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Enhanced Dehydrogenation Properties of Modified Mg(NH2)2-LiBH4 Composites X. Y. Chen, Y. H. Guo, and X. B. Yu* Department of Materials Science, Fudan UniVersity, Shanghai 200433, China ReceiVed: May 18, 2010
Promotion of dehydrogenation based on a combination of [BH] and [NH] sources has been demonstrated to be an effective approach in developing an advanced borohydride/amide multicompound hydrogen storage system. In this article, the hydrogen storage properties of Mg(NH2)2-LiBH4 composite are studied systematically. It has been shown that the Mg(NH2)2-LiBH4 binary system exhibits an onset dehydrogenation temperature of 250 °C, accompanied by an emission of 5.5 mol % ammonia. Further investigation has revealed that addition of metal chlorides results in significant improvement of the dehydrogenation of Mg(NH2)2-LiBH4. In particular, with NiCl2 addition up to 20 wt %, the modified composite exhibits more favorable thermodynamics, as demonstrated by an onset dehydrogenation temperature as low as 60 °C, accompanied by the suppressed evolution of byproduct NH3 (2 mol %), which enables this modified system to release 7.3 wt % hydrogen in the temperature range of 60-400 °C, which is vastly superior compared with the dehydrogenation of Mg(NH2)2-LiBH4 alone. X-ray diffraction results indicate that the presence of Mg(NH2)2 results in an accelerated exchange reaction between LiBH4 and NiCl2, which may be the dominant reason for the improved thermodynamics and kinetics in the NiCl2-added Mg(NH2)2-LiBH4 system. 1. Introduction The increasing demand for energy, in association with the environmental problems from using fossil fuels, is driving society toward using renewable forms of energy. Hydrogen is widely considered to be an alternative energy carrier and an environmentally friendly fuel. One of the major obstacles to the widespread use of hydrogen as an energy carrier is the lack of safe and efficient means for hydrogen storage.1 In recent years, the metal-N-H system has been receiving serious attention as a potential candidate for hydrogen storage,2–6 since the pioneering work reported by Chen et al. on the LiH/LiNH2 system,2 in which Li3N was shown to reversibly store a high amount of hydrogen (10.4 wt %) through the following twostep reaction:
Li3N + 2H2 T Li2NH + LiH + H2 T LiNH2 + 2LiH (1) Unfortunately, the first step reaction needs a much higher standard enthalpy change of desorption (148 kJ/mol H2) than the second step reaction (44.5 kJ/mol H2).2 So, the Li-N-H system requires further improvement of its kinetic and thermodynamic properties for practical applications. More recently, efforts have focused on the development of the Li-based ternary nitride/amide system. It is recognized that, although Mg(NH2)2 mainly generates NH3 upon decomposition, it has the potential to work in coordination with other hydrogen storage candidates with negatively charged H, based on the reaction of H+ + H- f H2 (∆H ) -17.37 eV), for which the abnormally high reaction enthalpy is the driving force, leading to the expected dehydrogenation taking place under moderate conditions.7,8 Therefore, the utilization of Mg(NH2)2 and its related system may also be a promising technique for on-board * To whom correspondence should be addressed: E-mail: yuxuebin@ fudan.edu.cn (X.B.Y.).
hydrogen storage. A successful example is the LiH/Mg(NH2)2 system, in which Mg(NH2)2-2LiH composite has received most extensive study, because of its combined advantages of favorable thermodynamics, good reversibility, and significant decrease in NH3 emission.4,9–12 Although considerable work has been done on structural identification and kinetic improvement in the LiH/Mg(NH2)2 system, it is further hypothesized that the Mg(NH2)2/[BH4]system would also have favorable dehydrogenation properties due to the substitution of the [BH4]- anion for the covalently bonded H-, based on a comparison between the Li-B-N-H and Li-N-H systems.2,13,14 However, the presence of [NH2]in the composite would also facilitate the decomposition of [BH4]- because the strong covalent bonding between H and B in the [BH4]- ligand entails very high hydrogen desorption enthalpy on monovalent borohydrides.15 The comparatively shorter HB-NH distance between the [BH] and [NH] ligands of the composite would facilitate the dehydrogenation and trap N more effectively to reduce the emission of NH3. In other words, on the basis of the formation of B-N bonds, the combination of [BH] and [NH] would proceed adequately due to the short-range effect, and hence, it would facilitate the generation of H2 molecules. So, nonisothermal dehydrogenation experiments were carried out in mixtures of Mg(NH2)2 and metal borohydrides. After its identification as the best performing system, Mg(NH2)2-LiBH4 was chosen as a representative in the present study of amide/borohydride composites, and the main purpose of this manuscript is to clarify its thermal decomposition mechanism in the presence of additives. To this end, the thermodynamic, kinetic, structural, and chemical properties of Mg(NH2)2-LiBH4 system were characterized by temperatureprogrammed desorption (TPD), mass spectroscopy (MS), X-ray diffraction (XRD), nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopy. Furthermore, various additives were mixed into the Mg(NH2)2-LiBH4 system for a comprehensive study of their modifying effects on the amide/borohydride system and the underlying mechanisms.
