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Metal loaded Ti-pillared clays for selective catalytic reduction of NO by propylene
Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
723
Metal loaded Ti-pillared clays for selective catalytic reduction of NO by propylene +. J. L. Valverde*, A. de Lucas, P. S~inchez, F. Dorado and A. Romero Department of Chemical Engineering, Faculty of Chemistry. Castilla-La Mancha University. Campus Universitario s/n. 13071-Ciudad Real. Spain.
Metal loaded Ti-PILCs have been used as catalysts for the selective reduction of NO by propylene. Cu, Ni, and Fe ion exchanged Ti-PILCs were prepared. The influence of the metal loading was studied. When the metal loading increases, the catalytic activity also increases reaching a maximum of NO conversion and then decreased. Cu-TiPILs exhibited the highest NO conversion. Cu-PILCs prepared by impregnation were compared with those prepared by ion exchange. In general the ion exchange method resulted to be more adequate for the preparation of the catalyst. The presence of Cu 2+ species in the ion-exchanged samples could be the responsible of this behaviour.
1. INTRODUCTION. Pillared clays constitute one of the most studied families among the new groups of microporous materials developed by molecular engineering. These materials, also known as cross-linked clays or pillared inter-layered clays (PILCs), are synthesised by exchanging the inter-layered cations of the clay with inorganic polyoxocations, followed by calcination. The polyoxocations are then converted to metal oxide clusters by dehydration and dehydroxylation, leading to a permanently opening of the clay layers [ 1]. Properties as acidity, surface area, and pore size distribution of PILCs offer new shape selective catalysts similar to the zeolites. Nevertheless, thermal stability, lower than zeolites, limits their use as catalysts to specific reactions at relatively low temperatures. Emphasize in special titanium pillared clays (Ti-PILCs) by its catalytic activity in the selective reduction (SCR) of NOx of great importance from the environmental point of view. +Financial support from European Commission (ContractERK5-CT-1999-00001) and DGICYT (Direcci6n General de Investigaci6nCientificay T6cnica, Project 1FD97-1791, Ministryof Education, Spain) is gratefully acknowledged. *Corresponding author: Fax (+34) 926 29 53 18; e-mail:[email protected]
724 These materials showed an excellent thermal stability, high surface area and acidity, and its activity is almost unchanged in the presence of the poisons SO2 and H20, which are present in NOx containing streams [2]. Potential applications of PILCs in catalytic processes of a redox nature would require the clay structure to accommodate transition metal ions that are known to easily change their oxidation state [3]. A large number of catalyst, such as V 2 0 5 - W O 3 (or MoO3)/TiO2, other transition metals oxides (e.g., Fe, Cr, Co, Ni, Cu, Nb, etc.), and doped catalyst, as well as zeolite-type catalyst (e.g., H-ZSM-5, Fe-Y, Cu-ZSM-5), have been found active in this reaction. Despite the high activity of vanadium-based catalysts [4], major disadvantages remain, such as their toxicity and high activity for the oxidation of SO2 to SO3. In this work Ni, Fe and Cu have been used as metals for the preparation of active catalysts (metal-Ti-PILCs) in the SCR NOx reaction. The influence of the metal loading method for the Cu-Ti-PILCs catalyst preparation is also described.
2. EXPERIMENTAL.
2.1. Catalyst preparation. The starting clay was a purified grade bentonite (Fisher Company), with a particle size <2 ~tm and a cation exchange capacity of 97 meq g-1 dry clay. Ti-PILCs was prepared as follows. A pillaring solution was formed dissolving titanium metoxide in 5M HC1 until obtaining a molar relation HC1/metoxide of 2.5. This mixture was stirred at to room temperature for 3 h. The pillaring solution was then dropped to an aqueous clay suspension until obtaining 15 mmoles Ti/g clay. The mixture was kept under vigorous stirring for 12 hours at room temperature. Finally, the product was washed, dried and calcined for 2 h at 500~ Metal loaded samples (Ni, Fe and Cu) were obtained by ion exchange, using metal salts solutions. Cu was also introduced by the impregnation method. The resulting catalyst was calcined for 2 h at 400~ Ion exchanged samples were obtained adding 1g of sample to 200 mL of 0.05 M Cu acetate solution, under stirring for 15 h at room temperature. The ion-exchanged product was collected by centrifugation and washed several times with deionized water. Each sample had different metal loading, depending on how many times it was subjected to ion exchange. Impregnation samples were obtained pouring to the Ti-PILC the minimum amount of Cu(NO3)2 solution required to wet the solid. The slurry was placed in a glass vessel and kept under vacuum at room temperature until the solvent was evaporated. Each sample had different metal loading, depending on the Cu(NO3)2 solution concentration used.
