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 NN 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 ℃ 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.