Fischer-Tropsch synthesis (FTS) is a well-known process, which permits to produce clean fuels from a synthetic gas (CO+H₂). The syngas can be obtained from natural gas or by gaseification of lignocellulosic biomass, providing an interesting environment-friendly alternative to oil. It is generally admitted that the catalytic properties (activity and selectivity) of the cobalt-based catalysts used for the FTS depend on the size distribution of the cobalt nanoparticles [1]. However, in high-loaded catalysts, nanoparticles tend to form large aggregates, modifying the quantity of accessible active sites. Their impact of the catalytic properties is still uncertain.

The aim of this study is thus to lead to a detailed description of the multi-scale structure of Cobalt-based catalysts. A multi-technique approach is proposed combining transmission electron microscopy in different modes (dark field, HAADF-STEM and electron tomography) and Anomalous Small-Angle X-Ray Scattering (ASAXS). The project focuses on two Co-based catalysts, supported on alumina and containing 15%wt Co. After impregnation, the catalyst was dried and calcined in air. The first sample is the oxide catalyst. The second one is obtained after reduction under pure H₂ flow.

BF-TEM, DF-TEM and HAADF-STEM were used to characterize both the Co crystallites size and the aggregates size distribution. Electron tomography acquisitions were performed in HAADF-STEM mode [4] in order to obtain a 3D visualization of the Co aggregates. Quantitative analysis of the segmented aggregates was performed.

The ASAXS technique is yet little used for the characterization of the active phases of catalysts [2, 3]. Hence, ASAXS experiments have been performed on SWING Beamline of SOLEIL Synchrotron, slightly below the Co K-edge at 3 different energies. The exploitation of the ASAXS data was successfully performed by means of using X-Ray scattering theory, and allowed calculating the size distribution of the spherical nanoparticles and aggregates.

First, the complementarity between ASAXS and electron tomography techniques has been demonstrated: they bring new insights on the multi-scale structure of Cobalt-based catalysts.

The ASAXS technique has permitted to characterize simultaneously the size distribution of the nanoparticles and the aggregates, from a few angstrom scale to a scale of several hundreds of nanometers (Figure 1). Size distribution of the aggregates are in good agreement with HAADF-STEM results (figure 3).

Secondly, the advantage of the electron tomography has been highlighted. Electron tomography gives access to a unique 3D visualization of the aggregates (see figure 2). It has revealed that the morphology of the aggregate is changing during reduction: in the oxide state, they are compact, even if internal porosity is observed, whereas in the reduced state, they present a porous sponge-like structure, more or less compact. Calculation of porous volume, pore size and mean size of walls (nanoparticles forming the aggregates) can also be carried out, for a quantitative comparison.

Even if the sintering of the particles is often evoked during the reduction step, it appears that another phenomenon occurs at the aggregate scale : ASAXS and electron tomography show that, contrary to their size, the internal density of the aggregates does evolve. Indeed, the aggregates of the reduced sample are airier with a larger porosity. Is it due to the change of the crystallographic structure from Co₃O₄ to Co⁰ or to internal stresses that break and open the aggregate structure ?

Aknowledgments :

Electron tomography acquisitions were partially performed at « Centre Technologique des Microstructures de l’Université Lyon I». The authors thank X. Jaurand for his support.

References :

[1] G.L. Bezemer, et al., Journal of the American Chemical Society, 128 (2006).

[2] H. Brumberger, et al., Journal of Applied Crystallography, 38 (2005).

[3] P. Georgopoulos, J.B. Cohen, Journal of Catalysis, 92 (1985).

[4] I. Arslan, et al., Journal of the American Chemical Society, 130 (2008).

[5] P. Munnik, P.E. de Jongh, K.P. de Jong, Journal of the American Chemical Society, 136 (2014).

[6] P. Munnik, et al., ACS Catalysis, 4 (2014).

Figures:

ASAXS curves and the calculated particle size distribution

HAADF-STEM image of Co aggregates in the reduced sample

3D reconstruction obtained by electron tomography of a Co aggregate. Left : in the oxide sample; right: in the reduced sample.

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