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FAU zeolites for catalytic oxidation of VOCs
Applied Catalysis B: Environmental 70 (2007) 377–383 www.elsevier.com/locate/apcatb
Influence of the exchanged cation in Pd/BEA and Pd/FAU zeolites for catalytic oxidation of VOCs H.L. Tidahy a, S. Siffert a,*, J.-F. Lamonier a, R. Cousin a, E.A. Zhilinskaya a, A. Aboukaı¨s a, B.-L. Su b, X. Canet c, G. De Weireld c, M. Fre`re c, J.-M. Giraudon d, G. Leclercq d a
Laboratoire de Catalyse et Environnement, E.A. 2598, Universite´ du Littoral Coˆte d’Opale, 145 Avenue Schumann, 59140 Dunkerque, France Laboratoire de Chimie des Mate´riaux Inorganiques, Faculte´s Universitaires Noˆtre Dame de la Paix, 61 rue de Bruxelles, 5000 Namur, Belgium c Laboratoire de Thermodynamique Physique-Mathe´matique, Faculte´ Polytechnique de Mons, 31 Boulevard Dolez, 7000 Mons, Belgium d Laboratoire de Catalyse de Lille, UMR CNRS 8010, Universite´ des Sciences et Technologies de Lille, Baˆt C3, 59655 Villeneuve d’Ascq, France b
Available online 30 June 2006
Abstract 0.5 wt% palladium supported on exchanged BEA and FAU zeolites were prepared, characterized and tested in the total oxidation of volatile organic compounds (VOCs). The BEA and FAU zeolites were exchanged with different cations to study the influence of alkali metal cations (Na+, Cs+) and H+ in Pd-based catalysts on propene and toluene total oxidation. The exchange with different cations (Na+, Cs+) and H+ led to a decrease of the surface area and the micropore volume. All Pd/BEA and Pd/FAU zeolites were found to be powerful catalysts for the total oxidation of VOCs. They were active at low temperature and totally selective for CO2 and H2O. However, their activity depends significantly on the type of zeolite and on the nature of the charge-compensating cation. The activity order for propene and toluene oxidation on FAU catalysts, Pd/CsFAU > Pd/ NaFAU > Pd/HFAU, is the reverse of the activity order on BEA catalysts: Pd/HBEA > Pd/NaBEA > Pd/CsBEA. The catalytic activities can be rationalized in terms of the influence of the electronegativity of the charge-compensating cation on the Pd particles, the Pd dispersion, the PdO reducibility and the adsorption energies for VOCs. # 2006 Elsevier B.V. All rights reserved. Keywords: Total oxidation of VOC; Palladium; Exchanged zeolite; Adsorption
1. Introduction The increasing environmental awareness in the last two decades has prompted the emergence of stricter regulations covering automobile and industrial activities. Among these, the reduction of volatile organic compounds (VOCs) is particularly important because VOCs represent a serious environmental problem. The deep catalytic oxidation of these pollutants to carbon dioxide and water has been identified as one of the most efficient ways to destroy VOCs at low concentrations and to meet the increasingly stringent environmental regulations. In practice, the catalytic oxidation process requires heating large amounts of gas containing low concentrations of VOCs to the oxidation temperature. Therefore, highly active catalysts which work at lower temperatures are required. Noble metals which * Corresponding author. Tel.: +33 328 65 82 68/56; fax: +33 328 65 82 39. E-mail address: [email protected] (S. Siffert). 0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2006.02.027
possess high activity for total oxidation have been widely applied to the low-temperature complete oxidation [1–5]. Moreover, it was shown that the support may have an important impact on the catalytic activity, and zeolites were widely used as powerful catalytic supports [6–10]. A crystalline structure with uniform pore diameters and a high ion exchange capability are the unique properties of zeolites. The exchanged cations and the oxygen atoms of the zeolite framework are acid–base pairs [11], and the more electropositive the cations are, the higher is the basicity of the zeolite. Some articles described the influence of the ion exchanged on the catalytic properties, for example, in hexane reforming [12] or in methyl-isobutyl-ketone oxidation [13]. Moreover, the cations can affect the dispersion of the noble metal [14] and the adsorption properties [15]. Adsorption plays an important role for the catalytic process but also for a further concept of dual functional adsorbent/catalyst system for VOC removal [16]. Indeed, VOCs can be removed only by adsorption in a first step and, when the adsorbent is empty, in a second step the VOCs can be oxidized by catalysis.
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However, the influence of the charge-compensating alkali metal cations of the zeolite in noble metal catalysts was scarcely studied in the VOC oxidation [17], and not at all in the total oxidation of aromatics and alkenes. Therefore, the largepore zeolites FAU and BEA exchanged with different alkali metal cations were prepared, and 0.5 wt% of palladium was incorporated into these supports. The influence of the nature of the alkali metal cation (Na+, Cs+) and H+ in palladium-based catalysts for propene and toluene oxidation was studied. Propene and toluene were chosen as probe molecules for the catalytic oxidation, because they are often found in industrial exhausts and present high photochemical ozone creation potentials (POCP) [18,19]. Their kinetic diameters are irrespectively, 0.45 and 0.58 nm [20]. Therefore, the pore diameters of the zeolites (BEA: 0.75 nm 0.57 nm and 0.65 nm 0.56 nm; FAU: 0.75 nm) should not bring about any shape selectivity effects in our reactions. Moreover, adsorption studies were carried out on FAU zeolites, and the Henry constants for toluene were determined. 2. Experimental 2.1. Synthesis and characterization of the catalysts The HBEA and HFAU zeolites were prepared from NaBEA zeolite (framework Si/Al ratio of 25 from P.Q. Corporation) and NaFAU zeolite (framework Si/Al ratio of 2.43 from Union Carbide), respectively, by ion exchange with a 2 M solution of NH4NO3 (Acros) at 80 8C. Afterwards, the samples were filtered, washed and dried overnight at 100 8C. CsBEA and CsFAU were prepared from the same parent zeolites by exchanging seven times with a 0.5 M solution of CsCl (Prolabo) at 60 8C. After each ion exchange the samples were filtered, washed and heated at 350 8C. Finally, the obtained solids were ground. Calcination of all zeolite samples was performed in an air flow with a heating rate of 1 8C min1 from ambient temperature till 500 8C with a hold of 4 h. Pd/BEA catalysts were prepared by ion exchange of HBEA, NaBEA and CsBEA using a Pd(NO3)2xH2O solution (5.44 103 mol L1, Johnson Matthey), which was continuously stirred for 18 h at 60 8C. Afterwards, the samples were filtered and washed. Pd/FAU catalysts were prepared by ion exchange using the same salt as a palladium precursor but the catalyst was dried (and not filtered and washed) after the stirring step. Therefore, if the exchange is not complete, as it should be, all the palladium is deposed on the solid by impregnation. All catalyst samples were dried overnight at 100 8C and calcined at 400 8C in an air flow at 1 8C min1 for 4 h.
