Stability of Catalyzed Magnesium Hydride Nanocrystalline During

Article pubs.acs.org/JPCC

Stability of Catalyzed Magnesium Hydride Nanocrystalline During Hydrogen Cycling. Part II: Microstructure Evolution Chengshang Zhou,† Zhigang Zak Fang,*,† Robert C. Bowman, Jr.,† Yang Xia,† Jun Lu,‡ Xiangyi Luo,‡ and Yang Ren§ †

Department of Metallurgical Engineering, The University of Utah, 135 South 1460 East, Room 412, Salt Lake City, Utah 84112-0114, United States ‡ Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South, Cass Avenue, Argonne, Illinois 60439, United States § X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, 9700 South, Cass Avenue, Argonne, Illinois 60439, United States ABSTRACT: In Part I, the cyclic stabilities of the kinetics of catalyzed MgH2 systems including MgH2−TiH2, MgH2− TiMn2, and MgH2−VTiCr were investigated, showing stable kinetics at 300 °C but deteriorations of the hydrogenation kinetics at temperatures below 150 °C. The present Part II describes the characterization of uncycled and cycled catalyzed MgH2 by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) analysis. XRD analysis shows the crystallite sizes of the Mg and MgH2 significantly increased after the cycling. The mean crystallite sizes of the catalysts (TiH2 and VTiCr) increased moderately after the cycling. SEM and TEM imaging were used to compare the microstructures of uncycled (as-milled) and cycled materials, revealing a drastic change of the microstructure after 100 cycles. In particular, results from energy-dispersive spectroscopy (EDS) mapping show that a change of distribution of the catalyst particles in the Mg and MgH2 phase occurred during the cycling.

1. INTRODUCTION In Part I,1 the cyclic stability of the kinetic behavior of catalyzed MgH2 was investigated using the pressure−composition− isothermal (PCI) method. The cycling measurements demonstrated that at high temperature (300 °C) the hydrogenation and dehydrogenation kinetics were stable after the hydrogen cycling for 100 times. Results also showed that the lowtemperature (25−150 °C) hydrogenation kinetics suffered a severe degradation after the cycling. For the systems doped with different catalysts (TiH2, TiMn2, and VTiCr), the degradations of room-temperature hydrogenation kinetics are similar. Among the three systems, the MgH2−VTiCr exhibited slightly better cyclic properties. Comprehensive analysis from Part I indicates that the low-temperature kinetic degradation is mainly attributed to the extended hydrogenation−dehydrogenation reactions. The underlying mechanism of the kinetics degradation, however, could not be deduced from these experiments alone. To understand the changes of the kinetics during the cycling, it is necessary to first review the prior hypothesis of the catalytic effect on MgH2. According to the reported literature,2−4 hydrogenation of magnesium involves several steps taking place in sequence: (1) hydrogen molecule absorption and dissociation on the surface of particles, (2) H diffusion in the metal, and (3) nucleation and growth of the hydride phase. Since the diffusion rate of hydrogen through Mg metal is sufficiently high,2,5,6 the rate-limiting step for hydrogenation is likely to be © XXXX American Chemical Society

the step (1) or (3). When magnesium hydride desorbs hydrogen, a nearly reverse order can be defined,6 namely: (1) nucleation of Mg and transformation from the hydride to the metal phase, (2) movement of H atoms to the surface by diffusion, and (3) association of hydrogen atoms to gaseous hydrogen molecules. For the dehydrogenation process, kinetics and modeling studies suggested that the reaction rate is controlled by three-dimensional growth of Mg nuclei, as the best fitting provided by the use of the Johnson−Mehl−Avrami (JMA) model.7−11 Although the role of the catalyst is still unclear and under debate, it is considered that catalysts can enhance the hydrogen dissociation on the surface12 and/or reduce the activation energy of nucleation and growth of nuclei.13 In order to achieve a fast kinetic rate, a nanostructure MgH2−catalyst composite having effective catalytic nanoparticles homogeneously dispersed is favorable.14−20 In order to clarify the mechanism of the cycling behavior of the kinetics, a structure analysis of the MgH2 material in terms of size effect, catalyst location, morphology, and other factors is necessary. In this paper, TEM, SEM, and XRD were used to analyze milled and cycled products of the catalyzed MgH2 systems. Received: June 28, 2015 Revised: September 4, 2015

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DOI: 10.1021/acs.jpcc.5b06192 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. X-ray diffraction patterns of the as-milled, partially hydrogenated, fully hydrogenated, and dehydrogenated Mg−TiH2 system. (a) MgH2−5 mol % TiH2 system and (b) MgH2−5 mol % VTiCr system. Crystal structures: Mg: Hexagonal, P63/mmc; β-MgH2: Tetragonal, P42/mm; γ-MgH2: Orthorhombic, Pbcn; VTiCr: Cubic, Im3̅m; TiH2: Cubic, Fm3m ̅ .

