Self‐organized Ruthenium–Barium Core–Shell Nanoparticles on a Mesoporous Calcium Amide Matrix for Efficient Low‐Temperature Ammonia Synthesis

Abstract

A low‐temperature ammonia synthesis process is required for on‐site synthesis. Barium‐doped calcium amide (Ba‐Ca(NH2)2) enhances the efficacy of ammonia synthesis mediated by Ru and Co by 2 orders of magnitude more than that of a conventional Ru catalyst at temperatures below 300 °C. Furthermore, the presented catalysts are superior to the wüstite‐based Fe catalyst, which is known as a highly active industrial catalyst at low temperatures and pressures. Nanosized Ru–Ba core–shell structures are self‐organized on the Ba‐Ca(NH2)2 support during H2 pretreatment, and the support material is simultaneously converted into a mesoporous structure with a high surface area (>100 m2 g−1). These self‐organized nanostructures account for the high catalytic performance in low‐temperature ammonia synthesis.

Enhanced Ammonia Synthesis Activity of Ceria-Supported Ruthenium Catalysts Induced by CO Activation

“…The turnover frequencies for Ru catalysts with CO activation are comparable to those of Ru catalysts reported in literatures (Table S2). 7,9,[52][53][54][55][56][57] As shown in Figure S9, Ru/CeO 2 -C250H and Ru/CeO 2 -C500H show stability better than that of Ru/CeO 2 -H. The estimated activation energies are 60, 67 and 72 kJ mol -1 for Ru/CeO 2 -H, Ru/CeO 2 -C250H and Ru/CeO 2 -C500H, respectively, suggesting that the significant discrepancy in ammonia synthesis activity among the three cannot be directly attributed to the change of activation energy. The reaction orders of N 2 are 1.0, 1.2 and 1.3 for Ru/CeO 2 -H, Ru/CeO 2 -C250H and Ru/CeO 2 -C500H, respectively, and the corresponding reaction orders of NH 3 are -0.72, -0.75 and -0.79.…”

Compositional flexibility in Li–N–H materials: implications for ammonia catalysis and hydrogen storage

“…19 The broader significance of compositional variation in M-N-H materials is illustrated by recent reports of their use as strong promoters for ammonia synthesis in ruthenium catalysts. [20][21][22] The formation of solid solutions of electride material dicalcium nitride (Ca 2 N) and calcium nitride hydride (Ca 2 NH) under ammonia synthesis conditions has been hypothesised to promote high activity through (i) the formation of a defect structure which lowers the material work function, increasing electron donation to the ruthenium particles and (ii) the ability to act as a reservoir for hydrogen, avoiding the inhibition of catalytic activity by hydrogen which is often observed for ruthenium catalysts. 22 It is clear that the impressive performance of the M-N-H materials in these applications relates to compositional flexibility, which derives from the close structural relationships between the phases involved.…”

Recent progress towards mild-condition ammonia synthesis

“…In addition to LiH, the alkaline earth metal hydrides, such as BaH 2 , could also synergize with Co or Mn 4 N leading to activities that are approximately 20-and 2-times higher than that of highly active Cs-Ru/MgO at 250 °C, respectively [62,63] . Similarly, Ru supported on amides/imides catalysts which were found highly active for ammonia decomposition also performed well for ammonia synthesis [64,65] .…”