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The utilization of saturated gas-solid reactions in the preparation of heterogeneous catalysts
PREPARATION OF CATALYSTSVI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
The utilization of satm'ated heterogeneous catalysts
gas-solid
reactions
957
in
the
preparation
of
S.Haukka, A.KytOkivi, E-L.Lakomaa, U.Lehtovirta, M.Lindblad, V.Lujala, T.Suntola Microchemistry Ltd., P.O.Box 45, 02151 Espoo, Finland
ABSTRACT Saturated gas-solid reactions known from Atomic Layer Epitaxy (ALE) were used to process various catalysts. Good homogeneity of metal species was verified both along the entire catalyst bed and inside the particles. A variety of volatile metal compounds including metal chlorides, alkoxides and 13-diketonates were successfully used as reactants. The ALE processing is described with reference to examples demonstrating the achievement of surface saturation, reproducibility of processes, selection of process parameters, growth of oxides to modify the support and the binding of two metal compounds.
1. INTRODUCTION Gas-solid reactions are being widely explored for their potential in the manufacture of structurally well-defined catalytic surfaces. According to 1UPAC recommendations [1], deposition taking place by adsorption or reaction from the gas phase is called chemical vapour deposition (CVD). Examples of the use of CVD in tailoring support surfaces and in binding active components to the support can be found in references 2 - 7. Interesting features have been introduced to catalysts by these methods. Molecular level control and uniformity through the particles have not always been achieved, however. We describe here means to a better controlled preparation of catalysts by making use of gas-solid reactions and the property of the surface to saturate itself with the reactant as suggested in the growth of layer by layer structures in Atomic Layer Epitaxy (ALE) [8]. In the ALE technique, a better control of the build-up of surface structures is achieved by the sequential introduction of the active components, and other surface-modifying agents, in saturating gas-solid reactions. ALE can be classified as a special mode of CVD, since strict demands are made upon the conditions under which the gas-solid reactions are carried out. Uncontrolled deposition through condensation of the reactants or their decomposition products is prevented by the choice of reaction temperature. Not only must the solid surface be saturated with the chemisorbed species, but it must be stabilized before each reaction sequence. This means that physisorbed molecules such as water must be removed from the starting support surface by heating, and after each reaction any unreacted reactant must be removed, normally by inert gas purge.
958 For well-defined structures to be produced on the support surface chemisorption is required. The surface species then occupy their final bonding sites at the outset and there is no need for alter-treatment at elevated temperature. Through saturation of the surface, the surface density obtained during each reactant sequence is controlled by the surface itself. The capacity of the surface to chemisorb the reactant, not the dosing of the reactant, determines the amount bonded. The processing of the catalyst is not sensitive to a precise dosing of the reactant; all that is required is that the dose be sufficient for the reaction of all binding sites. This selfcontrolling feature of ALE allows a homogeneous (uniform) distribution of surface species throughout the porous support and gives good reproducibility in obtaining a desired saturation level. The self-control is also a good feature for scale-up. The saturation density obtained in a reaction sequence depends on the number of the bonding sites, the size and chemical form of the reactant molecule and the reaction temperature. If part of the original ligands of the reactants remain present in the surface complex the saturation density will usually be less than the full monolayer coverage of the corresponding oxide. Various means to regulate the saturation level have been reviewed in [9]. Active catalysts for a variety of reactions have been processed by the ALE method [ 10-13 ]. We shall present some examples of how the surface saturation proceeds using 5 - 1000 g of alumina or silica supports and of the reproducibility of ALE in binding a single metal compound on the surface. As well, the selection of reaction conditions will be described, and examples will be given of modifying the surface with a sublayer of a metal oxide and of binding two different metal species. 2. EXPERIMENTAL
2.1. Equipment The catalysts were processed in flow-type reactors with heated zones for the reaction vessel and for vaporizing solid reagents [9, 14]. Processing was done at a pressure of 6-10 kPa or at ambient pressure in nitrogen atmosphere. Reaction vessels of 0.1 - 2 1 were used. The heating and gas valving were computer controlled. 2.2. Reagents The support materials were 7-A1203 (AKZO Alumina 000-1.5E) and silica (Grace 955) with surface areas of 200 and 270 m2/g, respectively. Alumina was used as extrudates with a particle diameter of 1.5 mm and a length between 2 and 20 mm and as crushed and sieved to a particle size of 0.15 - 0.35 mm. The particle size of the silica was 40-80 ~tm. ZrCI 4 (Fluka), TiC14 (Merck), WOCI4 (Aldrich), Cr(acac)3 (Riedel-de-Haen,), Ni(acac)2 (Merck), Mg(thd)2 and Ti(OC3H7) 4 (Merck) were used as reactants without further purification. Mg(thd)2 was synthetized according to [ 15]. 2.3. Procedure The number of bonding sites was stabilized to a selected level by preheating the support at temperatures of 200-850 ~ The reactants were then volatilized from liquids or solids and led to the top of the solid support bed held at a selected temperature. The reaction temperature
959 was selected so that the activation energy for chemisorption was exceeded and the decomposition or condensation of the reactant was prohibited. The dose of the reactant was kept high enough to exceed the number of bonding sites available. The reaction time required was calculated so that an overdose of the reactant as compared with the number of bonding sites was brought into the reaction vessel. A purge with inert gas followed the chemisorption, at the same temperature. The efficiency of transport of the reactant vapour into the reactor is determined by the vapour pressure of the reactant at the vaporization temperature selected and the rate of the nitrogen flow. The process can consist of one chemisorption stage or of several stages each followed by an inert gas purge to avoid the presence of two reactants in the reaction chamber at the same time.
