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Catalytic activation of ceramic filter elements for combined particle separation, NOx removal and VOC total oxidation
Applied Catalysis B: Environmental 70 (2007) 370–376 www.elsevier.com/locate/apcatb
Catalytic activation of ceramic filter elements for combined particle separation, NOx removal and VOC total oxidation Manfred Nacken a,*, Steffen Heidenreich a, Marius Hackel b, Georg Schaub b a
Pall SeitzSchenk Filtersystems GmbH, D-74564 Crailsheim, Germany b Universita¨t Karlsruhe (TH), D-76131 Karlsruhe, Germany Available online 30 June 2006
Abstract The development of a catalytically active filter element for combined particle separation and NOx removal or VOC total oxidation, respectively, is presented. For NOx removal by selective catalytic reduction (SCR) a catalytic coating based on a TiO2–V2O5–WO3 catalyst system was developed on a ceramic filter element. Different TiO2 sols of tailor-made mean particle size between 40 and 190 nm were prepared by the sol–gel process and used for the impregnation of filter element cylinders by the incipient wetness technique. The obtained TiO2-impregnated sintered filter element cylinders exhibit BET surface areas in the range between 0.5 and 1.3 m2/g. Selected TiO2-impregnated filter element cylinders of high BET surface area were catalytically activated by impregnation with a V2O5 and WO3 precursor solution. The obtained catalytic filter element cylinders show high SCR activity leading to 96% NO conversion at 300 8C, a filtration velocity of 2 cm/s and an NO inlet concentration of 500 vol.ppm. The corresponding differential pressures fulfill the requirements for typical hot gas filtration applications. For VOC total oxidation, a TiO2impregnated filter element support was catalytically activated with a Pt/V2O5 system. Complete oxidation of propene with 100% selectivity to CO2 was achieved at 300 8C, a filtration velocity of 2 cm/s and a propene inlet concentration of 300 vol.-ppm. # 2006 Elsevier B.V. All rights reserved. Keywords: Catalytic filter; NOx reduction; VOC oxidation; TiO2–V2O5–WO3
1. Introduction More stringent regulations in terms of dust, NOx and VOC emission levels on one side and the demand for cost effective and energy-efficient gas cleaning technologies on the other side have forced activities to provide novel gas cleaning solutions. In conventional hot gas cleaning of gases from combustion plants [1] and advanced fluid catalytic cracking (FCC) units [2] a multistep gas cleaning procedure is performed for particle and NOx removal. In these cleaning systems particulates were usually separated before the DeNOx catalyst unit consisting of SCR honeycombs. In case of combustion plants an additional desulfurization step is usually performed after particle separation depending on the kind of feedstock used [1,3]. A disadvantage of this two- or three-step cleaning procedure, respectively, is the reheating of the particle-free gas to the required SCR catalyst operating temperature. Therefore, a combination of filtration and catalytic reaction in one unit using * Corresponding author. E-mail address: [email protected] (M. Nacken). 0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2006.02.030
a so-called catalytic filter [1,4–7] would allow using the high energy content of the gas. In this way processing costs as well as investment and maintenance costs are strongly reduced by combination of two units in one unit. By using ceramic hot gas filter elements with a fine filtering outer membrane, in which a catalyst is integrated as catalytic layer (Fig. 1), the latter would be sufficiently protected against particle deposition. This allows dry scrubbing by injected sorbents upsteam of the catalytic filter for removal of SOx and other potential gaseous catalyst poisons like HCl. Moreover, the integrated catalytic layer can be tailored in such a way to allow not only catalytic NOx removal but also catalytic oxidation of volatile organic compounds (VOC). A prerequisite for applying catalytic filter elements is a high catalytic activity combined with an as low as possible differential pressure at the optimum catalyst operating temperature and a filtration velocity typical for hot gas filtration. First results on the preparation of catalytic filters for NOx or VOC removal, respectively, were reported by Saracco et al. [5,6] by using a cylindrical alumina tube with a filter wall thickness of only 1.5 mm.
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Table 1 Mean particle size of the prepared TiO2 sols and a SiO2 sol, measured by photocorrelation spectroscopy
Fig. 1. Structure of a catalytic filter element.
In the present work a ceramic hot gas filter element with a thickness of 10 mm is used to allow a longer residence time in order to provide high catalytic activities at as high as possible superficial velocities. A catalytic activation of ceramic hot gas filter elements by impregnation techniques entails a differential pressure increase by pore confinement. Therefore, a minimum loading of the ceramic filter element support with catalyst allowing a high catalytic activity has to be adjusted. One concept to provide a highly active catalyst surface with a relatively low differential pressure of the ceramic filter element, that is followed in this work, is the formation of a TiO2 nanoparticle layer as catalyst support using a tailor-made TiO2 sol accessible by the sol–gel process. In this process hydrolysis and condensation of a titanium alkoxide is better controlled, if complexing agents like acetylacetone or acetic acid are used [8,9]. Based on the results of several studies on the SCR reaction [10–16] for the catalytic activation of a TiO2-modified ceramic filter element, a V2O5–WO3 catalyst system was selected. For integration of the catalyst in the porous ceramic filter element the incipient wetness technique was applied to allow the highest possible specific surface area and the lowest possible pore confinement by deposition of the catalyst on the pore wall. For the preparation of a catalytically activated filter element for VOC abatement, an Mn2O3-based catalyst was selected, because this system was proven to lead to high conversions and selectivities in the total oxidation of 2-propanol [17]. In addition, the TiO2-modified ceramic filter element was catalytically activated with a Pt/V2O5-based system for comparison. 2. Experimental 2.1. TiO2 sol synthesis Five TiO2 sols of different mean particle size, denoted as Ti, TiAc, TiAcAc, Ti0.5AcAc, and Ti0.25AcAc were synthesized
Sol
Mean particle size (nm)
Solid content (wt.%)
Ti TiAc TiAcAc Ti0.5AcAc Ti0.25AcAc TiK SiK
188.6 142.3 42.5 48.9 48.9 267.9 79.8
8.1 7.9 7.3 5.7 6.2 6.8 3
