Ind. Eng. Chem. Res. 2003, 42, 1863-1869
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Ceramic Foam Monoliths as Catalyst Carriers. 1. Adjustment and Description of the Morphology Florina C. Buciuman and Bettina Kraushaar-Czarnetzki* Institute of Chemical Process Engineering CVT, University of Karlsruhe, Kaiserstrasse 12, 76128 Karlsruhe, Germany
Many properties of open-cell ceramic foam monoliths prepared by means of the sponge replication method can be controlled by adjusting the viscosity of the ceramic slip to the pore count of the templating polymer sponge. Catalyst carriers with tailored strut thicknesses, relative densities, and void fractions in the range of 0.77-0.95 can thus be produced. A structural model based on the dense packing of tetrakaidekahedra is useful for an approximate description of the morphology of the foams. With this model, it is possible to estimate the mean pore diameter, the mean thickness of the struts, and the minimum surface area from two macroscopic parameters that are easily accessible by simple experimental methods. A comparison between the calculated minimum surface area and the BET surface area can provide information about the roughness of the struts. Introduction Ceramic foams have been developed for about three decades and are used now commercially as filters for molten metals or hot gases. In recent years, however, other applications have been explored as well. For instance, the potential of ceramic foams as catalyst carriers, burner heads, or fuel-cell electrodes has gained increasing interest. Ceramic foams can be highly suitable catalytic carriers when a low pressure drop is mandatory. In comparison to honeycomb monoliths, they offer the additional advantages of radial mixing within the body and enhanced mass and heat transfer due to the turbulence of the flow. Foam carriers exhibit a high contact surface between the gas phase and the solid and can be manufactured in different geometries, allowing for the adjustment of axial or radial flow patterns in the reactor. Pioneering work reported in the field of catalysis with foam carriers concerned partial oxidation of hydrocarbons,1-3 catalytic combustion,4,5 and removal of soot from diesel exhaust.6-8 There are several routes to manufacture ceramic foams. Among these, the polymer sponge replication method of Schwarzwalder and Somers9 is most suitable for the fabrication of oxidic, reticulated foams with easily accessible, open cells.10,11 This method consists of coating an elastic polymer sponge having open cells with a slip containing the precursors of the ceramic material together with other additives. Coating is achieved by immersing the polymer sponge into the slip, expelling the excess (e.g., by compression), and drying. Subsequently, the green body is exposed to a temperature treatment. In a first stage, the organic skeleton is burned off in air. At temperatures above 1400 °C, the particles forming the ceramic replica are sintered. The cell size of the finished foam is dependent on the socalled pore count (reported as ppi ) pores per linear inch) of the original polymer sponge and the degree of
shrinkage during drying and sintering of the green body. The phase composition of the ceramic is established by choosing the appropriate combination of raw materials and sinter temperature. A broad variety of ceramic foams with different compositions is described in the patent literature. However, details regarding the control of other essential features of the foams, such as their density and porosity, which, in turn, determine their mechanical strength and permeability, are very scarce. An approach to controlling the porosity by varying the compression upon removal of excess slip was reported by Salvini et al.12 In their study, data on the mechanical strength and pressure drop were correlated with the porosities of the foams. However, only 10 ppi foams (Al2O3-SiC) were investigated. Many other authors (see Montanaro et al.13 and references therein) noticed that the slip rheology affects the quality of the ceramic foams. For instance, thixotropic behavior appears to be advantageous for uniform wetting of the polymer sponge and can be realized by using various additives.14,15 Quantitative relations regarding the influence of the slip rheology and the characteristics of the foams, however, are not currently available. Our interest in ceramic foams is focused on their applications as catalyst carriers. In this context, morphological, mechanical, and surface chemical properties need to be adjusted. This part of our study is devoted to the control of the morphology of ceramic foams through adjustment of the slip rheology in relation to the structure of the polymer sponge. It was also our aim to compose a morphological model that can serve as a basis for the quantitative description of the hydrodynamic properties and the derived correlations for heat and mass transfer in ceramic foam carriers. Further reports on the hydrodynamic and mechanical properties and, finally, on the surface modification and catalytic functionalization of ceramic foam carriers are in preparation. Experimental Section
* To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: +49-721-3947/ 4133. Fax: +49-721-608 6118.
