Elementary reaction steps of the Fischer-Tropsch reaction

For long chain growth with formation of long hydrocarbons in the Fischer-Tropsch reaction a high surface concentration of reaction intermediates is needed. Hence the rate of initiation has to be fast compared to the chain growth rate in the Fischer-Tropsch catalytic cycle. As is now generally accepted for Co and Ru we will consider CO dissociation to be the initiation step. For the situation of high surface coverage of intermediates it is not obvious whether within microkinetic considerations the activation energy of CO with respect to its adsorbed state or with respect to the gas phase has to be compared with the activation energies of the other elementary reaction steps. A similar question arises for the chain growth reactions when reactions on steps are compared with those on terraces. We will present analytical expressions for the Flory-Schulz chain growth parameter derived within the microkinetics approach that resolve these issues. The relevant activation energies are to be defined with respect to the reactant adsorbed state. Next we will discuss the structure dependence of the activation energies for CO dissociation. Not only will we focus on the dependence of surface structure, but also compare this with the reactivity of very small metal clusters. Not withstanding the strong interaction of atoms as C or O with small clusters we will see that CO dissociation is inhibited. As we will demonstrate surface reconstruction effects are found to be dominated by carbide overlayer formation (1). This is important because surface reconstruction effects are responsible for faceting, with generation of edge sites. We will conclude with a short summary of the work we did on the mechanism of deactivating grapheme formation.