Live view into catalyst materials shows changes in pore structure
A new X-ray study offers unprecedented possibilities for analysing catalysts and other materials with a porous structure. The work marks an important step towards tailor-made porous materials and can thus contribute to saving emissions and raw materials in a more sustainable chemistry, as the team led by Thomas Sheppard from the Karlsruhe Institute of Technology (KIT) reports in the journal Advanced Science. Researchers from the Paul Scherrer Institute in Switzerland, the MAX IV research centre in Sweden, the universities of Hamburg and Leipzig, KIT and DESY are involved in the study.
Porous materials have a wide variety of technical applications and are omnipresent in nature. The 3D pore structure of these materials can be compared with a sponge. The pore system plays a major role in determining the physical properties of the material, and provides a large surface area on which adsorption or chemical reactions can occur. For example, some technical catalysts or adsorption materials can have approximately the same surface area as a football field in just one gram of catalyst.
In addition to the large surface area, the diameter of the pores and their interconnection into a larger network have a decisive role on the physicochemical properties of the material. In particular the pore network controls the transport of molecules within the pore system, which is referred to as mass transport. The mass transport properties of a catalyst are crucial for its performance and an important design parameter in the search for better catalysts, especially in industrial applications.
As the majority of all industrial chemical processes involve a catalyst at some stage during production, already small improvements in efficiency have a big influence on, e.g., the required process energy and linked CO2 emissions, or the formation of undesired secondary products. Therefore, the optimization of the pore structure can provide an important contribution to more sustainable chemical synthesis and processes.
However, the preparation of materials with adapted pore structures often requires complex synthesis methods, which are often based on experience and 'trial and error' approaches, and are in general not fully understood. Additionally, changes of the pore structure under reaction conditions can usually only be studied in comparison to the initial and final state, but not during the reaction as the material performs its chemical function. However, to optimize the pore system in a knowledge-based way, it is important to gain access to information of the changes during synthesis or reaction, which is currently limited by conventional and established methods.
The use of modern hard X-ray microscopy methods at synchrotron radiation sources like DESY's PETRA III is one such method to study complex pore structures, which typically exist on length scales from nanometres (nm; millionths of a millimetre) to micrometres (µm; thousandths of a millimetre). One of these modern methods is called X-ray ptychography, which can be performed in 2D as X-ray nanomicroscopy, or in 3D as X-ray nanotomography, at synchrotron radiation sources. X-ray ptychography is characterized by high resolution and routinely allows spatial resolutions of 50 nm with sample diameters of about 50 µm. In addition, tomography can be used to obtain a quantitative 3D image of the electron density of a sample.
This technique was used to study the evolution of the pore structure of a catalyst for chemical energy storage from CO2 with a tailored pore structure during calcination. Calcination is typically the final step in the synthesis of porous materials. For this purpose, in situ 2D X-ray nanomicroscopy experiments were first performed together with researchers at the DESY in Hamburg. This involved the use of a nanoreactor in which the temperature and gas atmosphere can be controlled. In these experiments, the change of the catalyst from room temperature to 800 °C and different gas conditions could be followed almost in real time. In situ 2D X-ray nanomicroscopy allows to identify morphological changes. However, since the pore system and the sample have a 3D structure, the quantification of the changes is limited in 2D.
Therefore, to quantify the change of the pore system, complementary X-ray nanotomography studies were performed in collaboration with researchers at the Paul Scherrer Institut (PSI) in Switzerland. Identical particles of the catalyst were examined before and after calcination. The obtained tomograms, which are essentially 3D digital models of the sample structure, were then used to describe the pore structure of the catalysts and thus quantify the changes that were also observed in the in situ experiments.
In principle, the tomography experiments can also be performed in the nanoreactor used for the in situ experiments, but this is currently limited by the required experimental time. The obtained information from the analysis of the tomograms can be directly applied in complex simulations and modelling of the mass transport properties. This is an important basis for a future optimization of the pore structure in such materials.
The developed method of X-ray nanoimaging offers the potential to obtain a much more detailed understanding of the evolution of pore structures than was previously possible using conventional methods such as porosimetry or sorption studies. This is relevant for the chemical industry in the development of new catalysts, but also in the field of adsorption materials, membranes, insulation materials or batteries, where detailed information about the 3D structure are important. The methods shown bring us one step closer to the ultimate goal of being able to design and tailor a catalyst from scratch for a specific process. This can make a significant contribution to save emissions and resources, and thus supports a more sustainable chemistry.