Structure and Dynamics of Molecular Hydrogen in the Interlayer Pores

Oct 10, 2014 - The right-hand axis shows H2 uptake determined from the pressure of H2 gas .... Calculated from D = D0 exp(−Ea/kBT) where D0 = 8.58 Ã...
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Structure and Dynamics of Molecular Hydrogen in the Interlayer Pores of a Swelling 2:1 Clay by Neutron Scattering Jacqueline S. Edge,†,# Neal T. Skipper,*,†,∥,# Felix Fernandez-Alonso,§,∥ Arthur Lovell,§,⊥ Gadipelli Srinivas,‡ Stephen M. Bennington,§,⊥ Victoria Garcia Sakai,§ and Tristan G. A. Youngs§ †

London Centre for Nanotechnology, 17-19 Gordon Street, London WC1H 0AH, United Kingdom Department of Chemistry, University College of London, 20 Gordon Street, London WC1H 0AJ, United Kingdom § ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, United Kingdom ∥ Department of Physics and Astronomy, University College of London, Gower Street, London WC1E 6BT, United Kingdom ⊥ Cella Energy Ltd, Rutherford Appleton Laboratory, Didcot OX11 0QX, United Kingdom ‡

S Supporting Information *

ABSTRACT: Neutron scattering has been used to reveal the structure and dynamics of molecular H2 physisorbed into the two-dimensional pores of sparingly hydrated Ca-laponite clay. Thermal pretreatment of the clay at 415 K under vacuum yielded an interlayer composition in the 1.0−1.5 water molecules per Ca2+ cation range and provided a vacant gallery height of 2.82 Å. This value is very well matched to the diameter of molecular hydrogen and allows intercalation of H2 up to the point where a liquid-like monolayer is formed within the clay. At a low coverage of 0.1 H2 per cation the isosteric heat of adsorption is 9.2 kJ mol−1. Quasielastic neutron scattering experiments conducted at 40−100 K reveal two populations of H2 within the clay. First, we find molecules that are localized close to the partially hydrated Ca2+ cations. Second, we identify a more mobile liquid-like population whose motion is captured by jump diffusion. At 40 K, the H2 diffusion coefficient is 2.3 ± 0.5 × 10−5 cm2 s−1. This is an order of magnitude slower than the value extrapolated from bulk liquid H2.

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INTRODUCTION The properties of molecular H2 physisorbed in solid-state nanoporous media are currently highly topical, with applications including gas adsorption and nuclear waste containment.1−3 In the context of hydrogen storage, physisorption of H2 offers the advantages of rapid kinetics, reversibility, and stability of the host to long-term recycling.2−5 Materials currently under investigation include nanostructured carbons,6−10 inorganic sorbents such as zeolites,11 metal−organic frameworks (MOFs),2,12−14 and polymers of intrinsic microporosity (PIMs).4 These systems offer large accessible surface areas and favorable host densities. However, they suffer from rather low enthalpies of H2 adsorption, typically in the range of 5−8 kJ mol−1. This restricts their use to cryogenic temperatures and/or high pressures. For operation close to ambient conditions, it has been estimated that host materials would require a binding energy for molecular hydrogen in the range in 15−25 kJ mol−1.15 Strategies for achieving such interaction energies with porous substrates include provision of open metal (Kubas) binding sites, creation of strong electrostatic fields within the cavity through a charged framework and counterions, and reduction of the pore size toward the adsorbate diameter (around 2.9 Å for H2) to allow contact with more than one sorbate surface.1,2,4,5 With these factors in mind, two-dimensional graphene-based nanostructures have been identified as potential targets for hydrogen storage.16 These materials allow metal © XXXX American Chemical Society

