Environ. Sci. Technol. 2001, 35, 1252-1258
Photocatalytic Oxidation and Decomposition of Acetic Acid on Titanium Silicalite GUN DAE LEE,† VU A. TUAN, AND JOHN L. FALCONER* Department of Chemical Engineering, University of Colorado, Boulder, Colorado 80309-0424
Transient reaction of adsorbed monolayers of acetic acid was used to characterize the photocatalytic properties of titanium silicalite zeolites (TS-1). The TS-1 zeolites having Si/Ti ratios of 5, 12.5, and 50 are effective catalysts at room temperature for both photocatalytic oxidation (PCO) and decomposition (PCD) of acetic acid. The rates of PCO are higher than the rates of PCD for each catalyst. Acetic acid oxidized photocatalytically in 0.2% O2 to form gasphase CO2 and CH4 and adsorbed H2O on the TS-1 catalysts, whereas no CH4 formed on Degussa P25 TiO2. Isotope labeling showed that, on both TiO2 and TS-1 catalysts, the R-carbon formed CO2 whereas the β-carbon formed CH4 and CO2. The rates of oxidation of the two carbons have different dependencies on UV intensity. The catalysts with higher Si/Ti ratios adsorbed significantly more acetic acid, and the PCO rates per gram of titanium are highest on the TS-1 catalyst with the lowest Ti content, apparently because a larger fraction of the Ti atoms are surface atoms on this catalyst. During PCD in an inert atmosphere, CO2, CH4, and C2H6 formed on TiO2 and on the catalyst with a Si/Ti ratio of 5, but C2H6 was not detected on the other catalysts. The CO2/CH4 selectivity during PCD increased with increasing Si/Ti ratio. The first step in PCO and PCD on TS-1 catalysts appears to be similar and involves formation of a CH3 radical.
Introduction Heterogeneous photocatalytic oxidation (PCO) of organic pollutants is a promising process for air decontamination. A wide range of organics in dilute systems can be oxidized to CO2 and H2O at ambient temperature and pressure on semiconductor photocatalysts under UV illumination. The rates of PCO are high for many organics so that PCO has the potential to be an efficient method for removing low concentrations of organics. Band gap excitation of semiconductors generates electron-hole pairs that can initiate redox processes. An efficient semiconductor photocatalyst must have suitable band-gap energy and stability over long irradiation times, and TiO2 is the most active catalyst for organic oxidation (1-3). Putting titanium within the zeolite cavity or the zeolite framework, as in titanium silicalite (TS1), may have advantages however because zeolites have nanoscale pores, high adsorption capacities, and ionexchange capacities (2, 3). Titanium silicalite has the MFI * Corresponding author e-mail:
[email protected]; fax: (303)492-4341. † Permanent address: Division of Chemical Engineering, Pukyong National University, Pusan 608-739, Korea. 1252
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structure, which is the same structure that ZSM-5 and silicalite zeolites have. Yamashita et al. (2) found that titanium oxides anchored in the Y-zeolite cavity had high activity for photocatalytic decomposition (PCD) of NO into N2, O2, and N2O at 275 K and high selectivity for N2 formation. The best catalysts were prepared by ion exchange, and they attributed the high activity to highly dispersed, tetrahedral titanium oxide species. The specific photocatalytic activities, normalized by the amount of TiO2 in the catalyst, were much higher than for bulk TiO2. Anpo et al. (3) used titanium oxides anchored within Y-zeolite for photocatalytic reduction of CO2 with H2O. The titanium oxide species that were highly dispersed within the zeolite cavities existed in a tetrahedral coordination, and they had high selectivity for CH3OH formation. In contrast, catalysts with aggregated, octahedrally coordinated titanium oxide and powdered TiO2 catalysts had high selectivities for CH4 formation. Yamagata et al. (4) used TS-1 with a Si/Ti ratio of 33.2 as a photocatalyst to form CH4 by H2 reduction of CO2. They concluded that all Ti atoms in the TS-1 framework acted as photocatalytic sites. From IR