J. Phys. Chem. C 2009, 113, 6485–6490
Hyperpolarized
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Xe NMR Investigation of Ammonia Borane in Mesoporous Silica
Li-Qiong Wang,* Abhi Karkamkar, Tom Autrey, and Gregory J. Exarhos Fundamental & Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354 ReceiVed: December 12, 2008; ReVised Manuscript ReceiVed: February 13, 2009
Hyperpolarized (HP) 129Xe NMR was used to probe the porosity of mesoporous silica (MCM) infused with ammonia borane (AB). Variable-temperature HP 129Xe NMR measurements have been systematically carried out on a series of MCM-41 materials with AB loading ranging from 33 to 75 wt % (1:2 to 3:1 AB:MCM). Three distinct types of pore environments are clearly evident: pristine mesopores, pores coated with AB inside the meso-channels, and interparticle spacing formed from AB aggregates outside the meso-channels. We found similarly uniform coating of AB on mesoporous silica channels with 1:2 and 1:1 AB:MCM loading (ratio of weight percent). When the loading of AB to MCM is greater than 1:1, AB starts to aggregate outside the meso-channels. Further increases in loading (g3:1) result in the formation of partially blocked mesochannels as a result of excessive AB. The detailed information obtained from this study on how supported AB resides in nanoporous channels and how it evolves with the increase of AB loading is helpful for the rational design of novel materials with optimal hydrogen storage and release properties. Introduction There has been a recent interest in developing new approaches to store hydrogen for fuel cell power applications. Approaches that provide methods to modify thermodynamics and enhance kinetics will be necessary to provide efficiencies required for on-board storage for fuel cell powered vehicles. Several groups have demonstrated that nanophase metal hydrides show enhanced kinetics for reversible hydrogen storage relative to the bulk materials;1-3 however, the kinetic enhancement is diminished for some materials after a few hydriding/dehydriding cycles, as they lose nanophase structure.4 Recently, it has been demonstrated that the properties of conventional hydrogen storage materials infused into mesoporous scaffolds can be enhanced relative to the bulk materials. One advantage of the porous scaffolds is the potential to preserve the nanophase properties through several cycles of hydrogen uptake and release. Gross and co-workers have shown that the temperature onset for hydrogen release from a complex hydride, LiBH4, is shifted to lower temperatures when infused into carbon aerogels. The smaller the diameter of the pores the lower the temperature onset for H2 release.5 Zheng et al. demonstrated that space confinement of NaAlH4 in mesoporous silica enhanced the kinetics of hydrogen desorption and resorption.6 In previous studies we discovered that ammonia borane (AB), infused into mesoporous silica (AB-MCM), has multiple benefits compared to the bulk material. At a saturated loading, i.e, 1:1 weight to weight loading of AB on mesoporous silica, the on-set temperature for hydrogen release is lower, the enthalpy of hydrogen release decreases, and the purity of hydrogen is enhanced.7 However, the benefits are at the expense of the added weight and volume of the scaffold leading to a decrease in the available hydrogen. In subsequent work we found that temperature onset for hydrogen release depended on the loading level of AB infused in mesoporous silica above a 1:1 weight-to-weight ratio.8 However, we were lacking any significant insight into how the * To whom correspondence
[email protected].
should
be
addressed.
