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Thermal Stability of Catalyst Supports
C.H. Bartholomew and J.B. Butt (Editors),Catalyst DeactLvatLon 1991 1991 Elsevier Science Publishers B.V., Amsterdam
29
THERMAL STABILITY OF CATALYST SUPPORTS
D.L. TRIMM School of Chemical Engineering and Industrial Chemistry, University of South Wales, Kensington, NSW 2033, Australia
New
SUMMARY Thermal sintering of catalysts and supports is a major cause of irreversible catalyst deactivation. In this presentation, the mechanisms of sintering are reviewed with particular emphasis on the importance of foreign ions and of ambient atmosphere - factors of interest in the context o f heterogeneous catalysis. It is shown, using the example of alumina, that controlled sintering can lead to supports that maintain desired texture for longer periods of time Calcining in the presence of steam is shown to produce a support particularly suitable f o r partial oxidation catalysis. Calcining under the influence of sulphur oxides is found to produce a high temperature stable support suitable for application in car exhaust catalysts. INTRODUCTION The overall efficiency of a catalyst is defined in terms of activity, selectivity and life. Of these, catalyst life has received by far the least attention in the literature, partly because life testing makes heavy demands on resources and partly as a result of the fact that accelerated testing may be unreliable. Nonetheless, catalyst life is a factor that can control the economic viability of industrial processes 111. Catalyst deactivation can result from poisoning [ 2 ] , from fouling [ 3 ] o r from sintering [ 4 ] . Catalyst poisons may or may not be removable, depending on the nature of the poison. One common foulant, coke deposited on catalysts, can be removed to regenerate the catalyst. Sintering, on the other hand, is largely irreversible and necessitates replacement of the catalyst. Sintering is not necessarily totally undesired, in that calcination during the preparation of a catalyst may result in sintering to produce a desired surface area or porosity [ 5 ] . Once such a desired texture is produced, however, subsequent reorganisations should be minimised. This article focuses on two systems in which it has been found to be possible to produce a desired surface by controlled calcination and to minimise subsequent sintering o f the solid. Both are based on alumina supports - one used for partial oxidation catalysts and the other used in the total oxidation car exhaust catalyst.
30 Sinterina of single Dhase solids Heterogeneous catalysis is an interface phenomenum and it is advantageous to prepare catalysts with the highest possible surface area for a given amount of material. Thermodynamically, this is an unfavourable state since minimal surface free energy is associated with material coalesced into a sphere. As a result, there is always a tendency for catalysts to sinter. Coalescence is, however, an activated process and sintering becomes more significant at higher temperatures. When it occurs, surface area is reduced, porosity changes and the catalysts lose activity. In general, the observed sequence of events during the sintering of single phase solids as temperature is increased includes a) Surface diffusion, which becomes important near to the Huttig temperature ( 0 . 2 - 0.3 x melting point (T,K)) 161 leading to surface smoothinq. b) Concave links or necks formed between close neighbour particles [7] c) Aggregate consolidation, perhaps via volume diffusion which becomes important near to the Tamman temperature (0.5 x T K) [8]. m d) Particle equilibration, with particles moving faceted spherical shape.
towards a
spherical or a
e) Particle vaporisation or melting. The first four stages of sintering have been observed in many catalytic systems of which supported silver is only one example [7]. The last stage is rarely observed except in systems such as catalytic combustors where excess heat may be generated [ 9 ] . Rates of sintering have functions of the type [lo] dD - -dt
dA or Dm-I dt
kl -
=
traditionally been
correlated by
power
law
kZA"
where D is the particle size, A is the surface area, t is the time and m and n are constants. Attempts to relate values of m and n to the mechanism of sintering have not been universally successful, partly because most catalysts are not single phase solids and partly because sintering may involve more than one mechanism (see below). Much attention has been focused on the sintering of supported metal particles [4,11].Sintering has been suggested to result from the migration of particles across the support [12], from the migration of metal atoms across the support [ l l l , from evaporation - condensation of metal atoms as, for example, oxides or chlorides [13] or from a combination of all of these.
31 Sophisticated models reflecting more closely the nature catalysts have also been developed [ 4 ] .
of
"real"
supported
Catalyst and suuuort slnterinp. : General A support is used in order to produce a high surface catalyst. Support materials are carefully chosen for particular systems and thermal stability is an important consideration [ 1 4 ] . Nonetheless, thermal deactivation of the catalyst may result either from sintering of the catalyst or of the support, particularly in situations where more desirable features of a support for a given application are such as to negate a less than desirable thermal stability performance.
The choice of support materials is made on the basis of a range of chemical and physical characteristics (Figure 1) [ 1 4 ] , of which surface area, porosity and thermal stability are particularly important. The range of materials that meet the desired standards is limited (Table l), but desired properties may be enhanced by, for example, doping the support, Physical Factors
Chemical Factors
Is catalytic activity required?
Does the support show desired surface area + porosity?
+
Are chemical catalyst-support interactions possible? Are they desired or undesired? Does the support interact with reactants or products?
Is the thermal conductiv tY acceptable? Is the support mechanica strong?
+
Can the support be poisoned?
I
Can the catalyst be deposited on the support in the desired form?
*
Is the support stable?
I
-
Is the support in the desired shape?
Fig 1. Factors affecting choice of a support. Despite the fact that supports may be composite materials, many of t h e processes affecting sintering have much in common with processes controlling the thermal reorganisation of pure single phase materials. Support sintering can involve a) Surface diffusion, which becomes significant near to the Huttig temperature ( 0 . 3 T K). Since sintering involves solid state diffusion, the rate of m sintering will be an activated process with a diffusion coefficient that is exponentially dependent on temperature.
32
b) Volume diffusion, which becomes significant near to the Tamman temperature (0.5 T K) and which may involve migration of interstitials (Frenkel defects) m or vacancies (Shottky defects). Again, the process is activated. c) Evaporation-condensation processes. As with metals, evaporationcondensation is usually enhanced by the presence of different gases. Thus, for example, silica migration in the presence of steam involves evaporation condensation of the hydroxide [ 1 5 ] . d) Grain Boundary diffusion, which involves the migration of along grain boundaries i n the lattice [16].
atoms or
ions
e) Phase transformation. All crystal lattices contain defects and vacancies, and it is due to these defects that solid state diffusion occurs. Atoms, ions or vacancies can move via these defects to a lower energy state, and such movements may be associated with phase transformations. TABLE 1.
Typical support materials Low surface area
High surface area >I mz g-
essentially non-porous
ground glass alundum (a-A1 0 ) silicon carbi8e3
porous
kieselguhr pumice
essentially non-porous
natural silica-alumina carbon black titania zinc oxide
porous
natural clays synthetic silica-aluminas 7-alumina magnesia activated carbon silica
The importance of phase transformations is easily seen by reference to alumina. The most stable phase is a-alumina,with a surface area of ca 2 2 -1 m g . 7-alumina is widely used as a support, with a surface area of ca 1 5 0 300 m'g-'. It is obvious that phase transformation of 7-A1 0 to a-A1 0 will 2 3 2 3 result in extensive loss of surface area, and it is fortunate that the process is activated, becoming significant only at higher temperatures [17,18]. The majority of polymorphic transformations can be described in terms of nucleation and growth [17]. The nuclei of a new phase are formed at one rate, and then grow at a second rate. Phase transformation is observed when these rates become significant, a process usually controlled by temperature.
33
In the immediate vicinity of the transition temperature, the free energies of the two phases are equal. The formation of the new phase is caused by fluctuations due to thermal excitation, which moves atoms/ions to new positions corresponding to the new phase. Subsequent growth involves the transfer of atoms to the nuclei by any of the processes listed above. Grain boundaries, dislocation sites, vacancies or other imperfections facilitate diffusion to the nuclei and accelerate sintering/phase transformation [lo]. The kinetics of the process depend on the type of rearrangement needed to form the new phase. Displacive transformations demand only the displacement of atoms in a lattice without bond breakage or change in coordination number. The transformation is rapid. Reconstructive transformations require the breakage and subsequent reforming of inter-atomic bonds. This process is usually much slower, since more energy is required to form the new lattice structure. Sintering and phase transformation is further complicated by the fact that supports are used in catalysis. This means that foreign ions (the catalyst) are brought into close contact with the support material and that the system can operate in a variety of atmospheres. Both of these factors can influence thermal reorganisation [10,19,20]. In general terms, the role o f foreign ions is easy to explain. Thermal reorganisation involves diffusion via defects and vacancies [lo]. If foreign ions occupy these sites, the rate of sintering will be affected. Alternatively, if foreign ions react with the host lattice to form a new chemical compound, then the reorganisation of the new compound will occur at a different rate. The effect of the gas atmosphere on sintering is also easy to understand in general terms. Modification of the stoichiometry of solid phases - be it by heat, atmosphere or solid state reaction can influence structure reorganisation. For example, if zinc oxide loses oxygen by heating or by reduction, interstitial zinc atoms or oxygen vacancies are formed which assist sintering. Similarly, metal ions in their lowest valencies become metal deficient in oxidising atmospheres and sinter faster. In the context of support materials sintering, however, it is perhaps the effect of steam that is most noticeable. The formation and removal of hydroxides on the surface leads to accelerated sintering in most supports. Turning from general statements to particular examples, it is useful to consider the behaviour of alumina, in that this support is used widely and exhibits many of the sintering phenomena listed above. Phase transformations in alumina Different phases of alumina may be prepared by calcination of aluminium tri-hydroxides (gibbsite and bayerite) or oxy-hydroxides (boehmite and
34
diaspore). The phases produced depend on the starting material and the temperature of calcination, as shown in Figure 2 [17,18]. Although this diagram is simplified, it provides a useful guide to the inter-relationships in the system.
