@article{032d0ffa85a846afaef5bada1c31100e,
title = "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",
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.",
keywords = "CO vs CO, Fischer-Tropsch synthesis, cobalt, density functional theory, microkinetic modeling",
author = "Bart Zijlstra and Broos, \{Robin J.P.\} and W. Chen and G.J. Bezemer and Filot, \{Ivo A.W.\} and Hensen, \{Emiel J.M.\}",
year = "2020",
month = aug,
day = "21",
doi = "10.1021/acscatal.0c02420",
language = "English",
volume = "10",
pages = "9376--9400",
journal = "ACS Catalysis",
issn = "2155-5435",
publisher = "American Chemical Society",
number = "16",
}