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Effects of sintering on the active site distribution on promoted catalysts
Applied Catalysis, 45 (1988) 115-130 Elsevier Science Publishers B.V., Amsterdam -
115 Printed in The Netherlands
Effects of Sintering on the Active Site Distribution on Promoted Catalysts M. BOWKER Leverhulme Centre for Innovative Catalysis, Department of Inorganic, Physical and Industrial Chemistry, University of Liverpool, PO Box 147, Liverpool L69 3BX (U.K.) (Received 26 May 1988, revised manuscript received 8 July 1988)
ABSTRACT Perhaps the most important property of any catalytic system in the applied context is the magnitude of the catalytic decay with time on stream. Even though this is the case it is very much more common for fundamental catalytic research to concern itself with the features of initial catalyst performance. In this paper the effects of catalyst sintering on catalytic behaviour are considered for the case where the catalyst has been doped with a promoter to enhance its initial performance. Two mechanisms of sintering are considered - coalescence and Ostwald ripening. It is shown that the long term effect of promotion is strongly dependent on the surface level of the promoter and that promotion can lead to very different decay rates of activity with time than for the unpromoted case, showing both greatly increased, and reduced rates of decline depending upon the mechanism of sintering and the range of the promoter action.
INTRODUCTION
A major factor in determining the cost effectiveness of many catalytic industrial processes is the loss of catalyst efficiency with time on stream. Very few catalytic systems do not show such decay and the associated time constant varies from days to years for different processes. Several factors contribute to this decay - poisoning, pressure drop problems due to attrition, loss of promoter efficiency and sintering. In the catalytic context the latter refers to the phenomenon of active particle enlargement which results in loss of total active area, the reaction rate being generally proportional to the latter parameter. Such loss can occur in several ways which are outlined below. Particle sintering is very often the main cause of catalyst decay and as such it is rather surprising that so little attention has been devoted to the subject. Evidence for the latter statement is the paucity of articles in a journal such as Catalysis Reviews Science and Engineering dealing with the subject of catalyst sintering. Although the fundamental effort in this area then is somewhat lacking it is nevertheless a subject of considerable concern and attention for the industri-
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alist. In general, the aim of such workers is to reduce catalyst decay bit by bit and the empirical route to good achievement is usually preferred; that is (i) novel catalyst preparation method, or variant of customary method, or additive (ii) test new formulation in microreactor (iii) if successful on to development or, if not, try again at (i). Often an appreciation of which of the factors above is the main cause of decay is absent and if the cause is known to be sintering an appreciation of what the sintering is doing and how it is effected is incomplete. Part of the reason for these difficulties with the sintering process is its complexity. Many factors can affect it and so it is often difficult to determine which one factor in any particular process may be most amenable to beneficial change. In the present paper the major concern is to examine the effect of catalyst sintering on the active site distribution in promoted catalysts since, to the author’s knowledge, this has received no attention in the literature thus far. This is of great importance since many of the high tonnage, heterogeneously catalysed industrial processes use promoted catalysts; these include ammonia synthesis, ethylene epoxidation, Fischer-Tropsch oxygenate production, some reforming systems and many other syntheses. In what follows the dependence of the total number of active sites on promoter coverage, and the effect of sintering (both coalescent and surface mediated ‘Ostwald ripening”) on that distribution for the cases where the promoter species is completely mobile over the whole catalyst and where it is immobilised on each particle is described. Thus the effect of sintering on parameters of more significance for catalytic chemists - activity and selectivity - is determined. SINTERING
MECHANISMS
Firstly it must be noted that the word sintering is more properly used in a metallurgical sense to describe the coalescence of metal particles which are in contact with one another, that is in powders, and this usually occurs thermally. In the catalytic sense sintering includes the loss of surface area of supported materials (usually metals) which are often highly dispersed on the support and not in contact with one another. In the catalytic environment sintering often occurs at temperatures very much lower than would occur in vacuum or an inert atmosphere. Thus supported silver catalysts sinter rather quickly during the initial stages of ethylene epoxidation at z 500 K, whereas treatment at 850 K in vacuum appears to have no appreciable effect over a similar time period [ 11.Thus the chemical nature of the catalyst can, as might be expected, strongly affect the kinetics of the sintering process. This can be due to (i) formation of compounds of high volatility (ii) formation of compounds of high surface diffusivity (with high mobility across the support medium) or (iii) alteration of the support surface to aid diffusion. In all these cases the diffusion is of the molecular form with a flux of species taking place between catalyst
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particles. Case (i) above is the type of sintering known as Ostwald ripening [ 2-41 in which the vapour pressure above small particles is higher than that above larger particles (due to the lower average coordination number of surface species); thus large particles grow at the expense of small ones. Case (ii) is really the two-dimensional analogue of the same phenomenon, but is likely to occur at lower temperature since in general lower activation barriers are involved in surface diffusion as compared with evaporation. Thus sintering is most likely to be caused in this way (here called surface mediated Ostwald Ripening) than by evaporative sintering. However, another possible major route to sintering is by coalescence. This too can occur in several ways. Firstly, in the metallurgical sense, by the “necking” and merging of particles densely packed onto a support. This is likely to be the initial phase of sintering in high loading catalysts; clearly after some such sintering has taken place particles will no longer be in immediate proximity and therefore such coalesence will cease. Secondly a process referred to elsewhere as the “crystallite migration model” [ 21 in which the mass of a catalyst particle, as a whole, migrates across a surface and finally collides with another causing sintering. This is only likely to be the situation for very small catalyst particles and has been observed in the field ion microscopy for several metal systems adsorbed on other metals [ 51. Nevertheless many catalytic processes use very highly dispersed metal particles and so this possibility must be considered. Sintering by these differing mechanisms has been shown very nicely in recent work by Ruckenstein and co-workers for model suported silver [9] and platinum [lo] catalysts using transmission electron microscopy. Thus in what follows the two basic mechanisms of catalyst particle sintering have been considered, both the coalescence and the Ostwald ripening type of mechanism. In the next immediate section the effects of increasing the coverage of promoter species upon the distribution of activated and unaffected sites will be described. EFFECTS OF PROMOTER LEVEL
On site distribution The effect of promoter surface coverage (or,, where 13=1 corresponds to saturation of all surface sites) upon the coverage of promoted sites (eA) and unaffected sites (0,) is shown in Figs. 1 and 2. Fig. 1 represents that relationship for an ordered surface with the parameter n differentiating the four curves being the number of affected sites - the affected “ensemble”. In this case it is assumed that the promoter atoms are always completely ordered with equal surface separation (repulsive interactions ). The active site dependence on promoter coverage shows a discontinuity at a critical coverage 0, where
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O-O 0.2 0.4 O-6 0.8 1.0 PROMOTER COVERAGE Fig. 1. The dependence 1,2,4
of promoted
and 8 in order of increasing
0.2
0.0
site fraction
maximum
0.4 O-6 PROMOTER COVERAGE
on promoter
coverage
for several values of n of
for an ordered system.
