CO2 to CO Electroreduction, Electrocatalytic H2 Evolution, and Catalytic Degradation of Organic Dyes Using a Co(II) meso-Tetraarylporphyrin

The meso-tetrakis(4-(trifluoromethyl)phenyl)porphyrinato cobalt(II) complex [Co(TMFPP)] was synthesised in 93% yield. The compound was studied by 1H NMR, UV-visible absorption, and photoluminescence spectroscopy. The optical band gap Eg was calculated to 2.15 eV using the Tauc plot method and a semiconducting character is suggested. Cyclic voltammetry showed two fully reversible reduction waves at E1/2 = -0.91 V and E1/2 = -2.05 V vs. SCE and reversible oxidations at 0.30 V and 0.98 V representing both metal-centred (Co(0)/Co(I)/Co(II)/Co(III)) and porphyrin-centred (Por2-/Por-) processes. [Co(TMFPP)] is a very active catalyst for the electrochemical formation of H2 from DMF/acetic acid, with a Faradaic Efficiency (FE) of 85%, and also catalysed the reduction of CO2 to CO with a FE of 90%. Moreover, the two triarylmethane dyes crystal violet and malachite green were decomposed using H2O2 and [Co(TMFPP)] as catalyst with an efficiency of more than 85% in one batch.

Cobalt Tetrabutano- and Tetrabenzotetraarylporphyrin Complexes: Effect of Substituents on the Electrochemical Properties and Catalytic Activity of Oxygen Reduction Reactions

Three series of cobalt tetraarylporphyrins were synthesized and characterized by electrochemistry and spectroelectrochemistry. The investigated compounds have the general formula (TpYPP)Co, butano(TpYPP)Co^II, and benzo(TpYPP)Co^II, where TpYPP represents the dianion of the meso-substituted porphyrin, Y is a CH₃, H, or Cl substituent on the para position of the four phenyl rings, and butano and benzo are respectively the β- and β'-substituted groups on the four pyrrole rings of the compound. Each porphyrin undergoes one or two reductions depending upon the meso substituent and solvent utilized. Two irreversible reductions are observed for (TpYPP)Co^II and butano(TpYPP)Co^II in CH₂Cl₂ containing 0.1 M tetra-n-butylammonium perchlorate; the first leads to the formation of a highly reactive cobalt(I) porphyrin, which can then rapidly react with a solvent to give a Co^IIICH₂Cl as the product. Only one reversible reduction is seen for benzo(TpYPP)Co^II under the same solution conditions, and the one-electron-reduction product is assigned as a cobalt(II) porphyrin π-anion radical. Three oxidations can be observed for each examined compound in CH₂Cl₂. The first oxidation is metal-centered for the (TpYPP)Co and benzo(TpYPP)Co^II derivatives, leading to generation of a cobalt(III) porphyrin with an intact π-ring system, but this redox process is ring-centered in the case of butano(TpYPP)Co^II and gives a Co^II π-cation radical product. Each porphyrin was also examined as a catalyst for oxygen reduction reactions (ORRs) when adsorbed on a graphite electrode in 1.0 M HClO₄. The number of electrons transferred (n) during ORRs is 2.0 for the butano(TpYPP)Co^II derivatives, consistent with only H₂O₂ being produced as a product for the reaction with O₂. However, the reduction of O₂ using the cobalt benzoporphyrins as catalysts gave n values between 2.6 and 3.1 under the same solution conditions, thus producing a mixture of H₂O and H₂O₂ as the reduction product. This result indicates that the β and β' substituents have a significant effect on the catalytic properties of the cobalt porphyrins for ORRs in acid media.

Hydrophobicity effects in iron polypyridyl complex electrocatalysis within Nafion thin-film electrodes

Four polypyridyl redox catalysts Fe(bp)₃²⁺, Fe(ph)₃²⁺, Fe(dm)₃²⁺, and Fe(tm)₃²⁺ (with bp, ph, dm, and tm representing 2,2′-bipyridine, 1,10-phenanthroline, 4,4′-dimethyl-2,2′-bipyridine, and 3,4,7,8-tetramethyl-1,10-phenanthroline, respectively) are investigated for the electrocatalytic oxidation of three analytes (nitrite, arsenite, and isoniazid).