Influence of Meso-Substitution of the Porphyrin Ring on Enhanced Hydrogen Evolution in a Photochemical System

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• 作者：Ekaterina Koposova ; Xiao Liu ; Andrey Pendin ; Björn Thiele ; Galina Shumilova ; Yury Ermolenko ; Andreas Offenhäusser ; Yulia Mourzina
• 刊名：Journal of Physical Chemistry C
• 出版年：2016
• 出版时间：July 7, 2016
• 年：2016
• 卷：120
• 期：26
• 页码：13873-13890
• 全文大小：851K
• 年卷期：0
• ISSN：1932-7455

文摘

This study establishes the relationships between the structure of a series of meso-substituted tin(IV) porphyrins and their efficiency as photosensitizers for hydrogen generation in the Sn(IV)P/Pt–TiO<sub>2sub> nanocomposite system. The electrochemical properties of a series of SnPs, the catalytic performance of Pt nanomodifications, and the morphology of the Pt/TiO<sub>2sub> nanocomposites were characterized by electrochemical and electron microscopy methods. The dependence of photocatalytic performance on the structure for a series of Sn(IV) meso-substituted phenyl porphyrins was studied, and possible mechanisms are discussed employing the results of the electrochemical studies. It was found that the time course and type of the photochemically reduced species of Sn(IV)Ps, which are essential intermediates, are important factors and depend on the electronegativity of the metal center, the character of meso-substituents of the porphyrin ring, and pH and are correlated with the redox potential sequence of the respective Sn(IV)Ps: SnTMPyP > SnTPyP > SnTPPS > SnTPPC. Optimization of the experimental parameters was performed with regard to the SnPs with different functional groups, pH values, concentrations of Pt/TiO<sub>2sub>, light intensity, and Pt nanoparticles with different surface stabilizers. Finally, the maximum hydrogen yield under visible light was obtained from the system of Sn(IV) meso-tetra(4-pyridyl)porphyrin dichloride (SnTPyP) sensitized TiO<sub>2sub>/Pt prepared by the citrate method/EDTA at pH 9.0. This demonstrates that the photochemically reduced species of SnTPyP are relatively long lived, so they have enough time to complete electron transfer to TiO<sub>2sub> and/or Pt. The adsorption of SnTPyP on the TiO<sub>2sub>/Pt surface is therefore not essential for hydrogen generation. Moreover, this study demonstrates for the first time the synergic effect of the excitation of TiO<sub>2sub> and mostly Q-bands of Sn(IV)P (wavelength range 390–650 nm), which enhances the efficiency of photocatalytic hydrogen generation in the system. The Soret band of Sn(IV)TPyP was found to produce a minor (about 23%) contribution to the photocatalytic activity of the porphyrin sensitizer in this system. Possible processes involved are discussed, and mechanisms are proposed explaining different aspects of a series of photocatalytic systems with SnPs and Pt catalysts for hydrogen production under visible light. These structure–function relationships are essential to effectively harness solar energy for hydrogen production as well as for a wide range of energy and environmentally related problems.