The preparation of ACFs from PAN fibers under various conditions and the method to load Pd on ACFs were

described. Chemical activation of the fibers increased the surface areas of the fibers from about 64 m2/g to 381 m2/g.

SEM micrographs of Pd-loaded indicated the diameters of the fibers were in the range of 1.0-10.0 mm. Diameters of

metallic Pd particles on the fibers changed between 80 nm and 100 nm. 4.5% (by wt) metallic palladium was deposited

on the ACFs. This high percentage of palladium deposited on ACFs is useful for hydrogen storage, since Pd-H system

is established under a wide range of pressure and temperature

Baş, Serkan

Dumanlı, Ahu Gümrah

Yürüm, Yuda

English

Sabanci University Research Database

PAN-Based Pd-doped Activated Carbon Fibers for Hydrogen Storage:Preparation, A new Method for Chemical Activation andCharacterization of FibersSerkan Baş1, Özgül Hakli2, Ahu Gümrah Dumanli1 and Yuda Yürüm1*1Faculty of Engineering and Natural Sciences, Sabanci University, Tuzla, 34956 Istanbul, Turkey2Department of Chemistry, Celal Bayar University, Muradiye, 45030 Manisa, TurkeyAbstractThe preparation of ACFs from PAN fibers under various conditions and the method to load Pd on ACFs weredescribed. Chemical activation of the fibers increased the surface areas of the fibers from about 64 m2/g to 381 m2/g.SEM micrographs of Pd-loaded indicated the diameters of the fibers were in the range of 1.0-10.0 mm. Diameters ofmetallic Pd particles on the fibers changed between 80 nm and 100 nm. 4.5% (by wt) metallic palladium was depositedon theACFs. This high percentage of palladium deposited onACFs is useful for hydrogen storage, since Pd-H systemis established under a wide range of pressure and temperature.Key Words: Polyacrylonitrile fibers, activated carbon fibers, BET surface area, scanning electron microscopy.INTRODUCTIONHydrogen storage on carbon materials has recentlybeen attracting interest because of the significanceof hydrogen as an energy carrier in automotiveproducts. In order to upgrade the hydrogen storagecapacity of carbon materials, numerous techniqueshave been proposed [1–4].Particularly carbon materials, such as activatedcarbon, carbon fibers, carbon nanotubes andcarbon nanofibers have caught the attention ofscientists of different disciplines. The mechanism ofhydrogen storage and the interaction between thecarbon surface and hydrogen are not sufficientlycomprehended. Carbon materials adsorb hydrogenthrough physical interactions, and the bindingenergy between the solid and the H2 molecule isextremely inadequate to accomplish noteworthylevels of adsorption at room temperature. Only atcryogenic temperatures the intensity of theinteraction increases noticeably, and physicaladsorption gives the hydrogen capacity that isrequired for storage [5]. In order to improve thehydrogen sorption capacity of carbon materials atroom temperature, it is essential to raise thestrength of the gas–solid interaction. Alternatively,the increase in the extent of the binding energy mustbe reasonable, because too strong interactions formHACETTEPE JOURNAL OF BIOLOGYAND CHEMISTRY Research ArticleHacettepe J. Biol. & Chem., 2008, 36 (3), 247-253247* Correspondence to: Yuda Yürüm,Faculty of Engineering and Natural Sciences, SabanciUniversity, Tuzla, Istanbul 34956, TurkeyTel: +90216 483 9512 Fax: +90216 483 9550E-mail: yyurum@sabanciuniv.edu248irreversible bonds, as in the case of chemisorption.Evidently, an irreversible H2 sorption avoids the useof the material for storage purposes since hydrogenmust be straightforwardly desorbed from the storagesystem.Oxygen functional groups have also been revealedto be a factor in physisorption of hydrogen onactivated carbon: increasing oxidation functionalgroups of an activated carbon caused an increase inphysisorption [6]. The outcome of surfacefunctionalities on physisorption of hydrogen tocarbon has not been extensively investigated, buthydrogen adsorption has been demonstrated tochemically alter functional groups on carbon tocreate basic carbons [7]. Without a doubt, the widediversity of existing carbon structures gives aremarkable occasion to methodically clarify theeffect of carbon properties, including pore size,graphitic content and surface functionalities, onhydrogen adsorption.Recently, metal doping has drawn much interest assome selected metals increase hydrogen storagebecause of the formation of active spots on thecarbon materials, improving hydrogen adsorption[8–11]. The interaction of noble metals, such aspalladium and platinum with carbon nanotubesworks as the prototypical transition metal–CNTinteraction and for that reason, is one of the mostexamined systems [12-15]. In spite of this, thehydrogen storage capacities of such systems areinadequately understood, rationalizing theircomplexity.Takagi et al. [16] described that the hydrogenstorage capacities of Pt and Pd-doped activatedcarbon resulted from the sorption of hydrogen onmetal and carbon phases, telling a simplecumulative effect. In some examples, the variation inthe hydrogen storage capacities of metal dopedcarbon materials can be completely attributed to thesize of metal particles involved [17]. Thus at thisphase, additional experiments are essential, toprovide more complete understanding of the metal-mediated hydrogen storage development in carbonnanotubes.In the present report, we describe the structuralcharacteristics of raw, surface-treated andpalladium-doped carbon fibers. First, the carbonfibers were characterized in terms of physical andchemical properties in order to extend thisinformation to hydrogen adsorption. Secondly,carbons were doped with palladium in an attempt tooptimize a carbon-metal system and find apromising material for hydrogen storage goals. Thepalladium nanoparticles were doped on to carbonfibers via impregnation and in situ condensed phasereduction method. The preparation of CFs from PANfibers under various conditions and the method toload Pd on ACFs was investigated. Structure of theCFs were explored by surface analysis, FTIRspectrometry and scanning electron microscopy(SEM) methods. The next paper in the series willreport hydrogen storage measured at 298 K and apressure range of 0.66–2.22 MPa, technicallyrelevant for practical hydrogen storage applications.EXPERIMENTALProduction and chemical activation of CFs.Commercial 1.1 decitex PAN fibers (weight of10,000 m of single fiber in gram is 1 decitex)obtained from AKSA Acrylic Chemical Industries,Yalova, Turkey, were used in the present work. Fibersamples were stabilized, while it was stretched, for48 hours under a static air atmosphere in an oven at250oC. Stretching of the fibers prevented curling ofthe stabilized fibers. The stabilized fibers werecarbonized at 1100oC for 3 hours under a nitrogenatmosphere. Carbonized fibers retained their fibrousstructure and their color became black after249carbonization. A new method of chemical activationof carbon fibers is proposed in the present study. 