Pd2+ -Initiated Formic Acid Decomposition: Plausible Pathways for C-H Activation of Formate

Abstract

Formic acid (HCOOH, FA) has long been considered as a promising hydrogen-storage material due to its efficient hydrogen release under mild conditions. In this work, FA decomposes to generate CO₂ and H₂ selectively in the presence of aqueous Pd²⁺ complex solutions at 333 K. Pd(NO₃ )₂ was the most effective in generating H₂ among various Pd²⁺ complexes explored. Pd²⁺ complexes were in situ reduced to Pd⁰ species by the mixture of FA and sodium formate (SF) during the course of the reaction. Since C-H activation reaction of Pd²⁺ -bound formate is occurred for both Pd²⁺ reduction and H₂ /CO₂ gas generation, FA decomposition pathways using several Pd²⁺ species were explored using density functional theory (DFT) calculations. Rotation of formate bound to Pd²⁺ , β-hydride elimination, and subsequent CO₂ and H₂ elimination by formic acid were examined, providing different energies for rate determining step depending on the ligand electronics and geometries coordinated to the Pd²⁺ complexes. Finally, Pd²⁺ reduction toward Pd⁰ pathways were explored computationally either by generated H₂ or reductive elimination of CO₂ and H₂ gas.

Keywords: decomposition pathway; density functional theory; formic acid; hydrogen storage; palladium.