Pt/MnO<sub>2</sub> catalyst for high-efficiency catalytic wet oxidation of formaldehyde wastewater at low temperature

Formaldehyde wastewater, which is mainly from plastics, paper, resin and other industries, not only causes serious environmental problems, but also gives harmful effects to human health. With the increasing environmental concerns, the abatement of formaldehyde wastewater has become an urgent problem. Compared with the conventional technologies, such as the biological treatment, catalytic wet air oxidation (CWAO) has been shown to be an effective and environmentally-friendly way to remove the formaldehyde in water. CWAO technology uses air or oxygen as the oxidant, where the organic can be directly oxidized into CO₂ and H₂O or partially converted to the less toxic compounds by suitable catalysts at elevated temperature and pressure. The key issue of CWAO is to develop a promising catalyst that can work efficiently at low temperature. MnO₂ is widely applied in the complete oxidation reactions due to its variable valence and abundant crystal structures, but is rarely used in the treatment of formaldehyde wastewater. In this article, Pt/R-MnO₂ (Pt supported on rod-like-MnO₂) with very low precious metal content (Pt, 0.1wt%) was prepared by impregnation method. For comparison, Pt/TiO₂ and Pt/CeO₂ catalysts were also prepared. The three catalysts with different supports were investigated in the CWAO of formaldehyde wastewater. Pt/TiO₂ showed the lowest activity and the TOC conversion is less than 35% even at the temperature of 100°C. Pt/CeO₂ exhibited better catalyst performance. As the reaction temperature increased from 30 to 100°C, the TOC conversion rose from 19.1% to 83.0%. Pt/R-MnO₂ displayed the most excellent catalytic performance compared with Pt/TiO₂ and Pt/CeO₂. At the reaction temperatures of 50 and 80°C, the TOC conversions of Pt/R-MnO₂ are 82.2% and 89.0%, respectively. MnO₂ possesses abundant morphologies, which may affect its catalytic performance. The effect of MnO₂ morphologies on the catalytic performance of Pt/MnO₂ in formaldehyde wastewater was investigated and the cocoon-like- and sheet-like-MnO₂ were also prepared. It was found that Pt/C-MnO₂ catalyst (Pt supported on cocoon-like-MnO₂) exhibited the best catalytic performance among the prepared Pt/MnO₂ catalysts. At the temperature of 50°C, Pt/C-MnO₂ can achieve TOC conversion of 88.1%. TOC conversion can be retained at a level higher than 80% even after four consecutive runs. Pt/S-MnO₂ (Pt supported on sheet-like-MnO₂) and Pt/R-MnO₂ (Pt supported on rod-like-MnO₂) showed lower activity and quicker deactivation with the reaction proceeding. After four consecutive runs, TOC conversion is decreased from 82.4% and 82.2% to 35.6% and 16.3% for Pt/S-MnO₂ and Pt/R-MnO₂, respectively. The Pt/C-MnO₂ showed the most excellent activity and stability for CWAO of aqueous formaldehyde at low temperature. Various characterizations were used to analyze the catalysts of Pt/C-MnO₂, Pt/R-MnO₂ and Pt/S-MnO₂. The measurement of dispersion indicated that Pt/C-MnO₂ catalyst has better dispersion, which is up to 36.0%, higher than 21.2% for Pt/S-MnO₂ and 17.3% for Pt/R-MnO₂. The results of XPS and O₂-TPD showed that Pt/C-MnO₂ contains more reactive oxygen species. H₂-TPR revealed that Pt/C-MnO₂ can be reduced at lower temperature and thus possesses better oxidizing ability. ICP analysis implied that Pt/C-MnO₂ has a better ability to inhibit the leaching of Pt. The Pt content of Pt/S-MnO₂ and Pt/R-MnO₂ is severely lost after four reactions, which caused a sharp decrease in the activity of the two catalysts. Better dispersion, more reactive oxygen species, higher oxidizing ability and better anti-leaching capability contribute to the excellent performance of Pt/C-MnO₂ in CWAO of aqueous formaldehyde reaction.