sites marked in Figure 2a, which are located on the terraces, are the active sites for oxygen evolution
on MnO2.44 DFT calculations suggest this to be the case for RuO2 and other rutile oxides.13,14,51 It
follows that a viable approach for stabilizing such a surface would be to selectively block the undercoordinated
sites with a compound that is stable at highly anodic potentials; in principle, this should
not come at the cost of catalytic activity, as we expect these sites to be inactive.
The notion that the undercoordinated sites could be selectively blocked is supported by numerous
reports in the literature. For Ru(0001) surfaces Behm and coworkers observed that Au or Cu would
selectively adsorb on step edges from which larger islands could grow.52,53 Interestingly, the same
phenomena was observed for Ru growth on Au(111).54 Specifically for N2 activation on Ru, selectively
blocked step edges had enormous impact on the activity.55 Selective adsorption has also been
observed for Sulphur deposition on Ni(111) and for Oxygen on Ruthenium.56,57 A relevant example
for oxides was reported by Stensgaard and co-workers, who showed that Pd nucleates preferentially
on the step edges of Al2O3.58 Finally, for electrochemical systems Bi and Te have been reported to
decorate steps of Pt(775) 59 and Cu bind the strongest to Pt steps and kinks.60
Herein, using DFT, we simulate several MnO2 structures with different guest oxides placed at the
undercoordinated sites, with the configurations shown in Figure 2b and c. The guest oxides, in their
bulk form, have more positive dissolution potentials than any MnO2 compounds. Our approach allows
us to elucidate trends in surface segregation on MnO2, and to predict whether heteroatoms may
stabilize the surface of the catalyst. The calculations are performed at the Generalized Gradient
Approximation (GGA) level, with an RPBE functional61. Similar calculations have previously been
reported to be sufficient for finding bulk properties of oxides,62 surface stability and activity of Mn
oxides,44 trends in overpotential for OER catalysts 13,14,51,63 and dissolution phenomena.50,64
The structures used for the model are shown in Figure 2b and 2c. As a first order approach, we will
define the energy of termination as the stability of the guest material versus its own bulk structure
using Equation (2):
Δ���� = ����term − ����ref − (����unit,guest − ����unit,ref) Equation (2)