2.4. Characterization Inert sampling could be done when desired. Zr, W and Ni were determined by XRF, Ti and Cr by neutron activation analysis (NAA), Mg by AAS, C with a Leco carbon analyzer and CI by potentiometric titration. FTIR in diffuse reflectance mode was used to follow the chemisorption and to detect possible decomposition of the reactant. Scanning electron microscopy with an energy dispersive spectrometer (SEM/EDS) was used to determine element concentrations through the particles. The specific surface area and pore volume were determined by means of nitrogen adsorption and condensation with lk,ficromeritics ASAP 2400 equipment. Detailed experimental conditions used in the characterization are in Ref. 16.
3. RESULTS
3.1. Surface saturation and catalyst homogeneity The saturation of the support surface with the reactant was followed by taking samples from the top and bottom parts of the support bed. Figure 1 shows the achievement of surface saturation as a function of reactant dose. Saturation of bonding sites proceeds from the top of the support bed towards the bottom, i.e. in the direction of the reactant flow.
0
E E tO
0.6 0
o
0.4
Top
t__ t| 0
Bottom
0.2
0 0
|
0.0
0.2
0.4
0.6
0.8
1.0
Reactant dose (mmol/g)
Figure 1. Surface saturation (mmol/g) as a function of the reactant dose (mmol/g). Metal determinations in samples taken from the top and the bottom of the fixed bed were made.
960 The homogeneity of saturated samples was also verified for a larger scale processing with 300 -1000 g of support. Figure 2 shows the variation in Zr saturation density in different parts of a
Zr/SiO 2 catalyst bed containing 300 g of the catalyst.
l
120 o~ >,, e-.
~
t-
100 80 60
O
~
I
q/
6 cm
=
40
=
20
0
1 ~
15 cm >
2
3
4
5
Sampling point
Figure 2. Macroscopic homogeneity of Zr in larger scale processing from ZrC14 on SiO 2.
Macroscopic homogeneity does not rule out the existence of a concentration gradient within the particles. Uniformity within the particles was therefore investigated by embedding the catalyst in epoxy resin, cutting cross-sections of particles with a microtome and analysing by SEM/EDS. Macroscopically homogeneous W/A1203extrudate samples were found to contain an uneven distribution of W because of a too fast flow rate of the reactant in processing. The combination of extrudate support and a fast WOCI4 reactant flow rate resulted in the loss of the reactant through the bed. Table 1 shows the tungsten concentration of W/A1203 samples with two different flow rates of the reactant through powder and extrudate beds. Reaction time was constant. The lower tungsten content for the catalyst processed from extrudates at the higher reactant flow rate was due to an eggshell distribution, as revealed in SEM/EDS analysis.
961
Table 1 Tungsten and chloride concentration of extrudate and powder samples with fast and slow WOC14 flow rate. The preheating of alumina and the reaction with WOC14 were carried out at 200~ Particle size Flow rate W (wt-%) CI (wt-%) powder powder extrudates extrudates
fast slow fast slow
11 11 6 11
4.8 4.8 2.3 5.1
3.2 Reproducibility The reproducibility from run to run was investigated by determining the metal concentration in samples taken from the top and bottom of the fixed catalyst bed. Table 2 shows the reproducibility of binding a single metal compound to the support surface for different metal compound/support pairs. The reproducibility was within the accuracy of the element determination methods used ( XRF, NAA, AAS).
Table 2 Reproducibility of saturated metal concentrations (mean value _+ standard deviation) in different runs. Samples were taken from the top and bottom of the catalyst bed. Metal compound/ Preheating/ Metal (wt-%) Metal (wt-%) Number support pair reaction top bottom of temperature process (~ runs ZrCI4/SiO 2 300 / 300 6.6 + 0.2 6.6 + 0.2 5 ZrCI4 / SiO2 600 / 450 2.7 + 0.1 2.7 _+ 0.1 6 WOCI4 / AI20 3 200 / 200 10.6 + 0.4 10.6 + 0.3 10 Ni(acac)2/AI20 3 200 / 200 4.7 + 0.5 4.2 + 0.2 5 Cr(acac)3/SiO 2 820 / 200 0.75+ 0.03 0.70+ 0.0 3 Mg(thd)2/SiO 2 600 / 250 1.2 + 0.1 1.0 + 0.1 6
3.3. Process parameters Several ZffSiO 2 catalysts were processed by using three different reactors: two small-scale reactors operating under vacuum and at ambient pressure and a bench-scale reactor operating at ambient pressure. Figure 3 describes the control of the Zr concentration in Zr/SiO 2 as a function of the preheating temperature of SiO2. The results for processes carried out in the different reactors are in good agreement, demonstrating that surface saturation was achieved
962
o-.O. v
tO Ira. 4-o
C:
ID O C: o o
0 200
t___
N
400
600
800
Preheating temperature (~ Figure 3. The Zr concentration as a function of the preheating temperature Of SiO 2 in ZrCl4/SiO 2 processed with different reactors. Reaction temperature 300 ~ - V - 6 kPa, (5-10 g) + ambient pressure, 1 kg and 10 g , reaction temperature 450 ~ --o-- 10 kPa, (5-10 g) 9 ambient pressure in two different equipment (5-10 g).
A
5
10
C/Ti ratio
Ti (wt-%)
10
~o O~O
~
0
_ _
100 |
*
. . . .
3900
|
.
