Hydrogen adsorption on 3d transition-metal-doped organosilica complexes

Chemical Physics Letters 488 (2010) 7–9

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Hydrogen adsorption on 3d transition-metal-doped organosilica complexes Min Hee Park, Yoon Sup Lee * Department of Chemistry, KAIST, Daejeon 301-750, Republic of Korea

a r t i c l e

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Article history: Received 30 December 2009 In final form 29 January 2010 Available online 4 February 2010

a b s t r a c t The adsorption of H2 on a series of 3d transition-metal (TM)-doped organosilica complexes is investigated using density functional calculations. We show that a modified benzene-silica (MBS) model with the 3d TM atoms can adsorb H2 as dihydride or dihydrogen configurations except the model with Fe, Co, or Ni, which can store hydrogen as dihydrogen forms only. The maximum numbers of H2 molecules adsorbed are one to three for the various TM atoms, with the average binding energies of 0.4–0.9 eV. We propose that the TM–MBS (TM = Sc, Ti, and V) complexes can be the building blocks in designing hydrogen storage materials. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Hydrogen is widely regarded as an abundant, nonpolluting, renewable, and potential cost-effective energy alternative that could one day replace petroleum fuels [1,2]. However, problems such as hydrogen storage capacity with high gravimetric and volumetric density and safety to transport effectively this highly combustible gas are still challenging [3,4]. Recent reports from the Departments of Energy (DOE) announce that a threshold weight percent (wt.%) of hydrogen capacity, 6.5 wt.% is required to be economically viable [5]. No material so far has been successful in combining the hydrogen storage capacity for on-board systems such as automotive engineering and the ability to reversibly adsorb and desorb hydrogen under proper kinetic and thermodynamic conditions [6–10]. Although several hydrogen storage methods such as storing in the form of gas, cryogenic liquid, or adsorbed gas within solid materials have been suggested, the storage of compressed or liquid hydrogen is unsuitable for automotive engineering due to cost, safety, and low density although the kinetics is fast enough. In spite of energy efficiency and safety, storing hydrogen in solid materials also has problems such as high weight and high temperatures required. Another direction of research to develop materials is the use of hydrogen adsorbent which consists of lightweight materials with very high surface areas to adsorb H2 molecules. These materials such as carbon-based materials (graphite, graphite nanofibers, and carbon nanotubes) and metal–organic frameworks (MOFs) do not yet satisfy the target of DOE [11,12]. Recently advanced researches to raise a binding energy of adsorbed dihydrogen suggest several ways to modify the surfaces of materials or to dope metal atoms [13–19]. Materials with transition-metal

(TM) atoms can chemisorb atomically or physisorb molecularly hydrogen mainly by the Kubas interaction [20,21] that is characterized by forward donation of the bonding electron in H2 to a partially filled metal d orbital and back donation from the metal atom to the r* antibonding orbital of H2 molecule. Zhao et al. [14] predicted that 12 Sc atoms deposited on the C60 and C48B12 can reversibly store 11 hydrogen atoms per TM (9 wt.%). Yildirim and coworkers [15] found that single ethylene molecule with two Ti atoms (C2H4Ti2) can absorb 14 wt.% hydrogen. Kiran et al. [16] indicated that a Ti atom on C4H4, C5H5, or C8H8 rings can store up to 9 wt.% hydrogen with an average binding energy of about 0.55 eV/H2 molecule. Weck et al. [17] calculated that the hydrogen storage capacity of organometallic compounds consisting of Sc, Ti, and V transition-metal atoms bound to C4H4, C5H5, or C6H6 rings can reach the maximum retrievable hydrogen uptake level of 9.3 wt.% for ScC4H4 + (H2)5 complex, with the average binding energy of 0.33 eV/H2. These indicate that TM bound aromatic compounds can be promising candidates as hydrogen storage materials. Therefore, we examined the H2 adsorption on a modified benzene-silica (MBS) model, TM–C6H4(SiH3)2 complex for the 3d series of metals from Sc to Ni. It is known that organosilica materials can store hydrogen effectively from the experimental results [22] as well as the theoretical simulations [18]. Density functional theory (DFT) calculations were employed to show the possibility of TM– C6H4(SiH3)2 complexes as hydrogen storage materials. Our work would aid designing and understanding operational principles of multidecker hydrogen-storage systems consist of metal–organic building blocks.

