First-Principles Study of Electronic Structure and Hydrogen Adsorption

Mar 7, 2012 - Open-site paddle wheels, comprised of two transition metals bridged with four carboxylate ions, have been widely used for constructing m...
2 downloads 11 Views 3MB Size
Download PDF
Article pubs.acs.org/JPCC

First-Principles Study of Electronic Structure and Hydrogen Adsorption of 3d Transition Metal Exposed Paddle Wheel Frameworks Ji Hyun Bak,†,⊥,¶ Viet-Duc Le,‡,⊥ Joongoo Kang,§ Su-Huai Wei,§ and Yong-Hyun Kim*,‡,† †

Department of Physics, KAIST, Daejeon 305-701, Korea Graduate School of Nanoscience and Technology (WCU), KAIST, Daejeon 305-701, Korea § National Renewable Energy Laboratory, Golden, Colorado 80401, United States ‡

S Supporting Information *

ABSTRACT: Open-site paddle wheels, comprised of two transition metals bridged with four carboxylate ions, have been widely used for constructing metal−organic frameworks with large surface area and high binding energy sites. Using firstprinciples density functional theory calculations, we have investigated atomic and electronic structures of various 3d transition metal paddle wheels before and after metal exposure and their hydrogen adsorption properties at open metal sites. Notably, the hydrogen adsorption is impeded by covalent metal−metal bonds in early transition metal paddle wheels from Sc to Cr and by the strong ferromagnetic coupling of diatomic Mn and Fe in the paddle wheel configurations. A significantly enhanced H2 adsorption is predicted in the nonmagnetic Co2 and Zn2 paddle wheel with the binding energy of ∼0.2 eV per H2. We also propose the use of two-dimensional Co2 and Zn2 paddle wheel frameworks that could have strongly adsorbed dihydrogen up to 1.35 wt % for noncryogenic hydrogen storage applications.

■

INTRODUCTION The physisorption of hydrogen is nondissociative thus making the storage process just as prompt and reversible as the traditional tank-type storages.1 Because the physisorption of hydrogen takes place on the surface of the adsorbent, it is crucial for a storage medium to have a high specific surface area, preferably with a nanoporous structure, as well as a large H2 binding energy.2,3 Doping carbon nanostructures with metal atoms has been proposed as a means to enhance the dihydrogen binding while maintaining high specific surface areas.4,5 Nanoporous metal−organic frameworks (MOFs) have been emerging as a promising hydrogen storage medium in the physisorption approach. MOFs, made of various metal cations and organic linkers, are expected to be particularly practical because their surface areas and binding energies are not only uniquely large but also tunable by the choices of organic linkers and metal cations. An MOF comprises many secondary building units (SBUs) with metal binding sites in either a closed-shell or an open-site configuration. The metal−dihydrogen interaction in a closedshell MOF is necessarily the van der Waals type, which is not strong enough for room-temperature hydrogen storage. There have been attempts to enhance the metal−dihydrogen interaction by constructing open-site or metal-exposed MOFs.6−8 Paddle wheel frameworks (PWFs), in which © 2012 American Chemical Society

carboxylate bridged Cu2 paddle wheels (PWs) (see Figure 1a) are used as an SBU, have attracted much attention lately because of their unique capability to store much hydrogen.9−12 The as-synthesized Cu2−PW is a closed-shell SBU with 6-fold coordinated Cu sites; each Cu sits at the center of an almost octahedral cage consisting of four in-plane oxygen atoms, the other Cu, and a hydrating water molecule. After a necessary degassing process at 0 indicates a favorable or exothermic adsorption of the molecule on TM2−PWs. For the H2 adsorption, we found that the side-on adsorption in the D2h symmetry is most favorable compared to other configurations including the head-on configuration in the D4h symmetry (Figure S2 of the Supporting Information).

