Theoretical Study of Hydrogen and Ammonia Adsorption on Magnesium OXIDE(100) Surfaces.

The physical properties of hydrogen and ammonia adsorbed at various surface sites of MgO(100) were investigated by means of the Density Functional Theory (DFT). These calculations from first principles complement ongoing experimental studies. In this study, a finite MgO cluster is used to represent the MgO(100) surface. This model is motivated by the idea that an adsorbate interacts chemically with a small number of surface ions. However, due to effects of electron delocalization, atoms must be included that are not directly bonded to the adsorbate. Furthermore, MgO is a highly ionic crystal, so long range electrostatic interactions must be included. To address these issues and also to reduce the computational effort, the surface model used MgO clusters of several atoms surrounded by an array of fixed point charges to account for the electrostatic interaction between ions. The effects of long range electrostatic interactions have not been previously considered though they have a substantial effect on adsorption. It was found that hydrogen molecules physisorb on a flat MgO surface, though dissociative adsorption occurs at the kink site with an adsorption energy of more than 14 kcal/mol H_2. A type of adsorption at step sites is not definite and it may be either molecular physisorption or dissociative adsorption. Several different structures have been found for ammonia bound at flat, step, and kink surface sites. Ammonia weakly chemisorbs at Magnesium sites and physisorbs at Oxygen sites. The calculated adsorption energy on a flat surface is 15-17 kcal/mol, which agrees well with the experimental values at low coverage, 17 kcal/mol. The predicted activation energy for surface diffusion is 12-16 kcal/mol, while experiments provide only an estimated lower bound of 5 kcal/mol. There is also evidence that ammonia dissociates at kink sites, with an adsorption energy of 42 kcal/mol.