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VOC's abatement: Photocatalytic oxidation of toluene in vapour phase on anatase TiO2 catalyst
3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 9 1997 Elsevier Science B.V. All rights reserved.
663
VOC's ABATEMENT: PHOTOCATALYTIC OXIDATION OF T O L U E N E I N V A P O U R P H A S E O N A N A T A S E TiO~ C A T A L Y S T V. Augugliaro a, S. Coluccia b, V. Loddo ~, L. Marchese b, G. Martra b, L. Palmisano ~, M. Pantaleone ~ and M. Schiavello a aDipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universith degli Studi di Palermo, Viale delle Scienze, 90128 Palermo, Italy bDipartimento di Chimica I.F.M., Universit/l degli Studi di Torino, Via P. Giuria 7, 10125 Torino, Italy
Photocatalytic oxidation of toluene has been carried out in a gas-solid regime by using polycrystalline anatase TiO2. A fixed bed continuous photoreactor of cylindrical shape was used for performing the photoreactivity runs; the Pyrex glass photoreactor was irradiated by a medium pressure Hg lamp. Air containing toluene and water vapours in various molar ratios was fed to the photoreactor. Toluene was mainly photooxldised to benzaldehyde although benzene, benzyl alcohol and traces of benzoic acid and phenol were also detected. The presence of oxygen was essential for the occurrence of the photoreaction while water played an important role in the mamtainance of the catalytic activity. The results obtained in a preliminary Fourier transform infrared (FT-IR) investigation indicate that toluene is weakly stabilised on the TiO2 particles by hydrogenbonding between the aromatic ring and surface hydroxyl groups.
1. I N T R O D U C T I O N Volatile organic compounds, VOC's, are an important class of mr pollutants usually found in the atmosphere of all urban and industrial areas. Toluene is one of these compounds and, due to its noxious nature, several strategies have been identified in order to reduce its presence in indoor and industrial emissions. Among the methods effective to oxidize toluene, heterogeneous photocatalysis is one of the most attractive, due to the mild conditions under which this process is usually carried out. Photocatalysis has been largely used for the photooxidation of many organic molecules in the hquid-solid regime [1-5], but a few papers report photoreactions in the gas-solid regime [6-10]. The photocatalytic oxidation of toluene in the presence of water was performed m the gas-solid regime by Ibusuki and Takeuchi [6] at room temperature by using TiO2. They found that the presence of water was beneficial in order to achieve the almost complete photo-omdation of toluene to CO2 and HzO, in fact,
664 only very small amounts of benzaldehyde, which is the main product of toluene partial oxidation, were detected. The present paper reports the results of the toluene photooxidation reaction using polycrystalline anatase TiO2 as catalyst. The photoreactivity runs were carried out in a continuous photoreactor fed by a mixture of air, toluene, and water in various molar ratios and irradiated in the near-LW region. The influence of toluene concentration, gas flow rate, and water presence on the photoprocess performance was investigated. A preliminary investigation of the interaction between toluene and the catalyst surface was carried out by Fourier transform infrared (FT-IR) spectroscopy.
2. E X P E R I M E N T A L The set-up of the experimental apparatus is reported in Figure 1. The reactivity experiments were carried out in a flow apparatus using a Pyrex cylindrical reactor whose dimensions were: internal diameter, 1 cm; external diameter, 1.2 cm; and height, 30 cm. A porous frit at the bottom of the cylinder was used to support the fixed bed of powder and to distribute the inlet gaseous mixture. The reactor was vertically positioned inside a thermostatted chamber and was irradiated through a circular window made on a wall of the chamber and covered by a Pyrex glass sheet. An aluminum parabolic reflector was located behind the photoreactor in order to mcrease the illumination. The radiation source was a 400 W medium pressure Hg lamp (Polymer GN ZS, Helios Italquartz) put at approximate 30 cm from the reactor. The radiation power impinging on the photoreactor was measured by usmg a radiometer UVX Digital and its mean value was 5 mWcm 2. The catalyst was polycrystalline TiO2 (Merck, anatase, BET surface area: 10 m2gl); the powder was classified by sieving and the fraction with particle size in the 45-90 ~m range was used. The amount of catalyst was 8 g, corresponding to a fixed bed height of ca. 10 cm. The reactant mixture was generated by bubbling air through saturators containing water and toluene at room temperature. For some runs benzoic acid was used instead of toluene. The gas flows were then mixed and fed to the photoreactor. The temperature of the reactor was always 413 K. The gas flow rates were m the 0.17-10 cm3s-1 range and the toluene molar fraction ranged from 4.0x10 4 to 1.3x10 2. The water molar fraction was held constant to 2.5x10 2. The catalyst was irradiated only when steady state conditions were achieved in the system, i.e. after about 24 h from the beginning of the photoreactor feeding. The runs lasted 170, 350 or 470 h. The gas leaving the photoreactor was periodically analyzed by a gas chromatograph (Varian, Vista 6000), equipped with FID detector. A 0.1% AT-1000 on Carbograph column (2 m x 2 ram) and a Porapak QS column (2 m x 2 ram) were used. For some experiments the gas exiting from the photoreactor was continuously bubbled in liquid acetomtrile. The resulting solution was analyzed by high pressure liquid chromatography (HPLC) (Varian 9050) in order to detect
665 compounds produced in small quantities. At the end of each run, the catalyst was held for 24 h in twice distilled water or acetonitrile in order to dissolve products adsorbed on the surface; the resulting solutions were analyzed by HPLC.
