Energetics and Thermodynamics of the Initial Stages of Hydrogen

Feb 24, 2014 - We study the energetics and thermodynamics of the initial stages of hydrogen storage by spillover on prototypical metal–organic frame...
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Energetics and Thermodynamics of the Initial Stages of Hydrogen Storage by Spillover on Prototypical Metal−Organic Framework and Covalent−Organic Framework Materials Eric Ganz* and Matthew Dornfeld Department of Physics, University of Minnesota, Minneapolis, Minnesota 55455, United States ABSTRACT: We study the energetics and thermodynamics of the initial stages of hydrogen storage by spillover on prototypical metal−organic framework (MOF) and covalent−organic framework (COF) materials. We use density functional theory on periodic frameworks to achieve reliable and accurate predictions for these materials. For pure IRMOF-1, it has been suggested that there is an enthalpy barrier to the addition of the first hydrogen per benzene and that this barrier is removed by hole doping. We do not observe this enthalpy barrier. However, we do observe that the binding energy for the first hydrogen is unfavorable and creates a kinetic barrier without hole doping. Therefore, hole doping by Zn vacancies or other means is still necessary for the hydrogen storage process to proceed. We observe that the optical absorption for the Zn vacancy model is different from that of the pure model; therefore, the presence of Zn vacancies could be experimentally tested. For COF-5, we find that the energy barrier is not resolved by doping. This may explain why it has been difficult to achieve significant hydrogen spillover on COF materials.

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INTRODUCTION Hydrogen storage remains one of the main challenges in the implementation of a hydrogen-based energy economy. Although several different approaches are being pursued, sorption onto a porous high surface area material is one contender. There is great interest in finding porous solid materials that can store hydrogen for use in fuel cell vehicles. Ideally, these materials would adsorb large amounts of hydrogen gas reproducibly at room temperature and moderate pressure. Suh et al. recently reviewed the use of metal−organic framework (MOF) materials for hydrogen storage.1 The spillover process works using nanoscale metal catalysts distributed through the porous substrate material to break the molecular hydrogen gas into physisorbed atomic hydrogen.2 The atomic H then diffuses across and chemisorbs to the substrate. As H covers the nearby surface area, further H diffuses across the saturated areas and spills over onto remaining areas. The best results have been on substrates based on metal− organic framework materials.2 These MOF and covalent−organic framework (COF) materials are relatively easy to fabricate, porous, lightweight, and have extremely high surface area. Spillover has been a topic of investigation for many years; we will concentrate only on hydrogen storage by spillover onto COF and MOF materials. Hydrogen storage by spillover has been reviewed by Wang and Yang.3 Li and Yang found that IRMOF-8 with bridged Pt catalysts can reversibly store 4 wt % of hydrogen at room temperature and 100 bar pressure by spillover.4 Ingeniously, they used commercial Pt catalysts mounted on amorphous carbon substrates © 2014 American Chemical Society

(Pt/AC). These were then mixed with IRMOF-8 crystals and sucrose and annealed to form amorphous carbon bridges between the Pt/AC and the IRMOF-8. Yang’s group has also tested several other high surface area substrates, but none of these have yet reached the storage capacity of bridged IRMOF-8. Recently, Yang’s group has published the details of the bridging methods and tested variations.5 Independently, Tsao et al. have extended the IRMOF-8 result to 4.7 wt % in equilibrium at 70 bar.6 This is the largest value to date for hydrogen storage by spillover at room temperature and is close to the 2010 DOE gravimetric storage targets of 6 wt %. Some experimentalists have found it difficult to repeat these results. For example, Luzan and Talyzin7,8 and Hirscher9,10 have reported difficulty in reproducing the results of Yang et al. Li and Yang have written that creation of these bridged IRMOF-8 samples remains an art.11 We are convinced that the results of Li and Yang are correct but are difficult to reproduce. We note that Tsao et al. independently reproduced the results in 2009.6 Miller et al. also were able to achieve 2.5 wt % storage with bridging on IRMOF-8.12 In recent years, experimentalists still struggle to extend these results, for example, see Ardelean et al.13 Experimentalists have also found it very difficult to extend these spillover techniques to other substrates and materials. Yang et al. have achieved 0.7 wt % on COF-1, 1.1 wt % on HKUST-1, and 1.5 wt % on MOF-177.3 Kalidindi et al. are able Received: October 24, 2013 Revised: February 19, 2014 Published: February 24, 2014 5657

