In Situ X-ray Absorption Spectroscopy–X-ray Diffraction Investigation

Article pubs.acs.org/JPCC

In Situ X‑ray Absorption Spectroscopy−X-ray Diffraction Investigation of Nb−H Nanoclusters in MgH2 during Hydrogen Desorption C. Maurizio,*,† R. Checchetto,‡ A. Trapananti,§ A. Rizzo,§ F. D’Acapito,§ and A. Miotello‡ †

Physics and Astronomy Department and CNISM, University of Padova, via Marzolo 8, I-35131 Padova, Italy Physics Department, University of Trento, via Sommarive 14, 38123 Povo (Trento) Italy § CNR-IOM-OGG c/o ESRFThe European Synchrotron, 71 Avenue des Martyrs, CS 40220 F-38043 Grenoble, France ‡

ABSTRACT: A combined in situ X-ray diffraction (XRD) and Xray absorption spectroscopy (XAS) experiment was carried out to monitor hydrogen desorption from NbH0.9 nanoclusters embedded into MgH2. Just after the MgH2 → Mg transition, a NbHx bcc nanophase is detected, whose lattice parameter measured by XRD is significantly longer than the one inferred from XAS. This difference is explained considering the broad niobium hydride cluster size distribution and in particular the fact that the XRD signal, differently from the XAS one, is dominated by larger NbHx crystalline structures. The results indicate that, during the hydride to metal phase transformation of the matrix, the NbHx cluster composition depends on the cluster size. It is shown for the first time for embedded nanoparticles that faster (and complete) Nb dehydrogenation is favored for small (1.5−4 nm) clusters with respect to larger (∼20 nm) ones. The role of the matrix and of the annealing atmosphere in the stability of the Nb-related nanophases is discussed.

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INTRODUCTION Fast hydrogen sorption kinetics in Mg-based nanocomposite materials for hydrogen storage are obtained by mixing nanostructured transition metal or transition metal oxide nanoclusters (NCs) with the hydride forming matrix.1−5 The metal dopant acts as catalyst, significantly improving the sorption kinetics of the Mg-based matrix. The cluster−matrix interaction certainly plays a crucial role in different processes occurring at nanoscale level during sorption, such as the nucleation of the Mg phase in the hydride matrix, the onset of gateways for fast H transport,6 and the stability of the different hydrogen-related nanophases.7,8 The dynamics of these processes also depend on the NC catalyst size: a faster hydrogen desorption kinetics from nanostructured MgH2 has been observed when the Nb catalyst was dispersed into the matrix in the form of ∼1 nm NCs rather than in larger (∼15− 20 nm) particles.9 More recently, a size-dependent hydrogen sorption kinetics was observed for pure, unsupported Pd clusters, being faster for smaller NCs.10,11 Anyway, to optimize the catalytic effect of metal nanoclusters in practical hydrogen storage materials, the catalytic clusters are in contact with and often embedded into the light matrix. Therefore, due to the mutual interaction of the nanocatalyst with the matrix,12 both undergoing to hydrogen sorption processes,13 the behavior of unsupported nanocatalysts cannot be transferred directly to embedded clusters. At present few data are available on © 2015 American Chemical Society

hydrogenation/dehydrogenation of embedded nanocatalysts, despite its relevance for a through understanding and optimization of composite nanomaterials for hydrogen storage. In this paper we have monitored by in situ synchrotron radiation X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) the structure of niobium hydride nanoclusters embedded into a MgH2 matrix during the MgH2 → Mg phase transformation. In the presence of a broad cluster size distribution of the nanocatalyst, both techniques give information on the NC related structure, being complementary in terms of sensitivity to the cluster size. In fact, the XRD peak broadening of smaller structures combined with their lower diffracting power makes their signal less evident, while XAS measures the average short-range structure around Nb sites irrespective of the cluster size. With respect to XRD, XAS thus brings information on the smaller clusters, when a large fraction of them is present. The combined results allow monitoring of the structural evolution of the nanocatalyst (large and small Nb−H particles) as well as the role of the matrix and of the annealing atmosphere in the stability of the Nb-related nanophases. Received: January 9, 2015 Revised: March 20, 2015 Published: March 20, 2015 7765

