Fischer-Tropsch activation energy

Bian, G., Mochizuki, T., Fujishita, N., Nomoto, H., and Yamada, M. 2003. Activation and catalytic behavior of several Co/Si02 catalysts for Fischer-Tropsch synthesis. Energy Fuels 17 799-803. 

O Brien, R.J., Xu, L., Spicer, R.L., and Davis, B.H. 1996. Activation study of precipitated iron Fischer-Tropsch catalysts. Energy Fuels 10 921-26. 

The catalytic partial oxidation of methane for the production of synthesis gas is an interesting alternative to steam reforming which is currently practiced in industry [1]. Significant research efforts have been exerted worldwide in recent years to develop a viable process based on the partial oxidation route [2-9]. This process would offer many advantages over steam reforming, namely (a) the formation of a suitable H2/CO ratio for use in the Fischer-Tropsch synthesis network, (b) the requirement of less energy input due to its exothermic nature, (c) high activity and selectivity for synthesis gas formation. 

Activation Energy, and Collision Factor, Arlt of Carbon Nanomaterial-Supported Co Catalysts and Commercially Used Fischer-Tropsch Catalysts... 

In general, TPR measurements are interpreted on a qualitative basis as in the example discussed above. Attempts to calculate activation energies of reduction by means of Expression (2-7) can only be undertaken if the TPR pattern represents a single, well-defined process. This requires, for example, that all catalyst particles are equivalent. In a supported catalyst, all particles should have the same morphology and all atoms of the supported phase should be affected by the support in the same way, otherwise the TPR pattern would represent a combination of different reduction reactions. Such strict conditions are seldom obeyed in supported catalysts but are more easily met in unsupported particles. As an example we discuss the TPR work by Wimmers et al. [8] on the reduction of unsupported Fe203 particles (diameter approximately 300 nm). Such research is of interest with regard to the synthesis of ammonia and the Fischer-Tropsch process, both of which are carried out over unsupported iron catalysts. 

Fischer-Tropsch synthesis, 28 80, 97, 103, 30 166-168, 34 18, 37 147, 39 221-296 activation energy and kinetics, 39 276 added olefin reactions, 39 251-253 bed residence time effects on chain growth probability and product functionality, 39 246-250... 

As described below, this approach is being applied by several groups to analyze elementary reaction steps that are part of the Fischer-Tropsch reaction scheme. A detailed imderstanding of the relationship between activation energies and site structure is becoming possible hence, the factors that control activity as well as selectivity can be identified. 

We have summarized these developments in two recent papers the first addresses the topic of structure sensitivity in combination with BEP relationships (14) and the second addresses the Sabatier principle (27). Sabatier-type volcano relationships can be deduced for activity as a function of adsorption energy, and they can also be used to predict trends regarding deactivation of Fischer-Tropsch catalysts by C—C recombination reactions (28). We refer to these texts as background information to the material presented here. 

For illustration purposes, we briefly discuss oxygen removal on cobalt. The Fischer-Tropsch reaction on this catalyst is known to be only weakly suppressed by the product water (41). The available computational results indicate that the activation energy for the reaction of adsorbed hydrogen, Hads/ with Oads to adsorbed OH species, OHads/ on cobalt is about 166 kj/mol for the flat Co(OOOl) surface and 70 kj/mol for sfepped cobalf surfaces (42). For comparison, fhe activation energy for fhis reaction on rhodium is 90 kJ/mol (43) Subsequent water formation occurs by recombination of OHads with Hads this reaction has a barrier of befween 5 and 10 kj/mol. 

Equations (12a) through (12c) are of great significance because they allow formulation of quantitative relationships between the activation energies of the elementary steps of the Fischer-Tropsch reaction that have to be satisfied for a high chain-growth selectivity. 

Recent simulations by Marin and coworkers (56,57) seem to confirm Equations (12b) and (12c). A single-event microkinetics (SEMK ) model was used to analyze data characterizing Fischer-Tropsch catalysis on iron. The authors reported an activation energy of only 57 kj/mol for CO dissociation, whereas activation energies for the chain-growth reaction and termination reaction leading to alkane or alkene formation were found to be 45,118, and 97 kJ/mol, respectively. 

To suppress graphene formation, a Fischer-Tropsch catalyst needs to have sites that activate hydrogen. Graphene will not form on step edges because of their high interaction energies with adsorbed C. Graphene formation and methanation occur preferentially on surface terraces. 

Fig. 12. Fischer-Tropsch reaction at 1 atm is first-order in CO, with an activation energy of 27 kcal/ mole (Lancet, 1972). Rate in a flow system is 10 times faster than in the static system used here. Dashed line shows extrapolation to solar nebula, assuming that the rate is proportional to (PcoIIPhj) . Reaction proceeds at an undetectable rate when the Bruderheim L6 chondrite is used as a catalyst. Apparently the high-temperature minerals in this meteorite (olivine, orthopyroxene, troilite, and nickel-iron) do not catalyze the hydrogenation of CO. Thus CO can survive in the solar nebula down to 400 K, when catalytically active minerals first from (Fig. 1 and 10)...

Fig. 23. The effect of catalyst structural parameter ix) on Fischer-Tropsch synthesis activation energy and kinetics (473 K, 2000 kPa, H2/CO = 2.1 55-65% CO conversion, > 24 h onstream).

The Fischer-Tropsch synthesis temperature is around 250°C. Elements to the left of the lines in Fig.(3.26) will dissociate CO at room temperature. The increase in temperature to the right implies an increase in the activation energy of CO dissociation. In consequence, we have a smaller dissociation activation energy for CO to the left of the periodic system than to the right in agreement with the the Polanyi relation (3.41). 

A considerable interest has been expressed in using the SBCR to carry out FTS particularly for the conversion of stranded natural gas into liquids. Currently, the Center for Applied Energy Research (CAER) is utilizing a Prototype Integrated Process Unit (PIPU) system for scale-up research of the FTS. The purpose of this study was to compare the performance and activity decline of a precipitated Fe/K Fischer Tropsch Synthesis (FTS) catalyst in a revamped slurry bubble colurtm reactor (SBCR) to that of previous CSTR and SBCR rans using the same catalyst and operating conditions. The activity decline measured in the revamped SBCR system was shown to be similar to that of the CSTR experiments. The apparent activity decline in a previous SBCR run was due a transient startup effect from the slurry filtration system. 

Kinetic expressions similar to that of Equation 3 and similar activation energies have been reported for methanation over a cobalt-alumina catalyst (4) and for Fischer-Tropsch reaction over a cobalt-thoria catalyst (5). This similarity, despite appreciably different product distributions in the three cases, argues for a common rate-controlling step in the mechanisms. 

Florea I, Liu Y, Ersen O, Meny C, Pham-Huu C. (2013) Microstructural analysis and energy-filtered tern imaging to investigate the structure-activity relationship in Fischer-Tropsch catalysts. ChemCatChem, 5 2610-2620. 

The universal, macrokinetic equation of the Fischer-Tropsch reaction is not known. For each type of catalyst and reaction, separate equations are written which are only valid for a specific range of parameters. The activation energy is high (80-105 kJ mol ) and it governs the reaction rate the reaction rate is not determined by the diffusion of reagents to the surface of the catalyst or by the reverse process of diffusion of products. 

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