Engineering Pd-based Charge-Asymmetrical Metal Pair Sites to Promote *CHO-CHO Coupling for Selective CO2 Photoreduction to C2H4

The photoreduction of CO₂ to C₂₊ products is primarily limited by the kinetic challenges of C-C coupling. Here, we engineer Pd-based charge-asymmetrical metal pair sites to accommodate the energetically favourable CHO-CHO coupling pathway, accomplishing the topmost activity and selectivity toward C₂H₄. The as-designed Pd-loaded CdS nanospheres Pd/CdS-Sv) featured the Pd-Cd charge-asymmetrical sites co-manipulated by variable Pd loading and sulfur vacancies. They afford the C₂H₄ evolution rate as high as 14.2 μmol·g^-1·h^-1, with a selectivity of up to 81.6%, which outperform most of reported photocatalysts. In situ diffuse reflectance infrared Fourier transform spectra distinctly identify the favourable CHO-CHO coupling pathway on Pd/CdS-Sv, which benefits from the obviously shortened C-C bond of 1.453 Å on the Pd-Cd sites as compared to that in CO dimerization 3.508 Å) according to theoretical calculations. The introduction of Pd promotes water dissociation and provides sufficient H to enable the conversion of CO to CHO, and more importantly lowers the energy barrier of the CHO-CHO coupling on the charge-asymmetrical pair sites from 0.37 eV to -0.29 eV, thereby avoiding the sluggish CO-CO dimerization. Gaining new insights into engineering charge-asymmetrical sites to effectively perform C-C coupling pathways, this work will expedite the catalyst exploitation for CO₂ photoreduction.