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Theoretical Study of the Hydrogen Absorption Mechanism into a Palladium Nanocube Coated with a Metal−Organic Framework Yusuke Nanba,*,† Tatsuki Tsutsumi,‡ Takayoshi Ishimoto,∥ and Michihisa Koyama*,†,‡,∥,§ †

INAMORI Frontier Research Center, ‡Graduate School of Engineering, and §International Institute for Carbon-Neutral Energy Research, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ∥ Institute of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8527, Japan S Supporting Information *

ABSTRACT: We analyzed theoretically the hydrogen absorption properties and kinetics into Pd nanocubes coated with copper(II) 1,3,5benzenetricarboxylate (HKUST-1), which is a type of metal−organic framework, using density functional theory. We prepared an interface model consisting of the Pd(100) surface and Cu-edged HKUST-1 structure and calculated the hydrogen adsorption and absorption energies in a Pd nanocube model. To discuss the kinetics of the hydrogen absorption, we also evaluated the hydrogen diffusion barrier near the interface. Compared with bare Pd, the hydrogen diffusion barrier from the surface to the subsurface decreased. We found that the adsorption of HKUST-1 on the Pd nanocube leads to chemical and steric effects for the diffusion rate increase of hydrogen. As a chemical effect, hydrogen adsorption was destabilized by the change of electronic structure of the Pd surface because of the atomic charge displacement between the Pd and Cu atoms in HKUST-1. As a steric effect, a new hydrogen diffusion path from the unique Pd5Cu octahedral site was created.

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INTRODUCTION Enhancement of hydrogen storage properties is one of the important tasks for achieving a hydrogen economy. Pd is wellknown as one of the hydrogen storage metals under normal temperature and pressure. The hydrogen adsorption and absorption properties of Pd have been investigated for decades by various methods, such as low-energy electron diffraction,1−3 electron energy loss spectroscopy,4 and transmission channeling.5 It is regarded that the site with high coordination number is favorable for hydrogen adsorption in the Pd surface. Neutron experiments revealed that the hydrogen atoms absorbed in Pd prefer the octahedral site to the tetrahedral site.6,7 Calculations based on density functional theory (DFT) could reproduce these characteristics of bulk Pd.8−13 To enhance the hydrogen absorption property, Pd-based alloys,14,15 Pd nanoparticles (Pd-NPs),16,17 and Pd-based solidsolution NP alloys18,19 have been extensively studied. For example, DFT calculations were used to understand the hydrogen adsorption and absorption properties of Pd-based alloys.20−22 Even if the hydrogen atom is located at sites with the same coordination number, the potential energies of the hydrogen adsorption and absorption are affected by the local environment, such as nearest-neighbor transition metal and second-nearest-neighbor configuration. This is because the electronic structure of Pd is changed by the alloying metals.22 As another approach, the combined system of Pd-NP coated with metal−organic framework (MOF) has been synthesized.23−25 MOFs, which are composed of a metal ion and © XXXX American Chemical Society

organic linker, have advantages of high porosity and high periodicity. MOFs have been applied to many research areas, such as catalyst,26 separation,27 and storage.28 It is generally regarded that the metal NPs coated with MOF show not only the synergetic effects of the two materials but also new reaction and selectivity functions.29−33 The hydrogen adsorption and absorption properties of metal NPs coated with MOF have also been studied.23−25,34,35 The physical mixture of Pt/C and MOF enhanced the hydrogen adsorption compared with the sum of their properties.34,35 The Pd-NP coated with Al3O(OH)(H2O)2[BTC]2·24H2O (MIL-100 (Al), BTC = 1,3,5-benzenetricarboxylate) or [Zn3(NTB)2]n (SNU-3, NTB = 4,4′,4″nitrilotrisbenzoate) also showed enhancement of the hydrogen adsorption property and hydrogen uptake.23,24 The enhancements of the hydrogen adsorption properties of these materials are caused by the hydrogen spillover effect. Recently, Li et al. synthesized a Pd-NP coated with Cu3(BTC)2 (HKUST-1; copper(II) 1,3,5-benzenetricarboxylate).25 Pd-NP coated with HKUST-1 (Pd@HKUST-1) showed larger hydrogen storage capacity than Pd-NP, although pure HKUST-1 did not absorb hydrogen at all. This result indicates that HKUST-1 enhances the hydrogen absorption capacity of Pd-NP. Furthermore, the kinetics of the hydrogen absorption of Pd was also enhanced by the HKUST-1. To investigate the electronic structure of NP Received: April 3, 2017 Revised: June 12, 2017 Published: June 14, 2017 A

