Evaluating　the　effect　of　metal　surface　density　on　catalytic　performance　is　critical　for　designing　high-activity　metal-based　catalysts.　In　this　study,　a　series　of　ceria　(CeO2)-supported　Ru　catalysts　(Ru/CeO2)　were　prepared　to　analyze　the　effect　of　Ru　surface　density　on　the　catalytic　performance　of　Ru/CeO2　for　ammonia　synthesis.　For　the　Ru/CeO2　catalysts　with　Ru　surface　densities　lower　than　0.68　Ru　nm−2,　the　Ru　layers　were　in　close　contact　with　CeO2,　and　electrons　were　transferred　directly　from　the　CeO2　defect　sites　to　the　Ru　species.　In　such　cases,　the　adsorption　of　hydrogen　species　on　the　Ru　sites　in　the　vicinity　of　O　atoms　was　high,　leading　to　a　high　ammonia　synthesis　activity　and　strong　hydrogen　poisoning.　In　contrast,　the　preferential　aggregation　of　Ru　species　into　large　particles　on　top　of　the　Ru　overlayer　resulted　in　the　coexistence　of　Ru　clusters　and　particles,　for　catalysts　with　a　Ru　surface　density　higher　than　1.4　Ru　nm−2,　for　which　Ru　particles　were　isolated　from　the　direct　electronic　influence　of　CeO2.　Consequently,　the　Ru−CeO2　interactions　were　weak,　and　hydrogen　poisoning　can　be　significantly　alleviated.　Overall,　electron　transfer　and　hydrogen　adsorption　synergistically　affected　the　synthesis　of　ammonia　over　Ru/CeO2　catalysts,　and　catalyst　samples　with　a　Ru　surface　density　lower　than　0.31　Ru　nm−2　or　exactly　2.1　Ru　nm−2　exhibited　high　catalytic　activity　for　ammonia　synthesis.　©　2021　Dalian　Institute　of　Chemical　Physics,　the　Chinese　Academy　of　Sciences