Experimental Research On Degradation Of VOCs And PCDD/Fs Catalyzed By Powder And Honeycomb VO_x-MoO_x/TiO₂

The vast majority of site soil pollution has become a hard nut to crack in the field of soil remediation due to the high concentration of pollutants and poor biochemical properties.Thermal desorption technology has the advantages of short remediation cycle and strong universality and has been widely used in soil organic contaminated sites.However,volatile organic compounds（VOCs）in soil thermal desorption tail gas and polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans（PCDD/Fs）pollution that may be synthesized are the main key and difficult problems in the application of this technology.Therefore,aiming at VOCs and PCDD/Fs in thermal desorption tail gas,this thesis develops powder and honeycomb catalysts for efficient degradation of VOCs and PCDD/Fs,providing theoretical guidance and technical support for thermal desorption remediation technology of organic contaminated site soil.In this thesis,the traditional VO_x/Ti O₂ catalyst was doped with Mo,and the powder VO_x-Mo O_x/Ti O₂ catalyst was prepared by wet impregnation method with（NH₄）₆Mo₇O₂₄ as Mo source.Powder VO_x-Mo O_x/Ti O₂ catalyst was prepared by wet impregnation method,and honeycomb VO_x-Mo O_x/Ti O₂ catalyst was prepared by small catalyst forming device,to achieve efficient catalytic degradation of VOCs（including toluene,chlorobenzene,1,2-dichlorobenzene,the same below）and PCDD/Fs at relatively low temperature（150～190℃）.The main conclusions of this thesis are as follows:（1）Through XRD,XPS and other catalyst characterization methods,it is found that compared with VO_x-WO_x/Ti O₂ and VO_x-Ce O_x/Ti O₂ catalysts,Mo doped VO_x-Mo O_x/Ti O₂ catalysts have more surface catalytic active sites,good dispersion of active components on the surface of catalyst support,low initial reduction temperature,more active oxygen content,large specific surface area,good morphology and better catalytic characteristics.（2）Compared with other metal doped five kinds of powder catalysts（V5-W5-Ti、V5-Ti、V5-Ce5-Ti、V5-Mn5-Ti、V5-Fe5-Ti）,powder V5-Mo5-Ti（5%VO_x-5%Mo O_x/Ti O₂）has the best catalytic effect on VOCs at low temperature and 5%is the best loading capacity.The conversion of V5-Mo5-Ti powder catalyst to toluene with an initial stable concentration of about 650 ppm at 275℃is more than 90%,the conversion of chlorobenzene with an initial stable concentration of about 150 ppm at 230℃is more than 90%,and the conversion of 1,2-dichlorobenzene with an initial stable concentration of about 150 ppm at 250℃is more than 90%.When the temperature was increased from 150℃to 190℃,the catalytic removal efficiency of V5-Mo5-Ti powder catalyst for PCDD/Fs was increased from 86.3%to 90.8%,and the degradation efficiency was increased from 74.0%to 79.5%.（3）When the space velocity ratio is 10000 h^-1,the initial toluene stable concentration is about 650 ppm,and the initial stable concentration of chlorobenzene and 1,2-dichlorobenzene is about 150 ppm,the conversion rates of toluene,chlorobenzene and1,2-dichlorobenzene over honeycomb V5-Mo5-Ti catalyst were 78.5%,54.5%and 41.3%respectively at 150℃.Compared with powder catalyst,the conversion rates of toluene,chlorobenzene and 1,2-dichlorobenzene over honeycomb catalyst were increased by12.1%,0.5%and 22.0%respectively.At 150℃,the catalytic removal efficiency of V5-Mo5-Ti honeycomb catalyst was 79.0%and the catalytic degradation efficiency was32.1%.Although the catalytic degradation efficiency of honeycomb V5-Mo5-Ti catalyst at 150℃is half that of powder catalyst,its catalytic removal efficiency of PCDD/Fs is only 7.3%lower than that of powder catalyst,and its conversion rate of VOCs is higher than that of powder catalyst.At the same time,honeycomb V5-Mo5-Ti catalyst has a stable structure as a finalized industrial catalyst,while the powder catalyst will diffuse with the gas when the actual thermal desorption tail gas passes,resulting in difficulty in finalization and affecting the catalytic efficiency.Overall,the prepared V5-Mo5-Ti honeycomb catalyst is suitable for catalytic purification of thermal desorption tail gas.