Adsorption enthalpies, factors

We again assume that the pre-exponential factor and the entropy contributions do not depend on temperature. This assumption is not strictly correct but, as we shall see in Chapter 3, the latter dependence is much weaker than that of the energy in the exponential terms. The normalized activation energy is also shown in Fig. 2.11 as a function of mole fraction. Notice that the activation energy is not just that of the rate-limiting step. It also depends on the adsorption enthalpies of the steps prior to the rate-limiting step and the coverages. 

Adsorption enthalpies are also very similar but quite different pre-exponential factors are obtained. This indicates that the same sites are involved for the adsorption of NO but their number has increased. The increase of the number of active sites can be confirmed by TPD experiments obtaining that the total amount of CO and CO2 increased considerably after treatment of the raw sample (Table 2). On the other hand, the amount of different type of surface groups varies in a low extent after treatment of the raw sample, except for the carbonyls that increases in concentration by approximately a factor of three. So, these structures could be the responsible of the enhancement of the NO conversion once determined that diffusion coefficients are similar for both samples. 

When using more active emulsifiers of the C-10 type (Yeliseyeva, 1966) the effective rate and the equilibrium value of adsorption increase in the polymerization of polar monomers, which reduces the intensity of the processes of initial flocculation. Moreover, in this case the charge density is not the only stabilizing factor it is su(f>lemented by the enthalpy factor which arises from hydration of oxyethylenic chains of emulsifier molecules. The presence of these emulsifiers creates conditions that enable tbe obtain-ment of polar polymer latexes of high stability. 

Adsorption enthalpy Wide range, related to the chemical bond strength— typically 40-800 kJ/mol Related to factors like molecular mass and polarity but typically 5 0 kJ/mol (i.e. heat of liquefaction)... 

If the adsorption enthalpy itself does not vary with temperature, one expects a Bnwir relationship between the logarithm of tte retention factor and the inverse... 

Functionalising MIL-53(A1) with amino groups leads to an increased selectivity for CO2 over CH4 [32]. The amino-MIL-53(Al) was synthesised using 2-aminoter-ephthalic acid as a linker [33, 34], and the initial CO2 adsorption enthalpy in amino-MIL-53(A1) is 38.4 kJ mol significantly higher than that for CH4 (20 kJ mol ). Such differences result in a separation factor of 60 for CO2 over CH4,5 times higher than the parent material M1L-53(A1). 

In most cases, the separation of enantiomers on CSPs is controlled by the difference of adsorption enthalpies and a increases with decreasing temperature. The effects and thus the separation factors are small (which means unfortunately that nonchiral retention is dominating). An a of 1.05 corresponds to a free energy difference of adsorption of 120 J mol and a minimum of 8000 theoretical plates is necessary to obtain good resolution (gas constant, R = 1 with ki = 10 the calculation is performed by means of the chromatographic resolution equation). 

Table 1 summarises the steady state experiments over the zeolite crystals and pellets. The activation energy for the intrinsic rate constant kmtr was 34.4 kJ/mol. The low value indicates that the adsorption enthalpy is of the same order as the reaction enthalpy, i.e. (ERx ,obs = Ekr + EAds). The diffusion coefficient under steady state conditions was found to be an order of magnitude higher than that calculated from Knudsen diffusion (2x10 cmVs) using an average pore size of 3.8x10 cm measured by BET and a tortuosity factor of 4. The estimated... 

Table 5 Proportionality factors C(E) connecting the adsorption enthalpy of noble-gas like elements on metals with the adsorption enthalpy of Xe on these metals...

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