COMPUTATIONAL METHOD We have performed first-principles calculations of TM exposed PWs on the basis of the spin-polarized DFT with the generalized gradient approximation of Perdew−Burke−Ernzerhof (PBE)22 as implemented in the Vienna ab initio simulation package (VASP).23 We used a plane-wave basis set with a cutoff energy of 500 eV and the all-electron-like projector-augmented wave potentials. For VASP calculations of molecular PW units, a supercell approximation was employed with a cubic cell with the lattice constant of 20 Å. The PW unit could be modeled by using the TM2−tetrabenzenecarboxylate (TBC) model as we did in our previous publication14 (see Figure 1b for TM2− TBC), but we further simplified the benzene ring of the benzenecarboxylate group by a hydrogen termination [see Figure 1c for TM2−(HCOO)4] to facilitate calculations for all 3d TM PWs. This simplification can be justified by several reasons: (1) Enhanced dihydrogen adsorption occurs locally at exposed TM sites by coupling between H2 σ and TM d orbitals; (2) the TM−TM bond lengths and the TM−H2 binding energies do not sensitively depend on H- and benzene-ring terminations, or 2D network, as listed in Tables 1 and 2; (3) local 3d electronic structures of Co2−TBC and Co2− (HCOO)4 before and after H2 adsorption, as shown in Figure S1 of the Supporting Information, are almost unchanged with including the identical highest occupied molecular orbital− lowest unoccupied molecular orbital (HOMO−LUMO) gap. The same simplification has also been employed in other theoretical studies24,25 of paddle wheel structures. The nonmagnetic (NM), ferromagnetic (FM), and antiferromagnetic (AFM) spin configurations of the two metal ions were considered for the ground state, and the stable PW geometry was obtained after full structural relaxations with residual atomic forces less than 0.02 eV/Å. We also constructed 2D TM PWFs with TM2−PW units and a short organic linker by directly connecting the C terminal of the carboxylic group as shown in Figure 1a. The planar PWFs were modeled in a slab configuration with the vacuum separation of 10 Å. To see the experimental feasibility of 3d TM exposed PWs, we also calculated their aqueous formation energies as we did in our previous publication14 if the experimental formation enthalpies of aqueous TM ions were available.26,27 Experimental formation enthalpies of Sc, Ti, and V aqueous ions were not available because of their high oxidation states.

■

RESULTS AND DISCUSSION A carboxylate ion (RCOO−) is typically an anion with the charged state of (−1), and a divalent 3d TM atom tends to stay in the charged state of (+2) by losing its two 4s electrons. Then, a TM2−PW that is made of two TM atoms and four carboxylate ions would be energetically stable according to a simple electron counting model. To see this explicitly, we performed first-principles DFT calculations for metal-exposed PWs of all 3d TMs from Sc to Zn, and we elucidated their atomic and electronic structures, formation energetics, and hydrogen adsorption characteristics as summarized in Tables 1 and 2. For comparison, we have considered fully coordinated PWs with pyridine, which is a stronger Lewis base than H2. The calculated aqueous formation energies in Table 1 indicate that the Cu− and Cr−PWs without axial ligands are very likely to exist, which is consistent with experiments. On the other hand, the Mn−PW shows the highest formation energy indicating its synthetic difficulty in experiment. Also, Table 2 clearly shows that the metal−metal distance in the PW configuration widely varies depending on the metal−metal bonding character as we will individually discuss below. Early Transition-Metal PWs (from Sc to Cr). When the two TM centers are exposed, Sc-to-Cr PWs are all nonmagnetic as listed in Table 1. The V−V and Cr−Cr distances are 7388

dx.doi.org/10.1021/jp210985a | J. Phys. Chem. C 2012, 116, 7386−7392

The Journal of Physical Chemistry C

Article

noticeably short, less than 2 Å, implying the existence of strong metal−metal multiple covalent bonds (Table 2). The calculated Cr−Cr distance of 1.69 Å agrees reasonably with experimental reports of the ultrashort distances that range from 1.74 to 1.86 Å.15 The Sc−Sc distance (2.93 Å) is also shorter than that of the bulk Sc metal (3.25 Å). In contrast, the Ti−Ti distance (2.83 Å) is very close to the value of hexagonal close-packed Ti metal (2.89 Å) implying that the Ti−Ti bonding is rather weak. The local density of states (LDOS) analyses for the Sc2− to Cr2−PWs show that the single-to-multiple d−d covalent bond is the origin of the short metal−metal distance and the nonmagnetic spin configuration. In the diatomic paddle wheel configuration, the two-atom-centered dz2 orbitals can form a σ bond because their directional electron lobes point to and overlap significantly with each other. Likewise, depending on the extent of the d orbital overlap, the doubly degenerate dxz and dyz orbitals can form a doubly degenerate π bond, and the dxy and dx2−y2 orbitals can form, respectively, δxy and δx2−y2 bonds. In general, the σ bond is the strongest, and the δ bond is the weakest. The exposed Sc2−PW should be only stabilized by the ddσ bond between the two occupied dz2 orbitals (Figure S3 of the Supporting Information). Thus, the Sc−Sc bond is a single covalent bond. As the number of valence electrons increases from Sc to Ti, V, and Cr, the LDOS plots show double, triple, and quadruple d−d covalent bonds between the TMs. It is a little peculiar, however, that the Ti2−PW is stabilized by a rather weak σ−δxy double bond instead of by the σ−π double bond as shown in Figure 2. The

