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Hydrogen Adsorption in the Ti-Doped Mesoporous Silicate SBA-15 A. I. Acatrinei,† M. A. Hartl,† Juergen Eckert,†,‡ Eduardo H. L. Falcao,‡ G. Chertkov,†,§ and L. L. Daemen*,† Manuel Lujan, Jr. Neutron Scattering Center, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, Materials Research Laboratory, UniVersity of California, Santa Barbara, California 93106, and Department of Chemistry and Biochemistry, UniVersity of California, Santa Barbara, California 93106 ReceiVed: December 15, 2008; ReVised Manuscript ReceiVed: May 31, 2009
While metal-doped mesoporous silicate SBA-15 has been studied extensively for its catalytic properties, the adsorption of molecular hydrogen in this system has not been considered. We report herein that the titaniumdoped material (10% Ti) adsorbs nearly twice the amount of hydrogen at 77 K and a pressure of 25 bar than pure SBA-15, Hydrogen adsorption isotherms also show that binding energies in the Ti-doped material are more than 50% greater than in SBA-15. Neutron vibrational spectroscopy shows clear evidence for interaction of hydrogen with Ti, which is responsible for part of the increase in hydrogen sorption in the material. Introduction Since the initial report of the synthesis of the mesoporoius silicate SBA-15 in 19981 this material has been studied extensively in a variety of contexts, most notably with respect to catalysis. The material displays large cylindrical, hexagonal pores, the size of which can be tailored with relative ease during the material synthesis.2 Metal-doped SBA-15 has proved a useful catalyst for a number of applications.3 However there have been few fundamental investigations of the interaction of small molecules with the inner surface of the SBA-15 pores. This is most likely because the principal attraction of such mesoporous materials is their use with larger molecules. To the best of our knowledge, no study addresses the interaction of molecular hydrogen with pure or doped SBA-15. Among the two principal, current candidates for potential hydrogen storage materials for use in mobile applications, those based on chemical storage possess very high hydrogen densities but suffer from the large expense in energy that is required to retrieve hydrogen from the material, and even more so, the regeneration of the storage material. On the other hand, materials with high porosity that use the sorption of molecular hydrogen are handicapped by the weakness of the guest-host interaction, which in turn imposes the use of high pressures and low temperatures to achieve practical storage capacities. A considerable effort is therefore being made to improve the understanding of the nature of the interaction of hydrogen with porous hosts and their surfaces and to investigate more systematically the parameters that may enhance he binding of hydrogen in such materials. A large number of studies to date have focused on graphitic materials4 and hybrid materials such as metal-organic frameworks.5 In this work we propose the use of metal-doped SBA-15 as a platform for the study of many of the factors that may enhance hydrogen binding in porous materials with a large surface area, as it possesses an adjustable microstructure and is easily doped in controllable manner6 with a variety of metals. * To whom correspondence should be addressed. E-mail:
[email protected]. † Los Alamos National Laboratory. ‡ Materials Research Laboratory, University of California, Santa Barbara. § Department of Chemistry and Biochemistry, University of California, Santa Barbara.
