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Nov 21, 2016 - Moreover, in the hydrogenated state some nanometric islands of ZrH2 can be formed [23,26,27]. ... at a fixed p decreases as T increases; for example, for p = 1 bar one observes a hydrogen content of ..... Nevada, Reno.
membranes Article

Hydrogen Induced Abrupt Structural Expansion at High Temperatures of a Ni32Nb28Zr30Cu10 Membrane for H2 Purification Oriele Palumbo 1 , Francesco Trequattrini 1,2 , Madhura Hulyalkar 3 , Suchismita Sarker 3 , Narendra Pal 3 , Dhanesh Chandra 3 , Ted Flanagan 4 , Michael Dolan 5 and Annalisa Paolone 1, * 1 2 3

4 5

*

Consiglio Nazionale delle Ricerche, Istituto del Sistemi Complessi, U.O.S. La Sapienza, Piazzale A. Moro 5, Rome 00185, Italy; [email protected] (O.P.); [email protected] (F.T.) Department of Physics, Sapienza University of Rome, Piazzale A. Moro 5, Rome 00185, Italy Department of Chemical and Materials Engineering, University of Nevada, Reno, NV 89557, USA; [email protected] (M.H.); [email protected] (S.S.); [email protected] (N.P.); [email protected] (D.C.) Department of Chemistry, University of Vermont, Burlington, VT 05405, USA; [email protected] Commonwealth Scientific and Industrial Research Organisation, Queensland Centre for Advanced Technologies, Energy, 1 Technology Court, Pullenvale 4069, Australia; [email protected] Correspondence: [email protected]; Tel.: +39-6-4991-4400

Academic Editor: Angelo Basile Received: 19 October 2016; Accepted: 16 November 2016; Published: 21 November 2016

Abstract: Ni-Nb-Zr amorphous membranes, prepared by melt-spinning, show great potential for replacing crystalline Pd-based materials in the field of hydrogen purification to an ultrapure grade (>99.999%). In this study, we investigate the temperature evolution of the structure of an amorphous ribbon with the composition Ni32 Nb28 Zr30 Cu10 (expressed in atom %) by means of XRD and DTA measurements. An abrupt structural expansion is induced between 240 and 300 ◦ C by hydrogenation. This structural modification deeply modifies the hydrogen sorption properties of the membrane, which indeed shows a strong reduction of the hydrogen capacity above 270 ◦ C. Keywords: lattice expansion; amorphous membranes; hydrogen sorption; XRD; DTA; hydrogenation enthalpy PACS: 61.05.cp; 61.43.Dq; 65.60.+a

1. Introduction For purification of hydrogen to an ultrapure grade (>99.999%), crystalline Pd and Pd-Ag (100–200 µm thickness) membranes have been employed for several decades [1,2]. Usually, one side of the membrane is exposed to the pressure of a gas containing H2 . The membrane splits the hydrogen molecule, and the atomic hydrogen is absorbed in the metal and diffuses towards the lower H2 concentration side. On the low-pressure face, the hydrogen atoms recombine to form H2 molecules and pure hydrogen is released thanks to the high selectivity of the metallic membranes. For the Pd-based membranes, H2 flux values in excess of 1 mol·m−2 ·s−1 have been reported [3]. However, Pd is an expensive material and in certain conditions can suffer from embrittlement, due to the formation of hydrides. In order to prevent embrittlement, alloying of Pd with Ag, Au or Ru is usually performed [4]. To mitigate the economic impact of Pd, thin layers of Pd-Ag alloys (about 50 µm) were shown to have both complete selectivity to hydrogen and good durability [5]. More recently, a different approach was proposed, in which small particles of Pd (200 nm) were mixed with silica substrates [6]. Nevertheless, palladium is also a strategic material and, therefore,