(and solvent) intercalation with concomitant tunability of the layer−layer spacing, toward the theoretical optimum of around 6 Å (the layer−layer spacing of unintercalated graphite itself is 3.35 Å).17 For example, the stage-II graphite intercalation compound (GIC) KC24 physisorbs up to 2 H2 molecules per cation, equivalent to 1.3% hydrogen by weight, with an isosteric heat of absorption of approximately 9 kJ mol−1.17,18 In principle, GICs incorporating calcium should be more favorable for hydrogen physisorption because this metal interacts more strongly with H2.19 However, in practice, Ca-based GICs are unstable to the formation of metal hydride plus graphite.20 An alternative class of tunable two-dimensional materials, and the ones studied here, are the swelling 2:1 clays. This group of phyllosilicates, which includes smectites and vermiculites, are composed of negatively charged mica-like sheets held together by charge-balancing counterions, such as Na+ and Ca2+.21 Water molecules readily diffuse into the interlayer region of these clays causing lattice expansion22 and thereby provide an ideal pillar to facilitate H2 intercalation. Moreover, the hydration state of the interlayer cations of clays can be controllably reduced by thermal treatment, leaving them exposed for direct binding to multiple H2 molecules. Unlike many zeolites, which are prone to pore-blocking by larger cations,11 clay interlayers provide a Received: August 14, 2014 Revised: October 9, 2014

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outgassed in situ by evacuating the sealed sample container to a vacuum of around 10−6 mbar for several hours. For neutron scattering experiments, the samples were predeuterated by repeated exchange in 99.99% pure D2O and were then dosed with ultrahigh purity (UHP) 99.999% H2 gas in situ on the beamline using a calibrated volume. In situ neutron diffraction was conducted on the NIMROD instrument at the ISIS pulsed neutron source (Rutherford Appleton Laboratory, UK).26 Approximately 1.3 g of predried predeuterated Ca-laponite powder was loaded into a flat-plate geometry null-scattering titanium/zirconium alloy cell of wall thickness 1 mm, aligned normal to the incident neutron beam. The sample volume was 30 × 30 × 1 mm3 thick; this geometry minimizes multiple scattering and absorption effects. The temperature of the samples was maintained to an accuracy of ±0.1 K using a closed cycle refrigerator and cartridge heaters. Typical counting times were ∼2 h for each state point. To allow data correction and calibration, scattering patterns were also collected from the empty instrument (with and without the empty sample cell) and an incoherent scattering vanadium standard slab of thickness 3.0 mm. Background, multiple scattering, absorption, detector deadtime, and normalization correction procedures were implemented by the Gudrun suite of programs,27 to give the differential scattering cross-section (DCS) for each sample over an approximate momentum transfer, Q, range of 0.05−25 Å−1, where Q = (4π sin θ)/λ = 2π/d. For all samples, the “top-hat” deconvolution method was used to remove inelastic self-scattering.27 Samples were studied at 40 K and at a range of H2 loadings from 0 to 7.67 H2 per Ca2+. Under these conditions, the out-of-plane d-spacing is approximately 12.0−12.5 Å (Figure 1), giving rise to a measurable (001) Bragg peak at Q001 ≈ 0.5 Å−1. This peak was fitted to a single Gaussian, and the small-angle background scattering was removed via a Q−4 power law. Hydrogen uptake by the dehydrated outgassed Ca-laponite was determined by Sievert’s method. Excess volumetric H2 isotherms were measured up to 1 bar at liquid nitrogen and liquid argon temperatures (77 and 87 K, respectively) using a Quantachrome Autosorb iQ2 sorption analyzer and were converted to absolute isotherms. Quasi-elastic neutron scattering (QENS) experiments were conducted on the indirect-geometry time-of-flight spectrometer IRIS28 at the ISIS pulsed neutron facility (Rutherford Appleton Laboratory, UK). The instrument was operated in offset mode using the (002) Bragg reflection of the pyrolytic graphite analysers. In this configuration the elastic energy resolution of the instrument was 17.5 μeV and the energy transfer, ℏω, ran from −0.3 to +1.2 meV. IRIS’s 49 functional detectors were grouped into 17 spectral groups, spanning scattering angles, 2θ, from 27 to 158° and giving Q from 0.46 to 1.84 Å−1. A 6.68 g sample of predried Ca-laponite powder was loaded into an annular cylindrical aluminum sample holder with inner diameter 10 mm, wall thickness 1 mm, and sample space thickness 2 mm. The temperature of the samples was maintained to an accuracy of ±0.1 K using a closed cycle refrigerator and belt heaters. Typical counting times were ∼6 h for each state point. In addition to the samples themselves, spectra were collected from the empty instrument, a vanadium standard, and the undosed clay substrate at 6 K. The QENS data were analyzed using the MODES software package (see Supporting Information). Spectra were corrected for selfabsorption and detector efficiency and were normalized by reference to the vanadium standard to yield the incoherent

