Abstract
The increasing availability of quantum-chemical data on surface reaction intermediates invites one to revisit unresolved mechanistic issues in heterogeneous catalysis. One such issue of particular current interest is the molecular basis of the Fischer-Tropsch reaction. Here we review current molecular understanding of this reaction that converts synthesis gas into longer hydrocarbons where we especially elucidate recent progress due to the contributions of computational catalysis. This perspective highlights the theoretical approach to heterogeneous catalysis that aims for kinetic prediction from quantum-chemical first principle data. Discussion of the Fischer-Tropsch reaction from this point of view is interesting because of the several mechanistic options available for this reaction. There are many proposals on the nature of the monomeric single C atom containing intermediate that is inserted into the growing hydrocarbon chain as well as on the nature of the growing hydrocarbon chain itself. Two dominant conflicting mechanistic proposals of the Fischer-Tropsch reaction that will be especially compared are the carbide mechanism and the CO insertion mechanism, which involve cleavage of the C-O bond of CO before incorporation of a CHx species into the growing hydrocarbon chain (the carbide mechanism) or after incorporation into the growing hydrocarbon chain (the CO insertion mechanism). The choice of a particular mechanism has important kinetic consequences. Since it is based on molecular information it also affects the structure sensitivity of this particular reaction and hence influences the choice of catalyst composition. We will show how quantum-chemical information on the relative stability of relevant reaction intermediates and estimates of the rate constants of corresponding elementary surface reactions provides a firm foundation to the kinetic analysis of such reactions and allows one to discriminate between the different mechanistic options. The paper will be concluded with a short perspective section dealing with the needs for future research. Many of the current key questions on the physical chemistry as well as computational study of heterogeneous catalysis relate to particular topics for further research on the fundamental aspects of Fischer-Tropsch catalysis.
Highlights
Synthesis gas derived from coal, with a low H2 to CO ratio, for instance, is preferentially converted by Fe based catalysts, whereas Co is the preferred catalytic material for Fischer– Tropsch processes involving synthesis gas with a higher H2/CO ratio derived from natural gas
By decomposing 13CO on a Ru catalyst and exposing this surface to a mixture of synthesis gas with 12CO, Biloen et al found that more than one 13C isotope label was incorporated into the growing chain, which proves that ‘‘C1’’ generated by decomposition of CO is the monomer that is inserted into the growing hydrocarbon chain
In the Fischer–Tropsch reaction, CO activation generates the CHx species that is inserted into the growing hydrocarbon chain
Summary
This perspective will describe advances in our current understanding, based on molecular catalysis research of the past decade,[11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36] of the mechanism of one particular heterogeneous catalytic reaction of significant current interest, that is, the Fischer–Tropsch reaction. It provides information on the relative stability of reaction intermediates as well as elementary rate constant parameters as activation energy barriers and transition state entropies The availability of such catalyst surface structure and composition dependent quantitative data makes microkinetics simulations useful for catalyst performance studies. In this review we will limit ourselves to mechanistic and kinetics considerations on static surfaces, except that the quantum-chemical calculations include local changes in metal–metal bond distances when reaction intermediates change With these limitations, microkinetics simulations based on first principle input of elementary reaction rates and relative stability or reaction intermediates can give predictions of catalyst performance as a function of surface structure and composition. In the final perspective section we will evaluate the current status of fundamental insights into the Fischer–Tropsch reaction and indicate open questions for future research
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