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.
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