Al-Doped magnetite encapsulated in mesoporous carbon: a long-lasting Fenton catalyst for CWPO of phenol in a fixed-bed reactor under mild conditions

Abstract

Al-Doped magnetite spinel nanoparticles encapsulated in mesoporous carbon (MC) were identified as a promising heterogeneous Fenton catalyst towards phenol degradation in a continuous system for practical applications. Under the working conditions inside the fixed-bed reactor hercynite in the fabricated 21%γ-Fe₂O₃/28%FeAl₂O₄@MC material reacts with H₂O₂. In this reaction Al ions occupied the vacant octahedral cationic sites in the framework of the γ-Fe₂O₃ component converting it to Al substituted magnetite spinel. The Al in the obtained Fe³⁺_0.66Fe²⁺_0.33(Fe²⁺_0.33Fe³⁺_0.33Al³⁺_0.33)₂O₄@MC polarizes electrons of iron ions through its Lewis acid properties imparting more positive charge on iron ions (Fe^n+(δ+)). This expedites the rate of the challenging reduction reaction Fe³⁺ → Fe²⁺ with H₂O₂ to produce HOO˙ and reinforces the bonding of iron ions in the spinel improving their activity and stability. Hence, under mild operating conditions (pH 5, 40 °C, 8.6 ml water per ml cat h⁻¹, 0.036 mol H₂O₂, 200 ppm phenol) the in situ produced catalyst Fe(Fe_0.66Al_0.33)₂O₄@MC of 35 nm containing 19.9%Fe and 2.4%Al with a surface area of 335 m² g⁻¹ exhibited long-lasting high catalytic activity and stability during a 500 h run. 80% TOC conversion and ∼1 ppm of leached Fe in treated water were obtained without a visible change of catalytic performance.