Theoretical Explanation of Hydrogen Spillover in Metal−Organic

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Theoretical Explanation of Hydrogen Spillover in Metal-Organic Frameworks George M. Psofogiannakis and George E. Froudakis* Department of Chemistry, University of Crete, P.O. Box 2208, Voutes, Heraklion Crete, 710 03, Greece ABSTRACT: The thermodynamics and kinetics of hydrogen spillover in the IRMOF-1 (MOF-5) structure were studied via density functional theory. Addition of six H atoms is thermodynamically favored with respect to molecular H2, which is close to the maximum capacity that has been observed experimentally by spillover in IRMOF-1 (∼2.6 wt % excess). Through modeling, mechanistic details of the spillover process are proposed: During hydrogen spillover, atomic hydrogen cannot migrate in the chemisorbed state in the pore network but rather diffuses in the gas phase. During diffusion, atomic hydrogen can stick on all available binding sites on the linkers with negligible energy barriers. However, diffusing hydrogen atoms can also abstract adsorbed hydrogen atoms via Eley-Rideal pathways to form H2. Diffusion of atomic H in the MOF is greatly assisted by pore defects, and spillover would be plugged in ideal MOF structures. Experimental observations for spillover in MOFs are discussed in view of these results, and possible ways of enhancing spillover capacities are presented.

’ INTRODUCTION Hydrogen storage is still a bottleneck for the utilization of hydrogen as an energy source for fuel-cell vehicles. Sorption in solid sorbents via physisorption or chemisorption pathways is being extensively investigated. Up to date, no material or method has been able to satisfy reproducibly the widely accepted gravimetric uptake target set by DOE, which is 6 wt % for 2010. One of the most promising pathways for hydrogen adsorption utilizes the so-called spillover mechanism. The hydrogen storage capacity of a variety of adsorbents, such as graphite nanofibers, activated carbons, carbon nanotubes, metal-organic frameworks, and oxidized graphites, can be enhanced by doping them with small amounts of transition metals (Pt, Pd).1-6 Hydrogen storage on such materials has been described through H spillover, a mechanism that involves the dissociation of H2 on the metal particles, atomic H diffusion from the particles to the sorbent, and chemisorption of atomic hydrogen on the material’s binding sites.7,8 A very important attribute of this mechanism is that it is operative at ambient temperature as opposed to storage of H2 by physisorption, which requires very low temperatures. Li and Yang have done pioneering work on hydrogen spillover in MOFs.9 They mixed a small quantity of supported Pt catalyst (Pt/AC) with IRMOF-1 and with IRMOF-8 and used a carbonated sugar to provide better contact between the materials. They measured hydrogen uptakes close to 4 wt % for IRMOF-8 and close to 3 wt % for IRMOF-1 at 100 bar and 298 K. Importantly, they noticed that adsorption of H2 was reversible. Furthermore, the measured adsorption isotherms had close to linear shapes. Later, Miller et al.10 obtained very similar results for H2 adsorption on Pt/AC- carbon bridge-IRMOF-8 mixtures and further identified multiple H-chemisorption sites by laser thermal-desorption mass spectrometry. Recently, Tsao et al.11 measured 4.7 wt % at 69 bar and 298 K for IRMOF-8 in which r 2011 American Chemical Society

they had created a partially collapsed MOF network. All these values are among the highest ever reported in the literature for room-temperature adsorption of H2. Furthermore, the experimental data so far show that defective MOF structures are necessary for spillover enhancement. As a result, the spillover mechanism for H2 storage deserves further examination. Despite the promising results, there is controversy in the research community regarding the plausibility of the spillover mechanism and its usefulness as a hydrogen storage mechanism. These controversies mainly arise from the limited understanding of the process that can be afforded through experiment. Theoretical-computational work can be very useful in this case. In the past, we successfully used accurate ab initio methods to study the elementary steps of hydrogen spillover on carbon-based materials.8 In the present work, we use ab initio methodology to examine the thermodynamic and kinetic features of the interaction of IRMOF-1 with atomic hydrogen in order to gain enhanced understanding of the spillover mechanism in MOFs. In particular, in this work we aimed to (a) Identify the thermodynamically stable structures for the hydrogenated MOF and their stability with respect to molecular H2. (b) Identify the nature of diffusive transport of atomic H within the structure. (c) Probe the kinetics of H sticking on the available binding sites. (d) Probe the kinetics of Eley-Rideal H-abstraction reactions by gas-phase H from the hydrogenated structures.

