The chemical essences of catalysis

The role of catalyst in catalytic reactions is just to reduce the reaction activation energy that is necessary for cracking chemical bonds and forming new bonds during chemical reactions. 

The role of a catalyst is, first, to form a complex molecule with reactant molecule coordination through its appropriate frontal orbital and thus fully weaken the H H bond and triple N=N bond. For example, H-H changes to two coordinated hydrogens and then may easily form new bond with the activated reaction intermediates. In other words, catalyst can participate in the reaction and form unstable intermediate complexes with reactants and form products at last. The activation energies required for every step are much lower than that for the reaction without catalyst, and therefore the reaction rate is accelerated. Consequently, the reaction pathway is changed by catalysts. For the case of ammonia synthesis reaction on Fe (111), Ertl, a winner of the Nobel chemistry prize in 2007, proposed a thermochemical kinetic profile, as shown in Fig. 2.1. 

1 Thermo chemical kinetic profile of ammonia synthesis on Fe (111) 

It can be seen that the energy requirement for the transformation of N2 and H2 to N and H is very high. 

The reaction mechanism for ammonia synthesis on industrial iron catalyst is still uncertain. Maybe it can be said that the reaction mechanism for ammonia synthesis are well understood when the remaining uncertainties in Fig. 2.1 are resovled.  

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