C-O Bond Activation and C-C Bond Formation Paths in Catalytic CO Hydrogenation
- Author(s): Loveless, Brett
- Advisor(s): Iglesia, Enrique
- et al.
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(Ru201) 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.
Kinetic responses of CO conversion turnover rates and oxygen rejection selectivities (in the form of H2O/CO2 formation rate ratios) are consistent with CO activation occurring predominantly via H-assisted paths on Ru catalysts, irrespective of Ru cluster size. CO adsorption equilibrium constants and CO conversion activation
energies, measured during FTS reactions, are the same for both small and large Ru clusters, suggesting that CO activation occurs in similar environments on both particle sizes. Turnover rates are smaller on small (~ 1 nm) Ru clusters than rates on large (~ 7
nm) Ru clusters when rates are normalized by the total number of exposed Ru atoms. Such differences in turnover rates become much smaller, however, when rates are normalized by the number of exposed Ru atoms in low-index (111) planes; CO activation
reactions, as a result, likely occur on Ru atoms in high-coordination environments on both small and large Ru clusters.
Increases in H2 pressures increases CH4 selectivities and decreases C5+
selectivities because chain termination is proportional to the concentration of surface hydrogen atoms (H*); chain growth reactions are favored at high CO pressures where termination is suppressed. Chain termination parameters for C1 species (to CH4) are much larger than such parameters for C2+ species; these parameters depend exponentially on the difference in chain termination and chain propagation barriers for each carbon number. DFT-derived activation barriers for termination reactions (via hydrogenation of
surface alkyl species) are small and independent of chain length because such reactions are exothermic and proceed via early transition states; chain propagation reactions (via alkyl migration to vicinal CO* species), however, proceed through late transition states
where surface-alkyl bonds are broken completely. Surface methyl (CH3*) species are bound to surfaces much more strongly than ethyl (C2H5*) and propyl (C3H7*) species; C1
propagation barriers, as a result, are much larger than such barriers for C2+ hydrocarbons, a result that is consistent with measured chain termination parameters on supported Ru catalysts. A steady-state treatment of reactive intermediates shows that CO conversion rate equations must include terms for CO consumption via CO activation as well as for
consumption via chain growth with CO* monomers. 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.