Abstract
Sabatier’s principle suggests, that for hydrogen evolution a plot of the rate constant versus the hydrogen adsorption energy should result in a volcano, and several such plots have been presented in the literature. A thorough examination of the data shows, that there is no volcano once the oxide-covered metals are left out. We examine the factors that govern the reaction rate in the light of our own theory and conclude, that Sabatier’s principle is only one of several factors that determine the rate. With the exception of nickel and cobalt, the reaction rate does not decrease for highly exothermic hydrogen adsorption as predicted, because the reaction passes through more suitable intermediate states. The case of nickel is given special attention; since it is a 3d metal, its orbitals are compact and the overlap with hydrogen is too low to make it a good catalyst.
Highlights
Sabatier’s principle [1] is one of the oldest rules in catalysis
The ascending branch of Trasatti’s volcano plot is quite convincing; on the descending branch, there are only metals which are covered by an oxide film during hydrogen evolution, a fact that was not known at that time
Our group has developed its own theory of hydrogen electrocatalysis, based on a model Hamiltonian, quantum statistics and density functional theory (DFT), which we have reviewed in [8]
Summary
Sabatier’s principle [1] is one of the oldest rules in catalysis. For a two-step reaction passing through an adsorbed intermediate, like the hydrogen reaction, it states that the adsorption energy should be neither too high nor too low. If Sabatier’s principle is the only factor that governs a reaction, a plot of the reaction rate versus the free energy of adsorption of the intermediate results in a volcano curve.
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