0.8
13
Fig. 2. The fraction of promoted sites (solid lines) and unaffected sites (dotted lines) as a function of promoter coverage for a disordered system. There are five curves for each case representing ensembles increasing
of 2,4,6, ensemble
8 and 10 sites affected by a central promoter moiety. For the promoted sites size increases the site fraction, whereas the opposite is the case for unaffected
sites.
&=(rz+l)-l
(1)
n being the total number of promoted sites in the ensemble of n+ 1 total sites
119
(the promoter species being assumed to OCCUPY a single site). To the low COVside of this curve the number of activated sites is equal to n%, whereas above 8, the only effect of adding promoter is site blockage, equal to 1- 0~. Above 0, there are no unaffected sites on the surface. The situation illustrated in Fig. 1 is unlikely to be encountered in catalytic systems due to a variety of factors. Firstly, the promoter is more likely to be randomly distributed, except perhaps at the highest coverages. Secondly, even if such ordered structures were present on a catalyst the lineshape of Fig. 1 would be smoothed due to 6) crystallographic heterogeneity giving regions of differing n values, (ii ) differing promoter coverages on different planes and possibly (iii ) defects, dislocations and edges at the surface. Thirdly, the catalyst surface itself couldbe lacking in order except over very short distances. For these reasons the situation illustrated in Fig. 2 is more likely to resemble the kind of promoter effect which would be measured on real catalyst surfaces. The most significant difference here from that in Fig. 1 is that some unpromoted sites remain up to considerably higher coverages of promoter. These curves are generated from the function given in eqn. 3 which is derived from the following considerations, for a random distribution of promoter species. The average number of unaffected sites (19,) is as follows
erage
Bu= (l-HJn+l
(2)
the right hand side representing the statistical product of probabilities of finding any site unaffected by either blocking of the promoter atom itself, or by the promotion affect by virtue of its adjacence to such a species. The coverage of Promoted sites (0~) is then given simply by the difference between total sites (unity) and promoter sites and unaffected sites: e*=l-e,-s,=l-e,-(l-s,)n+l=y(l-~n)
(3)
where Y= I- 0,. The resulting distribution of promoted sites shows a much broader, smoother shape than in Fig. 1 with the maximum coverage of such sites being Produced at significantly higher promoter coverages, especially for higher order effects. Thus for the ordered system with 8 affected sites the max_ imum is at 0, = 0.11, whereas for the random distribution case it is at 0.23. For 2 affected sites the respective figures are 0.33 and 0.42. Furthermore the max_ imum proportion promoted sites is significantly lower for the random com_ pared with the ordered situation.
of
On catalyst performance The major interest of all this for the catalytic worker is to understand how promotion will affect the catalytic performance. In general promoters can act in two ways: (i) as an activity promoter (enhanced reactivity of promoted sites for a 100% selective reaction ) and (ii) as a selectivity promoter. The latter can
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occur by either reduction of the rate of the non selective route, promotion of the selective route or combinations of such effects. Often the catalytic activity can be diminished in such a situation. A well known example of this is selectivity promotion in ethylene epoxidation catalysis over silver in which chlorine adatoms are deposited on the metal surface and enhance selectivity to the desired product from z 50 to 7O%, while significantly decreasing the catalytic activity [ 6,7]. An example of what is thought to be a simple activity promoting system is ammonia synthesis conducted over potassium (in compound form) doped iron catalysts. The effects of varying promoter coverage on activity for case (i) and on selectivity for a type (ii) system (in which selectivity and activity are enhanced) has been examined. These data are presented in Figs. 3 and 4. They are based on the following relationships. First, for case (i) AR =&Au
+ @,A,
(4)
where Au and AA are the individual site activities for unpromoted and promoted states and AR is a relative activity based on the zero promoter activity. For case (ii) the selectivity is determined from the following:
where S is the fractional selectivity to desired product, and SU and SA are the selectivities for unaffected and affected sites respectively. In eqn. 5 the numerator represents the activity to desired product, while the denominator is the total activity. Fig. 3 has been derived assuming that promoter affected sites have quadrupled activity and it can be seen that there is the expected strong activity enhancement, except above 8,=0.75 [ (l-0,) =AU/AA]. This shape is reminiscent of those seen for several real catalytic systems [ 81. At the top of Fig. 3 is shown the potassium promoter loading in wt.-% as a percentage of the total catalyst weight which would be required to promote a hypothetical iron catalyst of surface area 10 m2 gg’. The scale in the figure is derived from the following general relationship: 0,= WNl/s
(6)
where W is the weight ratio of promoter species added, N1 is Avogadros number per unit weight of promoter and s is the number of sites available on the catalyst per unit weight and this assumes a homogeneous distribution of the promoter species on the catalyst surface. From the figure it is clear that low coverages of promoter are required for optimum effect, the value being around 0, = 0.2. However, if sintering is a significant feature of the process then the considerations outlined below (section on effect.s of sintering on the behaviour of promoted catalysts) may alter the preferred choice of promoter loading.
121
00
O-2
0.4 O-6 O-8 PROMOTER COVERAGE
1
I.(3
Fig. 3. The variation of catalytic activity with promoter coverage for (in order of increasing activity) affected ensembles of 2,4,6,8 and 10 sites respectively within the disordered adsorbate model. The parameters used were AA = 4 and Au = 1.The upper calibration axis is the potassium loading (w/w) for a hypothetical catalyst of total surface area 10 m2 g-’ composed of pure iron.
