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Preparation and characterization of porous silica spheres containing a copper(oxide) catalyst
Catalysis, 65 (1990) 225239 Elsevier Science Publishers B.V.. Amsterdam
Applied
225
Preparation and characterization of porous silica spheres containing a copper(oxide) catalyst C.J.G. van der Grift*, A.Q.M. Boon, A.J.W. van Veldhuizen, H.G.J. Trommar and J.W. Geus Department
of Inorganic
3522 AD Utrecht
Chemistry,
(The Netherlands),
University
of Utrecht,
tel. (+31-30)
Croesestraat
778,
890819
J.F. Quinson Universiti
Claude Bernard
U.A. CNRS 417,43
Lyon Z, Laboratoire
Bd. du 11 Novembre
de Chimie Appliquei
1918,69622
Villeurbanne
et Genie Chimique, (France)
and M. Brun Ecole Centrale
de Lyon, Laboratoire
des Machines
Thermiques,
69130 Ecully (France)
(Received 27 March 1990, revised manuscript received 11 June 1990)
ABSTRACT Porous catalyst (sorbent) spheres have been prepared by incorporation of a finely divided catalyst precursor into a silica gel matrix. The newly developed preparation procedure facilitates independent control over the dispersion of the active material and the porous structure of the support. As an example the preparation of porous copper-on-silica spheres is described. Scanning electron microscopy shows that the incorporated catalyst is homogeneously distributed throughout the silica gel matrix. The texture of the porous spheres has been investigated by means of water thermoporometry and nitrogen adsorption. The pore structure of the spheres can be modified by application of a hydrothermal treatment. Under prolonged hydrothermal treatment the mean pore size gradually increases from 3 to 30 nm, whereas the specific surface area simultaneously decreases from 550 to 100 m*/g. The effect of the texture of the silica matrix on the accessibility of the incorporated copper catalyst has been investigated by means of temperatureprogrammed reduction and by catalytic oxidation of carbon monoxide to carbon dioxide. The results indicate that the accessibility of the incorporated catalyst is improved by the presence of wider pores in the silica matrix. silica spheres, pore size, copper/silica, catalyst characterization (TPR), carbon monoxide oxidation, catalyst preparation (support).
Keywords:
INTRODUCTION
Supported metal catalysts and inorganic sorbents usually exhibit a specific surface area of some hundreds of square meters per gram. In many reactions a
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high surface area per unit volume is required in order to provide a large contact area between the active solid and the gaseous reactants. Application of catalysts or sorbents in chemical reactors calls for the formation of solid bodies of a particular size (1 cm to 1 mm) and shape (spheres, trilobes, etc.) depending on the type of reactor used. Due to these requirements the desired high specific surface areas can only be established with bodies of a significant porosity. As a result, a large fraction of the active surface is located inside the porous structure. To achieve a good performance of the catalyst (sorbent), the porous structure of the bodies should facilitate a rapid transport of the reactants to and products from the active material. In addition to the texture of the bodies, the dispersion and the distribution of the active material over the surface of the support affect the performance of the catalyst system. Generally, a refractory inorganic support of high surface area is used to disperse and to stabilize the active component. In order to take full advantage of the large specific surface area of the support, a homogeneous distribution of the active material, or of its precursor, should be achieved during the preparation. Furthermore, to achieve a good performance of the catalyst (sorbent) the dispersion of the active material must be optimized. With the preparation of catalyst (sorbent) particles it is thus of prime importance to control both the porous structure of the bodies and the distribution of the active material over the support. Usually, however, it is hard to prepare catalysts exhibiting both the desired porous structure and the required distribution of the active material over the support. Using a finely divided support the active material can be deposited onto the carrier by precipitation, adsorption or ion-exchange from a homogeneous aqueous solution. Although the catalyst precursor thus obtained may exhibit a homogeneous distribution of the active material, control over the texture of the final catalyst body is generally poor. Alternatively, a pre-shaped support of a preferred texture may be loaded with active material by impregnation, adsorption or ion-exchange. However, especially with higher metal loadings a homogeneous distribution of the active material over the support bodies is difficult to achieve. In this paper we will describe a newly developed preparation procedure for the production of porous catalyst spheres. The preparation of porous spheres to be used as a catalyst support is usually brought about by sol-gel techniques which allow fixation of any desired shape of the bodies. Porous spheres are prepared by spraying a sol of the support with simultaneous conversion of the sol droplets into rigid gel spheres. Subsequent loading of the porous support spheres generally provides little control over the distribution of the active material. Here, we follow an alternative approach which is based on the homogeneous incorporation of a separately prepared catalyst precursor into porous (silica) gel spheres. The catalyst precursor is prepared by deposition-precipitation of the active material onto silica (primary support). A suspension of the catalyst precursor thus obtained is homogenized with a silica sol which is sub-
227
sequently sprayed into a mixture of hot paraffinic hydrocarbons to form the gel spheres. The catalyst precursor is thus incorporated into a porous silica gel matrix (secondary support). Following the formation of the spheres, the texture of the silica matrix can be adapted by a hydrothermal treatment. This preparation route enables us to produce porous catalyst spheres of which both the dispersion of the active material and the texture of the support can be controlled independently. As an example, we will describe the production of copper-on-silica spheres. The importance of shaping of copper-on-silica catalysts for the hydrolysis of acrylonitrile to acrylamide was recently demonstrated by Stammbach et al. [ 11. The porous catalyst spheres were prepared by sol-gel conversion of an aqueous mixture containing a copper-on-silica catalyst prepared by homogeneous deposition-precipitation [ 21 and a silica sol. The morphology of the porous spheres and the distribution of the supported copper catalyst throughout the silica spheres was investigated by means of scanning electron microscopy. To investigate the effect of a hydrothermal treatment on the porous structure of the catalyst spheres, batches of initially identical spheres (taken from the same ‘mother batch’) were subjected to various treatments. The porous structure of the various samples was assessed by means of thermoporometry [ 3-51. This technique enabled us to investigate the texture of the wet porous structure, whereas nitrogen adsorption was used to characterize the texture of the dried spheres. Thus, by comparing the results of these two techniques the effect of drying on the pore structure can be monitored. The oxidation of carbon monoxide was used to assess the catalytic performance of a series of coppercontaining spheres of different pore dimensions. EXPERIMENTAL
