13 - Noble metals-based nanocatalysts for electrochemical NNR

Abstract

Ammonia is an important raw material, which is widely applied in industry and agriculture fields. Due to the relatively high hydrogen content, ammonia also shows application potential as a hydrogen storage material. However, the domain industrial method for ammonia synthesis is the Haber–Bosch process while the synthesis process has to be carried out at high temperature and high pressure, which requires a lot of energy. At the same time, hydrogen, which is one of the reactants, needs to be obtained by steam reforming of natural gas and the CO2 by-product is the main greenhouse gas. In order to solve the shortcomings of traditional Haber–Bosch process, scientists have focused on the electrochemical catalytic synthesis of ammonia in recent years. Ammonia can be facilely synthesized from N2 and H2O in a mild environment and the electrochemical nitrogen reduction reaction (NRR) is therefore recognized as a clean and reproducible method for ammonia synthesis. However, since N2 is relatively very stable in ambient condition, the ammonia generation rate and Faraday efficiency of electrochemically catalyzed NRR are relatively low. There is still a long way to reach the standard of industrial synthetic ammonia. Therefore, catalysts with high catalytic activity and selectivity toward NRR are widely studied. Among the new developed catalysts, the precious metal catalyst exhibits relatively high ammonia synthesis rate and Faraday efficiency. At the same time, it has relatively great stability compared with cost-effective catalysts. This chapter mainly reviews the research progress of noble metal catalysts and the performance and mechanism of each catalyst in the electrochemical synthesis of ammonia. Finally, some perspectives are made on the shortcomings of noble metal catalysts and future research and development.