TiO2. One can clearly observe that there are three reduction peaks in the TPR profiles of TiO2-loaded Cr-MCM-41 samples with Si/ Cr ratio from 320 to 40. The first reduction peak at 270 8C is due to the reduction of Ti4+ to Ti3+ because of its interaction with chromia, and the second reduction peak at 317 8C is due

Fig. 3. TPR profiles of the samples prepared from Si/Cr = 320–5: (A) CrMCM-41 (Si/Cr = 320–20), CrSi10Ox (Si/Cr = 10) and CrSi5Ox (Si/Cr = 5) samples. (B) TiO2/Cr-MCM-41 (Si/Cr = 320–20), TiO2/CrSi10Ox (Si/ Cr = 10) and TiO2/CrSi5Ox (Si/Cr = 5) samples.

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to the reduction of Cr6+ to Cr5+ because of its interaction with titania. As shown in Fig. 3B, the amount of hydrogen consumed for the first reduction peak (270 8C) is exactly equal to that for the second one (317 8C). Accordingly, it is calculated that the oxidation state change for both Ti and Cr in Ti–O–Cr is 1. Because of the covering of Cr-MCM-41 by TiO2, Cr5+ may not be further reduced in the temperature range applied. That is why TiO2-loaded systems show lower H2 consumption values (Fig. 4A) than those corresponding Cr-MCM-41 samples (Fig. 4B), in which Cr6+ was completely reduced to Cr3+. The third reduction peak at 390 8C is due to the reduction of Cr6+ free from the interaction with Ti4+. The first two peaks in the TPR curve are combined into one for the samples with Si/Cr = 30 and 20. Therefore, only two reduction transition peaks are observed for these two samples. The atomic ratio of Ti to Cr in 25% TiO2/Cr-MCM-41 (Si/Cr = 320) sample is 80.4, whereas that in 25% TiO2/Cr-MCM-41 (Si/Cr = 20) is 5.4. For the TiO2/Cr-MCM-41 (Si/Cr = 320–20) materials, the number of Ti atoms is much higher than that of Cr, each of the Cr6+ ions present in these materials can interact with the TiO2 loaded on the surface inside Cr-MCM-41 pores. However, this layer of TiO2 stops a fraction of Cr6+ in the framework from being reduced by H2 as indicated by the decrease of peak intensity in TPR (Fig. 3) and the decrease of Cr dispersion (Table 2) after TiO2 loading. Hence, the

Fig. 4. H2 consumption values calculated from TPR study vs. Cr/Si ratio: (A) (a) values corresponding to the interaction of Cr6+ and Ti4+ (the first and second peaks for TiO2/Cr-MCM-41 (Si/Cr = 320–40), the first peak for TiO2/Cr-MCM-41 (Si/Cr = 30 and 20), the first peak for TiO2/CrSi10Ox, the first and second peaks for TiO2/CrSi5Ox from Fig. 3B); (b) values corresponding to the free Cr6+ (the third peak for TiO2/Cr-MCM-41 (Si/ Cr = 320–40), the second peak for TiO2/Cr-MCM-41 (Si/Cr = 30 and 20), the second peak for TiO2/CrSi10Ox, the third and fourth peaks for TiO2/CrSi5Ox from Fig. 3B). (B) Total consumption values obtained form samples prepared from Si/Cr = 320–5 and TiO2-loaded samples.

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Table 2 Cr-dispersion, Cr-surface area and active particle size for Cr-MCM-41 and 25% TiO2/Cr-MCM-41 materials determined by oxygen pulse chemisorption measurements Si/Cr

320 120 80 40 30 20 10b 5c a b c

25% TiO2/Cr-MCM-41

Cr-MCM-41 Cr dispersion (%)a

Cr surface area (m2/g catalyst)

Active particle size (nm)

Cr dispersion (%)a

Cr surface area (m2/g catalyst)

Active particle size (nm)