Preparation. The ceramic foams carriers were made by means of the polymer foam replication method.9
10.1021/ie0204134 CCC: $25.00 © 2003 American Chemical Society Published on Web 04/05/2003
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Table 1. Coating Scheme for Alumina-Mullite Foams pore count (ppi) 10 20 30 45 60
Table 2. Properties of Alumina-Mullite Precursor Slips slip no.
slip no. 1
2
3
4
b b
b b b
b b b b
b b b b b
5
b b b
1
2
3
4
5
solid content (vol %) 29 25 20 16 12 density (g‚cm-3) 1.51 1.44 1.34 1.28 1.21 6 4 1 -1 viscosity at τ ) 20 Pa 4 × 10 3 × 10 3 × 10 6 × 10 2 × 10-1 (Pa‚s)
Results and Discussion Polymer sponges (polyester, Koepp AG) having different pore sizes in the range of 10-60 ppi, cut in pieces of 20-50 mm diameter, were used. Most of the work reported here was carried out with alumina-mullite foams. For the preparation of the respective slip, the ceramic precursors (boehmite, kaolin, and silica in a weight ratio of 3.5:2.5:1) were suspended in water containing poly(vinylpyrrolidone) (Luviskol, BASF) as an organic binder and Dolapix PC 67 (Zschimmer & Schwarz) as a deflocculant. The mixtures were milled with alumina balls for 30 min. Several slips having different concentrations of solid material and, therefore, different viscosities were employed. The viscosity measurements were performed on a Haake RS150 rheometer in a cone-plate configuration. The slips were used to coat polymer sponges with different pore counts between 10 and 60 ppi. The various combinations produced are summarized in Table 1. The coating was performed by immersing the sponge pieces in the slurry, squeezing them, and passing them through rollers preset at 80% compression to expel the excess slip. The bodies were dried for 24 h at room temperature and heated at 1 °C/min to 500 °C and then further at 5 °C/min to the final temperature (1600 °C), which was held for 300 min to achieve sintering of the ceramic. To produce China porcelain foams, the same procedure was employed, but the ceramic precursors were ball clay, kaolin, nepheline syenite, talc, and silica in the weight ratio 10:10:24:1:2. The sintering of China foams was performed at 1150 °C. Characterization. The phase composition of the ceramic foams was analyzed with a Siemens D-500 diffractometer after a previous ball milling. The apparent densities of the foams were derived from the weights and volumes of the samples. To determine the bulk density of the sintered ceramic, several nonporous disks (20 mm in diameter, 2 mm in height) were prepared by casting the ceramic slurry into cylindrical, flat forms followed by drying and sintering. The density of the disks was measured with a water picnometer. The foam porosity (or voidage) was calculated from the apparent and bulk densities. Krypton physisorption according to the BET method (Micromeritics, ASAP 2010) was carried out to determine the surface area of the ceramic foams. The structure of the foams was examined with an optical microscope. The diameters of the cell windows (“pore diameters”) and thicknesses of the struts were measured by means of the sizing technique16 with feature-to-feature scanning. Care was taken to determine the widths of the windows rather than those of the cell sections and to take into account only the window axes that are oriented parallel to the observation plane. The scanning was performed along the foam pieces in both the horizontal and vertical directions. The data reported represent mean arithmetic values. The morphology of the strut surface was inspected using scanning electron microscopy (Hitachi S-4500).
