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Catalytic combustion of volatile organic compounds over group IB metal catalysts on Fe2O3
Catalysis Communications 2 (2001) 229±232
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Catalytic combustion of volatile organic compounds over group IB metal catalysts on Fe2O3 Salvatore Scire a,*, Simona Minic o a, Carmelo Crisafulli a, Signorino Galvagno b b
a Dipartimento di Scienze Chimiche, Universit a di Catania, Viale A. Doria 6, I-95125 Catania, Italy Dipartimento di Chimica Industriale e Ingegneria dei Materiali, Universit a di Messina, Salita Sperone 31, 98166 Messina, Italy
Received 6 June 2001; received in revised form 16 July 2001; accepted 16 July 2001
Abstract Catalytic combustion of methanol, 2-propanol, and toluene was investigated on co-precipitated Au=Fe2 O3 , Ag=Fe2 O3 and Cu=Fe2 O3 catalysts in the presence of excess of oxygen. In the range of temperature investigated 40±400 °C the activity towards the oxidation of volatile organic compounds (VOCs) has been found to be in the order: Au=Fe2 O3 Ag=Fe2 O3 > Cu=Fe2 O3 > Fe2 O3 . This trend of activity has been explained on the basis of the capacity of the IB metal to weaken the Fe±O bond thus increasing the mobility of the lattice oxygen which is involved in the VOCs oxidation through a Mars±van Krevelen reaction mechanism. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Volatile organic compounds; Catalytic combustion; Gold; Silver; Copper; Iron oxide; Methanol; 2-propanol; Toluene
1. Introduction Volatile organic compounds (VOCs), emitted from many industrial processes and transportation activities, are considered as an important class of air pollutants [1]. Catalytic combustion is one of the most promising abatement technology for VOCs, due to its de®nitive character and save of energy [1]. VOCs can be, in fact, oxidized over a catalyst at temperatures much lower than those of thermal oxidation. Supported noble metals (Pt, Pd, Rh) or metal oxides (Cu, Cr, Mn) are typical catalysts for such an application [1±7]. In the last years Au=Fe2 O3 catalysts have been found to be very active in the low temperature CO oxidation [8±10]. More recently we have shown
*
Corresponding author. Fax: +39-095-580138. E-mail address: [email protected] (S. Scire).
that the Au/iron oxide system presents a good activity towards the VOCs oxidation [11]. In this paper we now report a comparative study on the oxidation of some representative VOCs (methanol, 2-propanol and toluene) on dierent M=Fe2 O3 catalysts, where M is a metal of the IB group (i.e. Au, Ag and Cu). 2. Experimental Catalysts were prepared by co-precipitation from Fe NO3 3 9H2 O and the corresponding precursor of the IB metal (Table 1). An aqueous mixture of the precursors was poured at 7.5 ml/ min rate into an aqueous solution of Na2 CO3 (1 M and pH 11.9) under vigorous stirring (500 rpm); the precipitation temperature was maintained at 75 °C. The co-precipitated samples obtained were kept digesting for about 24 h, washed several times
1566-7367/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 5 6 6 - 7 3 6 7 ( 0 1 ) 0 0 0 3 5 - 8
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S. Scire et al. / Catalysis Communications 2 (2001) 229±232
Table 1 Code, metal content, BET surface area and metal precursors of M/Fe2 O3 catalysts Code
Metal content (wt%)
BET surface area (m2 =g
Metal precursor
Au=Fe2 O3 Ag=Fe2 O3 Cu=Fe2 O3 Fe2 O3
3.5 3.2 3.3 ±
245 252 240 230
HAuCl4 AgNO3 Cu NO3 2 3H2 O ±
(until disappearance of nitrate and chloride), then dried under vacuum at 70 °C and ®nally ground before use. For comparison a sample of Fe2 O3 only was prepared by precipitation from Fe NO3 3 9H2 O following the same procedure employed for co-precipitated catalysts. The metal content in the catalysts, measured by atomic absorption, is reported in Table 1. Catalytic activity tests were performed in a continuous-¯ow ®xed-bed microreactor ®lled with catalysts (80±140 mesh) diluted with an inert glass powder. The feed composition was 0.7% of organic compound and 10% of O2 . Helium was used as diluent. A space velocity of 7:6 10 3 moles h 1 gcat1 was always used. Before the catalytic activity tests the samples were pretreated by ¯owing a 10% O2 stream diluted in He at 300 °C for 1 h. The euent gases were analyzed on-line by a gas chromatograph equipped with a packed column with 10% FFAP on Chromosorb W and FID detector, and by a quadrupole mass spectrometer (Sensorlab VG quadrupoles). Temperature programmed reduction (TPR) tests were carried out in a conventional ¯ow apparatus with a TCD detector, at heating rate of 5 °C/min using 5 vol% H2 in Ar. Surface area was measured with a Sorptomatic (mod. 1800 Carlo Erba) by adsorption of N2 at liquid nitrogen temperature, with BET method. 3. Results and discussion Fig. 1 shows the conversion of methanol as a function of reaction temperature on all tested catalysts. It must be reminded that CO2 and water were the only products formed in all range of
Fig. 1. Methanol conversion as a function of reaction temperature on Au=Fe2 O3 r, Ag=Fe2 O3 , Cu=Fe2 O3 and Fe2 O3 samples.
