The development of more active catalysts for the production of ammonia will have a worldwide impact on the energy cost of producing fertiliser. Composites of lithium hydride and transitions metals/metal nitrides have recently been identified as highly active ammonia synthesis catalysts. Their activity has been explained by a nitrogen-shuttling mechanism between the metal/metal nitride and lithium hydride, resulting in the formation of lithium amide. We request four days on POLARIS to measure neutron powder diffraction data on these catalyst composites during the ammonia synthesis reaction. These data will provide insights into the active form of the catalysts and the nature of their remarkable catalytic effect on this critical industrial reaction.