Adjustment of Solids Distribution and Morphology. The coating of the polymer sponge with the ceramic slip is a crucial step in the manufacturing of ceramic foams. It is important to achieve complete penetration of the polymer sponge and homogeneous coverage of all of its struts with slip. Interestingly, in numerous experiments, we could observe that the suitable combination of slip viscosity and sponge pore count determine the product quality, whereas the chemical nature of both the ceramic slip and the polymer sponge are of minor importance. Sponges made of polyurethane instead of polyester, for instance, can be employed as well, but they evolve toxic gases upon burnoff. As to the ceramic materials, the study presented here refers to aluminamullite and porcelain foams. However, the results can be transferred to other slip compositions as well. The viscosities of the mullite precursor slips were measured at a constant shear stress of 20 Pa after a shear time of 60 s. Slip designations, together with their solid contents, densities, and viscosities, are summarized in Table 2. As to the applicability of the slips, a general rule can be established that the larger the cells of the polymer sponge are, the thicker the slip must be to obtain a homogeneous coverage. As an example, Figure 1 shows images of ceramic foams produced with 20 ppi sponges and three different alumina-mullite slips. The most viscous slip could not completely penetrate the sponge and plugged cells at the outer regions of the body (Figure 1a). When the viscosity was too low, the slip drained and accumulated at the bottom part of the green body, resulting in an inhomogeneous distribution along the height. In Figure 1c, the upper part of such a product (after sintering) with very tiny and fragile struts is shown. Insufficient coverage of polymer struts frequently results in a collapse of the structure upon removal of the organic material during calcination. Figure 1b, in contrast, shows a piece of a stable foam in which the apparent density is not graded along the length or the radius of the body. The magnitude of the slip viscosity employed here had a medium value between those for the other foams (Figure 1a and c). Microscopic images of green bodies after drying are depicted in Figure 2. The photographs show 10 ppi sponges coated with three slips of different viscosities. The most viscous slip (Figure 2a, slip no. 1) yields very thick struts, whereas a more watery slip results in increasingly poorer coverage: first, the edges become uncoated (Figure 2b, slip no. 2), followed by even larger portions of the polymer sponge (Figure 2c, slip no. 4). This is clearly visible because the polyester sponge has a black color and the ceramic coat is white. During the burnoff of the polyester frame, the last structure will collapse. For every cell size, there exists an optimum viscosity or viscosity range of the slip that allows for complete penetration and regular coverage of the polymer sponge. The best combinations of slip viscosities and pore counts were found to be the following: 106-107 Pa‚s for 10 ppi,
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Figure 1. Images of alumina-mullite ceramic foams produced with 20 ppi sponges and (a) slip 1, (b) slip 2, and (c) slip 4; for slip properties, see Table 2.
Figure 3. Details of a ceramic foam structure
Figure 2. Struts of dried green bodies produced with blackcolored 10 ppi sponges and (a) slip 1, (b) slip 2, and (c) slip 4. Black dots and edges indicate incomplete coverage with slip.
104-105 Pa‚s for 20 ppi, 101-102 Pa‚s for 30 ppi, 10 Pa‚ s for 45 ppi, and 10-1 Pa‚s for 60 ppi polymer sponges. Morphological Model Using Density and Pore Count as Input Parameters. The application of
ceramic foam monoliths in catalytic reactors requires quantitative descriptions of the flow and heat and mass transfer. Material properties on both the microscopic and the macroscopic scale must be known and included in the corresponding transport correlations. The thermal conductivity, for instance, depends on the composition of the material and on the microscopic-scale morphological properties such as length, thickness, and strut arrangement.17 The permeability of the ceramic foams to gas flow can be related to macroscopic properties such as the number of pores per unit length, the apparent density, or the void fraction.18 These macroscopic properties are accessible in routine measurements. Therefore, it was our aim to provide a morphological model of open-cell ceramic foams that relates