temperature investigated. It is possible to note that the presence of the group IB metal enhances the rate of methanol oxidation compared to the Fe2 O3 sample. The order of activity observed is Au=Fe2 O3 Ag=Fe2 O3 > Cu=Fe2 O3 > Fe2 O3 . In particular on the most active catalyst Au=Fe2 O3 the oxidation of methanol starts at about 60 °C reaching nearly a total conversion at about 160 °C. On the Fe2 O3 sample the same reaction starts at about 200 °C and reaches a total conversion only at T > 300 °C. The conversion of 2-propanol and the yield to CO2 and to acetone, as a function of reaction temperature, are reported in Fig. 2. From this ®gure a positive eect of the IB metal on the conversion of 2-propanol can be observed, with the same activity trend seen above. It is important to underline that acetone is formed at low temperature with a nearly 100% selectivity. This selectivity falls quickly to zero at higher temperatures with a corresponding increase of the selectivity to CO2 . Therefore it seems evident that acetone is an intermediate in 2-propanol combustion [2,11]. In Fig. 3 the results of the catalytic oxidation of toluene are reported. CO2 and water were the only products revealed. The ®gure shows the same activity order of the M=Fe2 O3 catalysts reported both for methanol and 2-propanol. However it can be noted that the light-o temperature of toluene is considerably higher than that required for methanol and 2-propanol. From the ®gure it is also
S. Scire et al. / Catalysis Communications 2 (2001) 229±232
231
Fig. 3. Toluene conversion as a function of reaction temperature on Au=Fe2 O3 r, Ag=Fe2 O3 , Cu=Fe2 O3 and Fe2 O3 samples.
Fig. 2. Conversion of 2-propanol and yields to acetone and to CO2 on M=Fe2 O3 samples.
clear that the catalytic eect of the IB metal is minor than in the oxidation of alcohols with a light-o temperature which is, on the most active catalyst Au=Fe2 O3 , only 40 °C lower than that
measured on Fe2 O3 . The higher reactivity of the alcohols compared to toluene is in accordance to the results reported in the literature [2,11,14,15]. This can be presumably related to the dierent interaction between the organic substrate and the catalytic surface, considering that alcohols absorb on the iron oxide surface in a higher extent than aromatics [2,11,16]. VOC oxidation on metal oxides is thought, in fact, to occur via the Mars±van Krevelen mechanism, implying lattice oxide ions as the active oxygen species [2,5,11]. This mechanism is in accordance with the fact that in absence of oxygen in the feed we observed that the reaction proceeds for a very short time and then the activity drops to zero. The catalytic data showed that, for all the organic molecules taken into consideration, the activity is in the order: Au=Fe2 O3 Ag=Fe2 O3 > Cu=Fe2 O3 > Fe2 O3 . In order to better clarify this trend it is important to take into consideration TPR pro®les carried out on the catalysts and reported in Fig. 4. On the Fe2 O3 sample two main reduction peaks are visible with a maximum at about 390 °C P1 and 630 °C P2 . According to the literature [11±13] the P1 peak can be attributed to the reduction of Fe2 O3 to Fe3 O4 (magnetite) and the P2 peak to the further transformation of magnetite to FeO. The hydrogen consumption measured, calculated from the area under the peaks, is in good agreement with these attributions. It must be noted that a shoulder appears on the low temperature side of the P1 peak. According
232
S. Scire et al. / Catalysis Communications 2 (2001) 229±232
Fig. 4. TPR pro®les of M=Fe2 O3 samples.
