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Heterogeneous photocatalytic oxidation of benzene, toluene, cyclohexene and cyclohexane in humidified air: comparison of decomposition behavior on photoirradiated TiO2 catalyst
Applied Catalysis B: Environmental 38 (2002) 215–225
Heterogeneous photocatalytic oxidation of benzene, toluene, cyclohexene and cyclohexane in humidified air: comparison of decomposition behavior on photoirradiated TiO2 catalyst Hisahiro Einaga* , Shigeru Futamura, Takashi Ibusuki National Institute of Advanced Industrial Science and Technology, AIST Tsukuba West, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan Received 30 November 2001; received in revised form 3 January 2002; accepted 4 March 2002
Abstract Gas–solid heterogeneous photocatalytic decomposition of benzene, toluene, cyclohexane and cyclohexene over TiO2 was studied at room temperature, and their reactivities were compared. Catalyst deactivation was ascribed to the formation of the carbon deposits on TiO2 surface, and the formation and decomposition behavior of the carbon deposits affected the decomposition rate. Deactivated TiO2 catalysts were photochemically regenerated in the presence of water vapor, and the carbon deposits were decomposed to COx . © 2002 Elsevier Science B.V. All rights reserved. Keywords: Photocatalysis; TiO2 ; Hydrocarbon; Catalyst deactivation; Catalyst regeneration
1. Introduction Photocatalytic reactions are useful for the treatment of air polluted with volatile organic compounds (VOCs) [1–3]. Hitherto, many researchers have investigated the photooxidation of VOCs and have reported that a variety of compounds, including hydrocarbons, alcohols, chlorohydrocarbons and amines in gas phase are decomposed on the UV-irradiated TiO2 under ambient conditions. Photocatalytic reactions have an advantage over other reactions, such as thermal incineration and catalytic incineration in that they can efficiently decompose low concentrations of VOCs under mild conditions. It has been reported, however, that TiO2 catalyst is generally deactivated during the course of the VOC photooxidation [4–8]. In many ∗ Corresponding author. Tel: +81-298-61-8679; fax: +81-298-61-8266. E-mail address: [email protected] (H. Einaga).
cases, catalyst deactivation has been ascribed to the accumulation of the intermediates on TiO2 surfaces [9]. Regeneration processes of deactivated TiO2 catalyst have been investigated. In the case of TCE photooxidation, the catalyst is regenerated only by flowing humid air over the catalyst [5]. The other method of catalyst regeneration is heat treatment. Recently, Suib and co-workers have investigated the deactivation behavior of TiO2 in the toluene photooxidation and have shown that the catalysts are completely recovered by the heating above 420 ◦ C [6]. In this treatment, the intermediates on the catalyst are decomposed at elevated temperatures. Deactivated TiO2 catalysts have also been regenerated by the prolonged photoirradiation in air. Peral and Ollis have reported that the deactivated TiO2 catalyst used for the 1-butanol photooxidation are regenerated by the irradiation in flowing pure air [7]. Ameen and Raupp have reported that water vapor is needed for the efficient regeneration of the deactivated TiO2 ,
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which have been used for the xylene photooxidation [8]. In our previous work on benzene photooxidation with TiO2 catalyst, it was shown that the water vapor in the reaction gas inhibited the formation of carbon deposits on the catalyst surface and accelerated their decomposition [10]. These findings imply that the irradiation in humidified air is a useful technique for the regeneration of deactivated TiO2 catalyst. Hydrocarbons are one of the air pollutants to be removed from the flue gases, since not only some of them are toxic, but also they trigger tropospheric ozone increase. It is important to investigate the photooxidation of various types of hydrocarbons with TiO2 , in order to practically utilize the photocatalytic oxidation. In this study, the photooxidation of hydrocarbons, alkane, alkene, and aromatic compounds was carried out and the photoirradiation process was adopted for the regeneration of deactivated catalyst. The objective of this work is to compare the reactivities of hydrocarbons in their TiO2 -catalyzed photooxidation, and the behavior of catalyst deactivation and regeneration.
2. Experimental section 2.1. Catalysts and substrates Commercially available TiO2 P-25 powder (Japan Aerosil Co. Ltd.) was used as catalyst. BET surface area of the catalyst was 43 m2 g−1 . Benzene (514 ppm, N2 balance), toluene (499 ppm), cyclohexane (493 ppm) and cyclohexene (523 ppm) were purchased from Takachiho Kogyo Co. Ltd. and used without further purification.
impurities on TiO2 surface. The reaction temperature was maintained at 303 K. After the gas–solid adsorption equilibrium of the substrate was achieved in the reactor in the dark, the reactor was irradiated to start the reaction. The reaction gas was humidified by passing N2 through a water bubbler. W/F was defined as the weight of catalyst divided by the molar flow rate of hydrocarbons. Simultaneous determination of CO2 and CO concentrations in the effluent gas was performed with a gas chromatograph (GL Sciences GC-390B) equipped with a thermal conductivity detector (TCD), a flame ionization detector (FID) and a methane converter. Concentrations of hydrocarbons were determined with an FID–gas chromatograph (Shimadzu GC-14A). 2.3. Spectroscopic measurement Diffuse reflectance UV–VIS spectra of TiO2 samples were obtained using a Hitachi U-3010 spectrometer equipped with a diffuse reflectance accessory. After TiO2 catalysts were used for the photoreaction, they were scraped from the inner rod of the photoreactor and a fraction of them was diluted with a-Al2 O3 powder by a factor of 1/25 by weight. The spectra were taken by using a-Al2 O3 as a reference sample. Diffuse reflectance FT-IR spectra of TiO2 samples were measured on a JASCO FT-IR-430 spectrometer equipped with a diffuse reflectance attachment JASCO DR-81. Before the measurement, the catalyst samples were purged with the flow of dry N2 for 30 min at room temperature.
3. Results and discussion 2.2. Photocatalytic reactions Reactions were carried out with a flow-type photochemical reactor. Details of the reactor were described elsewhere [10]. The reactor was composed of an inner glass rod (8 mm diameter, 500 mm length) and an outer glass tube (13 mm i.d.) fabricated from Pyrex glass. The reactor was irradiated by four 20 W black light bulbs (Toshiba FL20S BLB-A) surrounding the reactor. The catalyst was coated onto the inner rod from an aqueous slurry and then the rod was dried at 383 K. The catalyst was pretreated by the irradiation in a humidified air in order to decompose the organic
3.1. Photoreaction and regeneration processes in humidified air Fig. 1 compares the time courses for the photooxidation of benzene, toluene, cyclohexane, and cyclohexene (concentration 250 ± 10 ppm) with TiO2 catalyst in humidified air (relative humidity = 50%). The reactions were carried out separately. W/F was 9.4 × 104 g min mol−1 for benzene, toluene and cyclohexane. The cyclohexene conversion was much higher than that for the other substrates and reached almost 100% under the same condition. Therefore, W/F
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Fig. 1. Time courses for the photooxidation of cyclohexane (䊐), cyclohexene (䊏), benzene (䊊) and toluene (䊉) with TiO2 in air. Concentration of hydrocarbon = 250 ± 10 ppm; relative humidity = 50%; W/F = 9.4 × 104 g min mol−1 (cyclohexane, benzene and toluene) and 2.4 × 104 g min mol−1 (cyclohexene).