2.2. Catalyst test. Activity tests of the catalysts were carried out in a fixed bed reactor. 1000 ppm NO, 1000 ppm C3H6, 5% 02 were used as flue gas component and He was used as a balance gas at a total flow-rate of 125 ml/min. The flow rates were controlled by calibrated Brooks flowmeters. The space velocity of the feed was 15000 h l (GHSV). The reaction was studied in the 200-400 ~
725 temperature interval. The outlet gases were analysed using a gas chromatograph equipped with a TCD detector and a 1010 Carboxen column (Supelco) for the separation of 02, N2, N20, C3H6, CO2 and CO2 and a chemiluminiscence NOx analyzer (Eco Physics CLD 700 EL ht) for NO and NO2. 2.3. Catalyst characterization. X-ray diffraction (XRD) patterns were measured with a Philips model PW 1710 diffractometer using Ni-filtered CuK~ radiation. To maximize the (001) reflection intensities, oriented specimens were prepared by spreading them on a glass slide. Surface area and pore size distributions were determined by using nitrogen as the sorbate at 77 K in a static volumetric apparatus (Micromeritics ASAP 2010 sorptometer). Pillared clays were previously outgassed at 180~ for 16 h under a vacuum of 6.6 x 10 -9 bar. Specific total surface areas were calculated by using the BET equation, whereas specific total pore volumes were evaluated from nitrogen uptake at P/Po = 0.99. The Horvath-Kawazoe method was used to determine the micropore surface area and volume. Total acid-site density of the samples was measured by a temperature programmed desorption (TPD) of ammonia, by using a Micromeritics TPD/TPR analyzer. Samples were housed in a quartz tubular reactor and pretreated in flowing helium (99.999%) while heating at 15 ~ min 1 up to 500 ~ After 0.5 h at 500 ~ the samples were cooled to 180 ~ and saturated for 0.25 h in an ammonia (99.999%) stream. The sample was then allowed to equilibrate in a helium flow at 180 ~ for 1 h. Finally; ammonia was desorbed using a linear heating rate of 15 ~ rain 1. Temperature and detector signals were simultaneously recorded. The average relative error in the acidity determination was lower than 3%. Temperature programmed reduction (TPR) measurements were carried out with the same apparatus previously described. After loading, the sample was outgassed by heating at 20 ~ min -~ in an argon flow to 500 ~ This temperature was kept constant for 30 min. Next, it was cooled to 25 ~ and stabilized under an argon/hydrogen (99.999%, 83/17 volumetric ratio) flow. The temperature and detector signals were continuously recorded while heating at 20 ~ min 1. A cooling trap placed between the sample and the detector retained the liquids formed during the reduction process. TPR profiles were reproducible with an average relative error in the determination of the reduction maximum temperatures lower than 2%. The metallic content (wt %) was determined by atomic absorption measurements by using a SpectrAA 220 FS analyzer. In all cases, calibrations from the corresponding patron solutions were performed.
3. RESULTS AND DISCUSSION. 3.1. Characterization of the catalysts. Figure 1 shows XRD patterns of the starting clay and the titanium pillared clay (Ti-PILC). It can be seen that the basal (001) peak around 2~=9 ~ (characteristic of the natural bentonite)
726 shift towards lower values of 28 on the pillared samples. This result clearly would indicate an enlargement of the basal spacing of the clay as a consequence of the pillaring process. A basal spacing about 24 .A was obtained in the titanium-pillared sample. It should also be noted from Figure 1 a second peak at about 7~ corresponding to an interlayer spacing ranging from 3-4 A. The presence of polymeric titanium species, smaller in size, could explain the emergence of this second peak. Moreover a homogeneous pillars distribution was reached as indicate the relative high intensity of the (001) XRD diffraction peak. Table 1 shows the metal loading, acidity and textural properties of samples. It can be observed the great increase on surface and micropore area of the Ti-PILC sample as compared with the starting clay. Obviously, this result is a consequence of the pillaring process. An increase of the metal loading led to a decrease of the surface area (mainly the micropore area) of the catalyst. This result could be explained by the partial blocking of the pillaring matrix by the metal species [5]. On the other hand, the acidity of Ti-PILC increased due to the increase on the micropore area (better accessibility of the NH3 molecules) and the presence of the pillaring titanium species with acid character. Moreover, the acidity of ion exchanged pillared clays depended on both the ion exchange transition metal and the metal loading. Cu and Fe ion exchanged samples showed an increase of the acidity with the metal loading. Nevertheless, the nickel loading reached in the preparation of Ni ion exchanged samples was lower, and accordingly the acidity kept practically constant:
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28 (~ Figure 1. XRD patterns of the starting clay and the titanium pillared sample (Ti-PILC).
