Preparation Of Nanocatalysts Derived From Metal Organic Framworks And Their Effects On Hydrogen Absorption/Desorption Properties Of NaAlH₄

Developing safe, high-efficiency and revisible solid-state hydrogen storage technologies is critical to promote the practical utilization of hydrogen energy. Sodium alanate (NaAlH4) with a theoretical hydrogen storage capacity of 7.6 wt% has attracted intense interest as a high-density hydrogen storage material. However, high dehydrogenation temperatures, slow reaction kinetics and limited reversibility retard its practical applications. Catalyst doping is one of the most effective approaches to improve the de/hydrogenation performance of NaAlH4. On the basis of previous studies, we proposed a novel strategy for the preparation of ultrafine nanocrystalline metal oxide supported on carbon in this work. The Ti-, Ce- and Zr-based nanocrystalline catalysts have been synthesized successfully. The catalytic effects of the as-synthesized catalysts on de/hydrogenation behavior of NaAlH4 were systematically investigated. The catalytic active species and the corresponding mechanisms were also revealed.First, a Ti-based metal organic framework (MIL-125-(Ti)) was synthesized using a solvothermal method. The catalytic effects of MIL-125-(Ti) on the de/hydrogenation behavior of NaAlH4 were systematically investigated. The results revealed that adding MIL-125-(Ti) significantly improved the hydrogen storage performance of NaAlH4. At 140℃, approximately 4.66 wt% of hydrogen was released from the NaAlH4-5 wt% MIL-125-(Ti) sample within 10 h. The dehydrogenated sample took up approximately 4.5 wt% of hydrogen while being heated from room temperature to 150℃ under 100 bar of hydrogen, exhibiting a good cycling stability. Mechanistic investigations revealed that the Ti4+ in MIL-125-(Ti) additive was reduced to zero-valent Ti during the ball milling process, which further combines with Al to form Ti-Al species in dehydrogenation process. The newly formed Ti-Al species and remaining carboxyl ligands dispersed on the surfaces of NaAlH4 particles and acted as active centers, which facilitate the dissociation and recombination of hydrogen molecules, consequently improving the hydrogen storage performance of NaAlH4.A nanocrystalline TiO2 supported on nanoporous carbon (referred to TiO2@C) was synthesized by using MIL-125-(Ti) as the precausor and furfuryl alcohol (FA) as carbon source. The effect of carbonization temperatures on the catalytic activity of as-synthesized composites was investigated. It was observed that the particle sizes of nanoporous carbon supported TiO2 were 1～10 nm after calcinating the FA-containing MIL-125-(Ti) under 700-1000℃. The nanocrystalline TiO2@C composites distinctly improved the dehydrogenation kinetics of NaAlH4, and the material carbonized at 900℃ showed the optimal catalytic performance. The onset temperature of hydrogen desorption from the NaAlH4-9 wt% TiO2@C sample was 75℃ which was 80℃ lower than that of the pristine NaAlH4. The rehydrogenation of the dehydrogenated producted was completed at 115℃ under 100 bar of hydrogen pressure. The compositional and structural characterization revealed that the Ti element in TiO2@C underwent a reduction process of Ti4+â†'Ti3+â†'Ti2+â†'Ti during the ball milling and dehydrogenation, and further converted to Ti hydrides and/or formed Ti-Al solid state solution after rehydrogenation, which is the most important reason for the improved de/hydrogenation performance of NaAlH4. Comparison revealed that the catalytic activities of Ti-based catalytic species decreased in the order:Al-Ti-solid state solution>TiH0.71>TiH2>TiO2.Subsequently, a low-temperature activation process at 150℃ under 100 bar H2 was proposed for NaAlH4-9 wt% TiO2@C sample to further improve the hydrogen storage performance. The results revealed that the as-acivated NaAlH4-9 wt% TiO2@C sample released hydrogen starting from 63℃ and re-absorbed starting from 31℃, which were reduced by 12℃ and 7℃ relative to those of unactivated sample, respectively. At 140℃, approximately 4.2 wt% of hydrogen was released within 10 min, representing the fastest dehydrogenation kinetics of any presently known NaAlH4 system. More importantly, the dehydrogenated sample could be fully hydrogenated under 100 bar H2 even at temperatures as low as 50℃, thus achieving ambient-temperature hydrogen storage. The synergetic effect of the Al-Ti solid state solution and carbon contributed to the significantly improved performance.Afterward, a nanocrystalline CeO2@C was synthesized using Ce-BTC as precausor. The CeO2@C-containing NaAlH4 was prepared by in-situ hydrogenating the NaH/Al mixture, and the corresponding catalytic effect of CeO2@C was also investigated. To further improve the hydrogen storage performance, especially for the desorption kinetics of CeO2@C-containing NaAlH4, a small quantity of excess Al was added. The results revealed that CeO2@C-containing NaAlH4 was in-situ synthesized by ball milling the CeO2@C-added NaH/Al composite under hydrogen pressure. The de/hydrogenation temperatures of the as-prepared composites were reduced and the 7 wt% CeO2@C-added sample possessed optimal hydrogen storage properties. Structural investiagtion on the samples collected after different milling durations indicated that metallic Al was consumed during the hydrogenation of NaH/Al composite. The depletion of metallic Al was the main reason for the elevated dehydrogenation temperature of the prolong-milled sample. After that, the effect of adding excess Al on the de/hydrogenation kinetics of NaAlH4-CeO2@C composite was further investigated. It was found that the on-set temperature of [NaH-Al-7 wt% CeO2@C]-0.04Al sample was 49℃ lower than that of the CeO2@C single-added sample. At 160℃, approximately 4.5 wt% of hydrogen was released from the [NaH-Al-7 wt%CeO2@C]-0.04Al sample within 15 min. The dehydrogenated sample was fully hydrogenated within 35 min at 100℃ and 100 bar of hydrogen. Compositional and structural analyses revealed that CeO2 was converted to CeH2 during ball milling and the newly formed CeH2 worked with the excess Al to synergistically improve the de/hydrogenation performance of NaAlH4 by reducing the operating temperature and enhancing the de/hydrogenation kinetics.Finally, a nanocrystalline ZrO2@C catalyst was synthesized by using Uio-66-(Zr) as precausor and the effects of adding ZrO2@C on de/hydrogenation behavior of NaAlH4 were investigated. It was found that the NaAlH4-7 wt%ZrO2@C sample possessed optimal performance and 5.0 wt% of H2 was released from NaAlH4-7 wt% ZrO2@C sample at the temperature range from 127℃ to 210℃. At 140℃, approximately 3.1 wt% of hydrogen was released within 30 min. The rehydrogenation was completed at 150℃ under 100 bar of hydrogen pressure and hydrogen uptake amounted to 4.9 wt% H2. In particular, the operating temperature was further reduced for hydrogen desorption from the ZrO2@C-added sample after the first de/hydrogenation cycle. The main reason was ascribed to that a minor amount of ZrO2 was reduced to metallic Zr with higher catalytic activity at a higher dehydeogenation temperature.