uniform pore size with no bottleneck apertures. Although not competitive in terms of their likely gravimetric hydrogen uptake, the swelling 2:1 clays nevertheless provide an ideal arena in which to study H2 physisorption in layered hosts. Further motivation for studying these systems stems from the fact that the nuclear and toxic waste industries employ smectite clay minerals as a key component of their multibarrier containment mechanisms for toxic wastes. A number of toxic waste products, such as bitumen and radioactive incinerator ash,23,24 release significant quantities of H2 during their decay lifetime, as do the metallic containers themselves by corrosion.25 We present the results of neutron scattering studies of the structure and dynamics of H2 molecules absorbed into the interlayer region of sparingly hydrated Ca-exchanged laponite synthetic clay. We show that water acts as an ideal pillar to allow the two-dimensional clay galleries to accommodate a monolayer of interlayer H2 at cryogenic temperatures (Figure 1). These hydrogen molecules are able to coordinate directly to the partially solvated Ca2+ cations and move through the interlayer region via a jump diffusion mechanism.

Figure 1. Co-intercalation of water and hydrogen molecules into the interlayer galleries of Ca-laponite clay. This molecular graphics snapshot was obtained from an equilibrated Monte Carlo simulation and shows sparingly hydrated Ca-laponite with a layer−layer basal plane d-spacing of 12.43 Å (see Supporting Information and Figure 2). Each Ca2+ cation is coordinated to 1 or 2 water molecules, which act as pillars between the two-dimensional clay layers. Co-intercalation of eight hydrogen molecules per cation then gives rise to a twodimensional H2 monolayer of bulk liquid-like density. Color scheme: hydrogen, light gray; oxygen, blue; calcium, red; silicon, dark gray; magnesium, green.

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EXPERIMENTAL METHODS Laponite RD/EL is a synthetic clay with platelet diameter of approximately 25 nm and was prepared in its calciumexchanged form via three exchanges with 1 M solution of CaCl2 using presoaked Visking dialysis tubing with a molecular weight cutoff of 12 000−14 000 Da. The water content of the Ca-laponite was measured as a function of temperature using thermogravimetric analysis on a Hiden Isochema Intelligent Gravimetric Analyzer (see Supporting Information). Before use, all samples were dried under vacuum at 415 K before being ground to a fine powder and then redried. For each hydrogen uptake or neutron scattering experiment, samples were then B

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dynamic structure factor, S(Q, ω). The scattering due to adsorbed H2 was obtained by subtracting the background scattering from the undosed clay substrate.