ion exchange were not determined for H+ and Na+ but the exchanges are quite easy and total for these cations. The degrees of ion exchange for Cs+ are about 75% for CsBEA and 90% for CsFAU. The structures of the solids were analyzed by powder X-ray diffraction (XRD) at room temperature with a Bruker diffractometer using Cu Ka radiation. Nitrogen adsorption analysis was performed on a Sorptomatic 1990 apparatus at 196 8C, and the specific surface areas of the solids were determined by the BET method. Temperature-programmed reduction experiments were carried out in an Altamira AMI-200 apparatus. The TPR profiles were obtained by passing a 5% H2/Ar flow (30 mL min1) through the calcined sample (about 100 mg). The temperature was increased from 40 to 200 8C at a rate of 5 8C min1. The hydrogen concentration in the effluent was continuously monitored by a thermoconductivity detector (TCD). Pulse chemisorption measurements were performed using an Altamira AMI-200 instrument. The samples were pretreated for 2 h in a flow of 5% H2/Ar at 200 8C, in order to reduce the Pd in the catalyst. The samples were then cooled to 100 8C in a stream of argon. Pulse chemisorption measurements were performed at this temperature with 5% H2/Ar. In the adsorption studies on FAU zeolites the Henry constants for toluene were determined for temperatures ranging from 225 to 400 8C using the pulse chromatography technique [21]. The Van’t Hoff law (Eq. (1)) was used to calculate the adsorption energy. K ¼ K 0 exp
DU RT
(1)
Here, K is the dimensionless Henry constant, K0 the Van’t Hoff pre-exponential factor and DU is the adsorption energy. The usual equation of Van’t Hoff (where K0 denotes Henry constant) is: dln K0 /dT = DH/RT2. Integration gives ln K0 = DH/RT + cst or K 0 ¼ K 0 0 expðDH=RTÞ. As DH = DU + p DV or DH = DU + Dn RT for a perfect gas, the same calculation gives K = K0 exp(DU/RT), with K = K0 /RT. Electron paramagnetic resonance (EPR) measurements were performed at 196 and 25 8C on a Bruker EMX spectrometer. A cavity operating with a frequency of 9.5 GHz (X band) was used. Precise g values were determined from precise frequency and magnetic field values. Thermal analysis measurements were performed using a Netzsch STA 409 instrument equipped with a microbalance differential analysis (DTA) and a flow gas system. The dried catalysts were treated under air; the temperature was raised at a rate of 5 8C min1 from room temperature to 1000 8C.
2.2. Characterization techniques 2.3. Catalytic experiments The palladium content was determined by optical emission spectroscopy with an inductively coupled plasma and mass spectroscopy (ICP/OES/MS Thermo Jarrell Ash) after dissolution of the Pd/zeolite in a mixture of HF and HNO3. The Pd content was close to 0.5 wt% for all catalysts. The degrees of
Propene or toluene oxidation was studied in a conventional fixed bed microreactor using 100 mg of catalyst at temperatures between 25 and 300 8C (1 8C min1). The gas stream (100 mL min1) was composed of air and 6000 ppm of
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gaseous propene or 1000 ppm of gaseous toluene. The combustion products were analyzed in a Varian 3600 chromatograph or in a Perkin-Elmer autosystem equipped with a TCD and an FID. Before the catalytic test, the solid was calcined in a flow of air (2 L h1) at 400 8C (1 8C min1) and reduced for 2 h in a hydrogen flow (2 L h1) at 200 8C (1 8C min1). 3. Results and discussion The surface areas and micropore volumes of the FAU and BEA zeolites are given in Table 1. Both the surface area and the micropore volume decrease as Na in the parent zeolites is exchanged for Cs or H. For H zeolites, it is well known that dealumination may occur and bring about a partial breakdown of the framework. As far as Cs is concerned, this cation is bigger than Na, and the exchange is rather difficult [13]. Therefore, several exchanges are needed, and as the sample is heated many times, a partial breakdown of the zeolites could occur. The loss of surface area is more evident for the FAU zeolite, it decreases from 731 m2 g1 for NaFAU before ion exchange to 485 m2 g1 for CsFAU. The Si/Al ratio is lower for the FAU zeolite (2.43) than for the BEA zeolite (25), therefore the cation exchange (number of compensating cations) is higher for FAU zeolites and then leads to a higher decrease of the BET surface area. Figs. 1 and 2 show the XRD patterns of the FAU and BEA zeolites, respectively. The cristallinities of the HBEA and CsBEA zeolites (Fig. 2) are only slightly altered after ion exchange. For the FAU zeolites, the cristallinity loss upon ion exchange seems to be more pronounced (Fig. 1). It is well known that the introduction of a larger cation such as Cs+ into the zeolite can distort the zeolite framework and hence, of the XRD pattern [22]. The comparison of both XRD patterns of NaFAU and CsFAU shows some shifts in the 2u values and intensity variations of the peaks. A sharp decrease in the intensity of the diffraction lines at low angles is observed (Fig. 1). The crystallites of the starting NaFAU zeolite (not shown) are multi-facetted spherulites, which are not altered by the ion exchange. In the case of HFAU, it seems that a partial breakdown of the zeolite framework has occurred. The upper part of Table 1 shows pronounced differences in the specific surface areas for the palladium-free FAU zeolites. Table 2 gives the surface areas, the palladium dispersions and the palladium particle sizes for the metalcontaining zeolites.
Fig. 1. XRD patterns of zeolites FAU.
The order of palladium dispersions (Pd/CsBEA > Pd/ NaBEA > Pd/HBEA) is surprising since it is the reverse of the order of the specific surface areas, whereas the order of palladium dispersions (Pd/NaFAU > Pd/HFAU > Pd/CsFAU) follows the order of the specific surface areas. Pd/CsBEA which has the lowest specific surface area exhibits the highest palladium dispersion. The partial breakdown of the zeolite framework during ion exchange indicates a more open structure of the zeolite, by the appearance of some new ‘‘holes’’, which is more accessible for the palladium particles. Besides, the palladium particle sizes ranging from 4.4 to 9.9 nm imply that these particles are exclusively located at the outer surface of the zeolite. The channels and pores of both zeolites are too small to accommodate such palladium particles, unless the framework
Table 1 Characterization of the Pd-free zeolites after calcination at 500 8C Sample
SBET (m2 g1)
Micropore volumea (cm3 g1)
HFAU NaFAU CsFAU HBEA NaBEA CsBEA
547 731 485 551 584 487
0.200 0.291 0.190 0.160 0.140 0.132
a
Determined by the t-plot method.
