The Preparation Of Structured Catalysts And The Application For The Removal Of Environmental Contaminants

In order to decompose volatile organic compounds (VOCs) and solve the problems caused by VOCs to the environment, catalysts for the catalytic oxidation of toluene were prepared by sol-gel and impregnation methods, usingγ-Al₂O₃ or SiO₂ as carriers, V₂O₅, TiO₂ or V₂O₅-TiO₂ as active components. Dynamic catalytic oxidation reaction system was established, from which, the activity of catalysts and reaction conditions were investigated. The results indicated that V₂O₅-TiO₂/SiO₂ illustrates the best ability for the degradation of toluene. The influence of carriers' size, precursor and loading amount on the activity of catalysts was experimented and analyzed. The best size ofγ-Al₂O₃ was 60-80 mesh; the best precursor was ammonium vanadate in an oxalic acid solution; the best loading amount of V₂O₅ was 1wt. %; and the molar ratio of nV/nTi was 0.127 in terms of treatment efficiency and costs.Three main factors in catalytic oxidation reaction: reacting temperature, space velocity, inlet concentration were investigated. The experimental results indicated that V₂O₅-TiO₂/SiO₂ had low ignition temperature, and efficiency of toluene conversion rose quickly as temperature increases, proving that V₂O₅-TiO₂/SiO₂ possess similar temperature characteristics to noble metal. When space velocity and concentration were changed, efficiency of toluene conversion on V₂O₅-TiO₂/SiO₂ did not change too much. By contrast, efficiency of toluene conversion on V₂O₅-TiO₂/Al₂O₃ descended as space velocity and concentration increased. Compared with V₂O₅-TiO₂/Al₂O₃, V₂O₅-TiO₂/SiO₂ shows better resistance against impact.The prepared catalysts were characterized on structure and morphology by X-ray diffraction (XRD), Scan Electron Microscope (SEM), energy dispersion X-ray energy spectrum (EDX) and X-ray photoelectron energy spectrum (XPS). According to the result of activity test and characterization, the relationship between catalyst structure and catalytic activity was analyzed. The results showed that vanadium ion was imbedded in the crystal structure of TiO₂, instead of creating V₂O₅ crystal form because of limited V₂O₅ loaded amount. The chemical bonds of V-O-Ti and Ti-O-Si were formed. However, there was only physical combination betweenγ-Al₂O₃ and TiO₂. The result of activity test indicated that these chemical bonds might be the active sites of catalysts which increase the catalytic capability of catalysts and strengthen the binding strength among the components of catalysts.The CuO-CeO₂/Al₂O₃ catalysts were prepared by impregnation methods, usingγ-Al₂O₃ or SiO₂ as carriers, CuO as active component, and with addition of Ce. Using the catalytic oxidation of toluene and xylene as the model reaction, dynamic catalytic oxidation reaction system was established from which the activity of catalysts and reaction conditions were investigated. In all kinds of catalysts using different carriers, the one usingγ-Al₂O₃ (60～80mesh) as carrier had the best ability on the degradation of toluene. Compared with CuO/Al₂O₃ catalyst, CuO-CeO₂/Al₂O₃ catalyst which was added with Ce had lower initial combustion temperature and complete oxidation. The prepared process was optimized and the results showed that the catalyst with 5 wt.% Cu loading amount, molar ratio of Cu/Ce 1:1 and calcined at 500℃for 6 hours has the highest catalytic activity.Three main factors such as reacting temperature, space velocity, inlet concentration in the catalytic oxidation reaction were researched for toluene and xylene degradation. In test range, the optimal space velocity of toluene on CuO-CeO₂/Al₂O₃ was 8000h-1, and 6000h-1 for CuO/Al₂O₃. The results of influencing factors experiment on catalytic capability of xylene were similar to that of toluene, and the optimal space velocity of xylene on CuO-CeO₂/Al₂O₃ and CuO/Al₂O₃ was 8000h-1. Compared with the catalytic oxidation of toluene, the xylene removal efficiency on CuO/Al₂O₃ was preferable, and it was over 90% in different space velocity and inlet concentration. The removal purpose of xylene could be achieved but it was not preferable compared to CuO-CeO₂/Al₂O₃. CuO-CeO₂/Al₂O₃ has better resistance against impact