to Munteanu et al. [13], this shoulder can be related to the reduction of hydroxylated iron oxide species. Addition of IB metals shifts the P1 peak to lower temperature whereas the temperature of P2 peak remains practically unchanged. The extent of the shift of the P1 peak is in the order: Au=Fe2 O3 > Ag=Fe2 O3 > Cu=Fe2 O3 . It must be underlined that on M=Fe2 O3 catalysts the amount of H2 consumed, related to the P1 peak, is greater than that necessary for the partial reduction of the Fe2 O3 to Fe3 O4 suggesting that the reduction of the IB metal overlaps with that of the support (Fe2 O3 ). These TPR experiments suggest that on co-precipitated samples a strong interaction between the IB metal and Fe2 O3 occurs. Gold, silver and copper enhance the reducibility of iron species favoring essentially the reduction of Fe2 O3 to Fe3 O4 . The higher reducibility cannot be explained with an increase of the BET surface area of the IB metal catalysts; in fact all catalysts present very similar surface area values (about 230±250 m2 =g), which are consistent with an amorphous structure as revealed by XRD. It could be proposed that the enhanced reducibility of M=Fe2 O3 catalysts, compared to Fe2 O3 alone, is due to the ability of the IB metal to weaken the Fe±O bond. The presence of IB metals could lead to a decrease in the strength of the Fe±O bond located nearby the metal atoms [11]. This would increase the lattice oxygen mobility as a consequence of
higher structural defects induced by the metals [12]. A larger size of the metal should cause a larger extent of lattice distortion. This could justify the order of reducibility observed, i.e. Au=Fe2 O3 > Ag=Fe2 O3 > Cu=Fe2 O3 and consequently be able to explain the activity trend reported for M=Fe2 O3 catalysts. However it cannot be ruled out that the enhanced reducibility of the M=Fe2 O3 catalyst is related to the activation of the H2 molecule on the IB metal. It has been, in fact, reported in the literature that hydrogen can be dissociated on nanosized gold particles even at room temperature [17,18]. Work is in progress to clarify this point.
References [1] J.J. Spivey, Ind. Eng. Chem. Res. 26 (1987) 2165. [2] M. Baldi, E. Finocchio, F. Milella, G. Busca, Appl. Catal. B Environ. 16 (1998) 43. [3] J.M. Gallardo-Amores, T. Armaroli, G. Ramis, E. Finocchio, G. Busca, Appl. Catal. B Environ. 22 (1998) 249. [4] P. Papaefthimiou, T. Ioannides, X.E. Verykios, Appl. Catal. B Environ. 13 (1997) 175. [5] E.M. Cordi, P.J. O'Neill, J.L. Falconer, Appl. Catal. B Environ. 14 (1997) 23. [6] E.M. Cordi, J.L. Falconer, J. Catal. 162 (1996) 104. [7] N.E. Quaranta, J. Soria, V. Cortes Corberan, J.L.G. Fierro, J. Catal. 171 (1997) 1. [8] M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M.J. Genet, B. Delmon, J. Catal. 144 (1993) 175. [9] A.M. Visco, A. Donato, C. Milone, S. Galvagno, React. Kinet. Katal. Lett. 61 (1997) 219. [10] S. Minic o, S. Scire, C. Crisafulli, A.M. Visco, S. Galvagno, Catal. Lett. 47 (1997) 273. [11] S. Minic o, S. Scire, C. Crisafulli, R. Maggiore, S. Galvagno, Appl. Catal. B Environ. 28 (2000) 245. [12] G. Neri, A.M. Visco, S. Galvagno, A. Donato, M. Panzalorto, Thermochim. Acta 329 (1999) 39. [13] G. Munteanu, L. Ilieva, D. Andreeva, Thermochim. Acta 291 (1997) 171. [14] J. Hermia, S. Vigneron, Catal. Today 349 (1993) 17. [15] A. O'Malley, B.K. Hodnett, Catal. Today 54 (1999) 31. [16] E. Finocchio, G. Busca, V. Lorenzelli, V.S. Escribano, J. Chem. Soc., Faraday Trans. 92 (1996) 1587. [17] G.C. Bond, D.T. Thompson, Catal. Rev. Sci. Eng. 41 (1999) 319. [18] F. Boccuzzi, A. Chiorino, M. Manzoli, D. Andreeva, T. Tabakova, J. Catal. 188 (1999) 176.