Fig. 2. XRD patterns of zeolites BEA.
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Table 2 Palladium dispersions and metal particle sizes of the Pd/FAU and Pd/BEA catalysts after calcination at 400 8C and their surface areas (SBET) before and after propene conversion Catalyst
SBET before propene conversion (m2 g1)
SBET after propene conversion (m2 g1)
Palladium dispersionb (%)
Palladium particle sizeb (nm)
Pd/HFAU Pd/NaFAU Pd/CsFAU Pd/HBEA Pd/NaBEA Pd/CsBEA
589 714 414 590 550 500
231 675 347 564 520 450
21 25 12 18 19 28
5.3 4.4 9.9 6.2 5.8 3.9
b
Determined from H2 chemisorption at 100 8C.
became more open after ion exchange (Pd/CsBEA). Moreover, the electronegativity of the cations could be another factor for rationalizing the observed differences in the dispersion [14]. The catalysts were tested in the total oxidation of propene and toluene. It should be noted that CO could not be detected in the reaction products, but only CO2 and H2O. It is, however, known that palladium is a good active phase for CO oxidation [23]. Figs. 3 and 4 show the propene conversion as a function of temperature over the supported Pd catalysts. The catalysts based on zeolite FAU are more active than the BEA zeolites, except for Pd/HBEA which is more active than Pd/HFAU. The activity order for the FAU catalysts is Pd/ CsFAU > Pd/NaFAU > Pd/HFAU. For these catalysts, the activity may be dependent on the palladium dispersion. The more active catalyst Pd/CsFAU has the lower Pd dispersion. However, no real correlation exists between the activity and the surface area of the catalysts. This activity order could, however, have to do with the electronegativity of the cations and the acid– base properties of the samples. Pinard et al. [17] observed the same activity order for Pt/FAU zeolites exchanged with Cs+, Na+ and H+ in the total oxidation of chloromethane and explained this by the basicity which increases from the
protonated Pt/HFAU to the Pt/CsFAU sample. But of course, the reaction should be different due to the differences between our probe molecules and chloromethane. It was shown also by XPS that an electronic shift occurs from the zeolite framework to the Pt particles which become stronger with increasing basicity from Pt/HBEA via Pt/NaBEA to Pt/CsBEA [12]. However, with the zeolite BEA samples the activity order for propene total oxidation is opposite to that of the FAU catalysts: Pd/ HBEA > Pd/NaBEA > Pd/CsBEA. Therefore, this activity order could be also explained by the order of the Pd dispersions which follows (Table 2) reversibly the activity order and the reducibility of the palladium particles. Thus, H2-TPR was used to characterize the reducibility of the palladium species which could be an important factor for the catalytic oxidation reactions. The TPR profiles of Pd/BEA and Pd/FAU zeolites are displayed in Figs. 5 and 6, respectively. According to the literature [24,25], the hydrogen consumed indicates the reduction of PdO to metallic Pd. In our samples, one to three peaks could be observed for PdO reduction corresponding to three different types of PdO species. For all samples, the Pd species are reduced below 50 8C except for Pd/CsBEA in which PdO reduction occurs till 120 8C. If we compare the activity order for propene oxidation
Fig. 3. Propene conversion over the Pd/FAU zeolites.
Fig. 4. Propene conversion over the Pd/BEA zeolites.
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Fig. 7. Toluene conversion over the Pd/FAU zeolites.
Fig. 5. TPR profiles of the Pd/FAU zeolites.
and the TPR profiles, it seems that the quantity of PdO species reducible at around 20–25 8C is correlated to the activity in propene oxidation. In fact, the quantity of PdO species reducible at around 20–25 8C for FAU zeolite increases from Pd/HFAU via Pd/NaFAU to Pd/CsFAU, whereas it is the inverse
Fig. 8. Toluene conversion over the Pd/BEA zeolites.
for the BEA zeolites increasing from Pd/CsBEA to Pd/NaBEA to Pd/HBEA. This type of species could be in a special interaction with the zeolite leading to highly active Pd particles. The toluene conversions over the Pd/BEA and Pd/FAU catalysts are displayed in Figs. 7 and 8, respectively. For the Pd/ BEA systems, the activity order for toluene oxidation is the same as for propene oxidation, but the toluene oxidation proceeds at lower temperature (lower T501 difference: 35 8C for Pd/HBEA). For the Pd/FAU catalysts, the activity order is also the same for the toluene and propene oxidation. However, Pd/ CsFAU and Pd/NaFAU catalysts present similar performances. In fact, Na and Cs have similar electronegativities (0.9 and 0.7, respectively) which are much lower than that of H (2.1).
Fig. 6. TPR profiles of the Pd/BEA zeolites.
1
T50 is the temperature corresponding to 50% VOC conversion.
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Fig. 9. Determination of the adsorption Henry constants for toluene.
