Ammonia Synthesis Over an Iron Catalyst with an Inverse Structure.

Achieving a substantial increase in the ammonia productivity of the Haber-Bosch (HB) process at low temperatures has been a significant challenge for over 100 years. However, the iron catalyst designed over 100 years ago remains at the forefront of this process because it is difficult to exceed the industrial iron catalyst in terms of the ammonia synthesis rate/catalyst volume that determines ammonia productivity in a reactor. Here, a new catalyst with an inverse structure of a supported metal catalyst that consists of metallic iron particles loaded with an aluminum hydride species is reported. The iron catalyst is readily prepared from an α-FeO precursor and ammonia could be synthesized at more than twice the ammonia synthesis rate/catalyst volume of the conventional industrial iron catalyst, even though the specific surface area of the former is only half that of the latter. In addition, ammonia synthesis over the catalyst is observed with a small apparent activation energy at 50 °C. Mechanistic studies suggested that an increase in the active sites with strong electron-donating capability on the iron catalyst significantly increased the ammonia synthesis rate/catalyst surface area, which resulted in high catalytic activity/catalyst volume.