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A multifactor study of catalyzed hydrolysis of solid NaBH4 on cobalt nanoparticles: Thermodynamics and kinetics

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A multifactor study of catalyzed hydrolysis of solid NaBH4 on cobalt nanoparticles: Thermodynamics and kinetics
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  A multifactor study of catalyzed hydrolysis of solid NaBH 4 on cobalt nanoparticles: Thermodynamics and kinetics  Je´ roˆme Andrieux a, *, Dariusz Swierczynski b , Laetitia Laversenne a , Anthony Garron b , Simona Bennici b , Christelle Goutaudier a , Philippe Miele a , Aline Auroux b , Bernard Bonnetot a,1 a Universite´  de Lyon, Laboratoire des Multimate´ riaux et Interfaces, CNRS UMR 5615, F-69622, Villeurbanne, France b Universite´  de Lyon, Institut de Recherches sur la Catalyse et l’Environnement de Lyon, CNRS UMR 5256, F-69622, Villeurbanne, France a r t i c l e i n f o Article history: Received 28 July 2008Received in revised form16 September 2008Accepted 17 September 2008Available online 30 November 2008 Keywords: Hydrogen storageSolid sodium borohydrideHydrolysis reactionCatalysisCobaltKineticsThermodynamics a b s t r a c t In the present work, hydrogen generation through hydrolysis of a NaBH 4(s)  /catalyst (s)  solidmixture was realized for the first time as a solid/liquid compact hydrogen storage systemusing Co nanoparticles as a model catalyst. The performance of the system was analysedfrom both the thermodynamic and kinetic points of view and compared with the classicalcatalyzed hydrolysis of a NaBH 4  solution. The kinetic analysis of the NaBH 4(s)  /catalyst (s)  /H 2 O (l)  system shows that the reaction is first order with respect to the catalyst concen-tration, and the activation energy equal to 35 kJmol NaBH4  1 . Additionally, calorimetricmeasurements of the heat evolved during the hydrolysis of NaBH 4  solutions evidence theglobal process energy (  217 kJmol NaBH4  1 ). Characterization of the cobalt nanoparticlesbefore and after the hydrolysis associated with the calorimetric measurements suggeststhe ‘‘in situ’’ formation of a catalytically active Co x B phase through ‘‘reduction’’ of an outerprotective oxide layer that is regenerated at the end of reaction. ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rightsreserved. 1. Introduction Now that technologies to use hydrogen as a clean fuel arereadily available, like the Proton Exchange Membrane FuelCell (PEMFC), and can be developed at an industrial scale,research mainly focuses on the barrier of development whichis hydrogen storage for delayed use.The potential applications of sodium borohydride havebeen widely studied and have been recently reviewed byC¸akany ı ld ı r ı m and Gu ¨ ru ¨  [1]. Most of the research efforts andindustrial devices developed up to now have been aimed atthe catalyzed hydrolysis of sodium borohydride solutions(stabilised with NaOH), according to the reaction (1)BH  4 ð aq Þ  þ  4H 2 O ð l Þ    ! cat B ð OH Þ  4 ð aq Þ þ 4H 2 ð g  Þ  (1)Thehydrogenstoragecapacityofsodiumborohydridesolutionsdepends on the quantity of water involved in the whole storagesystem.Assumingaquantitativereaction,astandardcommercialsolutioncontaining20wt.%ofNaBH 4 allowsastoragecapacityof only 4.2 wt.% H 2 . Moreover, in order to prevent self-hydrolysis of thefuel,NaOHisusuallyaddedinaconcentrationof1–5wt.%thatfurtherlowersthe reactivity and the storage capacity. *  Corresponding author.  Tel.:  þ 33 472431234; fax:  þ 33 472440618.E-mail address: jerome.andrieux@univ-lyon1.fr (J. Andrieux). 