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 H-2 selectively in the presence of aqueous Pd2+ complex solutions at 333K. Pd(NO3)(2) was the most effective in generating H-2 among various Pd2+ complexes explored. Pd2+ complexes were insitu reduced to Pd-0 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 H-2/CO2 gas generation, FA decomposition pathways using several Pd2+ species were explored using density functional theory (DFT) calculations. Rotation of formate bound to Pd2+, -hydride elimination, and subsequent CO2 and H-2 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 Pd-0 pathways were explored computationally either by generated H-2 or reductive elimination of CO2 and H-2 gas.