2. EXPERIMENTAL SECTION MgH2 with additives in 5% molar ratios were milled using a custom-made ultrahigh-energy−high-pressure (UHEHP) planetary ball milling machine under a 150 bar hydrogen pressure.

The ball-to-powder ratio was 10:1 by volume, and the milling was carried out for 4 h at room temperature. All material handling was carried out in a glovebox filled with purified argon (99.999%), with water vapor and oxygen concentrations both at B


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background subtraction, peak integration, and use of the particular set of relative sensitivity factors on our instrument.

less than 1 ppm. The synthesis of materials as well as the apparatus and procedures for the cycling experiments and kinetic measurements were presented in detail in Part I.1 After cycling experiments, the samples were taken out from the Sieverts-type machine (Hy-energy LLC. PCTPro 2000) and preserved in the glovebox. A partially hydrogenated sample was prepared by interrupting the hydrogen sorption process at reaction about 3 wt % hydrogen content. The HE-XRD measurements were carried out at the 11-ID-C beamline of the Advanced Photon Source (APS), Argonne National Laboratory. The X-ray wavelength used in the 11-IDC beamline was 0.413684 Å. The XRD patterns were collected in the transmission mode using a PerkinElmer large area detector. The collected 2D patterns were then integrated into conventional 1D patterns (intensity vs 2θ) for final data analysis using the Fit2d software. Prior to the measurements, all samples were completely covered with kapton tape as a protective film in the glovebox to minimize undesired reactions with an ambient atmosphere. Using the Scherrer equation,21 the crystallite size was calculated based on XRD peak broadening.

3. RESULTS 3.1. XRD Analysis. In order to understand the phase transformation of catalyzed MgH2 during hydrogen cycles, high-resolution (synchrotron) XRD analysis was conducted using MgH2−5 mol % TiH2 samples that were prepared by interrupting the first cycle at different stages of the reaction process. The four MgH2−5 mol % TiH2 samples are as-milled, fully dehydrogenated, partially rehydrogenated, and fully rehydrogenated MgH2−TiH2, and their XRD spectra are shown in Figure 1(a). The partially rehydrogenated sample has approximately 3 wt % hydrogen. It can be seen that the TiH 2 catalyst phase appears to be stable during the dehydrogenation and rehydrogenation process, suggesting that the catalyst did not participate in the dehydrogenation reaction MgH2 = Mg + H2, nor did it participate in the hydrogenation reaction. Moreover, peaks of MgH2 can be found in the spectra of the dehydrogenated specimen, although those peaks are significantly weaker than those of the hydrogenated sample. As reported in Part I,1 the loss of hydrogen capacity can be related to the presence of the unreacted MgH2 phase in the dehydrogenated sample, which indicates that the dehydrogenating transition was not completely achieved in the cycling condition. Figure 1(b) compares the XRD patterns of as-milled and 100 cycled (partially hydrogenated) samples of the MgH2−5 mol % VTiCr system. According to the result reported in Part I,1 the rehydrogenated sample after 100 cycles contained only 0.7 wt % hydrogen, and therefore the majority composition in the sample is Mg. It also can be seen that both as-milled MgH2−5 mol % VTiCr and MgH2−5 mol % TiH2 present the significantly broadened peaks compared to those of the cycled samples, indicating a much finer crystalline size and high lattice disorder due to the high energy ball milling. On the basis of the XRD peaks broadening, the average crystallite sizes of the samples were estimated and are listed in Table 1. The average crystallite sizes of MgH2 from the as-