1 Deceased December 2006. Available at www.sciencedirect.comjournal homepage: www.elsevier.com/locate/he 0360-3199/$ – see front matter  ª  2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.ijhydene.2008.09.102 international journal of hydrogen energy 34 (2009) 938–951  Higher hydrogen storage capacities can be achieved by thereactionofstoichiometricquantityofwaterwithsolidsodiumborohydride according to the Eq. (2) (NaBH 4(s)  /catalyst (s)  /H 2 O (l) system), which leads to the formation of solid hydratedmetaborates and the release of four moles of hydrogen [2,3].NaBH 4 ð s Þ  þ ð 2 þ  x Þ H 2 O ð l Þ    ! cat NaBO 2 $ x H 2 O ð s Þ  þ 4H 2 ð g  Þ  (2)where  x ¼ 0, 2 and 4 [3].The first advantage of using the solid NaBH 4 –liquid watersystem is that both reagents are stored separately, thusavoiding the problem of instability encountered for NaBH 4 solutions. In this case, the storage capacity depends on theboratehydrationdegree[3]. x ¼ 0leadstoatheoreticalamountof generated H 2  equivalent to 10.8 wt.% (117 kgH 2 m  3 )considering the fuel (NaBH 4 þ H 2 O).IfthewatercontentintheNaBH 4(s)  /catalyst (s)  /H 2 O (l) systemis high enough to dissolve the metaborate product at roomtemperature (25   C), the advantages of this configuration arethe possibility to store the reagents separately and to removethe products of the reaction from the system in liquid form.Schlesinger et al. were among the first to study NaBH 4 hydrolysis [4] from stabilised NaBH 4  solution. They haveshownthatnon-catalyzedsodiumborohydridehydrolysishasa very slow kinetics, making it inappropriate for a solution toproduce hydrogen on demand. A linear evolution of hydrogenas a function of time is obtained at the beginning of thehydrolysis. For the first step of the reaction, the activationenergy is found to be approximately 134 kJmol NaBH4  1 [5]. Asecond step is then observed where the rate of hydrogenrelease falls down rapidly to reach a plateau corresponding tothe end of the reaction. The shape of the experimentalhydrogenyieldasafunctionoftimehasbeenclearlyexplained.ThisbehaviourcouldbeexplainbyanincreaseofthepHofthesolutionduringthecourseofthereaction,duetotheformationof the strongly basic metaborate ion B(OH) 4  [4,6,7].The potential outlets of this technology have motivatedmany studies on different catalysts with the aim to improvethe kinetics and the yield of NaBH 4  hydrolysis.A literature review concerning kinetic studies, focused ontransitionmetalcatalysts,issummarized inTable 1and Fig.1. It showsfirstofallthe diversityofconfigurations testedby theauthorsofthesestudies.Thekineticstudieshavebeencarriedout on aqueous solutions with NaBH 4  concentrations varying from 0.05 to 25 wt.%, stabilized by NaOH (0.1–10 wt.%) or non-stabilized, using batch reactors. The activation energies andthe hydrogen generation rates reported by the authors arepresented in Table 1. Hydrolysis experiments have beencarriedoutinatemperaturerangefrom10to50   Conaverage,pointingout a commontemperaturedomaineasytorealize forindustrial applications. Hydrogen generation rates have beenrecalculated from the literature data in Lmin  1 g  catalyst  1 andLmin  1 g  metal  1 for comparison, and show the influence of thesupport on the catalyst efficiency.Noble metal-based catalysts under the form of slurrydispersed in the solution have been the most widely studied.Platinum supported on LiCoO 2  presents the highest hydrogengeneration rate at room temperature [8]. Other noble metal-based catalysts, stabilised ruthenium nanoclusters andpalladium supported on activated carbon, show interesting properties: activation energies of 28.5 and 28 kJmol NaBH4  1 ,respectively, and hydrogen generation rates at room temper-ature of 3.65 and 0.5 Lmin  1 g  metal  1 , respectively [9,10].AsindicatedinTable1thequantityofcatalystusedinmoststudies is quite high with respect to the quantity of NaBH 4 .Studies have thus also focused on less expensive catalystswith the