The measurement of Henry constants for toluene adsorption from 225 to 400 8C (Fig. 9) leads to the determination of the Van’t Hoff parameters (pre-exponential factors and adsorption energies) collected in Table 3. The results show an increase of the Henry constant with increasing cation size. The ionexchanged zeolites have significantly lower Henry constants than the respective original zeolites. The average reduction in the slope of the adsorption isotherm is 41.7% for Pd/HFAU, 26.1% for Pd/NaFAU and 36.5% for Pd/CsFAU. Moreover, the adsorption of the reactant is often directly correlated to the catalytic activity and the adsorption energies could then be important to know. In fact, it is interesting to observe that the activity order for VOC oxidation follows the order of the adsorption energy. The lower the adsorption energy for toluene, the higher is the catalytic activity. This result could also explain the activities of our catalysts for VOC oxidation. The samples were also characterized after the catalytic tests. Table 2 shows a small decrease of the surface areas after use in the propene oxidation except for Pd/HFAU. The conditions of the catalytic test (water formation during the reaction) could lead to a partial breakdown of the zeolites and an agglomeration of palladium particles and also to a formation of carbonaceous deposits (coke) on the zeolites. EPR measurements (Fig. 10) made coke deposits visible on samples after propene oxidation, although no mass loss was observed for these solids by TGA. The quantity of coke should be too low to observe a mass loss by TGA but sufficient to be observed by EPR because of its high sensitivity (> 1013 spin). Iron impurities (Fe3+ signal) and palladium (Pd3+ signal) were also detected on the samples. Table 3 Adsorption energies and pre-exponential factors for toluene Sample
K0
DU (kJ mol1)
HFAU Pd/HFAU NaFAU Pd/NaFAU CsFAU Pd/CsFAU
0.00098396 0.00032093 0.01159015 0.00407453 0.00932832 0.31553048
64.97 67.53 61.76 65.82 68.32 47.44
Fig. 10. EPR spectra of the Pd-based catalysts after propene oxidation obtained at 77 K. Table 4 Thermal analysis under air after toluene oxidation Catalyst
Mass loss (%) after toluene oxidation
Pd/HBEA Pd/NaBEA Pd/CsBEA Pd/HFAU Pd/NaFAU Pd/CsFAU
3.3 2.4 3.9 3.8 2 1.2
After propene oxidation, Pd/HBEA, Pd/NaBEA and Pd/HFAU show a signal of carbon radicals observed at g = 2.003 [26] corresponding to coke. The acidic character of these zeolites seems to favor coke deposition [5]. This coke EPR signal is observed on all samples after toluene oxidation [27], and the quantity of coke deposits could be evaluated by TGA. The weight loss (%) corresponding to coke combustion at ca. 400 8C is shown in Table 4. De´ge´ et al. [28] also observed the combustion of coke coming from xylene total oxidation on Pd/HFAU at about 400 8C. Coke is always more strongly retained on acidic forms of zeolites but also on less active catalysts. These results can be correlated to the highest adsorption energy for toluene observed for Pd/HFAU (Table 3). 4. Conclusions 0.5 wt% palladium supported on ion-exchanged BEA and FAU zeolites were tested in the total oxidation of propene and toluene. The exchange with different cations (Na+, Cs+) and H+ led to a decrease in the surface area and the micropore volume. However, Pd/BEA and Pd/FAU zeolites were found to be powerful catalysts for the total oxidation of VOCs which are active at low temperatures and totally selective for CO2 and H2O. The activity in the total oxidation of propene and toluene was found to depend on the type of zeolite and cation in the zeolite.
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For the BEA catalysts, the activity order PdHBEA > PdNaPdNaBEA > PdCsBEA was observed and explained in terms of the Pd dispersion of the catalysts and the PdO reducibility (TPR measurements). The reverse activity order for both molecules was found for the FAU zeolites: PdCsFAU > PdNaFAU > PdHPdHFAU. For these last samples, the catalytic activity is explained in terms of the adsorption energies for VOCs, the electronegativity of the cations on the Pd particles, the Pd dispersion and the PdO reducibility. Acknowledgements The authors would like to thank Mr. Fabrice Cazier (CCM, Dunkerque) for elemental analysis. We thank the European community through an Interreg IIIa France-Wallonie-Flandre project and Walloon Region for financial supports. References [1] J.J. Spivey, Ind. Eng. Chem. Res. 26 (1987) 2165. [2] P. Ge´lin, M. Primet, Appl. Catal. B: Environ. 39 (2002) 1. [3] T. Maillet, C. Solleau, J. Barbier, D. Duprez, Appl. Catal. B: Environ. 14 (1997) 85. [4] M.-W. Ryoo, S.-G. Chung, J.-H. Kim, Y.S. Song, G. Seo, Catal. Today 83 (2003) 131. [5] M. Guisnet, P. De´ge´, P. Magnoux, Appl. Catal. B: Environ. 20 (1999) 1. [6] Y. Li, J.L. Armor, Appl. Catal. B: Environ. 51 (1994) 275. [7] J. Tsou, P. Magnoux, M. Guisnet, J.J.M. Orfao, J.L. Figueiredo, Appl. Catal. B: Environ. 57 (2005) 117.
383
[8] D. Jaumain, B.-L. Su, J. Mol. Catal. A: Chem. 197 (2003) 263. [9] L. Becker, H. Fo¨rster, Appl. Catal. B: Environ. 17 (1998) 43. [10] R. Lopez-Fonseca, B. de Rivas, J.I. Gutie´rrez-Ortiz, A. Aranzabal, Appl. Catal. B: Environ. 41 (2003) 31. [11] D. Barthomeuf, J. Phys. Chem. 88 (1984) 42. [12] S. Siffert, J.-L. Schmitt, J. Sommer, F. Garin, J. Catal. 184 (1999) 19. [13] J. Tsou, P. Magnoux, M. Guisnet, J.J.M. Orfao, J.L. Figueiredo, Appl. Catal. B: Environ. 51 (2004) 129. [14] Y. Yazawa, H. Yoshida, S. Komai, T. Hattori, Appl. Catal. A: Gen. 233 (2002) 113–124. [15] X. Canet, M. Fre`re, H.L. Tidahy, S. Siffert, B.-L. Su, Stud. Surf. Sci. Catal. 158 (2006) 225. [16] S.-W. Baek, J.-R. Kim, S.-K. Ihm, Catal. Today 93–95 (2004) 575. [17] L. Pinard, J. Mijoin, P. Magnoux, M. Guisnet, C. R. Chim. 8 (2005) 457. [18] R.G. Derwent, M.E. Jenkin, S.M. Saunders, Atmos. Environ. 30 (1996) 181. [19] R.G. Derwent, M.E. Jenkin, S.M. Saunders, M.J. Pilling, Atmos. Environ. 32 (1998) 2429. [20] D.W. Breck, Zeolite Molecular Sieves, Structure, Chemistry and Use, Wiley, New York, 1974. [21] X. Canet, J. Nokerman, M. Fre`re, Adsorption 11 (2005) 213. [22] A. de Mallmann, Thesis of Paris VI University, 1989. [23] Y. Bi, G. Lu, Appl. Catal. B: Environ. 41 (2003) 279. [24] S.T. Homeyer, W.M.H. Sachtler, J. Catal. 117 (1989) 91. [25] W.J. Shen, M. Okumura, Y. Matsumura, M. Haruta, Appl. Catal. A: Gen. 213 (2001) 225. [26] C.L. Li, O. Novaro, E. Munoz, J.L. Boldu, X. Bokhimi, J.A. Wang, T. Lopez, R. Gomez, Appl. Catal. A: Gen. 199 (2000) 211. [27] J. Jacquemin, S. Siffert, J.-F. Lamonier, E.A. Zhilinskaya, A. Aboukaı¨s, Stud. Surf. Sci. Catal. 142 (2002) 699. [28] P. De´ge´, L. Pinard, P. Magnoux, M. Guisnet, Appl. Catal. B: Environ. 27 (2000) 17.