D = kλ /β cos θ

where D is the mean size of the crystallite; k is a shape factor which is taken as 0.9; λ is the X-ray wavelength; β is the full width half-maximum of the peak (fwhm) in radians with the instrumental broadening effect subtracted; and θ is the Bragg angle. Lattice parameters were analyzed using GSAS software.22 The scanning electron microscopy (SEM) analysis was performed on an FEI NovaNano 630, which was equipped with a high-resolution field emission gun. The SEM powder samples were prepared by dispersion of the powders onto a carbon tape supported by a sample stage. The microstructure characterization of the materials was also carried out using transmission electron microscopy (TEM). TEM, STEM, images and energydispersive X-ray spectroscopy (EDS) spectra were collected using a JEOL 2800 scanning transmission electron microscope, operated at 200 kV with a 1.0 nm probe size. The EDS spectra and spectral maps were acquired in STEM mode using a dual SDD detector system. To analyze the EDS spectra, the software NSS 3.2 (Thermo Fischer Scientific) was used. The mapping result was extracted using the net counts method. Before the TEM analysis, the powder samples were dispersed in heptane to create a suspension. A drop of solvent with powder was placed onto a carbon film supported by a copper grid. Then vacuum was applied to evacuate the heptane. The above operation was done in an Ar glovebox before TEM analysis. The samples were exposed to air for a very short time during loading onto the TEM holder. X-ray photoelectron spectroscopy (XPS) was performed with a Kratos Axis Ultra DLD instrument, using a monochromatic Al Kα (1486.7 eV) X-ray source and an analysis area of 300 × 700 μm. Wide-energy survey spectra were collected at a pass energy of 160 eV, with high-resolution spectra of individual photoelectron peak areas collected at 40 eV. In both cases, the hemispherical analyzer was operated in FAT (Fixed Analyzer Transmission) mode. Spectra were collected after 60 s Ar+ etching. For Ar+ etching, the ion gun was run at 4 keV to facilitate rapid removal of sample material. By using the program SRIM (Stopping and Range of Ions in Matter), we can estimate that the amount of material removed from the surface after Ar+ sputtering is on the order of 10 nm. Quantification was performed by using the standard method of Shirley-type

Table 1. Estimated Average Crystallite Size of Uncycled and Cycled Samples average crystallite size, nm samples MgH2−TiH2

MgH2−VTiCr

process status as-milled dehydrogenated, 300 °C partially rehydrogenated, 300 °C fully rehydrogenated, 300 °C as-milled after 100 cycles, Partially hydrogenated at rt

Mg

MgH2

catalyst

− 184.6 215.4

8.5 − 195.1

9.7 15.1 17.4

−

225.8

14.9

− 319.0

10.5 48.5

8.6a 9.4a

a Calculation of VTiCr crystallite size used in spectra of pure vanadium.

milled MgH2−VTiCr and MgH2−TiH2 are 8.5 and 10.5 nm, respectively. After as-milled MgH2−TiH2 was dehydrogenated, the Mg crystallite size (dehydrogenated MgH2−TiH2) is 184.6 nm, and the crystallite size of the TiH2 catalyst is 15.1 nm. Further, the sample rehydrogenated at 300 °C had an average MgH2 crystallite size around 200 nm. For the MgH2−VTiCr C


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The Journal of Physical Chemistry C Table 2. Estimated Lattice Parameters of Mg and MgH2 in the Uncycled and Cycled Samples lattice parameter, Å Mg samples MgH2−TiH2

MgH2−VTiCr

MgH2

process status

a

c

a

c

as-milled dehydrogenated, 300 °C partially rehydrogenated, 300 °C fully rehydrogenated, 300 °C as-milled after 100 cycles, partially hydrogenated at rt

3.211 3.211 3.214 3.210 3.210

5.213 5.214 5.217 5.212 5.212

4.504 4.517 4.517 4.517 4.510 4.517 4.517

3.031 3.020 3.021 3.021 3.017 3.020 3.021

standard lattice parameters, ref 23

Figure 2. SEM images of as-milled MgH2−VTiCr and the sample after 1 cycle (hydrogenated) and 100 cycles (dehydrogenated).

and VTiCr) since the MgH2 lattice of MgH2−TiH2 shrinks in the a direction and expands in the c direction while the MgH2 lattice of MgH2−VTiCr shrinks in the c direction and expands in the a direction. Table 2 also shows that after dehydrogenation and during rehydrogenation the lattice structures of Mg and MgH2 phases present no change and are consistent with reported pure Mg and MgH2 crystal structures.23 3.2. SEM Imaging. Figure 2 provides the high-resolution SEM images, showing the evolution of the microscale morphology of VTiCr catalyzed MgH2 during the cycling. It can be found that the microstructure of the material radically changed after 100 cycles. First, the aggregate size of as-milled sample and the sample after 1 cycle is in a range of several microns to 20 μm. After 100 cycles, the size of the aggregates was in a range of 30 to 60 μm, showing an agglomeration of the aggregates. Second, the microstructure of the sample after 100 cycles changed to a porous, sponge-like morphology. Furthermore, it is interesting that a large number of secondary particles with a size of few nanometers was found on the surface of the material after 100 cycles. These bright secondary particles are likely to be the catalyst phase containing V and Ti transition metals. 3.3. TEM and EDS Analysis. Figure 3(a) and (b) compares TEM micrographs of as-milled MgH2−TiMn2 and the sample after 100 cycles. The significant morphology difference between as-milled and cycled samples shows a similar morphology from the SEM observation (see Figure 2). Compared to the as-milled sample, the aggregate after cycles presents an irregular edge, which is decorated with the nanosized particles. Figure 4 shows

