Preparation Of Noble Metal-based Catalysts And Their Catalytic Performance For C₆ Hydrocarbons Combustion

Volatile organic compounds(VOCs),as precursors of O3 and PM2.5,pose great threats to human beings and environment.Compared with conventional treatment techniques,such as adsorption and thermal incineration method,the catalytic combustion approach gains advantages of high removal efficiency,relatively low operation costs as well as less secondary pollutions.Pt and Pd have been widely applied in the catalytic combustion of industrial VOCs.Nonetheless,the price of Pt and Pd are expensive and the activity of Pt is prone to decline for the reason of sintering.Therefore,it is highly desirable to lower the content of Pt and Pd,and prolong their lifespan.In the current research,the Pt-Pd and Pt-Sn catalysts,supported on y-Al2O3 with cordierite honeycomb ceramics as the first carrier,were prepared by incipient wetness impregnation method.The crystal structure,surface morphology and chemical state of the catalysts were characterized by X-ray diffraction,transmission electron microscope,temperature programmed reduction,etc.Four representative C6 hydrocarbons(benzene,cyclohexanone,cyclohexane,n-hexane)were selected as model VOCs to evaluate the performances of the catalysts.Besides,the adsorption of C6 hydrocarbons on catalysts surface was explored by density functional theory calculations.Based on above,the structure-activity relationship between catalyst structure,molecular configuration and reactivity was established.The Pt-Pd/y-Al2O3 honeycomb catalysts were prepared via incipient wetness co-impregnation method.Results showed that the catalytic activity of Pt for four C6 hydrocarbons was higher than that of Pd,and the T90(the temperature when a 90%conversion is attainted)increased gradually with the decrease of Pt content.The T90 of that four C6 hydrocarbons followed the order of:cyclohexane>n-hexane>cyclohexanone>benzene.Density functional theory calculations revealed that benzene and cyclohexane were chemically adsorbed and firmly attached on Pt and Pd surface,which was beneficial to their further oxidation.On the contrary,cyclohexanone and n-hexane were physically adsorbed and hard to be activated.The inferior activity of Pd might be ascribed to the fact that the d-band center of Pd is closer to the Fermi level,thus Pd bonds strongly with the C6 hydrocarbons and restrains products desorption.The Pt-Sn/y-Al2O3 honeycomb catalysts were prepared by incipient wetness sequence impregnation procedure.The Sn,acting as promoter,improved the catalytic activity of Pt slightly.The optimal Pt/Sn ratio was 3/1(w/w),where the T90 of it decreased about 20 ?.In addition,the cyclohexane conversion over Pt/Sn=3/1 catalyst displayed no sign of decline,while the Pt activity dropped about 1.5%,after 720 h of consecutive run.Part of Sn alloyed with Pt and formed Pt3Sn alloy.In the alloy structure,Sn charge transferred to Pt sites and increased the Pt electron density,thereby weakening the adsorption strength of unsaturated hydrocarbons and reducing surface coke deposition.Besides,Sn divided Pt into smaller clusters and improved the dispersion of Pt.Sn also anchored Pt onto the support,inhibiting the size growth of Pt clusters.Consequently,the Pt/Sn=3/1 catalyst exhibited enhanced catalytic activity and prolonged service life.