- Email: [email protected]
79 Microwaves assisted regeneration of catalytic filter for soot particulate
Science and Technology in Catalysis 2002 Copyright 9 2003 by Kodansha Ltd.
367
79 Microwaves Assisted Regeneration of Catalytic Filter for Soot Particulate
Paolo CIAMBELLI*, Vincenzo PALMA, Paola RUSSO and Matteo D'AMORE Department of Chemical and Food Engineering, University of Salerno, via Ponte Don Melillo, 84084 Fisciano (SA), Italy. *Corresponding Author: tel. +39089964151 - fax +39089964057 e-mail: [email protected] Abstract In this work a microwave source has been employed for the regeneration of a ceramic foam used as a filter for soot trapping in the exhaust of a gas-oil burner. A specially designed single mode microwave cavity with a parabolic-like profile of the electromagnetic field has been constructed. We have found that a combined system of catalytic filter and microwave heating is able to perform filter regeneration at the temperature typical of diesel exhaust. In particular, the presence of catalyst assures lower soot ignition temperature, higher selectivity to CO2, higher soot combustion rate and higher microwave energy saving. 1. INTRODUCTION Diesel engine vehicles are worldwide used, however the presence of soot particulate in diesel exhaust is hamafitl for health and environment [ 1]. In order to meet the 2007 standards for particulate matter reduction soot filtration is the only proven method [2]. A number of filtering devices have been studied to this purpose, wall flow filter being generally accepted as the most suitable choice [2]. Recently, ceramic foam has been investigated as effective alternative to wall flow filter [3,4]. However, as the filter gets progressively blocked, raise in the total backpressure effects negatively engine's performance and, consequently, requires periodic cleaning of the filter. On the other hand, filter self-regeneration is not possible because of the high ignition temperature of diesel soot (773-923K) to be compared to the temperature of the exhaust (<673K). Catalytic filters, able to reduce the ignition temperature of soot, could perform regeneration at less severe conditions [5,6]. Nevertheless, also catalysed oxidation requires temperatures higher than the exhaust. Therefore, additional energy input is needed to clean the filter from the trapped soot. In the case of external heat sources, such as fuel burners or electrical resistance heaters, the high temperature required for soot oxidation and the uneven or uncontrolled combustion can cause incomplete filter regeneration and filter breakage or melting [7]. Microwave heating greatly differs from conventional heating, being a volume phenomenon. Microwaves penetrate the material at a depth depending on the dielectric permittivity, and heat it up from the inside. The different permittivity of the materials leads to the seducing possibility of selectively heating them. With respect to this features, soot filter regeneration by microwave heating should be a very effective operation, as soot is a strong absorber of microwaves whereas most ceramics are virtually transparent to them. This method allows improved control over the soot combustion and lower thermal stresses within the filter [8]. Moreover, as the catalyst itself can be a good microwave energy receptor [9], it can bej formulated to optimize microwaves absorption. Thus combining microwave heating with catalytic combustion seems to be an attractive way to diesel soot abatement [ 10]. As in the microwave heating application, the geometry of the microwave cavity is crucial since
368 P. Ciambelli etal.
the temperature within the material undergoing microwave heating is' inherently linked to the distribution of the electric field within the applicator [11], in a previous work a single mode microwave cavity was specially designed so as to have a well defined electromagnetic field acting on a soot-loaded foam to be regenerated [12]. In this work the regeneration by microwave irradiation of catalytic and tmcatalytic ceramic foam used as filter for soot emissions at the exhaust of a gas-oil burner has been investigated. 2. EXPERIMENTAL 2.1. Materials The filter was a 76mm diameter, 15mm thick alumina foam disk (Vesuvius Hi-Tech Ceramics) with 92% porosity and 65ppi. The catalytic filter was prepared by deposition of CuN/K/C1 catalyst over the alumina foam surface, through several cycles of impregnation with aqueous solution of catalyst precursor salts, drying and calcination at 973K overnight [6]. Soot particulate was generated by a gas-oil burner and deposited directly on the filter located at the burner exhaust [12].