system, the crystallite size of Mg increases to 319.0 nm after 100 hydrogen cycles at 300 °C. However, the size of MgH2 crystallite was 48.5 nm, probably because the hydrogenation was performed at room temperature (25 °C) and low pressure (1 bar). It has to be pointed out that the accuracy of estimating a material with crystallite sizes above 200 nm based on XRD peak broadening is limited. Nevertheless, there is no doubt that the sizes of Mg or MgH2 crystallites after one dehydrogenation or hydrogenation reaction at 300 °C increased significantly compared to that of the as-milled MgH2. For the catalyst particles, the growth after the cycling is moderate. The mean size of TiH2 catalyst particles increased from 9.7 nm to about 15 nm after the dehydrogenation and hydrogenation reactions. The size of the VTiCr catalyst maintained relatively stable, i.e., 8.6 nm from the as-milled sample compared to 9.4 nm from the sample cycled 100 times. It has to be pointed out that the MgH2−VTiCr system showed better cyclic performance at room temperature than that of MgH2−TiH2, which can be attributed to the fact that the nanosized VTiCr catalyst was more stable during the cycling. Using the XRD spectra present in Figure 1, lattice parameters of MgH2 and Mg for the as-milled and cycled samples were extracted, as listed in Table 2. It shows that the lattice parameters of as-milled MgH2−TiH2 and MgH2−VTiCr systems shifted compared to the reference data.23 These differences can be attributed to lattice strain that was created due to high-energy milling together with the effect of additive. It is interesting to observe that the directions of lattice strains were different under the effect of the different catalysts (TiH2 D


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particles was in a range from a few nanometers to 100 nm. Few large catalyst grains with a size of about 100 nm are embedded in the particle. Despite the size difference of catalyst grains, in general, the catalysts are homogeneously distributed over the milled MgH2. The microstructure is similar to that of ballmilled MgH2−NbHx, which was reported earlier by Zhang et al.20 Furthermore, STEM and EDS analysis for the same system (MgH2−TiMn2) after 100 cycles was conducted, as shown in Figure 6. A feature observation from the mapping results is a significant change of the distribution of the catalyst particles in the Mg matrix. For example, the catalyst, despite diversity of its size, presents a relatively uniform dispersion in the as-milled aggregates. For the cycled sample on the other hand, there are large areas (circled in Figure 6) showing significant concentrations of Ti, V, and Mn. Several large Mg particles (>1 μm) were found that show a low concentration of the catalytic elements compared to that of the catalyst-rich region. Figure 7 shows detailed EDS mapping of as-milled MgH2− VTiCr (Figure 7a) and the sample that cycled 100 times (Figure 7b), as well as the SEI and TEM images for the same area. The evolution of the microstructure and the catalyst distribution during cycling was further confirmed. In Figure 7a, it is noted that some catalyst particles (arrowed) cannot be found on the SEI image, implying that these catalysts are embedded or on the other side of the aggregate. In contrast, for the cycled aggregate, most of the catalytic element-rich area can be correlated to the surface features of the aggregate (e.g., the areas arrowed in Figure 7b), implying these catalysts appeared on the surface of the cycled sample. The EDS mapping of the cycled sample (Figure 5) shows accumulative oxygen concentration over the catalyst-rich area, which is opposite to the as-milled sample. The as-milled sample presents a layer of oxide that covers the surface of the MgH2 particles but less oxygen content on the catalyst grains. The present observation of this oxide layer on the as-milled sample confirms results given in previous reports.24−26 Generally, magnesium oxide (MgO) and magnesium hydroxide (Mg(OH)2) are considered to be diffusion barriers that block the hydrogen motion into the interior regions of the grains.27,28 3.4. XPS Analysis. X-ray photoelectron spectroscopy (XPS) analysis was employed to determine the status of the surface of as-milled and cycled MgH2−VTiCr samples. Table 3 provides elemental content of the surface, showing Mg, V, and Cr contents are 98.76, 0.86, and 0.39 wt %, respectively, for the as-milled sample surface, and 96.93, 1.96, and 1.11 wt %, respectively, for the cycled sample surface. Figure 8 shows a comparison of V spectra for the sample before and after the cycling. The result indicates that the content of Mg decreased and the content of catalytic elements (V and Cr) increased after cycling reactions. It can assume that in the as-milled structure some VTiCr catalyst particles exist on the surface, while the rest of the catalysts embedded in the aggregates. After the hydrogen cycling, some previously embedded catalysts appeared on the surface. Another observation from XPS measurement is that the position of the V and Cr peak shifted to a low binding energy after cycling, as shown in Table 3 and Figure 8. This can be attributed to a higher oxidized state of these elements, which coincides with the increased oxygen contents from the EDS results (Figure 6).29

