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Hydrogen Storage in Mesoporous Metal Oxides with Catalyst and External Electric Field Xiang Sun, Jiann-Yang Hwang,* and Shangzhao Shi Department of Materials Science and Engineering, Michigan Technological UniVersity, Houghton, Michigan 49931 ReceiVed: NoVember 3, 2009; ReVised Manuscript ReceiVed: March 2, 2010
The effects of catalyst and external electric field on hydrogen storage were evaluated for mesoporous nickel oxide and magnesium oxide. When Pt was introduced into the metal oxides, the capacity of hydrogen adsorption increased remarkably. Furthermore, the measurements of hydrogen adsorption were also carried out on metal oxides by using piezoelectric materials PMN-PT to generate the external electric field. It was observed that the external electric field can increase hydrogen uptake by 37.5% for nickel oxide and 25% for magnesium oxide in a pressure range from 0 to 60 bar. Our DFT calculations support this electrical field enhancement. 1. Introduction The use of porous materials for hydrogen storage has been hindered by weak gas-surface interactions. Although significant storage capacity has been achieved at 77 K with highly porous carbon materials,1-5 minerals,4,6-9 and metal organic frameworks,4,9-21 at ambient temperature, these materials barely adsorb hydrogen due to weak interaction between hydrogen and the solid materials. Such weak physisorption is not able to fulfill the hydrogen storage task for real applications. Recently, efforts have been focused on hydrogen spillover in porous materials,22-26 which drastically improved the storage capacity to 4 wt % at room temperature and 10 MPa in Pt/C-doped IRMOF-8.27 The intrinsic principle relies on the strong interaction between atomic hydrogen and the adsorbent surface. However, slow adsorption/ desorption kinetics have been reported28,29 and need to be further improved. Another strategy to increase the interaction energy refers to the modifications of the chemical properties of the exposed adsorbent sites, such as surface treatment,30 ion exchange,6,7,31 doping,32-35 or change of the coordinative centers.36-38 Few experiments have been carried out to prove the feasibility of this strategy.39,40 In addition to the intrinsic modifications and hydrogen spillover, an externally applied electric field could be a promising means to enhance adsorption enthalpy. Metal oxides still attract attention for adsorption due to their wide application in catalysis,41-49 gas storage,50-52 and sensors.53-56 However, with respect to the usage in physisorption of hydrogen, few research outcomes have been reported,57,58 since there remains the difficult task of synthesizing highly porous metal oxides with unsaturated metal centers, which account for the strong interaction with hydrogen.59-61 Nickel oxides and magnesium oxides are polar materials with an intrinsic dipole moment. The externally applied electric field could further enhance the dipole moment so that the hydrogen could be more strongly adsorbed. Here, we report a study on hydrogen storage in porous nickel and magnesium oxides under a PMN-PT-generated electric field. Neither of these oxides is conductive under normal conditions, where the electric field strength could be preserved inside the materials and the induced enhancement of adsorption could be * To whom correspondence should be addressed. E-mail: jhwang@ mtu.edu.
