Size-Dependent Activity of Palladium Nanoparticles: Efficient Conversion of CO₂ into Formate at Low Overpotentials

Remarkable size-dependent activity of palladium nanoparticles (PdNPs) towards formate production is evident at very low overpotentials (-0.1 to -0.5 V vs. RHE). Size-selective PdNPs, chemically synthesized at sizes of 3.8-10.7 nm, effected an electrochemical CO₂ reduction reaction in aqueous 0.5 m NaHCO₃ . The faradaic efficiency of formate production (FE_formate ) on 3.8 nm PdNPs exceeded 86 % at E=-0.1 V versus RHE, whereas on 6.5 nm PdNPs an even higher FE_formate of 98 % was observed. However, FE_formate decreased for larger PdNPs. The superior efficiency towards formate production at low overpotentials is rationalized in terms of a changed catalytic pathway through PdH phases. The observed maximum in the formate efficiency for a mean particle size of about 6.5 nm is discussed in terms of counterbalancing the size-dependent effects of a competing CO₂ reduction reaction and a parasitic hydrogen evolution reaction. Production rates of formate are also remarkably high at -0.3 V versus RHE with 539.9 and 452.3 ppm h^-1  mg_Pd^-1 for the 6.5 and 3.8 nm PdNPs, respectively.