Received: October 5, 2010 Revised: January 22, 2011 Published: February 22, 2011 4047

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Figure 1. DFT optimized structures of hydrogenated IRMOF-1 linkers. L1-L5: large models (RI-DFT/BP86/def2-TZVP), S1-S5: small models (DFT/B3LYP/6-31G**). L1, S1: Pristine linker; L2, S2: Linker with 4 additional chemisorbed H atoms on Ca carbons; L3, S3: Linker with 6 additional H atoms on Ca, Cb; L4, S4: Linker with 10 additional H atoms on Ca, Cb, Od; L5, S5: Linker with 12 additional atoms on Ca, Cb, Cc, Od.

(e) Clarify the mechanism of spillover in MOFs as a collection of elementary steps, explain the observed adsorption isotherms, and propose ways of enhancing the efficiency of the mechanism. IRMOF-1 was chosen as a representative member of the IRMOF family because of its simpler linker structure that allows for a complete set of calculations. Furthermore, the majority of previous experimental and theoretical works connected to spillover have been performed with IRMOF-1 and IRMOF-8. On the basis of the similar structures of the IRMOF family members, we can expect that the basic spillover mechanism should be quite similar on other MOFs as well.

’ COMPUTATIONAL METHODOLOGY To simulate the periodic framework of IRMOF-1, we used the cluster approach. Figure 1 shows the density functional theory (DFT) optimized structures of two different cluster models for IRMOF-1 (L1, S1) together with some hydrogenated analogues. The larger cluster, labeled L, contains two Zn4O metal centers, the surrounding carboxylate groups, and one BDC linker between them. In the optimizations, the remaining carboxylate groups were saturated with one H atom each (not shown in the figure). The Zn4O cluster atoms of the hydrogenated structures were kept fixed at the optimized coordinates of the pristine structure (L1) in order to avoid unnatural deformation of the entire structure upon saturation with H. Optimizations of the large clusters were done using the ri-DFT/BP86/def2-TZVP level of theory in Turbomole.12

However, to perform the numerous time-consuming optimizations and internal reaction coordinate scans that will be described in the following, it was necessary to use a smaller efficient model for the IRMOF-1 linker (model S in Figure 1). In this model of the BDC linker, the O atoms of the carboxylic groups interact with a Li atom, which provides a better description of the actual electronic distribution by polarizing it. Li and O atoms were kept fixed in the hydrogenated structures of the S model to avoid unnatural deformations. It was noticed that the coordinates of O atoms in the large model changed insignificantly upon hydrogenation, which justifies our choice. All optimizations for model S, as well as further calculations described in this work, were done at the DFT/B3LYP/6-31G** level using Gaussian03.13 The small model has been extensively used to describe energetics of H and H2 interactions with IRMOF-1.14,15 Figure 1 also shows the nomenclature used throughout the paper for the various atoms of the linker. H adsorbed on Ca, Cb, Cc, and Od will be labeled Ha, Hb, Hc, and Hd, respectively.

’ RESULTS AND DISCUSSION Calculation of Theoretical Hydrogen Capacity. In the following, we consider the energetics of atomic H addition on the BDC linkers of IRMOF-1. In structures L2 and S2, four equivalent Ha atoms were added to the π-system. Although the structure shown is not the minimum-energy configuration for four added H atoms, as will be clarified below, it serves as a comparison for H binding on Ca between the two models. In 4048

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Table 1. Atomic H Binding Energies per H Atom Corresponding to the Structures of Figure 1a energy per H (eV) large model

energy per H (eV) small model

Ha

-2.1

-2.0

Hb Hd

-4.0 -0.9

Hc

-2.6

Table 2. Incremental H Binding Energies and Incremental H2 Adsorption Energies for Sequential H Addition on the IRMOF-1 Linker in the Order Shown in Figure 2a incremental H binding

incremental H2 adsorption

-4.4 -1.0

H atom #

energy EB,H (eV)

energy EB,H2 (eV)

