Structure sensitivity of the Fischer-Tropsch reaction; molecular kinetics simulations

Strong interest in the Fischer–Tropsch reaction that converts synthesis gas into hydrocarbons is reappearing because it is basic to one of the major routes that convert natural gas into liquid energy carriers. For catalytic science it provides amongst others an opportunity to revisit still open mechanistic issues of the Fischer–Tropsch conversion reaction. New approaches as computational advances and development of model systems are tools that may provide new insights. In this paper we will review our current understanding of the kinetics and its relation to catalyst structural parameters that determine the selectivity of the reaction. In the introductory section we formulate the key questions that we will address. Especially we will discuss the reason for particle size dependence of metallic catalysts as Co and Ru when particles are in the nanosize range and also the apparent paradox that step–edge sites are necessary for the chain growth reaction, where the CO molecule has to dissociate but that such sites should not be poisoned by the presence of the growing hydrocarbon chains or deactivating carbonaceous residue. One of the main selectivity issues of this reaction is the desire to produce long chain hydrocarbon molecules, without co-production of light gas molecules as methane. We will begin the presentation with the elementary kinetic expressions that enable calculation of the selectivity from a microkinetics reaction scheme. This will highlight that the rate of chain growth termination has to be one of the slow reaction steps and also that CO dissociation has to be fast. Since the past decade has seen major advances in the understanding of the structure sensitivity of transition metal catalysed surface reactions, the kinetic analysis helps to understand how structure sensitivity affects Fischer–Tropsch selectivity. Quantum chemical computational studies now can be used to analyse reaction paths and estimate reaction intermediate adsorption energies. Also activation free energies can be deduced for elementary surface reactions. We will illustrate this by discussing a so-called dual site model of the reactive catalyst center. On this reaction center we will discuss in detail CO dissociation and initiation of the chain growth reaction. It appears that synchronized subsequent reaction events involving reaction intermediate diffusion to different positions at the reaction center leads to accommodation of hydrocarbon chain growth while CO dissociation is not suppressed. Ultimately the mechanistic model deduced from the quantum-chemical studies will have to be used in kinetic equations to predict overall catalytic conversion rates. We will demonstrate how this can be done in a Kinetic Monte Carlo scheme, in which no assumption on the rate limiting step has to be made. We will present the results of some initial simulations where we allow for growth of short hydrocarbon chains. These results can be used in an insightful way using the kinetic model equations presented earlier in the paper. The paper is concluded by discussing the implications of these model results for our general mechanistic understanding of the Fischer–Tropsch reaction.