Here, noble metal doped two-dimensional metal oxide nanosheets are designed to realize selective CO 2 photoreduction to CH 4 . As a prototype, Pd-doped CeO 2 nanosheets are fabricated, where the active sites of Pd δ+ (2 < δ < 4) and Ce 3+ -O v are revealed by quasi in situ X-ray photoelectron spectra and in situ electron paramagnetic resonance spectra. Moreover, in situ Fourier-transform infrared spectra of D 2 O photodissociation and desorption verify the existence of the Pd-OD bond, implying that Pd δ+ sites can participate in water oxidation to deliver H* species for facilitating the protonation of the intermediates. Furthermore, theoretical calculations suggest the Pd doping could regulate the formation energy barrier of the key intermediates CHO* and CH 3 O*, thus making CO 2 reduction to CH 4 become the favorable process. Accordingly, Pd-doped CeO 2 nanosheets achieve nearly 100% CH 4 selectivity of CO 2 photoreduction, with the raising CH 4 evolution rate of 41.6 μmol g -1 h -1 .