Kappa
Alpha
D e l t a Theta Alpha
Bayerite
a
*
Theta
Eta
Alpha
Alpha
Diaspore
Fig. 2 . Alumina phases produced at different temperatures [18]. Path a is favoured for fine crystals, Path b is favoured for moist/coarse particles. TABLE 2 Crystal structure of the aluminas. Phase
Crystal System
Unit Cell Parameters (Angstroms) a
b
C
Hydrated aluminas : Gibbsite Bayerite Boehmite Diaspore
Monoclinic Monoclinic Orthorhombic Orthorhombic
8.641 4.716 2.868 4.396
Cubic Cubic (spinel) Tetragonal Tetragonal Monoclinic Orthorhombic Rhombohedra1
7.95 7.90 7.95 7.967 5.63 8.49 4.758
5.070 8.679 12.227 9.426
9.720 5.060 3.700 2.844
7.95 7.697 2.95 12.73
7.79 23.47 11.86 13.39 12.991
Transition aluminas: Chi Eta Gamma Delta Theta Kappa Alpha alumina
35 These phase changes are accompanied by changes in the crystal structure (Table 2). The changes are quite significant, and it is not surprising that phase transformation leads to loss of surface area. The sequence of phase transformations shown in Figure 2 is an approximation, largely because process variables such as time, atmosphere and properties of precursor hydroxides are not included. Thus, for example, bayerite and gibbsite may be converted to boehmite and thence to 7-alumina during calcination if the particle size is large and the precipitate is moist [181,
Such factors have led to contradictory reports. Thus, for example, Vinnitova et al. [21] reported the conversion of boehmite to 7-alumina at 3 0 0 4 0 0 " C , the 7-phase being stable to ca 900 C when conversion to @-alumina occurred. The a-phase was then produced at temperatures as low as 1000 C. These results are certainly in contradiction to those reported by Wefers and Bell [18] and summarised in Figure 2. Part of the problem appears to lie in the small differences in the X-ray diffraction patterns of the various phases of alumina [17] and it is not inconceivable that &-aluminawas present in the samples examined by Vinnikova but was not observed. Similar disagreements have been reported concerning the mechanism of sintering during the initial stages of reorganisation. Prochazka and Coble [22], based on activation energy data, suggested that surface diffusion was the governing process of the initial stages of neck growth between sintering alumina particles. Raja Rao and Cutler [ 2 3 ] reported that volume diffusion predominates over surface diffusion in these initial stages. Johnson [24], on the other hand, has produced an interesting new model in which hydroxyl groups on the surface of alumina may play an important role in the initial stages of sintering. It is well known that the addition of salts such as aluminium formate to alumina can have a significant effect on porosity and particle strength [ 5 ] . This is thought to result from the formation of 0-A1-0 bridges between particles. Johnson suggested that a similar process may occur with hydroxyl bonds, residing on adjacent particles, eliminating water to give A1-0-A1 linkages and neck formation between particles [24]. Accelerated sintering in the presence of water (see below) was described in terms of surface hydration leading to more opportunities for interparticle bridging. Although explaining the initial formation of inter-particle bridges, subsequent consolidation of the neck must involve other than links treated through hydroxyl groups. In view of the fact that the temperatures at which this occurs are low as compared to the melting temperature of alumina, surface diffusion as suggested by Prochazka and Coble [22] seems the more probable route for neck consolidation. Although these initial stages of sintering are important, it is perhaps the phase transformation of 8 to 7 - A 1 0 which is of more interest in a catalyst 2 3
36 support, since this phase change has been reported to account for the greatest loss of surface area. Badkar and Bailey [ 2 5 ] reported that the phase change involves a nucleation and growth mechanism, with grain boundary diffusion being the controlling migratory factor. Although unable directly to observe the nucleation of the a-phase, they concluded that nucleation occurred uniformly throughout the 8-A1203 matrix on a scale of approximately one o nucleus per 10 B grains. Part of the problem in determining an exact mode of sintering of alumina lies in the fact that catalyst supports are prepared to give high surface areas. A s a result, the arrangement of particles one to another is very far from ordered, and sintering may often depend on points of contact within the disordered structure. Thus, for example, Oberlander points out that the development of porosity in pseudoboehmite depends either on the agglomeration of small particles or on the agglomeration of fibril-like particles [ 2 4 ] . Small particles have many points of contact, while fibrils should tend to form bundles with large interstices between the bundles. Subsequent sintering patterns would be expected to be very different, despite the fact that the starting material is similar. Thus it can be suggested that the geometry of different particles can have a significant effect on sintering. This is a finding which will be shown later to have considerable ramifications. Before considering these, however, it is useful to review the effect of the presence of foreign ions and the ambient atmosphere on the sintering of alumina. The effect of foreign ions The presence of small amounts of impurities can either accelerate or decelerate phase transformations in alumina. The presence of platinum at concentrations from 0.1 to 3 w% has been reported to accelerate sintering, with the production of the 0 phase, in particular, being favoured (Figure 3) [ 2 7 ] . On the other hand, 3% Ni was found to lead to the initial stabilisation o f alumina surface area, followed by rapid collapse of the structure (Figure 4 ) [191.
The effects o f metals are, however, less significant than the effect of metal oxides, which are used widely to stabilise alumina [ 2 8 ] . Lanthana, in particular, is widely used to maintain alumina surface area in automobile catalysis [ 2 9 ] . The addition o f the oxides of Li, K or Mg to alumina has been found to increase stability when added at up to ca 2 - 4 mol% [30]. Above this level, stability decreases, as confirmed by Penichon & Durupty [31]. Lanthana, on the other hand, increases the area of alumina calcined at 873-1373 K when added at
37
Temperature w
4 0 0 I
0 0 1
I
Gamma
Gamma
O
W
A
0 0 I
I
Theta
Alpha
Theta
Delta
0
ifc
Alpha
ThetaAlpha
Phase transformations of alumina in the absence and
0 Fig. 4 .
J
0 0
Gamma
N -
Fig. 3.
u l
("C)
8
16 Time (h)
24
of
Pt
32
Surface area loss on heating A 1 0 and 3% N i / A 1 0 2 3
presence
-
2 3
in hydrogen.
1 mol% [ 3 2 ] , but, in the presence of steam, improvements are observed up to 5 the oxides of P, Si, Ba, Sr or Sn increases stability, at least up to about 700 p mol/g
m o l % [ 3 3 ] . Johnson has reported that increasing concentrations of
L241.
Johnson suggests that stabilising elements slow phase transformations by replacing surface hydroxyls and, as a result, slow the formation of A1-0-A1 bridges [ 2 4 ] . Silica, at least, has been found to interact with surface hydroxyls on alumina and this is suggested to reduce sintering by reducing such bridges. In general, however, the evidence is rather weak, and further investigation is needed to prove or disprove the suggestion. What is certain is that the addition of foreign ions can lead to new compounds. Nickel aluminate has been shown to be formed from the calcination of Ni/A1 0 catalysts, and the aluminate has been suggested to be responsible, 2 3 at least in part, for the observed extra stability [19, 19(a)l. Lanthanum aluminate is also known to be formed on calcination of La doped aluminas [ 2 8 , 2 9 ] . Ozawa et al. [ 2 8 ] report that La is inserted into the crystal lattice of alumina with a spinel structure, while Ce remains on the surface in the form of CeO . The activation energy for the formation of a-A1 0 was found to 2 3 be 635-655 W/mol for alumina modified with La, Sm and Y b , as opposed to 5 8 1 583 kJ/mol for pure alumina with and without CeO additions. Although these values seem high, the effect of incorporation of foreign ions in the alumina lattice versus deposition on the lattice is obvious. At the same conference, Chen et al. [ 3 4 ] suggested that ca 10’’ atoms/mz of alumina surface were available for filling by cations, but this measure took no account of the possibility of e.g. La ions moving into the alumina lattice or vice versa. Nor was any estimate given of the importance of octahedral and tetrahedral sites on the lattice substitution [ 3 4 ] , despite the inference that tetrahedral sites could well be of importance to the process. Substitution of such ions in the surface or bulk of alumina may be beneficial in several ways. Firstly, as suggested by Johnson [ 2 4 ] , the hydroxyl surface concentration may be reduced with a subsequent reduction of inter-particlebridging. Secondly, the foreign ion may occupy a vacancy in the alumina lattice, thereby reducing solid state diffusion. Finally, the production of a new compound - either at the surface or in the bulk - may result in greater support stability. The effect of atmosphere Steam is by far the most deleterious atmosphere for the sintering of most stoichiometric oxides used as supports [14,19].At the same time, however, steam is an essential reactant in many processes [ 3 5 ] . It is not surprising, then, to find in-depth studies of the effect of steam on the sintering of e.g. Ni/A1 0 steam reforming catalysts [ 1 9 , 2 0 ] . 2 3
The long term effects of steam and hydrogen on the properties of this catalyst have been studied by Williams et a1 [ 2 0 ] . The catalyst contained only 25% 7-A1203, and may be considered almost as alumina stabilised nickel. The catalyst was found to sinter rapidly in a 9 : l steam : hydrogen atmosphere (25 atmospheres pressure) but, after an initial period, sintering slowed down. The
39 initial sintering increased with increasing steam content at temperatures between 400 and 800"C, the majority of the final a-alumina content being formed during this period. The effect of steam was many times greater than any effect due to nickel. Similar conclusions were reached by Harris et al. [ 3 6 ] , who found that 3% Ni/Al2O3 sintering was much more dependent on the presence of steam than on effects due to Ni (Figure 5). However, the initial loss of surface area was found to be associated more with the transformation of y - A l 0 to the 6 and 0 2 3 phases (Table 3 : compare with Figure 5). The production of a-A1 0 was found 2 3 to be associated more with the slower decrease in surface area at longer times on line.