3400
9
,
,
i
r3~O
150
o
0
200
R e a c t i o n t e m p e r a t u r e (~
2900
Wavenumber (cm "1) Figure 4. (a) FTIR-spectra, in the O-H- and C-H-vibration regions, of Ti(OPr)4/),-A1203 prepared at 100, 110, 150, 170 and 190~ (from the top to the bottom of the picture) compared with 800~ alumina, and (b) the corresponding C/Ti ratios and Ti concentrations.
963 Volatility and stability of the reactants at the chemisorption temperature is a prerequisite for ALE processing. Metal chlorides can often withstand high temperatures, whereas metals with organic ligands often need milder reaction conditions. Even though the reactant as such can withstand elevated temperatures, the support surface may catalyse decomposition of the reactant already at somewhat lower temperatures. The decomposition of titanium isopropoxide, Ti(OPr)4, on alumina is an example of this. The reaction of Ti(OPr)4 was followed by FTIR and element determinations at different temperatures as shown in Figure 4. The decomposition of the reactant is seen as the gradual decrease in the intensity of the C-H vibration bands (2800-3000 cm -1) of the ligand and in the atomic C/Ti ratio of the samples with increasing reaction temperature. At 190~ all carbon was removed from the sample already during the binding of Ti(OPr) 4 to alumina.
3.4 Modification of the surface by growing oxides Reaction sequences of a metal compound and a ligand-removing reactant can be used to modify the support with oxides, sulfides and so on. Useful metal compounds are chlorides, alkoxides or 13-diketonates and the reactant for ligand removal can be water vapour, H2S or 02. We have grown TiO2 on silica [14, 17], WO 3 on alumina [18] and ZrO 2 on silica and alumina [19] by the ALE method. The number of reaction cycles is selected according to the desired modification. Each reaction sequence is led to surface saturation. One requirement of such growth is that suitable bonding sites be available for the next chemisorption reaction of the metal compound. A lack or decrease of bonding sites halts the layer growth. 3.5 Binding of two metal compounds Two or several metal compounds can be bound selectively as long as bonding sites are available. The second metal compound can be brought onto a modified surface, for example Cr onto silica modified with TiO 2, or two different metal compounds can simply be brought onto a support alternately, with no ligand removing reaction in between. The pulsing order of the reactants to the surface may change the surface saturation density, as shown in Table 3.
Table 3 Saturation densities of Zr and Ti on SiO2 preheated at 450~ Chemisorption temperature of 300~ was used for ZrCI4 and 200~ for Ti-isopropoxide. Reactant and pulsing order Zr saturation density Ti saturation density ( at/nm2) (at/nm 2) ZrCI 4 + Ti-isopropoxide Ti-isopropoxide + ZrC14 ZrCI 4 alone Ti-isopropoxide alone
0.8 1.3 1.1 -
0.4 0.4 1.4
964 4. DISCUSSION
The progress of the surface saturation was followed for each reactant by determining the metal concentration of the samples taken from the top and bottom of the fixed bed. When the metal concentrations of the two samples are the same, the macroscopic surface saturation is assured. To confirm the penetration of the metal compound into the pores requires a determination by SEM/EDS of the metal in particle cross-sections. The use of fixed bed thus provides a means to check that the surface saturation is complete. Although a fluidized bed can be used, the information on the surface saturation cannot then be obtained by element determinations alone. In a fixed bed the unsaturated situation can easily be demonstrated by using low dosing of the reactant, either by vaporizing an underdose or by keeping the reaction time too short to reach the saturation. An unsaturated situation may also occur due to diffusion limitations, which might happen, when using a fast reactant flow in combination with extrudates. Gas phase methods relying on dosing are a common means of processing catalysts [5-7]. Many of the papers describing experiments in which the reactant is dosed note the difficulty of achieving good homogeneity even at macroscopic level. The macroscopic homogeneity can be achieved by using fluidized bed [5] or by rotating the whole catalyst bed [6]. However, whether the reactant penetrates into all possible bonding sites has not been carefully studied alter the mixing of the samples. Dosing most o~en results in inhomogeneous metal distribution, and the method cannot be considered as ready for scale-up. The use of surface saturation conditions offers advantages in this respect due to the selfcontrolling feature. Good homogeneity of metal content even in 1 kg scale, as well as, good reproducibility from run to run is obtained by ALE. The use of various reactant/support pairs shows that a wide variety of catalytic surfaces can be processed. A prerequisite for good reproducibility is that the number of bonding sites is stabilized to a selected level, the reactant is stable at the reaction temperature used and the reactant dose is high enough for surface saturation. A suitable dose for achieving surface saturation can be calculated once the chemisorption mechanism and the number of bonding sites are known. Regulation of the metal content, however, demands other means [9] than those commonly used in impregnation. In routine use, element determinations can be used to check the surface saturation and scaleup without the need for strict dose control makes the process facile. The pressure of the reaction chamber had no effect on the surface saturation, which is as expected since the saturation density is determined by the number of bonding sites and the energy available to produce chemisorption to these sites. The transport of the reactant into the reaction chamber is determined by the vapour pressure, and the flow rate of the vapour to the support bed. Once the reactant is inside a pore it will continue to react so long as bonding sites are still available. The same surface saturation was achieved by using three different reactor set-ups and either a lower pressure of 6-10 kPa or ambient pressure in nitrogen flow. In ALE processing the reaction conditions are selected to lead to chemisorption. Differing from many CVD processes, in which thermal decomposition of the reactant ot~en is a desired part of the reaction, in ALE processing decomposition of the reactant is prohibited. The