The parent material is inexpensive, lightweight, and versatile and can be synthesized in large quantities in the laboratory. It remains to be seen whether this material can be developed into a practical hydrogen storage medium. The principal purpose of this work, however, is to determine fundamental details of the interaction of H2 with the pore surface of Ti-doped SBA-15 as a preliminary step to assess the potential of this and related materials for such an application. We demonstrate that doping SBA-15 with titanium atoms increases the hydrogen storage capacity of the material and does so more dramatically with moderate hydrogen pressure (up to 50 bar in our study). We have used neutron scattering vibrational spectroscopy to obtain some detailed information on the binding of hydrogen in these materials and on the role of the Ti dopant in increasing the capacity and binding energies for hydrogen. Experimental Details The first step in the synthesis of SBA-15 is the preparation of an aqueous solution of P123 surfactant (BASF) by dissolving 18 g of P123 in 270 mL of H2O. (Moderate heating was used to dissolve most of the surfactant.) A solution of 135 mL of concentrated hydrochloric acid in 540 mL of H2O is added. After stirring for a few minutes, this solution is filtered to remove any undissolved surfactant material. The resulting solution was divided into six equal portions. We found out that smaller batches produce better material and give somewhat better yields than a single large batch in subsequent operations. Each portion was placed in a 250-mL Teflon Erlenmeyer flask. The Erlenmeyer flasks were then placed in a water bath at 50 °C. After waiting for the solutions to reach the temperature of the water bath, 5.9 mL of tetraorthosilicate (TEOS) was added rapidly to each flask. The flask was stirred by hand for a few seconds and then returned to the water bath. After a few minutes the solutions become cloudy, which marks the onset of TEOS hydrolysis. At this juncture 0.8 mL of titatnium isopropoxide (approximately a 1:10 ratio of Ti to Si) was added to each flask. After stirring for a few seconds, each flask was returned to the water bath. The flasks were stoppered and heating at 50 °C was continued overnight (approximately 14 to 15 h). The content of each flask was filtered on a coarse filter paper and washed with 1 L of
10.1021/jp8110875 CCC: $40.75 2009 American Chemical Society Published on Web 08/10/2009
Ti-Doped Mesoporous Silicate SBA-15
Figure 1. Bragg peak in the XRD pattern of SBA-15 and Ti-doped (10%) SBA-15.
boiling water to remove as much surfactant as possible before the calcination step. The resulting gel was dried under vacuum at 150 °C until it crumbled easily to a fine powder under the pressure of a spatula. This solid material was ground to a fine powder and calcined in air at 550 °C for 2 to 3 h to burn the residual surfactant in the pores. The preparation of pure SBA15 was identical to that described above but without the addition of titanium isopropoxide. In each case we collected approximately 9 g of material after calcination (typically 85 to 90% yield, based on the amount of TEOS used). Figure 1 shows the X-ray diffraction (XRD) patterns of both materials. XRD data were collected on a Rigaku Ultima III powder X-ray diffractometer immediately after calcination. The pore spacing estimated from the position of the main Bragg peak [100] is 90 Å for Ti-SBA-15 and 75 Å for SBA-15. X-ray fluorescence measurements with a PanAnalytical instrument show that approximately 5% Ti is retained in the structure so that the Ti:Si ratio is closer to 1:20 rather than the 1:10 ratio used during the synthesis. This is in agreement with previous reports in the literature that at most 6.5% or so Ti can be incorporated in the SBA-15 framework.3,6 X-ray diffraction shows no TiO2 residual material. High-pressure hydrogen adsorption isotherms were collected at 77 K on a VTI Corp. gas sorption apparatus with 1.8 g (TiSBA-15) and 3.1 g (SBA-15) of material. The material was heated under an ultimate 10-7 mbar vacuum at 200 °C for several hours prior to gas adsorption measurements. The hydrogen gas used was ultrahigh purity, research grade hydrogen from Matheson. Surface areas were determined from nitrogen adsorption isotherms at 77 K, with a Micromeritics TriStar instrument. The Brunauer-Emmett-Teller (BET) model was used for data fitting. Samples were degassed in a FlowPrep sample station at 200-250 °C under a low nitrogen gas flow before measurements. For the acquisition of low pressure hydrogen adsorption isotherms, samples were degassed under vacuum at 200-250 °C for several hours prior to measurements. A Micromeritics ASAP 2010 was used for the measurements, at 77 and 87 K. Neutron scattering vibrational spectra were collected with the pure SBA-15 and Ti-SBA-15 materials, and with hydrogen that was adsorbed at several hydrogen pressures. Powder samples
