Synthesis And Properties Of Carbon Supported Metal Catalysts Based On Strong Metal-support Interaction

Noble metal catalysts often display distinct activity and selectivity in various important chemical reactions owing to their specific electronic and geometric structures.Due to the high cost of noble metals,the noble metals are usually immobilized on porous carriers to prepare supported catalysts for recycling.As only the exposed surface metal atoms of the heterogeneous catalysts are directly involved in the catalytic process,downsizing the size of metals is effective to improve metal atoms utilization and thus can greatly enhance the mass activity.With the rapid development of nanotechnology during the last decades,the particle sizes of metals have been decreased to nanoscale and even atomic scale,realizing the maximizing utilization of noble metal catalysts.Meanwhile,the electronic and geometric structures of nanoscale particles are different from their bulk counterparts,thus endowing them with unexpected catalytic performance.On the other hand,with the decrease of metal particle size,metals are more easily tend to aggregate into larger particles due to the sharply increased surface free energy.This thesis aims to develop general synthesis strategies of noble metal catalysts via strong metal-support interactions(SMSIs)between the metals and porous carbons doped with heteroatoms or defects.The as-prepared catalysts are applied to electrocatalysis and heterogeneous catalysis reactions to demonstrate their superior catalytic performances.The main achievements of the thesis are summarized as follows:1.A general synthesis strategy toward high-loading atomically dispersed noble metal catalysts is developed based on the strong interactions between sulfur-doped mesoporous carbons(meso_S-C)and noble metals.The meso_S-C supports are first prepared by the previously developed method by our group,that is,the transition metal-assisted carbonization of organic molecules.With the meso_S-C supports,5 different atomically dispersed noble metal catalysts with high metal loading(up to.10 wt%)including Pt,Ir,Pd,Rh,and Ru are then prepared via the traditional impregnation/H2-reduction method.X-ray absorption fine structure(XAFS)characterization confirms the strong chemical interaction between noble metals and the sulfur atoms doped in meso_S-C.The control syntheses prove that the doped high-content sulfur atoms and mesoporous structures of meso_S-C play the pivotal roles for the preparation of high-loading atomically dispersed noble metal catalysts.The prepared atomically dispersed Pt/meso_S-C catalysts exhibit 30-fold higher activity than the commercial Pt/C for electrocatalyzing formic acid oxidation(FAOR).The high FAOR activity of Pt/meso_S-C can be ascribed to the Pt atom utilization and the isolated Pt sites that can induce the direction FAOR route and prevent catalysts poisoning.The atomically Ir/meso_S-C catalysts exhibit 20-fold higher activity than the commercial Ir/C catalysts for catalyzing quinoline hydrogenation.Besides the high Ir atom utilization,the electronic interaction between Ir and S species make the electron-deficient Ir sites tend to preferentially adsorb the N-containing rings.Besides,the Hδ+/Hδ-pairs coming from heterolytic dissociation of H2 at the isolated Ir sites can improve the catalytic activity.2.A synthesis strategy toward Pd-based alloy cluster catalysts is developed based on the strong interactions between nitrogen-doped carbon(N-C)supports and Pd.With the N-C supports prepared by the molten salt method,a series of ultrasmall Pd-based(Pd-M,M=Ru,Rh,Fe,Ga,Ge,Sn,Sb,Pb,and Bi)alloy cluster catalysts are prepared with traditional impregnation/H2-reduction method.The doped nitrogen species in N-C can efficiently suppress the agglomeration of metal nanoparticles during the high-temperature(400-500℃)H2-reduction process,achieving the preparation of small-sized alloy clusters.X-ray photoelectron spectroscopy(XPS)and XAFS characterizations confirm the strong chemical interactions between metal and nitrogen species of the N-C supports.The prepared bimetallic Pd-Pb/N-C nanocluster catalyst exhibits a high turnover frequency of 1092 h-1 for the semihydrogenation of phenylacetylene under a mild condition(30℃,5 bar H2),much higher than that commercial Ketjenblack EC-600JD supported Pd-Pb nanoparticle catalyst(TOF=208 h-1).Furthermore,the styrene selectivity reached to 93%under the full conversion on the Pd-Pb/N-C nanocluster catalyst.The superior performance of Pd-Pb/N-C is associated to the alloying and small-size effects.3.A defect graphene shell-confined synthesis strategy is developed toward small-sized Pd3Pb intermetallic compound nanoparticle catalysts.The pristine nanodiamond materials are first annealing under high temperature of 900℃ to obtain a hybrid carbon supports composed of nanodiamond core and one or two defect-rich graphene layers(ND@G).The strong interactions between ND@G and Pd can efficiently suppress the Pd sintering at high temperature up to 650℃,which allows the synthesis of small-sized(<5 nm)atomically ordered Pd3Pb intermetallic compound nanoparticle catalysts.Atomic resolution HADDF-STEM observations show the atomically ordered structure of the Pd3Pb catalysts without segregation of Pd atoms on the particle surface.The Pd3Pb/ND@G catalyst exhibits a high activity with TOF of 932 h-1 for semihydrogenation of phenylacetylene under a mild condition(25 ℃,3 bar H2),which is much higher than that of Pd3Pb/OLC(338 h-1),Pd3Pb/XC(146 hour-1)and Lindlar(338 h-1)catalysts.Moreover,the Pd3Pb/ND@G catalyst shows a high selectivity(94%)under the full phenylacetylene conversion.XPS analyses indicate that the electrons transfer from the ND@G support to Pd3Pb,making the Pd3Pb surface equipped with an electron-enriched state and increase the adsorption ability to the alkynes,which could greatly improve the reaction activity.The atomically ordered alloy structures of Pd3Pb can not only prevent the formation of highly active subsurface β-PdH species but also decrease the adsorption ability to C=C bond,thus leading to the high selectivity for phenylacetylene semihydrogenation.