Controllablefabricationofcarbonaerogels.docx

controllable fabrication of carbon aerogelsfen g yaning1) , miao lei1) , tan emura s akae1) , tan emura masaki1) , and su zu ki kenzi2)1) department of environmental technology & urban planning , nagoya institute of technology , showa2ku , nagoya ,46628555 , j apan2) ecotopia science institute , nagoya university , chikusa2ku , nagoya , j apan( received 2006206228)abstract : nano2pore carbon aerogels were prepared by the sol2gel polymerization of resorcinol ( 1 , 32di2 hydroxybenzene) ( c6 h4 (oh) 2 ) with formaldehyde ( hcho) in a slightly basic aqueous solution , followed by super2critical drying under liquid carbon dioxide as super2critical media and carbonization at 700 under n2 gas atmosphere . the key of the work is to fabricate carbon aerogels with controllable nano2pore structure ,which means extremely high surface area and sharp pore size distribution. aiming to investigate the effects of preparation conditions on the gelation process , the bulk density , and the physical and chemical structure of the resultant carbon aerogels , the molar ratio of rc ( resorcinol to catalyst) and the amount of distilled water were varied , consequently two different sets of samples , with series of rc ratio and rfw ( resorcinol2formaldehyde to water , or the content of reactant) ratio , were prepared. the result of n2 adsorptiondesorp2tion experiment at 77 k shows that the pore sizes decreasing from 11 . 4 down to 2 . 2 nm with the increasing of the molar ratio of rc from 100 to 400 , andor , the pore sizes decreasing from 3 . 8 down to 1 . 6 nm with the increasing of reactant content from 0 . 4 to 0 . 6 .key words : resorcinol2formaldehyde aerogels ; carbon aerogels ; super2critical drying ; n2 adsorptiondesorp2tion this project was f inancially supported by a grant f rom the nitech 21st century co e program ,“world ceramics center f or environmental harmony ”.5 investigated the multi2scale structure of thesamples. tamon h. et al . 6 27 synthesized the mesoporous carbon cryogels by freeze2drying with t2butanol . reynolds g. a. m. et al . 8 synthesized the ordered macroporous rf and carbon aerogels with template2directed method. it is well known from the viewpoint of their ap2plications that the control of nanoporous struc2ture of rf and carbon aerogels is very impor2 tant . litter work has laid stress on this point . the objective of this work was to propose a con2 trollable procedure of nanostructure aerogels by taking account of the synthesis recipe of the rf aerogels.based upon previous work 9 in which the molar ratio of rf and the gelation process were investigated , in present work , the molar ratio of1 . introductionas carbon aerogels being promising materi2 als in such applications as thermal and phonic insulators , adsorbents , capacitors , and fuel cell and catalyst support , it has seen an explosion of interest over the last few years , and many re2 searches have fabricated by various processes1 28 . among them , pekala et al . 2 firstsynthesized carbon aerogels via resorcinol2formaldehyde system aerogels. cf system and cf cresol2formaldehyde aerogels were success2 fully synthesized and characterized by wencuiet al . 4 investigat2l . et al . 3 . czakkel o.ed the influence of drying method on the struc2ture and porosity of the rf aerogels and their carbonized derivatives. berthon2fabry s. et al .e2ma il : miao . lei nitech. ac . jpcorresponding a uthor : miao leifeng y. n . et al . , controlla ble fa brication of carbon aerogels285rc and the content of reactants ( resorcinol andformaldehyde) were mainly concerned. further2 more , the bulk densities and the liner shrinkage of the rf aerogels were calculated. the thermal characterization of the rf aerogels was deter2 mined by thermogravimetric analyzer ( tga) . the synthesised porous samples were character2 ized by n2 adsorptiondesorption analysis or brunauer2emmett2teller ( b et) method. the effects of recipe on the nanostructure and therinsed by ethanol ( 10 times of sample in vol2ume) for three times during 2 d ; then trans2 ferred into a pressure vessel and processed un2 der the super2critical drying by use of liquid co2 as super2critical media by applying the heating at 40 and the pressurizing at 8 mpa to the autoclave for 2 h in time duration. the organic aerogels were firstly obtained. the cor2 responding carbon derivatives were obtained bythe pyrolyzing of the organic aerogels up to 9733- 1porosity properties of thewere investigated.synthesizedsamplesk in n2 flow ( 100 cm min ) . the heatingrate was 1 . 