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AB resides in porous channels of silica at different loading. Unfortunately conventional experimental techniques such as BET and TEM fail to provide direct insight into the observed phenomena as a function of AB loading in silica. BET does not provide reliable results for all AB-coated MCM samples because a relatively high vapor pressure of AB prevents accurate measurements. TEM is not successful at imaging the ammonia borane in the scaffold as a result of weak scattering of electrons by the light elements B and N. Therefore, a novel approach to investigate the porosity of hybrid materials is needed. The insight obtained can be useful in designing new materials with optimal hydrogen storage properties. Over the years 129Xe NMR has developed into a powerful and robust method for studying porous solids.9-12 The large chemical shift range of 129Xe is strongly dependent on environmental and chemical factors such as the composition of the adsorbent, the nature and concentration of coadsorbed molecules, and the shape and size of resident void spaces. Since spinpolarized Xe gas percolates through the interconnected pores and samples the local pore environments, 129Xe NMR has the unique advantage of directly probing not only the size and resident structure but also the connectivity between the pores that are not accessible by other pore characterization techniques. The use of optical pumping techniques for the production of hyperpolarized (HP) xenon allows a dramatic increase in the sensitivity of 129Xe NMR to a factor of 104.13 With HP xenon produced under continuous flow (CF) conditions, measurements are possible at very low concentrations of xenon, which makes the contribution to the chemical shift of Xe-Xe interactions on the surface very small. The observed 129Xe chemical shifts can be assigned principally to the interactions between the xenon atoms and the surface of the porous materials. We have successfully applied HP 129Xe NMR to characterize a variety of mesoporous materials including ordered cylindrical pore channels in functionalized mesoporous silica from our earlier work,14 pore channels in precise replication of wood microstructure in a ceramic,15 molecular-like pores in organic frameworks,16 and open pores in polymer aerogel.17 By using similar
10.1021/jp810994p CCC: $40.75 2009 American Chemical Society Published on Web 03/30/2009
6486 J. Phys. Chem. C, Vol. 113, No. 16, 2009 approaches, the local pore environment and connectivity of pores in AB:MCM materials will be investigated in this study. This paper presents a first study of nanophase ammonia borane in mesoporous silica with use of HP 129Xe NMR. The objective is to obtain insight into how supported AB resides in nanoporous channels of MCM-41 and how it evolves with increase of AB loading for a better understanding of the improved hydrogen storage and release properties for AB:MCM materials. Variable-temperature HP 129Xe NMR is applied to systematically examine a series of AB:MCM materials at four different loadings using an innovative HP 129Xe NMR polarizer. Detailed information such as the uniformity of the AB coating and if AB is inside or outside the mesoporous channels obtained from this study is important for rational design of novel materials with optimal hydrogen storage and release properties. Experimental Section Materials and Physical Properties. The MCM-41 silica materials were purchased from Mobil Corporation. A typical procedure for preparing 1:1 AB:MCM samples using the wetness incipient approach follows: A solution of ammonia borane (50 mg) in tetrahydrofuran (THF) (1 mL) was added to a sample of MCM-41 (50 mg) in small aliquots. The solution appeared to fill the internal channels of the mesoporous scaffold through a capillary action. The “wet” MCM-41 was dried under vacuum to produce a sample with an internal coating of ammonia borane (approximately 1:1 AB to MCM-41 by weight). Similar procedures were used for preparing 1:2 and 3:1 AB: MCM samples with use of appropriate quantities of ammonia borane and MCM-41. A thin section of the specimen was prepared for highresolution TEM (HRTEM) by standard epoxy embedding followed by ultramicrotoming to a thickness of less than 50 nm. The HRTEM analysis was carried out with a Jeol JEM 2010 microscope with a specified point-to-point resolution of 0.194 nm. The operating voltage of the microscope was 200 keV. All images were recorded digitally with a slow-scan CCD camera (image size: 1024 × 1024 pixels), and image processing was carried out by using a digital micrograph (Gatan). Attempts to obtain clear TEM images for AB:MCM materials were unsuccessful due to the weak scattering of electrons by the light elements B and N. BET data show type-IV isotherms with a rather narrow distribution of mesopore diameter of 4.0 nm for the pristine MCMsamples.TheadsorptiondataindicateBrunauer-Emmett-Teller (BET) surface areas of 900 m2/g for the neat MCM silica. HP129Xe NMR Measurements. HP 129Xe NMR experiments were carried out on a Chemagnetics spectrometer operating at 82.98 MHz (magnetic field 7.05 T) with use of a homemade variable-temperature double-resonance magic angle spinning (MAS) probe under a continuous flow (CF) of HP xenon. For 129 Xe NMR experiments, a single-pulse (SP) Bloch-decay method was used and samples were loaded into 7.5-mm Zirconia PENCIL rotors. SP spectra were collected with a 4.5-µs (90°) 129 Xe pulse and a repetition delay of 1 s. The number of transients was 100. The HP Xe gas was produced with use of a homemade HP 129 Xe polarizer. In collaboration with Prof. Brian Saam’s group at the University of Utah, we have recently designed and constructed a highly efficient HP Xe 129Xe polarizer. This Xe polarizer has a unique design based upon a report by Hersman’s group.18 Unlike most of the HP xenon polarizers available in other research laboratories, a low-pressure environment is used in the new polarizer to avoid quenching the efficient transfer of