PROMOTER COVERAGE
0
Fig. 4. As for Fig. 3 except showing the dependence of selectivity on promoter coverage; S, = 0.5 and S, = 1, and the smallest affected ensemble shows the lowest selectivity. Fig. 4 shows the results for selectivity dependence on promoter coverage. In this case it has been assumed that the selectivity of unpromoted sites is 0.5 (the value at zero promoter coverage) and promoted sites are 1.0 (the high promoter coverage asymptote) and the relative activities are as above. There is a smooth transition between these values at fairly low coverages of promoter, for large ensembles the optimum selectivity is essentially achieved at very low promoter coverages (S, = 0.1, for n = 8). Clearly in such a case increasing coverages of promoter only reduce the catalyst activity without significant gain of selectivity and is therefore undesirable. Of course, the assumption in this treatment is that all promoter affected sites are affected in an identical way, which
122
is not necessarily the case, especially when selectivity modifications are associated with altered transition state geometrical requirements. However, it is beyond the scope of this work, both in terms of intelligibility and length, to consider all possibilities within such complex systems, though such different interactions are treatable; the main aim here is to show how sintering can affect simple promoter distributions and their associated sites. EFFECTS OF SINTERING ON THE BEHAVIOUR OF PROMOTED CATALYSTS
Having described above the relationship between promoter surface coverage, site distribution and catalyst performance, the effect of sintering on these parameters will now be considered. The two types of sintering mechanism to be considered, coalesence and surface mediated Ostwald ripening have been outlined in the section on sintering mechanisms above. In addition to this, in what follows, the possibility of two different ways in which the promoter can distribute itself over the catalyst surface have been included. These are: (1) the promoter is not migratory, that on the active component (metal phase, for instance) cannot diffuse to the support phase, or the latter is only a minor constituent (ammonia synthesis catalysts, for example ref. 8); (2) the promoter is homogeneously distributed over the active and support phase by relatively rapid surface diffusion. In the latter case, if the total area changes little during sintering (as in ethylene epoxidation catalysts, where the metal area is a small fraction of the total [ 61) then the surface coverage of promoter on the active phase will change little with the consequences described further below. In the former case the situation is more complex as outlined immediately below. Immobile promoter Particle coalescence The effects of particle coalescence on the coverage of U and A sites are shown in Figs. 5 and 6 for two promoter coverages. The curves are derived from the following relationship assuming an immobile promoter distribution: SA =N(r)-2nr2
(7)
where SA is the total surface area of the particles, N(r) is the number of particles (r dependent during sintering) of radius r, all assumed to be hemispheres with no shape change during sintering. The constraint on the system is conservation of total particle volume V during sintering, thus V=constant=N(r).2nr3/3
(6)
and insertion of this in eqn. 7 gives SA =3V/r
(9)
123
SURFACE AREA Fig. 5. The effect of coalescent sintering on the fraction of unaffected sites at the surface of a catalyst. The solid lines are for an initial promoter coverage of 0.05 and the dotted curves for 0.2 coverage. The five curves are for n values of (in order of decreasing initial site fraction) 2,4,6,8 and 10 respectively. The parameters used were 500 initial particles of 10 units radius. Sintering to half the initial area reduces the number of particles to 54 and increases their radius to 21.
o.ooi-IO
SURFACE AREA
Fig. 6. As for Fig. 5, but representing the fraction of promoted sites. Increasing site fraction corresponds with increasing n.
and so surface area shows a reciprocal dependence on r. This relationship is a well known one. Another important relationship in this context is that between surface area and particle number density and this is given below for the unimodal distribution case: s*=
[27cN(r)]q3V)+
(10)
124
The coverage of promoter species during this sintering process is
(11) where 0: and Si are the initial values of promoter coverage and surface area for the modelled system. Figs. 5 and 6 show the effect of sintering on unpromoted and promoted site distribution for two initial promoter coverages of 0.05 and 0.2. In the case of the promoted sites at the lower promoter coverage the surface fraction of activated sites increases during sintering (although the total promoted sites in the sample decrease due to reduced particle number density) whereas at the high coverage lower order ensembles show a maximum in A sites. For U sites sintering always reduces their coverage due to increased site blockage/promotion. The effect of sintering on the catalyst performance is shown in Fig. 7, again for two values of initial promoter coverage of 0.05 and 0.2. The catalyst activity is given by: A= (B,A,
+&A,)N(r)/N*(r)
(12)
and can show dramatic decline in activity during the sintering process. Thus for the initial promoter coverage of 0.2 the activity for an ensemble of 4 affected sites declines by 4 for only a halving of the surface area (latter stages of sintering). During the sintering process the promoted sites become more concentrated and therefore, for a selectivity promoting system the selectivity will increase to the maximum, except for cases of high promoter coverage where all available I.0 /‘/
/‘/
/
.<* go.5;&d& //
/’
NC/’ ’ ,// /’
//,’
,v/
/
0.0
/’
0.2
I
A’
’
4’ I
0.b
I
I
O-6
I
SURFACE AREA
I
O-8
I
I.0
Fig. 7. The effect of coalescent sintering on catalyst activity referred to the highest activity as unity. Full lines for 8,=0.05, dotted lines for 0,= 0.2. Parameters are AA=4, AU= 1. The five curves for each promoter coverage are for n = 2,4,6,8 and 10 in order of increasing initial activity.
125
I 0.5 0.2
I I I I 0.1, 06 SURFACE AREA
Fig. 8. AS for Fig. 7, but showing
I 0-a selectivity
I
I.0 changes
during sinking. Parameters are &=0.5, with increasing IZ.
SA = 1,AU = 1,AA = 4. The five curves increase in selectivity
sites are already promoting. Fig. 8 illustrates these points and is derived using eqn. 5. Thus, in general for this kind of sintering a promoted catalyst can show severe declines of activity and continual increases in selectivity. In the following section the alternative form of sintering is considered in this context.
Ostwald ripening The mechanism of this sintering process has been described above. There are many parameters which come into play here, such as the initial particle size distribution, the density of particles at the surface, the promoter loading, process parameters and so forth. For simplicity in what follows a catalyst system of bimodal particle sizes is considered and each of these are delta functions. In this case the surface area of the particulate system is given by SA =2NrcR2+2nnr2
(13)
where N and R are the number and radius of large particles in the model system and n and r are the same parameters for the smaller ones. During sintering the larger particles act as nuclei and get larger at the expense of the small particles. As above the constraint on the system is the conservation of matter represented here as volume and so V= $NnR”+
$nm3
If sintering begins and the smaller particle larger particles increase to radius R, where
(14) diminish
in size to rl, then the
126
SURFACEAREA Fig. 9. The distribution ofpromoted sites during sintering by an Ostwald ripening type mechanism for two different coverages of promoter. The solid lines are for an initial promoter coverage of 0.05, dotted lines 0.2. The five curves are for n values of 2,4,6,8 and 10 in order of increasing site fraction. The parameters used were R= 12, r= lo! N= 103, n= 104. The fraction of sites is normalised with respect to the total number of initial sites as unity.
4
;::I
--_--*
__----____-EO-& --------------- ---_______-----E? a
I___-_-____~~_~~_____~I-------
5o,2
-_
O-0’ 0.6 I
---I
I
08 07 SURFACE AREA
---_-----I
0.9
--__
I I.0
Fig. 10. As for Fig. 9 except representing the distribution of unaffected sites during Ostwald ripening type sintering. In this case increasing n gives curves of reduced site fraction.
