Hydrogen absorption of slurry system composed of different MgNi alloy and benzene

Vol.13 No. 1Trans. Nonferrous Met. Soc. ChinaFeb.2003Article ID: 1003 - 6326(2003)01 - 0029 - 04Hydrogen absorption of slurry system composedof different MgNi alloy and benzeneCAI Guan-ming(蔡官明), CHEN Chang pin(陈长聘)，AN Yue(安趣)，XU Guo-hua(徐国华)，CHEN Li-xin(陈立新), WANG Qi-dong(王启东)(ollege of Materials Science and Chemistry Engineerng, Zhejiang University, Hangzhou 310027, China)Abstract: The hydrogen absorption anount and kinetics o[ the slurry formed by suspending the MgNi aly powderin liquid benzene were studied. It is discovered that hydrogen is absorbed by both the solid phase(ally) and liquidphase(C% He) and the hydrogen absorption rate varies with the temperature and the content of the Mg Ni in the slur-ry. Most hydrogen absorption curves of the slurry fall into two regijions, in which the mechanism of hydriding reac-tion in the slurry system is dfferent. In the former region, the bhydriding of the alloy proceeds with hydrogen diffu-sing through C% H. The part in the second region is the outcome of the hydrogenation of Cs H&. At 548K and underthe hydrogen pressure of 4.5 MPa the saturation capacity for the slurry of 80% Co Hs (mass fraction) + 20% MgNi(mass fraction) is 5. 9% (mass fraction) hydrogen, which is 97% of the theoretic capacity of the slurry system,The bydride of the alloy MgNi, which is only the hydride of Mgz Ni phase, Mgz NiH&, is an eficient catalyst for thehydrogenation of C。H, into C; Hx (C。Hs +3H2- +CHu2) in the slurry system.Key words: MgNi alloy ; slurry, benzene; eyclohexane; metal hydrideCLC number: TG 139.7Document code: A1 INTRODUCTIONthe form of a film was used as the catalyst, the a-mount of cyclohexene formed was very small withMagnesium is known to have very high hydro-the selectivity around 19%. Traditionally the hy-gen storage capacity(mass fraction 7. 6%), but itsdrogenation of CH。is only performed in gaseoushydrogen absorption/ desorption kinetics is bad andstate with Pt and Ni as the primary catalyst. Theits reaction temperature is high. Magnesium ab-sorbs hydrogen at 573 - 673 K and desorbs hydro-hydrogenation of C% H; in liquid state has not evengen at atmospheric pressure only above 673 K. Inbeen extensively studied. Reilly et al[6.7J first pro-recent years many attempts were made to improveposed the concept of the metal hydride slurry. B:the bydrogen absorption/ desorption kinetics ofsuspending metal hydride particles in the chemical-Mg-based hydrogen storage materials10. AO etinert solvents(e. g. n-octane, rundecane ，siliconal[6] reported that the hydriding/ dehydriding kinet-oil and so on)[8.9], some serious problems, such asics of the magnesium was improved after beingthe deformation or rupture of the vessels due to thetreated with some organic compounds such asparticle fragmentation and the poor heat transfer a-Aetrabydrofuran ( THF ). As Mg2Ni is a wellbility of gas-solid systems, can be easily occurredknown magnesium-based hydrogen storage materi-in a wel-stirred slurry system. However, in one ofal, with a hydrogen storage capacity of 3.6% and aour early studies the hydrogen absorption proper-absorption/ desorption temperature lower than theties of the slurry of MINis alloy suspended in ben-pure magnesium. It occurs to us that Mg: Ni whichzene were investigated d.1. It was discovered thatreacts reversibly with hydrogen may be used as athe La-rich mischmetal nickel hydrogen storage al-catalyst to hydrogenate some hydrocarbons such asloy MINi; had good hydrogen absorption behaviorbenzene and toluene. A few reacting products wereformed in the hydrogenation of Cr Hs. With cyclo-when being suspended in C; Hg, and the hydrogenhexane and cyclohexene being the main productsabsorption thermodynamics of the MINis alloy infrom a literature review, we learned that cyclohex-the slurry system was similar to that of MINis inene was first detected in the low temperature andthe gas-solid system; at the same time, the MINislow pressure hydrogenation of CgH。