Adlayer Dynamics Drives CO Activation in Ru-Catalyzed Fischer Tropsch Synthesis

The first step of the Fischer-Tropsch synthesis (FTS) consists of the carbon monoxide activation at ca. 200 degrees C on catalyst metal surfaces covered with dense adlayers. Based on first-principles calculations, two main mechanisms for the C-O bond cleavage have been proposed for Ru catalysts: the direct and the hydrogen-assisted routes on step-edges and flat surfaces, respectively. However, commonly used static density functional theory (DFT) methods describe FTS adlayers using nonmobile adsorbed CO (CO*) species, while under reaction conditions the adsorbates diffuse and interact with each other. Here, we use ab initio molecular dynamics (AIMD) simulations on Ru flat and stepped model surfaces covered with CO* and H* to interrogate the effect of adlayer dynamics on the preferred reaction mechanisms. We show that hydrogen-assisted CO activation mechanisms via hydroxyl-carbonyl (COH*) intermediates formed on step-edges are the most favored according to AIMD simulations. Therefore, both step-edges and surface hydrogen play a key role in CO cleavage during Ru-catalyzed FTS at high CO* coverage. Direct comparison with static DFT results reveals that the dynamic adlayer significantly affects the relative stability of reaction intermediates and shows that the mobility of adsorbed molecules modulates the reaction paths, calling for a systematic analysis of reaction networks on complex systems at high coverages using AIMD simulations.