(b)
(c) ~1
(a)
(e)
_i (h)
ill
vent
(i) GC ._1
(mt
(1)
I~176I
(o) (n)
Figure 1. Photoreactivity set-up. (a) Air cylinder, (b) control valves, (c) bottle with water, (d) bottle with toluene, (e) switch valves, (f) thermostatted chamber, (g) parabolic reflector, (h) cylindrical photoreactor, (i) lamp, (1) power supply, (m) water filter, (n) gas chromatograph, (o) bubbling bottle containing acetonitrile. Thick line was electrically warmed in order to avoid product condensation.
For selected runs the gas at the outlet of the photoreactor was bubbled in a saturated aqueous solution of barium hydroxide in order to trap CO2 as BaCO3. The IR spectra were obtained with a Bruker IFS 48 spectrometer. The catalyst powders, as self-supporting pellets, were placed m an infrared cell allowing adsorption-desorption experiments to be carried out in situ. Prior to the adsorption of toluene, the cell was evacuated (1.0xl0 -~ Torr) at room temperature.
3. R E S U L T S AND D I S C U S S I O N Blank reactivity tests were performed at the same experimental conditions used for the photoreactivity experiments but in the absence of catalyst, oxygen or light. Other runs were carried out by using COz or Nz instead of 02. No reactivity
666 was observed in all these cases so that it may be concluded that 02, catalyst, and irradiation areneeded for the occurrence of the photoprocess. The photoreactivity results showed that the reactor reaches steady state conditions after a long period of time (ca. 70 h) from the beginning of the irradiation. At steady state conditions the mare photooxidation product was benzaldehyde but also benzyl alcohol and traces of benzoic acid and phenol were detected at all the experimental conditions used. During the transient period, benzene together with CO2 were also produced. No significant evidence of CO2 production was observed at steady state conditions. These results obviously indicate that at the experimental conditions used the photoprocess does not give rise to a complete degradation of toluene. In Figure 2 the experimental data of benzaldehyde steady state production rate are reported as a function of the gas flow rate. An increase in the reaction rate occurred above a flow rate of 1.7 cm3s1, and remained constant at higher values. This indicates that external mass transfer limitations occur for flow rates less than 1.7 cmas ' . Because of this all the runs carried out for investigating the influence of toluene concentration were performed at a flow rate of 2.5 cm3s -1.
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6
4
~~
3
.~~
2
"~
1
o 0
5
10
F l o w r a t e [cm~s -1] Figure 2. Benzaldehyde steady state production rate versus the gas flow rate. Toluene molar fraction: 4.0x10-4. Catalyst amount: 8 g. Photon flux: 5 mWcm 2.
The inlet toluene concentration greatly affected the benzaldehyde production rate. The runs performed at higher molar fraction of toluene (4.0x10 4, 7,0x10 4, and 1.3x10 -2) showed an increasing benzaldehyde production rate (4.1x10 a, 6.2x10 "a, and 3.4"x10 2 ~molsl).
667 The occurrence of catalyst deactivation and of the role played by water, was investigated by performing very long reactivity runs at the flow rate of 0.42 cm3s 1. Deactivation is not affected by the presence of mass transfer limitations, while the use of a low flow rate allows variations of outlet gas composition to be detected more accurately. Figure 3 reports the fractional conversion of toluene to benzaldehyde versus irradiation time for two long runs carried out in the presence and in the absence of water vapour. For the run in the presence of water, a maximum conversion of 0.19 (corresponding to an
O O
0 r cD
0.1
A
0
AA A m
(D ,.Q
A O
m
A
0.01 0.1
1
10
100
1000
T i m e [h] Figure 3. Toluene fractional conversion to benzaldehyde versus irradiation time for runs carried out in the presence of water vapour (m) and in the absence of water vapour (A). Flow rate: 0.42 cmasl; toluene molar fraction" 1.3x10 -~. Catalyst amount: 8 g. Photon flux: 5mWcm -2.
oxidation rate of 5.6x10 -2 ~mols -1) was achieved after 2 h, while a steady state conversion of 0.08 was achieved after 70 h of irradiation (corresponding to an oxidation rate of 2.4x10 -2 ~mols-i). No decrease of the photoreactivity was observed after 350 h. A different behaviour can be observed for the run carried out in the absence of water. Indeed, a maximum conversion of 0.1 (corresponding to an oxidation rate of 2.9x10 -2 ~mols -1) was reached after ca. 12 h. After that time, the photo-reactivity continuously decreased down to negligible values. In Figure 4 the fractional conversion of toluene to benzene is reported versus irradiation time for the same runs reported in Fig. 3. The presence of water was beneficial for benzene production, but benzene virtually disappeared after 3-4 h of irradiation, independent of the presence of water.