1.6g of carbon fibers (CF) were immersed into 30 mL,0.3 M aqueous solution of Al(OH)3 and stirred for 6hours at room temperature. CFs separated byfiltration were dried at room temperature andactivated at 1100oC under an argon atmosphere for3 hours. The probable activation reactions were asfollow:2 Al(OH)3 g Al2O3 + 3 H2O (1)2 Al2O3 + 3 C g 3 CO2 + 4 Al (2)CO2 + C g 2 CO (3)Reactions (2) and (3) are the reactions that CFs lostcarbon material and the surface of the fibers thuslost original smoothness by the creation of newporosity. Aluminum residues that attached on thesurface of activated carbon fibers (ACFs) weredissolved by stirring the fibers in 10% nitric acid for2 hours, then the fibers were rinsed thoroughly withwater and dried at 100oC in an oven.Doping of ACFs with Pd.0.5 g of ACFs was stirred with 50 ml of aqueoushydrazine solution (hydrazine:water = 1:200, byvolume) for 10 hours (Solution 1). 0.0417 g PdCl2(Aldrich, 99.999 %) was dissolved in 20 mLKOH:NH4OH (by wt) solution at room temperature(Solution 2). Solution 1 was added to the Solution 2as reducing agent for Pd2+. Metallic Pd appeared onthe surface of ACFs after several hours of reactiontime. The Pd-doped-ACFs were then washed withdistilled water to clean the ACFs from the residualmatter and organics, and dried in an oven at 70oCunder a nitrogen atmosphere. Washing of the fiberswith fresh distilled water leached negligible amountsof metallic Pd particles.Surface analysis.Surface areas of CFs, ACFs were measured byQuantachrome NOVA 2200e series SurfaceAnalyzer. The determination is based on themeasurements of the adsorption isotherms ofnitrogen at 77 K. Surface area of the samples weredetermined by using BET equation in the relativepressure range of between 0.05 to 0.3, sevenadsorption points and BJH (Barrett-Joyner-Halenda)method was utilized for the measurement of poresize distributions. Before all of the measurements,moisture and gases such as nitrogen and oxygenadsorbed on the surface or held in the open pores,were removed under reduced pressure at 100oC for5 h.FTIR spectrometry.FT-IR spectra of CFs, ACFs and Pd-doped-ACFswere measured with a Bruker EQUINOX 55 FT-IRspectrometer. Activated carbon samples were driedunder a nitrogen atmosphere at 110oC for 24 hours.KBr pellets were prepared by grinding 2.5 mg of drysample with 200 mg of dried KBr. Spectra wereobtained with 200 scans at a resolution 2 cm-1. Theassignment of the bands in the infrared spectra wasaccording to early reports [18,19].SEM analysis.The CFs, ACFs and Pd-doped-ACFs wereexamined with a Leo G34-Supra 35VP scanningelectron microscope. Imaging was generally doneat 2-5 keV accelerating voltage, using the secondaryelectron imaging technique.RESULTS AND DISCUSSIONFTIR spectra of carbon fibers.The FTIR spectra of carbon fibers stabilized at250oC for 1 hour and carbonized for 1 hour at 600oC,700oC, 800oC and 1100oC are presented in Figure1. All of the spectra were similar, the main differencewas the low intensity bands due to nitrogen andoxygen functional groups as the carbonizationtemperature was increased to 1100oC indicating that250more carbonaceous material was formed at highertemperatures. The major bands observed in theFTIR spectra were the following: weak bands near3655 cm-1 of free –O–H stretching vibrations due toprobably humidity adsorbed during measurements;twin peaks at 2982 cm-1 and 2891 cm-1 due tosymmetric stretching vibrations of –CH2 andstretching vibrations of –CH, respectively. Theintensity of these twin peaks decreased as thecarbonization temperature was increased to 1100oC.The strong bands at 1564 cm-1 were due to –N–Hbending and –C–N= vibrations. The intensity ofthese functionalities also decreased very sharply asthe carbonization temperature was increased to1100oC. As a matter of fact in the FTIR spectrum ofthe sample produced at 1100oC it is quite impossibleto observe these bands. The same is true for thebands at 1400 cm-1 of –N–N=O asymmetricstretching and –O–H bending vibrations and for thebands at 1254 cm-1 due to –C–N– stretch and –CH3rocking vibrations. The bands at 1164 cm-1, at 1090cm-1 and at 964 cm-1 were as a result of –CH3rocking, –C–OH