Eco-innovation in action: Using rice husk-derived silica for highly dispersed iron catalysts in the oxidation of phenol

Highly dispersed iron catalysts on rice husk silica are obtained by deposition-precipitation assisted by hydrolysis with urea, controlling the hydrolysis (OH/metal molar ratio 5:1) up to pH 4.5 and low load of Fe³⁺ as active phase (0.25–5 %). X-ray diffraction analysis presumes a good dispersion of the metallic phase, while H₂ temperature-programmed reduction suggests a reducibility from 49 to 100 % (5 to 0.25%wt Fe). Thermogravimetric analysis reveals good thermal stability of the catalysts. The IR analysis evidences the ≡Si-O-Fe interaction through a reduction in the bands of silanol groups, and an in situ diffuse reflectance infrared spectroscopy study with carbon monoxide on the reduced catalysts confirms the high dispersion of the metal phase in the catalysts. The catalytic activity was investigated in the wet oxidation reaction of phenol (5 × 10⁻⁴ M) with H₂O₂. The catalysts achieve phenol conversions >80 % and high selectivity to CO₂ (37–52 %) in just 20 min, which greatly exceeds the catalyst obtained by conventional impregnation, which only achieves 54 % conversion and 26 % selectivity.