965 reaction temperature in ALE must nevertheless be high enough to avoid condensation of the reactant. The decomposition of many reactants, for example alkoxides and carbonyls, makes them in some cases unsuitable for ALE processes. In fixed bed, the first sign of decomposition was often an increase in the metal content in the top of the bed as compared with the bottom. Mixing of the bed by fluidization or stirring would thus destroy the first signs of decomposition of the reactant. Chlorides and oxychlorides are not very sensitive to decomposition, but a check of the decomposition temperature should be made for metal compounds with organic ligands. An example of the selection of the reaction temperature is the deposition of Tiisopropoxide on alumina. FTIR revealed a partial decomposition of the ligand, and this decomposition increased with the reaction temperature. The commencement of decomposition may remain undetected if samples are not thoroughly analysed. The reaction temperature is not only determined by the thermal behaviour of the reactant but also by the decomposition catalysed by the support. Thus the reaction temperature range within which Ti-isopropoxide can be bonded to an alumina support is narrower than the one usable for silica. The binding of one metal compound may be followed by an oxidation or reduction to change the oxidation state of the metal species. The support can also be modified by treating the surface with several cycles of metal compound and air/water. Modifying the surface often has important advantages. For example, the favourable mechanical properties of a support like alumina can be combined with the favourable chemical nature of the new surface species created, to obtain a catalyst that does not cause cracking or other undesired side reactions in catalysis. The surface areas of the support is not significantly reduced when modification is done by ALE. Thus materials that are difficult to produce with large surface area can be grown on various supports by ALE. Other surface species can be synthesised after the first metal compound has been bound. The new surface with a single metal compound serves as a support for the second reactant, which may be another metal or some other compound promoting the catalytic function. The second reactant may react with sites energetically unfavourable to the first reactant. It may also replace part of the sites already occupied by the first reactant or bind straight to the first metal or to its ligands. The bonding mode depends on the type of reactant/support pair. The pulsing order of the reactant was seen to have an effect on the saturation densities of the metals. When ZrC14 and Ti-isopropoxide were used as reactants and Ti-isopropoxide pulsed first, the saturation density of Zr was greater than that obtained with ZrC14 alone. When ZrC14 is subsequently pulsed to the Ti/silica surface, it replaces the main part of the Ti species and the volatile Ti compounds are vaporized. The tailoring of the catalytic surfaces becomes possible when more than one reactant is used, and the reactants and their pulsing order is selected so that desired surface density and proximity of the different metals is achieved. 5. SUMMARY Some basic features of the application of ALE to the processing of catalysts have been described. We successfully processed several types of active catalysts by the method, and easily achieved good homogeneity for several metal compounds. The controllability of the catalyst preparation is good so long as the proper reaction conditions are maintained. The advantages
966 of ALE reactions are even more obvious when it is desirable to have more than one metal compound bound to the surface or more complex surface structures.
REFERENCES
1. 2. 3. 4. 5.
J. Haber, Pure and Appl. Chem. 63 (1991) 1227. K. Asakura, M. Aoki and Y. Iwasawa, Catal. Lett. 1 (1988) 395. M. Niwa, N. Katada and Y. Murakami, J. Phys. Chem. 94 (1990) 6441. D. Mehandjiev, S. Angelov and D. Damyanov, Stud. Surf. Sci. Catal. 3 (1979) 605. M.P. McDaniel and P.M. Stricklen, CO reduced chromylhalide on silica catalyst. US Patent 4 439 543 (1984). 6. S. Sato, M. Toita, T. Sodesawa and F. Nozaki, Appl. Catal. 62 (1990) 73. 7. J. NicE, D. Dutoit, A. Baiker, U. Scharf and A. Wokaun, Appl. Catal. A: General 98 (1993) 173. 8. T. Suntola, Mater. Sci. Rep. 4 (1989) 261. 9. E-L. Lakomaa, Appl. Surf. Sci. 75 (1994) 185. 10. L-P. Lindfors, E. Rautiainen and E-L. Lakomaa, Catalyst for Aromatization of Light Hydrocarbons, US Patent 5 124 293 (1992). 11. H. Knuuttila and E-L. Lakomaa, Method for Preparing a Catalyst for Polymerization of Olefins. US Patent 5 290 748 (1994). 12. J. Hietala, P. Knuuttila and A. Kyt6kivi, Metathesis Catalyst for Olefins, FI Patent 87891 (1993). 13. L.P. Lindfors, M. Lindblad and U. Lehtovirta, Method for Manufacturing a Catalyst Suited for Hydrogenation of Aromatics, FI Patent 90632 (1994). 14. S. Haukka, E-L. Lakomaa and T. Suntola, Thin Solid Films 225 (1993) 280. 15. G. S. Hammond, D.C. Nonhebel and C-H. S. Wu, Inorg. Chem. 2 (1963)73. 16. S. Haukka, Characterization of Surface Species Generated in Atomic Layer Epitaxy on Silica, Diss. Helsinki Univ., J-Paino Ky, Helsinki, 1993.46 p + 8 App. 17. E-L. Lakomaa, S. Haukka and T. Suntola, Appl. Surf. Sci. 60/61 (1992) 742. 18. M. Lindblad and L.P. Lindfors, Proc. 10th Int. Conf. on Catalysis, July 19-24, 1992, Budapest. Hungary, L. Guczi, F. Solymosi and P. Tetenyi (Eds.), Akademiai Kiado, Budapest, 1993, p. 1763. 19. A. Kyt6kivi and E-L. Lakomaa, Proc. Europa-CAT-l, Sep. 12-17, 1993, Montpellier, France, Book of Abstracts 1, p. 499. ACKNOWLEDGEMENTS Mirja Rissanen is thanked for her contribution to the processing experiments.