2.2. Apparatuses and Experimental Procedure In Fig. 1 the scheme of the apparatus for filter regeneration is reported. It consists of: i) mass flow controllers operating on each gas; ii) a single mode microwave applicator; iii) a Nz ~~-,-i ~ Applicator NDIR and a paramagnetic continuous analyser for measurements of, respectively, CO, CO2 and O2 concentrations at the reactor outlet: The single mode applicator basically consists of a stainless steel resonant cavity, equipped with a 900W magnetron (2.45GHz) as a power source, a cooling fan, MFC controller Corn puter a PID controller and a thermocouple properly shielded. The system and the Fig. 1. Scheme of the apparatus for filter regeneration. controller are independent. The system temperature is continuously checked and kept under the desired set point value. More details are reported elsewhere [12]. The foam disk was placed in the microwave cavity with its axis coincident with the cavity axis. Temperature programmed combustion tests of the soot deposited on catalytic and uncatalytic filter were performed in the single mode cavity employing the temperature controller. The inlet gas, containing 5%vol 02 in N2, was fed with a flow rate 2000Ncm3/min. In each test the temperature was raised from 293 to 973K with a heating rate of 10K/min. The operating pressure was 10 lkPa. 3. RESULTS Preliminary tests, performed with the soot loaded ceramic foam in the microwave cavity without any temperature control and at the maximum microwave power (900W) showed that in only 2 min of microwave heating, the temperature reached a value larger than 1273K, while the 02 concentration decreased, reaching a zero value. This behaviour is likely due to the fast oxidation of soot achieved by the rapid microwave heating, resulting in complete regeneration but also breaking of filter. This result outlines the need for a temperature controlled system. Therefore, temperature programmed tests of the soot loaded ceramic foams were performed in the presence of 02. In Fig. 2 the outlet concentration of CO, CO2 and 02, the relevant temperatures, and the microwave power supplied are reported as functions of time. At temperatures higher than 650K, CO and CO2 concentrations in the outlet gas start to increase whereas 02 decreases conforming the occurrence of soot oxidation. At the maximum temperature value of 850K, CO and CO2 concentrations have a maximuna, whereas 02 has a minimum. Afterwards, the temperature slightly decreases for about 15 rain, then goes down to 620K, with a higher decreasing rate. Correspondingly, the conversion rate of soot oxidation decreases. In all the temperature range CO and CO2 concentration values are very similar. These results suggest that oxidation rate and
369 temperature are functions of the mass of soot present on the filter. In particular, when the mass of soot is too low, the filter is no more able to maintain the temperature value of 850K. This latter decreases to a lower value, and soot 1~o ~ does not oxidize anymore. 8_ & The inspection of filter before ~'. 0 o.! 0 20000 microwave exposure shows that a hNtlng rate = 10 K/mln 211111111 ,,-.- - . + . , . . . ,,,+ very good soot distribution was E .," ~, - 800 Teml~mbarl : achieved during the soot deposition Q, A ot a" ,,,'" ~Je* step. After regeneration by 4) co 0 o 15800 9 ~ .,"' ~ ""'. " e+ microwave, the filter was completely free of soot in the central zone, i.e. the ~ 10000 o zone where the electromagnetic field o4 has a maximum. The outer part of the 8i 0 I~00 filter surface shows a significant 0 reduction in the amount of soot left over, likely due to the electromagnetic 0 20 40 60 80 field distribution of the cavity and the time, mln relatively high heat transfer from the Fig. 2. Temperature programmedmicrowaveheating of the soot filter border to the cavity cold wails. loaded ceramic foam. Feed gas: 5%v0102, N2; dT/dt =10K/min. A comparison of regeneration effectiveness in the case of the soot loaded catalytic filter was performed & -0 0 in the same operating conditions as L previous one. The catalytic filter was 3~176176176 II ' - - "" " II ?~:~"'" -., loaded with higher amount of soot E ,p,- .. s,o (0.75g) with respect to the tmcatalytic o 9 co2 ,filter (0.42g). Results reported in Fig. 4k CO t 2oooo I _ I, , t ~' I so, 3 indicate that the catalytic filter promotes soot combustion by | lowering the ignition temperature by !8 400 about 120K with respect to the uncatalytic filter. The presence of catalyst enhances the soot oxidation O~ ] . . . . . " ---.. L 200 rate in the overall range of 0 20 40 60 80 time, rain temperatures investigated. In Fig. 3. Temperatureprogrammedmicrowaveheating of the soot particular, it is observed that at loaded catalytic foam. Feed gas: 5%vol 02, N2; dT/dt= l 0K/min. temperature of 700K the catalytic soot combustion rate is about six times greater than the uncatalytic one. Moreover, the catalyst strongly increases the production of carbon dioxide with respect to carbon monoxide, allowing a CO2/CO ratio eight time greater than that relevant to the uncatalytic filter. Furthermore, the comparison between catalytic and uncatalytic filter shows that in the catalytic case the specific power required for complete filter regeneration is half than that relevant to the uncatalytic one. Therefore, the synergism between microwaves field and catalysis assures a further energy saving. In the case of soot loaded catalytic foam filter the effect of microwave heating and, consequently, of filter regeneration, is the result of a different power absorption by the different materials involved, such as ceramic foam, supported catalyst and soot. In order to give an evidence of the different interaction with microwaves, the effect of catalyst deposition and soot loading on the absorption of microwaves is shown in Fig. 4. In this figure, the measured temperature profiles relevant to the catalytic and uncatalytic foam with or without soot during temperature programmed microwave heating are compared. It is observed that the ceramic foam reaches a maximum temperature of 350K after 4 min test, afterwards it remains constant even if the maximum power is continuously supplied to the magnetron. In the presence of the catalyst a linear behaviour is observed
|
.//
i:]ill
J,i
i
"
1,.|
i"10000_.,"" .+'++::'I! /]
370 P. Ciambelli et
al.
