Theoretical　studies　have　demonstrated　that　metal　atomic　clusters　are　highly　efficient　in　catalytic　ammonia　synthesis　under　mild　conditions.　However,　activation　of　N-2　over　atomic　clusters　from　an　experimental　viewpoint　has　not　been　systematically　investigated.　Here,　for　the　first　time,　cobalt　atomic　dimers　on　a　nitrogen-carbon　support　were　synthesized　for　NH3　synthesis.　It　is　disclosed　that　N-2　activation　over　Co-2　atomic　clusters　(Co-2　ACCs)　is　entirely　different　from　that　over　Co　single-atom　catalysts　(Co　SACs)　or　Co　nanoparticles　(Co　NPs).　A　suite　of　in　situ　spectroscopy　studies　reveal　that　Co-2　ACCs　not　only　provide　active　sites　to　promote　strong　metal-support　interactions　but　also　initiate　strong　interclustering　interactions,　resulting　in　electron　transfers　from　Co　d-orbitals　to　antibonding　orbitals　of　N-2　molecules.　As　such,　the　activation　energy　of　the　Co-2　ACC　catalyst　in　NH3　synthesis　is　small.　Consequently,　the　catalytic　activity　of　the　developed　Ba-promoted　Co-2　ACC　catalyst　reached　19.42　mmol(NH3)　g(cat)(-1)　h(-1)　at　400　degrees　C　and　1　MPa,　which　is　much　superior　than　that　of　other　Co-based　catalysts　under　such　mild　conditions.　More　importantly,　the　synthesis　strategy　of　Co-2　ACCs　has　general　applicability　as　demonstrated　in　the　preparation　of　Fe-2　and　Mn-2　atomic　clusters.