1 (Ha)

-1.36

-2.7

2 (Ha)

-3.33

0.16

3 (Hb)

-2.35

-0.84

4 (Ha) 5 (Ha)

-3.73 -2.03

-1.23 -0.91

6 (Hb)

-3.81

-1.00

7 (Hd)

-0.22

0.81

8 (Hc)

-3.93

0.69 -0.14

a

Ha denotes each of the four additional H’s added in structures L2, S2; Hb denotes each of the two additional H’s added in structures L3, S3; Hd denotes each of the four additional H’s added in structures L4, S4. Negative numbers represent binding.

Figure 2. The order of hydrogenation of the BDC linker by atomic H.

structures labeled L3, S3, two additional Hb atoms were added on the substituted carbon atoms of the ring (Cb). The optimized structures resemble the chair conformation of cyclohexane. It was found that H cannot be added to the C atoms of the carboxylate groups (Cc) when only the benzene ring is hydrogenated as the adsorption is endothermic by about 0.6 eV/H atom. The O atoms of the carboxylates can be hydrogenated. The optimized structure is labeled L4 and S4 for the two models, respectively. When the O atoms are hydrogenated, H can be added to the Cc atoms. The final structures obtained by complete hydrogenation of the ring are labeled L5, S5 in Figure 1. In all cases, the structures of the two models are in good agreement. The binding energy per H atom corresponding to structures L2-L5 and S2-S5 is shown in Table 1. The binding energies for Hb are those of the two additional H atoms of structures L3, S3 compared to structure L2, S2 per H atom. The binding energies for Hd are those of the four additional H atoms of structures L4, S4 compared to structures L3, S3 per H atom. The binding energies for Hc are those of the two additional H atoms of structures L5, S5 compared to structures L4, S4 per H atom. The calculated binding energies for the two models approximately agree with each other, which justifies the choice of using the smaller cluster for the remaining calculations. Furthermore, the structures and energies show that 12 H atoms can be added to the linker, which translates to a maximum theoretical 4.5 wt % H2 excess uptake within the MOF structure (36 H atoms per formula unit Zn4O(BDC)3). Thermodynamics of Hydrogen Addition to IRMOF-1 Linker . The order of hydrogenation of the BDC linker, as predicted by DFT optimizations of the partially hydrogenated possible configurations of the linker using the S model, is shown in Figure 2. Same-side Ca ring atoms are hydrogenated first. Cb atoms are hydrogenated third and sixth in sequence. For the ring atoms (1-6), this order has also been verified in previous computational work.10 One carboxylate Od atom is hydrogenated and then the seventh, which is necessary for H sticking on the Cc atom (eighth). The existence of the O-H bond turns the Cc-H bond formation from endothermic to highly exothermic as the Cc atom adopts a geometry that is favorable for tetrahedral coordination. Additional stability is then gained by hydrogenation of the other Od atom. The process of hydrogenation continues in the same order on the other carboxylate group.

9 (Hd)

-1.06

10 (Hd)

þ0.61

4.40

11 (Hc)

-4.70

0.76

12 (Hd)

-0.29

-0.14

For the nth atom: EB,H = EN - EN-1 - EH and EB,H2 = EN - EN-2 EH2 where EN is the electronic energy of the linker with N hydrogen atoms adsorbed. Calculated using model S with DFT/B3LYP/6-31G**. a

