Bicarbonate-Dependence for Pd-Catalyzed CO ₂ Hydrogenation to Formate over an Electronegativity-Induced Bimetallic Center

Palladium exhibits a near-zero overpotential for CO₂ reduction to formate via an electrohydrogenation pathway, but it undergoes a rapid deterioration due to surface CO accumulation. Herein, we conduct a systematic investigation into the bicarbonate electrolyte's proton-donating capacity in adjusting CO poisoning kinetics over an electronegativity-induced PdCu bimetallic center. The surface-H adsorption and lattice-H absorption features of Pd(alloy) in varying electrolytes are determined and quantified. Theoretical calculations incorporating electronic structure analyses reveal an electronegativity-driven charge redistribution, inducing negatively charged Pd and *H adatoms over the Pd(Cu)H with a downshifted d-band center, which greatly weakens *CO and *H adsorption/absorption onto Pd sites. Potentiodynamic X-ray diffraction, X-ray absorption spectroscopy, and anodic voltammetric scans confirm that the undesired electrochemical phase transition from α-PdH to β-PdH is significantly retarded by the incorporation of Cu. Moreover, the weakened *H-Pd interaction on the PdCu system enables the bicarbonate-rich electrolyte to enhance surface-H adsorption rather than subsurface-H absorption. Combining in situ infrared spectroscopy with differential mass spectrometry, it is experimentally identified that the concentrated bicarbonate electrolyte favors the maintenance of high *OCHO coverage, thereby delaying the formation of CO in a wider potential interval (-0.3 to -0.8 V versus a reversible hydrogen electrode). Based on the free energy profiles, we reveal that the preferred route of chemical hydrogenation (CH) and proton-coupled electron transfer (PCET) is closely correlated with the surface-H coverage. Moreover, the competition between the CH route and the PCET route on monometallic Pd and PdCu systems exhibits different responses to the degree of surface-H coverage. Our findings establish the bicarbonate electrolyte to be a unique regulatory factor for promoting Pd(alloy)-catalyzed CO₂ hydrogenation to formate.