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oxidation at more cathodic potentials as observed with XAS. It is therefore noteworthy that the Au particles
used in the study by Kuo et al. have an average size of 4 nm,33 which are likely to contain less undercoordinated
sites. These observations indirectly indicate that Au terrace sites are important for the interaction with Mn
oxides. From the theoretical model proposed in 30 *OH adsorbed to the terrace sites of bulk Au(111) was used
as proton acceptor. A particle size dependency on the activity could suggest that these sites are indeed
important and play a role in the reaction mechanism.
However, to fully establish such an effect more specific measurements much be carried out. From these
measurements it is, however, clear that gold promotes the activity towards OER and leads to higher oxidation
state of Mn under reaction conditions. In future studies it would also be of great interest to mimic this
interaction and beneficial effect using a non-noble metal activity promoter instead of Au.
4. Conclusion
In this study we have investigated mixed Au-MnOx films as catalysts for the oxygen evolution reaction. Films
with 30 % Au exhibited modest increase in current density over the pure Mn3O4 film . At 50 % Au more than
five times higher current densities were measured. From X-ray Diffraction measurements it was found that Au
forms particles in the MnOx matrix, with sizes dependent on the Au concentration. At 30 % the crystallite size
was approximately 2 nm, while at 50 % Au it increased to 3 nm. The films were studied with in-situ X-ray
Absorption where the Mn K-edge and Au LIII-edge were probed as a function of electrochemical potential. At
increasingly anodic potentials all films showed a shift of the Mn K-edge towards higher photon energies.
However, the films with Au showed a significantly larger shift, indicating that the Mn atoms on average reach a
higher oxidation state. The Au LIII-edge revealed that at a low Au concentration, 30 %, significant oxidation
could be seen, which was not the case at 50 %. The combined study suggests that a beneficial interaction
between Mn oxide and Au depends on more than just number of neighboring Mn-Au sites. It is likely that the
type of Au sites available next to active Mn sites has a strong impact on the increase in OER activity.
Acknowledgements
The authors gratefully acknowledge financial support from the Danish Ministry of Science’s UNIK initiative,
Catalysis for Sustainable Energy. The Center for Individual Nanoparticle Functionality is supported by the
Danish National Research Foundation (DNRF54).
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