39 39 34 25 30 31 7 3

0.78 2.06 2.67 3.79 6.08 9.16 4.13 3.19

2.89 2.92 3.31 4.60 3.78 3.68 15.32 35.40

43 34 31 19 16 15 3 3

0.63 1.35 1.80 2.16 2.35 3.45 1.37 2.08

2.63 3.34 3.70 6.07 7.33 7.33 34.60 40.71

Cr dispersion was measured based on the stoichiometric factor used for oxygen to chromium equal to 1. CrSi10Ox (this sample did not form Cr-MCM-41 structure). CrSi5Ox (this sample also did not form Cr-MCM-41 structure).

hydrogen consumption corresponding to the third peak induced by reduction of Cr6+ in Fig. 3B remains small until Si/Cr decreases to 20 (Fig. 4A). In contrast, the H2 consumption corresponding to the first and second peaks in Fig. 3B increased linearly with the amount of Cr incorporated (Fig. 4A). The total H2 consumed by either Cr-MCM-41 or TiO2/Cr-MCM-41 catalyst samples during TPR studies increases also linearly with the Cr amount increasing from 320 to 20 (Fig. 4B), which implies that the oxidation states of Cr in these catalysts are the same. The TPR results combined with the UV–vis absorption results above clearly show that Cr exists as Cr6+ in the samples prepared with the Si/Cr ratio ranging from 320 to 20. The TPR profile of 25% TiO2/CrSi10Ox sample show two reduction peaks (Fig. 3B). The first broad peak at 246 8C corresponds to reduction of Cr6+ and Ti4+ interacting with each other and also with silica. The second peak at 390 8C corresponds to the reduction of bulk CrO3. In contrast, the TPR profile of 25% TiO2/CrSi5Ox shows four reduction peaks (Fig. 3B). The first peak at 223 8C corresponds to the reduction of Ti4+ interacting with silica and chromia. The second peak at 255 8C is related to the reduction of Cr6+ interacting with titania and silica. The third and fourth peaks are due to the stepwise reduction of bulk chromium oxide. However, the intensity of these peaks clearly demonstrate that the consumptions of H2 by 25% TiO2/CrSi10Ox and 25% TiO2/CrSi5Ox are much less than those by 25% TiO2/CrMCM-41 (Si/Cr  20). This means that most of the Cr in the TiO2-loaded CrSi10Ox and CrSi5Ox samples is in Cr2O3 phase, which cannot be reduced at selected reduction temperature. The lower H2 consumption by the catalysts with Si/Cr ratios of 10 and 5 was consistent with the existence of different Cr oxidation states indicated by UV– vis characterization results. 3.5. O2 pulse chemisorption The O2 pulse chemisorption experiments (Table 2) show that the dispersion of Cr in Cr-MCM-41 decreases and the Cr surface area increases almost all along with decreasing the

ratio of Si to Cr. This was because a large amount of chromium oxide is stabilized in the pores and at the mouth of the MCM-41 pores as well as being dispersed in the MCM41 structure. The ability of SiO2 to accept Cr ions in MCM41 structure is limited because Si4+ and Cr6+ sizes are different. The possibility of chromium oxide congregating together becomes larger with more Cr incorporation. The results ensued are the increase of Cr particle size and the decrease of Cr dispersion. At the same time, the original/ siliceous MCM-41 structure has a higher tendency to collapse with more Cr incorporation, as proved by the results of BET surface area and XRD characterization. Interestingly, the Cr dispersion of TiO2/Cr-MCM-41 (Si/Cr = 320) is higher than that of the Cr-MCM-41 (Si/Cr = 320). This may be because the amount of TiO2 reduced during the H2 pretreatment for O2 pulse chemisorption measurement is comparable to that of Cr sites in the sample with Si/ Cr = 320. The dispersion of Cr in other 25% TiO2/Cr-MCM41 samples is less than that of their corresponding Cr-MCM41 materials. The Cr dispersion decreases monotonically with decreasing Si/Cr. Correspondingly, the Cr surface area and Cr active particle size increases all along from 0.63 m2/g sample and 2.63 nm at Si/Cr = 320 to 3.45 m2/g sample and 7.33 nm at Si/Cr = 20, respectively. The Cr surface area decreases dramatically with further Cr incorporation because of the mixed oxidation states [18] in 25% TiO2/ CrSi10Ox and 25% TiO2/CrSi5Ox samples. The calculated active particle size increased to 40.7 nm. The changes of the characterization results after TiO2 loading also indicate the interaction between Cr and TiO2. 3.6. Catalytic activity 3.6.1. Catalytic study on various amounts of Crincorporated 25% TiO2/Cr-MCM-41 catalysts Fig. 5A shows the curves of total organic carbon (TOC) concentration with respect to time during 4-chlorophenol photodegradation under visible light. No decrease of TOC concentration is observed with TiO2/MCM-41. In comparison, the activity of TiO2/Cr-MCM-41 increases with