measurements, they determined that -CH2 and -CH3 were left on the catalyst after reaction. Yamashita et al. (5) used TS-1 (Si/Ti ) 100) with some titanium oxide dispersed within zeolite cavities as photocatalysts for CO2 reduction by H2O to produce CH4 and CH3OH. They observed that Ti-mesoporous molecular sieves exhibited much higher photocatalytic activity for CH3OH formation than powered TiO2. The high activity was attributed to both high dispersion of titanium oxide species and the large pore size. In the current study, PCO and PCD of gas-phase acetic acid were compared on three TS-1 catalysts with different Si/Ti ratios. These catalysts are of interest because of their activity in selective oxidation of organic compounds with hydrogen peroxide under mild conditions (6) and because they have been used for other photocatalytic reactions as described above. The titanium in TS-1 is in the framework or present as extra framework oxide, and TS-1 zeolites have high surface areas for adsorption. Thus, these catalysts were studied to determine their effectiveness for complete oxidation of organic pollutants in air and to determine if they have any advantages over TiO2 photocatalysts. Photocatalytic reaction of acetic acid is of interest because it is a common product of biological digestion (7). Also, acetic acid is a reaction intermediate during oxidation of ethanol and acetaldehyde, and it exhibits similar rates to these molecules during PCO on TiO2. Photocatalytic oxidation rates were measured, and subsequent temperature-programmed oxidation (TPO) helped determine the saturation coverages. Transient photocatalytic reactions were carried out by adsorbing a monolayer of acetic acid on the catalyst, flushing excess organic from the gas phase, and then exposing the catalyst to UV illumination in both the presence (PCO) and absence (PCD) of gas-phase O2. The reaction products were detected by a mass spectrometer. For comparison, PCO and PCD experiments were carried out on powdered TiO2 (Degussa P25). In some experiments, 13C-labeled acetic acid (CH313COOH) was used to monitor the reactivity of the Rand β-carbons separately.
Experimental Methods Catalyst Preparation and Characterization. Three titanium silicalite (TS-1) catalysts with Si/Ti atomic ratios of 5, 12.5, and 50 were prepared by in-situ crystallization. To obtain 10.1021/es001400p CCC: $20.00
2001 American Chemical Society Published on Web 02/10/2001
TABLE 1. Molar Compositions of Gel for TS-1 Synthesis sample
Si/Ti ratio
Ti wt %
TPAOH
2-propanol
TEOT
TEOS
H 2O
TS-1-A TS-1-B TS-1-C
5 12.5 50
21 9.6 2.6
0.5 0.5 0.5
8.0 5.0 3.0
0.20 0.08 0.02
1.0 1.0 1.0
36.0 36.0 36.0
high dispersion of Ti within the MFI framework, clear synthesis solutions were used. The Si source was tetraethyl orthosilicate (TEOS), and the Ti source was tetraethyl orthotitanate (TEOT). Tetrapropylammonium hydroxide (TPAOH) was used as the template. The required amount of TEOT was diluted with 2-propanol while stirring, TEOS was then added, and the solution was stirred for 30 min at room temperature. A solution of water and TPAOH was then added to the mixture, which was stirred for an additional 2 h at room temperature to obtain a clear, homogeneous gel. The gel was crystallized in a Teflon-lined autoclave at 448 K for 5 days. The zeolite was separated from the liquid phase by centrifugation, dried at 373 K, and then calcined at 773 K for 8 h. Gel compositions are presented in Table 1. Under these synthesis conditions, the Si/Ti ratio in the TS-1 crystals is expected to be the same as in the synthesis mixture. Degussa P25 TiO2 was used as a reference TiO2 catalyst. X-ray diffraction (XRD) analyses of the TS-1 samples were preformed on a Siemens diffractometer using Cu KR radiation. Scanning electron micrographs (SEM) were obtained on a JEOL 8600 microscope. Transient Reaction Measurements. The apparatus used for PCD, PCO, and TPO was described previously (8). Approximately 30-50 mg of catalyst was coated as a thin layer on the inside surfaces of an annular Pyrex reactor so that all the catalyst