DOI: 10.1021/acs.jpcc.5b00252 J. Phys. Chem. C 2015, 119, 7765−7770

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The Journal of Physical Chemistry C

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EXPERIMENTAL AND DATA ANALYSIS Nb (5 atom %)-doped Mg films were deposited by magnetron sputtering and activated with a series of sequential hydrogen sorption cycles at T = 350 °C. A combined in situ XRD and XAS experiment was performed at the Italian beamline GILDA of the European Synchrotron ESRF (Grenoble, France). The monochromator was equipped with a couple of Si(311) crystals and worked in dynamic focusing configuration. Harmonics rejection and vertical focusing were achieved by a couple of Ptcoated mirrors (incidence angle = 2.7 mrad). Nb-doped MgH2 powder was inserted into a 1 mm quartz capillary, and in vacuum conditions, the temperature was linearly increased by a N2 gas blower up to about 350 °C (ramp step, time duration of 1 h), where the MgH2 → Mg phase transition occurs. The sample temperature was then maintained constant (annealing step) and the evolution of the sample structure was monitored by both XAS and XRD, using the setup sketched in Figure 1. A

parameters, the ratio of the two coordination numbers CN1/ CN2 was set to the bulk value (8/6), as well as the ratio of the corresponding interatomic distances R1/R2 (2.86 Å/3.31 Å). The same Debye−Waller parameter was used for both coordinations; the use of an asymmetric interatomic distance distribution was also tested but did not lead to a significant improvement of the fit.

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RESULTS AND DISCUSSION In Figure 1 a typical two-dimensional (2D) XRD collected pattern is reported: the diffraction rings recorded indicate that the crystallites pertaining to both the matrix and the catalyst are randomly oriented. In Figure 2a the radially integrated XRD

Figure 1. Experimental setup for the in situ XAS−XRD experiment.

Figure 2. (a) XRD radially integrated powder diffraction patterns recorded during annealing at 350 °C, before (1) and after (2) MgH2 to Mg phase transition. (b) Zoom in the range 2θ = 14.5−17.0° of the spectrum recorded after the MgH2 → Mg phase transition (data indicated by symbols, fit by a solid line). The positions of the (110) diffraction peaks for the bcc Nb and NbH0.9 phases are also marked.

very similar treatment was also performed in air (open capillary), where the MgH2 → Mg transition was found to occur at higher temperature (about 370 °C). XRD patterns were recorded every ∼2 min with the X-ray beam at λ = 0.653 Å and a MAR3450 2D detector. An XRD pattern of LaB6 crystalline standard compound was also measured and used for calibration. Nb K-edge XAS spectra were recorded in fluorescence mode during annealing using a Si PIN photodiode. Each XAS spectrum lasted ∼15 min. To measure the XAS spectrum before the MgH2 → Mg phase transition, the sample temperature was kept about 10−20 °C below the transition temperature for about 15 min. No difference was observed between the two XRD patterns recorded immediately before and immediately after each XAS spectrum presented. The XRD analysis of the radially integrated diffraction patterns was performed with a Rietveld refinement by using the MAUD software.14 The crystallite size was estimated by the Scherrer formula, accounting for the instrumental broadening (obtained by measuring the powder diffraction pattern of a LaB6 standard with the same setup). The XAS analysis, done by using the IFEFFIT package,15 was based on the first shell fitting of the two Nb−Nb coordinations present in the NbHx solid solution bcc phase.9,13 It has to be remarked that the XAS signals from orthorhombic and bcc NbH0.9 structures are too close to be distinguished. To reduce the number of fitting

patterns recorded at T = 350 °C just before and after the MgH2 → Mg transition are shown. Before hydrogen desorption Mg is mainly in MgH2 crystalline phase, with a minor oxidized (MgO) fraction. The other visible crystalline phase is compatible with orthorhombic NbH0.9, as previously observed on similar systems.7,13,16 After hydrogen desorption the major part of the matrix is metallic Mg, while the positions of the diffraction peaks corresponding to the Nb-related phase gradually shift to higher scattering angles, indicating progressively smaller interplanar distances, connected with a partial hydrogen desorption from the catalyst during isothermal annealing. The five diffraction peaks recorded, pertinent to the Nb-related phase, can be indexed as a randomly oriented NbHx bcc phase, for which a Debye−Scherrer analysis indicates an approximate grain size of 20 nm that does not vary significantly during the desorption process. Moreover, during isothermal annealing the peak positions of the Mg and MgH2 phases do not change. In Figure 2b a zoom (data and fit) of the diffraction range where the signal from the Nb-related clusters 7766