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Table 1. Energy E, binding energy ΔE, vibrational entropy Svib, configurational entropy Sconfig, enthalpy H, change in enthalpy ΔH, Gibbs free energy G, and change in Gibbs free energy ΔG with and without configurational entropy for IRMOF-1 models H ΔH vs H2 ΔE vs H Svib Sconfig (random, G ΔG vs H2 (per H; eV) (cal/(mol K)) cal/(mol K)) (kcal/mol) (per H; kcal/mol) (kcal/mol) (per H; kcal/mol)

ΔG vs H2 with Sconfig (per H; kcal/mol)

name

E (Ha)

IRMOF-1 + 1H + 2H vacancy: IRMOF-1 + 1H + 2H doped: IRMOF-1 + 1H + 2H

−5832.753 −5833.300 −5833.917

−1.38 −2.34

337.34 336.57 337.81

6.30 4.90

457.30 462.75 470.35

1.30 2.37

356.73 362.41 369.63

6.38 7.15

4.50 5.69

−5578.814 −5579.383 −5579.981

−1.99 −2.37

341.67 342.40 346.31

6.30 4.90

461.15 468.09 474.38

2.79 2.46

359.28 366.01 369.96

7.43 6.04

5.55 4.58

−5832.552 −5833.120 −5833.709

−1.94 −2.23

335.34 332.74 332.57

6.30 4.90

454.18 460.61 466.65

2.28 2.09

354.19 361.40 367.49

7.91 7.35

6.03 5.89

H H2

−0.496 −1.161

−1.40

8.30

to achieve only 0.4 wt % on Pd-doped COF-102.14 Storage densities up to 1.3 wt % have been achieved for MIL-101.15,16

role of a dangling bond state in the gap of pure IRMOF-1 that is mitigated by hole doping. For the +1H model, they found ΔH = +46 kcal/mol H2 for pure IRMOF-1 and −16 kcal/mol for doped IRMOF-1. The doping also dramatically reduced the diffusion barrier. Cao et al. have extended this idea. They found that decoration of the BDC linker with electrophilic groups enhanced the hydrogen uptakes by spillover at low loading.23 Recently, Guo et al. published a DFT study of spillover on COF materials.24 They observe a hydrogen storage saturation capacity of 6 wt % onto COF-8. They also study the diffusion barrier and recombination mechanisms. There has also been a multiscale theoretical investigation of hydrogen storage in covalent organic frameworks.25 We will also discuss the addition of H to covalent−organic frameworks.26 Two important members of this group are COF-1 and COF-5. These were found to have expanded porous graphitic layers with staggered (COF-1, P63/mmc) or eclipsed (COF-5, P6/mmm) structures. Cote et al. found that COF-1 and COF-5 have high thermal stability (to temperatures up to 500 and 600 °C, respectively), permanent porosity, and high surface areas (711 and 1590 m2 per gram, respectively). We will use COF-5 as a prototype for the entire COF series. In this paper we will use DFT calculations of energy, enthalpy, and Gibbs free energy to explore whether it is favorable to add the first and second hydrogen atoms to specific IRMOF and COF materials. We will also consider the role of hole doping in modifying this process. In particular, we will make calculations on IRMOF-1 and COF-5 in an effort to extend or confirm results of Lee et al.,22 i.e., we want to determine whether Zn or vacancy doping can lead to a thermodynamically accessible pathway for H sorption onto these materials.