A copper-on-silica catalyst was prepared by homogeneous deposition-precipitation [ 2,6]. The hydrolysis of urea was used to raise the pH in a vigorously stirred aqueous suspension containing the silica support (ZOOV Aerosil, Degussa, F.R.G.) and dissolved copper nitrate (p.a. Merck). After complete precipitation of the copper ions onto the support, the catalyst precursor was washed extensively and concentrated to a suspension containing 0.11 g catalyst per g suspension. Characterization of the catalyst precursor indicates a high dispersion of the copper ions over the silica support as in the mineral copper hydrosilicate chrysocolla, making it a suitable precursor for incorporation into the gel matrix [ 71. The copper-on-silica catalyst had a metal loading of 20 wt.-% [lOO%*gCu/(gCu+ gSiOa)]. The silica sol was prepared by addition of a sodium silicate solution (approximately 3.5 M) to 40 ml of nitric acid (approximately 2.5 M) at 273 K until a pH of 1 was reached. At this pH level, 30 g of the catalyst suspension was added to the silica sol. Upon introduction of the (slightly basic, pH w 8)
228
catalyst suspension the pH should not rise above 5 to prevent premature gelation giving rise to undesired inhomogeneities. After homogenization of the silica sol and the catalyst suspension (preferably at pH E 3 ), the pH was carefully increased to 5 by further addition of the sodium silicate solution. Next, the mixture was sprayed as small droplets, 1 to 4 mm in diameter, into a hot (353 K) mixture of paraffrnic hydrocarbons to bring about the sol-gel conversion. A schematic drawing of the all-glass assembly is given in Fig. 1. The hydrogel spheres as obtained by this procedure were washed extensively. The copper loading of the thus prepared copper-on-silica spheres corresponds to approximately 3.4 wt.-% of the dry weight. Next, eight batches of the hydrogel spheres were separated and subjected to various treatments. Four batches were dried in air at 393 K to give the xerogel (X) samples, whereas the other four were stored in excess water as hydrogel (H) samples. Both X and H samples were subjected to a hydrothermal treatment at 173 K in an autoclave containing 4.5 g solid and 20.5 g water in a total volume of 48 ml. During the hydrothermal treatment the spheres were saturated with a 0.5-M sodium chloride solution to prevent destruction of the spherical shape by osmosis induced by the release of sodium ions during aging of the silica gel [8]. After the hydrothermal treatment the spheres were thoroughly washed to remove NaCl and coded X(h) or H(h) in which h is the duration (h) of the hydrothermal treatment. The wet samples obtained after hydrothermal treatment were characterized by water-thermoporometry according to procedures described by Quinson et
~
Stock-vessel
(0.07L~
T IOcm I
I
Fng paraffwcolumn
expanwn
water
_____
(51
rings
(2L)
Plug
Fig. 1. Equipment used for preparation of the catalyst spheres.
229
al. [ 31, Eyraud et al. [ 41 and Quinson and Brun [ 51. Both solidification and melting of water in the porous structure was investigated with a differential scanning calorimeter (DSC, model 101, Setaram). The equations used to calculate the textural data have been described previously [ 3-5 1. After drying (at 393 K in air), the texture of the same series of hydrothermally treated samples was investigated by means of nitrogen adsorption at 77 K (Omnisorb 100, Omicron). Prior to adsorption measurements the samples were outgassed in situ at 393 K and 0.13 Pa for 16 h. The morphology of the dried spheres and the distribution of copper (oxide) through the porous structure were studied by scanning electron microscopy (SEM ) (Cambridge 15OS, microscope ). The accessibility of the incorporated catalyst was investigated by means of temperature-programmed reduction. Approximately, 100 mg of dried catalyst spheres (1-2 mm) was subjected to in situ calcination (oxidation)-reduction cycles in a micro-reactor with an internal diameter of 8.0 mm. Calcination (oxidation) was performed by heating the dried catalyst spheres from ambient to 723 K at 5 K/min in a 2 vol.-% O,/He flow (50 ml/min). Temperatureprogrammed reduction of the calcined (oxidized) catalyst spheres was performed by heating from ambient to 723 K at 5 K/min in a 10 vol.-% HZ/N2 flow (50 ml/min). The performance of the incorporated catalyst in the oxidation of carbon monoxide was assessed by measuring the activity (conversion) of the catalyst spheres over a wide temperature range (300 to 900 K). Prior to the activity measurements the catalyst spheres were calcined in 10 vol.-% 0z/N2 at 723 K, reduced in 10 vol.-% Hz/N2 up to 973 K and re-oxidized in 10 vol.-% Oz/N2 at 723 K. At room temperature, the re-oxidized catalyst was then subjected to a gas flow of 400 ml/min (0.25 vol.-% CO, 1 vol.-% 02, 98.75 vol.-% N2)+ Subsequently, the catalytic oxidation of carbon monoxide was measured as a function of the temperature. Details of the equipment used for these measurements are given elsewhere [ 91. RESULTS AND DISCUSSION
Preparation of the spheres The preparation of the spheres requires a rapid sol-gel conversion in the droplets of the catalyst-sol mixture sprayed into the hot mixture of paraffinic hydrocarbons. Using the equipment shown in Fig. 1,5 to 7 s are available to bring about sol-gel conversion before the droplets reach the bottom of the column. The rate of the gel formation depends on a variety of parameters such as the temperature, the pH, the silica concentration and the ionic-strength of the sol [lo]. The preparation conditions should provide a homogeneous catalystsol mixture stable enough to refrain from premature gelation, but sufficiently
230
reactive to form hydrogel spheres once sprayed into the hot mixture of alkanes. The conditions described in the Experimental section allow the preparation of such a sol from which the catalyst hydrogel spheres can be formed. The amount of catalyst suspension that can be incorporated into the silica gel matrix (i.e. the metal loading of the final spheres) is limited by a number of parameters such as, e.g., the concentration and the basicity of the catalyst suspension as well as the silica concentration and the acidity of the silica sol. Clearly, the maximum amount of catalyst precursor that can be incorporated into the silica gel is set by the maximum concentration of the catalyst suspension and the minimum amount of silica needed to develop a rigid gel structure around the (individual) suspension particles. In order to achieve a minimum fluidity (needed for homogenization of the catalyst precursor with the silica sol) the solid concentration of the catalyst suspension is limited. Another limitation is set by the concentration of the silica sol. Decreasing the silica concentration of the sol gives rise to rather weak hydrogel spheres, whereas too high silica concentrations result in premature, inhomogeneous gel formation. The recipe used in this work is based on experience but may be subject to further optimization. Scanning electron microscopy Fig. 2a shows an electron micrograph of the copper-on-silica spheres. This picture shows the spherical shape of the catalyst particles (diameter 1 to 2 mm). A micrograph taken at a higher magnification shows the roughness of the external surface which depends on the applied hydrothermal treatment (Fig. 2b). The surface roughness is caused by shrinkage of the gel-spheres upon removal of water from the pores during drying. Since the catalyst precursor particles do not shrink as much as the surrounding silica gel matrix, drying gives rise to a ‘puckered’ surface. Prolonged hydrothermal aging of the silica gel matrix leads to smoothening of the external surface. Several spheres were cleaved to investigate the distribution of the catalyst particles throughout the spheres (Fig. 2b). The distribution of copper over the external surface and the cleavage plane shown in Fig. 2c was investigated by means of backscattered electron microscopy (BEI). The intensity of the backscattered electron beam is sensitive to the atomic mass of the local elements in the sample. Due to the higher atomic mass of copper as compared to silicon and oxygen, the catalyst suspension particles are seen as bright spots. BE1 shows that the catalyst particles are (10 to 20 ,um in diameter) homogeneously distributed over the investigated surfaces (Figs. 2c and d). It is thus concluded that the catalyst precursor is homogeneously dispersed throughout the body of the porous silica spheres.