R
(r3-rT)n+R3 i
=
1 [
N
1
(15)
and with new values of radii, the sintered surface area can be determined from eqn. 13 above. The promoter coverage after sintering is then given by eqn. 3 for each of the two sets of particles. Obviously, as sintering occurs SA associated with nuclei particles continuously increases and so the promoter coverage continuously diminishes, whereas the opposite is the case for the reducing particles. The number of promoted and unpromoted sites can then be determined by reference to Fig. 2 and eqns. 2 and 3 for each particle. Starting with the distribution shown in the figure legend, as the small particles sinter to zero area so the total surface area diminishes by 40% (increase
$_7 F
4 0.7 06
0.01 0.5
I
0.6
I
I
0.7 0.8 SURFACE AREA
I
0.9
i I
I
I
,!O 0.5' 0,6 0.8 SURFACE AREA
I
I
1.0
Fig. 11. The effect of Ostwald ripening type sintering on the activity of a promoted catalyst. Parameters are as for Fig. 9 with additionally AA=4, AK= 1, and the activity is normalised with respect to the highest initial activity. Increasing ensemble size n corresponds with increasing activity curves. Fig. 12. The effects of Ostwald ripening type sintering on the selectivity of a promoted catalyst. Parameters as in Fig. 9 with additionally A_*= 4, AK = 1, SA = 1, S, = 0.5. The curves are for increasing n values in order of increasing selectivity.
of 50% in size of large particles). In terms of the site distribution, at very high initial promoter coverages the total number of promoted sites shows a minimum during sintering whereas at moderate and lower coverages the promoter coverage continuously diminishes (Fig. 9). The unpromoted sites also diminish for low initial promoter levels, but at high levels show an increase with sintering and at intermediate coverage shows a minimum for this promoter set. These effects are shown in Fig. 10. The effects of these changes on activity are shown in Fig. 11 for the activity promoting case. The activity diminishes strongly with sintering at both sets of coverages, the effect being greater than would bk expected simply from the area decrease. Selectivity changes with sintering in this case are shown in Fig. 12, and are quite opposite to the coalescence case shown above in that generally the selectivity decreases with sintering. However, at low 0, and small ensemble size there is a slight initial increase in selectivity due to the concentration of selective sites on the smaller particle set. However, generally the effect is dominated by the opposite situation pertaining to the larger particles. Mobile promoter, homogeneously distributed
In what follows the above assumption of an immobile relaxed to the assumption of a homogeneously distributed
promoting layer is layer. In this case
128
the situation is envisaged in which the active phase is supported on an inactive phase which constitutes the large majority of the surface area; this is often the case in real catalytic systems of low metal loading. Coalescence
The effects of sintering by particle agglomeration on the site distribution, activity and selectivity in a promoted system are now quite simple. Since the promoter is homogeneously distributed there is always a constant ratio of A and U sites and therefore in a system which is selectivity promoting the selectivity does not change during this sintering process. For activity promoters the catalytic activity diminishes linearly with sintering, the activity being directly related t.o the area as follows: A=S:AL+S;AA
(16)
where Sk and Si are the unpromoted and promoted areas (in terms of the site area multiplied by total number of such sites), Since the promoter is homogeneously distributed the ratio Sy/Si is always constant and so the activity is linearly related to surface area. Ostwald ripening The effects here are similar to those described in the section on coalescence above. Once again selectivity is particle size independent, and the activity diminishes linearly with decreasing total area during sintering according to equation 16 above. TIME-DEPENDENCE
In terms of catalysis, the practitioner is interested in the effect of reactor running on the catalyst’s performance. In order to show this in relation to the data above empirical sintering can be considered. Thus the dispersion can be emprically represented in the following forms [4] : D=Ao exp( -ht) D= [C/(l+h’t]l’(“-1’
for m=l for m#l
(17) (16)
where C, Do, k and k’ are constants, m is the order of sintering and t is the time. The order often seems to be much greater than unity [4,11]. The dispersion is proportional to surface area and so the surface area dependences shown in the previous sections can be converted to time trends. The case of a mobile homogeneously distributed promoter will not be treated since the time dependence is simple, the activity showing the same time dependence as above. Assuming m = 4, a reasonable average value from literature data [4,11], then the time dependencies are represented in Fig. 13 for the case
129
TIME(arb. units) Fig. 13. A comparison of activity decay for promoted and unpromoted catalysts which sinter by coalescence. Promoter coverage is 0.05 (0 ), 0 ( x ), 0.2, ( 0 ) and 0.5 (0 1.
I
0
2
I
4 TIMElarb. units)
I
6
I
8
Fig. 14. As for Fig. 13 except sintering occurs by Ostwald ripening.
of coalescent sintering. It is seen that, compared with an unpromoted standard, the rate of activity decay is less for an initial promoter coverage of 0.05, but much faster for a coverage of 0.5 (note that the absolute initial activities have been normalised in the figure). Clearly the overpromotion of a catalyst in the first instance has a potentially disastrous effect on the total make of desired product (the time integral of activity) for coalescent sintering. The investigator trying to analyse catalyst decay under these circumstances would find unusual results; the activity for a catalyst with an initial coverage of 0.5 declines by a factor of 5 for a surface area decrease of only 40%. For the low promoter coverage the activity decline is of a lower order than the time sintering rate. Fig. 14 also shows the time dependence of activity for the Ostwald ripening case. Once again it must be stressed that this is modelled on a strictly bimodal (delta function) distribution which is not realistic. In reality there would be
130
some kind of broadened distribution of particle sizes. For the modelled case, once all the smaller particles have been lost by sintering, no further area loss occurs. Nevertheless the data of Fig. 14 indicate the qualitative trends of promoter effects on activity in this case. Once again the high promoter level catalyst shows severe activity decline, the initial activity loss being more acute than for the coalescence case. However, even the lowest promoter level shown here exhibits worse activities for longer sintering times. The most unrealistic part of these time dependences is the longer time scale decay. This is limited by the stop on sintering because of the delta function of the larger particles. In the real situation this size function would be more spread, sintering would contiue and so the decline would be more severe than shown for the long times in Fig. 14. In the real catalytic situation the sintering will probably occur through different mechanisms at different points in the process. Certainly for high loading, or very high dispersion catalysts it is likely that the initial sintering will be dominated by coalescence, while Ostwald ripening is likely to dominate when the interparticle distance becomes greater. Clearly a complete qualitative modelling of catalytic sintering in the presence of promoters would be an extremely difficult and lengthy problem and is probably not worth the effort because of the very many variables involved. In what has been presented here it is hoped that the gross effects of sintering on the surface properties and catalytic kinetics has been clarified and that attention has been drawn to the longer term detrimental effects of overpromotion on catalytic behaviour.
REFERENCES M. Bowker, unpublished data. B.K. Chakraverty, J. Phys. Chem. Solids, 28 (1967) 2401. P. Wynblatt and N. Gjostein, Prog. Solid State Chem., 9 (1985)
21.
4 5
SE. Wanke and P.C. Flynn, Catol. Rev.-Sci. Eng., 12 (1975) 93. G. Ehrlich, in J.L. de Segovia (Ed.), Proc. 9th Intern. Vat. Congress and 5th Intern. Conf.
6
Solid Surfaces, A.S.E.V.A. Madrid, 1983, p. 3 and references therein. P.A. Kilty, N. Rol and W. Sachtler, Proc. 5th Intern. Congr. Catalysis,
8
C.T. Campbell and M.T. Paffett, Appl. Surf. Sci., 19 (1984) 28. W.D. Mross, Catal. Rev.-Sci. Eng., 25 (1983), 591 and references therein.
9 10 11
E. Ruckenstein
and S.H. Lee, J. Catal., 109 (1988)
100.
I. Sushumna and E. Ruckenstein, J. Catal., 109 (1988) R.W. Rice and C.C. Chien, J. Catal., 103 (1987) 140.
433.