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Fig. 6. UV–vis spectra of: (A) deactivated TiO2/Cr-MCM-41 (Si/Cr = 320– 20), TiO2/CrSi10Ox and TiO2/CrSi5Ox samples. (B) Cr-MCM-41 (Si/ Cr = 20), CrSi20Ox-1 and CrSi20Ox-2 bulk oxides, TiO2/Cr-MCM-41 (Si/ Cr = 20), TiO2/CrSi20Ox-1 and TiO2/CrSi20Ox-2.

Fig. 5. (A) The time course of total carbon concentration during photocatalytic 4-chlorophenol decomposition with TiO2/Cr-MCM-41 (Si/ Cr = 320–20), TiO2/CrSi10Ox and TiO2/CrSi5Ox. The curves were correlated according to the proposed model. (B) Initial TOC removal rate with respect to Cr/Si ratio.

decreasing Si/Cr ratio from 320 to 20. This ratio reaches a maximum at Si/Cr = 20 and then decreases dramatically with further Cr incorporation. Unfortunately, all the catalysts deactivate gradually in the visible light. That is, the rate of TOC concentration decrease becomes smaller and then zero after a certain reaction time. The deactivation mechanism was proposed to be the gradual reduction of Cr6+ during photoreaction [13,19]. The TPR–TPO study of deactivated catalyst discussed below will show that the final Cr oxidation state after deactivation is +2. In order to single out the influence of the Cr6+ reduction on the reaction rate, we use the initial TOC degradation rate (Table 3 and Fig. 5B) to quantify the activity of the TiO2/Cr-MCM-41 catalysts. The initial rate increases with the Cr/Si ratio, namely

increasing from 0 to 0.05, and becomes smaller with the Si/ Cr ratio of 10 and 5. The comparison of Fig. 5B with Fig. 4A indicates that the activity of 25% TiO2/Cr-MCM-41 is highly related to the Cr6+ concentration in the final catalyst, and the Cr6+ concentration is the most critical for determining the activity. The low XRD peak and the highest activity of 25% TiO2/Cr-MCM-41 (Si/Cr = 20) seem to indicate that the uniform MCM-41 structure is not the most important factor for the total ability of 4-chlorophenol photodegradation under visible light. This will be discussed in the later paragraphs through comparison with the catalysts without MCM-41 structure. All the synthesized photocatalysts with different Si/Cr ratios deactivated after a period of time under visible light. The reaction suspensions were filtered and the deactivated catalysts were collected and studied by UV–vis spectrometer (Fig. 6A). The shoulders at 274 and 360 nm corresponding to Cr6+ in Fig. 2B for the original catalysts disappear after deactivation. Moreover, the above 600 nm wavelength region of the UV–vis spectra for the original catalysts (Fig. 2B) is leveled off to the base line; in comparison, the same region for deactivated catalysts (Fig. 6A) is not leveled off to the base line, which proves the existence of Cr3+ and Cr2+ ions in the deactivated catalysts. This will be proved further by the TPR–TPO study discussed later. The shoulder and peaks at 465 and 610 nm corresponding to Cr3+ survive in the spectra for 25% TiO2/CrSi10Ox and 25% TiO2/CrSi5Ox samples after deactivation. These results prove that the Cr6+ reduction is the reason for the catalysts’ deactivation, and Cr6+, instead of Cr3+, is the reason for the catalyst’s ability to oxidize 4chlorophenol under visible light. From the UV–vis spectra (Figs. 2B and 6A), one can see that only a part of Cr in CrSi10Ox, CrSi5Ox, TiO2/CrSi10Ox and TiO2/CrSi5Ox is Cr6+. The lower activity of TiO2/CrSi10Ox and TiO2/CrSi5Ox samples also indicates that the visible light absorption is only one prerequisite for the catalyst’s activity under visible light.