10.1021/jp1075753 2010 American Chemical Society Published on Web 09/28/2010
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2. Experimental Methods 2.1. Sample Preparation. Lithium borohydride (LiBH4) (95%, Acros Organics), magnesium hydride (MgH2) (90%, Acros Organics), calcium borohydride (Ca(BH4)2) (98%, SigmaAldrich), nickel(II) chloride (NiCl2) (ultra dry, 99.9%, Alfa Aesar), titanium(III) fluoride (TiF3) (Alfa Aesar), cobalt(II) chloride (CoCl2) (ultra dry, 99.9%, Alfa Aesar), iron(II) chloride (FeCl2) (98%, Alfa Aesar), palladium(II) chloride (PdCl2) (99.9%, Sigma-Aldrich), manganese(II) chloride (MnCl2) (99.99%, Sigma-Aldrich), aluminum chloride (AlCl3) (anhydrous, 99.985%, Alfa Aesar), and platinum(II) chloride (PtCl2) (98%, SigmaAldrich) were used as received. Magnesium amide (Mg(NH2)2) was synthesized according to the method reported in the literature.16 To prevent any oxidation of the amide, the synthesis was carried out in a glovebox under an inert atmosphere (5 wt % of hydrogen in 10 min (taking the weight percent of NiCl2 into account). However, the NiCl2-free sample only released ∼1.4 wt % of gas, even at a much higher temperature (340 °C). These results demonstrate that the addition of 20 wt % NiCl2 results in a significant improvement of the dehydrogenation kinetics of Mg(NH2)2-LiBH4. The MS results for Mg(NH2)2-LiBH4 samples with and without NiCl2 addition are shown in Figure 6. Mg(NH2)2-LiBH4 composite desorbs a large quantity of H2 above 250 °C, and the composite exhibits a desorption peak at 320 °C with a shoulder at 280 °C, accompanied by an ammonia evolution peak centered at 290 °C. In contrast, little NH3 byproduct is detected in the Mg(NH2)2-LiBH4 sample with 20 wt % NiCl2 added.
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Figure 6. MS results for: (a) Mg(NH2)2-LiBH4 ball milled with 20 wt % NiCl2 for 0.5 h; (b) Mg(NH2)2-LiBH4 ball milled for 0.5 h. The heating rate is 5 °C/min.
Figure 7. XRD patterns of: as-prepared Mg(NH2)2 annealed at 300 °C for 1 h (a); LiBH4 (b); Mg(NH2)2-LiBH4 (c); Mg(NH2)2-LiBH4 + 20 wt % NiCl2 before (d) and after heating to 250 °C (e) and 400 °C (f); and as-milled LiBH4 + 20 wt % NiCl2 (g).