between the dimensions of the clay pores and hydrogen molecules, the basal plane d-spacing of the pretreated Calaponite was then measured in situ at 40 K as a function of H2 loading (Figure 2). These data revealed a small increase in dspacing due to co-intercalation of hydrogen, up to 12.43 Å (dg = 2.98 Å) for n = 7.67 H2 per Ca2+ cation. Hence, the clay layer−layer expansion is, on average, around only 0.02 Å per H2 per cation. At this point, if we make no correction for the volume occupied by Ca2+ and D2O, the interlayer density of H2 is around 0.062 g cm−3. This compares to 0.071 g cm−3 in the bulk liquid at 23 K. In addition to the shifts in the position of the (001) Bragg peak evident in Figure 2, we note also that the intensity of this feature increases with increasing H2 content. This is due to the fact that hydrogen has a negative neutron scattering length (bH = −3.7406 fm), which contrasts with the positive scattering length density of the clay sheets. Thus, the (001) Bragg intensities confirm that the H2 is intercalated into the two-dimensional interlayer pores. The isosteric heat of adsorption, ΔHads, of hydrogen was obtained as a function of H2 uptake, n (eq 2), by fitting the experimental adsorption isotherms shown in Figure 3 to a

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RESULTS AND DISCUSSION Calcium-exchanged swelling 2:1 clays have a strong affinity for water,29−31 and under ambient conditions the interlayer galleries of Ca-laponite are occupied by fully (6-fold) hydrated calcium cations and excess water molecules (see Supporting Information). To allow effective absorption of molecular hydrogen into our clay, it was therefore necessary to reduce the water content to the point where the calcium cations were only sparingly hydrated while still retaining a sufficiently expanded pore structure to allow intercalation of H2. The structural formula of the Ca-laponite clay can be expressed as [(Si8Mg5.5Li 0.4)O20 (OH)4 ]−0.7 · [(Ca·x H 2O·nH 2)0.35 ]+0.7 Clay Layer

·

Interlayer (1)

where x and n represent the number of water and hydrogen molecules per cation, respectively. Thermogravimetric analysis showed that thermal pretreatment at 415 K under vacuum produced a starting material with water content per calcium cation in the range of x ≈ 1.0−1.5 (see Supporting Information). Neutron diffraction revealed that the basal-plane d-spacing of this thermally pretreated Ca-laponite starting material was 12.27 Å (Figure 2). This value compares with 9.45 Å in the

Figure 2. In situ neutron diffraction of thermally pretreated Calaponite, showing the differential scattering cross section (DCS) measured as a function of hydrogen loadings (open circles). The (001) Bragg peak occurs at around Q = 0.5 Å−1 and provides a direct measure of the basal-plane d-spacing via Q001 = 2π/d001. This Bragg feature was fitted to a single Gaussian (solid lines). The background was fitted to a Q−4 power law, representing Porod scattering (dashed lines). The residuals are shown on a separate axis below. Color is used to match the components for each loading as follows: red, 0.0 H2:Ca2+, d = 12.27 Å; orange, 2.76 H2:Ca2+, d = 12.36 Å; green, 4.83 H2:Ca2+, d = 12.37 Å; cyan, 6.55 H2:Ca2+, d = 12.41 Å; and blue, 7.67 H2:Ca2+, d = 12.43 Å.

Figure 3. Hydrogen uptake by the dehydrated outgassed Ca-laponite. (a) Absolute uptake volumetric isotherms measured at two different temperatures, 77 K (red) and 87 K (dark blue) for Ca-laponite pretreated at 415 K under vacuum, showing the fits to the fourth-order polynomial for both as the solid lines. (b) Isosteric heat of adsorption, ΔHads, calculated from the fitted isotherms using the Clausius− Clapeyron relation (eq 2). This gives heats of absorption of 9.2 and 5.2 kJ mol−1 at uptakes of 0.01 and 0.1 wt % H2, respectively. An uptake of 0.1 wt % H2 corresponds to 1.1 H2 per cation.

fourth-order polynomial and applying the Clausius−Clapeyron relation4

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uncharged anhydrous analogue, talc [Si8Mg6O20(OH)4]. Because of the pillaring effect of the intercalated water molecules, the interlayer gallery height of the sparingly hydrated Ca-laponite was therefore approximately dg = (12.27 − 9.45) Å = 2.82 Å. This value is very well matched to the kinetic diameter of H2 (2.89 Å). To confirm this compatibility