up to a temperature of about 450K, after that the slope of the temperature increase Q Soot ~ K I ~ ~ l y U o ~mm diminishes and a maximum temperature of 1000 9 c,t,~, f~. I 520K after 80 min test is reached. Finally, when soot is present, on both uncatalytic e" and catalytic foaln, a linear increase of = 800 temperature during the test time is 8. observed, in perfect agreement with the | ~60O programmed one. These results are in agreement with the expected different behaviour of the 4O0 components of the filtering system, due to their different permittivity. In fact they 0 2O 40 6O 8O 100 time, rain show that alumina based ceramic foam is Figure 4. Temperatureprogrammedmicrowaveheating of practically transparent to the microwaves, uncatalytic and catalytic foam with or without soot. Feed while the Cu/V/K/CI catalyst is a good gas: N2, dT/dt = 10K/min. microwaves absorber. Therefore, the catalytic foam results in an improved system able to assure higher temperatures and more effective heat transfer, it is evident also the remarkable ability of soot to selectively absorb microwaves. 1200
../_.:.:."--i;- ..............
r
4. CONCLUSIONS The soot oxidation is very fast with a single mode microwave applicator controlled by a PID acting on the microwaves generator duty cycle. The filter regeneration, even in cold reacting gas, is completed in a shorter time compared to that required from conventional techniques. A combined system of catalytic filter and microwave heating can be regenerate at a temperature typical of diesel exhaust. The presence of catalyst gets onset temperature lower than that of uncatalytic filter, higher selectivity to CO2 and higher soot combustion rate. With respect to conventional heating further energy is recovered when a catalytic filter/microwave heating combined system is employed. ACKNOWLEDGMENT This work was financially supported by MURST PRIN 2000 Project "Soot particulate and NOx abatement from diesel engines exhaust by means of catalytic filters" and by CNR Agenzia 2000 Project "Catalytic abatement of soot particulate assisted by microwaves". 5. REFERENCES
[1] J.L. Mauderly, "Environmental Toxicants. Human Exposure and their Health Effects", van Nostrand Reinhold, New York, 1992, p. 119. [2] S.T. Gulati, "Structured Catalysts and Reactors", Marcel Dekker Inc., New York, 1998, p. 510. [3] P. Ciambelli, V. Palma, P. Russo and S. Vaccaro, Catal. Today 73 (2002) 363. [4] B.A.A.L.van Setten, J. Brenmm:, S.J. Jelles, M. Makkee and J.A. Moulijn, Cam]. Today 53 (1999) 613. [5] J.P.A. Neefi, M. Makkee and J.A. Moulijn, Fuel Process. Technol. 47 (1996) 1. [6] P. Ciambelli, V. Palma, P. Russo and S. Vaccaro, Proc. 4th Intern. Congr. Catal. Autom. Poll. Cont., Brussels 1997, p. 313. [7] N. Higuchi, S. Mochida and M. Kojima, SAE Paper 830078 (1983). [8] F.B. Walton, P.J. Hayward and D.J. Wren, SAE Paper 900327 (1990). [9] J.K.S. Wan and M.S. Ioffe, Res. Chem. Intermed. 20 (1994) 115. [10]J. Ma, M. Fang, P. Li, B. Zhu, X. Lu and N.T. Lau, Appl. Catal. A: General 159 (1997) 211. [11]E.T. Thostenson and T.-W. Chou, Composites: Part A 30 (1999) 1055. [12] V. Pallm, M. d'Amore, P. Russo, A. D'Arco and P. Ciambelli in press on Comb. Sci. Tech. (2002).