1973, Vol. 2, p. 929.
115 Printed in The Netherlands
Effects of Sintering on the Active Site Distribution on Promoted Catalysts M. BOWKER Leverhulme Centre for Innovative Catalysis, Department of Inorganic, Physical and Industrial Chemistry, University of Liverpool, PO Box 147, Liverpool L69 3BX (U.K.) (Received 26 May 1988, revised manuscript received 8 July 1988)
ABSTRACT Perhaps the most important property of any catalytic system in the applied context is the magnitude of the catalytic decay with time on stream. Even though this is the case it is very much more common for fundamental catalytic research to concern itself with the features of initial catalyst performance. In this paper the effects of catalyst sintering on catalytic behaviour are considered for the case where the catalyst has been doped with a promoter to enhance its initial performance. Two mechanisms of sintering are considered - coalescence and Ostwald ripening. It is shown that the long term effect of promotion is strongly dependent on the surface level of the promoter and that promotion can lead to very different decay rates of activity with time than for the unpromoted case, showing both greatly increased, and reduced rates of decline depending upon the mechanism of sintering and the range of the promoter action.
INTRODUCTION
A major factor in determining the cost effectiveness of many catalytic industrial processes is the loss of catalyst efficiency with time on stream. Very few catalytic systems do not show such decay and the associated time constant varies from days to years for different processes. Several factors contribute to this decay - poisoning, pressure drop problems due to attrition, loss of promoter efficiency and sintering. In the catalytic context the latter refers to the phenomenon of active particle enlargement which results in loss of total active area, the reaction rate being generally proportional to the latter parameter. Such loss can occur in several ways which are outlined below. Particle sintering is very often the main cause of catalyst decay and as such it is rather surprising that so little attention has been devoted to the subject. Evidence for the latter statement is the paucity of articles in a journal such as Catalysis Reviews Science and Engineering dealing with the subject of catalyst sintering. Although the fundamental effort in this area then is somewhat lacking it is nevertheless a subject of considerable concern and attention for the industri-
0166-9834/88/$03.50
0 1988 Elsevier Science Publishers B.V.
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alist. In general, the aim of such workers is to reduce catalyst decay bit by bit and the empirical route to good achievement is usually preferred; that is (i) novel catalyst preparation method, or variant of customary method, or additive (ii) test new formulation in microreactor (iii) if successful on to development or, if not, try again at (i). Often an appreciation of which of the factors above is the main cause of decay is absent and if the cause is known to be sintering an appreciation of what the sintering is doing and how it is effected is incomplete. Part of the reason for these difficulties with the sintering process is its complexity. Many factors can affect it and so it is often difficult to determine which one factor in any particular process may be most amenable to beneficial change. In the present paper the major concern is to examine the effect of catalyst sintering on the active site distribution in promoted catalysts since, to the author’s knowledge, this has received no attention in the literature thus far. This is of great importance since many of the high tonnage, heterogeneously catalysed industrial processes use promoted catalysts; these include ammonia synthesis, ethylene epoxidation, Fischer-Tropsch oxygenate production, some reforming systems and many other syntheses. In what follows the dependence of the total number of active sites on promoter coverage, and the effect of sintering (both coalescent and surface mediated ‘Ostwald ripening”) on that distribution for the cases where the promoter species is completely mobile over the whole catalyst and where it is immobilised on each particle is described. Thus the effect of sintering on parameters of more significance for catalytic chemists - activity and selectivity - is determined. SINTERING
MECHANISMS
Firstly it must be noted that the word sintering is more properly used in a metallurgical sense to describe the coalescence of metal particles which are in contact with one another, that is in powders, and this usually occurs thermally. In the catalytic sense sintering includes the loss of surface area of supported materials (usually metals) which are often highly dispersed on the support and not in contact with one another. In the catalytic environment sintering often occurs at temperatures very much lower than would occur in vacuum or an inert atmosphere. Thus supported silver catalysts sinter rather quickly during the initial stages of ethylene epoxidation at z 500 K, whereas treatment at 850 K in vacuum appears to have no appreciable effect over a similar time period [ 11.Thus the chemical nature of the catalyst can, as might be expected, strongly affect the kinetics of the sintering process. This can be due to (i) formation of compounds of high volatility (ii) formation of compounds of high surface diffusivity (with high mobility across the support medium) or (iii) alteration of the support surface to aid diffusion. In all these cases the diffusion is of the molecular form with a flux of species taking place between catalyst
117
particles. Case (i) above is the type of sintering known as Ostwald ripening [ 2-41 in which the vapour pressure above small particles is higher than that above larger particles (due to the lower average coordination number of surface species); thus large particles grow at the expense of small ones. Case (ii) is really the two-dimensional analogue of the same phenomenon, but is likely to occur at lower temperature since in general lower activation barriers are involved in surface diffusion as compared with evaporation. Thus sintering is most likely to be caused in this way (here called surface mediated Ostwald Ripening) than by evaporative sintering. However, another possible major route to sintering is by coalescence. This too can occur in several ways. Firstly, in the metallurgical sense, by the “necking” and merging of particles densely packed onto a support. This is likely to be the initial phase of sintering in high loading catalysts; clearly after some such sintering has taken place particles will no longer be in immediate proximity and therefore such coalesence will cease. Secondly a process referred to elsewhere as the “crystallite migration model” [ 21 in which the mass of a catalyst particle, as a whole, migrates across a surface and finally collides with another causing sintering. This is only likely to be the situation for very small catalyst particles and has been observed in the field ion microscopy for several metal systems adsorbed on other metals [ 51. Nevertheless many catalytic processes use very highly dispersed metal particles and so this possibility must be considered. Sintering by these differing mechanisms has been shown very nicely in recent work by Ruckenstein and co-workers for model suported silver [9] and platinum [lo] catalysts using transmission electron microscopy. Thus in what follows the two basic mechanisms of catalyst particle sintering have been considered, both the coalescence and the Ostwald ripening type of mechanism. In the next immediate section the effects of increasing the coverage of promoter species upon the distribution of activated and unaffected sites will be described. EFFECTS OF PROMOTER LEVEL
On site distribution The effect of promoter surface coverage (or,, where 13=1 corresponds to saturation of all surface sites) upon the coverage of promoted sites (eA) and unaffected sites (0,) is shown in Figs. 1 and 2. Fig. 1 represents that relationship for an ordered surface with the parameter n differentiating the four curves being the number of affected sites - the affected “ensemble”. In this case it is assumed that the promoter atoms are always completely ordered with equal surface separation (repulsive interactions ). The active site dependence on promoter coverage shows a discontinuity at a critical coverage 0, where
118
O-O 0.2 0.4 O-6 0.8 1.0 PROMOTER COVERAGE Fig. 1. The dependence 1,2,4
of promoted
and 8 in order of increasing
0.2
0.0
site fraction
maximum
0.4 O-6 PROMOTER COVERAGE
on promoter
coverage
for several values of n of
for an ordered system.
