Design And Synthesis Of Ordered Meso/Microporous Carbons And Their Catalytic Performance On Oxygen Reduction Reaction

Ordered meso/microporous carbons have promising applications in secondary battery supercapacitor,fuel cell and catalyst support due to their 3D-ordered porous structure,high specific surface area,large pore volume,good electrical conductivity,rapid transfer of mass and electrons,and excellent physicochemical properties.Therefore,it is attracting much attention from the design,synthesis,surface functionalization,and application of ordered meso/microporous carbon.In this thesis,a series of(nitrogen-doped)ordered meso/microporous carbons have been successfully synthesized by direct chemical vapor deposition(CVD)using the modified inorganic ordered meso/microporous molecular sieves with good carbon deposition activity as templates.Moreover,large-scale synthesis of 3D ordered microporous carbon(ZTCs)at low temperature has been achieved.As a catalyst for the oxygen reduction reaction(ORR)of proton exchange membrane fuel cell,these ordered mesoporous/microporous carbons exhibit good catalytic properties.(1)Nitrogen-doped porous carbon has good catalytic performance as a metal-free ORR catalyst,but the low SSA,tedious and disordered pore structure casue low utilization of active sites and slow mass transfer processes in catalysts.In this thesis,the carbon deposition activity of the pore wall of ordered mesoporous silica(SBA-15)molecular sieve was enhanced by the silanization treatment,and a dual-ordered mesoporous structure and dual nitrogen-doped interfacial porous carbon(DN-OPC)has been successfully synthesized by acetylene(600?)/acetonitrile vapor(750?)CVD using silanized SBA-15as a hard template.The DN-OPC-4h catalyst with a large SSA(1430 m~2/g)and pore volume(1.83 cm~3/g)has been synthesized by optimizing the preparation conditions,which contains two types of ordered mesopores,a cylindrical pore(4.0?5.0 nm)inherited from the SBA-15 pore channel and a quasi-hexagonal pore(3.0?4.0 nm)formed by etching the SBA-15 pore wall.Both ordered mesopore pore walls are composed of nitrogen-doped carbon layers,forming a dual nitrogen-doped interface with a total nitrogen content of approximately 4.2 at%.Electrochemical characterization showed that DN-OPC-4h exhibited good catalytic performance as a metal-free ORR catalyst,with ORR onset potential and limiting current density of 0.84 V and 5.17 m A/cm~2,respectively,DN-OPC-4h exhibited superior stability and better tolerance to methanol than commercial20 wt%Pt/C catalysts.The good ORR electrocatalytic performance is attributed to its dual-ordered mesoporous structure,which accelerates mass transport.The dual nitrogen-doped interface provides more catalytic active sites,both of which work synergistically to improve active site utilization efficiency.(2)The microporous pores in nitrogen-doped porous carbons directly affect their performance as metal-free ORR catalysts.It remains a significant challenge to control their micropore size and dope efficient nitrogen heroatoms of porouscarbons at the same time.In this study,the carbon deposition activity of 3D-ordered microporous channels of zeolite Y was improved by silanization,and then using it as hard templates,the nitrogen-doped ordered microporous cabon(N-ZTC)has been successful synthesized by a direct acetonitrile vapor CVD.The N-ZTC-4h catalyst prepared by acetonitrile vapor CVD of 4h at 750?has a large SSA of 1322 m~2/g with ordered micropore arrangement and pore size mainly concentrated in the range of 1.0?2.0 nm,containing a high proportion of pyridinic-N(0.84 at%)and graphitic-N(2.60 at%)functional groups with a total nitrogen content of 5.2 at%.Electrochemical characterization showed that N-ZTC-4h,as a metal-free ORR catalyst,has an ORR onset potential and limiting current density of 0.90V and 4.27 m A/cm~2,respectively,with superior stability and better tolerance to methanol than commercial 20 wt%Pt/C catalysts.The good ORR catalytic performance of N-ZTC-4h is mainly attributed to its 3D-ordered micropore distribution for mass transfer,and the high proportion of pyridinic-N and graphitic-N in the nitrogen-containing functional groups providing more vigorous ORR catalytic activity.(3)Currently,common synthesis of zeolite-templated carbons(ZTCs)with 3D-ordered microporous structure have drawbacks of complex technology and difficut to produce in a large-scale.Although CVD method is simple,CVD at high-temperature generally causes random pyrolysis and carbon deposition of precusors,which results the blockage of the zeolite template pore entrance and thus make it impossible to establish a stable 3D-carbon skeleton inside the zeolite.Herein,cobalt ion-exchanged zeolite(Co Y zeolite)was prepared using the ion-exchange capacity of Na Y zeolite and used as hard templates,ZTCs with high-quality has been synthesized by direct acetylene CVD without water vapor assiatance at low-temperature synthesis.The results show that the cobalt ions confined in the Co Y zeolite can be used as catalytic sites and acetylene can be selectively pyrolytically deposited onto the inner part of zeolite skeleton rather than on the outer part of the particles at a CVD temperature of the only 400?by the d-?synergistic effect of transition metal ions and acetylene.By optimizing the synthesis conditions,ZTC-400-4h as prepared owns a perfect 3D ordered microporous structure,a large SSA of 2520 m~2/g and a pore volume of 1.11 cm~3/g.The increased order of the microporous structure is attributed to the enough amount of carbon inside the zeolite,facilitating the establishment of a stable ordered carbon skeleton structure.Moreover,since the advantages of low-temperature and direct CVD of the method,large-sacle synthesis of 3.0 g/batch of ZTCs with high-quality has been achived only by enlarging the amount of template(10.0g/batch).