Heterogeneous energetic aspects

Thus chemisorption and the associated energetic aspects play a crucial role in understanding heterogeneous catalysis [10]. The active centers on the catalyst surface are probably the result of free valences or electron defects, which weaken the bonds in the adsorbed molecules to such an extent that a reaction can readily occur. The course of a heterogeneously catalyzed reaction is compared to that of an uncatalyzed reaction in Figure 5-9. 

Some of the systems where the symmetry is low and the number of atoms relatively high are those occurring in the field of heterogeneous catalysis. The combination of a surface of a (in the ideal case, semi-infinite) crystal and one or more finite molecules (maybe even together with the presence of some medium) makes the study of the chemical reactions on the surfaces of crystals very complicated. In Section 6 we saw that such studies are becoming possible but still only for the simplest reactions on fairly idealized surfaces. Moreover, the calculations explore mainly energetical aspects and not kinetic aspects of the reactions. This means also that the time scales that can be treated with quantum-mechanical molecular-dynamics simulations are much too small (10 -10 s) compared with those that often are relevant for chemical reactions. 

In addition to purely energetical heterogeneity one should also take into account some basic aspects of possible heterogeneities resulting from geometrical effects. The simplest and yet experimentally quite important geometric effects are due to the finite size of crystallites. Experimental measurements ave clearly demonstrated that the size of typical crystallites may be quite small (of the order of 50-100 A [116,132] and quite large (of the order of 10 A [61]. 

The surface energetic heterogeneity determination constitutes an additional aspect of the present study. This was performed by means of the methylene chloride adsorption energy distribution functions (AEDF) computation, relating the number of interactive surface sites to the desorption energy of each individual site. The latter are displayed in Fig. 1. 

In discussing the relation between heterogeneous catalysis and the defect solid state in a general way, one must consider three aspects of the problem (1) structural, (2) atom transport, (3) energetics of electron processes. These are not rigorously separable, but can be discussed... 

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