727 60 A
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Figure 2. Influence of the metal loading on the catalytic activity of SCR NO.
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Figure 3. Influence of the preparation procedure of Cu-TiPILC samples on the SCR NO catalytic activity.
728 Table 1. Metal loading, acidity, and textural properties of samples. SAMPLE Bentonite TiPILC Fe-Ti-PILC Ion Exchange
Ni-Ti-PILC Ion Exchange
Cu-Ti-PILC Ion Exchange
Cu-Ti-PILC Impregnation
Metal Acidity Surface Area Micropore Area (wt %) (mmol NH3/g) (mZ/g) (m2/g) 0.0 0.0 5.8 8.0 12.6 15.5 1.6 2.9 3.4 3.6 4.6 7.4 9.0 9.5 4.6 8.0 8.6 9.7
0.132 0.529 0.441 0.469 0.608 0.668 0.467 0.468 0.470 0.472 0.620 0.731 0.894 0.766 0.500 0.502 0.508 0.515
35.2 273.2 244.3 217.5 201.3 197.4 260.3 247.3 236.3 222.7 241.6 234.3 201.8 198.8 259.3 228.0 226.6 225.0
15.1 224.5 195.2 154.8 143.1 149.8 195.7 180.2 173.5 160.4 202.2 189.1 153.7 137.5 227.1 202.9 198.7 197.4
Pore Volume (cm3/g) 0.069 0.266 0.234 0.234 0.241 0.200 0.269 0.278 0.265 0.257 0.226 0.236 0.219 0.233 0.229 0.207 0.198 0.180
Copper ion exchanged Ti-PILC showed the highest acidity due probably to the intrinsic acidity of the Cu 2+ species as demonstrated below by TPR analysis.
3.2. Catalytic Activity. Figure 2 shows the catalytic results achieved in the NO-SCR reaction by using Ti-PILCs ion exchanged with different amounts of Ni, Fe and Cu. Conversion of NO increased with the metal content until a maximum. On the other hand, Ni and Fe Ti-PILCs presented low conversions (under 35 %) as compared with Cu-Ti-PILC. Both the high acidity and the adequate redox characteristics of the copper species formed should explain this behaviour. It is important to note that the most effective temperature defined as the temperature of the maximum NO conversion was around 250~ over Cu ion exchanged samples, whereas Fe an Ni presented higher temperatures, 325 ~ and 425 ~ respectively. Since the metal providing the best results was the Cu, a thoroughly study comparing two ways to introduce this metal (ion exchange and impregnation) was carried out (Figure 3). When the Cu content is low the catalyst prepared by impregnation presents higher conversion than that obtained by ion exchange. Nevertheless, when the Cu content is high, similar conversion values are obtained. Sample prepared by ion exchange with 7.4 wt % of Cu presents the highest catalytic activity.
729 These results could be explained because both the preparation method and the Cu content influences the nature and the positions of the metal on the clay. TPR can be used to identify and quantify the metal species in samples. Figure 4 shows the Hz-TPR profiles of ion exchanged and impregnated Cu samples. The peak at the lowest temperature would be related with the presence of CuO aggregates [6]. The other two reduction peaks suggests a two-step reduction process of isolated Cu 2+ species [7]. The peak at the lower temperature would indicate that the Cu 2+ to Cu + reduction process occurred. The other peak at the highest temperature suggests that the produced Cu + was further reduced to Cu ~ As can be seen on Figure 4, the only peak that clearly appears on impregnated samples was the one at the lowest temperature with a small shoulder that could be related with the second reduction peak (Cu 2+ to Cu+), whereas the other two peaks are absent. On the contrary, Cu ion exchanged exhibited the three above-mentioned peaks. This result seem indicates that the increase on the catalytic activity observed in Cu ion exchanged samples as compared with the impregnated ones, is due to the presence of Cu 2+ species.