by the sol–gel process by hydrolysis and condensation reactions starting from titanium isopropoxide (TIP) as TiO2 precursor in 2-propanol. For the synthesis of the sols TiAc and TiAcAc, the equimolar amount of the complexing agents acetic acid or acetylacetone, respectively, was additionally added to the alcoholic solution at ambient temperature, whereas in the case of the synthesis of the sols Ti0.5AcAc and Ti0.25AcAc the half molar or quarter molar amount, respectively, related to the amount of TIP was used. Complete hydrolysis of the titanium alkoxide was performed to form TiO2 in situ. The as-prepared TiO2 sols were characterized in terms of their mean particle size by photocorrelation spectroscopy (Table 1). Additionally, a commercially available TiO2 powder was dispersed in 2-propanol. For comparison, a commercially available 30% SiO2 sol of pH 10 was diluted to 3% in deionisized water. The as-prepared sols, denoted as TiK or SiK, respectively, were also characterized by photocorrelation spectroscopy (Table 1). 2.2. Preparation of catalytically activated filter elements A SiC–Al2O3-based porous ceramic hot gas filter element cylinder of the dimension 60/40 50 mm (outer/inner diameter length) and 50% porosity with a mullite outer membrane was used for catalytic activation. The catalytic activation was performed in two steps. First, the porous filter element cylinder was impregnated by a TiO2 or the SiO2 sol applying the incipient wetness technique. In a second step the as-obtained TiO2 and SiO2-modified porous filter cylinders were impregnated with an aqueous solution of ammonium metavanadate (NH4VO3) and ammonium metatungstate hydrate [(NH4)6W12O39H2O] to adjust a V2O5 and a WO3 loading of 22.5 or 78 wt.%, respectively, after sintering at 500 8C related to a TiO2 and SiO2 loading of 1.4 g on the virgin filter element cylinder. The adjusted TiO2 loadings by using the different TiO2 sols (Table 1) depend on the type and solid content of the used sol and range between 1.7 and 2.2 wt.% for the sol–gel derived TiO2 sols and 1.3 wt.% for the sol TiK related to the virgin filter element cylinder. In case of the SiO2 sol a loading of 1.2 wt.% was adjusted. Catalytically activated filter elements for NOx removal were denoted with NC before the notation of the used TiO2 or SiO2 sol, respectively (e.g. NCTi), whereas TiO2 or SiO2 solimpregnated and sintered filter element cylinders, prepared for the determination of the BET surface area and the differential
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pressure, are denoted with S before the notation of the used sol (e.g. STi). Two TiO2 sol-impregnated filter element cylinders were catalytically activated for VOC abatement. The first sample, denoted as VCTiK, was prepared by using an aqueous solution of Mn(NO3)24H2O instead of the SCR catalyst precursor solution following the same impregnation procedure mentioned above. An Mn2O3 loading of 8 wt.% related to the TiO2 loading was adjusted. The second sample, denoted as NVCTiK, was prepared by using an aqueous solution of NH4VO3 and [(NH3)4PtCl2] to adjust a loading of 22.5 wt.% V2O5 and 2 wt.% Pt related to the TiO2 loading after reduction of the sample for 5 h at 400 8C in a H2/N2 mixture. 2.3. Characterization 2.3.1. Mean particle size The mean particle size of the prepared TiO2 sols and the SiO2 sol were measured as Z-average value with a relative measurement error of 5% using the photocorrelation spectrometer MAL500673 of the company Malvern Instruments. The sols were measured as prepared without dilution. 2.3.2. BET surface area For the determination of the BET surface area of the TiO2and SiO2-impregnated 50 mm long filter element cylinders, an 8 mm cylinder segment was cut out and broken into monolithic pieces, that were dried for 5 h under vacuum before the measurement. The measurements were performed on a Quantachrome Autosorb-3 instrument with N2 at 77 K with a measurement error of 0.03 m2/g.
parts of the cylinder and passing air with a defined volume flow in the range from 0.9 to 2.4 m3/h from the inside through the porous structure to the outside of the cylinder. The resulting flow resistance was measured as differential pressure with a measurement error of 0.2 mbar. 2.4. Measurement of the catalytic activity The catalytic activity of the impregnated filter elements was examined by measuring the conversion of NO and propene, as VOC model compound, in a laboratory test rig (Fig. 2). The test rig is designed for measurements of the reaction kinetics of the SCR reaction and the oxidation reactions of different volatile organic compounds. The measurement conditions are described in more detail in an earlier publication [18]. Model gas containing 500 vol.-ppm NO, 500 vol.-ppm NH3 and 3 vol.% O2 or 300 vol.-ppm propene and 3 vol.% O2 with N2 as carrier gas in both cases was used for the measurement of the catalytic conversion of NO or propene, respectively. The measurements were performed on a segment of the dimension 60/40 20 mm of the catalytically activated filter element cylinders in the temperature range between 100 and 400 8C at a total pressure of 1–1.1 bar by adjusting a constant filtration velocity of 2 cm/s at all temperature points. This corresponds to gas hourly space velocity (GHSV) values from 2000 to 11,500 h1. The NO and propene conversion was calculated from the corresponding inlet and outlet concentrations measured by means of an FTIR analyzer. 3. Results and discussion 3.1. Sol–gel derived TiO2 sols
2.3.3. Differential pressure The TiO2-impregnated and catalytically impregnated filter element cylinders of 50 mm length were examined on their differential pressure at 25 8C by adjusting a defined filtration velocity related to the outer surface of the filter element cylinder. The cylinder was vertically fixed in an appropriate differential pressure test-bench by sealing the front and end
In a series of TiO2 sol synthesis experiments acetylacetone and acetic acid were used as complexing agents to adjust different mean particle sizes in the TiO2 nanoparticle sols and to study the effect of metal alkoxide complexation on the mean TiO2 particle size of the particle sols. Using equimolar amounts of acetylacetone would lead to the formation of a titanium
Fig. 2. Flow scheme of the test rig (from [18]) for the measurement of NO or propene conversion with catalytically activated filter element segments at varying conditions.
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alkoxide complex species (Ti(OR)3AcAc) with an acetylacetonate ligand (AcAc) under release of one alcohol molecule according to Eq. (1) [19]. TiðORÞ4 þ HAcAc ! TiðORÞ3 AcAc þ ROH
(1)
In case of acetate ligands (Ac), an analogous complexation reaction can be formulated [9]. After hydrolysis of the asformed titanium alkoxide complex species Ti(OR)3AcAc and Ti(OR)3Ac the AcAc or Ac ligands, respectively, remain bonded, whereas the OR groups are hydrolyzed. In this way the number of hydrolytically instable OR groups is reduced from four to three by titanium alkoxide complexation, from which a slowing-down of the condensation reactions results to form a colloidal TiO2 sol instead of precipitating TiO2. For the synthesis of the TiO2 sols Ti, Ti0.25AcAc, Ti0.5AcAc and TiAcAc, the molar ratio HAcAc/Ti(OR)4 was varied from 0 to 1. For comparison, a TiO2 sol using acetic acid (HAc) in a molar ratio HAc/Ti(OR)4 = 1 was synthesized. The as-synthesized TiO2 sols were examined by photocorrelation spectroscopy to determine their mean particle size (Fig. 3). A strong decrease of the mean particle size from 189 to 49 nm is found after addition of 0.25 mol acetylacetone per mol titanium alkoxide. A higher amount of this complexing agent up to 1 mol acetylacetone leads only to a slightly decreased mean particle size of 42.5 nm. Compared with this the addition of an equimolar amount of acetic acid to the titanium alkoxide decreases the mean particle size from 189 to only 149 nm. Evidently, the acetylacetonate ligand is more strongly bonded to the Ti atom resulting in a stronger blocking effect in the course of the hydrolysis and condensation reactions in comparison to acetic acid. Moreover, even a relatively low molar amount of 0.25 mol acetylacetone per mole titanium leads to a strong decrease of the mean particle size by 75% of the initial TiO2 particle size. These findings support the aforementioned assumption of the stronger binding of AcAc ligands to the titanium atom. 3.2. TiO2 impregnated filter elements The synthesized TiO2 sols TiAc, TiAcAc, Ti0.5AcAc and Ti were used as prepared for the impregnation of the filter element cylinders. The TiO2-impregnated and sintered cylinders STiAc, STiAcAc, STi0.5AcAc and STi were examined on their BET surface area. In Fig. 4 a correlation of the corresponding BET
Fig. 4. Correlation of the BET surface area of the TiO2-impregnated filter elements STiAcAc, STi0.5AcAc, STiAc and STi with the mean particle size of the used TiO2 sol.
