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For a rough approximation, we can further assume that an adsorbed species or transition state has lost all degrees of freedom (it is completely locked in place on the surface), and the entropy is therefore zero. 68 J molÀ1 KÀ1, respectively, and we can now estimate the entropy changes between the initial states and the À1 À1 transition state as DS1z ¼ À192:77 J molÀ1 K and DS2z ¼ À130:68 J molÀ1 K . For the first step it is assumed that N2 loses all its gas-phase entropy upon entering the transition state and in the second step, the entropy loss of one H2 gas-phase molecule is considered when it reacts with N* on the surface.

In typical scenarios, the degree of rate control for transition states is positive, indicating that lowering the transition state energy increases the reaction rate, and the degree of rate control for surface intermediates is negative, which means that stabilizing the intermediate will poison the surface and result in a slower reaction rate. This can be interpreted more generally for heterogeneously catalyzed reactions. On the optimal catalyst the overall reaction proceeds along a ‘‘smooth’’ PES from the reactants to the products.

First, more attention should to be paid to parameters to which the model shows a high sensitivity, and these parameters need to be carefully measured or calculated. Non-sensitive parameters, on the other hand, may be roughly estimated or even guessed. Second, the sensitivity of a microkinetic model towards certain parameters can be interpreted as a measure of the degree to which a step is rate controlling (degree of rate control). This information can provide direction for useful catalyst modifications which could optimize performance (degree of catalyst control).