Figure 3. Local density of states (LDOS) of Cr2−PW before and after the dihydrogen adsorption.

adsorption energy of 0.02 eV per H2 indicate that the Cr−H2 binding is basically of a van der Waals type. For the Sc2−PW, the dihydrogen adsorption on the exposed metal site weakens the metal−metal covalency and induces as well a net magnetization or a spin-flip of the complex. The metal−metal distance is indeed increased from 2.93 to 3.08 Å after dihydrogen adsorption. For Ti2− and V2−PWs, although the H2 adsorption energy is small, the metal−metal distances are noticeably reduced indicating that the metal−metal covalency is unexpectedly enhanced. This can be understood by LDOS analyses. From the LDOS analyses, one can see that the dihydrogen σ orbital couples with the TM dz2 orbital upon hydrogen adsorption as observed in previous studies.28−30 The uplifted minority-spin dz2 orbital becomes unoccupied as a consequence of the dihydrogen adsorption, while the majorityspin dz2 orbital remains occupied. After the spin-flip process, the majority σ−π−δ states are evenly occupied for Ti2− and V2− PWs. This may result in the enhanced metal−metal covalency. The calculated Ti−H2 and V−H2 separations are 2.38 and 2.42 Å, respectively, which are shorter than van der Waals separations but are longer than covalent-bond distances. As we discussed before,14,28−30 the TM−H2 separation is a better indicator for enhanced (or Kubas-type) dihydrogen interaction than the traditional H−H distance, when the TM−H 2 interaction energy is small. For a typical Kubas dihydrogen interaction of about 1 eV,31 the H−H separation can vary up to 20%, and so it can be a good indicator for Kubas but so is the TM−H2 separation which is much shorter than their van der Waals distances. When the interaction is weak at 2.3 Å. From Sc to Cr, the pyridine adsorption energy is 1.20, 0.83, 0.53, and 0.31 eV, respectively. The stronger the covalent metal−metal interaction is, the smaller the adsorption energy is. Because of the reduced covalency in the pyridine-coordinated TM2−PWs, a magnetization appears in either FM or AFM spin configuration even for the most covalent Cr2−PW.

Figure 2. Local density of states (LDOS) of Ti2−PW before and after the dihydrogen adsorption. The Fermi level is marked by the dashed line. The energy zero indicates the vacuum level. The projected H2 and TM2 states are represented by the red and black solid lines, respectively.

fractional occupation of the doubly degenerate ddπ state in the σ−π double bond may not be energetically favored in the Ti2− PW. This accounts for the weak covalency of the Ti−Ti bond. The exposed V2− and Cr2−PWs exhibit the low-lying doubly degenerate ddπ orbitals, even lower than the ddσ in the orbital energy, as shown in Figure S4 of the Supporting Information and in Figure 3, respectively. The ultrashort Cr−Cr distance can thus be understood on the basis of the strong ddπ covalent couplings. The hydrogen adsorption on the exposed early TM PWs is almost insignificant with the adsorption energies of 0.02−0.08 eV per H2 because in general the covalency of the metal−metal bond opposes the dihydrogen−TM interaction. As shown in Figure 3, the most covalent Cr2−PW does not show any noticeable change in the LDOS upon hydrogen adsorption. The calculated Cr−H2 distance of 2.93 Å and the dihydrogen 7389

dx.doi.org/10.1021/jp210985a | J. Phys. Chem. C 2012, 116, 7386−7392

The Journal of Physical Chemistry C

Article

Late Transition Metal PWs (from Mn to Ni). For the exposed Mn2− and Fe2−PWs, the metal−metal antibonding states such as ddπ* and ddσ* start to be filled. This weakens the covalent character of the metal−metal bond. Also, the strong exchange interaction of Mn and Fe dominates the covalent metal−metal interaction so that the systems favor the FM spin configurations. The calculated hydrogen adsorption energies on Mn2− and Fe2−PWs are 0.03 and 0.02 eV per H2, respectively. The LDOS plot of Fe2−PW (Figure 4) shows insignificant change in the Figure 5. Local density of states (LDOS) of Co2−PW before and after the dihydrogen adsorption.