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alternative materials or novel geometries for hydrogen separation membranes are under consideration. Among them, separation by means of polymeric membranes is of great interest [7–9]. Gas separation by means of polymer membranes is already used in industrial processes, such as the production of technical-grade nitrogen, hydrogen separation in refineries and the petrochemical industry and the separation of CO2 from CH4 or N2 [8]. In these materials, the H2 molecules do not need to split into hydrogen atoms to diffuse through the polymeric membranes [9]. Polymer membranes can be used for gas separation in the temperature range below 100 ◦ C and, generally speaking, they are relatively cheap; however, usually their selectivity is relatively low and these membranes suffer from swelling and poisoning [1]. Another concern about polymer membranes is the need to recover the toxic by-products for their production [10]. Other classes of separation membranes are those based on ceramics or on porous carbon. The last one is brittle and has a selectivity for hydrogen in the range of 4–20 [1]. Also, ceramic membranes are brittle and in their dense form can be used above 600 ◦ C [1]. An important part of the research on hydrogen purification membranes deals with the use of other metallic materials. Indeed, some metals show equivalent or greater permeability than Pd [11]. Furthermore, many of these metals, such as Ni, Co, Nb, and Zr, cost in the range of 5–40 USD·kg−1 , making them of great interest as potential Pd alternatives. Among other options, amorphous alloys formed from a combination of Ni and one or more early transition metals show great potential for this application [12]. Due their low cost and high selectivity, amorphous membranes have been largely investigated. A review of the properties of amorphous alloy membranes for hydrogen purification is reported in Reference [13]. Amorphous alloy membranes can be produced by different methods, such as arc-melting, die-casting and melt-spinning [1]. Here, we will focus on ribbons formed directly by melt-spinning, i.e., by rapid solidification on the surface of a rolling copper wheel (∆T/∆t ~ 105 ◦ C·s−1 ). One of the main concerns about such membranes is the possible crystallization during operation at high temperature, because it leads to the reduction in hydrogen diffusion pathways and often induces brittleness in the ribbons. Therefore, care should be used to avoid operation close to the crystallization temperature. In this framework, knowledge of the temperature evolution of the structure of the amorphous membranes is extremely important. A promising class of amorphous alloy membranes is the Ni-Zr or Ni-Nb-Zr alloys. Hara et al. reported that Ni-Zr-based alloys are stable and do not become brittle in the temperature range between 200 and 350 ◦ C while the hydrogen permeability of the Ni64 Zr36 sample is on the order of 10−9 mol·m−1 ·s−1 ·Pa−0.5 [14–16]. Moreover, the hydrogen permeability of Ni-Nb-Zr amorphous samples is significantly enhanced by the addition of Zr to the alloys and it is comparable to that of the Pd-based alloys, i.e., 10−9 –10−8 mol·m−1 ·s−1 ·Pa−0.5 [17–21]. The crystallization temperature of Ni-Nb-Zr alloys is generally higher than 450 ◦ C [22,23], the maximum temperature at which permeation tests are usually conducted. These amorphous membranes can be used in the temperature range of around 400 ◦ C, compatible with the water shift reaction of syn-gas [13]. From a structural point of view, Ni-Nb-Zr membranes are quite complex. In their amorphous state, they are composed of icosahedral clusters with a random distribution of the three chemical elements on the vertexes [18,24,25]. Density functional theory (DFT) calculations indicate that Ni-centered clusters have lower energy than the Nb- or Zr-centered clusters and, moreover, clusters with smaller Nb-Ni coordination numbers have lower energy [25]. Additional DFT calculations and the comparison with EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge structure) data suggest that the most stable hydrogen site is formed by two tetrahedra consisting of two Zr and two Nb atoms. The next stable site is formed by two tetrahedra consisting of three Zr atoms and one Nb atom [24]. Moreover, EXAFS measurements suggest that the Zr-Zr distance drastically increases upon hydrogenation and that hydrogen may easily permeate between the Zr-Zr pairs where the Zr-Zr bond length is expanded by the hydrogen atoms, giving rise to the high permeability [18]. Yamaura et al. [18] therefore suggested a link between the structural and functional properties of Ni-Nb-Zr membranes. Moreover, in the hydrogenated state some nanometric islands of ZrH2 can be formed [23,26,27]. Finally,

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starting from the amorphous phase, heating the sample above the crystallization temperature, usually more thanheating one crystalline phase is formed [23]. phase, the sample above the crystallization temperature, usually more than one crystalline Furthermore, Yamaura et al. [28] investigated the changes induced by the addition of a different phase is formed [23]. elementFurthermore, in the Ni-Nb-Zr ternary alloys on hydrogenthe permeability; among considered elements Yamaura et al. [28] investigated changes induced by all thethe addition of a different (Al, Co, Cu, P, Pd, Si, Sn, Ta and Ti), the best permeability results were obtained by the copper element in the Ni-Nb-Zr ternary alloys on hydrogen permeability; among all the considered addition. The copper substitution notand increase thebest crystallization temperature, is considered elements (Al, Co, Cu, P, Pd, Si, does Sn, Ta Ti), the permeability results werewhich obtained by the copper addition.for Thethe copper substitution notmembranes; increase the crystallization a key parameter potential use ofdoes these however, fortemperature, (Ni0.6 Nb0.4 )which Cu5 45 Zr50 is considered a key parameter for the potential use of these membranes; however, for alloy, the copper substitution increases the permeability significantly. Yamaura et al. [28] showed (Nihydrogen 0.6Nb0.4)45Zrpermeability 50Cu5 alloy, the copper substitution increases the permeability significantly. Yamaura that decreases with the increasing Vickers hardness and concluded that al. [28] showed permeability alloys decreases with thepotential increasingasVickers hardness and theetNi-Nb-Zr-X (X that = Cohydrogen or Cu) amorphous have high hydrogen-permeable ◦ C the permeability −8 mol concluded Indeed, that the Ni-Nb-Zr-X (X = Co increases or Cu) amorphous alloys high membranes. at 400 from 0.11–1.30 × 10have ·m−1potential ·s−1 ·Pa−0.5asfor hydrogen-permeable membranes. Indeed, at 400 °C the permeability increases from 0.11–1.30 × 10−8 − 8 − 1 − 1 − 0.5 (Ni0.6 Nb0.4 )50 Zr50 to 2.34 × 10 mol·m ·s ·Pa for (Ni0.6 Nb0.4 )45 Zr50 Cu5 , which is comparable −1·s−1·Pa−0.5 for (Ni0.6Nb0.4)50Zr50 to 2.34 × 10−8 mol·m−1·s−1·Pa−0.5 for (Ni0.6Nb0.4)45Zr50Cu5, which is mol· m to the permeability of conventional Pd-Ag alloys. comparable to the permeability of conventional Pd-Ag alloys. In order to obtain materials with high permeability but low hydrogen embrittlement, knowledge In order to obtain materials with high permeability but low hydrogen embrittlement, of their hydrogenation properties is important as it can provide information about the hydrogen effect knowledge of their hydrogenation properties is important as it can provide information about the on the material structure. In this framework, in this paper, we will report a study of the structure hydrogen effect on the material structure. In this framework, in this paper, we will report a study of of a Ni32 Nb28 Zr30 Cu10 amorphous membrane, performed between 25 and 400 ◦ C, by means of high the structure of a Ni32Nb28Zr30Cu10 amorphous membrane, performed between 25 and 400 °C, by temperature XRD temperature measurements (HTXRD), either in a hydrogen or in atmosphere. We will means of high XRD measurements (HTXRD), either in aa helium hydrogen or in a helium show that uponWe hydrogenation, membranes display abrupt expansion the structure around atmosphere. will show that the upon hydrogenation, thean membranes display anofabrupt expansion of ◦ C. In correspondence with this structural modification, the hydrogen absorption properties 300the structure around 300 °C. In correspondence with this structural modification, the hydrogen change and the membrane absorbs much hydrogen in the much high temperature regime thanhigh in the absorption properties change and the less membrane absorbs less hydrogen in the low temperature one. than in the low temperature one. temperature regime 2. 2.Results Resultsand andDiscussion Discussion 2.1.2.1. Preliminarily PreliminarilyDTA DTAMeasurements Measurements AsAsthetheoccurrence the absorption absorption properties propertiesofofthe theNi-Nb-Zr Ni-Nb-Zr occurrenceofof crystallization crystallization changes changes the membranes, processby byDTA DTAmeasurements measurements membranes,we wepreliminary preliminaryinvestigated investigated the the crystallization crystallization process onon thethe NiNi Nb Zr Cu sample. In the similar compound Ni Nb Zr , which does not contain Cu, it has 10 sample. In the similar compound Ni42 Nb2828 Zr30, 30which does not contain Cu, it has 32 32Nb 2828Zr 3030Cu10 42 ◦ C [20,23,29]; however, been reported that crystallization occurs at temperatures higher than 480 been reported that crystallization occurs at temperatures higher than 480 °C [20,23,29]; however, an aninvestigation investigationofofthe thepresently presentlyinvestigated investigated composition still missing. Figure 1 reports DTA composition is is still missing. Figure 1 reports thethe DTA ◦ C/min. curves measured differenttemperature temperaturerates, rates, ranging ranging between between 15 curves measured atatdifferent 15and and30 30°C/min.

