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hectorite thin films layered catalyst
Journal of Colloid and Interface Science 299 (2006) 125–135 www.elsevier.com/locate/jcis
Photooxidation of dibenzothiophene on TiO2/hectorite thin films layered catalyst Jamie Robertson, Teresa J. Bandosz ∗ Department of Chemistry, The City College of New York, 138th Street and Convent Avenue, New York, NY 10031, USA Received 11 December 2005; accepted 7 February 2006 Available online 5 April 2006
Abstract A new titanium(IV) oxide–hectorite nanofilm photocatalyst was prepared on quartz slides. It was evaluated in the photooxidation of dibenzothiophene (DBT) in nonpolar organic solution (tetradecane), as a model for diesel fuel. A removal regimen was developed consisting of catalytic photooxidation followed by adsorption of products on silica gel. Photooxidation of DBT was performed with and without catalyst, at 254 and 300 nm. Comparison was made with a commercially available TiO2 catalyst, Degussa P25. The catalyst was analyzed by nitrogen adsorption, XRD, SEM, and TGA-DTA. DBT concentrations were measured by HPLC and UV spectrophotometry. Preliminary qualititative analysis of products was performed by UV and HPLC. Results indicated that the outlined process was effective in reducing sulfur levels to below 10 ppm sulfur. © 2006 Elsevier Inc. All rights reserved. Keywords: Titanium oxide; Dibenzothiophene; Photooxidation; Desulfurization
1. Introduction U.S. Environmental Protection Agency regulations slated to take effect in 2006 require deep desulfurization of both gasoline and diesel fuels. Specifically, new fuel standards call for eventual reduction in sulfur content to below 30 ppm sulfur by mass for gasoline and 15 ppm for diesel [1]. Sulfur in fuel leads to the release of SO2 upon combustion. High sulfur content is also associated with the poisoning of catalysts in catalytic converters [2] and fuel cells. Dibenzothiophene (DBT) and its derivatives occur naturally in coal and oil deposits. It is one of the most difficult sulfur compounds to remove from liquid fuels. The conventional desulfurization method involves reduction with hydrogen gas at high pressures. Unfortunately, this method is not very effective at removing DBT and its alkylated derivatives [3]. This necessitates the development of improved techniques for the removal of DBT from gasoline and diesel fuels. Recently, a number of researchers have investigated the use of photooxidation to re* Corresponding author. Fax: +1 212 650 6107.
E-mail address: [email protected] (T.J. Bandosz). 0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2006.02.011
move DBT and other heterocyclic sulfur compounds from both aqueous and organic solutions [4–11]. Most of these investigations have taken place in the presence of water. Some have been performed with the aid of catalysts, such as titanium(IV) oxide. The use of titanium(IV) oxide as a photocatalyst has received extensive coverage in Ref. [12]. TiO2 can absorb light throughout the UV range and has been demonstrated to be an effective photocatalyst for a number of reactions [12]. Recently, Dekany and co-workers have demonstrated the synthesis of multilayer photocatalysts containing metal oxides layered with clays [13,14]. The multilayer photocatalyst is affixed to a solid substrate. This prevents some common problems associated with heterogeneous catalysts in some other systems. Titanium(IV) oxide has been investigated as a catalyst for the oxidation of dibenzothiophene [11,15]. It has been found to greatly enhance the rate of photooxidation, particularly at higher wavelengths. At wavelengths above 300 nm, the extinction coefficient for DBT is much lower, so that photooxidation at these wavelengths is more likely due to indirect (catalyzed) photolysis than to direct photolysis. Much of the research with TiO2 photocatalysts has involved suspensions of the catalysts in either aqueous or polar organic solution. This method can pose a few logistical problems. Cat-
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alyst suspensions limit the amount of TiO2 that can be used, since agitation can only suspend a limited amount of catalyst. Also, suspended catalyst particles may actually block out ultraviolet light. This is particularly problematic for any scale-up of the process. Finally, suspended catalysts are costly and problematic to remove. Thus various methods and substrates have been used to make nanoscale TiO2 catalysts [13,16,17] with mixed effects. While some papers focus primarily on benzothiophenes in the natural environment [6,8,10], others describe the removal of these species from petroleum feedstocks [5,7,9]. When a 300-W high-pressure mercury lamp (unfiltered) was used to photoirradiate solutions of DBT in tetradecane (100 ml) or water (300 ml), DBT and its derivatives were completely removed over the course of less than 10 h. The rate of photolysis was highest for the most substituted DBT and lowest for the least substituted, which is the reverse of the reaction rate for hydrodesulfurization. Hirai et al. reported that when the tetradecane–DBT solution was irradiated without aqueous phase, yellow discoloration and increased viscosity occurred in the organic phase [5]. They also found that the absence of air or oxygen dramatically reduced the rate of photolysis. After irradiation, only residual DBT was present in the organic phase, without other sulfur compounds. Less than 1% of tetradecane was decomposed in the process. In the aqueous phase, the presence of sulfate ion (SO2− 4 ) was detected. Its concentration increased with a decrease in concentration of DBT, with 80% of the sulfur converted to SO2− 4 . In commercial light oil, the desulfurization was found to be 22% after 30 h, far less than predicted, which was attributed to presence of naphthalene in the oil. The acidic minor products of the DBT oxidations were investigated by Shiraishi et al. [9]. The formation of 2-sulfobenzoic acid anhydride and benzothiophene-2,3-dicarboxylic acid was reported. The objective of this research is to evaluate a two-stage process for removal of dibenzothiophene from nonpolar organic solvent. The project requires the synthesis of a multilayer TiO2 – hectorite photocatalyst and the study of this catalyst for the photooxidation of dibenzothiophene in organic solution. In determining the efficiency of the catalyst, studies are performed using light at different wavelengths, in the presence or absence of the catalyst. Comparative studies are also carried out between photooxidation of dibenzothiophene with the synthesized catalyst and with a commercially available TiO2 catalyst. 2. Experimental
The titanium oxide sol was prepared as in Lakshmi et al. [18]. Briefly, 5 ml of titanium(IV) isopropoxide was added to 25 ml ethanol. The solution was stirred in an ice bath. To another 25-ml aliquot of ethanol, 0.5 ml of deionized water and 0.5 ml of 0.1 M hydrochloric acid were added. The solution was removed from the ice bath and placed in a 15 ◦ C water bath Then the acid solution was slowly added to it. A milky white suspension formed within 1 min. The hectorite suspension was prepared as in Kotov et al. [13]. One gram of synthetic hectorite (Na0.3 (Mg,Li)3 Si4 O10 (OH)2 ) was added to 100 ml of deionized water. The mixture was placed in a sonication bath (Branson 1510) for 40 min. The mixture was centrifuged (Fisher Centrific, 3000 rpm) for 35 min. The supernatant was reserved for use in catalyst synthesis. The multilayer assembly process was a variation of that performed in Szabo et al. [14]. Substrate slides were immersed into the titanium sol for 60 s. Slides were immediately rinsed with running deionized water and air-dried. They were then immersed into the hectorite suspension for 60 s and again rinsed with deionized water and air-dried. Hectorite, as a light transparent mineral with high surface area, was chosen to spatially separate layers of titania oxide and thus increase the efficiency of the catalytic process. The layers of film were built via electrostatic adsorption. The experimental steps were repeated a total of 10 times. The mass of the dry, uncalcinated slides was recorded. The hectorite/titania oxide film was stable in tetradecane solution. In layering experiments, considerable variation was seen for the first four or five multilayers. By the time eight multilayers had been deposited, slide coatings were found to be quite consistent. Increase in mass was generally found to be proportional to surface area. The dry multilayered slides were heated in a horizontal tube furnace in an air atmosphere to 400 ◦ C with 1 ◦ C/min heating rate. Temperature was maintained at 400 ◦ C for 6 h and then it was reduced over 6 h to room temperature. The mass of calcinated slides was recorded. The material is hereafter referred to as CAT. The catalyst slides, as prepared, were analyzed by XRD and SEM. Due to space limitations, it was not possible to perform TGA-DTA or nitrogen adsorption analysis on the catalyst slides. Characterization for these analyses was performed using TiO2 powder obtained from the titanium sol. These samples were prepared by evaporating the ethanol at room temperature and then calcinating the resulting powder in the tube furnace using the program outlined above. The powder is referred to throughout this paper as TiO2 .
2.1. Synthesis of the catalyst
2.2. Methods
Quartz slides (1 mm thick) were cut to a size of 1.25 by 7.5 cm. Two small holes (about 2 mm in diameter) were drilled in each slide to facilitate mounting. The slides were cleaned by sonification (Branson 1510) for 20 min in detergent solution. As a next step they were rinsed with copious amounts of deionized water and dried in air. The slides were then immersed in chromic acid for 1 h, rinsed and dried again. The mass of each slide was recorded prior to multilayer assembly.
