Method of H₂ Transfer Is Vital for Catalytic Hydrodefluorination of Perfluorooctanoic Acid (PFOA)

The efficient transfer of H₂ plays a critical role in catalytic hydrogenation, particularly for the removal of recalcitrant contaminants from water. One of the most persistent contaminants, perfluorooctanoic acid (PFOA), was used to investigate how the method of H₂ transfer affected the catalytic hydrodefluorination ability of elemental palladium nanoparticles (Pd⁰NPs). Pd⁰NPs were synthesized through an in situ autocatalytic reduction of Pd²⁺ driven by H₂ from the membrane. The Pd⁰ nanoparticles were directly deposited onto the membrane fibers to form the catalyst film. Direct delivery of H₂ to Pd⁰NPs through the walls of nonporous gas transfer membranes enhanced the hydrodefluorination of PFOA, compared to delivering H₂ through the headspace. A higher H₂ lumen pressure (20 vs 5 psig) also significantly increased the defluorination rate, although 5 psig H₂ flux was sufficient for full reductive defluorination of PFOA. Calculations made using density functional theory (DFT) suggest that subsurface hydrogen delivered directly from the membrane increases and accelerates hydrodefluorination by creating a higher coverage of reactive hydrogen species on the Pd⁰NP catalyst compared to H₂ delivery through the headspace. This study documents the crucial role of the H₂ transfer method in the catalytic hydrogenation of PFOA and provides mechanistic insights into how membrane delivery accelerates hydrodefluorination.