in 1957[6]. Atalloy was also found to be a good catalyst for the318K and under the low pressure of 3 669 Pa(917hydrogenation of C.H6. On account of the resultsPa benzene and 2752 Pa hydrogen) when nickel inof our precious experiments, a study on the hydro-中国煤化工D Foundation item: Projct(2000026406)0 suprted by the Nationalrogram of Chinal Pro-jc(500701053) supporied by the National Natural Science Foundation of CHYHCNMHG'Recelved datc; 2002 -01 - 281 Accepted date: 2002 -05 -27Correspondence .CHEN Chang-pin, Professor, + 86-571-87951152, cepchen@zju. edu. cn.30●Trans. Nonferrous Met. Soc. ChinaFeb. 2003gen absorption properties of a slurry system com-that of Mgz Ni due to the dummy weight of MgNi.posed of MgNi alloy and benzene was initiated.Fig. 2 shows the hydrogen absorption kineticsMg- based alloy MgNi was chosen as Mg2Ni and itcurves of MgNi alloy. It can be seen that as thehas a higher hydrogen storage capacity than MINigtemperature is elevated, both the hydriding speedhas. An extra amount of nickel was added toand the storage capacity increase. Even at the low-Mg:Ni in expectation of the catalytic function oest temperature, 453 K, in this study the testedNi. Both the hydrogen absorption of the alloy inhydriding rate of Mg-Ni is still satisfactory. Thethe CiHe and the hydrogenation of benzene werehydrogen storage capacity finally arrives at 1. 2%investigated.(mass fraction) at 453 K and reaches 1. 7% (massfraction) at 523 K. Compared with the single phase2 EXPERIMENTALMg: Ni compound, the absorption kinetics of thediphase MgNi alloy composed of Mg:Ni + MgNiz isThe purity of raw materials, Mg and Ni, washigher. That is possibly due to the increase of the99. 9%. The purity of the hydrogen used in the ex-interface and the presence of more tunnels betweenperiment was 99. 99%. In this experiment the ana-the two phases for hydrogen diffusion.lytic pure benzene was used without any furtherpurification. The MgNi alloy was prepared by arcmelting a mixture of equal mole proportion(1↑1)◆-Mg2Niof Mg and Ni under a high purity argon atmos-▼-MgNi2 .phere. The XRD spectra of the alloy were obtainedona Philip X'Pert MPD type X-ray diffractometerwith CuK。radiation. The cast alloy ingot wascrushed to particles of 74 μm and then loaded intothe reactor for experimentation. The hydrogen ab-sorption properties of the alloy powder were meas-i ijMlai m.ured with an automatic Sieverts apparatus. Theexperiments for the hydrogen absorption of the1020”30405060708090slurry were carried out in a stainless steel reactor.20(°)The absorption kinetics of the slurry systems wasFig. 1 XRD spectrum of MgNi alloymeasured with the volumetric method. For investi-gating the difference in hydrogenation ability be-2.0tween the alloy and its hydride, alloy or its hydridewas tested separately in liquid into benzene in thei 1.8form of liquid suspension. The hydrogen absorp-tion curves for both cases were plotted. Slurrieswith different proportions of alloy in benzene(10%，20% and 50%) were tested. Gas chroma-i 0.8-■-523Ktographic examination on the liquid in the slurry●-483Kwas made after the liquid was hydrogenated.全0.4f▲-453K3 RESULTS AND DISCUSSION200 400 600 800 1000Time/sFig.1 shows the X-ray spectrum of MgNi alloy. Two intermetallic phases, Mg2Ni and MgNizFlg.2 Hydrogen absorption curves of MgNiare observed, among which Mg2Ni is a hydrogenalloy at various temperatures understorage alloy and hence responsible for its own hy-hydrogen pressure of 3. 5 MPadriding and breaking up the bonds of hydrogenmolecules and thereby activating the hydrogen forFig. 3 shows the hydriding kinetics curves ofhydrogenation of Co Hs. With the intention of get-the slurry with 10% MgNi, Compared with theting some free Ni on the surface of alloy, a surplushydrogen absorption curves for solid alloy parti-amount of Ni was added to the B- side of the MgzNicles, every hydriding curve of the slurry can bealloy to form a non-stoichiometric composition ofdivided into two parts. The first part denotes theMgso Nio as mentioned above. However, as showna中国煤化Tfirst 90 min by thein the same figure, no free Ni was observed, butslfen storage capacityonly MgNiz was detected together with MgzNi. AsofYHC N M H Ghis period of time,MgNi; is not a hydrogen storage alloy, the hydro-both the solid phase alloy and liquid phase benzenegen storage capacity of MgNi was found lower thanin the slurry were absorbing hydrogen, but theVol. 