This is consistent with the volumetric measurements results, which reveal that the molar ratio of NH3 in the gas generated from Mg(NH2)2-LiBH4 is 5.5 mol % (Table 1), but the percentage is only 2 mol % for the NiCl2-added sample. More importantly, the onset dehydrogenation temperature of the NiCl2added Mg(NH2)2-LiBH4 composite (60 °C) is substantially lower than that of the NiCl2-free sample (250 °C). However, the MS results also illustrate two main dehydrogenation stages with peaks at 125 and 330 °C, which result in termination of the dehydrogenation only above 350 °C. In this regard, better understanding of the effects of the NiCl2 addition on altering the reaction route may provide an important foundation for understanding the dehydrogenation mechanism in the Mg(NH2)2-LiBH4 system. 3.4. Structural Identification. To identify the changes in the composition and structure of NiCl2-added Mg(NH2)2-LiBH4 during dehydrogenation, FTIR, 11B NMR, and XRD measurements were carried out on the samples. Figure 7 shows the XRD patterns for Mg(NH2)2-LiBH4 and for NiCl2-added Mg(NH2)2LiBH4 before and after heating to 250 and 400 °C, compared with pure Mg(NH2)2 and LiBH4. In the case of the Mg(NH2)2-LiBH4 sample, LiBH4 and MgH2 phases are observed (part c of Figure 7), indicating that part of the amorphous Mg(NH2)2 has decomposed back into MgH2. After the addition of NiCl2, characteristic peaks assigned to LiBH4, NiCl2, MgH2, and LiCl are present (part d of Figure 7). The formation of LiCl
Modified Mg(NH2)2-LiBH4 Composites
Figure 8. FTIR spectra of: as-prepared Mg(NH2)2 annealed at 300 °C for 1 h (a); commercially purchased LiBH4 (b); as-milled Mg(NH2)2-LiBH4 before (c) and after heating to 400 °C (d); and as-milled Mg(NH2)2-LiBH4 + 20 wt % NiCl2 sample before (e) and after heating to 400 °C (f).
demonstrates that a reaction between LiBH4 and NiCl2 also occurs during milling. Surprisingly, in the patterns of samples heated to 250 and 400 °C, only LiCl phase can be observed, suggesting that a new amorphous intermediate hydride has been formed, accompanying the first stage dehydrogenation. The intermediate phase undergoes further decomposition until 400 °C, resulting in the second stage dehydrogenation. It should be noted that without the presence of Mg(NH2)2, the reaction between LiBH4 and NiCl2 can not proceed during milling, as demonstrated by the presence of characteristic peaks assigned to NiCl2 in part g of Figure 7. Hence, the promoting effect by Mg(NH2)2 on the reaction between LiBH4 and NiCl2 is significant. The broad peak packet around 2θ ) 45° for the sample heated to 400 °C may come from the oxidation of Mg at high temperature. XRD examination fails to give relevant information on some phases due to their severe amorphism, whereas the infraredactive N-H, B-H, or B-N stretching vibrations existing in these phases facilitate the application of infrared spectroscopy as an ancillary tool for the phase and structure identification. The IR results displayed in Figure 8 indicate that the N-H characteristic vibrations of the as-prepared Mg(NH2)2 appear at 1572, 3282, and 3370 cm-1, respectively (part a of Figure 8), and the asymmetrical Vas vibration at 3370 cm-1 is shifted to a higher wavenumber compared with the literature,20,21 which may be due to the differences in synthesis conditions and from the influence of the impurities on the N-H vibrations. For the Mg(NH2)2-LiBH4 sample, the presence of the amide vibrations after ball milling (part c of Figure 8) and the XRD results shown in part c of Figure 7 demonstrate that part of the Mg(NH2)2 has decomposed back into MgH2 during milling. In the spectrum of the sample after heating to 400 °C, the remaining weak peaks of B-H stretching vibrations represent residual [BH4]-. Also, the characteristic vibrations of N-H bonds do not diminish but are shifted to lower wave numbers (part d of Figure 8), which is consistent with the incomplete consumption of [BH4]-. In particular, the symmetrical vibration Vs shifts down to the region that is assigned to imide-like N-H vibration (part d of Figure 8).22 Because the hydrogen is desorbed from the mixture, and N-H is the only IR-active species in the mixture in the investigated region, this new structure should contain less hydrogen than Mg(NH2)2, that is, should be an imide.23,24 For
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Figure 9. 11B magic angle spinning (MAS) NMR spectra of 20 wt % added Mg(NH2)2-LiBH4 sample: as-milled (a); heated up to 400 °C (b). Spinning side bands are marked with asterisks.