stretching and –C=C–H out-of-plane C–H vibrations, respectively.Structural characteristics of the ACFs.The SEM micrographs of carbon fibers stabilized at250oC and carbonized at 1100oC for 1 hour arepresented in Figure 2. The shape of the fibers wascylindrical as they were before the carbonizationexperiments. Fibers of non-uniform shape wereproduced after stabilization and carbonizationexperiments. It seemed that some of the fiberssticked to each other due to softening of the fibersduring the thermal treatment. Surface of the fiberswas observed to contain some roughness due toactivation reactions which volatilized some of thecarbon material from the surface of the fibers andactivation of the fibers caused some material lossfrom the surface creating irregular channels on thesurface, Figure 3. An additional feature in the SEMof the fibers in Figure 3 was the presence ofaluminum residues. These were removed after nitricacid treatment.The BET surface areas measured before activationwere in the range of 64-75 m2/g. Activation of thefibers increased the surface areas of the fibers to381 m2/g. ACFs show higher apparent specificFigure 1. FTIR spectra carbon fibers stabilized at 250oC for 1 hour and carbonized for 1 hour at a) 600oC, b) 700oC,c) 800oC and d) 1100oC.251surface areas normally in the range of 1500–3000m2/g, The BET areas measured in the present workwere much lower than these values. The reason forthis was probably the new chemical methodproposed in the present work was a milder methodwhen compared with the other chemical activationmethods. The pore size distribution of the ACFs arepresented in Figure 4. The ACFs contained poreswith diameters of 2-5 nm, therefore the porosity ofthe fibers can be considered as mesoporous sincetypes of pores as defined by IUPAC are microporouswith width not exceeding 2 nm, mesoporous withwidth between 2 nm and 50 nm and macroporouswith width exceeding 50 nm [20].Pd-loaded ACFs.SEMmicrographs of Pd-loadedACFs are presentedin Figure 5. While diameters of the fibers weremeasured as 0.5-1.0 μm, the diameters of metallicPd particles loaded on the fibers changedapproximately between 80 nm and 100 nm. Theshape of the Pd particles seemed to be spherical onthe average though some non-spherical Pd particlesalso existed on the fibers. The diameters of the Pdparticles formed in the present study were greaterthan those reported by Bulushev et al [21]. Goldnanoparticles of 2–5 nm diameters formed onwoven fabrics of ACFs. To form highly dispersed 0.8% (by wt.) Au nanoparticles, gold was deposited onFigure 2. SEM micrographs of PAN fibers carbonized at1100oC before activation.Top figure, magnification = 1.00 KX, bottom figure,magnification = 10.00 KX.Figure 3. SEM micrographs of carbon fibers afterchemical activation, before aluminum deposition waswashed with nitric acid treatment. Top figure,magnification = 1.00 KX, bottom figure, magnification =10.00 KX.Figure 4. BJH pore distribution graph of ACFs.252activated carbon fibers (ACF) from [Au(en)2]Cl3solution. The reason for this difference was theutilization of a different method for the deposition ofthe Pd0. The percentage of the Pd0 deposited oncarbon fibers was 4.5 % (by wt), this value wasmuch greater than that of the value in Bulushev etal. [21]. This high percentage of palladium depositedon ACFs is useful for hydrogen storage, since Pd-Hsystem is established under a wide range ofpressure and temperature [22].CONCLUSIONSThe BET surface areas before activation were in therange of 64-75 m2/g. Activation of the fibers withcarbon dioxide increased the surface areas of thefibers to about 381 m2/g. TheACFs contained poreswith diameters of 2-5 nm, therefore the porosity ofthe fibers can be considered