Packing one atom in a cage within an atom framework for a new catalyst

Just like in the Russian wooden toy, a hull of 12 copper atoms encases a single tin atom. This hull is, in turn, enveloped by 20 further tin atoms. With their large surfaces these structures can serve as highly efficient catalysts. Source: TUM

A hull of twelve copper atoms encases a single tin atom. This hull is, in turn, enveloped by 20 further tin atoms. A new way for highly efficient catlysts and nanotubes with unusual symmetry was found. Professor Faessler's work group at the Institute of Inorganic Chemistry at the Technical University Munic (TUM) was the first to generate these spatial structures built up in three layers as isolated metal clusters in bronze alloys.

In the laboratory the substance is an unimpressive, fine, grayish-black powder, yet the structure models are in colour and in various nested shapes. These powders, with their large surfaces, are interesting as an interim step for catalysts that transfer hydrogen, for instance. Similar structures made of silicon could be used in solar cells to capture light from the sun more effectively.

The new bronzes from the Faessler laboratory are different. The PhD candidate Saskia Stegmaier melted a particularly pure form of copper wire and tin granulate under special conditions – protected from air and moisture in an argon atmosphere. The bronze produced in this manner was then sealed into an alkali metal such as potassium in an ampoule made of tantalum. The melting point of tantalum is 3,000 degrees Celsius, making it particularly well suited as a vessel for binging other metals into contact with each other.

This is how the new metal clusters, nested inside each other just like the Russian doll, came into existence. When bronze is heated, together with potassium or sodium, to 600 to 800 degrees Celsius, the alkali metals act like scissors that cut up the alloy grid and then edge their way between the pieces, thereby stabilising the isolated atomic clusters. On their own, these clusters cannot organize themselves into dense, uniformly structured layers to form crystals. They are made up of pentagons with 20 tin atoms in all – a constellation in which repetitive patterns are not possible under normal conditions. But "cheating" a little and using potassium atoms as glue can produce a seemingly normal crystal.

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