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SUMMARY TiCl3 has been considered as the best catalyst for the hydrogen desorption/re-absorption of NaAlH4 in terms of kinetic enhancement. However, the formation of NaCl as a by-product leads to the decrease in the reversible hydrogen capacity of NaAlH4. In this work, TiO2 and metallic Ti were selected as catalysts for the reaction to avoid the formation of the by-product. The comparison of the catalytic activity of Ti, TiCl3, TiO2 and Ti(OBu)4 on the hydrogen desorption/absorption NaAlH4 were carried out. It was found that TiO2 doped NaAlH4 exhibits similar behavior as TiCl3 doped NaAlH4 with the reversible hydrogen capacity about 3.8 wt% (H/M). In addition, TiO2 doped NaAlH4 exhibits the superior hydrogen re-absorption rate to the one doped with TiCl3. That may be due to the Ti3+ defect sites on the surface of TiO2 would facilitate the hydrogen dissociation. Moreover, high surface area of TiO2 prevents the segregation and the morphological change of the desorbed substances (NaH and Al). This benefits to the mass transfer into the hydride system. However, doping with TiO2 also produces sodium oxide and hydroxide as by-products. Unexpectedly, metallic Ti doped NaAlH4 shows the lowest hydrogen desorption/re-absorption among the tested samples. Its hydrogen reversible capacity is around 1 wt% (H/M). The formation of TiHx (1 < x < 2) was detected in the sample after the hydrogen desorption/reabsorption. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Sodium aluminum hydride or sodium alanate (NaAlH4) has been considered as a potential material for hydrogen storage. Although its theoretical hydrogen storage capacity is 5.5 wt.% at 250 °C, the material still has its drawback in the regeneration issue. With the use of certain catalysts, the regeneration problem can somewhat be alleviated with added benefits in the decrease in the hydrogen decomposition temperature and the increase in the decomposition rate. This work summarizes what we have learned from the decomposition of NaAlH4 with/without catalysts and co-dopants. The decomposition was carried out using a thermovolumetric apparatus. For the tested catalysts—HfCl4, VCl3, TiO2, TiCl3, and Ti—the decomposition temperature of the hydride decreases; however, they affect the temperature in the subsequent cycles differently and TiO2 appears to have the most positive effect on the temperature. Sample segregation and the morphological change are postulated to hinder the reversibility of the hydride. To prevent the problems, co-dopants—activated carbon, graphite, and MCM-41—were loaded. Results show that the hydrogen reabsorption capacity of HfCl4- and TiO2-doped NaAlH4 added with the co-dopants increases 10–50% compared with that without a co-dopant, and graphite is the best co-dopant in terms of reabsorption capacity. In addition, the decomposition temperature in the subsequent cycles of the co-dopant doped samples decreases about 10–15 °C as compared to the sample without a co-dopant. Porosity and large surface area of the co-dopant may decrease the segregation of bulk aluminum after the desorption and improve hydrogen diffusion in/out bulk of desorbed/reabsorbed samples. [ABSTRACT FROM AUTHOR]

Abstract: Although hydrogen has a great potential as clean energy, safe practical storage of hydrogen for applications such as fuel cells has been a major challenge. NaAlH4 is one of the metal hydrides, which are candidates for hydrogen storage in vehicles. However, the rather slow absorption/desorption kinetics is still a significant drawback. To alleviate this problem, purified NaAlH4 was ground with TiCl3, ZrCl4, or HfCl4. Desorption kinetics and capacities were observed under TPD-like operation. Absorption efficiency was determined by raising the temperature up to 125°C. Of the three doped metals investigated for the positive effect on facilitating NaAlH4 decomposition, TiCl3 assists the best on the first reaction while ZrCl4 and HfCl4 do for the second one. Despite the kinetics enhancement directly involves with the ZrCl4 amount, there is a threshold of ZrCl4-content which affects. 6% ZrCl4 is considered as an appropriate amount to improve the hydrogen release because it simultaneously decreases the desorption temperature and gives the outstanding rate. In hydrogen desorption, ZrCl4 provides the most amount of released hydrogen, but for hydrogen absorption TiCl3-doped NaAlH4 possesses the highest capacity. It is believed that the metal size is one of the key factors resulting in such the behavior. [Copyright &y& Elsevier]