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PREVIOUS COMPUTATIONAL STUDIES There have been several theoretical studies of the spillover process. Li et al. published a study of the kinetics and mechanics of this process on a metal−organic framework substrate using density functional theory (DFT).17 Their conclusion was that H atoms could bind by spillover at all sites studied (producing 6.5 wt % storage). This paper generated a reply by Mavrandonakis et al.18 and then a reply by Li et al.19 Cheng et al. studied the energetics and binding energies of hydrogen spillover on several graphitic materials with Pt catalysts using DFT calculations.2 They studied the dissociative chemisorption of gaseous H on a transition-metal catalyst, the migration of H atoms from the catalyst to the substrate, and the diffusion of H atoms on substrate surfaces. Miller et al. studied the addition of multiple hydrogen atoms to isolated benzenedicarboxylate and napthalenedicarboxylate linkers using DFT as models for hydrogen addition to IRMOF1 and IRMOF-8.12 They found that one can add up to 10 H to the isolated naphthalene molecule. Further DFT calculations were performed by Psofogiannakis.20 In a previous paper, we determined the saturation storage density for hydrogen on several MOF and COF materials.21 We found that one hydrogen can be stored at each C atom of the linker and an additional H for each CO2 group. For IRMOF-1 and IRMOF-8, we found reasonable agreement with the experimental results. For other materials such as COF-1 and MOF177, we found that the experiments could be dramatically improved. We also predicted the gravimetric and volumetric storage densities for several new materials, including IRMOF-9, IRMOF-993, MIL-101, PCN-14, COF-1, and COF-5. We found gravimetric storage densities up to 5.5 wt % and volumetric storage densities up to 44 g/L. Lee et al. (using DFT in periodic cells) have proposed a holemediated hydrogen spillover mechanism.22 They suggest that a population of Zn vacancies can dramatically change the energetics and diffusion barriers. This would provide a thermodynamic pathway for the addition of the first six H atoms to the IRMOF-1 framework. This was a breakthrough, as previous DFT calculations could not find a viable thermodynamic pathway to explain the experimental results. They described the key

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COMPUTATIONAL METHOD Density functional theory (DFT) calculations were performed in Materials Studio 6.1, using the DMol3 program.27 These calculations were performed on the Itasca supercomputer at the Minnesota Supercomputer Institute (MSI). We have followed the DFT method of Wang et al. developed for ZnO clusters.28 The generalized gradient approximation (GGA) with Perdew− Burke−Ernzerhof (PBE) parametrization29 was used to describe the exchange-correlation interaction. Density functional semicore pseudopotentials (DSPP) fitted to all-electron 5658

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relativistic DFT results and double numerical basis set including d-polarization functions (DND) were employed. The accuracy of this PBE/DSPP/DND scheme has been assessed via testing calculations on the ZnO molecule and wurtzite bulk by Wang et al.28 This PBE/DSPP/DND method was also used in our previous papers.21,30 All calculations were performed using periodic boundary conditions. For IRMOF-1, the cell size was optimized. CASTEP calculations (GGA, PBE, fine, and onthe-fly pseudopotentials) were used to determine optical absorption properties. As a test of accuracy, using the data presented in Table 1, we find a binding energy of 4.60 eV for the H2 molecule, which is quite close to the experimental value of 4.50 eV.31We follow the approach of Lee et al.22 to investigate the thermodynamic accessibility of the sequential addition of H to the substrate lattice. For IRMOF-1, we use the same 106atom primitive unit cell used by Lee et al. We extend this method by optimizing the cell parameters, and by performing the frequency calculations on the full 106-atom periodic unit cell. For COF-5, we use the same periodic cells developed in our previous paper.21 For the calculations, the energy threshold was set to 3 × 10−6 Ha, force to 3 × 10−4 Ha/Å, displacement to 0.003 Å, thermal smearing to 0.005, DIIS to 10, parameter “Vibration_steps 2 0.02”, and with 2 k-points, one at the Γ-point (4 k-points were needed for some of the models). The pure IRMOF-1 crystal model has Fm3̅m symmetry. For this paper, we converted this to P1 symmetry and used the primitive unit cell with 106 atoms. The lattice size was optimized at this point. However, the frequency calculations showed a negative (soft) mode. To resolve this negative mode, the mode was animated in Materials Studio and a geometry was selected that incorporated motion of this mode. The model was then reoptimized. We call this process warping. For the pure crystal, insignificant changes in energy were observed, but all of the other models required significant warping once H is added and symmetry is broken. Therefore, all of the calculations were performed with minimal symmetry, increasing reliability and accuracy at a cost of significantly increased computational time. Following the results of the RPI group,22 we considered hole doping of the 3D periodic IRMOF-1 lattice by addition of 1 Zn vacancy. This model is shown in Figure 1 and has charge 0. To be realistic, we terminated the vacancy with 1 H atom. Note: we started with an unterminated Zn atom and optimized the geometry. We then introduced a single H atom nearby and optimized this. We observed that the H readily attached to the unterminated vacancy. A further H atom did not attach, showing that the terminated Zn vacancy is stable in an experimental environment. We created doped IRMOF-1 models by changing the charge of the calculations to +1. The geometry is similar to that of the pure IRMOF-1 model (not shown). The doped IRMOF-1 model is shown in Figure 2. We also created a 3D periodic COF-5 model shown in Figure 3. For the COF-5 model, we used fixed cell size because of the limitations of the DFT theory following our previous work.21

