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Monolithic catalysts — non-uniform active phase distribution by impregnation
Applied Catalysis A: General 213 (2001) 179–187
Monolithic catalysts — non-uniform active phase distribution by impregnation Theo Vergunst1 , Freek Kapteijn∗ , Jacob A. Moulijn Industrial Catalysis, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands Received 20 July 2000; received in revised form 23 November 2000; accepted 25 November 2000
Abstract During preparation of monolithic catalysts via impregnation of monolithic structures, an inhomogeneous active phase distribution can evolve on the scale of the whole monolithic structure. This is demonstrated by the Ni/Al2 O3 /cordierite system, investigated as a model system. Macroscopic redistribution of the active phase precursor occurred during drying of the monolithic structure after impregnation due to capillary suction, resulting in an accumulation of the active phase in an outer shell of the monolithic structure. Other solvent removal techniques like freeze-drying or microwave drying, or preparation techniques like deposition–precipitation can prevent this accumulation by immobilization of the precursor phase, resulting in a uniform active phase distribution throughout the monolithic catalyst. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Impregnation; Monolithic catalyst; Metal distribution; Drying; Microwave drying; Freeze-drying; Deposition–precipitation
1. Introduction In general, a monolithic catalyst comprises a honeycomb-like ceramic structure, covered by a thin (<50 m) layer of a porous support material (washcoat) in which a catalytically active component is present. Articles dealing with monolithic catalysts in general and their coating can be found elsewhere [1–5]. The application of an active phase to a catalyst support material is often the final step in the preparation of a catalyst [6,7]. Also for monolithic catalysts, the support or washcoat is usually loaded with an active component after its application to the ceramic structure. It can occur that the active phase ∗ Corresponding author. E-mail address: [email protected] (F. Kapteijn). 1 Present address: Avantium Technologies, P.O. Box 2915, 1000 CX Amsterdam, The Netherlands.
is located in an outer shell of the monolithic structure. Fig. 1 shows a Pt/cordierite catalyst [8] and a Ni/Al2 O3 /cordierite catalyst [4] with a non-uniform active phase distribution. A similar phenomenon was observed by Wahlberg et al. [9] for copper on alumina and titania monoliths, illustrating the generality of the shown observations. For monolithic catalysts, the active phase loading, dispersion, and distribution are very important. Due to the lower catalyst content of a monolithic structure per unit reactor volume compared to a packed bed reactor, the active phase should be highly active. This can be obtained by a high loading, a high dispersion, and a uniform active phase distribution over the geometric surface area of the support. As shown in Fig. 1, the metal distribution on the scale of the whole monolith is not always uniform. Therefore, a study has been carried out to determine preparation conditions that ensure a uniform active phase distribution, and to explain the non-uniform metal distribution.
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Fig. 1. Active phase distribution obtained after preparing a Pt/cordierite (left [8]) and a Ni/Al2 O3 /cordierite (right [4]) catalyst by impregnation. Samples were cut in two after calcination. One half shows the exterior, the other half shows the inner surface. The metal concentration is proportional to the grayscale.
The system Ni/Al2 O3 /cordierite has been chosen because of the clearly visible active phase distribution. The effects of concentration of nickel in the impregnation solution, the drying method, and the suitability to apply deposition–precipitation were studied in a qualitative manner. The prepared catalysts were characterized with respect to metal distribution. 2. Catalyst preparation Preparation of catalysts, starting from pre-shaped support particles is quite common in catalyst manufacture. Generally, three steps can be distinguished: application of the active phase, drying of the support, and activation of the precursor. The first two steps are of importance here and will be discussed. 2.1. Impregnation methods Several methods are available to apply active phase precursors to pre-shaped support particles, such as impregnation methods, ion-exchange, and (deposition)–precipitation. Focussing on impregnation methods, two methods are distinguished, pore
volume or dry impregnation (incipient wetness), and wet impregnation. In pore volume impregnation, a liquid volume equal to the pore volume of the catalyst support is used, for wet impregnation excess liquid is used. Alternative methods to apply a component from a solution to a support material are the so-called deposition–precipitation method, based on the formation of an insoluble catalyst precursor complex [10], or ion-exchange with surface groups of the support to complex active phase precursors [7]. Deposition–precipitation can be performed as follows. A soluble metal and a precipitating agent are present in the impregnation solution together with the monolithic support. For instance, urea can be used as precipitating agent for nickel deposition. By heating this mixture to sufficiently high temperature (>333 K), the pH increases homogeneously throughout the solution, and the following reactions take place [10]: H2 N–CO–NH2 (aq) + 3H2 O +
T >333 K
→
CO2 (g)
−
+2NH4 (aq) + 2OH (aq) Ni2+ (aq) + 2OH− (aq) → Ni(OH)2 (s)
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In the presence of a support material, nickel hydroxide will deposit onto this support from the solution. Although ion-exchange phenomena can also occur during impregnation [11], a clear difference is that during impregnation, the major part of the active phase precursor remains dissolved in the impregnation fluid. 2.2. Drying of supports The impregnation step is usually followed by a drying step to remove the solvent from the catalyst
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support. The principle of drying a porous support can be illustrated by the drying of sand beds, which show a comparable structure to porous support particles [12–14]. Drying is a combined mass and heat transfer problem, in which the evaporation and heat transfer rate are coupled by the heat of evaporation. The processes happening during drying partially depend on the way the support is contacted with liquid. Fig. 2 illustrates the drying process schematically. When prior to contacting with liquid, the support body is evacuated (Fig. 2a–c), liquid is present
Fig. 2. Drying of porous support particles happens depending on the nature and pretreatment of the particle. Drying of a particle, which has been evacuated prior to impregnation (a–c): large pores are emptied in favor of small pores. Drying of a particle with air included during impregnation (d–f): the occluded air expands during drying pushing liquid outward. Drying of a monolithic structures (g–i): both mechanisms occur, although inclusion of air is limited and liquid is only pushed out in radial direction.