Figure 4. Local density of states (LDOS) of Fe2−PW before and after the dihydrogen adsorption.

electronic structure upon the hydrogen adsorption. The Mn− H2 and the Fe−H2 distances are 2.86 Å and 2.74 Å, respectively. Both Mn−H2 and Fe−H2 fall into the van der Waals regime.14,29,30 The weak adsorption energies can be understood in light of the strong exchange interaction of Mn and Fe: Because of the strong tendency to have a specific, particularly ferromagnetic, spin polarization, the TM−H2 coupling could not initiate any spin crossover by lifting the dz2 orbital.32 In addition, the energy difference between H2 σ and ddσ* orbitals for the Mn−PW is too big to produce any meaningful coupling (Figure S5 of the Supporting Information). The strong magnetic coupling of TM dimers can be altered only by introducing the stronger ligation with pyridine as shown in Tables 1 and 2. Pyridine causes a significant alternation in electronic structure and magnetization properties of late TM PWs. This is because the lone pair electrons of the N site could couple strongly enough with the ddσ* orbital to compete the TM exchange interaction. The pyridine adsorption energies range from 0.5 to 0.7 eV except ∼1 eV for the Mn2− PW. For Co- and Ni-based PWs, the energy difference between H2 σ and ddσ* states becomes smaller, and the exchange interaction of the cations becomes weaker. Particularly for Co2−PW, 14 3d electrons of the two Co cations occupy the σ(2), π(4), π*(4), δxy(2), and δxy*(2) orbitals but not the σ*(2), as shown in Figure 5, where the number in parentheses represents the degeneracy. The Co−Co has thus a single covalent bond and favors the nonmagnetic spin configuration. The H2 adsorption energy in this case is remarkably large, up to 0.18 eV per H2, through a Kubas-type coupling between the H2 σ and the unoccupied ddσ* (or antibonding dz2) orbitals. The Co−H2 distance is only 1.88 Å, and the adsorbed H2 is noticeably elongated up to 0.771 from 0.751 Å. This unique enhancement of hydrogen adsorption in the Co−PW is due to the antibonding character of the unoccupied ddσ* state. As shown in Figure 6, the ddσ* state spreads out to the vacuum

Figure 6. Squared charge density plots of the (a) unperturbed and (b) H2 σ-perturbed ddσ* states for Co2−PW. In b, the H2σ−ddσ* antibonding coupling is clearly seen. The contour line corresponds to 0.02 e/Å3. C, O, H, and Co atoms are indicated by green, red, white, and magenta colored balls, respectively.

space so that it can overlap effectively with the approaching hydrogen σ state. For Ni2−PW in which the ddσ* is halfoccupied (Figure S6 of the Supporting Information), the hydrogen adsorption energy is nearly halved to be 0.09 eV per H2 compared to the case of Co2−PW. Post-Transtion Metal PWs (Cu and Zn). After filling all low-lying d orbitals from ddσ to ddσ*, the valence electrons of post-TMs start to fill the two dx2−y2 orbitals as shown in Figure 7 and in Figure S7 of the Supporting Information. The d orbitals of Cu are half-occupied and thus are antiferromagnetically coupled, but the d orbitals of Zn are fully occupied, and so it may be subjective to a tetrahedral distortion (see Supporting Information of ref 14). Because the protruding dz2-related orbitals are all fully occupied, the σ−dz2 coupling could not

Figure 7. Local density of states (LDOS) of Zn2−PW before and after the dihydrogen adsorption. 7390

dx.doi.org/10.1021/jp210985a | J. Phys. Chem. C 2012, 116, 7386−7392

The Journal of Physical Chemistry C

Article

and Cu2−PWFs before and after hydrogen absorption. This information is available free of charge via the Internet at http:// pubs.acs.org.