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was observed in the original and modified HKUST-1 structures (Figure S4 (b)), and the interatomic distances between the Cu atoms and between the Cu and O atoms in BDC were much the same as those in BTC (Table S1). These results mean that the modification of the linker does not greatly affect the electronic structure of the Cu atom. Thus, the modified HKUST-1 is reasonable to adopt as the interface model. Figure 1 shows the interface model of Pd@HKUST-1 for the analysis

after coating with MOF, X-ray photoelectron spectroscopy (XPS) was performed. In the Pd 3d XPS study of Pd@ HKUST-1, the main peak shifted to the higher energy side by a few eV.23,25 These studies suggest that the electronic structure of Pd is changed by the MOF coating. It is important to understand the mechanism of the enhancement of the hydrogen absorption property of Pd-NPs coated with HKUST-1 based on electronic structure calculations. In this paper, we analyze the hydrogen adsorption and absorption energies and diffusion barrier of hydrogen in Pd of Pd@HKUST-1 using DFT calculations and compare with the bare Pd(100) model. In addition, we replaced Cu in HKUST-1 with other 3d transition metals and calculated the diffusion barrier to analyze the substitution effect of metals.

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METHODS Computational Details. All calculations were performed using the DMol3 software package based on DFT.36 The generalized gradient approximation with Perdew−Burke− Ernzerhof exchange-correlation functional37 was used. A double numerical plus polarization basis set implemented in DMol3 software36 was employed, and core electrons were treated by the effective core potential of the Stuttgart−Dersden group.38 The energy convergences, maximum force, and displacement were set as 2.0 × 10−5 Ha, 0.004 Ha/Å, and 0.005 Å, respectively. The complete Linear Synchronous Transit/ Quadratic Synchronous Transit method39 was used to search the transition state (TS) for the hydrogen diffusion process. Zero-point energy (ZPE) was derived from partial vibrational analyses for adsorbed and absorbed hydrogen atoms. Atomic charges were estimated using the Hirshfeld population method.40 Modeling of Pd@HKUST-1. It has been reported that the {100} facets of Pd nanocubes interact with HKUST-1,9 and hydrogen diffuses throughout these facets. We employed a periodic Pd(100) slab model of three Pd layers with a (4 × 5) unit cell (11.26 Å × 14.07 Å after geometry optimization). The relative energies of the hydrogen adsorption and absorption were calculated using the following equation 1 ΔE = E Pd − H − E Pd − E H2 (1) 2 where EPd−H, EPd, and EH2 represent the energies of Pd with an adsorbed or absorbed H atom, Pd without a H atom, and a H2 molecule, respectively. In addition, the hydrogen adsorption and absorption energies include the ZPE correction. HKUST-1 is composed of copper(II) and 1,3,5-benzentricarboxylate (BTC). The polymer framework of the HKUST141 is shown in Figure S1. The interface structure of HKUST-1 interacting with the Pd(100) surface was determined by the stability of the edge structure of HKUST-1 and the adsorption site. As shown in Figure S2, the Cu-edged and O-edged HKUST-1 cluster models were adsorbed on the top or 4-fold sites of the Pd surface. The interface model of 4-fold adsorption of the Cu-edged HKUST-1 on the Pd(100) surface is suitable to discuss the hydrogen adsorption and absorption of Pd@ HKUST-1 from the relative energy analysis (Figure S3). To reduce the model size of Pd@HKUST-1, the BTC, which is the linker in HKUST-1, was modified to benzenedicarboxylate (BDC), as shown in Figure S4 (a). The atomic charges of the Cu atoms in the original and modified HKUST-1 structures were estimated to verify the reliability of the modified linker. No difference of the average atomic charges in the Cu atoms

Figure 1. Pd@Cu(BDC) interface model. BTC in HKUST-1 is approximated as BDC, which is adsorbed on the 4-fold site of the Pd(100) surface. CuIF is a Cu at the interface between Pd and BDC, while CuIN is an internal Cu of HKUST-1. White, red, gray, and dark green spheres are hydrogen, oxygen, carbon, and palladium atoms, respectively.

of the hydrogen adsorption and absorption. The unit cell along the normal axis for a periodic Pd slab and the atomic positions were optimized. After the geometry optimization, the cell size along the normal axis is 24.23 Å. The Pd(100) slab models without and with adsorbed HKUST-1 are shown as the bare Pd and Pd@Cu(BDC), respectively, in this study.