2.2.1. Photooxidation Photooxidation trials were performed in a Rayonet Chamber reactor (RMR-600) using 253.7- and 300.0-nm UV lamps. Eight lamps were used in each photoirradiation. The nominal power of the 253.7-nm lamps was approximately 8 W each at that wavelength. The nominal power of the 300.0-nm lamps was 3.9 W each [19]. The sample tubes were mounted on a stationary carousel, with each tube aligned with one of the lamps at a
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distance of approximately 1 cm, to ensure equal UV intensity for the eight sample tubes mounted in the chamber. Sample tubes were composed of quartz silica, with a total tube volume of 15 ml. Ambient air was supplied at a rate of 0.2 L/min for the duration of the photooxidation. To increase the concentration of active radicals [15], the air was humidified by passing it through a water bubbler prior to its entering the reactor tubes. Stock solutions were made of DBT (98% purity) at a concentration of 200 ppm weight in n-tetradecane (purity 99%). DBT was supplied by Sigma–Aldrich. Tetradecane was obtained from VWR. Ten milliliters of solution were added to each sample tube. Trials without the catalyst were performed with 254-nm lamps. After 2-h of irradiation, two tubes of solution were removed and diluted back to their original volume with tetradecane. (Samples were found to lose less than 3% of their original volume over the course of 8 h of photoirradiation). Samples were placed in taped vials and stored in the dark prior to being analyzed by UV spectrophotometry. The sample tubes were replaced by fresh sample solutions. Additional sample tubes were removed at varying time intervals. In this way, two to four replicate samples at each interval were obtained with irradiation times of 2, 4, 6, and 8 h for each solution. Trials were numbered for record-keeping purposes (e.g., 1-254, 2-254, etc.). Trials at 300 nm without a catalyst were performed in a fashion identical to the 254-nm trials, except that 300-nm lamps were used. The 300-nm lamp produces a broader wavelength pattern centered on 300 nm, with the intensity dropping to near zero around 250 and 350 nm. The total photon intensity was approximately half that of the 254-nm trials. These trials were numbered 1-300 and 2-300. In a third trial, 1-DARK, everything was as in the above described experiments, but the tubes were wrapped in foil, to prevent any ambient light from entering the tubes. These trials were otherwise identical to the previous trials. Each photooxidation trial was repeated in the presence of the photocatalyst. One catalyst slide was placed in each sample tube, oriented so that one side of the slide would receive maximum irradiance from the lamps. These trials were labeled 1-254-CAT and 1-300-CAT, respectively. The trials were otherwise identical to the noncatalyzed trials. An additional sample, consisting of a foil-wrapped tube with a catalyst slide, was subjected to air bubbling for 8 h. An 8-h oxidation trial was performed using Degussa P25 as photocatalyst. This trial was labeled 1-254-P25. Three samples were prepared using 10.0 mg of P25. Three more were prepared using 20.0 mg. The samples were subjected to the same air bubbling and irradiation conditions as in trial 1-254. At the end of irradiation, samples were filtered to remove suspended TiO2 , and placed in taped vials. 2.2.2. Adsorption of DBT and products of its oxidation on silica An adsorption isotherm for DBT on silica was measured at room temperature. Silica gel (40 mesh) was obtained from Baker. Solutions were made with DBT concentration varying
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from 50 to 1000 ppm weight in tetradecane. Ten milliliters of each solution was placed in a 12-ml glass vial, along with 0.50 g of silica gel. The tubes were capped, shaken, and stored in the dark overnight. Samples were filtered using a syringe equipped with a 0.2-µm filter. Concentration of DBT was determined for each sample using UV spectrophotometry at 313 and 326 nm. 2.2.3. UV spectroscopy DBT concentrations were determined by UV spectrophotometry on a Cary 100 Bio. For DBT, absorbances at 313 and 326 nm were compared with calibration curves generated previously on the same instrument. To eliminate interference from oxidation products of DBT, 10.0 ml of each sample was placed in a closed vial with 0.50 g of silica gel for at least 12 h, prior to spectrum measurement. Adsorption experiments showed that this amount of silica would lead to a reduction in DBT concentration by no more than 5 ppm, indicating that DBT is not strongly adsorbed on its surface. The oxidation products, on the other hand, are strongly adsorbed on silica, allowing their removal from sample solutions. 2.2.4. HPLC DBT concentrations for trial 1-300 were obtained by reverse phase HPLC on a Agilent 1100 with a Merck SP-18 column with Lichrospher C-18 bonded packing and a UV detector at 245 nm. In preparation for HPLC, 1 ml of sample was diluted to 10 ml with ethanol. The mobile phase was a mix (75/25) of acetonitrile and water, with a flow rate of 0.8 ml/min. This solution was injected using an injection loop with an injection volume of 20 µl. An external standard of DBT was found to elute under these conditions at 3.9 min. DBT sulfone (97%, Aldrich) was found to elute around 1.4 min. Separate peaks were unresolved for other photooxidation products, with the exception of a peak that appeared consistently at 2.1 min for all of the oxidized samples. DBT concentrations were quantified using a calibration curve. During the course of these experiments, it became apparent that the tetradecane solvent, as supplied, contained aromatic impurities, possibly including DBT. This was evidenced by a peak with the same retention time as DBT that was present even during blank injections. This is hardly surprising, considering the presence of DBT in most hydrocarbon feedstocks. In order to determine the actual concentration of the solutions, rather than simply the amount of DBT added, the HPLC calibration was performed using only ethanol as a solvent, without tetradecane. Qualitative determination of photooxidation products was performed by reverse phase HPLC on a Waters 2695 with a Waters 996 photodiode array detector and an RP-18 column (LiChrospher 100 packing, 5-µm diameter, 150-mm length). The mobile phase was a mix (75/25) of acetonitrile and water, with a flow rate of 0.6 ml/min. Solutions were injected using an autoinjector with an injection volume of 10.0 µl. Catalyst slides and spent silica gel were rinsed with HPLC grade acetonitrile, to remove adsorbed products. These solutions were injected onto the HPLC column. Retention times were compared with standards of DBT and DBT sulfone, also in acetonitrile.
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2.2.5. Adsorption of nitrogen On the catalysts, sorption of nitrogen at its boiling point was carried out using ASAP 2010 (Micromeritics). Before the experiments, the samples were outgassed at 120 ◦ C to constant vacuum (10−4 Torr). From the isotherms, the surface areas (BET method), total pore volumes, Vt (from the last point of isotherm at relative pressure equal to 0.99), volumes of micropores, Vmic , mesopore volumes, and Vmes , along with pore size distributions, were calculated. The last three quantities were calculated using density functional theory, DFT [20,21]. 2.2.6. Thermal analysis Thermal analysis was carried out using TA Instrument Thermal Analyzer. The instrument settings were heating rate 10 ◦ C/min and an air or nitrogen atmosphere with 100 ml/min flow rate. For each measurement about 25 mg of a ground catalyst sample were used. 2.2.7. SEM Scanning electron microscopy was performed on a Hitachi S-4700 cold field emission instrument. The accelerating voltage was 2000 V. Scanning was performed in situ on a section of a catalyst slide (CAT), as well as on TiO2 powder and hectorite. 3. Results and discussion Nitrogen adsorption isotherms are presented in Fig. 1. Their shape indicates that significant differences in surface texture exist. Although the isotherms for titania samples are type II of the IUPAC classification, the Degussa P25 titania adsorbs much more nitrogen at higher pressure, indicating that its pores are much larger than those in TiO2 . Nitrogen uptake of the latter sample is greater at low pressure, which is related to the presence of larger pore volumes and surface area. The nitrogen adsorption isotherm on hectorite also represents type II but the volume of pores is much higher than for titanium oxide materials.
Fig. 1. Nitrogen adsorption isotherms.
The structural parameters calculated from the isotherms are listed in Table 1. The total pore volume of the Degussa P25 sample is twice that of TiO2 . In spite of this, the micropore volume and total surface area of TiO2 are about 20 and 37% greater, respectively. This is the result of 20% higher volume of micropores in the synthesized sample than in P25. The surface area and volume of pores are critical components of a photocatalyst, since catalysis is assumed to take place only for molecules on or near the surface. Pore diameter is also important, since molecules are expected to accommodate more effectively in pores whose diameter is close to that of the molecule. As expected based on the shape of isotherms, the hectorite sample has a high surface area, reaching 377 m2 /g, and a very high volume of micropores with small average diameter. The data indicate that the interlayer space of hectorite is accessible for the nitrogen molecule and the significant volume of pores is formed between the small hectorite particles. The mineral significantly differs from montmorillonite in its primary and secondary pore structure [22]. Fig. 2 shows the distribution of pore sizes in the two catalysts and the hectorite calculated using the DFT method. As expected, a majority of the pore volume of P25 is composed of large pores, with an average diameter of around 30 nm. Conversely, a large portion of the TiO2 pore volume consists of pores in a size range around 3–10 nm. These micropores account for the larger surface area of the catalyst, even while the material seems to be more dense than P25. In the case of hectorite the PSD indicates a high degree of heterogeneity for the pore structure of this materials. Besides micropores smaller than 2 nm, a significant volume exits in pores between 3 and 10 nm. Table 1 Structural parameters of the catalysts calculated from nitrogen adsorption isotherms Sample
SBET (m2 /g)
Vt (cm3 /g)
Vmic (cm3 /g)
Vmes (cm3 /g)
L (Å)
Degussa P25 Hectorite TiO2 catalyst
64 377 89
0.167 0.262 0.082
0.019 0.163 0.023
0.148 0.099 0.059
104 30 37
Fig. 2. Pore size distributions.
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Fig. 3. SEM images: CAT (A–D), P25 (E), and hectorite (F). Magnification bars: A—200 nm; B—100 µm; C—1 µm; D—200 nm; E—4.44 µm; F—2.22 µm.
Fig. 3 presents SEM images of the materials studied. As can be clearly seen, the surface of CAT cannot be described as a homogeneous layer of material (Figs. 3A and 3B). Rather, it is made up of agglomerations of nanoscale particles of hectorite (outer layer), stuck together into features, which are in some cases nearly a micrometer across. The individual crystals appear to span a range of about 15–40 nm. The micrographs presented in Fig. 3 are in marked contrast to the results of Kotov et al. [13]. One reason for this is in the use of hectorite instead of montmorillonite. As mentioned above, based on the high volume of large pores, the hectorite tends to form agglomerates of particles (Fig. 3F). This process hinders the formation of monolayers of TiO2 . Another reason is likely variation in the layering process itself. Even in the case of pure titania catalysts, Degussa P25 (Fig. 3E), the micrograph suggests crystal growth by progressive agglomeration. The resulting structures show treelike complexity, rather than the smooth layering observed by Kotov et al. When the film layer is broken (Fig. 3C), the spheres of TiO2 is revealed with sizes between 30 nm and 1 mm. They are located between the layers of hectorite, forming a sandwich structure. These complex structures are responsible
for the large surface area of the catalyst, while the small size of the individual crystals leads to greater microporosity. The phase composition of TiO2 on the catalyst slides was determined by X-ray diffraction. Because the catalyst was mounted on quartz, it was possible to insert it directly into the XRD instrument. Fig. 4 shows the X-ray diffraction spectrum of a catalyst slide over a 2Θ range from 3◦ to 49◦ . This range covers three of the most intense anatase peaks, as well as two of the most intense rutile peaks. As Fig. 4 shows, the catalyst reveals well-defined peaks corresponding to the anatase crystal structure, with no significant corresponding rutile peaks. The absence of the most intense rutile peak suggests that the catalyst material is essentially entirely anatase. The broad hump from about 16◦ to 30◦ indicates that at least some of the material is still in an amorphous state and may be related to the thin layer of hectorite. for which diffraction peaks are rather broad. In the case of Degussa P25 the mixture of anatase and rutile is detected. X-ray diffraction can also be used to estimate average crystallite size using the Scherer equation [23]. When the most intense peak for our catalysts at 25.4◦ , is used for calculation, the
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Fig. 4. XRD patterns for the samples studied.
