The Role of Composition for Cobalt Molybdenum Carbide in Ammonia Synthesis

Abstract

The performance of Co3Mo3N, Co3Mo3C, and Co6Mo6C for ammonia synthesis has been compared. In contrast to Co3Mo3N, which is active at 400 °C, a reaction temperature of 500 °C, which was preceded by an induction period, was necessary for the establishment of steady state activity for Co3Mo3C. Co6Mo6C was found to be inactive under the conditions tested. During the induction period, nitridation of the Co3Mo3C lattice was found to occur, and this continued throughout the period of steady state reaction with the material transforming in composition toward Co3Mo3N. Taken together, these observations demonstrate that ammonia synthesis activity in ternary cobalt molybdenum systems is associated with the presence of N in the 16c Wyckoff lattice site.

The role of interstitial species upon the ammonia synthesis activity of ternary Fe–Mo–C(N) and Ni–Mo–C(N) phases

“…Unlike its nitride counterpart, the η-6 carbide structured Fe 3 Mo 3 C was not active for ammonia synthesis at 400 °C. Activity was found to develop at 500 °C and CHN analyses and postreaction XRD analyses were consistent with the substitution of lattice C by N. As is the case for Co 3 Mo 3 C reported elsewhere [23] , it is currently not clear whether activity arises as a consequence of the presence of lattice N or whether lattice N results as a consequence of ammonia synthesis. The extent to which development of activity requires removal of a surface passivation layer and/or restructuring in the near surface region also remains to be established.…”

“…318 μmol h −1 g −1 is evident (690 μmol h −1 g −1 would correspond the thermodynamically limited yield of 0.129% under these conditions). This type of behaviour mirrors that of the isostructural Co 3 Mo 3 C phase, which we have discussed in detail elsewhere [23] . As for Co 3 Mo 3 C, the material is found to transform to a carbonitride upon reaction as evidenced by both the post-500 °C reaction CHN analysis as presented in Table 1 .…”

“…In addition, N vacancy sites have been shown to be important for N 2 activation on supported Co-Mo nanoparticles [22] . In an investigation of the isostructural Co 3 Mo 3 C system aimed at developing understanding of the potential role of lattice N in catalytic ammonia synthesis, onset of activity was found to occur at higher temperatures than that of Co 3 Mo 3 N and also to be associated with progressive substitution of lattice C by N during the course of the reaction [23] . Computational modeling studies have also indicated the importance of nitrogen vacancies in the Co 3 Mo 3 N system, with calculated vacancy concentrations being significant at temperatures of interest for ammonia synthesis and active for N 2 activation [24] .…”

Combination of theoretical and in situ experimental investigations of the role of lithium dopant in manganese nitride: a two-stage reagent for ammonia synthesis

“…While the mechanism of ammonia synthesis for nitride based materials is still not unequivocally elucidated, lattice nitrogen mobility and reactivity has been demonstrated in a number of nitride materials including mono-and bimetallic nitrides such as Ta 3 N 5 , Mg 3 N 2 and Co 3 Mo 3 N, leading to the possibility of ammonia production through pathways based upon the Mars-van Krevelen (MvK) mechanism. [1][2][3][4] In this mechanism, part of the lattice nitrogen can react directly with dihydrogen to produce ammonia resulting in the generation of nitrogen vacancies that can activate N 2 to complete the cycle.…”

The integration of experiment and computational modelling in heterogeneously catalysed ammonia synthesis over metal nitrides

“…Additional experimental evidence for the role of lattice nitrogen in the ammonia synthesis reaction comes from our studies of the Co 3 Mo 3 C and Co 6 Mo 6 C systems which are isostructural with their nitride counterparts. 31 Co 3 Mo 3 C is only active at temperatures above those required for Co 3 Mo 3 N and, following an initial lag period, the onset of activity is related the incorporation of lattice N in the 16c lattice site at 500 1C. Co 6 Mo 6 C, in which C is present in the 8a site, was not found to possess activity over prolonged testing periods.…”