Uses and abuses of Volcano plots in hydrogen electrocatalysis.

M.F. JUAREZ; P.QUAINO; E. SANTOS; W. SCHMICKLER

Simposio; International Sympsosium on Electrocatalysis ECAT 2014, ?Explorations of the Volcano Landscape?; 2014

Sabatier?s principle [1] states that for a two-step reaction passing through an adsorbedintermediate the adsorption energy should be neither too high nor too low. Therefore, if it is theonly factor that governs a reaction, a plot of the reaction rate versus the free energy of adsorptionof the intermediate results in a volcano curve [2-3].A thorough examination of the hydrogen evolution reaction shows, that there is no volcano oncethe oxide-covered metals are left out. It is difficult to find an electrode material that follows thedescending branch of the volcano curves, because the metals with a strong affinity to hydrogenusually have more than one adsorbed states, and the reaction passes through the morefavourable ones. With the exception of nickel and cobalt, the reaction rate does not decrease forhighly exothermic hydrogen adsorption as predicted, because the reaction passes through moresuitable 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.We investigate the factors that govern the reaction rate in the light of our own theory [4] andconclude that there are three aspects that determine the catalytic activity in an electrochemicalenvironment:- Sabatier?s principle (free energy of adsorption of the ?true? intermediate)- The distribution of electronic states of the system in both space and energy coordinates(extension of the orbitals interacting with the adsorbate and relative position to the Fermi level)- The degree of solvation of the intermediate state.Volcano plots attempt to compare the catalytic properties of a wide range of systems with the aidof a single descriptor, typically the energy of adsorption of a single intermediate. However, thekinetics of complex reactions is not so simple. The reactions can occur through differentmechanisms, involving either different steps or different intermediates.