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J. Phys. Chem. C 2009, 113, 3222–3231
Experimental and Theoretical Investigation Into Hydrogen Storage via Spillover in IRMOF-8 Michael A. Miller,*,†,‡ Cheng-Yu Wang,§ and Grant N. Merrill*,† Department of Chemistry, UniVersity of Texas at San Antonio, San Antonio, Texas 78249, Department of Materials Engineering, Southwest Research Institute, San Antonio, Texas 78228, and Institute of Nuclear Energy Research, Longtan 32546, Taiwan ReceiVed: August 3, 2008; ReVised Manuscript ReceiVed: December 7, 2008
The principal obstacle to the implementation of a hydrogen-based economy remains storage under ambient conditions of temperature and pressure. While most approaches to this problem have to date focused on physisorption, wherein useful uptake is realized only at low temperatures (e.g., 77 K), recent experiments have suggested the possibility of chemisorptive strategies based on hydrogen spillover. We report the results of an experimental and a theoretical investigation into the thermochemistry of dihydrogen chemisorption on a catalytically activated isoreticular metal organic framework (IRMOF). Experimental results demonstrate a spillover mechanism leading to 2.5 wt % uptake at 75 bar for Pt-catalyzed, carbon-bridged IRMOF-8 [Zn4O(naphthalene-2,6-dicarboxylate)3], while theoretical results suggest the thermochemical plausibility of this mechanism and offer upper limits for chemisorption in these materials. Laser thermal-desorption mass spectrometry measurements further reveal multiple binding sites occurring between 263 and 298 K, substantially higher than that for simple physisorbed dihydrogen (165 K) and consistent with the reported isosteric enthalpies based upon theoretical partition functions for the chemisorbed structures. 1. Introduction As the costs of petroleum products continue to rise, coupled with national security and pollution concerns, increased calls for alternative fuels can be heard. One highly touted alternative fuel is hydrogen. The potential of this alternative energy source can hardly be overstated; it is essentially unlimited in supply and does not need to be imported, and its use leads to little if any pollution.1 The largest obstacle to the implementation of a hydrogenbased economy is storage. This is particularly true for transportation applications. In an attempt to prod development of viable hydrogen-storage systems for vehicles, the U.S. Department of Energy has established a series of milestones.2 By the year 2010 storage systems must meet standards of 6% by weight and 45 kg/m3 by density. These targets increase to 9% by weight and 81 kg/m3 by density by 2015. It should also be noted that these weight percentages are for the total storage system; for example, the 2010 target actually amounts to about 8-12% by material weight alone to compensate for the total weight of the charged storage system. At present, no hydrogen-storage system meets these gravimetric or volumetric targets.3 Compressed or liquefied hydrogen will not meet these goals as liquefied hydrogen at 20 K and 1 atm amounts to a density of only 70 kg/m3. Most research has, therefore, turned to solid hydrogen-storage solutions. All of these approaches are predicated upon either physi- or chemisorptive strategies. Physisorptive materials have yielded rather disappointing gravimetric and volumetric results to date,4 and increased attention has, therefore, been focused on chemisorptive * To whom correspondence should be addressed. (G.N.M.) Fax: (210) 458-7428. E-mail:
[email protected]. (M.A.M.) Fax: (210) 522-6220. E-mail:
[email protected]. † University of Texas at San Antonio. ‡ Southwest Research Institute. § Institute of Nuclear Energy Research.
materials. Of all the chemisorptive materials studied, hydrides have received the most attention.5 These materials have evinced the greatest storage capacities to date, but they are plagued by high weight and, in many instances, slow kinetics. A number of researchers have investigated the storage of hydrogen in organic materials.6 Of particular note has been the work of Yang and co-workers,7 where their approach has made use of spillover of hydrogen chemisorbed on catalytic metal surfaces onto amorphous and crystalline substrates. The best results obtained by these researchers has involved a platinum (Pt) catalyst supported on activated carbon (AC), which is in turn coupled to an isoreticular metal organic framework (IRMOF) via a bridging compound composed of amorphous carbon (BC). The mechanism proposed for these Pt/AC/BC/IRMOF heterostructures can be summarized concisely: (1) adsorption of dihydrogen on the Pt surface, (2) dissociation of dihydrogen and chemisorption of atomic hydrogen on the surface, (3) migration of atomic hydrogen onto the AC support, and finally (4) chemisorptive spillover onto the IRMOF substrate. Spillover is facilitated through the use of an amorphous-carbon bridging compound between the Pt/AC complex and the IRMOF substrate. It is possible that modification of any of the elements of these heterostructures (e.g., choice of IRMOF) could lead to improved storage under near ambient conditions. While this mechanism may be endothermic (BDE(H2) ) 4.5 eV),8 higher than expected uptake still occurs reversibly at room temperature. In order to assess critically the viability of hydrogen-storage materials predicated upon spillover, a better understanding of the underlying spillover mechanism is required. This can only be accomplished by establishing the origins of kinetic and thermodynamic effects associated with these types of materials. In this paper, we outline an experimental and theoretical investigation into the storage capabilities of a Pt/AC/BC/IRMOF
10.1021/jp806916a CCC: $40.75 2009 American Chemical Society Published on Web 02/02/2009
Hydrogen Storage via Spillover in IRMOF-8 system, specifically, IRMOF-8 (containing a naphthalene-2,6dicarboxylate organic linker, I) with a glucose-derived bridging compound.