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730 Cu introduced by ion exchange firstly occupies very stable positions of the clay structure, practically inaccessible to the reagents or any molecule test. Once these positions are filled, the copper place in less stable but more accessible positions, which seem to be more catalytically active. The decrease on activity observed at high Cu content could indicate that the metal is deposited mainly as Cu oxide aggregates [6]. The impregnation method favours the deposition of Cu as Cu oxide on the surface, although it is accompanied of a simultaneous ion exchange process that lead the metal to accessible positions.
4. CONCLUSIONS. Metal loaded titanium pillared clays are active as catalysts in the SCR NO reaction. Cu loaded samples showed the highest activity as compared with Fe and Ni catalyst. The high acidity and mainly the redox nature of the Cu species are the responsible of this behaviour. The NO conversion increases with increasing the metal loading of samples, reaching a maximum and then, a decrease was observed. The preparation procedure of Cu loaded samples influences the catalytic activity. Cu ion exchanged samples showed the best results. This fact could be attributed to the presence of accessible Cu2§ species on the ion-exchanged samples. TPR result is in agreement with the higher acidity of theses samples.
REFERENCES
1. Gil, A.; Gandia, L.M., Catal. Rev-SCI. ENG, 42, (2000) 145. 2. A. Bahamonde, F. Mohino, M. Rd~oUar, M. Yates, P. Avila, S. Mendioroz, Catal. Today 69 (2001) 233. 3. K. Bahranowski, M. Gasior, A. Kielski, J. Podobinski, E.M. Serwicka, L.A. Vartikian, K. Wodnicka, Clays Clay Miner. 46 (1998) 98. 4. N.Y. Topsoe, H.Topsoe, J.A. Dumesic, J. Catal. 151 (1995) 241. 5. R.Q. Long, R.T. Yang, J. Catal. 196 (2000) 73. 6. R.T. Yang, N. Tharappiwattananon, R.Q. Long, App. Catal. B 19 0998) 289. 7. R. BulAnek, B. Wichtedovfi, Z. Sobalik, J. Tich:, Appl. Catal. B 31 (2001) 13.
723
Metal loaded Ti-pillared clays for selective catalytic reduction of NO by propylene +. J. L. Valverde*, A. de Lucas, P. S~inchez, F. Dorado and A. Romero Department of Chemical Engineering, Faculty of Chemistry. Castilla-La Mancha University. Campus Universitario s/n. 13071-Ciudad Real. Spain.
Metal loaded Ti-PILCs have been used as catalysts for the selective reduction of NO by propylene. Cu, Ni, and Fe ion exchanged Ti-PILCs were prepared. The influence of the metal loading was studied. When the metal loading increases, the catalytic activity also increases reaching a maximum of NO conversion and then decreased. Cu-TiPILs exhibited the highest NO conversion. Cu-PILCs prepared by impregnation were compared with those prepared by ion exchange. In general the ion exchange method resulted to be more adequate for the preparation of the catalyst. The presence of Cu 2+ species in the ion-exchanged samples could be the responsible of this behaviour.
1. INTRODUCTION. Pillared clays constitute one of the most studied families among the new groups of microporous materials developed by molecular engineering. These materials, also known as cross-linked clays or pillared inter-layered clays (PILCs), are synthesised by exchanging the inter-layered cations of the clay with inorganic polyoxocations, followed by calcination. The polyoxocations are then converted to metal oxide clusters by dehydration and dehydroxylation, leading to a permanently opening of the clay layers [ 1]. Properties as acidity, surface area, and pore size distribution of PILCs offer new shape selective catalysts similar to the zeolites. Nevertheless, thermal stability, lower than zeolites, limits their use as catalysts to specific reactions at relatively low temperatures. Emphasize in special titanium pillared clays (Ti-PILCs) by its catalytic activity in the selective reduction (SCR) of NOx of great importance from the environmental point of view. +Financial support from European Commission (ContractERK5-CT-1999-00001) and DGICYT (Direcci6n General de Investigaci6nCientificay T6cnica, Project 1FD97-1791, Ministryof Education, Spain) is gratefully acknowledged. *Corresponding author: Fax (+34) 926 29 53 18; e-mail:[email protected]
724 These materials showed an excellent thermal stability, high surface area and acidity, and its activity is almost unchanged in the presence of the poisons SO2 and H20, which are present in NOx containing streams [2]. Potential applications of PILCs in catalytic processes of a redox nature would require the clay structure to accommodate transition metal ions that are known to easily change their oxidation state [3]. A large number of catalyst, such as V 2 0 5 - W O 3 (or MoO3)/TiO2, other transition metals oxides (e.g., Fe, Cr, Co, Ni, Cu, Nb, etc.), and doped catalyst, as well as zeolite-type catalyst (e.g., H-ZSM-5, Fe-Y, Cu-ZSM-5), have been found active in this reaction. Despite the high activity of vanadium-based catalysts [4], major disadvantages remain, such as their toxicity and high activity for the oxidation of SO2 to SO3. In this work Ni, Fe and Cu have been used as metals for the preparation of active catalysts (metal-Ti-PILCs) in the SCR NOx reaction. The influence of the metal loading method for the Cu-Ti-PILCs catalyst preparation is also described.
