Ba-Ce-based perovskite makes ammonia under mild conditions

Perovskites, materials that share the ABX3 stoichiometry and calcium titanate (CaTiO₃) crystal structure, are well known in solar cells. Scientists have found that by replacing the oxygen in perovskites with other ions, they can tune the properties of these cheap materials and use them in other applications. By swapping in N^3- and H^-, Hideo Hosono, Masaaki Kitano, and coworkers at the Tokyo Institute of Technology synthesized a BaCeO_3−xN_yH_z perovskite that makes ammonia via a greener path than the traditional Haber-Bosch process (J. Am. Chem. Soc. 2019, DOI: 10.1021/jacs.9b10726). “Our catalysts show very high activity in the range of 240–400 °C at 0.9 MPa,” says Kitano—much milder conditions than the traditional Haber-Bosch process, which runs at around 500 °C and 20 MPa. The researchers took a bottom-up approach to make the perovskite, flowing ammonia over barium amide and cerium dioxide at 300–600 °C for about 6 h. Conventional methods for making similar perovskites typically run at over 800 °C. Adding nanoparticles such as cobalt and iron increases the cataytic activity beyond that of conventional Ru-based ammonia-synthesis catalysts, Kitano says. The team also finds that the perovskite lowers the activation energy of ammonia synthesis by more than 40 kJ/mol and shifts the rate-limiting step from breaking the N-N bond to forming the N-H bond.