No abstract

Thus far, essentially all heterogeneous catalysts of industrial importance have been developed by trial-and-error experimentation. The classic example of this approach is the discovery of the iron-based ammonia synthesis catalyst by Mittasch and co-workers who tested more than 2500 different catalysts in 6500 experiments. 1,2 Parallel testing methods, which can speed up catalyst screening significantly, have recently been introduced, 3 but a better scientific basis could make catalyst development substantially more efficient.We show that a rational catalyst development strategy can be developed on the basis of simple, physically motivated concepts. We use the ammonia synthesis reaction to illustrate the approach, but the general principles should be broadly applicable.The starting point is the volcano-shaped relation between the ammonia synthesis activity of different catalysts and their nitrogen adsorption energy shown in Figure 1. The curve shows (in complete agreement with experimental evidence 4 ) that Ru and Os are the best catalysts among the pure metals. The dependence of the catalytic activity on the nitrogen adsorption energy is a consequence of a linear (Brønsted-Evans-Polanyi) relationship 5 between the activation energy for the rate-limiting step, which is N 2 dissociation, 6,7 and the stability of adsorbed N on the surface. The reason for this relationship is that the transition state for N 2 dissociation is very final-state-like. Therefore, the transition-state energy essentially follows the nitrogen adsorption energy from one metal to the next.The volcano shape of the plot in Figure 1 implies that there is an optimum for the nitrogen adsorption energy. This optimum reflects a compromise between two mutually opposing ways of achieving a high activity: a small activation barrier for N 2 dissociation and a surface with low coverage of adsorbed atomic nitrogen during ammonia synthesis. This requires a strong and a weak N-surface interaction, respectively. At conditions relevant in industrial processes we get closest to the optimum by using Ru or Os as catalysts. However, these metals are very expensive and thus less commercially attractive compared to the third-best catalyst, Fe.A rational approach could be to construct a surface (active sites) with the desired intermediate nitrogen interaction energy by combining two metals: one with too high adsorption energy and one with too low adsorption energy. As indicated in Figure 1, a combination of Mo (which binds N too strongly) with Co (which binds N too weakly) should be close to optimum. This is exactly what was found experimentally. 8-10 A Co-Mo catalyst was developed using this principle, and it had an ammonia synthesis activity much better than that of the constituents and even better than those of both Fe and Ru at low NH 3 concentrations, see Figure 2. In the following, we will show why this is the case.We study the chemical behavior of alloy surfaces with mixedmetal sites for ammonia synthesis using plane wave DFT calculations. The RPBE exchange-co...

No abstract

No abstract