We initiate our investigation by examining the free energy diagram of CaTiO 3 . The most prominent thermodynamic energy barrier identified is associated with the initial oxidation step (∆G OH * = 2.9 eV), which leads to the formation of adsorbed hydroxyl groups (OH * ). Notably, the energy required for this step is larger than that of solvated hydroxyl radicals (∆G• OH(aq) = The black line shows the conventional proton-electron transfer on the surface while the red line represents H 2 O 2 evolution through the hemibonded structure.