Fundamental Research Of Chlorobenzenes And PCDD/Fs Decomposition Over Titanium-based Catalysts

With the high-speed development of human society, various pollutants are emitted inevitably during production and daily life, thus, causes increasingly serious environmental pollutants. In order to alleviate the contradiction between development and environmental pollution and achieve sustainable development, the development and improvement of the advanced pollutants emission control method and technology, has become one of the major problems to be solved and attracted highly public attention in recent years. Catalysis technology can overcome the drawback of traditional active carbon adsorption method and achieve the complete mineralization of organic compounds. Therefore, it has been considered as most potential end of the pipe control method. This paper employed carbon nanotubes modification, other active components addition, ultraviolet light/ozone external aids coupling to promote the catalytic activity of conventional catalyst. Furthermore, the promotion mechanism of carbon nanotubes and the multi-ways coupled decomposition mechanism of chlorinated aromatic compounds were discussed. The main conclusions were obtained as followed:(1) Active component content is not the only factor that determines the catalytic activity. The type, particle size and crystal phase are also the key factors influence the catalytic activity. In comparison with conventional TiO2, anatase TiO2 with particle size in nanoscale is more suitable as catalyst support. The tetrahedron molecular structure composed of a single V=O bond and three V-O-Ti bond is considered as the main active sites. The reduction of the reaction temperature and increase of the space velocity are neither benefit for the catalytic decomposition process. For different catalysts, the change of reactants initial concentration may make different effects. Activation energy needed by chlorobenzene decomposition over nano-TiO2 supported V2O5 catalyst is 19.8±3.7 kJ/mol, which is lower than that of benzene decomposition (25.9±4.3 kJ/mol). It confirms that chlorine substituent in benzene ring can make the catalytic decomposition of chlorinated aromatic compounds easier to occur.(2) When CNTs is used as catalyst support to modify Ti-based catalyst, the catalyst active components are uniformly coated on the outer walls of CNTs. The specific surface area of catalyst is enlarged and the particles are better dispersed because of the introduction of CNTs. Average oxidation state of metal vanadium, the concentration of surface chemical adsorption oxygen and the binding energy between lattice oxygen and metal atoms are all increased after catalyst modification by CNTs. Compared with conventional V/Ti catalyst,1,2-dichlorobenzene catalytic removal efficiency at low temperature (150℃) over V/Ti-CNTs catalyst is promoted by 37%. It should be attributed to the addition of CNTs, which increases the adsorption ability, changes the adsorption mode and improves the oxidation ability. Moreover, the CNTs with the diameter size of 20-30 nm and with surface carboxyl have the best promotion effect for catalytic activity.(3) 1,2-dichlorobenzene catalytic decomposition efficiencies have linear positive correlation with catalyst surface chemical adsorption oxygen concentration, lattice oxygen binding energy and metal V average oxidation state. The linear correlation relationship between 1,2-dichlorobenzene catalytic decomposition efficiencies and lattice oxygen binding energy is especially obvious.1,2-dichlorobenze molecular parallel adsorbs on the catalyst surface of V/Ti-CNTs. It may facilitate the benzene ring cleavage, while weaken the dechlorination process. During catalytic reaction process, catalyst surface chemical adsorption oxygen is firstly consumed. When chemical adsorption oxygen is exhausted, the lattice oxygen in catalyst is offered as oxidant agent. The oxygen in atmosphere can replenish the consumed oxygen atom and reoxidize the reduced V to higher valence state, achieving the catalyst circulation utilization. Therefore, oxygen from atmosphere can extend the life time of V/Ti-CNTs.(4) Although in the adverse environment, created by high space velocity (20, 000h-1) and high initial concentration (12.2ngTEQ/Nm3) of PCDD/Fs, V/Ti-CNTs still can remove gaseous PCDD/Fs with high efficiency and the catalytic removal efficiency reaches above 99%. It can be due to the presence of CNTs, which promotes the capture ability of V/Ti-CNTs for dioxin molecular. Adsorption is the main reason for PCDD/Fs catalytic removal at relatively low reaction temperature (150℃ or 220℃), the real destruction efficiency is low. When the reaction temperature is increased, the PCDD/Fs catalytic decomposition efficiency is lifted and their adsorption efficiency is dropped. On the whole, the removal efficiency of PCDD/Fs is decreased inversely. NO and NH3 may compete with dioxin molecular for adsorption. However, their presence promotes the PCDD/Fs decomposition efficiency obviously, implying that NO-NH3 reaction and PCDD/Fs decomposition call for different catalyst active sites. The addition of NO and NH3 in reaction atmosphere, brings down the activation energies needed for OCDD and OCDF decomposition to 3.6 kJ/mol and 5.4 kJ/mol and makes their destruction easier to occur.(5) Catalyst/UV/Ozone multi-ways coupling are applied to further improve the catalytic activity. According to reaction kinetic analysis, ozone addition can improve the reaction rate by 22 times. Hence, catalyst/ozone coupling is more effectively than catalyst/UV coupling. It may benefit from the strong oxidizing ability of super oxygen free radical O2-and oxygen ions O-, which are generated from ozone catalytic decomposition. The catalyst components, basic structure parameter, surface acidity, and element chemical state and REDOX ability are changed because of ozone addition. CNTs and MnOx can respectively improve the ability to capture ozone and decompose ozone. Otherwise, V-Mn/Ti-CNTs catalyst shows better catalytic activity than V/Ti-CNTs-COOH.