was exposed to UV light. The annular reactor had a 1-mm annular spacing to minimize mass transfer effects and rapidly flush gas-phase products from the reactor. The outside diameter of the reactor was 2 cm, and the reactor was 13 cm high so that sufficient catalyst mass was present to allow detection of reaction products by a mass spectrometer. Twelve 8-W UV lamps (Johnlite, F8T5BlB) surrounded the reactor. Transient reaction of acetic acid was carried out at room temperature with mass spectrometric detection. Before each experiment, the catalyst was held at 723 K for 30 min in 20% O2 in He and then cooled to room temperature to create a reproducible surface. Depending on the catalyst, two to four 1-µL pulses of acetic acid (Sigma, 99%) or 13C-labeled acetic acid (CH313COOH, Isotec, 99+% atom enrichment) were used to saturate the catalyst in the dark at 300 K prior to PCD or PCO. All experiments started with the surface saturated. After acetic acid was injected, the reactor was flushed for 2 h to remove it from the gas phase so that only reaction of the adsorbed monolayer was studied. Photocatalytic decomposition was carried out at room temperature in 100 cm3/min (STP) of He flow by turning the UV lights on and observing the products that formed with a Balzers QMG 421C quadrupole mass spectrometer. Reaction was stopped and started again by turning the lights off and on. A small m/e ) 32 signal was detected by the mass spectrometer, but that signal did not change when the lights were turned on with acetic acid adsorbed on the surface. Thus, this signal was due to background gas in the vacuum chamber, and O2 in the feed stream was below the detection limit, which we estimate to be 0.3 ppm (9). Photocatalytic oxidation was carried out in 0.2% O2 flow at room temperature for most experiments, although a few used 20% O2. The mass spectrometer monitored the reactor effluent immediately downstream of the reactor. A computer interfaced to the mass spectrometer recorded the amplitudes of
multiple mass peaks simultaneously and the output from a thermocouple in contact with the catalyst. The mass spectrometer was calibrated by injecting known volumes of gases into the flow downstream of the reactor, and signals were corrected for cracking in the mass spectrometer. After PCO or PCD, TPO was performed by heating the catalyst at 1 K/s to 723 K in 20% O2 and holding it at 723 K until no desorption products were detected. The TPO was used to determined how much acetic acid, reaction products, and intermediates remained on the surface after PCD or PCO.
Results and Discussion Characterization of TS-1 Catalysts. Figure 1 shows the XRD patterns of the three TS-1 catalysts. The XRD pattern for the Si/Ti ) 50 catalyst matches that reported for the pure MFI structure with good crystallinity. The upper limit for inclusion of titanium in the TS-1 framework is around 2.5 mol % (6). Above that value, the excess is extra framework titanium, which is usually present as anatase particles. Since catalysts TS-1-A and TS-1-B have loading higher than 2% (Table 1), they should have extra framework titanium, and thus the XRD pattern for the TS-1A (Si/Ti ) 5) catalyst is a combination of TS-1 and anatase patterns. A number of studies reported that, at Si/Ti ratios of 5-12, a mixture of TS-1 and anatase was obtained (17-20). As shown in Figure 2, SEM analysis indicates that the size and shape of crystallites changed with the Si/Ti ratio. The crystal size increased with increasing Ti content, probably due to gel dilution. For the catalyst with Si/Ti ) 50, most crystals were approximately 0.7 µm in diameter, whereas for the catalyst with Si/Ti ) 5, the crystals were rectangular with an approximate length of 5 µm and an aspect ratio of about 2.5. More 2-propanol was used in the synthesis as the Si/Ti ratio decreased (Table 1), and this lowered the gel viscosity, which seemed to favor the development of zeolite crystals along the a and c axes. Similarly, the crystal size of Ge-ZSM-5 zeolite has been reported to increase with decreasing Si/Ge ratio (14). Photocatalytic Oxidation. In previous transient PCO studies of a monolayer of acetic acid on P25 TiO2, gas-phase CO2 and formaldehyde formed immediately upon illumination of the catalyst, and the water product remained on the TiO2 surface (9). The same behavior was seen in the current experiments, but a higher light intensity was used so that rates are higher and the details are different. The higher intensity creates electron-hole pairs at a higher rate, and different steps in the reaction can be affected differently by the higher concentrations of electrons and holes. As shown in Figure 3, PCO (0.2% O2) of a monolayer of CH313COOH on P25 formed 13CO2, 12CO2, and CH2O. The water product is not shown because water adsorbs strongly on TiO2, whereas CO2 is weakly adsorbed. The rate of 13CO2 formation in Figure 3 rapidly reached a maximum after the P25 catalyst was illuminated, but the rate of 12CO2 formation increased slowly to a maximum after 460 s of illumination. As previously reported (9), this indicates that the R-carbon (labeled with 13C) in acetic acid preferentially oxidizes to CO2. The rates of 12CO2 and 13CO2 formation were only comparable at long times. The time delay before 12CO2 formation reached a maximum indicates that the β-carbon oxidized through an intermediate species, such as formate or formaldehyde. Indeed, 12CH2O but no 13CH O formed during PCO of CH 13COOH on P25 TiO . The 2 3 2 mass spectrometer was not calibrated for CH2O. The amount of CH2O was estimated from a carbon mass balance between 12C and 13C. During PCO for 3600 s, the rate of total CO 2 (12CO2 + 13CO2) formation decreased to 0.045 µmol (g of catalyst)-1 s-1. This final rate was 7% of the initial rate, and the acetic acid coverage was approximately 8% of a monolayer. The amount of acetic acid adsorbed on P25 was 417 VOL. 35, NO. 6, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. X-ray diffraction patterns of TS-1 powders with different Si/Ti ratios. µmol/g of TiO2. This was estimated from the amount of carbon dioxide formed during PCO and during a subsequent TPO. A comparison of the PCO results in Figure 3, which was for a UV intensity of 2.5 mW/cm2, to transient PCO on P25 at a lower UV intensity (0.3 mW/cm2) but also in 0.2% O2 (9) shows several differences. The maximum rate of 13CO2 formation was a factor of 2.5 higher for the higher light intensity, whereas the maximum rate of 12CO2 formation was 4.6 times higher. Thus, 12CO2 formed faster than 13CO2 formed for a short time period in Figure 3, but this was not observed at the lower light intensity. The oxidation rates of the two types of carbons have different dependencies on light intensity, and this may be because the oxidation rates of intermediates that form from the β-carbon have different dependencies. Moreover, the 12CO2 rate reached a maximum after 460 s in Figure 3, but at the lower light intensity, the maximum was not reached until 1050 s. This also indicates that both acetic acid and its intermediates oxidized faster at the higher UV intensity. In contrast to its behavior on P25 TiO2, acetic acid oxidized photocatalytically on TS-1 catalysts to form gas-phase CO2 and CH4, and CO2 formed significantly faster than CH4. As shown in Figure 4, on TS-1-A the rates of CO2 and CH4 formation reached maxima immediately after UV illumination, decreased relatively rapidly, and then decreased more slowly. The same behavior was observed for TS-1-B, but the rates were lower. For PCO of CH313COOH on TS-1-B (Figure 5), the R-carbon oxidized exclusively to 13CO2, whereas the β-carbon formed both 12CO2 and 12CH4. The 13CO2 and 12CH4 formation rates reached maxima almost as soon as the catalyst was exposed to UV. In contrast, the 12CO2 formation rate did not reach a maximum until approximately 2000 s, and the maximum was broad. The immediate maximum in the rate of 12CH4 formation and the delay before the maximum 1254
