Oxidation of cobalt based Fischer–Tropsch catalysts as a deactivation mechanism

Berge, P.J. van; Loosdrecht, J. van de; Barradas, S.; Kraan, A.M. van der

doi:10.1016/s0920-5861(00)00265-0

Cited by 269 publications

(163 citation statements)

References 22 publications

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“…In order to maximize the catalytic activity measured by CO conversion during FTS, prior catalyst reduction is paramount because the active species in Co-containing samples are construed to be metallic (Co • ) in nature. Some authors have asserted that the highest CO conversions in FTS are attributed to higher Co reducibility [33], while the oxidation of the Co metal leads to catalyst deactivation [21]. Figure 4 presents the XRD patterns of the fresh Co metal injected into the plasma, which comprised two phases as analysed by RQA: 62% having face centred cubic (FCC) structure, and 38% hexagonal closed packing (HCP) structure [7].…”

Section: Xrd and Rqa Analysismentioning

confidence: 99%

“…Other causes of catalyst deactivation include coking, surface restructuring of the Co metal phase in syngas, and sintering of the Co nanoparticles [20]. Some authors have equally suggested that Co-metal re-oxidation may also lead to catalyst deactivation [21], although there are some disagreements based on particle size effects as shown by empirical data [22].…”

Section: Introductionmentioning

confidence: 99%

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Effect of CO Concentration on the α-Value of Plasma-Synthesized Co/C Catalyst in Fischer-Tropsch Synthesis

Aluha

Abatzoglou

2017

Catalysts

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Abstract:A plasma-synthesized cobalt catalyst supported on carbon (Co/C) was tested for Fischer-Tropsch synthesis (FTS) in a 3-phase continuously-stirred tank slurry reactor (3-φ-CSTSR) operated isothermally at 220 • C (493 K), and 2 MPa pressure. Initial syngas feed stream of H 2 :CO ratio = 2 with molar composition of 0.6 L/L (60 vol %) H 2 and 0.3 L/L (30 vol %) CO, balanced in 0.1 L/L (10 vol %) Ar was used, flowing at hourly space velocity (GHSV) of 3600 cm 3 ·h −1 ·g −1 of catalyst. Similarly, other syngas feed compositions of H 2 :CO ratio = 1.5 and 1.0 were used. Results showed~40% CO conversion with early catalyst selectivity inclined towards formation of gasoline (C 4 -C 12 ) and diesel (C 13 -C 20 ) fractions. With prolonged time-on-stream (TOS), catalyst selectivity escalated towards the heavier molecular-weight fractions such as waxes (C 21+ ). The catalyst's α-value, which signifies the probability of the hydrocarbon-chain growth was empirically determined to be in the range of 0.85-0.87 (at H 2 :CO ratio = 2), demonstrating prevalence of the hydrocarbon-chain propagation, with particular predisposition for wax production. The inhibiting CO effect towards FTS was noted at molar H 2 :CO ratio of 1.0 and 1.5, giving only~10% and~20% CO conversion respectively, although with a high α-value of 0.93 in both cases, which showed predominant production of the heavier molecular weight fractions.

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Section: Xrd and Rqa Analysismentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Effect of CO Concentration on the α-Value of Plasma-Synthesized Co/C Catalyst in Fischer-Tropsch Synthesis

Aluha

Abatzoglou

2017

Catalysts

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“…Other cobalt species found in FTS catalysts include; cobalt oxide (both CoO and Co 3 O 4 ), cobalt carbide (Co 2 C) and cobalt-supported compounds (such as cobalt aluminate (CoAl 2 O 4 ) or cobalt titanate (CoTiO 3 )) [6]. The oxidation of cobalt catalysts by water to form cobalt oxides, first raised as an issue in work by A. Holmen [7], has been extensively discussed in the literature as research groups strive to determine whether oxidation is in fact a catalytic deactivation mechanism [7,8]. In the past, it has been assumed that oxidation triggers deactivation but XAFS measurements have suggested that water oxidation may not be an issue for these catalyst [9].…”

Section: Introductionmentioning

confidence: 99%

X-ray spectroscopic and scattering methods applied to the characterisation of cobalt-based Fischer–Tropsch synthesis catalysts

Herbert

Sénécal

Martin

et al. 2016

Catal. Sci. Technol.

