Measurement of adsorption heat

This measure of heat of adsorption is much higher than that derived from the fitting of the data with the Sips equation earlier, where we have found a value of 28750 J/mol for Q. This large difference should cause no alarm as the parameter Q is only the measure of adsorption heat. For example in the case of the Sips equation, Q is the isosteric heat of adsorption at a fractional loading of 0.5, while the parameter Q in the case of the Toth equation is the isosteric heat of adsorption at zero fractional loading as we shall show in the next section. 

As mentioned earlier, a main source of information about adsorption and its mechanism is, besides the colorimetric measurements of adsorption heats [65], the adsorption isotherm. It should be pointed out that the fact that the adsorption isotherm is integral characteristic of a concrete adsorption system is... 

It is not practical for catalyst development to look for the correlation of adsorption heats and catalytic activities from adsorption heat measurements. First, the measurement of adsorption heat is more difficult and more complicated than the evaluation of catalyst activity in general. Second the adsorption heat varies with surface coverage. Although it is known that catalytic activity does not depend on initial adsorption heat, it is still used for correlating catalytic activities of ammonia synthesis and ethylene hydrogenation because of the lack of experiment data. Therefore, there is a certain limit for the study of catalyst and catalytic reaction using adsorption heat. 

Measurement of adsorption heat. Quantitative measurement of adsorption heat can be used not only to distinguish physisorption from chemisorption, but also to estimate the type of chemical bonds and the adsorption intensity. The measurement of adsorption heat in different coverages can determine whether the surface is uniform. 

There are numerous references in the literature to irreversible adsorption from solution. Irreversible adsorption is defined as the lack of desotption from an adsoibed layer equilibrated with pure solvent. Often there is no evidence of strong surface-adsorbate bond formation, either in terms of the chemistry of the system or from direct calorimetric measurements of the heat of adsorption. It is also typical that if a better solvent is used, or a strongly competitive adsorbate, then desorption is rapid and complete. Adsorption irreversibility occurs quite frequently in polymers [4] and proteins [121-123] but has also been observed in small molecules and surfactants [124-128]. Each of these cases has a different explanation and discussion. 

Moreover, the use of heat-flow calorimetry in heterogeneous catalysis research is not limited to the measurement of differential heats of adsorption. Surface interactions between adsorbed species or between gases and adsorbed species, similar to the interactions which either constitute some of the steps of the reaction mechanisms or produce, during the catalytic reaction, the inhibition of the catalyst, may also be studied by this experimental technique. The calorimetric results, compared to thermodynamic data in thermochemical cycles, yield, in the favorable cases, useful information concerning the most probable reaction mechanisms or the fraction of the energy spectrum of surface sites which is really active during the catalytic reaction. Some of the conclusions of these investigations may be controlled directly by the calorimetric studies of the catalytic reaction itself. 

Pyridine sorption studies on EDTA-dealuminated Y zeolites at various temperatures (54,58), as well as measurements of differential heats of adsorption of ammonia on aluminum-deficient Y zeolites (57,59) have led to the conclusion that aluminum-deficient Y zeolites have stronger acid sites than the parent zeolite. 

Another thermal analysis method available for catalyst characterization is microcalorimetiy, which is based on the measurement of the heat generated or consumed when a gas adsorbs and reacts on the surface of a solid [66-68], This information can be used, for instance, to determine the relative stability among different phases of a solid [69], Microcalorimetiy is also applicable in the measurement of the strengths and distribution of acidic or basic sites as well as for the characterization of metal-based catalysts [66-68], For instance, Figure 1.10 presents microcalorimetry data for ammonia adsorption on H-ZSM-5 and H-mordenite zeolites [70], clearly illustrating the differences in both acid strength (indicated by the different initial adsorption heats) and total number of acidic sites (measured by the total ammonia uptake) between the two catalysts. 

The solid curve is the heat of adsorption of hydrogen alone in its dependence of coverage, and the points represent measurements of the heat of adsorption on the 40% of surface not covered by nitrogen when the latter is preadsorbed. It is seen that these values fall essentially on the curve as if the total surface were covered with hydrogen. Since the hydrogen measurements can be made with exceptionally great accuracy, it is quite possible that the small deviations from the curve toward lower heats of adsorption values are real and indicate that the... 

