Bixbyite-structured high-entropy oxide (CeZrYbErGd)₂O₃+δ for visible-light photocatalytic degradation of gallic acid: Performance and energy efficiency

This work reports the synthesis of a bixbyite-structured High Entropy Oxide (Ce₀.₂Zr₀.₂Yb₀.₂Er₀.₂Gd₀.₂)₂O₃₊δ via a co-precipitation route in an ammonia-based medium and its application as a visible-light-active photocatalyst for the degradation of gallic acid (GA), a model phenolic contaminant. Structural characterization demonstrated that calcination at 750 °C (CZYbEG-750) produced a nanocrystalline bixbyite-structured high-entropy oxide with lower crystallite size and defect-related vibrational features, while calcination at 1500 °C led to a well-crystallized system with bigger crystallites and very low surface area, detrimental to photocatalytic activity. Under optimal conditions (0.125 g L⁻¹ catalyst, 5 mg L⁻¹ GA initial concentration, pH 4.5), CZYbEG-750 achieved nearly complete GA removal (∼99%) and 72% mineralization after 180 min of visible light irradiation. The photocatalytic performance was strongly affected by solution pH, with spontaneous acidic conditions (pH 4.5) yielding the highest degradation efficiency. Response surface methodology (RSM) and ANOVA identified irradiation time and pH as the most significant factors influencing performance (R² = 0.9905). Mechanistic investigation based on reactive oxygen species (ROS) scavenger tests and band edge calculations identified photogenerated holes as the main oxidative species, with hydroxyl and superoxide radicals contributing synergistically to the degradation process. The catalyst showed good stability, maintaining activity over five reuse cycles with minimal loss of efficiency, and retained ∼85% degradation efficiency in tap water despite the presence of inhibitory inorganic ions. Compared to other photocatalytic systems, CZYbEG-750 exhibited competitive kinetics and remarkably low electrical energy consumption (∼40 kWh m⁻³), underscoring its energy efficiency. These findings highlight the potential of entropy-stabilized bixbyite-type high entropy oxides as robust and durable photocatalysts for the removal of recalcitrant, non-chromophoric pollutants in advanced water treatment applications.