values with the mean particle size of the used TiO2 sols is presented. Fig. 4 shows that the BET surface area depends strongly on the mean particle size of the used TiO2 sols. The highest BET surface area of about 1.3 m2/g is created with a medium mean particle size of about 140 nm under the applied sintering conditions. Smaller particle sizes of about 40 and 50 nm lead to relatively small BET surface areas of about 0.5 or 0.9 m2/g, respectively, whereas a larger mean particle size leads to a lower specific surface of 1.1 m2/g. These results can be explained by the strong dependency of the sintering temperature of the TiO2 nanoparticles on their size. Evidently, the use of nanoparticles in the range of 40–50 nm does not lead to a surface increasing effect, as the small particles are sintered together to form larger particles, probably in the range of sizes higher than 190 nm. A particle size of 140 nm seems to be the optimum particle size to prevent sintering into larger particles and exhibits the largest possible spherical surface for generating the highest BET surface area increase by TiO2 impregnation. The specific surface area of the virgin filter element is 0.03 m2/g, about two orders of magnitude lower. For comparison, one filter element cylinder was impregnated with an SiO2 sol and examined on its specific surface area after sintering. The as-prepared filter element cylinder SSiK exhibits a BET surface area of 1.8 m2/g. The higher specific surface area of SSiK in comparison to the BET value of sample STi is probably due to the smaller mean particle size (Table 1) and the higher sintering temperature of SiO2 nanoparticles in comparison to TiO2 nanoparticles of similar size. 3.3. Catalytic filter elements for NOx removal
Fig. 3. Mean particle size of the TiO2 particle containing sols Ti, TiAcAc, Ti0.25AcAc, Ti0.5AcAc and TiAc as a function of the molar ratio of added acetylacetone (HAcAc) or acetic acid (HAc) per Ti atom.
For the subsequent catalytic activation with the SCR catalyst, the TiO2-impregnated filter element cylinders STi and STiAc are the most suitable samples as they exhibit the largest BET surface area for fixation of the catalytically active species V2O5 and the promoter WO3 in high dispersion. Therefore, these two samples were prepared again and dried without the sintering step, commonly applied for the preparation of STi and STiAc. As after the subsequent catalytic impregnation with the ammonium metavanadate
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and ammonium metatungstate hydrate precursor solution the same sintering program is applied, a similar high specific surface of the catalytic layer should be obtained in this way. For comparison, the SiO2-impregnated filter element cylinder SSiK was also impregnated under comparable conditions to study the effect of the kind of support material on the catalytic activity in the SCR reaction. The catalytic activity of the as-prepared catalytically activated filter element cylinders NCTi, NCTiAc and NCSiK were determined by measuring the NO and NH3 conversion as a function of the reaction temperature (Fig. 5). The activated filter element cylinder NCTi exhibits the highest catalytic activity with an NO conversion of 96% followed by sample NCTiAc with 90% NO conversion, whereas sample NCSiK shows only poor activity with a conversion of 15% at the optimum catalyst operating temperature of 300 8C (Fig. 5(a)). Evidently, the BET surface is not the only activitydetermining parameter, as the TiO2-based catalyst support surface area of sample NCTi is by 0.2 m2/g lower than the BET surface area of sample NCTiAc (Fig. 4). Possibly, minor differences in the distribution of the active catalyst species over the 10 mm thick porous wall of the filter element cylinder are the reason for the measured activity differences. In case of sample NCSiK, a strong decrease in the catalytic activity is found in spite of using the same catalyst support loading and applying the same preparation method. Moreover, the BET surface area of the SiO2-impregnated filter element support (SSiK) is by a factor of 1.6 higher than the
TiO2-impregnated support STi. This should lead to a better dispersion of the SCR catalyst species V2O5 and WO3. Therefore, the substitution of TiO2 by SiO2 can be accounted responsible for the strong activity decrease, indicating the predominant influence of the kind and acidity of the support material on the catalytic activity. The acid sites of TiO2 seem to be strongly involved in the SCR reaction mechanism for NH3 adsorption. This is in line with the proposed SCR mechanism of Lietti et al. [13], where beside tungsten oxide TiO2 is also involved in the ammonia adsorption to provide a sufficient number of NH3 molecules to migrate to the Lewis acidic vanadyl sites for their activation. Moreover, it can be derived from these results, that the support is more involved in NH3 adsorption than tungsten oxide. According to the SCR stoichiometry equal conversion of NO and NH3 was obtained within the temperature range from 180 to 320 8C (Fig. 5(a) and (b)). The small differences between the NO and NH3 conversion values of the three examined catalyst systems at 180 8C are ascribed to a higher experimental error of 7% at lower conversions of NO and NH3 than the usual error of up to 5% for the corresponding conversions at higher reaction temperatures. The small differences between the NO and NH3 conversion values at 320 8C are due to the oxidation of small amounts of ammonia. The oxidation products of ammonia are NO, that is indicated by the slight decrease of the NO conversions above 300 8C in the three examined catalyst systems (Fig. 5(a)), and N2O. N2O is only formed in small amounts ranging from 12 vol.-ppm in the system NCSiK up to 17 vol.-ppm in the system NCTi at 320 8C. NO2 formation was not observed. 3.4. Differential pressure of the catalytic filter elements
Fig. 5. NO (a) and NH3 conversion (b) of the catalytically activated filter element cylinders NCTi, NCTiAc and NCSiK as a function of the reaction temperature at a filtration velocity of 2 cm/s.