Figure 1. Snapshot of part of two unit cells showing benzene and two Zn corners for the Zn vacancy IRMOF-1 model. The missing Zn atom is shown on the center left, where the saturating H atom is seen bonded to the central O. (Zn, blue; C, gray; O, red; H, white).

Figure 2. Snapshot of part of two unit cells showing benzene and two Zn corners for the doped IRMOF-1 model. (Zn, blue; C, gray; O, red; H, white).

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RESULTS First, we discuss the results for the pure IRMOF-1 model. We calculate the binding energy of each added H by comparing the energy of the +1H model to that of the clean model and an isolated H atom.

Figure 3. Snapshot of COF-5 model showing top view of unit cell. (B, peach; C, gray; O, red; H, white).

We find that the binding energy versus H per H is −1.38 eV for +1H and −2.34 eV for +2H. These results are shown in Table 1. We see that the binding energy for the first H is

ΔE( +nH) = E( +nH) − E(0H) − nE(H) 5659

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significantly lower than the binding energy for 2H. This is consistent with our previous results.21 In fact, this binding energy is too low and acts as an energetic barrier to the spillover process. Specifically, the binding energy must stay relatively close to the H2 binding energy (2.30 eV per H) for the process to proceed and be reversible. Therefore, the +1H step acts as a gateway for the entire process, which will ultimately include +3H, +4H, and on up to saturation. We have not calculated these higher cases. We also calculated the change in enthalpy ΔH versus H2 for this process. (1.30 and 2.37 kcal/mol for +1H and +2H respectively). Next we discuss our Zn vacancy IRMOF-1 model. We see in Table 1 that the Zn vacancy has a significant effect on the binding energy of the 1H model. It increases from 1.38 to 1.99 eV per H. This is larger and much closer to the 2.37 eV for the +2H model. Next we discuss the doped IRMOF-1 model. We observe in Figure 5 that this doping also removes the dangling bond state from the gap. We also see that doping also raises the +1H binding energy to 1.94 eV, which is close to the 2.23 eV result for +2H. This is now close enough not to create a kinetic barrier. For COF-5, we also observe a small increase in the +1H enthalpy result, but we do not see the removal of a large barrier here. To make an upper limit estimate of the configurational entropy, we will assume that the H atoms are distributed randomly through the crystal. Therefore, in each 106-atom unit cell there are 24 possible H binding sites. Per mole, we have S1H config = kB ln Ω = kB ln(24NA) = kBNA ln 24. Therefore, for 1H, the S1H config = 6.3 cal/mol·K. For the 2H case, we can assume that the second atom binds across from the first in the lowest-energy configuration; therefore, there will be 12 binding sites per unit cell, leading to S2H config = 4.9 cal/mol·K We note that in the case of ideal spillover, we expect that the actual number of configurations will be much smaller. In this case a hemisphere of filled states will have configurations at the surface. This will lead to a substantially lower entropy of configuration, so low that it can be ignored. Therefore, in real materials, we expect the actual configurational entropy to be between 0 and the maximum estimates calculated above. These configurational entropy values are then added to the calculation of ΔG w/Sconfig. We show calculated density of states for several IRMOF-1 models in Figure 4. We use 2 × 2 ×2 sampling, and there is substantial smoothing included by DMol3. We see that the Fermi level is right below a large band gap of 2.60 eV in the pure material. More details on the pure material are discussed in the previous DFT work of Yang et al.32 Yang et al. estimate a band gap of 2.5 eV. When we add 1 H to the pure material, we see a new dangling bond state in the gap. The Fermi level is now located at this state, which is occupied. This is in agreement with the results of Lee et al.22 This dangling bond state will be responsible for the reduction in binding energy for the pure material. We now consider the DOS results for hole-doped and Zn vacancy materials. The doped +1H model shows the Fermi level below the gap; therefore, the dangling bond state is not occupied. The doped +2H model (not shown) shows a Fermi level at the bottom of an empty gap, with no dangling bond state. The Zn vacancy +1H model also shows the Fermi level at the bottom of the gap with the dangling bond state not occupied. The Zn vacancy +2H model (not shown) also shows