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throughout the porous network. Initially, liquid is evaporated from the exterior of the support. The amount of liquid in both large and small pores at the exterior of the support reduces, and menisci at the pore mouths develop. Because of the larger capillary force in small pores due to the larger curvature of their menisci, these pores attract liquid from larger pores located at the interior of the catalyst particle, which will progressively be emptied. This situation is maintained as long as the exterior of the particle is connected to the interior via a continuous liquid film. This is characterized by a constant drying rate. When this connectivity is lost, wet regions become isolated and evaporation occurs from the interior of the particle, characterized by a decreasing drying rate. If the support particle prior to impregnation has not been evacuated (Fig. 2d–f), it is likely that there is gas trapped under pressure inside the particle, compressed by capillary forces [15]. In addition to the processes described above, the occluded gas is heated as well by the surrounding air during drying and expands, pushing the impregnation solution outward. Drying causes therefore an outward flow of liquid from the interior of the particle to the exterior. In case of a weak interaction between catalyst precursor and the catalyst support material, this outward flow of liquid transports the catalyst precursor to the exterior of the support. This leads to an accumulation of the active phase in the outer region of the catalyst support particle [16]. 2.3. Monolithic catalysts Monolithic structures differ in two ways from commonly used catalyst supports. Firstly, in the ‘particle size’ (characteristic length scales) and secondly, in the void fraction of the particle. The most frequently used monolithic supports consist of a monolithic structure made of a low surface area, macroporous inert material, e.g. cordierite, with a thin layer of a high surface area catalyst support coated to the wall (so-called washcoat). Monolithic structures show two characteristic length scales (Fig. 3): the coating thickness, which is usually ∼50 m, and the channel length, which can range up to 0.50 m. The phenomena occurring during impregnation of monolithic structures do not differ fundamentally from those occurring dur-
Fig. 3. Characteristic lengths of a monolithic channel in axial (=channel length) and in radial direction (coating/wall thickness).
ing that of commonly applied catalyst support particles. They can, however, take place on a much larger length scale. Profiling in axial and radial direction of the whole monolithic structure is undesired because it causes a lower utilization of the monolithic structure. Profiling of active phase material perpendicular to the coating layer, however, may have similar advantages as for common catalyst particles in certain applications [17–19]. The high void fraction of monolithic structures changes the way in which the pores are filled when contacted with liquid. When a washcoated monolithic structure is contacted with a liquid, the liquid is allowed to penetrate all pores in the structure, pushing all gas out. In contrast with commonly applied catalyst support particles, almost all gas present in the pores can escape from the structure and is not entrapped (Fig. 2g). It is noted that the impregnation liquid is not only present in the pores of the washcoat, but also in the pores of ceramic backbone of the monolithic structure.
3. Experimental 3.1. Materials Alumina washcoated monolithic supports with a cell density of 400 cpsi (cells per square inch ∼ = 62 cells per cm2 ) were obtained from Degussa AG (Hanau, Germany). Structures of ∅ = 20 mm and L = 40 mm, containing 100 g/l alumina (19.6 wt.%) were used for studying concentration profile development. Ni(NO3 )2 ·6H2 O (Merck, >99% [13478-00-6])
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and urea (J.T. Baker, >99% [57-13-6]) were used for the application of nickel.
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4. Results 4.1. Nickel concentration
3.2. Equipment For applying the active phase precursor to the washcoated monoliths common labware was used. For drying of the structures use was made of an oven at 363 K (=static drying), flowing air at room temperature, a Whirlpool Procombi AVM 840 microwave operated at 200 W, and a Modulyo Freeze Dryer operated at 223 K and 10 Pa. 3.3. Experimental procedures Impregnation was performed using a 0.5 M Ni(NO3 )2 ·6H2 O solution. After contacting the monolithic structure with the solution for 1 h, it was withdrawn and the channels were cleared by removing excess liquid with air. Samples were dried using either stationary air in an oven at 363 K, flowing air at room temperature, a microwave oven or a freeze dryer. All samples were calcined in air, using a heating rate of 10 K/min up to 723 K and maintaining this temperature for 2 h. To study the effect of the nickel concentration in the solution, catalysts have been impregnated using 0.2, 0.5, 1.0, and 2.0 M Ni(NO3 )2 ·6H2 O solutions. Samples were dried in stationary air as described above. Deposition precipitation was performed using a 0.2 M Ni(NO3 )2 ·6H2 O and a 2.0 M urea solution. The monolithic structure was placed in a tightly fitting glass cylinder and heated to 353 K with a heating rate of 1 K/min and maintaining this temperature for 12 h. The sample was then carefully washed and dried (static drying), followed by calcination as described previously. 3.4. Catalyst characterization The impregnation profiles were visually characterized after cutting the catalyst samples in two halves in axial direction. The average metal oxide content was estimated from the weight increase of the structures after calcination. Although weighing is not an accurate method, it is sufficient for the performed study.
