Wirth J, Apeleo Zubiri B, Wu M, Englisch S, Drobek D, Taccardi N, Wasserscheid P, Spiecker E (2021)
Publication Type: Conference contribution
Publication year: 2021
Liquid metal catalysts (LMC) are intermetallic compounds of low-melting main group elements (e.g. Ga, In) with a small percentage of catalytically active transition metal elements (e.g. Pt, Pd, Rh). These compounds possess complex intermetallic phases in solid state and become highly dynamic in liquid form. Liquid metal solutions show a superior activity, robust performance and selectivity in dehydrogenation reactions [1]. At typical process temperatures around 500°C, it is conceivable that the large surface/volume ratio of nanodroplets, supported in the open meso-/macroporous scaffold, provide a huge number of active sites, which are intrinsically robust to coke formation and degradation due to the highly dynamic nature of the liquid metal alloy. Therefore, structural information including phase stability, local composition and wetting behavior of the nanoalloys inside/on the scaffold as well as the degradation behavior are indispensable to capture the full picture of the catalytic performance. Such microscopic insights are hardly accessible with macroscopic catalytic tests and require a combination of surface science, scattering and high-resolution imaging/analytical techniques, if possible, in situ, in operando or in conditions close to it [2].
In this study, we applied analytical and in situ transmission electron microscopy (TEM) in combination with correlative tomography techniques to contribute to a comprehensive characterization of LMC systems. This is required to unravel the complex pore morphology and the spatial distribution and composition of individual alloy droplets containing the catalytically active sites and, furthermore, to reveal and understand microscopic changes occurring at realistic catalytic process temperatures.
Firstly, we report on a correlative three-dimensional (3D) characterization across different length scales of Pd-Ga supported catalytically active liquid metal solutions (SCALMS), which exhibit a complex material structure featuring a molecularly defined, catalytically active liquid droplet layer adsorbed on macroporous silica templates. X-ray nanotomography (Nano-CT) allows non-destructive 3D investigations of materials across multiple length scales. The ZEISS Xradia 810 Ultra laboratory instrument uses Fresnel zone plate optics to achieve 3D resolutions down to 50 nm and can be equipped with a Zernike phase ring enabling phase contrast imaging, which is beneficial to study weakly absorbing materials and porous structures. We used phase contrast Nano-CT large-field of view (LFOV) and multiple region-of-interest (ROI) high-resolution (HR) imaging mode to reconstruct the macroporous silica support and, at the same time, the positions of larger Pd-Ga droplets inside a LMC sample (Figure 1). Figure 1c shows evenly distributed droplets inside the scaffold, which is important for a uniform catalytic performance.
For correlative 3D studies at higher resolution using 360° electron tomography (ET), we prepared a pillar sample via focused ion beam (FIB) milling out of the Nano-CT sample (Figure 2a). We used a Fischione model 2050 on-axis rotation tomography holder inside a FEI Titan Themis 300 transmission electron microscope to acquire tilt series of projections with a 180° tilt-angle range to investigate the spatial distribution of Pd and Ga inside the porous silica network at higher resolution (Figure 2). Complementary selected area electron diffraction (SAED) and scanning TEM (STEM) energy-dispersive X-ray spectroscopy (EDXS) enable to distinguish between Ga- and Pd-rich phases (Figure 2b). Both Nano-CT and 360° ET allow investigating the pore interconnectivity and the pore size distribution by applying the maximum sphere inscription (MSI) algorithm [3] to the 3D reconstructions (Figure 2c,d). A precise knowledge of these parameters are of utmost importance since they highly influence the gas diffusion properties in porous catalyst supports.
In addition to scale-bridging 3D analyses of the porous network and the position of the nanodroplets, we applied advanced and in situ TEM methods using a Gatan Heating Holder (Model 652) to Ga-Rh nanoparticles as a model LMC deposited on SiNx membranes simulating realistic process temperatures (Figure 3) [2]. The analyses reveal a typical two-phase microstructure comprised of amorphous nanoparticles with laminate-shaped Rh-rich crystalline precipitates. In situ SAED upon heating clearly reveals the melting of the crystalline precipitates at a temperature of ~500 ºC, which is in accordance with complementary X-ray photoelectron spectroscopy (XPS) studies. These experiments help to explore the temperature-dependent phase behavior and thus the phase diagram of the Ga-Rh system in the low Rh concentration regime, which is not yet safely established. Moreover, for bimetallic nanoparticles thermodynamic properties like phase stability and melting may be altered by surface effects due to the small particle size. The investigation of such size effects are expected to be key to understand the relationship between superior catalytic performance on the one hand and preparation and process conditions on the other hand.
APA:
Wirth, J., Apeleo Zubiri, B., Wu, M., Englisch, S., Drobek, D., Taccardi, N.,... Spiecker, E. (2021). Correlative 3D nanotomography and in situ transmission electron microscopy studies on supported catalytically active liquid metal solutions. In Proceedings of the European Microscopy Congress 2020.
MLA:
Wirth, Janis, et al. "Correlative 3D nanotomography and in situ transmission electron microscopy studies on supported catalytically active liquid metal solutions." Proceedings of the European Microscopy Congress 2020 2021.
BibTeX: Download