calculated particle size is about 32 nm. This is in good agreement with the range of particle sizes seen in the SEM images (Fig. 3). In the case of P25 and hectorite the particle sizes are found to be about 12 and 30 nm, respectively, which is also in agreement with the SEM analysis. To get a better picture of the phase changes of the catalyst, differential thermal analysis was performed. Uncalcined TiO2 powder, P25 and hectorite were analyzed. Figs. 5 and 6 show the resulting DTG and DTA graphs, respectively. The DTG curve for TiO2 shows an initial decrease in mass represented by the peak at about 120 ◦ C, which represents the loss of adsorbed water from the surface of the powder. The final weight of the powder was consistently between 70 and 75% of the original weight. The typical weight loss of the coated catalysts was around 20 to 25%. Some of this difference is likely caused by the presence of hectorite on the slides, which contributes to the total mass but does not account for the mass change to the same degree. TG analysis of dry hectorite powder showed a 17% decrease in mass up to 400 ◦ C. In addition, it is possible that the coated slides do not recrystallize completely during calcination. The DTA curve for TiO2 shows two large exothermic peaks around 270 and 425 ◦ C, with a smaller peak at 350 ◦ C (Fig. 6). The 425◦ peak is accompanied by a sharp decrease in mass. This would normally indicate some sort of outside interference with the instrument. Repetition of the experiment showed the same results. The drop in mass is therefore believed to be characteristic of the material. Temperature spikes such as these may be caused either by changes of state or by chemical reactions occurring in the sample. The amorphous titania is known to undergo a reorganization of crystal structure to form the predominately anatase phase during calcination. The experiment was repeated using nitrogen, rather than air. Three temperature peaks were still apparent, though at slightly higher furnace temperatures. The relative intensity of the peaks changed, with the 375 ◦ C peak being the most prominent. The 450 ◦ C peak was considerably smaller, and there was no corresponding steep change in mass. This seems to indicate that the change in mass is linked to an oxidation reaction. The amorphous titania should include ethano, aqua, hydroxo, and isopropoxo ligands, which are removed during the calcination process. It is reasonable that such groups could be burned in the presence of air. The steep
Fig. 5. DTG curves in air.
Fig. 6. DTA curves in air.
decline in mass may be a result of these groups being released suddenly as the material recrystallizes. The results of TA analysis for P25 show a weight loss due to loss of adsorbed moisture, as well as a second loss around 270 ◦ C. This DTG peak, unlike the corresponding one for our catalyst, is related to an endothermic effect (Figs. 5 and 6). The mass leveled off near the minimum just above 400 ◦ C, though there is an increase in the intensity of the endothermic effects around 800 ◦ C. As XRD indicated (Fig. 4), Degussa P25 is a mixture of anatase and rutile phases of TiO2 . Unlike our catalyst, which forms as an amorphous powder, P25 is purchased as an essentially crystalline material. Hence, it did not exhibit the more complex behavior of the amorphous powder when exposed to heat treatment. To properly account for the effect of radiation on DBT removal, the dark trial, 1-DARK, was performed. In this trial, a 200 ppm solution of DBT was subjected to gas bubbling (air and moisture) in the absence of light and catalysts. No significant loss of DBT was found. Therefore, it is concluded that any decrease in DBT concentration in subsequent trials is the result of photooxidation reaction. The results of the photolysis experiments are summarized in Fig. 7 and Table 2. Final concentrations of DBT under trial
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(A)
Fig. 7. Changes in DBT concentration with time for various trials. Table 2 Summary of the photolysis experiments Trial 2-300
Order of reaction
First Zeroth 2-254 First 1-300-CAT Zeroth First 1-254-CAT First 1-254-P25-10 Unknown 1-254-P25-20 Unknown
Rate constanta
Final [DBT] Final [S] (ppm) (8 h) (ppm) (8 h)
% DBT removal
0.11 14.80 0.17 16.25 0.12 0.25 Unknown Unknown
79
14
60
53 69
9 12
73 66
29 40 43
5 7 7
86 80 78
(B)
a First-order rate constants are in ppm−1 h−1 . Zero-order rate constants are in h−1 .
1-300 averaged 80 ppm. This is the equivalent of about 14 ppm sulfur and constitutes reaction of 60% of starting DBT. Data presented in Fig. 7 suggest that the photooxidation of DBT in 1-300 is first order with respect to DBT, as expected for low concentration for Langmuir–Hinshellwood kinetics at low concentrations. This is in apparent contrast with the results of Hirai et al. [5], who used a starting concentration of DBT of 2–4 g/L (2650–5300 ppm of sulfur) and found a zero-order rate. This discrepancy is a result of the concentration differences. For semiconductor-catalyzed photoreactions zero order is expected at high concentrations. It is also significant that according to the results of Hirai et al. [5], the pure tetradecane solution appears to obey a first-order rate law. In fact, several of the figures depicting the biphasic system also appear to tail off near the end at very low concentrations. These low concentrations are more consistent with the conditions reported here. According to Salem [12], the mechanism of the reaction is likely the production of conduction-band holes in the TiO2 photocatalyst, which could then interact either with the organic substrate (in our case, DBT) or with an inorganic oxidant, including O2 , to form radicals.
(C) Fig. 8. UV spectra of the DBT solution before photooxidation (A), after photooxidation (B), and after photooxidation followed by adsorption onto silica (C).
HPLC results, in addition to being a tool for concentration determination, yield a hint to the products of photooxidation. The peak that appeared at a retention time of about 2.1 min grew in height with the course of the reaction, reaching a maximum
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near 30 mAU. The primary quantitative results were obtained using UV spectrophotometry. Fig. 8A shows a typical UV spectrum for DBT at a concentration of 200 ppm. Four main peaks are visible. Of these four, the 313- and 326-nm peaks show a linear dependence on DBT concentration over the range investigated. Following photooxidation, a number of changes are observed in the spectrum of the sample solutions (Fig. 8B). An increase in absorbance at 326 nm was recorded with increasing irradiation, along with the emergence of maxima at 304 and 310 nm. Due to the intensity and closeness of the peaks, however, it was not possible to resolve them. To test the separation method, the solution after irradiation was mixed with silica gel powder, shaken, and left for 75 min to allow adsorption of the oxidation products (DBT is not adsorbed onto silica). Fig. 8C shows the spectrum of the same solution as in Fig. 8B but after the adsorption process. The remaining four peaks are the same four peaks that are characteristic of DBT. It is important to note that it is not certain whether there are any compounds other than DBT remaining in solution after adsorption. In that event, the data presented here would represent an overestimate of the amount of DBT remaining in solution. However, the similarity of the spectra in Figs. 8A and 8C suggests that any contributions from other compounds are minimal. In addition to the evidence from UV spectroscopy, two other observations support the hypothesis of nearly complete product removal. During photooxidation, the sample solutions became increasingly yellow, in accord with the observations of Hirai et al. [5]. After adsorption onto silica, the solutions returned to a nearly colorless state (Fig. 9A). Moreover, during the process of photooxidation, some of the products adsorbed onto the catalyst slides. This adsorption of the products resulted in a yellowing (darkening in the photos) of the slides, as shown in Fig. 9B. The slide on the left is unused, while that on the right was used for 8 h of photooxidation at 254 nm. Adsorption likely occurs on hectorite owing to its high surface area and pore volume. This phenomenon can be technologically advantageous from the point of view of removal of oxidation product but, on the other hand, it may prevent the reuse of the catalyst. The results of trial 2-300, plotted in Fig. 7, indicate that the photooxidation may either follow a zero- or first-order rate law with respect to DBT. A zero-order fit again results in a slightly lower R 2 value (0.985). First-order dependence on the substrate is to be expected in the case of single-photon direct photolysis. In this sort of mechanism, the rate-limiting step is presumed to be n–π ∗ excitation of the substrate, followed by rapid reaction of the excited intermediate to form a series of products. Because oxidant concentrations and irradiation intensity were kept essentially constant, a first-order rate is reasonable. From a linear least-squares fit of the data in Fig. 7, the pseudo-firstorder rate constant under these conditions was determined to be 0.1149 ppm−1 h−1 . On the other hand, trial 2-254 shows a clearer first-order rate dependence than the 300-nm trials (Fig. 7). A greater decrease in the concentration of DBT over time is observed. The 8-h irradiated samples showed an average final concentration of 53 ppm DBT, which is equivalent to about 9 ppm sulfur. This represents a removal of 73% of the
Fig. 9. Changes in the color of the DBT solutions exposed to photooxidation and silica adsorption (A); changes in the color of the CAT after exposure to photooxidation (B).
DBT from the original solution. As with the 300-nm trial, there is some variation in the calculated concentrations of DBT at each irradiation time. Of note is the variation between the 4-h samples. It is significant that the concentrations appear to come in pairs, with samples 4A and 4B showing significantly different concentrations than 4C and 4D. Samples 4A and 4B were present at the same time in the reaction chamber, while 4C and 4D were present during a different 4-h period. This suggests that some unknown variable may be responsible for this discrepancy. Unlike the 300-nm trials, the rate for 2-254 shows clear first-order dependence on DBT. A zero-order fit gives an R 2 of only 0.90. The rate constant for the 254-nm reaction is calculated to be 0.1690 ppm−1 h−1 . For the 1-300-CAT, after 8 h of irradiation in the presence of the catalyst, the average concentration of DBT decreased to 69 ppm DBT (Fig. 7). This is equivalent to 12 ppm sulfur, a removal of 66% of the original DBT. Like the uncatalyzed trials, the catalyzed reaction could appear to follow either a zeroor first-order rate law with respect to DBT. However, the first order regression fit is in this case worse than the zero-order (R 2 = 0.977). The first-order rate constant is 16.25 ppm−1 h−1 .
