Efficient Ru/MgO–CeO2 catalyst for ammonia synthesis as a hydrogen and energy carrier

Abstract

CeO 2 -supported Ru (Ru/CeO 2 ) is considered an efficient catalyst for ammonia synthesis but the use of CeO 2 (expensive rare earth oxide) increases the cost of the catalyst limiting its application at large/plant scale. The objective of this study was to develop a relatively low-cost efficient catalyst for plant scale ammonia synthesis. For this purpose, in this study, a mixed oxide of MgO–CeO 2 (Mg/Ce molar ratio = 1/1) was prepared and used as a support for the Ru catalyst. The prepared 3 wt-% Ru/MgO–CeO 2 catalyst was evaluated for ammonia synthesis efficiency and was also compared with the Ru/CeO 2 . Though the Ru/MgO–CeO 2 had a lower CeO 2 concentration than Ru/CeO 2 , it showed equivalent ammonia synthesis activity. The activation energies, and H 2 , N 2 , and NH 3 orders for Ru/MgO–CeO 2 and Ru/CeO 2 were very close to each other which confirmed the equivalent efficiency of both catalysts for ammonia synthesis. Moreover, H 2 -TPR results showed very close temperature ranges for the appearance of reduction peaks for both catalysts. Containing a lower amount of CeO 2 , Ru/MgO–CeO 2 was evaluated as an efficient and cost-effective catalyst as compared to Ru/CeO 2 . The optimization study shows that H 2 /N 2 ratio, temperature, and pressure conditions were strongly dependent on each other affecting the ammonia synthesis efficiency. The reaction conditions were optimized to 375 °C, 2.5 MPa gauge pressure using a reactant feed with H 2 /N 2 ratio 1 for Ru/MgO–CeO 2 catalyst. • Ru/MgO–CeO 2 with Mg/Ce molar ratio 1/1 is an efficient catalyst for ammonia synthesis. • Ru/MgO–CeO 2 showed equivalent efficiency to that obtained with pure CeO 2 supported Ru. • Ea and H 2 , N 2, and NH 3 orders of both Ru/MgO–CeO 2 and Ru/CeO 2 were very close. • Optimization of reaction temperature, pressure, and H 2 /N 2 ratio was conducted. • H 2 /N 2 molar ratio strongly influenced the ammonia synthesis activity.