Abstract
We present a comprehensive structural and analytical characterization of the highly promising supported catalytically active liquid metal solutions (SCALMS) system. This novel catalyst shows excellent performance for alkane dehydrogenation, especially in terms of resistance to coking. SCALMS consists of a porous support containing catalytically active low-melting alloy particles (e.g., Ga-Pd) featuring a complex structure, which are liquid at reaction temperature. High-resolution 3D characterization at various length scales is required to reveal the complex pore morphology and catalytically active sites’ location. Nano X-ray computed tomography (nano-CT) in combination with electron tomography (ET) enables nondestructive and scale-bridging 3D materials research. We developed and applied a correlative approach using nano-CT, 360°-ET and analytical transmission electron microscopy (TEM) to decipher the morphology, distribution and chemical composition of the Ga-Pd droplets of the SCALMS system over several length scales. Utilizing ET-based segmentations of nano-CT reconstructions, we are able to reliably reveal the homogenous porous support network with embedded Ga-Pd droplets featuring a nonhomogenous elemental distribution of Ga and Pd. In contrast, large Ga-Pd droplets with a high Ga/Pd ratio are located on the surface of SCALMS primary particles, whereas the droplet size and the Ga/Pd ratio decreases while advancing into the porous volume. Our studies reveal new findings about the complex structure of SCALMS which are required to understand its superior catalytic performance. Furthermore, advancements in lab-based nano-CT imaging are presented by extending the field of view (FOV) of a single experiment via a multiple region-of-interest (ROI) stitching approach.
Highlights
With the main characteristics of a state-of-the-art catalyst system being the activity and selectivity combined with the reproducibility, hightemperature stability and capacity for easy regeneration [8], the supported ionic liquid-phase catalysis (SILP) concept expanded into a new concept published by Taccardi et al in 2017: supported catalytically active liquid metal solutions (SCALMS) [9]
We developed a correlative workflow by combining lab Nano X-ray computed tomography (nano-computed tomography (CT)), 360◦ -electron tomography (ET), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDXS) tailored to characterize the complex and scale-bridging structure of novel SCALMS systems (Figure 1)
This work combined 360◦ -ET, HRES and LFOV nano-CT complemented by scanning electron microscopy (SEM)/scanning transmission electron microscopy (STEM)
Summary
In heterogeneous catalysis, the catalytic process derives from the immediate surface area between different phases: for instance, solid catalysts acting within gaseous or liquid reactants [4]. A catalytically active phase is adsorbed onto a porous support material in the form of a film or droplets [6]. This concept was later improved in the so-called supported ionic liquid-phase catalysis (SILP) [7]. With the main characteristics of a state-of-the-art catalyst system being the activity and selectivity combined with the reproducibility, hightemperature stability and capacity for easy regeneration [8], the SILP concept expanded into a new concept published by Taccardi et al in 2017: supported catalytically active liquid metal solutions (SCALMS) [9]. The porous support material and the distribution and size of the catalytically active metal–metal complexes cover important structural and morphological features on different length scales, whose
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