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Catalytic combustion of volatile organic compounds over group IB metal catalysts on Fe2O3 Salvatore Scire a,*, Simona Minic o a, Carmelo Crisafulli a, Signorino Galvagno b b
a Dipartimento di Scienze Chimiche, Universit a di Catania, Viale A. Doria 6, I-95125 Catania, Italy Dipartimento di Chimica Industriale e Ingegneria dei Materiali, Universit a di Messina, Salita Sperone 31, 98166 Messina, Italy
Received 6 June 2001; received in revised form 16 July 2001; accepted 16 July 2001
Abstract Catalytic combustion of methanol, 2-propanol, and toluene was investigated on co-precipitated Au=Fe2 O3 , Ag=Fe2 O3 and Cu=Fe2 O3 catalysts in the presence of excess of oxygen. In the range of temperature investigated 40±400 °C the activity towards the oxidation of volatile organic compounds (VOCs) has been found to be in the order: Au=Fe2 O3 Ag=Fe2 O3 > Cu=Fe2 O3 > Fe2 O3 . This trend of activity has been explained on the basis of the capacity of the IB metal to weaken the Fe±O bond thus increasing the mobility of the lattice oxygen which is involved in the VOCs oxidation through a Mars±van Krevelen reaction mechanism. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Volatile organic compounds; Catalytic combustion; Gold; Silver; Copper; Iron oxide; Methanol; 2-propanol; Toluene
1. Introduction Volatile organic compounds (VOCs), emitted from many industrial processes and transportation activities, are considered as an important class of air pollutants [1]. Catalytic combustion is one of the most promising abatement technology for VOCs, due to its de®nitive character and save of energy [1]. VOCs can be, in fact, oxidized over a catalyst at temperatures much lower than those of thermal oxidation. Supported noble metals (Pt, Pd, Rh) or metal oxides (Cu, Cr, Mn) are typical catalysts for such an application [1±7]. In the last years Au=Fe2 O3 catalysts have been found to be very active in the low temperature CO oxidation [8±10]. More recently we have shown
*
Corresponding author. Fax: +39-095-580138. E-mail address: [email protected] (S. Scire).
that the Au/iron oxide system presents a good activity towards the VOCs oxidation [11]. In this paper we now report a comparative study on the oxidation of some representative VOCs (methanol, 2-propanol and toluene) on dierent M=Fe2 O3 catalysts, where M is a metal of the IB group (i.e. Au, Ag and Cu). 2. Experimental Catalysts were prepared by co-precipitation from Fe NO3 3 9H2 O and the corresponding precursor of the IB metal (Table 1). An aqueous mixture of the precursors was poured at 7.5 ml/ min rate into an aqueous solution of Na2 CO3 (1 M and pH 11.9) under vigorous stirring (500 rpm); the precipitation temperature was maintained at 75 °C. The co-precipitated samples obtained were kept digesting for about 24 h, washed several times
1566-7367/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 5 6 6 - 7 3 6 7 ( 0 1 ) 0 0 0 3 5 - 8
230
S. Scire et al. / Catalysis Communications 2 (2001) 229±232
Table 1 Code, metal content, BET surface area and metal precursors of M/Fe2 O3 catalysts Code
Metal content (wt%)
BET surface area (m2 =g
Metal precursor
Au=Fe2 O3 Ag=Fe2 O3 Cu=Fe2 O3 Fe2 O3
3.5 3.2 3.3 ±
245 252 240 230
HAuCl4 AgNO3 Cu NO3 2 3H2 O ±
(until disappearance of nitrate and chloride), then dried under vacuum at 70 °C and ®nally ground before use. For comparison a sample of Fe2 O3 only was prepared by precipitation from Fe NO3 3 9H2 O following the same procedure employed for co-precipitated catalysts. The metal content in the catalysts, measured by atomic absorption, is reported in Table 1. Catalytic activity tests were performed in a continuous-¯ow ®xed-bed microreactor ®lled with catalysts (80±140 mesh) diluted with an inert glass powder. The feed composition was 0.7% of organic compound and 10% of O2 . Helium was used as diluent. A space velocity of 7:6 10 3 moles h 1 gcat1 was always used. Before the catalytic activity tests the samples were pretreated by ¯owing a 10% O2 stream diluted in He at 300 °C for 1 h. The euent gases were analyzed on-line by a gas chromatograph equipped with a packed column with 10% FFAP on Chromosorb W and FID detector, and by a quadrupole mass spectrometer (Sensorlab VG quadrupoles). Temperature programmed reduction (TPR) tests were carried out in a conventional ¯ow apparatus with a TCD detector, at heating rate of 5 °C/min using 5 vol% H2 in Ar. Surface area was measured with a Sorptomatic (mod. 1800 Carlo Erba) by adsorption of N2 at liquid nitrogen temperature, with BET method. 3. Results and discussion Fig. 1 shows the conversion of methanol as a function of reaction temperature on all tested catalysts. It must be reminded that CO2 and water were the only products formed in all range of
Fig. 1. Methanol conversion as a function of reaction temperature on Au=Fe2 O3 r, Ag=Fe2 O3 , Cu=Fe2 O3 and Fe2 O3 samples.