Wang et al. spin from the Rb electron manifold to the Xe nucleus. Efficient use of the pump diode laser power of 80 W at 795 nm is affected by means of a long path length cell (1 m), high Rb vapor density, and counter flowing the spin exchanged optically pumped (SEOP) gas in a direction opposite to that of laser propagation. This unique arrangement achieves a remarkably high polarization rate (>35%), even at high flow velocity. With this enhanced sensitivity, our new 129Xe NMR polarizer enables our future studies of materials with relatively low surface area, thin porous membranes, and self-assembly structures in solution. A xenon-helium-nitrogen mixture with a volume composition of 1%-66%-33% was used in all CF HP experiments. The flow rate was kept constant in the range of about 500 scc/ min (gas flow normalized to standard conditions). In CF HP experiments, the HP xenon flow was delivered directly from the polarizer to the coil region of the NMR probe through 1.5mm i.d. plastic tubing. Variable-temperature NMR experiments in the 183-333 K range were performed with a homemade temperature controller. The temperature inside the NMR coil of the CF probe was calibrated by using the 207Pb resonance in Pb(NO3)2.19 All 129Xe NMR chemical shifts were referenced to xenon gas extrapolated to zero pressure (0 ppm). Results and Discussion A series of variable-temperature HP 129Xe NMR spectra for AB:MCM materials at four different loading levels are displayed in Figure 1. For all four samples, a sharp resonance peak at 0 ppm is observed and its peak position and line shape vary little with temperature, while the other peaks move toward the higher chemical shifts and their peak areas increase relative to the peak area at 0 ppm with decreasing the temperature. The 0 ppm peak is associated with the free Xe gas and the peaks at higher chemical shifts are from the adsorbed Xe on the surface of AB: MCM materials. The slower exchange between the gas phase and adsorbed Xe at reduced temperatures results in larger observed chemical shifts. Due to the increased adsorption at lower temperatures, a larger signal for Xe adsorbed on the pore surfaces is observed as compared to the signal associated with the free Xe gas. A single resonance peak associated with the adsorbed Xe inside the pristine silica meso-channels is observed in Figure 1 at 0% AB loading (0:1 AB:MCM). Its chemical shift position and temperature-dependent behavior are in agreement with the previous studies on MCM41.14,20 Upon increasing the AB loading to 33% (1:2 AB:MCM), the original resonance peak representing the pristine mesoporous channels becomes a small shoulder accompanied by a new large peak that appears at a lower chemical shift. These lower chemical shift peaks marked in blue in Figure 1 result from the meso-channels coated with ammonia borane. Only small shoulders associated with the bare silica are observed, suggesting that most meso-channels are coated with AB. As the AB loading is increased to 50% (1:1 AB-MCM), an additional peak at a much lower chemical shift (marked in red) starts to appear along with the other two peaks that are from the meso-channels coated with and without AB. Further increasing the loading to 75% (3:1 AB:MCM), this new low chemical shift peak (marked in red) moves toward the free gas Xe peak. At 333 K, this new peak almost overlaps with the free Xe gas peak at 0 ppm. We assign these peaks at near 0 ppm to the interparticle spacing formed by the excessive AB aggregated outside the meso-channels. Since most surface area is from the porous channels and the mesoporous silicas are large micron-sized particles, only a very small portion of the excessive AB would be deposited on the micron-sized particles of silica
Investigation of Ammonia Borane in Mesoporous Silica
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Figure 1. Variable-temperature continuous flow HP 129Xe spectra for 0:1, 1:2, 1:1, and 3:1 AB-MCM materials. The bottom spectrum was recorded at 183 K and the temperature was raised 20 K for each subsequent spectrum up to 333 K. The dashed lines are a visual guide only.
and the majority of the AB would aggregate themselves to form small particles made of AB. HP 129Xe NMR spectra (not shown here) for the pure AB solids did not give any resonance peaks besides the free Xe gas peak, confirming that bulk AB solids are nonporous. Therefore, the additional signals at low chemical shifts at higher loading are mostly from the interparticle spacing of AB aggregates formed outside the meso-channels. In general, when pore sizes are smaller as a result of the AB coating inside the mesoporous channels, we should have observed a higher chemical shift for AB-coated MCM-41 than for the pristine MCM-41. However, the lower chemical shift peaks marked in blue in Figure 1 start to appear at increased loading of AB and they are assigned to the meso-channels coated with AB. Similar lower chemical shifts upon adsorption of water and hydrocarbon chains14 have been observed previously for MCM-41 and were explained in terms of a screening of the surface micropores, or the “roughness”, of the internal surface of the mesoporous channels. The lower chemical shift with increased loading is expected when a small amount of relatively strong adsorption centers is present on the surface.21 These strong adsorption centers in the case of MCM-41 are likely to be shallow micropores with sizes very close to that of xenon. As compared to zeolites, the unexpectedly high chemical shift for Xe adsorbed on pristine mesoporous silica arises from the strong Xe interaction with the shallow micropores on the surface. The existence of such “micropores” in the walls of ordered mesoporous materials has been discussed already on the basis of synchrotron X-ray diffraction data.22 The coating of the AB in mesoporous channels blocks most of these strong micropore centers on the surface with the bound AB coated in mesoporous channels, resulting in lower chemical shift upon increasing the AB loading. Figure 2 displays the spectra plotted together for four samples at 283 and 243 K for a better visualization of different pore environments in AB:MCM materials. Three different pore environments for AB in MCM41 at four loading levels are
Figure 2. HP 129Xe NMR spectra taken for 0:1, 1:2, 1:1, and 3:1 AB-MCM materials at 283 and 243 K, respectively. Peaks are assigned to pristine mesoporous channels in MCM (green arrows), mesoporous channels coated with AB (blue arrows), and interparticle spacing due to the excessive AB aggregated outside the MCM meso-channels (red arrows).