A prerequisite for the application of catalytically activated filter elements in hot gas filtration is a relatively low differential pressure value under typical hot gas filtration velocities ranging from 2 to 4 cm/s. The measured differential pressures of the prepared catalytically activated filter element cylinders and the corresponding virgin ceramic support are plotted versus the filtration velocity at 25 8C (Fig. 6). It is shown, that the TiO2-based catalytic filter element cylinders NCTi and NCTiAc exhibit higher differential pressure values than the SiO2-based catalytic filter element sample after the two-step impregnation procedure. This is due to the larger mean particle size of the TiO2 particles in comparison to the SiO2 particles deposited on the pore wall (Table 1). Practically, no difference in the differential pressure is found if the mean TiO2 particle size differs only by about 50 nm. An extrapolation of the differential pressure lines of the samples NCTi, NCTiAc and NCSiK to a filtration velocity of 2 cm/s and calculation of the obtained values to 300 8C using Darcy’s equation results in differential pressure values of 21, 20.5 or 15 mbar, respectively. As the typical differential pressure limits in hot gas filtration applications are in the range of about 50–100 mbar, the catalytically most active filter
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Fig. 7) range from 93.6% to 98.7% for sample VCTiK. In case of sample NVCTiK complete oxidation of propene to CO2 is found at all temperatures. The superior activity of sample NVCTiK is due to the doping with Pt as also a TiO2-supported Pt-based catalyst system was found to be catalytically active in the total oxidation of propene [6]. The relatively high selectivity values of sample VCTiK indicate the performance of manganese oxide in the total oxidation of propene. Possibly, a higher amount or better dispersion of this catalytically active compound is necessary to increase the propene conversions, and this has to be examined in future work. Fig. 6. Differential pressure of NCTi, NCTiAc and NCSiK and the corresponding uncoated support materials as a function of the filtration velocity at 25 8C.
element NCTi would fulfill the criteria for its economic use in hot gas filtration. 3.5. Catalytic filter elements for VOC removal In special applications of hot gas cleaning, combined removal of particles and volatile organic compounds (VOC) is required to meet the stringent emission levels. Therefore, two catalytic ceramic filter element samples were prepared using in sample VCTiK a manganese oxide-based and in sample NVCTiK a Pt-doped V2O5-based catalyst system for the total oxidation of VOC. The catalytic activity of these two samples was determined by measuring the conversion of propene as a function of the reaction temperature at a filtration velocity of 2 cm/s (Fig. 7). A comparison of the propene conversion values of the samples VCTiK and NVCTiK shows that the Pt-doped V2O5based system NVCTiK shows independently of the reaction temperature in the temperature range from 260 to 320 8C constant conversion values of 95%, whereas the corresponding conversions of the manganese oxide based system VCTiK are relatively low and increase from about 20% at 260 to 55% at 320 8C. The corresponding selectivities to CO2 (not shown in
4. Conclusions Different TiO2 sols of a mean particle size between 40 and 190 nm were synthesized by the sol–gel process by varying the concentration and type of the complexing agent. The highest BET surface area of 1.3 m2/g in the series of TiO2 solimpregnated and sintered ceramic filter element samples was obtained by using a TiO2 sol of a medium mean particle size of about 140 nm. After catalytic activation of the TiO2-impregnated ceramic filter elements of high BET surface areas, maximum NO conversion of up to 96% at 300 8C, a filtration velocity of 2 cm/s and an NO inlet concentration of 500 vol.-ppm were achieved. The catalytic activity is strongly decreased by using SiO2 instead of TiO2 as support material. The catalytically most active filter element for NOx removal shows a differential pressure of 21 mbar at 300 8C and a filtration velocity of 2 cm/s and fulfills the requirements for the use in hot gas cleaning. Complete conversions in the total oxidation of propene with 100% selectivity to CO2 were achieved at the same filtration conditions using a Pt-doped TiO2–V2O5 system. With regard to the intrinsic SCR activity of this catalyst system future work will be focused on combined NOx and VOC removal as well as on an analysis of the corresponding competitive reactions. Acknowledgements The authors gratefully acknowledge the preparative and experimental work by Mrs. M. Knorn from the Universita¨t Karlsruhe in the framework of the project ‘‘Clean Energy Recovery from Biomass Waste and Residues’’ (BioWaRe, project number NNE-2001-00078) funded by the EC. References
Fig. 7. Propene conversion of the catalytic filter elements VCTiK and NVCTiK vs. reaction temperature at a filtration velocity of 2 cm/s.
[1] G. Saracco, V. Specchia, in: A. Cybulsky, J.A. Moulijn (Eds.), Structured Catalysts and Reactors, Marcel Dekker, New York, 1996, pp. 417–434. [2] P. Brook, B. Hagger, J. Wood, in: PTQ 2, Crambeth Allen Publ., Craven Arms, 1997, pp. 25–31. [3] S.R. Ness, G.E. Dunham, G.F. Weber, D.K. Ludlow, Environm. Progr. 14 (1) (1995) 69. [4] G. Saracco, L. Montanaro, Ind. Eng. Chem. Res. 34 (1995) 1471. [5] G. Saracco, S. Specchia, V. Specchia, Chem. Eng. Sci. 51 (1996) 5289. [6] G. Saracco, V. Specchia, Chem. Eng. Sci. 55 (2000) 897.
376
M. Nacken et al. / Applied Catalysis B: Environmental 70 (2007) 370–376
[7] J.H. Choi, S.-K. Kim, Y.-C. Bak, Kor. J. Chem. 18 (2001) 719. [8] C. Sanchez, J. Livage, New J. Chem. 14 (1990) 513. [9] C. Sanchez, J. Livage, M. Henry, F. Babonneau, J. Non-Cryst. Solids 100 (1988) 65. [10] H. Bosch, F. Janssen, Catal. Today 2 (1988) 369. [11] G.C. Bond, S.F. Tahir, Appl. Catal. 71 (1991) 1. [12] B.M. Weckhuysen, D.E. Keller, Catal. Today 78 (2003) 25. [13] L. Lietti, G. Ramis, F. Berti, G. Toledo, D. Robba, G. Busca, P. Forzatti, Catal. Today 42 (1998) 101.
[14] J.P. Chen, R.T. Yang, Appl. Catal. A: Gen. 80 (1992) 135. [15] M.D. Amiridis, I.E. Wachs, G. Deo, J.-M. Jehng, D.S. Kim, J. Catal. 161 (1996) 247. [16] J.A. Dumesic, N.-Y. Topsøe, Y. Chen, T. Slabiak, J. Catal. 163 (1996) 409. [17] E. Finocchio, M. Baldi, G. Busca, C. Pistarino, G. Romezzano, F. Bregani, G.P. Toledo, Catal. Today 59 (2000) 261. [18] M. Hackel, G. Schaub, M. Nacken, S. Heidenreich, Chem. Ing. Techn. 76 (2004) 1823. [19] A. Le´austic, F. Babonneau, J. Livage, Chem. Mater. 1 (1989) 240.