668 In order to determine if the catalyst deactivation was irreversible, a run was performed where first the wet reagent mixture was fed to the irradiated photoreactor and time was allowed for the achievement of a constant photoreactivity level. Then the dry reagent mixture was fed for a prolonged time; and finally the wet reagent mixture was again fed. In Figure 5 the results obtained in this run are reported as toluene fractional conversion to benzaldehyde versus irradiation time. In the absence of water a sharp decrease of toluene conversion occur from 0.08 to 0.04 after ca. 6 h and thereafter from 0.04 to 0.01 after ca. 180 h. When water vapour was again added to the reaction mixture, the photoreactivity initially increased but then slowly decreased until a
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A A o
0
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m
i
I
1
2
nI
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ILl 3
n I
n I
4
Time [h] Figure 4. Toluene fractional conversion to benzene versus irradiation time for the runs reported in Figure 3.
constant value of conversion was achieved. For this run benzene was produced only by the fresh catalyst in the first hours of irradiation; the partially spent catalyst did not produce benzene when water vapour was again present. The results reported in Figure 5 indicate that water is an essential reagent for the photoprocess. The highest activity is shown by the fresh catalyst working m the presence of water vapour; m the absence of water the catalyst progressively deactivates. The catalyst deactivation seems to be a partially reversible process; the restoration of water m the reaction mixture allows a partial recovery of the activity whose final level, however, is less than that of the fresh catalyst.
669 0.1 O O r~
.r,-I
',
;>
! ! ! ! ! ! ! !
-
O r
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~9 !
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~ . !
.!
~.
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.~
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150
250
350
450
Time [hi Figure 5. Effect of water on the photoreactivity.
Figure 6 reports FT-IR spectra obtained at various experimental conditions. The admission of toluene onto the catalyst causes the depletion of the band originally present in the background spectrum at 3670 cm -1 (curve a), due to a stretching mode of surface hydroxyl groups [11]. This band is transformed to a complex and much broader absorption in the 3600-3450 cm -I range (curve b). On the basis of such behaviour it can be concluded that surface OH groups are revolved in the adsorp.tion of toluene onto the catalyst, the resulting organic molecules are probably stabilized on the surface by hydrogen bonding between the aromatic ring and the hydroxyl groups [12]. IR bands assignable to adsorbed toluene appear in spectrum b, namely in the 3200-2800 cm 1 (CH stretchings) and 2000-1300 cm 1 (summation bands, ring stretchings). By simply outgassing at room temperature these bands disappear (spectra not reported) and the original spectral band of the surface hydroxyls is progressively fully restored (Figure 6, reset). Such reversibility suggests a weak character of the observed interaction between toluene and the catalyst. By taking into account all the results above reported, the foUowmg reaction mechanism for the production of benzaldehyde can be suggested. Under photoexcitation of the semiconductor oxide with band gap irradiation, electron-hole (e-h) pairs are photogenerated: TiO2 + hv -~ TiO2 + e- + h §
(1)
The pairs, once separated, can induce chemical redox transformations with the species adsorbed on the surface, subject to thermodynamic constraints. It is generally assumed that surface hydroxyls act as hole traps by producing OH" radicals:
670
2.0 1.3 1.2
!.5 "~ e~
1.0 0.9
l.O
0.8
r,~
0.7
<
3800 3700 3600 3500 3400 3300
0.5
Wavenumber [ cm -1 ]
0.0 3500
3000
2500
2000
1500
Wavenumber [ cm -1] Figure 6, IR spectra of toluene adsorbed on TiO2 catalyst: a) spectrum of the catalyst outgassed 30' at room temperature; b) after admission of 3 Torr of toluene vapour. Inset: a,b) the same as in the mare layer; c-h) desorption of toluene by outgassmg at room temperature for increasing times.
OH
(2)
+ h § -~ OH'.
The reactivity results indicate that oxygen is needed to sustain t h e toluene photooxidation; therefore, while OH groups act as traps for the photo-holes, adsorbed oxygen species act as traps for free photo-electrons and give rise to very reactive species by the following reactions [ 13]:
O2(gas phase) ~
O2(ads)
(3)
(ads)
(4)
02 (ads)+ H § -~ 02H"
(5)
202H"
(6)
O2(ads) "~" e -
---> 0 2
--~ 02
+
H202
H202(.~) +O2-(.~) --> OH- +
OH" + 02.
(7)
671 The radical species are very reactive and may attack toluene molecules according to the following reactions: OH"
+ C6H~CH3(.~)
-+ H20 + C6H5CH2"
C6HsCH2" + 02(,d~) --> C6HsCH200" C6HsCH2OO"
+ e
~
C~H~CHO + O H .
