RESUMEN
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 Mo2.6 W0.4 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 Mo2.8 W0.2 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.
RESUMEN
Single phase solid solutions based on copper and iron substitution into the nickel sites of Ni2Mo3N are produced using a citrate gel approach followed by ammonolysis. These metal nitrides show good ammonia synthesis activity at 500 °C and ambient pressure. The activity fell with copper content and increased with iron content, although these changes appear to be related to the surface areas of the catalysts. The oxide intermediates from the citrate gel process cleanly converted to the metal nitride under the ammonia synthesis gas stream (75% H2, 25% N2) providing more active catalysts than those pre-nitrided by ammonolysis.