the NiCl2 added sample, however, the neutralization between Hδ+ in Mg(NH2)2 and Hδ- in LiBH4 proceeds adequately during heating, regardless of the detailed reaction pathway, which is confirmed by the substantial disappearance of absorbances related to B-H and N-H in part f of Figure 8. The N-H absorbances assigned to Mg(NH2)2 are also absent in the spectrum shown in part e of Figure 8, indicating the conversion of [NH2]- into a new phase, which is well correlated with the XRD results. It is speculated that, instead of disappearing as a volatile byproduct, the greater part of the [NH] groups are trapped in the newly formed complex compound possessing a possible chemical composition of (NiNBH), the formation of which signals that a solid-state coordinated interaction was present in the NiCl2 destabilized amide/borohydride system during milling. This probably results from the fact that, although a reaction between LiBH4 and NiCl2 occurs during milling, which has been verified by the XRD measurements, the possibly formed Ni(BH4)2 is not a stable compound itself unless it is combined with ancillary ligands.19,25 The steric configuration described by Curtis26 provides good support to the speculation above and also gives an explanation for the diminishing of peaks for [NH2]- in the spectrum of the as-milled sample (part e of Figure 8). Furthermore, solid-state 11B NMR measurements provide some valuable hints toward understanding the evolution of the composition and hydrogen release process of the 20 wt % NiCl2 modified Mg(NH2)2-LiBH4 sample during the milling and heating processes. Part a of Figure 9 shows that only a single resonance at δ ) -40.705 ppm is observed for the as-milled sample, which should be assigned to [BH4]-, as reported in the literature.27,28 The absence of any chemical shift corresponding to the B-N environment for boron demonstrates that a compound containing B-N bonds is not generated during milling, which corroborates the results of the IR spectroscopy. Considering the absence of characteristic peaks assigned to LiBH4 and the presence of those assigned to LiCl in the XRD patttern for the 20 wt % NiCl2 modified Mg(NH2)2-LiBH4 (part d of Figure 7), conclusions can be drawn that the [BH4]- may combine with the Ni2+ anion during the milling process as discussed above. However, the spectrum of the postheated 20 wt % NiCl2 modified Mg(NH2)2-LiBH4 sample presents a broad line shape with three overlapping 11B peaks at 20.767 ppm, 11.709 ppm, and 2.156 ppm. The B species in the postheated sample is most likely in a BN3 and/or BN2 environment, and similar line shapes were also observed in previous studies.29,30 These results imply that the reaction process for the modified
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sample is first the combination of [BH4]- and the ligand [NH] group to form a new complex during the ball milling, and then the decomposition of the complex to generate BN polymer. Therefore, during ball milling, the main reaction product, which is responsible for the modified dehydrogenation, may be the newly formed complex [NiNBH], and the dehydrogenation properties characterized by MS and TPD investigations up to 400 °C come from the synergetic effects of MgH2 and [NiNBH], resulting in the formation of the BN polymer (part b of Figure 9). However, more effective detection methods are essential for clearly deciphering the reaction mechanism, due to the large number of elements present and because the reaction products have various degrees of crystallinity. 3.5. Results Analysis. It is clear that the NiCl2 added Mg(NH2)2/LiBH4 complex is an ideal hydrogen storage system that surpasses both LiBH4/NiCl2 and Mg(NH2)2/LiBH4 as discussed above, due to the synergetic effects that exist among the 3 components ([BH4]-, [NH2]-, and NiCl2). FTIR examination of the LiBH4-Mg(NH2)2 before and after heat treatment (parts c and d of Figure 8) reveals the incomplete neutralization between [NH2]- and [BH4]- after dehydrogenation up to 400 °C. However, according to the TPD profiles, the as-milled LiBH4-NiCl2 sample (part g of Figure 4) fails to exhibit significantly modified properties compared to pure LiBH4, and shows the presence of the characteristic peaks assigned to NiCl2 in the XRD pattern (part g of Figure 7), which reveals that NiCl2 alone scarcely has any modifying effect on the decomposition of [BH4]-. However, the modifying effect of NiCl2 on facilitating the B-N composite formation during heating cannot be negligible, and it contributes to the functioning of the dihydrogen bonding mechanism.31 In the present study, it