231
Fig. 2. SEM photographs: (a) spherical particles, (b) cleaved sphere, (c and d) morphology (EI) and composition (BEI) of cleaved sphere.
Thermoporometry and nitrogen adsorption Prior to the hydrothermal treatment the X samples were dried at 400 K (air) and subsequently re-immersed into the aqueous sodium chloride solution, whereas the H samples were treated in the NaCl solution without intermediate drying. The exact conditions of the hydrothermal treatment are given in the Experimental section. After the hydrothermal treatment the spheres were washed extensively with excess de-ionized water to remove dissolved salts. Subsequently, the texture of the wet spheres was characterized by thermoporometry, whereas the texture of the dried spheres (393 K, air) was investigated by means of nitrogen adsorption. The results of thermoporometry and nitrogen adsorption are summarized in Table 1. With prolonged duration of the hydrothermal treatment the pore volume does not change significantly, whereas the average pore size increases and the specific surface area decreases upon aging of the silica matrix [ 91. The increase of the average pore size is accompanied by a broadening of the pore size distribution (Table 1). The pore size distributions as obtained by ther-
232 TABLE 1 Textural characteristics of the catalyst spheres Catalyst
Nitrogen adsorption
HzO-thermoporometry
Sa(m2/g)
Wcm3/g)
Rf,(nm)
S (m”/g)
V,,(cm3/g)
R,(nm)
Ld(nm)
x0 X4 X7 X24
510 158 _e
0.8 0.8 -
2.8 13.0 -
105
0.8
10-30
514 104 71 37
0.8 0.8 0.8 0.8
3.8 19.6 20-30 30-50
0.7 2.2 3.8 7.8
HO H4 H7 H24
555 201 157 131
0.8 1.1 1.2 1.0
2.6 12.0 10-20 10-30
373 148 106 65
2.5 2.7 2.7 2.9
9.4
28-38 43-53 55-85
0.5 6.8 7.3 4.4
“S specific surface area. “VP specific pore volume. ‘R, main pore-radius. dL thickness adsorbed water layer. e- not determined.
moporometry are shown in Fig. 3. Detailed analysis of the nitrogen adsorption data according to procedures by Broekhoff-de Boer gave no evidence for the presence of micropores in any of the samples [ 11-161. The results presented in Table 1 and Fig. 3 clearly indicate the effect of the hydrothermal treatment on the texture of the spheres. Comparison of the results obtained by thermoporometry on the wet spheres with the results of nitrogen adsorption on the dried spheres shows the effect of drying on the porosity of the spheres. Upon drying, the pore volume of the X samples does not change, whereas drying the H samples causes a large decrease of the pore volume due to shrinkage of the porous structure. Since the X samples have been dried (shrunk) prior to the hydrothermal treatment no further shrinkage is observed upon drying of the treated spheres. After preparation, all catalyst spheres exhibit a pore volume of about 2.5 cm3/g and a pore size of approximately 9 nm, whereas after drying the pore volume is typically 0.8 to 1.0 cm3/g and the pore size is 3 to 30 nm depending on the hydrothermal treatment. With prolonged duration of the hydrothermal treatment the values obtained for the specific surface area and the main pore-radius of the X samples as obtained by thermoporometry and nitrogen adsorption show an increasing deviation (Table 1) . The difference between results obtained by both techniques is probably due the large amount of water which does not participate in the solidification-melting cycle with thermoporometry. This amount of water is determined by comparing the integrated heat of the waterice transition with the total amount of water in the sample [ 3-51. Water not
233 600 x0
zoo-
x24 0
I 20
0
I 40
60
80
I 100
pore-radius(nm)
250HO c G P
200-
k 6 5 2
i50-
H7
0 0
50
100
150
I 200 pore-radius(nm)
I 250
L
Fig. 3. Pore-size distribution of the wet spheres as determined by thermoporometry: effect of the hydrothermal treatment.
participating in the solidification-melting cycles is supposed to be located at the pore-wall creating a liquid adsorbed layer between the surface of the porous solid and the ice in the bulk of the pores. With most oxidic materials like silica and alumina this amount of water corresponds to a calculated layer thickness (L) of approximately 1 nm. With prolonged hydrothermal treatment of the present samples, however, the calculated layer thickness increases up to 7 nm (Table 1). In order to investigate the origin of the difference between the results ob-
234
tained by nitrogen adsorption and thermoporometry, we studied the applicability of both techniques on the two constituents of the catalyst spheres, viz., the silica gel matrix and the incorporated catalyst precursor. The texture of silica gel spheres can be accurately assessed by nitrogen adsorption as well as by thermoporometry. Both techniques show a reasonable agreement and with thermoporometry a calculated layer thickness of approximately 1 nm is observed independent of the hydrothermal treatment [ 31. In previous work it has been established that the plate-like structure of the catalyst precursor is hardly affected by the hydrothermal treatment [ 171. The texture of the pure dried catalyst precursor was investigated by nitrogen adsorption and thermoporometry. The pore-size distributions obtained by both techniques show a striking similarity (Fig. 4). Furthermore, the specific sur250 z 2
200 -
E UI 1503 -u IOO-
50 -
0 0
I
I 2
I 4
I 6
I 8
I / I 10 12 14 pore-radius (nm)
pore-radius
(nm)
Fig. 4. Pore-size distributions of dried (393 K) catalyst precursor as determined by: (a) thermo, porometry, (b) nitrogen adsorption.
