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Continued: Characterisation of acid sites in catalysts by the determination of the irreversible heats of NH3 adsorption, Table 1
Irreversability:
The first exposure results in 25·14 µM of ammonia adsorbed whereas the second exposure can only manage to adsorb 8·77 µM, probably because about 16 µM are chemisorbed on the sample during the first adsorption cycle. It is likely that this irreversible adsorption is on the most active sites which also appear to have restricted access. It should be noted that the profiles of both the heat evolution and adsorption peaks are very different for the second adsorption cycle in which the adsorption occurs predominantly on weak and easily accessible surface sites.
Figure 2 shows this discrepancy more clearly in the rate of adsorption and heat evolution during the initial cycle for a zeolite (Zeolite Y in the acid form) at 205°C. The differences between the profile in the top trace (thermal data) and the lower trace (matter transfer) are more apparent since we are looking at only the first adsorption.
Figure 2
 Click to see full size
An additional benefit of the flow–through calorimetric approach is the fact that sequential adsorption, desorption and repeat adsorption cycles in the one experiment, reveals the level of reversibility for the probe's interaction with the surface. Conventionally this comparison was made in the heats recorded for first and second exposures to the probe. Now we can include the matter transfer measurements as well and make a direct comparison between the molar enthalpies of adsorption for the first and second exposures to the probe.
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