ΔHads(n) = −R[TT 1 2 /(T2 − T1)] ln(p2 / p1 )

(2)

where T1 and T2 are 77 and 87 K, respectively and p1 and p2 are the pressures required to produce the given H2 uptake at 77 and 87 K, respectively. This procedure yielded isosteric heats of C

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absorption of 9.2 and 5.2 kJ mol−1 at coverages of 0.1 and 1 hydrogen molecules per cation, respectively. These values will be reduced by the energy required to expand the clay itself to give the d-spacings measured by in situ neutron diffraction (Figure 2). This correction was estimated from computer simulations to be approximately 2.9 kJ mol−1 per H2 at low coverage (see Supporting Information). The saturation uptake, nm, was then estimated by fitting the experimental isotherms to the Tóth equation4,33 n = nm[kpm /(1 + kpm )]1/ m

(3)

where k and m are empirical constants. This procedure yielded an nm of 0.82 wt % H2 at 77K, corresponding to 9.1 H2 per cations and a limiting interlayer H2 density of approximately 0.074 g cm−3. The structure and dynamics of the absorbed molecular hydrogen were then elucidated by the incoherent approximation for quasi-elastic neutron scattering (QENS). This technique exploits the fact that, of all the relevant elements, the incoherent neutron scattering cross-section of hydrogen (1H) is by far the largest. For example, for hydrogen, σinc(1H) = 80.27 barn, whereas for deuterium, σinc(2H) = 2.05 barn. We therefore predeuterated our starting Ca-laponite so that the measured incoherent QENS signal was dominated by the absorbed H2. Under such circumstances, QENS provides unique insight into the total uptake, the nature of any confinement, and the mechanisms and time scales of molecular diffusion. For this reason QENS has been widely applied to studies of the dynamics of water in clays 34−36 and H2 physisorbed in a variety porous of materials.37−41 After data correction and normalization, the quantity that is measured in a QENS experiment is the incoherent dynamic structure factor42

Figure 4. Structure of H2 confinement in Ca-laponite by QENS. (a) Q-dependence of the EISF at two temperatures (blue diamonds, 40 K; red circles, 100 K) with fits (solid lines) to the proposed model including localized two- and three-site rotational diffusion (eq 5 and Figure 5). (b) Temperature-dependence of the intensity measured during QENS, showing the relative proportions attributed to elastic (blue) and quasielastic (green) scattering, as well as the total scattering (red). The red dotted line is an exponential fitted to the decay of the total intensity. The right-hand axis shows H2 uptake determined from the pressure of H2 gas over the sample (open black squares), which was fitted to an exponential (black dotted line).

S(Q, ω) = [EISF(Q) δ(ω) + (1 − EISF(Q))L(Q, ω)] (4)

where Q is the scattering vector and ℏω the scattering energy transfer. EISF(Q) is the elastic incoherent structure factor, which is the fraction of the total scattered intensity that is due to elastic scattering within the instrumental energy resolution of 17.5 μeV. This term gives information about the localization of particles (H2 molecules in this case) that are confined to a limited volume over the instrument’s time scale of approximately 2−50 ps. L(Q, ω), on the other hand, is the quasielastic contribution to the dynamic structure factor. This term provides information about the motion of particles, for example diffusion, over the instrumental time-scale. The spectra were fitted to eq 4 using Bayesian analysis (see Supporting Information), in which the instrument’s asymmetric resolution function was convolved with a Lorentzian representing EISF(Q) and a single Lorentzian with Q-dependent fullwidth half-maximum (fwhm), Γ(Q), representing L(Q, ω). Considering first the elastic component of the scattering, Figure 4 shows the total scattered intensity as a function of temperature and the EISF(Q) for the sample held at 40 K. The former provides a measure of the total uptake of H2, in excellent agreement with our volumetric calibration on the beamline, and also the contributions from elastic and inelastic scattering. We note that, in contrast to bulk liquid H2 which shows no elastic scattering,39 our H2-infused Ca-laponite has a clear elastic component at all four supercritical temperatures studied, indicating that a significant population of H2 is immobilized on the time scales of the instrument (Table 1).

