How catalysts age
PSI researchers have developed a new tomography method with which they can measure chemical properties inside catalyst materials in 3D extremely accurately and faster than before. The application is equally important for research and industry.
The vanadium phosphorus oxide (VPO) group of materials is widely used as a catalyst in the chemical industry. Since as early as the 1970s, VPO have been used in the production of maleic anhydride; this in turn is the starting material for the manufacture of certain types of plastics, including increasingly biodegradable plastics. In industry, catalytic materials are used for several years because, although they play an important role in the course of chemical reactions, they are not themselves consumed in the process. Nevertheless, the VPO catalyst changes over time as a result of this use.
Researchers from two units of the Paul Scherrer Institute (PSI) - the Photon Research Division and the Energy and Environment Division - have now collaborated with ETH Zurich and the Swiss company Clariant AG to investigate the aging process of VPO in detail - and in the process have also developed a new experimental method.
Two methods...
Clariant AG is a global leader in specialty chemicals. Clariant provided PSI with two samples: First, a sample of previously unused VPO, and second, VPO that had been used as a catalyst in industrial operations for four years. It had long been known that VPO changes with the years of use and slightly loses its desired properties. However, it had not yet been fully clarified which processes in the nanostructure and at the atomic level were responsible for this.
The PSI researchers investigated this question using state-of-the-art material characterization methods. To make the chemical structure of the samples visible on the nanoscale, they combined two methods: First, a particular tomography method previously developed at PSI called ptychographic X-ray computed tomography, which uses X-rays from the Swiss Synchrotron Light Source SLS and can non-destructively image the interior of the sample in 3D and at nano-resolution. Second, the researchers added a local transmission spectroscopy method that additionally revealed the chemical properties of the material in each volume element of the tomograms.
"Basically, we collected 4-dimensional data," explains Johannes Ihli, a researcher at PSI and one of the study authors: "We reconstructed a high-resolution 3D representation of our sample, where the individual volume elements - called voxels - have an edge length of only 26 nanometers. In addition, we have a quantitative X-ray transmission spectrum for each of these voxels, the analysis of which tells us the chemistry right there."
Using these spectra, the researchers determined some of the most basic chemical quantities for each voxel: The electron density, the vanadium concentration and the degree of oxidation of the vanadium. Since the VPO catalysts studied are a so-called heterogeneous material, these quantities change at different scales throughout the sample volume. This in turn determines or limits the performance of the catalyst material.
... and a new algorithm
The step-by-step procedure to obtain this data was to first measure the sample for a 2D projection image, then rotate it a tiny bit, measure it again, and so on. This process was then repeated at different energies. Using the previous method, this would have required about fifty thousand individual 2D images, which would have been assembled into about a hundred tomograms. For each of the two samples, this would have meant about a week of pure measurement time.
"The experimental stations at the SLS are in great demand and booked up around the year," explains Manuel Guizar-Sicairos, also a PSI researcher and head of this study. "So we can't afford to make measurements that take that long." Data collection had to become more efficient.
Zirui Gao, first author of the study, achieved this in the form of a new principle of data acquisition and an associated reconstruction algorithm. "For 3D reconstruction of tomograms, you need images from many angles," Gao explains. "But our new algorithm manages to extract the required amount of information even if you increase the distance between angles about tenfold - that is, take only about one-tenth of the 2D images." In this way, the researchers succeeded in obtaining the required data in only about two days of measurement time and consequently saved a lot of time and thus also costs.
Larger pores and missing atoms
The measurements of the two samples showed: As expected, the fresh VPO had many small pores evenly distributed throughout the material. These pores are important because they provide the surface on which catalysis can take place. In contrast, in the VPO sample that had been in use for four years, the structure had remodeled on the nanoscale: There were larger voids and fewer voids. The material in between showed larger, elongated crystalline shapes.
Changes were also evident at the molecular level: Over time, voids, also called holes, had appeared in the atomic lattice. Their existence had previously only been suspected. With their newly obtained knowledge of sample chemistry on the nanoscale, the researchers were now able to confirm these holes and determine their exact location: at the site of certain vanadium atoms that are now missing. "That the relative content of vanadium decreases with time was already known before," Gao said. "But we have now been able to show for the first time at which position in the crystal lattice these atoms are missing. Together with our other results, this confirms a previous assumption: namely, that these missing sites in the atomic lattice can serve as additional active sites for the process of catalysis."
This means that these increasing voids are a welcome effect: They increase catalytic activity, at least partially counteracting the loss of activity that occurs when the number of pores decreases. "Our new, detailed results could help industrial companies optimize their catalysts and make them more durable," Gao says.
Original publication: Sparse ab initio X-ray transmission spectro-tomography for nanoscopic compositional analysis of functional materials; Z. Gao, M. Odstrcil, S. Böcklein, D. Palagin, M. Holler, D. Ferreira Sanchez, F. Krumeich, A. Menzel, M. Stampanoni, G. Mestl, J.A. van Bokhoven, M. Guizar-Sicairos, J. Ihli.
Science Advances June 9, 2021 (online).
DOI: 10.1126/sciadv.abf6971
