18 Electrocatalysis and the splitting of water
transition state that has a low probability of being formed. If, however, the
reactants bind to a suitable catalytic surface, intermediates are stabilized and
can react with lower energy barriers, shown as the red curve.
Figure 2.1: A cartoon depicting how catalysis works. The blue surface works
to facilitate binding of reactants and intermediates, which can then break and form
bonds. The overall energy barrier for the reaction can be lowered by the use of such
catalytic surfaces. Notice that this simple explanatory gure is not appropriate for
electrocatalysis, where the reactions can only proceed on electrode surfaces. However,
the nature of the surface interaction is equally important.
As a result, the reaction can proceed at faster rates or the same rate can be
achieved with less energy input. Thermal catalysis provides an intuitive example
where the energy input is temperature. A new catalyst that decreases the
operating temperature can make a chemical process much cheaper and a chemical
plant (more) protable. This simple explanation highlights the importance
of catalysts and gives an impression of how they work. It is the binding of reactants,
intermediates and products to the catalyst surface that lowers the energy
barriers. However, any given surface that binds these species is not necessarily
a good catalyst. In fact, it is crucial that the binding is balanced. This concept
is famously described by the Nobel prize winner Paul Sabatier, who stated that
the interactions between a catalyst and the reactants should be "just right" 47.
If the interaction is too weak reactants will not bind to the surface and no reaction
takes place. If the interaction is too strong either reactants or products
block the catalyst surface, causing little or no reaction to proceed. This concept
has been demonstrated for many reactions and an example is shown in gure
2.2 with catalysts for electrochemical hydrogen evolution. In this example the
energy of hydrogen adsorption is used as descriptor and the activity has a clear
optimum 48, 49.
The left part of gure 2.2 shows Parson's attempt to quantify and understand
the theoretical basis of the hydrogen evolution reaction, HER, from 1958 48.
He found that i0, exchange current density used to indicate catalyst activity,