Oxidation Degradation of Formaldehyde over Goethite Calcinated Product and Hexagonal Birnessite Composite at Ambient Temperature

Birnessite has exhibited moderate activity in room-temperature oxidation degradation of formaldehyde and is considered as the most promising material for formaldehyde degradation. However, birnessite also exposed insufficient surface active sites to meet the needs for long-term and sufficient degradation of formaldehyde. As a cheap and easily obtainable mineral, goethite not only has a large specific surface area and high oxidation-reduction activity, but also usually appears as nanoscale particles. Hematite can be prepared via calcination of goethite, which can maintain the original needle like structure of goethite, and has a larger specific surface area than that of goethite. It is speculated that using hematite obtained by heat treatment of goethite to support hexagonal birnessite might increase the surface active sites of the catalyst, thereby improving the formaldehyde degradation activity at room temperature. Therefore, in this study, goethite was used as a precursor to prepare hematite with different specific surface areas by calcining method at different temperatures. The structure and physicochemical properties of the catalyst, the activity and mechanism of the catalyst for the oxidation and degradation of low concentration formaldehyde at room temperature were studied.

The synthesis method of K0.6hbir was as follows: Dissolve KMnO₄ (4.00 g), MnCl₂ (8.35 g), and NaOH (2.88 g) in 130, 130 mL, and 140 mL of deionized water, respectively. Firstly, slowly added KMnO₄ solution to (within 5 min) NaOH solution and stirred for 1 h. After that, MnCl₂ solution was added into the above mixture within 35 min and stirred for 2 h, and then aged for 4 h. After wash and centrifuge for 3 times, the generated precipitate was dried at 70 ℃. The synthesis method of hematite and birnessite composite was as follows: Goethite was calcined in a muffle furnace at 200, 250, 300, 400 ℃, and 500 ℃ for 2 h, respectively. The obtained hematite was named as Hem-X, where X represents the calcination temperature. Add the above sample to 140 mL of NaOH solution (0.18 mol/L) and stir vigorously for 30 min. Then, synthesize hematite composite birnessite sample using the method of preparing birnessite. The amount of birnessite in the composite was 30% (mass fraction), and the sample was named as K0.6hbir@Hem-X. The phase and structure of the catalyst was characterized using X-ray diffractometer (XRD, Dandong Haoyuan, DX-2700), Raman spectrometer (RAMAN, LabRAM HR Evolution), high-resolution transmission electron microscopy (TEM, JEM-2100F), respectively. The specific surface area was measured using a BET adsorption desorption instrument (BET, NOVA 3000e). Surface elemental valence states of the sample was characterized by X-ray fluorescence spectroscopy (XPS, ESCALAB250Xi). The in-situ degradation process of formaldehyde was carried out in a Fourier transform infrared spectrometer (FTIR, VERTEX 70, Bruker). The dynamic experiment for catalytic oxidation of formaldehyde was carried out in a quartz tube. 1 mg/L formaldehyde gas can be stably obtained by volatilization of polyformaldehyde under constant temperature and circulating cooling water conditions at 10 ℃. The total gas flow rate was 1 L/min corresponding to a WHSV of 1200 L/(g·h). The formaldehyde concentration before and after the reaction were detected online by a formaldehyde detector (Interscan 4160-19.99 m). CO₂ was detected by gas chromatography (GC-2020) under the formaldehyde concentration of 10 mg/L.

Hematite obtained by calcining goethite at 300 ℃ shows a higher formaldehyde removal rate which can maintain a formaldehyde removal rate at about 90% within 2 h, as compared with the other hematite samples and goethite. Based on the results of specific surface area, the formaldehyde removal ability of hematite was closely related to its specific surface area. The K0.6hbir@Hem catalysts also exhibited the same trend as that of the bulk hematite, and had superior removal performance than hematite. K0.6hbir@Hem-300 had the best catalytic activity and maintained a removal rate of over 95% for formaldehyde within 300 min. In particular, the K0.6hbir@Hem-300 catalyst can maintain almost 100% formaldehyde removal rate in 24 h when the WHSV was 400 L/(g·h), but showed a significant decrease with the increasing of WHSV. The results of in-situ DRFITS and quenching experiments indicated that the ·O₂^– species promoted the transformation of DOM into formate, while the ·OH species had the effect on the completely conversion of the intermediates to CO₂ and H₂O during the oxidation degradation of formaldehyde.

K0.6hbir@Hem-300 had superior formaldehyde oxidation degradation performance, achieving almost 100% formaldehyde (1 mg/L) removal rate within 24 h at 400 L/(g·h) WHSV with no activity loss over 5 cycles. The relative humidity significantly affected the oxidation degradation performance of the composite. The hematite and hexagonal birnessite composite had higher CO₂ yield (55%) than that of the goethite and hexagonal birnessite composite (40%), indicating that the obtained hematite was a more suitable support than the original goethite. The specific surface area of K0.6hbir@Hem-300 (116.5 m²/g) further increased compared with that of Hem-300 (86.7 m²/g), which was beneficial to the adsorption of formaldehyde during the degradation processes. The introduction of hematite increased the contents of Mn (Ⅲ) and surface active oxygen species over the composite, leading to the enhancement of oxidation degradation performance of formaldehyde. In particular, the ·O₂^– species promoted the transformation of DOM into formate, while the ·OH species had the effect on the completely conversion of the intermediates to CO₂ and H₂O during the oxidation degradation of formaldehyde.