Synthesis And Application Of Carbon And Carbon-based Metal Catalysts

In the 21st century,new materials are the foundation of industry and one of the country's major strategies for emerging industries.New carbon nanomaterials and carbon-based metal materials have been widely used in the field of catalysis.However,this field is currently facing two major challenges:1)The traditional approaches to fabricate nanostructured carbon materials rely greatly on complex procedures and specific precursors.Small molecules are inexpensive and diverse in structure,making them ideal raw materials for the preparation of carbon materials.However,small molecules generally have a high vapor pressure and are easily volatilized by heat,which makes it difficult to pyrolyze into carbon.Starting from small molecular precursors,designing a simple and universal method for preparing new carbon materials is still challenging.2)Small-sized bimetallic nanoparticles that possess numerous accessible metal sites and optimal geometric/electronic structures show great promise for advanced synergetic catalysis.Nevertheless,the synthesis of ultrasmall carbon-supported bimetallic nanoparticles of<2 nm remains a substantial challenge with traditional wet-impregnation method,as metal species tend strongly to aggregate into larger particles due to the sharply increased surface free energy with the decrease of particle size.This thesis aims to develop novel synthetic methodology for preparing carbon materials based on transition metal-assisted carbonization of small organic molecules,and carbon-based small-size bimetallic catalysts based on the strong interaction between metal and support.Subsequently,this prepared catalysts were applied to organic catalysis,industrial catalysis,biomass conversion under harsh conditions.The major results achieved are summarized as follows:1.A strategy of transition metal-assisted carbonization of small organic molecules was developed to prepare functional carbon materials.To demonstrate the versatility of this synthetic methodology,we selected 15 different organic precursors and 9 different transition metal salts(Co(NO3)2,Fe(NO3)3,Cr(NO3)3,Cu(NO3)2,Mn(NO3)2,Ni(NO3)2,AgNO3,Zn(NO3)2,H2PtCl6),to synthesize a rich multifunctional carbon material system.This synthetic methodology can control the morphology,specific surface area,porosity,heteroatom doping and the degree of graphitization of carbon materials at the molecular level.We have verified the formation mechanism of carbon materials through ultraviolet-visible absorption spectroscopy(UV-vis),gel permeation chromatography(GPC),X-ray photoelectron spectroscopy(XPS)characterizations:thermally stable oligomer intermediates are formed under low temperature conditions,which then was converted to carbon materials at relatively high temperatures.In addition,the prepared carbons exhibit promising performance in elective oxidization of ethylbenzene to acetophenone,hydrogenation of nitrobenzene to aniline and hydrogen evolution reaction and oxygen reduction reaction.Due to the high surface area and porosity,and the large amount of nitrogen/metal doping,the CM-Phen/Co/SiO2 catalyst displayed the highest conversion of 91.4%and a good selectivity of 97.9%for selective oxidization of ethylbenzene to acetophenone.For electrocatalyzing the hydrogen evolution reaction(HER),the CM-DBrPhen/Co/SiO2 catalyst exhibited a low overpotential of 15 8 mV and 271 mV at the current density of 10 mA cm-2 in acidic and alkaline electrolytes,respectively.2.Based on the strong chemical interaction between the metal and the sulfur-doped carbon support,a universal method for preparing small-sized alloy nanoclusters was developed.With the sulfur-doped carbon supports,a library of bimetallic nanocluster catalysts was constructed,consisting of 27 combinations of 3 noble metals(that is,Pt,Rh,Ir)and 8 other metals(Al,Cr,Co,Cu,Ga,Ru,Sn,and Pb),with average particle sizes ranging from 0.7 to 1.4 nm.It was found that the strong metal-S interaction greatly suppresses the metal aggregation even at an elevated temperature and thus enables us to prepare well alloyed and small-sized bimetallic nanoparticles by the conventional impregnation process.Systematic characterizations,including powder X-ray diffraction(PXRD),H2-temperature-programmed-reduction(TPR),high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM),energy-dispersive X-ray spectroscopy(EDS)technique,XPS and X-ray absorption fine structure(XAFS)spectroscopy characterization strongly evidenced the formation of small-sized alloy catalysts.The enhanced catalytic properties of the ultrasmall bimetallic nanoparticles were demonstrated in the selective hydrogenation of nitroarenes,dehydrogenation of propane at high temperature and oxidative dehydrogenations of N-heterocycles.3.A hydrothermally stable catalyst was prepared by alloying Pt with a high-melting-point metal Nb on a high-surface-area carbon support.The Pt-Nb alloy catalysts were prepared by H2 reduction at a high temperature of 900 ? with a high-surface-area carbon black support,which can suppress metal sintering at high temperatures and thus lead to small-sized alloyed PtNb particles of only 2.2 nm.Taking the advantages of surface acid property provided by the Nb sites and the size effect,the prepared carbon-supported small-sized Pt-Nb alloy catalysts exhibited attractive activities for the hydrogenation of levulinic acid into y-valerolactone(TOF=0.66 s-1)and the water-gas shift reaction(2.51 × 10-2 molco s-1 molpt-1).More significantly,benefiting from the inherent stability of high-melting point Nb,the Pt-Nb alloy catalysts showed no activity loss under hydrothermal stability test compared to commercial Pt/C and Ru/C catalysts.