Synthesis Of Mesoporous Manganese Dioxide-based Catalysts And Their Excellent Performances For VOCs Catalytic Oxidation

In recent years,the urbanization and industrialization in China have improved dramatically.However,the ensuring air pollution problems cannot be underestimated.Volatile organic compounds（VOCs）are known as the major air contaminants in China,which should be treated effectively to avoid photochemical smog and ozone pollution.Catalytic oxidation is one of the technologies for efficient purification of VOCs,the development of low-cost,high-efficiency and stable catalysts is a prerequisite for the widespread industrial application of this technology.Among numerous non-noble metal catalysts,MnO₂-based catalysts are considered as a kind of potential catalytic materials for VOCs removal.However,most of the currently reported MnO₂ catalysts used in catalytic combustion of VOCs show poor activity,so for purpose of effectively improving the performance of MnO₂-based catalysts,in this paper,the research is conducted on the generation of MnO₂ pore structure,the regulation of defect concentration,metal cation doping and the construction of heterostructures.The catalytic oxidation reaction of toluene/o-xylene is used as the evaluation of catalyst performance.The main research contents of this paper are described as follows:（1）Three-dimensional ordered mesoporous Mn₂O₃ precursor was prepared via the hard template method and using KIT-6 as template,and the precursor was treated with dilute nitric acid solution to synthesize the mesoporousγ-MnO₂ catalyst（meso-γ-MnO₂）.The resultant meso-γ-MnO₂ has high specific surface area and abundant surface oxygen vacancies,which could increase the surface active sites and boosts the surface oxygen activities.Therefore,compared with the catalytic performances ofα-MnO₂nanorods andγ-MnO₂ micro-urchins,the meso-γ-MnO₂ exhibited better catalytic performance for toluene and o-xylene removal,and 90%conversion of toluene and o-xylene was achieved at 219 and 237 ^oC,respectively.The synthetic route of this experiment provides a new approach for the preparation of meso-γ-MnO₂.（2）Three-dimensional ordered mesoporousβ-MnO₂ with different Cr doping contents（1%,6%and 12%）were synthesized by the impregnation via adjusting the adding amounts of chromium nitrate.Theβ-MnO₂ with 1%Cr doping showed the better-activity in the toluene catalytic oxidation.It was found by XPS that Cr was incorporated intoβ-MnO₂ lattice existed as Cr³⁺species,leading to the increase of Mn³⁺species and surface active oxygen species.TPD results showed that the lattice oxygen ofβ-MnO₂ was enhanced by Cr³⁺doping.Besides,density function theory（DFT）calculations showed that the formation of oxygen vacancy decreases from 0.18 e V to-0.47 e V caused by Cr³⁺doping,and the oxygen adsorption energy is enhanced,which provides theoretical support for Cr³⁺doping to promote the catalytic performance ofβ-MnO₂.（3）The CuCeCo@MnO₂ core-shell heterostructure was fabricated by solvothermal method,which was applied in toluene catalytic oxidation.As a result,the activity sequence was CuCeCo@MnO₂,CuCeCo,δ-MnO₂,Ce Co,Cu Co and Co₃O₄,implying that the formation of CuCeCo@MnO₂ heterostructure promotes the catalytic oxidation of toluene.The N₂adsorption-desorption test showed that the heterostructure assembly resulted in mesopores and large specific surface area.XPS revealed the doping states of Cu and Ce were mainly as Cu²⁺and Ce⁴⁺,and the CuCeCo@MnO₂ contains rich Co³⁺and Mn³⁺,abundant surface adsorption oxygen and oxygen vacancies,and the synergistic effect between Co and Mn.The O₂-TPD result suggested CuCeCo@MnO₂ has more active oxygen species,and the abovementioned factors are contributed to accelerate catalytic performance of CuCeCo@MnO₂ core-shell heterostructure.