decreased to 2.4 × 104 g min mol−1 for cyclohexene so that the change of its conversion was easily compared with other hydrocarbons. The conversion for the first 5 min was 38% for benzene, 63% for toluene, 66% for cyclohexane and 41% for cyclohexene, respectively (stage I in Fig. 1). The conversions gradually decreased with time and after 2 h were 9% for benzene, 9% for toluene, 57% for cyclohexane and 28% for cyclohexene, respectively. Thus, conversion decreased greatly during the photooxidation of benzene and toluene. The color of the catalyst changed from white to brown after the photooxidation of benzene and toluene, indicating that the carbon deposits were formed on the TiO2 surface. Yellow-brownish carbon deposits were also formed in the cyclohexene photooxidation. It has been reported that less reactive intermediates are formed on irradiated TiO2 surface in the photooxidation of benzene and toluene [9,11]. We have suggested that the formation of carbon deposits is responsible for the catalyst deactivation [10]. In the case of cyclohexane where the conversion decrement was smallest, the color was almost unchanged after the photooxidation. After the 2 h photoreaction, irradiation was stopped and the gas flow was changed to humidified air without the feed of hydrocarbons, and the photoreactor was irradiated for 2–4 h. In this treatment, the carbon deposits decomposed and the catalyst color was changed
back to white. Then, the photoreaction was carried out again with the feed of the substrate (stage II in Fig. 1). The time courses for the conversions in this stage were similar to those in stage I. These observations indicate that deactivated TiO2 catalysts were regenerated by the irradiation in humidified air. In the regeneration processes, the carbon deposits on the catalyst surface were decomposed to COx . Fig. 2 shows the time course for the COx formation
Fig. 2. Time courses for the photooxidation of carbon deposits on TiO2 . Cyclohexane (䊐), cyclohexene (䊏), benzene (䊊) and toluene (䊉); relative humidity = 50%. Reactions had been carried out at the W/F = 9.4 × 104 g min mol−1 , the hydrocarbon concentrations of 250 ± 10 ppm and relative humidity of 50%.
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when the used TiO2 catalysts were irradiated in humidified air (relative humidity = 50%). Here, all the reactions had been carried out at W/F of 9.4 × 104 g min mol−1 . Formation ratios of CO2 to CO were more than 25. It is clearly seen that the decomposition behavior of carbon deposits, namely, the formation behavior of COx was much different for these compounds. In the decomposition of carbon deposits derived from cyclohexane and cyclohexene, the COx formation monotonously decreased with time and completely consumed within 60 min. On the other hand, the COx formation increased within about 10 min for benzene and from 10 to 25 min for toluene, showing that induction periods were observed for the decomposition of carbon deposits. In these cases, the amount of COx formed for the first 3 min was smaller as compared with cyclohexene. It took about 4 h before the carbon deposits were completely decomposed to COx . These findings indicate that the carbon deposits derived from aromatic compounds were much difficult to be decomposed compared with those from cyclohexane and cyclohexene. Table 1 summarizes the rates for the photooxidation of hydrocarbons and the amounts of the carbon deposits estimated by integrating the curves of the plots in Fig. 2. The rates were evaluated under the condition where the conversion was proportional to W/F. The initial rates of hydrocarbon decomposition could not be correctly determined by the analytical system used in this study. In order to evaluate these, the values at the first 5 min in the time course plots were adopted instead. The amount of carbon deposits was largest in toluene photooxidation. It decreased in the order of benzene, cyclohexene and cyclohexane. The C density per Ti site was estimated by taking the Ti density
of the single crystal surface of anatase (0 0 1) plane [11], which is the most atomically dense plane and is usually assumed to terminate the crystallites predominantly [12]. This leads to 0.86 (atom per Ti site) for benzene and 0.92 (atom per Ti site) for toluene, implying that the catalyst surface was thoroughly covered with carbon deposits after the photooxidation of benzene and toluene. On the other hand, the C density of 0.21 (atom per Ti site) implies that only a small amount of active site was covered with the carbon deposits for the cyclohexane photooxidation. Here, the durability factor was defined as the ratio of the reaction rates at 2 h to those at the first 5 min. The factor was smallest in the toluene photooxidation and increased with the order of benzene, cyclohexene and cyclohexane. The order for the amount of carbon deposits correlated with that for the deactivation level of the TiO2 catalyst. Thus, the formation and decomposition behavior of carbon deposits was the key factor for the TiO2 deactivation in the photooxidation of the hydrocarbons tested in this research. 3.2. COx selectivities We previously reported that the selectivities to CO2 and CO for benzene photooxidation with pure TiO2 were obtained in around 93 and 7%, respectively [10]. In this case, 97–103% of carbon mass balances were obtained. The COx selectivities were almost independent of W/F, and complete oxidation to CO2 could not be achieved. From the point of practical application, it is important to compare the COx selectivities in the photooxidation of cyclohexane, cyclohexene, and toluene with those in the benzene photooxidation. Fig. 3 shows the effect of W/F on the COx selectivity
Table 1 Catalytic activities for the photooxidation of hydrocarbons with TiO2 Rate for decomposition (×10−9 mol m−2 s−1 )a
Cyclohexane Cyclohexene Benzene Toluene
At 5 min
Steady state
3.876 7.093 1.744 3.643
3.217 4.109 0.360 0.376
Durability factorb
Amount of carbon deposits (mol g−1 catalyst)c
Carbon density (atom per Ti site)
0.83 0.58 0.21 0.10
103 247 431 462
0.21 0.49 0.86 0.92
Concentration of hydrocarbon = 250 ± 10 ppm in air, relative humidity = 50%. Durability factor = rate at the steady state/rate at 5 min. c Estimated from the total amount of CO evolved after the reactions at the W/F of 9.4×104 g min mol−1 . x a
b
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Fig. 3. COx selectivity vs. conversion in the photooxidation of cyclohexane (䊐, 䊏) cyclohexene (䉭, 䉱) and toluene (䊊, 䊉) with TiO2 in air; COx selectivity = [COx ]/([CO2 ] + [CO]). Concentration of hydrocarbon = 178 ± 5ppm; relative humidity = 45%.