Figure 3. TEM micrographs of a representative aggregate of (a) asmilled MgH2−TiMn2 (hydride state) (b) and MgH2−TiMn2 after 100 cycles (in the partially hydrogenated state).

Figure 4. STEM micrograph and selected area diffraction (SAD) from the same area. (A) As-milled MgH2−VTiCr (hydride state), (b) and MgH2−VTiCr after 100 cycles (in the partially hydrogenated state).

the selected area diffraction (SAD) patterns of the as-milled sample and the same sample cycled 100 times. The pattern for the MgO phase is seen as a ring in the SAD pattern of the asmilled sample, indicating that it is essentially amorphous or nanocrystalline probably as surface films. In the SAD of the cycled sample, rings with a number of spots indexed as MgH2 were observed, indicating a cluster of fine hydride crystallites was formed. Isolated Mg spot reflection is marked in the SAD. Figure 5 shows the high-resolution STEM and EDS analysis of as-milled MgH2−TiMn2. It can be seen that the catalyst was distributed over the matrix of MgH2. The size of catalytic E


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Figure 5. STEM image and EDS mapping of as-milled MgH2−TiMn2 aggregates (hydride state).

Figure 6. STEM images and EDS mapping of MgH2−TiMn2 aggregates (dehydrogenated state) after 100 cycles.

4. DISCUSSION From the TEM images obtained during the present study, we observed microstructure changes on Mg/MgH2−catalyst aggregates as the sample was cycled. It is realized that substrate phases (Mg/MgH2) must undergo significant changes due to the hydrogenation and dehydrogenation reactions during hydrogen cycling, which can be attributed to a substantial volume change between the Mg metal and MgH2 phases. The theoretical density of metallic Mg is 1.74 g/cm3, and the theoretical density of the hydride phase (MgH2) is 1.42 g/ cm3.23 Because of the substantial difference between the densities of Mg and MgH2, a significant volumetric expansion (33.1 vol %) and shrinkage (24.9 vol %) occurs when Mg is hydrogenated and MgH2 was dehydrogenated, respectively. One direct outcome is a severe change of the structure of Mg/ MgH2 after a number of cyclings. More importantly, the catalytic particles will likely suffer repeating stresses from the

surrounding Mg/MgH2 substrate as these reactions take place. The stress might be strong enough so that a “migration” of the catalyst particle is possible. Also the catalyst particle that closely contacted to the Mg/MgH2 substrate might be detached from the substrate due to the shrinkage during dehydrogenation. Figure 9 schematically depicts this hypothetical evolution of one catalyst particle that was attached on the surface of the Mg/MgH2 substrate. After a number of cycles, most of the embedded catalysts are eventually protruded from the substrate, which can lead to a drastic decrease of the catalytic effect. Overall, a schematic picturing a cycling-induced morphology evolution is shown in Figure 10. The figure depicts the course of the microstructure changes during hydrogen cycling. A nanostructure for catalyzed MgH2, which is generally referred to as a composition of nanosized Mg or MgH2 and homogeneously distributed nanoscale catalyst particles, is F


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Figure 7. STEM images and EDS mapping of MgH2−VTiCr aggregates. (A) As-milled sample (hydride state); (B) sample after 100 cycles (dehydrogenated state). BF: Bright-field image. DF: Dark-field image. SEI: Secondary electron image.

the kinetics. In the cycling analysis reported by Tan et al.,32 grain growths from an average 25 to 55 nm, 25 to 50 nm, and 3 to 4 nm, for Mg, MgH2, and Nb−V catalyst, respectively, were observed as the cycles accomplished 200 times. Tan’s result also examined another system of Mg−V, showing faster grain growths and also a faster kinetic degradation compared to Mg− Nb−V. Similarly, the VTiCr catalyst in the present study exhibited a slower growth rate compared to that of the TiH2 catalyst. Consequently, the cycling kinetics of the MgH2− VTiCr system present better behavior over those of the MgH2−TiH2 system. It has been proposed that catalyst phases in MgH2 or Mg may act as a grain growth inhibitor that can prevent the coarsening of Mg or MgH2 crystallites during the course of cycling reactions.33,34 This means that a uniform distribution of the catalysts inside the Mg/MgH2 particle should help preserve the nanosized Mg and MgH2 crystallites.