235
face area (330 and 379 m”/g) and specific pore-volume (0.67 and 0.68 cm3/g), as measured by thermoporometry and nitrogen adsorption, respectively, show a close resemblance. The calculated water layer thickness (L) is 1.2 nm, close to the values normally observed with thermoporometry on oxidic materials. These results indicate that nitrogen adsorption and thermoporometry yield similar results with the catalyst precursor, and with the pure porous silica samples. However, deviations between nitrogen adsorption and thermoporometry arise when the catalyst suspension particles containing narrow pores are incorporated into a wide-pore silica matrix. This is due to the fact that, the experimental conditions (extremely low cooling-rate over a limited temperature range just below 273 K [ 3-5 ] ) applied during thermoporometry aiming at determination of the wider pores in the silica matrix leave the water in the narrow pores of the catalyst precursor unaffected. Water in pores of 30 nm radius freezes at about 271 K, whereas water in pores of 3 nm only freezes at about 250 K [3]. Thus, the (constant) volume of water in the narrow pores of the catalyst precursor particles is not frozen upon measurement of the wider pores of the silica matrix. However, since the total surface area of the sample decreases a high apparent layer thickness (L) is then calculated. The hydrothermal treatment hardly affects the texture of the catalyst precursor, whereas the texture of the silica gel matrix is significantly modified by this treatment. The increasingly divergent pore dimensions in the catalyst precursor and the silica matrix lead to an increasing difference between the results of nitrogen adsorption and thermoporometry. Based on the large calculated layer thickness (L), indicating the presence of a considerable amount of unfrozen water in narrow pores, it seems justified to assume that, under the experimental conditions applied, the thermoporometry only measures the texture of the smoothened silica matrix, whereas nitrogen adsorption measures the complete texture of the spheres, viz., both the silica matrix and the catalyst precursor. It is concluded that hydrothermal aging of the catalyst spheres reduces the specific surface area and increases the pore size, whereas the specific pore volume remains nearly constant. The hydrothermal treatment leaves the texture of the catalyst precursor unaffected, whereas the texture of the silica matrix surrounding the incorporated catalyst is significantly modified. Accessibility of the incorporated catalyst The performance of a solid catalyst not only depends upon the distribution of the active material over the support surface, but also upon the accessibility of the active material in the porous structure of the support. The effect of the texture of the silica matrix on the accessibility of the incorporated copper catalyst has been investigated by means of temperature-programmed reduction and by the catalytic oxidation of carbon monoxide to carbon dioxide.
236
Regardless of the hydrothermal treatment, the hydrogen consumption profiles recorded during reduction of the catalyst spheres are very similar to the profile obtained during reduction of the pure catalyst precursor (Fig. 5) [ 181. Hydrothermal treatment of the spheres does not affect the reducibility of the incorporated catalyst: the temperature of reduction and the degree of reduction remain constant with prolonged duration of the hydrothermal treatment. These results indicate that the reducibility of the catalyst precursor is not affected by incorporation into the porous silica gel nor by modification of the texture of the spheres by means of a hydrothermal treatment. The oxidation of carbon monoxide has been used to assess the catalytic performance of a series of copper-containing spheres of different pore dimensions. Details of the kinetics of this reaction have been given elsewhere [9]. The accessibility of the copper catalyst is reflected in the temperature dependence of the carbon monoxide conversion. By measuring the carbon monoxide conversion over a wide temperature-range, differences in the accessibility become apparent from the temperature at which diffusion limitations start to affect the conversion. The higher the temperature at which diffusion limitation becomes apparent, the better the accessibility [ 191. The carbon monoxide conversion curves obtained on catalyst spheres of different pore diameters are shown in Fig. 6. The measurements were highly reproducible and the conversion curves obtained upon increasing the temperature coincide with the curves obtained upon decreasing the temperature. The results in Fig. 6 indicate that, the catalytic oxidation of carbon monoxide proceeds at temperatures above 380 K, where the carbon monoxide conversion increases with increasing temperatures. However, depending on the texture of the spheres, the carbon monoxide conversion levels off at temperatures above 460 K. The slow increase of the conversion with temperatures exceeding 460
400
500
600
Temperature (K)
Fig. 5. Temperature-programmed reduction profiles: (a) reduction of the pure catalyst precursor, (b) reduction of an incorporated catalyst precursor.
237
300
400
500
600
700
Temperature
800
900
1000
(K)
Fig. 6. Carbon monoxide conversion as a function of the temperature for catalyst spheres of different pore dimensions. The amount of copper is the same in all samples (see Experimental).
6
i.O-
b
‘Z
& z
0.8 -
1
s 0.6 -
a
0.4 -
0.2 -
0.0 1 300
-i 400
500
600 Temperature
700
800
900
1000
(K)
Fig. 7. Carbon monoxide conversion as a function of the temperature for catalyst H4: (a) intact spheres, (b) crushed spheres.
K suggests that the rate of diffusion limits the conversion. The influence of diffusion limitations on the conversion is further illustrated by measurement of the carbon monoxide conversion over crushed H4 spheres. Crushing the spheres in order to shorten the distance between the external gas flow and the incorporated catalyst give rise to a considerable increase of the carbon monoxide conversion (Fig. 7). The increase of the carbon monoxide conversion established by shortening the diffusion path suggests that diffusion limitations in the pores of the inert silica matrix control (restrict) the conversion. Since the distribution of active material through the silica spheres is homogeneous and identical for all samples, the different conversion levels observed with spheres of different textures indicate that the accessibility of the incorporated
238
catalyst increases with the pore dimensions. A more detailed investigation into the accessibility (effectiveness) of the incorporated catalyst is to be published shortly [ 201. CONCLUSIONS
Porous copper-on-silica spheres have been prepared by incorporation of a finely divided catalyst precursor into a silica gel matrix. The newly developed preparation procedure facilitates independent control over the dispersion of the active material and the porous structure of the support. The effect of the texture of the spheres on the accessibility of the incorporated copper catalyst has been investigated. The copper-on-silica catalyst precursor particles (diameter lo-20 pm) are homogeneously distributed throughout the silica gel matrix. The texture of the silica matrix around the incorporated copper catalyst can be modified by application of a hydrothermal treatment. Hydrothermal aging reduces the specific surface area and increases the pore size, while the specific pore volume remains nearly constant. The incorporated catalyst can be completely reduced indicating that the active material remains accessible for hydrogen molecules. The performance of the incorporated catalyst depends upon the porous structure of the silica matrix. At elevated temperatures, the catalytic oxidation of carbon monoxide increases with the pore size of the catalyst spheres indicating that the accessibility of the incorporated catalyst is improved by the presence of wider pores in the silica matrix.
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12
M.R. Stammbach, M. Valix, D.L. Trimm and M.S. Wainwright, Appl. Catal., 55 (1989) 109. J.W. Geus, in G. Poncelet, P. Grange and P.A. Jacobs (Editors), Preparation of Catalysts III, Elsevier, Amsterdam, 1983, p. 1. J.F. Quinson, J. Dumas and J. Serughetti, J. Non-Cryst. Solids, 79 (1986) 397. C. Eyraud, J.F. Quinson and M. Brun, in K.K. Unger, J. Rouquerol, K.S.W. Sing and H. Kral (Editors), Characterization of Porous Solids, Elsevier, Amsterdam, 1988, p. 295. J.F. Quinson and M. Brun, in K.K. Unger, J. Rouquerol, K.S.W. Sing and H. Kral (Editors), Characterization of Porous Solids, Elsevier, Amsterdam, 1988, p. 307. J. van der Meijden, Preparation and Carbon Monoxide Oxidation of Supported Copper (oxide) Catalysts, PhD Thesis, University of Utrecht, 1980. C.J.G. van der Grift, P.A. Elberse, A. Mulder and J.W. Geus, Appl. Catal., 59 (1990) 275. M.K. Titulaer, private communication. I.I.M. Tijburg, Preparation and Properties of Thermostable Alumina-supported Copper Catalysts, PhD Thesis, University of Utrecht, 1989. R.K. Iler, The Chemistry of Silica, Wiley, New York, 1979. J.C.P. Broekhoffand J.H. de Boer, J. Catal., 9 (1967) 8. J.C.P. Broekhoff and J.H. de Boer, J. Catal., 9 (1967) 15.
