Enthalpy desorption

If the desorption rate is second-order, as is often the case for hydrogen on a metal surface, so that appears in Eq. XVIII-1, an equation analogous to Eq. XVIII-3 can be derived by the Redhead procedure. Derive this equation. In a particular case, H2 on Cu3Pt(III) surface, A was taken to be 1 x 10 cm /atom, the maximum desorption rate was at 225 K, 6 at the maximum was 0.5. Monolayer coverage was 4.2 x 10 atoms/cm, and = 5.5 K/sec. Calculate the desorption enthalpy (from Ref. 110). 

Neopentane adsorbs dissociatively on the Pt surface. A C-H bond is broken and a Pt-C and Pt-H bond are formed. The cleavage of the neopentane C-H bond is not influenced by the support. Moreover, as argued above, the adsorption enthalpy of H2 is > 45 kJ/mol regardless of the support, and on a basic support an additional H2 molecule is formed prior to the adsorption of neopentane. Thus, according to equation (6) the difference in Eapp between the basic and acidic support is given by the sum of the difference in the intrinsic Pt-C bond energy (16 kJ/mol lower for the basic support) and the H2 desorption enthalpy limit at the reaction temperature ... 

Figure 5. Absorption and desorption enthalpies vs. composition at 245° A - absorption, - desorption.

Figure 7. Partial molar desorption enthalpy calculated from van t Hoff plot.

Properties of relevance for hydrogen storage applications, such as desorption enthalpies and weight and volume efficiencies, are summarized in Table 21. 

Table 21 Desorption enthalpies and hydrogen storage efficiencies...

While the reaction kinetics could be optimized significantly, the desorption enthalpy of NaAlH4 of 37 kj (mol H2) and Na3AlH6 of 47kj (mol H2) is still too large for many applications. 

As discussed above, due to the absence of any stable compound in the dehydrogenated state and the formation of a rather stable hydride the desorption enthalpy is not decreased compared to the original single alkali composed hydrides. Fossdal et al. [58] measured pressure-composition isotherms for TiF3-enhanced material in the temperature range 170-250 °C. From the van t Floff plot they determined a dissociation enthalpy of 56.4 0.4kJ (mol Fl2) and a corresponding entropy of 137.9 0.7 (mol H2). The respective value of the reaction enthalpy of reaction (7.4) is 47 kj (mol H2) [19]. 

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