Fig. 4 shows schematically the effect of concentration in the impregnation solution on the development of impregnation profiles using static drying of the monolithic structures. Using a 0.2 M Ni(NO3 )2 ·6H2 O solution yields a very thin layer of nickel near the exterior of the monolith. With increasing nickel concentration in the impregnation solution, the region in which nickel accumulates increases. With the highest nickel concentration (2.0 M Ni(NO3 )2 ·6H2 O) an almost uniform distribution is obtained for this size of monolith. The nickel content of the catalysts, expressed as NiO content was 0.29, 0.74, 1.42, and 3.16 wt.% respectively, after impregnation with a 0.2, 0.5, 1.0, and 2.0 M Ni(NO3 )2 ·6H2 O solution. The light parts of the monolithic structures also contain a small amount of nickel, deposited by means of ion-exchange with the alumina surface groups, e.g. [7,11]. 4.2. Effect of drying method The solvent removal step (drying) is a crucial step during the preparation of monolithic catalysts. Several methods have been investigated in order to prevent the redistribution of active phase precursors. Fig. 5 shows a photograph and a schematical representation of the results of the various drying methods. Static drying yields an eggshell distribution under the conditions investigated. Drying of monolithic structures with forced airflow through the channels accumulates the active phase at the inlet plane of the airflow into the monolith. Microwave drying yields a more uniform active phase distribution. Freeze-drying of monolithic structures for various times (1 and 24 h) shows different results (Fig. 5). At 1 h (followed by further static drying), a kind of egg white distribution is obtained. Outside the egg white distribution also a homogeneously distributed active phase is present, within the “egg white” layer, almost no active phase is present. Freeze-drying for 24 h yields a uniform active phase distribution. 4.3. Deposition–precipitation Fig. 5 also shows schematically a monolithic structure impregnated with nickel by means of deposi-
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Fig. 4. Schematic representation of the cross section of monolithic catalysts showing the effect of nickel concentration in the aqueous impregnation solution on the development of impregnation profiles, from left to right: 0.2, 0.5, 1.0, and 2.0 M Ni(NO3 )2 ·6H2 O, respectively. The nickel concentration is proportional to the grayscale.
tion precipitation. The catalyst shows a uniform active phase distribution. A loading of 2.0 wt.% was achieved.
5. Discussion Although monolithic catalysts belong to the most frequently applied type of catalyst [2,20], hardly any information in the open literature is available dealing with impregnation of these structures [9,17,21]. The
monoliths shown in Fig. 1 show that impregnation can result in a non-uniform active phase distribution, leading to a low utilization of the monolithic structure. A qualitative study has been undertaken to explain the development of non-uniform active phase distribution in monolithic catalysts. Although not stressed explicitly, the results presented in the previous section have been found highly reproducible. The effect of the drying conditions itself (temperature, humidity) have not been addressed in this study. Though these conditions have an effect on the final
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Fig. 5. (A) Photograph of the cross section of monolithic catalysts showing the impregnation profiles obtained after static drying (left), freeze-drying for 1 h, followed by static drying (center), and freeze-drying for 24 h (right). (B) Schematic representation of the cross section of monolithic catalysts showing the impregnation profiles obtained after static drying, forced flow drying, microwave drying, freeze-drying (1 h) followed by static drying, freeze-drying for 24 h, and deposition precipitation. The nickel concentration is indicated by the grayscale.
active phase distribution, they do not eliminate this redistribution. The result obtained with drying by forced convection through the monolithic channels proves that nickel migrates towards the surface where evaporation occurs. At the plane where the dry inlet air contacts the wet monolith, the driving force for evaporation is largest and the highest drying rates will occur. Indeed, nickel accumulates at this inlet plane
(Fig. 5). Drying of monolithic structures under differential conditions with wet air has been suggested. However, drying under such differential conditions requires a high gas flow rate to minimize the solvent vapor pressure increase of the gas during the contact between gas and wet monolith. These high flow rates enhance mass transfer and thus evaporation, making it even more difficult to maintain differential conditions.