(8) (9) (10)
The formation of benzyl alcohol may occur by the following reaction: C6HsCH2" + OH"
~
C6H~CH2OH.
(11)
The small amount of benzyl alcohol found as a product is understandable since two radicals are involved in reaction (11). Concerning the simultaneous appearance of benzene and C02, the experimental results suggest that the breakage of the bond between the CH3 group and the ct carbon of toluene does not occur. In this last case, in fact, CH4 and/or CH30H molecules should be produced by the reaction between CH3 radicals and OH groups present on the catalyst surface but neither CH4 or CH3OH were ever detected in our experiments. On this basis the occurrence of the following reaction steps may be suggested: C6HsCHO(.~) + OH" C6H~CO" + 0 2 ( ~ ) ~ C6H5C000"
-~ C6H~CO" + H20 C6H5C000"
+ C6HsCHO(a~)-~
C6H5C0" + C6HsCOOOH(a~)
(12) (13) (14)
C6HsCOOOH(.~) + C6HsCHO(.~)---> 2 C6H~COOH(.~)
(15)
C6H~COOH(~) --> C6H6 + C02.
(16)
According to reactions (12)-(16), CO2 results from the oxidation of toluene to benzoic acid whose traces were found in our experiments and its subsequent photodecarboxylation. It must be reported that some runs carried out by using benzoic acid at the same experimental conditions used for toluene photooxidation, afforded benzene and CO2 in large amounts. It is well known [7, 8] that ethanoic acid is easily photodecarboxylated in a gas-solid regime in the presence of irradiated polycrystallme semiconductor oxides. The small amounts of phenol are probably due to an attack of benzene by OH radicals (see eqn.(16)). The progressive deactivation of the catalyst in the absence of water could be due to some surface dehydroxylation and/or to the formation of stable intermediate species which can not evolve in the absence of water so that they remain strongly adsorbed on the catalyst surface. It is worth reporting that the
672 catalyst was found of ochre colour at the end of the run and this colour completely disappeared by irradiating the catalyst in liquid water. The presence on the catalyst surface of sites with different oxidation strength can explain the role played by water and the deactivation-activation pattern exhibited by the catalyst. The partial restoration of activity when water was restored to the reacting ambient could be due to the rehydroxylation of surface sites. The finding that benzene is produced only in the first hours of irradiation while the partially spent catalyst never produced benzene, may be justified by considering that the fresh catalyst has higher oxidant properties due to the presence of highly oxidant hydroxyl groups. Benzaldehyde, instead, can be obtained both on these sites and on less oxidant sites. The experimental results suggest that the more oxidizmg sites react irreversibly under irradiation. By concluding, the photo-oxidation of toluene mainly to benzeldehyde and benzene (as a transient product) was proved to occur in mild conditions in gassolid regime. The presence of sites with different oxidant properties is proposed and a preliminary FTIR mvestigation indicates a weak interaction between toluene molecules and TiO2 surface. ACKNOWLEDGEMENT The authors wish to thank the 'TIinistero delrUmversith e della Ricerca Scientifica e Tecnologica" (Rome) for financially supporting this work.
REFERENCES 1. M. Schiavello (ed.), Photocatalysis and Environment. Trends and Applications, Kluwer, Dordrecht, 1988. 2. V. Augugliaro, L, Palmisano, A. Sclafani, C. Mmero and E. Pelizzetti, Toxicol. Envir. Chem., 16 (1988) 89. 3. E. Pelizzetti and N. Serpone (eds.), Photocatalysis. Fundamentals and Applications, Wiley, New York, 1989. 4. E. Pelizzetti and M. Schiavello (eds.), Photochemical Conversion and Storage of Solar Energy, Kluwer, Dordrecht, 1991. 5. D. F, Ollis and H. A1-Ekabi (eds.), Photocatalytic Purification and Treatment of Water and Air, Elsevier, Amsterdam, 1993. 6. T. Ibusuki and K. Takeuchi, Atmos. Environ., 20 (1986) 1711. 7. L. Palmisano, M. Schiavello, A. Sclafani, S. Coluccia and L. Marchese, New J. Chem., 12 (1988) 847. 8. T. Matsuura and M. Anpo (eds.), Photochemistry on Solid Surfaces, Elsevier, Amsterdam, 1989. 9. M.L. Sauer and D. F. Ollis, J. Catal., 158 (1996) 570. 10. A. J. Malta Vidal, J. Soria, V. Augugliaro, V. Loddo, Chem. Biochem. Eng. Quart., 1997 (in press). 11. C. Morterra, J. Chem. Soc. Faraday Trans. I, 84 (1988) 1617. 12. E. A. Paukshits and E. N. Yurchenko, Russ. Chem. Rev., 52 (1983)242. 13. R. I. Bickley, G. Munuera and F. S. Stone, J. Catal., 31 (1973) 398.
