The application of nitrogen isotopic labelling in operando powder neutron diffraction studies of metal nitride ammonia synthesis catalysts

Recently we found that doping tungsten into Co 3 Mo 3 N reduces its ammonia synthesis activity, despite indications that nitrogen is still mobile in its lattice and similar energies for surface nitrogen vacancy formation. In this short paper, we demonstrate operando powder neutron diffraction (PND) to directly compare the behaviour of Co 3 Mo 3 N and W-doped Co 3 Mo 2.6 W 0.4 N as a method, to understand nitrogen exchange and mobility differences. PND is ideal for this purpose due to the large scattering length of nitrogen (9.37 fm natural abundance or 14 N 9.36 fm) and good contrast with 15 N (6.44 fm), that lead to sensitivity to nitrogen site behaviours during the changes. • Nitrogen environment was tracked with isotopic labelling powder neutron diffraction. • Nitrogen removal occurred more rapidly in W-doped Co 3 Mo 3 N. • All the lattice 14 N in the main Co 3 Mo 3 N type phase were replaced by 15 N.