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It should also be mentioned that rate-limiting steps can change depending on the catalyst and reaction conditions and they should not be considered as a constant in any given reaction mechanism. A more detailed discussion of rate-limiting steps is presented later in this chapter. E+01 –3 ax ax . 20 eV. observation here is that Pt and Pd are located closest to the top of the volcano, which is exactly what we expected. Given the similarly correct identification of the optimum catalysts for NH3 synthesis in the earlier example, the success of a Sabatier analysis can no longer be considered a coincidence!

In a computational screening study, the rate and equilibrium constants calculated using scaling and BEP relations are functions of the chosen descriptors. A volcano plot could be constructed by systematically sweeping through the parameter space for the descriptors. Root-finding methods can quickly solve View Online 30 Chapter 1 ΔE2 . 14 ΔE1 Suggested looping technique for the exploration of a two-dimensional descriptor space. the steady-state equations, but the rate equations in microkinetic models are highly non-linear and convergence is only achieved when good initial guesses are provided.

K. 56 dissociate N2, but the desorption step will be very difficult (note that in the desorption step only the energy of N* is affected by the catalyst because H2 and NH3 are gas phase species). In contrast, a catalyst interacting weakly with N allows for easy desorption, but the barrier for N2 dissociation will be large. Hence, there is an optimal binding energy of N*, EN, at which the dissociative adsorption of N2 and NH3 desorption are optimized simultaneously. We will now try to find an estimate of EN, by performing a Sabatier analysis for typical NH3 synthesis conditions.