Influence of the exchanged cation in Pd/BEA and Pd/FAU zeolites for catalytic oxidation of VOCs H.L. Tidahy a, S. Siffert a,*, J.-F. Lamonier a, R. Cousin a, E.A. Zhilinskaya a, A. Aboukaı¨s a, B.-L. Su b, X. Canet c, G. De Weireld c, M. Fre`re c, J.-M. Giraudon d, G. Leclercq d a
Laboratoire de Catalyse et Environnement, E.A. 2598, Universite´ du Littoral Coˆte d’Opale, 145 Avenue Schumann, 59140 Dunkerque, France Laboratoire de Chimie des Mate´riaux Inorganiques, Faculte´s Universitaires Noˆtre Dame de la Paix, 61 rue de Bruxelles, 5000 Namur, Belgium c Laboratoire de Thermodynamique Physique-Mathe´matique, Faculte´ Polytechnique de Mons, 31 Boulevard Dolez, 7000 Mons, Belgium d Laboratoire de Catalyse de Lille, UMR CNRS 8010, Universite´ des Sciences et Technologies de Lille, Baˆt C3, 59655 Villeneuve d’Ascq, France b
Available online 30 June 2006
Abstract 0.5 wt% palladium supported on exchanged BEA and FAU zeolites were prepared, characterized and tested in the total oxidation of volatile organic compounds (VOCs). The BEA and FAU zeolites were exchanged with different cations to study the influence of alkali metal cations (Na+, Cs+) and H+ in Pd-based catalysts on propene and toluene total oxidation. The exchange with different cations (Na+, Cs+) and H+ led to a decrease of the surface area and the micropore volume. All Pd/BEA and Pd/FAU zeolites were found to be powerful catalysts for the total oxidation of VOCs. They were active at low temperature and totally selective for CO2 and H2O. However, their activity depends significantly on the type of zeolite and on the nature of the charge-compensating cation. The activity order for propene and toluene oxidation on FAU catalysts, Pd/CsFAU > Pd/ NaFAU > Pd/HFAU, is the reverse of the activity order on BEA catalysts: Pd/HBEA > Pd/NaBEA > Pd/CsBEA. The catalytic activities can be rationalized in terms of the influence of the electronegativity of the charge-compensating cation on the Pd particles, the Pd dispersion, the PdO reducibility and the adsorption energies for VOCs. # 2006 Elsevier B.V. All rights reserved. Keywords: Total oxidation of VOC; Palladium; Exchanged zeolite; Adsorption
1. Introduction The increasing environmental awareness in the last two decades has prompted the emergence of stricter regulations covering automobile and industrial activities. Among these, the reduction of volatile organic compounds (VOCs) is particularly important because VOCs represent a serious environmental problem. The deep catalytic oxidation of these pollutants to carbon dioxide and water has been identified as one of the most efficient ways to destroy VOCs at low concentrations and to meet the increasingly stringent environmental regulations. In practice, the catalytic oxidation process requires heating large amounts of gas containing low concentrations of VOCs to the oxidation temperature. Therefore, highly active catalysts which work at lower temperatures are required. Noble metals which * Corresponding author. Tel.: +33 328 65 82 68/56; fax: +33 328 65 82 39. E-mail address: [email protected] (S. Siffert). 0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2006.02.027
possess high activity for total oxidation have been widely applied to the low-temperature complete oxidation [1–5]. Moreover, it was shown that the support may have an important impact on the catalytic activity, and zeolites were widely used as powerful catalytic supports [6–10]. A crystalline structure with uniform pore diameters and a high ion exchange capability are the unique properties of zeolites. The exchanged cations and the oxygen atoms of the zeolite framework are acid–base pairs [11], and the more electropositive the cations are, the higher is the basicity of the zeolite. Some articles described the influence of the ion exchanged on the catalytic properties, for example, in hexane reforming [12] or in methyl-isobutyl-ketone oxidation [13]. Moreover, the cations can affect the dispersion of the noble metal [14] and the adsorption properties [15]. Adsorption plays an important role for the catalytic process but also for a further concept of dual functional adsorbent/catalyst system for VOC removal [16]. Indeed, VOCs can be removed only by adsorption in a first step and, when the adsorbent is empty, in a second step the VOCs can be oxidized by catalysis.
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However, the influence of the charge-compensating alkali metal cations of the zeolite in noble metal catalysts was scarcely studied in the VOC oxidation [17], and not at all in the total oxidation of aromatics and alkenes. Therefore, the largepore zeolites FAU and BEA exchanged with different alkali metal cations were prepared, and 0.5 wt% of palladium was incorporated into these supports. The influence of the nature of the alkali metal cation (Na+, Cs+) and H+ in palladium-based catalysts for propene and toluene oxidation was studied. Propene and toluene were chosen as probe molecules for the catalytic oxidation, because they are often found in industrial exhausts and present high photochemical ozone creation potentials (POCP) [18,19]. Their kinetic diameters are irrespectively, 0.45 and 0.58 nm [20]. Therefore, the pore diameters of the zeolites (BEA: 0.75 nm 0.57 nm and 0.65 nm 0.56 nm; FAU: 0.75 nm) should not bring about any shape selectivity effects in our reactions. Moreover, adsorption studies were carried out on FAU zeolites, and the Henry constants for toluene were determined. 2. Experimental 2.1. Synthesis and characterization of the catalysts The HBEA and HFAU zeolites were prepared from NaBEA zeolite (framework Si/Al ratio of 25 from P.Q. Corporation) and NaFAU zeolite (framework Si/Al ratio of 2.43 from Union Carbide), respectively, by ion exchange with a 2 M solution of NH4NO3 (Acros) at 80 8C. Afterwards, the samples were filtered, washed and dried overnight at 100 8C. CsBEA and CsFAU were prepared from the same parent zeolites by exchanging seven times with a 0.5 M solution of CsCl (Prolabo) at 60 8C. After each ion exchange the samples were filtered, washed and heated at 350 8C. Finally, the obtained solids were ground. Calcination of all zeolite samples was performed in an air flow with a heating rate of 1 8C min1 from ambient temperature till 500 8C with a hold of 4 h. Pd/BEA catalysts were prepared by ion exchange of HBEA, NaBEA and CsBEA using a Pd(NO3)2xH2O solution (5.44 103 mol L1, Johnson Matthey), which was continuously stirred for 18 h at 60 8C. Afterwards, the samples were filtered and washed. Pd/FAU catalysts were prepared by ion exchange using the same salt as a palladium precursor but the catalyst was dried (and not filtered and washed) after the stirring step. Therefore, if the exchange is not complete, as it should be, all the palladium is deposed on the solid by impregnation. All catalyst samples were dried overnight at 100 8C and calcined at 400 8C in an air flow at 1 8C min1 for 4 h.