0.8
13
Fig. 2. The fraction of promoted sites (solid lines) and unaffected sites (dotted lines) as a function of promoter coverage for a disordered system. There are five curves for each case representing ensembles increasing
of 2,4,6, ensemble
8 and 10 sites affected by a central promoter moiety. For the promoted sites size increases the site fraction, whereas the opposite is the case for unaffected
sites.
&=(rz+l)-l
(1)
n being the total number of promoted sites in the ensemble of n+ 1 total sites
119
(the promoter species being assumed to OCCUPY a single site). To the low COVside of this curve the number of activated sites is equal to n%, whereas above 8, the only effect of adding promoter is site blockage, equal to 1- 0~. Above 0, there are no unaffected sites on the surface. The situation illustrated in Fig. 1 is unlikely to be encountered in catalytic systems due to a variety of factors. Firstly, the promoter is more likely to be randomly distributed, except perhaps at the highest coverages. Secondly, even if such ordered structures were present on a catalyst the lineshape of Fig. 1 would be smoothed due to 6) crystallographic heterogeneity giving regions of differing n values, (ii ) differing promoter coverages on different planes and possibly (iii ) defects, dislocations and edges at the surface. Thirdly, the catalyst surface itself couldbe lacking in order except over very short distances. For these reasons the situation illustrated in Fig. 2 is more likely to resemble the kind of promoter effect which would be measured on real catalyst surfaces. The most significant difference here from that in Fig. 1 is that some unpromoted sites remain up to considerably higher coverages of promoter. These curves are generated from the function given in eqn. 3 which is derived from the following considerations, for a random distribution of promoter species. The average number of unaffected sites (19,) is as follows
erage
Bu= (l-HJn+l
(2)
the right hand side representing the statistical product of probabilities of finding any site unaffected by either blocking of the promoter atom itself, or by the promotion affect by virtue of its adjacence to such a species. The coverage of Promoted sites (0~) is then given simply by the difference between total sites (unity) and promoter sites and unaffected sites: e*=l-e,-s,=l-e,-(l-s,)n+l=y(l-~n)
(3)
where Y= I- 0,. The resulting distribution of promoted sites shows a much broader, smoother shape than in Fig. 1 with the maximum coverage of such sites being Produced at significantly higher promoter coverages, especially for higher order effects. Thus for the ordered system with 8 affected sites the max_ imum is at 0, = 0.11, whereas for the random distribution case it is at 0.23. For 2 affected sites the respective figures are 0.33 and 0.42. Furthermore the max_ imum proportion promoted sites is significantly lower for the random com_ pared with the ordered situation.
of
On catalyst performance The major interest of all this for the catalytic worker is to understand how promotion will affect the catalytic performance. In general promoters can act in two ways: (i) as an activity promoter (enhanced reactivity of promoted sites for a 100% selective reaction ) and (ii) as a selectivity promoter. The latter can
120
occur by either reduction of the rate of the non selective route, promotion of the selective route or combinations of such effects. Often the catalytic activity can be diminished in such a situation. A well known example of this is selectivity promotion in ethylene epoxidation catalysis over silver in which chlorine adatoms are deposited on the metal surface and enhance selectivity to the desired product from z 50 to 7O%, while significantly decreasing the catalytic activity [ 6,7]. An example of what is thought to be a simple activity promoting system is ammonia synthesis conducted over potassium (in compound form) doped iron catalysts. The effects of varying promoter coverage on activity for case (i) and on selectivity for a type (ii) system (in which selectivity and activity are enhanced) has been examined. These data are presented in Figs. 3 and 4. They are based on the following relationships. First, for case (i) AR =&Au
+ @,A,
(4)
where Au and AA are the individual site activities for unpromoted and promoted states and AR is a relative activity based on the zero promoter activity. For case (ii) the selectivity is determined from the following:
where S is the fractional selectivity to desired product, and SU and SA are the selectivities for unaffected and affected sites respectively. In eqn. 5 the numerator represents the activity to desired product, while the denominator is the total activity. Fig. 3 has been derived assuming that promoter affected sites have quadrupled activity and it can be seen that there is the expected strong activity enhancement, except above 8,=0.75 [ (l-0,) =AU/AA]. This shape is reminiscent of those seen for several real catalytic systems [ 81. At the top of Fig. 3 is shown the potassium promoter loading in wt.-% as a percentage of the total catalyst weight which would be required to promote a hypothetical iron catalyst of surface area 10 m2 gg’. The scale in the figure is derived from the following general relationship: 0,= WNl/s
(6)
where W is the weight ratio of promoter species added, N1 is Avogadros number per unit weight of promoter and s is the number of sites available on the catalyst per unit weight and this assumes a homogeneous distribution of the promoter species on the catalyst surface. From the figure it is clear that low coverages of promoter are required for optimum effect, the value being around 0, = 0.2. However, if sintering is a significant feature of the process then the considerations outlined below (section on effect.s of sintering on the behaviour of promoted catalysts) may alter the preferred choice of promoter loading.
121
00
O-2
0.4 O-6 O-8 PROMOTER COVERAGE
1
I.(3
Fig. 3. The variation of catalytic activity with promoter coverage for (in order of increasing activity) affected ensembles of 2,4,6,8 and 10 sites respectively within the disordered adsorbate model. The parameters used were AA = 4 and Au = 1.The upper calibration axis is the potassium loading (w/w) for a hypothetical catalyst of total surface area 10 m2 g-’ composed of pure iron.
