Fischer-Tropsch synthesis, turnover frequency

According to the International Union of Pure and Applied Chemistry (IUPAC O)) the turnover frequency of a catalytic reac tion is defined as the number of molecules reacting per active site in unit time. The term active sites is applied to those sites for adsorption which are effective sites for a particular heterogeneous catalytic reaction. Because it is often impossible to measure the amount of active sites, some indirect method is needed to express the rate data in terms of turnover frequencies In some cases a realistic measure of the number of active sites may be the number of molecules of some compound that can be adsorbed on the catalyst. This measure is frequently used in the literature of the Fischer-Tropsch synthesis, where the amount of adsorption sites is determined by carbon monoxide adsorption on the reduced catalyst. However, it is questionable whether the number of adsorption sites on the reduced catalyst is really an indication of the number of sites on the catalyst active during the synthesis, because the metallic phase of the Fischer-Tropsch catalysts is often carbided or oxidized during the process. 

The turnover frequencies reported for the Fischer-Tropsch synthesis are small. In the publication of Vannice (2) the turnover frequencies for CO-conversion to hydrocarbons range from... 

A controversial issue related to cobalt catalysts in Fisher-Tropsch synthesis is the structure-sensitive character of this reaction. Iglesia and co-workers [126,127] reported a large increase in activity when the cobalt particle size was decreased from 200 nm to 9 nm, whereas the specific activity [turnover frequency (TOF)] was not influenced by the cobalt particle size. However, other authors have reported that the TOF suddenly decreased for catalysts with cobalt particle sizes smaller than 10 nm [122,128]. Bezemer et al. [125] were the first to investigate the influence of cobalt particle size in the range 2.6 to 27 nm on performance in Fischer-Tropsch synthesis on well-defined catalysts supported on carbon nanofibers. It was found that the TOF for CO hydrogenation was independent of cobalt particle size for catalysts with particles larger then 6 nm (at atmospheric pressure) or 8 nm (at 35 bar). But both the TOF and the C5+ selectivity decreased for catalysts with smaller particles. It was proposed that the cobalt particle size effects could be attributed to a strucmre-sensitivity characteristic of the reaction, together with a CO-indnced reconstmction of the cobalt surface. 

Note that this is the reaction rate or activity. However, this definition takes into account the reaction medium, be it volume, surface, or interface, and not exactly the active sites. Not all mass or surface is active, but part of its outer surface has active sites, which are truly the sites where the chemical reaction occurs. Therefore, rj in fact represents the apparent rate. An important example of reaction that allows to differentiate the apparent from the true rate is the hydrogenation of carbon monoxide to form methane, which is conducted with different catalysts. With iron and cobalt catalysts, the rate per unit of mass of catalyst, used as reference, has shown controversial values. The activity of the catalysts in the Fischer-Tropsch synthesis to form hydrocarbons would decrease according to the order Fe > Co > Ni. However, when the rate per active site was defined, the order of activity was different, i.e., Co > Fe > Ni. This controversy was resolved by Boudart, who defined the intrinsic activity, i.e., the rate per active site. To make it more clear, the turnover frequency (TOF) was defined. Thus, the intrinsic activity is determined, knowing the active sites, i.e. ... 

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