Mechanistic Relationships Between Homogeneous and Heterogeneous Catalysis: Some Concepts Arising from Surface Organometallic Chemistry.

Abstract

Publisher Summary This chapter presents various aspects of surface organometallic chemistry, which may be relevant to a mechanistic approach of heterogeneous catalysis. Reactivity of organometallic compounds with surfaces of oxides obeys the same principles and elementary steps as those already observed in molecular chemistry. On basic oxides such as hydroxylated magnesia, alumina, zinc oxide, or lanthanum oxide, carbonyls of group VIII metals such as M3(CO)12, have a tendency to undergo a nucleophilic attack by surface hydroxyl groups. When Ir4(CO)12 is chemisorbed on alumina, one observes the formation of a surface formate, and, simultaneously, the cluster is transformed into very small particles of iridium covered with CO. Electrophilic cleavage occurs when a metal-alkyl or metal-allyl bond is allowed to react with the surface of an oxide which has electrophilic OH groups. This kind of reactions is mostly observed on silica, titania, and sometimes alumina. When low valent metal clusters such as Ru3(CO)12 or Os3(CO)12 react with an oxide surface covered with acidic OH groups such as silanols, an oxidative addition occurs with formation of an hydrido cluster. Such kind of reaction corresponds probably to a precursor step in the oxidation of zerovalent metal particles on a support. It indicates that the surface OH groups of silica can make an electrophilic attack at the metal-metal bond of the particle, resulting in an oxidation process in a very localized region of the particle.