Figure DTA signalmeasured measuredatatdifferent differentheating heating rates. rates. The plot forfor thethe Figure 1. 1. DTA signal Theinset insetreports reportsthe theKissinger Kissinger plot three thermally activatedpeaks peaksshown shownby bythe the DTA DTA curves. curves. three thermally activated

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They present three subsequent thermally activated peaks, starting above 480 °C, centered ◦ C, centered around 520, 550 and 570subsequent °C, when measured ΔT/Δt =peaks, 15 °C/min. Theabove inset 480 of Figure 1 displays the They present three thermally at activated starting around ◦ ◦ Kissinger plot three peaks. The calculated energies the three processes are 370 ± 520, 550 and 570forC,the when measured at ∆T/∆t = 15 activation C/min. The inset ofofFigure 1 displays the Kissinger 30, 441 ± 2 and 470 ± 70 kJ/mol. Such values are on the same order of magnitude of those already plot for the three peaks. The calculated activation energies of the three processes are 370 ± 30, 441 ±2 reported for other Ni-Nb-Zr membranes [20,22,23,29]. The DTA measurements indicate that and 470 ± 70 kJ/mol. Such values are on the same order of magnitude of those already reported the for crystallization occurring in subsequent steps,measurements starts above 480 °C. that the crystallization other Ni-Nb-Zrprocess, membranes [20,22,23,29]. The DTA indicate process, occurring in subsequent steps, starts above 480 ◦ C. 2.2. Hydrogen Absorption Properties 2.2. Hydrogen Absorption Properties The hydrogen absorption properties of the membranes were measured by obtaining pressure-composition isotherms at different of temperatures in the range 400 °C; The hydrogen absorption properties the membranes were between measured158byand obtaining these temperatures are well below the crystallization temperature obtained by158 theand previously pressure-composition isotherms at different temperatures in the range between 400 ◦ C; reported DTA experiments. these temperatures are well below the crystallization temperature obtained by the previously reported The pressure-composition isotherms of the Ni32Nb28Zr30Cu10 membrane are reported in Figure 2. DTA experiments. We indicate the hydrogen concentration in the the Ni sample asZrH/M, i.e., the ratio are between the in number The pressure-composition isotherms of membrane reported Figureof 2. 32 Nb28 30 Cu10 hydrogen and the concentration number of metal measurements at low temperatures were We indicateatoms the hydrogen in theatoms. sampleThe as H/M, i.e., the ratio between the number of possible thanks the kinetics of theatoms. sorption in this membrane (at least 10 times faster hydrogen atoms to and thefast number of metal Theprocess measurements at low temperatures were possible than in to Nithe 42Nb 28Zrkinetics 30). Figure shows the absence plateau in the wholefaster temperature thanks fast of 2the sorption process of in any this pressure membrane (at least 10 times than in range. behavior has been already observed for Ni-Nb-Zr ribbons [29]. Pressure Ni Zr30 ). Figure 2 shows the absence of any pressure plateauamorphous in the whole temperature range. 42 NbA 28similar plateaus on the typically observed in crystalline hydrides. Moreover, the hydrogen A similar are, behavior hascontrary, been already observed for Ni-Nb-Zr amorphous ribbons [29]. Pressure plateaus capacity the Ni32typically Nb28Zr30Cu 10 membrane is higher at lower temperatures or, equivalently, are, on theofcontrary, observed in crystalline hydrides. Moreover, the hydrogen capacity of the hydrogen solubility at a fixed p decreases as T increases; for example, for p = 1 bar one Ni32 Nb28 Zr30 Cu10 membrane is higher at lower temperatures or, equivalently, the hydrogenobserves solubilitya hydrogen of ~0.52 at 158for °C example, and only for ~0.27 H/M at one 400 °C. Some single pointscontent along the at a fixed pcontent decreases as T H/M increases; p= 1 bar observes a hydrogen of pressure-composition in order verify thethereproducibility of the ~0.52 H/M at 158 ◦ C andisotherms only ~0.27were H/M duplicated at 400 ◦ C. Some single to points along pressure-composition measurements. isotherms were duplicated in order to verify the reproducibility of the measurements.