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Table 3 Initial TOC removal rate during 4-CP photodegradation with 25%/CrMCM-41 (Si/Cr = 320, 120, 80, 40, 30, 20, 10, 5) Si/Cr

Cr/Si

Initial TOC removal rate (102 ppm/min)

320 120 80 40 30 20 10 5 20a 20b 20c

0.003 0.008 0.013 0.025 0.033 0.050 0.100 0.200 0.050 0.050 0.050

1.0  0.3 3.1  0.3 3.6  0.6 5.8  0.6 6.3  1.1 8.1  1.1 5.2  0.9 5.7  0.3 9.3  1.1 8.9  0.8 8.2  0.9

a b c

Fig. 7. TPR profiles of: (A) (a) Cr-MCM-41 (Si/Cr = 20); (b) CrSi20Ox-2 (Si/Cr = 20), (c) CrSi20Ox-1 (Si/Cr = 20). (B) (a) TiO2/Cr-MCM-41, (b) TiO2/CrSi20Ox-2, (c) TiO2/CrSi20Ox-1.

Reactivated by calcination for the first time. Reactivated by calcination for the second time. Reactivated by calcination for the third time.

3.6.2. Comparison of 25% TiO2/Cr-MCM-41 (Si/Cr = 20) with two kinds of 25% TiO2-loaded CrSi20Ox bulk oxide catalysts In order to clarify the role of MCM-41 structure in 25% TiO2/Cr-MCM-41 for organic compound photodegradation under visible light, a comparative study was carried out between TiO2/Cr-MCM-41 (Si/Cr = 20) and two kinds of TiO2-loaded CrSi20Ox bulk oxide catalysts synthesized without the CTABr template which is necessary for the formation of the MCM-41 structure. The BET surface areas of CrSi20Ox-1, CrSi20Ox-2, TiO2/CrSi20Ox-1 and TiO2/ CrSi20Ox-2 (Table 4) are lower than those of Cr-MCM-41 and TiO2/Cr-MCM-41 (Si/Cr = 20). Fig. 6B shows the UV– vis spectra of CrSi20Ox-1, CrSi20Ox-2, TiO2/CrSi20Ox-1 and TiO2/CrSi20Ox-2. The peaks and shoulders corresponding to the Cr6+ species appear in the curves for all the four samples. However, the shoulder at 274 nm nearly disappears for 25% TiO2/CrSi20Ox-2. This may be because the catalyst, which is composed of very small particles, happens to have a higher diffraction of light with a wavelength less than 360 nm. 25% TiO2/CrSi20Ox-1 (Si/Cr = 20) also absorbs much less light in the above wavelength region than 25% TiO2/Cr-MCM-41 (Si/Cr = 20). As shown in Fig. 6B, the width of the shoulder at 360 nm for 25% TiO2/CrSi20Ox-1 and 25% TiO2/ CrSi20Ox-2 is less than that for 25% TiO2/Cr-MCM-41 (Si/Cr = 20), which indicates that the MCM-41 structure helps the Cr6+ formation in the final sample and thus the Cr6+