expected. We obtained significant storage improvement in both oxides. By combining the spillover and electric field effects, the overall storage capacity has been increased remarkably in Pt-deposited nickel oxides. Moreover, density functional theory (DFT) modeling of the hydrogen adsorption on oxide clusters was performed to support the experimental observations and reveal the underlying mechanism. 2. Experimental Details 2.1. Synthesis of Porous Oxides. The porous oxides were synthesized following previously developed techniques.62,63 Typically, for porous nickel oxides, 13.19 g of NiCl2 · 6H2O, 32.01 g of sodium dodecyl sulfate (SDS), 100 g of urea, and 60 mL of DI water were magnetically stirred at 40 °C for 1 h to obtain a transparent solution. For porous magnesium oxides, 10.17 g of MgCl2 · 6H2O, 28.84 g of SDS, 90 g of urea, and 54 mL of DI water were mixed. Then the mixture was transferred to an 80 °C oil bath and further reacted for 6 h. The obtained slurry was cooled and filtered and then dried at 120 °C overnight. The solids were then ground and washed several times with ethanol to remove the surfactant. The final products were dried in an oven and calcined for the desired time to obtain porous oxides. 2.2. Platinum Deposition. The platinum deposition on porous nickel oxide was prepared by the wet impregnationreduction method.64-66 Typically, 0.6 g of porous NiO was mixed with 80 mL of DI water in a 90 °C oil bath with a continuous argon gas purge. A 6.4 mL portion of 50 mM H2PtCl6 solution was added to the slurry. A 2 M NaOH solution was used to adjust the slurry pH to 11-13. Subsequently, a 3.75 mL HCHO solution (37%) was added dropwise and stirred continuously for 2 h. The final products were washed, filtered, and dried in air. 2.3. Characterization of the Synthesized Oxides. The surface area and pore size distribution were measured by using a Micromeritics ASAP2000 instrument. The samples were evacuated by heating at 200 °C under vacuum for 12 h before testing. X-ray diffraction (XRD) measurement was carried out by using a Scintag XDS2000 powder diffractometer at a scan rate of 0.08°/s with Cu radiation at 45 kV, 35 mA. A Hitachi S-4700 field emission-scanning electron microscope (FE-SEM) was used to examine the microstructure of the
10.1021/jp910506g 2010 American Chemical Society Published on Web 03/18/2010
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Figure 1. XRD patterns of (a) nickel oxides and (b) magnesium oxides.
Figure 2. Hydrogen adsorption isotherms at 298 K for (a) porous nickel oxide and PMN-PT and (b) porous magnesium oxide and PMN-PT.
samples. The incorporated energy dispersive analysis (EDS) was employed to examine the element concentrations. 2.4. Hydrogen Storage Measurements. Hydrogen adsorption at pressure between 0.1 and 140 bar was measured by using static volumetric techniques with an automatic Sieverts’ apparatus (PCT-Pro 2000 from Hy-Energy LLC) at room temperature. Previous to the test, samples were degassed extensively at 200 °C for 12 h and then carefully moved into the sample holder and evacuated again at 100 °C for 2 h to ensure the removal of any contaminations on the sample surface. Around 100-300 mg of the sample was used in each test, and ultrapure (99.999%) hydrogen and helium gases were used for all calibrations and measurements. A single-crystal PMN-PT disk, 5 mm in length and width and 1 mm in thickness, was acquired from Morgan Electro Ceramics. The positive side of the PMN-PT was covered by a nonconductive, thin BaTiO3 film made by BaTiO3 powders and glue to prevent electron transfer from the negative side to the positive side. The PMN-PT was immersed in the selected adsorbents. A glass container was used to ensure good electric insulation from the outside steel holder. In such an arrangement, the electrons from the PMN-PT were not able to transfer to the outside so that the electric field could be preserved inside the adsorbents. The piezoelectric voltage factor, g33, of the PMNPT is 15.8 × 10-3 Vm/N, indicating that the PMN-PT can generate significant voltage under pressure. 2.5. Computational Method. The computational clusters used here have been chosen to satisfy the criteria of being neutral and stoichiometric. The metal oxides were represented by various M5O5 clusters, where M indicates the cation and O indicates the oxygen anion. The clusters were embedded in an
alternating positive and negative point charge of equal value. The point charge arrays were arranged at a construction of 13 × 13 × 13 system. All the bonds, angles, and crystal data (CIF files) of oxide crystals were obtained from an online crystallography database.67 Geometry optimization of clusters was first performed at different values of surrounding point charges by varying the bond length of the lattice parameter while fixing all the bond angles so as to obtain the correct magnitude of point charge. The criteria used here are that the magnitude of point charges were varied until the optimized lattice parameter could well reproduce the parameter from the standard database.68 After obtaining the correct point charge value, the point charge array was moved down along the z-axis to expose the surface atoms with the charge magnitude and structure unchanged. For modeling of hydrogen adsorption, the oxide clusters were fixed at the initial structure while the hydrogen molecules were free to move. For the calculation of the effects of the external electric field, the applied field was applied along the vertical direction at a field strength of 0.010 and 0.015 au, which is equal to 5.14 × 109 and 7.71 × 109 V/m, respectively. The total binding energy was calculated by the difference between the total energy of the whole adsorption system and the total energy of the separate reactants before combination, as presented in following equation:
M5O5 + Hx f M5O5Hx
(1)
All the calculations were performed by using the B3LYP/ 6-31G(d)69,70 method in the Gaussian 03 software program.71 Density of state (DOS) analysis was visualized in Gausssum
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Figure 3. Nitrogen adsorption/desorption isotherms of Pt deposited porous NiO. Insert: BJH pore size distribution obtained by desorption isotherms.