13 No.lHydrogen absorption of slurry systemy. When the bydrogenation reaction stopped, asmall amount of the organic compound in the slur-▼一548 Kry was taken out and analyzed. The results show,■-523Kthat the content of the cyclohexane is above 99%，●-483 Kwhich means that nearly all benzene has beens-453 K .transformed into cyclohexane.6. ▼一- 548 K圣21●-483K4. 1-453K60120 180 240 300 360Time/minFig. 3 Hydriding kinetics of slurry system with10% Mg2 Ni at various temperaturesunder hydrogen pressure of 4. 5 MPa50 120 180 240 300 360Timc/minspeed for both was very slow. After 90 min the re-action changed into the second stage, in which theFig. 4 Hydrogen absorption curves of slurryrate of hydrogenation of the benzene accelerated. Itwith 20% MgNi at different temperaturescan be seen from the curves that the hydriding rateincreased significantly and was controlled by thetemperature of the slurry. At 453K, the rate ofThe hydriding curves of the slurry with differ-hydrogen absorption was still low and was nearly aent contents of MgNi are shown in Fig. 5. In theconstant. As the temperature increased to 483 Khydriding curve of the slurry with 50% MgNi thethe hydrogen absorption rate increased noticeably.slope of the curve is nearly a constant value and theThe temperature range in which the slurry startedslopes of the two parts of the curve do not showto absorb hydrogen rather quickly was very closemuch difference as compared with the slopes of theto that found in our previous investigations for thecurves for slurry systems with lower alloy con-slurry composed of the AB: type alloy and ben-tents. As the hydrogen storage capacity of MgNizeneFig.3 suggests also that the hydrogena-alloy is much smaller than that of benzene, the sat-tion of benzene does not take place noticeably untiluration hydrogen capacity of the slurry decreases asa definite amount of MgNi has transformed to athe alloy content in the slurry is increased. Thehydride(Mgr NiH、) on absorbing hydrogen or thatsaturation hydrogen capacity (4.2%，mass frac-the hydrogenation process of benzene is only cata-tion) of the slurry with 50% MgNi is smaller thanlyzed by the bydride Mgz NiH, in the slurry; as thethat(5.9%，mass fraction) of the slurry with 20%catalysis of hydrided MgNi proceeds, the rate ofMgNi. From Fig. 5, it can be seen that the absorp-hydrogenation of benzene increases rapidly. Fig. 4tion rate increases as the content of alloy increases.shows the hydrogen absorption curves of the slurryAccording to a study of Reilly et a][12] the hydrogenwith 20% MgNi at various temperatures under theabsorption rate of the alloy in the slurry composedhydrogen pressure of 4. 5 MPa. The absorptionof alloy and n-undecane or silicon oil is controlledrate of this slurry is evidently much higher com-by the mass transfer process of hydrogen in the liq-pared with that of the slurry with 10% MgNi. Theuid, which is determined by the temperature of theabsorption rate curves are also composed of twoslurry, the hydrogen pressure and the amount ofparts as shown in Fig. 4, but the difference in thesuspended alloy particles. As the content of the al-hydrogenation rates in the two parts is much smal-loy increases, therate of hydrogen transferredler than that of the curves for the slurry with 10%through the liquid is reduced or the rate of bydro-MgNi. The increase in the amount of hydrogen ab~gen of the alloy is reduced. However, in the pres-sorbed in the first part of the curve is from the in-ent experiment, the hydriding ( or hydrogen abcrease of the amount of alloy, which transforms in-sorbing) materials in the slurry are not only theto hydride in the slurry. The increase of hydrogen-hydrogen storage alloy but also benzene(C H。).ation rate with the increase of temperature is the中国煤化工slurry increases,same as that for the slurry with 10% MgNi. Atthe,YH二ride) and the con-548 K under 4. 