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step Eqs. (1)–(4) with and without the hydrogen transfer
mechanism. The overpotential for Mn2O3 is estimated to be
1.0 V, rendering this oxide inert for water oxidation. The overpotential
is lowered significantly with a rutile-type MnO2 catalyst,
the surface Mn atoms of which are more oxidized. This
weakens the Mn=O bond so that it breaks more easily on formation
of the MnOOH intermediate, thus, the related overpotential
decreases (see Figure 2 b). Even in this case the formation
of MnOOH remains potential-determining but the critical
Mn=O intermediate is significantly destabilized, lowering the
potential required to form the OO bond to 1.7 eV. This corresponds
to an overpotential of 0.5 V.
In both cases a stabilization of the *OOH would result in
a decrease of overpotential. Such an effect may be obtained
by a hydrogen transfer from *OOH to an adjacent acceptor
site.23 The hydrogen transfer could occur either to a Au=O acceptor
site at an adjacent nanoparticle or, assuming the possibility
of incorporating Au into the surface, to a MnOAu site.
Notably, the *OH binding energy for a Au nanoparticle is
modelled by the binding energy to Au(111). A visualization of
these effects can be seen in Figure 3, with the hydrogen transfer
to a nearby Au nanoparticle shown at the top and the incorporated
Au site at the bottom. Additionally, the possibility
of hydrogen transfer to a Mn=O unit needs to be considered.
The latter situation may also be present in pure MnO2.
Including hydrogen transfer to an adjacent Mn=O site on
MnO2 in the reaction mechanism results in the free energy diagram
shown in Figure 2b (c). There is a clear stabilization of
the MnOOH binding, the energy of which becomes 3.5 eV. At
this point, only 0.3 eV is required to facilitate OO bond formation.
Correspondingly, the oxidation of water to a hydroxide
Eq. (1) becomes potential-determining, resulting in a decrease
in the overall overpotential to only 0.4 V. Although the assumed
hydrogen transfer is thermodynamically favorable, it
would likely be blocked under the reaction conditions as the
required adjacent Mn=O sites are involved in the OER and thus
unavailable.
This is in contrast to the case with Au=O and MnOAu
sites, which are both inactive for water oxidation39 but show
favorable energetics as hydrogen acceptors. The cost for recovery
of Au=O species, assuming an *=O coverage of 1/3 at
a face-centered cubic (111) surface was reported to be
1.4 eV.4 In fact, there is likely a variety of different sites available
on Au nanoparticles that could act as hydrogen acceptors,
however, treating a full Au nanoparticle is outside the scope of
this investigation. Correspondingly, the binding energy of the
*OOH species decreases to 3.5 eV, rendering the initial formation
of *OH Eq. (2) potential-determining. Again the overpotential
is lowered to 0.4 V. Incorporation of Au into the MnO2
lattice, depicted in the lower highlight in Figure 3, can result in
the formation of a MnOAu site. In such a configuration the
Au is located in a bridging position.10 Assuming hydrogen
transfer to a MnOAu site again renders the oxidation of
water to MnOH potential-determining by lowering the binding
energy of MnOOH to 3.5 eV. Therefore, reaction (1) determines
the theoretical overpotential, which becomes 0.4 V.
Figure 2. Free energy diagrams for the OER at zero applied potential.
a) Mn2O3 without H transfer (c), with H transfer (c), and with H transfer
to an adjacent Au=O site (c). b) Rutile MnO2 without H transfer (c),
with H transfer (c), with H transfer to an adjacent Au=O acceptor (c),
and with H transfer to an MnOAu site (c); blue and purple lines coincide.
c) CoOOH (0 11 2) (c), (0 11 4) (c), and (0 00 1) (c) surfaces,
Co3O4 (c), and Co3O4 with H transfer to a Au=O acceptor (c). CoOOH
data are taken from Ref. 43.g: Energy levels for an ideal catalyst.
Figure 3. Model showing two different pathways for hydrogen transfer
during OER on a rutile (11 0) MnO2 surface. *: Au, *: Mn, *: lattice O,
*: reacting O, and *: H atoms. In the first pathway (upper highlight), the
hydrogen transfer is facilitated by an adjacent Au nanoparticle. In the
second pathway (lower highlight), the MnOAu site functions as hydrogen
acceptor, requiring Au to be incorporated into the MnO2. A similar situation
is possible for Co3O4 or the (0 11 4) facet of b-CoOOH, which both benefit
from Au=O as hydrogen acceptor.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemCatChem 0000, 00, 1–7 &3&
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