Fischer-Tropsch synthesis spectroscopy

Mossbauer spectroscopy is a specialist characterization tool in catalysis. Nevertheless, it has yielded essential information on a number of important catalysts, such as the iron catalyst for ammonia and Fischer-Tropsch synthesis, as well as the CoMoS hydrotreating catalyst. Mossbauer spectroscopy provides the oxidation state, the internal magnetic field, and the lattice symmetry of a limited number of elements such as iron, cobalt, tin, iridium, ruthenium, antimony, platinum and gold, and can be applied in situ. 

Temperature-programmed reduction combined with x-ray absorption fine-structure (XAFS) spectroscopy provided clear evidence that the doping of Fischer-Tropsch synthesis catalysts with Cu and alkali (e.g., K) promotes the carburization rate relative to the undoped catalyst. Since XAFS provides information about the local atomic environment, it can be a powerful tool to aid in catalyst characterization. While XAFS should probably not be used exclusively to characterize the types of iron carbide present in catalysts, it may be, as this example shows, a useful complement to verify results from Mossbauer spectroscopy and other temperature-programmed methods. The EXAFS results suggest that either the Hagg or s-carbides were formed during the reduction process over the cementite form. There appears to be a correlation between the a-value of the product distribution and the carburization rate. 

The aim of this work was to apply combined temperature-programmed reduction (TPR)/x-ray absorption fine-structure (XAFS) spectroscopy to provide clear evidence regarding the manner in which common promoters (e.g., Cu and alkali, like K) operate during the activation of iron-based Fischer-Tropsch synthesis catalysts. In addition, it was of interest to compare results obtained by EXAFS with earlier ones obtained by Mossbauer spectroscopy to shed light on the possible types of iron carbides formed. To that end, model spectra were generated based on the existing crystallography literature for four carbide compounds of... 

An investigation of bulk (a-WC, a-W2C) and supported tungsten carbide was carried out using the formation of alcohols in Fischer-Tropsch synthesis as a chemical probe. The active phases were characterized by X-ray photoelectron spectroscopy. 

Niemantsverdriet JW, Van der Kraan AM, Van Dijk WL, Van der Baan HS. Behavior of metallic iron catalysts during Fischer-Tropsch synthesis studied with Mossbauer-spectroscopy, X-ray-diffraction, carbon content determination, and reaction kinetic measurements. J Phys Chem. 1980 84(25) 3363-70. 

J.W. Niemantsverdriet, A.M. van der Kraan, W.L. van Dijk, and H.S. van der Baan, Behavior of Metallic Iron Catalysts During Fischer-Tropsch Synthesis Studied with Mossbauer Spectroscopy, X-ray Diffraction, Carbon Content Determination, and Reaction Kinetic Measurements, J. Phys. Chem. 84 (1980) 3363. 

Figure 4. Bulk composition of catalyst 100Fe/3.6Si/0.71K from Mossbauer spectroscopy as a function of time on stream. Negative TOS denote catalyst pretreatment positive TOS denote Fischer-Tropsch synthesis. S.P. denotes super paramagnetic.

Attempts to correlate the catalytic activity of unpromoted and potassium-promoted iron Fischer-Tropsch catalysts to their bulk phase compositions as determined using Mossbauer spectroscopy are reported. The CO-conversion of both catalysts declined with time-on-stream. Samples of the umnpromoted catalyst were primarily Fe C and this phase gradually decreased to become essentially pure Fe O after 450 hours time-on-stream. In contrast, the potassium-promoted catalyst was primarily Fe C after pretreatment and remained this phase during 400 hours of synthesis. Thus, both Fe C and Fe C phases are about equally active for Fischer-Tropsch synthesis but exhibit much different stability during the course of the synthesis reaction. 

Modem investigations of the Fischer-Tropsch synthesis by X-ray and UV photoelectron spectroscopy have shown the presence of surface carbides and oxygen atoms in the adsorption of CO on various metals. 

Cagnoli MV, Alvarez AM, Gallegos NG, et al. Mossbauer and XPS spectroscopies studies of SMSI effect on Fe/Nb205 catalysts fm the Fischer-Tropsch synthesis. Appl Catal A Gen. 2007 326 113-9. 

H.-B. Zhang and G. L. Schrader, Characterization of a fused iron catalyst for Fischer-Tropsch synthesis by in situ laser Raman spectroscopy, Journal of Catalysis, vol. 95, no. 1, pp. 325-332, 1985. 

When the Fe-MnO catalyst is analyzed after use in the Fischer-Tropsch reaction (the synthesis of hydrocarbons from CO and H2), the XRD pattern in Fig. 6.2 reveals that all metallic iron has disappeared. Instead, a number of weak reflections are visible, which are consistent with the presence of iron carbides, as confirmed by Mossbauer spectroscopy [7]. The conversion of iron to carbides under Fischer-Tropsch conditions has been studied by many investigators and has been discussed in more detail in Chapter 5 on Mossbauer spectroscopy. 

Investigations of functioning catalysts with Mossbauer spectroscopy have been performed for a wide range of samples and applications. The reactions include hydrodesulfuration 15), the Fischer-Tropsch reaction (20,180), selective oxidation or oxidative dehydrogenation (181-186), and acetonitrile synthesis (187). 

Iron, Cobalt and Nickel. - Despite the great importance of supported iron in the Fischer-Tropsch and NH3 synthesis, very few papers using conventional spectroscopies as surface tools have appeared recently. Papers here, however, appeared by Bradshaw and Pritchard concerning the adsorption of CO on films deposited in vacuo (2040-1960 cm ) and by Blyholder et concerning CO adsorption on Fe/Si02 (2020-1980-1925 cm ). 

Surface spectroscopy, as discussed in the previous section, has recently provided evidence that concurrent with the hydrocarbon synthesis a reactive carbidic overlayer develops, and that this reactive overlayer may contain the intermediates operative in the FT synthesis. However, the claims of surface spectroscopy regarding the relevance of carbidic intermediates have had a precedent in one of the very first publications of Fischer and Tropsch... 

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