2. Experimental Methods 2.1. Materials Used. Purified Gases. Hydrogen and helium (99.999%, as purchased) were purified in separate ultra-highpurity (UHP), stainless-steel gas manifolds (electropolished, 0.25 µm Ra) using redundant, high-pressure gas purifiers (Pall Microelectronics, East Hills, NY; model Gaskleen ST) to achieve contaminant levels S2(NDC-3a) ≈ S2(NDC-9a). This correlation was also seen for the BDC radicals: S2(BDC-1a) > S2(BDC-3a) > S2(BDC-5a). This observation may serve as a diagnostic on the quality of the wave function for radicals,31 and it is currently being explored in depth in our laboratory. Once again, given the lack of comparable experimental data it is not possible to state conclusively which level of theory is more reliable. Examination of the enthalpies of molecular hydrogen addition (Table 4) reveals the two levels of theory to be in qualitative agreement with one another. The DFT calculations predict that
3230 J. Phys. Chem. C, Vol. 113, No. 8, 2009
Miller et al.
TABLE 5: Comparison between Vibrational Energies Derived from the Theoretical Partition Functions for Hydrogenated NDC Structures (at the lower and upper desorption range of LTDMS measurements) and the Isosteric Enthalpies for Pt/AC/BC/IRMOF-8 at Low Hydrogen Concentrations vibrational energy from theoretical partition function (kcal/mol) hydrogen isotherm data (literature values)
T (K)
excess conc. (wt % H)
isosteric enthalpy (kcal/mol H2)
298 323 348
0.0078 0.0108 0.0137
-5.92 -5.09 -4.80
a
structure LTDMS-observed desorption rangea (K)
0a
2a
4a
6a
8a
10a
260 294
4.57 5.17
5.96 6.74
5.89 6.66
5.21 5.89
6.04 6.83
6.11 6.91
Measured hydrogen concentration ) 0.0097 wt % (see Table 1).
Figure 6. Plot of the computed ethalpies for sequential addition of 1 equiv of dihydrogen to NDC. Gray band indicates ideal range of binding energies for reversible uptake at room temperature.
the first addition of H2 to be mildly endothermic, corresponding to disruption of the quasi-aromatic naphthalene system, across the 1 and 4 positions. The second addition across the 2 and 3 positions is found to be exothermic as there is no aromatic penalty to be paid. The third addition must be made to the benzene moiety, and it is, therefore, predicted to be endothermic. Finally, the fourth and fifth additions are both predicted to be exothermic. The HF enthalpies mirror those calculated at the DFT level with the exception of the first addition, which is predicted to be slightly exothermic at the HF level of theory. Total enthalpies -47.4 and -70.6 kcal/mol are computed for addition of 5 equiv of molecular hydrogen at the DFT and HF levels of theory, respectively. These values correspond to average enthalpies of addition for 1 equiv of molecular hydrogen of -9.5 (DFT) and -14.1 (HF) kcal/mol. 3.5. Comparison of Experiment and Theory. The temperature range over which hydrogen was desorbed in the LTDMS measurements (260-294 K) for Pt/AC/BC/ IRMOF-8 can be transformed to the thermodynamic total energy of the chemical system by calculating the microcanonical partition functions for the hydrogenated NDC structures, which are determined from the ab initio calculations. In the present case we assume that the dynamics of the free NDC structures are representative of crystal-lattice dynamics and consider only vibrational dynamics as rotations and translations of NDC linkers in IRMOF-8 do not occur. The results are given in Table 5 as vibrational energies for the lower and upper temperature range of hydrogen desorption (LTDMS) compared with the isosteric enthalpies for Pt/AC/BC/IRMOF-8 previously reported by Li et al.32 Here,
we find that there is good agreement between the combined experimental (LTDMS) and theoretical (ab initio calculations) values and the independently measured isosteric enthalpies for similar hydrogen concentrations. Given the assumptions, this analysis suggests that up to 3 equiv of molecular hydrogen may be chemisorbed in some of the linkers of the bulk sample at low doses of hydrogen (0.0097 wt %). 