Effect of TiO2 Nanoparticles on the Hydrogen Sorption Characteristics of Magnesium Hydride

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Copyright © 2013 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol. 13, 5493–5499, 2013

Effect of TiO2 Nanoparticles on the HydrogenSorption Characteristics of Magnesium Hydride

Sunita K. Pandey, Ashish Bhatnagar, Rohit R. Shahi,M. Sterlin Leo Hudson, Milind K. Singh, and O. N. Srivastava∗

Hydrogen Storage Mission Mode MNRE Project Unit, Hydrogen Energy Centre,Department of Physics, Banaras Hindu University, Varanasi 221005, India

The present paper explores the enhancement in hydrogen sorption behavior of MgH2 with TiO2

nanoparticles. The catalytic effect of TiO2 nanoparticles with different sizes (7, 25, 50, 100 and250 nm) were used for improving the sorption characteristics of MgH2. The MgH2 catalyzed with50 nm of TiO2 exhibited the optimum catalytic effect for hydrogen sorption behavior. The desorp-tion temperature of MgH2 catalyzed through 50 nm TiO2 was found to be 310 �C. This is 80 �Clower as compared to MgH2 having a desorption temperature of 390 �C. It was noticed that thedehydrogenated MgH2 catalyzed with 50 nm TiO2 reabsorbed 5.1 wt% of H2 within 6 minutes attemperature and pressure of 250 �C and 50 atm, respectively. The 50 nm TiO2 catalyst lowered theabsorption activation energy of MgH2 from ∼92 to ∼52.7 kJ mol−1.

Keywords: TiO2 Nanoparticles, Catalysts, Hydrogen Sorption, Magnesium Hydride.

1. INTRODUCTION

Hydrogen storage is considered to be the crucial issueof the hydrogen economy, cutting across production, dis-tribution and applications.1 It is generally believed thatstorage in the form of hydrides is an efficient and safestorage mode with high hydrogen density.2 Recent stud-ies confirms the importance of nanomaterials in energystorage.3 Nowadays different studies also reveals that car-bon based nanostructured may also be used as hydro-gen storage materials.4–7 Earlier, the emphasis has beenon hydrogen fill-in and take-out type hydrides typified byintermetallics such as FeTi, LaNi5.

8–10 The emphasis hasrecently shifted on take-out and regenerate type of built-in hydrides such as MgH2, NaAlH4, LiNH2.

11–12 Out ofthese, MgH2 appears to be a promising material becauseof high storage capacity (7.6 wt% H2�, better reversibil-ity and plentiful availability of Mg in the earth crust.13

In order to make MgH2 as a viable hydrogen storage mate-rial for applications, two of the limitations associated withit (sorption at high temperatures and poor kinetics) have tobe overcome.14 To get over these limitations, several mate-rial tailoring routes have already been applied by differentresearch.15–17 Examples of these are, making compositeswith MgH2

18 as the primary ingredient, transforming MgH2

∗Author to whom correspondence should be addressed.

to its nano versions and employing effective catalysts.19

Several transition metals such as Ti, Fe, Ni, Co, Fe20–30 andoxides like TiO2, Nb2O5, V2O3, Cr2O3, Mn2O3, Fe3O

31–364

have already been utilized as a catalyst. Out of these, TiO2

appears to be a favoured catalyst since it is readily availableat low cost.37 For MgH2, it is also thought that the nanoversion of TiO2 (rutile) may be an effective catalyst. This isso since it can undergo reduction in hydrogen atmosphereand has the same tetragonal structure with the close latticeparameters as that of MgH2.

38

The effect of variation in concentration of catalyst onimproving the sorption characteristics of MgH2 has beenwidely investigated by various researchers.22�24�26�28�30�38

However, the effect of variation in size of the catalyst par-ticles on the sorption behaviour of MgH2 has only sparselybeen studied. Earlier studies on TiO2 catalyzed MgH2 hasbeen carried out by Jung et al.38 They have shown that outof three phases of TiO2 namely rutile, anatase and com-mercial P25, the optimum catalytic activity for improv-ing the sorption behaviour of MgH2 is exhibited by rutile.They have also studied that the variation on concentra-tion of rutile TiO2 on the sorption behaviour of MgH2. Animportant aspect of catalytic activity relates to the size ofthe catalyst which may lead to uniform distribution overand/or within MgH2. This will lead to select the opti-mum size of catalyst particle for efficient hydrogen sorp-tion for the case of MgH2. Here, we report the effect of the

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Effect of TiO2 Nanoparticles on the Hydrogen Sorption Characteristics of Magnesium Hydride Pandey et al.

size variation of TiO2 nanoparticles (np) and determina-tion of optimum size for improving the hydrogen sorptionbehaviour of MgH2.