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Table 3 Results of photolysis on Degussa P25 titanium oxide Sample
Mass of catalyst (mg)
Irradiation time (h)
Absorbance at 326 nm
[DBT] (ppm)
Absorbance at 313 nm
[DBT] (ppm)
[DBT] (Avg) (ppm)
Initial 1-254-P25-10A 1-254-P25-10B 1-254-P25-20A 1-254-P25-20B
0 10 10 20 20
0 8 8 8 8
0.4598 0.4316 0.4735 0.4743
36 34 37 37
0.4427 0.4009 0.4711 0.4496
47 43 50 48
199 42 38 44 43
If the rate of reaction is in fact independent of the concentration of DBT, it would be consistent with the general mechanism for TiO2 -catalyzed photooxidation given by Salem [12], as well as the mechanism proposed by Abdel-Wahab and Gaber [15]. Both of these mechanisms involve primary excitation of TiO2 electrons to produce photochemical holes and conduction-band electrons. DBT molecules adsorbed onto the surface of the catalyst then react with either holes or electrons to lead to oxidation products. The rate of this reaction mechanism is dependent on the light intensity and surface area of the catalyst, but not on the substrate concentration. Because DBT has a much lower absorbance around 300 nm, the primary mechanism for photooxidation at this wavelength appears to be indirect photolysis. In the case of trial 1-254-CAT, after 8 h of photolysis (followed by adsorption onto silica) the average concentration of DBT was reduced to 29 ppm. This is 5 ppm sulfur, a removal of 86% of the original DBT. This is a substantial reduction from the 53 ppm result from trial 2-254. This trial also showed significant variation between replicates, which can be attributed to both variation between the lamps and slight positioning variations for the catalyst slides. Interestingly, for this sample, the data cannot be fitted to a zero-order curve like that used for the 300-nm catalyzed data (R 2 = 0.847). A first-order rate expression, however, successfully explains the results. A first-order rate constant of 0.2497 ppm−1 h−1 was calculated for the conditions in this experiment. Table 3 collects the data from trial 1-254-P25, which was performed using Degussa P25 photocatalyst. Over the course of 8 h, the samples containing 10 mg of P25 reacted all but 40 ppm of sulfur. The amount of DBT remaining in these solutions is less than the 53 ppm remaining in the uncatalyzed samples, but more than the 29 ppm remaining after 8 h of irradiation with the quartz-supported catalyst. It is important to mention here that the maximum mass of the layered material present in any of the catalyst trials was 8.4 mg, where titania makes up only a fraction of this mass. Interestingly, the samples with much more P25 catalyst did not remove more DBT than those with 10 mg. This suggests that the surface area present in 10 mg was sufficient to perform whatever catalysis occurred. Alternately, it may simply be due to a limitation of the use of suspended TiO2 . It is difficult to maintain large amounts of suspended catalyst for long periods of time, even with constant agitation. Comparison of the UV spectra for a solution irradiated for 8 h at 254 nm, in the absence of catalyst, and 20 mg of P25 shows that peaks are nearly identical in shape, with only a slight difference in their height. This indicates that the major products
of the two samples are essentially the same compounds and thus supports the hypothesis that oxidation at 254 nm is primarily a result of direct photolysis. Table 2 summarizes the results of the photooxidation trials. Possible rate constants are listed for all fits yielding an R 2 value better than 0.95, with the best fit listed first. As demonstrated in the table, all of the catalyzed trials showed an improved rate of reaction over the corresponding uncatalyzed trials. Between the catalysts tested, the layered catalyst showed somewhat higher efficiency than Degussa P25 at 254 nm. Although the percentage of DBT removed was relatively close, it is significant that our catalyst slides contained no more than 9 mg of catalyst for any trial. All of the 254-nm trials showed first-order rate dependence on the concentration of DBT, while the 300-nm trials may be either first or zero order in DBT. In all cases, removal of DBT was greater with the 254-nm lamps than with the 300-nm ones. However, this result does not necessarily indicate the superiority of irradiation at 254 nm. The 254-nm lamps used in these experiments were estimated to have about twice the intensity of the 300-nm lamps. Assuming single-photon excitation of DBT, the rate would depend directly on the intensity of incident radiation, as well as on the absorptivity coefficient at the incident wavelength. The absorbance of DBT is somewhat higher at 254 nm than at either 313 or 325 nm. This would lead one to expect a rate constant for 254 nm at least double that of the 300 nm lamps. This was not the case in these experiments. The apparent first-order dependence on DBT concentration for the 254-nm reaction suggests a different mechanism than that proposed for the 300-nm reaction. The rate constant of the catalyzed reaction is 44% larger than that for the uncatalyzed trial. Hence, it is reasonable to conclude that the catalyst did have an effect. The dark trial with the catalyst demonstrates that the catalyst did not simply adsorb the additional DBT, since that sample showed no decrease in DBT concentration in the absence of light. One possible explanation of the result is that the reaction proceeds by multiple pathways under irradiation at 254 nm. The dominant pathway is the first-order direct photooxidation of DBT. In addition, the presence of the photocatalyst allows for additional indirect photolysis to occur on the surface of the TiO2 . This results in a higher overall removal rate, with predominately first-order rate dependence. To analyze these adsorbed products, spent catalyst slides and silica were rinsed with three times 3 ml of acetonitrile. The resulting solutions were analyzed by HPLC. This analysis confirmed the presence of products significantly more polar than the DBT starting material. The standards used were DBT and
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4. Conclusions
(A)
(B)
(C) Fig. 10. Examples of the chromatograms used for qualitative determination of the effects of photooxidation.
DBT sulfone. These compounds eluted at 8.1 and 2.5 min, respectively. In HPLC analysis of the compounds extracted from silica, an array of compounds was eluted in rapid succession. All of the compounds eluted before DBT, and some even earlier than DBT sulfone (Fig. 10). While the product peaks were not resolved, some qualitative information was obtained. Clearly, a number of compounds were produced as a result of photooxidation [9]. One peak corresponds well to the retention time of the sulfone standard. While most other peaks eluted later than the sulfone, at least two fell earlier. One of these peaks came shortly after the injection peak, meaning that the compound was barely retained by the column. This is characteristic of a compound significantly lower in molecular weight than the starting material. The peak at 3.15 min was seen in the rinsate from catalyst slides irradiated at both 254 and 300 nm, as well as the silica extract. The spectrum of this compound can be seen in Fig. 10C. This spectrum shows absorbances around 230 and 270 nm, which are consistent with absorbances for a conjugated benzoic acid [24]. The strong absorbance at 327 nm suggests an n–π ∗ excitation similar to that for the sulfur atom in DBT. A sulfobenzoic acid would be consistent with the findings of Traulsen et al. [8]. The large peak around 4.6 min remains unidentified.
The results presented here indicate that the applied procedure produces an effective photocatalyst for the removal of dibenzothiophene from nonpolar organic solvent. At 300 nm in the presence of catalyst, photooxidation appears to proceed primarily by indirect photolysis, according to a zero-order rate law. At 254 nm the rate is determined primarily by first-order direct photolysis, though the presence of the catalyst contributes to a greater overall rate of removal. Coupled with adsorption of oxidation products by silica adsorption, the process presented here is capable of reducing sulfur concentrations below the target level of 10 ppm. The catalyst synthesized was not, as previously expected, a series of alternating monolayers. Rather, the results indicate a complex structure of agglomerated nanoparticles. This structure leads to a high degree of microporosity, with a correspondingly high surface area. These features suggest the synthesized catalyst may perform with greater efficiency than the commercially available Degussa P25. Under the conditions tested, irradiation at 254 nm appears to be more effective than irradiation at 300 nm in removal of DBT. However, due to the uncertain order of the rate of the 300 nm reaction, further experiments are warranted to determine if a change of conditions would improve the photolysis yield of the 300 nm process. Specifically, higher intensity lamps and a higher starting concentration of DBT, as well as the presence of naphthalene, may favor the 300 nm reaction. Acknowledgments The authors gratefully acknowledge the assistance of Dr. James T. Yardley and Dr. Richard Harniman of the Columbia University Nanocenter in producing SEM images. We are also indebted to Dr. Jeffrey Steiner and Karin Block of the City College Earth and Atmospheric Sciences Department for assistance with XRD. The help of Dr. Stanislaus Wong (Brookhaven National Laboratory/SUNY), Tirandai Hemray-Benny of SUNY, and Dr. Jorge Morales of CCNY in obtaining the SEM images of CAT, Degussa P25, and hectorite is appreciated. References [1] U.S. Environmental Protection Agency, Control of Air Pollution from New Motor Vehicles: Heavy-Duty Engine and Vehicle Standards and Highway Diesel Fuel Sulfur Control Requirements. http://www.epa.gov/ fedrgstr/EPA-AIR/2001/January/Day-18/a01a.htm (accessed February 2005). [2] U.S. Environmental Protection Agency, Fuel Sulfur Effects on Exhaust Emissions. http://www.epa.gov/otaq/models/mobile6/m6ful001.pdf (accessed February 2005). [3] M. Houalla, D.H. Broderick, A.V. Sapre, N.K. Nag, V.H.J. de Beer, B.C. Gates, H. Kwart, J. Catal. 61 (1980) 523–527. [4] J.T. Andersson, S. Bobinger, Chemosphere 24 (1992) 383–389. [5] T. Hirai, K. Ogawa, I. Komasawa, Ind. Eng. Chem. Res. 35 (1996) 586– 589. [6] S. Bobinger, J.T. Andersson, Chemosphere 36 (1998) 2569–2579. [7] Y. Shiraishi, T. Hirai, I. Komasawa, Ind. Eng. Chem. Res. 37 (1998) 203– 211.
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[8] F. Traulsen, J.T. Andersson, M.G. Ehrhardt, Anal. Chim. Acta 392 (1999) 19–28. [9] Y. Shiraishi, T. Hirai, I. Komasawa, Ind. Eng. Chem. Res. 38 (1999) 3300– 3309. [10] H. Maki, T. Sasaki, S. Harayama, Chemosphere 44 (2001) 1145–1151. [11] S. Matsuzawa, J. Tanaka, S. Sato, T. Ibusuki, J. Photochem. Photobiol. A 149 (2002) 183–189. [12] I. Salem, Catal. Rev. 45 (2003) 205–296. [13] N.A. Kotov, I. Dekany, J.H. Fendler, J. Phys. Chem. 99 (1995) 13,065– 13,069. [14] T. Szabo, J. Nemeth, I. Dekany, Colloids Surf. A Physicochem. Eng. Aspects 230 (2004) 23–35. [15] A.M.A. Abdel-Wahab, A.E.A.M. Gaber, J. Photochem. Photobiol. A 114 (1998) 213–218. [16] J.C. Yu, J. Yu, W. Ho, J. Zhao, J. Photochem. Photobiol. A 148 (2002) 331–339.
