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Germany: Computation and Chemistry Catalyst Research: Molecular Probes Require Precise Calculations

Editor: Alexander Stark

Many technologies would not be possible without catalysts. To further improve heterogeneous catalysts, it is necessary to analyze the complex processes at their surface, where the active centers are located. Researchers at the Karlsruhe Institute of Technology (KIT), together with colleagues from Spain and Argentina, have made decisive progress in this area.

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Analysis of a cerium oxide catalyst with carbon monoxide probe molecules and infrared reflectance absorption spectroscopy.
Analysis of a cerium oxide catalyst with carbon monoxide probe molecules and infrared reflectance absorption spectroscopy.
(Source: IFG/KIT)

Karlsruhe/Germany — Many important technologies, for example for energy conversion, emission reduction or chemical production, only work with suitable catalysts. Therefore, highly efficient materials for heterogeneous catalysis are becoming increasingly important. In heterogeneous catalysis, the substance acting as a catalyst and the substances reacting with each other are present in different phases, for example solid and gaseous. The material compositions can already be reliably determined with the aid of various methods - the processes that take place directly at the catalyst surface, on the other hand, still elude most analytical methods. “But it is precisely the highly complex chemical processes at the very outer surface of the catalysts that are crucial,” explains Professor Christof Wöll, head of the Institute for Functional Interfaces (IFG) at KIT, “because that is where the active centers are located at which the catalyzed reaction takes place.”

Precise Analysis of the Surface of Powder Catalysts

Cerium oxides, which are compounds of the rare earth metal cerium with oxygen, are some of the most important heterogeneous catalysts. They exist as powders, consisting of nanoparticles with a controlled structure. The shape of the nanoparticles significantly affects the reactivity of the catalyst. To study the processes at the surface of such powder catalysts, researchers have for some time been using probe molecules, such as carbon monoxide molecules, that attach to the nanoparticles and measuring the samples with infrared reflectance absorption spectroscopy (IRRAS).

The infrared radiation excites molecular vibrations. Detailed information about the nature and composition of the catalytic centers can be obtained from the vibrational frequencies of the probe molecules. However, the interpretation of the experimental IRRAS data has been problematic until now. This is because, especially for the technologically relevant powder catalysts, many oscillation bands can be observed, the exact assignment of which is difficult. Theoretical calculations have not really been helpful in this respect so far, because the deviation from the experiment — even in the case of model systems - was so large that the experimentally observed oscillation bands could not be reliably assigned.

Long Computing Time — High Accuracy

Researchers at the Institute for Functional Interfaces (IFG) and the Institute for Catalysis Research and Technology (IKFT) of KIT have now identified and solved an important problem of theoretical analysis in an international cooperation with colleagues from Spain and Argentina, coordinated by Dr. M. Verónica Ganduglia-Pirovano of the Consejo Superior de Investigaciones Científicas (CSIC) in Madrid. As the researchers report in the journal Physical Review Letters, they have used systematic theoretical studies and validation of the results on model systems to show that the theoretical methods used so far have fundamental weaknesses.

Such weaknesses are generally observed in density functional theory (DFT) calculations, a method that can be used to determine the quantum mechanical ground state of a multi-electron system based on the density of electrons. As the research group discovered, the shortcomings can be overcome with so-called hybrid functionals that combine DFT and Hartree-Fock methods, an approximation technique in quantum chemistry. While this makes the calculations quite costly, it also makes them highly accurate. “It is true that the computation times required for these new methods are about a factor of 100 greater than those for conventional methods,” Wöll explains. “But this disadvantage is more than compensated by the excellent agreement with the experimental systems.” The researchers used nanoscale ceria catalysts as an example to demonstrate this advance, which can contribute significantly to making heterogeneous catalysts more effective and durable.

The results of the work also provide an important contribution to the new Collaborative Research Center (SFB) "Track Act — Tracking Active Sites in Heterogeneous Catalysts for Emission Control" at KIT.

References: Pablo G. Lustemberg, Philipp Pleßow, Yuemin Wang, Chengwu Yang, Alexei Nefedov, Felix Studt, Christof Wöll, and M. Verónica Ganduglia-Pirovano: Vibrational Frequencies of Cerium-Oxide-Bound CO: A Challenge for Conventional DFT Methods. Physical Review Letters, 2020. DOI: 10.1103/PhysRevLett.125.256101.