generate any energy gain. The enhanced dihydrogen adsorption in Cu and Zn PWs has been recently understood in terms of the σ−s coupling.14 The Zn2−(HCOO)4 model confirms once again that the appreciable energy gain of 0.19 eV per H2 is due to the unexpected orbital coupling of H2 σ with the Zn 4s derived states (Figure 7). The pyridine coordination stabilizes the Zn2−PW with a significant energy gain of 1.19 eV. The pyridine adsorption energy for the exposed Cu2−PW is about 0.6 eV. Considering that metal-exposed Cu2−PW can be readily prepared in experiments,9−12 we could expect that the metal exposure is very likely to happen for the fully coordinated V2−, Cr2−, Fe2−, Co2−, and Ni2−PWs of which pyridine adsorption energies are 0.3−0.7 eV. 2D Transition Metal PWFs. The simplest, densest, but hypothetical 2D PWF can be constructed by directly bridging the PW units with the C−C bond as shown in Figure 1a. Their calculated atomic, magnetic, and dihydrogen adsorption properties are colisted in Tables 1 and 2. Magnetic properties of 2D TM PWFs before H2 adsorption are almost the same with those of the molecular PW units except for the cases of Sc and V. The dihydrogen adsorption energies onto exposed 2D TM PWFs are slightly increased compared to the values for the molecular PWs although the orbital coupling schemes are almost identical. Because of the ideal packing of open-metal sites in 2D PWFs, the theoretical hydrogen storage capacity can be increased up to 1.3−1.5 wt % depending on the choice of transition metals. Particularly, a H2 capacity of 1.35 wt % with the high binding energy of 0.2 eV per H2 can be achieved in the 2D Co2− and Zn2−PWFs, which could be more desirable for noncryogenic, high-gravimetric, high-volumetric H2 storage application than the Cu2−PWF or HKUST-1.33

■

*E-mail: [email protected]. Present Address ¶

Department of Physics, Princeton University, Princeton, NJ 08544, United States.

Author Contributions ⊥

These two authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The work at KAIST was supported by the WCU program (R31-2008-000-10071-0) and the Basic Science Research program (2010-0006922) through the NRF of Korea and Korea Institute of Machinery and Materials. The work at NREL was supported by U.S. DOE/OS/BES and DOE/EERE under Contract No. DE-AC36-08GO28308.

■

REFERENCES

(1) Hydrogen, Fuel Cells & Infrastructure Technologies Program. http://www.eere.energy.gov/hydrogenandfuelcells/storage/. (2) Bénard, P.; Chahine, R. Scr. Mater. 2007, 56, 803−808. (3) Yaghi, O.; O’Keeffe, M.; Ockwig, N.; Chae, H. Nature 2003, 423, 705−714. (4) Zhao, Y.; Kim, Y.-H.; Dillon, a. C.; Heben, M. J.; Zhang, S. B. Phys. Rev. Lett. 2005, 94, 155504. (5) Kim, Y.-H.; Zhao, Y.; Williamson, A.; Heben, M. J; Zhang, S. B. Phys. Rev. Lett. 2006, 96, 016102. (6) Yan, Y.; et al. J. Am. Chem. Soc. 2010, 132, 4092−4094. (7) Dinca, M.; Dailly, A.; Liu, Y.; Brown, C. M.; Neumann, D. A.; Long, J. R. J. Am. Chem. Soc. 2006, 128, 16876−16883. (8) Dincă, M.; Long, J. R. Angew. Chem., Int. Ed. 2008, 47, 6766− 6779. (9) Lin, X.; et al. J. Am. Chem. Soc. 2009, 131, 2159−2171. (10) Peterson, V. K.; Liu, Y.; Brown, C. M.; Kepert, C. J. J. Am. Chem. Soc. 2006, 128, 15578−15579. (11) Chui, S. S.-Y.; Lo, S. M.-F.; Charmant, J. P. H; Orpen, A. G.; Williams, I. D. Science 1999, 283, 1148−1150. (12) Ma, L.; Mihalcik, D. J.; Lin, W. J. Am. Chem. Soc. 2009, 131, 4610−4612. (13) Choi, E.-Y.; Wray, C. A.; Hu, C.; Choe, W. CrystEngComm 2009, 11, 553. (14) Kim, Y.-H.; Kang, J.; Wei, S.-H. Phys. Rev. Lett. 2010, 105, 236105. (15) Mendiratta, A.; et al. Inorg. Chem. 2006, 45, 4328−4330. (16) Cotton, F. A.; Hillard, E. A.; Murillo, C. A.; Wang, X. Inorg. Chem. 2003, 42, 6063−6070. (17) Tsai, Y.-C.; Hsu, C.-W.; Yu, J.-S. K.; Lee, G.-H.; Wang, Y.; Kuo, T.-S. Angew. Chem. 2008, 47, 7250−7253. Nguyen, T.; Sutton, A. D.; Brynda, M.; Fettinger, J. C.; Long, G. J.; Power, P. P. Science 2005, 310, 844−847. Cotton, F. A.; Daniels, L. M.; Murillo, C. A.; Pascual, I.; Zhou, H.-C. J. Am. Chem. Soc. 1999, 121, 6856−6861. (18) Fu, Z.; Yi, J.; Chen, Y.; Liao, S.; Guo, N. Eur. J. Inorg. Chem. 2008, 2008, 628. (19) Lee, D.; Du Bois, J.; Petasis, D.; Hendrich, M. P.; Krebs, C.; Huynh, B. H.; Lippard, S. J. J. Am. Chem. Soc. 1999, 121, 9893−9894. (20) Benbellat, N.; Gavrilenko, K. S.; Gal, Y. L.; Cador, O.; Gohlen, S.; Gouasmia, A.; Fabre, J.-M.; Ouahab, L. Inorg. Chem. 2006, 45, 10440−10442. (21) Kounavi, K.; Manos, M. J.; Tasiopoulos, A. J.; Perlepes, S. P.; Nastopoulos, V. Bioinorg. Chem. Appl. 2010, 2010, 178034.