visible: pristine mesoporous channels in MCM41(green arrows); mesoporous channels coated with AB (blue arrows); and pores formed by interparticle spacing due to the excessive AB aggregated outside the MCM meso-channels (red arrows). As compared with TEM and BET that show no distinction between 1:1 and 3:1 AB-MCM, this study has demonstrated that variable-temperature HP 129Xe NMR can be used to detect different local pore environments that otherwise are indistinguishable by other pore characterization techniques. At a relatively low AB loading at 33% (1:2 AB:MCM), no excessive AB particles are observed in Figure 2, indicating that all AB are infused into the mesopore channels. With an increase in the loading to 50% (1:1 AB:MCM), we start to observe the excessive AB particles deposited outside the mesoporous channels, suggesting a sequential deposition process where AB is first infiltrated into the mesoprous channels and then starts to deposit outside the channels. It is understandable that AB prefers the mesoporous channels because of the capillary effect of these nanosized porous channels. With a further increase of
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Figure 3. Temperature-dependent chemical shift plots for the following: (a) three peaks assigned to pristine mesoporous channels in MCM (green), mesoporous channels coated with AB (blue), and interparticle spacing of AB deposited outside the MCM meso-channels for the 1:1 AB-MCM material; (b) the peak associated with AB inside the meso-channels for 1:2, 1:1, and 3:1 AB-MCM materials along with the pristine mesoporous silica (0:1 AB:MCM). The dots are experimental data and the solid lines are the fitted curves with use of eq 1.
the loading to 75% (3:1 AB:MCM), more AB are deposited outside the mesporous channels as shown in Figure 2 where a large resonance peak at near 0 ppm associated with the excessive AB outside the meso-channels is observed at 283 K. However, the resonance peak representing AB inside the meso-channels is only visible at temperatures below 243 K. It is surprising to observe that at a higher loading of 75% (3:1 AB:MCM) the resonance peaks associated with AB infused inside the meso-channels are only observable at lower temperatures. Above room temperature the peak is no longer observable. Due to the increased adsorption at lower temperatures, a larger signal for Xe adsorbed on the pore surfaces is detected as compared to the signal for Xe in void space. The signals associated with the smaller pores will be enhanced due to the higher surface area to free volume ratio. Therefore it is understandable that we observe the Xe signals associated with the AB inside the meso-channels at reduced temperatures for 3:1 AB:MCM. Figures 1 and 2 give no signals associated with the Xe inside the mesoporous channels for 3:1 AB:MCM but large signals for 1:1 and 1:2 AB:MCM above 243 K. On the basis of our experimental measurements, it appears that the AB saturation coverage of the mesoporous channels in MCM-41 occurs near 33 wt % AB. If we assume a saturation of MCM41 at 33 wt % AB, then the 1:1 ratio and the 3:1 AB:MCM would require ca. 17% and 42% of the AB be deposited “outside” the mesoporous channels of MCM-41. Accordingly the 3:1 AB:MCM sample could have at ca. 2.5 time more AB particles aggregates formed outside the meso-channels than 1:1 AB:MCM, resulting in a significant decrease in Xe signal. A large signal associated with the meso-channels for 1:1 AB:MCM is found in Figure 2 and its peak area from the meso-channels is about twice that from the interparticle spacing at 283 K. On the basis of the peak area of this signal for 1:1 AB:MCM at 283 K, signals for 3:1 AB:MCM would be reduced significantly but should still be observable at room temperature. At high AB loading the excess AB may block some of the entrances of the mesoporous channels, inhibiting Xe diffusion into the mesoporous channels. These partially blocked channels will also lead to a decrease in the observable signals associated with the Xe inside the mesoporous channels at temperatures above 243 K. Unlike BET, which gives a low surface area of
