THE ENERGETICS OF NH3 ADSORPTION AT THE MGO(001) SURFACE

Ab initio quantum-mechanical calculations based on density-functional theory and the pseudopotential method have been used to study the adsorption of the NH3 molecule at the MgO(001) surface. The calculations employ slab geometry and periodic boundary conditions, with the occupied orbitals expanded in plane waves. The reliability of the theoretical methods has been verified by calculations on the bare surface and the isolated molecule. Four different adsorption geometries have been studied, and in each case the equilibrium configuration has been determined by full relaxation of the system. The two most stable configurations have about the same adsorption energy, and this energy agrees well with the results of recent thermal desorption measurements. Intermolecular repulsion is found to be a dominant effect at monolayer coverage, but becomes small at coverages below 25%. It is shown that chemical effects are not significant, and that the adsorption mechanism is predominantly physisorption.