Synergistic-Tandem Active Sites Engineered in Pd-Doped Mo5N6 Nanosheets for Efficient CO2 Photoreduction to Acetic Acid

CO2 photoreduction into multicarbon products such as acetic acid represents a promising strategy for addressing the greenhouse effect and energy shortages. However, the process is limited by kinetically sluggish C–C coupling and inefficient proton-coupled electron transfer (PCET). Herein, we construct Nv-Pdδ+ tandem active sites in Mo5N6 nanosheets via low-valent Pdδ+ (2 < δ < 4) doping, which simultaneously introduces abundant stable nitrogen vacancies (Nv) and adjacent Pdδ+ centers. Combined experimental and theoretical analyses reveal that the Nv sites promote CO2 activation and reduce the energy barrier for *CO dimerization, while the neighboring Pdδ+ sites efficiently drive water splitting to supply H*, thereby facilitating the hydrogenation of key *OCCO intermediates. Accordingly, the optimized Pd0.5-Mo5N6 achieves a high acetic acid selectivity of 90.1% with an evolution rate of 33.24 μmol·g–1·h–1 under solar irradiation. This work establishes a tandem active-site design strategy for efficient and durable photocatalytic synthesis of multicarbon products from CO2.