Electronic factors in catalysis: the volcano curve and the effect of promotion in catalytic ammonia synthesis

The activity and selectivity of heterogeneous catalysts are determined by their electronic and structural properties. In many cases, the electronic properties are determined by the choice of both the catalytically active transition metal and promoter elements. Density functional theory is used to calculate how these two factors affect the energies of the intermediates and transition states in the ammonia synthesis reaction. We show that a linear relationship exists between the activation energy for N-2 dissociation and the binding energy of adsorbed nitrogen. The ammonia synthesis activity under industrial conditions can be determined as a function of the nitrogen-surface interaction energy by combining the calculated adsorption energy-activation energy relation with a micro-kinetic model. The result is a volcano curve and we illustrate such relationships for both the non-promoted and alkali metal promoted transition metals. We conclude that promotion is most effective for the best non-promoted catalysts and that promotion will always be essential for obtaining an optimal ammonia synthesis catalyst. Analysis of the micro-kinetic model show that the best catalysts are those with the lowest apparent activation energies and they exhibit reaction orders between two asymptotic behaviors. (C) 2001 Elsevier Science B.V. All rights reserved.