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Chemie - Ingenieur - Technik 94(11), 1591 - 1610 (2022). doi:10.1002/cite.202200082 special issue: "Katalyse", The structure and function of solid catalysts are inseparably linked at length scales from nm to cm and beyond. Hard X-ray tomography offers unique potential for spatially-resolved characterization by combining flexible spatial resolution with a range of chemical contrasts. However, the full capabilities of hard X-ray tomography have not been widely explored in the catalysis community. This review highlights modern advances in hard X-ray tomography using synchrotron radiation. Case studies from model to technical scale illustrate the bright future of X-ray tomography in catalysis research., Published by Wiley-VCH Verl., Weinheim

There is considerable motivation in the catalysis community and chemical industry to envision a future where rational catalyst design and targeted chemical process optimization become standard. Achieving this goal for heterogeneous catalysis requires a cultural shift centered around effective research data management. The core elements of modern catalysis research are synthesis, characterization, and testing, while all can be elevated by effective collection, correlation, interoperation, and exploitation of data between disciplines and stakeholders. Here, first steps are made towards a holistic picture of an industrial Ni/Al₂O₃ reference catalyst for CO₂ methanation. A range of conventional and advanced characterization tools are applied to probe metal particle size and pore characteristics of the support, selected as crucial parameters for catalyst performance. Challenges are shown with respect to current reporting of characterization data and metadata, which ultimately influences the development and reliability of digital twins in catalysis research. Furthermore, the cooperation and combined expertise of diverse research groups from different fields is recognized as essential to deliver meaningful progress towards the digital future of catalysis research.

Advanced science 9, 2105432 (1-12) (2022). doi:10.1002/advs.202105432, The synthesis of hierarchically porous materials usually requires complex experimental procedures, often based around extensive trial and error approaches. One common synthesis strategy is the sol���gel method, although the relation between synthesis parameters, material structure and function has not been widely explored. Here, in situ 2D hard X-ray ptychography (XRP) and 3D ptychographic X-ray computed tomography (PXCT) are applied to monitor the development of hierarchical porosity in Ni/Al$_2$O$_3$ and Al$_2$O$_3$ catalysts with connected meso- and macropore networks. In situ XRP allows to follow textural changes of a dried gel Ni/Al$_2$O$_3$ sample as a function of temperature during calcination, activation and CO$_2$ methanation reaction. Complementary PXCT studies on dried gel particles of Ni/Al$_2$O$_3$ and Al$_2$O$_3$ provide quantitative information on pore structure, size distribution, and shape with 3D spatial resolution approaching 50 nm, while identical particles are imaged ex situ before and after calcination. The X-ray imaging results are correlated with N$_2$-sorption, Hg porosimetry and He pycnometry pore characterization. Hard X-ray nanotomography is highlighted to derive fine structural details including tortuosity, branching nodes, and closed pores, which are relevant in understanding transport phenomena during chemical reactions. XRP and PXCT are enabling technologies to understand complex synthesis pathways of porous materials., Published by Wiley-VCH, Weinheim

Fischer‐Tropsch synthesis: Four types of bi‐functional catalysts with cobalt nanoparticles supported on meso‐ or microporous silicates or aluminosilicates are investigated regarding the obtained CO2 and CH4 selectivity under low‐temperature Fischer‐Tropsch reaction conditions. In situ x‐ray absorption spectroscopy results under industrially relevant conditions reveal that strong cobalt‐support interactions and oxidized cobalt species are the main factors determining the selectivity depending on the specific support material used. The production of liquid hydrocarbons from syngas (CO and H2) via the combined Fischer‐Tropsch (FT) synthesis and hydroprocessing (HP) is a promising strategy to provide valuable chemicals and fuels based on renewable feedstocks. High yields of liquid products are essential for industrial implementation since short‐chain side products like methane and CO2 reduce the overall carbon efficiency, which holds true especially for bi‐functional Co/zeolite catalysts. In order to investigate the influence of the support material properties on the methane and CO2 selectivities in the combined FT and HP reaction, we synthesized four well‐defined catalyst materials with similar cobalt particle sizes. The active material is supported on either meso‐ or microporous silicates or aluminosilicates. The catalytic properties are investigated in FT experiments at industrially relevant conditions (20 bar, 200–260 °C) and correlated with in situ x‐ray absorption spectroscopy results to determine the chemical environment responsible for the selectivity observed. The origin of the high methane selectivity detected for crystalline and amorphous aluminosilicate was mainly traced back to the strong cobalt‐support interactions. The high CO2 selectivity, observed only for crystalline zeolite materials, is driven by the presence of oxidized cobalt species, while the acidic support in combination with micropores and possible overcracking leads to the observed drop in the C5+ selectivity., publishedVersion