2. EXPERIMENTAL.
2.1. Catalyst preparation. The starting clay was a purified grade bentonite (Fisher Company), with a particle size <2 ~tm and a cation exchange capacity of 97 meq g-1 dry clay. Ti-PILCs was prepared as follows. A pillaring solution was formed dissolving titanium metoxide in 5M HC1 until obtaining a molar relation HC1/metoxide of 2.5. This mixture was stirred at to room temperature for 3 h. The pillaring solution was then dropped to an aqueous clay suspension until obtaining 15 mmoles Ti/g clay. The mixture was kept under vigorous stirring for 12 hours at room temperature. Finally, the product was washed, dried and calcined for 2 h at 500~ Metal loaded samples (Ni, Fe and Cu) were obtained by ion exchange, using metal salts solutions. Cu was also introduced by the impregnation method. The resulting catalyst was calcined for 2 h at 400~ Ion exchanged samples were obtained adding 1g of sample to 200 mL of 0.05 M Cu acetate solution, under stirring for 15 h at room temperature. The ion-exchanged product was collected by centrifugation and washed several times with deionized water. Each sample had different metal loading, depending on how many times it was subjected to ion exchange. Impregnation samples were obtained pouring to the Ti-PILC the minimum amount of Cu(NO3)2 solution required to wet the solid. The slurry was placed in a glass vessel and kept under vacuum at room temperature until the solvent was evaporated. Each sample had different metal loading, depending on the Cu(NO3)2 solution concentration used.
2.2. Catalyst test. Activity tests of the catalysts were carried out in a fixed bed reactor. 1000 ppm NO, 1000 ppm C3H6, 5% 02 were used as flue gas component and He was used as a balance gas at a total flow-rate of 125 ml/min. The flow rates were controlled by calibrated Brooks flowmeters. The space velocity of the feed was 15000 h l (GHSV). The reaction was studied in the 200-400 ~
725 temperature interval. The outlet gases were analysed using a gas chromatograph equipped with a TCD detector and a 1010 Carboxen column (Supelco) for the separation of 02, N2, N20, C3H6, CO2 and CO2 and a chemiluminiscence NOx analyzer (Eco Physics CLD 700 EL ht) for NO and NO2. 2.3. Catalyst characterization. X-ray diffraction (XRD) patterns were measured with a Philips model PW 1710 diffractometer using Ni-filtered CuK~ radiation. To maximize the (001) reflection intensities, oriented specimens were prepared by spreading them on a glass slide. Surface area and pore size distributions were determined by using nitrogen as the sorbate at 77 K in a static volumetric apparatus (Micromeritics ASAP 2010 sorptometer). Pillared clays were previously outgassed at 180~ for 16 h under a vacuum of 6.6 x 10 -9 bar. Specific total surface areas were calculated by using the BET equation, whereas specific total pore volumes were evaluated from nitrogen uptake at P/Po = 0.99. The Horvath-Kawazoe method was used to determine the micropore surface area and volume. Total acid-site density of the samples was measured by a temperature programmed desorption (TPD) of ammonia, by using a Micromeritics TPD/TPR analyzer. Samples were housed in a quartz tubular reactor and pretreated in flowing helium (99.999%) while heating at 15 ~ min 1 up to 500 ~ After 0.5 h at 500 ~ the samples were cooled to 180 ~ and saturated for 0.25 h in an ammonia (99.999%) stream. The sample was then allowed to equilibrate in a helium flow at 180 ~ for 1 h. Finally; ammonia was desorbed using a linear heating rate of 15 ~ rain 1. Temperature and detector signals were simultaneously recorded. The average relative error in the acidity determination was lower than 3%. Temperature programmed reduction (TPR) measurements were carried out with the same apparatus previously described. After loading, the sample was outgassed by heating at 20 ~ min -~ in an argon flow to 500 ~ This temperature was kept constant for 30 min. Next, it was cooled to 25 ~ and stabilized under an argon/hydrogen (99.999%, 83/17 volumetric ratio) flow. The temperature and detector signals were continuously recorded while heating at 20 ~ min 1. A cooling trap placed between the sample and the detector retained the liquids formed during the reduction process. TPR profiles were reproducible with an average relative error in the determination of the reduction maximum temperatures lower than 2%. The metallic content (wt %) was determined by atomic absorption measurements by using a SpectrAA 220 FS analyzer. In all cases, calibrations from the corresponding patron solutions were performed.