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in 12CO2 formation indicate a competition between hydrogenation and oxidation of adsorbed CH3, with CO2 forming through an intermediate. The 12C/13C ratio for the gas-phase products in Figure 5 is less than 1, indicating that some 12C-enriched products or intermediates were left on TS-1-B catalyst after PCO. This was confirmed by a subsequent TPO; the 12C/13C ratio during TPO was greater than 1, and thus the ratio of 12C/13C in the total products during PCO and TPO was close to 1. The rate dependence on time during transient PCO of acetic acid was different for the TS-1-C catalyst. As shown in Figure 6, the rates were much smaller than for the other catalysts. Note, however, that these rates are per gram of catalyst, and TS-1-C contains much less titanium than the other catalysts. The CO2 and CH4 formation rates rapidly increased upon UV illumination, but the rates and the CO2/ CH4 selectivity were almost constant during 3600 s of PCO. The constant rate may arise because most of the acetic acid was not adsorbed on active sites (i.e., those that contain titanium). As acetic acid on the titanium sites reacted, more diffused from unreactive sites to keep the active sites saturated and thus the rate approximately constant. The P25 had a higher initial rate per gram of catalyst than the TS-1 catalysts, whose rates decreased with increasing Si/Ti ratio. However, the initial rates per gram of TiO2 (Table 2) for the TS-1 catalysts were 2-3 times higher than that for P25. This difference may be due mostly to the higher dispersion of titanium in the TS-1 catalysts; i.e., more of the titanium is on the surface of the TS-1 catalysts than for P25. Since some titanium is not in the framework for the TS-1 catalysts with lower Si/Ti ratios, the TiO2 dispersion might be expected to increase as the Si/Ti ratio increased. The initial rate, per gram of TiO2, increased with Si/Ti ratio, as shown in Table 2. The acetic acid adsorbs on both titanium sites
FIGURE 4. Product formation rates during transient photocatalytic oxidation of a monolayer of CH3COOH on TS-1-A (Si/Ti ) 5) in 0.2% O2 in He flow.
FIGURE 2. Scanning electron micrographs of TS-1 powders with different Si/Ti ratios.
FIGURE 5. Product formation rates during transient photocatalytic oxidation of a monolayer of CH313COOH on TS-1-B (Si/Ti ) 12.5) in 0.2% O2 in He flow.
FIGURE 3. Product formation rates during transient photocatalytic oxidation of a monolayer of CH313COOH on TiO2 in 0.2% O2 in He flow. The UV lights were turned on (open triangles) and off (solid triangles) as indicated. and on other sites in the zeolite since it adsorbs on silicalite (15). As shown in Figure 7, when 20% O2 was used instead of 0.2% O2, the 13CO2 and 12CO2 rates were higher on TS-1-B and no 12CH4 was detected. A 100-fold increase in the O2 concentration increased the 13CO2 rate by only 25-50% and the 12CO2 rate by 100%. The 12CO2 rate increased more partly because no 12CH4 formed at the higher O2 concentration. Note that the 12CO2 and 13CO2 rates were still not equal; the R-carbon oxidizes faster than the β-carbon. The absence of CH4 formation indicates that during PCO on TS-1, methyl
FIGURE 6. Product formation rates during transient photocatalytic oxidation of a monolayer of CH3COOH on TS-1-C (Si/Ti ) 50) in 0.2% O2 in He flow. groups form on the surface and then react by parallel pathways to CH4 and CO2. Methane is not being formed and then oxidized at the higher O2 concentration; it has a low reactivity during PCO (16). Higher O2 concentrations shifts the selectivity toward CO2 formation. VOL. 35, NO. 6, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Initial Rates of CO2 and CH4 Formation during PCO of Acetic Acid on TiO2 and TS-1 initial rates (µmol (g of TiO2)-1 s-1) catalyst
CO2
CH4
total
TiO2 TS-1-A (Si/Ti ) 5) TS-1-B (Si/Ti ) 12.5) TS-1-C (Si/Ti ) 50)
0.64 1.05 1.35 1.54
0 0.19 0.21 0.38
0.64 1.24 1.56 1.92
FIGURE 8. Product formation rates during transient photocatalytic decomposition of a monolayer of CH3COOH on TiO2 in He flow.
FIGURE 7. Product formation rates during transient photocatalytic oxidation of a monolayer of CH313COOH on TS-1-B (Si/Ti ) 12.5) in 20% O2 in He flow.