DOI: 10.1021/acs.jpcc.5b00252 J. Phys. Chem. C 2015, 119, 7765−7770

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The Journal of Physical Chemistry C is more intense is reported, measured after the MgH2 → Mg phase transition. The evolution of the different crystalline fractions and of the NbHx lattice parameter a during the annealing step are reported in Figure 3. We note that although the MgH2 → Mg

Figure 4. (a) Nb K-edge XAS spectra for Nb-doped nanostructured MgH2 recorded in situ during hydrogen desorption, compared to the ones recorded at room temperature (RT) before and after desorption (gray). Vertical lines indicate the k-position of the main features. The Nb thermal expansion with respect to RT (Δa/a ∼ 0.25%) is below our XAS sensitivity. (b) Data (symbols) from sample A2 and corresponding first shell fit (solid line, fitting range k = 3.5−11.0 Å−1) in the first shell filtered k-space. The first shell signal simulated from a Nb structure with a lattice parameter of 3.366 Å (i.e., XRD result) is also shown for comparison (dashed line).

Figure 3. (a) Fraction of the different crystalline phases (±10%) related to the matrix and (b) lattice parameter (±0.005 Å) of the NbHx phase (from XRD) during hydrogen desorption. The beginning of XAS acquisitions are marked. t = 0 marks the beginning of the annealing step.

Table 1. Results of the EXAFS Analysis on Nb−Nb First Two Correlations, Recorded during Annealing of Nb-Doped MgH2 at 350 °Ca

transition was not complete at the end of the process, likely due to air exposure prior to the in situ studies, the duration of the desorption process of the matrix is well comparable to that measured at the same temperature in samples never exposed to air.17 After hydrogen desorption the lattice parameter of bcc NbHx is a = 3.366 ± 0.005 Å. Considering that the lattice parameter for bulk NbHx phase is (almost) linearly dependent on the hydrogen concentration in the alloy,18 the estimated lattice parameter corresponds to a NbH0.4 solid solution alloy. A metastable phase similar to this one was also detected in ballmilled Nb-doped Mg powders upon in situ desorption.7,16 With respect to the ball-milling system, here the matrix grain size is larger (about 70 nm) and the kinetics is slower, allowing monitoring of the catalyst structure by XAS. In Figure 4a the XAS spectra recorded at T = 350 °C before (spectrum A1) and after (A2) the MgH2 → Mg transition are compared to the XAS spectra measured ex situ at room temperature before and after desorption and relative to NbH0.9 NCs and Nb NCs, respectively:13 the signal of the Nb−Nb correlations is pretty visible in both cases. The positions of the main features in k-space are similar for sample A1 (A2) and for NbH0.9 NCs (Nb NCs), suggesting similar interatomic distances. XAS results are reported in Table 1. The first shell signal and superimposed fit for the spectrum measured just after MgH2 → Mg are reported in Figure 4b. In Figure 5 values of the lattice parameter measured by XAS and XRD are compared. Before hydrogen desorption the XAS estimation of the interatomic distances indicates a lattice parameter of 3.44 ± 0.02 Å, in agreement with XRD results and with XAS analysis on NbH0.9 NCs.13 The Debye−Waller factor of the Nb−Nb correlation (Table 1) is significantly higher than the one measured at room temperature13 because of thermal vibrations. Just after the MgH2 → Mg transformation the Nb−Nb interatomic distances are those of bulk Nb (a = 3.30 Å, see Table 1), not compatible with the value of 3.366 Å measured by

sample

CN1; CN2

A1 A2 B Nb NbH0.9

5.2; 3.9 ± 2.0 2.1; 1.6 ± 0.3 2.3; 1.8 ± 0.5 8; 6 8; 6

R1; R2 (±0.02 Å) 2.99; 2.86; 2.90; 2.86; 3.00;

