Insight into the preferred formation mechanism of long-chain hydrocarbons in Fischer–Tropsch synthesis on Hcp Co(10−11) surfaces from DFT and microkinetic modeling

Abstract

DFT calculations, together with microkinetic modeling, have been employed to probe into the preferred mechanism of hydrocarbon C–C chain growth on Co(10−11) surfaces during Fischer–Tropsch synthesis. The results show that both CH and CH₂ are favored CH_x (x = 1–3) monomers, and are much easier to be formed than CH₄ and CH₃OH. CH and CH₂ self-coupling via a carbide mechanism realizes the initial C–C chain formation, rather than via a CO/CHO insertion mechanism. Meanwhile, CH₃CH₂ is the favored C₂ monomer, and is predominantly formed via a carbide mechanism rather than via a CO/CHO insertion mechanism, leading to C₂H₅OH formation. Starting from CH₃CH₂ intermediates, CH₃CH₂ coupling with CH₂ to form CH₃CH₂CH₂ realizes further C–C chain growth from C₂ to C₃ species, instead of a CO/CHO insertion mechanism leading to C₃H₇OH formation. Thus, the proposed mechanism of C–C chain growth is that RCH₂CH₂ coupling with CH₂ to R′CH₂CH₂ (R′ = RCH₂) realizes C–C chain growth. Meanwhile, CHO insertion into RCH₂CH leads to RCH₂CHCHO, followed by its hydrogenation to an alcohol. However, microkinetic modeling shows that the effect of CH₄ formation on the production of C₂₊ hydrocarbons should be considered, whereas alcohols have a negligible effect on the selectivity of C₂₊ hydrocarbons. Our results confirm that Co(10−11) surfaces exhibit a better catalytic activity and selectivity toward C₂₊ hydrocarbon formation.