AN ANALYSIS OF FISCHER-TROPSCH SYNTHESIS BY THE BOND-ORDER-CONSERVATION-MORSE-POTENTIAL APPROACH

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 CH2 group into the carbon-metal bond of a CH3 group. The activation barrier for CO insertion into the metal-carbon bond of a CH3 group is greater than that for CH2 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 beta-elimination of hydrogen is found to be lower than that for alpha-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 x 2)C,O, it is shown that carburization of an Fe 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.