This is consistent with the relatively large values for ΔHads measured at low H2 coverages. Table 1. H2 Content of the Ca-Laponite Samples Studied by QENS T (K)

p (bar)

H2/Ca2+

wt % H2

adsorbate phase density (g cm−3)b

40 60 (77a 80 100

0.49 0.75 1.00 0.92 1.00

8.29 4.51 2.50 1.96 0.77

0.76 0.41 0.23 0.18 0.07

0.067 0.036 0.020) 0.016 0.006

a

Calculated from isotherms shown in Figure 3. bCalculated from the interlayer volume assuming the d-spacing expands from 9.45 Å (talc) to 12.43 Å (see Figure 2) to accommodate the H2 and making no correction for the volume occupied by the interlayer Ca2+ and D2O.

The Q-dependence of the EISF(Q) then allows us to deduce the geometry and length-scale of this confinement (Figure 4). A satisfactory fit to the experimental data can be obtained by combining the powder-averaged models for localized two-42 and three-site43 rotational diffusion. In this case, EISF decreases with increasing Q according to D

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3 ⎛ ⎛ n ⎞⎞⎤ c1 c ⎡ (1 + j0 (Qr1)) + 2 ⎢∑ j0 ⎜2Qr2 sin⎜ π ⎟⎟⎥ ⎝ 3 ⎠⎠⎥⎦ 2 3 ⎢⎣ n = 1 ⎝

lengths follow an exponential distribution (see Supporting Information)

(5)

ΓSS(Q) =

where c1 and c2 and r1 and r2 are constants representing the weighting and length-scales, respectively, and j0 is the zerothorder Bessel function. Physically, the first term in the right-hand side of eq 5 represents localized motion between two sites, with a characteristic radius of gyration of r1 ∼2.8 Å at 40 K that immediately suggests immobilization between sites around a single Ca2+ cation. We note here that the Ca2+−H2 distance has been estimated as 2.5 Å in graphite intercalation compounds.44 The second term on the right-hand side represents rotational diffusion between three sites, with a slightly larger radius of gyration r2 ∼ 3.0 Å, also suggesting that the sites lie around a single Ca2+. This model is illustrated schematically in Figure 5.

ℏDQ 2 1 + DQ 2τ0

(6)

where D is the diffusion coefficient given by D=

L2 6τ0

(7)

Figure 6 shows the SS fits to the data for all four temperatures, while the corresponding fitting parameters and

Figure 6. Diffusional dynamics of H2, as shown by the Q2-dependence of the broadening of the quasielastic components measured at four temperatures. Red circles, 100 K; orange squares, 80 K; cyan triangles, 60 K; blue diamonds, 40 K; corresponding solid lines show leastsquares linear regression fits to the Singwi and Sjölander jump diffusion model.45 Figure 5. Proposed geometric arrangement of the interlayer region of Ca-laponite at a hydrogen content of 2.3 H2 per cation (top view). The hexagonal geometry is formed by the Si−O tetrahedra, shown in detail for a single hexagon only at the bottom right. Interlayer Ca2+, shown as randomly distributed at approximately 0.35 per unit cell, forms square-planar complexes with H2O and H2. On the basis of the average composition of 1.0−1.5 water molecules per cation, some complexes are shown to have one water and three H2, whereas others have two of each. The laponite unit cell is represented by the red dashed-line rectangle, and its dimensions are given by the arrows below and to the left. The measured jump lengths from the fitted QENS data at all four temperatures are shown to scale on the right (see Table 2).

