An analysis of Fischer-Tropsch synthesis by the bond-order-conservation-Morse-potential approach

Abstract

The BOC-MP (bond-order-conservation-Morse-potential) approach has been used to calculate the heats of chemisorption of adspecies and activation barriers for elementary reaction steps envisioned to occur during Fischer-Tropsch (F-T) synthesis over the periodic series Fe/W(110), Ni(111), Pt(111), and Cu(111). Dissociative adsorption of CO to form carbidic carbon is projected to occur spontaneously on Fe/W(110) and with a small activation barrier on Ni(111). The calculated barrier heights for this reaction on Pt(111) and Cu(111) are high enough to preclude appreciable dissociation of CO. Hydrogen-assisted dissociation of CO s is found to have an even smaller activation barrier on Fe/W and Ni, but not on Pt or Cu. On all the metal surfaces, the energetically preferred path for initiation of alkyl chain growth is via insertion of a CH 2 group into the carbon-metal bond of a CH 3 group. The activation barrier for CO insertion into the metal-carbon bond of a CH 3 group is greater than that for CH 2 insertion. As a consequence, the acetyl group formed by CO insertion serves mainly as a precursor to oxygenated products. On Fe/W, Ni, and Pt the activation barrier for termination of alkyl chain growth by β-elimination of hydrogen is found to be lower than that for α-addition of hydrogen, and as a consequence olefins are projected to be formed more readily than paraffins. By using as examples Fe(100) and Fe(100)-c(2 × 2)C,O, it is shown that carburization of an Pe surface reduces the heats of adsorption of C, O, and CO, resulting in nondissociative chemisorption of CO, similar to that on Pt(111). The BOC-MP model projections are consistent with the available experimental data and contain claims that can be tested experimentally in the future.