Pd-incorporated polyoxometalate catalysts for electrochemical CO2 reduction

Abstract

Polyoxometalates (POMs), representing anionic metal–oxo clusters, display diverse properties depending on their structures, constituent elements, and countercations. These characteristics position them as promising catalysts or catalyst precursors for electrochemical carbon dioxide reduction reaction (CO₂RR). This study synthesized various salts—TBA⁺ (tetra-n-butylammonium), Cs⁺, Sr²⁺, and Ba²⁺—of a dipalladium-incorporated POM (Pd2, [γ-H₂SiW₁₀O₃₆Pd₂(OAc)₂]⁴⁻) immobilized on a carbon support (Pd2/C). The synthesized catalysts—TBAPd2/C, CsPd2/C, SrPd2/C, and BaPd2/C—were deposited on a gas-diffusion carbon electrode, and the CO₂RR performance was subsequently evaluated using a gas-diffusion flow electrolysis cell. Among the catalysts tested, BaPd2/C exhibited high selectivity toward carbon monoxide (CO) production (ca. 90%), while TBAPd2/C produced CO and hydrogen (H₂) with moderate selectivity (ca. 40% for CO and ca. 60% for H₂). Moreover, BaPd2/C exhibited high selectivity toward CO production over 12 h, while palladium acetate, a precursor of Pd2, showed a significant decline in CO selectivity during the CO₂RR. Although both BaPd2/C and TBAPd2/C transformed into Pd nanoparticles and WO_x nanospecies during the CO₂RR, the influence of countercations on their product selectivity was significant. These results highlight that POMs and their countercations can effectively modulate the catalytic performance of POM-based electrocatalysts in CO₂RR.