Nb-Gateway for Hydrogen Desorption in Nb 2 O 5 Catalyzed MgH 2 Nanocomposite

: The state of Nb-contained catalysts in MgH 2 nanocomposites was investigated during the full cycle. X-ray di ﬀ raction (XRD) results showed that Nb 2 O 5 and Nb reacted with MgH 2 during ball-milling, forming NbH 2 and NbH, respectively. In the following dehydrogenation, the (di)hydride decomposed, and Nb was produced. Then NbH was generated in both samples after rehydrogenation. Similar composition in both samples suggests that the catalytic e ﬀ ect follows the same mechanism, the Nb-gateway model, in which Nb facilitates the hydrogen transportation from MgH 2 to the outside. By contrast, NbO remained during the full cycle. Scanning and transmission electron microscopy (SEM and TEM) observations revealed that the Nb 2 O 5 -doped sample tended to be re ﬁ ned in size, compared to the Nb-doped and NbO-doped ones. Nb crystals in the Nb 2 O 5 -doped sample were observed to be highly dispersed in the sample, with 10 − 20 nm in size. Given all that, tiny Nb crystals distributed in the composites worked as the gateway facilitating hydrogen transportation and improving dehydrogenation properties.


■ INTRODUCTION
Hydrogen is a promising alternative energy carrier because of its prominent advantages such as high energy density, great variety of potential sources, light weight, and environmental friendliness; thus it has been highly regarded.As a medium for hydrogen storage, magnesium hydride has been well studied in the past decades.Having notable advantages such as high capacity (7.6 mass %), light weight, and low cost, it is considered as a promising candidate for hydrogen storage materials. 1However, the absorption and desorption processes require high temperatures of 300−400 °C, and the reactions are slow.By ball-milling with some transition metals, the kinetics of the reactions can be improved. 2−4 Huot et al. showed the result that MgH 2 nanoparticles catalyzed by 5 mol % Nb completed full desorption within 300 s at 300 °C. 4 They confirmed the formation of a short-lived metastable NbH x (x ≈ 0.6) phase during dehydrogenation, thus concluded that it acted as a gateway through which hydrogen from MgH 2 released. 5A similar mechanism was claimed later by Li et al. in a density functional theory calculation, in which the substitution of Nb at the Mg site followed by the clustering of H around Nb was a likely pathway for hydrogen desorption. 6On the other hand, Mg−Nb−O perovskite phase was also found in the dehydrogenated composite and believed to help with hydrogen transportation. 7ransition-metal oxides also show the catalytic effect on facilitating hydrogen absorption and desorption in nanostructured MgH 2 . 8−13 Among those catalysts studied so far, Nb 2 O 5 showed a superior catalytic effect.Barkhordarian et al.  reported that MgH 2 catalyzed by 0.5 mol % Nb 2 O 5 and ball-milled for 100 h finished desorption within 90 s at 300 °C. 10 Hanada et al. reported that the composite, MgH 2 and 1 mol % Nb 2 O 5 milled for 20 h, was able to absorb ∼4.5 wt % of hydrogen after full desorption, under a pressure of 1.0 MPa within 15 s at room temperature. 13However, the exact mechanism leading to this remarkable catalytic effect is still unclear.In the recent transmission electron microscopy (TEM) observations, the size of both MgH 2 and Nb 2 O 5 was found to be within nanoscale.Thus the refinement of size and the consequently homogeneous distribution of the additives may be responsible for the fast kinetics. 14,15It was also confirmed that in the cycled composites, Mg−Nb−O phases formed, which accelerated hydrogen desorption by providing special diffusion pathways. 16,17n our previous investigation on the chemical bonding state of Nb in Nb 2 O 5 catalyzed MgH 2 nanocomposites, it was found that Nb 2 O 5 was gradually reduced as the ball-milling time increased.The reduced Nb compounds were considered as the essential catalysts improving the desorption kinetics. 18In the present paper, we aimed at clarifying the exact state of the catalyst during a full cycle.The reactions between MgH 2 and three kinds of Nb-contained catalysts, Nb, NbO, and Nb 2 O 5 , were investigated and compared, in order to figure out the mechanism of the catalytic effect.We increased the amount of the additives to 50 wt % so that we were able to identify the state of the additives exactly at each stage.The morphology and the microstructure of the samples were also compared to draw the conclusion.

