Kinetic model for cooperative dissociative chemisorption and catalytic activity via surface restructuring

A kinetic model for catalytic reactions involving metal surface restructuring is proposed and discussed. The model is an extension of the equilibrium model for cooperative dissociative chemisorption [I. Oppenheim and R.D. Levine, Chem. Phys. Lett. 155 (1989) 168]. The model provides a possible explanation as to: (a) why strong binding sites (e.g. those at high surface roughness) can also exhibit a high turnover rate resulting in efficient catalytic activity, (b) why bond breaking occurs preferentially over a narrow temperature range and, (c) why this range is lower for more open surfaces. All these and other conclusions derive from the central result of the model that dissociative chemisorption of the physisorbed molecules is a cooperative process. It must, however, be emphasized that this behavior is due to the cooperative rearrangement of the substrate metal atoms upon the chemisorptive dissociation of the absorbate and not to the weaker forces that may operate among the adsorbates. The model is a kinetic one and depends on input from experiment. In particular, we take two general results as given. One is the well documented [G.A. Somorjai and M.A. Van Hove, Catal. Lett. 1 (1988) 433] restructuring of the surface upon bond breaking. The other, is that upon a bare, defect-free, smooth surface, the probability for dissociative chemisorption of an isolated physisorbed molecule, is low, [B. Poelsema and G. Comsa, Scattering of Thermal Energy Atoms (Springer, Berlin, 1989) p. 90].