6.2 Experimental investigation of Au-MnOx catalysts 109
Nordlund.
One of the hypotheses brought forward regarding Au and Mn oxide interactions
is a possible change in Mn oxidation state. Kuo et al. suggested that upon
Au modication the Mn oxidation state decreases and a higher concentration
of Mn3+ species can exist at the surface 226. This was concluded from ex-situ
XAS measurements. With this in-situ study dierences in the Mn oxidation
state can be documented for catalytically active lms while the reaction takes
place in the same environment as the activity measurements.
XAS mesaurements for the three lms were made for both dry conditions and
immersed in 1 M KOH. The measurements with electrolyte were further carried
out at open circuit, 0.8, 1.0, 1.2, 1.4 and 1.65 VRHE. It should be noted that
the results presented here have been processed and normalised to have an edge
jump of unity after linear backgrounds are subtracted. In gure 6.12 results
from the dry measurements are shown for the Mn K-edge of all three lms.
Mn3O4
+30%Au
Dryconditions
6540656065806600662066406660
1,6
1,4
1,2
1,0
0,8
0,6
0,4
0,2
0,0
+50%Au
NormalizedIntensity/arb.units
PhotonEnergy/eV
Figure 6.12: Mn K-edge XAS of the three Mn oxide lms under dry conditions.
Mn3O4 in black, Au(30%)-MnOx in blue and Au(50%)-MnOx in red. Error bars are
based on standard deviations from Poisson statistics on several scans.
The overall features are very similar. The dierences are mainly a slightly
lower white line (the highest peak) for the lms containing gold, which can be
due to fewer Mn atoms. Fewer Mn atoms yield a slightly lower signal to noise
ratio. Besides this small dierence the single pre-edge and shoulder around 6552
eV are close to identical for the lms. The shoulder feature has been reported