CO2　adsorption　is　a　critical　step　for　CO2　photoreduction,　especially　in　diluted　CO2,　whereas　an　in-depth　understanding　of　CO2　adsorption　effects　is　still　lacking.　Herein,　isostructural　NiCo2O4　and　MgCo2O4　hierarchical　hollow　nanocages　(NiCo2O4　HCs　and　MgCo2O4　HCs)　have　been　ingeniously　constructed　for　diluted　CO2　photoreduction.　NiCo2O4　HCs　exerts　an　apparent　quantum　yield　of　1.56%　with　CO　selectivity　of　89%,　exceeding　most　previous　inorganic　catalysts　in　pure　CO2.　Nevertheless,　MgCo2O4　HCs　is　almost　inert,　even　though　it　shows　higher　CO2　uptake　ability.　DFT　calculations　results　indicate　that　CO2　molecule　adsorbed　on　Ni　sites　can　be　readily　reduced　to　CO,　while　the　CO2　on　Mg　sites　cannot　participate　in　this　reaction.　Therefore,　only　adsorbed　CO2　which　can　participate　in　reduction　reactions,　named　active　adsorption,　can　accelerate　the　whole　reactions.　This　work　unearths　atomic-level　insights　into　the　relation　between　CO2　adsorption　and　reduction,　providing　fundamental　guidance　to　improve　photocatalytic　performance.　©　2019　Elsevier　B.V.