Table 2. Fit Parameters and Diffusion Coefficients for the Singwi and Sjölander Jump Diffusion Model T (K)

na

100 80 60 40

0.77 1.96 4.51 8.29

Lb (Å) 5.5 4.5 3.7 3.2

± ± ± ±

0.6 0.3 0.3 0.4

τ0c (ps) 1.3 1.8 2.9 7.6

± ± ± ±

0.1 0.1 0.2 0.7

Dd (Å2 ps−1)

Dbulke (Å2 ps−1)

± ± ± ±

5.5 4.9 4.1 2.8

3.8 1.9 0.8 0.2

0.6 0.2 0.1 0.1

a

H2 per cation (eq 1). bMean jump length. cMean residence time. Calculated from D = L2/(6τ). eCalculated from D = D0 exp(−Ea/ kBT) where D0 = 8.58 Å2 ps−1 and Ea = 44.8 K.42

d

Bearing in mind that our sample was prepared with around 1.0−1.5 water molecules per cation and that Ca2+ adopts square planar solvation in similar 2:1 clays,29−31 then a model in which each partially hydrated Ca2+ can offer either 2 or 3 solvation sites to H2 is satisfyingly plausible. Next, we discuss the QENS component of the scattering, L(Q, ω), arising from translational motion of the spatially unrestricted fraction of H2 molecules. We find that this term is well fitted by using a single Q-dependent Lorentzian, whose full-width at half-maximum, fwhm = Γ(Q), is consistent with jump diffusion, wherein molecules execute a series of discrete hops of average length L interspersed by short-range oscillatory motion for an average residence time τ0. A number of models have been developed to describe this type of molecular motion in two-dimensional galleries, of which the best fit is obtained with the Singwi and Sjölander (SS) model45 in which the hop

diffusion coefficients are listed in Table 2. For comparison, we have calculated the diffusion coefficient of bulk H2 by extrapolating into the supercritical regime from the parameters obtained for liquid H2, as determined by O’Reilly et al.46 We note that at 40 K the diffusion of molecular hydrogen confined within the clay is an order of magnitude lower that that extrapolated from the bulk. The average jump length, L, of H2 in our Ca-laponite is well matched to the underlying clay lattice and the distance between partially hydrated cations. The geometric implications of the data obtained from the jump diffusion fit are summarized in Figure 5. The distribution of the cations presented in this diagram has been estimated from the crystallographic structure of eq 1; each unit cell contains 0.35 Ca2+. At 100 K, the jump length is long enough for the H2 to jump between cation complexes. At all temperatures measured, E

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clays are not competitive in terms of their gravimetric hydrogen uptake. However, if the same principles of pillaring and cation solvation can be applied to graphite intercalates, this opens up the possibility of achieving the theoretically predicted H2 storage performance for graphite-based physisorption systems.16

the estimated jump length is sufficient for the H2 to jump between adjacent hexagons. The temperature dependence of the measured diffusion coefficient is shown in Figure 7. If an Arrhenius relation is

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CONCLUSIONS In this study we have demonstrated that by using water as a molecular pillar, we are able to tune the layer−layer spacing of Ca-laponite clay to match the diameter of hydrogen (H2). This allows us to co-intercalate up to a monolayer of hydrogen molecules into the clay interlayer region at liquid-like densities. In situ neutron diffraction showed that at 40 K this process leads to expansion of the clay layer−layer spacing to 12.43 Å at 7.67 H2 per cation. Hence, the clay layer−layer expansion is on average around only 0.02 Å per H2 per cation, confirming that the interlayer galleries are well optimized for interaction with molecular hydrogen. Adsorption isotherms yield isosteric heats of absorption of 9.2 and 5.2 kJ mol−1 at coverages of 0.1 and 1 hydrogen molecules per cation, respectively. At 77 K, the limiting uptake is around 9.1 H2 per cation (0.82 wt %), corresponding to a complete monolayer. Quasielastic neutron scattering experiments conducted from 40−100 K reveal two populations of absorbed hydrogen. First, we observe molecules that are localized close to the partially hydrated Ca2+ cations. Second, we identify a more mobile liquid-like population whose motion is well captured by jump diffusion. We find that the average H2 diffusion coefficient is 2.3 ± 0.5 × 10−5 cm2 s−1 at 40 K, which is an order of magnitude slower than the value extrapolated from bulk liquid H2. Arrhenius analysis of the data yields 1.53 kJ mol−1 for the activation energy of this hopping. The clay interlayer therefore offers a tunable two-dimensional environment in which to study the physisorption of molecular H2, with the opportunity to control layer charge and to study interactions with a wide variety of interlayer cations and pillar molecules.

