Preparation And Investigation On Adsorption/photocatalysis Performance Of VOCs For Carbon Based Composite Materials

Volatile Organic Compounds?VOCs?in the atmosphere are mainly featured with low concentration.It has been reported that prolonged exposure to VOCs in the atmosphere even at low concentrations may pose serious threat to human beings.Exploring efficient ways for the curbing of low concentration VOCs is an urgent issue.Adsorption is one of the most common and effective techniques for the treatment of VOCs with low concentration.Aiming at the problems of low adsorption capacity and poor adsorption selectivity of traditional adsorbents to VOCs under high humidity,this thesis proposed the design and development of nitrogen-doped graphitized porous carbon adsorptive materials with high specific surface,strong hydrophobicity and low cost,for the achievement of adsorbents with high adsorption performance on VOCs under high humidity.Moreover,in order to achieve the VOCs treatment along with green environmental protection,the newly emerged photocatalytic technology with great potential was coupled with and the efficient adsorptive-photocatalytic dual-function photocatalyst was built.In this thesis,the development of high-performance adsorbent and photocatalyst to realize the efficient management of VOCs was studied systematically.The main research contents and results obtained in this thesis are as follows:?1?Research on the preparation and adsorption of benzene series VOCs for the nitrogen-doped porous carbon with high surface area.Using dopamine biomolecule as carbon source,a nitrogen-doped microporous graphtized biocarbon?MGBC?with high surface area was synthesized under the control over the proportion of the active reagent and carbon precursor.As a result,its adsorption performance on VOCs at low concentration was improved.The Scanning Electron Microscope/Energy Dispersive Spectroscopy?SEM/EDS?,X-ray Diffraction Spectra?XRD?,Raman Spectroscopy,X-ray Photoelectron Spectroscopy?XPS?and surface area and pore structure analysis?ASAP?were separately conducted to characterize the surface morphology/elements,crystal structure,graphitized structure,elemental valence,specific surface area and pore size of the carbon material,and to explore the influence of its surface and structural properties on its adsorption performance towards benzene series VOCs.Results showed that the morphology of MGBC was spherical?750-800nm?with smooth surface and uniform particle size,and rich in N elements?8-10at%?and sp² C/N on the surface.After carbonization and hole expansion,MGBC exhibited with higher degree of graphitization?I_G/I_D:⁰.48-0.66?and higher specific surface area?>2080 m²/g?as well as narrower micropores?6.8-8.8??distribution.MGBC showed very high adsorption capacities which separately reached 5.8 and 5.2 mmol/g for benzene and toluene at ultralow vapor pressures?80 Pa?and 298 K.These uptakes are 4-6 times greater than that in previously reported zeolite and some MOFs adsorbents?i.e.,MIL-101?Chromium-?TerephthalicAcid?based?,HKUST-1?Copper-?BenzenetricarboxylicAcid?based?,andUIO-66?Zirconium-?Terephthalic Acid?based??under the same conditions.Even at 60RH%,78%of working adsorption capacity under dry condition was still kept on the MGBC,which showed 1.5-2.5 times of toluene breakthrough time longer than that of MIL-101 and activated carbon?SY-1?.This indicated that the high specific surface,uniform micropores and high graphitization of MGBC are beneficial to enhance its adsorption performance towards benzene series VOCs under ultralow pressures and high humidity.?2?Research on the preparation and competitive and selective adsorption of benzene series VOCs/H₂O for the nitrogen-doped graphitized porous carbon with enhanced hydrophobicity.On the basis of the previous work,different carbonization methods and hydrogenation post-treatment were proposed to prepare a hydrophobic N-doped porous carbon?HN-PC?,resulting in improved adsorption capacity and selectivity of p-xylene under high humidity.Various characterization techniques?SEM/EDS,Raman,Fourier Transform Infrared Spectrometer?FTIR?,XPS,ASAP,etc.?were applied to investigate the effects of different carbonization and hole expansion methods and hydrogenation strategies on the surface properties?morphology/element content,crystal structure,surface group,element valence,etc.?and pore structure of?H?N-PCs.The effects of surface chemical properties?oxygen content,N species,graphitized structure,sp² C/N content,etc.?on adsorption capacity of p-xylene/H₂O and adsorption selectivity were systematically investigated under various humidities.The results showed that compared with the N-PC1 obtained by one-step activation,the N-PC2 obtained by carbonization and hole expansion in two steps not only maintained a complete smooth spherical morphology,but also showed higher N content?8.9 at%?,basic N content?pyrrole/graphitized N?and graphitization degree?I_G/I_D=0.90?,as well as smaller pore size?