Table 3. XPS Quantitative Analysis of Mg, V, and Cr on the Surface of As-Milled and Cycled MgH2−VTiCr Samplesa

as-milled

cycled 100 times a

peak

position, binding energy (eV)

fwhm (eV)

raw area (CPS)

mass content, %

Mg 2p V 2p Cr 2p Mg 2p V 2p Cr 2p

50.5 513.0 574.1 46.8 508.1 569.7

2.069 0.843 0.640 5.198 1.180 0.396

14975.3 906.9 472.0 7238.1 1022.7 673.5

98.76 0.86 0.39 96.93 1.96 1.11

The C and O content were not considered in the analysis.

considered as a key factor that improves the kinetics of Mgbased hydride.15,30,31 There is little doubt that losing such a nanostructure for the catalyzed MgH2 results in deterioration of G


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compared to that in the as-milled sample. It is rational to suspect that the catalyst particles become more easily oxidized because they have an increased surface area when a significant amount of catalysts are protruded from the body of Mg/MgH2. Moreover, as the sample is progressively cycled, MgO and Mg(OH)2 on the surface of the MgH2 particles might aggregate to the catalyst particles. If a number of the catalysts were oxidized or surrounded by the inert phases, the catalysts will be deactivated, and therefore the catalytic effect will be diminished. According the cycling results reported in Part I,1 the hightemperature kinetics present a stable behavior as the cycling experiment progressed, in contrast to the deterioration of lowtemperature kinetics after the hydrogen cycling. Although the behavior of high-temperature kinetics should be more robust than those at lower temperatures, such a big difference still remains a question that has not been well elucidated. However, significant changes of the morphology were indeed observed after the hydrogen cycling. It seems that high-temperature kinetics were not vulnerable to the microstructure change. It implies that the roles of the catalyst work differently at the high temperatures and low temperatures.

Figure 8. XPS vanadium 2p (catalyst) spectra for as-milled MgH2− VTiCr samples (hydride state) and cycled MgH2−VTiCr (dehydrogeated state). Literature binding energy ranges (2p3/2 peak) for V, V2O3, and V2O5 are 512.1−512.9 eV, 513.1−515.8 eV, and 516.9− 517.7 eV, respectively.29

5. CONCLUSIONS In conclusion, XRD, SEM, TEM, and XPS characterizations of TiH2, TiMn2, and VTiCr catalyzed MgH2 were conducted to compare their phase compositions and microstructure before and after the cycling. XRD analysis indicates that the crystallite size of as-milled MgH2 was about 10 nm, and the crystallite sizes of Mg and MgH2 significantly increased to over 100 nm after the cycling. The crystallite sizes of TiH2 and VTiCr catalysts also increased during the cycling. SEM and TEM imaging show a drastic change of the morphology of the catalyzed MgH2 after 100 cycles. EDS mapping exhibits a change of the distribution of the catalyst particles in the Mg and MgH2 phase after cycling. In addition, XPS was used to analyze the elemental composition on the surface of the as-milled and cycled samples, showing that a higher concentration of catalytic elements appeared on the surface of the cycled sample. XPS analysis also shows the catalytic elements on the surface of the cycled sample are in a higher oxidation state compared to those in the as-milled sample.

Figure 9. Schematics show the evolution of the morphology of the catalyst particle in relation to the hydrogen-induced volume changes during cycling.

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As the catalysts migrated and agglomerated, it can be expected that the growth rate of Mg and MgH2 crystallites became faster, as illustrated in Figure 10. Another important observation is the oxidization of the catalysts after cycles. It is known that the detected oxygen can be attributed to the cycling experiment or the sample transferring of TEM characterization. However, the different oxygen distribution between the uncycled and cycled sample cannot be fully explained by the TEM sample transferring. Moreover, XPS analysis confirms that the metallic catalysts on the surface of the cycled sample are in higher oxidation states

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the U.S. Department of Energy (DOE) under contract number DE-AR0000173. The TEM work made use of University of Utah shared facilities of the

Figure 10. Schematic depicting the evolution of microstructure of the catalyzed MgH2 particle during cycling. H


