From nanoparticles to sub-nanoclusters: Pd catalysts unlock near-quantitative CO2-to-CO conversion.

Abstract

Electrochemical CO2 reduction to CO is a key technology for carbon utilization, but achieving high selectivity at low overpotentials is hindered by competitive hydrogen evolution, particularly on palladium-based catalysts. Here, we show that reducing palladium particle size to the sub-nanometer regime (∼1.2 nm) shifts selectivity from hydrogen evolution toward CO formation. Using a phenylazomethine dendrimer template, we synthesized size-controlled Pd sub-nanoclusters (Pd SNCs) supported on Vulcan carbon. Compared with conventional Pd nanoparticles, Pd SNCs exhibit high CO selectivity from -0.5 V vs. RHE and near-quantitative CO faradaic efficiency at -0.6 V, while suppressing H2 and formate formation. At -0.8 V, their total-Pd-normalized CO activity is approximately 5.5 times that of commercial Pd/C. Combined electrochemical, spectroscopic, and computational evidence supports an interpretation in which the finite, low-coordinate Pd environment facilitates *COOH formation, enabling the CO pathway to outcompete hydrogen evolution. The catalyst retained total current and CO selectivity during a 6 h continuous-electrolysis test. These findings identify control of local Pd coordination in the sub-nanometer regime as a route to selective and active catalysts for CO2-to-CO conversion.

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