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RESULTS AND DISCUSSION Hydrogen Adsorption and Absorption Energies in Bare Pd. We calculated the hydrogen adsorption and absorption energies in bare Pd. At first, molecular hydrogen adsorbs and dissociates spontaneously on the Pd(100) surface.1 Although the estimated energy barrier exists in the present study, the values were almost zero (Figure S5). Thus, we discuss adsorption and absorption of a hydrogen atom. A hydrogen atom was adsorbed on the 4-fold site of the Pd(100) surface and absorbed in the tetrahedral and octahedral sites, as shown in Figure 2. Table 1 shows the hydrogen adsorption and absorption energies with and without the ZPE correction in the bare Pd. The hydrogen adsorption energy in the present study is lower than those in the previous studies (Table S2) because the hydrogen adsorption to transition metal is affected by the number of layers.42 Before the ZPE correction, the hydrogen absorption energy to the tetrahedral site was lower than that of the octahedral site. The ZPE correction stabilized the hydrogen adsorption energy of the surface and the hydrogen absorption energy of the octahedral site, while the hydrogen absorption energy of the tetrahedral site became unstable. The experimental and calculated results indicate that the interstitial H atom occupies the octahedral site of the Pd.13,24,25 This result means that the ZPE correction is indispensable for analyzing the hydrogen in the Pd. Atomic Charge Analysis in Pd@Cu(BDC). An atomic charge analysis was performed to evaluate the atomic charge B

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of the Cu atoms. The atomic charge change by the HKUST-1 adsorption has already been investigated using Cu 2p and Pd 3d XPS.9 The peak in the Cu 2p and Pd 3d XPS spectra shifts to lower and higher binding energy regions, respectively. These results suggest an atomic charge displacement between Pd and Cu in Pd@HKUST-1. Therefore, we clearly found the same trend of atomic charge displacement between Pd and Cu as with the experimental Cu 2p and Pd 3d XPS studies in the Cuedge interface model of Pd@Cu(BDC). Hydrogen Adsorption and Absorption Energies in Pd@Cu(BDC). We analyzed the hydrogen adsorption and absorption energies in the Pd@Cu(BDC) model and compared them with the bare Pd. As hydrogen adsorption and absorption sites, 7 surface (Sx; Figure 3 (a)), 15 tetrahedral (Tx; Figure 3 (b)), and 10 octahedral (Ox; Figure 3 (c)) sites were prepared. The sites were assigned in the order of their distance from the Cu atom. For example, the nearest surface site from the Cu atom is expressed as S1, while S5 represents the surface site farthest from the Cu atom. The Cu(BDC) adsorption formed two Pd2Cu surface sites denoted as SA and SB. OA represents a Pd5Cu octahedral site formed by the Cu(BDC) adsorption. Figure 4 (a) shows the hydrogen adsorption energies in the Pd@Cu(BDC) model. The hydrogen adsorption energies of all surface sites in the Pd@(BDC) model became unstable compared with the bare Pd case. In particular, the S1, S2, and S3 sites showed weak hydrogen adsorption energies in the 4fold surface sites near adsorbed Cu in Cu(BDC). This result means that the hydrogen adsorption on the surface site was destabilized by the Cu(BDC) adsorption because the Cu(BDC) adsorption influences the electronic structure of the Pd surface. In contrast, the hydrogen adsorption energy of the S5 site, which is far from the Cu atom, has a similar value to the bare Pd. The hydrogen adsorption energy of the SB site was more unstable than that of the SA site because a large distortion of the SB site was found (Figure S7). The hydrogen absorption energies of the tetrahedral sites are shown in Figure 4 (b). The hydrogen absorption energies of the T1, T2, T3, and T4 sites were higher than those of the other tetrahedral sites. These tetrahedral sites are also close to the Cu atoms in Cu(BDC). On the other hand, the T10 and T15 sites, which are far from the Cu atom, showed lower absorption energies. Figure 4 (c) shows the hydrogen absorption energies of the octahedral sites in the Pd@Cu(BDC) model. The O1 site showed the highest hydrogen absorption energy in the octahedral site. This site is also close to the Cu atom in Cu(BDC). In contrast, the OA site showed the lowest hydrogen absorption energy in the octahedral site. This octahedral site was a newly formed surface site by adsorbed Cu(BDC). Hydrogen Diffusion Mechanism of Pd@Cu(BDC). We analyzed the hydrogen diffusion barrier of the bare Pd and Pd@ Cu(BDC) model and compared them. Figure 5 shows a potential energy diagram of the hydrogen diffusion in the bare Pd case (black line). In the bare Pd, the hydrogen atom diffuses from the surface to the octahedral site via the tetrahedral site (Figure 5 (a)). The hydrogen diffusion barriers of forward and backward reactions from the surface to the tetrahedral site were 0.41 and 0.07 eV, respectively. These values are close to those of the previous study.13 In the Pd@Cu(BDC) model, the hydrogen atom also diffused from the Pd 4-fold surface as well as the bare Pd case (Figure 5 (b)). Figure 5 (c) shows the potential energy diagram of hydrogen diffusion from the S1 sites (blue line). The hydrogen adsorption energy of the Pd 4-fold surface was changed by the HKUST-1 adsorption, as shown in

