Hot electrons are generated when an exothermic chemical reaction takes place on the surface of a metal catalyst. Detection of these electrons using a catalytic nanodiode based on a metal-semiconductor Schottky junction can shed light on the mechanisms for energy transfer between the reacting molecules and the catalyst. Here, we present a study on the isotope effect of hot electron generation during the catalytic water formation reaction on platinum nanoparticles. To elucidate the isotope effect of hot electrons and to distinguish the reaction steps responsible for the creation of hot electrons, we carried out H₂ and D₂ oxidation reactions. We also considered the dependence of hot electron flux across the nanodiode on the temperature and geometry of the catalyst. Based on these results, we conclude that the observed effect of hot electron creation is mainly associated with energy released during the surface reaction of adsorbed hydrogen atoms and hydroxyl radicals, i.e. at high gas pressure.