Figure 7. Arrhenius analysis showing the temperature dependence of the diffusion coefficient, measured by QENS, for H2 in Ca-laponite (RD) (magenta squares), giving Ea = 1.53 kJ mol−1. The diffusion coefficients for bulk liquid H2 extrapolated to the same temperatures are represented by the top two lines: a solid black line47,48 (Ea, 0.38 kJ mol−1) and a dashed black line46 (Ea, 0.37 kJ mol−1). Also shown are the data from two studies of hydrogen-adsorbing materials: an activated carbon, PFAC49 (Ea, 1.90 kJ mol−1), shown as black asterisks, and a carbon black, XC-7250 (Ea, 0.93 kJ mol−1), shown as black triangles. The dotted black lines passing through the latter two sets of data represent their regression analyses.

assumed, then the activation energy (Ea) is 1.53 kJ mol−1 (184 K) for H2 in Ca-laponite. This is similar to the 1.59 kJ mol−1 (191 K) value obtained from diffusion in bulk solid H2,47 whereas the value obtained for bulk liquid H2 is only 0.4 kJ mol−1 (44.8 K).48 It is also consistent with previous QENS experiments on MOFs MIL-53(Cr) and MIL-47(V), which gave values of 1.60 kJ mol−1 and 0.6 kJ mol−1, respectively.41 Of these two materials, only the former (MIL-53) contains hydroxyl groups at the metal−oxygen−metal links as occur on the laponite surface. In Figure 7 we also compare with data from two porous carbon-based materials.49,50 At this point it is worth noting again that at 40 K and 0.5 bar, the density of interlayer H2 absorbed into our Ca-laponite is around 0.067 g cm−3, if we assume dg = 2.98 Å and making no correction for the volume occupied by the interlayer Ca2+ and D2O (see Table 1). This is close to 0.071 g cm−3, which is the density of liquid H2 at 1 bar at the boiling point of 23 K. By analogy with the freezing point depression observed for water in the clay interlayer,51 this raises the intriguing prospect of supercooling liquid H2 below the predicted Bose−Einstein condensation temperature at 1 K.52 Our experiments have shown that coordinatively unsaturated metal centers (so-called “open metal” sites)1,5 can be created by partial solvation of the exchangeable cations in layered clay materials. Neutron scattering has confirmed metal−H 2 interactions, and at low H2 coverage we find an isosteric heat of adsorption, ΔHads, of 9.2 kJ mol−1, which is comparable to the values that can be achieved in MOFs2,12 and graphite intercalates.18 We have noted that uptake of H2 leads to a small increase in basal plane spacing in our Ca-laponite. Further increase in ΔHads is therefore possible by use of slightly larger solvent molecules, for example methanol, or by use of interlayer d-block cations.2 Because of the structure of their layers, 2:1

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ASSOCIATED CONTENT

S Supporting Information *

Details on the thermogravimetric analysis, Monte Carlo computer simulations, and analysis of quasielastic neutron scattering data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions #

These authors contributed equally (J.S.E and N.T.S.). The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are gratefully indebted to W. Spencer Howells for invaluable support with IRIS data analysis. This work was supported by the Engineering and Physical Science Research Council, through grant EP/P505224/1. We also thank Pat Jenness from Laporte Industries, Inc. UK, for the gift of the Laponite sample and Zhengxiao Guo for the use of the Quantachrome sorption analyzer. F

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