Summary Concepts in Catalysis

In this chapter we introduced the basic physical chemistry that governs catalytic reactivity. The catalytic reaction is a cycle comprised of elementary steps including adsorption, surface reaction, desorption, and diffusion. For optimum catalytic performance, the activation of the reactant and the evolution of the product must be in direct balance. This is the heart of the Sabatier principle. Practical biological, as well as chemical, catalytic systems are often much more complex since one of the key intermediates can actually be a catalytic reagent which is generated within the reaction system. The overall catalytic system can then be thought of as nested catalytic reaction cycles. Bifunctional or multifunctional catalysts realize this by combining several catalytic reaction centers into one catalyst. Optimal catalytic performance then requires that the rates of reaction at different reaction centers be carefully tuned. 

To predict catalyst performance, one needs to predict the rates of the elementary reaction steps at the catalyst surface. This must ultimately be integrated into a kinetic simulation which treats the interactions between the many different adsorbates present on the catalyst surface. In this chapter, we presented rate expressions derived from transition state reaction rate theory as a bridge to connect ab initio quantum mechanical information to reaction rate predictions. In Chapter 3, we present a more extensive treatment of kinetic simulations including many-body interactions and their influence on the catalytic performance. 

We use the constructs of transition state theory in order to define the Br0nsted-Evans-Polanyi (BEP) relationship, which relates the equilibrium thermodynamics (reaction enthalpy or free energy) with non-equilibrium thermodynamic features, namely the activation energy and activation entropy. A small value of the proportionality parameter in the BEP relationship, a, is identified with an early transition state, whereas values of a that are close to 1 relate to a late transition state. Microscopic reversibility ensures that if the forward reaction is an early transition state then the backward reaction must be a late transition state and vice versa. 

The rates of reaction that proceed through early transition states are rather insensitive to changes in the reaction enthalpy, and hence variations in the catalyst, provided that there is no change in the reaction path. On the other hand, reactions with late transition states depend strongly on the reaction energy and hence are quite sensitive to variations in 

The transition-state entropies for a surface reaction tend to be small because of the need for tight contact with the catalytic surface atoms. On the other hand, changes in the activation entropies are large for elementary reaction steps in which the reactants desorb from the surface. 

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