The utilization of satm'ated heterogeneous catalysts
gas-solid
reactions
957
in
the
preparation
of
S.Haukka, A.KytOkivi, E-L.Lakomaa, U.Lehtovirta, M.Lindblad, V.Lujala, T.Suntola Microchemistry Ltd., P.O.Box 45, 02151 Espoo, Finland
ABSTRACT Saturated gas-solid reactions known from Atomic Layer Epitaxy (ALE) were used to process various catalysts. Good homogeneity of metal species was verified both along the entire catalyst bed and inside the particles. A variety of volatile metal compounds including metal chlorides, alkoxides and 13-diketonates were successfully used as reactants. The ALE processing is described with reference to examples demonstrating the achievement of surface saturation, reproducibility of processes, selection of process parameters, growth of oxides to modify the support and the binding of two metal compounds.
1. INTRODUCTION Gas-solid reactions are being widely explored for their potential in the manufacture of structurally well-defined catalytic surfaces. According to 1UPAC recommendations [1], deposition taking place by adsorption or reaction from the gas phase is called chemical vapour deposition (CVD). Examples of the use of CVD in tailoring support surfaces and in binding active components to the support can be found in references 2 - 7. Interesting features have been introduced to catalysts by these methods. Molecular level control and uniformity through the particles have not always been achieved, however. We describe here means to a better controlled preparation of catalysts by making use of gas-solid reactions and the property of the surface to saturate itself with the reactant as suggested in the growth of layer by layer structures in Atomic Layer Epitaxy (ALE) [8]. In the ALE technique, a better control of the build-up of surface structures is achieved by the sequential introduction of the active components, and other surface-modifying agents, in saturating gas-solid reactions. ALE can be classified as a special mode of CVD, since strict demands are made upon the conditions under which the gas-solid reactions are carried out. Uncontrolled deposition through condensation of the reactants or their decomposition products is prevented by the choice of reaction temperature. Not only must the solid surface be saturated with the chemisorbed species, but it must be stabilized before each reaction sequence. This means that physisorbed molecules such as water must be removed from the starting support surface by heating, and after each reaction any unreacted reactant must be removed, normally by inert gas purge.
958 For well-defined structures to be produced on the support surface chemisorption is required. The surface species then occupy their final bonding sites at the outset and there is no need for alter-treatment at elevated temperature. Through saturation of the surface, the surface density obtained during each reactant sequence is controlled by the surface itself. The capacity of the surface to chemisorb the reactant, not the dosing of the reactant, determines the amount bonded. The processing of the catalyst is not sensitive to a precise dosing of the reactant; all that is required is that the dose be sufficient for the reaction of all binding sites. This selfcontrolling feature of ALE allows a homogeneous (uniform) distribution of surface species throughout the porous support and gives good reproducibility in obtaining a desired saturation level. The self-control is also a good feature for scale-up. The saturation density obtained in a reaction sequence depends on the number of the bonding sites, the size and chemical form of the reactant molecule and the reaction temperature. If part of the original ligands of the reactants remain present in the surface complex the saturation density will usually be less than the full monolayer coverage of the corresponding oxide. Various means to regulate the saturation level have been reviewed in [9]. Active catalysts for a variety of reactions have been processed by the ALE method [ 10-13 ]. We shall present some examples of how the surface saturation proceeds using 5 - 1000 g of alumina or silica supports and of the reproducibility of ALE in binding a single metal compound on the surface. As well, the selection of reaction conditions will be described, and examples will be given of modifying the surface with a sublayer of a metal oxide and of binding two different metal species. 2. EXPERIMENTAL
2.1. Equipment The catalysts were processed in flow-type reactors with heated zones for the reaction vessel and for vaporizing solid reagents [9, 14]. Processing was done at a pressure of 6-10 kPa or at ambient pressure in nitrogen atmosphere. Reaction vessels of 0.1 - 2 1 were used. The heating and gas valving were computer controlled. 2.2. Reagents The support materials were 7-A1203 (AKZO Alumina 000-1.5E) and silica (Grace 955) with surface areas of 200 and 270 m2/g, respectively. Alumina was used as extrudates with a particle diameter of 1.5 mm and a length between 2 and 20 mm and as crushed and sieved to a particle size of 0.15 - 0.35 mm. The particle size of the silica was 40-80 ~tm. ZrCI 4 (Fluka), TiC14 (Merck), WOCI4 (Aldrich), Cr(acac)3 (Riedel-de-Haen,), Ni(acac)2 (Merck), Mg(thd)2 and Ti(OC3H7) 4 (Merck) were used as reactants without further purification. Mg(thd)2 was synthetized according to [ 15]. 2.3. Procedure The number of bonding sites was stabilized to a selected level by preheating the support at temperatures of 200-850 ~ The reactants were then volatilized from liquids or solids and led to the top of the solid support bed held at a selected temperature. The reaction temperature
959 was selected so that the activation energy for chemisorption was exceeded and the decomposition or condensation of the reactant was prohibited. The dose of the reactant was kept high enough to exceed the number of bonding sites available. The reaction time required was calculated so that an overdose of the reactant as compared with the number of bonding sites was brought into the reaction vessel. A purge with inert gas followed the chemisorption, at the same temperature. The efficiency of transport of the reactant vapour into the reactor is determined by the vapour pressure of the reactant at the vaporization temperature selected and the rate of the nitrogen flow. The process can consist of one chemisorption stage or of several stages each followed by an inert gas purge to avoid the presence of two reactants in the reaction chamber at the same time.