P o \
0 A I 2 O 3 / p u r e He 0 A 1 2 0 3 / N i / p u r e H2 x A I 2 0 3 / 5 % steam A A 1 2 0 3 /Ni/5% steam 0 A1203/10% steam 0 A I p 0 3 / N i / 1 0 % steam
A
25
50
75
Time (h) Fig. 5 . Surface area loss on heating. Alumina and 3% Ni/A1203 i n hydrogen and steam.
40
TABLE 3 Phase constitution of A1 0 and 3% Ni/A1 0 as a function of time of annealing in a 5 % steamfiydrogen atmisphere at 950'C: ~~
A1203/5% steam
Time (hrs)
Phases Present 7
0 2 5
16
18 24 27
48 65 74
3% Ni/A1203/5% steam
+ + i
S
+ + + + + +
6
+ +
+
+ + +
Phases Present a
+
+ + + +
+ +
7
+ + +
S
+ +
+ + + +
6
a
____
+ +
+
+ + +
+ + +
+ f
+ +
Harris et al. [ 3 6 ] reported values of apparent activation energy based on decrease of surface area. Pure alumina sintering in hydrogen was found to have an apparent activation energy of ca 125 W/mol : in steam, the value was reduced to ca 70 k.J/mol. The effect of steam was suggested to result both from the production of extra hydroxyl groups on the surface and from the possibility of an increase in the number of cation vacancies induced by steam in the trivalent aluminium lattice. However, the evidence for either mechanism was sparse. It is clear from the above discussion that, despite the importance of alumina as a catalyst support, the detailed mechanism of sintering is far from clear. There is good understanding of the effect of different factors on sintering, but the detailed mechanism of phase transformations is, in many cases, open to considerable uncertainty. Factors which influence the thermal stability of the support are better understood. Particle size and shape is known to be important. The absence or presence of foreign ions and differing gas atmospheres significantly affects reorganisation. The kinetics of the process are known to be activated, although it is difficult or impossible to predict what they will be in different systems. Obviously it is possible to design and manufacture catalyst supports based on this knowledge and without detailed understanding of the processes involved in the phase transformations. The more important question is whether the design of supports can be improved even with the limited knowledge available. Two examples are given below to show that such improvements are indeed possible.
41
Preparation of reorvanisation
supports
with
desired
DroDerties by
thermal
The choice of a catalyst support depends on the physical and chemical characteristics of the catalyst and the support, on desired or undesired interactions with components of the system to be catalysed and on the necessity of controlling mass and heat transfer in the process (Figure 1) [ 1 4 ] . Often it is necessary to compromise between various demands or to dope the support to enhance or reduce given properties. Chlorine doping of alumina used in reforming catalysts is a case in point [ 3 7 ] . Supported vanadia, used to promote the selective oxidation of hydrocarbons, is another example. Vanadia, with or without promoters, may be supported on silica (naphthalene oxidation [ 3 8 ] ) , on titania o-xylene oxidation [ 3 9 ] ) or on a-alumina (benzene oxidation [ 4 0 1 ) . It was believed that supports should have open porosity (and associated lower surface area) in order to minimise over oxidation to carbon oxides. However, it was shown that reasonably high activities and selectivities could be obtained over vanadia supported on high surface area material and it was suggested that low selectivity was, in fact, primarily associated with high acidity on the support [ 4 1 ] . In agreement with this, vanadia supported on 7-alumina showed zero selectivity for the production of maleic anhydride from benzene. If this is the case, then a high surface area support with minimal acidity would be desired. With this in mind, attention was focused on modifying 7-aluminaby thermal treatment. It is known that 7-A1 0 can be transformed to a-A1 0 by heating 2 3 2 3 in steam [ 1 7 , 1 8 ] . It is also known that the process is activated, the rate of reaction depending on solid state diffusion. Therefore, it was argued that, if conditions were carefully adjusted, it might be possible to transform the 7 phase at the particle exterior, without transforming the gamma phase in the interior. In this way, the surface would have the properties o f a-alumina (with, in particular, low acidity) while relatively high surface area would be maintained by the underlying 7-phase. As expected, heating -y-Al 0 in steam produced a rapid decrease in surface 2 3 area (Figure 6) together with an associated increase in a-A12 03 as detected by X-ray diffraction. The nomenclature on Figure 7 , 8 and 9 is used only to distinguish samples. Measurement of surface acidity in terms of the rate constant for methanol dehydration showed, however, that the surface properties of the alumina were changing to a different pattern. On the assumption that decrease in acidity was associated with the conversion of 7 to a-A1 0 it is 2 3 seen from figures 7 and 8 that, as expected, acidity changes ascribed to phase transformations at the surface were occurring more rapidly than phase changes in the bulk.
42
T i m e (h)
Fig. 6. Surface area and amount of a - A l - 0 - as determined by X-ray diffraction as-a function of heating time in steam. ’TJ= 1027 C.
0
g -2 7
-
0
I
25
i
50
% a in bulk
cr-A1203
75
<
100
Fig. 7. Plot of logarithm of rate constant from methanol dehydration (used as a measure of surface acidity) vs amount a - A 1 0 in the bulk (as measured by X2 3 ray diffraction).
43
The effect of these changes on the selectivity of the catalyst for maleic anhydride production is seen in Figure 9 . It is clear that the selectivity of the reaction increases dramatically as the acidity of the support is reduced (Figure 9 ) . The conversion of ?-to a-A1 0 in the bulk, however, has little 2 1 subsequent effect on selectivity.
OO
25
50 % a i n bulk
75
100
Fig. 8 . Plot of % a - A L 2 0 3 on surface (from measurements dehydration) v s % a-AL2O3 in bulk (from X-ray diffraction).
loo
of me hanol
I
log 1 0
[ k2 x 1 O7
(L*rn~I-’rn-~s-~)]
Fig. 9 . Selectivity for maleic anhydride production from benzene as a function of surface acidity as measured by the rate constant for methanol dehydration 1411.
44
This is an interesting example of thermal transformations being used to produce a support of desired characteristics. The desired transformation is, effectively, a kinetic phenomenon, where the slow phase transformation resulting from solid state diffusion is arrested at a point where surface reorganisation is complete and bulk transformation - which would result in a massive loss of surface area and activity - is not. Subsequent phase transformations cannot, of course, be avoided, but the relative temperatures of support treatment (1027°C) and benzene oxidation (350°C) are such that the total conversion of the support to a-A1 0 is slow. 2 3 A s a result, the supported catalyst has a long life under normal operating conditions. The (bulk) 7-phase alumina ensures reasonably high surface area, while the (surface) a-phase alumina minimises undesired acidity at the interface. Minimisation of surface reornanlsation Supports sinter with time on line, and this process is accelerated at higher temperatures. This can present significant problems with reactions such as catalytic combustion [9], where excess heat is generated by the reaction. Unless this heat is rapidly removed, the catalyst/support system can overheat and sinter. The best known example involves catalysts used to clean up automobile exhausts [ 4 2 ] . Depending on operating conditions, the exhaust gases from an engine can include unburnt hydrocarbons, carbon monoxide and nitrogen oxides, as well as carbon dioxide, water, nitrogen and oxygen. An automobile exhaust catalyst operates by oxidising hydrocarbons and carbon monoxide and reducing nitrogen oxides to nitrogen, either in one or two bed systems [ 4 2 ] . Design of the catalysts is critical [ 4 2 ] . The volume flow rate of exhaust gas can be high, and pressure drop must be avoided. As a result, it is more common to use a monolithic structure as the base of the catalyst. However, pellets can and have been used. This base material is usually fabricated from cordierite or from a metal alloy [ 4 2 ] , these materials being chosen for their stability and thermal expansion characteristics. The surface area of these materials is low, and it is common practise to coat the base with a washcoat. The washcoat usually contains alumina, metal oxides and the catalyst which, in nearly every commercial case, involves one or more precious metals. Platinum and palladium are favoured for oxidation, while rhodium is the preferred material for selective reduction [ 4 2 ] . The alumina provides a relatively high surface area support for the catalyst, with small amounts of other metal oxides being added to the washcoat in order to stabilise the alumina against sintering [ 2 8 , 2 9 ] . Lanthana and baria are added far this reason : ceria, although added originally as an oxygen storage component [ 4 3 ] , also helps to stabilise the alumina to a small extent [ 2 8 , 2 9 ] .
