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Self-assembled catalyst has the excellent selectivity and activity at low concentrations of CO 2. [Display omitted] • The self-assembled catalyst of CC3/Pd is synthesized. • Catalyst has the excellent selectivity and activity at low concentrations of CO 2. • The size effect of Pd nanoparticles has been systematically studied. • The mechanism of catalyst photoreduction CO 2 to CH 4 is studied. Porous heterogeneous photocatalysts containing both CO 2 adsorption and catalytically active sites can be might beneficial to achieve photoreduction of low concentrations of CO 2 to methane. Here, we show a self-assembled catalyst by combining catalytically-active Pd nanoparticles and porous organic cages for the CO 2 -to-CH 4 photoreduction reaction in the presence a molecular photosensitizer and a sacrificial reagent. The Pd nanoparticles is vital for the high CH 4 selectivity (over 97%) for the CO 2 photoreduction reaction, but they suffer from the low catalytic activity. When these Pd nanoparticles are equipped with an underneath support of porous organic cages, it dramatically boosts the catalytic activity with a high production rate of 78.5 μmol g-1h−1, while the CH 4 selectivity is up to ∼ 98% (electron basis). Importantly, when the photoreduction is performed under low concentration of CO 2 (10% concentration), over 98% of CH 4 selectivity and nearly 50% of the catalytic activity for the Pd-based catalysts retains. [ABSTRACT FROM AUTHOR]