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J.C.P. Broekhoff and J.H. de Boer, J. Catal., 9 (1967) 153. J.C.P. Broekhoff and J.H. de Boer, J. Catal., 9 (1967) 368. J.C.P. Broekhoff and J.H. de Boer, J. Catal., 9 (1967) 377. J.C.P. Broekhoff and J.H. de Boer, J. Catal., 9 (1967) 391. C.J.G. van der Grift, A. Mulder and J.W. Geus, Colloids Surf., in press. C.J.G. van der Grift, A. Mulder and J.W. Geus, Appl. Catal., 60 (1990) 181. C.N. Satterfield, Mass Transfer in Heterogeneous Catalysts, MIT Press, Massachusetts, 1970. A.Q.M. Boon, C.J.G. van der Grift, A.J.W. van Veldhuizen and J.W. Geus, in F. RodriguezReinoso (Editor), Characterization of Porous Solids II, Studies in Surface Science and Catalysis, Elsevier, Amsterdam, in press.
Applied
225
Preparation and characterization of porous silica spheres containing a copper(oxide) catalyst C.J.G. van der Grift*, A.Q.M. Boon, A.J.W. van Veldhuizen, H.G.J. Trommar and J.W. Geus Department
of Inorganic
3522 AD Utrecht
Chemistry,
(The Netherlands),
University
of Utrecht,
tel. (+31-30)
Croesestraat
778,
890819
J.F. Quinson Universiti
Claude Bernard
U.A. CNRS 417,43
Lyon Z, Laboratoire
Bd. du 11 Novembre
de Chimie Appliquei
1918,69622
Villeurbanne
et Genie Chimique, (France)
and M. Brun Ecole Centrale
de Lyon, Laboratoire
des Machines
Thermiques,
69130 Ecully (France)
(Received 27 March 1990, revised manuscript received 11 June 1990)
ABSTRACT Porous catalyst (sorbent) spheres have been prepared by incorporation of a finely divided catalyst precursor into a silica gel matrix. The newly developed preparation procedure facilitates independent control over the dispersion of the active material and the porous structure of the support. As an example the preparation of porous copper-on-silica spheres is described. Scanning electron microscopy shows that the incorporated catalyst is homogeneously distributed throughout the silica gel matrix. The texture of the porous spheres has been investigated by means of water thermoporometry and nitrogen adsorption. The pore structure of the spheres can be modified by application of a hydrothermal treatment. Under prolonged hydrothermal treatment the mean pore size gradually increases from 3 to 30 nm, whereas the specific surface area simultaneously decreases from 550 to 100 m*/g. The effect of the texture of the silica matrix on the accessibility of the incorporated copper catalyst has been investigated by means of temperatureprogrammed reduction and by catalytic oxidation of carbon monoxide to carbon dioxide. The results indicate that the accessibility of the incorporated catalyst is improved by the presence of wider pores in the silica matrix. silica spheres, pore size, copper/silica, catalyst characterization (TPR), carbon monoxide oxidation, catalyst preparation (support).
Keywords:
INTRODUCTION
Supported metal catalysts and inorganic sorbents usually exhibit a specific surface area of some hundreds of square meters per gram. In many reactions a
0166-9834/90/603.50
0 1990 -
Elsevier Science Publishers B.V.
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high surface area per unit volume is required in order to provide a large contact area between the active solid and the gaseous reactants. Application of catalysts or sorbents in chemical reactors calls for the formation of solid bodies of a particular size (1 cm to 1 mm) and shape (spheres, trilobes, etc.) depending on the type of reactor used. Due to these requirements the desired high specific surface areas can only be established with bodies of a significant porosity. As a result, a large fraction of the active surface is located inside the porous structure. To achieve a good performance of the catalyst (sorbent), the porous structure of the bodies should facilitate a rapid transport of the reactants to and products from the active material. In addition to the texture of the bodies, the dispersion and the distribution of the active material over the surface of the support affect the performance of the catalyst system. Generally, a refractory inorganic support of high surface area is used to disperse and to stabilize the active component. In order to take full advantage of the large specific surface area of the support, a homogeneous distribution of the active material, or of its precursor, should be achieved during the preparation. Furthermore, to achieve a good performance of the catalyst (sorbent) the dispersion of the active material must be optimized. With the preparation of catalyst (sorbent) particles it is thus of prime importance to control both the porous structure of the bodies and the distribution of the active material over the support. Usually, however, it is hard to prepare catalysts exhibiting both the desired porous structure and the required distribution of the active material over the support. Using a finely divided support the active material can be deposited onto the carrier by precipitation, adsorption or ion-exchange from a homogeneous aqueous solution. Although the catalyst precursor thus obtained may exhibit a homogeneous distribution of the active material, control over the texture of the final catalyst body is generally poor. Alternatively, a pre-shaped support of a preferred texture may be loaded with active material by impregnation, adsorption or ion-exchange. However, especially with higher metal loadings a homogeneous distribution of the active material over the support bodies is difficult to achieve. In this paper we will describe a newly developed preparation procedure for the production of porous catalyst spheres. The preparation of porous spheres to be used as a catalyst support is usually brought about by sol-gel techniques which allow fixation of any desired shape of the bodies. Porous spheres are prepared by spraying a sol of the support with simultaneous conversion of the sol droplets into rigid gel spheres. Subsequent loading of the porous support spheres generally provides little control over the distribution of the active material. Here, we follow an alternative approach which is based on the homogeneous incorporation of a separately prepared catalyst precursor into porous (silica) gel spheres. The catalyst precursor is prepared by deposition-precipitation of the active material onto silica (primary support). A suspension of the catalyst precursor thus obtained is homogenized with a silica sol which is sub-
227
sequently sprayed into a mixture of hot paraffinic hydrocarbons to form the gel spheres. The catalyst precursor is thus incorporated into a porous silica gel matrix (secondary support). Following the formation of the spheres, the texture of the silica matrix can be adapted by a hydrothermal treatment. This preparation route enables us to produce porous catalyst spheres of which both the dispersion of the active material and the texture of the support can be controlled independently. As an example, we will describe the production of copper-on-silica spheres. The importance of shaping of copper-on-silica catalysts for the hydrolysis of acrylonitrile to acrylamide was recently demonstrated by Stammbach et al. [ 11. The porous catalyst spheres were prepared by sol-gel conversion of an aqueous mixture containing a copper-on-silica catalyst prepared by homogeneous deposition-precipitation [ 21 and a silica sol. The morphology of the porous spheres and the distribution of the supported copper catalyst throughout the silica spheres was investigated by means of scanning electron microscopy. To investigate the effect of a hydrothermal treatment on the porous structure of the catalyst spheres, batches of initially identical spheres (taken from the same ‘mother batch’) were subjected to various treatments. The porous structure of the various samples was assessed by means of thermoporometry [ 3-51. This technique enabled us to investigate the texture of the wet porous structure, whereas nitrogen adsorption was used to characterize the texture of the dried spheres. Thus, by comparing the results of these two techniques the effect of drying on the pore structure can be monitored. The oxidation of carbon monoxide was used to assess the catalytic performance of a series of coppercontaining spheres of different pore dimensions. EXPERIMENTAL