macroscopic properties to the mesostructure. A closeup of an open-cell foam is depicted in Figure 3. The structure consists of a network of struts of ceramic material. These struts are connected in vertices and surround the cells. Struts typically exhibit a triangular cross section and are located between three cells. As indicated in Figure 3, the strut geometry can be characterized by the strut length, l, and the distance between the edges of the triangle, t. The cells are irregular polyhedra. The voids inside are accessible through windows in the faces, also denoted as pores.18 In real foams, the shape of the pores is nearly circular because of the accumulation of material in the vertices. One can distinguish between the “inner” diameter of the pores, dP, and the face diameter measured between the strut axes, DP. These magnitudes are related by DP ) dP + t. Average values of both dP and t can be determined from microscope images by using the feature-
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Table 3. Morphological Properties of Alumina-Mullite Foams
foam pore count (ppi)/slip no. 10/1 10/2 10/3 20/1 20/2 20/3 20/4 30/2 30/3 30/4 45/3 45/4 45/5 60/4 60/5 a
relative density
geometric data on ceramic foams voidage
FA FB
FA )1FB
0.166 0.136 0.053 0.201 0.149 0.116 0.103 0.233 0.163 0.156 0.203 0.151 0.128 0.173 0.122
0.834 0.864 0.947 0.799 0.851 0.884 0.897 0.767 0.837 0.844 0.797 0.849 0.872 0.827 0.878
strut edge width t (mm)
pore diameter dP (mm)
mean
standard deviation
mean
standard deviation
face diameter DP (mm)a
0.68 0.55 0.43 0.30 0.28 0.27 0.26 0.22 0.20 0.18 0.15 0.09 0.07 0.09 0.04
0.32 0.33 0.27 0.17 0.14 0.11 0.08 0.14 0.15 0.13 0.10 0.07 0.03 0.06 0.02
1.50 1.55 1.58 0.95 0.95 0.94 0.98 0.58 0.62 0.63 0.34 0.36 0.37 0.21 0.25
0.41 0.35 0.34 0.23 0.25 0.27 0.29 0.15 0.23 0.19 0.11 0.12 0.12 0.07 0.12
2.18 2.10 2.01 1.25 1.23 1.21 1.25 0.80 0.82 0.81 0.49 0.45 0.44 0.30 0.29
reciprocal metric pore count of polymer spongeb (mm) 2.54 1.27
0.85 0.56 0.42
DP ) dP + t. b Data from the manufacturer.
to-feature technique. The pore diameter dP, as well as the face diameter DP, should be well distinguished from the reciprocal value of the “pore count”, which is the customary designation of the pore size used by the manufacturers of foams. The pore count represents the average number of holes per unit length (in inches) in a plane cut through the foam. This number includes not only windows parallel to the observation plane (1/DP, here in in.-1) but also windows lying in different space directions, causing foreshortening, and all kinds of sections of cells. In the present work, the pore count was used only to specify the virgin polymer sponges in accordance with the statements of the sponge manufacturers. It should also be stressed that the number of pores per unit length in the polymer sponges is not equal to the number of pores per unit length in the ceramic product. The green bodies undergo shrinkage during drying and sintering. Therefore, the magnitude of DP is smaller than the reciprocal metric pore count of the polymer sponge before coating. In Table 3, the morphological properties of the alumina-mullite composite foams are summarized. The first column indicates the combination of polymer sponge and slip used to produce the ceramic foam sample. The second column displays the relative densities, which represent the ratios of the apparent densities, FA, to the bulk density, FB, of the ceramic foam. In the case of alumina-mullite, the bulk density is FB ) 3.24 g/cm3. As could be expected, the relative density and the edge width of the struts increase with increasing viscosity of the slip used to produce the ceramic foam. The face diameter of the foams, DP, is almost invariant with the slip viscosity, but it strongly depends on the pore count of the polymer sponge used. Our assessment of a morphological model that connects macroscopic with mesoscopic features of ceramic foams is based on a work by Gibson and Ashby.17 On the basis of morphological analyses of numerous natural foams, they found that the average number of struts per cell window is about 5. A structure built by packed polyhedra with pentagonal faces would represent closely the foam morphology. However, a package of regular pentagonal dodecahedra does not fill the entire space. Alternatively, a dense package of tetrakaidekahedra will be considered (Figure 4). The tetrakaidekahedron can be described as a truncated octahedron, exhibiting six
Figure 4. Single tetrakaidekahedron (top) and a dense package (bottom).
