Differences in the Oxidizing Species on the Surface of Metal-Doped Ferrihydrite and Hematite in the Alkaline S(IV)-O₂ Solution

Abstract

Although inorganic Fe(IV)�O oxo and Fe(III)-OOH peroxo have attracted more attention in aqueous environments, direct evidence for the characterization and identification of those oxidizing species is still lacking. In this study, the doped Cu/Co increases the reactivity of ferrihydrite/hematite in oxidizing As(III) by S(IV)-O 2 under alkaline and dark conditions. Quenching experiments exclude the contributions of •OH, O 2 •− , SO 3 •− , SO 4 •− , SO 5 •−, HSO 5 − , and H 2 O 2 and confirm that peroxo and oxo are primary oxidizing species in As(III) oxidation. Electrochemical analyses indicate that Fe(IV)�O oxo and Fe(III)-OOH peroxo are primarily located on the surface of ferrihydrite and hematite. Selective oxidation of methyl phenyl sulfoxide and methyl phenyl sulfide to the corresponding oxidation products further reveals the presence of �Fe(IV)�O oxo on both ferrihydrite and hematite, and the doped Cu/ Co significantly decreases the produced �Fe(IV)�O oxo on ferrihydrite, while increases the produced �Fe(IV)�O oxo on hematite. Meanwhile, selective oxidation of benzyl alcohol to benzaldehyde indicates the presence of �Fe(III)-OOH peroxo on both ferrihydrite and hematite, and the doped Cu/Co apparently increases the produced �Fe(III)-OOH peroxo on ferrihydrite and hematite. Raman analysis further reveals the presence of �Fe(IV)�O oxo and �Fe(III)-OOH peroxo on pure and metal-doped ferrihydrite/hematite, which also confirms that the doped Cu/Co decreases the produced �Fe(IV)�O oxo on ferrihydrite and increases �Fe(IV)�O oxo on hematite, while apparently increasing �Fe(III)-OOH peroxo on both ferrihydrite and hematite. This study enriches our understanding on the oxidizing species in the S(IV)-mediated oxidation processes under alkaline conditions, providing new insight on the oxidation reactivity of surface peroxo and oxo species mediated by the doped metal ions.

Highly Selective Photooxidation of Benzyl Alcohol to Benzaldehyde by Bi–Mo/g-C3N4 in Aqueous Solution

“…In addition, the significant decrease in the conversion of benzyl alcohol to 20% in the presence of CAT also indicates the important contribution of the peroxo species (Figure a). Considering that oxidation of benzyl alcohol to benzaldehyde by Bi 2.15 WO 6 is a H-abstraction reaction, which is accompanied by the production of H 2 O 2 , the DPD colorimetric method was therefore used to determine variations of H 2 O 2 in the reaction system. , Figure a shows that the concentrations of H 2 O 2 increase from 46 to 230 μM with increasing concentrations of benzyl alcohol (0–80 mM) after 150 min of reaction, confirming that the H-abstraction pathway is also involved in the photooxidation of benzyl alcohol by Bi–Mo/CN-2. The positive proportion between the concentrations of benzyl alcohol and produced H 2 O 2 also confirms that benzyl alcohol is the H-donor in the production of H 2 O 2 (Figure b).…”

“…Alkaline conditions are favorable in initiating H-abstraction from benzyl alcohol to benzaldehyde and facilitate stabilization of the surface peroxo. , However, the strong alkaline conditions inevitably bring about problems in equipment corrosion and subsequent separations . As an ideal matrix, the N atoms in g-C 3 N 4 can act as alkaline sites and favor the stabilization of the peroxo intermediates, thus facilitating the two-electron oxygen reduction reaction for H 2 O 2 production .…”