The　construction　of　metal–organic　frameworks　that　possesses　the　tunable　metal　active　sites　and　synergistic　effect　for　the　reactant　activation　is　an　attractive　strategy　for　the　optimization　of　photocatalytic　performance.　Herein,　a　series　of　MIL-88A(Fe1-xCux)　were　synthesized　for　photocatalytic　nitrogen　reduction　reaction　(NRR).　It　was　characterized　that　the　Fe2+　and　oxygen　vacancies　were　produced　in　MIL-88A(Fe)　due　to　the　partly　reduction　of　ligand　fumaric　acid　in　synthesis　process.　The　substitution　of　Fe3+　with　Cu2+　induces　charge　imbalance　and　lattice　distortion,　further　increasing　the　content　of　Fe2+　and　oxygen　vacancies.　The　optimal　sample　MIL-88A(Fe0.95Cu0.05)　(CMA-5　%)　exhibits　the　highest　nitrogen　fixation　performance　of　68.6　μmol·g−1·h−1,　which　is　8　times　higher　than　that　of　the　pristine　material.　It　is　attributable　to　the　synergistic　effect　of　the　abundant　Fe2+　and　oxygen　vacancies　as　active　sites　to　promote　N2　coordination　and　photogenerated　carrier　separation,　resulting　in　the　efficient　conversion　of　activated　N2　to　NH3.　Furthermore,　the　in-situ　Diffuse　Reflectance　Infrared　Fourier　Transform　Spectroscopy　provide　clear　evidence　of　the　behavior　of　the　N2　adsorption　and　activation　on　the　catalyst.　Finally,　we　propose　a　potential　mechanism　at　molecular　level　regarding　the　relationship　between　the　synergistic　effect　and　the　nitrogen　fixation　activity.　©　2025　Elsevier　Inc.