QUANTUM-CHEMICAL STUDY OF ELECTROCHEMICAL PROMOTION IN CATALYSIS

We report a first-principles theoretical investigation of electrochemi cal promotion induced by controlled variation in catalyst-electrode po tential of metals interfaced with solid electrolytes. Cluster models o f oxygen adsorbed on Cu and Pt metal surfaces with coadsorbed positive and negative ions have been considered. The presence of the ions has been simulated also by using point charges and uniform electric fields . We have analyzed the dependence of the metal work function and of th e oxygen adsorption energy on the magnitude of the point charges or of the external electric fields. The theoretical results are compared wi th temperature-programmed desorption, TPD, and potentiometric work fun ction measurements for oxygen on Pt films supported on ZrO2, an O2- co nductor. The theoretical calculations show the same linear dependence of the O bond strength on the metal work function observed experimenta lly. This linear relationship is found also at the first-order perturb ation theory level by taking into account only the purely electrostati c interaction between the field induced by the ions and the polar meta l-oxygen bond. This suggests that the change in oxygen desorption ener gy is largely due to electrostatic effects. In addition to providing a direct explanation for the effect of electrochemical promotion, the p resent results may also provide a sound theoretical basis for the stud y and understanding of the role of promoters in heterogeneous catalysi s.