Fig. 4. Effect of humidity on the rate for photooxidation of cyclohexane (䊐), cyclohexene (䊏), benzene (䊊) and toluene (䊉) in air. Concentration of hydrocarbon = 152 ± 5 ppm.
for the photooxidation of hydrocarbons in humidified air. Here, CO2 selectivity was defined as the amount of CO2 divided by total amount of CO2 and CO. The selectivities to CO2 and CO were 94 and 6% for toluene, 98 and 2% for cyclohexene, 97 and 3% for cyclohexane, respectively. In these cases, the selectivities were almost independent of W/F, indicating that CO was not the intermediate of CO2 . It has been reported that benzaldehyde, benzoic acid are detected for the toluene photooxidation [13,14]. But their amount in gas phase is very small [14].
increased very much when the gas was switched from humidified air to dry air [10]. In this process, the water vapor could inhibit the formation of carbon deposits on the TiO2 surface and accelerated their decomposition. Table 2 summarizes the amount of carbon deposits on TiO2 surface after the photooxidation of cyclohexane, cyclohexene and toluene. In the photooxidation of toluene and cyclohexene, the amount of carbon deposits on the catalyst without water vapor were estimated to be 1053 and 766 mol g−1 , respectively. They were larger than those in the reaction with water vapor and corresponded to C density of 2.11 (atom per Ti site) for toluene, and that of 1.53 for cyclohexene. These findings indicate that the increasing amount of carbon deposits was responsible for the lowered activity of TiO2 for the photooxidation of benzene, toluene and cyclohexene in dry air.
3.3. Effect of humidity It has been reported that humidity in the reaction gas significantly affects the rate of VOC photooxidation [10,14–20], and it has both positive and negative effects. In our previous paper, we showed that the rate of benzene photooxidation decreased with decreasing the humidity. These findings urged us to investigate the effect of humidity on the photooxidation of toluene, cyclohexane and cyclohexene. Fig. 4 shows the dependence of their photooxidation rates on the relative humidity compared with that for benzene. The values were measured after the reaction reached almost the steady state. The reaction rates for toluene and cyclohexene increased with increasing the humidity, as was observed for benzene photooxidation [10]. In the benzene photooxidation, the amount of carbon deposits
Table 2 The amount of carbon deposits on TiO2
Cyclohexane Cyclohexene Toluene
Amount of carbon deposits (mol g−1 catalyst)a
Carbon density (atom per Ti site)
182 766 1053
0.36 1.53 2.11
a Estimated from the total amount of CO evolved after the x reactions at the W/F of 9.4×104 g min mol−1 and concentration of hydrocarbon of 250 ppm in dry air.
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In cyclohexane photooxidation, however, the reaction rates decreased with increasing humidity. In this reaction, the amount of carbon deposits in dry air increased from 103 to 182 mol g−1 in humidified air. The value corresponded to the C density of 0.37 (atom per Ti site), implying that the catalyst surface was not fully covered with carbon deposits. Therefore, dependency of reaction rate on humidity was not controlled by the formation behavior of carbon deposits in the cyclohexane photooxidation. It has been reported that the presence of water vapor inhibits the adsorption of ethylene [18,19] and trichloroethylene [20] on TiO2 and lowers the rate for their decomposition. The decrease of the reaction rate for the cyclohexane photooxidation may also be due to the inhibition of its adsorption on the catalyst surface. The deactivation behavior of TiO2 in cyclohexane in dry air was compared with that in humidified air. Fig. 5a shows the time course for the cyclohexane
Fig. 5. Photooxidation of cyclohexane with TiO2 in air. (a) Time course for cyclohexane conversion in dry air. Cyclohexane concentration = 247 ppm; W/F = 4.7 × 104 g min mol−1 . (b) Time course for COx formation when the used TiO2 was irradiated without cyclohexane in dry air, and then in humidified air.
photooxidation in dry air (concentration 250 ppm; W/F = 4.7 × 104 g min mol−1 ). The cyclohexane conversion for the first 5 min was 45%. The conversion decreased slightly with time, and it was 34% after 200 min. The total amount of cyclohexane consumed during the reaction was estimated to be 1.56 × 10−4 mol. The turnover number estimated by the Ti density of (0 0 1) plane was 3.26, showing that the reaction catalytically proceeded. During the cyclohexane photooxidation in dry air, the color of the catalyst was changed to pale yellow due to the formation of carbon deposits. After the cyclohexane photooxidation, the catalyst was irradiated in dry air without cyclohexane (Fig. 5b). In this stage, COx was formed and their formation decreased with time. Although the catalyst color was diluted, a yellowish color remained on the catalyst. Subsequently, the gas flow was changed to humidified air (relative humidity, 50%). The color of the catalyst changed back to white and a larger amount of COx was formed in this stage compared with that in Fig. 5b, showing that the residual yellowish carbon deposits were completely decomposed. Thus, water vapor is needed for the complete decomposition of yellowish carbon deposits, although the cyclohexane photooxidation proceeds catalytically without water vapor. It is noteworthy that the yellow-brownish carbon deposits derived from benzene, toluene and cyclohexene were not completely decomposed by the irradiation without water vapor. The dependency of the COx selectivities on humidity in the photooxidation of toluene, cyclohexane and cyclohexene was compared with that of benzene. Fig. 6 shows the effect of relative humidity on the COx selectivities in the photooxidation. Under the humidified condition, the selectivities to CO2 and CO were 94 and 6% for the toluene photooxidation, which were very similar to those for benzene photooxidation. The CO selectivity was 2% for cyclohexene and 3% for cyclohexane. They were almost independent of the humidity for all the reactions. 3.4. UV–VIS and FT-IR spectroscopic study for the TiO2 surface during the photoreaction The changes in the TiO2 surface during the photooxidation and regeneration processes were investigated in detail. Fig. 7 shows the diffuse reflectance
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Fig. 6. Effect of humidity on the COx selectivity in the photooxidation of cyclohexane (䊐, 䊏), cyclohexene (䉭, 䉱) and toluene (䊊, 䊉) with TiO2 in air. Concentration of hydrocarbon = 152±5 ppm.
UV–VIS spectra of the TiO2 samples. The band due to the absorption by TiO2 was observed in the region of wavelengths shorter than 400 nm for all the samples (Fig. 7a–e). In the samples used for the photooxidation of benzene, toluene and cyclohexene, the new absorption appeared in the visible light region (spectra A in Fig. 7c–e). Since no absorption band was observed for the reactant compounds in the visible light region, the bands can be attributed to the yellow-brownish carbon deposits on the TiO2 surface. These bands were completely diminished after the photoirradiation with water vapor (spectra B in Fig. 7c–e). In cyclohexane photooxidation, no obvious change was observed through the photooxidation and the regeneration processes (Fig. 7b). All these phenomena were consistent with the observation that carbon deposits formed in the photooxidation of benzene, toluene and cyclohexene were decomposed by the irradiation in humidified air, and that the color of the catalyst was unchanged through the cyclohexane photooxidation and the regeneration in humidified air. Fig. 8 shows the diffuse reflectance FT-IR spectra of the TiO2 samples. The spectra were obtained before and after the photoreaction, and after the regeneration. For the fresh sample, the band due to the adsorbed water was observed at around 1620 cm−1 (Fig. 8a). After the benzene photooxidation in humidified air for 2 h (benzene 250 ppm, relative humidity
Fig. 7. Diffuse reflectance UV–VIS spectra of TiO2 samples. (A) Samples after the photooxidation of hydrocarbon; (B) samples after the irradiation of used TiO2 in humidified air without hydrocarbon. Reactions were carried out at the W/F = 9.4 × 104 g min mol−1 .