software.72 The choice of the basis set was demonstrated to be reliable for such a cluster system from our previous calculations.73 3. Results and Discussion 3.1. Characterization of Porous Oxides. The as-synthesized nickel compound and magnesium compound were calcined for 3 h at 300 and 400 °C, respectively. The X-ray diffraction patterns of the nickel oxides and magnesium oxides are shown in Figure 1. The XRD analyses clearly show that only the single phase of each oxide is present with no other impurities,
Sun et al. indicating that the decomposition reaction was complete. The significant peak broadening revealed that the calcined particles have small crystal size. Brunauer, Emmett, and Teller theory (BET) analysis showed that porous nickel oxide has a total BET surface area of 382.56 m2/g with an average pore diameter of 7.08 nm, and porous magnesium oxide has a BET surface area of 363.59 m2/g with an average pore diameter of 3.87 nm. The high surface area of both mesoporous oxides makes them ideal candidates for this hydrogen adsorption study. 3.2. Hydrogen Adsorption. Hydrogen adsorption isotherms at 298 K for porous NiO and NiO + PMN-PT are shown in Figure 2a. The porous nickel oxide had a total hydrogen capacity of about 0.08 wt % at 135 bar. The shape of the adsorption curve was almost linear, which indicates a further increase in capacity could be obtained at a pressure >135 bar. The curve also presented a typical physisorption type caused by the low interaction force of the hydrogen molecule and nickel oxide surface. PMN-PT is a typical piezoelectric material that can generate electrical potential in response to external stress. Under high hydrostatic hydrogen gas pressure, substantial voltage potential could be generated across the positive and negative sides of the piezoelectric materials. By embedding the piezoelectric material into the ionic adsorbent, the electrostatic interaction could be further enhanced so that the total hydrogen storage could be remarkably increased. By simply embedding an electric field generating material PMN-PT into the porous nickel oxide, the hydrogen uptake had been remarkably enhanced to 0.11 wt % at 135 bar, which was about 37.5% increase. It should be noted that the PMN-PT alone had zero adsorption at the same conditions. Such significant enhancement
Figure 4. FE-SEM images of platinum deposited porous nickel oxide (a, b) and EDS spectrum (c).
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Figure 5. Hydrogen uptake of porous nickel oxide with spillover and PMN-PT at 298 K.
Figure 6. Computational modeling of hydrogen adsorption on nickel oxide at nickel site (left) and oxygen site (right). See Table 1 for optimized parameters. Point charges are omitted for clarity.