■ EXPERIMENTAL SECTION
MgH 2 powder was purchased from Alfa Aesar with a purity of 98%.Powder of Nb 2 O 5 (99.99%),NbO (99.9%), and Nb (99.9%) was purchased from the Kojundo Chemical Laboratory.All materials were used as received.For sample preparation, 150 mg MgH 2 and 150 mg additives (Nb 2 O 5 , NbO or Nb) were sealed into a steel vessel (30 cm 3 inner volume) together with 20 steel balls (7 mm in diameter).After evacuating the vessel, 1 MPa Ar was introduced into it.The mixture was mechanically milled at 400 rpm for 20 h using a planetary ball-milling apparatus (Fritsch P7).Dehydrogenation was performed by annealing the milled composites for 1 h at 300 °C under vacuum.Rehydrogenation was carried out at 200 °C for 2 h, under a 1 MPa H 2 atmosphere.X-ray diffraction (XRD) was performed at each stage, using a Philips X'Pert Pro powder diffractometer with Cu K α radiation.The morphology of the samples was observed via a field emission scanning electron microscope (FE-SEM, JEOL JSM-6500F).TEM observations were carried out using a 200 kV TEM (JEOL JEM-2010) and a 1250 kV high-voltage electron microscope (HVEM, JEM-ARM1300).The plastic bag method 19 was used to prevent oxidation of the samples during transport into TEM instruments.

