Fakultät für Chemie und Pharmazie

Binding of atoms: Repulsion is followed by attraction

What happens, for example, in the exhaust gas purification in the catalytic converter? How come less from the exhaust? Physicists study basic mechanisms of adsorption. Photo: imago images / xbizoo_nx Panthermedia

A team of physicists and quantum chemists is the first to measure the transition from a slight atomic bond to a strong one.

 

Atoms are the elementary building blocks of matter. The American physicist and Nobel laureate Richard Feynman once explained their meaning in his basic textbook as follows: "... all things are made up of atoms - small particles that are constantly moving, dressing when distanced from each other but repelled when you press them together ". The atomic bonds can exist with different equilibrium distances, as the physicist John Lennard-Jones has already established in 1932: There is a slight bond, called physisorption, and a strong chemical bond, called chemisorption. The former is responsible, for example, for the adhesion of dust to surfaces. The second is ten to a hundred times stronger. The interplay of these types of adsorption is important for the course of chemical reactions on surfaces, for example, in the exhaust gas purification in the autocatalyst or in the catalytic recovery of chemical feedstocks. The existence of these two types of adsorption is represented by an energy curve with two minima. Such graphics have been printed for decades in the textbooks of physical chemistry and surface physics, although they have been postulated only theoretically so far.

For the first time, a group led by Professor Franz J. Giessibl (Experimental Physics) at the University of Regensburg has succeeded in directly measuring the transition from physisorption to chemisorption. They did this by attaching a carbon monoxide molecule to the top of their atomic force microscope, approximating it to a single iron atom on a copper surface, and recording the force evolution as it approached. They were supported by a team of quantum chemists from LMU: Professor Hubert Ebert, Dr. Sergiy Mankovsky and Svitlana Polyesa. The LMU researchers contributed the theoretical explanation to the experiment: Overcoming the potential barrier after physisorption requires a so-called hybridization, which was explained and proved in the quantum-chemical calculations.
Science 2019