PSI researchers have developed a new method of tomography with which they can measure chemical properties inside catalyst materials in 3D extremely accurately and faster than ever before. The app is equally important for science and industry. The researchers published their findings today in the journal Scientists progress.
The vanadium phosphorus oxides (VPO) material group is widely used as a catalyst in the chemical industry. OPV has been used in the production of maleic anhydride since the 1970s. Maleic anhydride is in turn the starting material for the production of various plastics, including increasingly biodegradable ones. In industry, catalytic materials are usually used for several years, as they play an important role in chemical reactions but are not consumed in the process. Nevertheless, an VPO catalyst evolves over time following this use.
In a collaborative effort, scientists from two research divisions of the Paul Scherrer Institute PSI – the Photon Science Division and the Energy and Environment Division – as well as researchers from ETH Zurich and the Swiss company Clariant AG, have now studied in detail the aging process of VPO catalysts. During their research, they also developed a new experimental method.
Two methods …
Clariant AG is one of the world leaders in specialty chemicals. Clariant provided PSI with two samples: first, a sample of previously unused VPO catalyst; and second, a sample of VPO catalyst that had been used in industrial operations for four years. It has long been known that OPVs change over years of use and exhibit a slight loss of desired properties. Until now, however, it was not entirely clear which processes in the nanostructure and at the atomic scale were responsible for the observed decrease in performance.
PSI researchers have investigated this question with state-of-the-art material characterization techniques. To make the chemical structure of the samples visible at the nanoscale, they combined two methods: the first was a specific tomography method previously developed at PSI called ptychographic x-ray computed tomography, which uses x-rays from the Swiss light source SLS. and can non-destructively image the inside of the sample in 3D and with nanometric resolution. To this, on the other hand, the researchers added a method of local transmission spectroscopy which further revealed the chemical properties of the material in each volume element of the tomograms.
“Basically, we collected data in four dimensions,” explains Johannes Ihli, a researcher at PSI and one of the study’s authors: “We reconstructed a high-resolution 3D representation of our sample in which the volume elements Individuals – 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 about the local chemistry. “
These spectra have allowed scientists to determine some of the most basic chemical quantities for each voxel. These included the electron density, the concentration of vanadium and the degree of oxidation of the vanadium. Since the OPV catalysts examined are a so-called heterogeneous material, these quantities change at different scales throughout its volume. This in turn defines or limits the functional performance of the material.
… and a new algorithm
The step-by-step procedure to get this data was to measure the sample for a 2D projection image, then rotate it a little bit, measure again, etc. This process was then repeated at various other energies. With the previous method, around fifty thousand individual 2D images would have been necessary, and these would have been combined into around one hundred tomograms. For each of the two samples, this would have meant about a week of pure measurement time.
“The SLS experimental stations are in great demand and full all year round,” explains Manuel Guizar-Sicairos, also a researcher at PSI and principal investigator of this study. “So we cannot afford to take such long measurements.” Data collection needed to become more efficient.
Zirui Gao, lead 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 multiple angles,” Gao explains. “But our new algorithm manages to extract the required amount of information even if you increase the distance between the angles by about ten times, that is, if you only take about a tenth of the 2D images.” In this way, the researchers managed to obtain the required data in just about two measuring days, thus saving a lot of time and therefore also costs.
Bigger pores and missing atoms
This is what the measurements of the two samples showed: As expected, the fresh OPV had many small pores that were evenly distributed throughout the material. These pores are important because they provide the surface on which catalysis can take place. In contrast, the structure of the OPV sample that had been used for four years had changed at the nanoscale. There were larger and fewer cavities. The material between them showed larger and elongated crystalline shapes.
Changes were also found at the molecular level: Over time, voids, also called holes, had appeared in the atomic lattice. Their existence was previously only suspected. With the chemical information acquired at the nanoscale, the researchers were now able to confirm this hypothesis and also show exactly where the voids were: at the location of specific vanadium atoms that were now missing. “The fact that the relative vanadium content decreases over time was already known,” Gao explains. “But we have now been able to show for the first time how much of the crystal lattice these atoms are missing. Along with our other findings, this confirms the previous hypothesis that these holes in the atomic lattice may serve as additional active sites for the process. of catalysis. “
This also implies that the increase in these imperfections is a welcome effect: they reinforce the catalytic activity and thus at least partially compensate for the loss of activity caused by the reduction in the number of pores. “Our new detailed results could help industrial companies optimize their catalysts and make them more sustainable,” Gao said.