Utrecht University: Researchers solve mystery of ‘structure-insensitive’ catalytic reactions

The team of researchers at Utrecht University, in collaboration with Technion Institute of Technology, Eindhoven University, Oak Ridge National Laboratory, Stony Brook University, and the Paul Scherrer Institute, are publishing their results today in Nature Communications.

Never truly explained
Catalysts are widely used in the chemical industry to more efficiently make fuels, chemicals and materials, including plastics, coatings and pharmaceuticals. Catalysts are nanoparticles, only a millionth of a millimeter in size, that work by activating molecules on their surface. Since catalyst particles are so small, their performance is highly dependent on their shape: a very small particle has more irregular surfaces with single atoms ‘sticking out’, while larger particles have more ‘flat’ sides of atoms.

Knowing that molecules react differently on the regular versus irregular planes of atoms, one would thus expect that larger nanoparticles behave completely differently than smaller ones. However, reactions have been observed where this does not occur. “These ‘structure-insensitive’ reactions were never truly explained,” states Charlotte Vogt, first author of the paper.

Structure sensitive versus structure insensitive
Rapid restructuring
The researchers used a combination of advanced infrared spectroscopy, X-ray spectroscopy and electron microscopy to study a set of well-defined supported nickel catalysts and compared their activity for a structure-insensitive reaction, turning ethene into ethane, and a structure-sensitive one: turning carbon dioxide into methane.

In the supposedly structure-insensitive reaction, they observed a rapid surface restructuring of the metal nanoparticle. This restructuring happens faster as well as more pronounced for larger metal nanoparticles than for smaller ones. The most likely explanation for empirically observed structure insensitivity is thus not the absence of sensitivity to geometrical and electronic effects, which explains structure sensitivity, but the fact that the metal surface rapidly changes as a function of particle size leaving only very specific sites exposed. Therefore, the researchers propose to replace the term ‘structure insensitivity’ with ‘apparent structure insensitivity’.

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