80 Towards a stable and inexpensive catalyst for OER in acid
a new surface formation energy. The dierence in surface formation energies
for initial and nal state has to be considered in evaluating the probability of
the process. If a kinked site or a stepped row are removed, the surface left
behind has exactly same structure as before. In terms of surface formation
energy there is no change either. If instead an atom from a at terrace is
removed it leaves behind a vacancy, which has a very high surface formation
energy. The removal of atoms from the at surface is therefore associated with
an extra energy barrier and it is therefore likely that the undercoordinated sites
are removed at lower potentials compared to at surface sites. The activity
of Mn oxides have been investigated with DFT calculations on the at terrace
sites, which matched very well with experimental results 131. For other rutile
oxides the coordinatively undersaturated sites (CUS), located on at terraces,
have been used to predict the activity 90, 98. This leads to an interesting
opportunity: if dierent sites are indeed responsible for activity and stability it
could be possible to change the two properties independently from each other.
To modify the stability the undercoordinated sites must be protected. This
could be done by adding a more stable compound that would selectively block
or terminate the undercoordinated sites. The prospect of selectively blocking
some surface sites have been successfully carried out in numerous examples.
For Ru(0001), deposited Au or Cu atoms were observed to selectively block
the step edges 214, 215. This had enormous impact on such surfaces for N2
activation 216. A study from Stensgaard and co-workers is particularly relevant
for oxides, where it was observed that palladium would nucleate selectively on
the step edges of Al2O3. For an MnO2 surface this process was investigated
with density functional theory, DFT, in my master thesis, which I conducted
prior to this Ph.D project 217. Here a brief description of the results will be
given along with a short introduction to DFT.
5.2.1 Density Functional Theory
Density Functional Theory is a quantum mechanical theory that can be used to
calculate fundamental properties of materials. Since no DFT calculations have
been performed by me during this Ph.D only a short qualitative introduction
will be given. The reader is referred to a book about the subject 218 and my
master thesis 217. With DFT, many-electron systems can be treated based on a
functional of the electron density described with spatial coordinates. The theory
originates from the idea of solving the time independent Schrödinger equation,
which in principle facilitates calculating all relevant ground state information
about a physical system:
^H
n(R; r) = En n(R; r) (5.2)
En are eigenvalues and n(R; r) are the eigenfunctions. The Hamiltonian ^H
is a
complex operator which encompasses multicomponent, many body interactions