Role of Surface Charge in the Speciation and Photocatalytic Degradation of 4-Nitrophenol Using ZnO–CeO2–WO3 Photocatalysts

Understanding how the surface charge environment governs pollutant–catalyst interactions is essential for designing efficient photocatalysts. In this study, ZnO–CeO2–WO3 composite materials were synthesized through a simplex-centroid mixture design to evaluate their photocatalytic activity toward the degradation of 4-nitrophenol (4-NPOH) under UV irradiation. The materials were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), UV–Vis diffuse reflectance spectroscopy (DRS), photoluminescence (PL), nitrogen adsorption–desorption (BET/DFT) and scanning electron microscopy (SEM). Photocatalytic experiments were conducted without pH adjustment to analyze the intrinsic behavior of each oxide and their mixtures. The acid–base equilibrium of 4-NPOH (pKa = 7.2) allowed evaluating its deprotonation to 4-nitrophenolate (4-NP−) and its interaction with the catalyst surface, which depends on the point of zero charge (pHPzc) of ZnO, CeO2, and WO3. The Zn–W binary system (ZnWO4 phase) exhibited the highest activity, achieving 81% degradation efficiency and the largest apparent rate constant (k = 5.1 × 10−3 min−1). However, a 51% decrease in activity was observed after three reuse cycles, attributed to WO3 leaching induced by the interaction between 4-NPO− and zinc tungstate hydroxide (Zn[W(OH)8]). This work establishes a direct correlation between surface charge, pollutant speciation, and photocatalytic performance, providing a mechanistic framework for understanding pH-dependent degradation processes over multicomponent oxide composites.