Rhenium containing binary catalysts for ammonia synthesis Original Research Article

The rhenium nitride was prepared and applied for ammonia synthesis. Under the synthesis conditions (623 K, 0.1 MPa), the nitride was partly decomposed (Re3N+Re) but the activity was stable and higher (179 μmol h−1 g−1 at 623 K under 0.1 MPa) than the other transition metal nitrides (VN, NbN, Mo2N, and W2N), although the BET surface area was immeasurably low. The active phase was considered to be Re3N.The effect of transition metal addition to the rhenium catalyst for ammonia synthesis was investigated. Cobalt and iron were effective, but nickel, chromium, and copper were not effective. The best combination was Co-Re (1:4) (492 μmol h−1 g−1 at 623 K under 0.1 MPa, expressed as Co-Re4 catalyst). NH3 treatment of Co-Re4 at 973 K gave a new XRD pattern (probably some kind of rhenium nitride phase), which was suggested to be responsible to the high activity of Co-Re4 catalyst. The ammonia synthesis rate over Co-Re4 catalyst was increased under high pressure (2372 μmol h−1 g−1 at 623 K under 3.1 MPa), however, the rate overshot above 1.1 MPa. The kinetic analysis indicated that Re surface was strongly covered either by hydrogen or by nitrogen (ammonia). By the addition of cobalt, the degree of nitrogen (ammonia) poisoning was weakened, but at the same time, the degree of hydrogen poisoning was strengthened.