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Controlled Synthesis Of Close-contact Pd-ZnO Heteroaggregate Nanostructures And Their Applications In Catalytic Hydrogenations Of Substituted Nitrophenols

Posted on:2020-04-23Degree:MasterType:Thesis
Country:ChinaCandidate:Q LiFull Text:PDF
GTID:2381330572989571Subject:Chemical Engineering and Technology
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This thesis aims at controllably synthesizing Pd-ZnO heteroaggregate nanostructures and preparing new types of nanocatalysts that are superior to monometallic analogues in catalytic properties by employing metal-metal oxide interactions.Compared with monometallic catalysts synthesized in the same conditions,the obtained supported metal-metal oxide heteroaggregate nanocatalysts show enhanced catalytic performances in selective hydrogenations of substituted nitrophenols.The combination of characterizations and catalytic results suggests that the metal-metal oxide interactions controllably adjust the electronic structure of catalytically active centers so that their catalytic performances are enhanced.The detailed study includes the followings:(1)The controlled synthesis and characterizations of well-defined Pd-ZnO/Al2O3 heteroaggregate nanocatalysts.This study developed a method to synthesize Pd-ZnO heteroaggregate nanocatalysts by in-situ transformation of PdZn bimetallic nanoparticle on supports.In the typical synthesis of Pd-ZnO/Al2O3 catalysts,a co-reduction method is used to reduce Pd2+ and Zn2+ ions by n-butyllithium in liquid phase,resulting in PdZn ordered intermetallic nanoparticles.The following impregnation of PdZn alloy colloids with Al2O3 supports gives PdZn/Al2O3,which is further calcined at high temperatures and subsequently selectively reduced to form Pd-ZnO/Al2O3 heteroaggregate nanocatalysts.Controlling the ratio of Pd/Zn in Pd,Zny alloy precursors,Pdx-(ZnO)y/Al2O3 catalyst can be obtained by the same procedures.Various characterization techniques,such as diffuse reflectance infrared fourier transform spectroscopy with CO probes,transmission electron microscope,X-ray diffraction,hydrogen temperature programmed reduction and X-ray photoelectron spectroscopy reveal the followings:PdZn ordered intermetallic compounds are formed by using n-butyllithium as reducing agent at 240 ℃,and close-contact Pd-ZnO heteroaggragate nanostructures are obtained by in-situ transformation of PdZn nanoparticles on supports.(2)Catalytic performance of Pd-ZnO/Al2O3 for hydrogenations of substituted nitrophenols.This study selects the selective hydrogenations of substituted nitrophenols to investigate the relationship between the catalyst structures and their catalytic properties.Compared with monometallic Pd/Al2O3 catalysts,the obtained Pd-ZnO/Al2O3 heteroaggregate nanstructures show greatly enhanced catalytic performances for selective hydrogenations of various substituted nitrophenols,and the hydrogenation conversion was significantly improved.Under mild reaction conditions(45 ℃ and H2-0.1 MPa),the Pdx-(ZnO)y nanosturcture has the highest catalytic performance when the ratio of Pd/Zn in PdxZny alloy precursor is 1/1.The study suggests that the close-contact heteroaggregate nanostructures not only provide the highly active sites for hydrogenations but also prevent the active particles f-rom aggregation by anchoring Pd particles onto supports,resulting in enhanced catalytic activities and stability.Moreover,the interaction between Pd and ZnO can be precisely adj usted by adj usting the Pd/Zn ratios in PdxZny alloy precursors.
Keywords/Search Tags:nanoalloys, metal-metal oxide heteroaggregate nanostructures, selective hydrogenation, supported catalysts, substituted nitrophenols
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