Carbon-Oxygen Bond Activation and Carbon-Carbon Bond Formation Paths in Catalytic Carbon Monoxide Hydrogenation

Fundamental mechanistic details regarding C-O bond activation and C-C bond formation remain unknown in catalytic CO hydrogenation to heavy hydrocarbons (Fischer-Tropsch synthesis, FTS). This study combines infrared spectroscopic and density functional theory methods (DFT) to first identify relevant surface CO coverages during FTS; reaction energy profiles are then calculated using DFT to determine the most facile path for C-O bond activation on Ru cluster surfaces. Kinetic responses of CO conversion rates and product selectivities to changes in H2 and CO pressures are measured in a packed-bed reactor at differential conversions on supported Ru catalysts to develop kinetic rate equations for FTS and corresponding sets of elementary reaction steps consistent with such equations. The effects of Ru cluster size on CO turnover rates, CO adsorption equilibrium constants, and CO conversion activation energies are also investigated to identify the Ru coordination environment in which C-O bonds are activated during FTS. C-C bond formation paths are then probed by measuring the effects of H2 and CO pressures on chain termination parameters, which provide a direct comparison between chain termination and chain propagation rates for all hydrocarbon products. DFT simulations of chain termination and chain propagation reactions as a function of carbon number are also performed to elucidate relevant C-C bond formation paths and to explain the apparent difficulty in forming the initial C-C bond during FTS.;Calculated CO adsorption energies decrease with increasing CO coverage on flat extended Ru(0001) surfaces and on 201-atom Ru cluster surfaces(Ru 201) due to repulsive intermolecular CO-CO interactions; such repulsive interactions are alleviated on convex Ru201 cluster surfaces, which can also accommodate CO coverages above a stoichiometric monolayer by forming geminal di-carbonyl species (also identified by infrared spectroscopy of CO adsorbed on Ru/SiO2 catalysts) at under-coordinated corner and edge Ru atoms. DFT-derived activation barriers for H-assisted CO activation paths on Ru atoms in high-coordination environments are much smaller than such barriers for direct CO bond cleavage, suggesting that CO species can be activated via H-assistance on low-index surfaces. Direct CO activation barriers on under-coordinated corner and step-edge sites are larger than those on high-coordination sites because of unfavorable interactions between di-carbonyl species and vicinal C-O activation transition states. DFT results also show that C-O bond activation during FTS is irreversible, and any direct CO dissociation path, as a result, is inconsistent with the reported effects of H2 and COpressures on FTS rates.;This study unites experiment and theory to investigate fundamental C-O bond activation and C-C bond formation reactions on relevant catalyst surfaces during FTS. Kinetic and selectivity measurements remain vital in the understanding of elementary steps involved in bond making and bond breaking events in FTS reactions; theoretical tools can be used to investigate elementary reactions under conditions inaccessible to experimental techniques. Theoretical investigations of FTS reactions must be performed on cluster surfaces at high CO* coverages that prevail during relevant conditions because CO* coverages affect thermodynamic and kinetic parameters included in apparent activation energies. High CO* coverages also weaken Ru-CO bonds, consistent with quasi-equilibrated CO adsorption-desorption processes during FTS. This study is an important example of how theoretical calculations performed on relevant surfaces at relevant coverages are an invaluable compliment to experimental studies of metal-catalyzed chemical reactions. (Abstract shortened by UMI.).