Adsorption calorimetry

It is evident, however, from the preceding examples that the detailed analysis of surface interactions by means of adsorption calorimetry is... 

Concerning the acidity measured by NH3 and pyridine adsorption calorimetry the obtained results are summarized in Table 2. 

Microcalorimetry has gained importance as one of the most reliable method for the study of gas-solid interactions due to the development of commercial instrumentation able to measure small heat quantities and also the adsorbed amounts. There are basically three types of calorimeters sensitive enough (i.e., microcalorimeters) to measure differential heats of adsorption of simple gas molecules on powdered solids isoperibol calorimeters [131,132], constant temperature calorimeters [133], and heat-flow calorimeters [134,135]. During the early days of adsorption calorimetry, the most widely used calorimeters were of the isoperibol type [136-138] and their use in heterogeneous catalysis has been discussed in [134]. Many of these calorimeters consist of an inner vessel that is imperfectly insulated from its surroundings, the latter usually maintained at a constant temperature. These calorimeters usually do not have high resolution or accuracy. 

An apparatus with high sensitivity is the heat-flow microcalorimeter originally developed by Calvet and Prat [139] based on the design of Tian [140]. Several Tian-Calvet type microcalorimeters have been designed [141-144]. In the Calvet microcalorimeter, heat flow is measured between the system and the heat block itself. The principles and theory of heat-flow microcalorimetry, the analysis of calorimetric data, as well as the merits and limitations of the various applications of adsorption calorimetry to the study of heterogeneous catalysis have been discussed in several reviews [61,118,134,135,141,145]. The Tian-Calvet type calorimeters are preferred because they have been shown to be reliable, can be used with a wide variety of solids, can follow both slow and fast processes, and can be operated over a reasonably broad temperature range [118,135]. The apparatus is composed by an experimental vessel, where the system is located, which is contained into a calorimetric block (Figure 13.3 [146]). 

Heat of Adsorption and the Data Obtained FROM Adsorption Calorimetry... 

The data obtained directly from adsorption calorimetry measurements can be expressed in different ways (Figure 13.5 [155]) as follows ... 

FIGURE 13.5 Calorimetric and volumetric data obtained from adsorption calorimetry measurements Raw pressure and heat flow data obtained for each dose of probe molecule and Thermokinetic parameter (a), Volumetric isotherms (b), Calorimetric isotherms (c), Integral heats (d), Differential heats (e), Site Energy Distribution Spectrum (f). (From Damjanovic, Lj. and Auroux, A., Handbook of Thermal Analysis and Calorimetry, Further Advances, Techniques and Applications, Elsevier, Amsterdam, 387-438, 2007. With permission.)... 

Although it is a powerful and informative technique, adsorption calorimetry presents several inherent limitations that require its use in combination with other characterization methods [103,154]. 

Adsorption calorimetry allows the total number of adsorption sites and potentially catalytically active centers to be estimated the values obtained depend on the nature and size of the probe molecule. Appropriate probe molecules to be selected for adsorption microcalorimetry should be stable over time and with temperature. The probe adsorbed on the catalyst surface should also have sufficient mobility to equilibrate with active sites at the given temperature [103]. 

Extensive studies of the acidity and basicity of zeolites by adsorption calorimetry have been carried out over the past decades, and many reviews have been published [62,64,103,118,120,121,145,146,153,154]. For a given zeolite, different factors can modify its acidity and acid strength the size and strength of the probe molecule, the adsorption temperature, the morphology and crystallinity, the synthesis mode, the effect of pretreatment, the effect of the proton exchange level, the Si/Al ratio and dealumination, the isomorphous substitution, chemical modifications, aging, and coke deposits. 

In this study, we analyze this situation using Si-MAS-NMR spectroscopy and high-temperature ammonia-adsorption calorimetry. The acid strength will be determined from the heat of adsorption of ammonia. On adsorption of ammonia, the reaction. 

As we have seen, an adsorption isotherm is one way of describing the thermodynamics of gas adsorption. However, it is by no means the only way. Calorimetric measurements can be made for the process of adsorption, and thermodynamic parameters may be evaluated from the results. To discuss all of these in detail would require another chapter. Rather than develop all the theoretical and experimental aspects of this subject, therefore, it seems preferable to continue focusing on adsorption isotherms, extracting as much thermodynamic insight from this topic as possible. Within this context, results from adsorption calorimetry may be cited for comparison without a full development of this latter topic. 