The incremental binding energies per each additional H atom and per each additional H2 molecule, corresponding to the 12 sequential hydrogen additions in the order of Figure 2, are shown in Table 2. The results for the first six H additions (hydrogenation of the ring) are discussed first. As shown in the first column of Table 2, each of the first six H additions is exothermic and therefore thermodynamically favored over the high-energy H-radical state. The first H addition has the lowest exothermicity for the ring as it breaks the resonance state. In the second column of Table 2, the incremental energy change upon adsorption, per H2 molecule, is presented, corresponding to the sequential binding of H atoms. A negative value of this number for the nth atom shows the thermodynamic preference for H2 formation by abstraction of the (n - 1) atom to form H2 compared to the propensity for H addition. Addition of the first H2 molecule is overall endothermic (0.16 eV), while additions of the second and third H2 are very exothermic (-1.23 and -1.00 eV, respectively). It is a well-known result in the literature of hydrogenation of benzene that the first H2 addition is endothermic, while the other two additions are much more exothermic.16 Overall, completely hydrogenating the ring with three H2 molecules is quite exothermic and thermodynamically favored. Obviously, this link between electronic energies and thermormodynamic quantities (reaction energies) neglects entropy and thermal energy effects. These are, however, lower-order corrections. The thermodynamic propensity for complete hydrogenation of the benzene ring is assured by the large negative reaction energy in adding the second and third hydrogen molecule to the ring. The results for the hydrogenation of the -CO2 groups are presented next. The first column of Table 2 shows the calculated incremental binding energies of H atoms on the Od and Cc atoms of the BDC linker, where extra stability is gained upon adsorption of the H atoms on the Cc atoms. The binding energy of the seventh atom (the first adsorption on the -CO2 group) is slightly exothermic (-0.22 eV), while that of the eighth Hc atom is much more exothermic (-3.93 eV) and shows increased stability associated with occupation of the Cc atom when one OH bond is formed. The addition of the 10th H atom, the first on 4049

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The Journal of Physical Chemistry C the other side of the ring, is endothermic. This is due to the increased strain of the structure when the first -CO2 group is saturated with H. The linker tends to shrink upon oversaturation with H, which shows up as decreased binding strength because the dimensions of the cluster are fixed in the optimization to imitate the behavior of the surrounding MOF environment. The second column of Table 2 clearly shows that the addition of H2 molecules on the -CO2 groups is a very endothermic process, so that the thermodynamic driving force is toward formation of H2 and reversion to the conformation with six added H’s. Therefore, provided the kinetics are fast enough, a gas-phase H atom interacting with the Hd atom will tend to abstract it to form H2 because of the much lower O-H bond energies compared to the H2 bond energy. Overall, the structure that contains six additional H atoms (L3, S3) is the thermodynamic sink in the overall scheme. Adding six H atoms to the linker translates to an excess uptake of 2.3 wt %, very close to the maximum uptake that has been observed experimentally for IRMOF-1 (which is ∼2.6 wt % excess uptake because of spillover9). Calculations on the binding energies of atomic hydrogen on the IRMOF-1 linker have been previously also performed by other researchers. Our results on the thermodynamics of addition of the first three H2 molecules are in very good agreement with those of Miller et al.10 Our results on the binding energies of a single H atom on the various MOF sites are in good agreement with those of Mavrandonakis and Klopper,17 who have also elaborated on the computational inconsistencies of the model by Li et al.18 Suri et al.15 obtained similar binding energies for H on the linker sites but suggested, by counting 12 possible H binding sites on the BDC linker, that ∼4.5 wt % H2 could potentially be stored in IRMOF-1 by spillover. However, it was clearly shown in the calculations that structures with more than six added H atoms are unstable with respect to forming H2, so that such high capacities cannot be expected by IRMOF-1. H Diffusion within MOF Pores. The kinetics of the elementary processes that can take place during spillover was then examined. It is well-known that some transition metals, such as Pt and Pd, can dissociate H2, and previous work has shown that this hydrogen can be pumped out of the metal nanoparticles and chemisorb on the MOF.10 If we assume as reality that the increased uptake during spillover is indeed due to atomic hydrogen spillover and hydrogenation of the binding sites of the linkers, then atomic hydrogen has to enter the pore structure and diffuse within the pores. There are two ways this could potentially happen: (1) Diffusion of chemisorbed H. (2) Diffusion of gas-phase (or weakly physisorbed) atomic H. Even with simple examination of the large differences in binding strengths between the various binding locations in the MOF structure, diffusion in the chemisorbed state is very unlikely. For a single H atom, the binding energy of H on the four different binding sites for the cluster S1 was calculated to be -1.36 eV for Ha, -1.06 eV for Hb, þ0.34 eV for Hc, and -0.82 eV for Hd. For Hc, there is a local energy minimum, which, however, is very unstable as desorption of this H atom is exothermic. The binding energies are in good agreement with those of ref 17. We performed reaction coordinate calculations and located approximate transition states for the migration of a single chemisorbed H atom through the linker as shown in Figure 3. The results are presented as energy versus free H energy, and the

