The Vital Role of Step-Edge Sites for Both CO Activation and Chain Growth on Cobalt Fischer–Tropsch Catalysts Revealed through First-Principles-Based Microkinetic Modeling Including Lateral Interactions

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Abstract

Microkinetic modeling is a bottom-up approach that pinpoints activity and selectivity controlling elementary reaction steps. We have applied this method to the Fischer-Tropsch (FT) reaction by computing all relevant elementary reaction steps at Co(11"2" ̅1) step-edge and Co(0001) terrace sites. Our model includes important aspects such as the impact of coverage-related lateral interactions, different chain-growth mechanisms, and the migration of adsorbed species between the two surfaces in the dual-site model. We found that CHx–CHy coupling pathways relevant to the carbide mechanism have favorable barriers, while the overall barriers via CO insertion are much higher. A comparison with the CO dissociation barrier indicates why cobalt is such a good FT catalyst: CO bond scission and chain growth compete, while termination to olefins has a slightly higher barrier. The simulations predict kinetic parameters that correspond well with experimental kinetic data. They show that the Co(11"2" ̅1) model surface is highly active and selective for the FT reaction. Adding terrace Co(0001) sites in a dual-site model leads to a substantially higher CH4 selectivity at the expense of the C2+-hydrocarbons selectivity. The chain-growth probability decreases with increasing temperature and H2/CO ratio, which is caused by faster hydrogenation of the hydrocarbon chain. The elementary reaction steps for O removal and CO dissociation significantly control the overall CO consumption rate. Chain growth occurs almost exclusively at step-edge sites, while additional CH4 stems from CH and CH3 migration from step-edge to terrace sites. Replacing CO by CO2 as the reactant shifts the product distribution nearly completely to CH4. We show that the much higher H/CO coverage ratio during CO2 hydrogenation causes this high CH4 selectivity. These findings highlight the importance of a proper balance of CO and H surface species during the FT reaction and pinpoint low-reactive terrace sites near step-edge sites as the origin of unwanted CH4.

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