Effect Of Carrier On CeO₂-based Mixed Oxides Catalysts Towards VOCs Combustion

Volatile organic compounds are one of the major contributors to air pollution.The interest in those compounds is due to their contribution to the formation of photochemical smog and their hazardous effects on both atmospheric environment and human health.Among various available techniques,catalytic combustion is an effective environmental friendly and low cost technology for the abatement of VOCs.Catalyst is the key of catalytic combustion,so it is necessary to search for a kind of catalysts with less expensive,relatively high activity and thermal stability.Cu-Mn-Ce ternary mixed oxide catalyst is an effective catalyst for the combustion of VOCs and it has been researched for a long time in our group.SiO₂ with high surface area have always been considered as the most promising carrier and pore template material for metal oxide catalysts.In this dissertation,the effect of SiO₂ carrier for Cu-Mn-Ce mixed oxide catalyst were studied systematically.Catalysts were characterized by HTEM,H₂-TPR,BET and XRD,and their catalytic activities were used to analyze the effect mechanism of SiO₂.Some conclusions were drawn as follows:A series of Cu-Mn-Ce/SiO₂ mixed oxides with different Cu-Mn-Ce loading were prepared by impregnation method,and there had three types of SiO₂（KIT-6,SBA-15,silica gel）as the carrier.Their catalytic performances were studied by total oxidation of toluene.The results showed that three types of SiO₂ carrier exhibited similar effect:with low Cu-Mn-Ce loading and highly dispersed state,the catalysts appeared significant deactivation,and when the Cu-Mn-Ce loading increased,the catalyst activity was restored.XRD and H₂-TPR characterization results showed that SiO₂ carrier had no effect on the structure and redox properties of the catalyst.BET and HTEM characterization showed that SiO₂ carrier had no effect on the textural properties and morphology of the catalysts.The results indicate that SiO₂ carrier has a physical shielding effect on Cu-Mn-Ce catalysts,which result in the active ingredient lose its activity.It’s just like SiO₂ carrier with high specific surface area was covered with"trap",which will disable active ingredient.Only by filling the trap can make the ingredient active again.To further verify the physical shield"trap"effect,the mixed oxides were treated with a Na OH solution to wash the SiO₂ carrier,and surprisedly the catalytic activity gradually recovered again,especially for mixed oxides with low loading proportion.In addition,the mixed oxides showed high catalytic activity when calcined at a high temperature for the reason of the shrinked surface and removal of the trap.On the other hand,Cu-Mn-Ce/SiO₂ catalyst prepared by the mechanical mixing method show a higher catalytic activity over those prepared by the impregnation method.All results indicated that trap effect of SiO₂ carriers is due to the pore shielding impact,it’s a kind of physical effect and is reversible.Cordierite ceramic honeycomb-type Cu-Mn/CH、Cu-Mn-Ce/CH、Cu-Mn-Ce-Zr/CH and La-Mn/CH mixed oxides were prepared by a impregnation method.The results showed that Ceria-based composite oxides on SiO₂ carrier with high surface area appear the trap effect,but no such effect on cordierite surface with lower surface area（<2 m²/g）.