4. Conclusions The present study aimed at establishing the thermochemical plausibility for hydrogen uptake in catalytically doped IRMOFs via a chemisorptive pathway; specifically, the dissociative spillover of hydrogen at catalytic sites followed by hydrogenation of the organic linkers in the framework was studied. Using high-pressure gravimetric methods we corroborated the reversible uptake of hydrogen in Pt/AC/BC/IRMOF-8 at room temperature (2.5 wt % at 75 bar) and further showed by laserinduced thermal desorption mass spectrometry that hydrogen in this heterostructure is bound to multiple sites, which collectively occur well within the reversible chemisorptive regime at room temperature (-5 to -14 kcal/mol). This level of uptake is clearly far higher than that observed in pure IRMOF-8 (0.5 wt % at 100 bar)11 and cannot be explained by physisorption alone at room temperature. This uptake is, however, quite slow as noted by the 5-10 h periods required to reach gravimetric equilibrium at a given pressure. We also observed the unusual result that the isotherm curves are nearly linear over a broad range of equilibrium pressures. These
Hydrogen Storage via Spillover in IRMOF-8 characteristics are consistent with a spillover mechanism, where molecular hydrogen rapidly dissociates to atomic hydrogen on a catalytic site, followed by migration onto the framework. Diffusion of atomic hydrogen onto the organic framework is retarded with increased hydrogen uptake. While the experimental results are compelling, the proposed mechanism for hydrogen spillover would belie thermodynamic arguments unless the products for hydrogen addition to the framework are energetically favorable relative to the dissociation energy of molecular hydrogen in the gas phase. Given that hydrogen uptake in Pt/AC/BC/IRMOF-8 exhibits isothermal reversibility, we recognize that the degree to which hydrogen additions yield exothermic configurations must be tempered by the ease with which hydrogen can be removed during isothermal desorption. Ab initio calculations show that consecutive additions of molecular hydrogen to the organic linker in IRMOF-8 are mostly exothermic with some additions being only slightly endothermic if system aromaticity is disrupted (Table 4 and Figure 6 for B3LYP/6-31G(d)/HF/6-31G(d)). Addition of up to 4 equiv of molecular hydrogen (0a f 2a f 4a f 6a f 8a) represents the most saturated structure in which the enthalpy for incremental addition of 1 equiv (6a f 8a, -18 kcal/mol) is closest to the upper value of the range generally accepted as being conducive to reversible chemisorption at room temperature (-14 kcal/mol).33 This structure (8a, Figure 5) corresponds to a theoretical uptake of 2.56 wt % on the basis of an IRMOF-8 formula unit [Zn4O(2,6-NDC)3], which would represent the limiting uptake in these types of heterostructures at room temperature if it is assumed that chemisorptive uptake via spillover occurs only on the NDC linkers. The linearity of the measured isotherms at room temperature in our results indicates that the limiting uptake is higher than this value since saturation (i.e., plateau in the isotherm curve) is not observed. The implication of this is that other stable, though reversible, binding sites, such as on the zinc vertices of the framework, may also play a role in hydrogen uptake via spillover in IRMOF-8. It is important to point out that the present theoretical calculations do not consider the effects of the lattice on hydrogen uptake. While it is fairly certain that the deformations of the lattice will have an impact upon the thermodynamics and kinetics of hydrogen uptake, it is not altogether clear if they will be favorable or unfavorable. These very calculations are currently being carried out in our laboratory. Experimental evidence for the proposed spillover mechanism in catalytically doped IRMOFs may be immediately accessible by conducting in situ Raman, nuclear magnetic resonance (NMR), and inelastic neutron scattering measurements under hydrogen pressure to determine which type of linker atoms have actually undergone hydrogen addition. Finally, the diffusion limitations of hydrogen spillover may be overcome by synthesizing MOF-based heterostructures in which very small catalyst particles are captured within the periodic voids of the framework and, thus, permitting dissociative spillover to occur in the immediate vicinity of a linker for chemisorption. These experiments and synthetic approaches are also currently being undertaken in our laboratory. Acknowledgment. This work was supported in part by the U.S. Department of Energy under contract DEFC3602AL67619 and by Southwest Research Institute’s Internal Research and Development Program. Supporting Information Available: Table S1. Enthalpies and Gibbs Free Energies of Hydrogenation. Table S2. S2
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