Unraveling Kinetic Effects during Photoelectrochemical Mineralization of Phenols. Rutile:Anatase TiO2Nanotube Photoanodes under Thin-Layer Conditions

Abstract

The kinetics of the photoelectrochemical mineralization of p-nitrophenol (PNP) and p-methoxyphenol (PMP) on rutile:anatase TiO2 nanotube electrodes (% rutile: 0, 16, and 20) has been studied. Oxidative photocurrent enhances with the increase of both the electrode potential and the rutile content. Time-resolved photoluminescence measurements reveal that incorporation of rutile into anatase nanotubes has effects on recombination dynamics. From measurements of electrical charge as a function of time under thin-layer conditions under potentiostatic control, it was determined that the oxidation of PNP and PMP follows the apparent Langmuir-Hinshelwood rate law. Chemical oxygen demand measurements confirmed total oxidation. The reaction rate parameters that define the kinetic constant (k) due to the reaction of hydroxyl radicals with adsorbed phenols and the thermodynamic constant (K) due to the adsorption of phenols with the electrode surface have been decoupled. The interaction of PNP with the surface of all TiO2 electrodes is highly favored, K(PNP) > K(PMP), resulting in a surface concentration of PNP considerably higher than that of PMP. The specific rate of the surface reaction for the transformation of phenols also increases, but the trend is inverse, that is, for all electrodes, k(PMP) > k(PNP). Understanding these kinetic tendencies can lead to better transformation of light in redox reactions.