aim to get the same catalytic activity as noble metalcatalysts. Non-noble metal catalysts have been intensivelystudiedin the formof metal salts [2,4,11], Raney metals [11,12] ormetalborides[11–16].Thefirststudieshaveshownthepoorefficiency of iron or copper as catalysts in the hydrolysis of NaBH 4 , and reveal the better activity of nickel and cobalt.Kaufman and co-workers found an activation energy of 71 kJmol NaBH4  1 for bulk nickel catalyst, and 63 kJmol NaBH4  1 forRaney nickel [12]. Nickel based catalysts are more efficientunder micrometric or nanometric sizes, which demonstratesthe key influence of the catalyst surface area/volume ratio. Inordertomaximizethis parameter,Metinandco-workershavesynthesized water-dispersible nickel (0) nanoclusters [17]which were found to be highly active catalysts even at roomtemperature, with an activation energy of 54 kJmol NaBH4  1 .Cobalt as a catalyst for the hydrolysis of sodium borohydridewas first studied by Kaufman and co-workers [12]. Theyfound for this catalyst a quite high activation energy of 75 kJmol NaBH4  1 , but provided no details on the catalyst form ormorphology.Liuandco-workershavestudieddifferentcobalt-based catalysts [11]. A commercially available fine powder of cobalt showed a good catalytic activity with an activationenergy of 62.7 kJmol NaBH4  1 . Ye and co-workers have compareddifferent supported cobalt catalysts [18]. They found that a  g -Al 2 O 3  supported cobalt catalyst was very efficient, with anactivation energy of 32.63 kJmol NaBH4  1 and a rate of hydrogengeneration of1.15 Lmin  1 g  metal  1 . Asshown in Fig.1, this resultis close to the best results obtained for noble metal catalysts[9,10]. Recent studies have demonstrated also the efficiency of Co x B phase in the catalysis of NaBH 4  hydrolysis. A rate of 2.32 Lmin  1 g  metal  1 was measured with Co x B supported onactivated carbon [19] while a rate of 11 Lmin  1 g  metal  1 wasobtained for Co x B/Ni foam [20]. Finally, recent promising works have shown the efficiency of Co x B doped with othermetals. Pd–Co x B catalyst has been reported by Liang et al.with the hydrogen generation rate of 2.87 Lmin  1 g  metal  1 [21].Dai et al. have reported excellent performances of W–Co x B catalysts [22] with a hydrogen generation rateof 15 Lmin  1 g  metal  1 at room temperature and an activationenergyof29 kJmol NaBH4  1 ,equaltothebestresultobtainedwithnoble metal catalysts.Finally, the Co x B phase has been claimed to be the activephase in the hydrolysis [11,12,18–20,23,25]. However, in spiteof the abundance of the literature studies on Co-based cata-lysts, few details are given neither on the possible catalyticmechanism that occurs during the hydrolysis nor on theidentification of the active phase.For this purpose, we have started a detailed study ona nano-dispersed cobalt-based catalyst. 2. Experimental Experiments were carried out with commercial sodiumborohydride (Acros organics, 98% purity, powder, average international journal of hydrogen energy 34 (2009) 938–951  939  Table 1 – Comparison of chosen kinetic studies of catalyzed NaBH 4  hydrolysis from the literature and this work Nature of the catalystForm Hydrolysis configuration a Quantity of catalyst used(wt.% of NaBH 4 )Activationenergy(kJmol NaBH4  1 )Temperaturerange (  C) r catb (Lmin  1 g  catalyst  1 )(20   C < T < 25   C) r metalc (L.min  1 .g  metal  1 )(20   C  < T < 25   C)ReferenceNaBH 4  (wt%) NaOH(wt.%) N.C Pt/LiCoO 2 (1.5 wt.% Pt)LiCoO 2 supported metal 20 10 4.3 (Pt/LiCoO 2) 0.06 (Pt) N.C N.C 3.9 260.4 [8]Ru Nanoclusters 0.57 0 0.7 28.5 30–45 3.65 3.65 [9]Ru/IRA400(5 wt.% Ru)IRA 400 resinsupported metal 20 10 4.3 (Ru/IRA400)0.2 (Ru) 47 25–55 0.199 3.98 [24]Ru/IRA400(5 wt.% Ru)IRA 400 resinsupported metal 7.5 1 11 (Ru/IRA400)0.5 (Ru) 56 0–40 0.378 7.56 [7]Pd/C(10 wt.