Engineering Pd-based charge-asymmetrical metal pair sites to promote *CHO–CHO coupling for selective C ₂ photoreduction to C ₂ H ₄

The photoreduction of CO₂ to C_2⁺ 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 excellent activity and selectivity toward C₂H₄. The as-designed Pd-loaded CdS nanospheres (Pd/CdS-Sv) featured Pd-Cd charge-asymmetrical sites co-manipulated by variable Pd loading and sulfur vacancies. They afford a C₂H₄ evolution rate as high as 14.2 μmol g^-1 h^-1, with a selectivity of up to 81.6%, outperforming most reported photocatalysts. <i>In situ</i> 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 catalyst exploitation for CO₂ photoreduction.