Porphyrins are a group of heterocyclic organic compounds which are widely distributed in nature. Several metalloporphyrins have shown to be efficient catalysts for H2 generation both electrochemically and photochemically. A free base porphyrin by itself is very electroactive due to its macrocyclic conjugated system but there is not reported study of a metal-free porphyrin as electrocatalyst for H2 production. Here we present a 5, 10, 15, 20-tetrakis (pentafluorophenyl) porphyrin (H2TFPP) where the pentafluorophenyls serve as electron withdrawing groups to study its electrocatalysis for H2 generation. H2TFPP was successfully synthesized by Lindsey’s condensation and characterized via NMR and Mass Spectrum. Cyclic voltammetric measurements of H2TFPP displayed two reversible reduction waves at E1/2 = -0.76 V, -1.17 V (versus Ag/AgCl) in the absence of acid in THF. Upon addition of p-toluene sulfonic acid, a catalytic wave appeared at a potential around -0.85 V vs. Ag/AgCl, which indicated the reduction of H+ to evolve H2. The generation of H2 gas was later confirmed by gas chromatography with quantitative faradaic efficiency. By means of UV-Vis absorption, spectroelectrochemistry studies, Digisim simulations and DFT calculations, we were also able to study the mechanistic pathways during the catalysis and an EPPE (Electron- Proton-Proton-Electron) process was suggested to be the most favorable mechanism.

Fluorinated porphyrin as a metal-free electrocatalyst for hydrogen generation

251st National Meeting (2016)

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