Effect of the metal oxide nature on the catalytic selectivity of PANI@g-C3N4@metal oxide (TiO2 or Er2O3) nanocomposites toward organic pollutant reduction: a comparative study

The persistent discharge of synthetic dyes and nitroaromatic compounds into aquatic environments continues to challenge conventional wastewater treatment, motivating the rational design of multicomponent nanocatalysts able to operate under mild conditions. Two novel ternary nanocomposites, polyaniline@g‑C 3 N 4 @TiO 2 (PANI@CTI) and polyaniline@g‑C 3 N 4 @Er 2 O 3 (PANI@CER), were synthesized by thermal polymerization of melamine in the presence of the metal oxide followed by in situ oxidative polymerization of aniline. The materials were thoroughly characterized by XRD, FTIR, XPS, SEM, TEM/STEM, TGA, photoluminescence, and zeta potential measurements. The combined data confirmed the coexistence of crystalline anatase TiO 2 or cubic Er 2 O 3 , semicrystalline polyaniline, and g‑C 3 N 4 within a strongly coupled hybrid framework, with PANI@CER displaying a higher oxide loading and dispersion. Hybridization shifted the polyaniline emission from the red region toward intense violet and blue bands and displaced the isoelectric point from pH ≈ 4.94 (PANI) to 5.28 (PANI@CTI) and 6.91 (PANI@CER). The catalytic reduction revealed a marked substrate dependent selectivity following pseudo first order kinetics: PANI@CTI was the most active toward cationic MB (kapp = 1.1326 min –1 ), whereas PANI@CER outperformed all benchmark catalysts for anionic MO (0.6914 min –1 ) and 4‑NP (0.1682 min –1 ). The selectivity inversion was rationalized by correlating surface charge, oxide dispersion, and electronic structure with the ionic nature of the pollutant, establishing a versatile platform for selectivity engineered wastewater treatment.