The very low water adsorption by Graphon precludes reliable calculations of thermodynamic quantities from isotherms at two temperatures. By combining one adsorption isotherm with measurements of the heats of immersion, however, it is possible to calculate both the isosteric heat and entropy change on adsorption with Equations (9) and (10). If the surface is assumed to be unperturbed by the adsorption, the absolute entropy of the water in the adsorbed state can be calculated. The isosteric heat values are much less than the heat of liquefaction with a minimum of 6 kcal./mole near the B.E.T. the entropy values are much greater than for liquid water. The formation of a two-dimensional gaseous film could account for the high entropy and low heat values, but the total evidence 22) indicates that water molecules adsorb on isolated sites (1 in 1,500), so that patch-wise adsorption takes place. 

The natural line of inquiry is to study the progress of a given reaction on various catalytic surfaces, to determine the relative numbers of molecules adsorbed on each surface, and to seek a correlation between the heat of activation, using provisionally the apparent value as a sufficiently good approximation to the true value, and the velocity of change referred to equal numbers of adsorbed molecules. Unfortunately, no example has hitherto been found suitable for experimental investigation, in which both the adsorptions and the reaction velocities can be measured. Thus no really valid test can be made. The existence of centres of varying activity would still further complicate the interpretation even of direct measurements of adsorption. 

Khalif et al.141 conclude from measurements of the heat of adsorption of 02 and e.s.r. measurements that there is unactivated adsorption on vanadium ions with a low co-ordination number of 3 or 4, whereas activated adsorption is observed at higher temperatures on ions with square pyramidal coordination. 02 adsorption occurs only as O2- in (V=0) or (V—O—V) groups and not as radicals which, according to these authors, is different from the behaviour of catalysts obtained by impregnation. 

The type of the oxidation product on galena is independent of the chemical environment during preparation. Rao152) measured the adsorption heat of K amyl xanthate (KAX) on unactivated and Cu2+-activated pyrrhotite (FeS) and compared his results with heats of the reaction between KAX and Fe2+ or Cu2+ salts. With the unactivated mineral, the interaction involves a chemical reaction of xanthate with Fe2+ salts present at the interface (i.e. not bound to the crystal surface). The adsorption enthalpy is identical with the formation of Fe2+ amyl xanthate FeS04 + 2 KAX —> FeX2 + K2S04, and -AH = 97.45 kJ/mol Fe2+). As revealed from the enthalpy values and the analysis of anions released into the solution, the interaction of xanthate with Cu2+-activated pyrrhotite consists of xanthate adsorption by exchange for sulfate ions (formed by an oxidation of sulfides) at isolated patches (active spots), and by further multilayer formation of xanthate. The adsorption heat of KAX on pyrrhotite at the initial pH 4.5 was - AH (FeS unactivated) = 93.55 kJ/mol Fe2+ and - AH (FeS activated) = 70.03 kJ/mol Cu2+. 

On the other hand, during the dehydration process of the zeolite, in order to measure the adsorption heats, the Ca2+ cation hydrolyzes the zeolitic water and creates Bronsted acid sites in the zeolite (see Section 2.5.3) [53], As is well known, the NH3 molecule suffers a strong interaction with these acid sites [31]. 

The heat of adsorption may be determined by a method due to Ewing (1931) whose work on the measurement of the heat of wetting has already been cited. The determination is made by a calorimeter in which the sample is placed. and the adsorbent material admitted. If the adsorbent is a vapor, it is admitted through the device a of Figure 66, and if a gas by means of the device b. The iron rod which is raised by an external permanent magnet is used to break the tips of the tubes containing the adsorbent material. 

The simultaneous measurement of the heat of adsorption and the adsorbed amount of H20 was performed by means of a Tian-Calvet microcalorimeter, operating at 303 K, connected to a volumetric apparatus. The samples were pretreated in vacuo at the chosen temperature and subsequently transferred into the calorimeter without further exposure to air. Small doses of water were subsequently admitted onto the sample, the pressure being continuously monitored by a transducer gauge (Baratron MKS, 0-100 Torr). 

