Study On Catalytic Oxidation Of CVOCs Over TiO₂ Supported Catalysts

Chlorinated volatile organic compounds(CVOCs)are harmful to environment and human health.However,they are widely used in various industry fields.Catalytic oxidation method can efficiently decompose CVOCs into clean products.Catalysts are key to the applications of catalytic oxidation.TiO2 are common catalyst supports for preparation of variety of catalysts due to its low price,facile preparation process and abundant surface hydroxyl groups.In this thesis,the performance and deactivation mechanism of TiO2 catalysts for CVOCs catalytic oxidation were comprehensively studied.The highly active supported TiO2 catalysts were designed and the influence of different active components to the catalytic activity were discussed.Based on the powder catalyst synthesis process,the preparation of monolith metal foam catalysts was explored.The research contents and main results are as following:(1)Mechanism of synergism between CrOx and TiO2Cr/TiO2 and M-Cr/TiO2(M= Au,Ag,Pt,Pd)catalysts were prepared by a nonhydrolytic template-mediated sol-gel route,using chromium nitrate,noble metal salts(chloroplatinic acid,palladium nitrate,gold chloride or silver nitrate),TBOT and TiCl4 as the precursors.The results showed that Cr/TiO2 and M-Cr/TiO2 both possessed bimodal micro-mesoporous structures.The surface areas of the catalysts were between 250 m2/g to 350 m2/g.DCM could be totally decomposed at 350? and the products were mainly CO,CO2,HCl and H2O.Noble metals had little promotion to the catalytic activity.The stability test proved irreversible inactivation occurred on the catalysts.Cr=O oxygen vacancy and surface hydroxyl groups were proved to be the active sites by in-situ infrared spectra.It was concluded that the inactivation should be attributed to the adsorption of water on the active sites where Cr=O vacancy could be occupied.(2)Influence of noble metals,TiO2 crystalline structures and preparation methods on catalytic performanceAu,Ag,Pt,Pd were supported on the TiO2 and three different types of TiO2(anatase,rutile and P25)were used as the support.The influence of preparation methods(impregnation and deposition-precipitation)to the catalytic activity of dichloromethane oxidation was also discussed.Catalyst prepared by DP method using anatase TiO2 as the support was more active than P25 and rutile TiO2.Pt supported catalyst performed better than Pd,Au and Ag supported catalysts.The products were mainly CO2.The inactivation of the catalysts was attributed to the sintering and abscission of noble metals,which was determined by the comparison of TEM micrographs and XPS analysis between samples before and after reaction.The results showed great importance of support to the stability of the catalysts.(3)Preparation of highly active TiO2 supports and its the reaction mechanismA mesoporous anatase TiO2 was prepared via NTMSG method using TBOT and TiCl4 as the precursors.The results showed that DCM was totally decomposed into CO and CM at 350?under 1 vol.%humidity.The catalyst remained stable under high humidity.A competing mechanism between disproportionation and oxidation was proved by in-situ IR analysis.Water molecules would dissociatively adsorb at the(001)sites on anatase TiO2 and generated surface hydroxyl groups took part in the reaction.It was concluded that dissociative adsorption of water was the key to the active site regeneration.(4)Promotion mechanism of Pt on TiO2 and catalytic performance of Pt/TiO2 with low loading amountX wt.%Pt/TiO2 catalysts(X=1,0.1 and 0)were synthesized by DP method using chloroplatinic acid as the Pt precursor.The results showed that 1%Pt/TiO2 was able to decompose 90%of DCM,CB and TCE at 300? under 1 vol.%humidity,240000 mL/(g·h).And T90 of 0.1%Pt/TiO2 was 350 ? under 1 vol.%humidity.The 24h stability test of 1%Pt/TiO2 was carried out under 1 vol.%humidity,400? for CB and TCE and 350? for DCM.The catalyst remained stable after stability test.It proved that the deep oxidation capacity of the catalyst was promoted by Pt indicating that noble metal would have good promotion with TiO2.(5)Preparation and characterizations of monolith metal foamA novel monolith metal foam catalyst was prepared via impregnation and in-situ hydrolysis of TiO2 sol using TBOT and TiCl4 as the precursors.The TiO2 coated Ti foam was loaded with Pt by DP method.The results showed that TiO2 coatings possessed controllable thickness,high surface area and pore structures.The monolith catalyst could totally decompose DCM and 88%CB at 400? under 1 vol.%humidity with GHSV of 1580/h.In addition,the catalyst remained stable after 24h aging at 350 ?.