ion exchange were not determined for H+ and Na+ but the exchanges are quite easy and total for these cations. The degrees of ion exchange for Cs+ are about 75% for CsBEA and 90% for CsFAU. The structures of the solids were analyzed by powder X-ray diffraction (XRD) at room temperature with a Bruker diffractometer using Cu Ka radiation. Nitrogen adsorption analysis was performed on a Sorptomatic 1990 apparatus at 196 8C, and the specific surface areas of the solids were determined by the BET method. Temperature-programmed reduction experiments were carried out in an Altamira AMI-200 apparatus. The TPR profiles were obtained by passing a 5% H2/Ar flow (30 mL min1) through the calcined sample (about 100 mg). The temperature was increased from 40 to 200 8C at a rate of 5 8C min1. The hydrogen concentration in the effluent was continuously monitored by a thermoconductivity detector (TCD). Pulse chemisorption measurements were performed using an Altamira AMI-200 instrument. The samples were pretreated for 2 h in a flow of 5% H2/Ar at 200 8C, in order to reduce the Pd in the catalyst. The samples were then cooled to 100 8C in a stream of argon. Pulse chemisorption measurements were performed at this temperature with 5% H2/Ar. In the adsorption studies on FAU zeolites the Henry constants for toluene were determined for temperatures ranging from 225 to 400 8C using the pulse chromatography technique [21]. The Van’t Hoff law (Eq. (1)) was used to calculate the adsorption energy. K ¼ K 0 exp
DU RT
(1)
Here, K is the dimensionless Henry constant, K0 the Van’t Hoff pre-exponential factor and DU is the adsorption energy. The usual equation of Van’t Hoff (where K0 denotes Henry constant) is: dln K0 /dT = DH/RT2. Integration gives ln K0 = DH/RT + cst or K 0 ¼ K 0 0 expðDH=RTÞ. As DH = DU + p DV or DH = DU + Dn RT for a perfect gas, the same calculation gives K = K0 exp(DU/RT), with K = K0 /RT. Electron paramagnetic resonance (EPR) measurements were performed at 196 and 25 8C on a Bruker EMX spectrometer. A cavity operating with a frequency of 9.5 GHz (X band) was used. Precise g values were determined from precise frequency and magnetic field values. Thermal analysis measurements were performed using a Netzsch STA 409 instrument equipped with a microbalance differential analysis (DTA) and a flow gas system. The dried catalysts were treated under air; the temperature was raised at a rate of 5 8C min1 from room temperature to 1000 8C.
2.2. Characterization techniques 2.3. Catalytic experiments The palladium content was determined by optical emission spectroscopy with an inductively coupled plasma and mass spectroscopy (ICP/OES/MS Thermo Jarrell Ash) after dissolution of the Pd/zeolite in a mixture of HF and HNO3. The Pd content was close to 0.5 wt% for all catalysts. The degrees of
Propene or toluene oxidation was studied in a conventional fixed bed microreactor using 100 mg of catalyst at temperatures between 25 and 300 8C (1 8C min1). The gas stream (100 mL min1) was composed of air and 6000 ppm of
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gaseous propene or 1000 ppm of gaseous toluene. The combustion products were analyzed in a Varian 3600 chromatograph or in a Perkin-Elmer autosystem equipped with a TCD and an FID. Before the catalytic test, the solid was calcined in a flow of air (2 L h1) at 400 8C (1 8C min1) and reduced for 2 h in a hydrogen flow (2 L h1) at 200 8C (1 8C min1). 3. Results and discussion The surface areas and micropore volumes of the FAU and BEA zeolites are given in Table 1. Both the surface area and the micropore volume decrease as Na in the parent zeolites is exchanged for Cs or H. For H zeolites, it is well known that dealumination may occur and bring about a partial breakdown of the framework. As far as Cs is concerned, this cation is bigger than Na, and the exchange is rather difficult [13]. Therefore, several exchanges are needed, and as the sample is heated many times, a partial breakdown of the zeolites could occur. The loss of surface area is more evident for the FAU zeolite, it decreases from 731 m2 g1 for NaFAU before ion exchange to 485 m2 g1 for CsFAU. The Si/Al ratio is lower for the FAU zeolite (2.43) than for the BEA zeolite (25), therefore the cation exchange (number of compensating cations) is higher for FAU zeolites and then leads to a higher decrease of the BET surface area. Figs. 1 and 2 show the XRD patterns of the FAU and BEA zeolites, respectively. The cristallinities of the HBEA and CsBEA zeolites (Fig. 2) are only slightly altered after ion exchange. For the FAU zeolites, the cristallinity loss upon ion exchange seems to be more pronounced (Fig. 1). It is well known that the introduction of a larger cation such as Cs+ into the zeolite can distort the zeolite framework and hence, of the XRD pattern [22]. The comparison of both XRD patterns of NaFAU and CsFAU shows some shifts in the 2u values and intensity variations of the peaks. A sharp decrease in the intensity of the diffraction lines at low angles is observed (Fig. 1). The crystallites of the starting NaFAU zeolite (not shown) are multi-facetted spherulites, which are not altered by the ion exchange. In the case of HFAU, it seems that a partial breakdown of the zeolite framework has occurred. The upper part of Table 1 shows pronounced differences in the specific surface areas for the palladium-free FAU zeolites. Table 2 gives the surface areas, the palladium dispersions and the palladium particle sizes for the metalcontaining zeolites.
Fig. 1. XRD patterns of zeolites FAU.
The order of palladium dispersions (Pd/CsBEA > Pd/ NaBEA > Pd/HBEA) is surprising since it is the reverse of the order of the specific surface areas, whereas the order of palladium dispersions (Pd/NaFAU > Pd/HFAU > Pd/CsFAU) follows the order of the specific surface areas. Pd/CsBEA which has the lowest specific surface area exhibits the highest palladium dispersion. The partial breakdown of the zeolite framework during ion exchange indicates a more open structure of the zeolite, by the appearance of some new ‘‘holes’’, which is more accessible for the palladium particles. Besides, the palladium particle sizes ranging from 4.4 to 9.9 nm imply that these particles are exclusively located at the outer surface of the zeolite. The channels and pores of both zeolites are too small to accommodate such palladium particles, unless the framework
Table 1 Characterization of the Pd-free zeolites after calcination at 500 8C Sample
SBET (m2 g1)
Micropore volumea (cm3 g1)
HFAU NaFAU CsFAU HBEA NaBEA CsBEA
547 731 485 551 584 487
0.200 0.291 0.190 0.160 0.140 0.132
a
Determined by the t-plot method.