Figure 2. Hydrogen adsorption and absorption sites in the Pd(100) slab model. White and dark green spheres are hydrogen and palladium atoms, respectively.

Table 1. Hydrogen Adsorption and Absorption Energies for the Pd(100) Surface with and without ZPE Correctiona surface tetra octa a

with ZPE

without ZPE

−0.444 −0.097 −0.092

−0.337 −0.117 0.011

Energy unit is eV.

displacement between the interface of the Pd@Cu(BDC) model. In this model, two types of Cu atoms are included because of the different surrounding conditions. One is the Cu atom in Cu(BDC) interacting with the Pd(100) surface. The other is the internal Cu atom in Cu(BDC). The former and latter are denoted as CuIF and CuIN, respectively, in Figure 1. Table 2 shows the total atomic charge of Pd and average atomic Table 2. Total Atomic Charge of Pd and Average Atomic Charges of Cu in Pd@Cu(BDC)a Pd CuIN CuIF

before

after

0.000 0.435 0.328

0.713 0.444 0.142

a

The values before and after Cu(BDC) adsorption are shown. CuIF is a Cu atom at the interface between Pd and Cu(BDC), and CuIN is an internal Cu atom of Cu(BDC) (see Figure 1).

charges of the Cu atoms in Cu(BDC) before and after adsorption to the Pd(100) surface. The average atomic charge of CuIN after the Cu(BDC) adsorption was almost the same as that before the adsorption. However, the average atomic charge of the CuIF atom decreased, and the total atomic charge of the Pd atoms increased after the Cu(BDC) adsorption. This result indicates an atomic charge displacement between Pd and Cu in Pd@Cu(BDC). We also calculated the O-edge interface model and estimated the atomic charge of the Cu atom (Figure S6 (b)). No significant change in average atomic charge of the Cu atom was observed by the Cu(BDC) adsorption. The two interface models showed different results for the atomic charge C

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Figure 4. Hydrogen adsorption and absorption energies in the Pd@ Cu(BDC) model. The sites in Figure 3 correspond with (a) surface sites, (b) tetrahedral sites, and (c) octahedral sites. A and B sites mean hydrogen adsorption and absorption sites formed by the Cu(BDC) adsorption. These energies include the ZPE correction. The numerical values are shown in Table S3. The broken lines represent the hydrogen adsorption and absorption energies in the bare Pd(100) slab model.