J. Phys. Chem. C, Vol. 113, No. 35, 2009 15635 of 9 g each were loaded in a drybox under He atmosphere into standard, airtight aluminum sample holders, which had been dried overnight under vacuum at 100 °C. Hydrogen adsorption was carrried out in situ on the neutron spectrometer at 77 K after collection of data on the blank samples. The sample holder was attached to an external gas handling system by way of a capillary tube. In this manner the sample was allowed to adsorb as much hydrogen as it could at 2 atm over a period of 30 min (hydrogen sorption stopped after 6 to 7 min). Following the adsorption of hydrogen in the sample, temperature was maintained at 77 K for 30 min to allow hydrogen to diffuse throughout the material. The sample was subsequently cooled to 30 K (to remain above the boiling point of hydrogen) and the neutron scattering vibrational spectrum collected over a period of several hours. We stayed safely above the boiling point of H2 to prevent the accidental condensation of any residual gaseous H2 in the sample holder. For the same reason, the sample was briefly pumped at 30 K to minimize the amount of residual hydrogen gas in the sample holder. The 30 K temperature was chosen to avoid excessive broadening of the vibrational modes with temperature. While this temperature differs from the 77 K temperature used in the collection of isotherm data, our goal here was primarily to gain some insight in the H2-Ti or H2-framework interaction. This was work accomplished on the filter difference spectrometer (FDS) at the Manuel Lujan, Jr. Neutron Scattering Center at Los Alamos National Laboratory. The Ti-SBA-15 material was cooled in situ to 77 K (the temperature of the isotherms), and the sample holder was connected with a capillary tube to a small H2 reservoir at room temperature and atmospheric pressure. The sample was held at 77 K for 30 min to allow hydrogen to diffuse throughout the host material. It was subsequently cooled to 30 K and held at this temperature for the duration of the experiment. An identical procedure was followed for the SBA-15 sample. Results and Discussion Hydrogen adsorption isotherms at 77K for SBA-15 and TiSBA-15 at low pressure (e1 bar) and at high pressures are shown in Figures 2 and 3, respectively. N2 adsorption isotherms at 77 K were used to determine the surface areas of the two materials, which we found to be 520 m2/g for SBA-15 and 835 m2/g for the Ti-doped form. The latter value is similar to what has previously been reported by Melero et al.7 in a study, which found a wide range of surface areas for Ti-doped SBA-15 depending on the method of synthesis and doping level. A cursory examination of these results reveals that SBA-15 adsorbs appreciably less hydrogen per gram of material than does the Ti-doped material at all pressures. Ti-SBA-15 has a capacity for adsorbed H2 that is approximately 15% larger than the difference in surface areas of the two materials. This result gives some indication of the importance of Ti incorporation the framework. Of much greater significance, however, is the dramatic change in the isosteric heat of adsorption for the Tidoped material (Figure 4), which is more than 50% greater than that for normal SBA-15 over a wide range of H2 loadings. The maximum value of about 6.5 kJ/mol in Ti-SBA-15, however, is less than what has been reported in several porous materials with open metals sites,8 which suggests that the adsorbed hydrogen cannot interact directly with Ti in this material at low pressures. At higher pressures (Figure 3), however, we find that TiSBA-15 continues adsorbing H2 at a relatively large rate [2.4 cm3 (STP)/bar] while SBA-15 picks up H2 at a much lower
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Figure 4. Isosteric heats of adsorption for H2 in Ti-SBA-15 (black) and SBA-15 (red) derived from adsorption isotherms measured at 77 and at 87 K.
Figure 2. Low-pressure H2 adsorption isotherms at 77 K.
Figure 3. Hydrogen high-pressure adsorption isotherms (T ) 77 K) for SBA-15 and Ti-SBA-15 (10% Ti doping). The lines are a guide to the eyes. Notice that the slope of the Ti-SBA-15 isotherm at 25 bar is larger than the slope of the SBA-15 isotherm at the same pressure.
rate [1.3 cm3 (STP)/bar]. At 25 bar, Ti-SBA-15 had adsorbed approximately 0.95 wt % H2, which is an amount comparable to what has been reported for some simple graphitic materials.9 This observation clearly demonstrates an enhanced effect of the Ti in the silicate framework.
