pH-Control of the Photocatalytic Degradation Mechanism of Rhodamine B over Pb₃Nb₄O₁₃

Lead niobate Pb3Nb4O13 was synthesized via chemical coprecipitation method and additionally deposited on silica Aerosil nanoparticles. The thus synthesized catalysts were characterized by X-ray diffraction, UV−vis reflectance spectroscopy, light scattering, and electron microscopy. Several samples were tested for the photocatalytic rhodamine B degradation at different pH values focusing on a determination of the degradation mechanism, that is, either the direct oxidative mineralization to carbon dioxide and water or the deethylation yielding the 4-fold deethylated rhodamine 110. For unsupported lead niobates, the degradation rates increase with decreasing pH value, while pH variation has no influence on degradation rates of Aerosil supported samples. An adsorption model is introduced revealing that significant deethylation only proceeds provided that special interactions between the catalyst surface and the dye are existent. As the surface charge and the structure of the dye are both functions of the proton concentration, adjusting the pH value controls the deethylation process.