Das Verst��ndnis der Katalysatordesaktivierung durch Verkokung ist entscheidend f��r ein wissensbasiertes Katalysator- und Prozessdesign bei Reaktionen mit Kohlenstoffverbindungen. Die Katalysatorverkokung wird dabei typischerweise durch Post-Mortem-Analyse untersucht. In der vorliegenden Arbeit wird ptychographische R��ntgentomographie (PXCT) zur Analyse von k��nstlich verkokten Ni/Al2O3-Katalysatoren f��r die CO2-Methansierung und CH4-Trockenreformierung verwendet. PXCT liefert dabei 3D-Informationen der lokalen Elektronendichte mit ca. 80 nm Aufl��sung und erm��glicht somit die Visualisierung und Untersuchung der Auspr��gung der Verkokung in Katalysatorpartikeln mit einem Durchmesser von ca. 40 ��m. Die Verkokung wurde haupts��chlich im nanopor��sen Festk��rper identifiziert und konnte nicht in den aufgel��sten Makroporen gefunden werden. Die Kohlenstoffbildung wurde unabh��ngig dazu mittels operando Raman-Spektroskopie best��tigt. PXCT wird als aufkommende Charakterisierungstechnik hervorgehoben, die eine nanoskalige Identifizierung, Lokalisierung und m��glicherweise Quantifizierung von verschiedenen Desaktivierungsph��nomenen mit 3D-Aufl��sung in kompletten Katalysatorpartikeln erm��glicht.

Catalysts 11(4), 459 (1-18) (2021). doi:10.3390/catal11040459, Structure–activity relations in heterogeneous catalysis can be revealed through in situ and operando measurements of catalysts in their active state. While hard X-ray tomography is an ideal method for non-invasive, multimodal 3D structural characterization on the micron to nm scale, performing tomography under controlled gas and temperature conditions is challenging. Here, we present a flexible sample environment for operando hard X-ray tomography at synchrotron radiation sources. The setup features are discussed, with demonstrations of operando powder X-ray diffraction tomography (XRD-CT) and energy-dispersive tomographic X-ray absorption spectroscopy (ED-XAS-CT). Catalysts for CO$_2$ methanation and partial oxidation of methane are shown as case studies. The setup can be adapted for different hard X-ray microscopy, spectroscopy, or scattering synchrotron radiation beamlines, is compatible with absorption, diffraction, fluorescence, and phase-contrast imaging, and can operate with scanning focused beam or full-field acquisition mode. We present an accessible methodology for operando hard X-ray tomography studies, which offer a unique source of 3D spatially resolved characterization data unavailable to contemporary methods., Published by MDPI, Basel

CO$_{2}$ methanation is often performed on Ni/Al$_{2}$O$_{3}$ catalysts, which can suffer from mass transport limitations and, therefore, decreased efficiency. Here we show the application of a hierarchically porous Ni/Al$_{2}$O$_{2}$ catalyst for methanation of CO$_{2}$. The material has a well-defined and connected meso- and macropore structure with a total porosity of 78%. The pore structure was thoroughly studied with conventional methods, i.e., N$_{2}$ sorption, Hg porosimetry, and He pycnometry, and advanced imaging techniques, i.e., electron tomography and ptychographic X-ray computed tomography. Tomography can quantify the pore system in a manner that is not possible using conventional porosimetry. Macrokinetic simulations were performed based on the measures obtained by porosity analysis. These show the potential benefit of enhanced mass-transfer properties of the hierarchical pore system compared to a pure mesoporous catalyst at industrially relevant conditions. Besides the investigation of the pore system, the catalyst was studied by Rietveld refinement, diffuse reflectance ultraviolet-visible (DRUV/vis) spectroscopy, and H$_{2}$-temperature programmed reduction (TPR), showing a high reduction temperature required for activation due to structural incorporation of Ni into the transition alumina. The reduced hierarchically porous Ni/Al$_{2}$O$_{3}$ catalyst is highly active in CO$_{2}$ methanation, showing comparable conversion and selectivity for CH$_{4}$ to an industrial reference catalyst.