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[17] G. Yuan, Y. Wei, C. Baohua, M. Yongxiang, L. Hulin, Rare Met. 22 (2003) 137. [18] B.L. Lakshmi, P.K. Dorhout, C.R. Martin, Chem. Mater. 9 (1997) 857– 862. [19] Southern New England Ultraviolet Company, Spectral Density of UV Lamps, Branford, CT, 2001. [20] J.P. Olivier, W.B. Conklin, in: Seventh International Conference on Surface and Colloid Science, Compiengne, France, 1991. [21] Ch.M. Lastoskie, K.E. Gubbins, N. Quirke, J. Phys. Chem. 97 (1993) 4786. [22] R.E. Grim, Clay Mineralogy, McGraw–Hill, New York, 1968. [23] A.J.C. Wilson, Proc. Phys. Soc. London 80 (1962) 286. [24] National Institute of Standards and Technology, NIST Chemistry WebBook: NIST Standard Reference Database Number 69, March 2003. http://webbook.nist.gov/chemistry/ (accessed April 2005).
Photooxidation of dibenzothiophene on TiO2/hectorite thin films layered catalyst Jamie Robertson, Teresa J. Bandosz ∗ Department of Chemistry, The City College of New York, 138th Street and Convent Avenue, New York, NY 10031, USA Received 11 December 2005; accepted 7 February 2006 Available online 5 April 2006
Abstract A new titanium(IV) oxide–hectorite nanofilm photocatalyst was prepared on quartz slides. It was evaluated in the photooxidation of dibenzothiophene (DBT) in nonpolar organic solution (tetradecane), as a model for diesel fuel. A removal regimen was developed consisting of catalytic photooxidation followed by adsorption of products on silica gel. Photooxidation of DBT was performed with and without catalyst, at 254 and 300 nm. Comparison was made with a commercially available TiO2 catalyst, Degussa P25. The catalyst was analyzed by nitrogen adsorption, XRD, SEM, and TGA-DTA. DBT concentrations were measured by HPLC and UV spectrophotometry. Preliminary qualititative analysis of products was performed by UV and HPLC. Results indicated that the outlined process was effective in reducing sulfur levels to below 10 ppm sulfur. © 2006 Elsevier Inc. All rights reserved. Keywords: Titanium oxide; Dibenzothiophene; Photooxidation; Desulfurization
1. Introduction U.S. Environmental Protection Agency regulations slated to take effect in 2006 require deep desulfurization of both gasoline and diesel fuels. Specifically, new fuel standards call for eventual reduction in sulfur content to below 30 ppm sulfur by mass for gasoline and 15 ppm for diesel [1]. Sulfur in fuel leads to the release of SO2 upon combustion. High sulfur content is also associated with the poisoning of catalysts in catalytic converters [2] and fuel cells. Dibenzothiophene (DBT) and its derivatives occur naturally in coal and oil deposits. It is one of the most difficult sulfur compounds to remove from liquid fuels. The conventional desulfurization method involves reduction with hydrogen gas at high pressures. Unfortunately, this method is not very effective at removing DBT and its alkylated derivatives [3]. This necessitates the development of improved techniques for the removal of DBT from gasoline and diesel fuels. Recently, a number of researchers have investigated the use of photooxidation to re* Corresponding author. Fax: +1 212 650 6107.
E-mail address: [email protected] (T.J. Bandosz). 0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2006.02.011
move DBT and other heterocyclic sulfur compounds from both aqueous and organic solutions [4–11]. Most of these investigations have taken place in the presence of water. Some have been performed with the aid of catalysts, such as titanium(IV) oxide. The use of titanium(IV) oxide as a photocatalyst has received extensive coverage in Ref. [12]. TiO2 can absorb light throughout the UV range and has been demonstrated to be an effective photocatalyst for a number of reactions [12]. Recently, Dekany and co-workers have demonstrated the synthesis of multilayer photocatalysts containing metal oxides layered with clays [13,14]. The multilayer photocatalyst is affixed to a solid substrate. This prevents some common problems associated with heterogeneous catalysts in some other systems. Titanium(IV) oxide has been investigated as a catalyst for the oxidation of dibenzothiophene [11,15]. It has been found to greatly enhance the rate of photooxidation, particularly at higher wavelengths. At wavelengths above 300 nm, the extinction coefficient for DBT is much lower, so that photooxidation at these wavelengths is more likely due to indirect (catalyzed) photolysis than to direct photolysis. Much of the research with TiO2 photocatalysts has involved suspensions of the catalysts in either aqueous or polar organic solution. This method can pose a few logistical problems. Cat-
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alyst suspensions limit the amount of TiO2 that can be used, since agitation can only suspend a limited amount of catalyst. Also, suspended catalyst particles may actually block out ultraviolet light. This is particularly problematic for any scale-up of the process. Finally, suspended catalysts are costly and problematic to remove. Thus various methods and substrates have been used to make nanoscale TiO2 catalysts [13,16,17] with mixed effects. While some papers focus primarily on benzothiophenes in the natural environment [6,8,10], others describe the removal of these species from petroleum feedstocks [5,7,9]. When a 300-W high-pressure mercury lamp (unfiltered) was used to photoirradiate solutions of DBT in tetradecane (100 ml) or water (300 ml), DBT and its derivatives were completely removed over the course of less than 10 h. The rate of photolysis was highest for the most substituted DBT and lowest for the least substituted, which is the reverse of the reaction rate for hydrodesulfurization. Hirai et al. reported that when the tetradecane–DBT solution was irradiated without aqueous phase, yellow discoloration and increased viscosity occurred in the organic phase [5]. They also found that the absence of air or oxygen dramatically reduced the rate of photolysis. After irradiation, only residual DBT was present in the organic phase, without other sulfur compounds. Less than 1% of tetradecane was decomposed in the process. In the aqueous phase, the presence of sulfate ion (SO2− 4 ) was detected. Its concentration increased with a decrease in concentration of DBT, with 80% of the sulfur converted to SO2− 4 . In commercial light oil, the desulfurization was found to be 22% after 30 h, far less than predicted, which was attributed to presence of naphthalene in the oil. The acidic minor products of the DBT oxidations were investigated by Shiraishi et al. [9]. The formation of 2-sulfobenzoic acid anhydride and benzothiophene-2,3-dicarboxylic acid was reported. The objective of this research is to evaluate a two-stage process for removal of dibenzothiophene from nonpolar organic solvent. The project requires the synthesis of a multilayer TiO2 – hectorite photocatalyst and the study of this catalyst for the photooxidation of dibenzothiophene in organic solution. In determining the efficiency of the catalyst, studies are performed using light at different wavelengths, in the presence or absence of the catalyst. Comparative studies are also carried out between photooxidation of dibenzothiophene with the synthesized catalyst and with a commercially available TiO2 catalyst. 2. Experimental
The titanium oxide sol was prepared as in Lakshmi et al. [18]. Briefly, 5 ml of titanium(IV) isopropoxide was added to 25 ml ethanol. The solution was stirred in an ice bath. To another 25-ml aliquot of ethanol, 0.5 ml of deionized water and 0.5 ml of 0.1 M hydrochloric acid were added. The solution was removed from the ice bath and placed in a 15 ◦ C water bath Then the acid solution was slowly added to it. A milky white suspension formed within 1 min. The hectorite suspension was prepared as in Kotov et al. [13]. One gram of synthetic hectorite (Na0.3 (Mg,Li)3 Si4 O10 (OH)2 ) was added to 100 ml of deionized water. The mixture was placed in a sonication bath (Branson 1510) for 40 min. The mixture was centrifuged (Fisher Centrific, 3000 rpm) for 35 min. The supernatant was reserved for use in catalyst synthesis. The multilayer assembly process was a variation of that performed in Szabo et al. [14]. Substrate slides were immersed into the titanium sol for 60 s. Slides were immediately rinsed with running deionized water and air-dried. They were then immersed into the hectorite suspension for 60 s and again rinsed with deionized water and air-dried. Hectorite, as a light transparent mineral with high surface area, was chosen to spatially separate layers of titania oxide and thus increase the efficiency of the catalytic process. The layers of film were built via electrostatic adsorption. The experimental steps were repeated a total of 10 times. The mass of the dry, uncalcinated slides was recorded. The hectorite/titania oxide film was stable in tetradecane solution. In layering experiments, considerable variation was seen for the first four or five multilayers. By the time eight multilayers had been deposited, slide coatings were found to be quite consistent. Increase in mass was generally found to be proportional to surface area. The dry multilayered slides were heated in a horizontal tube furnace in an air atmosphere to 400 ◦ C with 1 ◦ C/min heating rate. Temperature was maintained at 400 ◦ C for 6 h and then it was reduced over 6 h to room temperature. The mass of calcinated slides was recorded. The material is hereafter referred to as CAT. The catalyst slides, as prepared, were analyzed by XRD and SEM. Due to space limitations, it was not possible to perform TGA-DTA or nitrogen adsorption analysis on the catalyst slides. Characterization for these analyses was performed using TiO2 powder obtained from the titanium sol. These samples were prepared by evaporating the ethanol at room temperature and then calcinating the resulting powder in the tube furnace using the program outlined above. The powder is referred to throughout this paper as TiO2 .
2.1. Synthesis of the catalyst
2.2. Methods
Quartz slides (1 mm thick) were cut to a size of 1.25 by 7.5 cm. Two small holes (about 2 mm in diameter) were drilled in each slide to facilitate mounting. The slides were cleaned by sonification (Branson 1510) for 20 min in detergent solution. As a next step they were rinsed with copious amounts of deionized water and dried in air. The slides were then immersed in chromic acid for 1 h, rinsed and dried again. The mass of each slide was recorded prior to multilayer assembly.