interaction with TiO2 in the presence of light absorption. The peaks corresponding to Cr3+ do not appear in the spectra for all the samples. The TPR studies reveal some interesting results as shown in Fig. 7A and B. Although CrSi20Ox-1, CrSi20Ox-2 and CrMCM-41 (Si/Cr = 20) begin to consume H2 at about 250 8C, the reduction peaks for CrSi20Ox-2 and CrSi20Ox-1 appear before and after the peak for Cr-MCM-41, respectively. A sharp and most intense reduction peak appears at 378 8C in the TPR profile of CrSi20Ox-1. In comparison, broad reduction peaks show up at 341 and 352 8C in the TPR profiles of CrSi20Ox-2 and Cr-MCM-41 (Si/Cr = 20), respectively. These results indicate that Cr6+ in CrSi20Ox2 and Cr-MCM-41 (Si/Cr = 20) are more evenly distributed than that in CrSi20Ox-1. The total H2 consumed by CrSi20Ox1, CrSi20Ox-2 and Cr-MCM-41 (Si/Cr = 20) (Table 4 and Fig. 7A) during the TPR studies are 20.8, 22.5, and 27.1 ml/g sample, respectively. The relative order of these numbers is consistent with the Cr dispersion results. Two peaks appear in each of the TPR profiles of 25% TiO2/CrSi20Ox-1, 25% TiO2/CrSi20Ox-2, and 25% TiO2/CrMCM-41 (Si/Cr = 20) (Fig. 7B). As discussed above, the first peak is due to the reduction of Ti4+ and Cr6+ interacting with each other. The hydrogen consumed corresponding to the first peak areas are 6.7, 3.4 and 10.5 STP ml/g sample, respectively. The TOC concentration curves during 4chlorophenol photodegradation under visible light are presented in Fig. 8A and the calculated initial TOC removal rates are summarized in Table 4. A good correlation is found

Table 4 The physico-chemical characteristics and catalytic performances of the CrSi20Ox (Si/Cr = 20) bulk oxides and 25% TiO2/CrSi20Ox Sample

BET surface area (m2/g sample)

Total H2 consumption (ml/g sample)a

Cr dispersion (%)b,c

Metal surface area (m2/g sample)b

Active particle size (nm)b

Initial TOC removal rate (102 ppm/min)

CrSi20Ox-1 CrSi20Ox-2 TiO2/CrSi20Ox-1 TiO2/CrSi20Ox-2

256 586 225 432

20.8 22.5 11.3 8.8

20.0 26.7 11.6 8.9

5.95 7.95 2.58 2.00

5.67 4.24 9.80 12.67

– – 6.2  0.5 3.2  0.1

a b c

STP, total H2 adsorption in the TPR measurements. These values calculated from the O2 measured with pulse O2 chemisorption measurements. Cr dispersion was measured based on the stoichiometric factor used for oxygen to chromium equal to 1.

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Fig. 9. (A) TPR profiles of fresh TiO2/Cr-MCM-41 (Si/Cr = 20), D(1): first time deactivated, R(1): first time reactivated, R(2): second time reactivated, R(3): third time reactivated, D(4): fourth time deactivated samples. (B) TPO profiles of fresh TiO2/Cr-MCM-41 (Si/Cr = 20), D(1): first time deactivated, R(1): first time reactivated, R(2): second time reactivated, R(3): third time reactivated, D(4): fourth time deactivated samples.

Fig. 8. (A) The time course of total organic carbon concentration during 4chlorophenol decomposition by TiO2/Cr-MCM-41 (Si/Cr = 20) and the two kinds of TiO2/CrSi20Ox bulk oxides. (B) The initial TOC removal rates with respecttoH2 absorbedcorrespondingtothe firstpeakoftheTPRcurvesinFig.7B.

between the initial TOC removal rate and the H2 consumed corresponding to the first peak of the TPR profiles as shown in Fig. 8B. In comparison, there is no good correlation between the activity and the total H2 consumed for the two reduction peaks in TPR studies. The results above indicate that the interaction of Cr6+ with TiO2 is crucial for a catalyst’s activity and the role of MCM-41 structure is to enlarge the interaction between TiO2 and Cr6+ compared with bulk silicon oxides [13,17]. Therefore, we conclude that the catalytic activity depends on the catalysts preparation method and the structure of the support material. 3.6.3. Regeneration of deactivated catalyst 25% TiO2/ Cr-MCM-41 (Si/Cr = 20) TiO2/Cr-MCM-41 (Si/Cr = 20) has been selected for the regeneration studies due to its highest activity towards the degradation of 4-chlorophenol. Fig. 9A and B shows the TPR and TPO results of the deactivated and reactivated TiO2/Cr-MCM-41 (Si/Cr = 20). The corresponding H2 consumptions of the original sample to the first and second peaks are 10.5 and 2.4 ml/g. One unit of Ti4+ and Cr6+

