Theoretical　studies　have　demonstrated　that　metal　atomic　clusters　are　highly　efficient　in　catalytic　ammonia　synthesis　under　mild　conditions.　However,　activation　of　N2　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　N2　activation　over　Co2　atomic　clusters　(Co2　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　Co2　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　N2　molecules.　As　such,　the　activation　energy　of　the　Co2　ACC　catalyst　in　NH3　synthesis　is　small.　Consequently,　the　catalytic　activity　of　the　developed　Ba-promoted　Co2　ACC　catalyst　reached　19.42　mmolNH3　gcat-1　h-1　at　400　°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　Co2　ACCs　has　general　applicability　as　demonstrated　in　the　preparation　of　Fe2　and　Mn2　atomic　clusters.　©　2022　American　Chemical　Society.