An unprecedented oxalate-functionalized Ta/W polyoxometalate enabling the self-assembly of a 2D composite for catalytic hydrogenation

Abstract

An oxalate-functionalized polyoxometalate Na₂H₃₈[P₈W₆₀Ta₁₂O₂₄₂(C₂O₄)₄]·56H₂O (C₂O₄-POM) was synthesized by the reaction of the Ta/W mixed-addenda precursor [P₂W₁₅Ta₃O₆₂]⁹⁻ with oxalic acid and fully characterized using various techniques. The C₂O₄-POM polyanion is a tetragonal cluster composed of four Wells–Dawson {P₂W₁₅Ta₃O₆₂} fragments and four oxalate ligands (C₂O₄²⁻), with each oxalate chelating a Ta atom in a bidentate fashion. The stability of C₂O₄-POM was studied using phosphorus nuclear magnetic resonance spectroscopy, solid-state and liquid-state infrared spectroscopy, and ultraviolet spectroscopy, which reveal that C₂O₄-POM can maintain its structure for at least six months over a pH range of 0.5 to 3.0. Based on the planar structure with an oxalate “antenna” and the high stability of C₂O₄-POM, a composite material (Pd/(C₂O₄-POM)@rGO) was synthesized by loading C₂O₄-POM and Pd particles into graphene through a one-pot photoreduction strategy. TEM measurements indicate the uniform distribution of C₂O₄-POM and palladium nanoparticles on the graphene. Theoretical calculations indicate that C₂O₄-POM may interact with graphene through covalent bonding. Pd/(C₂O₄-POM)@rGO showed good catalytic performance as a heterogeneous catalyst for the hydrogenation of olefin under mild conditions. The hydrogen spillover effect was found to play a crucial role in enhancing its catalytic activity.