Elsevier

Applied Catalysis A: General

Volume 481, 5 July 2014, Pages 116-126
Applied Catalysis A: General

Cobalt and iron species in alumina supported bimetallic catalysts for Fischer–Tropsch reaction

https://doi.org/10.1016/j.apcata.2014.04.047Get rights and content

Highlights

  • Different cobalt and iron species as a function of Co/Fe ratio.

  • Higher dispersion of cobalt and iron in bimetallic catalysts.

  • Much easier iron and more difficult cobalt reducibilities in bimetallic catalysts.

  • Cobalt–iron mixed species in both calcined and reduced catalysts.

Abstract

This paper focuses on the identification of cobalt and iron species in bimetallic alumina supported catalysts and their role in Fischer–Tropsch synthesis. The catalysts are prepared via aqueous coimpregnation of alumina with solutions of cobalt and iron nitrates followed by calcination in air and reduction in hydrogen. Qualitative and quantitative analyses of cobalt and iron phases were performed using XANES/EXAFS, XRD, TPR, XPS and magnetic method. The results show that Co3O4 and Fe2O3 are the major phases in the calcined monometallic cobalt and iron catalysts, while the bimetallic catalysts also exhibited the presence of mixed Co–Fe oxides. Reduction of these Co–Fe oxides leads to cobalt–iron bimetallic particles. The Fischer–Tropsch reaction rate increases with cobalt content in bimetallic catalysts. Formation of mixed cobalt–iron species increases selectivity to lighter olefins compared to cobalt monometallic catalyst but leads to lower Fischer–Tropsch reaction rates.

Introduction

Fischer–Tropsch synthesis (FT) produces clean fuels from syngas (mixture of CO, H2) [1], [2], [3], [4], [5]. The syngas can be manufactured from fossil and renewable feedstocks (natural gas, coal and biomass). The synthetic fuels obtained using FT synthesis have a high cetane number and do not contain sulfur or aromatics. They could be considered therefore as a real alternative to traditional fuels from crude oil. The contribution of renewable resources such as biomass to the global energy supply has been growing in recent years [3].

Cobalt catalysts are the catalysts of choice for synthesis of long-chain hydrocarbons via low temperature FT synthesis from syngas. Long-chain hydrocarbons generally are produced on cobalt catalysts supported on oxide supports such as alumina, silica or carbon materials [5], [6], [7], [8]. It is known that the catalytic conversion of carbon monoxide occurs on the cobalt metal sites and that FT reaction rate is a function of cobalt dispersion, reducibility and stability. Iron catalysts which are much cheaper are also used commercially for low temperature FT synthesis, however with somewhat lower activity than cobalt catalysts. In this context, cobalt–iron bimetallic catalysts with optimized catalytic performance seem to be a viable option for converting syngas into hydrocarbons [9], [10], [11], [12], [13]. In addition, iron species in bimetallic catalysts which may have a significant activity in the water gas shift (WGS) reaction [1] could be used to adjust H2/CO ratio in syngas and thus, make the FT technology more flexible to different types of feedstock.

Previously cobalt–iron mixed catalysts have been a subject of several investigations. The obtained results were however rather contradictory. De la Pena O'Shea et al. [14] observed lower activity of silica supported cobalt monometallic catalysts in fixed bed reactor compared to iron counterparts. The activity of cobalt and iron catalyst supported on HMS mesoporous materials was studied by Bragana et al. [15]. HMS supported Co–Fe catalyst showed the highest activity and C5+ hydrocarbon selectivity, while Co-Fe/SBA-15 catalyst revealed highest selectivity to alcohols. Mixed cobalt–iron particles supported on carbon nanotubes were studied by Tavasoli et al. [16]. The monometallic cobalt catalyst exhibited high selectivity (85.1%) toward C5+ liquid hydrocarbons, while addition of small amounts of iron did not significantly affect the product selectivity. Lögdberg et al. [9] found that alloys of cobalt with small amount of iron improved the FT activity compared to the monometallic iron catalyst.

The present work focuses on the identification of different cobalt and iron phases present on the alumina supported catalysts with different Co/Fe ratio during catalyst preparation and activation using a combination of techniques: nitrogen adsorption, ex situ and in situ X-ray diffraction (XRD), X-ray absorption (XANES/EXAFS), temperature programmed reduction (TPR), in situ magnetization measurements and X-ray photoelectron spectroscopy (XPS). Particular attention is paid to interaction between cobalt and iron species in bimetallic catalysts at different catalysts preparation stages. The catalytic performance in FT synthesis has been evaluated in a Flowrence high-throughput unit (Avantium).

Section snippets

Catalysts preparation

The monometallic cobalt or iron catalysts were synthesized via incipient wetness impregnation of alumina (Puralox SCCA 5/170 Sasol, SBET = 183 m2/g, pore diameter = 8 nm) using aqueous solutions of cobalt or iron nitrates obtained from the following precursors: Co(NO3)2·6H2O (Fluka) and Fe(NO3)3·9H2O (Fluka). The cobalt or iron content in the monometallic catalysts prepared in a single impregnation step was 10 wt.%.

