Molecular dynamics simulations of reactions on metal surfaces: rate constants for selected diatomic dissociation reactions

We have employed our normalized bond index (NBI) potential energy function in molecular dynamics simulations of the dissociation of various isotopes of (a) O-2 on Ag(111), Au(111), Pt(111) and Ni(111), (b) S-2 on Ni(111), and (c) C-2 on Ni(111) surfaces for the purpose of computing the zero coverage limit Arrhenius reaction rate constants. For O-16(2) on Pt(111) we repeated the rate constant calculations at a simulated coverage of 0.4, in order to compare our result with an experimental value that corresponds to a low but otherwise unknown coverage. Our computed rate constants for O-16(2) dissociation on Pt(111) are: (a) for the zero coverage limit, k = 5.42 x 10(12) s(-1) exp {-1.1 kcal/mol/RT}, (b) for moderate coverage, k = 7.43 x 10(11) s(-1) exp {-5.8 kcal/mol/RT}. The experimentally determined rate constant 9 x 10(11) s(-1) exp {-2.5 kcal/mol/RT} is bracketed by our calculated values. Our computed rate constants possess the general trend that the Arrhenius pre-exponentials decrease with increasing vibrational mass of the dissociating bond. Also, the Arrhenius pre-exponentials tend to decrease with decreasing atomic heat of adsorption. We emphasize that the Arrhenius activation barrier is an effective barrier that depends on several aspects of the system, and may not closely correspond with any single feature of the potential energy surface. (C) 1997 Elsevier Science B.V.