Direct Activation of Cobalt Catalyst by 12CaO·7Al2O3 Electride for Ammonia Synthesis

Cobalt is well-known as an active component of heterogeneous solid catalysts or metal-complexes for the reduction of dinitrogen into ammonia. However, the activity of bare Co metal itself is not high because of its low nitrogen adsorption energy. Here, we show that the ammonia synthesis activity of a Co catalyst can be significantly boosted by 12CaO·7Al2O3 electride (C12A7:e–) with a low work function of 2.4 eV at low reaction temperatures (200–400 °C) compared with that for typical Co catalysts. The ammonia formation reaction is initiated at 200 °C for Co/C12A7:e–, whereas the reaction initiation temperature for Co/C12A7:O2– without electrons is 400 °C. Therefore, Co/C12A7:e– has a much lower activation energy (ca. 50 kJ mol–1) than Co/C12A7:O2– (113 kJ mol–1), which is a similar level to that of the state-of-the-art Co-based catalysts such as Co3Mo3N and LiH-Co, in which ammonia formation over the Co catalyst is mediated by the formation of nitrides such as MoN x and LiNH. On the other hand, C12A7:e– directly enhances the activity of the Co catalyst for the N2 dissociation reaction without the aid of nitride formation, which results in the highest turnover frequency with low activation energy for ammonia synthesis at low reaction temperatures.