Adsorption differential calorimetric heat

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. 

It is evident that a knowledge of f° (rate of adsorption) and 0 (heat flow) is not enough to derive a continuous curve of differential enthalpy of adsorption. One must also know the dead volume Vc of the calorimetric cell proper and the derivative of the quasi-equilibrium pressure with time. Note that when this derivative is very small (i.e. in the nearly vertical parts of an adsorption isotherm), Equation 2.82 becomes simply ... 

M. M. Dubinin (Academy of Sciences of the USSR, Moscow, USSR) In the work under discussion, the authors for the first time undertook an extensive and systematic investigation of heats of immersion into water of various zeolites in different cation-exchange forms containing varied amounts of preadsorbed water. On the basis of their experiments, they calculate the dependence of differential molar heats of adsorption on the adsorption values of water. In principle, assuming that equilibrium states are reached, the curves obtained should coincide with similar curves determined calorimetrically in adsorption of water vapors or with... 

The effective pore diameter of Y zeolite is determined by the kind of cation that balances the negative charge on the structure. Table IV shows micro-calorimetric measurements of different probe molecules adsorbed on cation-exchanged Y zeolite. Adsorption microcalorimetry has also proved to be a useful technique to study cation migration in zeolites 152). Specifically, repeated adsorption-desorption calorimetric measurements increased the heat of CO adsorption on a Cu-exchanged Y zeolite, indicating that Cu " cations were migrating from inaccessible sites for CO to accessible sites. Previously it had been shown that addition of Cu to NaY increased the differential heat of CO adsorption on these materials. 

We have given an incomplete discussion since details are readily available in the original papers. However, it should be clear that there is no longer any uncertainty about the relations between the two-component (adsorbent plus adsorbed molecules) point of view of solution thermodynamics, leading naturally to differential quantities, and the one-component (adsorbed molecules) point of view of adsorption thermodynamics, leading naturally to the molar quantities of more direct statistical mechanical interest. Also, the connections between calorimetric heats and entropies of adsorption now seem to be straightened out. 

Tian-Calvet heat flow equipment (Setaram) was used for microcalorimetric measurements. Every sample was evacuated (10 Pa) overnight at 673 K before the successive introduction of small doses of the probe gas (ammonia or carbon dioxide). The equilibrium pressure relative to each adsorbed amount was measured by means of a differential pressure gauge (Datametrics). The run was stopped at a final equilibrium pressure of 133.3 Pa. The adsorption temperature was maintained at 353 K, in order to limit physisorption. After overnight outgassing at the same temperature, a second run was carried out up to 133.3 Pa. The adsorption and calorimetric isotherms were obtained from each adsorption/readsorption... 

Prof. Fowkes s flow calorimetric work led to important advances in the understanding of the interactions between mineral fillers, polymers, and polymer additives. The work of S. T. Joslin and F. M. Fowkes [29] in which they time-sliced the outputs from an FMC and its accompanying DSD enabled them to determine differential molar heats of adsorption as a function of progress in the adsorption process, and gave an indication of the heterogeneity of strength of the acid or base adsorption sites on the filler or pigment surfaces. 

Fig. 9 Calorimetric differential molar heats of adsorption of various probe molecules versus coverage for Na-X zeolite ammonia (x from [65], activation at 673 K, adsorption at 308 K), water ( , activation at 593 K, adsorption at 298 K), methanol ( , activation at 623 K, adsorption at 298 K), benzene (+, activation at 623 K, adsorption at 298 K), and p-xylene (0, activation at 623 K, adsorption at 303 K)...

Let us consider now the coadsorption of two gases or more, the definition of the calorimetric heat is exactly the same as for the adsorption of a single component. In this case, it corresponds obviously to the differential molar enthalpy of coadsorption. It is not possible to measure directly by calorimetry the differential enthalpy of adsorption of each component present in the mixture. Thus, for the coadsorption of two components A and B, the molar calorimetric coadsorption heat is equal to the molar differential enthalpy of coadsorption ... 

The integral heat of adsorption Qi may be measured calorimetrically by determining directly the heat evolution when the desired amount of adsorbate is admitted to the clean solid surface. Alternatively, it may be more convenient to measure the heat of immersion of the solid in pure liquid adsorbate. Immersion of clean solid gives the integral heat of adsorption at P = Pq, that is, Qi(Po) or qi(Po), whereas immersion of solid previously equilibrated with adsorbate at pressure P gives the difference [qi(Po) differential heat of adsorption q may be obtained from the slope of the Qi-n plot, or by measuring the heat evolved as small increments of adsorbate are added [123]. 

Fig. XVn-21. (a) Differential heat of adsorption of N2 on Graphon, except for Oand , which were determined calorimetrically. (From Ref. 89.) (b) Differential heat of adsorption of N2 on carbon black (Spheron 6) at 78.5 K (From Ref. 124).

