100 The benecial interaction between Au and MnOx
Figure 6.2: Cartoon illustrating the possible proton transfer mechanism for a rutile
(110) MnO2 surface in the presence of Au. The highlighted area in front shows an
Au atom incorporated at a Bridge site, which makes the bridging oxygen available
as a proton acceptor for the OOH intermediate. The highlighted area in the back
depicts another scenario where a gold nanoparticle acts as proton acceptor for the
OOH intermediate adsorbed on an active Mn site. It should be noted that this gure
is an illustration and does not represent the exact surface used for calculations.
Busch at CAMd, DTU Physics. For more details the reader is referred to the
appended paper II. Instead a summary of the most important conclusions will
be given here. First the binding energies of O, OH and OOH were calculated
for both MnO2 and Mn2O3, resulting in the free energy diagrams shown in
gure 6.3. The binding of a hydrogen atom to the nearby Au site was modelled
in a simple way, by using the Au(111) surface at a third ML coverage of O, as
reported in 89.
From the free energy diagrams it can be seen that introducing the proton transfer
signicantly decreases the required energy for reaching the OOH step. This
is true for both MnO2 and Mn2O3. For a pure Mn2O3 surface the predicted
overpotential would be around 1 V whereas the MnO2 surface would give around
0.5 V. When assuming proton transfer to a gold site these values decreases to
0.2 V for Mn2O3 and 0.4 V for MnO2. The reason for the drastic change of
overpotential for Mn2O3 is that the binding of O and OH is close to the ideal
values although a bit too strong. When the OOH binding is stabilized, all the
binding energies are very close to ideal and either the O-O bond formation or
the H-transfer to a nearby Au site becomes potential determining. It should be
noted that it is doubtful whether such a surface will be stable under reaction
conditions, since the MnO2 phase is expected to be the most stable phase at