Determining the chain-growth pathway is very important to understand the Fischer Tropsch Synthesis (FTS) mechanism. FTS consists of the following steps: CO activation, hydrogenation and O removal, chain growth and termination. Three different mechanisms have been proposed depending on how the C-C coupling is achieved. The carbene mechanism involves polymerization of CH2 intermediates to achieve C-C coupling, the hydroxy carbene mechanism proceeds via dimerization reaction between the adsorbed hydroxyl methylene intermediates and the CO insertion mechanism occurs through insertion of CO into adsorbed alkyl intermediates. However, the carbene mechanism is being supported by many experimental1-3 and theoretical4-6 works. There are two types of CO dissociation namely, unassisted CO dissociation and H-assisted CO dissociation. Unassisted CO dissociation involves the dissociation of CO into C and O while H-assisted dissociation does not involve the direct dissociation of CO7.
Unassisted CO dissociation
H-assisted CO dissociation
CO* --> C* + O*
C* + H* --> CH*
O* + H* --> OH*
OH* + H* --> H2O*
CO* + O* --> CO2*
CO* + H* --> HCO*
HCO* + H* --> HCOH*
HCOH* --> CH* + OH*
OH* + H* --> H2O*
Both pathways are believed to occur on Fe catalysts, but on Co the H-assisted CO dissociation has been shown to be the preferred route both experimentally and theoretically7.
Many promoters have been suggested for enhancing the FTS activity of Cobalt. In this paper we investigate how the promoters influence the catalytic pathways. The preferred CO activation pathway in the presence of Pt promoter is studied. We will discuss the energetics of assisted and unassisted CO dissociation pathways. A surface alloy model, where the promoter metal is dispersed on the top surface of the catalyst, is studied. In this work, VASP (Vienna Ab Initio Simulation package) code8-10 with Perdew–Burke–Ernzerhof (PBE) form of the generalized gradient approximation (GGA)11 functional is utilized for the exchange and correlation functional. The electron-ion interactions are modeled by the projector-augmented wave (PAW)12 method. The activation barrier and the transition states are calculated using the Climbing Image Nudged Elastic Band (CI-NEB) method 13-15.
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