■ RESULTS AND DISCUSSION
State of the Additives during the Full Cycle.
This result is consistent with the work of Huot et al., in which NbH was also found in the 5 mol % Nb catalyzed MgH 2 nanocomposite. 4The dehydrogenation resulted in the formation of Mg and Nb.Finally, the metals returned to MgH 2 and NbH state after rehydrogenation.Figure 3 shows the XRD profiles of the NbO-doped sample during the full cycle.Unlike the obvious interactions between MgH 2 and the additives in the previous ones, MgH 2 and NbO still remained after ball-milling.After dehydrogenation, the peaks of Mg were seen, due to the decomposition of MgH 2 .The latter reappeared after rehydrogenation.Comparing the XRD profiles during the whole cycle, the peaks from NbO did not change at all.As an aside, an unknown peak at 2θ = 44°, which is close to β-MgO (400), appeared during the full cycle.We have not found any description about the metastable β-MgO in the literature.Whether it helps the dehydrogenation or not is unclear.
Mechanism of the Catalytic Effect in MgH 2 −Nb 2 O 5 Nanocomposite.It must be pointed out that in both Nb 2 O 5doped and Nb-doped samples, the peaks of Nb in the XRD spectra of the dehydrogenated samples were a little shifted toward the lower angle.The positions, however, quite match  the metastable NbH x phase, which is a solid solution described in the references. 5,20For one thing, it reaffirms the gateway model that NbH x facilitates the hydrogen transportation in MgH 2 −Nb composites, described elsewhere. 4,5For another, it suggests the possibility that the Nb 2 O 5 -doped sample may also undergo a dehydrogenation process following the Nb-gateway model, resulting in the formation of NbH x solid solution as it does in the Nb-doped sample.
Comparing the XRD results of Nb 2 O 5 -doped and Nb-doped samples, it can be noticed that the composition was the same after one cycle, except that MgO as the byproduct of the reaction existed in the former.The role of MgO was investigated additionally.(Details can be found in the Supporting Information.)The dehydrogenation peak-temperatures in Nb-doped and Nb-MgO-doped samples are almost the same (254 and 257 °C, respectively, shown in Figure S1), revealing that the addition of MgO, though it makes the same composition as in the Nb 2 O 5 -doped sample, provided no contribution to the desorption property.Furthermore, SEM observations shown in Figure S2 present few changes in the morphology brought by the addition of MgO, compared to the Nb-doped sample.Both the results indicate that the existence of MgO is meaningless for the desorption properties.Therefore, we can safely conclude that the catalytic effect in MgH 2 −Nb 2 O 5 nanocomposites is attributed to the existence of Nb, as it does in the Nb-doped composite.In both samples, the catalytic effect follows the same mechanism during dehydrogenation, which is based on a Nb−gateway model for hydrogen release.First, niobium (di)hydride decomposes rapidly (because niobium hydride is thermodynamically unstable) and Nb forms.Then, hydrogen diffuses from MgH 2 to Nb, forming the NbH x solid solution.This metastable phase is stabilized by the hydrogen flow from MgH 2 to the outside, until MgH 2 exhausts finally.In both samples, Nb plays an important role in hydrogen transportation and acts as an essential catalyst improving the kinetics during dehydrogenation.
The Size Effect in the Desorption.As we stated that the catalytic effect of Nb 2 O 5 and Nb follows the same mechanism, the Nb-gateway model, one would doubt why Nb 2 O 5 -doped and Nb-doped samples exhibited different desorption properties (see Figure S1).In order to figure out the differences between them, we estimated the crystallite size of MgH 2 in the as-milled samples, as well as that of Mg and Nb after dehydrogenation, using the Scherrer equation: where K is the shape factor, typically 0.89, λ is the X-ray wavelength, 1.54 Å for Cu radiation, β is the full width at halfmaximun (fwhm), θ is the Bragg angle, and τ is the mean size of the crystallites. 21For the estimation of MgH 2 , we selected the strongest peak, MgH 2 (110), to apply eq 3.In the case of Mg and Nb, ( 110) and ( 200), respectively, rather than the strongest peaks were chosen, in order to avoid the peak overlapping.The measured fwhm and corresponding crystallite size are listed in Table 1.In order to compare the morphology of the composites, we observed the as-milled and hydrogenated samples by SEM.The typical images are shown in Figure 4.It can be seen that for all the composites, the morphology hardly changed after dehydrogenation.On the other hand, the difference in size can be seen obviously.The Nb 2 O 5 -doped sample shown in Figure 4a,d is distinctly small in size, while the particle size in the other two is much bigger than the former.It indicates that both MgH 2 and the additive could be pulverized more easily in the Nb 2 O 5 -doped composite.The same tendency was also observed in 1 mol % Nb 2 O 5 -doped and 2 mol % Nb-doped, as well as 2 mol % NbO-doped samples, as shown in Figure S2.This could be explained by the physical properties of the additives.As a ceramic, Nb 2 O 5 exhibits hard and brittle  character, while Nb metal and NbO, which show metallic properties, are ductile and not easy to be pulverized.Thus even though following the same mechanism during dehydrogenation as Nb 2 O 5 -doped and Nb-doped composites are, the smaller size leads to a faster reaction rate.In other words, the size effect is considered to play an important role during the catalystpromoted dehydrogenation.Figures 5 and 6 show the typical TEM images of the Nb 2 O 5doped and Nb-doped composites after dehydrogenation.The insets in both figures show the Debye rings of Nb, meaning that Nb is polycrystalline.Nb crystals can be seen in the dark field images taken by selecting parts of Nb (110) rings marked by the white circles in the diffraction pattern.For the Nb 2 O 5doped sample as shown in Figure 5b, it can be seen that Nb crystals, with a size around 10−20 nm, dispersed homogeneously in the Mg matrix.However, in the case of the Nbdoped sample, Nb crystals with a size of ∼100 nm can be observed, as shown in Figure 6b.We also found Nb singlecrystals with a size around 200 nm, shown in Figure 7.The inhomogeneity in the size of the Nb catalyst could hinder the catalytic effect and deteriorate the desorption properties.
