$Ab initio$ insights into the initial steps of the Oxygen Reduction Reaction on CoTPyP metal-organic framework.

Metalorganic molecules self-assembled on surfaces form a template to host regular arrays of metal atom caged inside the molecules. Each single metal atom act as a catalyst, whose selectivity and efficiency towards specific chemical reactions can be tuned changing either the atomic species or molecular residues, or else the substrate. We focus here on a monolayer of Cobalt TetraPyridyl Porphyrins (CoTPyP) grown on a graphene/Ir(111) substrate, showing that it is able to stabilize hydroperoxyl molecule (OOH), that is widely accepted to be a fundamental reaction intermediate in Oxygen Reduction Reactions (ORR) but has an elusive behavior and is difficult to be detected. Experiments and $ab initio$ density-functional theory modeling suggest the formation of a OOH-water cluster on top of the molecules at room temperature in $O_{2}+H_{2}O$ atmosphere. The detailed analysis of the electronic properties provides a clear picture of the bonding nature and the role of CoTPyP and water in the OOH stabilization process, showing that a surprising combination of charge transfer, dipole and H-bonds and water solvation determines the hydroperoxyl-water complex stability.