Tailoring Of The Coordination Structure Of Pt Single Atom Catalysts For Enhanced Catalytic Hydrogenation Performance

Supported metal catalysts are an important class of heterogeneous catalysts,which are widely used in the fields of catalytic hydrogenation,oxidation,methane reforming,water gas conversion,photocatalysis and electrocatalysis.Generally,catalytic reactions take place on the surface atoms of metal particles.Therefore,reducing the particle size to the subnano region,especially to the atomic dispersion,would maximize the utilization efficiency of metal atoms.Since Zhang and co-workers proposed the concept of single atom catalysts in 2011,single atom catalysis has become a research hotspot in the field of heterogeneous catalysis in recent years.Compared with supported metal particle catalysts,single atom catalysts are the bridge bewtween heterogeneous and homogeneous catalysis.They have relatively simple,clear and uniform structures of metal active sites,which can faciliate to reveal the structure-activity relationships and to achieve atomic level understanding of the molecular mechanism of catalytic reactions through experimental and theoretical approaches.For single atom catalysts,the coordination environment of the active metal center greatly affects its electronic and geometric structure,which in turn affects its catalytic performance.Therefore,it is particularly important to tailor the the coordination environment single atom catalysts for improving its activity and stability.In this doctoral thesis,the Pt1/CeO2 single atom catalyst system is controlled by P-doping,N-doping,and changing the morphology of CeO2 support to tailor the coordination environment of Pt single atoms,and the relationship between its electronic structure and catalytic performance in hydrogenation reaction was studied.The main results are as follows:(1)By P doping on nanorods CeO2,the local coordination environment and electronic structure around Pt single atoms are adjusted and the hydrogenation performance is optimized.Firstly,nanorods CeO2 was successfully prepared by the hydrothermal crystallization under alkaline conditions and then impregnated with P.secondly,Pt single atom catalysts(Pt1/CeO2-500 and Pt1/PO4-CeO2-500)were prepared on the above two supports by wet-impregnated method,respectively.The materials were characterized and analyzed by CO-DRIFTS,HAADF-STEM,and XAFS.It was confirmed that the Pt atoms in both samples were atomic dispersion.Compared with Pt1/CeO2-500,the Pt1/PO4-CeO2-500 sample after P modification showed more excellent hydrogenation activity and the hydrogenation activity of styrene increased by 10 times at 60℃ and 1 bar hydrogen.The TOF increased from 30h-1 to 316h-1.In the catalytic hydrogenation of cyclohexene,phenylacetylene and nitrobenzene,Pt1/CeO2-500 showed poor activity,but the hydrogenation activity of Pt single atoms after P doping has increased at least 3 times.Among them,the TOF of phenylacetylene increased from 44h-1 to 169h-1,and the TOF of nitrobenzene increased from 32h-1 to 273h-1.Through CO-DRIFTS,XAFS and XPS characterization,it was found that P doping changed the coordination environment and electronic characteristics of Pt single atoms,forming a Pt-O-P coordination structure,which increased the proportion of Pt4+from 23%to 51%.In addition,the change of the Pt coordination environment also enhances the catalyst adsorption of the reaction substrate and the hydrogen spillover effect,which could improve the performance of catalytic hydrogenation.(2)The nanorods CeO2 support was further doped with N and the effects of P and N non-metal doped nanorods CeO2 on Pt coordination environment and hydrogenation activity were compared.In this chapter,nanorods CeO2 is doped by N and then loaded with Pt atoms.The characterization of CO-DRIFTS showed that compared with Pti/CeO2-500,the linear absorption peak of CO shifted from 2094cm-1 to 2097cm-1 for the N doped Pt single atom catalyst.After P doping CeO2,the linear absorption peak of CO shifted from 2097cm-1 to 2106cm-1.The XPS results showed that after N doping,the electron transferred from N to Pt,the proportion of Pt4+ was reduced from 23%to 11%,while the electron transferred from Pt to P,the proportion of Pt4+increased from 23%to 51%after P doping,which is consistent with the results of CO-DRIFTS.This result reflects that the doping of the two types of non-metallic(P and N)has the opposite effect on the valence state of Pt single atoms.Surprisingly,in the hydrogenation of phenylacetylene under 1 bar hydrogen at 60℃,the activity of Pt single atom catalyst was significantly improved after N and P doping on nanorods CeO2.The TOF of phenylacetylene was increased by 9 times and 4 times.N doping is significantly better than P doping.(3)The hydrogenation performance is further optimized by adjusting the morphology of CeO2 and doping N.Spherical CeO2 was successfully prepared by solvothermal method.The impregnation method was used to support Pt single atoms on nanorods CeO2 and spherical CeO2 carriers.The linear adsorption peaks of CO on Pt atoms are located at 2097cm-1 and 2096cm-1,respectively,which proves that the Pt in these two samples are single atom dispersed.Due to the abundant corner and step sites on the surface of spherical CeO2,spherical CeO2 has higher hydrogenation activity than Pt supported on nanorods CeO2 with rich(110)crystal plane.The hydrogenation activity increased by about 8 times,and the TOF increased from 23h-1 to 163h-1.When the calcination temperature is adjusted to 200℃,the TOF of the Pt1/N-CeO2-200 single atom catalyst can reach 559h-1,which is the best performance.