Catalytic activation of ceramic filter elements for combined particle separation, NOx removal and VOC total oxidation Manfred Nacken a,*, Steffen Heidenreich a, Marius Hackel b, Georg Schaub b a
Pall SeitzSchenk Filtersystems GmbH, D-74564 Crailsheim, Germany b Universita¨t Karlsruhe (TH), D-76131 Karlsruhe, Germany Available online 30 June 2006
Abstract The development of a catalytically active filter element for combined particle separation and NOx removal or VOC total oxidation, respectively, is presented. For NOx removal by selective catalytic reduction (SCR) a catalytic coating based on a TiO2–V2O5–WO3 catalyst system was developed on a ceramic filter element. Different TiO2 sols of tailor-made mean particle size between 40 and 190 nm were prepared by the sol–gel process and used for the impregnation of filter element cylinders by the incipient wetness technique. The obtained TiO2-impregnated sintered filter element cylinders exhibit BET surface areas in the range between 0.5 and 1.3 m2/g. Selected TiO2-impregnated filter element cylinders of high BET surface area were catalytically activated by impregnation with a V2O5 and WO3 precursor solution. The obtained catalytic filter element cylinders show high SCR activity leading to 96% NO conversion at 300 8C, a filtration velocity of 2 cm/s and an NO inlet concentration of 500 vol.ppm. The corresponding differential pressures fulfill the requirements for typical hot gas filtration applications. For VOC total oxidation, a TiO2impregnated filter element support was catalytically activated with a Pt/V2O5 system. Complete oxidation of propene with 100% selectivity to CO2 was achieved at 300 8C, a filtration velocity of 2 cm/s and a propene inlet concentration of 300 vol.-ppm. # 2006 Elsevier B.V. All rights reserved. Keywords: Catalytic filter; NOx reduction; VOC oxidation; TiO2–V2O5–WO3
1. Introduction More stringent regulations in terms of dust, NOx and VOC emission levels on one side and the demand for cost effective and energy-efficient gas cleaning technologies on the other side have forced activities to provide novel gas cleaning solutions. In conventional hot gas cleaning of gases from combustion plants [1] and advanced fluid catalytic cracking (FCC) units [2] a multistep gas cleaning procedure is performed for particle and NOx removal. In these cleaning systems particulates were usually separated before the DeNOx catalyst unit consisting of SCR honeycombs. In case of combustion plants an additional desulfurization step is usually performed after particle separation depending on the kind of feedstock used [1,3]. A disadvantage of this two- or three-step cleaning procedure, respectively, is the reheating of the particle-free gas to the required SCR catalyst operating temperature. Therefore, a combination of filtration and catalytic reaction in one unit using * Corresponding author. E-mail address: [email protected] (M. Nacken). 0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2006.02.030
a so-called catalytic filter [1,4–7] would allow using the high energy content of the gas. In this way processing costs as well as investment and maintenance costs are strongly reduced by combination of two units in one unit. By using ceramic hot gas filter elements with a fine filtering outer membrane, in which a catalyst is integrated as catalytic layer (Fig. 1), the latter would be sufficiently protected against particle deposition. This allows dry scrubbing by injected sorbents upsteam of the catalytic filter for removal of SOx and other potential gaseous catalyst poisons like HCl. Moreover, the integrated catalytic layer can be tailored in such a way to allow not only catalytic NOx removal but also catalytic oxidation of volatile organic compounds (VOC). A prerequisite for applying catalytic filter elements is a high catalytic activity combined with an as low as possible differential pressure at the optimum catalyst operating temperature and a filtration velocity typical for hot gas filtration. First results on the preparation of catalytic filters for NOx or VOC removal, respectively, were reported by Saracco et al. [5,6] by using a cylindrical alumina tube with a filter wall thickness of only 1.5 mm.
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Table 1 Mean particle size of the prepared TiO2 sols and a SiO2 sol, measured by photocorrelation spectroscopy
Fig. 1. Structure of a catalytic filter element.
In the present work a ceramic hot gas filter element with a thickness of 10 mm is used to allow a longer residence time in order to provide high catalytic activities at as high as possible superficial velocities. A catalytic activation of ceramic hot gas filter elements by impregnation techniques entails a differential pressure increase by pore confinement. Therefore, a minimum loading of the ceramic filter element support with catalyst allowing a high catalytic activity has to be adjusted. One concept to provide a highly active catalyst surface with a relatively low differential pressure of the ceramic filter element, that is followed in this work, is the formation of a TiO2 nanoparticle layer as catalyst support using a tailor-made TiO2 sol accessible by the sol–gel process. In this process hydrolysis and condensation of a titanium alkoxide is better controlled, if complexing agents like acetylacetone or acetic acid are used [8,9]. Based on the results of several studies on the SCR reaction [10–16] for the catalytic activation of a TiO2-modified ceramic filter element, a V2O5–WO3 catalyst system was selected. For integration of the catalyst in the porous ceramic filter element the incipient wetness technique was applied to allow the highest possible specific surface area and the lowest possible pore confinement by deposition of the catalyst on the pore wall. For the preparation of a catalytically activated filter element for VOC abatement, an Mn2O3-based catalyst was selected, because this system was proven to lead to high conversions and selectivities in the total oxidation of 2-propanol [17]. In addition, the TiO2-modified ceramic filter element was catalytically activated with a Pt/V2O5-based system for comparison. 2. Experimental 2.1. TiO2 sol synthesis Five TiO2 sols of different mean particle size, denoted as Ti, TiAc, TiAcAc, Ti0.5AcAc, and Ti0.25AcAc were synthesized
Sol
Mean particle size (nm)
Solid content (wt.%)
Ti TiAc TiAcAc Ti0.5AcAc Ti0.25AcAc TiK SiK
188.6 142.3 42.5 48.9 48.9 267.9 79.8
8.1 7.9 7.3 5.7 6.2 6.8 3
by the sol–gel process by hydrolysis and condensation reactions starting from titanium isopropoxide (TIP) as TiO2 precursor in 2-propanol. For the synthesis of the sols TiAc and TiAcAc, the equimolar amount of the complexing agents acetic acid or acetylacetone, respectively, was additionally added to the alcoholic solution at ambient temperature, whereas in the case of the synthesis of the sols Ti0.5AcAc and Ti0.25AcAc the half molar or quarter molar amount, respectively, related to the amount of TIP was used. Complete hydrolysis of the titanium alkoxide was performed to form TiO2 in situ. The as-prepared TiO2 sols were characterized in terms of their mean particle size by photocorrelation spectroscopy (Table 1). Additionally, a commercially available TiO2 powder was dispersed in 2-propanol. For comparison, a commercially available 30% SiO2 sol of pH 10 was diluted to 3% in deionisized water. The as-prepared sols, denoted as TiK or SiK, respectively, were also characterized by photocorrelation spectroscopy (Table 1). 