Figure 2. Pressure-composition isotherms measured measured at at various various temperature temperature between between 158 158 and and 400 400 ◦°C. C.

Only a limited comparison of the hydrogen absorption properties of the presently investigated Only a limited comparison of the hydrogen absorption properties of the presently investigated Ni32Nb28Zr30Cu10 membrane with the previous literature is possible. Indeed, in many cases Ni-Nb Ni32 Nb28 Zr30 Cu10 membrane with the previous literature is possible. Indeed, in many cases Ni-Nb ribbons were hydrogenated by means of electrolytic charging [30,31], instead of the direct reaction ribbons were hydrogenated by means of electrolytic charging [30,31], instead of the direct reaction with H2 gas. Conic et al. [32] measured only the absorption kinetics at various temperatures of Zr with H2 gas. Conic et al. [32] measured only the absorption kinetics at various temperatures of Zr alloys containing Nb and Ta mixtures (10 wt % Nb, 12 wt %Ta and 10 wt %Nb and 12 wt %Ta). Hao alloys containing Nb and Ta mixtures (10 wt % Nb, 12 wt % Ta and 10 wt % Nb and 12 wt % Ta). et al. reported pressure-composition curves measured at 300 °C for Ni60Nb40, (Ni0.6Nb0.4)50Zr50 and Hao et al. reported pressure-composition curves measured at 300 ◦ C for Ni60 Nb40 , (Ni0.6 Nb0.4 )50 Zr50 (Ni0.6Nb0.4)70Zr30 [33]. The hydrogen content increases as the Zr content increases, reaching p ≈ 0.5 and (Ni0.6 Nb0.4 )70 Zr30 [33]. The hydrogen content increases as the Zr content increases, reaching MPa ~1.1 mass % in (Ni0.6Nb0.4)50Zr50 and ~0.7 mass % in (Ni0.6Nb0.4)70Zr30 [16]. Also Yamaura et al. p ≈ 0.5 MPa ~1.1 mass % in (Ni0.6 Nb0.4 )50 Zr50 and ~0.7 mass % in (Ni0.6 Nb0.4 )70 Zr30 [16]. Also measured a single pressure-composition curve of (Ni0.6Nb0.4)70Zr30 at 300 °C [18]. Recently, we Yamaura et al. measured a single pressure-composition curve of (Ni0.6 Nb0.4 )70 Zr30 at 300 ◦ C [18]. investigated the sorption properties of melt-spun (Ni0.6Nb0.4−yTay)100−xZrx with y = 0, 0.1 and x = 20, 30,

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Recently, we investigated the sorption properties of melt-spun (Ni0.6 Nb0.4−y Tay )100−x Zrx with y = 0, 0.1 between 300 400 °C (Ni0.6Nb0.4to )70(Ni Zr300.6 , the presently investigated membranes and x = 20, 30,and between 300[29]. andCompared 400 ◦ C [29].toCompared Nb0.4 )70 Zr30 , the presently investigated ◦ ◦ display a higher solubility by ~0.02 H/M at 300 °C and ~0.04 H/M at 400 °C. membranes display a higher solubility by ~0.02 H/M at 300 C and ~0.04 H/M at 400 C. sorption isotherms isothermsreported reportedininFigure Figure2 are 2 are almost parallel to one another; however, The sorption almost parallel to one another; however, one one can ◦ C °C can observe a large displacement between curve corresponding to 242 that corresponding observe a large displacement between the the curve corresponding to 242 andand that corresponding to ◦ ◦ to 270 °C, and between the latter one and the isotherm measured at 300 °C, while all other curves 270 C, and between the latter one and the isotherm measured at 300 C, while all other curves display a constant shiftlower toward lower H/Masvalues as T increases (see afterwards for a quantitative adisplay constant shift toward H/M values T increases (see afterwards for a quantitative analysis). analysis). Such a change in the absorption properties of the membranes is even more evident Such a change in the absorption properties of the membranes is even more evident considering the considering the as van’t Hoff plot, as reported in Figure 3. van’t Hoff plot, reported in Figure 3.