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Three methods have been investigated to prevent migration of nickel during the drying stage: (i) formation of an insoluble complex by means of deposition precipitation; (ii) heat supply throughout the structure, so that evaporation does not only occur at the exterior (microwave drying) but also in the interior and (iii) preventing the liquid from flowing to the exterior of the structure (freeze-drying). All three methods yield a more uniform metal distribution (Fig. 5). The obtained nickel profile obtained after 1 h freeze-drying followed by static drying (see Fig. 5) shows that freeze-drying also starts at the exterior of the monolithic structure. The drying front migrates to the interior of the monolith with time. If the freeze-drying is stopped too early and is switched to the static drying procedure, a new evaporation front arises, which attracts the liquid from inside this front, shown as the egg white distribution. Microwave drying resulted in a homogeneous metal distribution throughout the structure. During microwave drying, which is much faster than static drying and freeze-drying, care should be taken that the evaporated liquid is removed from the microwave oven quickly. Deposition–precipitation yielded uniform nickel profiles, but has the disadvantage that it cannot be applied with every metal. Fortunately, also other methods than pH increase can be performed to deposit a metal on a surface [10]. Ion-exchange is also a potential alternative but its feasibility also depends on the precursor and support under investigation. Both freeze-drying and microwave drying are not frequently applied methods in catalyst preparation. Microwave drying was applied in catalyst preparation of nickel on alumina catalysts [22,23]. Ours confirm those results by using microwave drying a more homogeneous metal distribution is obtained compared to using static drying. Microwave drying has also been applied previously in the preparation of a monolithic manganese oxide based hydrogen peroxide decomposition catalyst [24]. Freeze-drying was used before for drying clays after pillaring [25], but not for modifying impregnation profiles. Both methods seem, however, suitable for monolithic catalyst preparation, since they are well-established methods in large applications for drying, e.g. in food industry. Static drying of monolithic structures can be described as follows (Fig. 2g–i). After placing the wet structure in a hot surrounding, liquid starts to evap-
orate from the exterior of the monolith. Both from small and large pores liquid evaporates. Because of the higher capillary force, the small pores are refilled with liquid from larger pores present in the interior of the monolithic structure. The macropores of the structure will be emptied first because they are much larger than the pores in the washcoat. This process continues as long as a continuous liquid network exists (funicular state) throughout the monolithic structure. Because a monolith is a large single piece of material; this transport process can take place over a long distance, as evidenced by our observations. As soon as this liquid network is broken-up (pendicular state), the evaporation front progressively moves inward to the center of the monolithic structure until complete dryness. The isolated liquid pockets that exist in the pendicular state can only be emptied by evaporation and not by convection. The effect of the nickel concentration in the impregnation liquid can be explained as follows. Transport of nickel nitrate to the exterior of the structure occurs until saturation is reached. When the amount of liquid is decreased enough, the nickel nitrate will deposit on the support. With higher nickel concentration the region with saturated liquid is larger as can be seen from Fig. 4. At this point, it can be remarked that monolithic catalysts are basically not very different from other pre-shaped catalyst support particles. However, the transport of precursor solution towards the exterior of monolithic structures during drying by capillary forces occurs on a much larger scale than for commonly applied support particles (decimeters instead of millimeters). Furthermore, because of the presence of two characteristic length scales and the high void fraction of monolithic structures, the impregnation process occurs as if the monolithic structure had been evacuated prior to impregnation. Therefore, the effect described by Lee and Aris [15] of occluded gas pushing out liquid from support particles during drying may only occur locally in the washcoat perpendicular to this layer. This theory can therefore not be used to explain the macroscopic inhomogeneous metal distribution of Ni/Al2 O3 /cordierite monolith catalysts. It is noted that by impregnation, both the porous washcoat and the macroporous ceramic monolith structure are filled with impregnation liquid. Due to the macroporosity of the ceramic monolith structure, this will
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be emptied first upon drying, so that almost all precursor will end up in the washcoat layer. So, the effect of capillary forces ensures here deposition of all precursor onto the catalyst support material (washcoat).
6. Conclusions When preparing monolithic catalysts, care should be taken that a uniform distribution of active phase material is obtained. Due to capillary force differences during drying a redistribution of the active phase precursor can occur on the scale of monolithic dimensions, resulting in accumulation of the active phase in an outer shell of the monolith structure. Three methods have successfully been applied to prevent the development of an inhomogeneous metal distribution. 1. Formation of an insoluble catalyst precursor, by means of deposition precipitation. 2. Evaporation of the liquid throughout the structure, by means of microwave heating. 3. Preventing the liquid from flowing, by means of freeze-drying. Preparing monolithic catalysts with a uniform active phase distribution by means of impregnation requires that the active phase precursor is not able to move to the exterior of the structure after it has been applied. Based on the presented work, recommended methods are freeze-drying, microwave drying and deposition–precipitation, although any other method that applies an active phase without the chance on redistribution during the preparation procedure, e.g. ion-exchange, should in principle be adequate, too.
Acknowledgements The IOP Catalysis (Dutch Ministry of Economic Affairs) is acknowledged for the financial support of this research (project IKA94061). Degussa AG (Hanau, Germany) is acknowledged for the supply of alumina washcoated monolithic structures. William Anker is acknowledged for the experimental work.
