Based on the Volmer-Heyrovsky-Tafel mechanism, Frumkin adsorption and thermodynamic data of hydrogen (deuterium) absorption in Pd, effects of surface parameters on the hydrogen (deuterium) loading ratio into Pd in the hydrogen (deuterium) evolution reaction are discussed. There is a change of mechanism from the Volmer-Tafel route to the Volmer-Heyrovsky route when the current density rises, and...