3. Van’t Van’tHoff Hoffplot plotfor forthe theNi Ni Nb2828Zr Zr30 30Cu10 H/M ratios and Figure 3. membrane calculated calculated at different H/M 3232Nb 10 membrane best fit lines in the low- and and high-temperature high-temperature range. range.

In the thecase caseof of crystalline materials, for which a pressure is observed the p-c In crystalline materials, for which a pressure plateauplateau is observed in the p-cin isotherms, isotherms, the van’t Hoff plot reports ln(p) vs. 1/T, where p is the plateau pressure at temperature T the van’t Hoff plot reports ln(p) vs. 1/T, where p is the plateau pressure at temperature T (K) [26], (K) [26], theofslope of thefitbest fit lines provides the hydrogenation enthalpy, hyd. However, the and the and slope the best lines provides the hydrogenation enthalpy, ∆HΔH hyd . However, the pressure-composition isotherms of the presently studied amorphous materials do exhibit any any pressure-composition isotherms of the presently studied amorphous materials do not not exhibit plateau. In this case, one can still construct a van’t Hoff plot reporting ln(p) vs. 1/T, where p is the plateau. In this case, one can still construct a van’t Hoff plot reporting ln(p) vs. 1/T, where p is the pressure at at aa fixed fixed value value of of H/M H/M at estimate the the pressure at the the various various temperatures. temperatures. Also, Also, in in this this case, case, one one can can estimate hydrogenation enthalpy, ΔH hyd, from the slope of the plot [34]. In order to obtain the values of p at hydrogenation enthalpy, ∆Hhyd , from the slope of the plot [34]. In order to obtain the values of p at fixed values valuesof ofH/M H/Matatthe thevarious varioustemperatures, temperatures, interpolation of the experimental fixed anan interpolation of the experimental datadata usingusing beta beta splines was performed by means of a program implemented with the Labview routines. The splines was performed by means of a program implemented with the Labview routines. The van’t van’t Hoff plot for the Ni 32Nb28Zr30Cu10 membrane calculated for H/M from 0.30 to 0.42 in steps of Hoff plot for the Ni32 Nb28 Zr30 Cu10 membrane calculated for H/M from 0.30 to 0.42 in steps of 0.02 is 0.02 is reported in Figure 3, together with reported in Figure 3, together with the bestthe fit best lines.fit lines. Figure 3 clearly shows two different slopes, oneinin the temperature range between ◦C Figure 3 clearly shows two different slopes, one the temperature range between 158158 andand 242242 °C and another between 300 and 400 °C; however, an evident jump is present between 242 and 300 and another between 300 and 400 ◦ C; however, an evident jump is present between 242 and 300 ◦ C. °C. Even hydrogenation enthalpy material changeswhen whenpassing passingfrom fromthe thelow lowTT region region to to Even the the hydrogenation enthalpy of of thethe material changes the high temperature one. For example, for H/M = 0.30, ΔH hyd passes from 36 ± 1 kJ/mol at low T to the high temperature one. For example, for H/M = 0.30, ∆Hhyd passes from 36 ± 1 kJ/mol at low T 23 23 ± 1± kJ/mol at at high temperature. depends on the particular H/M to 1 kJ/mol high temperature.It Itcan canbe benoted noted that that ΔH ∆Hhyd hyd depends on the particular H/M considered, as reported in Table 1. considered, as reported in Table 1. Table 1. The hydrogenation enthalpy, ΔHhyd,calculated for various hydrogen concentrations, H/M, values in the low (T ≤ 242 °C) and high temperature (T ≥ 300 °C) region. ΔHhyd (kJ/mol) low T region high T region

H/M = 0.30 36 ± 1 23 ± 1

H/M = 0.32 31 ± 1 21 ± 1

H/M = 0.34 31 ± 1 20 ± 1

H/M = 0.36 29 ± 1 18 ± 1

H/M = 0.38 25 ± 1 17 ± 1

H/M = 0.40 21 ± 1 15 ± 1

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Table 1. The hydrogenation enthalpy, ∆Hhyd ,calculated for various hydrogen concentrations, H/M, values in the low (T ≤ 242 ◦ C) and high temperature (T ≥ 300 ◦ C) region. ∆Hhyd (kJ/mol)

