This PZC shift is not seen in the *S case. The hydrogenation of adsorbed *S to an adsorbed thiol is very unfavorable. Even at extreme potentials of -1 V vs SHE, the reaction is still endergonic. The hydrogenation of adsorbed *OH to H2O (aq) is less unfavorable,as in the case of adsorption, grand canonical effects significantly reduce the energy of *OH hydrogenation at negative potentials.