The simplest surface catalytic mechanism of species A reacting to species B depicted in Figure obeys the Sabatier principle regarding adsorption enthalpy (ΔH A , ΔH B ) and surface reaction activation energy, (E a ). Reactant molecule A adsorbs to the surface as A*, undergoes surface reaction to B*, and then desorbs to gas-phase product B; the overall turnover rate can potentially be limited by any one of these three steps. Reactant adsorption is a fast, barrierless step unless it is combined with surface reactions; a combined step of dissociative adsorption (e.g., N 2 ) is commonly rate limiting on some catalytic materials . The volcano curve therefore results from the sequential kinetics of surface reaction(s) and product desorption, as presented originally by Balandin and depicted in Figure , the transition between surface reaction-and desorption-control exhibits the characteristic 'volcano' two-kinetic-regime plot. As depicted in Figure , surface adsorbates desorb slowly on strong-binding materials, while surface reactions occur slowly on weak-binding materials; the kinetic balance of these two steps forms the optimum turnover frequency of the system (i.e., volcano peak) characteristic to materials only exhibiting the optimal binding energy .