0 kmin - 1 and a soak time being 4 h. carbon derivates were indicated by“c at2 tached before the name of the aerogel ”in thetext .2 . experimental2 . 1 . sa mple preparationresorcinol ( 99 . 5 %) ( r) with formalde2 hyde ( 37 %) ( f ) both from kishida , were mixed by stoichiometric rf molar ratio of 12 ;catalyzed by sodium carbonate ( naco3 ) ( c)with content of 0 . 1 or 1 moll - 1 and rc from50 to 800 ; diluted by distilled water with differ2ent mass content of reactants from 10 % to60 %. two sets of rf hydrogels were prepared as shown in table1 where the set number 1 fol2 lowed after s stands for the samples prepared under different rc ratio with constant rf cont2 ents , and the set number 2 does for the samples prepared under the different rf contents with constant rc ratio .the mixtures were stirred to obtain homo2geneous solutions then were cast into glass mol2 ds and aging at 70 for 225 d. since water and liquid co2 are immiscible , the solvent wa2ter embedded into the network structure of the gels should be exchanged with organic solvent of ethanol , in which both water and liquid co2 are miscible. the all monolith organic gels were2 . 2 . tga and a dsorptiondesorption analy2sis of aerogels and their carbon derivativesthe pylolytic chemical mechanism for the formation of rf aerogels was determined by tga ( thermo plus tg8120 by rigaku co . ltd. ) . the powder sample was exposed to a high purity nitrogen atmosphere with fluid speedof 50 mlmin - 1 , being heated up to 1273 k bya heating rate of 10 kmin - 1 and being main2tained at that temperature for 4 h.a commercial adsorption apparatus (bel2sorp 36sp by bel japan , inc) was used for the nitrogen adsorptiondesorption experiment for the samples s212 , s213 , s214 , s224 , s225 , and s226 , and their carbon derivatives cs212 , cs213 , cs214 , cs224 , cs225 , and cs226 (see table1) . the adsorption isotherms of nitrogen gas at77 k , specific surface areas ( sbet ) , the pore size distributions of the aerogels and their car2 bonization derivatives were evaluated. ta ble 1. recipes of the sa mples , the bulk densities and l iner shrinkage of the rf aerogels s211s212s213s214s215s221s222s223s224s225s226samplesrf- rc-rf contentsbulk density0. 520015 %01190. 55040 %01515010040080010 %011020 %012130 %014450 %015660 %0158011801290131nd liner shrinkage 27 % 21 % 14 % 12 % 7 % 2914 % 25 % 1916 % 1716 % 1417 % 1211 % 3 nd : no experiment data ; 3 3 the unit of“bulk density”is gcm - 3 .286rar e m etals , vol . 25 , spec . iss ue , oct 2006to investigate the relation between the fab2rication recipes and the nanostructure of aero2gel , both the specific surface area ( sb et) and3 . results and discussion3 . 1 . bulk density and liner shrinkageas shown in table 1 , for the first set of the samples , the liner shrinkage decreases with the increasing of the bulk density and rc ratio . the similar tendency is observed for the second set , when the reactant content ( rf content ) is increasing. this might be due to the accelation of polymerization reaction by the increasing of the amount of both catalysis and reactant .the pore size distribution ( rp )are determinedby nitrogen adsorptiondesorption measurement .sb et is calculated by b et method and rp is estimated by applying the d2h ( dollimore & heal) method to the adsorption isotherms. the results are summarized in table 2 .figs. 2 , 3 , 4 and 5 show the nitrogen ad23 . 