Catalytic reaction mechanisms for ammonia synthesis

Catalytic reaction mechanisms represent the elementary steps at molecular level, but the expression in detail at molecular level is difficult for the catalytic reactions on solid surfaces. So, the usual strategy is to propose applicable reaction mechanisms for key chemical processes occurring on catalyst surface, based on a series of assumptions that must be in agreement with experimental data and could be modified indispensably. 

A workable reaction mechanism must embrace all reactants, products and reaction intermediates produced and consumed in a catal3ftic reaction. In gas-phase reactions, two-molecule reaction is popular and three-molecule reaction is very less. For catalyst surfaces, multi-molecule steps become important as the concentration of chemical species is higher than that of gas-phase, and even in this case the reaction between more than three molecules is in general not common. 

The availability of the elementary steps in a reaction sequence is usually estimated by the number of chemical bonds broken and formed, and it is not reality that a step involves a change of several chemical bonds. The formation of a reaction mechanism according to the molecular processes taking place on catalyst surface should be based on the kinetic parameters for expression of reaction mechanism which needs trading-off the experimental measurements or theoretical predictions. 

The trade-off between detail levels of reaction mechanism and the estimations of kinetic parameter plays an important role in the analysis of microkinetics. 

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