Fazalurahman Kuttassery

A redox-noninnocent 2,2′-azobis(pyridine) (abpy) ligand-based molecular catalyst, [NiII(abpy)3][PF6]2 (Ni-Cat) has been developed for efficient and selective photochemical reduction of CO2 to CO. The molecular and electronic structures of [Ni(abpy)3]n (n = +2, +1, 0, and −1) were confirmed by single-crystal X-ray crystallography and DFT calculations. Ni-Cat catalyzes the photochemical reduction of CO2 to CO in the presence of [Ru(bpy)3](PF6)2 (Ru-PS) as a photosensitizer (λex = 470 nm) producing HCOOH and H2 as minor products. After 6 h of irradiation, the turnover number, turnover frequency, and selectivity for CO formation in DMA-TEOH (4:1 v/v) medium (TEOH = triethanolamine) were TONCO = 2358, TOFCO = 393 h–1, and SCO = ∼99%. The cyclic voltammogram of Ni-Cat in DMA-H2O (9:1 v/v) under argon displays redox waves due to [NiIIL3]2+/[NiII(L●–)L2]1+, [NiII(L●–)L2]1+/[NiII(L●–)2L], and [NiII(L●–)2L]/[NiII(L●–)3]− redox couples (L = abpy), respectively. However, in the presence of CO2, Ni-Cat undergoes reduction by 2e only. [Ru-PS]+ that is realized after irradiation reduces Ni-Cat by 2e producing an azoanion radical (L●–) complex, [NiII(L●–)2L]. Subsequently, L●– of [NiII(L●–)2L] reacts with CO2 producing hydrazine carboxylic acid that forms an ester with TEOH. Homolysis of the ester evolves CO gas releasing back L that undergoes reduction again and forms a catalytic cycle.

Faced with the new stage of water oxidation by molecular catalysts (MCs) in artificial photosynthesis to overcome the bottle neck issue, the “Photon-flux density problem of sunlight,” a two-electron oxidation process forming H₂O₂ in place of the conventional four-electron oxidation evolving O₂ has attracted much attention. The molecular characteristics of tin(IV)-tetrapyridylporphyrin (SnTPyP), as one of the most promising MCs for the two-electron water oxidation, has been studied in detail. The protolytic equilibria among nine species of SnTPyP, with eight pK_a values on the axial ligands’ water molecules and peripheral pyridyl nitrogen atoms in both the ground and excited states, have been clarified through the measurements of UV-vis, fluorescence, ¹H NMR, and dynamic fluorescence decay behaviour. The oxidation potentials in the Pourbaix diagram and spin densities by DFT calculation of the one-electron oxidized form of each nine species have predicted that the fully deprotonated species ([SnTPyP(O⁻)₂]²⁻) and the singly deprotonated one ([SnTPyP(OH)(O⁻)]⁻) serve as the most favourable MCs for visible light-induced two-electron water oxidation when they are adsorbed on TiO₂ for H₂ formation or SnO₂ for Z-scheme CO₂ reduction in the molecular catalyst sensitized system of artificial photosynthesis.

A novel dye-sensitized molecular photocathode with polypyrrole networks exhibits high efficiency and durability for photocatalytic CO₂ reduction by using water as reductant and visible light as energy.

Molecular catalyst sensitized system (MCSS), where an excited molecular catalyst adsorbed on semiconductor such as TiO2 injects electron to the conduction band of semiconductor leading to hydrogen evolution/CO2 reduction coupled with an oxi-dation of water on the molecular catalyst, has been one of the most probable candidates in the approach to artificial photo-synthesis. For a full utilization of visible light, however, a serious light scattering of the aqueous suspension of TiO2 in visible region, which is generally experienced, should be avoided. Here we report a preparation of optically transparent colloidal dispersion of TiO¬2 by sol/gel reaction of TiCl4 through progressive hydrolysis/condensation under the basic condition with-out any calcination processes. The TiO2 particles (TiO2(NPs)) obtained were characterized as an amorphous particle (~ 10-15 nm) having microcrystal domain of anatase within several nm by XRD, Raman spectroscopies, XRF, XAFS, TG/DTA, HRTEM, respectively. The energy resolved distribution of carrier electron traps in TiO2(NPs) as a fingerprint of TiO2 was characterized through a reversed double-beam photo-acoustic spectroscopy to have a close similarity with that of TiO2(ST-01) as well as the observation of carrier traps by transient absorption spectroscopy. Though the powder TiO2(NPs) itself was not dispersed well in aqueous solution, the wet TiO2(NPs) as prepared before being dried up provided a completely trans-parent aqueous dispersion under the acidic condition (1 M HCl). Addition of methanol enabled the colloidal dispersion (TiO2(NPs, MeOH/H2O, 0.1 M HCl)) to keep the optical transparency for longer than one year (550 days), which is the first example of TiO2 dispersion storable for such a long period. TiO2(NPs, MeOH/H2O) exhibited a moderate photocatalytic reac-tivity of H2 evolution with the quantum yield of ~ 2.6% upon 365 nm light irradiation. An optically transparent thin film of TiO2(NPs, MeOH/H2O) was also successfully prepared on a glass plate to exhibit an enhanced hydrophilicity upon UV light irradiation.

Two‐electron water oxidation initiated by one‐electron oxidation of aluminum porphyrins (AlTMPyP) is an alternative water oxidation to the conventional four‐electron pathway and could help to avoid the bottleneck subject of photon‐flux density in artificial photosynthesis. Here, a dramatic enhancement of the reactivity by bicarbonate ion in the two‐electron water oxidation to form H₂O₂ is reported. An addition of sodium carbonate (Na₂CO₃) controlled both catalytic current and product selectivity of the two‐electron water oxidation to enhance the activity of AlTMPyP at pH≈10–11. Controlled potential electrolysis experiments at different concentrations of Na₂CO₃ (10–100 mm) showed that peroxide selectivity was improved up to approximately 73 % by the increase of [Na₂CO₃] added to the system. The promotion of the reaction cycle was induced by an enhanced dynamic capturing of H₂O₂ from the hydroperoxy complex of AlTMPyP through an attack of a bicarbonate ion. The detailed electrochemical studies and product selectivity indicated that the bicarbonate ion served as a good cofactor for producing H₂O₂ from water. At stronger alkaline conditions (pH 12.5), however, a retardative effect of the addition of Na₂CO₃ on the catalytic reactivity was observed.