Revisiting group 4–7 transition metals for heterogeneous ammonia synthesis

Ammonia is a key small molecule for manufacturing nitrogen-based fertilizers and organic chemicals and equally important for renewable energy storage and conversion. The available Haber–Bosch ammonia synthesis process using fused iron catalysts operated under harsh conditions is, however, unsustainable. The development of alternative and more efficient approaches to sustainable ammonia production has garnered much attention recently. Most of the prior work has been devoted to the investigation of Fe, Ru or Co-based metal catalysts for ammonia synthesis. In comparison, there are very limited studies on group 4–7 transition metals, because they are prone to form metal nitrides, which are difficult to hydrogenate to ammonia. This mini-review summarizes recent advances in activating these metals for heterogeneous ammonia synthesis. We show that the potential properties of group 4–7 transition metals for ammonia synthesis should be revisited, which may lead to the development of more efficient materials or chemical processes for ammonia production under mild conditions.