For the measurement of the heat of adsorption, various types of calorimeter have been described, suitable for measurement of the comparatively large amounts of heat given off by the adsorption on porous solids 8 Roberts, however, has been successful in measuring the heat evolved from adsorption on a single tungsten wire, by employing the wire as its own calorimeter. Its temperature is raised by the heat evolution, and the rise of temperature is measured by the resistance of the wire. 

Figure 1. Determination of the isosteric heat of adsorpton from the measurement of adsorption isotherms for the system Xe/ Ni(100) [13].

Adsorption Data for Argon on Bone Mineral at —195°. In previous sections we have emphasized that the polarizability of the adsorbate on the polar bone mineral surface contributes to high heats of adsorption. For comparison we have made calorimetric measurements of the heat of adsorption of argon at —195° on the bare surface of bone mineral and on a methanol-covered surface. The data for differential heats of adsorption of argon at —195° are shown in Figure 3 and isotherms as measured on the pilot sample are recorded in Figure 4. 

Probably the most significant comparisons which can be made are of values of properties determined from calorimetric measurements with values calculated from adsorption isotherms. Two general methods are available for the comparison of values of enthalpies determined from experiments of the two types. One involves two differentiations The change in the partial molal enthalpy, AH2, of X2, for Process 4, is determined from the differentiation with respect to n2/n1 of the integral heat of adsorption measured in a series of calorimetric experiments of the type represented by Equation 1. The values of the differential heats of adsorption (heats corresponding to the differential Process 4) are compared with values determined from the temperature variation of AG2/T for a series of values of n2/n in Process 4. This type of comparison has been made successfully by several groups of authors (3, 5, 10). 

In fact, the differential energy of adsorption may be obtained directly by the calorimetric measurement of the heat evolved by adsorption (see Sections 2.7 and 2.8). We can define the differential enthalpy of adsorption, A as ... 

Microcalorimeters are well suited for the determination of differential enthalpies of adsorption, as will be commented on in Sections 3.2.2 and 3.3.3. Nevertheless, one should appreciate that there is a big step between the measurement of a heat of adsorption and the determination of a meaningful energy or enthalpy of adsorption. The measured heat depends on the experimental conditions (e.g. on the extent of reversibility of the process, the dead volume of the calorimetric cell and the isothermal or adiabatic operation of the calorimeter). It is therefore essential to devise the calorimetric experiment in such a way that it is the change of state which is assessed and not the mode of operation of the calorimeter. 

In some respects, adiabatic calorimetry provides information which is complementary to that provided by heat-flow calorimetry. The latter allows a study to be made of the full composition range at constant temperature, whereas the adiabatic calorimetry study is carried out over the prescribed range of temperature with a constant amount of adsorptive in the adsorption cell (of course, this does not mean that a constant amount is adsorbed). Adiabatic calorimetry allows direct measurements of the heat capacities of adsorbed films, although they are difficult to make accurately... 

Finally, it should be noted that calorimetric measurements can also be used to monitor adsorption phenomena at the solid-liquid interface (in a solvent). This method has been used to measure the adsorption heats evolved upon injection of dilute solutions of pyridine in alkanes ( -hexane, cyclohexane) onto an acidic solid itself in a slurry with -hexane. The amount of free base in solution is measured separately with a UV-Vis spectrometer, leading to an adsorption isotherm that is measured over the range of base addition used in the calorimetric titrations. The combined data from the calorimetric titration and adsorption measurements are analyzed simultaneously to determine equihbrium constants, quantities of sites per gram and acid site strengths for different acid sites on the solid. 

Calorimetric measurements of adsorption of CO2 at 303 K on different titania samples have provided evidence of their surface heterogeneity, as expected for oxides, with heats of adsorption ranging from -100 to 30 kj moT. Acidity measurements by ammonia adsorption microcalorimetry on the same samples gave rise to adsorption heats ranging between 150 and 60 kJ moT [47]. 

In a joint work with A. A. Isirikyan with the participation of G. U. Rakhmat-Kariyev, we carried out direct measurements of differential heats of adsorption of water vapors on crystalline and molded zeolite NaA at 22 °C using a Tian-Calvet-type calorimeter. The calorimetric installation enabled us to measure thermal effects for each point of the adsorption isotherm for a period of 300 hours and more (Figure 1). The squares and circles in the upper part of the graph denote our data for... 

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