3.44 3.30 3.33 3.31 3.45

σ2 (×10−3 Å2) 17 ± 4 11 ± 2 16 ± 2

CN is the coordination number, R is the interatomic distance, and σ2 is the Debye−Waller factor. Crystallographic data for bcc Nb and for NbH0.9 are reported for comparison. a

Figure 5. Lattice parameter a of the NbHx bcc phase measured by XAS and XRD during annealing. Data measured at room temperature before and after complete dehydrogenation are also reported (stars); the solid line is a guide for the eye.

XRD immediately before acquiring the XAS spectrum (see also Figure 3b). In Figure 4b the first shell XAS spectrum simulated for clusters with the lattice parameter estimated by XRD is compared to the corresponding experimental signal recorded after MgH2 → Mg, showing that XAS and XRD estimations for 7767

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relevant contribution from smaller clusters. That is, both XAS and XRD probe the same cluster population, but with different intrinsic sensitivities to long-range (XRD) or short-range (XAS) order. Therefore, the lattice parameter measured by XAS is pertaining mostly to the (numerous) subpopulation of small clusters, while the lattice parameter measured by XRD is mostly pertaining to the (minor) subpopulation of larger structures. A direct estimation of the size of the subpopulation of smaller clusters that dominate the XAS signal (characterized by a = 3.30 Å) is not trivial. Anyway, the following arguments can help to define a reasonable range of size. Indeed, tight binding simulations indicate that the split of the first Nb shell into the two distances typical of the bcc phase occurs for NCs larger than ∼1.5 nm.19 Our experimental data show that a double coordination distance is required for a good fit, indicating that the minimum size of the nanoclusters detected by XAS is likely in this range. Moreover, an estimation of an upper bound for the subpopulation of small NCs can be guessed considering that Nb crystalline clusters of 4−5 nm would give a significant XRD signal corresponding to a = 3.30 Å (see Figure 6), which on the contrary is not experimentally detected (see Figure 2b). From these considerations we suggest that the average cluster size of the nanocluster population with a = 3.30 Å is in the range 1.5−4 nm. Since the lattice parameter of NbHx alloy is related to the H/ Nb atomic ratio,18 we infer that just after matrix desorption 1.5−4 nm clusters are of metallic Nb, while ∼20 nm ones maintain a hydride character. It is unlikely that the lower lattice parameter of small clusters is an effect of lattice contraction purely related to the reduced cluster size: in fact, although experimental data for small Nb or NbH NCs are lacking, usually in 2 nm sized metal clusters the lattice contraction caused by the surface tension is in the range 0.01−0.04 Å,20,21 i.e., at least about half of what is measured here. Therefore, our data strongly suggest that in the desorption process small (∼1.5−4 nm) NbH0.9 clusters are less stable than larger (∼20 nm) ones and release hydrogen faster. We cannot completely rule out the possible formation of a H-poor shell around a NbH NC and its subsequent fragmentation due to the different lattice mismatch. Anyway, the XRD analysis indicates that the NbHx cluster size does not change significantly, suggesting that this does not occur. In a previous paper,9 we have shown that the hydrogenation kinetics of nanostructured Mg is faster if the matrix is doped with small Nb clusters rather than atomically dispersed Nb or large Nb nanoparticles. The results presented here focus on the dehydrogenation dynamics of the catalyst itself, showing that small Nb clusters release hydrogen faster than larger ones. In this respect, it can be observed that clusters with a “magic number” of atoms tend to be particularly stable,22 while the catalytic activity of a nanostructure can depend on the number of atoms of which it is formed.23 Calculations and experiments in this sense refer generally to few-atom clusters. The faster desorption of the smallest particles in our case could be related to the formation of small, very stable, clusters. Nevertheless, the very broad size distribution of the Nb nanocatalyst does not allow getting further insights into this interesting aspect. About the role of the matrix in the stability of the different nanophases during annealing, the XRD results here reported show that the NbH0.9 phase is stable up to 350 °C, while the Nb−H bulk phase diagram indicates that for T > 145 °C it should progressively lose hydrogen to generate NbHx bcc solid solution.7,18 Therefore, as also observed in similar systems

