6.2 Experimental investigation of Au-MnOx catalysts 113
are plotted as function of applied potential on the working electrode in gure
6.17.
0,00,81,01,21,41,6
6548,5
6548,0
6547,5
6547,0
6546,5
Au(50%)-MnOx
Au(30%)-MnOx
E0/eV
U/V(vs.RHE)
DryOCV
Mn3O4
Figure 6.17: Overall edge position for the Mn K-edge of the Mn3O4 (purple),
Au(30%)-MnOx (wine red) and Au(50%)-MnOx (green) as function of the applied
potential. Note that the rst two points are Dry and OCV conditions followed by the
actual potential scale. The error bars here are based on varying the integral range
from equation 3.5.
For the pure Mn3O4 there is very little edge shift observed before 1.2 VRHE.
Even at the highest potential the total shift is around 0.3 eV. For comparison
the edge shift observed when going from +3 at around 6546.9 eV to +4 at
around 6549.1 eV is 2.2 eV for reference crystalline Mn oxides 237. For the
mixed lms there seem to be a shift already at OCV and again at 1.0 VRHE.
At the nal potential, 1.65 VRHE, the spectrum for 30 % Au has shifted 0.7 eV
compared to the spectrum for dry conditions. The edge shifts found with this
experimental conguration take into account all Mn atoms in the thin lms.
This is important to realize, since only the atoms in contact with the electrolyte
are likely to oxidise due to the anodic potential. Thus, only a small fraction of
the Mn atoms are in fact changing their oxidation state and it is not meaningful
to assign a specic oxidation state to the catalytically active surface species.
Still there is a signicant increase in oxidation state observed due to the Au
modication.