Synthesis,Catalytic Property And "Structure-activity Relationship" Of Supported Noble Metallic Catalysts

With the rapid development of global economy,human’s demand for energy is also increasing,which causes environmental pollution and other problems.Catalysts have great development and utilization value for resolving these problems.Noble metallic nanocatalysts have been widely studied owing to the advantages of lower light-off temperature,high catalytic activity at low temperature,and the effect of nano size.The factors that affect the catalytic performance of noble metallic nanocatlysts are not only the type,size and valence of the metal,the type,morphology and crystal shape of the carrier,but also the metal-support interaction and the local structure of the active metal.Therefore,it is of great significance to understand the influence factors and“structure-activity relationships”of supported noble metal nanocatalysts in different catalytic reactions for the early design and synthesis of catalysts and industrial production.In this dissertation,the conventional simple preparation method is used to support one noble metal on the inert materials or bimetal on the reduced materidals.The“structure-activity relationship”between the structure of noble metallic nanocatalysts and catalytic performance was studied systematically by adjusting the reaction conditions,the size and morphology of different metals.Related findings in this paper are summarised as follows:（1）The effects of catalyst factors（precursor,carrier,load）,synthetic methods（sedition-precipitation,impregnation）,doping of transition metals,and reaction conditions（reduction temperature,reaction space）on the catalytic performance of selective hydrogenation of acetylene are studied.It is determined that the 0.1Pd/SiC catalyst prepared by deposition-precipitation method with the K₂PdCl₄ as the precursor shows the best comprehensive performance in hydrogenation of acetylene（acetylene conversion rate is 98%and ethylene selectivity is 78%）and excellent stability when the reaction air speed is 4,8000 mL/g·h after reduced by 500?C in one hour.（2）A series of platinum-based catalysts is synthesized by incipient wetness impregnation,the platinum loads designed as 0.1,0.5,1 and 5 wt%,the different materials（silicon carbide,silicon nitride,silicon dioxide,activated carbon,cerium dioxide,etc.）is used as support.The results of high resolution transmission electron microscopy（HRTEM）and X-ray diffraction（XRD）technology prove that the size of platinum particles increased with the increase of the loading.According to the results of catalytic performance of the catalysts,we find that the conversion rate of 2-butyny-1,4-diol over the catalyst supported by silicon carbide decreases with the increase of the size of platinum nanoparticles,but the selectivity of 2-butene-1,4-diol show a"volcanic type"relationship with the size of nanoparticles.And the results of XRD and HRTEM confirm the size of Pt nanopartcles are not change during reaction,which means the silicon carbide supported platinium catalysts have a hydrothermal stability.The results of X-ray absorption near edge structure（XANES）spectroscopy indicate that the cationic platinum species change to the reduced state during the reaction.The results of the fitting results of extend X-ray absorption fine structure（EXAFS）and temperature programme reduction by hydrogenation（H₂-TPR）show that there is no obvious interaction between platinum nanoparticles and the support.Finally,we further verify that the 2-3 nm reduced platinum nanoparticles are the active species of selective hydrogenation of butynediol by using HRTEM,XANES and EXAFS.（3）In order to study the effect of transition metal structure to noble metallic nanocatalysts,firstly,we have synthesized nanorod cerium dioxide by hydrothermal method as the support,then deposited gold and nickel on the cerium dioxide nanorods by deposition-precipitation method with the sodium bicarbonate as the precipitant.The gold-nickel bimetallic nanocatalyst is synthesized by adjusting the content of nickel.The XRD and the scanning transmission electron microscopy（STEM）equipped with energy dispersive X-ray spectroscopy（EDS）is used to confirm that gold and nickel were homodispersed on the surface of cerium dioxide before the reaction.And the results of X-ray photoelectron spectrum（XPS）indicate that nickel exists mainly in the Ni²⁺before and after the reaction,while gold exists in the oxidation state before the CO oxidation and water-gas shift reactions,but the gold exists mainly in the reduced state after the reactions.In addition,the study of catalytic performance shows that the addition of nickel is negative to the activity of the catalyst over the CO oxidation,but positive to the water-gas shift reaction.Combining XANES and EXAFS techniques,it confirms that the ionic Au^?+species is beneficial to the water-gas shift reaction,while the metallic Au⁰ species promoted the acitivity over CO oxidation reaction.The results of XANES shows that the increasing loading of nickel increases the number of Au^?+species,which indicates that the main contribution of Ni is to adjust the electronic structure of gold species,thus affecting the catalytic activity of the bimetallic catalysts.In summary,the supported noble metallic nanocatalysts are the research object in this work,we have ascertained the optimal preparation and reaction conditions for silicon carbide supported palladium catalysts in selective hydrogenation of acetylene,and identified the“structure-activity relationship”between silicon carbide supported platinum catalysts and selective hydrogenation of 2-butyny-1,4-diol,and made clear the doping effect of nickel transmition metal to the cerium oxide supported gold nanocatalysts and the“structure-activity relationship”over oxidation of carbon monoxide and water-gas transformation reaction.These research results not only help us to expand the application of silicon carbide supported noble metal nanocatalysts in hydrogenation,but also further understanding of transition metals in the electronic structure and catalytic performance of suppoted noble metal nanocatalysts.