45
Such catalysts perform well, with an anticipated life of about 5 years or more. However, there are problems existing and foreseen that may limit their lives to much shorter periods. Thus, for example, a plug misfire can lead to excess hydrocarbons reaching the catalyst, to enhanced oxidation and heat release and to sintering. The trend towards lean-burn engines, although advantageous in many respects, leads to more hydrocarbons in the exhaust gas and to greater heat generation. As a result, there is considerable pressure to develop catalysts that are more thermally stable. In addition, there is pressure to reduce the cost of the catalyst. Washcoat aluminas have desired surface areas and porosities and are thermally stable. They are best produced by calcination of particular precursors, and aluminium isopropoxide or boehmite have been suggested to be useful materials to calcine [ 4 2 ] . Both of these precursors are not cheap, and less expensive raw materials would be desired. Thus it is possible to define the need for a cheap alumina that has desired surface area and porosity properties, that is thermally stable - possibly with but preferably without added stabilisers - and which meets the requirements for preparation of washcoats. As would be expected, the solution to these requirements is not simple and it is perhaps most useful to present the convoluted path that led to one possible product. The route that led to the desired product involved initial experiments focused on sulphide odours that could be produced by car exhaust catalysts under some driving conditions. When a vehicle was cold started, for example, the rich exhaust gases were oxidised but odours of hydrogen sulphide or carbonyl sulphide could be produced. The amounts involved were much greater than would be expected from the sulphur content of the gasoline, and it was obvious that sulphur storage was occurring on the catalyst. Studies of this phenomenum showed that this was indeed the case. Sulphur dioxide was not adsorbed to any large extent but sulphur trioxide was adsorbed by ceria and y-alumina (components of the washcoat) as well as by the catalyst [ 4 4 ] . The amount adsorbed depended markedly on temperature, as shown in Figure 10. Up to a 500"C, the amount of trioxide adsorbed increased : above this temperature, trioxide was given off. Calculations showed that the amounts adsorbed or desorbed were much larger than would be anticipated on the basis of adsorption, and these results were explained in terms of the formation and decomposition of metal sulphates. 2Ce02
+
3S03
A1203
-k
3S03
=
Ce2(S04)3
-k
%02
Alz(S04)3
Thermodynamic calculations showed that the formation and decomposition of sulphates at low and high temperatures respectively was not unexpected.
46
t30
t20
400
I
500
I
600
I
700
Temperature ("C)
800
Fig. 10. The amount of sulphur trioxide taken up by ?-A1 0 ceria and 2 3 ' exhaust catalyst as a function of temperature.
a car
As a result of these findings, sulphide odours were suggested to arise from reduction of sulphur trioxide over the precious metals in the catalyst. Rich gas, arriving from the engine, was oxidised to produce heat and some residual reducing gas. The heat increased the temperature of the catalyst which, in turn, led to release of sulphur oxides. These were reduced by the residual reducing gases to give sulphides. In the course of these studies, the morphology and composition of differing species was examined. There was strong evidence of recrystallisation in the washcoat material, with needle like crystals being observed. Since sintering is known to be dependent on particle contact (see above), there appeared to be possibilities of increased thermal stability.
In that reduced cost was an objective, the possibilities of using gibbsite in similar experiments were examined [45].Gibbsite was reacted with sulphuric acid solutions to produce aluminium sulphate. This material was dried and calcined in two stages. In the first, the water of hydration was removed. In the second, aluminium sulphate was decomposed to form alumina. The alumina so produced was then heat treated in order to study the thermal stability of the product,
47
The results are summarised in Figure 11. It is seen that the sample prepared via the aluminium sulphate route (UNSWAL) had a higher surface area after calcining at 800 C than ? - A 1 0 but lower than a typical washcoat 2 3 alumina. At higher temperatures it was less stable than ?-A12 03 and washcoat a1.umina.
01
600
I
700
I
800
I
900
1
1000
Temperature
!il
1100
I 12OC
("c)
Fig. 11. Surface area of samples after heating to indicated temperatures for 4 hrs. Samples were then prepared to which was added 2 % and 4% lanthanum oxide and 4% barium oxide ; some care was necessary as to when the dopant was introduced into the preparation sequence. These results were all very similar, and are summarised in Figure 11. It is seen that, at lower temperatures, surface areas are lower whereas, at higher temperatures, surface area is maintained at the same relatively high level as washcoat alumina. The pore size distribution of UNSWAL before and after thermal treatment at 1000 C is shown in Figure 12. Such a pore size distribution appears to be suitable for washcoat preparation. The reasons for the enhanced stability of the product were then examined. X-ray diffraction patterns showed the presence of gamma, chi, delta and a small amount of kappa alumina, but no obvious reasons for the enhanced stability could be detected.
48
0 Fig. 12. Pore size distribution of alumina after calcining at and 1000 C (residual) for 4 hrs.
850°C
(fresh)
However, inspection of the morphology of the samples confirmed trends previously observed and offered possible reasons for the enhanced stability. A s seen from Figure 1 3 , gibbsite approximates to a faceted structure. 7 alumina has no distinct morphological features. In contrast UNSWAL shows clear evidence of needle like crystals. It is believed that these crystals, by offering a well defined structure with minimal inter-particlecontacts, leads to the minimisation of thermal sintering in the product. The approach of controlling morphology to control sintering offers several interesting possibilities, and these are actively under consideration. At least one involves heat resistant hexa-aluminate catalysts for catalytic combustion ( 4 6 1 . The sintering of supports and catalysts is an inescapable fact, but the process can be controlled to produce desired and relatively stable textures in heterogeneous catalysts. The two examples discussed demonstrate that understanding the processes involved can lead to materials better suited for catalyst preparation and use.
ACKNOWLEDGEMENT The author acknowledges with gratitude financial assistance from Panatam Pty. and Johnson Matthey Ltd., Catalytic Systems Division, Australia.
49
Fig. 13. Scanning electron microscope photographs. Magnification x 1000 (b) -y-A1203 (c) UNSWAL.
(a)
Calcined Gibbsite
50
REFERENCES
1
C.H. Bartholomew, Chemical Engineering, 12 November (1984) 96. C.H. Bartholomew, in B. Delmon & G.F. Froment (Eds), Catalyst Deactivation 1987, Studies in Surface Science & Catalysis, Elsevier, Amsterdam, 34 (1987) 81. 3 A. Nishijima, H. Shimadu, Y. Yoshimura, T. Sat0 and N. Matsubayashi, Ibid 39. 4 D.B. Dadyburjor, Ibid 21. 5 D.L. Trim and A. Stanislaus, Appl. Catal. 21 (1986) 215. G.F. Huttig, Disc. Farad. SOC. 8 (1950) 215. 6 7 A.E. Presland, G.L. Price and D.L. Trim, Surface Science (1972) 424,435. G. Tamman and Q.A. Mansuri, 2. Anorg. Allg. Chem. 126 (1923) 119. 8 9 D.L. Trimm, Appl. Catal. 7 (1983) 249. 10 G.C. Chinchen, in J.R. Jennings (Ed) Selected Developments in Catalysis, Critical Reports on Applied Chemistry, Blackwells Scientific Pub., Oxford 12 (1985) 1. 11 P.E. Flynn and S.E. Wanke, Catal. Rev. Sci. Eng. 12 (1975) 93. 12 E. Ruckenstein and B. Pulvermacher, J . Catal. 29 (1973) 224. 13 F. Delamare and G.E. Rhead, Compt. Rend. 270 (1970) 249. 14 D.L. Trimm, Design of Industrial Catalysts, Elsevier Scientific Publ., Amsterdam, (1980) 91-114. Books, London 15 G.W. Bridger in Catalyst Handbook : Wolfe Scientific (1970) 64. 16 R . J . Coble, J. Amer. Ceram. SOC. 41 (1958) 55. 17 W.H. Gitzen, Alumina as a Ceramic Material, h e r . Ceram. SOC. Columbus (1970). 18 K. Wefers and F.M. Bell, Oxides and Hydroxides of Aluminium, Tech. paper No. 19, Alcoa, Pittsburgh (1972). 19 D.J. Young, P. Udaja and D.L. Trim in B. Delmon and G.F. Froment (Eds) Catalyst Deactivation, Elsevier, Amsterdam (1980) 331. 19a C.H. Bartholomew, R.B. Parnell and R.W. Fowler, J. Catal. 79 (1983) 34. 20 A. Williams, G.A. Butler and J . Hammonds, J. Catal. 24 (1972) 352. 21 T . S . Vinnikova, V.A. Dzisko, L.M. Kefeli and L.M. Plysova, Kin. & Cat. 13 (1972) 149. 22 S . Prochazka and R.L. Coble, Phys. Sintering 2 (1970) 1. 23 W. Rao Raja and I.B. Cutler, J . Amer. SOC. 55 (1971) 170. 24 M.F.L. Johnson, J . Catal. 123 (1990) 245. 25 P.A. Badkar and J.E. Bailey, J. Mater. Sci. 11 (1976) 1794. 26 R.K. Oberlander in "Applied Industrial Catalysis", Academic, N.Y., 1 (1984) 63. 27 N.S. Kozlov, M. Ya Lazarev, L. Ya Mostovaya and I.P. Stremok, Kin. & Cat. 14 (1973) 1130. 28 M. Ozawa, M. Kimura, A. Isogai and S . Matsumoto, Tocat 1, Paper 1-12, 1 July, Tokyo, Japan (1990). 29 S . Matsumoto, N. Miyoshi, f i . Kimara and M. Ozawa, Ibid, Paper 1-11, 30 R.M. Levy and D.J. Baner, J. Catal. 9 (1967) 76. 31 V. Perrichon and M.C. Durupty, Appl. Catal. 42 (1988) 217. 2
