A Comparison of the Reactivity of the Lattice Nitrogen in Tungsten Substituted Co₃ Mo₃ N and Ni₂ Mo₃ N

The effect of the partial substitution of Mo with W in Co₃ Mo₃ N and Ni₂ Mo₃ N on ammonia synthesis activity and lattice nitrogen reactivity has been investigated. This is of interest as the coordination environment of lattice N is changed by this process. When tungsten was introduced into the metal nitrides by substitution of Mo atoms, the catalytic performance was observed to have decreased. As expected, Co₃ Mo₃ N was reduced to Co₆ Mo₆ N under a 3 : 1 ratio of H₂ /Ar. Co₃ Mo_2.6 W_0.4 N was also shown to lose a large percentage of lattice nitrogen under these conditions. The bulk lattice nitrogen in Ni₂ Mo₃ N and Ni₂ Mo_2.8 W_0.2 N was unreactive, demonstrating that substitution with tungsten does not have a significant effect on lattice N reactivity. Computational calculations reveal that the vacancy formation energy for Ni₂ Mo₃ N is more endothermic than Co₃ Mo₃ N. Furthermore, calculations suggest that the inclusion of W does not have a substantial impact on the surface N vacancy formation energy or the N₂ adsorption and activation at the vacancy site.