interacting together [13] loses one bonded oxygen atom [20– 22] and the ions become Ti3+ and Cr5+ in the temperature range of the first TPR peak. There are two possibilities corresponding to the temperature range of the second TPR peak. First, one unit of free Cr6+ loses one bonded oxygen atom and Cr6+ becomes Cr4+. Second, two units of adjacent Cr6+ lose one bonded oxygen atom and Cr6+ becomes Cr5+. Because the ratio of Cr to Si in the selected sample is 1:20, the probability of two Cr6+ to be in adjacent locations in the structure is low and thus it is not considered in our calculation. In this way, the ratio of the calculated number of moles of total consumed H2 to that of the original Cr6+ is 0.98:1. Only a fraction (43%) of the reduced sites can be oxidized back to the original states in the temperature range of the TPO study. This is because each oxygen molecule has to dissociate into two atoms first in order to oxidize the reduced sites isolated from each other, and right sites are needed for dissociation even at a high temperature [20–22]. With chromium being well dispersed in the catalyst, Cr4+ or one pair of Ti3+ and Cr5+ reduced tends to be far from each other, one oxygen molecule adsorbed first dissociates in one site, one oxygen atom stays and the other migrated to other sites, which takes time and energy [22]. The peaks in the TPR profile of the original catalyst do not appear in the TPR profile of the deactivated catalyst, which implies the loss of Cr6+. This agrees with the UV–vis spectra results presented in Fig. 10. A new peak centered at around 430 8C shows up in the TPR profile of the sample that deactivated for the first time (Fig. 9A) and the volume of H2 consumed is 8.2 ml/g sample. A large peak centered at 360 8C appears in the TPO figure for the deactivated catalyst (Fig. 9B) and the volume of O2 absorbed is 16.9 ml/g sample. Then, the average Cr oxidation state in the deactivated catalyst is calculated to be +2 assuming that Ti3+ reduced is oxidized back to Ti4+ and the Cr oxidation state is +6 after TPO. The ratio of the number of moles of carbons from destructed 4-chlorophenol to the number of moles of electrons formed during Cr

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Fig. 10. UV–vis spectra of fresh TiO2/Cr-MCM-41 (Si/Cr = 20), D(1): first time deactivated, R(1): first time reactivated, R(2): second time reactivated, R(3): third time reactivated, D(4): fourth time deactivated samples.

reduction in the photoreaction is 1.5  0.1 by assuming that six electrons are needed for each 4-chlorophenol molecule destructed. That is, each Cr cation is reduced and oxidized, between +6 and +2, 1.5 times before it totally deactivates. The deactivated catalysts are gray, which is different from the yellow color of Cr6+ and the green color of Cr3+. UV

TiO2 !hþ þ e

(1)

hþ þ Cr2þ ! Cr6þ

(2)

e þ O2 ! O 2

(3)



450 C

CrO þ O2 ! CrO3

(4)

UV irradiation was tried to reactivate the catalyst. But the UV–vis spectrum of the deactivated 25% TiO2/Cr-MCM-41 (Si/Cr = 20) after 24 h UV irradiation is identical to that before the UV irradiation (Fig. 10). And the color of the deactivated catalyst still remains gray after the irradiation. Therefore, the reactions in Eqs. (1)–(3) cannot go through, and UV irradiation cannot oxidize Cr2+ back to Cr6+ under the conditions used in the present study. Recalcination is the other way we tried for reactivating the deactivated catalyst. The reactivated 25% TiO2/Cr-MCM-41 (Si/Cr = 20) exhibits the same spectrum as the original active one but with a smaller shoulder at 360 nm. This may be because the recalcination temperature used is 450 8C for keeping the TiO2 phase unchanged, while 550 8C is the calcination temperature for preparing Cr-MCM-41 (Si/Cr = 20) prior to the TiO2 loading. The color of the catalyst changes from gray back to its original yellow after recalcination, which indicates that the Cr2+ is oxidized into Cr6+ (Eq. (4)). And the activities of reactivated 25% TiO2/Cr-MCM-41 (Si/ Cr = 20) for the first, second, and third times were tested with 4-chlorophenol again under visible light. These

