Examples of Activity Maps for Important Catalytic Reactions

The Sabatier analysis can in principle be performed for any reaction. In case that there are more than two reaction steps, the Sabatier volcano could be constructed in analogy to Equation (7.16) by assuming that all intermediates that go in the forward direction have optimal coverages and by calculating the approach to equihbrium for each forward rate from the given approach to equilibrium for the overall reaction (as done in Eq. 7.8 for the reaction earher) under the assumption that all other partial reactions are in equihbrium. This wiU give a first approximation to each forward rate of the reaction. The Sabatier volcano provides an upper limit to the total rate by setting this rate equal to the minimum of all forward rates  

The Sabatier volcano is in general useful if one wants to obtain an upper limit of the overall rate in cases where there are many competing reaction steps, and it is not a priori clear which steps are rate determining. Its limitation to small approaches to equilibrium, however, should be kept in mind. 

4 EXAMPLES OF ACTIVITY MAPS FOR IMPORTANT CATALYTIC REACTIONS 

We will now analyze more complex reactions than the simple generic catalytic reaction discussed earlier, starting with the ammonia synthesis reaction. The free energy diagram of ammonia synthesis has been discussed in Chapter 3 (see Figs. 3.9 and 3.10), and a microkinetic model for this reaction has been developed in Chapter 5. The microkinetic model deals with ammonia synthesis on a stepped Ru surface, and we wiU now use the same model and extend it to all possible (stepped) metal surfaces through the scaling relations for adsorbates and for transition states as introduced in Chapter 6. 

Gibbs free energy of the reaction step is exphcitly dependent on the reaction energy, AE., through AG. = LE. - TAS., which can be expressed via the scaling relations as 

---