new hydrogen diffusion path was obtained by the Cu(BDC) adsorption (Figure 5 (b)). The potential energy diagram from SB sites is shown in Figure 5 (d). In the diffusion path from the new Pd2Cu 3-fold site, the path from the surface to the tetrahedral site was divided into two steps. The new diffusion path from the Pd2Cu surface and Pd5Cu octahedral sites was followed in order by the SB, OA, T1, and O1 sites. The diffusion barriers from the SB to the OA sites and from the OA to the T1 sites were smaller than that from the surface to a tetrahedral site of the bare Pd. The stepwise mechanism resulted in a smaller hydrogen diffusion barrier. Thus, the Cu(BDC) adsorption on the Pd(100) surface led to a steric effect for the diffusion rate increase of hydrogen. Based on the obtained diffusion barrier, the kinetics of hydrogen diffusion was evaluated. According to the Arrhenius equation, the kinetic coefficient is expressed as

Figure 3. Hydrogen adsorption and absorption sites: (a) surface sites, (b) tetrahedral sites, and (c) octahedral sites. A and B sites mean the hydrogen adsorption and absorption sites formed by the Cu(BDC) adsorption. White, red, gray, and dark green spheres are hydrogen, oxygen, carbon, and palladium atoms, respectively.

k = k 0 exp( −ΔE /RT )

Figure 4 (a). In particular, the S1 and S3 sites, which are closer to the Cu atom, showed higher hydrogen adsorption energies. The hydrogen diffusion barrier from the S1 to the T6 sites was 0.28 eV, while that from the S3 to the T8 sites was 0.30 eV. Moreover, the hydrogen diffusion barrier from the T6 to the O5 sites (0.05 eV) was lower than that from the T8 to the O4 sites (0.13 eV). Compared with the bare Pd, the diffusion barrier from the surface to the tetrahedral site in the Pd@Cu(BDC) model became smaller due to the destabilization of hydrogen adsorption energy by the Cu(BDC) adsorption. The Cu(BDC) adsorption on the Pd(100) surface led to the chemical effect for the diffusion rate increase of hydrogen. On the other hand, a

(2)

where k0, ΔE, R, and T represent the preexponential factor (1/ s), the diffusion barrier (J mol−1·K−1), gas constant (= 8.31 J mol−1·K−1), and temperature (K). We estimated the ratio of the Boltzmann factor of the hydrogen diffusion from the surface to tetrahedral sites in the Pd@Cu(BDC) model relatively to the bare Pd case at 300 K. The ratio for hydrogen diffusion from S1 to T6 sites was 152 by the chemical effect. The ratios for hydrogen diffusion from SB to T1 sites and from O1 to T1 sites were 3362 and 1069, respectively. This result indicates that higher hydrogen diffusion and lower adsorption energies are obtained by the Cu(BDC) adsorption. When the surface sites D

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Pd@Cu(BDC) to compare the change of the potential energy of hydrogen diffusion. The geometry optimizations were performed under the same calculation condition, and the ZPE correction was considered. Figure 6 shows the potential

Figure 6. Potential energy diagrams of hydrogen adsorption and absorption of the Pd@Cu(BDC) and Pd@M(BDC) (M = Ni, Co, and Fe) from (a) Pd 4-fold surface and (b) Pd2M1 3-fold surface. The hydrogen diffusion pathways are the same as in Figure 5 (b). The values of the hydrogen diffusion barrier for Pd@Cu(BDC) and Pd@ M(BDC) are shown in Table 3. The potential energy diagram for the Pd(100) slab model is shown for comparison.

Figure 5. Hydrogen diffusion paths in (a) Pd(100) slab model and (b) Pd@Cu(BDC) model and potential energy diagrams of hydrogen diffusion from adsorption and absorption from (c) Pd 4-fold surface (blue line) and (d) Pd2Cu1 3-fold surface (red line). The potential energy diagram in the Pd(100) slab model is also shown (black line). The numerical values represent the hydrogen diffusion barrier in each process.

