Citric Acid-Assisted Stabilization of Cu–La/Al2O3 Catalysts for Catalytic Wet Peroxide Oxidation of Phenol

Copper-based catalysts supported on γ-Al2O3 were prepared by wet impregnation and evaluated for the catalytic wet peroxide oxidation (CWPO) of phenol. Citric acid was used as a complexing agent to enhance copper stabilization, and lanthanum was incorporated as a structural promoter. The effects of calcination temperature, heating rate, Cu loading, and La incorporation route on catalyst structure and performance were systematically investigated. Thermal treatment and La incorporation-controlled phase evolution and copper oxidation state. Calcination at 900 °C promoted the development of CuAl2O4- and CuAlO2-type phases, as suggested by XRD, while XPS showed that the Cu2+/Cu+ ratio increased progressively with temperature, consistent with stronger metal–support interactions. Citric acid, incorporated at a CA:Cu molar ratio of 1:1, reduced copper leaching by up to 50% compared to catalysts prepared without the complexing agent, regardless of calcination temperature. Co-impregnated Cu–La catalysts achieved complete phenol conversion within 20–30 min and TOC removals of 84–95%, depending on synthesis conditions. The combination of La incorporation, calcination at 900 °C, and citric acid-assisted impregnation yielded the best stability–activity balance, with Cu5.0/La-A-900-1 showing 91% TOC removal and only 18% Cu leaching after 2 h of reaction. XPS, catalytic performance, and leaching results indicate that CWPO activity is governed by the balance between redox accessibility (Cu2+/Cu+) and structural stabilization of copper species. The results indicate that CWPO proceeds through a combined surface-mediated and homogeneous Fenton-like pathway, where the relative contribution of each depends on copper stabilization and leaching.