2.2.1. Photooxidation Photooxidation trials were performed in a Rayonet Chamber reactor (RMR-600) using 253.7- and 300.0-nm UV lamps. Eight lamps were used in each photoirradiation. The nominal power of the 253.7-nm lamps was approximately 8 W each at that wavelength. The nominal power of the 300.0-nm lamps was 3.9 W each [19]. The sample tubes were mounted on a stationary carousel, with each tube aligned with one of the lamps at a
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distance of approximately 1 cm, to ensure equal UV intensity for the eight sample tubes mounted in the chamber. Sample tubes were composed of quartz silica, with a total tube volume of 15 ml. Ambient air was supplied at a rate of 0.2 L/min for the duration of the photooxidation. To increase the concentration of active radicals [15], the air was humidified by passing it through a water bubbler prior to its entering the reactor tubes. Stock solutions were made of DBT (98% purity) at a concentration of 200 ppm weight in n-tetradecane (purity 99%). DBT was supplied by Sigma–Aldrich. Tetradecane was obtained from VWR. Ten milliliters of solution were added to each sample tube. Trials without the catalyst were performed with 254-nm lamps. After 2-h of irradiation, two tubes of solution were removed and diluted back to their original volume with tetradecane. (Samples were found to lose less than 3% of their original volume over the course of 8 h of photoirradiation). Samples were placed in taped vials and stored in the dark prior to being analyzed by UV spectrophotometry. The sample tubes were replaced by fresh sample solutions. Additional sample tubes were removed at varying time intervals. In this way, two to four replicate samples at each interval were obtained with irradiation times of 2, 4, 6, and 8 h for each solution. Trials were numbered for record-keeping purposes (e.g., 1-254, 2-254, etc.). Trials at 300 nm without a catalyst were performed in a fashion identical to the 254-nm trials, except that 300-nm lamps were used. The 300-nm lamp produces a broader wavelength pattern centered on 300 nm, with the intensity dropping to near zero around 250 and 350 nm. The total photon intensity was approximately half that of the 254-nm trials. These trials were numbered 1-300 and 2-300. In a third trial, 1-DARK, everything was as in the above described experiments, but the tubes were wrapped in foil, to prevent any ambient light from entering the tubes. These trials were otherwise identical to the previous trials. Each photooxidation trial was repeated in the presence of the photocatalyst. One catalyst slide was placed in each sample tube, oriented so that one side of the slide would receive maximum irradiance from the lamps. These trials were labeled 1-254-CAT and 1-300-CAT, respectively. The trials were otherwise identical to the noncatalyzed trials. An additional sample, consisting of a foil-wrapped tube with a catalyst slide, was subjected to air bubbling for 8 h. An 8-h oxidation trial was performed using Degussa P25 as photocatalyst. This trial was labeled 1-254-P25. Three samples were prepared using 10.0 mg of P25. Three more were prepared using 20.0 mg. The samples were subjected to the same air bubbling and irradiation conditions as in trial 1-254. At the end of irradiation, samples were filtered to remove suspended TiO2 , and placed in taped vials. 2.2.2. Adsorption of DBT and products of its oxidation on silica An adsorption isotherm for DBT on silica was measured at room temperature. Silica gel (40 mesh) was obtained from Baker. Solutions were made with DBT concentration varying
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from 50 to 1000 ppm weight in tetradecane. Ten milliliters of each solution was placed in a 12-ml glass vial, along with 0.50 g of silica gel. The tubes were capped, shaken, and stored in the dark overnight. Samples were filtered using a syringe equipped with a 0.2-µm filter. Concentration of DBT was determined for each sample using UV spectrophotometry at 313 and 326 nm. 2.2.3. UV spectroscopy DBT concentrations were determined by UV spectrophotometry on a Cary 100 Bio. For DBT, absorbances at 313 and 326 nm were compared with calibration curves generated previously on the same instrument. To eliminate interference from oxidation products of DBT, 10.0 ml of each sample was placed in a closed vial with 0.50 g of silica gel for at least 12 h, prior to spectrum measurement. Adsorption experiments showed that this amount of silica would lead to a reduction in DBT concentration by no more than 5 ppm, indicating that DBT is not strongly adsorbed on its surface. The oxidation products, on the other hand, are strongly adsorbed on silica, allowing their removal from sample solutions. 2.2.4. HPLC DBT concentrations for trial 1-300 were obtained by reverse phase HPLC on a Agilent 1100 with a Merck SP-18 column with Lichrospher C-18 bonded packing and a UV detector at 245 nm. In preparation for HPLC, 1 ml of sample was diluted to 10 ml with ethanol. The mobile phase was a mix (75/25) of acetonitrile and water, with a flow rate of 0.8 ml/min. This solution was injected using an injection loop with an injection volume of 20 µl. An external standard of DBT was found to elute under these conditions at 3.9 min. DBT sulfone (97%, Aldrich) was found to elute around 1.4 min. Separate peaks were unresolved for other photooxidation products, with the exception of a peak that appeared consistently at 2.1 min for all of the oxidized samples. DBT concentrations were quantified using a calibration curve. During the course of these experiments, it became apparent that the tetradecane solvent, as supplied, contained aromatic impurities, possibly including DBT. This was evidenced by a peak with the same retention time as DBT that was present even during blank injections. This is hardly surprising, considering the presence of DBT in most hydrocarbon feedstocks. In order to determine the actual concentration of the solutions, rather than simply the amount of DBT added, the HPLC calibration was performed using only ethanol as a solvent, without tetradecane. Qualitative determination of photooxidation products was performed by reverse phase HPLC on a Waters 2695 with a Waters 996 photodiode array detector and an RP-18 column (LiChrospher 100 packing, 5-µm diameter, 150-mm length). The mobile phase was a mix (75/25) of acetonitrile and water, with a flow rate of 0.6 ml/min. Solutions were injected using an autoinjector with an injection volume of 10.0 µl. Catalyst slides and spent silica gel were rinsed with HPLC grade acetonitrile, to remove adsorbed products. These solutions were injected onto the HPLC column. Retention times were compared with standards of DBT and DBT sulfone, also in acetonitrile.
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2.2.5. Adsorption of nitrogen On the catalysts, sorption of nitrogen at its boiling point was carried out using ASAP 2010 (Micromeritics). Before the experiments, the samples were outgassed at 120 ◦ C to constant vacuum (10−4 Torr). From the isotherms, the surface areas (BET method), total pore volumes, Vt (from the last point of isotherm at relative pressure equal to 0.99), volumes of micropores, Vmic , mesopore volumes, and Vmes , along with pore size distributions, were calculated. The last three quantities were calculated using density functional theory, DFT [20,21]. 2.2.6. Thermal analysis Thermal analysis was carried out using TA Instrument Thermal Analyzer. The instrument settings were heating rate 10 ◦ C/min and an air or nitrogen atmosphere with 100 ml/min flow rate. For each measurement about 25 mg of a ground catalyst sample were used. 2.2.7. SEM Scanning electron microscopy was performed on a Hitachi S-4700 cold field emission instrument. The accelerating voltage was 2000 V. Scanning was performed in situ on a section of a catalyst slide (CAT), as well as on TiO2 powder and hectorite. 3. Results and discussion Nitrogen adsorption isotherms are presented in Fig. 1. Their shape indicates that significant differences in surface texture exist. Although the isotherms for titania samples are type II of the IUPAC classification, the Degussa P25 titania adsorbs much more nitrogen at higher pressure, indicating that its pores are much larger than those in TiO2 . Nitrogen uptake of the latter sample is greater at low pressure, which is related to the presence of larger pore volumes and surface area. The nitrogen adsorption isotherm on hectorite also represents type II but the volume of pores is much higher than for titanium oxide materials.
Fig. 1. Nitrogen adsorption isotherms.
The structural parameters calculated from the isotherms are listed in Table 1. The total pore volume of the Degussa P25 sample is twice that of TiO2 . In spite of this, the micropore volume and total surface area of TiO2 are about 20 and 37% greater, respectively. This is the result of 20% higher volume of micropores in the synthesized sample than in P25. The surface area and volume of pores are critical components of a photocatalyst, since catalysis is assumed to take place only for molecules on or near the surface. Pore diameter is also important, since molecules are expected to accommodate more effectively in pores whose diameter is close to that of the molecule. As expected based on the shape of isotherms, the hectorite sample has a high surface area, reaching 377 m2 /g, and a very high volume of micropores with small average diameter. The data indicate that the interlayer space of hectorite is accessible for the nitrogen molecule and the significant volume of pores is formed between the small hectorite particles. The mineral significantly differs from montmorillonite in its primary and secondary pore structure [22]. Fig. 2 shows the distribution of pore sizes in the two catalysts and the hectorite calculated using the DFT method. As expected, a majority of the pore volume of P25 is composed of large pores, with an average diameter of around 30 nm. Conversely, a large portion of the TiO2 pore volume consists of pores in a size range around 3–10 nm. These micropores account for the larger surface area of the catalyst, even while the material seems to be more dense than P25. In the case of hectorite the PSD indicates a high degree of heterogeneity for the pore structure of this materials. Besides micropores smaller than 2 nm, a significant volume exits in pores between 3 and 10 nm. Table 1 Structural parameters of the catalysts calculated from nitrogen adsorption isotherms Sample
SBET (m2 /g)
Vt (cm3 /g)
Vmic (cm3 /g)
Vmes (cm3 /g)
L (Å)
Degussa P25 Hectorite TiO2 catalyst
64 377 89
0.167 0.262 0.082
0.019 0.163 0.023
0.148 0.099 0.059
104 30 37
Fig. 2. Pore size distributions.
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Fig. 3. SEM images: CAT (A–D), P25 (E), and hectorite (F). Magnification bars: A—200 nm; B—100 µm; C—1 µm; D—200 nm; E—4.44 µm; F—2.22 µm.