PROMOTER COVERAGE
0
Fig. 4. As for Fig. 3 except showing the dependence of selectivity on promoter coverage; S, = 0.5 and S, = 1, and the smallest affected ensemble shows the lowest selectivity. Fig. 4 shows the results for selectivity dependence on promoter coverage. In this case it has been assumed that the selectivity of unpromoted sites is 0.5 (the value at zero promoter coverage) and promoted sites are 1.0 (the high promoter coverage asymptote) and the relative activities are as above. There is a smooth transition between these values at fairly low coverages of promoter, for large ensembles the optimum selectivity is essentially achieved at very low promoter coverages (S, = 0.1, for n = 8). Clearly in such a case increasing coverages of promoter only reduce the catalyst activity without significant gain of selectivity and is therefore undesirable. Of course, the assumption in this treatment is that all promoter affected sites are affected in an identical way, which
122
is not necessarily the case, especially when selectivity modifications are associated with altered transition state geometrical requirements. However, it is beyond the scope of this work, both in terms of intelligibility and length, to consider all possibilities within such complex systems, though such different interactions are treatable; the main aim here is to show how sintering can affect simple promoter distributions and their associated sites. EFFECTS OF SINTERING ON THE BEHAVIOUR OF PROMOTED CATALYSTS
Having described above the relationship between promoter surface coverage, site distribution and catalyst performance, the effect of sintering on these parameters will now be considered. The two types of sintering mechanism to be considered, coalesence and surface mediated Ostwald ripening have been outlined in the section on sintering mechanisms above. In addition to this, in what follows, the possibility of two different ways in which the promoter can distribute itself over the catalyst surface have been included. These are: (1) the promoter is not migratory, that on the active component (metal phase, for instance) cannot diffuse to the support phase, or the latter is only a minor constituent (ammonia synthesis catalysts, for example ref. 8); (2) the promoter is homogeneously distributed over the active and support phase by relatively rapid surface diffusion. In the latter case, if the total area changes little during sintering (as in ethylene epoxidation catalysts, where the metal area is a small fraction of the total [ 61) then the surface coverage of promoter on the active phase will change little with the consequences described further below. In the former case the situation is more complex as outlined immediately below. Immobile promoter Particle coalescence The effects of particle coalescence on the coverage of U and A sites are shown in Figs. 5 and 6 for two promoter coverages. The curves are derived from the following relationship assuming an immobile promoter distribution: SA =N(r)-2nr2
(7)
where SA is the total surface area of the particles, N(r) is the number of particles (r dependent during sintering) of radius r, all assumed to be hemispheres with no shape change during sintering. The constraint on the system is conservation of total particle volume V during sintering, thus V=constant=N(r).2nr3/3
(6)
and insertion of this in eqn. 7 gives SA =3V/r
(9)
123
SURFACE AREA Fig. 5. The effect of coalescent sintering on the fraction of unaffected sites at the surface of a catalyst. The solid lines are for an initial promoter coverage of 0.05 and the dotted curves for 0.2 coverage. The five curves are for n values of (in order of decreasing initial site fraction) 2,4,6,8 and 10 respectively. The parameters used were 500 initial particles of 10 units radius. Sintering to half the initial area reduces the number of particles to 54 and increases their radius to 21.
o.ooi-IO
SURFACE AREA
Fig. 6. As for Fig. 5, but representing the fraction of promoted sites. Increasing site fraction corresponds with increasing n.
and so surface area shows a reciprocal dependence on r. This relationship is a well known one. Another important relationship in this context is that between surface area and particle number density and this is given below for the unimodal distribution case: s*=
[27cN(r)]q3V)+
(10)
124
The coverage of promoter species during this sintering process is
(11) where 0: and Si are the initial values of promoter coverage and surface area for the modelled system. Figs. 5 and 6 show the effect of sintering on unpromoted and promoted site distribution for two initial promoter coverages of 0.05 and 0.2. In the case of the promoted sites at the lower promoter coverage the surface fraction of activated sites increases during sintering (although the total promoted sites in the sample decrease due to reduced particle number density) whereas at the high coverage lower order ensembles show a maximum in A sites. For U sites sintering always reduces their coverage due to increased site blockage/promotion. The effect of sintering on the catalyst performance is shown in Fig. 7, again for two values of initial promoter coverage of 0.05 and 0.2. The catalyst activity is given by: A= (B,A,
+&A,)N(r)/N*(r)
(12)
and can show dramatic decline in activity during the sintering process. Thus for the initial promoter coverage of 0.2 the activity for an ensemble of 4 affected sites declines by 4 for only a halving of the surface area (latter stages of sintering). During the sintering process the promoted sites become more concentrated and therefore, for a selectivity promoting system the selectivity will increase to the maximum, except for cases of high promoter coverage where all available I.0 /‘/
/‘/
/
.<* go.5;&d& //
/’
NC/’ ’ ,// /’
//,’
,v/
/
0.0
/’
0.2
I
A’
’
4’ I
0.b
I
I
O-6
I
SURFACE AREA
I
O-8
I
I.0
Fig. 7. The effect of coalescent sintering on catalyst activity referred to the highest activity as unity. Full lines for 8,=0.05, dotted lines for 0,= 0.2. Parameters are AA=4, AU= 1. The five curves for each promoter coverage are for n = 2,4,6,8 and 10 in order of increasing initial activity.
125
I 0.5 0.2
I I I I 0.1, 06 SURFACE AREA
Fig. 8. AS for Fig. 7, but showing
I 0-a selectivity
I
I.0 changes
during sinking. Parameters are &=0.5, with increasing IZ.
SA = 1,AU = 1,AA = 4. The five curves increase in selectivity
sites are already promoting. Fig. 8 illustrates these points and is derived using eqn. 5. Thus, in general for this kind of sintering a promoted catalyst can show severe declines of activity and continual increases in selectivity. In the following section the alternative form of sintering is considered in this context.
Ostwald ripening The mechanism of this sintering process has been described above. There are many parameters which come into play here, such as the initial particle size distribution, the density of particles at the surface, the promoter loading, process parameters and so forth. For simplicity in what follows a catalyst system of bimodal particle sizes is considered and each of these are delta functions. In this case the surface area of the particulate system is given by SA =2NrcR2+2nnr2
(13)
where N and R are the number and radius of large particles in the model system and n and r are the same parameters for the smaller ones. During sintering the larger particles act as nuclei and get larger at the expense of the small particles. As above the constraint on the system is the conservation of matter represented here as volume and so V= $NnR”+
$nm3
If sintering begins and the smaller particle larger particles increase to radius R, where
(14) diminish
in size to rl, then the
126
SURFACEAREA Fig. 9. The distribution ofpromoted sites during sintering by an Ostwald ripening type mechanism for two different coverages of promoter. The solid lines are for an initial promoter coverage of 0.05, dotted lines 0.2. The five curves are for n values of 2,4,6,8 and 10 in order of increasing site fraction. The parameters used were R= 12, r= lo! N= 103, n= 104. The fraction of sites is normalised with respect to the total number of initial sites as unity.
4
;::I
--_--*
__----____-EO-& --------------- ---_______-----E? a
I___-_-____~~_~~_____~I-------
5o,2
-_
O-0’ 0.6 I
---I
I
08 07 SURFACE AREA
---_-----I
0.9
--__
I I.0
Fig. 10. As for Fig. 9 except representing the distribution of unaffected sites during Ostwald ripening type sintering. In this case increasing n gives curves of reduced site fraction.
