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Environmental TEM studies of single iron nanoparticle carburization in synthesis gas Xi Liu, Chenghua Zhang, Yongwang Li, J. W. (Hans) Niemantsverdriet, Jakob B. Wagner, and Thomas W Hansen ACS Catal., Just Accepted Manuscript • Publication Date (Web): 16 Jun 2017 Downloaded from http://pubs.acs.org on June 16, 2017
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ACS Catalysis
Environmental TEM studies of single iron nanoparticle carburization in synthesis gas. Xi Liu*, †‡, Chenghua Zhang‡, Yong Wang Li‡ , J.W. (Hans) Niemantsverdriet§, ‡, Jakob B. Wagner†, Thomas W. Hansen† †
Center for Electron Nanoscopy, Technical University of Denmark, Lyngby, 2800, Denmark, SynCat@Beijing, Synfuels China Technology Co., Ltd, Beijing, 101407, China, § SynCat@DIFFER, Syngaschem BV, PO Box 6336, 5600 HH, Eindhoven, The Netherlands ‡
*Corresponding author.
[email protected] ABSTRACT: Structural evolution of iron nanoparticles involving the formation and growth of iron carbide nuclei in the iron nanoparticle was directly visualized at the atomic level using environmental transmission electron microscopy under reactive conditions mimicking Fischer-Tropsch synthesis. Formation of the iron carbide nuclei and surface reconstruction of the iron nanoparticle play an essential role in carburization of the iron nanoparticle and consequent formation of Fe5C2. Identification of carbide and oxide intermediates evidenced by high-resolution TEM images, electron diffraction patterns and electron energy-loss spectra provides a detailed picture from initial activation to final degradation of iron under synthesis gas.
KEYWORDS: Environmental TEM, iron carbide, Fischer-Tropsch synthesis, in-situ, EELS
Hydrogenation of CO by H2, known as Fischer-Tropsch synthesis (abbreviated as FTS) offers an attractive route to produce high quality fuels and chemicals via synthesis gas from coal, natural gas, or biomass, and offers an alternative to the use of crude oil. FTS is an essential step in coal-to-liquids (CTL) technology, and is of great interest in China as a clean process for the utilization of coal, and prevents the environmental pollution associated with direct combustion of coal in domestic life and in industry [1,2]. For 2017, Chinese CTL processes based on gasification and FTS are foreseen to account for a cumulative production capacity of over 4 million tons [2]. Although both cobalt and iron are used as FTS catalysts [3], iron is usually prefered in CTL owing to its abundance and low cost. Although FTS has been developed over 90 years, various key issues related to the activation of Fe catalysts and formation of iron carbides are still disputed.[3, 4] For example, we have a limited understanding of initiallyformed surface carbide clusters in Fe-based catalysts and the mechanism underlying the transformation of surface species to bulk iron carbide, whereas the nano- or sub-nano-structured carbide should exhibit unique physicochemical properties in contrast to the corresponding bulk material. The subject is of interest in both catalysis and nanotechnology since iron carbides have also be considered as active species for synthesis of nanocarbons.[5-7] What has complicated characterization studies of activated or spent Fe catalysts is the complex nature and high reactivity of the formed metallic iron and iron carbides as the surface structure is quickly restructured after expo-
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sure to even trace amounts of oxygen or water.[4, 8] To date, this task has been partially resolved by the development of in situ bulk and surface characterization techniques, such as XRD, X-ray absorption and Mössbauer spectroscopy, which allow investigation of structural evolution of iron-based precursors or chemical properties of activated catalysts under chemical conditions.[9-14] However, these studies are not spatially resolved meaning that we cannot probe any changes in structure or chemical properties of an individual Fe nanoparticle under reaction conditions at the (sub) nanoscale.[15-17] In order to identify the initial formation of the surface carbide species in Fe nanoparticles and monitor the carburization process at the nanoscale and under chemical conditions, an environmental transmission electron microscope (ETEM) equipped with aberration-correction was employed to characterize the structure and chemistry of individual iron nanoparticles at the atomic-level, under a reactive gaseous environment. Using this tool, reduction of iron oxide nanoparticles by H2 and subsequent carburization by syngas were studied, - the latter step has been regarded as a key activation step in FTS. In this work, we directly visualize the occurrence of carburization, oxidation and coke formation in the Fe metal particles in sequence at the subnanometer level under the well-controlled chemical environment and examine different carbon species in the activated particles free of contamination. During the experiment, fast formation of metastable iron carbide nuclei was observed, the nuclei either quickly disappeared via surface reconstruction or gradually consolidated into larger carbide grains, which resulted in final formation of Fe-Fe5C2 composites. This observation, which for example has implications for recent DFT studies on well-crystallized iron carbide surfaces, gives a new perspective for mechanistic studies.[18-21]
RESULTS AND DISCUSSIONS Figure 1a-c shows TEM images of the as-prepared iron oxide nanoparticles dispersed on SiNx windows. The relatively narrow size distribution of the nanoparticles gives an average size of about 20 nm. After in situ reduction, diameters of the formed metallic iron particles range from 5 nm to above 50 nm (Figure 1d), which is attributed to sintering of reduced nanoparticles at elevated temperature. For monitoring the effect of carburization, metallic iron particles with a diameter of about 20 nm were selected (Figure 1 e), whose thickness makes it easier to acquire good quality selected area electron diffraction patterns (SAED) as well as electron energy-loss spectra (EELS) with acceptable loss of atomic-scale structural information of the subjects. It is worth noting that, under the gaseous environment, smaller particles could not stand strong beam irradiation as we conducted repeated STEM-EELs analysis during the ETEM work, thus we did not choose the smaller particles (
