Role of Bi Adatoms in Bi@Pd Nanoparticles for Electrochemical CO₂ Reduction

Surface modification of Pd by foreign metals is an appealing approach to achieve electrochemical CO₂ reduction to formate (CO₂RR-formate) with an overpotential close to zero. However, the exact role of surface dopants is still under debate. Here, we provide direct experimental proof that Bi adatoms on Pd nanoparticle surface are electrochemically active and geometrically assist electrochemical CO₂RR, by utilizing element-selective in situ X-ray absorption spectroscopy (XAS), which reveals that the partly oxidized Bi adatoms as prepared are fully reduced and form a surface single-atom coordinated only with Pd nanoparticles at 0 V_RHE, and are oxidized to Bi³⁺ valence state at 1.1 V_RHE. Electrochemical measurements show that the onset potential of the formate formation for Bi@Pd/C (0.05 V_RHE) is 0.1 V higher than that of Pd/C (-0.05 V_RHE), with the local concentration of formate at -0.15 V_RHE increasing from 1.6 to 4.6 mM. An oxygenated species chemisorbed on the Bi adatoms during CO₂RR is identified by operando XAS and DFT calculations and is tentatively attributed to an active HC(O*-Bi)₂ intermediate with direct Bi-O bonds. Our results clarify the long-term mysterious role of Bi adatoms in the CO₂RR process.