A copper-on-silica catalyst was prepared by homogeneous deposition-precipitation [ 2,6]. The hydrolysis of urea was used to raise the pH in a vigorously stirred aqueous suspension containing the silica support (ZOOV Aerosil, Degussa, F.R.G.) and dissolved copper nitrate (p.a. Merck). After complete precipitation of the copper ions onto the support, the catalyst precursor was washed extensively and concentrated to a suspension containing 0.11 g catalyst per g suspension. Characterization of the catalyst precursor indicates a high dispersion of the copper ions over the silica support as in the mineral copper hydrosilicate chrysocolla, making it a suitable precursor for incorporation into the gel matrix [ 71. The copper-on-silica catalyst had a metal loading of 20 wt.-% [lOO%*gCu/(gCu+ gSiOa)]. The silica sol was prepared by addition of a sodium silicate solution (approximately 3.5 M) to 40 ml of nitric acid (approximately 2.5 M) at 273 K until a pH of 1 was reached. At this pH level, 30 g of the catalyst suspension was added to the silica sol. Upon introduction of the (slightly basic, pH w 8)
228
catalyst suspension the pH should not rise above 5 to prevent premature gelation giving rise to undesired inhomogeneities. After homogenization of the silica sol and the catalyst suspension (preferably at pH E 3 ), the pH was carefully increased to 5 by further addition of the sodium silicate solution. Next, the mixture was sprayed as small droplets, 1 to 4 mm in diameter, into a hot (353 K) mixture of paraffrnic hydrocarbons to bring about the sol-gel conversion. A schematic drawing of the all-glass assembly is given in Fig. 1. The hydrogel spheres as obtained by this procedure were washed extensively. The copper loading of the thus prepared copper-on-silica spheres corresponds to approximately 3.4 wt.-% of the dry weight. Next, eight batches of the hydrogel spheres were separated and subjected to various treatments. Four batches were dried in air at 393 K to give the xerogel (X) samples, whereas the other four were stored in excess water as hydrogel (H) samples. Both X and H samples were subjected to a hydrothermal treatment at 173 K in an autoclave containing 4.5 g solid and 20.5 g water in a total volume of 48 ml. During the hydrothermal treatment the spheres were saturated with a 0.5-M sodium chloride solution to prevent destruction of the spherical shape by osmosis induced by the release of sodium ions during aging of the silica gel [8]. After the hydrothermal treatment the spheres were thoroughly washed to remove NaCl and coded X(h) or H(h) in which h is the duration (h) of the hydrothermal treatment. The wet samples obtained after hydrothermal treatment were characterized by water-thermoporometry according to procedures described by Quinson et
~
Stock-vessel
(0.07L~
T IOcm I
I
Fng paraffwcolumn
expanwn
water
_____
(51
rings
(2L)
Plug
Fig. 1. Equipment used for preparation of the catalyst spheres.
229
al. [ 31, Eyraud et al. [ 41 and Quinson and Brun [ 51. Both solidification and melting of water in the porous structure was investigated with a differential scanning calorimeter (DSC, model 101, Setaram). The equations used to calculate the textural data have been described previously [ 3-5 1. After drying (at 393 K in air), the texture of the same series of hydrothermally treated samples was investigated by means of nitrogen adsorption at 77 K (Omnisorb 100, Omicron). Prior to adsorption measurements the samples were outgassed in situ at 393 K and 0.13 Pa for 16 h. The morphology of the dried spheres and the distribution of copper (oxide) through the porous structure were studied by scanning electron microscopy (SEM ) (Cambridge 15OS, microscope ). The accessibility of the incorporated catalyst was investigated by means of temperature-programmed reduction. Approximately, 100 mg of dried catalyst spheres (1-2 mm) was subjected to in situ calcination (oxidation)-reduction cycles in a micro-reactor with an internal diameter of 8.0 mm. Calcination (oxidation) was performed by heating the dried catalyst spheres from ambient to 723 K at 5 K/min in a 2 vol.-% O,/He flow (50 ml/min). Temperatureprogrammed reduction of the calcined (oxidized) catalyst spheres was performed by heating from ambient to 723 K at 5 K/min in a 10 vol.-% HZ/N2 flow (50 ml/min). The performance of the incorporated catalyst in the oxidation of carbon monoxide was assessed by measuring the activity (conversion) of the catalyst spheres over a wide temperature range (300 to 900 K). Prior to the activity measurements the catalyst spheres were calcined in 10 vol.-% 0z/N2 at 723 K, reduced in 10 vol.-% Hz/N2 up to 973 K and re-oxidized in 10 vol.-% Oz/N2 at 723 K. At room temperature, the re-oxidized catalyst was then subjected to a gas flow of 400 ml/min (0.25 vol.-% CO, 1 vol.-% 02, 98.75 vol.-% N2)+ Subsequently, the catalytic oxidation of carbon monoxide was measured as a function of the temperature. Details of the equipment used for these measurements are given elsewhere [ 91. RESULTS AND DISCUSSION
Preparation of the spheres The preparation of the spheres requires a rapid sol-gel conversion in the droplets of the catalyst-sol mixture sprayed into the hot mixture of paraffinic hydrocarbons. Using the equipment shown in Fig. 1,5 to 7 s are available to bring about sol-gel conversion before the droplets reach the bottom of the column. The rate of the gel formation depends on a variety of parameters such as the temperature, the pH, the silica concentration and the ionic-strength of the sol [lo]. The preparation conditions should provide a homogeneous catalystsol mixture stable enough to refrain from premature gelation, but sufficiently
230
reactive to form hydrogel spheres once sprayed into the hot mixture of alkanes. The conditions described in the Experimental section allow the preparation of such a sol from which the catalyst hydrogel spheres can be formed. The amount of catalyst suspension that can be incorporated into the silica gel matrix (i.e. the metal loading of the final spheres) is limited by a number of parameters such as, e.g., the concentration and the basicity of the catalyst suspension as well as the silica concentration and the acidity of the silica sol. Clearly, the maximum amount of catalyst precursor that can be incorporated into the silica gel is set by the maximum concentration of the catalyst suspension and the minimum amount of silica needed to develop a rigid gel structure around the (individual) suspension particles. In order to achieve a minimum fluidity (needed for homogenization of the catalyst precursor with the silica sol) the solid concentration of the catalyst suspension is limited. Another limitation is set by the concentration of the silica sol. Decreasing the silica concentration of the sol gives rise to rather weak hydrogel spheres, whereas too high silica concentrations result in premature, inhomogeneous gel formation. The recipe used in this work is based on experience but may be subject to further optimization. Scanning electron microscopy Fig. 2a shows an electron micrograph of the copper-on-silica spheres. This picture shows the spherical shape of the catalyst particles (diameter 1 to 2 mm). A micrograph taken at a higher magnification shows the roughness of the external surface which depends on the applied hydrothermal treatment (Fig. 2b). The surface roughness is caused by shrinkage of the gel-spheres upon removal of water from the pores during drying. Since the catalyst precursor particles do not shrink as much as the surrounding silica gel matrix, drying gives rise to a ‘puckered’ surface. Prolonged hydrothermal aging of the silica gel matrix leads to smoothening of the external surface. Several spheres were cleaved to investigate the distribution of the catalyst particles throughout the spheres (Fig. 2b). The distribution of copper over the external surface and the cleavage plane shown in Fig. 2c was investigated by means of backscattered electron microscopy (BEI). The intensity of the backscattered electron beam is sensitive to the atomic mass of the local elements in the sample. Due to the higher atomic mass of copper as compared to silicon and oxygen, the catalyst suspension particles are seen as bright spots. BE1 shows that the catalyst particles are (10 to 20 ,um in diameter) homogeneously distributed over the investigated surfaces (Figs. 2c and d). It is thus concluded that the catalyst precursor is homogeneously dispersed throughout the body of the porous silica spheres.
