| Selective hydrogenation of unsaturated compounds plays a key role in chemical processes.For example,the selective hydrogenation of aromatic nitro compounds and unsaturated aldehydes and ketones,are two kinds of reactions that have been widely studied and utilized.The aromatic amines produced from the former reactions are important intermediates,from which about 300 kinds of chemicals can be synthesized;and the unsaturated alcohols produced from the later reactions are important fine chemicals.Ordinary,heterogeneous catalysts are ordinarily used to catalyze hydrogenation reactions of organic compounds,as they can be separated and regenerated easily.In view of the complexity of the microstructure of heterogeneous catalysts,there still remain some problems that are not clear,such as the adsorption and activation modes of H2 and reactants,the effects of the microstructure and the active species on the catalytic selectivity,and so on.The solution of these scientific problems is very significant for designing efficient heterogeneous catalysts.Therefore,the study on the relationship between surface structure and catalytic performance,as well as the catalytic reaction mechanism,at atomic level is an appealing and hot topic in the field of catalysis.In this dissertation,Pt/C,Co/Pt/PAC(P-containing Activated charcoal),Pt/SiO2-TiO2 and Pt/TiH2 catalysts were prepared and applied to the model reactions of selective hydrogenation of aromatic nitro compounds and ?,?-unsaturated aldehydes.The key scientific issues remaining in those selective catalytic hydrogenation reactions were investigated and discussed in detail,such as,the nature of metal-support strong interaction(SMSI),the relationship between interface defects and geometric effects,the relationship between interface defects and electronic effects,the influence of defects on hydrogen cleavage and substrate adsorption behavior.Discussion was given to the relationships among the defects,electronic effect,geometric effect and active centers in the heterogeneous catalytic system.Based on the results obtained above,the catalytic mechanisms and selectivity and activity controlling mechanisms were discussed.(1)Several Pt/C catalysts have been prepared by using impregnation method,and the effects of surface oxygen functional groups(SOFGs)and heteroatom P on the selective hydrogenation of 3-nitrostyrene were discussed.The surface oxygen containing functional group,electronic structure of Pt,the nature of the strong interaction between Pt and carbon support were adjusted by varying the reduction temperature of Pt/C,and thus the catalytic selectivity was changed from C=C hydrogenations to the N=O hydrogenation.The catalyst Pt/C reduced at low temperature favored the hydrogenation of C=C bond and the one reduced at higher temperature favored C=O bond.Based on the results of catalyst characterization such as TEM,TPR,XPS,TPD-MS,DRIFTS and the reaction experimental data,for the catalyst reduced at low temperature,surface acidic groups present close to Pt nanoparticles,affecting the electronic state of Pt,promoting the intermolecular interaction between acidic groups and N=O,resulting in the preferential adsorption of C=C on the surface of Pt,which facilitates the hydrogenation of C=C bond;For the catalyst reduced at high temperature,however,SOFGs on the surface of carbon support decomposed,inducing P species to interact with Pt to form Pt-POx complex,facilitating the preferential adsorption and activation of N=O on the catalyst surface,and so enhance the selectivity the N=O bond.(2)The bimetallic supported catalysts of Co/Pt/PAC and CoPt/PAC were prepared by using continue-impregnation(step-by-step)and co-impregnation method,respectively.The effects and roles of Co on selective hydrogenation of 3-nitrostyrene were discussed.It was found that the Co effect is largely on the Co/Pt/PAC catalyst compared with CoPt/PAC catalyst.With comparison to Pt/PAC catalyst,the addition of Co enhanced the selectivity of N=O bond for Co/Pt/PAC catalyst,while it is not obvious for CoPt/PAC catalyst.The results of catalyst characterization and experimental data showed that an active species of Pt-POx was formed in Co/Pt/PAC catalyst,and which is the adsorbing and activating sites for N=O bond as the addition of Co varying the interaction of Pt with support.The continuous impregnation method promoted the formation of CoOx on the surface of Pt,and changed the coordination environment of the phosphine species on the surface of the catalyst,and promoted the formation of low coordination phosphine oxides(P-?),resulting in an increase of Pt-POx active center of and improve the hydrogenation selectivity of N=O.By contrast,the co-impregnation will induce the formation of little amount of CoPOx,leading to the decrease of phosphine oxides(P-?)and Pt-POx species formation.(3)Pt/TiO2-SiO2 catalyst was prepared by using impregnation,the effects of strong metal-support interaction(SMS I),electron enriched Pt particle and Ti2+defects on catalytic performances of cinnalmaldehye hydrogenation were discussed.The strong interactions(SMSI)formed during the reduction process,the migration of electrons and interfacial defects were benefical to hydrogenation activity and selectivity of the C=O bond.The results of characterization and experiment showed that the Pt-TiOx species formed via electron transfer process benefited the formation of electrons enriched Pt,which facilitates the vertical adsorption of trans-cinnamaldehyde(CAL)molecules through the terminal C=O bond.The Si atoms in the TiO2-SiO2 would promote the formation of Ti2+defects,which played a key role in the adsorption and activation of cinnamaldehyde(CAL)molecules.In addition,the three-dimensional ordered mesoporous channels of Pt/TiO2-SiO2 catalysts facilitated the dispersion of Pt nanoparticles and limited the migration and dissolution of metal Pt nanoparticles.(4)Pt/TiH2 catalyst was prepared by using impregnation,the key role of strong metal-support interaction(SMSI)between Pt and hydrogen storage carrier TiH2 in ionic hydrogenation of cinnalmaldehye was investigated.The Pt-TiH2 interface and heterolytic hydrogenation promoted the selectivity of C=O bond.Based on the results of catalyst characterization such as in-site DRIFTS,TPD-MS,TPR and the reaction experimental data,the interface between Pt nanoparticles and defects-rich TiH2 support was the active site of the dominant adsorption of C=O in the cinnamaldehyde.The defects-rich TiHxOy surface layer were key media for hydrogen spillover and H-species storage,migration,and stabilization of H+(H-)species.In addition,the active H· generated on the surface of Pt spillovered to the surface of TiH2,the defects-rich TiHxOy layer play a key role in H+ generation and surface spillover,while H-would be stored and rapidly migrated in TiH2.For the hydrogenation of the C=O adsorbed on the interface,the H-species migrated from the TiH2 support attacked the positively charged C atom in the C=O bond,and H+attacked the negatively charged O atom in the C=O bond,and then heterolytic hydrogenation progress was completely.The isotope experiment proves that the storage H in the TiH2 participated in the hydrogenation reaction,and the diffusion rate of the H species is the rate-determining step in the heterolytic hydrogenation reaction. |
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