Synthesis of acetonitrile from NH₃/syngas mixtures on molybdenum nitride: Insights into the reaction mechanism

Abstract

Owing to their metallic-like surface electronic properties and their capacity to act as reservoirs and solid transfer agents for active nitrogen, transition metal nitrides are interesting as solid catalysts for C-C and C-N coupling reactions for the bottom-up production of higher (C2+) nitrogenated chemicals from unconventional carbon resources. The catalytically active state and reaction mechanism for the direct synthesis of acetonitrile from syngas/ammonia mixtures are studied on an unsupported Mo catalyst from complementary experimental and computational approaches. Temperature resolved X-ray diffraction and X-ray photoemission spectroscopy verify that an oxidic MoO(3 )catalyst precursor undergoes in situ (near-surface) nitridation, upon exposure to reaction conditions at 723 K, rendering Mo2N the actual working catalyst. Density Functional Theory mechanistic investigations on a gamma- Mo 2 N(100) model surface point to a hydrogen-assisted CO dissociation on the nitride surface. Moreover, surface oxygen, evolved from CO dissociation, is predicted to play a central role as hydrogen acceptor, to enable the dehydrogenative NH3 dissociation. Direct condensation of CH and N adspecies proceeds with a low energy barrier of 33 kJ mol(-1), which makes C-N coupling preferred over full hydrogenation of CHx species, in agreement with the experimental modest selectivity to methane (ca. 10 %). Both experimental and computational results indicate that HCN is a major intermediate product along the reaction pathway to acetonitrile. No energetically feasible associative reaction pathways could be identified for C-C coupling from HCN. The dissociation of the latter intermediate product is predicted to precede the reaction of CN adspecies to CHx. Similarly to NH3 dissociation, dehydrogenative HCN activation on the Mo2N 2 N surface is predicted to be facilitated through hydrogen abstraction by surface oxygen species, yet subjected to a comparatively higher energy barrier (>120 kJ mol(-1)), therefore likely to control the overall kinetics. These findings suggest that the enhancement of HCN dissociation is a central design objective towards Mo2N-based 2 N-based catalysts with advanced performance.