the NbHx lattice parameter are significantly different. In Figure 5 values of the lattice parameter as measured by XAS and XRD are compared, from which the discrepancy found for the Nb−H metastable phase is evident (sample B will be discussed later on). It is worth noting that the X-ray focal spots for XRD and XAS are the same, so that the two results are two pieces of structural information pertinent to exactly the same region of the sample under the same thermodynamic conditions. To explain these results, it has to be considered that in the investigated system a broad size distribution of Nb-related NCs is present. In fact, XRD analysis detects the presence of ∼20 nm NCs; transmission electron microscopy (TEM) on the dehydrogenated sample showed NCs having size in the 5−25 nm range13 and likely the poor contrast with the crystalline matrix did not allow detection of smaller clusters. Moreover, from the XAS analysis the first shell coordination number (measured in situ and also ex situ after dehydrogenation in a vacuum) is significantly lower than that for the bulk. The reason has to be found both in the presence of small size clusters and in the fraction of Nb atoms that remain dispersed into the matrix and so do not contribute to populating the Nb shell. The discrepancy between XRD and XAS results is related to the peculiar sensitivity and complementarity of the two techniques in the investigation of a population of clusters with a broad cluster size distribution, in particular with a large fraction of small clusters and few larger structures. As an example, Figure 6 shows the extended X-ray absorption fine

Figure 6. XRD simulation for unstrained Nb bcc clusters of different sizes. The total cluster volume is the same in different simulations. Inset: simulation of the corresponding first shell EXAFS signals (same line style).

structure (EXAFS) and XRD signals simulated for the same amount of Nb atoms aggregated in unstrained metal nanocrystals of various sizes: it is clear that in the presence of a broad cluster size distribution the detectability of the XRD signal from smaller NCs can be poor, especially in the presence of other phases or diffuse scattering. Moreover, reconstruction phenomena at the cluster surface are certainly present and, together with possible defects and structural constraints imposed by the matrix, contribute to increasing the structural disorder13 and consequently are expected to further reduce the XRD signal of the smallest aggregates. By contrast, the amplitude of the XAS signals from 1.5 and 20 nm NCs are much less different (see inset in Figure 6) and the overall XAS signal is an average of all Nb sites. Therefore, in the present case, with a significant volumetric fraction of small Nb-related NCs, the XRD signal is mainly due to larger NCs (∼20 nm), while the XAS signal, averaging over all Nb atom sites, gets a 7768

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NbH0.9 → Nb phase transformation. The kinetics of the Nb hydride decomposition also depends on the annealing atmosphere and is modest upon air annealing. The structural results suggest that the effectiveness of the catalyst is likely related to the cluster−matrix interface that offers fast diffusion channels for hydrogen6 and nucleation sites for the Mg phase,13 rather than to Nb hydride thermal instability: upon annealing, hydrogen depletion from the interface likely promotes hydrogen release from NbH0.9 NCs and likely from the matrix.

