Enthalpies of desorption

A mechano-chemical synthesis in the Mg-Fe-H system was examined as well [2], Two types of hydrides have been observed, non-crystalline (possibly amorphous) and crystalline. By the addition of a catalytic metaF the enthalpy of desorption improves as shown in Figure 8. 

As shown in Fig. 2.43b, the enthalpy of absorption and desorption calculated from the Van t Hoff plots using the mid-plateau pressures of PCT curves in Fig. 2.43a, which are listed in Table 2.18, is equal to -72 and 83 kJ/mol, respectively. The value of entropy is 138 and 151 J/mol K for absorption and desorption, respectively. The enthalpy value for absorption is very close to the values found in the literature for MgHj as discussed in Sect. 2.1.2 and 2.1.3. Surprisingly, however, the enthalpy of desorption at 83 kJ/mol is much greater than the former and also greater than the enthalpy of desorption of the as-received and activated MgH as shown in Fig. 2.11. The coefficients of fit are excellent and give good credibility to the obtained values. 

In the context of the Rabkin and Skripnyuk model, which requires that desorption is better approximation of equilibrium then absorption, the high value of the enthalpy of desorption observed for a reactively synthesized MgH in Fig. 2.43b seems to be correct. If so, the hysteresis in Fig. 2.43a is due to higher enthalpy of desorption rather than to an inhomogeneity in the fraction transformed and non-transformed particles during the absorption process. 

From the Van t Hoff plot they calculated the enthalpy and entropy change of absorption as AH = -69.5 kJ/molH and AS = -129.6 J/moIH K, respectively. Correspondingly, for desorption they calculated AH = -83.2 kJ/molH and AS = -146.7 J/molH K. It is interesting that the enthalpy of desorption of ball-milled, nanocrystalline Mg CoH is higher than that for absorption. This is a very similar behavior to the one observed for the ball-milled, nanocrystalline MgH as discussed in Sect. 2.1.4 (Fig. 2.43). 

It has long been realized that the heat of adsorption can be calculated more accurately from determinations of heats of immersion than from equilibrium vapor pressures of adsorbates. Harkins and Boyd (8) and Jura and Harkins (10) have discussed the emersion process and have developed an expression for the enthalpy of desorption that is the negative of the one above. That the immersion process is equivalent to the process we are discussing can readily be shown with the aid of the following two-step process ... 

This work is a continuation of our earlier study [1] of the hydrogen interaction with intermetallic compound (IMC) AB2-type Tio.9Zro.1Mn . 3V0.5. The measurements were carried out in twin-cell differential heat-conducting Tian-Calvet type calorimeter connected with the apparatus for gas dose feeding, that permitted us to measure the dependencies of differential molar enthalpy of desorption (AHdes.) and equilibrium hydrogen pressure (P) on hydrogen concentration x (x=[H]/[AB2]) at different temperatures simultaneously. The measurements were carried out at 150°C, 170°C and 190°C and hydrogen pressure up to 60 atm. 

Co content. The integral enthalpy of desorption, H es was calculated from these... 

In order to determine quantitatively these specific interactions, consider, as a first approximation, that the specific interactions are simply added to the dispersive interactions defined previously. Therefore, the experimental point corresponding to a probe capable of specific interaction (that is, acid or base characteristics) always lies above the reference straight line of RT Ln VN versus a(yi,)l/2 correspondjneto the n-alkanes, as schematically illustrated in Figure 6. At a given value of a(YU > tiie difference of ordinates between the point corresponding to the specific probe and the reference line leads to the value of the free enthalpy of desorption AG ° p corresponding to specific acid - base interactions. 

In order to get at least a semi-quantitative approach to the acid-base surface properties of the solids, the enthalpy of desorption, AHSp, corresponding to the specific interactions was determined by studying the variation of AGSp with temperature T according to ... 

The activation energies q ), in general, are significantly different from the enthalpies of desorption 40, 41, 49, 50). This observation demonstrates that the jumping of a sorbed molecule from one equilibrium position (site) to an adjacent one involves a transition state that cannot be identified with a free gas molecule. 

Heat required for thermal regeneration = 2.5 X (enthalpy to heat the adsorbent bed + enthalpy of desorption) for organics = enough to heat the system to 30-50 °C above the boiling temperature of the highest boiling component in the mixture. 

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