Metal hydrogen capacity

In the first three cycles, the capacity of the alanate is high for both materials, the titanium metal-doped and the nitride-doped material. However, after 15 cycles the hydrogen capacity decreases significantly below 4% for the titanium metal-doped sample, whereas for the nitride-doped sample the capacity remains high at about 5% hydrogen. 

This reaction is not easily reversible onboard a vehicle. Hence, the by-products must be removed from the vehicle and regenerated offboard. The irreversible metal hydrides can include a transition metal, for example, Mg2FeH6 (5.5 wt% hydrogen capacity), a nontransition metal such as Be(BH4)2 (20.8 wt% hydrogen capacity), or NaAlH4. Various promising metal hydrides are discussed in the following sections. 

Figure 4. Diagram showing the enthalpy of formation and hydrogen capacity of known metal hydrides.

Direct utilization of metallic Ti as dopant to prepare Ti-doped NaAlH4 offers a clear potential to achieve high hydrogen capacity. Material structural/compositional design is a feasible way to further improve hydrogen storage performance of Ti-NaAIRt system. 

Metal- hydrogen system Hydride Theoretical maximum gravimetric H, capacity) (wt%) Theoretical reversible gravimetric capacity (wt%) Approx, desorption temperature range (°C)... 

In the third method called mechanochemical activation synthesis (MCAS), a mixtnre of metal componnd, viz. metal chloride, is ball-milled to induce a reaction to yield a high-hydrogen capacity hydride. 

Hydrogen capacity H in Fig. 1.24 can be expressed in either atomic HIM ratio (H - number of H atoms, M - number of metal atoms) or weight percent (wt%), both of which are commonly used [14]. It must be noticed that calculating wt% both mass of hydrogen and mass of metal (not only mass of metal) must be considered in the denominator. 

The complex hydride Mg CoH is very similar to Mg FeH. In the binary system of Mg-Co there is no solubility of Co in either solid or liquid Mg and no inter-metallic compound, Mg Co, exists in equilibrium with other phases. However, in contrast to the Mg-Fe system, the intermetallic compound MgCo exists in equili-brium in the Mg-Co binary system (e.g., [14, p. 251]). The theoretical hydrogen capacity of Mg CoH is only 4.5 wt% which is obviously lower than that of Mg FeHg due to the presence of the heavier Co element and one less H atom in the hydride formula. 

The main limitation of this type of reactor is the gradual accumulation of metals when heavy feedstocks are processed. The metals accumulate in the pores of the catalyst and gradually block access for hydrogenation and desulfurization. The length of operation is then dictated by the metal-holding capacity of the catalyst and the nickel and vanadium content of the feed. As the catalyst deactivates, the reactor feed temperature is gradually increased to maintain conversion. 

Nevertheless, as it seems to us, the high hydrogen capacity of new carbon nanomaterials is actual. In addition, it is essential that carbon nanotubes (CNTs) are inert in the environment and the heat of H2 adsorption on CNTs is considerably lower than the heat of metal hydrides formation. This allows one to look forward to a possibility of the use of carbon nanomaterials in the actual systems for hydrogen accumulation. 

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