  A general formalism is presented to describe the turnover frequency (TOF)
during heterogeneous catalysis beyond a mean field treatment. For every
elementary reaction we define its multiplicity as the number of times the
reaction can be performed in the current configuration of the catalyst surface,
divided by the number of active sites. It is shown that any change in the
multiplicity with temperature can be directly understood as a modification in
configurational entropy. Based on this, we determine the probability of
observing any particular elementary reaction, leading to a procedure for
identifying any Rate Controlling Step (RCS) as well as the Rate Determining
Step (RDS), if it exists. Furthermore, it is shown that such probabilities
provide a thorough description of the overall catalytic activity, enabling a
deep understanding of the relative importance of every elementary reaction.
Most importantly, we formulate a simple expression to describe accurately the
apparent activation energy of the TOF, valid even when adsorbate-adsorbate
interactions are included, and compare it to previous, approximate expressions,
including the traditional Temkin formula for typical reaction mechanisms
(Langmuir-Hinshelwood, Eley-Rideal, etc...). To illustrate the validity of our
formalism beyond the mean field domain we present Kinetic Monte Carlo
simulations for two widely-studied and industrially-relevant catalytic
reactions, namely, the oxidation of CO on RuO$_2$(110) and the selective
oxidation of NH$_{3}$ on the same catalyst.