5 MPa, after 260 min, the hydrogentactC N M H Gmetal hydride instorage capacity reached 5. 9% ( mass fraction),crease, which are both propitious to the increase ofwhich is 97% of the theoretic capacity of this slur- the hydrogenation rate of benzene. So the increase●32.Trans. Nonferrous Met. Soc. ChinaFeb. 2003entirely by the rate of hydrogen reacted with ben-zene catalyzed by the metal hydride ( MgNiH2 )■- Hydrideformed in the first part of reaction. As in MgNi●- Aloyprepared by us only the portion of Mg2Ni phase ishydrided into Mgz NiH, and acts henceforth as thecatalyst, and the role played by MgNi2 in the reac-tion is get to be studied.昌2REFERENCES10 50100150200250300350400[1] Ronnebro E, JensenJ O, Noreus D, et al. StructuralTime/minstudies of disordered Mg: NiH formed by mechanicalgrinding[J]. J Alloy Comp, 1999, 293 - 295; 146 -Fig.5 Hydrogen absorption curves of149.slurry with different alloy contents[2] Reule H, Hirscher M, WeiBhardt A, et al. Hydrogenat483Kunder4.5MPadesorption properties of mechanically alloyed MgH:composite materials[J]. J Alloys Comp, 2000, 305;246 - 252.of the whole hydriding rate of the slurry system,which depends mainly on the increase of the hydro-[3] l.iang G, HoutJ, BoilyS, et al. Hydrogen storageproperties of nanocrystalline Mgr.oTi,n Ni made by me-genation rate of benzene, increases as the contentchanical alloying[J]. J Alloys Comp, 1999, 282; 286 -of the alloy increases.290.The hydrogen absorption kinetics of the slurry4] YU Zhen-xing, LIU Zu yan, WANG Er de. Influencecomposed of C.H。and the MgNi alloy versus theof CrCl, on hydrogen storage properties of Mg-Ni sys-slurry composed of CH。and metal hydridetems[J]. The Chinese Journal of Nonferrous Metals,MgzNiH, was also compared. Fig. 6 shows the two2001，11(6); 1036 - 1039. (in Chinese)hydriding curves of the 10% MgNi+ CoH slurry[5] AO Ming, WANG Qirdong. Hydrogen storage prop-and the 10% MgNiH, (metal hydride) + C.H。erties of Mg modified with tetrahydrofuran[J]. ActMetallurgica Sinica, 1990, 3(6): 415- 418.slurry respectively. From Fig. 6 it can be seen that6] YE Dai-qi, PANG Xian-shen, HUANG Zhong-tao.the hydriding curve of the slurry with 10% MgNi-New technology for the production of polyamide- theH, (metal hydride) is a straight line without anyselective hydrogenation of benzene to cyclohexene[J].bent or change in curvature, which means the hy-Polymer Communication. 1993(3); 170 - 177. (in Chi-driding rate of benzene when catalyzed by the metalnese)hydride is nearly a constant. But the hydriding7] ReillyJ J, Grohse E W. Johnson J R. Metal hydrideslurries[A]. VerirogluT N. Proc 5* Miami Int Confcurve of the slurry composed of MgNi alloy andAlt Energy Sources[C]. Univ Miami, Coral Gablesbenzene falls into two distinct regions. The firstFI, 1982.region shows that the amount of hydrogen ab8] JohnsonJ R, Reilly J J Kinetics of hydrogen absorp-sorbed by the slurry is the amount of hydrogention by metal hydride suspensions; the systems La-used in hydriding the alloy; thesecond region isNi, H,/ nr octane and LaNi.n Aln,; H,/ rrundecane[J].the rate of absorbed hydrogen determined almostZeitschrift fur Physikalische Chemie Neue Folge,1986，147(s); 263- 267.[9] Holstvoogd R D, ven Swaej W P M，VersteegG F.Continuous absorption of hydrogen in metal hydride■- - Hydrideslurries[J]. Zeitschrift fur Physikalische Chemie Neue●- - AlloyFolge, 1989, 164; 1429 - 1434.[10] CAI Guan- Ming, CHEN Chang-Pin, AN Yue, et al.Hydrogen absorption thermodynamic properties ofrare earth based hydrogen storage alloy in benzene[J]. J Rare Earths, 2002, 20(1); 28-30.[11] AN Yue, CHEN ChangPin, XU (Giuo-Hua, etal. Astudy on the kinetics of hydrogen absorption by metalhydride slurries I . absorption of bydrogen by hydro-gen storage alloy MINis suspended in benzene[J]. J50 100 150 200 250 300 350 400Rare Farth2002, 20<2): 113- 115.[12)中国煤化iaite od the lhorrudecane suspensionsFig.6 Hydrogen absorption curvesTYHCNMHG1. 104: 175 - 190.of slurry with 90% benzene at.. HUANG Jin-song)483 K under 4. 5 MPa