Adsorption of nitrogen and ammonia at zirconia surfaces

Incorporation of nitrogen into the bulk material is the key step in the preparation of N-doped transition metal oxides. As a first attempt to understand the basic reaction mechanisms involved in the surface process, molecular and dissociative adsorption of nitrogen (N2) and ammonia (NH3) at the perfect and oxygen-deficient (1 1 1) surface of cubic zirconia has been studied theoretically at hybrid density functional level. Coverages θ=12 and θ=14 were considered. In agreement with experimental observation nitrogen does not interact with the nondefective surface. Anionic adsorbed species, mainly N2-, are observed in the presence of oxygen vacancies in the topmost layer independent of coverage. NH3 adsorbs molecularly also on the defect-free surface whereas at the defective surface both molecular and dissociative adsorption is found. After dissociation the anions NH2- and NH−, accompanied by surface bound OH− and H−, are formed.