2.4. Characterization Inert sampling could be done when desired. Zr, W and Ni were determined by XRF, Ti and Cr by neutron activation analysis (NAA), Mg by AAS, C with a Leco carbon analyzer and CI by potentiometric titration. FTIR in diffuse reflectance mode was used to follow the chemisorption and to detect possible decomposition of the reactant. Scanning electron microscopy with an energy dispersive spectrometer (SEM/EDS) was used to determine element concentrations through the particles. The specific surface area and pore volume were determined by means of nitrogen adsorption and condensation with lk,ficromeritics ASAP 2400 equipment. Detailed experimental conditions used in the characterization are in Ref. 16.
3. RESULTS
3.1. Surface saturation and catalyst homogeneity The saturation of the support surface with the reactant was followed by taking samples from the top and bottom parts of the support bed. Figure 1 shows the achievement of surface saturation as a function of reactant dose. Saturation of bonding sites proceeds from the top of the support bed towards the bottom, i.e. in the direction of the reactant flow.
0
E E tO
0.6 0
o
0.4
Top
t__ t| 0
Bottom
0.2
0 0
|
0.0
0.2
0.4
0.6
0.8
1.0
Reactant dose (mmol/g)
Figure 1. Surface saturation (mmol/g) as a function of the reactant dose (mmol/g). Metal determinations in samples taken from the top and the bottom of the fixed bed were made.
960 The homogeneity of saturated samples was also verified for a larger scale processing with 300 -1000 g of support. Figure 2 shows the variation in Zr saturation density in different parts of a
Zr/SiO 2 catalyst bed containing 300 g of the catalyst.
l
120 o~ >,, e-.
~
t-
100 80 60
O
~
I
q/
6 cm
=
40
=
20
0
1 ~
15 cm >
2
3
4
5
Sampling point
Figure 2. Macroscopic homogeneity of Zr in larger scale processing from ZrC14 on SiO 2.
Macroscopic homogeneity does not rule out the existence of a concentration gradient within the particles. Uniformity within the particles was therefore investigated by embedding the catalyst in epoxy resin, cutting cross-sections of particles with a microtome and analysing by SEM/EDS. Macroscopically homogeneous W/A1203extrudate samples were found to contain an uneven distribution of W because of a too fast flow rate of the reactant in processing. The combination of extrudate support and a fast WOCI4 reactant flow rate resulted in the loss of the reactant through the bed. Table 1 shows the tungsten concentration of W/A1203 samples with two different flow rates of the reactant through powder and extrudate beds. Reaction time was constant. The lower tungsten content for the catalyst processed from extrudates at the higher reactant flow rate was due to an eggshell distribution, as revealed in SEM/EDS analysis.
961
Table 1 Tungsten and chloride concentration of extrudate and powder samples with fast and slow WOC14 flow rate. The preheating of alumina and the reaction with WOC14 were carried out at 200~ Particle size Flow rate W (wt-%) CI (wt-%) powder powder extrudates extrudates
fast slow fast slow
11 11 6 11
4.8 4.8 2.3 5.1
3.2 Reproducibility The reproducibility from run to run was investigated by determining the metal concentration in samples taken from the top and bottom of the fixed catalyst bed. Table 2 shows the reproducibility of binding a single metal compound to the support surface for different metal compound/support pairs. The reproducibility was within the accuracy of the element determination methods used ( XRF, NAA, AAS).
Table 2 Reproducibility of saturated metal concentrations (mean value _+ standard deviation) in different runs. Samples were taken from the top and bottom of the catalyst bed. Metal compound/ Preheating/ Metal (wt-%) Metal (wt-%) Number support pair reaction top bottom of temperature process (~ runs ZrCI4/SiO 2 300 / 300 6.6 + 0.2 6.6 + 0.2 5 ZrCI4 / SiO2 600 / 450 2.7 + 0.1 2.7 _+ 0.1 6 WOCI4 / AI20 3 200 / 200 10.6 + 0.4 10.6 + 0.3 10 Ni(acac)2/AI20 3 200 / 200 4.7 + 0.5 4.2 + 0.2 5 Cr(acac)3/SiO 2 820 / 200 0.75+ 0.03 0.70+ 0.0 3 Mg(thd)2/SiO 2 600 / 250 1.2 + 0.1 1.0 + 0.1 6
3.3. Process parameters Several ZffSiO 2 catalysts were processed by using three different reactors: two small-scale reactors operating under vacuum and at ambient pressure and a bench-scale reactor operating at ambient pressure. Figure 3 describes the control of the Zr concentration in Zr/SiO 2 as a function of the preheating temperature of SiO2. The results for processes carried out in the different reactors are in good agreement, demonstrating that surface saturation was achieved
962
o-.O. v
tO Ira. 4-o
C:
ID O C: o o
0 200
t___
N
400
600
800
Preheating temperature (~ Figure 3. The Zr concentration as a function of the preheating temperature Of SiO 2 in ZrCl4/SiO 2 processed with different reactors. Reaction temperature 300 ~ - V - 6 kPa, (5-10 g) + ambient pressure, 1 kg and 10 g , reaction temperature 450 ~ --o-- 10 kPa, (5-10 g) 9 ambient pressure in two different equipment (5-10 g).
A
5
10
C/Ti ratio
Ti (wt-%)
10
~o O~O
~
0
_ _
100 |
*
. . . .
3900
|
.
