Bonding functions characterizing the effects of non-reducible cations upon catalytic properties over iron, palladium and platinum supported catalysts

Abstract

The significant effects of cations upon ammonia synthesis over Fe/Al 2O 3, CO hydrogenation to form methanol over Pd/SiO 2 and propene hydrogenation over Pt/SiO 2 have been reported by us and others. In this article, the nature of the effects is disclosed using bonding functions correlated to the mechanisms of the reactions. It is demonstrated that the cation effect might be the direct interaction between cation and metal atom, through the overlap of the wave function, the effect of electron image force, or dipole interaction. The cation might also stabilize the negative valence of the carrier's oxygen atom, which indirectly affects the oxidation states of the metal atom. In CO hydrogenation to methanol, the cation might be also an active center for stabilizing the intermediate state of the formate ion. For non-reducible transition metal ions, the d-d interaction is shown to be more important than that of s-s or p-d interactions. A fairly good agreement between theory and experimental data is given. The theory has been used to find new effective catalysts.