The　design　of　an　efficient　non-noble　metal　catalyst　is　of　burgeoning　interest　for　ammonia　synthesis.　Herein,　we　report　a　Mo2C/CeO2　catalyst　that　is　superior　in　ammonia　synthesis　activity.　In　this　catalyst,　molybdenum　carbide　coexisted　with　the　ceria　overlayers　which　is　from　the　ceria　support　as　the　strong　metal-support　interaction.　There　is　a　high　proportion　of　low-valent　Mo　species,　as　well　as　high　concentration　of　Ce3+　and　surface　oxygen　species.　The　presence　of　Mo2C　and　CeO2　overlayers　not　only　leads　to　enhancement　of　hydrogen　and　nitrogen　adsorption,　but　also　facilitates　the　desorption　and　exchange　of　adsorbed　species　with　the　gaseous　reagents.　Compared　with　the　Mo/CeO2　catalyst　prepared　without　carbonization,　the　Mo2C/CeO2　catalyst　is　more　than　sevenfold　higher　in　ammonia　synthesis　rate.　This　work　not　only　presents　an　explicit　example　of　designing　Mo-based　catalyst　that　is　highly　efficient　for　ammonia　synthesis　by　tuning　the　adsorption　and　desorption　properties　of　the　reactant　gases,　but　opens　a　perspective　for　other　elements　in　ammonia　synthesis.