Figure 4. Density of states of (left to right) pure IRMOF-1, pure + 1H, doped + 1H, and Zn vacancy + 1H. Arbitrary vertical units; energy in hartrees.

a Fermi level at the bottom of an empty gap, with no dangling bond state. Therefore, both the hole doping and the saturated Zn vacancy have similar effects in IRMOF-1. They are both effective at eliminating the role of the dangling bond state. They are both effective at eliminating the energy barrier for the +1H state. Thus, both of these methods will allow the smooth addition of hydrogen atoms to IRMOF-1 by spillover without energetic barriers. For the COF-5 models, we observe a binding energy of 1.58 eV/H for the +1H, and 2.23 eV/H for the +2H, as shown Table 2. This suggests that the clean COF-5 system will have an energetic barrier at the +1H location. When we dope the crystal or substitute a Br atom into the lattice at one of the H positions, we do not see much change in the energetics. Therefore, we conclude that hole doping does not correct the energy problem for the H spillover on COF-5. The enthalpy and Gibbs free energy results are also shown in Table 2. We see that there is not an enthalpy barrier. Instead, the Gibbs free energy should be used to predict whether the reaction will move forward as a function of pressure.

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DISCUSSION For the change in enthalpy with the addition of +1H and +2H to the clean IRMOF-1 model, we find 2.60 and 4.74 kcal/mol versus H2 per H2 respectively. We see that our ΔH values are much smaller than those calculated by Lee et al. They calculated 46 kcal/mol versus H2 per H2 for pure IRMOF-1 and −16 kcal/mol versus H2 per H2 for doped IRMOF-1for the +1H. Our ΔH values are also consistently positive, which we will see below is necessary for the reaction to be reversible as gas pressure is changed. One of the main differences between the calculations is that we do a full periodic frequency calculation to determine ΔH for the true periodic crystal. This is more expensive computationally, but also much more reliable. The Helmholtz free-energy values of Lee et al. are probably too large because of the use of molecular fragments in their calculation. Isolated molecules will also contain translation and rotation contributions to the free energy, which do not apply to the bulk case. Therefore, our periodic calculations will be much more accurate for the actual experimental situation. We have found that the Helmholtz free energy does not change significantly on hole doping for IRMOF-1. Instead, the change in 5660

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Table 2. Energy E, Change in Energy ΔE, Enthalpy H, Change in Enthalpy ΔH, Gibbs Free Energy G, and Change in Gibbs Free Energy ΔG for COF-5 Models name

E (Ha)

ΔE vs H (per H; eV)

H (kcal/mol)

ΔH vs H2 (per H; kcal/mol)

G (kcal/mol)

ΔG vs H2 (per H; kcal/mol)

COF-5 + 1H + 2H Br: COF-5 + 1H + 2H doped: COF-5 + 1H + 2H

−3122.062 −3122.616 −3123.219

−1.58 −2.23

447.95 454.37 462.13

2.27 2.94

386.64 393.17 400.12

7.23 7.44

−3465.327 −3465.880 −3466.483

−1.56 −2.23

442.68 449.06 456.27

2.22 2.64

379.57 385.87 391.30

7.00 6.56

−3121.985 −3122.540 −3123.145

−1.61 −2.28

447.91 454.46 462.09

2.39 2.93

386.12 392.70 399.76

7.28 7.52

Figure 5. Optical absorption for pure (left panel) and Zn vacancy IRMOF-1 (right panel). We observe extra absorption at low energies from states in the gap from Zn vacancies.

Figure 6. Change of Helmholtz free energy ΔH and Gibbs free energy ΔG as a function of H2 gas pressure for the addition of +2H atoms for the Zn vacancy model.