50%), the bands due to the carbon deposits on the catalyst surface were observed in the wave number region of 1300–1750 cm−1 (Fig. 8b, spectra A). The two bands at around 1700 cm−1 were assigned to the C=O stretchings. The presence of these bands indicate that the attack by the active oxygen species and O2 occurred during the formation of carbon deposits. In the photooxidation of cyclohexane, cyclohexene and toluene, the bands due to the carbon deposits on the catalyst surface were also observed in the wave number region of 1300–1750 cm−1 (spectra A in Fig. 8c–e). The bands due to the C=O stretchings were observed for all the samples. The C–H stretching
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Fig. 8. Diffuse reflectance FT-IR spectra of TiO2 samples. (A) Samples after the photooxidation of hydrocarbon; (B) samples after the irradiation of used TiO2 in humidified air without hydrocarbon. Reactions were carried out at the W/F = 9.4 × 104 g min mol−1 .
are also observed in the region of 2800–3100 cm−1 after the reactions. After the used catalyst was irradiated in humidified air without the feed of hydrocarbons, the bands due to the carbon deposits were diminished (spectra B in Fig. 7b–e). These observations were consistent with the fact from the catalytic reaction and UV–VIS spectroscopic study that the carbon deposits were decom-
posed by the irradiation in humidified air. The bands at around 1410 cm−1 , very probably ascribed to the carboxyl C–O stretchings, and those due to the C=O stretchings (around 1700 cm−1 ) remained after the regeneration. It is likely that, however, these groups did not affect the photooxidation of hydrocarbons, since the regenerated catalysts showed the catalytic activities similar to those with fresh catalyst.
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3.5. Reaction mechanism Irradiation of TiO2 with the near-UV light generates the pairs of highly reactive electron and hole (Eq. (1)). The hole subsequently oxidizes the surface hydroxyl groups: h+ + –OH → OH
(1)
h+ + Ti–OH → Ti + OH
(2)
Ti + H2 O → Ti–OH + H+
(3)
(denoted by –OH) to form the OH radicals (Eq. (2)). We proposed on the basis of the FT-IR analysis that the presence of H2 O regenerated the surface hydroxyl groups of TiO2 which were consumed in the photoreaction (Eq. (3)) [10]. The reactivity of OH radicals toward hydrocarbons has been well investigated [21]. The OH radicals add to the aromatic rings of benzene and toluene, and unsaturated C=C bonds of cyclohexene (Fig. 9a)). The radicals also abstract the H atoms
Fig. 9. Mechanisms for oxidation of hydrocarbons.
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from the methyl groups of toluene and saturated C–H bonds of cyclohexane and cyclohexene (Fig. 9b)). The resulting intermediate radicals are subsequently oxidized by molecular O2 (Fig. 9c)). These intermediates are decomposed to CO2 and CO via the subsequent oxidation processes or polymerize to give carbon deposits, as shown in Fig. 10. In the oxidation of benzene and toluene, the intermediate radicals tend to react with aromatic rings and form dimerized species. The radicals in the cyclohexene photooxidation can also add to C=C double bonds. These reactions are the initial steps of polymerization. On the other hand, only small amounts of carbon deposits were formed in the cyclohexane oxidation among the four hydrocarbons. In this reaction, the resulting radicals may not propagate the polymerization. The carbon deposits are also decomposed to COx by the irradiation in humidified air (Fig. 10). The extent of catalyst deactivation depends on the formation and decomposition behavior of carbon deposits. The carbon deposits reduce the catalytic activity of TiO2 by blocking active sites on it. The influence of the change of TiO2 surface itself on the deactivation was not investigated in this study. However, that cannot be the key factor in the catalyst deactivation, since the catalyst was recovered only by removing carbon deposits. The gas phase reaction rate with OH radicals at around 298 K decreases in the following order: cyclohexene (64.6 ± 2.5 × 1012 cm3 per molecule s−1 ) > cyclohexane (6.8 ± 1.7 × 1012 cm3 per molecule s−1 ) ≈ toluene (5.78 ± 0.54 × 1012 cm3 per molecule s−1 ) > benzene (1.24 ± 0.09 × 1012 cm3 per molecule s−1 ) [22]. This order correlates well with that in the decomposition rate of hydrocarbons for the first 5 min (Table 1). The OH radicals can be the active species for the hydrocarbon decomposition under our reaction conditions. The reactivities of hydrocarbons with OH radicals may be one of the key factors of the initial rate for their decomposition. As described above, water vapor is needed for complete decomposition of carbon deposits on TiO2 . We have suggested that the role of water vapor is to regenerate the OH groups on TiO2 surface consumed in the photooxidation, which are the precursors of OH radicals [10]. It is plausible that the OH radicals are the active species for the decomposition of carbon deposits. When the OH groups were completely consumed in the reaction without water vapor, OH radicals could
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Fig. 10. Reaction pathways for decomposition of hydrocarbons.
not be formed on TiO2 surface any more. Hence, the amount of carbon deposits increased without water vapor and the reaction rate was lowered in the photooxidation of benzene, toluene and cyclohexene. On the other hand, the reaction proceeded catalytically in the cyclohexane photooxidation without water vapor, while carbon deposits were accumulated on the TiO2 surface. It is likely that the OH groups on TiO2 surface were not consumed during the reaction or holes oxidized cyclohexane to COx . The reaction rate at steady state is a function of the initial reaction rate and the extent of catalyst deactivation. As described earlier, the reactivity with OH radicals may be one of the key factors of the initial reaction rate, and the extent of catalyst deactivation depends on the formation and decomposition behavior of carbon deposits. The formation behavior of the carbon deposits depends on the reactivities of the hydrocarbons toward polymerization in the photooxidation process. On the other hand, the decomposition behavior is affected by the presence of water vapor in the reaction gas. The role of water vapor is to regenerate the OH groups consumed in the reaction and to form the OH radicals, which are considered to decompose the carbon deposits [10].
4. Conclusions Gas–solid heterogeneous photooxidation of four hydrocarbons with TiO2 was investigated to compare the reactivity of alkane, alkene, and aromatic compounds. It was clarified that formation of carbon deposits on TiO2 and their decomposition to COx were the key steps for the catalyst deactivation and they strongly depended on the structures of hydrocarbons. Photoirradiation in humidified air decomposed the carbon de-
posits on TiO2 and regenerated the catalyst, as was confirmed by UV–VIS and FT-IR spectroscopic studies. The photooxidation rate for toluene and cyclohexene decreased with the decreasing the humidity, due to the increasing amount of carbon deposits on TiO2 . The dependency was the same as that for benzene. On the other hand, the rate for cyclohexane was increased with decreasing humidity. In this reaction, the amount of carbon deposits was smallest and their formation behavior was not so much affected by the humidity.
