Ammonia synthesis over the Ba-promoted ruthenium catalysts supported on boron nitride

Abstract.

Barium promoted ruthenium catalysts deposited on the boron nitride supports were characterised (XRD, O₂ and CO chemisorption) and tested in NH₃ synthesis. Prior to use, the raw BN materials marked as BNS (Starck, 96 m²/g) and HCV (Advanced Ceramics Corporation Cleveland USA, 40 m²/g) were heated in an ammonia stream at 700–800 °C for 120 h. As a result, the oxygen content was reduced from 7.0 at% (BNS) to 3.5 at% (BNS_NH3) and from 3.8 to 2.7 at% (HCV_NH3), as evidenced by XPS. The kinetic studies of NH₃ synthesis (63 or 90 bar; H₂:N₂ = 3:1) revealed that the catalysts based on the modified supports were more active, respectively, than those derived from starting nitrides, the difference being especially pronounced in the case of BNS and BNS_NH3. Studies of the catalysts activation have shown, in turn, that the stabilisation in a H₂:N₂=3:1 mixture at 1bar is very slow, i.e. the reaction rate increases slowly versus time on stream even at a high temperature of 550 – 600°C. Stabilisation is faster and the NH₃ synthesis rates are higher when the activation is performed with an ammonia rich mixture (10% NH₃ in H₂:N₂=3:1) flowing under high pressure of 90 bar. It is suggested that boron oxide (an impurity) acts as a deactivating agent for Ba–Ru/BN and that the reaction between NH₃ and B₂O₃ (B₂O₃+2NH₃=2BN +3H₂O) is responsible for the activity increase. A poisoning mechanism of B₂O₃ is discussed.