% Pd)Activated Carbonsupported metal 0.05 0.1 307 (Pd/C)30.7 (Pd) 28 N.C. 0.05 0.5 [10]Co Powder 0.4 8 13 75 0–35 0.373 0.373 [12]Co Powder 1 10 250 41.9 10–50 0.126 0.126 [11]Co Raney form 1 10 50 53.7 10–30 0.267 0.267 [11]Co/Al 2 O 3 (9 w.t%) g -Al 2 O 3 supported metal 5 5 73 (Co/Al 2 O 3 )6.6 (Co) 32.63 30–50 0.103 1.15 [18]Co/C (9 wt.%) Active carbonsupported metal 5 5 73 (Co/C)6.6 (Co) 45.64 30–50 0.018 0.2 [18]Co–B Powder 20 5 N.C. (0.05 g) 64.87 10–30 0.875 0.875 [23]Co–B/C(30 wt.%)Active carbonsupported metal 0.76 8 26.7 (Co–B/C)8 (Co–B) 57.8 25–40 0.53 2.32 [19]( continued on next page ) i  nte rnati   onal  j   o urnal  of hydr o ge ne ne r gy  3  4  (    2 0 0  9  )      9  3 8  –  9  51  9  4  0    Table 1 (  continued  ) Nature of the catalystForm Hydrolysis configuration a Quantity of catalyst used(wt.% of NaBH 4 )Activationenergy(kJmol NaBH4  1 )Temperaturerange (  C) r catb (Lmin  1 g  catalyst  1 )(20   C < T < 25   C) r metalc (L.min  1 .g  metal  1 )(20   C  < T < 25   C)ReferenceNaBH 4  (wt%) NaOH(wt.%) Co–B/Ni Foam(50 wt.%)Ni foamsupported metal 25 3 60 (Co–B/NiFoam)0.3 (Co–B) 45 20–40 0.55 1.11 [25]Co–B/NiFoam (N.C.)Ni foamsupported metal 20 10 N.C. (Co–B/NiFoam)2.56 (Co-B) 33 25–45 N.C 11 (30   C) [20]Co–W–B/NiFoam (N.C.)Ni foamsupported metal 20 5 N.C. (Co–W–B/NiFoam)2.25 (Co–W–B) 29 25–45 N.C 15 (30   C) [22]N.C. Co–B/Pd–NiFS(N.C.)Pd modifiedNi foam supportedmetal20 4 N.C. (32 mg Co–B) N.C N.C N.C 2.87 (30   C) [21]Ni Powder 0.4 8 15.8 71 0–35 0.114 0.114 [12]Ni Powder 1 10 250 62.7 10–50 0.02 0.02 [11]Ni Nanoclusters 0.57 0 1.45 54 25–45 5 5 [17]Ni Raney form 1 10 50 50.7 10–30 0.23 0.23 [11]Ni Raney form 0.4 8 7.9 63 0–35 0.156 0.156 [12]Ni–B Powder 1,5 10 33.3 38 20–60 0.155 0.155 [14]Co Powder w 10 nm 19 0 10 35 40–80 N.C N.C This workN.C. ¼ non-communicated.a The wt.% refers to the component content in aqueous solution.b  r cat  ¼ Hydrogen generation specific rate based on catalyst (metal þ support) mass (Lmin  1 g  catalyst  1 ).c  r metal  ¼ Hydrogen generation specific rate based on metal mass (Lmin  1 g  metal  1 ). i  nte rnati   onal  j   o urnal  of hydr o ge ne ne r gy  3  4  (    2 0 0  9  )      9  3 8  –  9  51  9  4 1   particle size: 200  m m). Prior to the experiments the powderwas pre-treated at 180   C for 2 h under primary vacuum( w 10  2 mbar) in order to avoid the presence of hydratedspecies(NaBH 4 $ 2H 2 O).Commercialsodiumhydroxide(Sigma–Aldrich, 98%) was used to stabilize aqueous solutions of sodium borohydride.Cobalt nanoparticles from Strem chemicals (ref. 27-0020)were used as catalyst in this study. These nanoparticles havea very homogeneous particle size distribution with an averagediameter of 10–12 nm, and are covered with a protective oxidelayer [26]. The catalyst was mixed mechanically with the pre-treated NaBH 4  powder in a mortar under Ar atmosphere priorto the experiments of catalyzed hydrolysis of solid NaBH 4 .Hydrolyses have been carried out with 1, 5, and 10 wt.% of cobalt with respect to sodium borohydride. The fresh Conanoparticles will be referred to as ‘‘ nCoF ’’. In order to under-stand theevolutionofthecatalyst during hydrolysis ofsodiumborohydride, the catalyst has been characterized after a stan-dard test with 10 wt.% of catalyst at 30   C. The mass of pre-catalyzedmixturewas3 gandthehydrolysiswascarriedoutina 250 mL flask connected to a reflux condenser to avoid waterloss during the hydrolysis. After test, the nanoparticles werewashedfivetimeswithdistilled/deoxygenatedwater,andthendried under vacuum. These nanoparticles after hydrolysis testwill be named ‘‘ nCoT ’’. Finally, as received Co nanoparticles( nCoF ) have been oxidized in air in ambient conditions for 50days in order to study the influence of the oxide layer. Thesenanoparticles after oxidation will be referred to as ‘‘ nCoFox ’’. 