Introduction-to-Deactivation-of-Catalyst-_1988_Activation--Deactivation--and

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PART II 
Deactivation of Catalyst Pellets: 
Macroscopic Processes 
Catalysts exhibit intrinsic activity that declines with age, but many factors 
other than the intrinsic decay can influence the overall deactivation process. 
The time scale of the decline can vary by many orders of magnitude 
depending on the system and the operating conditions, as will be seen in 
the following chapters. In Part I we explored the fundamental processes of 
decay and the various mechanisms by which it occurs. In order to be utilized 
in reactors, catalysts have various physical forms that can require the 
consideration of certain physical rate processes in conjunction with the 
purely chemical rate processes examined to this point. We shall now discuss 
the physical bases for these added complications. 
Consider a catalyst whose intrinsic properties permit it to catalyze a 
given reaction at a rate of £% gram-moles per second per square centimeter 
of surface at a specified composition and temperature of reaction mixture. 
The most effective utilization of this catalyst would be to develop the area 
within the catalytic material to as large a value as possible, since the greater 
surface area enhances the catalytic effect. Of course, this is done. There are 
two common ways to increase the specific surface area of a material: (i) 
reduce the particle size by grinding or pulverizing or (ii) develop a network 
of fine pores within the material. The latter method is used almost exclusively 
because large porous pellets are separated from the reaction mixture much 
more easily than are fine powders and because it is very difficult to grind 
materials fine enough to obtain large specific surface areas. Those familiar 
with such technology will recognize that 1-2 m2 per gram of material is 
generally regarded as a low specific surface area; moderate to high surface 
area materials may range from 200-300 to 1000 m2/g, respectively. In 
general, then, heterogeneous catalysts are porous. 
The specific surface area of a porous material is roughly inversely propor­
tional to the pore radius and, accordingly, heterogeneous catalysts also 
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generally have small pores, often as small as 3 nm or even less. But now, 
to utilize all of this area, reactants must be transported into, and products 
out of, the interior of the catalyst through the pore structure. The more 
extensive the surface the more active the catalyst pellet per unit weight or 
volume, but also the smaller the pores and the greater the difficulty of 
transporting material into and out of the particle. The net result is that the 
interior of the pellet can be exposed to compositions and temperatures 
different from those at the exterior of the pellet and, as a consequence, the 
contribution to the overall activity of the pellet of an element of area in 
the interior can differ from that of the same size element on the exterior. 
This is, of course, a familiar problem in catalytic reaction engineering, often 
referred to as the "Thiele-Zeldovich" problem. Our pupose here is not to 
discuss this problem per se, as it is amply treated elsewhere,1 but to relate 
the many ways in which such phenomena are affected by deactivation.2 It 
is clear that if reactivity may be nonuniform throughout the pellet, deactiva­
tion may be also. The coupling between such nonuniformities greatly compli­
cates the interpretation of deactivation data and the subsequent design of 
reactors employing a heterogeneous catalyst phase. We must, therefore, 
treat this problem rather generally to learn how rates of transport, reaction, 
and deactivation can interact. In so doing in Part II, we shall learn that the 
interaction is a mixed blessing in the sense that it is neither all good nor 
all bad. For example, experiments in this regime can lead to mechanistic 
insights difficult to identify unequivocally under gradientless conditions. 
We shall learn further that nonuniformity of deactivation throughout the 
pellet can, under certain circumstances, be preferable to uniform deactiva­
tion in operation. However, from a mathematical point of view one must 
deal with gradients of concentration, activity, and temperature that can 
evolve in shape, position, and time and the dominant feature in describing 
the overall pellet behavior is a considerable increase in the complexity of 
analysis and design. 
1 R. Aris, "The Mathematical Theory of Diffusion and Reaction in Permeable Catalysts." 
Oxford Univ. Press (Clarendon), London and New York, 1975; E. E. Petersen, "Chemical 
Reaction Analysis," Prentice-Hall, Englewood Cliffs, New Jersey, 1965; C. N. Satterfield, 
"Mass Transfer in Heterogeneous Catalysis," MIT Press, Cambridge, Massachusetts, 1970; 
A. Wheeler, Adv. Catal 3, 250 (1950). 
2 R. Hughes, "Deactivation of Catalysts." Academic Press, London, 1984.