square and eight hexagonal faces. The average number of struts per face is 5.14, which is close to the corresponding value for a pentagonal dodecahedron. The struts have the shape of triangular prisms. Gibson and Ashby17 derived the following equation (eq 4) for the relative density of a package of tetrakaidekahedra as a function of the strut dimensions, where l represents the length and t the edge width of the struts
FA t2 ) 1.06 FB l
()
(1)
The length of the struts l is difficult to assess. More convenient would be an equation containing the face diameter DP as an input parameter. In the tetrakaidekahedron, the faces are not equal, but an average area of faces can be calculated as follows
Aface )
6Asq + 8Ahex 26.80l2 ) ) 1.91l2 14 14
(2)
Considering now a circular window of diameter DP and the same area, it follows that
π
DP2 ) 1.91l2 w l ) 0.64DP 4
(3)
This simple analogy allows the strut length in eq 1 to be replaced by the face diameter of the foam DP to give
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Figure 5. Determination of the constant C (eq 4) from the relative densities of sintered alumina-mullite foams as a function of their geometrical characteristics.
(
FA t ) 1.06 FB 0.64DP
)
2
) 2.59
( ) ( ) t DP
2
)C
t DP
2
(4)
To test this model, the relative densities of all foams are plotted versus (t/DP)2 in Figure 5. A value of 2.5 was obtained for the constant C. Keeping in mind that the irregular structure of real foams has been approximated by a package of regular bodies, the close agreement of the experimental constant with the theoretical value (2.59) can be considered as very satisfactory. However, the scattering of the data around the regression line cannot be overlooked. It results from both the difficulty of correctly assessing the mean values of the structural parameters (strut widths and pore diameters) and the imperfections in the morphology of real foams. For instance, real struts are hollow rather than dense because the polymer template was included before burnout and sintering. On the other hand, some faces or even complete cells in real foams are closed because the slip can form bubbles in the cells upon coating. It should be noted that these imperfections have opposing effects with respect to the ratio FA/FB. On average, the corresponding errors compensate each other to a certain extent. Provided that the scatter of the data is taken into account by inspecting a large foam volume (or many small pieces), eq 4 can be used to calculate the mean edge width of the struts, t, when the relative density and the face diameter of a ceramic foam are known. For a rough estimate, the reciprocal pore count of the ceramic foam can be used instead of the face diameter. However, a calculation on the basis of the pore count of the polymeric template would yield the poorest approximation. Upon calculating the strut width, the mean inner pore diameter can also be obtained as dP ) DP - t. Comparison of Experimental and Theoretical Surface Areas. The exposed geometric surface areas of all struts in a tetrakaidekahedron unit cell can be calculated. The cell volume is (11.31l3), and the unit consists of 36 struts. Each edge belongs to three cells, which yields an average of 12 struts/cell. The surface area of a strut (with the shape of a triangular prism and considering only the lateral area) amounts to 3tl. This results in a surface area of 36tl per cell or, expressed in area per unit volume
Sg )
t 36tl ) 3.18 2 11.31l3 l
(m2/m3)
(5)
Figure 6. Volumetric surface areas of ceramic foams as a function of the reciprocal mean face diameter expressed in units of in.-1. The lines indicate values calculated on the basis of a tetrakaidekahedron package.