663
VOC's ABATEMENT: PHOTOCATALYTIC OXIDATION OF T O L U E N E I N V A P O U R P H A S E O N A N A T A S E TiO~ C A T A L Y S T V. Augugliaro a, S. Coluccia b, V. Loddo ~, L. Marchese b, G. Martra b, L. Palmisano ~, M. Pantaleone ~ and M. Schiavello a aDipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universith degli Studi di Palermo, Viale delle Scienze, 90128 Palermo, Italy bDipartimento di Chimica I.F.M., Universit/l degli Studi di Torino, Via P. Giuria 7, 10125 Torino, Italy
Photocatalytic oxidation of toluene has been carried out in a gas-solid regime by using polycrystalline anatase TiO2. A fixed bed continuous photoreactor of cylindrical shape was used for performing the photoreactivity runs; the Pyrex glass photoreactor was irradiated by a medium pressure Hg lamp. Air containing toluene and water vapours in various molar ratios was fed to the photoreactor. Toluene was mainly photooxldised to benzaldehyde although benzene, benzyl alcohol and traces of benzoic acid and phenol were also detected. The presence of oxygen was essential for the occurrence of the photoreaction while water played an important role in the mamtainance of the catalytic activity. The results obtained in a preliminary Fourier transform infrared (FT-IR) investigation indicate that toluene is weakly stabilised on the TiO2 particles by hydrogenbonding between the aromatic ring and surface hydroxyl groups.
1. I N T R O D U C T I O N Volatile organic compounds, VOC's, are an important class of mr pollutants usually found in the atmosphere of all urban and industrial areas. Toluene is one of these compounds and, due to its noxious nature, several strategies have been identified in order to reduce its presence in indoor and industrial emissions. Among the methods effective to oxidize toluene, heterogeneous photocatalysis is one of the most attractive, due to the mild conditions under which this process is usually carried out. Photocatalysis has been largely used for the photooxidation of many organic molecules in the hquid-solid regime [1-5], but a few papers report photoreactions in the gas-solid regime [6-10]. The photocatalytic oxidation of toluene in the presence of water was performed m the gas-solid regime by Ibusuki and Takeuchi [6] at room temperature by using TiO2. They found that the presence of water was beneficial in order to achieve the almost complete photo-omdation of toluene to CO2 and HzO, in fact,
664 only very small amounts of benzaldehyde, which is the main product of toluene partial oxidation, were detected. The present paper reports the results of the toluene photooxidation reaction using polycrystalline anatase TiO2 as catalyst. The photoreactivity runs were carried out in a continuous photoreactor fed by a mixture of air, toluene, and water in various molar ratios and irradiated in the near-LW region. The influence of toluene concentration, gas flow rate, and water presence on the photoprocess performance was investigated. A preliminary investigation of the interaction between toluene and the catalyst surface was carried out by Fourier transform infrared (FT-IR) spectroscopy.
2. E X P E R I M E N T A L The set-up of the experimental apparatus is reported in Figure 1. The reactivity experiments were carried out in a flow apparatus using a Pyrex cylindrical reactor whose dimensions were: internal diameter, 1 cm; external diameter, 1.2 cm; and height, 30 cm. A porous frit at the bottom of the cylinder was used to support the fixed bed of powder and to distribute the inlet gaseous mixture. The reactor was vertically positioned inside a thermostatted chamber and was irradiated through a circular window made on a wall of the chamber and covered by a Pyrex glass sheet. An aluminum parabolic reflector was located behind the photoreactor in order to mcrease the illumination. The radiation source was a 400 W medium pressure Hg lamp (Polymer GN ZS, Helios Italquartz) put at approximate 30 cm from the reactor. The radiation power impinging on the photoreactor was measured by usmg a radiometer UVX Digital and its mean value was 5 mWcm 2. The catalyst was polycrystalline TiO2 (Merck, anatase, BET surface area: 10 m2gl); the powder was classified by sieving and the fraction with particle size in the 45-90 ~m range was used. The amount of catalyst was 8 g, corresponding to a fixed bed height of ca. 10 cm. The reactant mixture was generated by bubbling air through saturators containing water and toluene at room temperature. For some runs benzoic acid was used instead of toluene. The gas flows were then mixed and fed to the photoreactor. The temperature of the reactor was always 413 K. The gas flow rates were m the 0.17-10 cm3s-1 range and the toluene molar fraction ranged from 4.0x10 4 to 1.3x10 2. The water molar fraction was held constant to 2.5x10 2. The catalyst was irradiated only when steady state conditions were achieved in the system, i.e. after about 24 h from the beginning of the photoreactor feeding. The runs lasted 170, 350 or 470 h. The gas leaving the photoreactor was periodically analyzed by a gas chromatograph (Varian, Vista 6000), equipped with FID detector. A 0.1% AT-1000 on Carbograph column (2 m x 2 ram) and a Porapak QS column (2 m x 2 ram) were used. For some experiments the gas exiting from the photoreactor was continuously bubbled in liquid acetomtrile. The resulting solution was analyzed by high pressure liquid chromatography (HPLC) (Varian 9050) in order to detect
665 compounds produced in small quantities. At the end of each run, the catalyst was held for 24 h in twice distilled water or acetonitrile in order to dissolve products adsorbed on the surface; the resulting solutions were analyzed by HPLC.