51 32 H. Schaper, E.B.M. Doesburg and L.L. van Reijen, Appl. Catal. 7 (1983) 211. 33 H. Schaper, D.J. Amesz, E.B.M. Doesburg and L.L. van Reijen. Appl. Catal. 9 (1984) 129. 34 Y. Chen, L.F. Zhang, J . F . Lin and Y.S. Jin, Tocat 1, Paper 0-34, 1 July, Tokyo, Japan (1990). 35 J. Rostrup-Neilsen, Steam Reforming Catalysts, Teknisk Forlag A / S , Copenhagen (1975). 36 D.J. Harris, D.L. T r i m and D.J. Young, Proc. 10th Aust. Chem. Eng. Conf. (1982) 175. 37 B.C. Gates, J.R. Katzer and G.C.A. Schuit, Chemistry of Catalytic Processes, McGraw-Hill, N.Y. (1979) 184. 38 D.W.B. Westerman, N.R. Foster and M.S. Wainwright, Appl. Catal. 3 (1982) 151. 39 I.E. Wachs, R.Y. Saleh, S . S . Chan and C.C. Chersich, Appl. Catal. 15 (1985) 339. 40 J . Lucas, D. Vanderwell and K.C. Waugh, J. Chem. S O C . Trans. 77 (1981) 15,31. 41 M.Y.H. Kwan, N.W. Cant, D.L. T r i m and M.S. Wainwright, Appl. Catal. 31 (1987) 25. 42 J. Wei, Adv. Catal. 24 (1975) 57. 43 J.C. Schlatter and P.J. Mitchell, Ind. Eng. Chem. Prod. Res. Dev. 19 (1980) 288. 44 R. Di Mascio and D.L. T r i m , unpublished work. 45 A. Walpole, Y. Huang and D.L. T r i m , unpublished work. 46 H. Arai, K. Eguchi, M. Machida and T. Shiomitsu, Tocat 1, Paper 0-18, 1 July, Tokyo, Japan (1990).
29
THERMAL STABILITY OF CATALYST SUPPORTS
D.L. TRIMM School of Chemical Engineering and Industrial Chemistry, University of South Wales, Kensington, NSW 2033, Australia
New
SUMMARY Thermal sintering of catalysts and supports is a major cause of irreversible catalyst deactivation. In this presentation, the mechanisms of sintering are reviewed with particular emphasis on the importance of foreign ions and of ambient atmosphere - factors of interest in the context o f heterogeneous catalysis. It is shown, using the example of alumina, that controlled sintering can lead to supports that maintain desired texture for longer periods of time Calcining in the presence of steam is shown to produce a support particularly suitable f o r partial oxidation catalysis. Calcining under the influence of sulphur oxides is found to produce a high temperature stable support suitable for application in car exhaust catalysts. INTRODUCTION The overall efficiency of a catalyst is defined in terms of activity, selectivity and life. Of these, catalyst life has received by far the least attention in the literature, partly because life testing makes heavy demands on resources and partly as a result of the fact that accelerated testing may be unreliable. Nonetheless, catalyst life is a factor that can control the economic viability of industrial processes 111. Catalyst deactivation can result from poisoning [ 2 ] , from fouling [ 3 ] o r from sintering [ 4 ] . Catalyst poisons may or may not be removable, depending on the nature of the poison. One common foulant, coke deposited on catalysts, can be removed to regenerate the catalyst. Sintering, on the other hand, is largely irreversible and necessitates replacement of the catalyst. Sintering is not necessarily totally undesired, in that calcination during the preparation of a catalyst may result in sintering to produce a desired surface area or porosity [ 5 ] . Once such a desired texture is produced, however, subsequent reorganisations should be minimised. This article focuses on two systems in which it has been found to be possible to produce a desired surface by controlled calcination and to minimise subsequent sintering o f the solid. Both are based on alumina supports - one used for partial oxidation catalysts and the other used in the total oxidation car exhaust catalyst.
30 Sinterina of single Dhase solids Heterogeneous catalysis is an interface phenomenum and it is advantageous to prepare catalysts with the highest possible surface area for a given amount of material. Thermodynamically, this is an unfavourable state since minimal surface free energy is associated with material coalesced into a sphere. As a result, there is always a tendency for catalysts to sinter. Coalescence is, however, an activated process and sintering becomes more significant at higher temperatures. When it occurs, surface area is reduced, porosity changes and the catalysts lose activity. In general, the observed sequence of events during the sintering of single phase solids as temperature is increased includes a) Surface diffusion, which becomes important near to the Huttig temperature ( 0 . 2 - 0.3 x melting point (T,K)) 161 leading to surface smoothinq. b) Concave links or necks formed between close neighbour particles [7] c) Aggregate consolidation, perhaps via volume diffusion which becomes important near to the Tamman temperature (0.5 x T K) [8]. m d) Particle equilibration, with particles moving faceted spherical shape.
towards a
spherical or a
e) Particle vaporisation or melting. The first four stages of sintering have been observed in many catalytic systems of which supported silver is only one example [7]. The last stage is rarely observed except in systems such as catalytic combustors where excess heat may be generated [ 9 ] . Rates of sintering have functions of the type [lo] dD - -dt
dA or Dm-I dt
kl -
=
traditionally been
correlated by
power
law
kZA"
where D is the particle size, A is the surface area, t is the time and m and n are constants. Attempts to relate values of m and n to the mechanism of sintering have not been universally successful, partly because most catalysts are not single phase solids and partly because sintering may involve more than one mechanism (see below). Much attention has been focused on the sintering of supported metal particles [4,11].Sintering has been suggested to result from the migration of particles across the support [12], from the migration of metal atoms across the support [ l l l , from evaporation - condensation of metal atoms as, for example, oxides or chlorides [13] or from a combination of all of these.
31 Sophisticated models reflecting more closely the nature catalysts have also been developed [ 4 ] .
of
"real"
supported
Catalyst and suuuort slnterinp. : General A support is used in order to produce a high surface catalyst. Support materials are carefully chosen for particular systems and thermal stability is an important consideration [ 1 4 ] . Nonetheless, thermal deactivation of the catalyst may result either from sintering of the catalyst or of the support, particularly in situations where more desirable features of a support for a given application are such as to negate a less than desirable thermal stability performance.
The choice of support materials is made on the basis of a range of chemical and physical characteristics (Figure 1) [ 1 4 ] , of which surface area, porosity and thermal stability are particularly important. The range of materials that meet the desired standards is limited (Table l), but desired properties may be enhanced by, for example, doping the support, Physical Factors
Chemical Factors
Is catalytic activity required?
Does the support show desired surface area + porosity?
+
Are chemical catalyst-support interactions possible? Are they desired or undesired? Does the support interact with reactants or products?
Is the thermal conductiv tY acceptable? Is the support mechanica strong?
+
Can the support be poisoned?
I
Can the catalyst be deposited on the support in the desired form?
*
Is the support stable?
I
-
Is the support in the desired shape?
Fig 1. Factors affecting choice of a support. Despite the fact that supports may be composite materials, many of t h e processes affecting sintering have much in common with processes controlling the thermal reorganisation of pure single phase materials. Support sintering can involve a) Surface diffusion, which becomes significant near to the Huttig temperature ( 0 . 3 T K). Since sintering involves solid state diffusion, the rate of m sintering will be an activated process with a diffusion coefficient that is exponentially dependent on temperature.
32
b) Volume diffusion, which becomes significant near to the Tamman temperature (0.5 T K) and which may involve migration of interstitials (Frenkel defects) m or vacancies (Shottky defects). Again, the process is activated. c) Evaporation-condensation processes. As with metals, evaporationcondensation is usually enhanced by the presence of different gases. Thus, for example, silica migration in the presence of steam involves evaporation condensation of the hydroxide [ 1 5 ] . d) Grain Boundary diffusion, which involves the migration of along grain boundaries i n the lattice [16].
atoms or
ions
e) Phase transformation. All crystal lattices contain defects and vacancies, and it is due to these defects that solid state diffusion occurs. Atoms, ions or vacancies can move via these defects to a lower energy state, and such movements may be associated with phase transformations. TABLE 1.