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Figure 3. Energy diagram for the migration of atomic H on the IRMOF1 linker in chemisorbed state. The barriers are shown in red color. The binding energies of the H atom on the various atoms are shown in blue. All energies are given versus the free H energy (dotted line).

figure summarizes the energy barriers for the corresponding diffusion elementary steps. These calculations were done using the cluster model S. For the migration of the Hd hydrogen to a proximate O atom (Hd to Hd, not shown in the figure), a model containing one metal cluster and two BDC linkers was used. It was found that for the migration of chemisorbed H between two O atoms, the transition state required large deformation of the structure (unrealistic in the rigid MOF framework), and even then the energy barrier to diffusion was larger than the desorption energy. This is due to the large distance between the O atoms. We conclude that the Hd hydrogen must desorb to be transferred to a different linker. From Figure 3 we can make the following observations: (a) The energy barriers for many of the elementary diffusion steps are very high (up to about 1.60 eV). (b) H atoms in all binding sites can desorb easier than diffuse in chemisorbed state. Most energy barriers are between 1.3 and 1.6 eV. In fact, similar energy barriers have also been calculated for H diffusion on graphitic surfaces.8,19 The similarities stem from the fact that in both cases migration requires breaking the C-H bond of sp3 carbon and forming a new C-H bond. Theoretical and experimental studies have shown that chemisorbed H cannot diffuse on a graphitic surface in chemisorbed form.8,19,20 On the basis of the calculated energy barriers and binding energy differences, the chemisorptive pathway of H diffusion is unrealistic for room-temperature spillover on ideal MOF structures. These observations suggest that that atomic H possibly diffuses either in the gas phase or in a weakly physisorbed state within the pores of defective MOF structures. A radically different possibility has also been recently suggested: The authors of ref 21 have proposed a hole-mediated mechanism whereby it is argued that the traditional diffusion of chemisorbed H atoms within the defective MOF is assisted by the existence of electron holes that are presumably created by defects in the 4050

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Figure 4. Energy vs C-H distance for sticking of a free H atom on Ca for the IRMOF-1 linker model S. The inset shows the final (left) and the initial (right) structures. The energy barrier for sticking is 0.08 eV. (The line is a guide to the eyes.)

Figure 5. Energy vs H-H bond length for Eley-Rideal abstraction of the fourth adsorbed H atom from the IRMOF-1 linker. The inset shows the final (left) and the initial (right) structures. The energy barrier for abstraction is 0.11 eV. (The line is a guide to the eyes.)

structure. However, we will show in the following that diffusion of H in the gas phase is plausible and can explain qualitatively many observations associated with spillover, including the necessity of pore defects. The gas-phase spillover of H atoms has been previously proposed by Baumgarten and co-workers22,23 to explain reactions caused by spillover H that was generated in a physically isolated compartment in the reaction setup. A recent molecular dynamics simulation24 of hydrogen spillover on a Pt/γ-alumina catalyst identified a pathway whereby a H2 molecule dissociates on Pt and one of the two H atoms is ejected from the metal to the gas phase while the other H atom binds to the surface on a kink Pt atom of the nanoparticle. Essentially, the ejected H atom gains kinetic energy because of the dissociation of the H-H bond. This pathway can prove very important to the overall spillover mechanism. The gas-phase H spillover can naturally explain the transfer of H atoms between dissimilar surfaces, which would require very high energy barriers if H diffuses between them in chemisorbed form. For example, chemisorbed H migration from the metal to a carbon-based support is a highly activated process.8 Furthermore, it has been found that atomic H cannot diffuse on a graphitic surface in chemisorbed form because of very high energy barriers.8,19 Thus, it is difficult to justify how chemisorbed H can escape the support and migrate to the MOF crystals in the traditional mechanism. The migration of atomic H on surfaces that bind H strongly, and the transfer of H atoms between dissimilar surfaces, has long been difficult to justify theoretically,25 while the proposed mechanism gives a plausible explanation. The spillover mechanism has been found to be functional on several materials, such as metal-doped carbons, oxides, and zeolites.25 The generation of gas-phase H atoms gives a general explanation of H transfer to distant binding sites irrespective of the chemical nature of the materials (as long as a transition metal is present) without excluding the possibility that gas-phase H atoms can subsequently bind on surfaces and surface-diffuse. H atoms directly ejected from the nanoparticles may diffuse in the gas phase or get trapped in physisorption wells or bind on strong chemisorptions sites. In fact, physisorbed H atoms have been directly identified on a Pt/C catalyst by inelastic neutron scattering.26 As H atoms are very reactive, it is necessary that the nanoparticles and the final binding sites are in close contact, which justifies the importance of good mixing of the spillover source and receptor materials. In this mechanism, the ratelimiting step in the overall process is probably the generation of airborne H atoms.

