Catalysis in the solid state: fluctuations open the way

The use of effective catalytic agents is what makes many technical procedures possible in the first place. Indeed, the synthesis of more than 80% of the products generated in the chemical industry requires the contribution of specific catalysts. Most of these are catalysts in the solid state and the reactions they allow occur between molecules that adsorb to their surfaces.

The specific properties of the catalyst allow the starting molecules to interact and accelerate the reaction between them, without consuming or modifying the catalyst itself. However, efficient catalysis also requires effective mixing, so that the reagents must be able to diffuse laterally to the catalyst surface to maximize the chances of undergoing the desired reaction. Under conditions used in industrial processes, however, the surface of the catalyst is usually so dense in adsorbed particles that it has not been clear how the molecules could actually diffuse. Researchers led by Professor Joost Wintterlin of the Chemistry Department of the Ludwig-Maximilian-Universitaet University (LMU) have now shown that, although the reagents actually remain trapped on the surface of the catalyst, local fluctuations in the Occupation often offers job change opportunities. New discoveries appear in the reference log Science.

In order to better understand the molecular processes involved in a solid state catalyst, Wintterlin and his colleagues used tunneling microscopy (VTS) to monitor the mobility of individual oxygen atoms. on a ruthenium catalyst (Ru) densely loaded adsorbent carbon monoxide (CO) molecules. "We chose this system because CO 2 CO 2 oxidation on platinum group metals is a well-studied model for solid state catalysis in general," explains Wintterlin. However, conventional scanning tunneling microscopy would have been unable to capture the surface dynamics of this reaction system. But the team managed to improve the rate of data acquisition, eventually reaching rates of up to 50 frames per second, enough to make videos of particle dynamics on the catalyst.

The STM images revealed that the oxygen atoms are completely surrounded by triangular cages formed by CO molecules adsorbed on the Ru catalyst surface. The video analysis showed that a single atom of oxygen can rock only between three positions formed by the interstices of the Ru atoms. "But, to our surprise, we also found that an atom can escape from its cage and suddenly begin to diffuse through the carbon monoxide matrix at a rate almost as high as it can be. he was on a completely empty surface, "said Ann. -Kathrin Henß, first author of the research paper. In collaboration with Professor Axel Groß of the Institute of Theoretical Chemistry at the University of Ulm, Munich researchers have been able to relate this phenomenon to fluctuations in the local density of CO on the surface. regions in which the molecules are more or less important. less tightly packed together. When such a fluctuation occurs in the vicinity of an oxygen atom, the latter can escape from its cage and make its way to a new position. In fact, this "gate opening mechanism" opens diffusion pathways so rapidly that the movement of oxygen atoms across the matrix is ​​not significantly impeded. This explains why they can almost always find a new binding partner for the catalyst-facilitated reaction.

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More information:
Ann-Kathrin Henß et al, Density Fluctuations as Opener for Diffusion on Cluttered Surfaces, Science (2019). DOI: 10.1126 / science.aav4143