2. EXPERIMENTAL DETAILS

2.1. Ball-Milling

Commercial MgH2 (98%, Alpha Aesar having tetragonalphase with lattice parameter of a = 4�517 Å, and c =3�020 Å) and TiO2 nanoparticles with rutile phase (tetrag-onal phase with lattice parameter of a = 4�593 Å, andc = 2�959 Å) of different sizes as 7, 25, 50, 100 and250 nm (MK Nano Impex, Canada) were used for thepresent investigations. We have also employed an optimumconcentration of 5 wt% TiO2 nanoparticles as a catalystfor improving the hydrogen sorption behaviour of MgH2.MgH2 together with TiO2 (np) were ball-milled using aplanetary ball-miller (Retsch, PM400) on predeterminedprogramme parameters at an operating speed of 170 rpmunder 5 atm hydrogen pressure for 24 hours. The ball topowder ratio was kept at 30:1 for the present investiga-tions. Handling of samples was performed in an argonfilled glove box (mBRAun MB10 compact) with H2O andO2 less than 1 ppm.

2.2. Structural and Microstructural Characterization

The samples were characterized by powder X-ray diffrac-tion (XRD) using a X’Pert PRO X-ray diffractometer withCuK� radiation (�= 1�5402 Å). To prevent contaminationof the samples by air or moisture during X-ray diffrac-tion measurement, the specimen holder was covered bya thin layer of parafilm (Pechiney plastic packing). Thesurface microstructure of de/re-hydrogenated samples werecharacterized through scanning electron microscopy (SEM)(QUANTA-200). The transmission electron microscopy(TEM) observations were carried out on a TECHNAI 20 G2

microscope operating at an accelerated voltage of 200 kVfor structural and microstructural characterizations.

2.3. Hydrogen Sorption Measurement

Temperature-programmed desorption (TPD), hydrogena-tion kinetics and pressure-composition isotherm (PCI)measurements of the samples were done by using an auto-mated four-channel Sievert’s type apparatus (AdvancedMaterials Corporation, Pittsburgh, USA). For each mea-surement, 200 mg of sample was loaded in the speci-men chamber. TPD of the samples was performed usingthe programmable furnace (Thermcraft, Germany) at 2 �Cmin−1 heating rate. Hydrogen absorption/desorption mea-surements were done under isothermal condition at theoperating temperatures of 152, 202 and 262 �C. In orderto understand the catalytic activity of TiO2 in MgH2, thetemperature-programmed reduction (TPR) of the sample

was also carried out at the heating rate of 5 �C min−1 usingTPDRO 1100 series (Thermo Scientific).

3. RESULTS AND DISCUSSION

3.1. Structural and Microstructural Characterization

Figure 1 shows the XRD of the ball-milled MgH2 andMgH2 catalyzed with different sizes of 5 wt% TiO2 (7, 25,50, 100 and 250 nm). The XRD analysis reveals thatthe phase identity is maintained with the ball-milling.Figures 1(G)–(I) represents the XRD patterns of ball-milled, dehydrogenated and after 3 de/re-hydrogenationcycles of MgH2 catalysed with 50 nm TiO2 respec-tively. The XRD peaks in ball-milled sample are indexedwith �-MgH2 tetragonal phase (as shown in Fig. 1(G)).After dehydrogenation, the MgH2 phase has been trans-formed to the hexagonal Mg (shown in Fig. 1(H)). Itmay also be noticed that after dehydrogenation and 3de/re-hydrogenation cycles, some MgO phase is appeared

Fig. 1. XRD patterns of (A) ball-milled MgH2 and MgH2 catalyzedthrough TiO2 nanoparticles of sizes (B) 7 nm (C) 25 nm (D) 50 nm(E) 100 nm (F) 250 nm, (G) ball-milled MgH2+TiO2 (50 nm), (H) dehy-drogenated MgH2 +TiO2 (50 nm), and (I) MgH2 +TiO2 (50 nm) after3 cycles of hydrogenation.