H/M = 0.30

H/M = 0.32

H/M = 0.34

H/M = 0.36

H/M = 0.38

H/M = 0.40

low T region high T region

36 ± 1 23 ± 1

31 ± 1 21 ± 1

31 ± 1 20 ± 1

29 ± 1 18 ± 1

25 ± 1 17 ± 1

21 ± 1 15 ± 1

The dependence of the hydrogenation enthalpy on the hydrogen content has already recently been reported for some other amorphous membranes for hydrogen purification, namely (Ni0.6 NbMembranes )1006,−48 )7011Zr30 the 6 of 0.4−y Tay2016, x Zrx (x = 0.2, 0.3; y = 0, 0.1) membranes [29]. In the case of (Ni0.6 Nb0.4 enthalpy decreases from 41 kJ/mol for H/M = 0.34 to 33 kJ/mol for H/M = 0.42 [29]. This trend was of the hydrogenation enthalpy on the hydrogen content has already recently attributed toThe thedependence different interstitial sites available for hydrogen trapping during the progression of been reported for some other amorphous membranes for hydrogen purification, namely the absorption process: at the beginning, the deepest energy levels are occupied, while only shallower (Ni0.6Nb0.4−yTay)100−xZrx (x = 0.2, 0.3; y = 0, 0.1) membranes [29]. In the case of (Ni0.6Nb0.4)70Zr30 the energy levels are available at the higher [29].for H/M = 0.42 [29]. This trend was enthalpy decreases from 41 kJ/mol forhydrogen H/M = 0.34content to 33 kJ/mol Forattributed the presently studied Ni Nb Zr Cu membrane, the calculated to the different interstitial during the ∆H progression of in the 32 sites 28 available 30 10 for hydrogen trapping hyd both the absorption process: at the beginning, the deepest energy levels are occupied, while only lowtemperature and in the high T regions decreases as H/M increases, similar to the other energy levels are available at the higher hydrogen content [29]. (Ni0.6 Nbshallower 0.4−y Tay )100−x Zrx membranes [29]. However, for the low temperature phase, ∆Hhyd is much For the presently studied Ni32Nb28Zr30Cu10 membrane, the calculated ΔHhyd both in the higher than that of the high temperature phase, showing the different sorption properties of the lowtemperature and in the high T regions decreases as H/M increases, similar to the other compound two temperature ranges. Indeed, in Figure 2 we report a piece of the p-c curve at (Ni0.6in Nbthe 0.4−yTay)100−xZrx membranes [29]. However, for the low temperature phase, ΔHhyd is much 300 ◦ C calculated the of the phase, low temperature curves, i.e., using ∆Hhyd obtained for higher than as that of extrapolation the high temperature showing the different sorption properties of the two temperature ranges. Indeed, in Figure 2 we is report a piece the p-cH/M curve by at about the low compound T region. in It the is well evident that the experimental curve shifted to aoflower 0.2–0.23.300 °C calculated as the extrapolation of the low temperature curves, i.e., using ΔHhyd obtained for low T region. It is well evident that the experimental curve is shifted to a lower H/M by about Thethe observed change of the absorption properties around 240 ◦ C could be related to a structural 0.2–0.23. modification of the sample. However, the membrane should retain its amorphous structure, at least The observed change of the absorption properties around 240 °C could be related to a structural within the detectionoflimit of the previously DTA measurements, which structure, do not show any peak modification the sample. However, thereported membrane should retain its amorphous at least in this temperature range.limit To further study this hypothesis, we performed an XRD the high within the detection of the previously reported DTA measurements, which do notstudy show of any peak in this temperature Tostructure. further study this hypothesis, we performed an XRD study of the temperature evolution of the range. sample high temperature evolution of the sample structure.

2.3. XRD Study of the High Temperature Evolution of the Structure 2.3. XRD Study of the High Temperature Evolution of the Structure

The XRD pattern of the pristine membrane, measured at room temperature in a helium The XRD pattern of the pristine membrane, measured at room temperature in a helium atmosphere, is reported in in Figure presents the typical features of an amorphous atmosphere, is reported Figure 4. 4. ItItpresents the typical features of an amorphous structure, withstructure, a ◦ . This confirms the amorphous state of the investigated with a broad peak centered around 2θ ~ 39 broad peak centered around 2θ ~39°. This confirms the amorphous state of the investigated membrane, indeed, producedby by melt-spinning, melt-spinning, in in order to intentionally disruptdisrupt the membrane, which,which, indeed, waswas produced order to intentionally the crystalline orderrender and render its structure amorphous (see (see Section crystalline order and its structure amorphous Section3).3).

XRD spectrum the pristine NiNb 32Nb28 Zr30 30Cu measured at room temperature Figure 4.Figure XRD4.spectrum of theofpristine Ni32 Cu1010membrane membrane measured at room temperature 28 Zr in a He atmosphere. in a He atmosphere.

The temperature dependence of the structure of the Ni32Nb28Zr30Cu10 membrane was investigated by means of X-ray diffraction, heating the sample in a hydrogen (Figure 5) atmosphere between 25 and 400 °C in subsequent steps.

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The temperature dependence of the structure of the Ni32 Nb28 Zr30 Cu10 membrane was investigated by means of X-ray diffraction, heating the sample in a hydrogen (Figure 5) atmosphere Membranes 25 2016, 6, 48 7 of 11 between and 400 ◦ C in subsequent steps.

Figure 5. X-ray diffractograms measured on increasing temperature with the sample kept in a H2 atmosphere (p ~ 9 bar).