TABLE 3. Amounts Formed during 3600-s PCO and Subsequent TPO of Acetic Acid on TiO2 and TS-1 amt formed during PCO (µmol/g of catalyst) catalyst
CO2
CH4
TiO2 TS-1-A (Si/Ti ) 5) TS-1-B (Si/Ti )12.5) TS-1-C (Si/Ti ) 50)
723 414 369 144
56 48 29
a
HCHO 42a
CO2 amt during TPO (µmol/g of catalyst)
total acetic acid adsorbed (µmol/g of catalyst)
69 450 1177 1376
417 460 797 775
Estimated from carbon mass balance between
12C
and
13C.
Table 3 shows the amounts of gas-phase products detected during 3600 s of PCO and during a subsequent TPO for the four catalysts. Because of calibration difficulties, the amount of CH2O formed was estimated so that the 12C/13C ratio was 1 in the products of PCO plus TPO. The total acetic acid adsorbed (417 µmol/g of catalyst for P25) agrees well with the value of 415 µmol/g of catalyst measured previously using TPD and subsequent TPO (17). As shown in Table 3, the TS-1 catalysts have higher adsorption capacities than P25. This might be expected since the TS-1 zeolites have higher surface areas than P25 (50 m2/ g). The two TS-1 catalysts with higher Si/Ti ratios have higher adsorption capacities than TS-1-A. The presence of extra framework titania in TS-1-A may block access to adsorption sites (18). The amounts of CO2 that formed during 3600 s of PCO decreased as the Ti content decreased, as shown in Table 3. The fraction of the adsorbed acetic acid that reacted during PCO decreased dramatically. For P25 TiO2, 92% of the acetic acid oxidized in 3600 s, whereas on TS-1-C only 9% oxidized. Much of the acetic acid was not adsorbed on titania sites on TS-1-C, and thus the fraction that reacted was lower. When normalized by the amount of titanium in the catalyst, 1256
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then 10 times as much CO2 formed on TS-1-C as on P25 in 3600 s. The rate per gram of titanium is so much higher on TS-1-C because more of its titanium is on the surface than for P25. Photocatalytic Decomposition. Since the TS-1 catalysts are less active per gram of catalyst, they may not have any advantages over P25 TiO2 in applications to PCO of organic contaminants. However, the TS-1 catalysts have much higher adsorption capacities for organics, and under conditions where the organic concentrations vary widely, the TS-1 catalysts might have some advantages. The rates of product formation during PCD (no O2 in the gas phase) were lower than during PCO, and additional products were seen. As shown in Figure 8, acetic acid decomposed photocatalytically on P25 to form CO2, CH4, and C2H6. Upon UV illumination, the rates of CO2 and C2H6 formation reached maxima and decreased quickly. The C2H6 formation rate decreased to almost 0 after 250 s of PCD (the baseline apparently drifted slightly upward in Figure 8), but the CO2 and CH4 formation rates were similar after 250 s. All rates dropped to 0 when the UV lights were turned off. When they were turned back on after 1920 s, the rates of formation of products were higher than before the lights were turned off. The CO2 and CH4 rates increased by factors of 1.7-1.8, but the C2H6 rate increased a factor of 10. This behavior, for a UV intensity of 2.5 mW/cm2, is similar to that reported on P25 for a UV intensity of 0.3 mW/cm2 (9). However, the initial ratio of C2H6 to CH4 rates was higher at the higher UV intensity. Moreover, the CH4 rate was higher after a dark time than before the lights were turned off, as shown in Figure 8. No increase in CH4 rate was seen during a dark time following PCD at the lower UV intensity. As reported previously (9), acetic acid decomposed on TiO2 by parallel pathways to form CO2, CH4, C2H6, and H2O during PCD:
CH3COOH f CO2 + CH4
(1)
2CH3COOH + O(l) f C2H6 + 2CO2 + H2O
(2)
Reaction 1 does not require oxygen, whereas lattice oxygen is extracted from the TiO2 surface in reaction 2. As the TiO2 surface is reduced, the rate of C2H6 formation rapidly decreases because the remaining lattice oxygen is not as easy to remove. Methane and CO2 still form on the reduced surface, although their rates appear to be affected by the reduced surface since they increase after a dark time. During the dark time, lattice oxygen diffuses from the TiO2 bulk to replenish the surface oxygen vacancies (16), and thus the C2H6 formation rate increased dramatically after a dark time.