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Monolithic catalysts — non-uniform active phase distribution by impregnation Theo Vergunst1 , Freek Kapteijn∗ , Jacob A. Moulijn Industrial Catalysis, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands Received 20 July 2000; received in revised form 23 November 2000; accepted 25 November 2000
Abstract During preparation of monolithic catalysts via impregnation of monolithic structures, an inhomogeneous active phase distribution can evolve on the scale of the whole monolithic structure. This is demonstrated by the Ni/Al2 O3 /cordierite system, investigated as a model system. Macroscopic redistribution of the active phase precursor occurred during drying of the monolithic structure after impregnation due to capillary suction, resulting in an accumulation of the active phase in an outer shell of the monolithic structure. Other solvent removal techniques like freeze-drying or microwave drying, or preparation techniques like deposition–precipitation can prevent this accumulation by immobilization of the precursor phase, resulting in a uniform active phase distribution throughout the monolithic catalyst. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Impregnation; Monolithic catalyst; Metal distribution; Drying; Microwave drying; Freeze-drying; Deposition–precipitation
1. Introduction In general, a monolithic catalyst comprises a honeycomb-like ceramic structure, covered by a thin (<50 m) layer of a porous support material (washcoat) in which a catalytically active component is present. Articles dealing with monolithic catalysts in general and their coating can be found elsewhere [1–5]. The application of an active phase to a catalyst support material is often the final step in the preparation of a catalyst [6,7]. Also for monolithic catalysts, the support or washcoat is usually loaded with an active component after its application to the ceramic structure. It can occur that the active phase ∗ Corresponding author. E-mail address: [email protected] (F. Kapteijn). 1 Present address: Avantium Technologies, P.O. Box 2915, 1000 CX Amsterdam, The Netherlands.
is located in an outer shell of the monolithic structure. Fig. 1 shows a Pt/cordierite catalyst [8] and a Ni/Al2 O3 /cordierite catalyst [4] with a non-uniform active phase distribution. A similar phenomenon was observed by Wahlberg et al. [9] for copper on alumina and titania monoliths, illustrating the generality of the shown observations. For monolithic catalysts, the active phase loading, dispersion, and distribution are very important. Due to the lower catalyst content of a monolithic structure per unit reactor volume compared to a packed bed reactor, the active phase should be highly active. This can be obtained by a high loading, a high dispersion, and a uniform active phase distribution over the geometric surface area of the support. As shown in Fig. 1, the metal distribution on the scale of the whole monolith is not always uniform. Therefore, a study has been carried out to determine preparation conditions that ensure a uniform active phase distribution, and to explain the non-uniform metal distribution.
0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 0 ) 0 0 8 9 6 - 6
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Fig. 1. Active phase distribution obtained after preparing a Pt/cordierite (left [8]) and a Ni/Al2 O3 /cordierite (right [4]) catalyst by impregnation. Samples were cut in two after calcination. One half shows the exterior, the other half shows the inner surface. The metal concentration is proportional to the grayscale.
The system Ni/Al2 O3 /cordierite has been chosen because of the clearly visible active phase distribution. The effects of concentration of nickel in the impregnation solution, the drying method, and the suitability to apply deposition–precipitation were studied in a qualitative manner. The prepared catalysts were characterized with respect to metal distribution. 2. Catalyst preparation Preparation of catalysts, starting from pre-shaped support particles is quite common in catalyst manufacture. Generally, three steps can be distinguished: application of the active phase, drying of the support, and activation of the precursor. The first two steps are of importance here and will be discussed. 2.1. Impregnation methods Several methods are available to apply active phase precursors to pre-shaped support particles, such as impregnation methods, ion-exchange, and (deposition)–precipitation. Focussing on impregnation methods, two methods are distinguished, pore
volume or dry impregnation (incipient wetness), and wet impregnation. In pore volume impregnation, a liquid volume equal to the pore volume of the catalyst support is used, for wet impregnation excess liquid is used. Alternative methods to apply a component from a solution to a support material are the so-called deposition–precipitation method, based on the formation of an insoluble catalyst precursor complex [10], or ion-exchange with surface groups of the support to complex active phase precursors [7]. Deposition–precipitation can be performed as follows. A soluble metal and a precipitating agent are present in the impregnation solution together with the monolithic support. For instance, urea can be used as precipitating agent for nickel deposition. By heating this mixture to sufficiently high temperature (>333 K), the pH increases homogeneously throughout the solution, and the following reactions take place [10]: H2 N–CO–NH2 (aq) + 3H2 O +
T >333 K
→
CO2 (g)
−
+2NH4 (aq) + 2OH (aq) Ni2+ (aq) + 2OH− (aq) → Ni(OH)2 (s)
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In the presence of a support material, nickel hydroxide will deposit onto this support from the solution. Although ion-exchange phenomena can also occur during impregnation [11], a clear difference is that during impregnation, the major part of the active phase precursor remains dissolved in the impregnation fluid. 2.2. Drying of supports The impregnation step is usually followed by a drying step to remove the solvent from the catalyst
181
support. The principle of drying a porous support can be illustrated by the drying of sand beds, which show a comparable structure to porous support particles [12–14]. Drying is a combined mass and heat transfer problem, in which the evaporation and heat transfer rate are coupled by the heat of evaporation. The processes happening during drying partially depend on the way the support is contacted with liquid. Fig. 2 illustrates the drying process schematically. When prior to contacting with liquid, the support body is evacuated (Fig. 2a–c), liquid is present
Fig. 2. Drying of porous support particles happens depending on the nature and pretreatment of the particle. Drying of a particle, which has been evacuated prior to impregnation (a–c): large pores are emptied in favor of small pores. Drying of a particle with air included during impregnation (d–f): the occluded air expands during drying pushing liquid outward. Drying of a monolithic structures (g–i): both mechanisms occur, although inclusion of air is limited and liquid is only pushed out in radial direction.
