Heyrovsky-Horiuti mechanism

The initial step, reaction (20), in the HER is charge transfer, leading to the formation of an adsorbed intermediate MH containing the metal M. The MH can either recombine to form molecular hydrogOT by reaction (21) or interact with H+ and an electron to form hydrogen by reaction (22). The former step is termed the recombination or Tafel reaction and the latter, 4he electrdchemical desorption or Horiuti-Heyrovsky mechanism. 

When the electrochemical desorption mechanism (the Horiuti-Heyrovsky step) becomes rate controlling,... 

The Heyrovski-Horiuti mechanism is based on fast discharge and slow electrochemical desorption ... 

We now have V3 = 1 and 2A2 + A3 = A. If we now assume that in mechanism (158) applied to a particular metal-solution system step IV is rate-determining, we would be dealing with the so-called Heyrovsky-Horiuti mechanism. 

---