2.2. Preparation of catalytically activated filter elements A SiC–Al2O3-based porous ceramic hot gas filter element cylinder of the dimension 60/40 50 mm (outer/inner diameter length) and 50% porosity with a mullite outer membrane was used for catalytic activation. The catalytic activation was performed in two steps. First, the porous filter element cylinder was impregnated by a TiO2 or the SiO2 sol applying the incipient wetness technique. In a second step the as-obtained TiO2 and SiO2-modified porous filter cylinders were impregnated with an aqueous solution of ammonium metavanadate (NH4VO3) and ammonium metatungstate hydrate [(NH4)6W12O39H2O] to adjust a V2O5 and a WO3 loading of 22.5 or 78 wt.%, respectively, after sintering at 500 8C related to a TiO2 and SiO2 loading of 1.4 g on the virgin filter element cylinder. The adjusted TiO2 loadings by using the different TiO2 sols (Table 1) depend on the type and solid content of the used sol and range between 1.7 and 2.2 wt.% for the sol–gel derived TiO2 sols and 1.3 wt.% for the sol TiK related to the virgin filter element cylinder. In case of the SiO2 sol a loading of 1.2 wt.% was adjusted. Catalytically activated filter elements for NOx removal were denoted with NC before the notation of the used TiO2 or SiO2 sol, respectively (e.g. NCTi), whereas TiO2 or SiO2 solimpregnated and sintered filter element cylinders, prepared for the determination of the BET surface area and the differential
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pressure, are denoted with S before the notation of the used sol (e.g. STi). Two TiO2 sol-impregnated filter element cylinders were catalytically activated for VOC abatement. The first sample, denoted as VCTiK, was prepared by using an aqueous solution of Mn(NO3)24H2O instead of the SCR catalyst precursor solution following the same impregnation procedure mentioned above. An Mn2O3 loading of 8 wt.% related to the TiO2 loading was adjusted. The second sample, denoted as NVCTiK, was prepared by using an aqueous solution of NH4VO3 and [(NH3)4PtCl2] to adjust a loading of 22.5 wt.% V2O5 and 2 wt.% Pt related to the TiO2 loading after reduction of the sample for 5 h at 400 8C in a H2/N2 mixture. 2.3. Characterization 2.3.1. Mean particle size The mean particle size of the prepared TiO2 sols and the SiO2 sol were measured as Z-average value with a relative measurement error of 5% using the photocorrelation spectrometer MAL500673 of the company Malvern Instruments. The sols were measured as prepared without dilution. 2.3.2. BET surface area For the determination of the BET surface area of the TiO2and SiO2-impregnated 50 mm long filter element cylinders, an 8 mm cylinder segment was cut out and broken into monolithic pieces, that were dried for 5 h under vacuum before the measurement. The measurements were performed on a Quantachrome Autosorb-3 instrument with N2 at 77 K with a measurement error of 0.03 m2/g.
parts of the cylinder and passing air with a defined volume flow in the range from 0.9 to 2.4 m3/h from the inside through the porous structure to the outside of the cylinder. The resulting flow resistance was measured as differential pressure with a measurement error of 0.2 mbar. 2.4. Measurement of the catalytic activity The catalytic activity of the impregnated filter elements was examined by measuring the conversion of NO and propene, as VOC model compound, in a laboratory test rig (Fig. 2). The test rig is designed for measurements of the reaction kinetics of the SCR reaction and the oxidation reactions of different volatile organic compounds. The measurement conditions are described in more detail in an earlier publication [18]. Model gas containing 500 vol.-ppm NO, 500 vol.-ppm NH3 and 3 vol.% O2 or 300 vol.-ppm propene and 3 vol.% O2 with N2 as carrier gas in both cases was used for the measurement of the catalytic conversion of NO or propene, respectively. The measurements were performed on a segment of the dimension 60/40 20 mm of the catalytically activated filter element cylinders in the temperature range between 100 and 400 8C at a total pressure of 1–1.1 bar by adjusting a constant filtration velocity of 2 cm/s at all temperature points. This corresponds to gas hourly space velocity (GHSV) values from 2000 to 11,500 h1. The NO and propene conversion was calculated from the corresponding inlet and outlet concentrations measured by means of an FTIR analyzer. 3. Results and discussion 3.1. Sol–gel derived TiO2 sols
2.3.3. Differential pressure The TiO2-impregnated and catalytically impregnated filter element cylinders of 50 mm length were examined on their differential pressure at 25 8C by adjusting a defined filtration velocity related to the outer surface of the filter element cylinder. The cylinder was vertically fixed in an appropriate differential pressure test-bench by sealing the front and end
In a series of TiO2 sol synthesis experiments acetylacetone and acetic acid were used as complexing agents to adjust different mean particle sizes in the TiO2 nanoparticle sols and to study the effect of metal alkoxide complexation on the mean TiO2 particle size of the particle sols. Using equimolar amounts of acetylacetone would lead to the formation of a titanium
Fig. 2. Flow scheme of the test rig (from [18]) for the measurement of NO or propene conversion with catalytically activated filter element segments at varying conditions.
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alkoxide complex species (Ti(OR)3AcAc) with an acetylacetonate ligand (AcAc) under release of one alcohol molecule according to Eq. (1) [19]. TiðORÞ4 þ HAcAc ! TiðORÞ3 AcAc þ ROH
(1)
In case of acetate ligands (Ac), an analogous complexation reaction can be formulated [9]. After hydrolysis of the asformed titanium alkoxide complex species Ti(OR)3AcAc and Ti(OR)3Ac the AcAc or Ac ligands, respectively, remain bonded, whereas the OR groups are hydrolyzed. In this way the number of hydrolytically instable OR groups is reduced from four to three by titanium alkoxide complexation, from which a slowing-down of the condensation reactions results to form a colloidal TiO2 sol instead of precipitating TiO2. For the synthesis of the TiO2 sols Ti, Ti0.25AcAc, Ti0.5AcAc and TiAcAc, the molar ratio HAcAc/Ti(OR)4 was varied from 0 to 1. For comparison, a TiO2 sol using acetic acid (HAc) in a molar ratio HAc/Ti(OR)4 = 1 was synthesized. The as-synthesized TiO2 sols were examined by photocorrelation spectroscopy to determine their mean particle size (Fig. 3). A strong decrease of the mean particle size from 189 to 49 nm is found after addition of 0.25 mol acetylacetone per mol titanium alkoxide. A higher amount of this complexing agent up to 1 mol acetylacetone leads only to a slightly decreased mean particle size of 42.5 nm. Compared with this the addition of an equimolar amount of acetic acid to the titanium alkoxide decreases the mean particle size from 189 to only 149 nm. Evidently, the acetylacetonate ligand is more strongly bonded to the Ti atom resulting in a stronger blocking effect in the course of the hydrolysis and condensation reactions in comparison to acetic acid. Moreover, even a relatively low molar amount of 0.25 mol acetylacetone per mole titanium leads to a strong decrease of the mean particle size by 75% of the initial TiO2 particle size. These findings support the aforementioned assumption of the stronger binding of AcAc ligands to the titanium atom. 3.2. TiO2 impregnated filter elements The synthesized TiO2 sols TiAc, TiAcAc, Ti0.5AcAc and Ti were used as prepared for the impregnation of the filter element cylinders. The TiO2-impregnated and sintered cylinders STiAc, STiAcAc, STi0.5AcAc and STi were examined on their BET surface area. In Fig. 4 a correlation of the corresponding BET
Fig. 4. Correlation of the BET surface area of the TiO2-impregnated filter elements STiAcAc, STi0.5AcAc, STiAc and STi with the mean particle size of the used TiO2 sol.