Fig. 3 presents SEM images of the materials studied. As can be clearly seen, the surface of CAT cannot be described as a homogeneous layer of material (Figs. 3A and 3B). Rather, it is made up of agglomerations of nanoscale particles of hectorite (outer layer), stuck together into features, which are in some cases nearly a micrometer across. The individual crystals appear to span a range of about 15–40 nm. The micrographs presented in Fig. 3 are in marked contrast to the results of Kotov et al. [13]. One reason for this is in the use of hectorite instead of montmorillonite. As mentioned above, based on the high volume of large pores, the hectorite tends to form agglomerates of particles (Fig. 3F). This process hinders the formation of monolayers of TiO2 . Another reason is likely variation in the layering process itself. Even in the case of pure titania catalysts, Degussa P25 (Fig. 3E), the micrograph suggests crystal growth by progressive agglomeration. The resulting structures show treelike complexity, rather than the smooth layering observed by Kotov et al. When the film layer is broken (Fig. 3C), the spheres of TiO2 is revealed with sizes between 30 nm and 1 mm. They are located between the layers of hectorite, forming a sandwich structure. These complex structures are responsible
for the large surface area of the catalyst, while the small size of the individual crystals leads to greater microporosity. The phase composition of TiO2 on the catalyst slides was determined by X-ray diffraction. Because the catalyst was mounted on quartz, it was possible to insert it directly into the XRD instrument. Fig. 4 shows the X-ray diffraction spectrum of a catalyst slide over a 2Θ range from 3◦ to 49◦ . This range covers three of the most intense anatase peaks, as well as two of the most intense rutile peaks. As Fig. 4 shows, the catalyst reveals well-defined peaks corresponding to the anatase crystal structure, with no significant corresponding rutile peaks. The absence of the most intense rutile peak suggests that the catalyst material is essentially entirely anatase. The broad hump from about 16◦ to 30◦ indicates that at least some of the material is still in an amorphous state and may be related to the thin layer of hectorite. for which diffraction peaks are rather broad. In the case of Degussa P25 the mixture of anatase and rutile is detected. X-ray diffraction can also be used to estimate average crystallite size using the Scherer equation [23]. When the most intense peak for our catalysts at 25.4◦ , is used for calculation, the
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Fig. 4. XRD patterns for the samples studied.
calculated particle size is about 32 nm. This is in good agreement with the range of particle sizes seen in the SEM images (Fig. 3). In the case of P25 and hectorite the particle sizes are found to be about 12 and 30 nm, respectively, which is also in agreement with the SEM analysis. To get a better picture of the phase changes of the catalyst, differential thermal analysis was performed. Uncalcined TiO2 powder, P25 and hectorite were analyzed. Figs. 5 and 6 show the resulting DTG and DTA graphs, respectively. The DTG curve for TiO2 shows an initial decrease in mass represented by the peak at about 120 ◦ C, which represents the loss of adsorbed water from the surface of the powder. The final weight of the powder was consistently between 70 and 75% of the original weight. The typical weight loss of the coated catalysts was around 20 to 25%. Some of this difference is likely caused by the presence of hectorite on the slides, which contributes to the total mass but does not account for the mass change to the same degree. TG analysis of dry hectorite powder showed a 17% decrease in mass up to 400 ◦ C. In addition, it is possible that the coated slides do not recrystallize completely during calcination. The DTA curve for TiO2 shows two large exothermic peaks around 270 and 425 ◦ C, with a smaller peak at 350 ◦ C (Fig. 6). The 425◦ peak is accompanied by a sharp decrease in mass. This would normally indicate some sort of outside interference with the instrument. Repetition of the experiment showed the same results. The drop in mass is therefore believed to be characteristic of the material. Temperature spikes such as these may be caused either by changes of state or by chemical reactions occurring in the sample. The amorphous titania is known to undergo a reorganization of crystal structure to form the predominately anatase phase during calcination. The experiment was repeated using nitrogen, rather than air. Three temperature peaks were still apparent, though at slightly higher furnace temperatures. The relative intensity of the peaks changed, with the 375 ◦ C peak being the most prominent. The 450 ◦ C peak was considerably smaller, and there was no corresponding steep change in mass. This seems to indicate that the change in mass is linked to an oxidation reaction. The amorphous titania should include ethano, aqua, hydroxo, and isopropoxo ligands, which are removed during the calcination process. It is reasonable that such groups could be burned in the presence of air. The steep
Fig. 5. DTG curves in air.
Fig. 6. DTA curves in air.
decline in mass may be a result of these groups being released suddenly as the material recrystallizes. The results of TA analysis for P25 show a weight loss due to loss of adsorbed moisture, as well as a second loss around 270 ◦ C. This DTG peak, unlike the corresponding one for our catalyst, is related to an endothermic effect (Figs. 5 and 6). The mass leveled off near the minimum just above 400 ◦ C, though there is an increase in the intensity of the endothermic effects around 800 ◦ C. As XRD indicated (Fig. 4), Degussa P25 is a mixture of anatase and rutile phases of TiO2 . Unlike our catalyst, which forms as an amorphous powder, P25 is purchased as an essentially crystalline material. Hence, it did not exhibit the more complex behavior of the amorphous powder when exposed to heat treatment. To properly account for the effect of radiation on DBT removal, the dark trial, 1-DARK, was performed. In this trial, a 200 ppm solution of DBT was subjected to gas bubbling (air and moisture) in the absence of light and catalysts. No significant loss of DBT was found. Therefore, it is concluded that any decrease in DBT concentration in subsequent trials is the result of photooxidation reaction. The results of the photolysis experiments are summarized in Fig. 7 and Table 2. Final concentrations of DBT under trial
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(A)
Fig. 7. Changes in DBT concentration with time for various trials. Table 2 Summary of the photolysis experiments Trial 2-300
Order of reaction
First Zeroth 2-254 First 1-300-CAT Zeroth First 1-254-CAT First 1-254-P25-10 Unknown 1-254-P25-20 Unknown
Rate constanta
Final [DBT] Final [S] (ppm) (8 h) (ppm) (8 h)
% DBT removal
0.11 14.80 0.17 16.25 0.12 0.25 Unknown Unknown
79
14
60
53 69
9 12
73 66
29 40 43
5 7 7
86 80 78
(B)
a First-order rate constants are in ppm−1 h−1 . Zero-order rate constants are in h−1 .
1-300 averaged 80 ppm. This is the equivalent of about 14 ppm sulfur and constitutes reaction of 60% of starting DBT. Data presented in Fig. 7 suggest that the photooxidation of DBT in 1-300 is first order with respect to DBT, as expected for low concentration for Langmuir–Hinshellwood kinetics at low concentrations. This is in apparent contrast with the results of Hirai et al. [5], who used a starting concentration of DBT of 2–4 g/L (2650–5300 ppm of sulfur) and found a zero-order rate. This discrepancy is a result of the concentration differences. For semiconductor-catalyzed photoreactions zero order is expected at high concentrations. It is also significant that according to the results of Hirai et al. [5], the pure tetradecane solution appears to obey a first-order rate law. In fact, several of the figures depicting the biphasic system also appear to tail off near the end at very low concentrations. These low concentrations are more consistent with the conditions reported here. According to Salem [12], the mechanism of the reaction is likely the production of conduction-band holes in the TiO2 photocatalyst, which could then interact either with the organic substrate (in our case, DBT) or with an inorganic oxidant, including O2 , to form radicals.
(C) Fig. 8. UV spectra of the DBT solution before photooxidation (A), after photooxidation (B), and after photooxidation followed by adsorption onto silica (C).
HPLC results, in addition to being a tool for concentration determination, yield a hint to the products of photooxidation. The peak that appeared at a retention time of about 2.1 min grew in height with the course of the reaction, reaching a maximum
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near 30 mAU. The primary quantitative results were obtained using UV spectrophotometry. Fig. 8A shows a typical UV spectrum for DBT at a concentration of 200 ppm. Four main peaks are visible. Of these four, the 313- and 326-nm peaks show a linear dependence on DBT concentration over the range investigated. Following photooxidation, a number of changes are observed in the spectrum of the sample solutions (Fig. 8B). An increase in absorbance at 326 nm was recorded with increasing irradiation, along with the emergence of maxima at 304 and 310 nm. Due to the intensity and closeness of the peaks, however, it was not possible to resolve them. To test the separation method, the solution after irradiation was mixed with silica gel powder, shaken, and left for 75 min to allow adsorption of the oxidation products (DBT is not adsorbed onto silica). Fig. 8C shows the spectrum of the same solution as in Fig. 8B but after the adsorption process. The remaining four peaks are the same four peaks that are characteristic of DBT. It is important to note that it is not certain whether there are any compounds other than DBT remaining in solution after adsorption. In that event, the data presented here would represent an overestimate of the amount of DBT remaining in solution. However, the similarity of the spectra in Figs. 8A and 8C suggests that any contributions from other compounds are minimal. In addition to the evidence from UV spectroscopy, two other observations support the hypothesis of nearly complete product removal. During photooxidation, the sample solutions became increasingly yellow, in accord with the observations of Hirai et al. [5]. After adsorption onto silica, the solutions returned to a nearly colorless state (Fig. 9A). Moreover, during the process of photooxidation, some of the products adsorbed onto the catalyst slides. This adsorption of the products resulted in a yellowing (darkening in the photos) of the slides, as shown in Fig. 9B. The slide on the left is unused, while that on the right was used for 8 h of photooxidation at 254 nm. Adsorption likely occurs on hectorite owing to its high surface area and pore volume. This phenomenon can be technologically advantageous from the point of view of removal of oxidation product but, on the other hand, it may prevent the reuse of the catalyst. The results of trial 2-300, plotted in Fig. 7, indicate that the photooxidation may either follow a zero- or first-order rate law with respect to DBT. A zero-order fit again results in a slightly lower R 2 value (0.985). First-order dependence on the substrate is to be expected in the case of single-photon direct photolysis. In this sort of mechanism, the rate-limiting step is presumed to be n–π ∗ excitation of the substrate, followed by rapid reaction of the excited intermediate to form a series of products. Because oxidant concentrations and irradiation intensity were kept essentially constant, a first-order rate is reasonable. From a linear least-squares fit of the data in Fig. 7, the pseudo-firstorder rate constant under these conditions was determined to be 0.1149 ppm−1 h−1 . On the other hand, trial 2-254 shows a clearer first-order rate dependence than the 300-nm trials (Fig. 7). A greater decrease in the concentration of DBT over time is observed. The 8-h irradiated samples showed an average final concentration of 53 ppm DBT, which is equivalent to about 9 ppm sulfur. This represents a removal of 73% of the
Fig. 9. Changes in the color of the DBT solutions exposed to photooxidation and silica adsorption (A); changes in the color of the CAT after exposure to photooxidation (B).