and can be better applied in the practice.The prepared catalysts were characterized by XRD, SEM, EDX and XPS. The results indicated that surface particle diameter of CuO-CeO₂/Al₂O₃ catalyst became smaller when Ce added, and the dispersion was well proportioned. The presence of CeO₂ increased the Cu+ content on the surface of CuO/Al₂O₃ catalysts. So the activity of catalysts for toluene and xylene catalytic oxidation could be improved. To realize the selective catalytic reduction (SCR) of NOx with NH3, a new type of V₂O₅/TiO₂/Al₂O₃/Al wire-mesh honeycomb (WMH) catalyst was developed. The wire-mesh honeycomb made of aluminum was first oxidized as anode electrochemically to form porous layer, and then TiO₂ was adsorbed by sol-gel method, and V₂O₅/TiO₂ catalyst powder was wash-coated onto the wire surface of the honeycomb. V₂O₅/TiO₂/Al₂O₃/Al/WMH catalyst was applied for SCR of NOx with NH3. The catalyst shown high mass transfer coefficient and moderate pressure drop because of its porous structure. Wire-mesh has high extendibility and flexibility, and is suitable to reactors with different structures.The selective catalytic reduction activity of two types of catalysts, V₂O₅/TiO₂/Al₂O₃/Al WMH catalyst and powder catalyst, were investigated, and the results showed that they had wide active temperature range and high treatment efficiency within the temperature range of 200⁴00℃, the highest conversion of NO over V₂O₅/TiO₂/Al₂O₃/Al WMH catalyst being up to 86% and over powder catalyst 93.8%. They could both get high treatment efficiency in a wide range of space velocity. Increasing molar ratio nNH3/nNO and concentration of influent gas could enhance NO conversion for both of them. And the treatment effectiveness over powder catalyst was better than that over WMH catalyst, but bed pressure drop of powder catalyst was severely high which would limit its application. The manufactured catalysts were characterized by XRD, SEM, EDX and XPS and the results showed that TiO₂ was well combined with Al₂O₃ and V₂O₅ was fully dispersed. Reaction kinetics of two types of catalysts was primarily studied, and the results indicated that SCR of NOx with NH3 over V₂O₅/TiO₂ catalyst accorded with Eley-Rideal mechanism and belonged to first order reaction. Reaction over WMH catalyst was mainly controlled by chemical kinetics at low temperature and was affected by mass transfer characteristics at high temperature. For powder catalyst with 60⁸0 meshes, the reaction was controlled by reaction kinetics and not by mass transfer.To investigate the feasibility to combine the electrochemically catalytic oxidation process and photo catalytic process to decompose Reactive Black 5, the efficiency of the electrochemically catalytic oxidation process and photo-electric catalytic process for the degradation of Reactive Black 5 were studied on three different wire-mesh electrodes. They are Ti coated wire-mesh electrode sintered at 800℃for 10 hours with H₂; TiO₂/Ti coated wire-mesh electrode sintered at 800℃for 10 hours with H₂ and calcined at 600℃for 3 hours with air and fresh titanium wire-mesh electrode. The experiment result shown that with HCl as supporting electrolyte and additive, the coloring removing efficiency of Reactive Black 5 reach as high as 85% less than 20 minutes at Ti coated wire-mesh electrode sintered at 800℃for 10 hours with H₂ and at TiO₂/Ti coated wire-mesh electrode sintered at 800℃with H₂ for 10 hours and calcined at 600℃for 3 hours with air. Electrochemically promoted photo catalytic degradation of Reactive Black 5 was studied with UV irradiation on TiO₂/Ti wire-mesh electrodes. It is found that the introduction of UV irradiation to the electrochemical system would enhance the color removing efficiency for Reactive Black5 degradation especially on TiO₂/Ti coated wire-mesh sintered at 800℃for 10 hours with H₂ and calcined at 600℃for 3 hours with air. It is the TiO₂ formed after calcined at 600℃for 3 hours with air that photo-catalyzed the degradation decomposition of Reactive Black 5 with UV irradiation. Compared with decolourization efficiency obtained by individual application of electrochemical and photo catalytic oxidation procedures, a significant synergic function from these two processes could be concluded.