The bimetallic catalysts were synthesized via one step incipient wetness

Calcined samples

The XRD patterns of calcined catalysts are shown in Fig. 1. In addition to γ-alumina, they confirm the presence of Co3O4 and Fe2O3 phases in the monometallic cobalt and iron catalysts. The mean size of Co3O4 crystallites in the monometallic catalysts was calculated using the Scherrer equation [18]. The size of Co3O4 crystallites was about 11 nm in 10Co/Al2O (Table 1). The XRD peaks of Fe2O3 were too broad for the evaluation of crystallite sizes using the Scherrer equation. This suggests a high

Conclusion

A combination of characterization techniques allowed obtaining qualitative and quantitative information about the structure of iron–cobalt bimetallic catalysts supported on alumina support for FT reaction. The results have clearly shown considerable differences in the nature of oxide and reduced phases in bimetallic cobalt–iron catalysts as a function of Co/Fe ratio.

Iron forms much more dispersed species in monometallic catalysts than cobalt. Iron reduction seems to be much difficult that of

Acknowledgements

The authors thank A.-S. Mamede, M. Trentesaux, P. Abdala and O. V. Safonova for assistance with characterization techniques. The European Synchrotron Radiation Facility is acknowledged for the use of synchrotron radiation. The REALCAT platform is benefiting from a Governmental subvention administrated by the French National Research Agency (ANR) within the frame of the ‘Future Investments’ program (PIA), with the contractual reference ‘ANR-11-EQPX-0037’. The Nord-Pas-de-Calais Region and the

References (62)

  • M.E. Dry

    Catal. Today

    (2002)
  • E. Iglesia et al.

    Adv. Catal.

    (1993)
  • E. Iglesia

    Appl. Catal. A

    (1997)
  • M.E. Dry

    Appl. Catal. A

    (1996)
  • R. Oukaci et al.

    Appl. Catal.

    (1999)
  • W. Chu et al.

    J. Catal.

    (2007)
  • S. Lögdberg et al.

    Appl. Catal. B

    (2009)
  • L.F.F.P.G. Bragança et al.

    Appl. Catal. A

    (2012)
  • L. Wang et al.

    Appl. Catal. B

    (2012)
  • V.A. de la Peña O‘Shea et al.

    Appl. Catal. A

    (2007)
  • A. Tavasoli et al.

    Fuel Process Technol.

    (2009)
  • M. Rønning et al.

    Catal. Today

    (2010)
  • P.A. Chernavskii et al.

    Appl. Catal. A

    (2006)
  • A.Y. Khodakov et al.

    J. Catal.

    (1997)
  • A. Khodakov et al.

    J. Catal.

    (2002)
  • J.P. Bonnelle et al.

    J. Electron Spectr.

    (1975)
  • J.E. Thomas et al.

    Geochim. Cosmochim. Acta

    (1998)
  • S. Shwan et al.

    Appl. Catal. B

    (2014)
  • F. Tihay et al.

    Appl. Catal. A

    (2001)
  • H.-J. Wan et al.

    Catal. Commun.

    (2007)
  • H.-J. Wan et al.

    J. Nat. Gas Chem.

    (2007)
  • B. Ernst et al.

    Catal. Today

    (1998)
  • A. Jean-Marie et al.

    Catal. Today

    (2011)
  • G. Jacobs et al.

    Catal. Today

    (2013)
  • G. Jacobs et al.

    Appl. Catal. A

    (2007)
  • X. Zhang et al.

    Fuel Process Technol.

    (2010)
  • A.J.H.M. Kock et al.

    J. Catal.

    (1985)
  • A.F.H. Wielers et al.

    J. Catal.

    (1989)
  • D.J. Duvenhage et al.

    Appl. Catal. A

    (1997)
  • S. Storsæter et al.

    J. Catal.

    (2005)
  • O. Borg et al.

    J. Catal.

    (2007)
  • Cited by (57)

    • Pyrolysis and in-line catalytic decomposition of excavated landfill waste to produce carbon nanotubes and hydrogen over Fe- and Ni-based catalysts – Investigation of the catalyst type and process temperature

      2022, Chemical Engineering Journal
      Citation Excerpt :

      Additionally, a pronounced peak at a temperature higher than 850 °C for Fe/Al_Sg indicates the presence of barely reducible Fe-Al spinel structures. A merged series of peaks for FeNi/Al_Sg at 600–900 °C (Fig. 3) indicate the presence of different mixed Fe oxides and Fe-Al spinels [74,75]. A slight peak shift to lower temperatures can be observed for the bimetallic catalysts compared to the Fe-based catalysts, as they occur at 425 °C and 450 °C for FeNi/Al_Im and FeNi/Al_Sg catalysts, respectively.

    View all citing articles on Scopus
    View full text