Preparation,characterization And Mechanism Of Manganese Dioxide Catalysts For Maldehyde Purification At Low Temperatures

Formaldehyde is one of the main pollutants in indoor air.Long-term exposure can seriously harm people’s health.Studies have shown that catalytic oxidation is regarded as the most efficient and clean way,and manganese dioxide has great potential in formaldehyde oxidation because of its good oxidation,strong environmental adaptability and low cost.People usually improve the performance of oxidized formaldehyde by adjusting the morphology,structure,crystal phase and atomic doping of manganese dioxide.In this paper,the catalytic oxidation performance and mechanism of manganese dioxide to formaldehyde under photocatalysis and thermal catalysis were studied by preparing catalysts with different crystal phases and crystal faces.The main conclusions are as follows:1.Nano-flowersγ-MnO₂ catalysts with exposed（300）,（160）and（131）crystal faces were synthesized,and different exposed crystal faces showed different physicochemical properties.Compared withγ-MnO₂-160 andγ-MnO₂-131 catalysts,γ-MnO₂-300 catalysts had more oxygen vacancies and high surface lattice oxygen activity,and the surface adsorbed oxygen content is high,showing the highest formaldehyde oxidation performance.The mechanism analysis showed that HCHO was adsorbed on the surface of MnO₂ and activated to obtain dioxymethylene（DOM）,which existed in the form of Mn-O-CH₂-O-Mn.Then DOM was oxidized to formate by surface lattice oxygen.Finally,formate was oxidized to CO₂ and H₂O by surface lattice oxygen and adsorbed oxygen.2.Using hydrothermal method and low-temperature,controlled precipitation by redox（CPR）method to synthesize nano-flowerδ-MnO₂ andγ-MnO₂,nano-rod-likeβ-MnO₂catalyst,oxidation of formaldehyde at room temperature has excellent performance.Theδ-MnO₂ catalyst has the best performance and achieves complete conversion of formaldehyde at the lowest 35℃,δ-MnO₂have the weakest Mn-O bond,the most oxygen vacancy,and the surface lattice oxygen and surface adsorbed oxygen activity were the strongest,which significantly improved the performance of formaldehyde oxidation reaction.The mechanism analysis shows that formaldehyde adsorbed onδ-MnO₂ forms intermediate product dioxymethylene（DOM）,and DOM is oxidized by surface lattice oxygen and surface adsorbed oxygen to form formaldehyde species,resulting in CO and H₂O products.The adsorbed CO is further oxidized to CO₂,andγ-MnO₂ andβ-MnO₂ are consistent with the path ofδ-MnO₂,but formate may decompose directly into CO₂.3.We preparedδ-MnO₂,γ-MnO₂ andβ-MnO₂ catalysts,and studied their photocatalytic performance of formaldehyde.δ-MnO₂,γ-MnO₂ andβ-MnO₂ completely transformed formaldehyde at 10℃,25℃and 75℃,respectively.Compared withγ-MnO₂ andβ-MnO₂,δ-MnO₂ has narrower band gaps,better light absorption and more oxygen vacancies,resulting in the formation of more superoxide radical and hydroxyl groups,which helps to improve the catalytic oxidation performance of formaldehyde.