Preparation Of Supported Catalysts Based On Manganese-cerium Composite Oxide And Its Catalytic Oxidation Of HCHO

HCHO is a very hazardous and common volatile gas.Among the various methods for HCHO removal,catalytic oxidation is the most promising technology.Compared with noble metal catalysts,inexpensive and readily available transition metal catalysts have become the focus of current research,especially manganese oxide catalysts with high catalytic activity and diverse oxidation states,etc.However,this type of catalyst often has low activity and needs to be improved by chemical modifications.In this paper,birnessite-type manganese dioxide was taken as the research object.First,the manganese-cerium oxide prepared by metal doping showed high catalytic activity.On the basis of the above experiments,in order to improve the stability of the catalyst,this paper used a carrier of Ti O₂ to support the manganese-cerium oxide,and the prepared catalyst had good stability.At the same time,in order to further prepare an inexpensive manganese-based catalyst that could efficiently realize the catalytic oxidation of HCHO,the resource-rich sepiolite was used to support manganese-cerium oxide,and the obtained catalyst had good HCHO catalytic activity.At the same time,the catalysts were characterized and analyzed by a series of means such as XRD,BET,XPS,SEM,and H₂-TPR,which further provided theoretical guidance for the follow-up work.（1）The doping of metal ions can adjust the physical and chemical properties of manganese dioxide.In this paper,metals of potassium and cerium were introduced to enhance the activity of Mn O₂ for catalytic oxidation of HCHO.On the one hand,the catalytic oxidation performance of Mn O₂ was investigated by alkali metal modification（K）.Compared with the K_0.01Mn catalyst obtained by the one-step hydrothermal method,HK_0.01Mn catalyst obtained via acid treatment followed by potassium modification could completely convert about 550 ppm of HCHO at 140^oC.On the other hand,the effect of different metals（Cu,Co,Ce）doping on the catalyst activity was investigated and the doping ratio was preferred,and the results showed that Ce_0.5Mn₁₀could completely convert HCHO at 100^oC and the complete conversion temperature was about 70^oC lower than that under Mn O₂.The stability test of Ce_0.5Mn₁₀ was carried out for 32 h.In the first 20 h,the conversion rate of HCHO hardly decreased,but then,the conversion rate gradually decreased.The results showed that the introduction of a small amount of cerium could greatly promote the activity of manganese dioxide,but its stability needed to be further improved.（2）Based on the research of the optimal manganese-cerium catalyst,the activity and stability of the catalyst were further improved by loading the catalyst on a conventional carrier.It was found that Mn Ce O_x（20）/Ti O₂ showed 70%conversion at100^oC,while Mn Ce O_x（20）/Al₂O₃,Mn Ce O_x（20）/Si O₂and Mn Ce O_x（20）/HZSM-5 showed only 16%,22%and 17%conversion at 100^oC,respectively.By investigating the loading,it was found that Mn Ce O_x（30）/Ti O₂ at 30%loading showed 100%conversion at 90^oC,and the catalyst had good stability in the 32 h stability test.The BET test results showed that the specific surface area of Mn Ce O_x（30）/Ti O₂ increased to 146 m2/g after the manganese-cerium oxide was supported by Ti O2,and the active sites on the catalyst were more easily exposed.At the same time,XPS test results showed that the ratios of Mn³⁺/Mn⁴⁺and O_ads/O_latt of Mn Ce O_x（30）/Ti O₂ increased from 0.675 and 0.732（Mn Ce Ox）to 0.944 and 0.887,respectively,which might be the main reason for their better catalytic activity.（3）A series of Mn Ce Ox/sep catalysts were successfully synthesized by hydrothermal method by modifying sepiolite with a huge specific surface area and abundant hydroxyl groups.The results showed that the raw sepiolite-supported catalyst Mn Ce Ox/raw sep could completely convert about 550 ppm of HCHO at about 180^oC,72000 m L/g·h.After the sepiolite was modified with hydrochloric acid,Mn Ce Ox/3H sep could completely convert HCHO at 150^oC.Mn Ce O_x/3H-0.018C sep catalyst was obtained by continuing to use CTAB to modify the acid-treated sepiolite,which showed a better HCHO removal rate and could completely convert HCHO at about 110^oC.The catalyst had good stability in the 32 h stability test During the reaction,the hydroxyl groups in sepiolite possibly promoted the adsorption of HCHO on the catalyst surface and might accelerate the formation and decomposition of the formate.Characterization analysis by BET,XPS,H2-TPR,etc.showed that compared with catalyst Mn Ce Ox/raw sep and Mn Ce Ox/3H sep,Mn Ce Ox/3H-0.018C sep had the largest specific surface area（198 m²/g）,higher ratios of O_ads/O_latt,Mn³⁺/Mn⁴⁺（0.792,0.782）and excellent low-temperature reducibility.They were the main reasons for its excellent catalytic activity.These findings suggested that the supported manganese-based catalyst with sepiolite as the carrier had a good potential for catalytic oxidation of HCHO.