Combining eq 5 with eqs 3 and 4 gives the following relationship, in which the geometric surface area is expressed as a function of the relative density and the face diameter DP
1 Sgeo ) 4.82 DP
x
FA FB
(m2/m3)
(6)
In Figure 6, the lines represent the calculated geometric surface areas of foams with relative densities of 0.12 and 0.25 as a function of the reciprocal face diameter in in.-1. In the same graph, the closed symbols represent data obtained from physisorption measurements on several alumina-mullite foams having relative densities around 0.20. For this comparison, the mass-specific BET surface areas (m2/g) were multiplied by the apparent densities FA of the ceramic foams to obtain the corresponding volumetric values. Obviously, the surface areas of the mullite foams are considerably higher than the theoretical values, albeit that the trendline of the experimental data runs almost parallel to the dotted lines of the calculated surface areas. At first sight, this deviation might suggest that the morphological model is not suitable. However, the tetrakaidekahedron model takes into account only the spatial arrangement and dimensions of the struts rather than their specific surface characteristics. In the case of alumina-mullite, the foam skeleton is formed upon diffusive sintering of the particles. As can be seen in the scanning micrograph in Figure 7a, the resulting struts are very rough. Hollows and clefts are visible, and the internal surface of the hollow struts is accessible for gas adsorption. Although the particles are interconnected by sinternecks, they can still be distinguished from each other. Figure 7b, in contrast, shows the strut of a China porcelain foam. This material undergoes viscous sintering, and the resulting surfaces are much smoother. The voids inside the struts are not accessible. The volumetric surface areas of such foams should be close to the calculated values if the tetrakaidekahedron model is applicable to describe the geometric surface area of ceramic foams. This is, indeed, the case, as confirmed by the data for a series of China foams (open symbols) displayed in Figure 6. The characteristic data for these foams are reported in Table 4. The difference between the experimental and calculated surface areas, therefore, results from the internal surface of the hollow struts, cracks, and surface roughness. Our current and future work is directed toward an assessment of the effects of surface roughness on
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using accessible input parameters, i.e., average pore size and relative density. With respect to the measurement of the average pore size, the recommended method is feature-to-feature sizing of microscope images. Rough estimates of the pore size are obtained by using the reciprocal value of the so-called pore count. Acknowledgment This study was financially supported by the Deutsche Forschungsgemeinschaft. We thank Dr. B. Hochstein for the viscosity measurements. List of Symbols Used Aface, Asq, Ahex ) average areas of a face, a square face, and a hexagonal face of a tetrakaidekahedron, respectively C ) constant DP ) mean face diameter of a ceramic foam, DP ) dP + t, mm dP ) mean inner pore diameter of a ceramic foam, mm l ) mean strut length, mm ppi ) pore count expressed as pores per linear inch, in.-1 Sgeo ) geometric surface area per unit volume, m2/m3 t ) mean edge width of struts, mm ) foam voidage, ) 1 - FA/FB FA ) apparent density of a ceramic foam, kg/m3 FB ) bulk density (of struts), kg/m3 FA/FB ) relative foam density Figure 7. SEM images of the strut surfaces of ceramic foams: (a) alumina-mullite, (b) China porcelain Table 4. Morphological Properties of China Porcelain Foams relative density nominal pore count (ppi)
FA FB
10 20 30 45 60
0.116 0.151 0.178 0.198 0.226
voidage
)10.884 0.849 0.822 0.802 0.774
FA FB
face diameter, DP (mm) mean
standard deviation
2.321 1.243 0.825 0.487 0.333
0.49 0.25 0.21 0.08 0.06
pressure drop and heat and mass transfer and on the possibilities for modifying and functionalizing the surfaces of ceramic foams by coatings. Conclusions We have discussed how the quality and macroscopic properties of ceramic foams can be controlled during the polymer sponge replication process. The pore count of the templating polymer sponge determines the viscosity range of the ceramic slip that is suitable for obtaining uniform coverage. Within this range, it is possible to adjust the thickness of the struts in the finished ceramic foams and, accordingly, their density, voidage, and derived properties. The structure of many foams, although irregular, can be described to a good approximation on the basis of a dense package of regular tetrakaidekahedra. We have shown that this morphological model can be applied to open-cell ceramics as well. With the necessary cautions regarding the methods used to assess the average pore size and the representativeness of the samples, this model provides information on the mean strut thickness, the geometric surface area and the surface roughness
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Received for review May 28, 2002 Revised manuscript received February 3, 2003 Accepted February 11, 2003 IE0204134