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Pandey et al. Effect of TiO2 Nanoparticles on the Hydrogen Sorption Characteristics of Magnesium Hydride

Fig. 2. (a) Dark field image of MgH2 catalyzed with TiO2 and (b) SEMmicrograph of MgH2 catalyzed with TiO2 (50 nm). Inset of (a) representscorresponding SADP.

(as shown in Fig. 1(I)). It is pointed out that the XRDof 5 wt% TiO2 catalyzed MgH2 does not show any TiO2

diffraction lines. This is possibly due to very small con-centration of TiO2.

Figure 2(a) brings out a representative dark field TEMimage taken with (211) rutile reflection of TiO2 nanopar-ticles. As it can be seen from figure the TiO2 nanoparti-cles (with bright contrast) are nearly uniformly distributedinto MgH2 large particles with the TiO2 nanoparticles ofsize 50 nm. The corresponding selected area diffractionpattern is shown in the inset of Figure 2(a). The diffrac-tion pattern shows spots corresponding to large MgH2

particles and ring pattern for TiO2 nanoparticles. Themicrostructural features of MgH2–TiO2 (50 nm) has alsobeen analyzed from secondary electron image employingSEM (as shown in Fig. 2(b)). At the resolution level ofSEM, TiO2 nanoparticles cannot be visualized. However,the contrast in the SEM picture suggests the presence oflarge particles together with very small particles on andaround the large particles. Presumably large particles areMgH2 and the smaller ones are TiO2 nanoparticles.

3.2. Temperature-Programmed Desorption

Figure 3 brings out the representative TPD curves of MgH2

catalyzed with different sizes of TiO2 nanoparticles as

Fig. 3. TPD profile of MgH2 catalyzed with different sizes of TiO2

nanoparticles. The overall desorption profile is divided in two broad cat-egory (b)–(d) and (e)–(g).

a function of temperature. TPD was carried out at theheating rate of 2 �C/min from room temperature to 450 �C.It can be clearly observed from the TPD curves that thedehydrogenation temperature of MgH2 catalyzed with var-ious sizes of TiO2 (np) has shifted to a lower onset des-orption temperature than that of the ball-milled MgH2

and as-received MgH2. One significant result emanatingfrom Figure 3 is that, desorption from MgH2 catalyzedwith TiO2 (np) of distinct sizes is different. The TPDcurves can be divided into two broad categories. First cat-egory corresponds to the curves (b)–(d), i.e., TPD curvesof the ball-milled MgH2 and MgH2 catalyzed with TiO2

(np) (> 50 nm). Whereas, the second category correspondsto the curves (e–g) i.e., TPD curves of MgH2 catalyzedwith TiO2 (np) (≤ 50 nm). TiO2 (np) with size (≤ 50 nm)exhibit better desorption in regard to lowering of desorp-tion temperature and higher desorption capacity at partic-ular temperature.The onset desorption temperature for the ball-milled

MgH2 as well as for MgH2 catalyzed with TiO2 nanopar-ticles having particle sizes 250 nm and 100 nm (for thefirst category) is found to be ∼ 335 �C. This temperatureis ∼ 55 �C lower than received MgH2, for which the onsettemperature is at ∼ 390 �C. Whereas the onset desorptiontemperature for the second category is up to ∼310 �C andlower. Thus for MgH2 catalyzed with TiO2 (np) of sizes7, 25 and 50 nm, the onset desorption temperatures are300, 305 and 310 �C respectively. The desorption temper-atures for the second category as represented by the curves(e, f and g) are again similar and are within the rangeof ∼ 10 �C. It is clear from figure, the second categoryexhibits a higher degree of lowering of the desorption tem-perature. Here, the lowering of the temperature is in therange of ∼ 80–90 �C as compared to the received MgH2.The onset desorption temperature for the ball-milled MgH2

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Fig. 4. Re-hydrogenation kinetics of MgH2 catalyzed with differentsizes of TiO2 nanoparticles at 250 �C at 30 atm of hydrogen pressure.

is 335 �C. Therefore, as compared to this, the tempera-tures for the second category are lowered by ∼ 25–35 �C.Also, the desorption capacity from MgH2 catalyzed withTiO2 (50 nm) particle is highest, which is ∼ 6.5 wt% of H2.In view of this, TiO2 with 50 nm size may be considered asthe optimum catalyst for hydrogen desorption from MgH2.

