Hybrid Nano-structures Derived From Metal–organic Frameworks For Photocatalytic Degradation Of VOCs

Volatile organic compounds?VOCs?are a major source of air pollution.They have caused environmental pollution to different degrees and seriously threatened human health and ecological environment.Therefore,the development of efficient scientific and technological methods to eliminate VOCs in the atmosphere has become the focus of environmental protection researchers.Photocatalytic oxidation has been considered as one of the most promising technology for the removal of VOCs due to its simple process,less energy consumption,mild reaction condition,as well as the ability to mineralize various VOCs into innocuous CO₂ and H₂O.Among various exploited photocatalysts,TiO₂ has attracted extensive attention due to its excellent photocatalytic oxidation performance,nontoxicity and low cost.However,due to the wide band gap of TiO₂,photocatalytic reaction can only occur in the ultraviolet light region which account for less than 5%of solar energy.Moreover,its low surface area and high recombination rate of electron-hole often result in low efficiency.These greatly limit its practical application for photocatalytic removal of VOCs..Based on the thermal instability of MOFs,N-doped mesoporous carbon confined TiO₂ was obtained in this thesis via a pyrolysis strategy by employing an amine functionalized Ti-based MOFs as sacrificial templates.The effects of N-doping,interfacial contact between TiO₂ and carbon as well as the formed anatase-rutile TiO₂ phase junction on the photocatalytic performance for VOCs degradation were investigated in detail and the degradation reaction mechanism was also tentatively proposed.The main research contents are as follow:1)Preparation of N-doped mesoporous carbon confined TiO₂ composites?TiO₂@C-N?x??.The TiO₂@C-N composite was prepared by direct pyrolysis of a typical titanium-based MOF material,NH₂-MIL-125,under Ar atmosphere at 600°C for 6 h.Subsequently,the TiO₂@C-N composite was further treated in CO₂ atmosphere for a certain time to obtain the TiO₂@C-N?x?samples.Their surface physicochemical properties including surface composition,phase structure,morphology and specific surface area et al.were analyzed by X-ray diffraction,transmission electron microscopy and N₂ adsorption-desorption,etc.The results showed that the weak oxidative CO₂ can not only decompose amorphous carbon of TiO₂@C-N to increase the proportion of graphite carbon,but also can be served as an anatase-rutile phase transformation promoter.2)Investigation of photocatalytic oxidation performance of TiO₂@C-N?x?.The photocatalytic performance of the synthesized samples were investigated using our custom-made cubic quartz glass reactor in a continuous flow mode by the degradation of gaseous styrene.The results showed that both the photocatalytic activities and mineralization efficiencies of TiO₂@C-N composites with CO₂ treatment are significantly improved.Among them,TiO₂@C-N?30?exhibited optimal photocatalytic performance.The mineralization efficiency reached 51.9%at 62.4%of styrene degradation within 240 min of visible-light irradiation by using the optimal TiO₂@C-N?30?nanocomposites as compared with only 19.7%mineralization efficiency at 31.0%of styrene degradation under the same conditions of TiO₂@C-N.3)The exploration of mechanism of photocatalytic degradation of VOCs by TiO₂@C-N?x?composites.XPS,photoelectrochemical test and electron paramagnetic resonance spectroscopy were used to reveal the surface composition of the catalyst,the light absorption performance,the separation ability of photogenerated electron-hole pairs as well as the primary free radicals involved in photocatalytic degradation.Additionally,the possible degradation intermediates during the photocatalytic process were monitored by reaction time-of-flight mass spectrometry?PTR-ToF-MS?.The results indicated that TiO₂@C-N?x?could generate significantly more strong oxidative·O₂^-and·OH radicals than TiO₂@C-N under the synergistic effect of the strong interface interaction between TiO₂ and its outer highly graphitized carbon shell as well as the formed anatase-rutile phase junction.Thus,the TiO₂@C-N?x?samples exhibited remarkably superior photocatalytic activity and high mineralization ability in gaseous-phase oxidation of styrene under visible light irradiation.