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.
Highlights
The reactants for ammonia synthesis have been studied, employing density functional theory (DFT), with respect to their adsorption on tantalum nitride surfaces
We have previously shown via a semi-empirical thermodynamic DFT approach that the number of nitrogen vacancies on Co3Mo3N surfaces can be of the order of 1013 per cm[2] even at ambient temperature.[1]
We investigate the effect of cobalt promoters on the nitrogen vacancy formation energy on Ta3N5-(100), (010) and (001) surfaces
Summary
The reactants for ammonia synthesis have been studied, employing density functional theory (DFT), with respect to their adsorption on tantalum nitride surfaces. At bridging nitrogen sites it adsorbs dissociatively forming 4NH groups with an exothermic formation energy of À175 kJ molÀ1 per H2. The effect of co-adsorbed cobalt atoms with respect to the adsorption of H2 and N2 has not been previously studied theoretically. We investigate the effect of cobalt promoters on the nitrogen vacancy formation energy on Ta3N5-(100), (010) and (001) surfaces. We study the chemisorption of H2 and N2 on the Co promoted and plain Ta3N5 surfaces in order to understand the effect of Co promoters with respect to the chemisorption of the ammonia synthesis reactants
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