MAS NMR analysis

Due to the possible formation of amorphous compounds, the employment of the MAS NMR technique has been necessary to further elucidate the 2NaH+MgB2 hydrogen absorption reaction. Several specimens with different hydrogenation degrees were prepared at a hydrogen pressure of 50 bar in a HP-DSC Netzsch DSC 204 HP Phoenix and then characterized. The single pulse ¹¹B{¹H} and ²³Na {¹H}NMR analysis are shown in figure 3.10 and figure 3.11 respectively. The ¹¹B{¹H} spectra collected for the as milled material (figure 3.10 A) shows a peak at 96.51 ppm, due to the presence of MgB2 and although small, a further signal is visible at -42.00 ppm. The presence of this last signal (at -42.00 ppm), suggests a partial formation of NaBH4 already during milling, however it does not find confirmation in the ²³Na{¹H} MAS NMR analysis of figure 3.11 A. In fact, despite the signal observed at -42.00 ppm in figure 3.10 A, the ²³Na{¹H} spectrum of the as milled 2NaH+MgB2 (figure 3.11 A) shows only the signal related to NaH (10.84 ppm). Therefore, the signal at -42 ppm is not due to the formation of NaBH4. The investigation of the phase generating the signal at -42.00 ppm (figure 3.11 A) is still in progress.

A first hydrogen charged specimen was prepared by heating the as milled material from room temperature up to a final temperature of 300 °C (heating rate 5 °C/min) under a pressure of 50 bar of hydrogen, and subsequently cooling it down to room temperature. According to the HP-DSC analysis of figure 3.4 and the SR-PXD data shown in figure 3.6, this sample is expected to contain only the observed unknown crystalline phase together with the starting reactants. The presence of this additional phase among the starting reactants is confirmed by both ¹¹B{¹H} and ²³Na NMR analysis. In the ¹¹B{¹H} spectra (figure 3.10 B) an intensity increment of the already present peak at -42.00 ppm is clearly visible in addition to the signal at 96.51 ppm of the B- atoms contained in the MgB2. Moreover with respect to the as milled material (figure 3.11 A), the ²³Na{¹H} spectra (figure 3.11 B) shows the presence of two more signals at -11.71 and at -15.85 ppm. These two new peaks hint to the presence of the unknown crystalline phase. The last sample was prepared by heating the as milled material from room

temperature up to a final temperature of 400 °C (heating rate 5 °C/min) under 50 bar of hydrogen, and finally keeping it at 400 °C for 2 hours. For this material the ¹¹B{¹H} NMR spectra (figure 3.10 C) clearly shows at -42.29 ppm a peak relative to the NaBH4 presence, plus an additional signal at -15.97 ppm. Analyzing the region of positive shifts, in addition to the peak of the remaining MgB2 at 96.51 ppm, three more signals are visible. A broad signal at 3.09 ppm (overlapped with the spinning sideband of MgB2), and two sharp peaks at 6 and 18 ppm. Although, most likely the peak observed at 3.09 ppm is due to the formation of amorphous boron, due to the low signal proportion (less than 1% of total boron signal) an assignment for the three remaining B-containing species (-15.97, 6.16 and 18.20 ppm) is rather difficult. For this reason, a direct spectral sensitivity comparison between, equally recorded, proton decoupled boron experiment ¹¹B{¹H} and proton coupled boron experiment

11B was performed. This analysis allows distinguishing between species which contain boron atoms strongly coupled to protons (directly bonded) and those which are not. Several strategies are known for performing hetero nuclear decoupling. Herein, for CPD (Composite Pulse Decoupling) we employed TPPM technique (Two Pulse Phase Modulation). Figure 3.12 shows the ¹¹B MAS NMR spectra of as milled 2NaH+MgB2 hydrogenated at 50 bar and 400 °C with proton CDP (blue line) and without proton CDP (black line). Clearly the proton decoupling leads to an enhancement of the signal relative to the [BH4]^- anion at -42.29 ppm and of its spinning sidebands at 144.31, 61.62 and -135.33 ppm. Different behaviour is observed for the signals at 18.20, 6.16, 3.09 and -15.97 ppm. For these signals the application of the proton CDP does not influence the intensity of the signals. This behaviour is due to the fact that these species contain B-atoms which are not strongly coupled to hydrogen atoms.

This suggests the formation of species without B-H bonds. Spectrum C in figure 3.11 shows the ²³Na{¹H}NMR analysis of the material hydrogenated at 400 °C and 50 bar of hydrogen. It is possible to observe the peak of remaining NaH at 10.84 ppm and the signal of the Na contained in NaBH4 at -15.85 ppm. A further broad signal is observed at 2.32 ppm. Most likely this signal is related with the peaks observed at 18.20, 6.16 and -15.97 ppm in the

11B{¹H} NMR analysis of figure 3.10 spectrum C.

Figure 3.10: A) ¹¹B{¹H} MAS (12 kHz) single pulse NMR spectrum of: as milled 2NaH+MgB2. B) as milled 2NaH+MgB2 heated from room temperature up to a final temperature of 300 °C under a pressure of 50 bar of hydrogen. C) as milled 2NaH+MgB2

heated from room temperature up to a final temperature of 400 °C and then kept at 400 °C for 2 hours under a pressure of 50 bar of hydrogen.

Figure 3.11: A) ²³Na{¹H} MAS (12 kHz) single pulse NMR spectrum of: as milled 2NaH+MgB2. B) as milled 2NaH+MgB2 heated from room temperature up to a final temperature of 300 °C under a pressure of 50 bar of hydrogen. C) as milled 2NaH+MgB2

heated from room temperature up to a final temperature of 400 °C and then kept at 400 °C for 2 hours under a pressure of 50 bar of hydrogen.

Figure 3.12: ¹¹B{¹H} MAS (12 kHz) single pulse NMR spectrum of as milled 2NaH+MgB2

heated from room temperature up to a final temperature of 400 °C and then kept at 400 °C for 2 hours under a pressure of 50 bar of hydrogen (black line) and ¹H decoupled ¹¹B MAS (12 KHz) single pulse NMR spectrum of as milled 2NaH+MgB2 heated from room temperature up to a final temperature of 400 °C and then kept at 400 °C for 2 hours under a pressure of 50 bar of hydrogen