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Topic: Alkane activation by modified oxide catalysts

Speaker: Prof. Horia Metiu

            Department of Chemistry & Biochemistry University of California, USA

Abstract：

We use density functional theory to examine how oxide catalysts break the C-H bond to activate alkanes. Over the past five years we have documented a rule which states that the energy of adsorbates on an oxide is lowered if one species donates electrons (is a Lewis base) and the other accepts them (is a Lewis acid). This Lewis acid-base rule controls the binding sites of the fragments formed by hydrogen dissociative adsorption, or by alkane dissociative adsorption. If CH₃ binds alone to the oxide surface it will prefer to bind to an oxygen atom. However if CH₃ is coadsorbed with a H atom, H atom will bind to and oxygen atom and the alkyl to the cation. This unexpected binding configuration is preferred because the two fragments can act as a Lewis acid-base pair: H donates electron charge (is a base) and CH₃ accepts it (is an acid). This structure is more stable than the one in which H and CH₃ are both bound to the oxygen atoms of the oxide surface, even though when CH₃ binds alone (in the absence of H) it prefers to bind to oxygen. The Lewis acid-base rule can be used to anticipate how various modification of the oxide surface will change the energy of the dissociative adsorption of an alkane. Since the H atom and the alkyls are radicals, they tend to act as electron donors. Therefore, their dissociative adsorption energy is increas by making the oxide surface a Lewis acid. This can be done by using a lower-valence dopant (e.g. Li-doped MgO), or by the presence on the surface of O₂, or I₂, or Br2, or Cl₂ because all these are Lewis acids. This is a possible explanation of the observation that the addition of small amounts of halogen can improve the activation of an alkane by an oxide. These acid-base rules also offer guidance regarding the adsorption and the activation of oxygen. Since O₂ is a strong Lewis acid, the presence of a Lewis base on the surface, such as an oxygen vacancy, a hydroxyl, an alkyl, a higer-valence dopant (e.g. Zr-doped La₂O₃), will increase the binding energy of O₂ to the surface and will also convert it into a negatively charge O₂ molecule, which is more reactive than the neutral oxygen. This talk gives various examples which document the validity of these Lewis acid-base rules.