Direct and precursor-mediated dissociation

The dissociation of NH3 on Ru(0001) is the competing back reaction to NH3 synthesis on Ru(0001). In addition, NH3 has been proposed as a viable hydrogen storage/transport medium in a hydrogen economy [345] so that its dissociation liberating N2 and H2 are also important. 

Two DFT studies have calculated the stepwise addition of adsorbed H to adsorbed N on Ru(0001) terraces to form NH3. The last step is the reverse of NH3 dissociation on the terraces. One study finds W = 1.3 eV and V = 1.4 eV (for NH3 dissociation) [350,351] while the other finds W = 0.89 eV and V = 0.8 eV [352]. The second study is in better agreement with experiments in the direct dissociation regime. However, only Refs. [350,351] study dissociation of NH3 at step sites and find W = 1.76 eV and Ec—Ed = —0.5 eV. This agrees reasonably with experiment for the precursor channel. Therefore, DFT studies do qualitatively support the picture suggested by the experiments. 

The scenario described here, i.e., activated dissociation at terrace sites and precursor-mediated dissociation at step or defect sites, is likely to be a very general one since barriers to dissociation are generally much lower at step sites [353]. There are already many known examples C2H6 dissociation on Ir(lll) [354], CH3OH dissociation on Pt(lll) [355] and neopentane dissociation on Pt(lll) [356], etc. 

---

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.

---