Sodyum amid-sodyum borohidrit sisteminin geçiş metalleri ilavesiyle hidrojen verme özelliklerinin geliştirilmesi ve yeni amid-borohidrit sistemlerinin hidrojen depolama malzemeleri olarak incelenmesi

The present study consists of two main parts. The first one is focused on improvement of the hydrogen release behavior of NaNH2-NaBH4 system. For this purpose, samples with different molar ratios of the constituents -sodium amide and sodium borohydride- were prepared by using ball-milling technique and transition metal additives as catalysts. In all cases, the product obtained after ball-milling was ? -Na2BNH6 exhibiting the same thermal decomposition behavior as the previously synthesized one by the thermal method. According to the results of Mass Spectroscopy and DTA/TG, the addition of ~ 5 wt% of Pt, Pd, PtCl2 and PdCl2 decreased the dehydrogenation temperature of NaNH2-NaBH4 system approximately by 30-60 K. The most significant decrease in the hydrogen release temperature was observed in the presence of Pt/Vulcan carbon and Pd/Black Pearl. With the addition of 5.6 wt% Pt/Vulcan carbon, the temperature for hydrogen release decreased from 553 to 397 K, and a marked shift of 121 K was detected for the addition of 2.9 wt% Pd/Black Pearls. The hydrogen uptake experiments were performed at 473 K and 400 bar H2 for pure Na3BN2 and at 373 K and 350 bar H2 pressure for samples with 5.6 wt% Pt/Vulcan carbon addition, respectively. No change was observed in the XRPD diagrams of the samples after hydrogen loading, except for the formation of NaH. The results indicate that the hydrogenation process for the sodium amide-borohydride system is not reversible under the given conditions.In the second part of the study, the reaction conditions of the mixed alkali metal complex hydride systems of LiNH2-NaBH4, LiNH2-KBH4 and NaNH2-KBH4 (2:1 and 1:1 molar ratio) were investigated by using thermal and mass spectroscopic methods. The resulting intermediate and final products were characterized by X-ray powder diffraction, DTA/TG and vibrational spectroscopy (FT-Raman, FT-IR). According to the results of the DTA/TG and Mass spectroscopic measurements, the hydrogen release for the LiNH2-NaBH4 and LiNH2-KBH4 systems starts at 621 K and 682 K, respectively. While the dehydrogenation process of the LiNH2-NaBH4 samples occurred in two steps, the latter decomposed in a single step, accompanied by high amount of NH3 release. For both cases, the decomposition products identified were practically the same: Li3BN2, Na or K (trace) and amorphous boron.Unlike the aforementioned two systems, the dehydrogenation for the NaNH2-KBH4 mixture starts at 597 K and peaking out at 700 K. The final product after the dehydrogenation process turned out surprisingly to be a hitherto unknown ternary nitridoborate, Na2KBN2, which crystallizes in the tetragonal space group I 4/mmm (No.139) with a = 4.2359(0) Å, c = 10.3014(2) Å and Z = 2. The crystal structure is characterized by elongated rhombic dodecahedra formed by 8 Na and 6 K atoms each which are centered by the linear [NBN]-3 moieties. The (BN) bond lengths are 1.357(4) Å. The vibrational spectra of Na2KBN2 were measured and interpreted based on the D ? h symmetry of the linear [NBN]-3 groups.