Selected presentation abstracts, with an informal introduction from the speakers.
Diamond I18 / DESY P06
The overall aim of this work was to investigate a hierarchically structured bifunctional catalyst, used in one-step synthesis of DME from syngas. DME is a valuable chemical product and precursor, and also has applications as an alternative fuel source.
Quite often as Chemists, we focus more on the products and yield coming from a catalytic reaction, and less on how the material is actually working. To this end, in situ spectroscopy and microscopy offer a look inside the 'black box' which typifies many catalytic processes. By observing catalysts in their normally applied form and under realistic operating conditions, we can gain useful insights into their mechanism and active site.
Although synchrotron-based imaging has achieved extremely high resolution in 2D, this is often limited to relatively small fields of view, or thin slices of catalyst. On the other hand, 3D rendering of structures using tomography is non-invasive, allowing us to visualise complete catalyst particles in their real form and without any abstract sample preparation. Even more interesting is the ability to monitor the catalysts in situ, producing images of the working material, and offering unprecedented insight into the catalyst's function and mechanism. In situ tomography applied to catalysis is a developing field, which will no doubt become more prominent as the applications and results of such analysis are explored further.
One of the big issues with X-ray microscopy in general is the long acquisition times which can be required to obtain detailed and high resolution images. The development of next generation light sources in Europe in the coming years will certainly have a big impact on this technology, pushing the boundaries of resolution, acquisition time and image quality to new heights. This is good not only for those of us who like to see high quality results quickly, but also for us who don't like working through too many graveyard shifts during beamtime.
The experiments in this study were mainly performed at the I18 beamline of Diamond, using a newly developed reaction cell for in situ tomo. Despite many broken quartz capillaries and samples gaining their freedom up the ventilation of the fume cupboard, we were very pleased with the results and the possibilities enabled by this useful setup.
Top: Hierarchical core-shell structure and bifunctional catalyst operation of CuZnO/Al2O3@ZSM-5; Bottom: Orthographic cut of 3D CuZnO core structure resolved by XRF-C
IN SITU X-RAY MICROSCOPY IN CATALYSIS: DIRECT SYNTHESIS OF DME FROM SYNGAS ON A CORE-SHELL STRUCTURE
T. Sheppard1,2, S. Baier2, F. Benzi2, S. Price3, M. Klumpp4, W. Schwieger4, J.-D. Grunwaldt1,2
1. Institute of Catalysis Research and Technology, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany
2. Institute for Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology (KIT), Engesserstr. 20, D-76131 Karlsruhe, Germany
3. Science Division, Diamond Light Source, Didcot, Oxon, OX11 0DE, United Kingdom
4. Institute of Chemical Reaction Engineering, University Erlangen-Nuremberg (FAU), Egerlandstr. 3, D-91058 Erlangen, Germany
Dimethyl Ether (DME) is a promising alternative fuel and an environmentally friendly substitute for diesel. DME is normally synthesised in a multi-step process : biomass or fossil fuel derived syngas (CO + H2) is used to form methanol, followed by dehydration to DME. Recently, Cu/Zn oxides mixed with acidic supports like zeolites have proven to be effective catalysts for DME synthesis. Furthermore, composite core-shell structures (copper-zinc oxide core, zeolite shell) act as bifunctional materials, which are capable of converting syngas to DME via methanol in a single step, thus improving yield and selectivity .
At first the core-shell structure of CuO/ZnO/Al2O3@ZSM-5 was identified using ex situ XRD-CT and XRF-CT (Figure 1). In a next step information on the behaviour of the catalyst, and particularly the stability of the functional core-shell under reaction conditions is required. Understanding the relationship between structure and function is critical to the iterative process of designing and optimising future catalysts . In situ studies of active catalysts under operating conditions are therefore an attractive option. In particular, X-ray tomography at modern synchrotron radiation sources offers a unique, non-invasive approach to structural analysis on the micro- and nanoscale, allowing a combination of sample measurement techniques , which can also be applied in situ .
Here we present an ex situ and in situ tomography study on a novel bifunctional CuO/ZnO/Al2O33@ZSM-5 core-shell catalyst, used for direct synthesis of DME from syngas. Measurements were performed at beamline I18 of Diamond Light Source, offering an in situ setup consisting of a hot air blower and gas dosing system. Elemental mapping was provided by fluorescence detection (XRF), while phase information and differentiation between the metal oxide core and zeolite shell was performed by diffraction (XRD). Tomographic reconstruction was performed using in-house software. Tomograms were obtained on single catalyst grains (~100 μm diameter, 5 μm thick shell) under ambient conditions, then at 250 °C in reducing atmosphere (H2) and DME synthesis conditions (H2:CO:CO2 16:8:1). Changes in the structure and stability of the core-shell under operating conditions were of particular interest. The 3D information obtained complements previous Environmental Transmission Electron Microscopy (ETEM) and in situ hard X-ray ptychography studies on the same system, allowing correlation of the nanoscale structural changes observed to be expanded to a complete catalyst particle.
Figure 1: (left) Ex situ XRD-CT of two slices of calcined core-shell catalyst, differentiating between core and shell material; (right) XRF-CT of Cu (red) and Zn (green) species in the core.