367
79 Microwaves Assisted Regeneration of Catalytic Filter for Soot Particulate
Paolo CIAMBELLI*, Vincenzo PALMA, Paola RUSSO and Matteo D'AMORE Department of Chemical and Food Engineering, University of Salerno, via Ponte Don Melillo, 84084 Fisciano (SA), Italy. *Corresponding Author: tel. +39089964151 - fax +39089964057 e-mail: [email protected] Abstract In this work a microwave source has been employed for the regeneration of a ceramic foam used as a filter for soot trapping in the exhaust of a gas-oil burner. A specially designed single mode microwave cavity with a parabolic-like profile of the electromagnetic field has been constructed. We have found that a combined system of catalytic filter and microwave heating is able to perform filter regeneration at the temperature typical of diesel exhaust. In particular, the presence of catalyst assures lower soot ignition temperature, higher selectivity to CO2, higher soot combustion rate and higher microwave energy saving. 1. INTRODUCTION Diesel engine vehicles are worldwide used, however the presence of soot particulate in diesel exhaust is hamafitl for health and environment [ 1]. In order to meet the 2007 standards for particulate matter reduction soot filtration is the only proven method [2]. A number of filtering devices have been studied to this purpose, wall flow filter being generally accepted as the most suitable choice [2]. Recently, ceramic foam has been investigated as effective alternative to wall flow filter [3,4]. However, as the filter gets progressively blocked, raise in the total backpressure effects negatively engine's performance and, consequently, requires periodic cleaning of the filter. On the other hand, filter self-regeneration is not possible because of the high ignition temperature of diesel soot (773-923K) to be compared to the temperature of the exhaust (<673K). Catalytic filters, able to reduce the ignition temperature of soot, could perform regeneration at less severe conditions [5,6]. Nevertheless, also catalysed oxidation requires temperatures higher than the exhaust. Therefore, additional energy input is needed to clean the filter from the trapped soot. In the case of external heat sources, such as fuel burners or electrical resistance heaters, the high temperature required for soot oxidation and the uneven or uncontrolled combustion can cause incomplete filter regeneration and filter breakage or melting [7]. Microwave heating greatly differs from conventional heating, being a volume phenomenon. Microwaves penetrate the material at a depth depending on the dielectric permittivity, and heat it up from the inside. The different permittivity of the materials leads to the seducing possibility of selectively heating them. With respect to this features, soot filter regeneration by microwave heating should be a very effective operation, as soot is a strong absorber of microwaves whereas most ceramics are virtually transparent to them. This method allows improved control over the soot combustion and lower thermal stresses within the filter [8]. Moreover, as the catalyst itself can be a good microwave energy receptor [9], it can bej formulated to optimize microwaves absorption. Thus combining microwave heating with catalytic combustion seems to be an attractive way to diesel soot abatement [ 10]. As in the microwave heating application, the geometry of the microwave cavity is crucial since
368 P. Ciambelli etal.
the temperature within the material undergoing microwave heating is' inherently linked to the distribution of the electric field within the applicator [11], in a previous work a single mode microwave cavity was specially designed so as to have a well defined electromagnetic field acting on a soot-loaded foam to be regenerated [12]. In this work the regeneration by microwave irradiation of catalytic and tmcatalytic ceramic foam used as filter for soot emissions at the exhaust of a gas-oil burner has been investigated. 2. EXPERIMENTAL 2.1. Materials The filter was a 76mm diameter, 15mm thick alumina foam disk (Vesuvius Hi-Tech Ceramics) with 92% porosity and 65ppi. The catalytic filter was prepared by deposition of CuN/K/C1 catalyst over the alumina foam surface, through several cycles of impregnation with aqueous solution of catalyst precursor salts, drying and calcination at 973K overnight [6]. Soot particulate was generated by a gas-oil burner and deposited directly on the filter located at the burner exhaust [12].