(b)
(c) ~1
(a)
(e)
_i (h)
ill
vent
(i) GC ._1
(mt
(1)
I~176I
(o) (n)
Figure 1. Photoreactivity set-up. (a) Air cylinder, (b) control valves, (c) bottle with water, (d) bottle with toluene, (e) switch valves, (f) thermostatted chamber, (g) parabolic reflector, (h) cylindrical photoreactor, (i) lamp, (1) power supply, (m) water filter, (n) gas chromatograph, (o) bubbling bottle containing acetonitrile. Thick line was electrically warmed in order to avoid product condensation.
For selected runs the gas at the outlet of the photoreactor was bubbled in a saturated aqueous solution of barium hydroxide in order to trap CO2 as BaCO3. The IR spectra were obtained with a Bruker IFS 48 spectrometer. The catalyst powders, as self-supporting pellets, were placed m an infrared cell allowing adsorption-desorption experiments to be carried out in situ. Prior to the adsorption of toluene, the cell was evacuated (1.0xl0 -~ Torr) at room temperature.
3. R E S U L T S AND D I S C U S S I O N Blank reactivity tests were performed at the same experimental conditions used for the photoreactivity experiments but in the absence of catalyst, oxygen or light. Other runs were carried out by using COz or Nz instead of 02. No reactivity
666 was observed in all these cases so that it may be concluded that 02, catalyst, and irradiation areneeded for the occurrence of the photoprocess. The photoreactivity results showed that the reactor reaches steady state conditions after a long period of time (ca. 70 h) from the beginning of the irradiation. At steady state conditions the mare photooxidation product was benzaldehyde but also benzyl alcohol and traces of benzoic acid and phenol were detected at all the experimental conditions used. During the transient period, benzene together with CO2 were also produced. No significant evidence of CO2 production was observed at steady state conditions. These results obviously indicate that at the experimental conditions used the photoprocess does not give rise to a complete degradation of toluene. In Figure 2 the experimental data of benzaldehyde steady state production rate are reported as a function of the gas flow rate. An increase in the reaction rate occurred above a flow rate of 1.7 cm3s1, and remained constant at higher values. This indicates that external mass transfer limitations occur for flow rates less than 1.7 cmas ' . Because of this all the runs carried out for investigating the influence of toluene concentration were performed at a flow rate of 2.5 cm3s -1.
CD
A v
6
4
~~
3
.~~
2
"~
1
o 0
5
10
F l o w r a t e [cm~s -1] Figure 2. Benzaldehyde steady state production rate versus the gas flow rate. Toluene molar fraction: 4.0x10-4. Catalyst amount: 8 g. Photon flux: 5 mWcm 2.
The inlet toluene concentration greatly affected the benzaldehyde production rate. The runs performed at higher molar fraction of toluene (4.0x10 4, 7,0x10 4, and 1.3x10 -2) showed an increasing benzaldehyde production rate (4.1x10 a, 6.2x10 "a, and 3.4"x10 2 ~molsl).
667 The occurrence of catalyst deactivation and of the role played by water, was investigated by performing very long reactivity runs at the flow rate of 0.42 cm3s 1. Deactivation is not affected by the presence of mass transfer limitations, while the use of a low flow rate allows variations of outlet gas composition to be detected more accurately. Figure 3 reports the fractional conversion of toluene to benzaldehyde versus irradiation time for two long runs carried out in the presence and in the absence of water vapour. For the run in the presence of water, a maximum conversion of 0.19 (corresponding to an
O O
0 r cD
0.1
A
0
AA A m
(D ,.Q
A O
m
A
0.01 0.1
1
10
100
1000
T i m e [h] Figure 3. Toluene fractional conversion to benzaldehyde versus irradiation time for runs carried out in the presence of water vapour (m) and in the absence of water vapour (A). Flow rate: 0.42 cmasl; toluene molar fraction" 1.3x10 -~. Catalyst amount: 8 g. Photon flux: 5mWcm -2.
oxidation rate of 5.6x10 -2 ~mols -1) was achieved after 2 h, while a steady state conversion of 0.08 was achieved after 70 h of irradiation (corresponding to an oxidation rate of 2.4x10 -2 ~mols-i). No decrease of the photoreactivity was observed after 350 h. A different behaviour can be observed for the run carried out in the absence of water. Indeed, a maximum conversion of 0.1 (corresponding to an oxidation rate of 2.9x10 -2 ~mols -1) was reached after ca. 12 h. After that time, the photo-reactivity continuously decreased down to negligible values. In Figure 4 the fractional conversion of toluene to benzene is reported versus irradiation time for the same runs reported in Fig. 3. The presence of water was beneficial for benzene production, but benzene virtually disappeared after 3-4 h of irradiation, independent of the presence of water.