Typical support materials Low surface area
High surface area >I mz g-
essentially non-porous
ground glass alundum (a-A1 0 ) silicon carbi8e3
porous
kieselguhr pumice
essentially non-porous
natural silica-alumina carbon black titania zinc oxide
porous
natural clays synthetic silica-aluminas 7-alumina magnesia activated carbon silica
The importance of phase transformations is easily seen by reference to alumina. The most stable phase is a-alumina,with a surface area of ca 2 2 -1 m g . 7-alumina is widely used as a support, with a surface area of ca 1 5 0 300 m'g-'. It is obvious that phase transformation of 7-A1 0 to a-A1 0 will 2 3 2 3 result in extensive loss of surface area, and it is fortunate that the process is activated, becoming significant only at higher temperatures [17,18]. The majority of polymorphic transformations can be described in terms of nucleation and growth [17]. The nuclei of a new phase are formed at one rate, and then grow at a second rate. Phase transformation is observed when these rates become significant, a process usually controlled by temperature.
33
In the immediate vicinity of the transition temperature, the free energies of the two phases are equal. The formation of the new phase is caused by fluctuations due to thermal excitation, which moves atoms/ions to new positions corresponding to the new phase. Subsequent growth involves the transfer of atoms to the nuclei by any of the processes listed above. Grain boundaries, dislocation sites, vacancies or other imperfections facilitate diffusion to the nuclei and accelerate sintering/phase transformation [lo]. The kinetics of the process depend on the type of rearrangement needed to form the new phase. Displacive transformations demand only the displacement of atoms in a lattice without bond breakage or change in coordination number. The transformation is rapid. Reconstructive transformations require the breakage and subsequent reforming of inter-atomic bonds. This process is usually much slower, since more energy is required to form the new lattice structure. Sintering and phase transformation is further complicated by the fact that supports are used in catalysis. This means that foreign ions (the catalyst) are brought into close contact with the support material and that the system can operate in a variety of atmospheres. Both of these factors can influence thermal reorganisation [10,19,20]. In general terms, the role o f foreign ions is easy to explain. Thermal reorganisation involves diffusion via defects and vacancies [lo]. If foreign ions occupy these sites, the rate of sintering will be affected. Alternatively, if foreign ions react with the host lattice to form a new chemical compound, then the reorganisation of the new compound will occur at a different rate. The effect of the gas atmosphere on sintering is also easy to understand in general terms. Modification of the stoichiometry of solid phases - be it by heat, atmosphere or solid state reaction can influence structure reorganisation. For example, if zinc oxide loses oxygen by heating or by reduction, interstitial zinc atoms or oxygen vacancies are formed which assist sintering. Similarly, metal ions in their lowest valencies become metal deficient in oxidising atmospheres and sinter faster. In the context of support materials sintering, however, it is perhaps the effect of steam that is most noticeable. The formation and removal of hydroxides on the surface leads to accelerated sintering in most supports. Turning from general statements to particular examples, it is useful to consider the behaviour of alumina, in that this support is used widely and exhibits many of the sintering phenomena listed above. Phase transformations in alumina Different phases of alumina may be prepared by calcination of aluminium tri-hydroxides (gibbsite and bayerite) or oxy-hydroxides (boehmite and
34
diaspore). The phases produced depend on the starting material and the temperature of calcination, as shown in Figure 2 [17,18]. Although this diagram is simplified, it provides a useful guide to the inter-relationships in the system.
Kappa
Alpha
D e l t a Theta Alpha
Bayerite
a
*
Theta
Eta
Alpha
Alpha
Diaspore
Fig. 2 . Alumina phases produced at different temperatures [18]. Path a is favoured for fine crystals, Path b is favoured for moist/coarse particles. TABLE 2 Crystal structure of the aluminas. Phase
Crystal System
Unit Cell Parameters (Angstroms) a
b
C
Hydrated aluminas : Gibbsite Bayerite Boehmite Diaspore
Monoclinic Monoclinic Orthorhombic Orthorhombic
8.641 4.716 2.868 4.396
Cubic Cubic (spinel) Tetragonal Tetragonal Monoclinic Orthorhombic Rhombohedra1
7.95 7.90 7.95 7.967 5.63 8.49 4.758
5.070 8.679 12.227 9.426
9.720 5.060 3.700 2.844
7.95 7.697 2.95 12.73
7.79 23.47 11.86 13.39 12.991
Transition aluminas: Chi Eta Gamma Delta Theta Kappa Alpha alumina
35 These phase changes are accompanied by changes in the crystal structure (Table 2). The changes are quite significant, and it is not surprising that phase transformation leads to loss of surface area. The sequence of phase transformations shown in Figure 2 is an approximation, largely because process variables such as time, atmosphere and properties of precursor hydroxides are not included. Thus, for example, bayerite and gibbsite may be converted to boehmite and thence to 7-alumina during calcination if the particle size is large and the precipitate is moist [181,
Such factors have led to contradictory reports. Thus, for example, Vinnitova et al. [21] reported the conversion of boehmite to 7-alumina at 3 0 0 4 0 0 " C , the 7-phase being stable to ca 900 C when conversion to @-alumina occurred. The a-phase was then produced at temperatures as low as 1000 C. These results are certainly in contradiction to those reported by Wefers and Bell [18] and summarised in Figure 2. Part of the problem appears to lie in the small differences in the X-ray diffraction patterns of the various phases of alumina [17] and it is not inconceivable that &-aluminawas present in the samples examined by Vinnikova but was not observed. Similar disagreements have been reported concerning the mechanism of sintering during the initial stages of reorganisation. Prochazka and Coble [22], based on activation energy data, suggested that surface diffusion was the governing process of the initial stages of neck growth between sintering alumina particles. Raja Rao and Cutler [ 2 3 ] reported that volume diffusion predominates over surface diffusion in these initial stages. Johnson [24], on the other hand, has produced an interesting new model in which hydroxyl groups on the surface of alumina may play an important role in the initial stages of sintering. It is well known that the addition of salts such as aluminium formate to alumina can have a significant effect on porosity and particle strength [ 5 ] . This is thought to result from the formation of 0-A1-0 bridges between particles. Johnson suggested that a similar process may occur with hydroxyl bonds, residing on adjacent particles, eliminating water to give A1-0-A1 linkages and neck formation between particles [24]. Accelerated sintering in the presence of water (see below) was described in terms of surface hydration leading to more opportunities for interparticle bridging. Although explaining the initial formation of inter-particle bridges, subsequent consolidation of the neck must involve other than links treated through hydroxyl groups. In view of the fact that the temperatures at which this occurs are low as compared to the melting temperature of alumina, surface diffusion as suggested by Prochazka and Coble [22] seems the more probable route for neck consolidation. Although these initial stages of sintering are important, it is perhaps the phase transformation of 8 to 7 - A 1 0 which is of more interest in a catalyst 2 3
36 support, since this phase change has been reported to account for the greatest loss of surface area. Badkar and Bailey [ 2 5 ] reported that the phase change involves a nucleation and growth mechanism, with grain boundary diffusion being the controlling migratory factor. Although unable directly to observe the nucleation of the a-phase, they concluded that nucleation occurred uniformly throughout the 8-A1203 matrix on a scale of approximately one o nucleus per 10 B grains. Part of the problem in determining an exact mode of sintering of alumina lies in the fact that catalyst supports are prepared to give high surface areas. A s a result, the arrangement of particles one to another is very far from ordered, and sintering may often depend on points of contact within the disordered structure. Thus, for example, Oberlander points out that the development of porosity in pseudoboehmite depends either on the agglomeration of small particles or on the agglomeration of fibril-like particles [ 2 4 ] . Small particles have many points of contact, while fibrils should tend to form bundles with large interstices between the bundles. Subsequent sintering patterns would be expected to be very different, despite the fact that the starting material is similar. Thus it can be suggested that the geometry of different particles can have a significant effect on sintering. This is a finding which will be shown later to have considerable ramifications. Before considering these, however, it is useful to review the effect of the presence of foreign ions and the ambient atmosphere on the sintering of alumina. The effect of foreign ions The presence of small amounts of impurities can either accelerate or decelerate phase transformations in alumina. The presence of platinum at concentrations from 0.1 to 3 w% has been reported to accelerate sintering, with the production of the 0 phase, in particular, being favoured (Figure 3) [ 2 7 ] . On the other hand, 3% Ni was found to lead to the initial stabilisation o f alumina surface area, followed by rapid collapse of the structure (Figure 4 ) [191.
The effects o f metals are, however, less significant than the effect of metal oxides, which are used widely to stabilise alumina [ 2 8 ] . Lanthana, in particular, is widely used to maintain alumina surface area in automobile catalysis [ 2 9 ] . The addition o f the oxides of Li, K or Mg to alumina has been found to increase stability when added at up to ca 2 - 4 mol% [30]. Above this level, stability decreases, as confirmed by Penichon & Durupty [31]. Lanthana, on the other hand, increases the area of alumina calcined at 873-1373 K when added at
37
Temperature w
4 0 0 I
0 0 1
I
Gamma
Gamma
O
W
A
0 0 I
I
Theta
Alpha
Theta
Delta
0
ifc
Alpha
ThetaAlpha
Phase transformations of alumina in the absence and
0 Fig. 4 .
J
0 0
Gamma
N -
Fig. 3.
u l
("C)
8
16 Time (h)
24
of
Pt
32
Surface area loss on heating A 1 0 and 3% N i / A 1 0 2 3
presence
-
2 3
in hydrogen.