Fig. 11. The time course of total carbon concentration during 4-chlorophenol decomposition over fresh TiO2/Cr-MCM-41 (Si/Cr = 20), first time reactivated, second time reactivated and third time reactivated catalysts.

catalysts demonstrate nearly the same photoactivities as the fresh catalyst (Table 3 and Fig. 11). As shown in Fig. 9, the pattern of TPR–TPO profiles of deactivated catalysts (D(1) and D(4)) is different from those of fresh and reactivated catalysts (Fresh, R(1), R(2), and R(3)). The BET surface area, pore volume, and pore size increased a little after several reactivations and the unit cell parameter kept the same (Table 1). These results proved that the structure of the catalyst was stable. 3.6.4. Cr-leaching study In the UV–vis spectra of the sample solutions (Fig. 12), the peak at 350 nm is attributed to Cr6+ species leached in the solution and the peak at 279 nm is due to both Cr6+ and 4-chlorophenol. No other peak is detected. As shown in

Fig. 12. UV–vis spectra of the sample solutions taken during 4-chlorophenol photodegradation over TiO2/Cr-MCM-41 (Si/Cr = 20) catalyst at various time intervals.

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that the interaction of Cr6+ with loaded TiO2 is crucial for the catalyst’s activity. The MCM-41 structure enlarges the interaction between TiO2 and Cr6+ compared with the bulk oxide materials. All the TiO2/Cr-MCM-41 catalysts deactivate gradually under visible light due to the reduction of Cr6+ in the catalysts. The deactivated 25% TiO2/CrMCM-41 (Si/Cr = 20) can be fully reactivated by recalcination at 450 8C with airflow for 3 h. The structure of the reactivated catalysts is very stable. The rate of Cr leaching is low after 10 h of operation under visible light.

Acknowledgements

Fig. 13. Reaction time vs. concentrations of Cr6+ (280 nm) and 4-chlorophenol (350 nm) obtained from UV–vis spectroscopy from Fig. 12.

Fig. 12, the intensities of both peaks decrease continuously along with time. The concentrations of Cr6+ and 4chlorophenol in the filtered solutions are presented in Fig. 13. Both of them decrease with reaction time. The amount of Cr6+ present in the samples collected becomes very small after 800 min of reaction time. The 100% reactivation of 25% TiO2/Cr-MCM-41 (Si/Cr = 20) after complete deactivation also proves that Cr6+ leached after filtration is negligible from another aspect. The samples taken in the experiments with the reactivated catalysts are also measured in the same way and the corresponding UV–vis spectra of each series of sample solutions (not shown) follow the same pattern as the fresh catalyst.

4. Conclusions The 25% TiO2-loaded Cr-incorporated MCM-41 catalyst is active for the photodegradation of 4-chlorophenol under visible light. The optimum atomic ratio of Si to Cr in the catalyst is 20. The activity of the catalysts increases with the Cr concentration up to the optimum amount of Cr in the catalyst. However, the activity decreases abruptly with further Cr incorporation due to the decreasing Cr6+ concentration in the catalyst. Cr6+ is the form of chromium in Cr-MCM-41 and 25% TiO2/Cr-MCM-41 with Si/Cr ratio larger than 20. Mixed chromium oxides (Cr6+ and Cr3+) form in the samples with Si/Cr less than 20 due to the destruction of MCM-41 structure. Cr6+, instead of Cr3+, is found to be the active form. The catalyst’s activity is highly related to the Cr6+ concentration in the catalyst. 25% TiO2/ Cr-MCM-41 (Si/Cr = 20) is more active for 4-chlorophenol photodegradation under visible light than 25% TiO2-loaded CrSi20Ox bulk oxides with the same chromium concentration. The characterization and performance studies show

The authors wish to acknowledge the NSF and the US Department of Army for partial support for this work through the grants CTS-0097347 and DAAD 19-00-1-0399, respectively. 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.

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