A screening study of the catalytic performance of ternary alloy nanoparticles containing nickel, iron and gallium supported on silica for methanol synthesis from CO$_{2}$ and H$_{2}$ was performed. Catalysts were prepared by incipient wetness impregnation and subsequently reduced in H$_{2}$ before catalytic testing. Ni$_{2}$FeGa showed the best performance of the tested catalysts in terms of methanol yield. An optimization of the preparation was done to improve activity and selectivity, reaching a performance close to that of commercially available Cu/ZnO/Al$_{2}$O$_{3}$/MgO at low reaction temperatures and pressure. Extensive in situ characterisation using environmental TEM, in situ XRD and in situ EXAFS of the formation of the Ni$_{2}$FeGa catalyst explains an optimal reduction temperature of 550 °C: warm enough that the three atomic species will form an alloy while cold enough to prevent the catalyst from sintering during the formation.

Encapsulated cobalt particles live another day: Heterogeneously catalyzed reactions like the CO hydrogenation exhibit harsh conditions leading to catalyst deactivation. The cobalt particles investigated via ex situ and in situ TEM are located inside and outside the ZSM-5 support, undergoing realistic and model conditions. The encapsulated cobalt particles are found to be highly resistant towards sintering. Designing stable materials for processes operating under harsh reaction conditions, like CO hydrogenation, is a challenging topic in catalysis. These may provoke several deactivation mechanisms simultaneously, like thermal sintering, oxidation or poisoning of the active sites. We report HZSM-5 supported cobalt catalysts, exhibiting cobalt nanoparticles encapsulated inside, or located at the exterior of the ZSM-5 support. The materials were studied by a combination of ex situ and in situ electron microscopy with respect to the growth of the cobalt particles. After 1200 h time on stream under CO hydrogenation conditions, the spent catalyst showed minimal sintering of encapsulated cobalt particles. In situ environmental TEM experiments under model reduction and CO hydrogenation conditions indicate the presence of cobalt nanoparticles, which appear highly resistant towards sintering even up to 700 °C. These results provide a first indication for the preparation of sinter stable catalysts suitable for operating in harsh reaction environments., publishedVersion

Multicomponent Bi-Mo-Fe-Co oxide catalysts prepared via flame spray pyrolysis were tested for selective propylene oxidation, showing high conversion (&gt, 70%) and selectivity (&gt, 85%) for acrolein and acrylic acid at temperatures of 330 &deg, C. During extended time-on-stream tests (5&ndash, 7 days), the catalysts retained high activity while undergoing diverse structural changes. This was evident on: (a) the atomic scale, using powder X-ray diffraction, Raman spectroscopy, X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy, and (b) the microscopic scale, using synchrotron X-ray nanotomography, including full-field holotomography, scanning X-ray fluorescence, and absorption contrast imaging. On the atomic scale, sintering, coke formation, growth, and transformation of active and spectator components were observed. On the microscopic scale, the catalyst life cycle was studied at various stages through noninvasive imaging of a ~50-&micro, m grain with 100-nm resolution. Variation of catalyst synthesis parameters led to the formation of notably different structural compositions after reaction. Mobile bismuth species formed agglomerates of several hundred nanometres and segregated within the catalyst interior. This appeared to facilitate the formation of different active phases and induce selectivity for acrolein and acrylic acid. The combined multiscale approach here is generally applicable for deconvolution of complex catalyst systems. This is an important step to bridge model two-component catalysts with more relevant but complex multicomponent catalysts.