Interactions among adsorbates on solid surfaces play a critical role in heterogeneous catalysis, electro-catalysis, self-assembly, nucleation, growth, and almost every other phenomenon governed by chemical or physical surface processes. For example, alkali promoters (Li, Na, K, Rb, Cs) enhance activity and/or selectivity of various catalytic processes, such as Fisher-Tropsch synthesis, water-gas shift reactions, ammonia synthesis, olefin epoxidation, and automotive three way converters. Although the impact of alkalis in heterogeneous catalysis has been recognized for decades, not much is known about the underlying physical mechanisms that govern the promotion.
We have utilized density-functional theory (DFT) calculations to develop a physically transparent model for the analysis of the interactions between adsorbates on metal surfaces. We have applied this model to investigate underlying mechanisms that govern the interaction between Cs promoters and oxygen adsorbates on the Ag(111) surface. We have also employed an atomistic thermodynamics approach and Monte Carlo simulations to extend the results of DFT calculations to relevant catalytic conditions (T, p). We found that the underlying mechanisms of chemical promotion by alkali promoters change significantly as a function of external conditions.