3. RESULTS AND DISCUSSION. 3.1. Characterization of the catalysts. Figure 1 shows XRD patterns of the starting clay and the titanium pillared clay (Ti-PILC). It can be seen that the basal (001) peak around 2~=9 ~ (characteristic of the natural bentonite)
726 shift towards lower values of 28 on the pillared samples. This result clearly would indicate an enlargement of the basal spacing of the clay as a consequence of the pillaring process. A basal spacing about 24 .A was obtained in the titanium-pillared sample. It should also be noted from Figure 1 a second peak at about 7~ corresponding to an interlayer spacing ranging from 3-4 A. The presence of polymeric titanium species, smaller in size, could explain the emergence of this second peak. Moreover a homogeneous pillars distribution was reached as indicate the relative high intensity of the (001) XRD diffraction peak. Table 1 shows the metal loading, acidity and textural properties of samples. It can be observed the great increase on surface and micropore area of the Ti-PILC sample as compared with the starting clay. Obviously, this result is a consequence of the pillaring process. An increase of the metal loading led to a decrease of the surface area (mainly the micropore area) of the catalyst. This result could be explained by the partial blocking of the pillaring matrix by the metal species [5]. On the other hand, the acidity of Ti-PILC increased due to the increase on the micropore area (better accessibility of the NH3 molecules) and the presence of the pillaring titanium species with acid character. Moreover, the acidity of ion exchanged pillared clays depended on both the ion exchange transition metal and the metal loading. Cu and Fe ion exchanged samples showed an increase of the acidity with the metal loading. Nevertheless, the nickel loading reached in the preparation of Ni ion exchanged samples was lower, and accordingly the acidity kept practically constant:
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28 (~ Figure 1. XRD patterns of the starting clay and the titanium pillared sample (Ti-PILC).
727 60 A
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6 8 10 Ni, Fe, Cu ( w t % )
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Figure 2. Influence of the metal loading on the catalytic activity of SCR NO.
60 A
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Figure 3. Influence of the preparation procedure of Cu-TiPILC samples on the SCR NO catalytic activity.
728 Table 1. Metal loading, acidity, and textural properties of samples. SAMPLE Bentonite TiPILC Fe-Ti-PILC Ion Exchange
Ni-Ti-PILC Ion Exchange
Cu-Ti-PILC Ion Exchange
Cu-Ti-PILC Impregnation
Metal Acidity Surface Area Micropore Area (wt %) (mmol NH3/g) (mZ/g) (m2/g) 0.0 0.0 5.8 8.0 12.6 15.5 1.6 2.9 3.4 3.6 4.6 7.4 9.0 9.5 4.6 8.0 8.6 9.7
0.132 0.529 0.441 0.469 0.608 0.668 0.467 0.468 0.470 0.472 0.620 0.731 0.894 0.766 0.500 0.502 0.508 0.515
35.2 273.2 244.3 217.5 201.3 197.4 260.3 247.3 236.3 222.7 241.6 234.3 201.8 198.8 259.3 228.0 226.6 225.0
15.1 224.5 195.2 154.8 143.1 149.8 195.7 180.2 173.5 160.4 202.2 189.1 153.7 137.5 227.1 202.9 198.7 197.4
Pore Volume (cm3/g) 0.069 0.266 0.234 0.234 0.241 0.200 0.269 0.278 0.265 0.257 0.226 0.236 0.219 0.233 0.229 0.207 0.198 0.180
Copper ion exchanged Ti-PILC showed the highest acidity due probably to the intrinsic acidity of the Cu 2+ species as demonstrated below by TPR analysis.
