Photoactivity of Metal-Phenylporphyrins Adsorbed on TiO2 Under Visible Light Radiation: Influence of Central Metal

Abstract

A set of Co, Cu, Zn and metal-free phenylporphyrins were studied by spectroscopic (UV-vis, FTIR) and quantum-chemical methods. The Q and Soret bands were identified in the UV-vis spectra of solid samples. In all the complexes the frontier molecular orbitals (OMs) predict that the electronic processes sites are localized on the ligand rather than in the metal atom. Metal ion has a largely influence on energy of OMs. The calculated values of electronic transitions between the OMs are in good agreement with the UV-vis data. Phenyl porphyrins were attached onto TiO2 to be evaluated as photocatalysts, under visible light irradiation. The interaction of porphyrins with TiO2 surface was investigated using UV-vis and FT-IR spectroscopies and it was found that the dyes were adsorbed to the semiconductor by carboxylate groups. Degradation of luminol and photooxidation of terephthalic acid to 2-hydroxyterephthalic acid (TAOH) were employed as probe reactions. Luminol was degraded from 20 – 60% in presence of O2. In the same way, the formation of TAOH values of comprises between 0.12 – 0.17 mM, in presence of H2O2 and O2 was obtained. It was found that superoxide anion radicals (O2•_) are primarily formed and they are the precursor for the hydroxyl radicals (•OH) production when H2O2 is added to suspension. Influence of metal on photoactivity was analyzed, specifically, in terms of: the nature of metal (number of d electrons), photophysical properties and energies of frontier molecular orbitals (OMs). Apparently, energy of OMs is an important factor which could affect photoactivity of sensitizers attached on TiO2