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Heterogeneous photocatalytic oxidation of benzene, toluene, cyclohexene and cyclohexane in humidified air: comparison of decomposition behavior on photoirradiated TiO2 catalyst Hisahiro Einaga* , Shigeru Futamura, Takashi Ibusuki National Institute of Advanced Industrial Science and Technology, AIST Tsukuba West, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan Received 30 November 2001; received in revised form 3 January 2002; accepted 4 March 2002
Abstract Gas–solid heterogeneous photocatalytic decomposition of benzene, toluene, cyclohexane and cyclohexene over TiO2 was studied at room temperature, and their reactivities were compared. Catalyst deactivation was ascribed to the formation of the carbon deposits on TiO2 surface, and the formation and decomposition behavior of the carbon deposits affected the decomposition rate. Deactivated TiO2 catalysts were photochemically regenerated in the presence of water vapor, and the carbon deposits were decomposed to COx . © 2002 Elsevier Science B.V. All rights reserved. Keywords: Photocatalysis; TiO2 ; Hydrocarbon; Catalyst deactivation; Catalyst regeneration
1. Introduction Photocatalytic reactions are useful for the treatment of air polluted with volatile organic compounds (VOCs) [1–3]. Hitherto, many researchers have investigated the photooxidation of VOCs and have reported that a variety of compounds, including hydrocarbons, alcohols, chlorohydrocarbons and amines in gas phase are decomposed on the UV-irradiated TiO2 under ambient conditions. Photocatalytic reactions have an advantage over other reactions, such as thermal incineration and catalytic incineration in that they can efficiently decompose low concentrations of VOCs under mild conditions. It has been reported, however, that TiO2 catalyst is generally deactivated during the course of the VOC photooxidation [4–8]. In many ∗ Corresponding author. Tel: +81-298-61-8679; fax: +81-298-61-8266. E-mail address: [email protected] (H. Einaga).
cases, catalyst deactivation has been ascribed to the accumulation of the intermediates on TiO2 surfaces [9]. Regeneration processes of deactivated TiO2 catalyst have been investigated. In the case of TCE photooxidation, the catalyst is regenerated only by flowing humid air over the catalyst [5]. The other method of catalyst regeneration is heat treatment. Recently, Suib and co-workers have investigated the deactivation behavior of TiO2 in the toluene photooxidation and have shown that the catalysts are completely recovered by the heating above 420 ◦ C [6]. In this treatment, the intermediates on the catalyst are decomposed at elevated temperatures. Deactivated TiO2 catalysts have also been regenerated by the prolonged photoirradiation in air. Peral and Ollis have reported that the deactivated TiO2 catalyst used for the 1-butanol photooxidation are regenerated by the irradiation in flowing pure air [7]. Ameen and Raupp have reported that water vapor is needed for the efficient regeneration of the deactivated TiO2 ,
0926-3373/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 0 2 ) 0 0 0 5 6 - 5
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which have been used for the xylene photooxidation [8]. In our previous work on benzene photooxidation with TiO2 catalyst, it was shown that the water vapor in the reaction gas inhibited the formation of carbon deposits on the catalyst surface and accelerated their decomposition [10]. These findings imply that the irradiation in humidified air is a useful technique for the regeneration of deactivated TiO2 catalyst. Hydrocarbons are one of the air pollutants to be removed from the flue gases, since not only some of them are toxic, but also they trigger tropospheric ozone increase. It is important to investigate the photooxidation of various types of hydrocarbons with TiO2 , in order to practically utilize the photocatalytic oxidation. In this study, the photooxidation of hydrocarbons, alkane, alkene, and aromatic compounds was carried out and the photoirradiation process was adopted for the regeneration of deactivated catalyst. The objective of this work is to compare the reactivities of hydrocarbons in their TiO2 -catalyzed photooxidation, and the behavior of catalyst deactivation and regeneration.
2. Experimental section 2.1. Catalysts and substrates Commercially available TiO2 P-25 powder (Japan Aerosil Co. Ltd.) was used as catalyst. BET surface area of the catalyst was 43 m2 g−1 . Benzene (514 ppm, N2 balance), toluene (499 ppm), cyclohexane (493 ppm) and cyclohexene (523 ppm) were purchased from Takachiho Kogyo Co. Ltd. and used without further purification.
impurities on TiO2 surface. The reaction temperature was maintained at 303 K. After the gas–solid adsorption equilibrium of the substrate was achieved in the reactor in the dark, the reactor was irradiated to start the reaction. The reaction gas was humidified by passing N2 through a water bubbler. W/F was defined as the weight of catalyst divided by the molar flow rate of hydrocarbons. Simultaneous determination of CO2 and CO concentrations in the effluent gas was performed with a gas chromatograph (GL Sciences GC-390B) equipped with a thermal conductivity detector (TCD), a flame ionization detector (FID) and a methane converter. Concentrations of hydrocarbons were determined with an FID–gas chromatograph (Shimadzu GC-14A). 2.3. Spectroscopic measurement Diffuse reflectance UV–VIS spectra of TiO2 samples were obtained using a Hitachi U-3010 spectrometer equipped with a diffuse reflectance accessory. After TiO2 catalysts were used for the photoreaction, they were scraped from the inner rod of the photoreactor and a fraction of them was diluted with a-Al2 O3 powder by a factor of 1/25 by weight. The spectra were taken by using a-Al2 O3 as a reference sample. Diffuse reflectance FT-IR spectra of TiO2 samples were measured on a JASCO FT-IR-430 spectrometer equipped with a diffuse reflectance attachment JASCO DR-81. Before the measurement, the catalyst samples were purged with the flow of dry N2 for 30 min at room temperature.
3. Results and discussion 2.2. Photocatalytic reactions Reactions were carried out with a flow-type photochemical reactor. Details of the reactor were described elsewhere [10]. The reactor was composed of an inner glass rod (8 mm diameter, 500 mm length) and an outer glass tube (13 mm i.d.) fabricated from Pyrex glass. The reactor was irradiated by four 20 W black light bulbs (Toshiba FL20S BLB-A) surrounding the reactor. The catalyst was coated onto the inner rod from an aqueous slurry and then the rod was dried at 383 K. The catalyst was pretreated by the irradiation in a humidified air in order to decompose the organic
3.1. Photoreaction and regeneration processes in humidified air Fig. 1 compares the time courses for the photooxidation of benzene, toluene, cyclohexane, and cyclohexene (concentration 250 ± 10 ppm) with TiO2 catalyst in humidified air (relative humidity = 50%). The reactions were carried out separately. W/F was 9.4 × 104 g min mol−1 for benzene, toluene and cyclohexane. The cyclohexene conversion was much higher than that for the other substrates and reached almost 100% under the same condition. Therefore, W/F
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Fig. 1. Time courses for the photooxidation of cyclohexane (䊐), cyclohexene (䊏), benzene (䊊) and toluene (䊉) with TiO2 in air. Concentration of hydrocarbon = 250 ± 10 ppm; relative humidity = 50%; W/F = 9.4 × 104 g min mol−1 (cyclohexane, benzene and toluene) and 2.4 × 104 g min mol−1 (cyclohexene).
