Fischer-Tropsch synthesis, transition metal surfaces, CO and CH~ reactivity, electronic features in F-T synthesis, bond strength, activation energy, chemisorption, CO dissociation, extended Hiickel method, ASED method, quantum chemistry.The electronic features determining the reactivity of CO and CH x on transition metal surfaces are reviewed. Focus is on the relevant features that control the Fischer-Tropsch synthesis. The CO dissociation reaction path is controlled by the interaction with the CO bond strength weakening 2~r* orbitals. CH 3 fragment adsorption is controlled by o type molecule fragment orbitals. This directs the CH 3 fragment to the atop adsorption site on those late transition metals that have strongly interacting d-valence electrons. Adsorbed C and O have a stronger bond strength than CH 3 because they have also unoccupied atomic p orbitals available to bonding. Because the bond strength of adsorbed C and O increases more rapidly with depletion of d-valence electron occupation than that of CO, the activation energy for CO dissociation decreases for the corresponding transition metals towards the left of the periodic system. The rate of methanation versus chain growth is controlled by the strength of the M-CH 3 bond versus the activation energy of carbon-carbon bond formation. The first appears to be more sensitive to variations in metal carbon bond strength than the latter.

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Results of extended Huckel calculations show that coordination of CO to Group VIII transition-metal surfaces depends on a subtle balance of the interaction with the CO 5cr orbital, that tends to direct the CO molecule to the atop position and the CO 2 7 ~" orbital, that directs the molecule to higher coordination sites. In the atop position, the changes in bonding to different surfaces of the same metal can be mainly attributed to differences in the interaction with the CO SCT molecular orbital. The favoured dissociation path is such that carbon and oxygen atoms end in high coordination sites, sharing as few surface metal atoms as possible. The CO bond is activated by the metal atoms that are crossed upon dissociation.