668 In order to determine if the catalyst deactivation was irreversible, a run was performed where first the wet reagent mixture was fed to the irradiated photoreactor and time was allowed for the achievement of a constant photoreactivity level. Then the dry reagent mixture was fed for a prolonged time; and finally the wet reagent mixture was again fed. In Figure 5 the results obtained in this run are reported as toluene fractional conversion to benzaldehyde versus irradiation time. In the absence of water a sharp decrease of toluene conversion occur from 0.08 to 0.04 after ca. 6 h and thereafter from 0.04 to 0.01 after ca. 180 h. When water vapour was again added to the reaction mixture, the photoreactivity initially increased but then slowly decreased until a
0.005 o o o0
-mmq
mm
o c~
mm
c~
mm
A A o
0
m m A
m
i
I
1
2
nI
|
ILl 3
n I
n I
4
Time [h] Figure 4. Toluene fractional conversion to benzene versus irradiation time for the runs reported in Figure 3.
constant value of conversion was achieved. For this run benzene was produced only by the fresh catalyst in the first hours of irradiation; the partially spent catalyst did not produce benzene when water vapour was again present. The results reported in Figure 5 indicate that water is an essential reagent for the photoprocess. The highest activity is shown by the fresh catalyst working m the presence of water vapour; m the absence of water the catalyst progressively deactivates. The catalyst deactivation seems to be a partially reversible process; the restoration of water m the reaction mixture allows a partial recovery of the activity whose final level, however, is less than that of the fresh catalyst.
669 0.1 O O r~
.r,-I
',
;>
! ! ! ! ! ! ! !
-
O r
;= ~9
O r
- H~O
+ H20
t~ (D
(D ! !
~9 !
.l
!
!.
~ . !
.!
~.
l . ~
.~
.~
l.
! . ~
.!
~.
l . ~
.J
~.
l
~
~
O
150
250
350
450
Time [hi Figure 5. Effect of water on the photoreactivity.
Figure 6 reports FT-IR spectra obtained at various experimental conditions. The admission of toluene onto the catalyst causes the depletion of the band originally present in the background spectrum at 3670 cm -1 (curve a), due to a stretching mode of surface hydroxyl groups [11]. This band is transformed to a complex and much broader absorption in the 3600-3450 cm -I range (curve b). On the basis of such behaviour it can be concluded that surface OH groups are revolved in the adsorp.tion of toluene onto the catalyst, the resulting organic molecules are probably stabilized on the surface by hydrogen bonding between the aromatic ring and the hydroxyl groups [12]. IR bands assignable to adsorbed toluene appear in spectrum b, namely in the 3200-2800 cm 1 (CH stretchings) and 2000-1300 cm 1 (summation bands, ring stretchings). By simply outgassing at room temperature these bands disappear (spectra not reported) and the original spectral band of the surface hydroxyls is progressively fully restored (Figure 6, reset). Such reversibility suggests a weak character of the observed interaction between toluene and the catalyst. By taking into account all the results above reported, the foUowmg reaction mechanism for the production of benzaldehyde can be suggested. Under photoexcitation of the semiconductor oxide with band gap irradiation, electron-hole (e-h) pairs are photogenerated: TiO2 + hv -~ TiO2 + e- + h §
(1)
The pairs, once separated, can induce chemical redox transformations with the species adsorbed on the surface, subject to thermodynamic constraints. It is generally assumed that surface hydroxyls act as hole traps by producing OH" radicals:
670
2.0 1.3 1.2
!.5 "~ e~
1.0 0.9
l.O
0.8
r,~
0.7
<
3800 3700 3600 3500 3400 3300
0.5
Wavenumber [ cm -1 ]
0.0 3500
3000
2500
2000
1500
Wavenumber [ cm -1] Figure 6, IR spectra of toluene adsorbed on TiO2 catalyst: a) spectrum of the catalyst outgassed 30' at room temperature; b) after admission of 3 Torr of toluene vapour. Inset: a,b) the same as in the mare layer; c-h) desorption of toluene by outgassmg at room temperature for increasing times.
OH
(2)
+ h § -~ OH'.
The reactivity results indicate that oxygen is needed to sustain t h e toluene photooxidation; therefore, while OH groups act as traps for the photo-holes, adsorbed oxygen species act as traps for free photo-electrons and give rise to very reactive species by the following reactions [ 13]:
O2(gas phase) ~
O2(ads)
(3)
(ads)
(4)
02 (ads)+ H § -~ 02H"
(5)
202H"
(6)
O2(ads) "~" e -
---> 0 2
--~ 02
+
H202
H202(.~) +O2-(.~) --> OH- +
OH" + 02.
(7)
671 The radical species are very reactive and may attack toluene molecules according to the following reactions: OH"
+ C6H~CH3(.~)
-+ H20 + C6H5CH2"
C6HsCH2" + 02(,d~) --> C6HsCH200" C6HsCH2OO"
+ e
~
C~H~CHO + O H .