temperature investigated. It is possible to note that the presence of the group IB metal enhances the rate of methanol oxidation compared to the Fe2 O3 sample. The order of activity observed is Au=Fe2 O3 Ag=Fe2 O3 > Cu=Fe2 O3 > Fe2 O3 . In particular on the most active catalyst Au=Fe2 O3 the oxidation of methanol starts at about 60 °C reaching nearly a total conversion at about 160 °C. On the Fe2 O3 sample the same reaction starts at about 200 °C and reaches a total conversion only at T > 300 °C. The conversion of 2-propanol and the yield to CO2 and to acetone, as a function of reaction temperature, are reported in Fig. 2. From this ®gure a positive eect of the IB metal on the conversion of 2-propanol can be observed, with the same activity trend seen above. It is important to underline that acetone is formed at low temperature with a nearly 100% selectivity. This selectivity falls quickly to zero at higher temperatures with a corresponding increase of the selectivity to CO2 . Therefore it seems evident that acetone is an intermediate in 2-propanol combustion [2,11]. In Fig. 3 the results of the catalytic oxidation of toluene are reported. CO2 and water were the only products revealed. The ®gure shows the same activity order of the M=Fe2 O3 catalysts reported both for methanol and 2-propanol. However it can be noted that the light-o temperature of toluene is considerably higher than that required for methanol and 2-propanol. From the ®gure it is also
S. Scire et al. / Catalysis Communications 2 (2001) 229±232
231
Fig. 3. Toluene conversion as a function of reaction temperature on Au=Fe2 O3 r, Ag=Fe2 O3 , Cu=Fe2 O3 and Fe2 O3 samples.
Fig. 2. Conversion of 2-propanol and yields to acetone and to CO2 on M=Fe2 O3 samples.
clear that the catalytic eect of the IB metal is minor than in the oxidation of alcohols with a light-o temperature which is, on the most active catalyst Au=Fe2 O3 , only 40 °C lower than that
measured on Fe2 O3 . The higher reactivity of the alcohols compared to toluene is in accordance to the results reported in the literature [2,11,14,15]. This can be presumably related to the dierent interaction between the organic substrate and the catalytic surface, considering that alcohols absorb on the iron oxide surface in a higher extent than aromatics [2,11,16]. VOC oxidation on metal oxides is thought, in fact, to occur via the Mars±van Krevelen mechanism, implying lattice oxide ions as the active oxygen species [2,5,11]. This mechanism is in accordance with the fact that in absence of oxygen in the feed we observed that the reaction proceeds for a very short time and then the activity drops to zero. The catalytic data showed that, for all the organic molecules taken into consideration, the activity is in the order: Au=Fe2 O3 Ag=Fe2 O3 > Cu=Fe2 O3 > Fe2 O3 . In order to better clarify this trend it is important to take into consideration TPR pro®les carried out on the catalysts and reported in Fig. 4. On the Fe2 O3 sample two main reduction peaks are visible with a maximum at about 390 °C P1 and 630 °C P2 . According to the literature [11±13] the P1 peak can be attributed to the reduction of Fe2 O3 to Fe3 O4 (magnetite) and the P2 peak to the further transformation of magnetite to FeO. The hydrogen consumption measured, calculated from the area under the peaks, is in good agreement with these attributions. It must be noted that a shoulder appears on the low temperature side of the P1 peak. According
232
S. Scire et al. / Catalysis Communications 2 (2001) 229±232
Fig. 4. TPR pro®les of M=Fe2 O3 samples.