3.2. Catalytic Activity. Figure 2 shows the catalytic results achieved in the NO-SCR reaction by using Ti-PILCs ion exchanged with different amounts of Ni, Fe and Cu. Conversion of NO increased with the metal content until a maximum. On the other hand, Ni and Fe Ti-PILCs presented low conversions (under 35 %) as compared with Cu-Ti-PILC. Both the high acidity and the adequate redox characteristics of the copper species formed should explain this behaviour. It is important to note that the most effective temperature defined as the temperature of the maximum NO conversion was around 250~ over Cu ion exchanged samples, whereas Fe an Ni presented higher temperatures, 325 ~ and 425 ~ respectively. Since the metal providing the best results was the Cu, a thoroughly study comparing two ways to introduce this metal (ion exchange and impregnation) was carried out (Figure 3). When the Cu content is low the catalyst prepared by impregnation presents higher conversion than that obtained by ion exchange. Nevertheless, when the Cu content is high, similar conversion values are obtained. Sample prepared by ion exchange with 7.4 wt % of Cu presents the highest catalytic activity.
729 These results could be explained because both the preparation method and the Cu content influences the nature and the positions of the metal on the clay. TPR can be used to identify and quantify the metal species in samples. Figure 4 shows the Hz-TPR profiles of ion exchanged and impregnated Cu samples. The peak at the lowest temperature would be related with the presence of CuO aggregates [6]. The other two reduction peaks suggests a two-step reduction process of isolated Cu 2+ species [7]. The peak at the lower temperature would indicate that the Cu 2+ to Cu + reduction process occurred. The other peak at the highest temperature suggests that the produced Cu + was further reduced to Cu ~ As can be seen on Figure 4, the only peak that clearly appears on impregnated samples was the one at the lowest temperature with a small shoulder that could be related with the second reduction peak (Cu 2+ to Cu+), whereas the other two peaks are absent. On the contrary, Cu ion exchanged exhibited the three above-mentioned peaks. This result seem indicates that the increase on the catalytic activity observed in Cu ion exchanged samples as compared with the impregnated ones, is due to the presence of Cu 2+ species.
o
~
Impregnation
r/l
~ 250~ Ion Exchange
J '
0
I
200
'
I
400
'
I
600
Temperature (~ Figure 4. TPR profiles of Cu ion exchanged and impregnated samples.
730 Cu introduced by ion exchange firstly occupies very stable positions of the clay structure, practically inaccessible to the reagents or any molecule test. Once these positions are filled, the copper place in less stable but more accessible positions, which seem to be more catalytically active. The decrease on activity observed at high Cu content could indicate that the metal is deposited mainly as Cu oxide aggregates [6]. The impregnation method favours the deposition of Cu as Cu oxide on the surface, although it is accompanied of a simultaneous ion exchange process that lead the metal to accessible positions.
4. CONCLUSIONS. Metal loaded titanium pillared clays are active as catalysts in the SCR NO reaction. Cu loaded samples showed the highest activity as compared with Fe and Ni catalyst. The high acidity and mainly the redox nature of the Cu species are the responsible of this behaviour. The NO conversion increases with increasing the metal loading of samples, reaching a maximum and then, a decrease was observed. The preparation procedure of Cu loaded samples influences the catalytic activity. Cu ion exchanged samples showed the best results. This fact could be attributed to the presence of accessible Cu2§ species on the ion-exchanged samples. TPR result is in agreement with the higher acidity of theses samples.
REFERENCES
1. Gil, A.; Gandia, L.M., Catal. Rev-SCI. ENG, 42, (2000) 145. 2. A. Bahamonde, F. Mohino, M. Rd~oUar, M. Yates, P. Avila, S. Mendioroz, Catal. Today 69 (2001) 233. 3. K. Bahranowski, M. Gasior, A. Kielski, J. Podobinski, E.M. Serwicka, L.A. Vartikian, K. Wodnicka, Clays Clay Miner. 46 (1998) 98. 4. N.Y. Topsoe, H.Topsoe, J.A. Dumesic, J. Catal. 151 (1995) 241. 5. R.Q. Long, R.T. Yang, J. Catal. 196 (2000) 73. 6. R.T. Yang, N. Tharappiwattananon, R.Q. Long, App. Catal. B 19 0998) 289. 7. R. BulAnek, B. Wichtedovfi, Z. Sobalik, J. Tich:, Appl. Catal. B 31 (2001) 13.