The Brillouin zone is sampled with a Monkhorst-Pack grid 71 of 4×4×1 and 8×8×1 for the largest and smallest cells respectively. We used the RPBE functional 72 coupled with the D3 correction as it showed improved energetics for oxygenated and hydrogenated systems on Pt(111). Geometry optimizations were done keeping the two bottom layers held fixed, the force criterion was set to be less than 10 -3 Ry/Bohr, along with an energy criterion of 10 -5