Predicting the kinetics of the dissociative adsorption of homonuclear molecules on metal surfaces in gas phase and solution: I. Rate constant of the elementary step of adiabatic reaction

A model is presented to describe the kinetics of the dissociative adsorption of homonuclear molecules X2 on a metal catalyst in the gas phase and a solvent. The following reaction coordinates are used to construct crossing diabatic potential energy surfaces (PES): the distance y between the X fragments of X2 molecule, the distance x of the X2 reactant from the surface, the set of coordinates describing possible structural reorganization of the metal surface under adsorption, and the set of coordinates characterizing the state of the solvent polarization if the adsorption proceeds on the metal/solvent interface and is accompanied by a charge transfer. The adiabatic PES is determined by a standard procedure taking into account the resonance splitting 2V of the crossing diabatic surfaces. The activation energy is calculated beyond Condon approximation with due account of the dependence of the splitting 2V on the coordinates x and y. The vibration frequencies of the system at the transition configuration (needed to calculate the pre-exponential factor of the rate constant) are calculated by expanding in a series the adiabatic potential energy at the saddle point on this potential surface. Application of the general equations to a system characterized by specific form of the interactions potentials is given. In the following paper our approach is applied to the dissociative adsorption of oxygen on a Pd(1 1 1) surface.