Detailed insights at BESSY II into the oxidation processes of copper

Before pure copper oxide builds up, complex superstructures such as ‘29’Cu_xO, are formed. The new results on the '29' Cu_xO superstructure have relevance for catalyst research and corrosion protection in the development of safe copper containers for nuclear waste repositories.

Copper roofs lose their reddish colour over time because a pale green layer of copper oxide forms on their surface, protecting the roof from further corrosion for many years. But what exactly happens during the first steps of this process? Might it even be possible to stop it? This question has become increasingly urgent since copper containers have come to be regarded as one of the best options for the storage of nuclear waste. 

Steps to oxidation

Before pure copper oxide builds up, complex superstructures consisting of copper and oxygen atoms, such as ‘29’Cu_xO, are formed. The oxidation state of copper in this complex compound has been highly controversial and could not be measured directly, since conventional spectroscopic methods were barely able to detect any differences between pure copper and the partially oxidised surface. Now, a team led by Professor Dr Alexander Föhlisch has succeeded in doing so by using Auger photoelectron coincidence spectroscopy (APECS), at BESSY II. This extremely surface-sensitive method allowed for a clear distinction between the chemical states.

Surprising insights at BESSY II

The result: copper atoms in the well-known ‘29’ surface oxide are very close to the metallic state; their oxidation state is only around 0.3. This means that the surface resembles metallic copper much more closely than copper oxide (Cu₂O), even though it contains a great deal of oxygen. “This is surprising and truly remarkable, because the oxygen concentration at the surfaces is very high,” says Swarnshikha Sinha, who carried out the measurements. “We have also identified the distribution of oxygen species that make up the protective surface oxide,” she points out. Their study demonstrates that the amount of oxygen on a surface does not automatically determine the extent to which the copper atoms are oxidized.

Catalysts and copper containers

These insights are helpful for two different fields of application. Copper is used as a catalyst, for example, in CO₂ reduction, methanol synthesis or oxidation reactions. Often, the active sites are not fully oxidized CuO,  Cu₂O or pure metal, but partially oxidized surfaces. If these surfaces are electronically almost metallic, this explains why they often exhibit different activity and selectivity compared to pure oxides. This knowledge could boost the targeted development of catalysts.

The second area of application is a more precise understanding of corrosion processes on copper surfaces. This is particularly relevant to the development of safe copper containers for the final disposal of spent nuclear fuel elements.

Uppsala-Berlin Joint Laboratory

All experiments for this work were carried out within the framework of Uppsala–Berlin Joint Laboratory at the UE52 PGM beamline (CoESCA endstation) of BESSY II. The first principle calculations were performed with help of collaborators at Stockholm University, Sweden.

“Under the flagship of Uppsala–Berlin Joint Laboratory (UBjL), we were able to combine modern synchrotron spectroscopy techniques at BESSY II with theoretical methods to uncover chemical information that is not accessible using conventional methods,” says Föhlisch.