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throughout the porous network. Initially, liquid is evaporated from the exterior of the support. The amount of liquid in both large and small pores at the exterior of the support reduces, and menisci at the pore mouths develop. Because of the larger capillary force in small pores due to the larger curvature of their menisci, these pores attract liquid from larger pores located at the interior of the catalyst particle, which will progressively be emptied. This situation is maintained as long as the exterior of the particle is connected to the interior via a continuous liquid film. This is characterized by a constant drying rate. When this connectivity is lost, wet regions become isolated and evaporation occurs from the interior of the particle, characterized by a decreasing drying rate. If the support particle prior to impregnation has not been evacuated (Fig. 2d–f), it is likely that there is gas trapped under pressure inside the particle, compressed by capillary forces [15]. In addition to the processes described above, the occluded gas is heated as well by the surrounding air during drying and expands, pushing the impregnation solution outward. Drying causes therefore an outward flow of liquid from the interior of the particle to the exterior. In case of a weak interaction between catalyst precursor and the catalyst support material, this outward flow of liquid transports the catalyst precursor to the exterior of the support. This leads to an accumulation of the active phase in the outer region of the catalyst support particle [16]. 2.3. Monolithic catalysts Monolithic structures differ in two ways from commonly used catalyst supports. Firstly, in the ‘particle size’ (characteristic length scales) and secondly, in the void fraction of the particle. The most frequently used monolithic supports consist of a monolithic structure made of a low surface area, macroporous inert material, e.g. cordierite, with a thin layer of a high surface area catalyst support coated to the wall (so-called washcoat). Monolithic structures show two characteristic length scales (Fig. 3): the coating thickness, which is usually ∼50 m, and the channel length, which can range up to 0.50 m. The phenomena occurring during impregnation of monolithic structures do not differ fundamentally from those occurring dur-
Fig. 3. Characteristic lengths of a monolithic channel in axial (=channel length) and in radial direction (coating/wall thickness).
ing that of commonly applied catalyst support particles. They can, however, take place on a much larger length scale. Profiling in axial and radial direction of the whole monolithic structure is undesired because it causes a lower utilization of the monolithic structure. Profiling of active phase material perpendicular to the coating layer, however, may have similar advantages as for common catalyst particles in certain applications [17–19]. The high void fraction of monolithic structures changes the way in which the pores are filled when contacted with liquid. When a washcoated monolithic structure is contacted with a liquid, the liquid is allowed to penetrate all pores in the structure, pushing all gas out. In contrast with commonly applied catalyst support particles, almost all gas present in the pores can escape from the structure and is not entrapped (Fig. 2g). It is noted that the impregnation liquid is not only present in the pores of the washcoat, but also in the pores of ceramic backbone of the monolithic structure.
3. Experimental 3.1. Materials Alumina washcoated monolithic supports with a cell density of 400 cpsi (cells per square inch ∼ = 62 cells per cm2 ) were obtained from Degussa AG (Hanau, Germany). Structures of ∅ = 20 mm and L = 40 mm, containing 100 g/l alumina (19.6 wt.%) were used for studying concentration profile development. Ni(NO3 )2 ·6H2 O (Merck, >99% [13478-00-6])
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and urea (J.T. Baker, >99% [57-13-6]) were used for the application of nickel.
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4. Results 4.1. Nickel concentration
3.2. Equipment For applying the active phase precursor to the washcoated monoliths common labware was used. For drying of the structures use was made of an oven at 363 K (=static drying), flowing air at room temperature, a Whirlpool Procombi AVM 840 microwave operated at 200 W, and a Modulyo Freeze Dryer operated at 223 K and 10 Pa. 3.3. Experimental procedures Impregnation was performed using a 0.5 M Ni(NO3 )2 ·6H2 O solution. After contacting the monolithic structure with the solution for 1 h, it was withdrawn and the channels were cleared by removing excess liquid with air. Samples were dried using either stationary air in an oven at 363 K, flowing air at room temperature, a microwave oven or a freeze dryer. All samples were calcined in air, using a heating rate of 10 K/min up to 723 K and maintaining this temperature for 2 h. To study the effect of the nickel concentration in the solution, catalysts have been impregnated using 0.2, 0.5, 1.0, and 2.0 M Ni(NO3 )2 ·6H2 O solutions. Samples were dried in stationary air as described above. Deposition precipitation was performed using a 0.2 M Ni(NO3 )2 ·6H2 O and a 2.0 M urea solution. The monolithic structure was placed in a tightly fitting glass cylinder and heated to 353 K with a heating rate of 1 K/min and maintaining this temperature for 12 h. The sample was then carefully washed and dried (static drying), followed by calcination as described previously. 3.4. Catalyst characterization The impregnation profiles were visually characterized after cutting the catalyst samples in two halves in axial direction. The average metal oxide content was estimated from the weight increase of the structures after calcination. Although weighing is not an accurate method, it is sufficient for the performed study.