values with the mean particle size of the used TiO2 sols is presented. Fig. 4 shows that the BET surface area depends strongly on the mean particle size of the used TiO2 sols. The highest BET surface area of about 1.3 m2/g is created with a medium mean particle size of about 140 nm under the applied sintering conditions. Smaller particle sizes of about 40 and 50 nm lead to relatively small BET surface areas of about 0.5 or 0.9 m2/g, respectively, whereas a larger mean particle size leads to a lower specific surface of 1.1 m2/g. These results can be explained by the strong dependency of the sintering temperature of the TiO2 nanoparticles on their size. Evidently, the use of nanoparticles in the range of 40–50 nm does not lead to a surface increasing effect, as the small particles are sintered together to form larger particles, probably in the range of sizes higher than 190 nm. A particle size of 140 nm seems to be the optimum particle size to prevent sintering into larger particles and exhibits the largest possible spherical surface for generating the highest BET surface area increase by TiO2 impregnation. The specific surface area of the virgin filter element is 0.03 m2/g, about two orders of magnitude lower. For comparison, one filter element cylinder was impregnated with an SiO2 sol and examined on its specific surface area after sintering. The as-prepared filter element cylinder SSiK exhibits a BET surface area of 1.8 m2/g. The higher specific surface area of SSiK in comparison to the BET value of sample STi is probably due to the smaller mean particle size (Table 1) and the higher sintering temperature of SiO2 nanoparticles in comparison to TiO2 nanoparticles of similar size. 3.3. Catalytic filter elements for NOx removal
Fig. 3. Mean particle size of the TiO2 particle containing sols Ti, TiAcAc, Ti0.25AcAc, Ti0.5AcAc and TiAc as a function of the molar ratio of added acetylacetone (HAcAc) or acetic acid (HAc) per Ti atom.
For the subsequent catalytic activation with the SCR catalyst, the TiO2-impregnated filter element cylinders STi and STiAc are the most suitable samples as they exhibit the largest BET surface area for fixation of the catalytically active species V2O5 and the promoter WO3 in high dispersion. Therefore, these two samples were prepared again and dried without the sintering step, commonly applied for the preparation of STi and STiAc. As after the subsequent catalytic impregnation with the ammonium metavanadate
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and ammonium metatungstate hydrate precursor solution the same sintering program is applied, a similar high specific surface of the catalytic layer should be obtained in this way. For comparison, the SiO2-impregnated filter element cylinder SSiK was also impregnated under comparable conditions to study the effect of the kind of support material on the catalytic activity in the SCR reaction. The catalytic activity of the as-prepared catalytically activated filter element cylinders NCTi, NCTiAc and NCSiK were determined by measuring the NO and NH3 conversion as a function of the reaction temperature (Fig. 5). The activated filter element cylinder NCTi exhibits the highest catalytic activity with an NO conversion of 96% followed by sample NCTiAc with 90% NO conversion, whereas sample NCSiK shows only poor activity with a conversion of 15% at the optimum catalyst operating temperature of 300 8C (Fig. 5(a)). Evidently, the BET surface is not the only activitydetermining parameter, as the TiO2-based catalyst support surface area of sample NCTi is by 0.2 m2/g lower than the BET surface area of sample NCTiAc (Fig. 4). Possibly, minor differences in the distribution of the active catalyst species over the 10 mm thick porous wall of the filter element cylinder are the reason for the measured activity differences. In case of sample NCSiK, a strong decrease in the catalytic activity is found in spite of using the same catalyst support loading and applying the same preparation method. Moreover, the BET surface area of the SiO2-impregnated filter element support (SSiK) is by a factor of 1.6 higher than the
TiO2-impregnated support STi. This should lead to a better dispersion of the SCR catalyst species V2O5 and WO3. Therefore, the substitution of TiO2 by SiO2 can be accounted responsible for the strong activity decrease, indicating the predominant influence of the kind and acidity of the support material on the catalytic activity. The acid sites of TiO2 seem to be strongly involved in the SCR reaction mechanism for NH3 adsorption. This is in line with the proposed SCR mechanism of Lietti et al. [13], where beside tungsten oxide TiO2 is also involved in the ammonia adsorption to provide a sufficient number of NH3 molecules to migrate to the Lewis acidic vanadyl sites for their activation. Moreover, it can be derived from these results, that the support is more involved in NH3 adsorption than tungsten oxide. According to the SCR stoichiometry equal conversion of NO and NH3 was obtained within the temperature range from 180 to 320 8C (Fig. 5(a) and (b)). The small differences between the NO and NH3 conversion values of the three examined catalyst systems at 180 8C are ascribed to a higher experimental error of 7% at lower conversions of NO and NH3 than the usual error of up to 5% for the corresponding conversions at higher reaction temperatures. The small differences between the NO and NH3 conversion values at 320 8C are due to the oxidation of small amounts of ammonia. The oxidation products of ammonia are NO, that is indicated by the slight decrease of the NO conversions above 300 8C in the three examined catalyst systems (Fig. 5(a)), and N2O. N2O is only formed in small amounts ranging from 12 vol.-ppm in the system NCSiK up to 17 vol.-ppm in the system NCTi at 320 8C. NO2 formation was not observed. 3.4. Differential pressure of the catalytic filter elements
Fig. 5. NO (a) and NH3 conversion (b) of the catalytically activated filter element cylinders NCTi, NCTiAc and NCSiK as a function of the reaction temperature at a filtration velocity of 2 cm/s.
