Pd-incorporated polyoxometalate catalysts for electrochemical CO₂ reduction

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 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.