DFT-D3 study of H2 and N2 chemisorption over cobalt promoted Ta3N5-(100), (010) and (001) surfaces

Abstract

The reactants for ammonia synthesis have been studied, employing density functional theory (DFT), with respect to their adsorption on tantalum nitride surfaces. The adsorption of nitrogen was found to be mostly molecular and non-activated with side-on, end-on and tilt configurations. At bridging nitrogen sites (Ta–N–Ta) it results in an azide functional group formation with a formation energy of 205 kJ mol⁻¹. H₂ was found also to chemisorb molecularly with an adsorption energy in the range −81 to −91 kJ mol⁻¹. At bridging nitrogen sites it adsorbs dissociatively forming >NH groups with an exothermic formation energy of −175 kJ mol⁻¹ per H₂. The nitrogen vacancy formation energies were relatively high compared to other metal nitrides found to be 2.89 eV, 2.32 eV and 1.95 eV for plain, surface co-adsorbed cobalt and sub-surface co-adsorbed cobalt Ta₃N₅-(010). Co-adsorption of cobalt was found to occur mostly at nitrogen rich sites of the surface with an adsorption energy that ranged between −200 to −400 kJ mol⁻¹. The co-adsorption of cobalt was found to enhance the dissociation of molecular hydrogen on the surface of Ta₃N₅. The studies offer significant new insight with respect to the chemistry of N₂ and H₂ with tantalum nitride surfaces in the presence of cobalt promoters.