Dissociative adsorption precursor well

In this model, the rates of adsorption, migration and desorption from the intrinsic precursor are fea[A2], km [A ], and fc A ], respectively, and it follows that the normalised rate coefficients defined earlier are identical with the probabilities fc, fm and fd defined by King and Wells [46] and equation (65) is readily transformed into their rate expression for dissociative adsorption derived by the statistical method. 

FIGURE 23 One-dimensional potential energy curves for dissociative adsorption through a precursor or physisorbed state (a) adsorption into the stable state with no activation energy and (b) adsorption into the chemisorption well with activation energy A E. ... 

Figure 12.5 Schematic potential energy surface for dissociative adsorption-desorption [adapted from ErtI (1982)]. In the drawing, the barrier to dissociation-recombination occurs when the molecule is already at the surface and is along the bond distance. This will lead to vibrationally excited desorbed molecules. Note also the precursor well along the approach to the (physical) surface. This well will slow down the approaching reactant but may not be deep enough to insure that it fully accommodates to the surface.

Figure 3.3. Schematic of direct and precursor-mediated dissociation processes on a typical adiabatic PES (given by the solid line). Solid arrow labeled S represents direct dissociation and that labeled a represents trapping into a molecular adsorption well. Dashed arrows represent competing thermal (Arrhenius) rates for desorption (kd) and dissociation (kc) from the molecular well.

Figure 3.34. Schematic ID PES and dynamics for 02 dissociation on Pt(lll). Eiigh incident energies allow adsorption directly into the molecularly chemisorbed states, which then act as precursors to dissociation. At lower incident energies, 02 first adsorbs in the physisorption well and then proceeds through sequential precursors to dissociation. From Ref. [320].

The interaction of N2 with transition metals is quite complex. The dissociation is generally very exothermic, with many molecular adsorption wells, both oriented normal and parallel to the surface and at different sites on the surface existing prior to dissociation. Most of these, however, are only metastable. Both vertically adsorbed (y+) and parallel adsorption states (y) have been observed in vibrational spectroscopy for N2 adsorbed on W(100), and the parallel states are the ones known to ultimately dissociate [335]. The dissociation of N2 on W(100) has been well studied by molecular beam techniques [336-339] and these studies exemplify the complexity of the interaction. S(Et. 0n Ts) for this system [339] in Figure 3.36 (a) is interpreted as evidence for two distinct dissociation mechanisms a precursor-mediated one at low E and Ts and a direct activated process at higher These results are similar to those of Figure 3.35 for 02/ Pt(lll), except that there is no Ts... 

The promoter action of adsorbed potassium has been attributed both to a stabilization of the molecular a-species as well as to an enhancement of the sticking coefficient for the formation of this a-species. On Fe(lll) the adsorption energy for a-N2 is increased locally in the vicinity of an adsorbed potassium atom from 31 to 44 kJ moP. Parallel to this increase in the adsorption energy for the precursor state its activation energy for dissociation is lowered, as becomes apparent from inspection of Fig. 3.10. Once the nitrogen molecule has become dissociated, the adsorbed N atoms formed near the K atoms diffuse away to the bare iron patches (when the temperature is above 400 K), so that finally the whole available surface is covered by Nad-... 

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