Taking advantage of high-resolution electron microscopy, we were able to confirm the existence and the size of NbH 2 and Nb in the 1 mol % Nb 2 O 5 -doped composite.Figure 8a shows the high resolution image of the 1 mol % Nb 2 O 5 -doped composite before dehydrogenation.In the fast Fourier-transformation (FFT) image (Figure 8b) taken from the selected area, a pair of spots corresponding to NbH 2 (111) was identified.The size of NbH 2 was around 20 nm, as can be seen in the inverse fast    The Mechanism of NbO-Catalyst.While strong interaction between MgH 2 and Nb 2 O 5 , as well as Nb, was confirmed, the unexpected stability of NbO is obscure.Considering NbO is hard to be pulverized during mechanical milling, this poor reactivity may be attributed to the size effect.Although a similar reaction, such as MgH 2 + NbO → NbH 2 + MgO, is predicted by thermodynamics, large particle size and poor distribution of NbO limit the kinetics of the reaction.Therefore, NbO itself as the main phase is considered to have the catalytic effect, following some mechanism rather than the Nb-gateway model.In a recent study, NbO (111) was found to show effective catalytic activities, 22 which lend support to such consideration.On the other hand, TEM observations on NbOdoped composite also provide us with a hint to the mechanism of the catalytic effect.In the inset of Figure 10, single-crystal pattern of NbO, as well as MgNb 2 O 3.67 , has been indexed.The dark field image in Figure 10b was taken from MgNb 2 O 3.67 (120).By comparing it with the corresponding bright field image in Figure 10a, we can easily find out the single crystals of MgNb 2 O 3.67 , which show a dark contrast, with a size of ∼200 nm.In addition, a halo ring corresponding to an unknown amorphous phase(s), with the d-spacing range of 2.1−2.6 Å, was seen.It is implied that some reactions between MgH 2 and NbO, with poor kinetics, did occur during the milling process, forming MgNb 2 O 3.67 and the unknown amorphous phase(s).As it was stated previously, the Mg−Nb−O phase was considered to help with hydrogen diffusion.It was pointed out that MgO is almost impermeable for hydrogen. 23The Mg− Nb−O phase was proved able to absorb hydrogen 24 and even improve desorption properties of MgH 2 as a catalyst. 25Here the existence of MgNb 2 O 3.67 may provide pathways for hydrogen diffusion through the MgO scale covered on the surface of MgH 2 , improving the desorption properties during dehydrogenation.The results for MgH 2 catalyzed by 1 mol % Nb 2 O 5 , 2 mol % Nb, 2 mol % NbO, as well as 2 mol % Nb and 5 mol % MgO, including dehydrogenation properties and morphology, can be found in the Supporting Information.This material is available free of charge via the Internet at http://pubs.acs.org/.
Figure 1 shows the XRD profiles of MgH 2 −Nb 2 O 5 nanocomposite during the full cycle.In the as-milled sample, Nb 2 O 5 was not detected.Instead, the peaks from NbH 2 and MgO can be seen, indicating that Nb 2 O 5 reacted with MgH 2 during the ballmilling process.Nb (II) hydride and MgO as the products of the reaction were generated then.The reaction equation can be written as follows: the common phenomenon that MgO is always found in the milled MgH 2 −Nb 2 O 5 composite.Addi-tionally, the reduction of Nb 2 O 5 agrees with our previous work, 18 implying that the reduction product, NbH 2 , may be the essential catalyst that improves the desorption kinetics.According to the mole ratio of the reactants, a small amount of MgH 2 should remain after ball-milling.The absence of the peaks of MgH 2 suggests that the residual MgH 2 is microcrystalline.In the dehydrogenated sample, since both MgH 2 and NbH 2 have decomposed, the XRD profile shows the coexistence of Mg, Nb, and MgO.After rehydrogenation, the peaks of MgH 2 and NbH were confirmed in the XRD profile.It indicates that, after one cycle, the initial additive, Nb 2 O 5 , was converted into the niobium(I) hydride state.Similar reactions were found in the MgH 2 −Nb nanocomposite, except that NbH rather than NbH 2 appeared after ball-milling, as shown in the XRD profiles in Figure 2. The initial additive was changed into NbH following the equation below:
The size of MgH 2 in the Nb-doped and NbO-doped samples after ball-milling was around 70 Å, while that in the Nb 2 O 5 -doped sample was microcrystalline, since the peak was not detected in the XRD spectrum.Therefore, the size of MgH 2 in the Nb 2 O 5 -doped sample is extremely smaller than those in the others.The size of Nb and Mg in the Nb 2 O 5 -doped sample after dehydrogenation was evaluated as 38 Å and 88 Å respectively, roughly half those in the Nb-doped sample (68 Å and 190 Å, respectively).It was suggested that when MgH 2 is milled with Nb 2 O 5 , the crystallites tend to be refined.The decrease in size can lead to a fast decomposition, thus partially explaining the better desorption property of the Nb 2 O 5 -doped sample, compared to the others.On the other hand, being comparable in size compared to the Nb-doped sample, the NbO-doped sample showed better desorption property.It implies that there are other factors determining the desorption kinetics in the NbOdoped sample.