While adsorption in SBA-15 is governed by the relatively weak interaction of H2 with hydroxyl groups and oxygen atoms that line the surface of the pores in SBA-15, the above results do suggest that H2 also interacts in some way with Ti in the doped material. We have used neutron vibrational spectroscopy in an attempt to obtain more detailed information on the nature of this interaction. As a result of the fact that the (incoherent) neutron-proton scattering cross section is more than an order of magnitude larger than that of any other nuclide, the signal from vibrational modes involving hydrogen tend to dominate the vibrational spectrum, which makes inelastic neutron scattering (INS) an ideal tool for the study of the dynamics of hydrogen in materials. In an effort to eliminate spectral features originating from H2 weakly physisorbed by hydroxyl groups, we subtracted from the Ti-SBA-15+H2 spectrum a SBA-15+H2 vibrational spectrum collected at the same temperature (30 K) and same gas loading conditions. Much of the background is due to the vibrations of the SBA-15 framework, including silanol groups. By use of pure SBA-15 or Ti-SNBA-15 for background subtraction did not change our results appreciably. This difference spectrum (Figure 5) therefore should largely exhibit vibrational bands which arise from hydrogen interacting in some form with the Ti-doped framework. While this differential INS spectrum is somewhat noisy, we can identify several prominent features. The most intense peak at around 880 cm-1 is accompanied by shoulders at 790 and 650 cm-1, while two weaker peaks are evident at frequencies of approximately 1245 and 1480 cm-1. To assign these bands we need to consider the manner in which H2 or H binds around the Ti site in Ti-SBA-15, which in turn requires some knowledge of the structure of the active site. Unfortunately, the available structural information on the Ti site is limited because of the amorphous nature of this as well as related mesoporous materials such as Ti-MCM-41. It is only by means of X-ray absorption spectroscopies coupled with computational studies that a tentative picture of the Ti site can be developed.10 It is generally, although not universally agreed that Ti substitutes for Si in the silicate framework in a tetrahedral arrangement (as it does in TS-1) at least at low levels of Ti doping, while at higher levels titania clusters may be formed in
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Figure 5. Neutron vibrational spectrum of H2 adsorbed on Ti-SBA15. The figure shows the difference between the spectrum collected for H2 adsorbed on Ti-SBA-15 and H2 adsorbed on SBA-15. This removes from the spectrum the contribution from the interaction of H2 with the SBA-15 material and highlights the Ti-hydrogen interaction.
the pores.11 An alternate four-coordinate model for a Ti site at the surface involves a hydroxyl ligand along with three Ti-O-Si bonds. Adsorption H2 adsorption above about 10 bar becomes partially irreversible, and this effect increases with higher maximum adsorption pressures (Figure 6). This hysteresis becomes rather pronounced at 25 and 55 bar maximum adsorption pressure. These observations suggest that at those pressures a small fraction of the adsorbed H2 may bind dissociatively upon interaction at the Ti site. Formation of a dihydrogen complex as proposed by Iniguez et al.12 for Ti-decorated carbon nanotubes does not seem likely in the present case, as it requires Ti to be highly under-coordinated and exposed. We may assume, therefore, that it is possible for the H to form bridging hydroxyls (e.g., Ti-O(H)-Si), and additional Ti-OH groups resulting from hydrogen pressure introduced reconstruction of bonds around the Ti site. These conclusions can be qualitatively supported by reference to some computational and optical spectroscopic studies13,14 on titanium oxide/hydroxide clusters and on Ti(IV) hydroxides. The broadband between 800 and 900 cm-1 can accordingly be attributed to bending modes of bridging hydroxyls on Ti-O-Si, or perhaps Ti-O-Ti bridges. The Ti concentration in the material is sufficiently high for some of those dimeric species to occur. Ignatyev et al.14 calculate δ(Ti-OHbridge) to occur at 837 cm-1, while this mode is known to occur at 1060 cm-1 for a Si-O(H)-Si bridge.15 Hydroxyl bending modes on mixed Si-O-Ti bridges could therefore readily fall in the range of the broad INS band at 890 cm-1 depending on their actual geometry. The shoulders at 790 and 650 cm-1 then correspond to Ti-OH stretching modes, which will become visible in the INS because of the associated displacement of the H on the (Ti)O. Ti-H moieties do not appear to be present in significant numbers, as there is little evidence for the associated bending mode expected below or near 500 cm-1.16 The present system may therefore be viewed as another example of so-called hydrogen spillover, where molecular hydrogen dissociates at an active site on a surface (such as a metal cluster) and the resulting atomic H diffuses to and binds at other surfaces sites17 or possibly in the interior of the material.
