6.1 Theoretical model of Au-MnOx interaction 99
by ex-situ X-ray Absorption Near Edge Spectroscopy, XANES, measurements,
where they observed a small shift of the Mn K-edge threshold towards lower energies
when Au is present. Such an energy shift would be consistent with a small
decrease of the average oxidation state of Mn. However, from DFT calculations
and the Pourbaix diagram of Mn in aqueous environment, it is not expected that
a high concentration of Mn3+ can be sustained at potentials above 1.4 VRHE
where MnO2 is the most stable oxide 131, 138. At the same time the binding
energy of O to Mn2O3 is too strong so that the formation of OOH requires
a large overpotential 98, 131. On MnO2 the binding energies are actually distributed
a bit better for oxygen evolution.
For Ni and Co hydroxides supported on gold another explanation was proposed
by Yeo et al. based on charge transfer from the transition metal to the noble
metal 229, 230. Au is a very electronegative metal and therefore it is feasible
that such a charge transfer could occur. Nickel based hydroxides were studied
in 230, where the authors observed that submonolayer thin lms supported on
Au were signicantly more active than when it was supported on Pd. In their
discussion it was argued that the Au facilitated oxidation of the Ni. For Co
oxides, a series of noble metal substrates were tested and it was again found
that using a Au substrate resulted in the highest activity 229. The authors
argued that a 4+ state of Co was important for the OER activity and that Au
facilitated the oxidation of the Co sites. Similar eects could be valid for MnOx.
However, if the early oxidation to 4+ is critical it is surprising that MnO2 is not
signicantly outperforming lower oxides 162. Furthermore, the MnO2 catalyst
reported by Kuo et al. can still be improved with Au particles. The combined
studies at this point therefore suggests that Au assumes a role beyond charge
transfer.
In a study on RuO2 doped with Ni or Co, an explanation for the improved
activity has been proposed on the basis of proton transfer from OOH at the
catalytic site to a nearby proton acceptor 108. For RuO2 a lower overpotential
could be realized if the OOH intermediate was bound a bit stronger to the
surface. If a proton acceptor is present at the surface, right next to the active
catalytic site, it is possible that the OOH intermediate would be formed with
a larger probability due to instant transfer of H and conversion into an oxygen
molecule that can leave the surface. For RuO2, oxidised Ni or Co atoms placed in
a bridging position on the rutile (110) surface could function as proton acceptors
108. Similarly to RuO2, MnO2 would benet from such a proton transfer
mechanism, since the OOH step is potential determining. In gure 6.2 the
possible proton transfer mechanism is illustrated schematically for MnO2 in the
presence of Au. On the illustration two scenarios are depicted, one where Au
is present as a particle and one where Au is present as part of the surface. It
should be noted that the illustration is purely for explanatory purposes and does
not represent the actual surface for which DFT calculations were carried out.
The proton transfer mechanism was therefore investigated for a MnO2 system
based on DFT calculations. The calculations were performed by Dr. Michael