231
Fig. 2. SEM photographs: (a) spherical particles, (b) cleaved sphere, (c and d) morphology (EI) and composition (BEI) of cleaved sphere.
Thermoporometry and nitrogen adsorption Prior to the hydrothermal treatment the X samples were dried at 400 K (air) and subsequently re-immersed into the aqueous sodium chloride solution, whereas the H samples were treated in the NaCl solution without intermediate drying. The exact conditions of the hydrothermal treatment are given in the Experimental section. After the hydrothermal treatment the spheres were washed extensively with excess de-ionized water to remove dissolved salts. Subsequently, the texture of the wet spheres was characterized by thermoporometry, whereas the texture of the dried spheres (393 K, air) was investigated by means of nitrogen adsorption. The results of thermoporometry and nitrogen adsorption are summarized in Table 1. With prolonged duration of the hydrothermal treatment the pore volume does not change significantly, whereas the average pore size increases and the specific surface area decreases upon aging of the silica matrix [ 91. The increase of the average pore size is accompanied by a broadening of the pore size distribution (Table 1). The pore size distributions as obtained by ther-
232 TABLE 1 Textural characteristics of the catalyst spheres Catalyst
Nitrogen adsorption
HzO-thermoporometry
Sa(m2/g)
Wcm3/g)
Rf,(nm)
S (m”/g)
V,,(cm3/g)
R,(nm)
Ld(nm)
x0 X4 X7 X24
510 158 _e
0.8 0.8 -
2.8 13.0 -
105
0.8
10-30
514 104 71 37
0.8 0.8 0.8 0.8
3.8 19.6 20-30 30-50
0.7 2.2 3.8 7.8
HO H4 H7 H24
555 201 157 131
0.8 1.1 1.2 1.0
2.6 12.0 10-20 10-30
373 148 106 65
2.5 2.7 2.7 2.9
9.4
28-38 43-53 55-85
0.5 6.8 7.3 4.4
“S specific surface area. “VP specific pore volume. ‘R, main pore-radius. dL thickness adsorbed water layer. e- not determined.
moporometry are shown in Fig. 3. Detailed analysis of the nitrogen adsorption data according to procedures by Broekhoff-de Boer gave no evidence for the presence of micropores in any of the samples [ 11-161. The results presented in Table 1 and Fig. 3 clearly indicate the effect of the hydrothermal treatment on the texture of the spheres. Comparison of the results obtained by thermoporometry on the wet spheres with the results of nitrogen adsorption on the dried spheres shows the effect of drying on the porosity of the spheres. Upon drying, the pore volume of the X samples does not change, whereas drying the H samples causes a large decrease of the pore volume due to shrinkage of the porous structure. Since the X samples have been dried (shrunk) prior to the hydrothermal treatment no further shrinkage is observed upon drying of the treated spheres. After preparation, all catalyst spheres exhibit a pore volume of about 2.5 cm3/g and a pore size of approximately 9 nm, whereas after drying the pore volume is typically 0.8 to 1.0 cm3/g and the pore size is 3 to 30 nm depending on the hydrothermal treatment. With prolonged duration of the hydrothermal treatment the values obtained for the specific surface area and the main pore-radius of the X samples as obtained by thermoporometry and nitrogen adsorption show an increasing deviation (Table 1) . The difference between results obtained by both techniques is probably due the large amount of water which does not participate in the solidification-melting cycle with thermoporometry. This amount of water is determined by comparing the integrated heat of the waterice transition with the total amount of water in the sample [ 3-51. Water not
233 600 x0
zoo-
x24 0
I 20
0
I 40
60
80
I 100
pore-radius(nm)
250HO c G P
200-
k 6 5 2
i50-
H7
0 0
50
100
150
I 200 pore-radius(nm)
I 250
L
Fig. 3. Pore-size distribution of the wet spheres as determined by thermoporometry: effect of the hydrothermal treatment.
participating in the solidification-melting cycles is supposed to be located at the pore-wall creating a liquid adsorbed layer between the surface of the porous solid and the ice in the bulk of the pores. With most oxidic materials like silica and alumina this amount of water corresponds to a calculated layer thickness (L) of approximately 1 nm. With prolonged hydrothermal treatment of the present samples, however, the calculated layer thickness increases up to 7 nm (Table 1). In order to investigate the origin of the difference between the results ob-
234
tained by nitrogen adsorption and thermoporometry, we studied the applicability of both techniques on the two constituents of the catalyst spheres, viz., the silica gel matrix and the incorporated catalyst precursor. The texture of silica gel spheres can be accurately assessed by nitrogen adsorption as well as by thermoporometry. Both techniques show a reasonable agreement and with thermoporometry a calculated layer thickness of approximately 1 nm is observed independent of the hydrothermal treatment [ 31. In previous work it has been established that the plate-like structure of the catalyst precursor is hardly affected by the hydrothermal treatment [ 171. The texture of the pure dried catalyst precursor was investigated by nitrogen adsorption and thermoporometry. The pore-size distributions obtained by both techniques show a striking similarity (Fig. 4). Furthermore, the specific sur250 z 2
200 -
E UI 1503 -u IOO-
50 -
0 0
I
I 2
I 4
I 6
I 8
I / I 10 12 14 pore-radius (nm)
pore-radius
(nm)
Fig. 4. Pore-size distributions of dried (393 K) catalyst precursor as determined by: (a) thermo, porometry, (b) nitrogen adsorption.