is clearly demonstrated that significant synergetic effects exist between NiCl2, [NH2]-, and [BH4]- upon reaction. It is postulated that the Ni2+ cations, which have better affinity toward electrons, can attach themselves to [BH4]- and [NH2]- and activate them by promoting the formation of a more stable N-Ni-B-H compound rather than nickel borohydride during continuous ball milling.26 Consequently, the possibly formed [NiBNH] composite and the regenerated MgH2 from the decomposition of Mg(NH2)2 are responsible for the remarkably enhanced dehydrogenation properties of the LiBH4-Mg(NH2)2-NiCl2 sample. Moreover, the continuous dehydrogenation process over a wide temperature range is worthy of deep exploration. As an increasing NiCl2 content could not improve the second-step dehydrogenation of Mg(NH2)2-LiBH4, the Mg(NH2)2-LiBH4NiCl2 system may intrinsically have a two-step dehydrogenation. The XRD results clearly demonstrate the occurrence of an exchange reaction between LiBH4 and NiCl2, indicating that NiCl2 does not serve as a catalyst in the system, but functions as a reactant. LiBH4 + 20 wt % NiCl2 corresponds to a mole ratio of NiCl2:LiBH4 ) 1:6 in the complex. As a six-coordinate geometry is stable for nickel(II),19,26 this might be one reason for the superior performance of the sample with NiCl2 addition of 20 wt %, so it is proposed that the possibly formed Ni-N-B-H complex might have a six-coordinated environment for nickel(II). Although the second reaction stage can be modified in the present study by increasing the relative amount of LiBH4 in the complex system (part h of Figure 4), to a larger extent, the wide temperature range of the first dehydrogenation stage is due to the nature of its slow reaction rate, which is demonstrated by the effect of other metal chlorides as shown in Figure 10, in which, no matter what the chloride is (FeCl2, CoCl2, AlCl3, MnCl2, PdCl2, PtCl2, etc.), all of the samples show
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Figure 10. Desorption profiles of Mg(NH2)2-LiBH4 and Mg(NH2)2LiBH4 with 20 wt % addition of various metal chlorides. The heating rate is 5 °C/min.
significant improvement of the dehydrogenation but with slow dehydrogenation rates during heating. From the viewpoint of practical applications, further efforts should be devoted to the modification of the Mg(NH2)2LiBH4 system with related material, for in the present study, the performance of LiBH4-Mg(NH2)2 is far from satisfactory without the presence of NiCl2, which causes substantial elimination of byproduct NH3 in dehydrogenation and sheds some light on the hydrogen storage potential of this system. Nevertheless, there is a long way to go before the properties of the system can be optimized. The issue of the extensive dehydrogenation temperature range is the first one waiting to be addressed, and next, the failure to realize good reversibility under moderate conditions, because all efforts toward recycling the multicompound system under 40 bar H2 and 400 °C have failed. Thus, a complete understanding of the dehydrogenation reaction mechanism of amide/ borohydride and the role played by the additives relies on more combined theoretical and experimental explorations. 4. Conclusions In the present study, Mg(NH2)2-LiBH4 was chosen for study as a representative for the amide/borohydride system. It has been demonstrated that the Mg(NH2)2-LiBH4 system, releasing hydrogen from 250 °C, shows a significant improvement in the dehydrogenation kinetics and thermodynamics over those of pure LiBH4. Of particular interest, the 20 wt % NiCl2 modified Mg(NH2)2-LiBH4 has an onset dehydrogenation at a temperature as low as 60 °C. Simultaneously, the emission of ammonia is suppressed to 2 mol %, significantly lower than for the NiCl2free mixture (5.5 mol %). In the temperature range of 60-400 °C, the Mg(NH2)2-LiBH4-20 wt % NiCl2 presents a hydrogen desorption capacity of 7.3 wt %. Our various experiments have provided significant evidence of the synergetic behavior of the [BH4]-/[NH2]- system upon adding destabilizers, which will bring further insights for developing the B-N-H hydrogen storage system. Acknowledgment. This work was partially supported by the Program for New Century Excellent Talents in Universities (NCET-08-0135), the PhD Programs Foundation of the Ministry of Education of China (20090071110053), the Shanghai Leading Academic Discipline Project (B113), and the Shanghai Pujiang Programs (08PJ14014). References and Notes (1) Schlapbach, L.; Zu¨ttel, A. Nature 2001, 414, 353–358.
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