was due to the PMN-PT-generated electric field. Moreover, it is noted that the magnitude difference between the two lines continuously increased from 0 to 60 bar and then remained almost constant at pressures higher than 60 bar. This implies that there was a saturation point from which further enhancement of the electrical strength has no effect on the hydrogen adsorption. The hydrogen uptake curves of porous magnesium oxide at 298 K are shown in Figure 2b. The curve also followed a linear trend, indicating that no apparent saturation point was observed. It had very low hydrogen uptake at pressure below 30 bar, whereas the uptake increased vigorously when the pressure reached over 30 bar. The optimum adsorption at 90 bar reached about 0.2 wt %. With the assistance from the PMN-PT, the hydrogen uptake had improved to 0.25 wt % at 90 bar, about a
25% increase. The curve still preserved the linear trend, and no saturation was observed. The enhancement increased with increasing hydrogen pressure, and at over 60 bar, the magnitude of the enhancement became almost constant. It can be seen from Figure 2 that by simply embedding PMNPT inside oxide materials, the hydrogen uptakes have been significantly enhanced. In normal ionic compounds, the charge quadrupole and charge induced dipole electrostatic interactions are the major contributors to the bonding with hydrogen. However, there are other electronic interactions for transition metals, such as H2 σ orbital donation or metal dπ orbital back-donation.59,60 The oxide surface is anticipated to have active sites for hydrogen adsorption, and complex interaction might occur especially for unsaturated nickel on the surface. However, the role of PMN-PT here is aimed only to increase the electrostatic interaction between the hydrogen molecules and oxide surfaces. In addition to the intrinsic dipole moment of the metal oxides, the PMN-PT-generated electric field could further increase the electrostatic force so that the hydrogen molecules are strongly attracted. The experimental observations clearly supported our hypothesis. Furthermore, computational modeling work can better reveal the underlying mechanisms, as described in later sections. 3.3. Catalytic Effects. It is believed that hydrogen molecules would dissociate into atomic hydrogen when they are in contact or in close range with the catalytic surface. The atomic hydrogen then spills over to the adsorbent surface via surface diffusion.22 Spillover has been successfully used for hydrogen storage in carbon materials,22 metal-organic frameworks,25-29 and porous inorganic materials.24 The bonding between atomic hydrogen and the materials surface is much stronger than that of molecular hydrogen so that significant adsorption could be realized, especially at room temperature. The possible drawbacks are blocking of the pores by the deposited catalysts and also the decreased surface area for adsorption. The nitrogen adsorption/desorption of the porous nickel oxide, on which platinum was dispersed, is shown in Figure 3. The nickel oxide with platinum still maintains its mesorporous characteristics, which were determined from the hysteresis loop of the adsorption/desorption isotherms and the pore size distribution (insert). However, the total surface area dropped by 35%. Figure 4 shows the FE-SEM images and the EDS analyzed elemental compositions of the platinum deposited nickel oxides. The tiny spheroids on the surface were mainly deposited platinum particles at a size of ∼50-100 nm. Such small particle size made the platinum very reactive where the hydrogen dissociation would easily occur on the platinum surface. The EDS spectrum shows that the deposited platinum is about 20 wt %. The XRD analysis (shown in the Supporting Information)
TABLE 1: Geometric Parameters and Binding Energies for Nickel Oxide and Hydrogen Complex under Various Strength of Electric Field bond length (Å)
bond angle (°)
Mulliken charge
H2 adsorption site
E field (au)
H1-H2
H1-Ni
H2-Ni
H1-Ni-H2
H1
H2
Ni
binding energy (kJ/mol)
Ni
0 0.01 0.015
0.7429 0.7436 0.7441
3.4904 3.3670 3.2840
3.5260 3.3745 3.2917
12.1418 12.6645 12.9941
0.011 0.016 0.020
0.008 0.017 0.021
0.396 0.516 0.562
1.484 3.053 4.204
O
0 0.01 0.015
H1-H2
H1-O
H2-O
H1-O-H2
H1
H2
O
0.7439 0.7443 0.7454
3.1600 3.1508 3.1102
3.9019 3.8935 3.8553
175.3126 175.8978 178.3819
-0.004 -0.066 -0.097
0.008 0.075 0.110
-0.98 -0.997 -0.10
1.353 2.749 4.763