decreased to 2.4 × 104 g min mol−1 for cyclohexene so that the change of its conversion was easily compared with other hydrocarbons. The conversion for the first 5 min was 38% for benzene, 63% for toluene, 66% for cyclohexane and 41% for cyclohexene, respectively (stage I in Fig. 1). The conversions gradually decreased with time and after 2 h were 9% for benzene, 9% for toluene, 57% for cyclohexane and 28% for cyclohexene, respectively. Thus, conversion decreased greatly during the photooxidation of benzene and toluene. The color of the catalyst changed from white to brown after the photooxidation of benzene and toluene, indicating that the carbon deposits were formed on the TiO2 surface. Yellow-brownish carbon deposits were also formed in the cyclohexene photooxidation. It has been reported that less reactive intermediates are formed on irradiated TiO2 surface in the photooxidation of benzene and toluene [9,11]. We have suggested that the formation of carbon deposits is responsible for the catalyst deactivation [10]. In the case of cyclohexane where the conversion decrement was smallest, the color was almost unchanged after the photooxidation. After the 2 h photoreaction, irradiation was stopped and the gas flow was changed to humidified air without the feed of hydrocarbons, and the photoreactor was irradiated for 2–4 h. In this treatment, the carbon deposits decomposed and the catalyst color was changed
back to white. Then, the photoreaction was carried out again with the feed of the substrate (stage II in Fig. 1). The time courses for the conversions in this stage were similar to those in stage I. These observations indicate that deactivated TiO2 catalysts were regenerated by the irradiation in humidified air. In the regeneration processes, the carbon deposits on the catalyst surface were decomposed to COx . Fig. 2 shows the time course for the COx formation
Fig. 2. Time courses for the photooxidation of carbon deposits on TiO2 . Cyclohexane (䊐), cyclohexene (䊏), benzene (䊊) and toluene (䊉); relative humidity = 50%. Reactions had been carried out at the W/F = 9.4 × 104 g min mol−1 , the hydrocarbon concentrations of 250 ± 10 ppm and relative humidity of 50%.
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when the used TiO2 catalysts were irradiated in humidified air (relative humidity = 50%). Here, all the reactions had been carried out at W/F of 9.4 × 104 g min mol−1 . Formation ratios of CO2 to CO were more than 25. It is clearly seen that the decomposition behavior of carbon deposits, namely, the formation behavior of COx was much different for these compounds. In the decomposition of carbon deposits derived from cyclohexane and cyclohexene, the COx formation monotonously decreased with time and completely consumed within 60 min. On the other hand, the COx formation increased within about 10 min for benzene and from 10 to 25 min for toluene, showing that induction periods were observed for the decomposition of carbon deposits. In these cases, the amount of COx formed for the first 3 min was smaller as compared with cyclohexene. It took about 4 h before the carbon deposits were completely decomposed to COx . These findings indicate that the carbon deposits derived from aromatic compounds were much difficult to be decomposed compared with those from cyclohexane and cyclohexene. Table 1 summarizes the rates for the photooxidation of hydrocarbons and the amounts of the carbon deposits estimated by integrating the curves of the plots in Fig. 2. The rates were evaluated under the condition where the conversion was proportional to W/F. The initial rates of hydrocarbon decomposition could not be correctly determined by the analytical system used in this study. In order to evaluate these, the values at the first 5 min in the time course plots were adopted instead. The amount of carbon deposits was largest in toluene photooxidation. It decreased in the order of benzene, cyclohexene and cyclohexane. The C density per Ti site was estimated by taking the Ti density
of the single crystal surface of anatase (0 0 1) plane [11], which is the most atomically dense plane and is usually assumed to terminate the crystallites predominantly [12]. This leads to 0.86 (atom per Ti site) for benzene and 0.92 (atom per Ti site) for toluene, implying that the catalyst surface was thoroughly covered with carbon deposits after the photooxidation of benzene and toluene. On the other hand, the C density of 0.21 (atom per Ti site) implies that only a small amount of active site was covered with the carbon deposits for the cyclohexane photooxidation. Here, the durability factor was defined as the ratio of the reaction rates at 2 h to those at the first 5 min. The factor was smallest in the toluene photooxidation and increased with the order of benzene, cyclohexene and cyclohexane. The order for the amount of carbon deposits correlated with that for the deactivation level of the TiO2 catalyst. Thus, the formation and decomposition behavior of carbon deposits was the key factor for the TiO2 deactivation in the photooxidation of the hydrocarbons tested in this research. 3.2. COx selectivities We previously reported that the selectivities to CO2 and CO for benzene photooxidation with pure TiO2 were obtained in around 93 and 7%, respectively [10]. In this case, 97–103% of carbon mass balances were obtained. The COx selectivities were almost independent of W/F, and complete oxidation to CO2 could not be achieved. From the point of practical application, it is important to compare the COx selectivities in the photooxidation of cyclohexane, cyclohexene, and toluene with those in the benzene photooxidation. Fig. 3 shows the effect of W/F on the COx selectivity
Table 1 Catalytic activities for the photooxidation of hydrocarbons with TiO2 Rate for decomposition (×10−9 mol m−2 s−1 )a
Cyclohexane Cyclohexene Benzene Toluene
At 5 min
Steady state
3.876 7.093 1.744 3.643
3.217 4.109 0.360 0.376
Durability factorb
Amount of carbon deposits (mol g−1 catalyst)c
Carbon density (atom per Ti site)
0.83 0.58 0.21 0.10
103 247 431 462
0.21 0.49 0.86 0.92
Concentration of hydrocarbon = 250 ± 10 ppm in air, relative humidity = 50%. Durability factor = rate at the steady state/rate at 5 min. c Estimated from the total amount of CO evolved after the reactions at the W/F of 9.4×104 g min mol−1 . x a
b
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Fig. 3. COx selectivity vs. conversion in the photooxidation of cyclohexane (䊐, 䊏) cyclohexene (䉭, 䉱) and toluene (䊊, 䊉) with TiO2 in air; COx selectivity = [COx ]/([CO2 ] + [CO]). Concentration of hydrocarbon = 178 ± 5ppm; relative humidity = 45%.
