Preparation Of Supported Noble Metal Catalysts And Their Structure-activity Relationship For Benzene Oxidation

Catalytic oxidation technology has been considered as a highly efficient approach to degradation of volatile organic compounds(VOCs),and the efficiency of the such processes highly depends on the performance of the catalysts.Among various catalysts,supported noble metal catalysts are extremely interesting owing to their excellent activity.However,the commercialization of such noble metal catalysts has been hindered by their environmental hazards,high cost,tedious preparation,especially when those relatively expensive transition metal oxides are employed as the support.In a closely related issue,in recent decades plant-mediated biosynthesis of noble metal nanoparticles(NPs)has been proved as a simple and effective approach which avoids the introduction of additional protective agents;particularly,thus obtained noble metal NPs in general have been observed with high stability compared with th eir chemically synthesized counterparts.In this study,preparation and structure-activity relationship of supported noble metal catalysts for benzene oxidation were investigated based on three aspects.First,using 3D order mesoporous material as catalytic support,supported Pd catalyst was prepared through plant-mediated biosynthesis.Second,using waste eggshells with nature pore channel and hierarchical structure as catalytic support,supported Ag catalyst was prepared through impregnation method.Last,using such waste eggshells as catalytic support,supported Pt catalyst was further fabricated through plant-mediated biosynthesis.The detailed results were summarized below.First,a biogenic method was employed to synthesize Pd NPs with tunable sizes by using Cacumen platycladi(CP)leaf extract as a reducing agent.The Pd NPs were then anchored over a support which was made from KIT-6-templated three-dimensionally ordered mesoporous(3DOM)CeO2(i.e.,kit-CeO2)to form xPd/kit-CeO2 catalysts with various Pd loadings(x denotes Pd loading,wt%).The morphology of the samples show that both the support and catalyst exhibit a highly three-dimensional ordered mesostructure as a replica of the KIT-6,and demonstrate that the Pd NPs were well dispersed on kit-CeO2.The BET specific surface areas of the support and catalysts varied in the range of 105-109 m2/g.Density functional theory(DFT)calculation indicated that the interactions between Pd NPs and porous CeO2 would facilitate the activation of benzene adsorbed on Pd NPs.In the benzene oxidation catalyzed by 0.5Pd/kit-CeO2,the temperature required for 90%benzene conversion(T90%)was as low as 187?(20,000 mL/g/h);The biogenic method is better than the chemical method to synthesize Pd NPs with higher catalytic activity while the kit-CeO2 is better than the commercial CeO2 and KIT-6-templated transition metal oxides(e.g.,Fe2O3 and Co3O4)as a supporting material.Furthermore,the 0.5Pd/kit-CeO2 showed remarkable catalytic stability,which could operate steadily for at least 150 h in on-stream reaction.And the characterization results of the catalyst reflect that the structure of the Pd catalyst did not change during long-time reaction.Besides,XPS and H2-TPR results can demonstrate that high Pd0 concentration,abundant oxygen adspecies and low temperature reducibility are beneficial to the catalytic activity of the catalystSecond,Ag NPs loaded-eggshell catalysts were successfully synthesized by a simple impregnation method,in which waste eggshells were used as a support.SEM,XRD and XPS results show that Ag0 can be observed on the Ag catalyst and a different number of Ag NPs are uniformly distributed on the surface of eggshells,which are defined as Ag1/eggshell(10.8 wt%),Ag2/eggshell(19.9 wt%)and Ag3/eggshell(34.3 wt%)catalysts according to the amounts of loaded Ag NPs.When used for the catalytic oxidation of benzene,the eggshell-supported catalysts exhibit superior catalytic activity compared with the pure unsupported Ag NPs or those supported on the commercial CaCO3(com-CaCO3).The Ag2/Eggshell catalyst calcined at 500? exhibited excellent catalytic activity with a T90%as low as 257?(100,000 mL/g/h),which is assigned to the unique channel structure of eggshell,high dispersion of particles(size effects)on the surface of eggshell as well as the strong interaction between the Ag NPs and eggshell.Furthermore,the Ag2/eggshell catalyst also exhibited remarkable stability,as little degradation in the activity occurred even after 200 h of on-stream reaction.The characterization results of the catalyst after 200 h reaction are much the same as the unreacted one.The reaction mechanism is put forward based on the in situ FTIR experimental results,in which some carboxylate intermediate species are confirmed.Last,the waste eggshell was used to prepare the supported Pt catalysts through a plant-mediated biosynthesis method in which Pt precursor was reduced to Pt NPs with CP leaf extract.The temperature and atmosphere for thermal treatment of such eggshell-supported Pt catalysts were assessed to understand their effect on the catalytic perfromance towards benzene oxidation.The optimal Pt/Eggshell-Ar(calcined at 400? in Ar)demonstrate a T90%as low as 178?(80,000 mL/g/h)and could operate steadly for at least 300 h of on-stream reaction.The characterization results of the catalyst after 300 h in on-stream reaction reflect that the structure of the Pt catalyst are much the same as the unreacted one.TEM,TG and XPS results show that the Pt NPs are uniformly dispersed on the surface of eggshells,and the calcination conditions of the samples have an important influence on the residual CP leaf extract,the mean diameter of Pt NPs and the ratio of Pt0/Pt2+ over the catalysts.DFT calculation indicated that the interactions between Pt NPs and porous CaCO3 would facilitate the activation of benzene adsorbed on Pt NPs,and the larger pore structure was beneficial to the benzene activation.And thus bioreduced Pt catalyts were proved to overtake the chemically reduced counterparts in terms of their catalytic performance,and both these bioreduced and chemically reduced Pt NPs supported on the eggshell exhibited higher catalytic activity than the commercial Pt/C(com-Pt/C)catalyst.On this basis,immobolizing biosynthesized noble metal active components on the eggshell-based support could be a promising approach for the preparation of supported noble metal catalysts with high catalytic performance for VOCs oxidation.