A prerequisite for the application of catalytically activated filter elements in hot gas filtration is a relatively low differential pressure value under typical hot gas filtration velocities ranging from 2 to 4 cm/s. The measured differential pressures of the prepared catalytically activated filter element cylinders and the corresponding virgin ceramic support are plotted versus the filtration velocity at 25 8C (Fig. 6). It is shown, that the TiO2-based catalytic filter element cylinders NCTi and NCTiAc exhibit higher differential pressure values than the SiO2-based catalytic filter element sample after the two-step impregnation procedure. This is due to the larger mean particle size of the TiO2 particles in comparison to the SiO2 particles deposited on the pore wall (Table 1). Practically, no difference in the differential pressure is found if the mean TiO2 particle size differs only by about 50 nm. An extrapolation of the differential pressure lines of the samples NCTi, NCTiAc and NCSiK to a filtration velocity of 2 cm/s and calculation of the obtained values to 300 8C using Darcy’s equation results in differential pressure values of 21, 20.5 or 15 mbar, respectively. As the typical differential pressure limits in hot gas filtration applications are in the range of about 50–100 mbar, the catalytically most active filter
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Fig. 7) range from 93.6% to 98.7% for sample VCTiK. In case of sample NVCTiK complete oxidation of propene to CO2 is found at all temperatures. The superior activity of sample NVCTiK is due to the doping with Pt as also a TiO2-supported Pt-based catalyst system was found to be catalytically active in the total oxidation of propene [6]. The relatively high selectivity values of sample VCTiK indicate the performance of manganese oxide in the total oxidation of propene. Possibly, a higher amount or better dispersion of this catalytically active compound is necessary to increase the propene conversions, and this has to be examined in future work. Fig. 6. Differential pressure of NCTi, NCTiAc and NCSiK and the corresponding uncoated support materials as a function of the filtration velocity at 25 8C.
element NCTi would fulfill the criteria for its economic use in hot gas filtration. 3.5. Catalytic filter elements for VOC removal In special applications of hot gas cleaning, combined removal of particles and volatile organic compounds (VOC) is required to meet the stringent emission levels. Therefore, two catalytic ceramic filter element samples were prepared using in sample VCTiK a manganese oxide-based and in sample NVCTiK a Pt-doped V2O5-based catalyst system for the total oxidation of VOC. The catalytic activity of these two samples was determined by measuring the conversion of propene as a function of the reaction temperature at a filtration velocity of 2 cm/s (Fig. 7). A comparison of the propene conversion values of the samples VCTiK and NVCTiK shows that the Pt-doped V2O5based system NVCTiK shows independently of the reaction temperature in the temperature range from 260 to 320 8C constant conversion values of 95%, whereas the corresponding conversions of the manganese oxide based system VCTiK are relatively low and increase from about 20% at 260 to 55% at 320 8C. The corresponding selectivities to CO2 (not shown in
4. Conclusions Different TiO2 sols of a mean particle size between 40 and 190 nm were synthesized by the sol–gel process by varying the concentration and type of the complexing agent. The highest BET surface area of 1.3 m2/g in the series of TiO2 solimpregnated and sintered ceramic filter element samples was obtained by using a TiO2 sol of a medium mean particle size of about 140 nm. After catalytic activation of the TiO2-impregnated ceramic filter elements of high BET surface areas, maximum NO conversion of up to 96% at 300 8C, a filtration velocity of 2 cm/s and an NO inlet concentration of 500 vol.-ppm were achieved. The catalytic activity is strongly decreased by using SiO2 instead of TiO2 as support material. The catalytically most active filter element for NOx removal shows a differential pressure of 21 mbar at 300 8C and a filtration velocity of 2 cm/s and fulfills the requirements for the use in hot gas cleaning. Complete conversions in the total oxidation of propene with 100% selectivity to CO2 were achieved at the same filtration conditions using a Pt-doped TiO2–V2O5 system. With regard to the intrinsic SCR activity of this catalyst system future work will be focused on combined NOx and VOC removal as well as on an analysis of the corresponding competitive reactions. Acknowledgements The authors gratefully acknowledge the preparative and experimental work by Mrs. M. Knorn from the Universita¨t Karlsruhe in the framework of the project ‘‘Clean Energy Recovery from Biomass Waste and Residues’’ (BioWaRe, project number NNE-2001-00078) funded by the EC. References
Fig. 7. Propene conversion of the catalytic filter elements VCTiK and NVCTiK vs. reaction temperature at a filtration velocity of 2 cm/s.
[1] G. Saracco, V. Specchia, in: A. Cybulsky, J.A. Moulijn (Eds.), Structured Catalysts and Reactors, Marcel Dekker, New York, 1996, pp. 417–434. [2] P. Brook, B. Hagger, J. Wood, in: PTQ 2, Crambeth Allen Publ., Craven Arms, 1997, pp. 25–31. [3] S.R. Ness, G.E. Dunham, G.F. Weber, D.K. Ludlow, Environm. Progr. 14 (1) (1995) 69. [4] G. Saracco, L. Montanaro, Ind. Eng. Chem. Res. 34 (1995) 1471. [5] G. Saracco, S. Specchia, V. Specchia, Chem. Eng. Sci. 51 (1996) 5289. [6] G. Saracco, V. Specchia, Chem. Eng. Sci. 55 (2000) 897.
376
M. Nacken et al. / Applied Catalysis B: Environmental 70 (2007) 370–376
[7] J.H. Choi, S.-K. Kim, Y.-C. Bak, Kor. J. Chem. 18 (2001) 719. [8] C. Sanchez, J. Livage, New J. Chem. 14 (1990) 513. [9] C. Sanchez, J. Livage, M. Henry, F. Babonneau, J. Non-Cryst. Solids 100 (1988) 65. [10] H. Bosch, F. Janssen, Catal. Today 2 (1988) 369. [11] G.C. Bond, S.F. Tahir, Appl. Catal. 71 (1991) 1. [12] B.M. Weckhuysen, D.E. Keller, Catal. Today 78 (2003) 25. [13] L. Lietti, G. Ramis, F. Berti, G. Toledo, D. Robba, G. Busca, P. Forzatti, Catal. Today 42 (1998) 101.
[14] J.P. Chen, R.T. Yang, Appl. Catal. A: Gen. 80 (1992) 135. [15] M.D. Amiridis, I.E. Wachs, G. Deo, J.-M. Jehng, D.S. Kim, J. Catal. 161 (1996) 247. [16] J.A. Dumesic, N.-Y. Topsøe, Y. Chen, T. Slabiak, J. Catal. 163 (1996) 409. [17] E. Finocchio, M. Baldi, G. Busca, C. Pistarino, G. Romezzano, F. Bregani, G.P. Toledo, Catal. Today 59 (2000) 261. [18] M. Hackel, G. Schaub, M. Nacken, S. Heidenreich, Chem. Ing. Techn. 76 (2004) 1823. [19] A. Le´austic, F. Babonneau, J. Livage, Chem. Mater. 1 (1989) 240.