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2 3 4
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Continued:
Figure 3 shows a full two cycle run for ammonia on a zeolite surface at 205°C. The first exposure generated 4·238 Joules for the adsorption of 44·07 µmol of ammonia. The second exposure (after the first desorption cycle) generated 1·731 Joules during the adsorption of 20·60 µmol of ammonia. So the first adsorption indicates the molar enthalpy of adsorption was 96·17 kJ/Mol, while the second was only 84·03 kJ/Mol. The difference is a function of the chemisorbed (or irreversible) component of the interaction. However, since we are looking at a distribution of energy in the zeolite surface sites, this only tells us part of the story.
In Figure 3 above, the matter transfer (lower trace) tells us that approximately half of the ammonia is chemisorbed on the zeolite, and this figure is very significant for the performance of the zeolite material when it is used as a catalyst.
There is one further set of information we can obtain directly from these data. By time–slicing the heat and matter transfer profiles across each integral peak, the differential molar enthalpies of adsorption can be plotted against time into the interaction. This is useful in comparing the sorptive characteristics between two or more materials in that it shows us how the surface coverage progresses during the adsorption process.
Comparing different 'Lots' of materials:
In the following figure (Figure 4), the same experiment as in Figure 1 previously was performed on a K10 catalyst from another source (K10F).
Figure 4
 Click to see full size
Comparing the profiles qualitatively, shows how the kinetics vary between the two samples. Comparing the integral heat and matter transfer values gives an indication of the differences in the levels of saturation and reversibility of the interactions between the two samples.
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