3.3. Re-Hydrogenation Kinetics

Re-hydrogenation characteristics of the completely de-hydrogenated MgH2, i.e., re-hydrogenation of magnesiumand TiO2 (np) catalyzed magnesium has been studied. There-hydrogenation was carried out at 250 �C and under 30atmospheres of hydrogen pressure. Figure 4 exhibits repre-sentative re-hydrogenation kinetic curves of Mg and TiO2

(np) catalyzed Mg. As can be seen, the better reabsorptionkinetic is achieved for Mg catalyzed with 50 nm of TiO2.The Mg–TiO2 (50 nm) reabsorbs 5.1 wt% of hydrogenwithin 6 minutes. During the same time period, the sam-ple catalyzed with TiO2 (25 nm) reabsorbs only 4.2 wt%of H2 and the samples catalyzed with 7, 100 and 250 nmof TiO2 reabsorb merely 3 wt% of H2. In addition to re-hydrogenation kinetics, the ultimate hydrogen capacity of5.9 wt% for 50 nm TiO2 catalyzed Mg has been achieved.This is also the highest reabsorption capacity out of theball-milled Mg or TiO2 nanoparticles with sizes 7, 25, 100and 250 nm catalyzed Mg. It can thus be said that evenfor re-hydrogenation, the TiO2 (np) of 50 nm is the mosteffective catalyst.

3.4. Evaluation of Activation Energy

We have also evaluated the activation energy for the re-hydrogenation reaction of MgH2 catalysed with optimumsize of TiO2 (50 nm) by using Arrhenius plot, which ismost suitable model for evaluation of activation energy forthe case of MgH2. Figure 5(a) brings out the representative

Fig. 5. (a) Re-hydrogenation kinetic curves of MgH2 catalyzed withTiO2 (50 nm) at different temperatures and (b) Johnson–Mehl–Avramiplot of MgH2 catalyzed with TiO2 (50 nm). Inset shows the lnk versus1/T plot.

re-hydrogenation kinetic curves of MgH2 catalyzed with50 nm TiO2 at different temperatures 152, 202, 262 �Cand 30 atm of hydrogen pressure respectively. Since, theabsorption curve for MgH2 is not linear (as shown inFig. 5(a)) therefore, the rate constant k was derived byusing the Johnson–Mehl–Avrami equation (Eq. (1))39

�− ln1−��1/n = kt (1)

where, � is the extent of the reaction which can be identi-fied with a normalized hydrogen wt% (range from 0–1), tis the time and k and n are constants (at constant temper-ature).Figure 5(b) shows the plot between ln�− ln1−�� ver-

sus lnt� and the inset represents the corresponding lnk�versus 1/T plot (where k—rate constant derived fromln�− ln1−�� versus lnt� plot). The activation energyfor absorption of TiO2 (50 nm) catalyzed Mg is foundto be ∼ 52.7 kJ mol−1. Whereas, for ball-milled Mg, theactivation energy for absorption is ∼ 92.0 kJ mol−1.40




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Fig. 6. (a) PCI absorption/desorption curves of MgH2 catalyzed withTiO2 (50 nm), and (b) representative the corresponding vant’s Hoff plotof PCI-absorption curves.

Thus, there is significant reduction in activation energy by∼ 39.3 kJ mol−1 for 50 nm TiO2 catalyzed Mg.

3.5. Pressure-Composition Isotherms

Figure 6(a) shows the pressure-composition isotherms(PCI) for absorption/desorption of MgH2 catalyzed with50 nm TiO2 measured up to 20 atm hydrogen at 270, 290,310 and 330 �C, respectively. The sample absorbs uptocapacity of ∼ 6 wt% H2 at 290 �C. The plateau regionswere found to be nearly flat with equilibrium pressures of

Table I. Absorption/desorption plateau pressure and hysteresis loss of MgH2 catalyzed with TiO2 (50 nm).

Absorption Desorption plateau Hysteresis factor Hysteresis (free energy)Temperature (K) plateau (Pa) (Pd) (Hf ) (Pa/Pd) loss 1/2�RT lnPa/Pd�

543.29 1�99 0.65 3�06153846 2527�1786312562.83 3�25 1.69 1�92307692 1530�0735042584.15 5�72 3.18 1�79874214 1425�7173598605.67 8�931 6.09 1�4665 964�05832374

∼ 1.99, ∼ 3.26, ∼ 5.72 and ∼ 8.931 atm at 270, 290, 310and 330 �C, respectively.