In the following, we examine the reactivity of gas-phase H atoms that come in contact with the MOF linkers. Two distinct reactions can be postulated for the interaction of H atoms with the MOF: H atoms can directly stick on the linker sites or can recombine with adsorbed H atoms of hydrogenated linkers in Eley-Rideal (ER) type reactions. We examine the kinetics of these reactions and their implications on the spillover mechanism. Kinetics of H Sticking on the Linker Sites. The kinetics of atomic H sticking on the binding sites of the linker was examined next. To realize the thermodynamic driving force for formation of the structure with six additional hydrogen atoms, it is necessary that barriers to sticking are low enough. We performed internal reaction coordinate (constrained optimizations) calculations for the adsorption of H on each of the six sites of the ring to form the sequentially hydrogenated structures in the order of Figure 2 (up to six H atoms). Although DFT does not treat dispersion interactions accurately, an attempt to do MP2 calculations resulted in significant spin-contamination problems as has been also noted by other researchers.10 However, very accurate results were unnecessary. In fact, it was observed that only the first hydrogen addition, that has the smallest adsorption energy, has a small energy barrier (0.08 eV), which is easily overcome at very low temperatures. As a representative curve, the energy versus C-H distance graph for the first H addition is shown in Figure 4. All other H additions were completely barrierless. Thus, the thermodynamic driving force for formation of the hydrogenated structure would be quickly realized if the linker is brought into contact with H atoms. Kinetics of H Abstraction through Eley-Rideal Pathways. Another elementary step of great importance to the spillover mechanism is the abstraction of adsorbed H atoms by gas-phase hydrogen atoms through Eley-Rideal (ER) type recombination reactions. We performed DFT internal reaction coordinate (constrained optimizations) calculations for the abstraction of H atoms via the ER mechanism from the sequentially hydrogenated structures of Figure 2 (up to six H atoms). One such representative energy curve for the abstraction of the fourth H atom from a linker containing four added hydrogens is shown in Figure 5. The energy barrier for this reaction is 0.11 eV. The abstraction reactions of the structures with one, three, and five H atoms were found to be completely barrierless, while the barriers of the structures with two, four, and six H atoms were 0.04, 0.11, and 0.06 eV, respectively. Such energy barriers are negligible at room temperature. The abstraction of the seventh H (Hd) atom from the O atom of the -CO2 group is also without any energy barrier. It has already been mentioned that the thermodynamic 4051

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The Journal of Physical Chemistry C driving force during H spillover is toward the formation of hydrogenated linkers with six additional H atoms. However, the obviously very fast kinetics of ER abstraction reactions in this process has the following consequences: (a) This mechanism may be responsible for decreased penetration of atomic H into the MOF pores. We can accept that H atoms during spillover will quickly saturate the linkers close to the entrance pore channels forming hydrogenated linkers by H sticking on the binding sites. Then, additional H atoms would have to diffuse through H-saturated pores in order to be able to chemisorb on free sites. During this diffusion, the probability of the diffusing H to undergo ER abstraction reactions would be very high, limiting the penetration depth of H in the material pores. (b) The ER abstraction reactions would be responsible for quickly reverting overhydrogenated linkers (linkers with more that six adsorbed H atoms for IRMOF-1 linkers) back to the thermodynamic minimum with respect to H2.

