We present a different approach 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 changes in the multiplicity with temperature can be directly understood as modifications in configurational entropy. The multiplicities allow formulating the probability of observing any particular elementary reaction in the steady state, thus providing a complete understanding of the relative importance of every elementary reaction in the overall catalytic reaction. In addition, this allows identifying the Rate Determining Step (RDS), if it exists, as well as the Rate Controlling Steps (RCSs). In this manner, monitoring the multiplicities provides a straightforward alternative to computationally-expensive approaches based on the Degree of Rate Control and the Rate Sensitivity. The multiplicities also allow formulating a simple expression to describe the temperature dependence of the apparent activation energy of the TOF (Ea), valid even when adsorbate-adsorbate interactions are included. Even in the simplest case, when Ea remains constant within some temperature range, we show that it does not correspond to the largest elementary activation energy available in the system, as still believed by some researchers. In fact, Ea does not even correspond to the elementary activation energy of the RDS, when it exists, as also amply believed. In addition to the activation barrier of the RDS, Ea contains an important, unbound configurational entropy contribution from the temperature dependence of the multiplicity of the dominating reaction (i.e. the coverage for those surface sites participating in the RDS). Due to this contribution, Ea may depart from a constant value even when a single RDS is controlling the overall reaction. In comparison, the traditional description of Ea in terms of adsorption heats is limited in practice by difficulties in determining the required reaction orders, while conceptually it neglects the actual configurational origin and entropic character of an important part of Ea. To illustrate the validity of our approach beyond the mean field domain we present Kinetic Monte Carlo simulations for some widely-studied and industrially-relevant catalytic reactions.

Another approach to heterogeneous catalysis

2019 Spring National Meeting

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