With the increase of temperature, the hysteresis loss getsdecreased for MgH2-TiO2 (50 nm). The hysteresis factorHf � described in this study has been determined usingEq. (2).

Hf = 1/2RT lnPa/Pd� (2)

where, Pa and Pd are the absorption and desorption plateaupressures, respectively.Table I shows the calculated hysteresis factor and free

energy loss between formation and dissociation of MgH2

catalyzed with TiO2 (50 nm).From the PCI-absorption plateaus, using vant’s Hoff

equation (Eq. (3)), we have determined the thermodynam-ical parameter enthalpy of formation (�H ).

lnP = �H/RT −�S/R (3)

The enthalpy of formation of MgH2 when catalyzed with50 nm TiO2 was determined from the plot of lnP versus1/T . The vant’s Hoff plot for MgH2 formation is derivedfrom Figure 6(a). The corresponding representative vant’sHoff plot is shown in Figure 6(b). The slope of the vant’sHoff plot gives the formation enthalpy of MgH2. It hasbeen evaluated to be 66.50 kJ mol−1. Thus catalyzingwith 50 nm TiO2, the formation enthalpy of MgH2 getsdecreased by ∼ 10 kJ mol−1.41 It can therefore be said that50 nm TiO2 may be an effective catalyst for production ofMgH2 from Mg.

3.6. Feasible Catalysis Process of TiO2

In the present sorption studies, MgH2 catalyzed with TiO2

(50 nm) is annealed up to a temperature of ∼ 340 �C, theTiO2 is expected to undergo reduction leading to formationof non-stoichiometric TiO2. In order to check the reduc-tion of TiO2, we have studied temperature-programmedreduction (TPR) under hydrogen atmosphere at the heat-ing rate of 5 �C min−1. Figure 7 brings out a represen-tative TPR curve of MgH2 catalyzed with TiO2 (50 nm).From Figure 7, it can be observed that reduction of TiO2

in MgH2–50 nm TiO2 system by hydrogen takes place attemperature of ∼ 340 �C. The inset of Figure 7 representsthe TPR characteristics of 50 nm TiO2 under hydrogenatm. From the characteristics curve it can be observed thatthe reduction of TiO2 starts at temperature ∼ 325 �C. Thistemperature is close to the reaction temperature employed




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Fig. 7. TPR profile of MgH2 catalyzed with TiO2 (50 nm) at the heatingrate of 5 �C min−1. The inset shows the TPR curve of 50 nm TiO2.

for the present sorption studies. It may be pointed out thateven at temperatures lower than 340 �C, partial reductionmay take place. Keeping the observed results in the presentinvestigations, it can be said that TiO2 can undergo reduc-tion. TiO2−x being more electronegative (Pauling scaleelectronegativity of TiO2 is 1.9 and for MgH2, it is 1.4),

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will weaken the Mg H bond by attracting the electronsfrom MgH2. As a result of this, decomposition of MgH2

will occur at a lower temperature leading to fast desorptionkinetics. While for absorption, the reverse reaction willtake place, which leads to lower the absorption activationenergy.

4. CONCLUSIONS

Based on the results emanating from the present studies,we can say that the size of the catalyst plays a crucialrole in improving the hydrogen sorption characteristics ofMgH2. For MgH2–TiO2 (np) system, TiO2 nanoparticleswith the size of 50 nm is found to be the most effectivecatalyst. For Mg catalyzed with 50 nm TiO2, the activa-tion energy for rehydrogenation reaction is found to be∼ 52.7 kJ mol−1. This is one of the lowest values reportedfor catalyzed MgH2. Thus, 50 nm TiO2 may also be usedfor large-scale production of MgH2 from Mg.

Acknowledgments: Thanks are due to MNRE (MissionMode Hydrogen Storage Project), UGC, DST and DAE(HWB) for financial support. Thanks are also due to Pro-fessor B. Vishwanathan, Professor S. Srivanasa Murthy,Professor R. S. Tiwari, Dr. M. A. Shaz and Dr. T. P. Yadavfor helpful discussions.

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Received: 30 January 2013. Accepted: 4 March 2013.


Effect of TiO2 Nanoparticles on the Hydrogen Sorption Characteristics of Magnesium Hydride

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