Selective CO 2 -to-CH 4 Photoconversion Enabled by High-Valent Pd 4+ Species on PdO-Anchored Co 3 O 4 Nanosheets

Abstract Photocatalytic CO2 methanation offers a direct route to renewable energy storage, yet its efficiency is hampered by rapid charge recombination and inert CO2 activation. Herein, we report the rational design of PdO-anchored Co3O4 nanosheets featuring high-valent Pd4+ species. Synchrotron-radiation X-ray absorption fine structure spectroscopy, X-ray photoelectron spectroscopy, and Bader charge calculations confirm the formation of the high-valent Pd4+ species. In situ X-ray photoelectron spectroscopy unveils that the CO2 reduction sites are switched from Co species to highly oxidized Pd species over the PdO/Co3O4 nanosheets. In situ Fourier-transform infrared spectroscopy and electron paramagnetic resonance spectra further elucidate that Pd4+ accelerates the critical *COOH-to-*OCH3 hydrogenation step. Density functional theory calculations indicate that the introduction of Pd4+ lowers the excitation energy (from 1.62 to 1.03 eV) and reduces the *CO2-to-*COOH energy barrier by nearly half (from 1.27 to 0.70 eV). Consequently, the best-performing catalyst achieves a CH4 evolution rate of 136.6 μmol g–1 h–1 with 93.8% selectivity. This study offers deep insights into the mechanism of high-valent species in steering CO2 hydrogenation pathways.