Fig. 4. Effect of humidity on the rate for photooxidation of cyclohexane (䊐), cyclohexene (䊏), benzene (䊊) and toluene (䊉) in air. Concentration of hydrocarbon = 152 ± 5 ppm.
for the photooxidation of hydrocarbons in humidified air. Here, CO2 selectivity was defined as the amount of CO2 divided by total amount of CO2 and CO. The selectivities to CO2 and CO were 94 and 6% for toluene, 98 and 2% for cyclohexene, 97 and 3% for cyclohexane, respectively. In these cases, the selectivities were almost independent of W/F, indicating that CO was not the intermediate of CO2 . It has been reported that benzaldehyde, benzoic acid are detected for the toluene photooxidation [13,14]. But their amount in gas phase is very small [14].
increased very much when the gas was switched from humidified air to dry air [10]. In this process, the water vapor could inhibit the formation of carbon deposits on the TiO2 surface and accelerated their decomposition. Table 2 summarizes the amount of carbon deposits on TiO2 surface after the photooxidation of cyclohexane, cyclohexene and toluene. In the photooxidation of toluene and cyclohexene, the amount of carbon deposits on the catalyst without water vapor were estimated to be 1053 and 766 mol g−1 , respectively. They were larger than those in the reaction with water vapor and corresponded to C density of 2.11 (atom per Ti site) for toluene, and that of 1.53 for cyclohexene. These findings indicate that the increasing amount of carbon deposits was responsible for the lowered activity of TiO2 for the photooxidation of benzene, toluene and cyclohexene in dry air.
3.3. Effect of humidity It has been reported that humidity in the reaction gas significantly affects the rate of VOC photooxidation [10,14–20], and it has both positive and negative effects. In our previous paper, we showed that the rate of benzene photooxidation decreased with decreasing the humidity. These findings urged us to investigate the effect of humidity on the photooxidation of toluene, cyclohexane and cyclohexene. Fig. 4 shows the dependence of their photooxidation rates on the relative humidity compared with that for benzene. The values were measured after the reaction reached almost the steady state. The reaction rates for toluene and cyclohexene increased with increasing the humidity, as was observed for benzene photooxidation [10]. In the benzene photooxidation, the amount of carbon deposits
Table 2 The amount of carbon deposits on TiO2
Cyclohexane Cyclohexene Toluene
Amount of carbon deposits (mol g−1 catalyst)a
Carbon density (atom per Ti site)
182 766 1053
0.36 1.53 2.11
a Estimated from the total amount of CO evolved after the x reactions at the W/F of 9.4×104 g min mol−1 and concentration of hydrocarbon of 250 ppm in dry air.
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In cyclohexane photooxidation, however, the reaction rates decreased with increasing humidity. In this reaction, the amount of carbon deposits in dry air increased from 103 to 182 mol g−1 in humidified air. The value corresponded to the C density of 0.37 (atom per Ti site), implying that the catalyst surface was not fully covered with carbon deposits. Therefore, dependency of reaction rate on humidity was not controlled by the formation behavior of carbon deposits in the cyclohexane photooxidation. It has been reported that the presence of water vapor inhibits the adsorption of ethylene [18,19] and trichloroethylene [20] on TiO2 and lowers the rate for their decomposition. The decrease of the reaction rate for the cyclohexane photooxidation may also be due to the inhibition of its adsorption on the catalyst surface. The deactivation behavior of TiO2 in cyclohexane in dry air was compared with that in humidified air. Fig. 5a shows the time course for the cyclohexane
Fig. 5. Photooxidation of cyclohexane with TiO2 in air. (a) Time course for cyclohexane conversion in dry air. Cyclohexane concentration = 247 ppm; W/F = 4.7 × 104 g min mol−1 . (b) Time course for COx formation when the used TiO2 was irradiated without cyclohexane in dry air, and then in humidified air.
photooxidation in dry air (concentration 250 ppm; W/F = 4.7 × 104 g min mol−1 ). The cyclohexane conversion for the first 5 min was 45%. The conversion decreased slightly with time, and it was 34% after 200 min. The total amount of cyclohexane consumed during the reaction was estimated to be 1.56 × 10−4 mol. The turnover number estimated by the Ti density of (0 0 1) plane was 3.26, showing that the reaction catalytically proceeded. During the cyclohexane photooxidation in dry air, the color of the catalyst was changed to pale yellow due to the formation of carbon deposits. After the cyclohexane photooxidation, the catalyst was irradiated in dry air without cyclohexane (Fig. 5b). In this stage, COx was formed and their formation decreased with time. Although the catalyst color was diluted, a yellowish color remained on the catalyst. Subsequently, the gas flow was changed to humidified air (relative humidity, 50%). The color of the catalyst changed back to white and a larger amount of COx was formed in this stage compared with that in Fig. 5b, showing that the residual yellowish carbon deposits were completely decomposed. Thus, water vapor is needed for the complete decomposition of yellowish carbon deposits, although the cyclohexane photooxidation proceeds catalytically without water vapor. It is noteworthy that the yellow-brownish carbon deposits derived from benzene, toluene and cyclohexene were not completely decomposed by the irradiation without water vapor. The dependency of the COx selectivities on humidity in the photooxidation of toluene, cyclohexane and cyclohexene was compared with that of benzene. Fig. 6 shows the effect of relative humidity on the COx selectivities in the photooxidation. Under the humidified condition, the selectivities to CO2 and CO were 94 and 6% for the toluene photooxidation, which were very similar to those for benzene photooxidation. The CO selectivity was 2% for cyclohexene and 3% for cyclohexane. They were almost independent of the humidity for all the reactions. 3.4. UV–VIS and FT-IR spectroscopic study for the TiO2 surface during the photoreaction The changes in the TiO2 surface during the photooxidation and regeneration processes were investigated in detail. Fig. 7 shows the diffuse reflectance
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Fig. 6. Effect of humidity on the COx selectivity in the photooxidation of cyclohexane (䊐, 䊏), cyclohexene (䉭, 䉱) and toluene (䊊, 䊉) with TiO2 in air. Concentration of hydrocarbon = 152±5 ppm.
UV–VIS spectra of the TiO2 samples. The band due to the absorption by TiO2 was observed in the region of wavelengths shorter than 400 nm for all the samples (Fig. 7a–e). In the samples used for the photooxidation of benzene, toluene and cyclohexene, the new absorption appeared in the visible light region (spectra A in Fig. 7c–e). Since no absorption band was observed for the reactant compounds in the visible light region, the bands can be attributed to the yellow-brownish carbon deposits on the TiO2 surface. These bands were completely diminished after the photoirradiation with water vapor (spectra B in Fig. 7c–e). In cyclohexane photooxidation, no obvious change was observed through the photooxidation and the regeneration processes (Fig. 7b). All these phenomena were consistent with the observation that carbon deposits formed in the photooxidation of benzene, toluene and cyclohexene were decomposed by the irradiation in humidified air, and that the color of the catalyst was unchanged through the cyclohexane photooxidation and the regeneration in humidified air. Fig. 8 shows the diffuse reflectance FT-IR spectra of the TiO2 samples. The spectra were obtained before and after the photoreaction, and after the regeneration. For the fresh sample, the band due to the adsorbed water was observed at around 1620 cm−1 (Fig. 8a). After the benzene photooxidation in humidified air for 2 h (benzene 250 ppm, relative humidity
Fig. 7. Diffuse reflectance UV–VIS spectra of TiO2 samples. (A) Samples after the photooxidation of hydrocarbon; (B) samples after the irradiation of used TiO2 in humidified air without hydrocarbon. Reactions were carried out at the W/F = 9.4 × 104 g min mol−1 .