R
(r3-rT)n+R3 i
=
1 [
N
1
(15)
and with new values of radii, the sintered surface area can be determined from eqn. 13 above. The promoter coverage after sintering is then given by eqn. 3 for each of the two sets of particles. Obviously, as sintering occurs SA associated with nuclei particles continuously increases and so the promoter coverage continuously diminishes, whereas the opposite is the case for the reducing particles. The number of promoted and unpromoted sites can then be determined by reference to Fig. 2 and eqns. 2 and 3 for each particle. Starting with the distribution shown in the figure legend, as the small particles sinter to zero area so the total surface area diminishes by 40% (increase
$_7 F
4 0.7 06
0.01 0.5
I
0.6
I
I
0.7 0.8 SURFACE AREA
I
0.9
i I
I
I
,!O 0.5' 0,6 0.8 SURFACE AREA
I
I
1.0
Fig. 11. The effect of Ostwald ripening type sintering on the activity of a promoted catalyst. Parameters are as for Fig. 9 with additionally AA=4, AK= 1, and the activity is normalised with respect to the highest initial activity. Increasing ensemble size n corresponds with increasing activity curves. Fig. 12. The effects of Ostwald ripening type sintering on the selectivity of a promoted catalyst. Parameters as in Fig. 9 with additionally A_*= 4, AK = 1, SA = 1, S, = 0.5. The curves are for increasing n values in order of increasing selectivity.
of 50% in size of large particles). In terms of the site distribution, at very high initial promoter coverages the total number of promoted sites shows a minimum during sintering whereas at moderate and lower coverages the promoter coverage continuously diminishes (Fig. 9). The unpromoted sites also diminish for low initial promoter levels, but at high levels show an increase with sintering and at intermediate coverage shows a minimum for this promoter set. These effects are shown in Fig. 10. The effects of these changes on activity are shown in Fig. 11 for the activity promoting case. The activity diminishes strongly with sintering at both sets of coverages, the effect being greater than would bk expected simply from the area decrease. Selectivity changes with sintering in this case are shown in Fig. 12, and are quite opposite to the coalescence case shown above in that generally the selectivity decreases with sintering. However, at low 0, and small ensemble size there is a slight initial increase in selectivity due to the concentration of selective sites on the smaller particle set. However, generally the effect is dominated by the opposite situation pertaining to the larger particles. Mobile promoter, homogeneously distributed
In what follows the above assumption of an immobile relaxed to the assumption of a homogeneously distributed
promoting layer is layer. In this case
128
the situation is envisaged in which the active phase is supported on an inactive phase which constitutes the large majority of the surface area; this is often the case in real catalytic systems of low metal loading. Coalescence
The effects of sintering by particle agglomeration on the site distribution, activity and selectivity in a promoted system are now quite simple. Since the promoter is homogeneously distributed there is always a constant ratio of A and U sites and therefore in a system which is selectivity promoting the selectivity does not change during this sintering process. For activity promoters the catalytic activity diminishes linearly with sintering, the activity being directly related t.o the area as follows: A=S:AL+S;AA
(16)
where Sk and Si are the unpromoted and promoted areas (in terms of the site area multiplied by total number of such sites), Since the promoter is homogeneously distributed the ratio Sy/Si is always constant and so the activity is linearly related to surface area. Ostwald ripening The effects here are similar to those described in the section on coalescence above. Once again selectivity is particle size independent, and the activity diminishes linearly with decreasing total area during sintering according to equation 16 above. TIME-DEPENDENCE
In terms of catalysis, the practitioner is interested in the effect of reactor running on the catalyst’s performance. In order to show this in relation to the data above empirical sintering can be considered. Thus the dispersion can be emprically represented in the following forms [4] : D=Ao exp( -ht) D= [C/(l+h’t]l’(“-1’
for m=l for m#l
(17) (16)
where C, Do, k and k’ are constants, m is the order of sintering and t is the time. The order often seems to be much greater than unity [4,11]. The dispersion is proportional to surface area and so the surface area dependences shown in the previous sections can be converted to time trends. The case of a mobile homogeneously distributed promoter will not be treated since the time dependence is simple, the activity showing the same time dependence as above. Assuming m = 4, a reasonable average value from literature data [4,11], then the time dependencies are represented in Fig. 13 for the case
129
TIME(arb. units) Fig. 13. A comparison of activity decay for promoted and unpromoted catalysts which sinter by coalescence. Promoter coverage is 0.05 (0 ), 0 ( x ), 0.2, ( 0 ) and 0.5 (0 1.
I
0
2
I
4 TIMElarb. units)
I
6
I
8
Fig. 14. As for Fig. 13 except sintering occurs by Ostwald ripening.
of coalescent sintering. It is seen that, compared with an unpromoted standard, the rate of activity decay is less for an initial promoter coverage of 0.05, but much faster for a coverage of 0.5 (note that the absolute initial activities have been normalised in the figure). Clearly the overpromotion of a catalyst in the first instance has a potentially disastrous effect on the total make of desired product (the time integral of activity) for coalescent sintering. The investigator trying to analyse catalyst decay under these circumstances would find unusual results; the activity for a catalyst with an initial coverage of 0.5 declines by a factor of 5 for a surface area decrease of only 40%. For the low promoter coverage the activity decline is of a lower order than the time sintering rate. Fig. 14 also shows the time dependence of activity for the Ostwald ripening case. Once again it must be stressed that this is modelled on a strictly bimodal (delta function) distribution which is not realistic. In reality there would be
130
some kind of broadened distribution of particle sizes. For the modelled case, once all the smaller particles have been lost by sintering, no further area loss occurs. Nevertheless the data of Fig. 14 indicate the qualitative trends of promoter effects on activity in this case. Once again the high promoter level catalyst shows severe activity decline, the initial activity loss being more acute than for the coalescence case. However, even the lowest promoter level shown here exhibits worse activities for longer sintering times. The most unrealistic part of these time dependences is the longer time scale decay. This is limited by the stop on sintering because of the delta function of the larger particles. In the real situation this size function would be more spread, sintering would contiue and so the decline would be more severe than shown for the long times in Fig. 14. In the real catalytic situation the sintering will probably occur through different mechanisms at different points in the process. Certainly for high loading, or very high dispersion catalysts it is likely that the initial sintering will be dominated by coalescence, while Ostwald ripening is likely to dominate when the interparticle distance becomes greater. Clearly a complete qualitative modelling of catalytic sintering in the presence of promoters would be an extremely difficult and lengthy problem and is probably not worth the effort because of the very many variables involved. In what has been presented here it is hoped that the gross effects of sintering on the surface properties and catalytic kinetics has been clarified and that attention has been drawn to the longer term detrimental effects of overpromotion on catalytic behaviour.
REFERENCES M. Bowker, unpublished data. B.K. Chakraverty, J. Phys. Chem. Solids, 28 (1967) 2401. P. Wynblatt and N. Gjostein, Prog. Solid State Chem., 9 (1985)
21.
4 5
SE. Wanke and P.C. Flynn, Catol. Rev.-Sci. Eng., 12 (1975) 93. G. Ehrlich, in J.L. de Segovia (Ed.), Proc. 9th Intern. Vat. Congress and 5th Intern. Conf.
6
Solid Surfaces, A.S.E.V.A. Madrid, 1983, p. 3 and references therein. P.A. Kilty, N. Rol and W. Sachtler, Proc. 5th Intern. Congr. Catalysis,
8
C.T. Campbell and M.T. Paffett, Appl. Surf. Sci., 19 (1984) 28. W.D. Mross, Catal. Rev.-Sci. Eng., 25 (1983), 591 and references therein.
9 10 11
E. Ruckenstein
and S.H. Lee, J. Catal., 109 (1988)
100.
I. Sushumna and E. Ruckenstein, J. Catal., 109 (1988) R.W. Rice and C.C. Chien, J. Catal., 103 (1987) 140.
433.
1973, Vol. 2, p. 929.