114 The benecial interaction between Au and MnOx
6.2.3.1 Au L3-Edge
As mentioned above, the Au L3-edge was also investigated in-situ for the samples
containing gold. It is interesting to see if there are dierences for the two
concentrations since the dierences in capacitance and Mn oxidation under reaction
conditions are small but the dierences in activity are rather large. From
XRD measurements the size of gold domains was dierent for the two concentrations,
2 nm at 30 % and 3 nm at 50 %. In this size range Au nanoparticles
have been reported to change reactivity quite drastically due to the increased
ratio of undercoordinated sites vs. at terraces 240, 241. So even though the
size estimation from XRD is not completely accurate there is certainly a difference
which is interesting in terms of reactivity. It is therefore important to
check if these domains change oxidation state upon anodic polarisation. Spectra
obtained for in-situ measurements can be seen in gures 6.18 at OCV, 6.19 at
1.20 VRHE and 6.20 at 1.65 VRHE for the two Au concentrations. Up to 1.20
VRHE the Au edge for the two samples looks very similar and any dierences are
within the error margin, as seen on gure 6.18. However, at 1.20 VRHE a small
dierence in the main edge feature is observed, see gure 6.19. The white line is
slightly higher for 30 % Au compared to the 50 %. At 1.65 VRHE this dierence
is very clear and the white line feature for the 30 % sample is signicantly higher
than for the 50 %. A higher white line is related to Au in a 3+ state 242, 243.
1,2
1,0
0,8
0,6
0,4
0,2
0,0
Au(30%)-MnOx
Au(50%)-MnOx
119001192011940119601198012000
NormalizedIntensity/arb.units
PhotonEnergy/eV
OCV
Figure 6.18: Au L3-edge for Au(30%)-MnOx (blue) and Au(50%)-MnOx (red) at
OCV conditions. Error bars are based on standard deviations from Poisson statistics
on several scans.