Enhancement of the ammonia synthesis activity of a Cs- or Ba-promoted ruthenium catalyst supported on barium niobate

Barium niobates with different crystalline structures and morphologies were prepared via a hydrothermal method and applied as a support for a ruthenium catalyst in ammonia synthesis. The sample synthesized with a nominal Ba/Nb ratio = 2.0, having a pure Ba5Nb4O15 crystalline phase and uniform flake-like structure, exhibited the best performance as a support in ammonia synthesis. The flake-like substrate favored the uniform distribution of ruthenium on its surface, which could promote ruthenium to expose more B5 sites. Addition of a Ba- or Cs-promoter enhanced the activity of the Ru/Ba5Nb4O15 catalyst markedly. The highest rate of ammonia synthesis over 2Cs- and 1Ba-4 wt% Ru/Ba5Nb4O15 was 4900 and 3720 (mu mol g-1cat h-1) at 0.1 MPa and 623 K, respectively. Both catalysts were stable during the reaction for 72 h at 673 K and 0.1 MPa. Thus, the synthesized Ba5Nb4O15 is expected to be a promising oxide support for ruthenium catalysts for ammonia synthesis. Ba- or Cs-Ru/Ba5Nb4O15 catalysts demonstrated a significant improvement of the ammonia synthesis rate.