to Munteanu et al. [13], this shoulder can be related to the reduction of hydroxylated iron oxide species. Addition of IB metals shifts the P1 peak to lower temperature whereas the temperature of P2 peak remains practically unchanged. The extent of the shift of the P1 peak is in the order: Au=Fe2 O3 > Ag=Fe2 O3 > Cu=Fe2 O3 . It must be underlined that on M=Fe2 O3 catalysts the amount of H2 consumed, related to the P1 peak, is greater than that necessary for the partial reduction of the Fe2 O3 to Fe3 O4 suggesting that the reduction of the IB metal overlaps with that of the support (Fe2 O3 ). These TPR experiments suggest that on co-precipitated samples a strong interaction between the IB metal and Fe2 O3 occurs. Gold, silver and copper enhance the reducibility of iron species favoring essentially the reduction of Fe2 O3 to Fe3 O4 . The higher reducibility cannot be explained with an increase of the BET surface area of the IB metal catalysts; in fact all catalysts present very similar surface area values (about 230±250 m2 =g), which are consistent with an amorphous structure as revealed by XRD. It could be proposed that the enhanced reducibility of M=Fe2 O3 catalysts, compared to Fe2 O3 alone, is due to the ability of the IB metal to weaken the Fe±O bond. The presence of IB metals could lead to a decrease in the strength of the Fe±O bond located nearby the metal atoms [11]. This would increase the lattice oxygen mobility as a consequence of
higher structural defects induced by the metals [12]. A larger size of the metal should cause a larger extent of lattice distortion. This could justify the order of reducibility observed, i.e. Au=Fe2 O3 > Ag=Fe2 O3 > Cu=Fe2 O3 and consequently be able to explain the activity trend reported for M=Fe2 O3 catalysts. However it cannot be ruled out that the enhanced reducibility of the M=Fe2 O3 catalyst is related to the activation of the H2 molecule on the IB metal. It has been, in fact, reported in the literature that hydrogen can be dissociated on nanosized gold particles even at room temperature [17,18]. Work is in progress to clarify this point.
References [1] J.J. Spivey, Ind. Eng. Chem. Res. 26 (1987) 2165. [2] M. Baldi, E. Finocchio, F. Milella, G. Busca, Appl. Catal. B Environ. 16 (1998) 43. [3] J.M. Gallardo-Amores, T. Armaroli, G. Ramis, E. Finocchio, G. Busca, Appl. Catal. B Environ. 22 (1998) 249. [4] P. Papaefthimiou, T. Ioannides, X.E. Verykios, Appl. Catal. B Environ. 13 (1997) 175. [5] E.M. Cordi, P.J. O'Neill, J.L. Falconer, Appl. Catal. B Environ. 14 (1997) 23. [6] E.M. Cordi, J.L. Falconer, J. Catal. 162 (1996) 104. [7] N.E. Quaranta, J. Soria, V. Cortes Corberan, J.L.G. Fierro, J. Catal. 171 (1997) 1. [8] M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M.J. Genet, B. Delmon, J. Catal. 144 (1993) 175. [9] A.M. Visco, A. Donato, C. Milone, S. Galvagno, React. Kinet. Katal. Lett. 61 (1997) 219. [10] S. Minic o, S. Scire, C. Crisafulli, A.M. Visco, S. Galvagno, Catal. Lett. 47 (1997) 273. [11] S. Minic o, S. Scire, C. Crisafulli, R. Maggiore, S. Galvagno, Appl. Catal. B Environ. 28 (2000) 245. [12] G. Neri, A.M. Visco, S. Galvagno, A. Donato, M. Panzalorto, Thermochim. Acta 329 (1999) 39. [13] G. Munteanu, L. Ilieva, D. Andreeva, Thermochim. Acta 291 (1997) 171. [14] J. Hermia, S. Vigneron, Catal. Today 349 (1993) 17. [15] A. O'Malley, B.K. Hodnett, Catal. Today 54 (1999) 31. [16] E. Finocchio, G. Busca, V. Lorenzelli, V.S. Escribano, J. Chem. Soc., Faraday Trans. 92 (1996) 1587. [17] G.C. Bond, D.T. Thompson, Catal. Rev. Sci. Eng. 41 (1999) 319. [18] F. Boccuzzi, A. Chiorino, M. Manzoli, D. Andreeva, T. Tabakova, J. Catal. 188 (1999) 176.