Highly efficient subnanometer Ru-based catalyst for ammonia synthesis via an associative mechanism

Abstract

• N-doped carbon supported sub-nano Ru clusters (Ru-SNCs) catalyst was prepared. • Ru-SNCs catalyst showed a higher NH 3 synthesis rate than nanoparticle Ru catalyst. • Ru-SNCs catalyst can follow an associative route for NH 3 synthesis. The industrial manufacture of ammonia (NH 3 ) using Fe-based catalyst works under rigorous conditions. For the goal of carbon-neutrality, it is highly desired to develop advanced catalyst for NH 3 synthesis at mild conditions to reduce energy consumption and CO 2 emissions. However, the main challenge of NH 3 synthesis at mild conditions lies in the dissociation of steady N N triple bond. In this work, we report the design of subnanometer Ru clusters (0.8 nm) anchored on the hollow N-doped carbon spheres catalyst (Ru-SNCs), which effectively promotes the NH 3 synthesis at mild conditions via an associative route. The NH 3 synthesis rate over Ru-SNCs (0.49% (mass) Ru) reaches up to 11.7 mmol NH 3 ·(g cat) −1 ·h −1 at 400 °C and 3 MPa, which is superior to that of 8.3 mmol NH 3 ·(g cat) −1 ·h −1 over Ru nanoparticle catalyst (1.20% (mass) Ru). Various characterizations show that the N 2 H 4 species are the main intermediates for NH 3 synthesis on Ru-SNCs catalyst. It demonstrates that Ru-SNCs catalyst can follow an associative route for N 2 activation, which circumvents the direct dissociation of N 2 and results in highly efficient NH 3 synthesis at mild conditions.