energy diagram of the hydrogen diffusion from Pd 4-fold and Pd2M 3-fold sites in the Pd@M(BDC) model together with the bare Pd and Pd@Cu(BDC). The hydrogen diffusion barriers from the surface to the tetrahedral sites in Pd@Cu(BDC) and Pd@M(BDC) are shown in Table 3. In Figure 6 (a), the

are far from the adsorbed Cu atom in BDC, the kinetics of hydrogen diffusion become closer to the bare Pd(100) surface case. We clearly reproduced the diffusion rate increase of hydrogen in Pd@HKUST-1 qualitatively, although the consideration of actual coverage of HKUST-1 and the difference of the preexponential factor with path are necessary to discuss the observed kinetics quantitatively. Substitution Effect for Pd@Cu(BDC). We also analyzed the substitution effect of Cu in the Pd@Cu(BDC) model for designing the hydrogen absorption properties. The substitution of a transition metal is one of the simple approaches to evaluate the characteristics based on the known properties. In this study, the Cu atom in HKUST-1 was replaced with Ni, Co, and Fe, and the hydrogen diffusion barriers were estimated. Hereafter, the used models are denoted as Pd@M(BDC) (M = Ni, Co, Fe). We analyzed the same hydrogen diffusion pathways as

Table 3. Hydrogen Diffusion Barrier from Surface to Tetrahedral Sites in Pd@Cu(BDC) and Pd@M(BDC) Models (M = Ni, Co, and Fe)a S1 → TS → T6 Cu Ni Co Fe

0.28 0.36 0.47 0.35

eV eV eV eV

SB → TS → OA 0.20 0.28 0.32 0.45

eV eV eV eV

OA → TS → T1 0.23 0.22 0.19 0.21

eV eV eV eV

a

The potential energy diagrams of hydrogen adsorption and absorption of the Pd@Cu(BDC) and Pd@M(BDC) correspond to Figure 6.

E

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ment between Pd and the 3d transition metal occurs in Pd@ M(BDC). Thus, the excess atomic charge displacement in Pd@ M(BDC) (M = Ni, Co, and Fe) has an adverse effect on the diffusion rate increase of hydrogen. Controlling appropriate atomic charge displacement is important in the design of nanoparticles coated with MOF for the kinetics of hydrogen diffusion.

hydrogen adsorption energies of the S1 site in Pd@M(BDC) were almost the same as that in Pd@Cu(BDC). However, the diffusion barrier from the S1 to T6 sites in Pd@Co(BDC) was larger than that from the surface to the tetrahedral sites in the bare Pd. In the hydrogen diffusion from the Pd2M site (Figure 6 (b)), the hydrogen transfer from the surface to the tetrahedral sites was divided into a stepwise mechanism. The two diffusion barriers in the Pd@Ni(BDC) and Pd@Co(BDC) were lower than that from the surface to the tetrahedral sites in the bare Pd. On the other hand, the diffusion barrier from the SB to the OA sites in the Pd@Fe(BDC) was larger than that from the surface to the tetrahedral sites in the bare Pd. In both the hydrogen diffusion from Pd 4-fold and Pd2M 3-fold sites, the Pd@Ni(BDC) is expected to show diffusion rate increase of hydrogen. Compared with the Pd@Cu(BDC), the diffusion barrier from the OA to the T1 sites in Pd@Ni(BDC) was smaller, while that from the SB to the OA sites was larger. Because the maximum diffusion barrier in Pd@Cu(BDC) was smaller than those in Pd@Ni(BDC), replacing the Cu atom in HKUST-1 with Ni is not conducive to the development of the hydrogen diffusion rate. In Pd@Cu(BDC), the atomic charges of the Pd and Cu atoms were changed by the Cu(BDC) adsorption. We estimated the atomic charges of Pd@M(BDC) (Table S4) and compared them. While the average atomic charge of the MIF atom decreased, the total atomic charge of Pd increased after the M(BDC) adsorption. The average atomic charges of the MIN atoms did not vary greatly after the M(BDC) adsorption. The atomic charge displacement between Pd and M in Pd@M(BDC) was larger than that in Pd@ Cu(BDC). The atomic charge displacement between Pd and Cu is one of the critical factors for varying the diffusion barrier in Pd@Cu(BDC). The large diffusion barrier may be attributed to the excess atomic charge displacement between Pd and M in Pd@M(BDC). These results suggest that the excess atomic charge displacement between Pd and M in Pd@M(BDC) has an adverse effect. The electronic structure control by atomic charge displacement near the interface is important to obtain the diffusion rate increase of hydrogen in Pd@HKUST-1.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b03137. Details of model making, hydrogen adsorption and absorption energies, the total charge of Pd, and average charge of M in the Pd@M(BDC) model (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(Y.N.) E-mail: [email protected]. Tel.: +81-92-8016969. Fax: +81-92-801-6969. *(M.K.) E-mail: [email protected]. Tel.: +81-92-8016968. Fax: +81-92-801-6968. ORCID

Yusuke Nanba: 0000-0002-1692-4465 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by CREST, JST. Activities of INAMORI Frontier Research Center are supported by KYOCERA Corporation.