(8) (9) (10)
The formation of benzyl alcohol may occur by the following reaction: C6HsCH2" + OH"
~
C6H~CH2OH.
(11)
The small amount of benzyl alcohol found as a product is understandable since two radicals are involved in reaction (11). Concerning the simultaneous appearance of benzene and C02, the experimental results suggest that the breakage of the bond between the CH3 group and the ct carbon of toluene does not occur. In this last case, in fact, CH4 and/or CH30H molecules should be produced by the reaction between CH3 radicals and OH groups present on the catalyst surface but neither CH4 or CH3OH were ever detected in our experiments. On this basis the occurrence of the following reaction steps may be suggested: C6HsCHO(.~) + OH" C6H~CO" + 0 2 ( ~ ) ~ C6H5C000"
-~ C6H~CO" + H20 C6H5C000"
+ C6HsCHO(a~)-~
C6H5C0" + C6HsCOOOH(a~)
(12) (13) (14)
C6HsCOOOH(.~) + C6HsCHO(.~)---> 2 C6H~COOH(.~)
(15)
C6H~COOH(~) --> C6H6 + C02.
(16)
According to reactions (12)-(16), CO2 results from the oxidation of toluene to benzoic acid whose traces were found in our experiments and its subsequent photodecarboxylation. It must be reported that some runs carried out by using benzoic acid at the same experimental conditions used for toluene photooxidation, afforded benzene and CO2 in large amounts. It is well known [7, 8] that ethanoic acid is easily photodecarboxylated in a gas-solid regime in the presence of irradiated polycrystallme semiconductor oxides. The small amounts of phenol are probably due to an attack of benzene by OH radicals (see eqn.(16)). The progressive deactivation of the catalyst in the absence of water could be due to some surface dehydroxylation and/or to the formation of stable intermediate species which can not evolve in the absence of water so that they remain strongly adsorbed on the catalyst surface. It is worth reporting that the
672 catalyst was found of ochre colour at the end of the run and this colour completely disappeared by irradiating the catalyst in liquid water. The presence on the catalyst surface of sites with different oxidation strength can explain the role played by water and the deactivation-activation pattern exhibited by the catalyst. The partial restoration of activity when water was restored to the reacting ambient could be due to the rehydroxylation of surface sites. The finding that benzene is produced only in the first hours of irradiation while the partially spent catalyst never produced benzene, may be justified by considering that the fresh catalyst has higher oxidant properties due to the presence of highly oxidant hydroxyl groups. Benzaldehyde, instead, can be obtained both on these sites and on less oxidant sites. The experimental results suggest that the more oxidizmg sites react irreversibly under irradiation. By concluding, the photo-oxidation of toluene mainly to benzeldehyde and benzene (as a transient product) was proved to occur in mild conditions in gassolid regime. The presence of sites with different oxidant properties is proposed and a preliminary FTIR mvestigation indicates a weak interaction between toluene molecules and TiO2 surface. ACKNOWLEDGEMENT The authors wish to thank the 'TIinistero delrUmversith e della Ricerca Scientifica e Tecnologica" (Rome) for financially supporting this work.
REFERENCES 1. M. Schiavello (ed.), Photocatalysis and Environment. Trends and Applications, Kluwer, Dordrecht, 1988. 2. V. Augugliaro, L, Palmisano, A. Sclafani, C. Mmero and E. Pelizzetti, Toxicol. Envir. Chem., 16 (1988) 89. 3. E. Pelizzetti and N. Serpone (eds.), Photocatalysis. Fundamentals and Applications, Wiley, New York, 1989. 4. E. Pelizzetti and M. Schiavello (eds.), Photochemical Conversion and Storage of Solar Energy, Kluwer, Dordrecht, 1991. 5. D. F, Ollis and H. A1-Ekabi (eds.), Photocatalytic Purification and Treatment of Water and Air, Elsevier, Amsterdam, 1993. 6. T. Ibusuki and K. Takeuchi, Atmos. Environ., 20 (1986) 1711. 7. L. Palmisano, M. Schiavello, A. Sclafani, S. Coluccia and L. Marchese, New J. Chem., 12 (1988) 847. 8. T. Matsuura and M. Anpo (eds.), Photochemistry on Solid Surfaces, Elsevier, Amsterdam, 1989. 9. M.L. Sauer and D. F. Ollis, J. Catal., 158 (1996) 570. 10. A. J. Malta Vidal, J. Soria, V. Augugliaro, V. Loddo, Chem. Biochem. Eng. Quart., 1997 (in press). 11. C. Morterra, J. Chem. Soc. Faraday Trans. I, 84 (1988) 1617. 12. E. A. Paukshits and E. N. Yurchenko, Russ. Chem. Rev., 52 (1983)242. 13. R. I. Bickley, G. Munuera and F. S. Stone, J. Catal., 31 (1973) 398.