We have proposed that coadsorption or dissociative adsorption on oxide surfaces is governed by a Lewis acid-base rule: the two species would have the lowest binding energy if one becomes a Lewis acid and the other a Lewis base. In the first part of this talk I will give examples of density functional calculations that support this rule. I will show that a hydrogen atom adsorbed on a La2O3 surface lowers its energy by donating an electron to the oxide to form a polaron, which consists of an electron shared by four La ions. In this case the La4 cluster is an acid and H is a base. When an H2 molecule adsorbs dissociatively on La2O3, one H atom binds to an oxygen and donates an electron to form a polaron; the other atom binds at the location of the polaron and becomes a hydride ion. In this example one H atom is a Lewis base and the other is a Lewis acid. A similar acid-base rule applies to the dissociative adsorption of methane on La2O3. CH3 or H both prefer to bind to a surface oxygen atom, if each is alone on the surface. However, if H and CH3 are formed as a pair, by CH4 dissociative adsorption, the H binds to an oxygen atom and CH3 binds to a cation; H donates electron charge and CH3 accepts it, in agreement with the acid-base rule. The same situation is observed in calculations of CH4 dissociative adsorption on CaO and MgO. The fact that the dissociation fragments affect each other so strongly suggests that the dissociative adsorption of methane is likely to be affected by the presence of other adsorbates on the surface. Calculations show that if a hydrogen atom is present on a La2O3 surface, in the form of a hydroxyl, the lowest energy pathway for methane dissociation is to form a gas phase methyl and a hydrogen bound to a surface oxygen. A more interesting situation occurs when methane adsorbs dissociatively on an oxide surface on which O2 was preadsorbed. The presence of O2 affects deeply the dissociation of methane. For TiO2 the CH4 dissociates more readily and the mechanism is changed: instead of a heterolytic dissociation (in the absence of O2) the H and the CH3 are both electron donors to the O2 and to a Ti4+. In the case of a stepped MgO surface the presence of O2 does not affect CH4 dissociation but it facilitates the desorption of CH3 from the surface. While a variety of mechanisms are observed in the situations described above, the lowest energy pathway is always in agreement with the acid-base rules.

Activation of the carbon-hydrogen bond by oxides and halides

254th National Meeting (2017)

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