’ DISCUSSION OF THE SPILLOVER MECHANISM IN MOFS The transport of spillover H in the gas phase and the apparently nonactivated nature of sticking and ER recombination reactions will have profound implications on the mechanism of spillover. On the basis of these, it becomes easy to explain why spillover H cannot hydrogenate ideal (perfect) MOF crystal structures. Hydrogen atoms generated on the catalyst and diffusing into the perfect crystals can only hydrogenate the edge crystal atoms of the entrance pores. Any additional H atoms subsequently diffusing into the crystal will either recombine with the chemisorbed H at the edges or stick on any available binding sites. Both of the reactions are essentially instantaneous. Thus, we can expect that only a few network cells around the H-entrance pores would accommodate chemisorbed H. Perfect MOF crystal structures contain generally the narrow nanopores of the lattice, and the diffusing H atom unavoidably interacts with the lattice sites. Thus, because of the high reactivity of the H atom, it cannot penetrate deep into the crystal. A realistic explanation of the effect of the crystal defects on H-spillover can be afforded on the basis of the above argument. Tsao et al.11 concluded that the partially collapsed pore network with intracrystallite mesoporous channels and laterally narrow micropores are ideal for enhancing H-spillover capacities of MOFs. Mesopore channels are expected to accommodate gasphase atomic H diffusion and to increase the penetration depth of H in the crystal. In general, wider pore channels would increase the penetration depth of gas-phase diffusing H because of decreased interaction with the pore walls and therefore would allow a greater utilization of the MOF structure under H spillover conditions. In essence, the collapsed pore network dramatically increases the number and distribution of entrance channels for H atoms into the crystal lattice. Although this mechanism provides a plausible explanation of the effect of wider pores in the defective lattice structure, we cannot exclude that other factors may play a role in spillover of H in defective MOFs. Such factors may be inclusion defects, such as metal oxide clusters or leftover solvent and water species, various products due to the spillover H reactions, formation of trapped

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molecular H2, and other factors all of which are beyond the scope of this work and require further experimental evidence. The effect of the hydrogen pressure on the thermodynamics of spillover can be explained by the H2 pressure dependence of the Eley-Rideal recombination reactions (Hgas þ Hads f H2,gas). At low H2 pressures, ER recombination reactions would dominate especially because the non-hydrogenated BDC linker is a stable thermodynamic minimum with respect to the first H2 addition. However, as the H2 pressure is increased, the equilibrium of the ER recombination reaction is shifted because it becomes energetically unfavorable to squeeze more hydrogen molecules in the MOF cavities. As a result, at higher pressures, a larger fraction of the pores will follow the route to complete hydrogenation. Furthermore, as the pressure is increased, the penetration depth of atomic H into the crystal lattice will also increase because the higher pressure diminishes the ER recombination reactions that are responsible for underutilization of the MOF as explained above. Therefore, the pressure of H2 will affect both the thermodynamic equilibrium of hydrogen addition to the linkers (the degree of hydrogenation of the linkers that come in contact with atomic H) as well as the penetration depth of atomic H into the MOF crystal. Trapped H2 might also be responsible for increased uptake. For example, if atomic H can penetrate through H2 inaccessible narrow pores (such as pores with inclusion defects or isolated cavities in defective structures), then pressure can build up inside these cavities where H2 cannot back-diffuse. This would promote additional hydrogenation of the linkers in the isolated cavities as high pressure reduces ER recombination reactions, and hydrogenation of the linkers is promoted. Careful examination of the proposed spillover mechanism shows that the uptake of H2 in MOFs via spillover could be improved by (a) Modifying the MOF material toward the direction of increasing the number of chemisorption binding sites in order to overcome the thermodynamic driving force for H2 formation. For example, using linkers with multiple benzene rings is an obvious improvement over BDC linkers. Since the maximum theoretical capacity due to chemisorbed H can be predicted computationally, ab initio studies can be helpful in this respect. We need to stress that because of the easiness of recombination reactions by Eley-Rideal pathways, it is necessary to predict stable structures with respect to dihydrogen and not simply with respect to atomic H. (b) Dispersing the metal nanoparticles within the MOF structure would be ideal for increasing penetration depths of H inside the material. On the other hand, the embedded nanoparticles should not impede the diffusion of atomic H into the pore channels. (c) Altering the pore network in such a way as to provide more diffusion paths and entrance pores for H penetration into the MOF structure would be expected to be beneficial, an approach that has already been used for significant enhancement in storage capacity.11 The kinetics of hydrogenation of the MOF by spillover H may not be related to the intracrystallite structure of the MOF or even to the chemical identity of the MOF. As gas-phase (or physisorbed) H atom diffusion is very fast, the rate-limiting step in the overall process would be the generation of airborne atomic H and its supply from the catalyst to the MOF crystal. In this respect, the mechanism of atomic H generation and transfer to 4052

