Microscopy Conference 2023

Prof. Dr.

Session chairs

Prof.

Operando SEM and XPS insights of self-sustained oscillatory dynamics of ethylene to syngas over nickel catalysts

Abstract text (incl. figure legends and references) As scientists, it is our duty to pave the way for the development of a sustainable economy based on renewable energy resources and to reduce greenhouse gas emissions with an expected market value of 48Bn $ by 2027 [1], the catalyst—whether heterogeneous, homogeneous, or enzymatic—represents one of the pillars on which modern society is built. However, apart from the simplest model reactions, the scientific community is still unable to fully comprehend the underlying mechanism of catalysis and the atomistic details of the active state. The objective is to obtain mechanistic insights into the role of surface species present in the catalytic process of partial oxidation of ethylene to syngas on a model polycrystalline nickel foil. Using a combination of operando scanning electron microscopy (OSEM) for real-time observation of structural dynamics and ambient pressure x-ray photoelectron spectroscopy (APXPS) we correlate the state of the catalyst with its activity. Although challenging to study, the self-sustained oscillating reaction mode enables detailed analysis of the successive elementary steps of the catalytic act (Figure S1). The obtained insights are envisioned to expand the research on the self-regenerating operation of real-world catalysts. We were able to extend the functionality of a TFS Quattro S ESEM by developing a mobile in-situ that enabled the ambient pressure x-ray photoemission spectroscopy and operando scanning electron microscopy studies to be performed under identical conditions. This unit is composed of a modified sample holder that can achieve working temperatures of up to 1250C in reactive gas atmospheres, an automated heating unit and an automated gas feeding system. By maintaining a constant gas composition in isothermal conditions, a synchronized self-sustained oscillation mode was identified, in which the reaction`s product distribution reflects the catalyst morphology and chemical state. We were able to pinpoint the catalytic behavior down to competing species that influence catalyst deactivation with excellent spatial resolution and to discriminate between local and non-local mechanisms that impact the collective dynamics usually reported using traditional approaches. The combination of spatially resolved real-time imaging [2], [3] with integral surface-sensitive spectroscopy enables the chemical identification of surface species and simultaneous visualization of their evolution as a function of reaction conditions. We were able to provide insight into structure sensitivity and coupling phenomena between reactive gas-phase and catalyst states that are related to the emergence of catalytic activity in a synchronized self-sustained oscillatory mode. The here described structure-activity correlations are not accessible through integral characterization techniques or by studying steady-states alone, highlighting the importance of real-time, laterally resolved direct observation of active catalysts. Figure S1: Time series of a Ni surface during ethylene to syngas conversion. HFOV is 33.5µm. [1] Catalyst Market Size, Share &amp; Trends Analysis Report By Raw Material 2020 – 2027 [2] Danilatos,Gerasimos D. "Environmental scanning electron microscopy and microanalysis." Microchimica Acta 114.1 (1994): 143-155 [3] Sandoval-Diaz, Luis, et al. "Visualizing the importance of oxide-metal phase transitions in the production of synthesis gas over Ni catalysts." Journal of Energy Chemistry (2020).

IM 3: SEM and FIB developments

MS 1: Energy-related materials and catalysts

Abstract talk

Evaluating charge carrier mobility of organic semiconductor using ultra-low voltage spectral SEM

