Molecular adsorption of NH3 on MgO(001) and hydrogen abstraction from NH3 on gaseous LiO and Li‐doped MgO(001). A computational study

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Abstract
Accurate a b i n i t i o quantum‐mechanical calculations are presented for the molecular and dissociativeadsorption of ammonia on the pure and Li‐doped MgO(001) surface, and also for the hydrogen abstraction from ammonia on gas‐phase LiO. The surface is described in terms of embedded clusters. In its most stable physisorbed state the ammonia is found to be doubly coordinated, with the electrostatic attraction to a cation being the important feature for bonding. Heterolytic dissociation of the N–H bond is neither found to be feasible on the pure MgO(001) surface nor on the Li‐doped surface. Likewise, no hydrogen abstraction from ammonia takes place on the perfect (001) surface. On the Li‐doped surface, partial optimization of the transition state renders a low reaction barrier of 7 kcal/mol for the dissociativeadsorption of ammonia. The gas‐phase hydrogen abstraction starting from free reactants may take place with no barrier to a product LiOH–NH2 complex. However, the reactants may be trapped in a stable complex in which ammonia is doubly coordinated to the oxide, and from this state the reaction barrier is 13 kcal/mol. The end‐on abstraction, having as products gas‐phase LiOH and NH2, has a barrier of 11 kcal/mol.