1 mol% [ 3 2 ] , but, in the presence of steam, improvements are observed up to 5 the oxides of P, Si, Ba, Sr or Sn increases stability, at least up to about 700 p mol/g
m o l % [ 3 3 ] . Johnson has reported that increasing concentrations of
L241.
Johnson suggests that stabilising elements slow phase transformations by replacing surface hydroxyls and, as a result, slow the formation of A1-0-A1 bridges [ 2 4 ] . Silica, at least, has been found to interact with surface hydroxyls on alumina and this is suggested to reduce sintering by reducing such bridges. In general, however, the evidence is rather weak, and further investigation is needed to prove or disprove the suggestion. What is certain is that the addition of foreign ions can lead to new compounds. Nickel aluminate has been shown to be formed from the calcination of Ni/A1 0 catalysts, and the aluminate has been suggested to be responsible, 2 3 at least in part, for the observed extra stability [19, 19(a)l. Lanthanum aluminate is also known to be formed on calcination of La doped aluminas [ 2 8 , 2 9 ] . Ozawa et al. [ 2 8 ] report that La is inserted into the crystal lattice of alumina with a spinel structure, while Ce remains on the surface in the form of CeO . The activation energy for the formation of a-A1 0 was found to 2 3 be 635-655 W/mol for alumina modified with La, Sm and Y b , as opposed to 5 8 1 583 kJ/mol for pure alumina with and without CeO additions. Although these values seem high, the effect of incorporation of foreign ions in the alumina lattice versus deposition on the lattice is obvious. At the same conference, Chen et al. [ 3 4 ] suggested that ca 10’’ atoms/mz of alumina surface were available for filling by cations, but this measure took no account of the possibility of e.g. La ions moving into the alumina lattice or vice versa. Nor was any estimate given of the importance of octahedral and tetrahedral sites on the lattice substitution [ 3 4 ] , despite the inference that tetrahedral sites could well be of importance to the process. Substitution of such ions in the surface or bulk of alumina may be beneficial in several ways. Firstly, as suggested by Johnson [ 2 4 ] , the hydroxyl surface concentration may be reduced with a subsequent reduction of inter-particlebridging. Secondly, the foreign ion may occupy a vacancy in the alumina lattice, thereby reducing solid state diffusion. Finally, the production of a new compound - either at the surface or in the bulk - may result in greater support stability. The effect of atmosphere Steam is by far the most deleterious atmosphere for the sintering of most stoichiometric oxides used as supports [14,19].At the same time, however, steam is an essential reactant in many processes [ 3 5 ] . It is not surprising, then, to find in-depth studies of the effect of steam on the sintering of e.g. Ni/A1 0 steam reforming catalysts [ 1 9 , 2 0 ] . 2 3
The long term effects of steam and hydrogen on the properties of this catalyst have been studied by Williams et a1 [ 2 0 ] . The catalyst contained only 25% 7-A1203, and may be considered almost as alumina stabilised nickel. The catalyst was found to sinter rapidly in a 9 : l steam : hydrogen atmosphere (25 atmospheres pressure) but, after an initial period, sintering slowed down. The
39 initial sintering increased with increasing steam content at temperatures between 400 and 800"C, the majority of the final a-alumina content being formed during this period. The effect of steam was many times greater than any effect due to nickel. Similar conclusions were reached by Harris et al. [ 3 6 ] , who found that 3% Ni/Al2O3 sintering was much more dependent on the presence of steam than on effects due to Ni (Figure 5). However, the initial loss of surface area was found to be associated more with the transformation of y - A l 0 to the 6 and 0 2 3 phases (Table 3 : compare with Figure 5). The production of a-A1 0 was found 2 3 to be associated more with the slower decrease in surface area at longer times on line.
P o \
0 A I 2 O 3 / p u r e He 0 A 1 2 0 3 / N i / p u r e H2 x A I 2 0 3 / 5 % steam A A 1 2 0 3 /Ni/5% steam 0 A1203/10% steam 0 A I p 0 3 / N i / 1 0 % steam
A
25
50
75
Time (h) Fig. 5 . Surface area loss on heating. Alumina and 3% Ni/A1203 i n hydrogen and steam.
40
TABLE 3 Phase constitution of A1 0 and 3% Ni/A1 0 as a function of time of annealing in a 5 % steamfiydrogen atmisphere at 950'C: ~~
A1203/5% steam
Time (hrs)
Phases Present 7
0 2 5
16
18 24 27
48 65 74
3% Ni/A1203/5% steam
+ + i
S
+ + + + + +
6
+ +
+
+ + +
Phases Present a
+
+ + + +
+ +
7
+ + +
S
+ +
+ + + +
6
a
____
+ +
+
+ + +
+ + +
+ f
+ +
Harris et al. [ 3 6 ] reported values of apparent activation energy based on decrease of surface area. Pure alumina sintering in hydrogen was found to have an apparent activation energy of ca 125 W/mol : in steam, the value was reduced to ca 70 k.J/mol. The effect of steam was suggested to result both from the production of extra hydroxyl groups on the surface and from the possibility of an increase in the number of cation vacancies induced by steam in the trivalent aluminium lattice. However, the evidence for either mechanism was sparse. It is clear from the above discussion that, despite the importance of alumina as a catalyst support, the detailed mechanism of sintering is far from clear. There is good understanding of the effect of different factors on sintering, but the detailed mechanism of phase transformations is, in many cases, open to considerable uncertainty. Factors which influence the thermal stability of the support are better understood. Particle size and shape is known to be important. The absence or presence of foreign ions and differing gas atmospheres significantly affects reorganisation. The kinetics of the process are known to be activated, although it is difficult or impossible to predict what they will be in different systems. Obviously it is possible to design and manufacture catalyst supports based on this knowledge and without detailed understanding of the processes involved in the phase transformations. The more important question is whether the design of supports can be improved even with the limited knowledge available. Two examples are given below to show that such improvements are indeed possible.
41
Preparation of reorvanisation
supports
with
desired
DroDerties by
thermal
The choice of a catalyst support depends on the physical and chemical characteristics of the catalyst and the support, on desired or undesired interactions with components of the system to be catalysed and on the necessity of controlling mass and heat transfer in the process (Figure 1) [ 1 4 ] . Often it is necessary to compromise between various demands or to dope the support to enhance or reduce given properties. Chlorine doping of alumina used in reforming catalysts is a case in point [ 3 7 ] . Supported vanadia, used to promote the selective oxidation of hydrocarbons, is another example. Vanadia, with or without promoters, may be supported on silica (naphthalene oxidation [ 3 8 ] ) , on titania o-xylene oxidation [ 3 9 ] ) or on a-alumina (benzene oxidation [ 4 0 1 ) . It was believed that supports should have open porosity (and associated lower surface area) in order to minimise over oxidation to carbon oxides. However, it was shown that reasonably high activities and selectivities could be obtained over vanadia supported on high surface area material and it was suggested that low selectivity was, in fact, primarily associated with high acidity on the support [ 4 1 ] . In agreement with this, vanadia supported on 7-alumina showed zero selectivity for the production of maleic anhydride from benzene. If this is the case, then a high surface area support with minimal acidity would be desired. With this in mind, attention was focused on modifying 7-aluminaby thermal treatment. It is known that 7-A1 0 can be transformed to a-A1 0 by heating 2 3 2 3 in steam [ 1 7 , 1 8 ] . It is also known that the process is activated, the rate of reaction depending on solid state diffusion. Therefore, it was argued that, if conditions were carefully adjusted, it might be possible to transform the 7 phase at the particle exterior, without transforming the gamma phase in the interior. In this way, the surface would have the properties o f a-alumina (with, in particular, low acidity) while relatively high surface area would be maintained by the underlying 7-phase. As expected, heating -y-Al 0 in steam produced a rapid decrease in surface 2 3 area (Figure 6) together with an associated increase in a-A12 03 as detected by X-ray diffraction. The nomenclature on Figure 7 , 8 and 9 is used only to distinguish samples. Measurement of surface acidity in terms of the rate constant for methanol dehydration showed, however, that the surface properties of the alumina were changing to a different pattern. On the assumption that decrease in acidity was associated with the conversion of 7 to a-A1 0 it is 2 3 seen from figures 7 and 8 that, as expected, acidity changes ascribed to phase transformations at the surface were occurring more rapidly than phase changes in the bulk.
42
T i m e (h)
Fig. 6. Surface area and amount of a - A l - 0 - as determined by X-ray diffraction as-a function of heating time in steam. ’TJ= 1027 C.
0
g -2 7
-
0
I
25
i
50
% a in bulk
cr-A1203
75
<
100
Fig. 7. Plot of logarithm of rate constant from methanol dehydration (used as a measure of surface acidity) vs amount a - A 1 0 in the bulk (as measured by X2 3 ray diffraction).