Fig. 2. XRD patterns of zeolites BEA.
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Table 2 Palladium dispersions and metal particle sizes of the Pd/FAU and Pd/BEA catalysts after calcination at 400 8C and their surface areas (SBET) before and after propene conversion Catalyst
SBET before propene conversion (m2 g1)
SBET after propene conversion (m2 g1)
Palladium dispersionb (%)
Palladium particle sizeb (nm)
Pd/HFAU Pd/NaFAU Pd/CsFAU Pd/HBEA Pd/NaBEA Pd/CsBEA
589 714 414 590 550 500
231 675 347 564 520 450
21 25 12 18 19 28
5.3 4.4 9.9 6.2 5.8 3.9
b
Determined from H2 chemisorption at 100 8C.
became more open after ion exchange (Pd/CsBEA). Moreover, the electronegativity of the cations could be another factor for rationalizing the observed differences in the dispersion [14]. The catalysts were tested in the total oxidation of propene and toluene. It should be noted that CO could not be detected in the reaction products, but only CO2 and H2O. It is, however, known that palladium is a good active phase for CO oxidation [23]. Figs. 3 and 4 show the propene conversion as a function of temperature over the supported Pd catalysts. The catalysts based on zeolite FAU are more active than the BEA zeolites, except for Pd/HBEA which is more active than Pd/HFAU. The activity order for the FAU catalysts is Pd/ CsFAU > Pd/NaFAU > Pd/HFAU. For these catalysts, the activity may be dependent on the palladium dispersion. The more active catalyst Pd/CsFAU has the lower Pd dispersion. However, no real correlation exists between the activity and the surface area of the catalysts. This activity order could, however, have to do with the electronegativity of the cations and the acid– base properties of the samples. Pinard et al. [17] observed the same activity order for Pt/FAU zeolites exchanged with Cs+, Na+ and H+ in the total oxidation of chloromethane and explained this by the basicity which increases from the
protonated Pt/HFAU to the Pt/CsFAU sample. But of course, the reaction should be different due to the differences between our probe molecules and chloromethane. It was shown also by XPS that an electronic shift occurs from the zeolite framework to the Pt particles which become stronger with increasing basicity from Pt/HBEA via Pt/NaBEA to Pt/CsBEA [12]. However, with the zeolite BEA samples the activity order for propene total oxidation is opposite to that of the FAU catalysts: Pd/ HBEA > Pd/NaBEA > Pd/CsBEA. Therefore, this activity order could be also explained by the order of the Pd dispersions which follows (Table 2) reversibly the activity order and the reducibility of the palladium particles. Thus, H2-TPR was used to characterize the reducibility of the palladium species which could be an important factor for the catalytic oxidation reactions. The TPR profiles of Pd/BEA and Pd/FAU zeolites are displayed in Figs. 5 and 6, respectively. According to the literature [24,25], the hydrogen consumed indicates the reduction of PdO to metallic Pd. In our samples, one to three peaks could be observed for PdO reduction corresponding to three different types of PdO species. For all samples, the Pd species are reduced below 50 8C except for Pd/CsBEA in which PdO reduction occurs till 120 8C. If we compare the activity order for propene oxidation
Fig. 3. Propene conversion over the Pd/FAU zeolites.
Fig. 4. Propene conversion over the Pd/BEA zeolites.
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Fig. 7. Toluene conversion over the Pd/FAU zeolites.
Fig. 5. TPR profiles of the Pd/FAU zeolites.
and the TPR profiles, it seems that the quantity of PdO species reducible at around 20–25 8C is correlated to the activity in propene oxidation. In fact, the quantity of PdO species reducible at around 20–25 8C for FAU zeolite increases from Pd/HFAU via Pd/NaFAU to Pd/CsFAU, whereas it is the inverse
Fig. 8. Toluene conversion over the Pd/BEA zeolites.
for the BEA zeolites increasing from Pd/CsBEA to Pd/NaBEA to Pd/HBEA. This type of species could be in a special interaction with the zeolite leading to highly active Pd particles. The toluene conversions over the Pd/BEA and Pd/FAU catalysts are displayed in Figs. 7 and 8, respectively. For the Pd/ BEA systems, the activity order for toluene oxidation is the same as for propene oxidation, but the toluene oxidation proceeds at lower temperature (lower T501 difference: 35 8C for Pd/HBEA). For the Pd/FAU catalysts, the activity order is also the same for the toluene and propene oxidation. However, Pd/ CsFAU and Pd/NaFAU catalysts present similar performances. In fact, Na and Cs have similar electronegativities (0.9 and 0.7, respectively) which are much lower than that of H (2.1).
Fig. 6. TPR profiles of the Pd/BEA zeolites.
1
T50 is the temperature corresponding to 50% VOC conversion.
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Fig. 9. Determination of the adsorption Henry constants for toluene.