2. Computation details * Corresponding author. Fax: +82 42 350 2810. E-mail address: [email protected] (Y.S. Lee). 0009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2010.01.070

Spin polarized all-electron calculations of the total energies and geometry optimization were performed with the generalized

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M.H. Park, Y.S. Lee / Chemical Physics Letters 488 (2010) 7–9

Table 1 Spin multiplicities (2s + 1) of TM–MBS complexes (TM = Sc, Ti, V, Cr, Mn, Fe, Co, and Ni). Multiplicity

Sc

Ti

V

Cr

Mn

Fe

Co

Ni

TM TM–MBS

2 2

3 3

4 2

7 5

6 4

5 3

4 2

3 1

gradient approximation (GGA) using the parameterization of Perdew–Wang (PW91) [23] as implemented in GAUSSIAN 03 package [24]. The 6–31 + G(d,p) basis sets were used. Geometry optimizations were carried out without any symmetry constraints using a normal threshold corresponding to root mean square (RMS) residual forces smaller than 104 au for the optimal geometry. The 2H or H2 adsorptions to TM–C6H4(SiH3)2 complexes (TM = Sc, Ti, V, Cr, Mn, Fe, Co, and Ni) were obtained for a number of different orientations. The TM–MBS binding was assumed present for the binding energy larger than 0.05 eV. The average binding energies ðDEB Þ of nH2 molecules adsorbed on TM–C6H4(SiH3)2 complexes were evaluated by the formula

DEB ¼

" n X ðTM—C6 H4 ðSiH3 Þ2 ðH2 Þn1 Þ þ EðH2 Þ n1

#

 EðTM—C6 H4 ðSiH3 Þ2 ðH2 Þn Þ =n;

ð1Þ

where a positive binding energy denotes that the complexes are stable against the H2 dissociation.

3. Results and discussion To explore hydrogen adsorption properties of the TM–MBS materials, the binding energies between TM–MBS complexes and H2 molecules were evaluated. The TM–MBS complex is formed with a MBS model (C6H4(SiH3)2) and a single transition metal atom for the 3d series of metals (Sc, Ti, V, Cr, Mn, Fe, Co, and Ni). All the metal atoms bind with MBS in a g6 fashion, with the metal atoms lying 1.5–1.7 Å from the center of MBS. The calculated TM–MBS binding energies are 2.00, 2.42, 2.01, 0.27, 0.95, 1.95, 2.20, and 2.07 eV for TM = Sc, Ti, V, Cr, Mn, Fe, Co, and Ni, respectively. The interaction between TM and MBS is stronger than that of TM-Benzene complex [25] due to –SiH3 substitution instead of –H which is electron withdrawing group and affected electron lack of MBS ring. The Cr and Mn atoms bind weakly with MBS, implying that the TM–MBS (TM = Cr and Mn) complexes would not be useful as the hydrogen storage unit, at least for the neutral-state complexes. Despite the apparent weak interaction for some of the species, all the 3d TM–MBS complexes were considered here to find the trend in the period. The preferred spin multiplicities of TM and TM–MBS are summarized in Table 1. Table 2 lists the binding energies for consecutive adsorption of additional H2 species to the TM–MBS complexes for 3d TM elements along with the average binding energies ðDEB Þ. As in the TM–MBS complex, two ways of bonding between the complex and hydrogen are possible, i.e., hydrogen binding to a metal as either a dihydride atomic form or a dihydrogen molecular form [16]. The first H2 molecule added (n = 1) is found to dissociate to form a dihydride bound to the Sc atom with a binding energy of 0.65 eV and the Sc–H bond length of 1.84 Å, while to form dihydrogen absorbed to ScC6H4(SiH3)2 with a bind-

Table 2 Calculated binding energies for added 2H or H2 on TM–MBS complexes (TM = Sc, Ti, V, Cr, Mn, Fe, Co, and Ni) along with the binding energies ðDEB Þ. Units are in eV. TM–MBS + 2H(H2)n1