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confirmed that the sample contained only platinum and nickel oxide phases. The hydrogen adsorption curves of Pt-deposited NiO at 298 K are shown in Figure 5. Hydrogen spillover on nickel oxide improved the uptake to 0.22 wt % at 120 bar, which was ∼3 times the capacity of the nickel oxide without platinum. No apparent saturation point was approached for the Pt-deposited sample, and the adsorption trend followed a linear line, implying a further increase in total capacity could be expected at higher pressure. Under the PMN-PT-generated electric field, the total hydrogen uptake was tremendously improved. It reached about 0.31 wt % at 120 bar, which was another 50% increase over the spillover nickel oxides. There was also no apparent saturation value for adsorption. The results proved that the external electric field imposed on the nickel oxide could also attract atomic hydrogen and enhance the bonding interaction. The enhancement from PMN-PT was small at low pressure, but it increased as the pressure increased. At about 20 bar, the magnitude of the enhancement became constant, indicating that the significant effect of the electric field on atomic hydrogen adsorption occurred only at pressures lower than 20 bar. By combining the enhancement from hydrogen spillover and a PMN-PTgenerated electric field, the total capacity was increased remarkably by 400% (a factor of 4). One should notice that the enhancement for atomic hydrogen storage was larger than for the hydrogen molecule under the electric field. It is speculated that the σ bond between the hydrogen molecules is very strong and may not be easily affected, even by an extreme external force. However, atomic hydrogen has only an unbounded electron, which might be easier to be influenced by the electric field. Such an interesting discovery might be further tested and understood in MOF and MOF/Pt materials. 3.4. Computational Modeling. Figure 6 shows the modeling sketches for hydrogen adsorption on nickel oxides. The nominal charge for nickel oxide is 2; however, the point charge used to reproduce the lattice parameter is 1.2648. Carefully selecting the point charge value is critical for reliable modeling results. Both nickel and oxygen sites have been calculated. The external electric field was applied along the vertical directions. The optimized geometries and corresponding binding energies are shown in Table 1. Hydrogen molecules could be adsorbed onto the nickel site in nickel oxide following a side-on model (Yeager model)74 with a small binding energy. It is noted that the free hydrogen molecules optimized by B3LYP/6-31G(d) DFT calculations had a bond distance of H-H at 0.74279 Å. The
Sun et al.
Figure 8. Computational modeling of hydrogen adsorption on magnesium oxide at magnesium site (left) and oxygen site (right). See Table 2 for optimized parameters. Point charges are omitted for clarity.
hydrogen molecules are slightly elongated, indicating a small perturbation from the nickel atom. When the electrical field is applied, the hydrogen molecules are more perturbed and attracted closer to the nickel atom, indicating a stronger interaction. The shorter bond distance of H1-Ni or H2-Ni also matched well with the energy observations. The effects increase consistently with the increasing electrical field strength, and the highest binding energy at 0.015 au increased 3 times. These remarkable phenomena are in good agreement with the experimental observations. Hydrogen adsorption on oxygen sites exhibited end-on adsorption (Pauling model),74 which is a typical characteristic of gas-anion interaction. The small interaction energy is increased remarkably under the applied electric field. The corresponding bond length, angle, and charges confirmed the energy observations. One way to look at the detailed electron interactions is to use density of state analysis. Figure 7 shows the DOS spectrum of the hydrogen orbital of the adsorbed hydrogen molecules. Free hydrogen molecules have only one peak at around -12 eV, indicating the localization of the electrons. When they are adsorbed onto a nickel site, there are two more peaks occurring between -10 and -2 eV, indicating part of the electrons moving to higher energy state. Such electrons overlap with the electrons from the nickel outer shell so that the adsorption bonding is formed. When an electrical field is applied, there are more peaks
Figure 7. DOSs of orbitals of hydrogen molecules that adsorbed on a nickel site (a) and oxygen site (b) of the nickel oxide under the applied electric field. Insert: higher magnification of the DOSs between -10 and -2 eV.
