Synthesis, Characterization And Catalytic Application Of Single-Site Catalysts

Catalysis plays an important role in many fields such as petroleum industry,pharmaceutical industry and energy industry etc..The evaluation dimensions of catalyst performance include catalytic activity,selectivity and stability.In recent years,the emerging“single-atom catalyst”has bridged the gap between as well as took the advantages of homogeneous catalysis and heterogeneous catalysis.While metal species were atomic dispersed and attached onto the support,the stability and recyclability of as-prepared single-atom catalyst would significantly been improved compared with homogeneous precursors,the cost of the catalyst would be reduced,and the number of active sites were also increased.At the same time,the catalytic activity and selectivity can be optimized by tuning the metallic active site and the coordination environment of the support.In this thesis,the synthesis methods,characterization methods and catalytic applications of single-atom catalysts are summarized first as background introduction.Novel single-atom catalysts such as“Rh₁@MSNS-NH₂”and“MOF-derived CoN_xC”were synthesized and their catalytic properties were also studied.In addition,the relationship between the structural characteristics of single-atom catalysts and their corresponding catalytic activities were comprehensively studied by spectroscopy and electron microscopy characterization techniques.The relationship of"synthesis-structure-performance"of MOF-derived CoN_xC catalysts was established,which would provide the guilding strategy for further catalyst design.The progress has been made in my Ph.D research includes the following three aspects:1.Synthesis of Rh₁@MSNS-NH₂ single-site catalysts and their catalytic activity and selectivity in catalysis.A variety of single-atom catalysts,denoted as Rh₁@MSNS-Ligand,were synthesized by ligand exchange method between Rh salt precursors and three-dimensional dendritic mesoporous silica nanospheres（MSNSs）modified by different ligands.Several rhodium catalyst（Rh₁@MSNS-Ligand）modified by different ligands were applied to the catalytic reduction of 4-nitrophenol.It was found that different ligand environments have a significant effect on the catalytic rate of single-atom Rh catalysis.We also apply Rh₁@MSNS-NH₂ to hydrosilylation reaction with terminal alkyne,unlike most reported hydrosilylation catalyzed by nano-metal catalysts which tend to result with thermodynamically stableβ-（E）alkenyl siliconization products.The hydrosilylation reaction catalyzed by Rh₁@MSNS-NH₂ can alternatively realizeα-alkenyl siliconization as kinetic hindrance and thermodynamic instability of products by Markov addition.The product indicated that Rh₁@MSNS-NH₂ also has specific site selectivity for some catalytic reactions.2.Three-dimensional Electron tomography study of Au@MSNS-NH₂ samples.The Au@MSNS-NH₂ sample was photographed by two-dimensional projection with transmission electron at different tilt angles in TEM.The obtained images were reconstructed by weighted back projection resulting electronic three-dimensional tomography,and the internal cross-section image of Au@MSNS-NH₂ sample was analyzed.The 3D reconstructed results successfully give direct evidence that the gold nanoparticles are located inside the channels of the mesoporous silica nanosphere rather than on the outer surface,indicating that the mesoporous material has“spatial confinement effect”on their supported metal nanoparticles.3."Synthesis-Structure-Performance"relationship based on MOF-derived Co-N-C non-precious metal ORR catalyst.CoCl₂ as cobalt source,1,10-Phen as nitrogen source and ZIF-8 as support were used as catalyst supports,and then been polysized in argon atmosphere to obtain non-precious metal ORR catalyst.By different mixing methods of CoCl₂-Phen-ZIF8 catalyst precursor and subsequently after heat treatment,Co-N-C catalysts with dual-sites Co@C-CoN_xC and single-site CoN_xC could be obtained with controlled synthesis respectively.The ORR polarization curve test was carried out under acidic conditions with dual-sites CoN_xC-Co@C-1000℃and single-site CoN_xC-1000℃,The test results showed that the Co-N-C catalyst without Co@C site had no significant decrease in ORR performance under acidic medium,implies that CoN_xC site would be the real ORR catalytic active site instead of Co@C site.We also studied the"temperature effect"in the calcination activation process of catalyst precursor.We resort XAS,TEM as well as other characterization methods to comprehensively study the structural characteristics of the catalysts,combine the corresponding ORR catalytic performance transformed with calcination temperatures,and the"Sythesis-Structure-Performance"relationship among Co@C-CoN_xC and CoN_xC catalysts calcined at different temperatures was established.