3400
9
,
,
i
r3~O
150
o
0
200
R e a c t i o n t e m p e r a t u r e (~
2900
Wavenumber (cm "1) Figure 4. (a) FTIR-spectra, in the O-H- and C-H-vibration regions, of Ti(OPr)4/),-A1203 prepared at 100, 110, 150, 170 and 190~ (from the top to the bottom of the picture) compared with 800~ alumina, and (b) the corresponding C/Ti ratios and Ti concentrations.
963 Volatility and stability of the reactants at the chemisorption temperature is a prerequisite for ALE processing. Metal chlorides can often withstand high temperatures, whereas metals with organic ligands often need milder reaction conditions. Even though the reactant as such can withstand elevated temperatures, the support surface may catalyse decomposition of the reactant already at somewhat lower temperatures. The decomposition of titanium isopropoxide, Ti(OPr)4, on alumina is an example of this. The reaction of Ti(OPr)4 was followed by FTIR and element determinations at different temperatures as shown in Figure 4. The decomposition of the reactant is seen as the gradual decrease in the intensity of the C-H vibration bands (2800-3000 cm -1) of the ligand and in the atomic C/Ti ratio of the samples with increasing reaction temperature. At 190~ all carbon was removed from the sample already during the binding of Ti(OPr) 4 to alumina.
3.4 Modification of the surface by growing oxides Reaction sequences of a metal compound and a ligand-removing reactant can be used to modify the support with oxides, sulfides and so on. Useful metal compounds are chlorides, alkoxides or 13-diketonates and the reactant for ligand removal can be water vapour, H2S or 02. We have grown TiO2 on silica [14, 17], WO 3 on alumina [18] and ZrO 2 on silica and alumina [19] by the ALE method. The number of reaction cycles is selected according to the desired modification. Each reaction sequence is led to surface saturation. One requirement of such growth is that suitable bonding sites be available for the next chemisorption reaction of the metal compound. A lack or decrease of bonding sites halts the layer growth. 3.5 Binding of two metal compounds Two or several metal compounds can be bound selectively as long as bonding sites are available. The second metal compound can be brought onto a modified surface, for example Cr onto silica modified with TiO 2, or two different metal compounds can simply be brought onto a support alternately, with no ligand removing reaction in between. The pulsing order of the reactants to the surface may change the surface saturation density, as shown in Table 3.
Table 3 Saturation densities of Zr and Ti on SiO2 preheated at 450~ Chemisorption temperature of 300~ was used for ZrCI4 and 200~ for Ti-isopropoxide. Reactant and pulsing order Zr saturation density Ti saturation density ( at/nm2) (at/nm 2) ZrCI 4 + Ti-isopropoxide Ti-isopropoxide + ZrC14 ZrCI 4 alone Ti-isopropoxide alone
0.8 1.3 1.1 -
0.4 0.4 1.4
964 4. DISCUSSION
The progress of the surface saturation was followed for each reactant by determining the metal concentration of the samples taken from the top and bottom of the fixed bed. When the metal concentrations of the two samples are the same, the macroscopic surface saturation is assured. To confirm the penetration of the metal compound into the pores requires a determination by SEM/EDS of the metal in particle cross-sections. The use of fixed bed thus provides a means to check that the surface saturation is complete. Although a fluidized bed can be used, the information on the surface saturation cannot then be obtained by element determinations alone. In a fixed bed the unsaturated situation can easily be demonstrated by using low dosing of the reactant, either by vaporizing an underdose or by keeping the reaction time too short to reach the saturation. An unsaturated situation may also occur due to diffusion limitations, which might happen, when using a fast reactant flow in combination with extrudates. Gas phase methods relying on dosing are a common means of processing catalysts [5-7]. Many of the papers describing experiments in which the reactant is dosed note the difficulty of achieving good homogeneity even at macroscopic level. The macroscopic homogeneity can be achieved by using fluidized bed [5] or by rotating the whole catalyst bed [6]. However, whether the reactant penetrates into all possible bonding sites has not been carefully studied alter the mixing of the samples. Dosing most o~en results in inhomogeneous metal distribution, and the method cannot be considered as ready for scale-up. The use of surface saturation conditions offers advantages in this respect due to the selfcontrolling feature. Good homogeneity of metal content even in 1 kg scale, as well as, good reproducibility from run to run is obtained by ALE. The use of various reactant/support pairs shows that a wide variety of catalytic surfaces can be processed. A prerequisite for good reproducibility is that the number of bonding sites is stabilized to a selected level, the reactant is stable at the reaction temperature used and the reactant dose is high enough for surface saturation. A suitable dose for achieving surface saturation can be calculated once the chemisorption mechanism and the number of bonding sites are known. Regulation of the metal content, however, demands other means [9] than those commonly used in impregnation. In routine use, element determinations can be used to check the surface saturation and scaleup without the need for strict dose control makes the process facile. The pressure of the reaction chamber had no effect on the surface saturation, which is as expected since the saturation density is determined by the number of bonding sites and the energy available to produce chemisorption to these sites. The transport of the reactant into the reaction chamber is determined by the vapour pressure, and the flow rate of the vapour to the support bed. Once the reactant is inside a pore it will continue to react so long as bonding sites are still available. The same surface saturation was achieved by using three different reactor set-ups and either a lower pressure of 6-10 kPa or ambient pressure in nitrogen flow. In ALE processing the reaction conditions are selected to lead to chemisorption. Differing from many CVD processes, in which thermal decomposition of the reactant ot~en is a desired part of the reaction, in ALE processing decomposition of the reactant is prohibited. The