Abstract text (incl. figure legends and references) Introduction The pivotal property of organic semiconductors (OSCs) to achieve high performance devices is a high charge carrier mobility. However, the mobility is commonly measured from a model device, most often a field-effect transistor, making the mobility value material- and device-dependent. Both, the intrinsic factors - such as energy levels of all materials involved, molecular packing of the OSC, its crystalline domain size and orientation - and the extrinsic factors - likewise device structure, morphological defect, contact defect - will influence the final result. On the other hand, computational methods can provide intrinsic mobilities, which deviate frequently over several orders of magnitudes from the experimental data. Thus, material scientists need a reliable method to (re-)evaluate the existing and newly developed materials in order to gain a rational structure-function-relationship. Objectives Herein, we present an electron-spectroscopic method to evaluate the charge carrier mobility directly from thin-films or microcrystals. Such samples can easily be obtained for most materials. With the proposed method the effects of the extrinsic impact factors are reduced to only one - the contact resistance. This factor remains, as it is necessary to ground the sample for electron spectroscopic data collection. We show that the relative lateral mobilities between different OCSs can thus be directly determined. Materials &amp; methods Polycrystalline thin-films of small molecules were prepared via thermal evaporation under vacuum. Microcrystals of small molecules and polymers were obtained by wet techniques. We use a demonstrator of an electron-spectroscopic SEM, i.e. Zeiss Delta-SEM, for recording the secondary electron (SE) emission spectrum. The dynamic charging map, generated from the SE spectra on each pixel, is used for the mobility evaluation (see Figure 1). Results First, the beam-material interaction was investigated and the electrically neutral energy points were determined under various charging conditions. Depending on n-type or p-type OSCs, suitable charging conditions that results in negative or positive charging was selected.1 With step-wise increase of charging intensity, the OSCs turned from electrically neutral to severely charged. By comparing the stage currents among different OSCs, which exhibit the same/similar charging maps, one can evaluate OSCs in terms of lateral charge carrier mobility (example see Figure 2). The horizontal mobility is also accessible by analyzing the charging maps near and on the metal contacts. In addition, space charge, impacts from crystal orientation and molecular packing on mobility can be observed. Conclusion The lateral mobilities estimated with the presented method agree very well with the theoretical calculation (eg. transfer integral)2. Contact between OSC and electrode can be individually tested and thus a suitable device structure can be proposed according to the intrinsic lateral mobility. Ref. [1] Adv. Electron. Mater. 2021, 2100400. [2] Chem. Eur. J. 2015, 21, 17691-17700.

MS 5: Functional organic materials

Prof. Dr. rer. nat.

Hydrogen uptake and phase change in Pd nanoparticles at nanometer scale with in-situ TEM

Abstract text (incl. figure legends and references) Introduction Palladium-based nanomaterials play an important role in hydrogen technology. Unique properties of Pd, such as high affinity to hydrogen, good solubility of up to H/Pd = 1 atomic ratio and fast hydrogen absorption and desorption at room temperature are very attractive for various applications. Besides catalysis, Pd nanoparticles can assist in hydrogen delivery into other materials for hydrogen storage. Pd-based materials are also used as hydrogen purification membranes and hydrogen detectors. Objectives The hydrogenation and dehydrogenation process of Pd nanoparticles is hence of high interest in the applications mentioned above. Nanoscale systems reveal significant thermodynamic deviations from the bulk due to higher surface to volume ratio, the absence of grain boundaries, and different behavior of defects. In this work we investigate the behavior of Pd nanoparticles and the formation of PdHx in real time with in-situ gas TEM at room temperatures and elevated temperatures which are required by many applications. Materials &amp; methods The experiment was carried out at 80 kV on the ThermoFisher Scientific Themis Z equipped with Gatan GIF Continuum. In-situ hydrogen loading and unloading of Pd nanoparticles was accomplished with the gas system from Protochips. The sample was tightly sealed between two chips which both have electron transparent SiN windows, and analyzed at pressures of up to half atmosphere and temperatures of up to 200°C. Results and Conclusions We can observe the local phase change at different temperatures and pressures by measuring the position of the Pd bulk plasmon shift in low loss EELS [1]. With this technique we can study the direction of hydrogenation and dehydrogenation, hysteresis, size and shape related effects of the nanoparticles at various temperatures and pressures. In this work we also discuss the distinction of the quasi-planar surface plasmon modes in Pd nanoparticles from the PdHx signal. Fig.1. EELS mapping of the Pd NPs at 140 °C and 13 kPa H2 showing the separation of Pd-H phases: solid solution (blue) and palladium hydride (orange). [1] A. Baldi, et al, Nature Materials, 2014, DOI: 10.1038/NMAT4086

