Effect of activation method on Fe FTS catalysts: investigation at the site level using SSITKA
Introduction
Extensive phase changes of Fe Fischer–Tropsch (FT) catalysts during activations and especially during FTS make Fe the most complicated system among FT catalysts (including Ni, Co, and Ru). The catalytically active phase of the other metals is well known to be the metal state. Several phases of iron have been found to coexist during the FT reaction [1], [2], [3], [4], including metallic Fe (α-Fe), Fe oxides, and Fe carbides 5. The proportion of these Fe phases can be varied, depending upon reaction conditions and activation procedures, which determine the initial state of the catalyst before reaction. The catalytically active phase(s) in a working Fe catalyst for FTS has been debated extensively by researchers. The active Fe phases have been concluded by different researchers to be mainly Fe oxides (especially Fe3O4) [6], [7], [8], [9], [10], Fe carbides [11], [12], [13], [14], or Fe metal 4. However, other possible active Fe phases have also been suggested, such as a surface phase on Fe3O4 15.
Due to the above complexity, investigation into the active forms of Fe in a working catalyst requires an in situ technique with sufficient spatial resolution. Unfortunately, most of the techniques used to study iron catalysts in the past, including Mössbauer spectroscopy, XRD, and XPS, are not capable of providing such resolution 5. The conclusion has been reached by some [16], [17], [18] that the exact relationship between Fe phase composition and reactivity of the catalyst may not be able to be made.
The focus of the research reported here was on characterizing the kinetic nature of the active sites of an Fe catalyst pretreated in different ways. The effects of different activations (H2, CO, or syngas) were investigated. It was also desired to determine how the active sites generated changed with reaction time on stream (TOS). Steady state isotopic-transient kinetic analysis (SSITKA), first developed by Happel 19, Bennett 20, and Biloen 21, is a powerful technique capable of assessing the surface kinetics of catalytic reactions in situ. Previously, this isotopic tracing technique had been successfully used to study the product chain growth during CO hydrogenation on Fe [22], [23] and the carbon pathways on Fe/Al2O3 24. However, neither of these studies investigated the effect of pretreatment on Fe activity. The results of this study permit us to better understand activity development at the site level of an Fe catalyst after activation and during FTS. By using this isotopic tracing technique, the intrinsic site activities and concentrations of surface intermediates developing with TOS during Fe FTS are revealed for the first time.
Section snippets
Catalyst
The Fe catalyst used for this study was prepared by precipitation and then spray drying. The relative compositions by weight percentage were 100Fe/5Cu/4.2K/11SiO2. The details of catalyst preparation have been given elsewhere [25], [26], [27]. Briefly, a mixture containing the desired ratios of Fe, Cu, and Si was precipitated at room temperature with ammonium hydroxide solution. The resulting precipitate was filtered, washed, and then mixed with the desired ratio of KHCO3 solution. The
Catalyst properties
Table 1 shows the N2 physisorption properties and the major phases of Fe after different pretreatments. XRD patterns of all the catalyst samples studied are shown in Fig. 1, with the most intense diffraction peaks for each Fe phase evident indicated. As expected, the fresh calcined catalyst as prepared was in the form of hematite, Fe2O3. The major Fe phases of [H] were found to be Fe metal and magnetite, Fe3O4, while those of [CO] and [S] were mostly Fe carbides with only a small trace of Fe3O4
Conclusion
This study explored for the first time the effect of activation and TOS on site activity and concentration of surface reaction intermediates on an Fe FT catalyst, as determined by SSITKA. It was found that activity was primarily determined by the number of active intermediates, which were quite different for differently pretreated samples during the initial stage of the reaction. However, at steady state, the concentration of methane surface intermediates on [CO] and [S] were quite similar
Acknowledgements
Financial support by the Royal Thai Government of K.S. is gratefully acknowledged. Financial support was also provided in part by the US Department of Energy (Grants DE-FG26-99FT40619 and DE-FG26-01NT41360).
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