965 reaction temperature in ALE must nevertheless be high enough to avoid condensation of the reactant. The decomposition of many reactants, for example alkoxides and carbonyls, makes them in some cases unsuitable for ALE processes. In fixed bed, the first sign of decomposition was often an increase in the metal content in the top of the bed as compared with the bottom. Mixing of the bed by fluidization or stirring would thus destroy the first signs of decomposition of the reactant. Chlorides and oxychlorides are not very sensitive to decomposition, but a check of the decomposition temperature should be made for metal compounds with organic ligands. An example of the selection of the reaction temperature is the deposition of Tiisopropoxide on alumina. FTIR revealed a partial decomposition of the ligand, and this decomposition increased with the reaction temperature. The commencement of decomposition may remain undetected if samples are not thoroughly analysed. The reaction temperature is not only determined by the thermal behaviour of the reactant but also by the decomposition catalysed by the support. Thus the reaction temperature range within which Ti-isopropoxide can be bonded to an alumina support is narrower than the one usable for silica. The binding of one metal compound may be followed by an oxidation or reduction to change the oxidation state of the metal species. The support can also be modified by treating the surface with several cycles of metal compound and air/water. Modifying the surface often has important advantages. For example, the favourable mechanical properties of a support like alumina can be combined with the favourable chemical nature of the new surface species created, to obtain a catalyst that does not cause cracking or other undesired side reactions in catalysis. The surface areas of the support is not significantly reduced when modification is done by ALE. Thus materials that are difficult to produce with large surface area can be grown on various supports by ALE. Other surface species can be synthesised after the first metal compound has been bound. The new surface with a single metal compound serves as a support for the second reactant, which may be another metal or some other compound promoting the catalytic function. The second reactant may react with sites energetically unfavourable to the first reactant. It may also replace part of the sites already occupied by the first reactant or bind straight to the first metal or to its ligands. The bonding mode depends on the type of reactant/support pair. The pulsing order of the reactant was seen to have an effect on the saturation densities of the metals. When ZrC14 and Ti-isopropoxide were used as reactants and Ti-isopropoxide pulsed first, the saturation density of Zr was greater than that obtained with ZrC14 alone. When ZrC14 is subsequently pulsed to the Ti/silica surface, it replaces the main part of the Ti species and the volatile Ti compounds are vaporized. The tailoring of the catalytic surfaces becomes possible when more than one reactant is used, and the reactants and their pulsing order is selected so that desired surface density and proximity of the different metals is achieved. 5. SUMMARY Some basic features of the application of ALE to the processing of catalysts have been described. We successfully processed several types of active catalysts by the method, and easily achieved good homogeneity for several metal compounds. The controllability of the catalyst preparation is good so long as the proper reaction conditions are maintained. The advantages
966 of ALE reactions are even more obvious when it is desirable to have more than one metal compound bound to the surface or more complex surface structures.
REFERENCES
1. 2. 3. 4. 5.
J. Haber, Pure and Appl. Chem. 63 (1991) 1227. K. Asakura, M. Aoki and Y. Iwasawa, Catal. Lett. 1 (1988) 395. M. Niwa, N. Katada and Y. Murakami, J. Phys. Chem. 94 (1990) 6441. D. Mehandjiev, S. Angelov and D. Damyanov, Stud. Surf. Sci. Catal. 3 (1979) 605. M.P. McDaniel and P.M. Stricklen, CO reduced chromylhalide on silica catalyst. US Patent 4 439 543 (1984). 6. S. Sato, M. Toita, T. Sodesawa and F. Nozaki, Appl. Catal. 62 (1990) 73. 7. J. NicE, D. Dutoit, A. Baiker, U. Scharf and A. Wokaun, Appl. Catal. A: General 98 (1993) 173. 8. T. Suntola, Mater. Sci. Rep. 4 (1989) 261. 9. E-L. Lakomaa, Appl. Surf. Sci. 75 (1994) 185. 10. L-P. Lindfors, E. Rautiainen and E-L. Lakomaa, Catalyst for Aromatization of Light Hydrocarbons, US Patent 5 124 293 (1992). 11. H. Knuuttila and E-L. Lakomaa, Method for Preparing a Catalyst for Polymerization of Olefins. US Patent 5 290 748 (1994). 12. J. Hietala, P. Knuuttila and A. Kyt6kivi, Metathesis Catalyst for Olefins, FI Patent 87891 (1993). 13. L.P. Lindfors, M. Lindblad and U. Lehtovirta, Method for Manufacturing a Catalyst Suited for Hydrogenation of Aromatics, FI Patent 90632 (1994). 14. S. Haukka, E-L. Lakomaa and T. Suntola, Thin Solid Films 225 (1993) 280. 15. G. S. Hammond, D.C. Nonhebel and C-H. S. Wu, Inorg. Chem. 2 (1963)73. 16. S. Haukka, Characterization of Surface Species Generated in Atomic Layer Epitaxy on Silica, Diss. Helsinki Univ., J-Paino Ky, Helsinki, 1993.46 p + 8 App. 17. E-L. Lakomaa, S. Haukka and T. Suntola, Appl. Surf. Sci. 60/61 (1992) 742. 18. M. Lindblad and L.P. Lindfors, Proc. 10th Int. Conf. on Catalysis, July 19-24, 1992, Budapest. Hungary, L. Guczi, F. Solymosi and P. Tetenyi (Eds.), Akademiai Kiado, Budapest, 1993, p. 1763. 19. A. Kyt6kivi and E-L. Lakomaa, Proc. Europa-CAT-l, Sep. 12-17, 1993, Montpellier, France, Book of Abstracts 1, p. 499. ACKNOWLEDGEMENTS Mirja Rissanen is thanked for her contribution to the processing experiments.