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(18) Hanada, N.; Hirotoshi, E.; Ichikawa, T.; Akiba, E.; Fujii, H. SEM and TEM Characterization of Magnesium Hydride Catalyzed with Ni Nano-Particle or Nb2O5. J. Alloys Compd. 2008, 450, 395−399. (19) Reich, J. M.; Wang, L.-L.; Johnson, D. D. Surface and ParticleSize Effects on Hydrogen Desorption from Catalyst-Doped MgH2. J. Phys. Chem. C 2012, 116, 20315−20320. (20) Zhang, L.; Xiao, X.; Xu, C.; Zheng, J.; Fan, X.; Shao, J.; Li, S.; Ge, H.; Wang, Q.; Chen, L. Remarkably Improved Hydrogen Storage Performance of MgH2 Catalyzed by Multi-Valence NbHx Nanoparticles. J. Phys. Chem. C 2015, 119, 8554. (21) Patterson, A. L. The Scherrer Formula for X-Ray Particle Size Determination. Phys. Rev. 1939, 56, 978−982. (22) Rietveld, H. A Profile Refinement Method for Nuclear and Magnetic Structures. J. Appl. Crystallogr. 1969, 2, 65−71. (23) Nayeb-Hashemi, A. A.; Clark, J. B. Phase Diagrams of Binary Magnesium Alloys; ASM International Metals Park: OH, 1988. (24) Grzech, A.; Lafont, U.; Magusin, P. C. M. M.; Mulder, F. M. Microscopic Study of TiF3 as Hydrogen Storage Catalyst for MgH2. J. Phys. Chem. C 2012, 116, 26027. (25) House, S. D.; Vajo, J. J.; Ren, C.; Rockett, A. A.; Robertson, I. M. Effect of Ball-Milling Duration and Dehydrogenation on the Morphology, Microstructure and Catalyst Dispersion in Ni-Catalyzed MgH2 Hydrogen Storage Materials. Acta Mater. 2015, 86, 55−68. (26) Friedrichs, O.; Sánchez-López, J. C.; López-Cartes, C.; Dornheim, M.; Klassen, T.; Bormann, R.; Fernández, A. Chemical and Microstructural Study of the Oxygen Passivation Behaviour of Nanocrystalline Mg and MgH2. Appl. Surf. Sci. 2006, 252, 2334−2345. (27) Hjort, P.; Krozer, A.; Kasemo, B. Hydrogen Sorption Kinetics in Partly Oxidized Mg Films. J. Alloys Compd. 1996, 237, 74−80. (28) Ostenfeld, C. W.; Chorkendorff, I. Effect of Oxygen on the Hydrogenation Properties of Magnesium Films. Surf. Sci. 2006, 600, 1363−1368. (29) Naumkin, A. V.; Kraut-Vass, A.; Gaarenstroom S. W.; Powell C. J. NIST X-ray Photoelectron Spectroscopy Database, NIST: Maryland, 2012. (30) Zhou, C.; Fang, Z. Z.; Lu, J.; Zhang, X. Thermodynamic and Kinetic Destabilization of Magnesium Hydride Using Mg−In Solid Solution Alloys. J. Am. Chem. Soc. 2013, 135, 10982−10985. (31) Zhou, C.; Fang, Z. Z.; Sun, P. An Experimental Survey of Additives for Improving Dehydrogenation Properties of Magnesium Hydride. J. Power Sources 2015, 278, 38−42. (32) Tan, X.; Zahiri, B.; Holt, C. M. B.; Kubis, A.; Mitlin, D. A TEM Based Study of the Microstructure During Room Temperature and Low Temperature Hydrogen Storage Cycling in MgH2 Promoted by Nb−V. Acta Mater. 2012, 60, 5646−5661. (33) Lu, J.; Choi, Y. J.; Fang, Z. Z.; Sohn, H. Y.; Rönnebro, E. Hydrogen Storage Properties of Nanosized MgH2−0.1TiH2 Prepared by Ultrahigh-Energy−High-Pressure Milling. J. Am. Chem. Soc. 2009, 131, 15843−15852. (34) Ponthieu, M.; Cuevas, F.; Fernandez, J. F.; Laversenne, L.; Porcher, F.; Latroche, M. Structural Properties and Reversible Deuterium Loading of MgD2-TiD2 Nanocomposites. J. Phys. Chem. C 2013, 117, 18851−18862.

Micron Technology Foundation Inc. Microscopy Suite sponsored by the College of Engineering, Health Sciences Center, Office of the Vice President for Research, and the Utah Science Technology and Research (USTAR) initiative of the State of Utah. The TEM work made use of University of Utah USTAR shared facilities supported, in part, by the MRSEC Program of the NSF under Award No. DMR-1121252. The authors thank Dr. Brian van Devener for TEM and XPS analysis at Utah Nanofab, University of Utah.

