Size‐Dependent Activity of Palladium Nanoparticles: Efficient Conversion of CO 2 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 CO 2 reduction reaction in aqueous 0.5  m NaHCO 3 . 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 CO 2 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.