How Acidic Are Atoms?

The degree of acidity or alkalinity of a substance is crucial for its chemical behavior. The decisive–factor is so-called proton affinity, which indicates how easily an entity accepts or releases one proton. While it’s easy to calculate this for molecules, it’s not been possible for surfaces. this is often important because atoms on surfaces have very different proton affinities, depend on where they sit. Researchers at TU Wien have now succeeded in making this important physical quantity experimentally accessible for 1st time: using a specially modified atomic force microscope, it’s possible to review the proton affinity of individual atoms. this could help to analyse catalysts on an atomic scale. The results are published within the scientific journal Nature.

Precision rather than average

“All previous measurements of surface acidity had one severe drawback,” says Prof. Ulrike Diebold from the Institute of Applied Physics at TU Wien. “Although the surface atoms behave chemically differently, one could only ever measure the avg. value.”

Thus it’s not known which atoms contributed to chemical reactions, and to what extent, which makes it impossible to regulate the atomic scale of the surface to favor certain chemical reactions. But that’s exactly what’s needed, for instance , when trying to find effective catalysts for hydrogen production.

“We analyzed surfaces made from indium oxide. they’re particularly interesting because there are 5 differing types of OH groups with different properties on the surface,” says Margareta Wagner, who administered these measurements in Prof. Diebold’s lab.

With a special trick it had been possible to review these OH groups individually: The researchers placed one OH group at the tip of an atomic force microscope. This tip was then positioned specifically over one particular atom on the surface. A force then acts between the OH group on the tip and therefore the OH group directly below it on the indium oxide surface, and this force depends sensitively on the space between them.

“We vary the space between the tip and therefore the surface and measure how this changes the force,” explains Margareta Wagner. “This gives us a characteristic force curve for every OH group on the surface of a material.” shape of this force curve provides information about how well the respective oxygen atoms on the indium oxide surface hold their protons — or how easily they’re going to release them.

In order to get an actual value for the proton affinity, theoretical work was necessary. This was administered by Bernd Meyer at the Friedrich-Alexander-University Erlangen-Nürnberg, Germany. In elaborate computer simulations it had been possible to point out how the force curve of the atomic force microscope are often translated in simple and precise way into those quantities that are needed in chemistry.

Nanostructure determines the quality of catalysts

“This is quite-of crucial for the further development of catalysts,” says Bernd Meyer. “We know that atoms of same type behave quite differently counting on their atomic neighbors and therefore the way they’re incorporated into the surface.” for instance , it can make an enormous difference whether the surface is perfectly smooth or has steps on an atomic scale. Atoms with a smaller number of neighbors sit at such step edges, and that they can potentially significantly improve or worsen chemical reactions.

“With our functionalized scanning force microscope tip, we could now precisely investigate such questions for the 1st time,” says Ulrike Diebold. “This means we not need to believe trial and error, but can precisely understand and improve chemical properties of surfaces.”

The findings are reported on Vienna University of Technology