2.2. Apparatuses and Experimental Procedure In Fig. 1 the scheme of the apparatus for filter regeneration is reported. It consists of: i) mass flow controllers operating on each gas; ii) a single mode microwave applicator; iii) a Nz ~~-,-i ~ Applicator NDIR and a paramagnetic continuous analyser for measurements of, respectively, CO, CO2 and O2 concentrations at the reactor outlet: The single mode applicator basically consists of a stainless steel resonant cavity, equipped with a 900W magnetron (2.45GHz) as a power source, a cooling fan, MFC controller Corn puter a PID controller and a thermocouple properly shielded. The system and the Fig. 1. Scheme of the apparatus for filter regeneration. controller are independent. The system temperature is continuously checked and kept under the desired set point value. More details are reported elsewhere [12]. The foam disk was placed in the microwave cavity with its axis coincident with the cavity axis. Temperature programmed combustion tests of the soot deposited on catalytic and uncatalytic filter were performed in the single mode cavity employing the temperature controller. The inlet gas, containing 5%vol 02 in N2, was fed with a flow rate 2000Ncm3/min. In each test the temperature was raised from 293 to 973K with a heating rate of 10K/min. The operating pressure was 10 lkPa. 3. RESULTS Preliminary tests, performed with the soot loaded ceramic foam in the microwave cavity without any temperature control and at the maximum microwave power (900W) showed that in only 2 min of microwave heating, the temperature reached a value larger than 1273K, while the 02 concentration decreased, reaching a zero value. This behaviour is likely due to the fast oxidation of soot achieved by the rapid microwave heating, resulting in complete regeneration but also breaking of filter. This result outlines the need for a temperature controlled system. Therefore, temperature programmed tests of the soot loaded ceramic foams were performed in the presence of 02. In Fig. 2 the outlet concentration of CO, CO2 and 02, the relevant temperatures, and the microwave power supplied are reported as functions of time. At temperatures higher than 650K, CO and CO2 concentrations in the outlet gas start to increase whereas 02 decreases conforming the occurrence of soot oxidation. At the maximum temperature value of 850K, CO and CO2 concentrations have a maximuna, whereas 02 has a minimum. Afterwards, the temperature slightly decreases for about 15 rain, then goes down to 620K, with a higher decreasing rate. Correspondingly, the conversion rate of soot oxidation decreases. In all the temperature range CO and CO2 concentration values are very similar. These results suggest that oxidation rate and
369 temperature are functions of the mass of soot present on the filter. In particular, when the mass of soot is too low, the filter is no more able to maintain the temperature value of 850K. This latter decreases to a lower value, and soot 1~o ~ does not oxidize anymore. 8_ & The inspection of filter before ~'. 0 o.! 0 20000 microwave exposure shows that a hNtlng rate = 10 K/mln 211111111 ,,-.- - . + . , . . . ,,,+ very good soot distribution was E .," ~, - 800 Teml~mbarl : achieved during the soot deposition Q, A ot a" ,,,'" ~Je* step. After regeneration by 4) co 0 o 15800 9 ~ .,"' ~ ""'. " e+ microwave, the filter was completely free of soot in the central zone, i.e. the ~ 10000 o zone where the electromagnetic field o4 has a maximum. The outer part of the 8i 0 I~00 filter surface shows a significant 0 reduction in the amount of soot left over, likely due to the electromagnetic 0 20 40 60 80 field distribution of the cavity and the time, mln relatively high heat transfer from the Fig. 2. Temperature programmedmicrowaveheating of the soot filter border to the cavity cold wails. loaded ceramic foam. Feed gas: 5%v0102, N2; dT/dt =10K/min. A comparison of regeneration effectiveness in the case of the soot loaded catalytic filter was performed & -0 0 in the same operating conditions as L previous one. The catalytic filter was 3~176176176 II ' - - "" " II ?~:~"'" -., loaded with higher amount of soot E ,p,- .. s,o (0.75g) with respect to the tmcatalytic o 9 co2 ,filter (0.42g). Results reported in Fig. 4k CO t 2oooo I _ I, , t ~' I so, 3 indicate that the catalytic filter promotes soot combustion by | lowering the ignition temperature by !8 400 about 120K with respect to the uncatalytic filter. The presence of catalyst enhances the soot oxidation O~ ] . . . . . " ---.. L 200 rate in the overall range of 0 20 40 60 80 time, rain temperatures investigated. In Fig. 3. Temperatureprogrammedmicrowaveheating of the soot particular, it is observed that at loaded catalytic foam. Feed gas: 5%vol 02, N2; dT/dt= l 0K/min. temperature of 700K the catalytic soot combustion rate is about six times greater than the uncatalytic one. Moreover, the catalyst strongly increases the production of carbon dioxide with respect to carbon monoxide, allowing a CO2/CO ratio eight time greater than that relevant to the uncatalytic filter. Furthermore, the comparison between catalytic and uncatalytic filter shows that in the catalytic case the specific power required for complete filter regeneration is half than that relevant to the uncatalytic one. Therefore, the synergism between microwaves field and catalysis assures a further energy saving. In the case of soot loaded catalytic foam filter the effect of microwave heating and, consequently, of filter regeneration, is the result of a different power absorption by the different materials involved, such as ceramic foam, supported catalyst and soot. In order to give an evidence of the different interaction with microwaves, the effect of catalyst deposition and soot loading on the absorption of microwaves is shown in Fig. 4. In this figure, the measured temperature profiles relevant to the catalytic and uncatalytic foam with or without soot during temperature programmed microwave heating are compared. It is observed that the ceramic foam reaches a maximum temperature of 350K after 4 min test, afterwards it remains constant even if the maximum power is continuously supplied to the magnetron. In the presence of the catalyst a linear behaviour is observed
|
.//
i:]ill
J,i
i
"
1,.|
i"10000_.,"" .+'++::'I! /]
370 P. Ciambelli et
al.