SUMMARY TiCl3 has been considered as the best catalyst for the hydrogen desorption/re-absorption of NaAlH4 in terms of kinetic enhancement. However, the formation of NaCl as a by-product leads to the decrease in the reversible hydrogen capacity of NaAlH4. In this work, TiO2 and metallic Ti were selected as catalysts for the reaction to avoid the formation of the by-product. The comparison of the catalytic activity of Ti, TiCl3, TiO2 and Ti(OBu)4 on the hydrogen desorption/absorption NaAlH4 were carried out. It was found that TiO2 doped NaAlH4 exhibits similar behavior as TiCl3 doped NaAlH4 with the reversible hydrogen capacity about 3.8 wt% (H/M). In addition, TiO2 doped NaAlH4 exhibits the superior hydrogen re-absorption rate to the one doped with TiCl3. That may be due to the Ti3+ defect sites on the surface of TiO2 would facilitate the hydrogen dissociation. Moreover, high surface area of TiO2 prevents the segregation and the morphological change of the desorbed substances (NaH and Al). This benefits to the mass transfer into the hydride system. However, doping with TiO2 also produces sodium oxide and hydroxide as by-products. Unexpectedly, metallic Ti doped NaAlH4 shows the lowest hydrogen desorption/re-absorption among the tested samples. Its hydrogen reversible capacity is around 1 wt% (H/M). The formation of TiHx (1 < x < 2) was detected in the sample after the hydrogen desorption/reabsorption. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: The segregation of bulk Al after the hydrogen desorption may be one of many reasons why there is the incomplete hydrogen re-absorption and the need for higher desorption temperature in subsequent desorption of HfCl4- and TiO2-doped NaAlH4. In this work, we attempted to improve hydrogen storage capacity of 4mol% HfCl4- and 4mol% TiO2-doped NaAlH4 by adding a co-dopant (graphite, activated carbon, and MCM-41) using ball milling technique. It was found that the co-dopants significantly affect the hydrogen desorption/re-absorption of the hydride. Results show that the hydrogen re-absorption capacity of HfCl4- and TiO2-doped NaAlH4 added with the co-dopants increases 10–50% as compared with that without a co-dopant, and graphite seems to be the best co-dopant. Moreover, the hydrogen desorption temperature in the subsequent cycle of co-dopant doped samples decreases about 10–15°C compared to the sample without a co-dopant. It could be explained that high porosity and large surface area of the co-dopants would decrease the segregation of bulk Al after the desorption and improve hydrogen diffusion in/out bulk of desorbed/re-absorbed samples. [Copyright &y& Elsevier]

The main objective of this work was to investigate the effect of two transition metals (0–9mol% ZrCl4 and HfCl4) on hydrogen desorption/absorption of NaAlH4 and LiAlH4. The hydrogen desorption was carried out over a wide temperature range of 25– while the hydrogen absorption took place at and between 50– for NaAlH4 and LiAlH4, respectively, with the same pressure of 11MPa. The result revealed that the transition metals (Zr and Hf) could improve the kinetics of the hydrogen desorption on NaAlH4. NaAlH4 doped with HfCl4 released hydrogen at the lower temperature than that of ZrCl4-doped NaAlH4. In addition, the rate of hydrogen desorption increased with increasing the amount of HfCl4 doping. However, the maximum hydrogen capacity of was obtained from the first desorption, and dropped to 2.2–2.6wt% in the subsequent cycles. This may be because of, after the hydrogen desorption, the hydrides melt due to the high temperature resulted in agglomeration of the hydrides and segregation of Al. XRD results showed the formation of NaCl after milling of NaAlH4 with 4mol% HfCl4 and no evidence of any Hf-containing phase. However, with 10mol% HfCl4, the formation between Al and Hf in the form of Al3Hf was observed. This compound may act as catalyst in the reversible hydrogen desorption/absorption. For LiAlH4, ZrCl4 or HfCl4 also enhanced the kinetics of desorption of LiAlH4. Moreover, it was observed that LiAlH4 released hydrogen during the milling. After the hydrogen desorption from LiAlH4, no hydrogen absorption was observed for the undoped hydride or that doped with HfCl4 or ZrCl4. This may be because of the instability of LiAlH4. Moreover, milling and dopant may also destabilize the structure of LiAlH4 causing the irreversibility of hydrogen desorption/absorption on LiAlH4. [Copyright &y& Elsevier]

Abstract: The main objective of this work was to investigate the different effects of transition metals (TiO2, VCl3, HfCl4) on the hydrogen desorption/absorption of NaAlH4. The HfCl4 doped NaAlH4 showed the lowest temperature of the first desorption at 85°C, while the one doped with VCl3 or TiO2 desorbed at 135°C and 155°C, respectively. Interestingly, the temperature of desorption in subsequent cycles of the NaAlH4 doped with TiO2 reduced to 140°C. On the contrary, in the case of NaAlH4 doped with HfCl4 or VCl3, the temperature of desorption increased to 150°C and 175°C, respectively. This may be because Ti can disperse in NaAlH4 better than Hf and V; therefore, this affected segregation of the sample after the desorption. The maximum hydrogen absorption capacity can be restored up to 3.5wt% by doping with TiO2, while the amount of restored hydrogen was lower for HfCl4 and VCl3 doped samples. XRD analysis demonstrated that no Ti-compound was observed for the TiO2 doped samples. In contrast, there was evidence of Al–V alloy in the VCl3 doped sample and Al–Hf alloy in the HfCl4 doped sample after subsequent desorption/absorption. As a result, the V- or Hf-doped NaAlH4 showed the lower ability to reabsorb hydrogen and required higher temperature in the subsequent desorptions. [Copyright &y& Elsevier]