Sc Ti V Cr Mn Fe Co Ni a

DE n

ðDEB Þ

n=1

n=2

n=3

n=4

0.65 0.54 0.43 0.76 0.64

0.33 0.56 0.92 0.95

0.30 0.42 0.82

0.31

a a a

0.40 0.51 0.72 0.85 0.64

TM–MBS + (H2)n

Sc Ti V Cr Mn Fe Co Ni

DE n

ðDEB Þ

n=1

n=2

n=3

n=4

0.45 0.58 0.69 0.52 0.66 0.90 1.34 1.38

0.45 0.71 0.94 1.30

0.51 0.43 0.70

0.24

0.07 0.25 0.07

The initial TM–MBS + 2H(H2)n1 structures were optimized to the TM–MBS + (H2)n forms in the optimization process.

Fig. 1. Optimized geometries of TM–MBS complexes with H2 molecules (TM = Sc, Ti, V, Cr, Mn, Fe, Co, and Ni).

0.41 0.57 0.78 0.91 0.66 0.49 0.79 0.73

M.H. Park, Y.S. Lee / Chemical Physics Letters 488 (2010) 7–9

ing energy of 0.45 eV. All the successive H2 molecules added bind in the form of dihydrogen molecules (see Fig. 1 Sc–C6H4(SiH3)2). After the first H2 molecule binds atomically to Sc, the binding energies of successive second, third, and fourth H2 molecules are 0.33, 0.30, and 0.31 eV, respectively. The binding energies are in the range between the chemisorption and the physisorption energies, and the transition metal complexes satisfy the thermodynamic and kinetic requirements of the practical hydrogen storage materials [16,17]. We think that the Sc-MBS complex can store three H2 molecules as in the form of (2H)Sc(H2)3–MBS. For Ti- and V-MBS complexes, the first, second, and third H2 species adsorbed as the molecular dihydrogen form with the DEB values of 0.57 eV and 0.78 eV for Ti and V, respectively. The CrMBS complex has stronger bindings with two H2 molecules than other TM-MBS complexes, while the Cr-MBS interaction is too week at 0.27 eV. Only one H2 molecule is adsorbed to the MnC6H4(SiH3)2 complex as a dihydrogen form. The remaining TM-MBS (TM = Fe, Co, and Ni) complexes bind first two hydrogen molecules as dihydrogen forms. The first hydrogen molecule does not dissociate when binds to the heavier Fe, Co, and Ni atoms (see Table 2). The first H2 molecules are strongly adsorbed on TM-MBS (TM = Fe, Co, and Ni) with the binding energies of 0.90– 1.38 eV, while the second H2 molecules have weak bindings with the binding energies of 0.07–0.25 eV. These TM–MBS (TM = Fe, Co, and Ni) complexes need not be considered as reasonable hydrogen storage materials, in comparison to the early TM analogues. To conclude, we propose that the TM–MBS (TM = Sc, Ti, and V) complexes can be the building blocks in designing hydrogen storage materials. 4. Conclusions We have shown that the 3d transition-metal (TM)-doped organosilica complexes can adsorb hydrogen molecules as dihydride or dihydrogen forms. The modified benzene-silica (MBS) model with the 3d TM atoms, Sc, Ti, V, Cr, and Mn can bind the first H2 as dihydride or dihydrogen configurations, but the MBS model with Fe, Co, or Ni can store hydrogen only as dihydrogen forms. The maximum numbers of H2 molecules that can be adsorbed are one to three for the various TM atoms, with the average binding

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energies of 0.4–0.9 eV. We propose that the TM–MBS (TM = Sc, Ti, and V) complexes can be the building blocks in designing hydrogen storage materials which could provide high hydrogen gravimetric density at ambient thermodynamic conditions. Acknowledgement This work was supported by National Research Foundation of Korea Grant funded by the Korean Government (2009-0076263, R11-2007-012-03001-0), EEWS program of KAIST, and by a Grant from the supercomputing center of KISTI (KCS-2009-S02-0015). The authors thank Dr. Y.K. Han for discussion. References [1] [2] [3] [4] [5]

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