ELECTRONIC STRUCTURE, MAGNETISM AND DYNAMICS
Mobility and versatility of the liquid bismuth promoter in the working iron catalysts for light olefin synthesis from syngas, B. Gu (a), D.V. Peron (a), A.J. Barrios (a), M. Bahri (b), O. Ersen (b), M. Vorokhta (c), B. Šmíd (c), D. Banerjee (d,e), M. Virginie (a), E. Marceau (a), R. Wojcieszak (a),
V.V. Ordomsky (a) and A.Y. Khodakov (a), Chem. Sci. 24, 6167-6182 (2020); https:// doi.org/10.1039/D0SC01600D. (a) CNRS, UMR 8181 - UCCS - Unité de Catalyse et Chimie du Solide, Univ. Lille, Lille (France) (b) IPCMS-UMR 7504 CNRS, Université de Strasbourg, Strasbourg (France)
(c) Department of Surface and Plasma Science, Faculty of Mathematics and Physics, Charles University, Prague (Czech Republic) (d) ESRF (e) Department of Chemistry, KU Leuven, Leuven (Belgium)
 V.V. Ordomsky et al., ACS Catal. 7, 6445-6452 (2017).  T. Daeneke et al., Chem. Soc. Rev. 47, 4073-4111 (2018).
PRINCIPAL PUBLICATION AND AUTHORS
Traditionally, the catalysts are doped with transition or noble metals, which are immobile and localised on the specific sites over the catalyst surface. In contrast, the use of metals as promoters, which are liquid under the reaction conditions, is rather unconventional. Liquid metals have recently appeared as a new emerging and rapidly growing class of materials . They have recently gained renewed attention from the scientific community and found numerous applications in microfluidics, soft electronics, sensing and therapeutics. The extraordinary properties of liquid metals and alloys come from their electron-rich metallic cores, interfaces and interaction with the environment.
A higher mobility is the most important specificity of doping of Fischer-Tropsch catalysts with liquid metals. The structure of iron catalyst doped with extremely mobile bismuth dramatically evolves during the catalyst activation and Fischer-Tropsch reaction. The conventional ex- situ catalyst characterisation techniques were therefore unable to follow the genesis of the active phase in these catalysts.
In this work, a unique combination of in-situ characterisation techniques operating under the reaction gaseous atmosphere and temperatures: in-situ synchrotron-based X-ray absorption near- edge structure (XANES) at beamline BM26A, in- situ scanning transmission electron microscopy (STEM), and near-atmospheric-pressure X-ray photoelectron spectroscopy (NAP-XPS), have provided complementary information about the localisation, dynamic nature of the bismuth promoter and genesis of the active phase.
It was found that during the catalyst activation, the supported iron oxide species converted into iron carbide nanoparticles. STEM and NAP-XPS measurements performed in the atmosphere of reacting gases were indicative of the remarkable migration of the bismuth species to the active iron carbide phase (Figure 94). This migration resulted in formation of iron-bismuth core- shell nanoparticles and bismuth sintering. In- situ XANES uncovered continuous oxidation and reduction cycles of the bismuth promoter at the interface with the iron carbide phase during the Fischer-Tropsch reaction (Figure 95). These bismuth redox cycles facilitated carbon monoxide dissociation via oxygen scavenging and resulted in the substantial enhancement in the catalytic performance.
Fig. 95: a) XANES spectra showing re-oxidation of bismuth
on the surface of metallic iron during cooling under CO.
b) CO dissociation over iron carbide nanoparticles facilitated
by bismuth and bismuth re- oxidation cycle.
Fig. 94: a) Variation of the intensity of Bi 4f NAP-XPS signal during the catalyst activation in CO suggesting bismuth sintering. b) STEM-EDX images of iron-bismuth core shell nanoparticles in the activated catalysts.