■

CONCLUSION We have presented a comprehensive study of atomic, electronic, magnetic, and gas adsorptive properties of 3d transition-metal-exposed carboxylate-bridged PWs on the basis of first-principles spin-polarized density functional theory calculations. The general chemical trends of H2 adsorption on these PWs are successfully explained. We demonstrate that the weak H2 adsorption on the early TM PWs from Sc to Cr is due to the existence of multiple covalent metal−metal bonds for these PWs, and the chemical trends can be explained by the bond-breaking processes. This conclusion is further supported by the fact that the gas adsorptive interaction is more significant for pyridine (a stronger Lewis base) than for H2 (a weaker Lewis base). For the late TMs from Mn to Ni, the strong exchange interaction caused the weak hydrogen adsorption. The Co2−PW is the only sweet spot for appreciably enhanced dihydrogen adsorption (0.18 eV per H2). For the post-TMs like Cu and Zn, we confirmed that the enhanced hydrogen adsorption was indeed enabled by the previously predicted coupling between the H2 σ and post-TM 4s derived orbitals. Finally, the 2D paddle wheel frameworks were proposed for noncryogenic hydrogen storage applications with a maximized density of enhanced binding sites.

■

AUTHOR INFORMATION

Corresponding Author

ASSOCIATED CONTENT

S Supporting Information *

Comparison of electronic structure between Co2(TBC) and Co2(HCOO)4 before and after hydrogen absorption; three possible configurations of hydrogen absorption on Co2(TBC); and local density of states (LDOS) of Sc2−, V2−, Mn2−, Ni2−, 7391

dx.doi.org/10.1021/jp210985a | J. Phys. Chem. C 2012, 116, 7386−7392

The Journal of Physical Chemistry C

Article

(22) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. (23) Kresse, G.; Joubert, D. Phys. Rev. B 1999, 59, 1758. (24) Andersson, K.; Bauschlicher, C. W.; Persson, B. J. Chem. Phys. Lett. 1996, 2614. (25) Cotton, F. A.; Feng, X. J. Am. Chem. Soc. 1997, 119, 7514− 7520. (26) Wilson, B.; Georgiadis, R.; Bartmess, J. E. J. Am. Chem. Soc. 1991 , 113, 1762−1766. (27) Lide, D. R. CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL, 2006. (28) Sun, Y. Y.; Kim, Y.-H.; Zhang, S. B. J. Am. Chem. Soc. 2007, 129, 12606−12607. (29) Kim, Y.-H.; Sun, Y. Y.; Choi, W. I.; Kang, J.; Zhang, S. B. Phys. Chem. Chem. Phys. 2009, 11, 11400−11403. (30) Choi, W. I.; Jhi, S.-H.; Kim, K.; Kim, Y.-H. Phys. Rev. B 2010, 81, 085441. (31) Kubas, G. J.; Ryan, R. R.; Swanson, B. I.; Vergamini, P. J.; Wasserman, H. J. J. Am. Chem. Soc. 1984, 106, 451−452. (32) Sun, Y. Y.; Kim, Y.-H.; Lee, K.; West, D.; Zhang, S. B. Phys. Chem. Chem. Phys. 2011, 13, 5042−5046. (33) Vitillo, J. G.; Regli, L.; Chavan, S.; Ricchiardi, G.; Spoto, G.; Dietzel, P. D. C.; Bordiga, S.; Zecchina, A. J. Am. Chem. Soc. 2008, 130, 8386−8396.

7392

dx.doi.org/10.1021/jp210985a | J. Phys. Chem. C 2012, 116, 7386−7392