HYDROPHILIC/HYDROPHOBIC CHARACTER MONITORED BY ADSORPTION CALORIMETRY... 

Quantitative information on the number of sites may be obtained from a volumetric or gravimetric study of the adsorption of oxidizing or reducting molecules. Adsorption calorimetry can be used to determine the energetics of the site distribution, as in the case of acidic and basic sites. 

Even when the solid-state physical methods do not indicate that properties of A-in-B are very different from those of A-in-A, it can still be possible that small changes in the electronic structure (a ligand effect on A) can be important enough for chemisorption and catalysis [25]. This should in principle be seen by (i) IR spectra of adsorbed molecules (ii) adsorption calorimetry (iii) changes in the activation energy of a simple catalytic reaction. There is currently experimental information available on all three points. 

If we wish to study die adsorbent-adsorbate interactions we must undertake adsorption calorimetry or analysis of the isotherm data at very low surface coverage. It is only under these conditions that we can eliminate, or at least minimize, the adsorbate-adsorbate interactions. At higher coverage, an additional (self-potential) term, Em, must be added to E0 to allow for the latter interactions. 

We shall examine here the two major procedures for gas adsorption calorimetry (cf. Section 3.3.3). Both procedures make use of a diathermal, heat-flowmeter, Tian-Calvet microcalorimeter (cf. Section 3.2.2). 

In some respects the technique is less demanding than gas adsorption calorimetry. For example, the calorimeter and pre-adsorption rig are separate and therefore easy to handle. 

Phase-change adsorption calorimetry. This was the earliest type of diathermal-conduction calorimetry and was originally developed in the form of ice calorimetry by Lavoisier and Laplace (1783), who weighed the liquid water, and by Bunsen (1870), who measured the change of volume. Dewar (1904) devised an elegant adsorption calorimeter at liquid air temperature the heat was evaluated from the volume of air vaporized. Of course, the temperature of the calorimeter is imposed by the temperature of the phase change. Because these calorimeters lack adaptability and cannot be readily automated, they are mainly of historical interest. 

The first experiments of gas adsorption calorimetry by Favre (1854) were made with an isoperibol calorimeter. More recently, refinements were introduced by Beebe and his co-workers (1936) and by Kington and Smith (1964). Because of the uncontrolled difference between the temperature of the sample and that of the surroundings, Newton s law of cooling must be applied to correct the observed temperature rise of the sample. In consequence, any slow release of heat (over more than, say, 30 minutes), which would produce a large uncertainty in the corrective term, cannot be registered. For this reason, isoperibol calorimetry cannot be used to follow slow adsorption equilibria. However, its main drawback is that the experiment is never isothermal during each adsorption step, a temperature rise of a few kelvins is common. The corresponding desorption (or lack of adsorption) must then be taken into account and, after each step, the sample must be thermally earthed so as to start each step at the same temperature. In view of these drawbacks,... 

Gravimetry can be associated with adsorption calorimetry, either by using two samples (one in the microcalorimeter, the other in the microbalance) in contact with the same atmosphere of adsorptive (Gravelle, 1972) or using a single sample, located in the cylindrical pan of a microbalance and surrounded by a Tian-Calvet thermopile (LeParlouer, 1985). 

The determination of the energy of adsorption is the most direct way of studying surface heterogeneity, but as adsorption calorimetry is experimentally more demanding than the measurement of the isotherm, this approach has inevitably attracted less attention. However, as will become evident in subsequent chapters, there is much to be gained by employing the two experimental techniques in combination. 

Finally, when integral molar energies of adsorption are directly measured by gas adsorption calorimetry, it is possible to obtain the corresponding integral molar entropies of adsorption from Equations (2.65) and (2.66). 

Since the data provided by the above type of immersion calorimetry experiment can be directly related to the results obtained by gas adsorption calorimetry the question arises which type of experiment is to be preferred In fact, immersion calorimetry, although time-consuming, has certain advantages because of the difficulty in handling easily condensable vapours in adsorption manometry. 

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