Fig. 4 shows schematically the effect of concentration in the impregnation solution on the development of impregnation profiles using static drying of the monolithic structures. Using a 0.2 M Ni(NO3 )2 ·6H2 O solution yields a very thin layer of nickel near the exterior of the monolith. With increasing nickel concentration in the impregnation solution, the region in which nickel accumulates increases. With the highest nickel concentration (2.0 M Ni(NO3 )2 ·6H2 O) an almost uniform distribution is obtained for this size of monolith. The nickel content of the catalysts, expressed as NiO content was 0.29, 0.74, 1.42, and 3.16 wt.% respectively, after impregnation with a 0.2, 0.5, 1.0, and 2.0 M Ni(NO3 )2 ·6H2 O solution. The light parts of the monolithic structures also contain a small amount of nickel, deposited by means of ion-exchange with the alumina surface groups, e.g. [7,11]. 4.2. Effect of drying method The solvent removal step (drying) is a crucial step during the preparation of monolithic catalysts. Several methods have been investigated in order to prevent the redistribution of active phase precursors. Fig. 5 shows a photograph and a schematical representation of the results of the various drying methods. Static drying yields an eggshell distribution under the conditions investigated. Drying of monolithic structures with forced airflow through the channels accumulates the active phase at the inlet plane of the airflow into the monolith. Microwave drying yields a more uniform active phase distribution. Freeze-drying of monolithic structures for various times (1 and 24 h) shows different results (Fig. 5). At 1 h (followed by further static drying), a kind of egg white distribution is obtained. Outside the egg white distribution also a homogeneously distributed active phase is present, within the “egg white” layer, almost no active phase is present. Freeze-drying for 24 h yields a uniform active phase distribution. 4.3. Deposition–precipitation Fig. 5 also shows schematically a monolithic structure impregnated with nickel by means of deposi-
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Fig. 4. Schematic representation of the cross section of monolithic catalysts showing the effect of nickel concentration in the aqueous impregnation solution on the development of impregnation profiles, from left to right: 0.2, 0.5, 1.0, and 2.0 M Ni(NO3 )2 ·6H2 O, respectively. The nickel concentration is proportional to the grayscale.
tion precipitation. The catalyst shows a uniform active phase distribution. A loading of 2.0 wt.% was achieved.
5. Discussion Although monolithic catalysts belong to the most frequently applied type of catalyst [2,20], hardly any information in the open literature is available dealing with impregnation of these structures [9,17,21]. The
monoliths shown in Fig. 1 show that impregnation can result in a non-uniform active phase distribution, leading to a low utilization of the monolithic structure. A qualitative study has been undertaken to explain the development of non-uniform active phase distribution in monolithic catalysts. Although not stressed explicitly, the results presented in the previous section have been found highly reproducible. The effect of the drying conditions itself (temperature, humidity) have not been addressed in this study. Though these conditions have an effect on the final
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Fig. 5. (A) Photograph of the cross section of monolithic catalysts showing the impregnation profiles obtained after static drying (left), freeze-drying for 1 h, followed by static drying (center), and freeze-drying for 24 h (right). (B) Schematic representation of the cross section of monolithic catalysts showing the impregnation profiles obtained after static drying, forced flow drying, microwave drying, freeze-drying (1 h) followed by static drying, freeze-drying for 24 h, and deposition precipitation. The nickel concentration is indicated by the grayscale.
active phase distribution, they do not eliminate this redistribution. The result obtained with drying by forced convection through the monolithic channels proves that nickel migrates towards the surface where evaporation occurs. At the plane where the dry inlet air contacts the wet monolith, the driving force for evaporation is largest and the highest drying rates will occur. Indeed, nickel accumulates at this inlet plane
(Fig. 5). Drying of monolithic structures under differential conditions with wet air has been suggested. However, drying under such differential conditions requires a high gas flow rate to minimize the solvent vapor pressure increase of the gas during the contact between gas and wet monolith. These high flow rates enhance mass transfer and thus evaporation, making it even more difficult to maintain differential conditions.