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(2016) 'X-ray spectroscopic and scattering methods applied to the characterisation of cobalt-based FischerTropsch synthesis catalysts.', Catalysis science and technology., 6 (15). pp. 5773-5791. Further information on publisher's website:http://dx.doi.org/10.1039/C6CY00581KPublisher's copyright statement:Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Abstract: This review aims to critically assess the use of X-ray techniques, both of a scattering (e.g. X-ray diffraction (XRD), Pair Distribution Function (PDF)) and spectroscopic nature (X-ray Absorption spectroscopy (XAFS)), in the study of cobalt-based Fisher-Tropsch Synthesis (FTS) catalysts. In particular, the review will focus on how these techniques have been successfully used to describe the salient characteristics of these catalysts that govern subsequent activity and selectivity, as well as to afford insight into deactivation phenomena that have seemingly stifled their application. We discuss how these X-ray-based techniques have been used to yield insight into the bulk structure, the catalyst surface, oxidation states, local (cobalt) geometry, and elemental composition of particles, primarily from a 1D perspective but we also highlight how, with recent developments in advanced X-ray characterisation methods, crucial information can now be obtained in 2D and 3D. The examples chosen focus on data acquired in situ/operando, under realistic operating conditions and during activation which often allow for obtaining a more relevant perspective on the changes in catalyst structure that accompany a change in catalyst performance. We conclude with a perspective on some of the challenges that beset the Co-based FTS technology and discuss how X-ray based techniques could be used to solve them.2

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“…Air oxidation/reduction treatments are observed to increase the initial activity in FT catalysts [5,6]. Thus, oxidation can either lead to significant deactivation [1][2][3][4] or to improved activity [5,6]. Several ex-situ TEM studies were undertaken to reveal the effects of these different oxidation environments on the metal particle morphology in an experimental supported Co-based FTS catalyst.…”

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confidence: 99%

Using Ex-situ TEM to Understand the Effect of Different Oxidative Environments on Small Metal Particle Morphology

2018

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Metal particle oxidation is an important factor in catalysis. Water-induced oxidation is well-known to occur during Fischer-Tropsch synthesis (FTS) and can lead to deactivation [1][2][3][4]. Air oxidation/reduction treatments are observed to increase the initial activity in FT catalysts [5,6]. Thus, oxidation can either lead to significant deactivation [1][2][3][4] or to improved activity [5,6]. Several ex-situ TEM studies were undertaken to reveal the effects of these different oxidation environments on the metal particle morphology in an experimental supported Co-based FTS catalyst.In all cases, catalyst was prepared for TEM by crushing it into fines using an agate mortar and pestle. The fines were dusted onto a 200 mesh, holey-alumina-coated TEM grid. The grid was transferred into a specially designed ex-situ reactor where it was treated and then moved via an inert transfer protocol into a Philips CM200F for examination [7]. Metal particles were imaged in the bright field (BF) TEM mode at an accelerating voltage of 200 kV, and areas of interest were "mapped" so that the same metal particles could be re-examined subsequent to each reactor treatment.In the case of the dry (lab) air oxidation, the material was given an initial 400 C, 4 h treatment under H2. The reduction was followed by a 350 °C, 3 h air oxidation. Examination of the reduced material revealed distinct Co metal particles on the support (Figure 1a). Similar to previous observations [8][9][10] the air oxidation treatment resulted in the formation of torus-shaped structures (Figure 1b). This hollow dome morphology results from diffusion rate differences between the cobalt and the oxygen ions at elevated temperatures.The effect of a water (steam)-based oxidation was studied next. The experimental catalyst was first reduced in the reactor (1.2 MPa H2, 400 C, 8 h) and then held under FTS conditions (2 MPa H2:CO ~2.1:1, 215 °C) for 1 h. The gas hourly space velocity (GHSV) was specifically controlled to create high CO conversion (high water partial pressure) conditions around the TEM grid. The TEM grid was then given a low temperature reduction (1.2 MPa H2, 215 C, 4 h) followed by another 1 h FTS treatment.Significant morphological changes accompanied the water-based oxidation (Figures 2a-2c). Instead of forming the torus presented during a dry air oxidation, a metal oxide/hydroxide species developed. This species wet the support so effectively that it was essentially "invisible". However, subsequent rereduction of the metal oxide/hydroxide resulted in distinct metal particles. It is the spreading of this oxide/hydroxide species that is well-known to allow adjacent metal particles to "reach" each other and ultimately coalesce resulting in deactivation during FTS [4].

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Oxidation of cobalt based Fischer–Tropsch catalysts as a deactivation mechanism

Cited by 269 publications

References 22 publications

Effect of CO Concentration on the α-Value of Plasma-Synthesized Co/C Catalyst in Fischer-Tropsch Synthesis

Effect of CO Concentration on the α-Value of Plasma-Synthesized Co/C Catalyst in Fischer-Tropsch Synthesis

X-ray spectroscopic and scattering methods applied to the characterisation of cobalt-based Fischer–Tropsch synthesis catalysts

Using Ex-situ TEM to Understand the Effect of Different Oxidative Environments on Small Metal Particle Morphology

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