DBT from the original solution. As with the 300-nm trial, there is some variation in the calculated concentrations of DBT at each irradiation time. Of note is the variation between the 4-h samples. It is significant that the concentrations appear to come in pairs, with samples 4A and 4B showing significantly different concentrations than 4C and 4D. Samples 4A and 4B were present at the same time in the reaction chamber, while 4C and 4D were present during a different 4-h period. This suggests that some unknown variable may be responsible for this discrepancy. Unlike the 300-nm trials, the rate for 2-254 shows clear first-order dependence on DBT. A zero-order fit gives an R 2 of only 0.90. The rate constant for the 254-nm reaction is calculated to be 0.1690 ppm−1 h−1 . For the 1-300-CAT, after 8 h of irradiation in the presence of the catalyst, the average concentration of DBT decreased to 69 ppm DBT (Fig. 7). This is equivalent to 12 ppm sulfur, a removal of 66% of the original DBT. Like the uncatalyzed trials, the catalyzed reaction could appear to follow either a zeroor first-order rate law with respect to DBT. However, the first order regression fit is in this case worse than the zero-order (R 2 = 0.977). The first-order rate constant is 16.25 ppm−1 h−1 .
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Table 3 Results of photolysis on Degussa P25 titanium oxide Sample
Mass of catalyst (mg)
Irradiation time (h)
Absorbance at 326 nm
[DBT] (ppm)
Absorbance at 313 nm
[DBT] (ppm)
[DBT] (Avg) (ppm)
Initial 1-254-P25-10A 1-254-P25-10B 1-254-P25-20A 1-254-P25-20B
0 10 10 20 20
0 8 8 8 8
0.4598 0.4316 0.4735 0.4743
36 34 37 37
0.4427 0.4009 0.4711 0.4496
47 43 50 48
199 42 38 44 43
If the rate of reaction is in fact independent of the concentration of DBT, it would be consistent with the general mechanism for TiO2 -catalyzed photooxidation given by Salem [12], as well as the mechanism proposed by Abdel-Wahab and Gaber [15]. Both of these mechanisms involve primary excitation of TiO2 electrons to produce photochemical holes and conduction-band electrons. DBT molecules adsorbed onto the surface of the catalyst then react with either holes or electrons to lead to oxidation products. The rate of this reaction mechanism is dependent on the light intensity and surface area of the catalyst, but not on the substrate concentration. Because DBT has a much lower absorbance around 300 nm, the primary mechanism for photooxidation at this wavelength appears to be indirect photolysis. In the case of trial 1-254-CAT, after 8 h of photolysis (followed by adsorption onto silica) the average concentration of DBT was reduced to 29 ppm. This is 5 ppm sulfur, a removal of 86% of the original DBT. This is a substantial reduction from the 53 ppm result from trial 2-254. This trial also showed significant variation between replicates, which can be attributed to both variation between the lamps and slight positioning variations for the catalyst slides. Interestingly, for this sample, the data cannot be fitted to a zero-order curve like that used for the 300-nm catalyzed data (R 2 = 0.847). A first-order rate expression, however, successfully explains the results. A first-order rate constant of 0.2497 ppm−1 h−1 was calculated for the conditions in this experiment. Table 3 collects the data from trial 1-254-P25, which was performed using Degussa P25 photocatalyst. Over the course of 8 h, the samples containing 10 mg of P25 reacted all but 40 ppm of sulfur. The amount of DBT remaining in these solutions is less than the 53 ppm remaining in the uncatalyzed samples, but more than the 29 ppm remaining after 8 h of irradiation with the quartz-supported catalyst. It is important to mention here that the maximum mass of the layered material present in any of the catalyst trials was 8.4 mg, where titania makes up only a fraction of this mass. Interestingly, the samples with much more P25 catalyst did not remove more DBT than those with 10 mg. This suggests that the surface area present in 10 mg was sufficient to perform whatever catalysis occurred. Alternately, it may simply be due to a limitation of the use of suspended TiO2 . It is difficult to maintain large amounts of suspended catalyst for long periods of time, even with constant agitation. Comparison of the UV spectra for a solution irradiated for 8 h at 254 nm, in the absence of catalyst, and 20 mg of P25 shows that peaks are nearly identical in shape, with only a slight difference in their height. This indicates that the major products
of the two samples are essentially the same compounds and thus supports the hypothesis that oxidation at 254 nm is primarily a result of direct photolysis. Table 2 summarizes the results of the photooxidation trials. Possible rate constants are listed for all fits yielding an R 2 value better than 0.95, with the best fit listed first. As demonstrated in the table, all of the catalyzed trials showed an improved rate of reaction over the corresponding uncatalyzed trials. Between the catalysts tested, the layered catalyst showed somewhat higher efficiency than Degussa P25 at 254 nm. Although the percentage of DBT removed was relatively close, it is significant that our catalyst slides contained no more than 9 mg of catalyst for any trial. All of the 254-nm trials showed first-order rate dependence on the concentration of DBT, while the 300-nm trials may be either first or zero order in DBT. In all cases, removal of DBT was greater with the 254-nm lamps than with the 300-nm ones. However, this result does not necessarily indicate the superiority of irradiation at 254 nm. The 254-nm lamps used in these experiments were estimated to have about twice the intensity of the 300-nm lamps. Assuming single-photon excitation of DBT, the rate would depend directly on the intensity of incident radiation, as well as on the absorptivity coefficient at the incident wavelength. The absorbance of DBT is somewhat higher at 254 nm than at either 313 or 325 nm. This would lead one to expect a rate constant for 254 nm at least double that of the 300 nm lamps. This was not the case in these experiments. The apparent first-order dependence on DBT concentration for the 254-nm reaction suggests a different mechanism than that proposed for the 300-nm reaction. The rate constant of the catalyzed reaction is 44% larger than that for the uncatalyzed trial. Hence, it is reasonable to conclude that the catalyst did have an effect. The dark trial with the catalyst demonstrates that the catalyst did not simply adsorb the additional DBT, since that sample showed no decrease in DBT concentration in the absence of light. One possible explanation of the result is that the reaction proceeds by multiple pathways under irradiation at 254 nm. The dominant pathway is the first-order direct photooxidation of DBT. In addition, the presence of the photocatalyst allows for additional indirect photolysis to occur on the surface of the TiO2 . This results in a higher overall removal rate, with predominately first-order rate dependence. To analyze these adsorbed products, spent catalyst slides and silica were rinsed with three times 3 ml of acetonitrile. The resulting solutions were analyzed by HPLC. This analysis confirmed the presence of products significantly more polar than the DBT starting material. The standards used were DBT and
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4. Conclusions
(A)
(B)
(C) Fig. 10. Examples of the chromatograms used for qualitative determination of the effects of photooxidation.
DBT sulfone. These compounds eluted at 8.1 and 2.5 min, respectively. In HPLC analysis of the compounds extracted from silica, an array of compounds was eluted in rapid succession. All of the compounds eluted before DBT, and some even earlier than DBT sulfone (Fig. 10). While the product peaks were not resolved, some qualitative information was obtained. Clearly, a number of compounds were produced as a result of photooxidation [9]. One peak corresponds well to the retention time of the sulfone standard. While most other peaks eluted later than the sulfone, at least two fell earlier. One of these peaks came shortly after the injection peak, meaning that the compound was barely retained by the column. This is characteristic of a compound significantly lower in molecular weight than the starting material. The peak at 3.15 min was seen in the rinsate from catalyst slides irradiated at both 254 and 300 nm, as well as the silica extract. The spectrum of this compound can be seen in Fig. 10C. This spectrum shows absorbances around 230 and 270 nm, which are consistent with absorbances for a conjugated benzoic acid [24]. The strong absorbance at 327 nm suggests an n–π ∗ excitation similar to that for the sulfur atom in DBT. A sulfobenzoic acid would be consistent with the findings of Traulsen et al. [8]. The large peak around 4.6 min remains unidentified.
The results presented here indicate that the applied procedure produces an effective photocatalyst for the removal of dibenzothiophene from nonpolar organic solvent. At 300 nm in the presence of catalyst, photooxidation appears to proceed primarily by indirect photolysis, according to a zero-order rate law. At 254 nm the rate is determined primarily by first-order direct photolysis, though the presence of the catalyst contributes to a greater overall rate of removal. Coupled with adsorption of oxidation products by silica adsorption, the process presented here is capable of reducing sulfur concentrations below the target level of 10 ppm. The catalyst synthesized was not, as previously expected, a series of alternating monolayers. Rather, the results indicate a complex structure of agglomerated nanoparticles. This structure leads to a high degree of microporosity, with a correspondingly high surface area. These features suggest the synthesized catalyst may perform with greater efficiency than the commercially available Degussa P25. Under the conditions tested, irradiation at 254 nm appears to be more effective than irradiation at 300 nm in removal of DBT. However, due to the uncertain order of the rate of the 300 nm reaction, further experiments are warranted to determine if a change of conditions would improve the photolysis yield of the 300 nm process. Specifically, higher intensity lamps and a higher starting concentration of DBT, as well as the presence of naphthalene, may favor the 300 nm reaction. Acknowledgments The authors gratefully acknowledge the assistance of Dr. James T. Yardley and Dr. Richard Harniman of the Columbia University Nanocenter in producing SEM images. We are also indebted to Dr. Jeffrey Steiner and Karin Block of the City College Earth and Atmospheric Sciences Department for assistance with XRD. The help of Dr. Stanislaus Wong (Brookhaven National Laboratory/SUNY), Tirandai Hemray-Benny of SUNY, and Dr. Jorge Morales of CCNY in obtaining the SEM images of CAT, Degussa P25, and hectorite is appreciated. References [1] U.S. Environmental Protection Agency, Control of Air Pollution from New Motor Vehicles: Heavy-Duty Engine and Vehicle Standards and Highway Diesel Fuel Sulfur Control Requirements. http://www.epa.gov/ fedrgstr/EPA-AIR/2001/January/Day-18/a01a.htm (accessed February 2005). [2] U.S. Environmental Protection Agency, Fuel Sulfur Effects on Exhaust Emissions. http://www.epa.gov/otaq/models/mobile6/m6ful001.pdf (accessed February 2005). [3] M. Houalla, D.H. Broderick, A.V. Sapre, N.K. Nag, V.H.J. de Beer, B.C. Gates, H. Kwart, J. Catal. 61 (1980) 523–527. [4] J.T. Andersson, S. Bobinger, Chemosphere 24 (1992) 383–389. [5] T. Hirai, K. Ogawa, I. Komasawa, Ind. Eng. Chem. Res. 35 (1996) 586– 589. [6] S. Bobinger, J.T. Andersson, Chemosphere 36 (1998) 2569–2579. [7] Y. Shiraishi, T. Hirai, I. Komasawa, Ind. Eng. Chem. Res. 37 (1998) 203– 211.
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