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REFERENCES

(1) Behm, R. J.; Christmann, K.; Ertl, G. Adsorption of Hydrogen on Pd(100). Surf. Sci. 1980, 99, 320−340. (2) Skottke, M.; Behm, R. J.; Ertl, G.; Penka, V.; Moritz, W. LEED Structure Analysis of the Clean and (2 × 1)H Covered Pd(110) Surface. J. Chem. Phys. 1987, 87, 6191−6198. (3) Felter, T. E.; Sowa, E. C.; Van Hove, M. A. Location of Hydrogen Adsorbed on Palladium (111) Studied by Low Energy Electron Diffraction. Phys. Rev. B: Condens. Matter Mater. Phys. 1989, 40, 891− 899. (4) Nyberg, C.; Tengstål, C. G. Vibrational Excitation of p(2 × 2) Oxygen and c(2 × 2) Hydrogen on Pd(100). Solid State Commun. 1982, 44, 251−254. (5) Besenbacher, F.; Stensgaard, I.; Mortensen, K. Adsorption Position of Deuterium on the Pd(100) Surface Determined with Transmission Channeling. Surf. Sci. 1987, 191, 288−301. (6) Worsham, J. E., Jr.; Wilkinson, M. K.; Shull, C. G. Neutron Diffraction Observation on the Palladium Hydrogen and Palladium Deuterium System. J. Phys. Chem. Solids 1957, 3, 303−310. (7) Cser, L.; Török, G.; Krexner, G.; Prem, M.; Sharkov, I. Neutron Holographic Study of Palladium Hydride. Appl. Phys. Lett. 2004, 85, 1149−1151. (8) Tománek, D.; Sun, Z.; Louie, S. G. Ab Initio Calculation of Chemisorption System: H on Pd(001) and Pd(110). Phys. Rev. B: Condens. Matter Mater. Phys. 1991, 43, 4699−4712. (9) Wilke, S.; Hennig, D.; Löber, R.; Methfessel, M.; Scheffler, M. Ab Initio Study of Hydrogen Adsorption on Pd(100). Surf. Sci. 1994, 307−309, 76−81. (10) Wilke, S.; Hennig, D.; Löber, R. Ab Initio Calculations of Hydrogen Adsorption on (100) Surfaces of Palladium and Rhodium. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 2548−2560.

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CONCLUSIONS We have analyzed theoretically the effect of HKUST-1 adsorption on the hydrogen adsorption and absorption of Pd@HKUST-1. The atomic charge was displaced between Pd and Cu in HKUST-1 by the HKUST-1 adsorption on the Pd surface. When the hydrogen adsorption site was close to the Cu atom, the hydrogen adsorption energy became higher. The diffusion barrier became smaller by the destabilization of the hydrogen adsorption. Moreover, the HKUST-1 adsorption formed a new hydrogen diffusion path. Hydrogen diffusion from the surface to the tetrahedral site was divided into a stepwise mechanism. Both the diffusion barriers from the Pd2Cu 3-fold surface to the Pd5Cu octahedral sites and from the Pd5Cu octahedral to the tetrahedral sites were smaller than that from the surface to the tetrahedral sites in the bare Pd. These results suggest the faster kinetics of hydrogen diffusion. Thus, the surface modifications, such as the chemical effect (atomic charge displacement) and steric effect (formation of a new path), lead to the diffusion rate increase of hydrogen in Pd@HKUST-1. In this study, replacing the Cu atom in HKUST-1 with Ni, Co, and Fe has also been analyzed for a deeper understanding. The combinations with Ni, Co, and Fe do not show an increased effect to overcome Pd@HKUST-1. Compared with Pd@Cu(BDC), large atomic charge displaceF

DOI: 10.1021/acs.jpcc.7b03137 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.7b03137 J. Phys. Chem. C XXXX, XXX, XXX−XXX