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TABLE 2: Geometric Parameters and Binding Energies for Magnesium Oxide and Hydrogen Complex under Various Strength of Electric Field bond length (Å)
bond angle (°)
H2 adsorption site
E field (au)
H1-H2
H1-Mg
H2-Mg
H1-Mg-H2
H1
H2
Mg
binding energy (kJ/mol)
Mg
0 0.01 0.15
0.7447 0.7456 0.7465
2.6807 2.4936 2.4267
2.6942 2.5042 2.4281
15.9237 17.1578 17.6916
0.026 0.035 0.044
0.013 0.034 0.045
0.567 0.651 0.641
5.134 6.567 9.111
O
0 0.01 0.15
Mulliken charge
H1-H2
H1-O
H2-O
H1-O-H2
H1
H2
O
0.7434 0.7450 0.7465
3.9277 3.4362 3.3248
4.6569 4.1810 4.0700
167.7338 178.4848 178.3676
-0.024 -0.091 -0.122
0.029 0.102 0.136
-0.87 -0.895 -0.91
presented due to more electrons’ overlapping facilitated by the external electric field. Accordingly, better interaction between hydrogen and nickel has resulted. Under a higher field strength, more peaks and higher peak intensity appear. The overall effects increase with increasing electrical field strength. Such a trend of improvement consistently matches the binding energy observations. The DOS analysis of hydrogen adsorption on oxygen sites (Figure 7b) also showed that the electron clouds were pulled over to the oxygen site, and the bonding was formed due to the overlapped electrons. It is noted that there is no peak splitting of hydrogen orbitals, indicating that it is still a weak interaction. Although the nickel atom has surplus d orbital electrons, which are found to be essential for bonding with polar gas molecules,75 the interaction with the nonpolar hydrogen molecule remains weak. The application of an external electric field solely forces the electrons to be overlapped and increase the electrostatic interaction; however, a turn bond is not formed. Figure 8 shows the modeling sketches for hydrogen adsorption on magnesium oxides. Here, the point charge used to reproduce the lattice parameter is calculated to be 0.706. Table 2 shows the optimized geometries and corresponding binding energies. The interaction enthalpy is in good agreement with the reported experimental thermodynamic measurements,76 which indicates the efficiency of the computational model. The same trend is observed for both the magnesium and oxygen sites. With the increase in the electric field strength, the hydrogen molecules become more perturbed along with the increased binding energy. The H-H bond length, H-O distance, and charges support these observations. The corresponding DOS spectra of hydrogen molecules are shown in Figure 9. For the free hydrogen molecules, there was only one peak, which indicated the localization of the s orbital. As the hydrogen molecule is adsorbed, a small peak appears at
0.522 3.657 6.416
a higher energy level. When the applied electric field strength increases, more peaks emerge, and the intensity of the peaks increases. The appearance of the new peaks indicated the overlap of electrons between hydrogen and adsorbent sites. The trend appears to be similar to that in nickel oxides. Unlike the nickel atom, magnesium does not have any d orbital electrons. The back-donation effect is not anticipated to occur. Again, the interaction with hydrogen is solely electrostatic, and the external field tremendously increases such effects so that the binding energy improves significantly. The modeling results are in good agreement with the experimental observations. Under an electric field, both NiO and MgO exhibit a stronger affinity for hydrogen molecules. With a strengthened electric field interaction, the overall storage capacity could be remarkably enhanced. The interaction energies from the computational results were on the order of 5-10 kJ/ mol, which was below the room temperature reversible adsorption enthalpy of 15 kJ/mol,77 indicating that the adsorbed hydrogen could be totally released at ambient temperature. 4. Conclusion In conclusion, we have demonstrated that hydrogen storage on mesoporous nickel oxide and magnesium oxide could be significant enhanced by a PMN-PT-generated electric field. The achieved enhancement was 37.5% for nickel oxide and 25% for magnesium oxide. The electric field was also proven to be effective for adsorption of atomic hydrogen, where another 50% adsorption increase was observed in Pt-doped nickel oxide. The overall effects of the catalyst and electric field could substantially increase the total adsorption by a factor of 4. The corresponding computer simulations on oxide clusters revealed that the electric field causes the electron clouds of the hydrogen molecules to
Figure 9. DOSs of orbitals of hydrogen molecules that adsorbed on the magnesium site (a) and oxygen site (b) of the magnesium oxide under the applied electric field. Insert: higher magnification of the DOSs.
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