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Three methods have been investigated to prevent migration of nickel during the drying stage: (i) formation of an insoluble complex by means of deposition precipitation; (ii) heat supply throughout the structure, so that evaporation does not only occur at the exterior (microwave drying) but also in the interior and (iii) preventing the liquid from flowing to the exterior of the structure (freeze-drying). All three methods yield a more uniform metal distribution (Fig. 5). The obtained nickel profile obtained after 1 h freeze-drying followed by static drying (see Fig. 5) shows that freeze-drying also starts at the exterior of the monolithic structure. The drying front migrates to the interior of the monolith with time. If the freeze-drying is stopped too early and is switched to the static drying procedure, a new evaporation front arises, which attracts the liquid from inside this front, shown as the egg white distribution. Microwave drying resulted in a homogeneous metal distribution throughout the structure. During microwave drying, which is much faster than static drying and freeze-drying, care should be taken that the evaporated liquid is removed from the microwave oven quickly. Deposition–precipitation yielded uniform nickel profiles, but has the disadvantage that it cannot be applied with every metal. Fortunately, also other methods than pH increase can be performed to deposit a metal on a surface [10]. Ion-exchange is also a potential alternative but its feasibility also depends on the precursor and support under investigation. Both freeze-drying and microwave drying are not frequently applied methods in catalyst preparation. Microwave drying was applied in catalyst preparation of nickel on alumina catalysts [22,23]. Ours confirm those results by using microwave drying a more homogeneous metal distribution is obtained compared to using static drying. Microwave drying has also been applied previously in the preparation of a monolithic manganese oxide based hydrogen peroxide decomposition catalyst [24]. Freeze-drying was used before for drying clays after pillaring [25], but not for modifying impregnation profiles. Both methods seem, however, suitable for monolithic catalyst preparation, since they are well-established methods in large applications for drying, e.g. in food industry. Static drying of monolithic structures can be described as follows (Fig. 2g–i). After placing the wet structure in a hot surrounding, liquid starts to evap-
orate from the exterior of the monolith. Both from small and large pores liquid evaporates. Because of the higher capillary force, the small pores are refilled with liquid from larger pores present in the interior of the monolithic structure. The macropores of the structure will be emptied first because they are much larger than the pores in the washcoat. This process continues as long as a continuous liquid network exists (funicular state) throughout the monolithic structure. Because a monolith is a large single piece of material; this transport process can take place over a long distance, as evidenced by our observations. As soon as this liquid network is broken-up (pendicular state), the evaporation front progressively moves inward to the center of the monolithic structure until complete dryness. The isolated liquid pockets that exist in the pendicular state can only be emptied by evaporation and not by convection. The effect of the nickel concentration in the impregnation liquid can be explained as follows. Transport of nickel nitrate to the exterior of the structure occurs until saturation is reached. When the amount of liquid is decreased enough, the nickel nitrate will deposit on the support. With higher nickel concentration the region with saturated liquid is larger as can be seen from Fig. 4. At this point, it can be remarked that monolithic catalysts are basically not very different from other pre-shaped catalyst support particles. However, the transport of precursor solution towards the exterior of monolithic structures during drying by capillary forces occurs on a much larger scale than for commonly applied support particles (decimeters instead of millimeters). Furthermore, because of the presence of two characteristic length scales and the high void fraction of monolithic structures, the impregnation process occurs as if the monolithic structure had been evacuated prior to impregnation. Therefore, the effect described by Lee and Aris [15] of occluded gas pushing out liquid from support particles during drying may only occur locally in the washcoat perpendicular to this layer. This theory can therefore not be used to explain the macroscopic inhomogeneous metal distribution of Ni/Al2 O3 /cordierite monolith catalysts. It is noted that by impregnation, both the porous washcoat and the macroporous ceramic monolith structure are filled with impregnation liquid. Due to the macroporosity of the ceramic monolith structure, this will
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be emptied first upon drying, so that almost all precursor will end up in the washcoat layer. So, the effect of capillary forces ensures here deposition of all precursor onto the catalyst support material (washcoat).
6. Conclusions When preparing monolithic catalysts, care should be taken that a uniform distribution of active phase material is obtained. Due to capillary force differences during drying a redistribution of the active phase precursor can occur on the scale of monolithic dimensions, resulting in accumulation of the active phase in an outer shell of the monolith structure. Three methods have successfully been applied to prevent the development of an inhomogeneous metal distribution. 1. Formation of an insoluble catalyst precursor, by means of deposition precipitation. 2. Evaporation of the liquid throughout the structure, by means of microwave heating. 3. Preventing the liquid from flowing, by means of freeze-drying. Preparing monolithic catalysts with a uniform active phase distribution by means of impregnation requires that the active phase precursor is not able to move to the exterior of the structure after it has been applied. Based on the presented work, recommended methods are freeze-drying, microwave drying and deposition–precipitation, although any other method that applies an active phase without the chance on redistribution during the preparation procedure, e.g. ion-exchange, should in principle be adequate, too.
Acknowledgements The IOP Catalysis (Dutch Ministry of Economic Affairs) is acknowledged for the financial support of this research (project IKA94061). Degussa AG (Hanau, Germany) is acknowledged for the supply of alumina washcoated monolithic structures. William Anker is acknowledged for the experimental work.
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