FIGURE 9. Product formation rates during transient photocatalytic decomposition of a monolayer of CH3COOH on TS-1-A (Si/Ti ) 5) in He flow.
FIGURE 10. Product formation rates during transient photocatalytic decomposition of a monolayer of CH3COOH on TS-1-B (Si/Ti ) 12.5) in He flow. Indeed, when O2 was injected in the dark, the rate of PCD was much higher when the lights were turned back on (19). The TS-1-A catalyst exhibited similar PCD behavior to P25. The main gas-phase products were CO2, CH4, and C2H6, as shown in Figure 9. The rates of formation of all products rapidly reached maxima upon UV illumination. The CO2 rate decreased rapidly initially, but then its rate was almost constant. In addition, when the UV lights were turned off, the CO2 and CH4 rates rapidly decreased at first, but then slowly dropped to 0. When the UV lights were turned on after 1920 s, the formation rates of all products were higher
FIGURE 11. Product formation rates during transient photocatalytic decomposition of a monolayer of CH3COOH on TS-1-C (Si/Ti ) 50) in He flow. than just before the lights were turned off, although the increase in the CH4 rate was slight. The PCD behavior on TS-1-B was similar to that on TS1-A. On TS-1-B, CH313COOH was used so the pathways of the two carbons could be followed. Acetic acid decomposed photocatalytically to 13CO2 and 12CH4, as shown in Figure 10. This is similar to the behavior during PCO; R-carbons were exclusively oxidized to CO2, and β-carbons only produced CH4. The rates of product formation on TS-1-B were less than half those on TS-1-A, and neither 12CO2 nor 13CH4 were detected. A lower rate is expected since TS-1-B has only half as much titanium as TS-1-A. Ethane, if it formed, had too low a rate to be detected. The rapid increase and decrease in the 13CO2 rate and the fact that the CO2 rate was initially much higher than the CH4 rate (the same behavior observed on TS-1-A) indicate that C2H6 probably formed on TS-1-B. Note also that the 12CH4/13CO2 ratio was less than 1. As on TS-1-A, the rates decreased quickly on TS-1-B when the lights were turned off, but they did not drop to 0. The slow decrease in rates over 1000 s is unlikely to be due to catalytic reaction at room temperature in the absence of UV illumination. Instead, the detection of CO2 and CH4 in the dark is probably because these species adsorb more strongly on TS-1 than on TiO2 and also because they are limited by diffusion out of the small TS-1 pores. On TS-1-C, the rates of CO2 and CH4 formation were lower than on TS-1-B since TS-1-C contains less titanium. No C2H6 was detected, although as on catalyst B, C2H6 may have formed in low concentrations and not been detected. More CO2 than CH4 formed, and as shown in Figure 11, the CO2 rate had two distinct maxima with time. When the lights were turned off, the rates dropped more slowly than for the
TABLE 4. Products Distribution of PCO in 0.2% O2 and PCD of Acetic Acid on TiO2 and TS-1 product distribution (%) catalyst TiO2 TS-1-A (Si/Ti ) 5) TS-1-B (Si/Ti ) 12.5) TS-1-C (Si/Ti)50) a
CO2 PCD PCO PCD PCO PCD PCO PCOb PCD PCO
CH4
52 (0.20)a 95 (0.64) 57 (0.09) 88 (0.22) 60 (0.03) 88 (0.13) 100 (0.20) 69 (0.01) 83 (0.04)
C2H6
40 (0.11)
8 (0.08)
29 (0.05) 12 (0.04) 40 (0.02) 12 (0.02)
14 (0.02)
HCHO 5
31 (