50%), the bands due to the carbon deposits on the catalyst surface were observed in the wave number region of 1300–1750 cm−1 (Fig. 8b, spectra A). The two bands at around 1700 cm−1 were assigned to the C=O stretchings. The presence of these bands indicate that the attack by the active oxygen species and O2 occurred during the formation of carbon deposits. In the photooxidation of cyclohexane, cyclohexene and toluene, the bands due to the carbon deposits on the catalyst surface were also observed in the wave number region of 1300–1750 cm−1 (spectra A in Fig. 8c–e). The bands due to the C=O stretchings were observed for all the samples. The C–H stretching
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Fig. 8. Diffuse reflectance FT-IR spectra of TiO2 samples. (A) Samples after the photooxidation of hydrocarbon; (B) samples after the irradiation of used TiO2 in humidified air without hydrocarbon. Reactions were carried out at the W/F = 9.4 × 104 g min mol−1 .
are also observed in the region of 2800–3100 cm−1 after the reactions. After the used catalyst was irradiated in humidified air without the feed of hydrocarbons, the bands due to the carbon deposits were diminished (spectra B in Fig. 7b–e). These observations were consistent with the fact from the catalytic reaction and UV–VIS spectroscopic study that the carbon deposits were decom-
posed by the irradiation in humidified air. The bands at around 1410 cm−1 , very probably ascribed to the carboxyl C–O stretchings, and those due to the C=O stretchings (around 1700 cm−1 ) remained after the regeneration. It is likely that, however, these groups did not affect the photooxidation of hydrocarbons, since the regenerated catalysts showed the catalytic activities similar to those with fresh catalyst.
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3.5. Reaction mechanism Irradiation of TiO2 with the near-UV light generates the pairs of highly reactive electron and hole (Eq. (1)). The hole subsequently oxidizes the surface hydroxyl groups: h+ + –OH → OH
(1)
h+ + Ti–OH → Ti + OH
(2)
Ti + H2 O → Ti–OH + H+
(3)
(denoted by –OH) to form the OH radicals (Eq. (2)). We proposed on the basis of the FT-IR analysis that the presence of H2 O regenerated the surface hydroxyl groups of TiO2 which were consumed in the photoreaction (Eq. (3)) [10]. The reactivity of OH radicals toward hydrocarbons has been well investigated [21]. The OH radicals add to the aromatic rings of benzene and toluene, and unsaturated C=C bonds of cyclohexene (Fig. 9a)). The radicals also abstract the H atoms
Fig. 9. Mechanisms for oxidation of hydrocarbons.
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from the methyl groups of toluene and saturated C–H bonds of cyclohexane and cyclohexene (Fig. 9b)). The resulting intermediate radicals are subsequently oxidized by molecular O2 (Fig. 9c)). These intermediates are decomposed to CO2 and CO via the subsequent oxidation processes or polymerize to give carbon deposits, as shown in Fig. 10. In the oxidation of benzene and toluene, the intermediate radicals tend to react with aromatic rings and form dimerized species. The radicals in the cyclohexene photooxidation can also add to C=C double bonds. These reactions are the initial steps of polymerization. On the other hand, only small amounts of carbon deposits were formed in the cyclohexane oxidation among the four hydrocarbons. In this reaction, the resulting radicals may not propagate the polymerization. The carbon deposits are also decomposed to COx by the irradiation in humidified air (Fig. 10). The extent of catalyst deactivation depends on the formation and decomposition behavior of carbon deposits. The carbon deposits reduce the catalytic activity of TiO2 by blocking active sites on it. The influence of the change of TiO2 surface itself on the deactivation was not investigated in this study. However, that cannot be the key factor in the catalyst deactivation, since the catalyst was recovered only by removing carbon deposits. The gas phase reaction rate with OH radicals at around 298 K decreases in the following order: cyclohexene (64.6 ± 2.5 × 1012 cm3 per molecule s−1 ) > cyclohexane (6.8 ± 1.7 × 1012 cm3 per molecule s−1 ) ≈ toluene (5.78 ± 0.54 × 1012 cm3 per molecule s−1 ) > benzene (1.24 ± 0.09 × 1012 cm3 per molecule s−1 ) [22]. This order correlates well with that in the decomposition rate of hydrocarbons for the first 5 min (Table 1). The OH radicals can be the active species for the hydrocarbon decomposition under our reaction conditions. The reactivities of hydrocarbons with OH radicals may be one of the key factors of the initial rate for their decomposition. As described above, water vapor is needed for complete decomposition of carbon deposits on TiO2 . We have suggested that the role of water vapor is to regenerate the OH groups on TiO2 surface consumed in the photooxidation, which are the precursors of OH radicals [10]. It is plausible that the OH radicals are the active species for the decomposition of carbon deposits. When the OH groups were completely consumed in the reaction without water vapor, OH radicals could
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Fig. 10. Reaction pathways for decomposition of hydrocarbons.
not be formed on TiO2 surface any more. Hence, the amount of carbon deposits increased without water vapor and the reaction rate was lowered in the photooxidation of benzene, toluene and cyclohexene. On the other hand, the reaction proceeded catalytically in the cyclohexane photooxidation without water vapor, while carbon deposits were accumulated on the TiO2 surface. It is likely that the OH groups on TiO2 surface were not consumed during the reaction or holes oxidized cyclohexane to COx . The reaction rate at steady state is a function of the initial reaction rate and the extent of catalyst deactivation. As described earlier, the reactivity with OH radicals may be one of the key factors of the initial reaction rate, and the extent of catalyst deactivation depends on the formation and decomposition behavior of carbon deposits. The formation behavior of the carbon deposits depends on the reactivities of the hydrocarbons toward polymerization in the photooxidation process. On the other hand, the decomposition behavior is affected by the presence of water vapor in the reaction gas. The role of water vapor is to regenerate the OH groups consumed in the reaction and to form the OH radicals, which are considered to decompose the carbon deposits [10].
4. Conclusions Gas–solid heterogeneous photooxidation of four hydrocarbons with TiO2 was investigated to compare the reactivity of alkane, alkene, and aromatic compounds. It was clarified that formation of carbon deposits on TiO2 and their decomposition to COx were the key steps for the catalyst deactivation and they strongly depended on the structures of hydrocarbons. Photoirradiation in humidified air decomposed the carbon de-
posits on TiO2 and regenerated the catalyst, as was confirmed by UV–VIS and FT-IR spectroscopic studies. The photooxidation rate for toluene and cyclohexene decreased with the decreasing the humidity, due to the increasing amount of carbon deposits on TiO2 . The dependency was the same as that for benzene. On the other hand, the rate for cyclohexane was increased with decreasing humidity. In this reaction, the amount of carbon deposits was smallest and their formation behavior was not so much affected by the humidity.
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