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 employed to predict catalytic turnover rates, product distributions, preferred mechanistic pathways, and rate- and selectivity-controlling elementary reaction steps for the Fischer-Tropsch (FT) reaction. We considered all relevant elementary reaction steps on Co(112̅1) step-edge and Co(0001) terrace sites as well as such important aspects as coverage-related lateral interactions, different chain-growth mechanisms, and the migration of adsorbed species between the two surfaces in the dual-site model. CHx–CHy coupling pathways relevant to the carbide mechanism have favorable barriers in comparison to the overall barriers for the CO insertion mechanism. A comparison of reaction barriers 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 predicted kinetic parameters correspond well with experimental kinetic data. The Co(112̅1) model surface is highly active and selective for the FT reaction. Adding terrace Co(0001) sites in a dual-site model approach 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, caused by faster hydrogenation of the hydrocarbon chains. 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, which is related to the much higher H/CO coverage ratio during CO2 hydrogenation in comparison to CO hydrogenation. These findings highlight the importance of a proper balance of CO and H surface species during the FT reaction and pinpoint step-edge sites as the locus of the FT reaction with low-reactive terrace sites near step-edge sites being the origin of unwanted CH4.