The measurement of Henry constants for toluene adsorption from 225 to 400 8C (Fig. 9) leads to the determination of the Van’t Hoff parameters (pre-exponential factors and adsorption energies) collected in Table 3. The results show an increase of the Henry constant with increasing cation size. The ionexchanged zeolites have significantly lower Henry constants than the respective original zeolites. The average reduction in the slope of the adsorption isotherm is 41.7% for Pd/HFAU, 26.1% for Pd/NaFAU and 36.5% for Pd/CsFAU. Moreover, the adsorption of the reactant is often directly correlated to the catalytic activity and the adsorption energies could then be important to know. In fact, it is interesting to observe that the activity order for VOC oxidation follows the order of the adsorption energy. The lower the adsorption energy for toluene, the higher is the catalytic activity. This result could also explain the activities of our catalysts for VOC oxidation. The samples were also characterized after the catalytic tests. Table 2 shows a small decrease of the surface areas after use in the propene oxidation except for Pd/HFAU. The conditions of the catalytic test (water formation during the reaction) could lead to a partial breakdown of the zeolites and an agglomeration of palladium particles and also to a formation of carbonaceous deposits (coke) on the zeolites. EPR measurements (Fig. 10) made coke deposits visible on samples after propene oxidation, although no mass loss was observed for these solids by TGA. The quantity of coke should be too low to observe a mass loss by TGA but sufficient to be observed by EPR because of its high sensitivity (> 1013 spin). Iron impurities (Fe3+ signal) and palladium (Pd3+ signal) were also detected on the samples. Table 3 Adsorption energies and pre-exponential factors for toluene Sample
K0
DU (kJ mol1)
HFAU Pd/HFAU NaFAU Pd/NaFAU CsFAU Pd/CsFAU
0.00098396 0.00032093 0.01159015 0.00407453 0.00932832 0.31553048
64.97 67.53 61.76 65.82 68.32 47.44
Fig. 10. EPR spectra of the Pd-based catalysts after propene oxidation obtained at 77 K. Table 4 Thermal analysis under air after toluene oxidation Catalyst
Mass loss (%) after toluene oxidation
Pd/HBEA Pd/NaBEA Pd/CsBEA Pd/HFAU Pd/NaFAU Pd/CsFAU
3.3 2.4 3.9 3.8 2 1.2
After propene oxidation, Pd/HBEA, Pd/NaBEA and Pd/HFAU show a signal of carbon radicals observed at g = 2.003 [26] corresponding to coke. The acidic character of these zeolites seems to favor coke deposition [5]. This coke EPR signal is observed on all samples after toluene oxidation [27], and the quantity of coke deposits could be evaluated by TGA. The weight loss (%) corresponding to coke combustion at ca. 400 8C is shown in Table 4. De´ge´ et al. [28] also observed the combustion of coke coming from xylene total oxidation on Pd/HFAU at about 400 8C. Coke is always more strongly retained on acidic forms of zeolites but also on less active catalysts. These results can be correlated to the highest adsorption energy for toluene observed for Pd/HFAU (Table 3). 4. Conclusions 0.5 wt% palladium supported on ion-exchanged BEA and FAU zeolites were tested in the total oxidation of propene and toluene. The exchange with different cations (Na+, Cs+) and H+ led to a decrease in the surface area and the micropore volume. However, Pd/BEA and Pd/FAU zeolites were found to be powerful catalysts for the total oxidation of VOCs which are active at low temperatures and totally selective for CO2 and H2O. The activity in the total oxidation of propene and toluene was found to depend on the type of zeolite and cation in the zeolite.
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For the BEA catalysts, the activity order PdHBEA > PdNaPdNaBEA > PdCsBEA was observed and explained in terms of the Pd dispersion of the catalysts and the PdO reducibility (TPR measurements). The reverse activity order for both molecules was found for the FAU zeolites: PdCsFAU > PdNaFAU > PdHPdHFAU. For these last samples, the catalytic activity is explained in terms of the adsorption energies for VOCs, the electronegativity of the cations on the Pd particles, the Pd dispersion and the PdO reducibility. Acknowledgements The authors would like to thank Mr. Fabrice Cazier (CCM, Dunkerque) for elemental analysis. We thank the European community through an Interreg IIIa France-Wallonie-Flandre project and Walloon Region for financial supports. References [1] J.J. Spivey, Ind. Eng. Chem. Res. 26 (1987) 2165. [2] P. Ge´lin, M. Primet, Appl. Catal. B: Environ. 39 (2002) 1. [3] T. Maillet, C. Solleau, J. Barbier, D. Duprez, Appl. Catal. B: Environ. 14 (1997) 85. [4] M.-W. Ryoo, S.-G. Chung, J.-H. Kim, Y.S. Song, G. Seo, Catal. Today 83 (2003) 131. [5] M. Guisnet, P. De´ge´, P. Magnoux, Appl. Catal. B: Environ. 20 (1999) 1. [6] Y. Li, J.L. Armor, Appl. Catal. B: Environ. 51 (1994) 275. [7] J. Tsou, P. Magnoux, M. Guisnet, J.J.M. Orfao, J.L. Figueiredo, Appl. Catal. B: Environ. 57 (2005) 117.
383
[8] D. Jaumain, B.-L. Su, J. Mol. Catal. A: Chem. 197 (2003) 263. [9] L. Becker, H. Fo¨rster, Appl. Catal. B: Environ. 17 (1998) 43. [10] R. Lopez-Fonseca, B. de Rivas, J.I. Gutie´rrez-Ortiz, A. Aranzabal, Appl. Catal. B: Environ. 41 (2003) 31. [11] D. Barthomeuf, J. Phys. Chem. 88 (1984) 42. [12] S. Siffert, J.-L. Schmitt, J. Sommer, F. Garin, J. Catal. 184 (1999) 19. [13] J. Tsou, P. Magnoux, M. Guisnet, J.J.M. Orfao, J.L. Figueiredo, Appl. Catal. B: Environ. 51 (2004) 129. [14] Y. Yazawa, H. Yoshida, S. Komai, T. Hattori, Appl. Catal. A: Gen. 233 (2002) 113–124. [15] X. Canet, M. Fre`re, H.L. Tidahy, S. Siffert, B.-L. Su, Stud. Surf. Sci. Catal. 158 (2006) 225. [16] S.-W. Baek, J.-R. Kim, S.-K. Ihm, Catal. Today 93–95 (2004) 575. [17] L. Pinard, J. Mijoin, P. Magnoux, M. Guisnet, C. R. Chim. 8 (2005) 457. [18] R.G. Derwent, M.E. Jenkin, S.M. Saunders, Atmos. Environ. 30 (1996) 181. [19] R.G. Derwent, M.E. Jenkin, S.M. Saunders, M.J. Pilling, Atmos. Environ. 32 (1998) 2429. [20] D.W. Breck, Zeolite Molecular Sieves, Structure, Chemistry and Use, Wiley, New York, 1974. [21] X. Canet, J. Nokerman, M. Fre`re, Adsorption 11 (2005) 213. [22] A. de Mallmann, Thesis of Paris VI University, 1989. [23] Y. Bi, G. Lu, Appl. Catal. B: Environ. 41 (2003) 279. [24] S.T. Homeyer, W.M.H. Sachtler, J. Catal. 117 (1989) 91. [25] W.J. Shen, M. Okumura, Y. Matsumura, M. Haruta, Appl. Catal. A: Gen. 213 (2001) 225. [26] C.L. Li, O. Novaro, E. Munoz, J.L. Boldu, X. Bokhimi, J.A. Wang, T. Lopez, R. Gomez, Appl. Catal. A: Gen. 199 (2000) 211. [27] J. Jacquemin, S. Siffert, J.-F. Lamonier, E.A. Zhilinskaya, A. Aboukaı¨s, Stud. Surf. Sci. Catal. 142 (2002) 699. [28] P. De´ge´, L. Pinard, P. Magnoux, M. Guisnet, Appl. Catal. B: Environ. 27 (2000) 17.