Elsevier

Journal of Catalysis

Volume 213, Issue 2, 25 January 2003, Pages 204-210
Journal of Catalysis

Effect of activation method on Fe FTS catalysts: investigation at the site level using SSITKA

https://doi.org/10.1016/S0021-9517(02)00010-6Get rights and content

Abstract

Proper activation of Fe catalysts is an important step in determining their activity for the Fischer–Tropsch synthesis (FTS). The results of this study reveal for the first time the effect of activation and time on stream (TOS) on intrinsic site activity and concentration of surface intermediates during CO hydrogenation (methanation) on an Fe FTS catalyst. The catalyst was activated under identical conditions but with different pretreatment gases: CO ([CO]), H2 ([H]), or syngas ([S]). Lifetime (τM) and concentration of methane surface intermediates (NM) were measured in situ using isotopic tracing (SSITKA) of CO hydrogenation under methanation conditions (H2 : CO = 10:1, T=280 °C, P=1.8 atm). Fe phases after activation were found by XRD to be Fe0 + Fe3O4 for [H] and Fe carbides + Fe3O4 for both [CO] and [S]. Reaction and SSITKA results showed that the rate and abundance of surface intermediates on the [H]-pretreated catalyst developed with TOS, reaching a maximum at ca. 1 h, and then declined to steady-state values at 21 h, still significantly higher than for the other pretreated samples. Activity was shown by SSITKA to be primarily determined by the number of active intermediates (related to the number of surface sites). Measures (1/τM) of the intrinsic site activity on the differently activated catalyst samples were not significantly different, suggesting that the active sites were all identical. Given the similarity in the activity of the sites and the increase in the concentration of active sites (and rate) of [H] and [CO] during the initial reaction period, it can be concluded that the active sites are probably on a (partially?) carburized Fe surface.

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).

References (35)

  • H. Jung et al.

    J. Catal.

    (1992)
  • H. Jung et al.

    J. Catal.

    (1993)
  • M.D. Shroff et al.

    J. Catal.

    (1995)
  • F. Blanchard et al.

    J. Mol. Catal.

    (1982)
  • J.P. Reymond et al.

    J. Catal.

    (1982)
  • C.S. Kuivila et al.

    J. Catal.

    (1989)
  • R.A. Dictor et al.

    J. Catal.

    (1986)
  • G.B. Raupp et al.

    J. Catal.

    (1979)
  • G.B. Raupp et al.

    J. Catal.

    (1979)
  • J. Happel

    Chem. Eng. Sci.

    (1978)
  • P. Biloen

    J. Mol. Catal.

    (1983)
  • D.M. Stockwell et al.

    J. Catal.

    (1988)
  • K. Jothimurugesan et al.

    Catal. Today

    (2000)
  • K. Jothimurugesan et al.
  • J.S. Hwang et al.

    Appl. Catal. A

    (2001)
  • B. Chen et al.

    J. Catal.

    (1995)
  • D.S. Kalakkad et al.

    Appl. Catal. A

    (1995)
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