Heating the sample in H2 (Figure 5), the angular position of the peak remains unaltered between 25 and 242 °C, while abruptly, at 270 increasing °C, the peak shifts to with a lower angular position, which Figure Figure 5. 5. X-ray X-ray diffractograms diffractograms measured measured on on increasing temperature temperature with the the sample sample kept kept in in aa H H22 afterward stays fixed up to 400 °C. It is worth noting that the persistence of the single broad feature atmosphere atmosphere (p (p ~ ~ 99 bar). bar). in the diffractogram even at high temperatures suggests that the amorphous nature of the membrane is retained up tointhe T investigated here (400 °C) and excludes aremains transition toward Heating thesample sample H2highest (Figure 5),angular the angular of remains the peakunaltered unaltered Heating the in H2 (Figure 5), the positionposition of the peak between 25 a crystalline state. For conducted similar temperature-dependent XRD experiments ◦ C,we between 25 while and 242 °C,comparison, while abruptly, 270shifts °C, the shifts to a position, lower angular and 242 ◦ C, abruptly, at 270 the at peak to apeak lower angular which position, afterwardwhich stays on a Ni32Nb 28Zr30Cu10 membrane under a helium atmosphere (p ~ 1 bar) and the results are displayed afterward fixed to 400 °C. It is worth noting that thesingle persistence of the single feature fixed up to stays 400 ◦ C. It isup worth noting that the persistence of the broad feature in the broad diffractogram in Figure 6; in this case,even no abrupt peak shift is detectable withthat the increasing temperature. This is in the diffractogram at high temperatures suggests the amorphous nature thea even at high temperatures suggests that the amorphous nature of the membrane is retained upof to the clear indication that the occurring in thehere sample structure are mainly inducedtoward by the membrane is retained upmodifications to(400 the◦highest T investigated (400 °C) excludes a transition highest T investigated here C) and excludes a transition toward aand crystalline state. For comparison, presence of hydrogen. a crystalline state. For temperature-dependent comparison, we conducted similar temperature-dependent XRD we conducted similar XRD experiments on a Ni32 Nb28 Zr30 Cu10experiments membrane The angular position of the broad HTXRD peak is linked(p to~the mean distance between atoms in on a Ni 32 Nb 28 Zr 30 Cu 10 membrane under a helium atmosphere 1 bar) and the results are displayed under a helium atmosphere (p ~ 1 bar) and the results are displayed in Figure 6; in this case, no abrupt theFigure amorphous and thepeak displacement towards lower 2θ valuestemperature. suggests anThis abrupt in in thisstructure case, no the abrupt is detectable the increasing is a peak shift 6; is detectable with increasingshift temperature. Thiswith is a clear indication that the modifications expansion of the structure at high temperatures. clear indication that the modifications occurring in the sample structure are mainly induced by the occurring in the sample structure are mainly induced by the presence of hydrogen. presence of hydrogen. The angular position of the broad HTXRD peak is linked to the mean distance between atoms in the amorphous structure and the displacement towards lower 2θ values suggests an abrupt expansion of the structure at high temperatures.

Figure Figure 6. 6. X-ray X-ray diffractograms diffractograms measured measured with with increasing increasing temperature temperature with with the the sample sample kept kept in in aa He He atmosphere (p ~ 1 bar). atmosphere (p ~ 1 bar).

Figure 6. X-ray diffractograms measured with increasing temperature with the sample kept in a He atmosphere (p ~ 1 bar).

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The angular position of the broad HTXRD peak is linked to the mean distance between atoms in the amorphous structure and the displacement towards lower 2θ values suggests an abrupt expansion of the structure at high temperatures. To obtain a more quantitative picture, it must be noticed that the peak position at the highest intensity represents Membranes 2016, 6, 48the main average interatomic distance, in either clusters or atoms in the 8 ofNi-rich 11 amorphous matrix. To obtain a more quantitative picture, it must be noticed that the peak position at the highest The 2θ values and the average interatomic distances obtained at the highest intensity are reported intensity represents the main average interatomic distance, in either clusters or atoms in the Ni-rich in Table 2, for both the X-ray diffraction patterns measured under H2 or He. In Figure 7, the average amorphous matrix. interatomic distances measured under hydrogen are plotted as a function of temperature: a significant The 2θ values and the average interatomic distances obtained at the highest intensity are ◦ C, in agreement with the temperature expansion of the structure is observed and 300 reported in Table 2, for both the X-ray between diffraction200 patterns measured under H2 or He. In Figure 7, the (242 ◦average C), where a changedistances in the hydrogen properties is detected by theofpreviously reported interatomic measuredabsorption under hydrogen are plotted as a function temperature: a pressure-composition isotherms. On theiscontrary, heating in 300 He, °C, a normal thermal expansion, significant expansion of the structure observed when between 200 and in agreement with the temperature (242 °C),iswhere a change in the2).hydrogen absorption properties is detected by the without abrupt changes, detected (see Table previously reported pressure-composition isotherms. On the contrary, when heating in He, a normal thermal without abrupt changes, is detected (see Table 2). measured under either H2 or The 2θ value and interatomic distances at various temperatures, Table 2. expansion, He atmosphere. Table 2. The 2θ value and interatomic distances at various temperatures, measured under either H2 or He atmosphere.

T (◦ C) 25 100 158 200 242 270 300 350 400

Under Hydrogen (~9 bar)

Under Hydrogen (~9 bar) (Å) 2θ Interatomic Distance T (°C) 2θ Interatomic Distance (Å) 38.92738.927 2.3117 25 2.3117 38.93 2.3122 100 38.93 2.3122 38.85238.852 2.3161 158 2.3161 38.92638.926 2.3118 200 2.3118 38.34338.343 2.3456 242 2.3456 37.86637.866 2.3741 270 2.3741 37.81137.811 2.3774 300 2.3774 37.74837.748 2.3812 350 2.3812 37.72637.726 2.3826 400 2.3826

Under Helium (~1 bar)

Under Helium (~1 bar) Distance (Å) 2θ Interatomic 2θ Interatomic Distance (Å) 39.287 39.287 2.2914 2.2914 39.235 39.235 2.2943 2.2943 39.221 39.221 2.2951 2.2951 39.223 2.295 39.223 2.295 39.183 39.183 2.2973 2.2973 39.152 2.299 39.152 2.299 39.127 2.3004 39.127 2.3004 39.059 39.059 2.3042 2.3042 39.207 39.207 2.2959 2.2959

Figure 7. Average interatomic distances themaxima maximaof of the the X-ray X-ray diffraction 5) 5) taken Figure 7. Average interatomic distances atatthe diffractionpeak peak(Figure (Figure taken at at 9 bar H2 pressure. 9 bar H2 pressure.