prepared by ball-milling, this increased stability of the Nb hydride phase is related to the nanocluster interaction with the hydride matrix as well as likely to the high hydrogen concentration at the cluster/matrix interface.7 When the thermal energy is sufficient to activate hydrogen outdiffusion from the Nb−H/MgH2 interface, then hydrogen desorption from small niobium hydride NCs is faster than that for larger NCs likely due to the higher surface to volume ratio. This last point is in agreement with the experimental results observed for unsupported Pd clusters, whose hydrogen sorption kinetics is significantly faster for smaller clusters.10,11 Reducing the radius of the cluster, then the fraction of atoms on the cluster surface increases, thus favoring the desorption process of hydrogen which, in small clusters, is poorly dependent on the bulk diffusion.10 For the system under investigation, it is relevant to stress for possible practical applications that the MgH2 → Mg process is connected with hydrogen desorption from 1.5−4 nm NbH0.9 NCs, while the desorption time of large NbH0.9 clusters is significantly longer. Therefore, in this respect, the data presented here suggest that hydrogen outdiffusion from the catalyst/matrix interface triggers hydrogen desorption both from MgH2 and from Nb−H nanoclusters. We have also performed a very similar XRD−XAS experiment on a part of the Nb-doped MgH2 powder during annealing in air. In this case, during the heating ramp the fraction of crystalline MgH2 decreases by a few percent, due to the progressive formation of crystalline MgO that is observed in the diffraction pattern (not shown). The MgH2 → Mg transition occurs at higher temperature (T = 370 °C), very likely related to the formation of MgO and Mg(OH)2 layers partially covering MgH2 grains that actively block outdiffusion of H 2 molecules and hydrogen atoms. 24 After matrix desorption, which lasts for about the same time as upon vacuum annealing, the rate of formation of crystalline MgO increases, likely due to the matrix fragmentation induced by the hydrogen desorption process. About the structural changes induced on the catalyst-related nanostructures, as upon vacuum annealing, during the MgH2 → Mg phase transition the NbH0.9 nanocrystals progressively lose hydrogen and a metastable NbHx phase forms. The lattice parameter measured after matrix desorption by XAS is 3.35 ± 0.02 Å (corresponding to x ≃ 0.3), while the one measured by XRD is 3.408 ± 0.005 Å (corresponding to x ≃ 0.6), as shown in Figure 5 and Table 1 (sample B). That is, as for hydrogen desorption in a vacuum, after phase transition of the matrix the H/Nb atomic ratio (i.e., the NbHx lattice parameter) is lower for smaller NbHx clusters. Anyway, it has to be noted that in the presence of O a more modest hydrogen desorption from NbH0.9 NCs is detected, which does not seem to have evident effects on the MgH2 → Mg phase transition. Considering that in this case a higher concentration of O is present at the grain boundaries and at the NC/matrix interface, it is likely that the hydrogen outdiffusion is partially hindered. Moreover, we note that in the NbO cubic phase the Nb−Nb shortest distance is 2.98 Å, very similar to the Nb−Nb nearest neighbor distance of larger NbHx NCs (2.95 Å, for a = 3.408 Å); this hydride composition could be stabilized by an oxide layer growing on the cluster surface. Globally, overall these results indicate that the dehydrogenation kinetics of NbH0.9 clusters embedded in MgH2/Mg strongly depends on the cluster size, being significantly faster for small (1.5−4 nm) clusters with respect to larger ones (∼20 nm). Moreover, hydrogen desorption of the MgH2 matrix catalyzed by Nb hydride NCs is not connected with a complete

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CONCLUSIONS We have investigated the structural modifications occurring during hydrogen desorption of Nb-doped nanostructured MgH2 by in situ X-ray diffraction and X-ray absorption spectroscopy. This combined experiment allowed obtaining new insights into the structural modifications of both the catalytic nanocluster population and the MgH2 matrix, occurring during hydrogen desorption. It is shown that fast hydrogen desorption of the MgH2 matrix catalyzed by Nb hydride NCs is not connected with a complete NbH0.9 → Nb phase transformation. In particular, just after the MgH2 → Mg transition, a NbHx bcc metastable nanophase is detected, whose composition depends on the cluster size and on the annealing atmosphere. It is shown for the first time for embedded nanoparticles that faster and (in vacuum) complete dehydrogenation of NbH0.9 is favored for small (1.5−4 nm) clusters with respect to larger (∼20 nm) ones. Moreover, the structural results obtained upon vacuum and air annealing suggest that the effectiveness of the catalyst in the MgH2 → Mg phase transition is likely related to the cluster−matrix interface that offers fast diffusion channels for hydrogen and nucleation sites for the Mg phase.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS G. Mattei (University of Padova) is acknowledged for fruitful discussion. This work was partially supported by the PAT (Provincia Autonoma di Trento) project (ENAM) in cooperation with Istituto MCB of CNR (Italy). GILDA is funded by the Italian CNR and INFN (Proposal No. 08-01660).

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