Molecular probes require very precise calculations

Catalysts are essential for many technologies. To further improve heterogeneous catalysts, it is necessary to analyze the complex processes at their surfaces, where the active sites are. Scientists at the Karlsruhe Institute of Technology (KIT), as well as colleagues from Spain and Argentina, have now achieved breakthroughs: as reported in Physical Review Letters, they use calculation methods with so-called functions hybrids for reliable interpretation of experimental data.

Many important technologies, such as processes for converting energy, reducing emissions or producing chemicals, only work with the right catalysts. For this reason, highly efficient materials for heterogeneous catalysis are gaining in importance. In heterogeneous catalysis, the material acting as a catalyst and the reacting substances exist in different phases as a solid or a gas, for example. Compositions of materials can be reliably determined by various methods. However, the processes taking place on the surface of the catalyst cannot be detected by virtually any analytical method. “But it is these very complex chemical processes on the outermost surface of the catalyst that are of decisive importance,” says Professor Christof Wöll, director of the Institute for Functional Interfaces (IFG) at KIT. “There are the active sites, where the catalyzed reaction takes place. “

Precise examination of the surface of powder catalysts

Among the most important heterogeneous catalysts are cerium oxides, i.e. compounds of the rare earth metal cerium with oxygen. They exist in powder form and consist of nanoparticles of controlled structure. The shape of the nanoparticles considerably influences the reactivity of the catalyst. To study the processes on the surface of these powdered catalysts, researchers have recently started using probe molecules, such as carbon monoxide molecules, which bind to nanoparticles. These probes are then measured by infrared reflection absorption spectroscopy (IRRAS). Infrared radiation causes molecules to vibrate. From the vibration frequencies of the probe molecules, detailed information can be obtained on the type and composition of the catalytic sites. Until now, however, the interpretation of experimental IRRAS data has been very difficult, as technologically relevant powder catalysts have many vibration bands, the exact attribution of which is difficult. Theoretical calculations were of no help, as the deviation from experience, also in the case of model systems, was so large that the vibration bands observed experimentally could not be accurately assigned. .

Long computing time – High precision

Researchers from the Institute of Functional Interfaces (IFG) and the Institute for Research and Technology in Catalysis (IKFT) of KIT, in cooperation with colleagues from Spain and Argentina coordinated by Dr M. Verónica Ganduglia- Pirovano of the Consejo Superior de Investigaciones Científicas (CSIC) in Madrid, have now identified and solved a major problem of theoretical analysis. As noted in Physical Review Letters, systematic theoretical studies and validation of results using model systems have revealed that the theoretical methods used so far have fundamental weaknesses. In general, such weaknesses can be observed in calculations using Density Functional Theory (DFT), a method by which the basic state of quantum mechanics of a multielectronic system can be determined based on the density of electrons. The researchers found that the weaknesses can be overcome with so-called hybrid functions that combine DFT with the Hartree-Fock method, an approximation method in quantum chemistry. This makes the calculations very complex, but also very precise. “The calculation times required by these new methods are longer by a factor of 100 than for conventional methods,” explains Christof Wöll. “But this disadvantage is more than compensated by the excellent agreement with the experimental systems.” Using nanoscale cerium oxide catalysts, the researchers demonstrated these advancements that could help make heterogeneous catalysts more efficient and durable.

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