The　design　of　an　efficient　catalyst　utilizing　non-precious　metals　is　a　top　priority　in　ammonia　synthesis,　and　Mo　is　found　to　be　inferior　in　ammonia　synthesis　due　to　strong　nitrogen　adsorption.　The　carburization　of　Mo　species　might　be　an　ideal　strategy　for　optimizing　nitrogen　adsorption　energy　and　ammonia　synthesis　activity.　In　this　work,　we　prepare　two　molybdenum　carbides　with　different　crystal　structures　(α-MoC　and　β-Mo2C).　The　result　shows　that　there　is　a　larger　number　of　Mo-C　bonds　and　lower　Mo/C　ratio　in　α-MoC,　leading　to　the　strong　adsorption　of　hydrogen　and　ammonia　species　on　α-MoC.　In　contrast,　the　desorption　energies　of　hydrogen　and　ammonia　are　lower　for　β-Mo2C,　and　the　spillover　and　migration　of　hydrogen　species　are　boosted.　The　facile　desorption　of　hydrogen　and　ammonia　species　also　limits　the　accumulation　of　H　adatoms　and　ammonia　on　the　surface　of　β-Mo2C,　releasing　of　active　sites　available　for　the　subsequent　activation　of　hydrogen　and　nitrogen　as　well　as　ammonia　synthesis　reaction.　As　a　result,　the　ammonia　synthesis　activity　of　β-Mo2C　catalyst　is　2.5-fold　higher　than　that　of　α-MoC,　and　a　small　negative　value　of　NH3　reaction　order　also　would　be　obtained　for　β-Mo2C.　This　work　not　only　reports　the　effect　of　molybdenum　carbide　phases　on　the　adsorption–desorption　properties　of　reactants　and　catalytic　activities　for　ammonia　synthesis,　but　also　opens　a　perspective　for　the　design　and　development　of　efficient　metal　based　heterogeneous　catalysts　by　tuning　the　adsorption　and　desorption　properties　of　the　reactant　gases.　Keywords:　Molybdenum　Carbide;　Phase　effect;　Adsorption–desorption　property;　Mo　catalyst;　Ammonia　synthesis　©　2022　Elsevier　Ltd