Quasi in situ TEM investigation on the activation of commercial wüstite precursors

Abstract text (incl. figure legends and references)

Introduction : The development of in situ and operando transmission electron microscopy (TEM) has substantialy improved our understanding on the materials during their operation at realistic conditions which are generally different from the vacuum. However, the atmospheric pressure mainly used by in situ and operando TEM may be still too low and therefore unsuitable for investigating lots of catalytic reactions such as ammonia synthesis, methanol synthesis and Fischer-Topsch synthesis. Hence the quasi in situ TEM[1] approach, – which consists of studying the sample before and after the treatment at relevant conditions (high pressure and temperature) – appears as a powerfull tool to investigate the local changes that occur during the activation and operation of the catalysts. In this case, the sample is treated in a special TEM grid-reactor outside the TEM, then later transfered to the TEM without exposing to ambient air.

Objectives : In order to understand the morphology and microstructure of the iron catalyst used in the NH₃-synthesis, we have investigated by quasi in situ TEM the reduction of iron oxide precursors so called wüstite. This is crucial for optimizing the active catalyst before the NH3 synthesis process.[2,3]

Materials and Methods : We have developped a high-pressure TEM grid-reactor for the quasi in situ studies. This has allowed us to investigate the reduction of a commercial wüstite sample from Sigma-Aldrich under relevant activation condition (H2/N2=3:1,10 bar, 500 °C). In addition, we conducted identical location imaging (ILI) using a double Cs corrected Jeol ARM 200F electron microscope that was operated at 200 kV.

Results : The wüstite precursors presented inhomogenous grains size (from 0.2 to few micrometer) with defective and polycristalline FeO structure (Fig. 1a-c). After the activation/reduction treatment (H2/N2=3:1, 10 bar, 500 °C), ILI revealed morpholological changes corresponding to the formation of core-shell-like structures due to the fragmentation of pristine particle into smaller particles agglomerates (Fig. 1d-e). Electron diffraction analysis (Fig. 1f) indicates the full reduction of wüstite into metallic iron.

Conclusions : Quasi in situ TEM study using a high-pressure TEM grid-reactor is powerful for investigating local changes on the morphology and structure of the catalysts at relevant activation conditions. Furthermore, this approach will be extended to study the pressure dependencies during the reductive activation.

References

[1] L. Masliuk, M. Swoboda, G. Algara-Siller, R. Schlögl, T. Lunkenbein, Ultramicroscopy 2018, 195, 121–128.

[2] J. R. Jennings, Ed., Catalytic Ammonia Synthesis: Fundamentals and Practice, Springer US, 1991.

[3] H. Liu, W. Han, Catalysis Today 2017, 297, 276–291.

Figure 1 : (a) Identical location TEM investigation and electron diffraction of a commercial wüstite sample before (a, b and c) and after (d, e and f) exposure to a total flow of 4 mL/min of a H₂/N₂ =3:1 mixture at 10 bar and 500 °C.