⁶.8??and lower oxygen content?6.0 at%?.Its specific surface area was 2189 m²/g which was lower than that of N-PC1?2503 m²/g?,but the morphology of N-PC2 has been seriously damaged which may affect its adsorption performance.After hydrogenation,HN-PC2 showed 4.0 at%of the oxygen content lower than that of N-PC2,and the highest sp²/sp³ C ratio?2.5?as well as N content?⁹.5 wt%?among all materials.Owing to the enhanced non-polar surface properties with the highest sp² C?graphitized C?,basic N?pyrrole/graphitized N?content and oxygen deficit,HN-PC2 exhibited the strongest special force on weakly polar VOCs and the lowest adsorption capacity on polar water molecules among all materials,and higher vapor adsorption equilibrium rate than that of HKUST-1.In the p-xylene breakthrough experiments,HN-PC2 still maintained a very high p-xylene adsorption amount?5.6 mmol/g?even at 80 RH%,which was 1-3 times higher than that of MIL-101?¹.33 mmol/g,60 RH%?.HN-PC2 showed excellent adsorption performance of VOCs with low concentration under high humidity.?3?Dopamine bridging N-doped graphene/TiO₂ photocatalyst with high degradation performance on aldehyde VOCs at room temperature.A novel synergistic strategy of N-doped and N-wrapping/bridging was proposed in this thesis,aimed at enhancing the interfacial interaction between TiO₂ and graphene and improving the photocatalytic degradation performance of gaseous formaldehyde.A hydrogenated dopamine-bridged and melamine-doped graphene/TiO₂ composite?H-TiO₂@MG-D?was prepared by in-situ hydrothermal synthesis and hydrogenation with tetrbutyl titanate as titanium source,melamine as nitrogen source,GO as co-catalyst and dopamine as interface"electron bridge"precursor.In addition,the synergistic effect of N doped and N wrapping/bridging,self-doping of oxygen vacancy or Ti³⁺defect as well as coupling with co-catalyst on photocatalytic degradation of gaseous formaldehyde by graphene/TiO₂ composites was studied.Combined with experimental and simulated results,the mechanism of degradation of formaldehyde by H-TiO₂@MG-D along the photocatalytic process was proposed.Results showed that there is a dopamine molecule bridge linking between the TiO₂ photocatalytic center and N-modified graphene co-catalyst by chemical bonding?such as Ti-N,Ti-O and Ti-C etc.?in the H-TiO₂@MG-D,resulting in its enhancing interfacial contact.Under the synergistic effect of N-doped and N-wrapping/bridging,oxygen vacancy or Ti³⁺defect self-doping as well as coupling with co-catalyst strategy,the H-TiO₂@MG-D showed the narrowest band gap?2.49 eV?,highest charge separation efficiency,and highest amount of active radicals?·O₂^-and·OH?.As a result,its photodegradation activity??=92%?and kinetic rate(k=1.504?10^-3 min^-1)of formaldehyde were4.1 and 9.2 times higher than that of original TiO₂,respectively.In this study,an effective N-doped and N-wrapping/bridging strategy was developed,which is expected to be applied to the modification of other photocatalyst systems to improve its photocatalytic performance.?4?Research on the preparation of Ti₃C₂T_x?MXene?/CdS composite photocatalyst with high dispersity and intimate contact interface.To realize the organic solvent soluble CdS nanorods and Ti₃C₂T_x with water solubility and negative charge closely assembled in the aqueous phase,an ease one-step"ligand exchange"method with cysteamine as ligand was applied in this thesis.The CdS was modified into a water-soluble and positively charged nanorod before the preparation of photocatalyst?Ti₃C₂T_x/CdS_M-C-90%?with good dispersivity and intimate interfacial contact.UV-vis absorption spectrum?UV-vis?,SEM,Transmission Electron Microscopy?TEM?,element Mapping and other technologies were separately applied to systematically characterize its composition,morphology,crystal structure and surface element distribution.Electrochemical workstation and three-electrode system were used to test and measure its separation efficiency of photo-generated carriers,and compared with the in situ synthesized Ti₃C₂T_x/TiO₂-DA.Results showed that obvious absorption peaks of Ti₃C₂T_x and CdS_M-C were both presented in UV-vis absorption spectrum of Ti₃C₂T_x/CdS_M-C-90%,confirming the successful recombination of Ti₃C₂T_x and CdS_M-C.SEM/TEM/Mapping results showed that numerous CdS_M-C nanorods?with a length of about 75 nm?were evenly anchored on the surface of Ti₃C₂T_x in the Ti₃C₂T_x/CdS_M-C-90%with highly dispersive elements?Ti,C,Cd and S?distribution.With free-bias voltage for carriers accelerating,the average photocurrent density of Ti₃C₂T_x/CdS_M-C-90%was about 24 uA/cm²,which was 2.4 times of Ti₃C₂T_x/TiO₂-DA at higher bias voltage?0.3 V?.The photocurrent of Ti₃C₂T_x/CdS_M-C-90%increased linearly under the instant illumination,which was more than 1 time higher than that of Ti₃C₂T_x/TiO₂-DA,indicating higher photo-induced carrier kinetic rate of Ti₃C₂T_x/CdS_M-C-90%.These were attributed to the synergy of Ti₃C₂T_x co-catalyst and ligand exchange strategy,which resulted at enhanced electrical conductivity,interface contact and good monodispersity,as well as improved separation efficiency of photo-generated carriers of Ti₃C₂T_x/CdS_M-C-90%.