Hydrides formation enthalpy

Fig. 1.25 Hydride formation enthalpy, AH, per mole H as a function of the plateau temperature at 1 bar. The plateau temperature is calculated from reported thermodynamic parameters using the Van t Hoff equation [162]...

Shao et al. [62] used Mg and Co nanoparticles with the size of 200-300 and 5-60 nm, respectively, for the synthesis of nanoparticulate Mg CoH (particle size 50-300 nm range) by pressing 2 1 powder mixtures of Mg and Co into pellets and then very complicated sintering under a hydrogen scheme. They observed the formation of two hydride phases Mg CoH and MgjCoH. From the Van t Hoff plot the formation enthalpy and enttopy were estimated as -82.3 kJ/molH and -138.8 kJ/molH K for Mg CoH, and -73.2 kJ/molH and -123.0 kJ/molH K for MgjCoHj, respectively. 

The thermodynamic aspects of hydride formation from gaseous hydrogen are described by means of pressure-composition isotherms in equilibrium (AG = 0). While the solid solution and hydride phase coexist, the isotherms show a flat plateau, the length of which determines the amount of H2 stored. In the pure P-phase, the H2 pressure rises steeply vfith increase in concentration. The two-phase region ends in a critical point T, above which the transition from the a- to the P-phase is continuous. The equilibrium pressure peq as a function of temperature is related to the changes AH° and AS° of enthalpy and entropy ... 

As a general rule, it can be stated that all elements with electronegativity in the range 1.35-1.82 do not form stable hydrides [34]. Exemptions are vanadium (1.45) and chromium (1.56), which form hydrides, andmolybdenum (1.30) and technetium (1.36), where hydride formation would be expected. The adsorption enthalpy can be estimated from the local environment of the hydrogen atom on the interstitial site. 

The stability of metal hydrides is presented in the form of van t Hoff plots. The most stable binary hydrides have enthalpies of formation of AHf= —226kJ mol H2, for example, H0H2. The least stable hydrides are FeHo.s, NiFlo.s and M0H0.5, with enthalpies of formation of AHf = + 20, + 20 and + 92 kj mol Fl2, respectively [42]. 

Fig. 8. Relationship between the activation energy for hydrogen evolution (1MKOH, 30 °C) on LaMs and the enthalpy of hydride formation calculated by means of equation (12) (later on in Section 4.7). The nature of M is specified by the points. After ref. 226, by permission of Elsevier Sequoia.

FeTi with the addition of 6% Nb, or 10% Mn or Ni, has been studied with respect to hydrogen absorption [553]. The presence of Nb enhances hydrogen absorption but the catalytic activity appears to be lower than that of NiTi. Impedance and voltammetric measurements have been used. The presence of 10% Mn in FeTi increases its hydrogen storage capacity slightly, whereas 20% Ni decreases it by ca. 20-30%. The enthalpies of hydride formation have been determined [537]. 

The formation enthalpies for the C6()H2n hydrides were calculated from the results of the B3LYP/6-31G calculations using homodesmic reaction (4.4) with fullerene C60, adamantane C10H16 and cyclohexane C6H12 ... 

The formation enthalpies for gaseous fullerene hydrides were calculated above from quantum chemical calculations. Since the experimental sublimation enthalpies are available only for fullerene C60... 

Complex metal hydride solid-state compound containing homoleptic, anionic metal-hydrogen complexes d, d, d etc electron configuration of transition element h.H enthalpy of hydride formation as determined from pressure-composition isotherms during desorption by using van t Hoff s method... 

As will be shown in the following sections many attempts have been made to favorably modify the enthalpy of hydride formation, not only of MgH2 but also of other hydrides, by alloy chemistry. One of the most prominent examples is Mg2NiH4 [18]. However, alloying in most cases leads to a substantial decrease in capacity. 

Nevertheless, Tsushio et al. [34] reported the hydrogenation properties of MgNio.86Mlo.o3 (Ml = Cr, Fe, Co, Mn) alloys. Using the van t Hoff equation based on the desorption plateaus and the standard entropy of hydrogen gas 130.9 J mol they determined an enthalpy of hydride formation of the Cr-substituted alloy of —50kJ (mol H2), which is, for Mg-based alloys, a very high value. 

The reaction yielded a reversible hydrogen capacity of 6.5 wt%. If the imide were subsequently decomposed, the overall hydrogen capacity of the amide-hydride pair would be 11.5 wt%. As with other systems, however, this total capacity has not been achieved reversibly. Furthermore, the formation enthalpy and hydrogen transport kinetics of this system require high temperatures ( 350°C) for hydrogen release at reasonable rates. Some improvement in hydrogen release kinetics was achieved by incorporating Ti catalysts. ... 

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