Dissociation Reactions on Surfaces

Elementary steps in which a bond is broken form a particularly important class of reactions in catalysis. The essence of catalytic action is often that the catalyst activates a strong bond that cannot be broken in a direct reaction, but which is effectively weakened in the interaction with the surface, as we explained in Chapter 6. To monitor a dissociation reaction we need special techniques. Temperature-programmed desorption is an excellent tool for monitoring reactions in which products desorb. However, when the reaction products remain on the surface, one needs to employ different methods such as infrared spectroscopy or secondary-ion mass spectrometry (SIMS). 

The middle panel of Fig. 7.12 shows the case when the surface is initially filled by 0.37 ML of NO molecules. If all molecules dissociated, the total coverage would become 0.75 ML, which is very high. This does not happen, because repulsive interactions between NO and the already formed atoms slows the dissociation reaction down, such that at about 400 K desorption becomes preferred over dissociation and part of the N O molecules leave the surface. The desorbed molecules leave space for the remaining molecules, which then dissociate instantaneously, as on the empty surface. 

If the surface is fully occupied by NO, all dissociation is blocked until temperatures are reached at which NO desorbs from the surface, after which dissociation follows instantaneously (right-hand panel of Fig. 7.12). Here dissociation is initially suppressed by site blocking rather than by lateral interactions. 

N or O atoms (speckled) on a grid of square sites that represent the Rh(lOO) surface. [Experiments adapted from M.j.P. Hopstaken and J.W. Niemantsverdriet, /. Phys. Chem. B 104 (2000) 3058 simulations courtesy of A.P. van Bavel, j. Lukkien and P.A.j. Hilbers, Eindhoven.  

---

Figure 7.12 neatly illustrates how dissociation reactions on surface are greatly affected by interactions with neighboring adsorbate species. This is because dissociations often require ensembles of sites in a specific arrangement. From our discus-... 

---