Figure 6. Adsorption/desorption isotherms for Ti-SBA-15. For each curve, the red line (squares) is the adsorption isotherm. The dashed blue line (triangles) is the desorption isotherm. A log-log scale was used to highlight the adsorption hysteresis. This hysteresis increases with the maximum pressure reached during adsorption. Three curves are shown with maximum adsorption pressures of 9.2, 28.4, and 54.9 bar. The last two curves were shifted up by 20 and 40 cm3/g, respectively, for clarity. The common portions of the adsorption isotherms lie on top of each other. The desorption isotherms, by contrast, do not lie on top of each other. All data were collected at 77 K with the same Ti-SBA-15 sample. The sample was heated under vacuum for 5 h at 200 °C before collecting each data set.
The latter process appears to be involved in some recent potential hydrogen storage materials with very large gravimentric hydrogen capacities at room temperature and elevated hydrogen pressures.18 Conclusions We have demonstrated that doping SBA-15 with Ti results in a significant improvement in the uptake of hydrogen at 77 K by the mesoporous silicate. Physisorption occurs at the hydroxyl groups and oxygen atoms of the pore surfaces. INS provides clear evidence for adsorption of H2 in the vicinity of the Ti sites. Despite the fact that these sites represent at most 5% of silica entities in the material, they appear to be at least as effective as the more numerous hydroxyl groups in adsorbing hydrogen. An increase in hydrogen pressure above 10 bar results in a significant enhancement of hydrogen adsorption, which exhibits a degree of hysteresis that increases with hydrogen pressure. INS vibrational spectroscopy suggests that this may be the result of activation of H2 near the Ti site followed by spillover to the immediate surroundings of that site. Our results open new possibilities to study the physisorption of H2 in another class of porous materials with high specific surface area under pressure. The effect of doping levels, specific
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surface area, functionalization of the hydroxyl groups, and nature of the dopant needs to be studied more systematically. Moreover, the possibility of pressure activated spillover phenomena that can be chemically tuned may well enhance the utilization of this type of material for hydrogen storage. This is a rich array of physical and chemical parameters to study the adsorptive interaction of H2 with the surface of doped SBA-15. Acknowledgment. This work has benefited from the use of the Manuel Lujan, Jr. Neutron Scattering Center at Los Alamos National Laboratory and funding from the U.S. Department of Energy’s Office of Basic Energy Sciences. Los Alamos National Laboratory is operated by Los Alamos National Security LLC under DOE Contract No. DE-AC52-06NA25396. Work at UCSB was supported by the Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy. References and Notes (1) Zhao, D. Y.; Feng, J. L.; Huo, Q. S.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; D Stucky, G. Science 1998, 279, 548. (2) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999, 38, 56. Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. Liu, J.; Yang, Q.; Zhao, X. S.; Zhang, L. Microporous Mesoporous Mater. 2007, 106, 62. (3) El Berrichi, Z.; Cherif, L.; Orsen, O.; Fraissard, J.; Tessonnier, J. P.; Vanhaecke, E.; Louis, B.; Ledoux, M. J.; Pham-Huu, C. Appl. Catal. 2006, 298, 194. Timofeeva, M. N.; Kholdeeva, O. A.; Jhung, S. H.; Chang, J. S. Appl. Catal. 2008, 345, 195. Perathoner, S.; Lanzafame, P.; Passalacqua, R.; Centi, G.; Schlo¨gl, R; Su, D. S. Microporous Mesoporous Mater. 2006, 90, 347. (4) Stan, G.; Cole, M. W. J. Low Temp. Phys. 1998, 110, 539. Stro¨bel, R.; Jo¨rissen, L.; Schliermann, T.; Trapp, V.; Schu¨tz, W.; Bohmhammel, K.; Wolf, G.; Garche, J. J. Power Sources 1999, 84, 221. Kajiura, H; Kadono, K.; Tsutsui, S.; Murakami, Y. Appl. Phys. Lett. 2003, 82, 1929.
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