43
The effect of these changes on the selectivity of the catalyst for maleic anhydride production is seen in Figure 9 . It is clear that the selectivity of the reaction increases dramatically as the acidity of the support is reduced (Figure 9 ) . The conversion of ?-to a-A1 0 in the bulk, however, has little 2 1 subsequent effect on selectivity.
OO
25
50 % a i n bulk
75
100
Fig. 8 . Plot of % a - A L 2 0 3 on surface (from measurements dehydration) v s % a-AL2O3 in bulk (from X-ray diffraction).
loo
of me hanol
I
log 1 0
[ k2 x 1 O7
(L*rn~I-’rn-~s-~)]
Fig. 9 . Selectivity for maleic anhydride production from benzene as a function of surface acidity as measured by the rate constant for methanol dehydration 1411.
44
This is an interesting example of thermal transformations being used to produce a support of desired characteristics. The desired transformation is, effectively, a kinetic phenomenon, where the slow phase transformation resulting from solid state diffusion is arrested at a point where surface reorganisation is complete and bulk transformation - which would result in a massive loss of surface area and activity - is not. Subsequent phase transformations cannot, of course, be avoided, but the relative temperatures of support treatment (1027°C) and benzene oxidation (350°C) are such that the total conversion of the support to a-A1 0 is slow. 2 3 A s a result, the supported catalyst has a long life under normal operating conditions. The (bulk) 7-phase alumina ensures reasonably high surface area, while the (surface) a-phase alumina minimises undesired acidity at the interface. Minimisation of surface reornanlsation Supports sinter with time on line, and this process is accelerated at higher temperatures. This can present significant problems with reactions such as catalytic combustion [9], where excess heat is generated by the reaction. Unless this heat is rapidly removed, the catalyst/support system can overheat and sinter. The best known example involves catalysts used to clean up automobile exhausts [ 4 2 ] . Depending on operating conditions, the exhaust gases from an engine can include unburnt hydrocarbons, carbon monoxide and nitrogen oxides, as well as carbon dioxide, water, nitrogen and oxygen. An automobile exhaust catalyst operates by oxidising hydrocarbons and carbon monoxide and reducing nitrogen oxides to nitrogen, either in one or two bed systems [ 4 2 ] . Design of the catalysts is critical [ 4 2 ] . The volume flow rate of exhaust gas can be high, and pressure drop must be avoided. As a result, it is more common to use a monolithic structure as the base of the catalyst. However, pellets can and have been used. This base material is usually fabricated from cordierite or from a metal alloy [ 4 2 ] , these materials being chosen for their stability and thermal expansion characteristics. The surface area of these materials is low, and it is common practise to coat the base with a washcoat. The washcoat usually contains alumina, metal oxides and the catalyst which, in nearly every commercial case, involves one or more precious metals. Platinum and palladium are favoured for oxidation, while rhodium is the preferred material for selective reduction [ 4 2 ] . The alumina provides a relatively high surface area support for the catalyst, with small amounts of other metal oxides being added to the washcoat in order to stabilise the alumina against sintering [ 2 8 , 2 9 ] . Lanthana and baria are added far this reason : ceria, although added originally as an oxygen storage component [ 4 3 ] , also helps to stabilise the alumina to a small extent [ 2 8 , 2 9 ] .
45
Such catalysts perform well, with an anticipated life of about 5 years or more. However, there are problems existing and foreseen that may limit their lives to much shorter periods. Thus, for example, a plug misfire can lead to excess hydrocarbons reaching the catalyst, to enhanced oxidation and heat release and to sintering. The trend towards lean-burn engines, although advantageous in many respects, leads to more hydrocarbons in the exhaust gas and to greater heat generation. As a result, there is considerable pressure to develop catalysts that are more thermally stable. In addition, there is pressure to reduce the cost of the catalyst. Washcoat aluminas have desired surface areas and porosities and are thermally stable. They are best produced by calcination of particular precursors, and aluminium isopropoxide or boehmite have been suggested to be useful materials to calcine [ 4 2 ] . Both of these precursors are not cheap, and less expensive raw materials would be desired. Thus it is possible to define the need for a cheap alumina that has desired surface area and porosity properties, that is thermally stable - possibly with but preferably without added stabilisers - and which meets the requirements for preparation of washcoats. As would be expected, the solution to these requirements is not simple and it is perhaps most useful to present the convoluted path that led to one possible product. The route that led to the desired product involved initial experiments focused on sulphide odours that could be produced by car exhaust catalysts under some driving conditions. When a vehicle was cold started, for example, the rich exhaust gases were oxidised but odours of hydrogen sulphide or carbonyl sulphide could be produced. The amounts involved were much greater than would be expected from the sulphur content of the gasoline, and it was obvious that sulphur storage was occurring on the catalyst. Studies of this phenomenum showed that this was indeed the case. Sulphur dioxide was not adsorbed to any large extent but sulphur trioxide was adsorbed by ceria and y-alumina (components of the washcoat) as well as by the catalyst [ 4 4 ] . The amount adsorbed depended markedly on temperature, as shown in Figure 10. Up to a 500"C, the amount of trioxide adsorbed increased : above this temperature, trioxide was given off. Calculations showed that the amounts adsorbed or desorbed were much larger than would be anticipated on the basis of adsorption, and these results were explained in terms of the formation and decomposition of metal sulphates. 2Ce02
+
3S03
A1203
-k
3S03
=
Ce2(S04)3
-k
%02
Alz(S04)3
Thermodynamic calculations showed that the formation and decomposition of sulphates at low and high temperatures respectively was not unexpected.
46
t30
t20
400
I
500
I
600
I
700
Temperature ("C)
800
Fig. 10. The amount of sulphur trioxide taken up by ?-A1 0 ceria and 2 3 ' exhaust catalyst as a function of temperature.
a car
As a result of these findings, sulphide odours were suggested to arise from reduction of sulphur trioxide over the precious metals in the catalyst. Rich gas, arriving from the engine, was oxidised to produce heat and some residual reducing gas. The heat increased the temperature of the catalyst which, in turn, led to release of sulphur oxides. These were reduced by the residual reducing gases to give sulphides. In the course of these studies, the morphology and composition of differing species was examined. There was strong evidence of recrystallisation in the washcoat material, with needle like crystals being observed. Since sintering is known to be dependent on particle contact (see above), there appeared to be possibilities of increased thermal stability.
In that reduced cost was an objective, the possibilities of using gibbsite in similar experiments were examined [45].Gibbsite was reacted with sulphuric acid solutions to produce aluminium sulphate. This material was dried and calcined in two stages. In the first, the water of hydration was removed. In the second, aluminium sulphate was decomposed to form alumina. The alumina so produced was then heat treated in order to study the thermal stability of the product,
47
The results are summarised in Figure 11. It is seen that the sample prepared via the aluminium sulphate route (UNSWAL) had a higher surface area after calcining at 800 C than ? - A 1 0 but lower than a typical washcoat 2 3 alumina. At higher temperatures it was less stable than ?-A12 03 and washcoat a1.umina.
01
600
I
700
I
800
I
900
1
1000
Temperature
!il
1100
I 12OC
("c)
Fig. 11. Surface area of samples after heating to indicated temperatures for 4 hrs. Samples were then prepared to which was added 2 % and 4% lanthanum oxide and 4% barium oxide ; some care was necessary as to when the dopant was introduced into the preparation sequence. These results were all very similar, and are summarised in Figure 11. It is seen that, at lower temperatures, surface areas are lower whereas, at higher temperatures, surface area is maintained at the same relatively high level as washcoat alumina. The pore size distribution of UNSWAL before and after thermal treatment at 1000 C is shown in Figure 12. Such a pore size distribution appears to be suitable for washcoat preparation. The reasons for the enhanced stability of the product were then examined. X-ray diffraction patterns showed the presence of gamma, chi, delta and a small amount of kappa alumina, but no obvious reasons for the enhanced stability could be detected.
48
0 Fig. 12. Pore size distribution of alumina after calcining at and 1000 C (residual) for 4 hrs.
850°C
(fresh)
However, inspection of the morphology of the samples confirmed trends previously observed and offered possible reasons for the enhanced stability. A s seen from Figure 1 3 , gibbsite approximates to a faceted structure. 7 alumina has no distinct morphological features. In contrast UNSWAL shows clear evidence of needle like crystals. It is believed that these crystals, by offering a well defined structure with minimal inter-particlecontacts, leads to the minimisation of thermal sintering in the product. The approach of controlling morphology to control sintering offers several interesting possibilities, and these are actively under consideration. At least one involves heat resistant hexa-aluminate catalysts for catalytic combustion ( 4 6 1 . The sintering of supports and catalysts is an inescapable fact, but the process can be controlled to produce desired and relatively stable textures in heterogeneous catalysts. The two examples discussed demonstrate that understanding the processes involved can lead to materials better suited for catalyst preparation and use.
ACKNOWLEDGEMENT The author acknowledges with gratitude financial assistance from Panatam Pty. and Johnson Matthey Ltd., Catalytic Systems Division, Australia.
49
Fig. 13. Scanning electron microscope photographs. Magnification x 1000 (b) -y-A1203 (c) UNSWAL.
(a)
Calcined Gibbsite
50
REFERENCES
1
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