Study On The Structual Property Of Pd-Ce Catalysts And Their Performance In CO Oxidation

Noble metal nanoparticles have a wide range of application prospects due to the excellent catalytic performance,while at the same time they faced the problem of poor stability under reaction conditions.The support could be employed to limit the agglomeration of nanoparticles,affect the structure of active sites and improve the activity and stability of catalyst,which had attracted extensive attention of researchers.In this dissertation,the structural regulation and catalytic improvement of Pd supported on Ce-based support were investigated.The particle size of Pd species on Ce-based support,along with their spatial location and distribution were controlled with appropriate preparation methods.The morphological evolutions of Pd on the catalysts were explored,which involved catalysts without spatial confinement,with partial confinement,and with complete confinement.After that,the effective regulation of the spatial distribution and active species of Pd was realized.On that basis,we applied these catalysts with different structures to CO oxidation to evaluate their catalytic activity,analyzed the structures of different Pd species and explored their catalytic mechanisms in CO oxidation.Finally,the structure-activity relationship of Pd/CeO2 catalyst for CO oxidation at low temperature was established,the reaction mechanism was elaborated and the structure of catalysts was developed.Firstly,Pd particles were dispersed on the external surface of CeO2,of which Pd was without spatial confinement.Pd species were present in form of PdO and Cel-xPdxO2-? solid solution.The Pd/CeO2 catalysts were thermal treated for varied duration and their effect on Pd species was explored.The results showed that Pd gradually incorporated into the CeO2 lattice to form Ce1-xPdxO2-? solid solution during the thermal treatment.The increasing concentration of Ce3+ on the surface was conducive to the formation of oxygen defects.When the doping concentration reached a maximum value,the catalyst showed a highest CO oxidation activity as 90%CO conversion at 84?after H2 reduction.At the same time,the mobility of oxygen ions was thus enhanced with the higher level of Ce4+reduction and larger oxygen vacancy concentration.Another elongation of thermal treatment led to the Pd extraction from CeO2 lattice in tandem with higher dispersion and smaller PdO particles on the surface,which was demonstrated by the results of XRD,ICP,TEM,CO pulse adsorption and CO-TPR as a redispersion phenomenon.The decrease in the diameter of PdO particles was beneficial for the improvement of catalytic activity.Secondly,we prepared Pd@CeO2 core-shell nanotube catalysts by solvothermal method using carbon nanotubes as a sacrificial template,on which Pd was anchored at the inner surface of CeO2 nanotubes and thus partially confined.The noble metal was preserved at high dispersion and small diameter due to the synergistic effect of physical-confinement and interaction-confinement.The confining degree was controlled by varying the thickness of ceria shell.The Pd-Ce interface,distribution of Pd species and interaction between Pd and Ce were regulated as a consequence.The interaction of Pd-Ce increased as the thickness of CeO2 shell increased,and the proportion of Pd atoms exposed on the surface decreased at the same time.In addition,a new model for calculating the numbers of exposed Pd atoms and Pd atoms interacted with ceria was proposed.The influence of different distribution for Pd species on the two turnover frequency(TOF)was investigated.The contributions of Pd atoms located at different positions to activity were found to be distinguished and the interfacial atoms were of most importance.When the thickness was appropriate,the ratio of the number of exposed Pd atoms to that of Pd atoms interacted with CeO2 was 0.26,and the highest perimeter TOF value(8.40 × 10-12 s-1)was displayed.The stability was also improved.In order to further spatially confine the size of Pd species and maximize Pd-Ce interface,we constructed Ce-based metal organic framework(CeMOF)encapsulating Pd nanoparticles catalyst,on which Pd was confined completely.The influences of diferent preparation methods and organic ligands on the spatial location,dispersion and Pd-Ce interaction were studied.The nanorod structure,which was formed when employing 1,3,5-benzene-tricarboxylic acid,was more favourable for dispersing Pd particles.While the spherical structure,which was formed when employing 1,4-benzenedicarboxylic acid,induced a negative effect.As for the latter,Pd was inclined to agglomerate during pyrolysis,and formed particles with larger size,low dispersion and weak interaction.In-situ assembly method was demonstrated to be an effective way to encapsulate Pd particles into the cavity of CeMOF.And the resulting Pd-CeMOF structure can enable the spatial restriction of Pd particles,promote high dispersion and prevent agglomeration during pyrolysis,A new Pd species,PdOx(s)/Pd-O-Ce(s)clusters appeared in the derivate's after pyrolysis,which was highly active for CO oxidation at low temperature.On the basis of above reserches that the morphology,size and dispersion of Pd nanoparticles could be regulated with the confined structure.Finally,the developed preparation of highly efficient Pd-Ce catalyst was realized in the embedded Pd-CeO2 nanosheet catalysts.Taken Pd-CeMOF as the precursor,the spatial location of Pd particles in derivatives could be controlled by changing the pyrolysis conditions.When Pd in the catalyst was at the critical point between confined state and non-confined state,it had the characteristics of high dispersion and small size due to the limitation of confinement while the accessibility of active sites was not effected by ceria.Besides,the content of highly active PdOx(s)/Pd-O-Ce(s)clusters was controlled by surface oxygen concentration.The high dispersion and abundant PdOx(s)/Pd-O-Ce(s)clusters in Pd-CeO2-900 led to a 90%CO conversion at 58?,which was the most active catalyst in this dissertation.Combined the evalutions and characterizations,PdOx(s)/Pd-O-Ce(s)clusters was found to be Pd species that was not completely oxidized when it was not completely oxidized under the limitation of confined structure and low surface oxygen concentration.It was revealed by O2-CO pulse experiment that PdOx(s)/Pd-0-Ce(s)clusters have high active oxygen species,which can react with CO at the first time.The reaction followed the Mars van Krevelen mechanism.PdOx(s)/Pd-O-Ce(s)clusters adsorbed and reacted with CO while CeO2 continuously supplied it with O through the interface,and CO2 was thus produced continuously.