2 . tgaa typical tga curve is exemplified in fig. 1 for sample s222 . the mass reduction of the sample is about 10 % when the temperaturechanges from 25 to 250 . this reduction ismainly due to the removal of the physically ad2 sorbed water vapor on the net2works and the or2 ganic compounds contaminated on the surface . another mass reduction is remarkable at thetemperature range between 250 to 600 ,where about 35 % mass reduction is caused by the evolution of a large amount of the chemically produced gases described below : it is well known that methane , hydrogen , carbon monox2 ide and carbon dioxide is generated during thepyrolysis exceeding 400 10 . methanecomes from methylene group s that do not partic2 ipate in the dehydration reaction. carbon mon2 oxide does from the decomposition of the bonds produced by dewatering reaction 10 . the rateof mass reduction becomes slower after 650 .consequently , no distinct physical and chemical reaction is observed up to 1000 .fig. 2.adsorption isotherms of samples s212 , s213and s214 .adsorption isotherms of samples s224 , andfig. 3.its carbon derivative.fig. 1.tg curve of sample s222 .3 . 3 . adsorptiondesorption mea surementfeng y. n . et al . , controlla ble fa brication of carbon aerogels287a network structure with meso2pores.we can observe that the maximum value of the adsorbed andor desorbed n2 volume de2 creases from 720 ( s212 ) , 400 ( s213 ) , to 310- 1(s214 ) ml g depending on the decreasingof catalyst content c under the constant amount of r (increasing of rc ratio) from fig. 2 . the possible reasons of this trend are as follows : if catalytic activation is reduced , the longer gela2 tion process is required for the completion of the polymerization of hydro2gel net2works. conse2 quently , pore size in net2works is becoming large enough to increase the total volume of n2 adsorbed andor desorbed.the maximum values of the adsorbed andordesorbed n2 volume of samples s224 , s225 and s2- 126 are about 930 , 880 and 600 mlg, respec2tively as shown in figs. 3 , 4 and 5 , and the values are increasing with the increasing of the content of reactant (the mass ratio of reactant of resorcinol and formaldehyde to distilled wa2 ter) . the increasing of water molecules embed2 ded on the network by the increasing of the re2 actant content results in the pore size increasing after the super critical drying process as shown in table 2 . consequently , the total volume of adsorbed andor desorbed n2 is enhanced. the n2 adsorptiondesorption volumes of all three samples are reduced after carbonization in n2flow at higher temperature . this is due to the drastic reduction of nano2pore size after carbon2fig. 4.adsorption isotherms of samples s225 , andits carbon derivative.ization. the estimated peak pore sizerpeak inthe pore2size distribution changes from 6 . 8 ( s224 ) to 3 . 8 ( cs224 ) , 6 . 0 ( s225 ) to 2 . 6 ( cs225 ) , and 2 . 9 ( s226 )( cs226 )re2to 1 . 6 nmadsorption isotherms of samples s226 , andfig. 5.spectively , as shown in table 2 . the observedsignificant mass increasing followed by the col2 lap sed carbon2carbon bonds in aerogel net works might results in the pore size decreasing as shown in table 2 .moreover , the specific area of the carbon2ized samples is reduced from that of rf samples by 34 % , 29 % and 22 % depending on the in2 creasing of rf content from table 2 . this might be due to the enhancement of water molecules embedded on the network by the increasing of the reactant content .its carbon derivative.sorptiondesorption isotherms of the dried aero2gels and their carbonization derivatives. the aerogels are generally of type iv isotherms in the iupac ( the international union on pure and applied chemistry ) classification 11 .the volume of n2 adsorbed and desorbed is plotted against the relative pressure pp0 ( p , pressure ; p0 , saturated vapor pressure) of n2 gas at 77 k. the hysteresis curves observed in n2 adsorption isotherms is directly attributed to288rar e m etals , vol . 25 , spec . iss ue , oct 2006 table 2. experimented peak pore size and the specif ic surface area of rf aerogels and their carbonized derivatives sbet(m2 g - 1 )rpeaknmsamplesrpeak%sbet%rf aerogelscarbonrf aerogelscarbon (a) aerogels (b) ( (a)2(b) )(b) (c) aerogels (d) ( (c)2(d) )(d) s224cs224s225cs2256. 86. 13. 82. 679 %60 %722. 1778. 9478. 5556. 934 %29 % s226cs226 2. 9 1. 6 45 % 731. 2 570. 4 22 % al . , j . non2cry . solids , 1992 , 145 : 90. pekala r. w. , united states patent : 4997804 , march 5 , 1991.li w. c. , and guo s. c. , letters to the editor ,carbon , 2000 , 38 : 1499.czakkel o. , marthi k. , geissler e. , et al . , mic. meso. materials , 2005 , 124 , in press. berthon2fabry s.