Gibbs free energy ΔG is the appropriate thermodynamic quantity to calculate. Lee et al. proposed that hole doping the IRMOF-1 lattice can remove the thermodynamic barrier that they observed in calculations of the change in enthalpy during this process.22 Hole doping removes the dangling bond state from the gap. Instead, we observe that the doping removes an energetic barrier that can block the process. To facilitate the experimental determination of whether Zn vacancies or hole doping exist in the actual bridged IRMOF-1 and IRMOF-8 samples, in Figure 5 we show the optical absorption from CASTEP calculations. We find that for pure

IRMOF-1, the first absorption is at 3.60 eV (346 nm), which is the band gap for this model. This compares well with the experimental band gap of 3.5 eV and the theoretical band gap of 3.4 eV discussed in Yang et al.32 For the Zn vacancy model, there are now states in the gap and therefore optical absorption at low excitation energies. We note that other types of defects or impurities in the real samples may also produce similar optical excitations. We find that the optical behavior of the Zn vacancy and doped materials will be significantly different than that of the pure IRMOF material; therefore, it should be possible to experimentally verify if Zn vacancies exist in these materials. The Zn vacancies may be created during the annealing 5661

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cycles that are part of the sample preparation. It is known that specific procedures and not just the purest samples gave the best results.33 Tsao et al. found that increasing the structural disorder by annealing could actually improve the performance of hydrogen spillover on bridged IRMOF-8.33 The change in Gibbs free energy determines whether the reaction will go forward or backward. (Gibbs free energy G = U + PV − TS.) Experimentally, the hydrogen storage capacity equilibrium is observed to grow roughly linearly from 0 to 3.8 wt % for IRMOF-1 as the pressure changes from 0 to 100 bar. In equilibrium, the +2H case will dominate over the +1H case, even at lower loading, where we would need to use a larger cell size. For the +2H model discussed above, we have a loading of 0.12 wt %. This corresponds to experimental pressure of roughly 3 bar (using linear approximation). Figure 6 plots ΔG as a function of pressure for the +2H Zn vacancy model compared to H2 gas at that pressure. We see that ΔG < 0 for P < 9 bar. We are able to qualitatively understand the linear dependence of the loading curve this way (the Gibbs free energy will scale linearly with number of +2H additions, and the ΔG of the H2 scales linearly with pressure), but we need more accurate entropy calculations. We see that the change in ΔH < 0 for P > 5 bar, which agrees with experiment. Therefore, at large H2 gas pressure, ΔH and ΔG will be negative and the reaction will proceed forward.

