Adsorption microcalorimetry chemisorption

The adsorption microcalorimetry has been also used to measure the heats of adsorption of ammonia and pyridine at 150°C on zeolites with variable offretite-erionite character [241]. The offretite sample (Si/Al = 3.9) exhibited only one population of sites with adsorption heats of NH3 near 155 kJ/mol. The presence of erionite domains in the crystals provoked the appearance of different acid site strengths and densities, as well as the presence of very strong acid sites attributed to the presence of extra-framework Al. In contrast, when the same adsorption experiments were repeated using pyridine, only crystals free from stacking faults, such as H-offretite, adsorbed this probe molecule. The presence of erionite domains in offretite drastically reduced pyridine adsorption. In crystals with erionite character, pyridine uptake could not be measured. Thus, it appears that chemisorption experiments with pyridine could serve as a diagnostic tool to quickly prove the existence of stacking faults in offretite-type crystals [241]. 

Chemisorption of gaseous bases, e.g., ammonia or pyridine, followed by adsorption microcalorimetry, FTIR, and/or TPD, can determine the concentration, strength, and type of surface acid sites. 

The most promising approach to this problem is the use of suitable probe molecules for the quantitative characterization of site density and strength by means of adsorption microcalorimetry. The best-known appUcations of chemisorption involve the use of bases such as NH3 or pyridine to probe the acidity of zeoUtes. Moreover, it is well known that adsorption influences all phenomena depending on surface properties, since it constitutes the primary step for every catalytic reaction involving sohd catalysts. Adsorption is generally exothermic (AH < 0) and the heat evolved is called heat of adsorption. This heat is related to the ability of the sites to interact with the probe molecule, i.e. to their basic or acidic character. 

A sample of mordenite (98% degree of ammonium-ion exchange) deam-moniated at various temperatures from 693 K to 923 K was studied at 573 K by NH3 adsorption microcalorimetry by Bankos et al. [203]. On increasing the pretreatment temperature, the number of acid sites passed through a maximum at 753 K as a result of simultaneous decationation and dehydroxylation. The heat of adsorption of NH3 on Bronsted acid sites formed by decationation was 110 -160 kJ mol During dehydroxylation, two types of Lewis sites were formed, characterized by heats of NH3 adsorption of 170-185kjmol and 95-100 kJ mol respectively, and on which dissociative chemisorption of ammonia was evidenced by IR [101]. 

Microcalorimetry can give erroneous results if adsorption equilibration is too slow, a particularly serious problem at low temperatures. The literature contains some controversial articles on this subject [39]. Generally speaking, the adsorption temperature should not be too low, in order to allow the detection of differences among the sites. Under certain circumstances, the evolved heat measured at low temperature can be merely an average value over various site populations of different strengths. Another important issue is that chemisorption must predominate over physisorption. 

For strongly chemisorbed species it can be difficult to obtain equilibrium uptakes below saturation at usual temperatures and measurable pressures (the adsorption isotherms rise very steeply with pressure to the monolayer coverage). Furthermore, chemisorption may be thermally activated, resulting in very long equilibration times. For these reasons other approaches are required to measure the thermodynamics of chemisorption. The two main ways are microcalorimetry and thermal desorption. 

As a conclusion, from these experimental results alone, it is impossible to decide in favour of either adsorption law So, adsorption studies have to resort to other methods than purely kinetic ones in order to unravel the intricacies of an adsorbate-adsorbent system In the case in point, microcalorimetry allows to exclude at least one model,since chemisorption heat is a linear function not of the adsorbed volume, as it should be if the ELOVICH-TEMKIN model were obeyed, but of its logarithm ... 

E. Garrone, G. Ghiotti, E. Giamello, B. Fubini, Entropy of adsorption by microcalorimetry. Part 1—Quasi-ideal chemisorption of CO onto various oxidic systems. 1. Chem. Soc., Faraday Trans. 1 Phys. Chem. Condensed Phases 77(11), 2613-2620 (1981)... 

Results obtained by temperature-programmed desorption suggested that in the case of bimetallic catalysts there was a reduction in the number of strong CO-adsorption sites. This finding allows conclusion that the alloying effect of these systems leads to the lowering of the CO heat of adsorption. This finding was confirmed by direct measurement of differential heat of CO chemisorption in the microcalorimetry experiment (see Fig. 4.20). 

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