235
face area (330 and 379 m”/g) and specific pore-volume (0.67 and 0.68 cm3/g), as measured by thermoporometry and nitrogen adsorption, respectively, show a close resemblance. The calculated water layer thickness (L) is 1.2 nm, close to the values normally observed with thermoporometry on oxidic materials. These results indicate that nitrogen adsorption and thermoporometry yield similar results with the catalyst precursor, and with the pure porous silica samples. However, deviations between nitrogen adsorption and thermoporometry arise when the catalyst suspension particles containing narrow pores are incorporated into a wide-pore silica matrix. This is due to the fact that, the experimental conditions (extremely low cooling-rate over a limited temperature range just below 273 K [ 3-5 ] ) applied during thermoporometry aiming at determination of the wider pores in the silica matrix leave the water in the narrow pores of the catalyst precursor unaffected. Water in pores of 30 nm radius freezes at about 271 K, whereas water in pores of 3 nm only freezes at about 250 K [3]. Thus, the (constant) volume of water in the narrow pores of the catalyst precursor particles is not frozen upon measurement of the wider pores of the silica matrix. However, since the total surface area of the sample decreases a high apparent layer thickness (L) is then calculated. The hydrothermal treatment hardly affects the texture of the catalyst precursor, whereas the texture of the silica gel matrix is significantly modified by this treatment. The increasingly divergent pore dimensions in the catalyst precursor and the silica matrix lead to an increasing difference between the results of nitrogen adsorption and thermoporometry. Based on the large calculated layer thickness (L), indicating the presence of a considerable amount of unfrozen water in narrow pores, it seems justified to assume that, under the experimental conditions applied, the thermoporometry only measures the texture of the smoothened silica matrix, whereas nitrogen adsorption measures the complete texture of the spheres, viz., both the silica matrix and the catalyst precursor. It is concluded that hydrothermal aging of the catalyst spheres reduces the specific surface area and increases the pore size, whereas the specific pore volume remains nearly constant. The hydrothermal treatment leaves the texture of the catalyst precursor unaffected, whereas the texture of the silica matrix surrounding the incorporated catalyst is significantly modified. Accessibility of the incorporated catalyst The performance of a solid catalyst not only depends upon the distribution of the active material over the support surface, but also upon the accessibility of the active material in the porous structure of the support. The effect of the texture of the silica matrix on the accessibility of the incorporated copper catalyst has been investigated by means of temperature-programmed reduction and by the catalytic oxidation of carbon monoxide to carbon dioxide.
236
Regardless of the hydrothermal treatment, the hydrogen consumption profiles recorded during reduction of the catalyst spheres are very similar to the profile obtained during reduction of the pure catalyst precursor (Fig. 5) [ 181. Hydrothermal treatment of the spheres does not affect the reducibility of the incorporated catalyst: the temperature of reduction and the degree of reduction remain constant with prolonged duration of the hydrothermal treatment. These results indicate that the reducibility of the catalyst precursor is not affected by incorporation into the porous silica gel nor by modification of the texture of the spheres by means of a hydrothermal treatment. The oxidation of carbon monoxide has been used to assess the catalytic performance of a series of copper-containing spheres of different pore dimensions. Details of the kinetics of this reaction have been given elsewhere [9]. The accessibility of the copper catalyst is reflected in the temperature dependence of the carbon monoxide conversion. By measuring the carbon monoxide conversion over a wide temperature-range, differences in the accessibility become apparent from the temperature at which diffusion limitations start to affect the conversion. The higher the temperature at which diffusion limitation becomes apparent, the better the accessibility [ 191. The carbon monoxide conversion curves obtained on catalyst spheres of different pore diameters are shown in Fig. 6. The measurements were highly reproducible and the conversion curves obtained upon increasing the temperature coincide with the curves obtained upon decreasing the temperature. The results in Fig. 6 indicate that, the catalytic oxidation of carbon monoxide proceeds at temperatures above 380 K, where the carbon monoxide conversion increases with increasing temperatures. However, depending on the texture of the spheres, the carbon monoxide conversion levels off at temperatures above 460 K. The slow increase of the conversion with temperatures exceeding 460
400
500
600
Temperature (K)
Fig. 5. Temperature-programmed reduction profiles: (a) reduction of the pure catalyst precursor, (b) reduction of an incorporated catalyst precursor.
237
300
400
500
600
700
Temperature
800
900
1000
(K)
Fig. 6. Carbon monoxide conversion as a function of the temperature for catalyst spheres of different pore dimensions. The amount of copper is the same in all samples (see Experimental).
6
i.O-
b
‘Z
& z
0.8 -
1
s 0.6 -
a
0.4 -
0.2 -
0.0 1 300
-i 400
500
600 Temperature
700
800
900
1000
(K)
Fig. 7. Carbon monoxide conversion as a function of the temperature for catalyst H4: (a) intact spheres, (b) crushed spheres.
K suggests that the rate of diffusion limits the conversion. The influence of diffusion limitations on the conversion is further illustrated by measurement of the carbon monoxide conversion over crushed H4 spheres. Crushing the spheres in order to shorten the distance between the external gas flow and the incorporated catalyst give rise to a considerable increase of the carbon monoxide conversion (Fig. 7). The increase of the carbon monoxide conversion established by shortening the diffusion path suggests that diffusion limitations in the pores of the inert silica matrix control (restrict) the conversion. Since the distribution of active material through the silica spheres is homogeneous and identical for all samples, the different conversion levels observed with spheres of different textures indicate that the accessibility of the incorporated
238
catalyst increases with the pore dimensions. A more detailed investigation into the accessibility (effectiveness) of the incorporated catalyst is to be published shortly [ 201. CONCLUSIONS
Porous copper-on-silica spheres have been prepared by incorporation of a finely divided catalyst precursor into a silica gel matrix. The newly developed preparation procedure facilitates independent control over the dispersion of the active material and the porous structure of the support. The effect of the texture of the spheres on the accessibility of the incorporated copper catalyst has been investigated. The copper-on-silica catalyst precursor particles (diameter lo-20 pm) are homogeneously distributed throughout the silica gel matrix. The texture of the silica matrix around the incorporated copper catalyst can be modified by application of a hydrothermal treatment. Hydrothermal aging reduces the specific surface area and increases the pore size, while the specific pore volume remains nearly constant. The incorporated catalyst can be completely reduced indicating that the active material remains accessible for hydrogen molecules. The performance of the incorporated catalyst depends upon the porous structure of the silica matrix. At elevated temperatures, the catalytic oxidation of carbon monoxide increases with the pore size of the catalyst spheres indicating that the accessibility of the incorporated catalyst is improved by the presence of wider pores in the silica matrix.
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