Design And Mechanism Of Zeolite Based Multi-component Catalyst For Catalytic Degradation Of Chlorinated VOCs

Catalytic oxidation is a mature and widely used technology for chlorinated volatile organic compounds（CVOCs）treatment,the core of which is the development of catalysts with excellent performance.The presence of chlorine in CVOCs tends to deactivate the active component of the catalyst by binding to chlorine,while the generation of chlorine-containing by-products such as dioxins at high temperatures can cause greater secondary pollution.Therefore,an ideal CVOCs catalyst requires not only low-temperature activity and high selectivity,but also excellent stability and high resistance to chlorine poisoning,which are often not available for singlecomponent catalysts.Based on the above,this paper developed a variety of composite component catalysts for the DCM catalytic combustion,and realized the high catalytic oxidation performance of DCM with the the synergistic effect between multiple components.Meanwhile,a monolithic catalyst pellets preparation method of with citric acid as binder was developed,which retained the activity of the original powder catalyst.The specific research work carried out in this paper are as follows:（1）Zeolites with different types and SiO₂/Al₃O₃ ratios were selected as support,three transition metal oxides（Co,Ni,Ce）as active components,and Ru as the noble metal active component to prepare a "Noble metal-Transition metal oxide-Zeolite" ternary catalyst for DCM catalytic combustion.It was found that the catalyst exhibited the best redox performance when Co₃O₄ was used as the active component of the transition metal oxide.The DCM oxidation activity of the catalyst was improved by introducing a trace amount of Ru due to the interaction between Ru and Co.Compared with Ru-Co/13 X and Ru-Co/β,Ru-Co/HZSM-5 catalyst exhibited the best DCM catalytic activity with a T90 of 275 °C and a CH3 Cl selectivity of less than 4%.This is due to the suitable Br（?）nsted/Lewis acid ratio of Ru-Co/HZSM-5 both before and after the reaction.Br（?）nsted acid can avoid catalyst poisoning deactivation,while Lewis acid is helpful to realize the complete oxidation of intermediate products.（2）Ru can accelerate the Deacon reaction and promote the production of Cl2 and polychlorinated by-products.To solve this problem,the noble metal Ru was replaced by Pt,and Pt-Co/HZSM-5 series catalysts were prepared by isovolume impregnation method.The synergistic effect of each component in the ternary catalyst was investigated in depth.The HZSM-5 provides a large number of acidic sites for DCM adsorption and dissociation;the presence of Co₃O₄ can effectively anchor Pt atoms and improve the noble metal dispersion;and the ultra-low content（0.01 wt.%）of noble metal Pt can increase the ratio of oxygen adsorbed on the surface of Co₃O₄ and enhance the redox performance of the catalyst.With the synergistic effect of Pt,Co₃O₄ and HZSM-5,the 0.01Pt-20Co/HZSM-5 catalyst with suitable acidity and sufficient surface adsorbed oxygen exhibited excellent DCM catalytic oxidation performance with T90 of 249 °C and CH3 Cl selectivity not more than 3.1 %.（3）The Co₃O₄ agglomeration in Pt-Co/HZSM-5 catalyst prepared by impregnation method is serious.In order to solve this problem,the preparation method of Co/HZSM-5 catalyst was improved.Cobalt-containing ZIF-67 was in-situ grown on the surface of HZSM-5 zeolite by one-pot method.After pyrolysis,ZIF-67 formed highly dispersed Co₃O₄ nanoparticles,and HZSM-5@Co₃O₄ catalysts were obtained.The performance of Pt/HZSM-5@Co₃O₄ catalyst after Pt loading was not significantly improved,so the pyrolysis atmosphere was further optimized to improve the DCM catalytic oxidation performance of HZSM-5@Co₃O₄.The results indicated that the 0.5HZSM-5@Co₃O₄-N500A350 catalyst obtained by “N2-air” two-step pyrolysis possessed strong redox ability and retained the acidity of HZSM-5,which the CH3 Cl selectivity was less than 2%.The improvement of catalyst performance is mainly due to the carbon-confinement effect generated by two-step pyrolysis,which results in smaller grain size and more oxygen vacancies of Co₃O₄.（4）The powder catalysts were prepared into monolithic catalyst pellets using citric acid as organic binder.The effects of different citric acid concentrations on the performance of the catalyst were investigated.Results indicated that 4M-PtCo/HZ exhibited excellent DCM catalytic activity and stability with a T90 of 270 °C.The byproduct selectivity of catalyst pellets could be further reduced by phosphoric acid external surface modification.The citric acid binder not only avoided the blockage of catalyst pore,but also increased their specific surface area,while retaining the physicochemical properties of original powder catalyst.Meanwhile,the catalyst pellets exhibited an ideal catalytic activity for common exhaust gases（DCM,acetone and benzene）in the pharmaceutical industry,providing a feasible idea for the synergistic treatment of industrial multi-component exhaust gas.