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SUMMARY

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AUTHOR INFORMATION

Article

REFERENCES

(1) Suh, M. P.; Park, H. J.; Prasad, T. K.; Lim, D.-W. Hydrogen Storage in Metal−Organic Frameworks. Chem. Rev. 2012, 112, 782− 835. (2) Cheng, H.; Chen, L.; Cooper, A. C.; Sha, X.; Pez, G. P. Hydrogen Spillover in the Context of Hydrogen Storage Using Solid-State Materials. Energy Environ. Sci. 2008, 1, 338. (3) Wang, L.; Yang, R. T. New Sorbents for Hydrogen Storage by Hydrogen Spillover − A Review. Energy Environ. Sci. 2008, 1, 268. (4) Li, Y.; Yang, R. T. Hydrogen Storage in Metal−Organic Frameworks by Bridged Hydrogen Spillover. J. Am. Chem. Soc. 2006, 128, 8136−8137. (5) Stuckert, N. R.; Wang, L.; Yang, R. T. Characteristics of Hydrogen Storage by Spillover on Pt-Doped Carbon and CatalystBridged Metal Organic Framework. Langmuir 2010, 26, 11963− 11971. (6) 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. Nanostructure and Hydrogen Spillover of Bridged Metal-Organic Frameworks. J. Am. Chem. Soc. 2009, 131, 1404−1406. (7) Luzan, S. M.; Talyzin, A. V. Hydrogen Adsorption in Pt Catalyst/ MOF-5 Materials. Microporous Mesoporous Mater. 2010, 135, 201− 205. (8) Luzan, S. M.; Talyzin, A. V. Comment to the “Response to “Hydrogen Adsorption in Pt Catalyst/MOF-5 Materials”” by Li et al. Microporous Mesoporous Mater. 2011, 139, 216−218. (9) Campesi, R.; Cuevas, F.; Latroche, M.; Hirscher, M. Hydrogen Spillover Measurements of Unbridged and Bridged Metal−Organic FrameworksRevisited. Phys. Chem. Chem. Phys. 2010, 12, 10457− 10459. (10) Hirscher, M. Remarks About Spillover and Hydrogen Adsorption − Comments on the Contributions of A.V. Talyzin and R.T. Yang. Microporous Mesoporous Mater. 2010, 135, 209−210. (11) Li, Y.; Wang, L.; Yang, R. T. Response to “Hydrogen Adsorption in Pt Catalyst/MOF-5 Materials” by Luzan and Talyzin. Microporous Mesoporous Mater. 2010, 135, 206−208. (12) Miller, M. A.; Wang, C.-Y.; Merrill, G. N. Experimental and Theoretical Investigation Into Hydrogen Storage via Spillover in IRMOF-8. J. Phys. Chem. C 2009, 113, 3222−3231. (13) Ardelean, O.; Blanita, G.; Borodi, G.; Mihet, M.; Coros, M.; Lupu, D. On the Enhancement of Hydrogen Uptake by IRMOF-8 Composites with Pt/Carbon Catalyst. Int. J. Hydrogen Energy 2012, 37, 7378−7384. (14) Kalidindi, S. B.; Oh, H.; Hirscher, M.; Esken, D.; Wiktor, C.; Turner, S.; Van Tendeloo, G.; Fischer, R. A. Metal@COFs: Covalent Organic Frameworks as Templates for Pd Nanoparticles and Hydrogen Storage Properties of Pd@COF-102 Hybrid Material. Chem.Eur. J. 2012, 18, 10848−10856. (15) Anbia, M.; Mandegarzad, S. Enhanced Hydrogen Sorption on Modified MIL-101 with Pt/CMK-3 By Hydrogen Spillover Effect. J. Alloys Compd. 2012, 532, 61−67. (16) Lin, K.-S.; Adhikari, A. K.; Su, Y.-H.; Shu, C.-W.; Chan, H.-Y. Synthesis, Characterization, and Hydrogen Storage Study by Hydrogen Spillover of MIL-101 Metal Organic Frameworks. Adsorption 2012, 18, 483−491. (17) Li, Y.; Yang, F. H.; Yang, R. T. Kinetics and Mechanistic Model for Hydrogen Spillover on Bridged Metal−Organic Frameworks. J. Phys. Chem. C 2007, 111, 3405−3411. (18) Mavrandonakis, A.; Klopper, W. Comment on “Kinetics and Mechanistic Model for Hydrogen Spillover on Bridged Metal-Organic Frameworks”. J. Phys. Chem. C 2008, 112, 3152−3154. (19) Li, Y.; Yang, F. H.; Yang, R. T. Reply to “Comment on ‘Kinetics and Mechanistic Model for Hydrogen Spillover on Bridged MetalOrganic Frameworks’”. J. Phys. Chem. C 2008, 112, 3155−3156. (20) Psofogiannakis, G. M.; Froudakis, G. E. Theoretical Explanation of Hydrogen Spillover in Metal−Organic Frameworks. J. Phys. Chem. C 2011, 115, 4047−4053.

For H spillover on pure IRMOF-1, we find that the binding energy for the first hydrogen is much weaker without hole doping, and this creates an energetic barrier. Therefore, hole doping by Zn vacancy or doping is necessary for the hydrogen storage process to proceed. We also see that the direction of the hydrogen sorption reaction as a function of pressure will be predicted by the change in Helmholtz free energy or Gibbs free energy. The change in Helmholtz free energy matches well with the experimental result. We believe that our results resolve some of the difficulties experimentalists have had extending hydrogen spillover to other substrates. Our results are consistent with the excellent hydrogen storage obtained on bridged IRMOF-1 and IRMOF-8. For COF-5, we do not observe a significant change in the energetic barrier on hole doping. Our results are consistent with the fact that the experiments have not been successful in storing large amounts of H (beyond 1 wt %) on COF-1 via spillover. The energy barrier that we observe for COF-5 leads to a kinetic barrier which is not overcome by increasing the gas pressure. Therefore, our results suggest that spillover will not proceed on either pure or hole-doped COF-5. This suggests that alternative methods may be necessary to modify the COF materials for H spillover.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Minnesota Supercomputer Institute for Advanced Computational Research for providing computational resources and support. 5662

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp4105322 | J. Phys. Chem. C 2014, 118, 5657−5663