Experiemtnal Study On Catalytic Oxidation Of DCM Over HZSM-5 Supported Transition Mental Oxides

As one kind of main air pollutants,the abatement of volatile organic compounds(VOCs)especially chlorinated volatile organic compounds(CVOCs),has attracted considerable attentions due to their risk on the environment and human health.Among the control technologies,CVOCs catalytic combustion to CO,CO2,HCl/Cl2 and H2O is considered as one of the most potential methods due to its efficient performance at low temperature and less production of secondary pollution.And,Cl species produced from CVOCs catalytic decomposion may poison catalysts,which makes the stability very important for industrial application.In this work,to achieve the synergy of acid and redox properties,HZSM-5 supported transition metal oxides catalysts were prepared and their catalytic stability for DCM abatement was evaluated.Carbon deposit(coke)and chlorine accumulation were studied and a possible reaction pathway was proposed.Based on the studies above,the activities and selectivities of Cr-O/HZSM-5 catalysts with different Cr loading were further investigated in order to provide some guidelines for catalysts design of CVOCs catalytic combustion.Firstly,three kinds of HZSM-5 supported transition metal(Cr,Fe and Cu)oxides catalysts were prepared by wet impregnation method and the stability performances for catalytic combustion of dichloromethane(DCM)were investigated at 320 ?.Different stabilities were observed for these three catalysts:It was found that Cr-O/HZSM-5 catalyst showed a good catalytic stability while Fe-O/HZSM-5 and Cu-O/HZSM-5 both suffered obvious deactivation.Based on the characterizations works,it was revealed that Fe-O/HZSM-5 suffered severe coke and Cu-O/HZSM-5 was mainly deactivated by chlorine poisoning.Then,a possible pathway of DCM catalytic decomposition over HZSM-5supported transition metal oxides catalysts was proposed.It was believed that the deactivation of Fe-O/HZSM-5 catalyst mainly resulted from the lack of synergy between acid sites and active oxygen species,leading to large amount of gas-phase CH3Cl production and severe cake deposition.In addition,a series of Cr-O/HZSM-5 catalysts with different Cr loading were prepared for an optimization for the catalytic oxidation of DCM.The results indicated that the surface acidity would decreased gradually with the rising of Cr content and a maximum value in redox ability emerged within the Cr loading investigated.The activity and selectivity of DCM catalytic degradation were found to be highly depended on the balance of surface acidity and redox property.It was found that 2 wt.%Cr-O/HZSM-5 sample was of the highest activity due to its strong acidty and the highest value in Cr(VI)/Cr ratio.The increased surface acidity could result in the enhancement of dissociated adsorption of DCM,which are beneficial to the catalytic activity.However,higher surface acidity meant lower Cr doping content,which may lead the adsorbed intermediates incompletely oxidized and the decrease in CO/CO2 selectivity due to the limit redox ability.