2.1. Calorimetric measurements for catalyzed hydrolysisof NaBH 4  solution A Differential Reaction Calorimeter (DRC, SETARAM) has beenused to quantify the heat effects and to follow the hydrolysisreaction by measurement of the total heat evolved at 30   C. Adetailed description of the experimental set-up was previ-ously described [27]. The volume of released gas has beenmeasured using a gas-meter coupled to the DRC. Detailsconcerning the use of DRC system are reported by Nogentet al. [28]. The common experimental protocol was thefollowing. Firstly 40 mg of catalyst was suspended in 20 mL of 1 M NaOH solution under nitrogen flow directly in the DRCsystem. The catalyst was then activated by addition of 10 mLof stabilised NaBH 4  solution containing 212 mg (2 wt.%,5.36 mmol) of sodium borohydride and 4 wt.% of NaOH. Thecatalytic–calorimetric tests were performed after generationoftheactivephasebyfoursuccessiveadditionsof10 mLofthestabilized solution of NaBH 4 .There are very scarce literature data concerning experi-mental calorimetric study of the reaction of borohydridehydrolysis. Davis et al. [29] measured the heat of reaction forsodium borohydride hydrolysis with hydrochloric acid giving the value of 267 kJmol NaBH4  1 . More recently Zhang et al. [30]measured the enthalpy of reaction of    212.1 kJmol NaBH4  1 cor-responding to the global reaction (3).The standard-state enthalpy (4) change for sodium boro-hydride hydrolysis in aqueous solution (4) can be calculatedfrom standard formation enthalpies at 25   C: 48.2 kJmol  1 (BH 4  ),   285.8 kJmol  1 (H 2 O),   1345.5 kJmol  1 (B(OH) 4  ) [31].BH  4 ð aq Þ  þ  4H 2 O ð l Þ / D r H  B ð OH Þ  4 ð aq Þ þ 4H 2 ð g  Þ  (3) D r H  ¼  250 : 5 kJ mol  1NaBH 4  (4) 2.2. Catalyzed hydrolysis of solid NaBH 4  /catalystmixture Hydrolysis experiments were performed in a 20 mL test-tubeclosed by a silicon stopper and placed in a thermostatic bath.No stirringis used in this configuration. Forthe determinationof kinetic parameters the overall system can be considered asa non-steady state slurry batch reactor. The H 2 O/NaBH 4  molarratio wasequalto9 (19wt.%ofsodiumborohydride)forallthetests. Prior to the experiment the mixture of catalyst andNaBH 4 waschargedintothereactorinagloveboxunderargonatmosphere. Distilled water was purged with argon prior touse in order to remove oxygen. In a typical experiment, 500  m lof water was injected into the reactor with a needle placeddirectlyinsidethebedofthepre-catalyzedmixturecontaining 110 mg of NaBH 4 . Automated burette was used to control thequantity of water. A second needle evacuated generatedgasesto the outlet tube connected to an inverted, water filled,graduated cylinder, situated in a water filled tank. Hydrogengenerated volumes were measured as a function of time, byvideomonitoringwaterdisplacedfromthegraduatedcylinderas the reaction proceeded. Thermal probe placed inside theNaBH 4  bed measured the temperature during the reaction. Asa consequence reactor’s temperature evolution can be recor-ded in relation with the hydrogen generated, as a function of time.The hydrogen generation yield is defined as the ratio of theexperimental hydrogen generated volume to the theoreticalone (the latter being a function of the initial number of reagent’s moles). Kinetic parameters were based on themeasurement of the rate of hydrogen generated, which isdefined as the slope in the catalytic step of the hydrolysis. For Fig. 1 – Comparison of activation energies obtained fordifferent catalysts in previous studies (  4  activation energyfrom the present work). international journal of hydrogen energy 34 (2009) 938–951 942
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