up to a temperature of about 450K, after that the slope of the temperature increase Q Soot ~ K I ~ ~ l y U o ~mm diminishes and a maximum temperature of 1000 9 c,t,~, f~. I 520K after 80 min test is reached. Finally, when soot is present, on both uncatalytic e" and catalytic foaln, a linear increase of = 800 temperature during the test time is 8. observed, in perfect agreement with the | ~60O programmed one. These results are in agreement with the expected different behaviour of the 4O0 components of the filtering system, due to their different permittivity. In fact they 0 2O 40 6O 8O 100 time, rain show that alumina based ceramic foam is Figure 4. Temperatureprogrammedmicrowaveheating of practically transparent to the microwaves, uncatalytic and catalytic foam with or without soot. Feed while the Cu/V/K/CI catalyst is a good gas: N2, dT/dt = 10K/min. microwaves absorber. Therefore, the catalytic foam results in an improved system able to assure higher temperatures and more effective heat transfer, it is evident also the remarkable ability of soot to selectively absorb microwaves. 1200
../_.:.:."--i;- ..............
r
4. CONCLUSIONS The soot oxidation is very fast with a single mode microwave applicator controlled by a PID acting on the microwaves generator duty cycle. The filter regeneration, even in cold reacting gas, is completed in a shorter time compared to that required from conventional techniques. A combined system of catalytic filter and microwave heating can be regenerate at a temperature typical of diesel exhaust. The presence of catalyst gets onset temperature lower than that of uncatalytic filter, higher selectivity to CO2 and higher soot combustion rate. With respect to conventional heating further energy is recovered when a catalytic filter/microwave heating combined system is employed. ACKNOWLEDGMENT This work was financially supported by MURST PRIN 2000 Project "Soot particulate and NOx abatement from diesel engines exhaust by means of catalytic filters" and by CNR Agenzia 2000 Project "Catalytic abatement of soot particulate assisted by microwaves". 5. REFERENCES
[1] J.L. Mauderly, "Environmental Toxicants. Human Exposure and their Health Effects", van Nostrand Reinhold, New York, 1992, p. 119. [2] S.T. Gulati, "Structured Catalysts and Reactors", Marcel Dekker Inc., New York, 1998, p. 510. [3] P. Ciambelli, V. Palma, P. Russo and S. Vaccaro, Catal. Today 73 (2002) 363. [4] B.A.A.L.van Setten, J. Brenmm:, S.J. Jelles, M. Makkee and J.A. Moulijn, Cam]. Today 53 (1999) 613. [5] J.P.A. Neefi, M. Makkee and J.A. Moulijn, Fuel Process. Technol. 47 (1996) 1. [6] P. Ciambelli, V. Palma, P. Russo and S. Vaccaro, Proc. 4th Intern. Congr. Catal. Autom. Poll. Cont., Brussels 1997, p. 313. [7] N. Higuchi, S. Mochida and M. Kojima, SAE Paper 830078 (1983). [8] F.B. Walton, P.J. Hayward and D.J. Wren, SAE Paper 900327 (1990). [9] J.K.S. Wan and M.S. Ioffe, Res. Chem. Intermed. 20 (1994) 115. [10]J. Ma, M. Fang, P. Li, B. Zhu, X. Lu and N.T. Lau, Appl. Catal. A: General 159 (1997) 211. [11]E.T. Thostenson and T.-W. Chou, Composites: Part A 30 (1999) 1055. [12] V. Pallm, M. d'Amore, P. Russo, A. D'Arco and P. Ciambelli in press on Comb. Sci. Tech. (2002).