Figure 4 .
Figure 4. SEM images showing the morphology of the as-milled and dehydrogenated composites.

Figure 5 .
Figure 5.Typical TEM micrograph of the Nb 2 O 5 -doped composite after dehydrogenation.(a) Bright field image.(b) Corresponding dark field image taken from the circled area.

Figure 6 .
Figure 6.Typical TEM micrograph of the Nb-doped composite after dehydrogenation.(a) Bright field image.(b) Corresponding dark field image taken from the circled area.

Figure 7 .
Figure 7. TEM images showing Nb particles in the Nb-doped composite after dehydrogenation.(a) Bright field image.(b) Diffraction pattern from the circled area.

■
CONCLUSIONSWe were able to clarify the state of the Nb-contained catalysts in MgH 2 nanocomposites.Nb 2 O 5 and Nb reacted with MgH 2 during the ball-milling process.NbH 2 and NbH, respectively, formed as the products of the reactions.During dehydrogenation, both NbH 2 and NbH decomposed into Nb and returned to the NbH state after rehydrogenation.It is suggested that both Nb 2 O 5 -doped and Nb-doped samples dehydrogenated following the Nb-gateway model in which Nb facilitates the hydrogen transportation from MgH 2 to the outside.On the other hand, reactions between NbO and MgH 2 were not confirmed.The promoted dehydrogenation may either be attributed to the catalytic effect of NbO itself or the existence of the MgNb 2 O 3.67 phase.In addition, we found that the Nb 2 O 5doped sample tended to be refined in size, compared to the Nbdoped and NbO-doped ones, due to the hard and brittle features of Nb 2 O 5 .This size effect partially leads to a better dehydrogenation property in the sample.NbH 2 and Nb, with 10−20 nm in size, were also observed in the composite doped by 1 mol % Nb 2 O 5 , validating the Nb-gateway model in the MgH 2 −Nb 2 O 5 system.All the results reveal that tiny Nb crystals highly dispersed in the composites act as the essential catalyst and improve the desorption properties following the Nb-gateway model.■ ASSOCIATED CONTENT * S Supporting Information

Figure 8 .
Figure 8.(a) High-resolution image, (b) FFT, and (c) IFFT images of the MgH 2 and 1 mol % Nb 2 O 5 composite after ball-milling.The FFT area is marked by the square in panel a.

Figure 9 .
Figure 9. (a) High-resolution image, (b) FFT, and (c) IFFT images of the MgH 2 and 1 mol % Nb 2 O 5 composite after dehydrogenation.The FFT area is marked by the square in panel a.

Figure 10 .
Figure 10.Typical TEM micrograph of the NbO-doped composite.(a) Bright field image.(b) Corresponding dark field image taken from the circled area.

Table 1 .
Crystalline Size (τ; in Å) of MgH 2 , Mg, and Nb in Each Sample