Herein, by modulating trivalent/tetravalent metallic elements, NiMLDHs (M = Al, Co, Fe, Mn, and Ti) were successfully prepared and evaluated in photocatalytic CO 2 reduction reaction (PCRR). Photocatalytic results declared that the electronic yields followed the order of NiTiLDH > NiCoLDH > NiFeLDH > NiMnLDH > NiAlLDH. Multiple characterizations affirmed that the introduction of various trivalent/tetravalent metallic elements could visibly affect the three critical aspects: (i) light harvesting; (ii) charge separation and transfer; and (iii) surface reactions, thus governing PCRR performance. Importantly, an in-depth mechanistic investigation was conducted by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments. These layered double hydroxides (LDHs) exhibited different adsorption/activation behaviors toward CO 2 molecule: NiAlLDH primarily converted CO 2 into b-CO 3 2- species; NiCoLDH, NiFeLDH, and NiMnLDH could induce c-CO 3 2- intermediate; NiTiLDH could generate a higher proportion of • CO 2 - species, which was an important intermediate to produce CO. More favorable carries separation and adsorption/activation process was presented upon NiTiLDH, thus more markedly enhancing photoactivity.