(284d) Connecting Molecular Processes to Macrokinetic Observables Using First-Principles Simulations
Quantitatively connecting observed reaction rates, rate orders, and activation energies with microscopic mechanisms remains one of the grand challenges of heterogeneous catalysis science. Such connections are difficult to make even for simple catalytic reactions at surfaces, where interactions between adsorbates can create a myriad of local reaction environments that collectively determine the observed reactivity. These adsorbate interactions influence coverage, adsorbate arrangements, and even the rates of elementary surface reactions. A proper description of these coverage effects on reaction rates requires a careful, quantitative treatment of adsorbate-adsorbate interactions and their effects on microscopic reaction rates. Here we describe a "basis site" approach to incorporating this coverage dependence that permits ready and accurate calculations of macroscopic kinetic parameters. We use as example NO oxidation by O2 on the Pt(111) surface. A minimal mechanism for this reaction includes the following steps:
NO + O* = NO2 + * (1) equilibrium
O2 + 2* → 2 O* (2) rate controlling
The rate of the latter reaction is a strong function of the available surface sites. We develop a DFT-parameterized cluster expansion to describe coverage-dependent O adsorption. Grand canonical Monte Carlo simulations on Reaction (1) provide equilibrated surface configurations as a function of reaction conditions. The rate of Reaction (2) is described using a DFT-developed Bronsted-Evans-Polyani relationship between activation barrier and final state binding energy. Combining these, we obtain condition-dependent rates suitable for differentiation and comparison with experiment. The approach highlights the subtle links between microscopic mechanism and observed kinetics.