Prevailing surface reactions in the plasma-catalytic ammonia synthesis with Ru/CeO2 and Ru/Ti-CeO2

Sustainable plasma-catalytic NH3 synthesis is receiving an increasing amount of interest as a greener alternative for the conventional Haber-Bosch (HB) process. However, the best energy efficiency for the plasma synthesis reported so far is still low compared to the one of HB. This research was conducted to experimentally investigate one of the plasma process rate-limiting steps, which is N2 bond breaking. This study aimed at identifying the enhancing-surface reactions as well as limiting surface reactions such as H-inhibition. Ru/CeO2 and Ru/Ti-CeO2 were tested in a packed-bed DBD reactor to investigate the influence of TiO2 addition, which possibly prevented H-inhibition, and simultaneously enhanced different surface reactions. Variations of feed gas ratios and designed operation schemes were studied to elucidate the effect of the catalyst composition. Results showed that Ru/Ti-CeO2 outperformed Ru/CeO2 in terms of energy consumption and a N2-rich atmosphere was needed to obtain minima of 84.5 MJ mol−1 and 126.5 MJ mol−1, respectively. The addition of TiO2 in the catalysts provided more surface sites for H2 to adsorb on, which was identified as main reason for improving the energy efficiency of the plasma-catalytic NH3 synthesis. Three detailed reaction mechanisms were proposed to elaborate on the behavior between N2-rich plasma and Ru/Ti-CeO2.