IM 7: In situ/operando electron microscopy

Structural evolution of a Cu/ZnO/Al2O3 catalysts revealed by operando TEM

Abstract text (incl. figure legends and references) Introduction: Cu–ZnO/Al2O3 catalysts have been used industrially since the mid-1960s for both the water-gas shift (WGS) reaction and methanol synthesis. Methanol produced from CO2 and H2 is believed to play a key role as an energy storage molecule [1]. The catalyst is activated by reduction in situ, which promotes the formation of a partially reduced ZnOx overlayer over Cu NPs through the strong metal-support interaction (SMSI). The SMSI has been observed in the spent catalyst after reaction [2], but has so far scarcely been investigated operando. Materials and Methods: The catalyst (Cu–ZnO/Al2O3) was synthesized by calcination of a zincian malachite precursor with a Cu:Zn ratio of 70:30 and 3 mol % Al, following a protocol published previously [3]. A sample was mounted in a DENS Climate environmental TEM holder, which was connected to our homebuilt gas feed setup. Selected area electron diffraction (SAED) temperature series were corrected for astigmatism and centered with sub-pixel accuracy[4], enabling us to track small lattice parameter changes and phase fractions from Rietveld refinement with the fast time resolution of SAED. Results: The calcined sample is first activated (pH2 = 79 mbar, 10% H2, 4K.min-1) during a 2-step process involving the segregation of CuO NPs from a (Cu,Zn) carbonate phase from 110C to 200C, then reduction and sintering of these NPs from CuII to Cu0. A CuI intermediate is detected as well between 200C and 250C, with all 3 oxidation states co-existing at these temperatures until only Cu0 remains after 2h at 250C. The SMSI ZnOx overlayer only partially covers few particles after the reduction is complete after 2h at 250C. Cooling the sample to 50C leads to a much fuller overlayer on a much larger NPs population, indicating that the SMSI overlayer is not stable at high temperatures and forms reversibly. This is compounded by results in MeOH synthesis conditions (H2:CO2:He = 3:1:0.5) and reverse water gas shift conditions (H2:CO2:He = 1:1:0.5), shedding tremendous insight into how Cu wetting by ZnOx is mediated by temperature as well as H2 and CO2 partial pressures (Figure 1.a) through a delicate balance of Cu-Zn alloy formation and reoxidation on Cu NPs surfaces [5]. Increasing the temperature to 400C showed a sudden increase of the Cu lattice parameter, which then leveled off to the expected Cu lattice parameter over 30 minutes, demonstrating the potential of operando SAED to track alloying and reoxidation in real time and revealing the time constant of these phenomena (Figure 1.b). Figure 1. a) Evolution of the catalyst throughout activation and operating conditions. Cooling after reduction leads to a full ZnOx coverage on the Cu NP. This overlayer is thicker and more stable as the CO2 fraction increases disappears reversibly as the temperature increases. b) Evolution of the Cu lattice parameter in RWGS conditions. The increase and decrease corresponds to Cu-Zn and re-oxidation. References A. Olah, A. Goeppert, G. K. Surya Prakash, in "Beyond Oil and Gas: The Methanol Economy", Wiley-VCH, Weinheim, 2006. Lunkenbein, T., Schumann, J. et al., Angewandte Chemie54, 15 (2015). Schumann, J., Lunkenbein, T. et al. ChemCatChem6, 10 (2014). Frisch, B., Wu, M., Ultramicroscopy 235 (2022) Amann, P., Klötzer, B. et al., Science 376, 6593 (2022).

copernicum

MS 1b

Energy-related materials and catalysts II

Materials science

MS 1b
Energy-related materials and catalysts II

Appointment

Session chairs

Programme

MS 1bEnergy-related materials and catalysts II

Appointment

Session chairs

Programme

MS 1b
Energy-related materials and catalysts II