From the present measurements, it is not possible to ascertain whether the entire icosahedra

From the present measurements, is not to ascertain the entire icosahedra composing the amorphous structureit or onlypossible some bonds betweenwhether certain atoms expand in correspondence with the abrupt thermal it is plausible it mayatoms be dueexpand to an composing the amorphous structure or expansion, only somebutbonds betweenthat certain in amorphous-to-amorphous phase thermal transition.expansion, This transition induced by hydrogen, is correspondence with the abrupt butwould it is be plausible that it may as beit due to not observed in the samplephase heated in He. AnThis expansion of the Zr-Zrbebond length by as it an amorphous-to-amorphous transition. transition would induced byinduced hydrogen, [18], but in the present case,length we areinduced able to by is nothydrogenation observed inwas thealready samplereported heatedatinroom He.temperature An expansion of the Zr-Zr bond show that an abrupt and unexpected change occurs as the membrane is heated at high temperature under H2 atmosphere. Moreover, in the present work, we show that the two phases, at low and at

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hydrogenation was already reported at room temperature [18], but in the present case, we are able to show that an abrupt and unexpected change occurs as the membrane is heated at high temperature under H2 atmosphere. Moreover, in the present work, we show that the two phases, at low and at high temperature, present different hydrogen sorption properties, with the lower temperature phase displaying a much higher value for the hydrogenation enthalpy. 3. Materials and Methods Ni (purity 99.99% purity), Nb (99.8%), Zr (99.98%) and Cu (99.999%) were purchased from MilliporeSigma (Billerica, MA, United States). An alloy button of Ni32 Nb28 Zr30 Cu10 was prepared by arc melting these metals in a purified argon atmosphere at AMES Laboratory Iowa, Ames, IA, USA. A melt spinning apparatus was used to fabricate the amorphous membranes at the CSIRO laboratory, Brisbane, Australia [16,19]. Typically the amorphous ribbons were ~30–70 µm thick and 30 mm wide [16,19]. The X-ray diffraction patterns were obtained using a PANalytical X’Pert pro θ-θ diffractometer (Almelo, The Netherlands). The ribbons were not coated with Pd. This instrument was equipped with a heating stage with Anton Paar XRK 900 (Graz, Austria). Hydrogen gas was introduced into the heating chamber via a home-made volumetric apparatus. The sample was placed inside the chamber and first evacuated using the turbo pump by Pfeiffer Vacuum model number TSH071E (Asslar, Germany). Simultaneous TGA-DTA measurements were conducted by means of a Setaram Sensys Evolution 1200 TGA system (Caluire, France) [35] under a high purity argon flux (60 mL/min) at ambient pressure. For each experiment, a sample mass of ~10 mg was used. Different temperature rates, between 10 and 30 ◦ C/min, were used for each sample in order to calculate the activation energy of the crystallization process. The hydrogen absorption curves were recorded by the home-made Sieverts apparatus at Sapienza University of Rome described in Reference [23,36]. The experiments were performed on specimens with a mass of ~300 mg. 4. Conclusions The hydrogen adsorption properties and the amorphous structure of a Ni32 Nb28 Zr30 Cu10 membrane are studied by means of pressure-composition isotherms and XRD measurements, respectively. In the temperature range between 242 and 300 ◦ C, an abrupt decrease of hydrogen absorption is observed; correspondingly, the hydrogen concentration measured at 7 bar is reduced from ~0.6 H/M at 242 ◦ C to ~0.45 H/M at 300 ◦ C. The X-ray diffractograms confirm that the membrane remains amorphous also at the highest investigated temperature (400 ◦ C), and clearly indicate an abrupt increase of the mean interatomic distance from ~2.31 Å at 200 ◦ C to ~2.38 Å occurs. This abrupt expansion is evidenced only when the samples are heated in a hydrogen atmosphere, while a much lower and non-abrupt thermal expansion is observed when samples are heated in an inert atmosphere. Here, for the first time, an abrupt change of the physical properties of amorphous membranes when heated in hydrogen at high temperatures is reported. Incidentally, these findings suggest that we should avoid extrapolations from properties measured close to room temperature. Acknowledgments: This research is supported by US DOE-NNSA grant, (US DE-NA0002004) at the University of Nevada, Reno. We would like to acknowledge Kent Coulter of SWRI (USA) for help with the sample preparation. Author Contributions: Michael Dolan produced the melt-spun membranes; Oriele Palumbo and Francesco Trequattrini performed the DTA measurements and analysis; Narendra Pal, Madhura Hulyalkar, Suchismita Sarker and Dhanesh Chandra performed the XRD measurements; Annalisa Paolone performed the Sieverts hydrogen sorption measurements and analysis; Dhanesh Chandra and Annalisa Paolone. wrote the paper; all authors discussed the data and share the conclusions. Conflicts of Interest: The authors declare no conflict of interest.

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