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REFERENCES

(1) Zhou, C.; Fang, Z. Z.; Bowman, R. C. Stability of Catalyzed Magnesium Hydride Nano-Crystalline During Hydrogen Cycling. Part I: Kinetic Analysis. J. Phys. Chem. C 2015, DOI: 10.1021/ acs.jpcc.5b06190. (2) Schimmel, H. G.; Huot, J.; Chapon, L. C.; Tichelaar, F. D.; Mulder, F. M. Hydrogen Cycling of Niobium and Vanadium Catalyzed Nanostructured Magnesium. J. Am. Chem. Soc. 2005, 127, 14348− 14354. (3) Zhou, C.; Fang, Z. Z.; Ren, C.; Li, J.; Lu, J. Effect of Ti Intermetallic Catalysts on Hydrogen Storage Properties of Magnesium Hydride. J. Phys. Chem. C 2013, 117, 12973−12980. (4) Bloch, J.; Mintz, M. H. Kinetics and Mechanisms of Metal Hydrides Formationa Review. J. Alloys Compd. 1997, 253−254, 529−541. (5) Yao, X.; Zhu, Z. H.; Cheng, H. M.; Lu, G. Q. Hydrogen Diffusion and Effect of Grain Size on Hydrogenation Kinetics in Magnesium Hydrides. J. Mater. Res. 2008, 23, 336−340. (6) Drozdov, I. V.; Va, R.; Stover, D. Modelling and Evaluation of Hydrogen Desorption Kinetics Controlled by Surface Reaction and Bulk Diffusion for Magnesium Hydride. RSC Adv. 2015, 5, 5363− 5371. (7) Sabitu, S. T.; Fagbami, O.; Goudy, A. J. Kinetics and Modeling Study of Magnesium Hydride with Various Additives at Constant Pressure Thermodynamic Driving Forces. J. Alloys Compd. 2011, 509, S588−S591. (8) Li, J.; Fan, P.; Fang, Z. Z.; Zhou, C. Kinetics of Isothermal Hydrogenation of Magnesium with TiH2 Additive. Int. J. Hydrogen Energy 2014, 39, 7373−7381. (9) Zahiri, B.; Danaie, M.; Tan, X.; Amirkhiz, B. S.; Botton, G. A.; Mitlin, D. Stable Hydrogen Storage Cycling in Magnesium Hydride, in the Range of Room Temperature to 300 °C, Achieved Using a New Bimetallic Cr-V Nanoscale Catalyst. J. Phys. Chem. C 2011, 116, 3188− 3199. (10) Lang, J.; Eagles, M.; Conradi, M. S.; Huot, J. Hydrogenation Rate Limiting Step, Diffusion and Thermal Conductivity in Cold Rolled Magnesium Hydride. J. Alloys Compd. 2014, 583, 116−120. (11) Mooij, L.; Dam, B. Nucleation and Growth Mechanisms of Nano Magnesium Hydride from the Hydrogen Sorption Kinetics. Phys. Chem. Chem. Phys. 2013, 15, 11501−11510. (12) Du, A. J.; Smith, S. C.; Yao, X. D.; Lu, G. Q. First-Principle Study of Adsorption of Hydrogen on Ti-Doped Mg(0001) Surface. J. Phys. Chem. B 2006, 110, 21747−21750. (13) Danaie, M.; Mitlin, D. TEM Analysis of the Microstructure in TiF3-Catalyzed and Pure MgH2 During the Hydrogen Storage Cycling. Acta Mater. 2012, 60, 6441−6456. (14) Zaluska, A.; Zaluski, L.; Ström-Olsen, J. O. Nanocrystalline Magnesium for Hydrogen Storage. J. Alloys Compd. 1999, 288, 217− 225. (15) Aguey-Zinsou, K.-F.; Ares-Fernandez, J.-R. Hydrogen in Magnesium: New Perspectives toward Functional Stores. Energy Environ. Sci. 2010, 3, 526−543. (16) Ma, T.; Isobe, S.; Wang, Y.; Hashimoto, N.; Ohnuki, S. NbGateway for Hydrogen Desorption in Nb2O5 Catalyzed MgH2 Nanocomposite. J. Phys. Chem. C 2013, 117, 10302. (17) Porcu, M.; Petford-Long, A. K.; Sykes, J. M. TEM Studies of Nb2O5 Catalyst in Ball-Milled MgH2 for Hydrogen Storage. J. Alloys Compd. 2008, 453, 341−346. I


Stability of Catalyzed Magnesium Hydride Nanocrystalline During

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