Synthesis Of Metal-oxide Heteroaggregate Nanocatalysts Using Core-Shell Nanoparticle Precursors And Their Application In Selective Hydrogenation Of Substituted Nitrobenzenes

This thesis aims to synthesize Rh-Co2O3 heteroaggregate nanocatalysts by a sequential reduction method and prepare new types of highly efficient nanocatalysts by the metal-oxide interaction.Compared with monometallic catalysts synthesized under the same conditions,the supported Rh-Co2O3 shows excellent catalytic activity and selectivity in the selective hydrogenation of substituted nitrobenzenes.The results indicate that the interaction between metal and oxide improve the catalytic performance.The detailed results are as follows:(1)Controllable synthesis of Rh-Co2O3/Al2O3 heteroaggregate nanocatalysts.In this work,poly(vinylpyrrolidone)stabilized core-shell Rh@M(M=Co,Ni)nanoparticles(NPs)were prepared by a sequential reduction method,which was further in situ transformed into metal-oxide heteroaggregate nanostructures on alumina supports.As an example of Rh-Co2O3/Al2O3,Rh NPs were prepared by reduction of Rh3+ precursor,and then Rh@Co NPs were obtained by a rapid reduction of Co2+ precursor onto Rh NPs.The following impregnation of Rh@Co colloids with Al2O3 supports gives Rh@Co/Al2O3 catalysts,which was further calcined at high temperatures and selectively reduced by H2 to give Rh-Co2O3/Al2O3 heteroaggregate nanocatalysts.Nanocatalysts with different Rh/M ratios can be prepared by adjusting the ratios of the starting materials.The characterization results of X-ray powder diffraction,dark field scanning transmission electron microscopy,X-ray photoelectron spectroscopy and Fourier infrared diffuse reflectance with CO probes show that Rh@M NPs consist of Rh cores and M shells,and the obtained Rh-MxOy using Rh@M precursors have close-contact metal-oxide heteroaggregate nanostructures.(2)Catalytic studies of the hydrogenation of substituted nitrobenzenes over Rh-MxOy/Al2O3 nanocatalysts.In this work,the selective hydrogenations of substituted nitrobenzenes were selected as probe reactions to investigate the relationship between the structure and performance of the catalysts.Compared with the monometallic Rh/Al2O3 catalysts,the catalytic activity and selectivity of Rh-MxOy/Al2O3 nanocatalysts are greatly improved for selective hydrogenations of several substituted nitrobenzenes.Under mild reaction conditions(45? and H2-0.1MPa),Rh-MxOy/Al2O3 nanocatalysts with a Rh/M ratio of 1/1 show the best catalytic performance and high stability.The results suggest that the improvement of Rh-MxOy is due to the interaction between Rh and MxOy,which not only anchors Rh NPs onto the support to avoid the agglomeration of Rh NPs but also provides highly reactive sites to improve the catalytic hydrogenation performance.In addition,the interaction between metal and oxide can be precisely adjusted by adjusting the ratios of Rh/M in the precursors.