as mesoporous. Itseemed that chemical activation process proposedwas not very efficient in producing high surface areafibers under the conditions applied in the presentstudy.The major bands observed in the FTIR spectra wereweak bands near 3655 cm-1 of free –O–H stretchingvibrations due to probably humidity adsorbed duringmeasurements; twin peaks at 2982 cm-1 and 2891cm-1 due to symmetric stretching vibrations of –CH2and stretching vibrations of –CH, respectively. Thestrong bands at 1564 cm-1 were due to –N–Hbending and –C–N= vibrations. The intensity ofthese functionalities decreased very sharply as thecarbonization temperature was increased to1100oC, as a matter of fact in the FTIR spectrum ofthe sample produced at 1100oC it is quite impossibleto observe these bands.SEM micrographs of Pd-loaded indicated thediameters of the fibers were in the range of 0.5-1.0μm. Diameters of metallic Pd particles loaded on thefibers changed approximately between 80 nm and100 nm. The shape of the Pd particles seemed tobe spherical on the average though some non-spherical Pd particles also existed on the fibers.4.5% (by wt) metallic palladium was deposited onthe ACFs.REFERENCES1. A. C. Dillon, K. E. H. Gilbert, J. L. Alleman, T.Gennett, K. M. Jones, P. A. Parilla, M.J. Heben, in: Proceedings of the 2001 DOEHydrogen Program Review, NREL/ CP-570-30535, 2001.2. E. David, J. Mater. Proc. Technol. 162/163(2005) 169.3. G. G. Tibbetts, G. P. Meisner, C. H. Olk, Carbon39 (2001) 292.4. H. M. Cheng, Q. H. Yang, C. Liu, Carbon 39(2001) 1447.5. L. Zhou, Y. Zhou, Y. Sun, Int. J. HydrogenEnergy 29 (3) (2004) 319.6. R. K. Agarwal, J. S. Noh, J. A. Schwarz, P.Davini, Carbon 25 (1987) 219.7. J. A. Menendez, L. R. Radovic, B. Xia, J.Phillips, J. Phys. Chem. 100 (1996) 17243.Figure 5. SEM micrographs of ACFs after Pd doping.Magnification = 20.00 KX.2538. P. Chen, X. Wu, J. Lin, K. L. Tan, Science 285(1999) 91.9. Z. H. Zhu, G. Q. Lu, S. C. Smith, Carbon 42(2004) 2509.10. S. Challet, P. Azais, R. J.-M. Pellenq, O. Isnard,J. -L. Soubeyroux, L. Duclaux, J.Phys. Chem. Solids 65 (2004) 541.11. K. Kim, H. Lee, K. Han, J. Kim, M. Song, M.Park, J. Lee, J. Kang, J. Phys. Chem. B 109(2005) 8983.12. S. Dag, Y. Ozturk, S. Ciraci, T. Yildirim, Phys.Rev. B 72 (2005) 155404.13. A. Lueking, R.T. Yang, J. Catal. 206 (2002)165.14. F.H. Yang, A.J. Lachawiec, R.T. Yang, J. Phys.Chem. B 110 (2006) 6236.15. A. Anson, E. Lafuente, E. Urriolabeitia, R.Navarro, A.M. Benito, W. K. Maser, M.T.Martinez, J. Phys. Chem. B 110 (2006) 6643.16. H. Takagi, H. Hatori, Y. Yamada, S. Matsuo, M.Shiraishi, J. Alloy. Compd. 385 (2004) 257.17. J. Ozaki, W. Ohizumi, A. Oya, M. J. Illan-Gomez, M. C. Roman- Martinez, A.Linares-Solano, Carbon 38 (2000) 775.18. G. Shevla, G. Comprehensive AnalyticalChemistry, Volume VI, Analytical InfraredSpectroscopy, Elsevier, Amsterdam, 1976, pp.334.19. Y. Yürüm, N. Altuntas, Fuel Science andTechnol. Int., 12 (1994) 1115.20. IUPAC Compendium of Chemical Terminology,second ed., vol. 46, 1997, p. 1976.21. D. A. Bulushev, I. Yuranov, E. I. Suvorova, P. A.Buffat, L. Kiwi-Minsker, J. Catalysis, 224(2004) 8.22. G. Sandrock, Hydrogen-Metal Systems, inHydrogen Energy System, Y. Yürüm (Ed.),NATOASI Series E, Vol. 295, Kluwer AcademicPublishers, Dordrecht, 1995, pp. 135.

PAN-based Pd-doped activated carbon fibers for hydrogen storage: preparation, a new method for chemical activation and characterization of fibers

http://research.sabanciuniv.edu/9451/1/JBChem2008_3_247-253.pdf

PAN-based Pd-doped activated carbon fibers for hydrogen storage: preparation, a new method for chemical activation and characterization of fibers

Abstract

Similar works

Full text

Available Versions

Sabanci University Research Database