Temkin-Pyzhev mechanism

Based on Temkin-Pyzhev mechanisms (steps 1- 3), the rates for ammonia decomposition can be expressed by Eq. (2.27), and its kinetic parameters calculated from equation V = fc (PnH3) /(Eh2) are given in Table 2.7. The value of parameter n/m in Table 2.7 is about 3/2, demonstrating that the Temkin Pjrahev mechanisms can be used for most metal catalysts, such as Re, Fe, Co, Ni, Ru and Rh, at the common conditions of ammonia decomposition. 

Table 2.8 Validation of Temkin Pyzhev mechanism for eunmonia synthesis.

The rate equation of ammonia decomposition was expressed in Eq. (7) according to the Temkin-Pyzhev mechanism (Eq. 1-3). The observed parameters in Table 3.9 show that n/m values are mostly 3/2, which indicates that the Temkin-Pyzhev mechanism is applicable on most catalysts (Re, Fe, Co, Ni, Ru and Rh) under normal ammonia decomposition conditions. 

As an example, the Temkin-Pyzhev rate expression for ammonia synthesis reproduces the experimentally observed kinetics quite well. However, this rate expression was originally derived from a proposed mechanism which had both the wrong key intermediates and the wrong rate-limiting step. 

An attempt was made by Ozaki et al to verify the mechanism, on which the Temkin-Pyzhev equation was derived, by comparing the rate of the following two reactions obtained on the same catalyst ... 

In a later article [74], Ferraris et al. again used the data by Nielsen et al. to test 23 different rate equations for ammonia synthesis. Some of the rate equations considered both associative and dissociative mechanisms. Others were purely empirical. They concluded that a large number of the models represented the data equally well, in fact better than the Temkin-Pyzhev equation. This again proves that it is extremely difficult, and in fact usually impossible, to draw definite conclusions about the mechanism from kinetic data alone. 

Adopting the mechanism involving steps (VII.2)-(VII.7) and introducing special assumptions in order to account for the heterogeneity of the surface, Temkin and Pyzhev (73) have deduced the rate equation... 

An important question for the reaction mechanism for NH3 synthesis is whether any H is added to N2 before the fission of the N-N bond. Mechanisms involving the additions of H before the fission of the N-N bond are referred to as associative while mechanisms involving N are called dissociative. Even if the kinetic expression by Temkin and Pyzhev was derived from considerations for an associative mechanism, the evidence for the associative mechanism is rather meager. 

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