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The Journal of Physical Chemistry C the MOF from the primary spillover source would have to be more carefully studied. Despite the possibility of increasing H2 uptake by methods b and c at a specified pressure, the maximum possible capacity would still be determined by the thermodynamics of H2 chemisorption to the MOF structures. If this maximum achievable capacity of a particular MOF material by chemisorbed H does not satisfy DOE targets, improvements due to b and c would have little practical benefit. Thus, to satisfy the storage DOE targets through the spillover mechanism in MOFs, new material structures with higher theoretical hydrogen capacities would have to be identified and evaluated. Finally, we suggest that the proposed mechanism of spillover, consisting of the generation of gas-phase atomic H on the metal catalyst, H transfer in gas phase or physisorbed form, sticking of H atoms on the binding sites of the receptor, and Eley-Rideal recombination reactions, might be general and applicable to various spillover materials and deserves further theoretical and experimental studies.

’ AUTHOR INFORMATION

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’ ACKNOWLEDGMENT Financial support from a European Union Marie-Curie International Re-Integration Grant (IRG) and partial funding by the European Commission on DG RTD (FP6 Integrated Project NESSHY, Contract SES6-518271) are gratefully acknowledged by the authors. ’ REFERENCES (1) Lueking, A.; Yang, R. T. AIChE J. 2003, 49, 1556–1568. (2) Lachawiec, A. J.; Qi, S. G.; Yang, R. T. Langmuir 2005, 21, 11418–11424. (3) Lueking, A.; Yang, R. T. Appl. Catal., A: General 2004, 265, 259–268. (4) Li, Y. W.; Yang, R. T. J. Am. Chem. Soc. 2006, 128, 726–727. (5) Li, Q.; Lueking, A. D. Prepr. Pap.—Am. Chem. Soc., Div. Fuel Chem. 2008, 53 (2). (6) Wang, J.; Yang, F. H.; Yang, R. T.; Miller, M. A. Ind. Eng. Chem. Res. 2009, 48, 2920–2926. (7) Chen, L.; Cooper, A. C.; Pez, G. P.; Cheng, H. J. Phys. Chem. C 2007, 111, 18995–19000. (8) Psofogiannakis, G. M.; Froudakis, G. E. J. Phys. Chem. C 2009, 113, 14908–14915. (9) Li, Y. W.; Yang, R. T. J. Am. Chem. Soc. 2006, 128, 8136–8137. (10) Miller, M. A.; Wang, C.-Y.; Merrill, G. N. J. Phys. Chem. C 2009, 113, 3222–3231. (11) Tsao, C.-S.; Yu, M.-S.; Wang, C.-Y.; Liao, P. Y.; Chen, H. L.; Jeng, U. S.; Tzeng, Y. R.; Chung, T. Y.; Wu, H.-C. J. Am. Chem. Soc. 2009, 131, 1404–1406. (12) Ahlrichs, R.; Bar, M.; Haser, M.; Horn, H.; Kolmel, J. Chem. Phys. Lett. 1989, 162, 165–169. (13) Frisch, M. J. et al. Gaussian 03 Package, rev. B.04; Gaussian, Inc.: Pittsburgh, PA, 2003. (14) Tylianakis, E.; Klontzas, E.; Froudakis, G. E. Nanotechnology 2009, 20, 204030/1–204030/9. (15) Suri, M.; Dornfeld, M.; Ganz, E. J. Chem. Phys. 2009, 131, 174703-1–174703-4. (16) Wiberg, K. B.; Nakaji, D. Y.; Morgan, K. M. J. Am. Chem. Soc. 1993, 115, 3527–3532. 4053

dx.doi.org/10.1021/jp109541n |J. Phys. Chem. C 2011, 115, 4047–4053