DFT-D3 Study of Molecular N2 and H2 Activation on Co3Mo3N Surfaces

Cobalt molybdenum nitride (Co3Mo3N) is one of the most active catalysts for ammonia synthesis, although the atomistic details of the reaction mechanism are currently unknown. We present a dispersion-corrected (D3) DFT study of the adsorption and activation of molecular nitrogen and hydrogen on Co3Mo3N-(111) surfaces to identify possible activation sites for ammonia synthesis. H2 was found to adsorb both molecularly on the Mo3N framework and dissociatively on Co8 clusters or Mo3 clusters that were exposed due to N-vacancies. We find that there are two possible activation sites for N2 where both N2 and H2 can coadsorb. The first is a Mo3 triangular cluster that resides at 3f nitrogen vacancies, and the second is a surface cavity where N2 is activated by a Co8 cluster, the second being a more efficient activation site. N2 was found to adsorb in three adsorption configurations: side-on, end-on, and an unusual tilt end-on (155°) configuration, and the existence of these three adsorption configurations is explained via MP2 calculations and the sphere-in-contact model.