Catalysis : an Integrated Approach to Homogeneous ...

Chapter 3
Chemical kinetics of catalysed
reactions
3.1 INTRODUCTION
Rate expressions are indispensable in the application of catalysed reactions, in
the design of chemical reactors, and their process control. Insight into the dependence
of the reaction rate on catalyst variables, the temperature and concentrations
of reactants, products and other relevant species are needed to predict
the sizes of catalytic reactors and the optimum operating conditions.
r =f(catalyst, T, pi ..., ki ..., Ki ...,
(3.1)
Catalytic reactions consist of a reaction cycle formed by a series of elementary
reaction steps. Hence the rate expression is in general a function of many param-
eters. In heterogeneously catalysed reactions reactant molecules are adsorbed on
the catalyst surface (characterized by equilibrium constants Kj ), undergo chemi-
cal modifications on the surface to give adsorbed products with rate constants kj
and these products finally desorb. The overall catalyst activity and selectivity is
determined by the composition and structure of its surface. Hence it is important
to relate constants, such as ki and Kj with the chemical reactivity of the catalyst
surface.
Since many different modes of adsorption and rearrangement of molecule
fragments are possible on catalyst surfaces, many reactions can occur in parallel.
Therefore these catalysts often have a low selectivity. Selectivity is increased by
the use of promotors or catalyst modifiers that create particular surface sites and
increase selectivity by suppressing undesired reaction paths.
In homogeneous catalysis ligands play a related role with respect to selectivity.
Many different complexes may be formed in a solution; the desired reaction
occurs only on a particular one. Ligand choice determines the particular complex
formed that results in the desired product.
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