Page 32 - Principles of Catalyst Development
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18 CHAPTER I
and N 2 • The 1/2 power comes from dissociation of hydrogen and nitrogen
molecules. We often make simplifying assumptions where justified, e.g.,
(1.26 )
which gives
( 1.27)
or more generally, the Power Rate Law
(1.28 )
This demonstration is given as an example only. The fact that (1.27)
does not fit experimental data is taken as evidence that reaction (1.23) is
not the rate-determining step.(301
Rate equations such as these, based on proposed mechanisms, can
become very complex. Often many alternative mechanisms fit equally well.
If confirmed, a given rate equation and its associated mechanism serve as
a guide only. Other techniques are always required to establish the validity
of the surface mechanism.
1.4.5. Product Steps
Steps (5), (6), and (7) in Fig. 1.5 are the reverse of (3), (2), and (1).
Although diffusion out is not likely to be different from diffusion in,
desorption of a product could be rate determining and must be handled
accordingly.
1.4.6. Global Rates
I f steps (1 )-( 4) are all first order and are combined as a series of
sequential events, then a first-order reaction with a "global" rate constant
3
ko describes the entire process. It can be shown thae !)
(1.29)
If a given step is much slower than the others (i.e., rate determining),
that appropriate term predominates in the global rate constant. If there is
more than one slow step, or if the surface reaction is not first order, then
a global rate expression is too complicated and cannot be solved analytically.