It is not surprising, in view of the material of the preceding section, that the heat of chemisorption often varies from the degree of surface coverage. It is convenient to consider two types of explanation (actual systems involving some combination of the two). First, the surface may be heterogeneous, so that a site energy distribution is involved (Section XVII-14). As an example, the variation of the calorimetric differential heat of adsorption of H2 on ZnO is shown in Fig. 

Fig. XVIII-11. Calorimetric differential heat of adsorption of H2 on ZnO. Dashed line differential heat of desorption. (From Ref. 104.)...

Fig. 2.11 Curves of the differential enthalpy of adsorption of nitrogen against surface coverage 0 (= for samples of Sterling carbon black heated at the following temperatures (a) 1500°C (fc) 1700°C (c) 2200 C (d) 2700°C. The curve for 2000°C was similar to (c). but with a lower peak. The calorimetric temperature was 77-5, 77-7, 77-4, 77-4 K in (a), (fc), (c) and (d) respectively.

All heat evolutions which occur simultaneously, in a similar manner, in both twin calorimetric elements connected differentially, are evidently not recorded. This particularity of twin or differential systems is particularly useful to eliminate, at least partially, from the thermograms, secondary thermal phenomena which would otherwise complicate the analysis of the calorimetric data. The introduction of a dose of gas into a single adsorption cell, containing no adsorbent, appears, for instance, on the calorimetric record as a sharp peak because it is not possible to preheat the gas at the exact temperature of the calorimeter. However, when the dose of gas is introduced simultaneously in both adsorption cells, containing no adsorbent, the corresponding calorimetric curve is considerably reduced. Its area (0.5-3 mm2, at 200°C) is then much smaller than the area of most thermograms of adsorption ( 300 mm2), and no correction for the gas-temperature effect is usually needed (65). 

It must be acknowledged, however, that the determination of the number of the different surface species which are formed during an adsorption process is often more difficult by means of calorimetry than by spectroscopic techniques. This may be phrased differently by saying that the resolution of spectra is usually better than the resolution of thermograms. Progress in data correction and analysis should probably improve the calorimetric results in that respect. The complex interactions with surface cations, anions, and defects which occur when carbon monoxide contacts nickel oxide at room temperature are thus revealed by the modifications of the infrared spectrum of the sample (75) but not by the differential heats of the CO-adsorption (76). Any modification of the nickel-oxide surface which alters its defect structure produces, however, a change of its energy spectrum with respect to carbon monoxide that is more clearly shown by heat-flow calorimetry (77) than by IR spectroscopy. 

The most reliable method for detecting the influence of internal diffusion upon the profile of Q-6 curves would be to determine calorimetrically and to compare the differential heats of adsorption of a given gas on the surface of similar samples with different porosities. But it would be very difficult... 

The stoichiometry of an interaction between gas molecules and preadsorbed species may thus be deduced from the modifications of the Q-6 curves for a given reactant which are produced by the presence of preadsorbed species on the solid. The results are, of course, particularly conclusive when the differential heats of adsorption of small doses of reactant are measured in a sensitive calorimeter. But, such a qualitative analysis of the calorimetric data, though very useful, does not allow definite conclusions. In the preceding example, for instance, a fraction of carbon dioxide may remain adsorbed on the solid ... 

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. 

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.)... 

Yoshizumi et al. (70) determined acid strength distributions on silica-alumina catalyst calorimetrically by measuring the heat adsorption of n-butylamine from benzene solution. They found that the differential heat of adsorption of n-butylamine ranged from 3.7 kcal/mole (weak acid sites) to 11.2 kcal/mole (strongest acid sites). 

In this paper, the chemical adsorption of NH3, using pulses, has been studied by combining the results of calorimetric measurement of heat released (in a differential scanning calorimeter) with the measurement of desorbed amount of base (by FTIR analysis of desorbed gases). In this way, the differential adsorption heat, representative of the aridity strength distribution of the deactivated catalyst, is obtained and the restrictions inherent to other techniques, which are affected by the measurement of coke degradation products, are avoided. 

In Figure 5 the curves of differential adsorption heat of NH3 for the fresh catalyst and the catalyst used for 2 and 12 h have been plotted for a reaction temperature of 350 °C and a contact time of 0.05 h. It is concluded that there is a severe decrease in total acidity (total amount of base used in the neutralization) and that the strongly acidic sites are those that are mostly affected by deactivation. The quality of the total acidity measurement obtained following the calorimetric technique has been contrasted with the desorption technique at programmed temperature (TPD), using the FTIR analysis for measurement of desorbed NH3. 

Fio. 32. Differential heats of adsorption for nitrogen on the oxidized (110), (100), and (111) single crystal faces and the polycrystalline surface of copper calculated from the adsorption isotherms by the author at 78.1-83.5, 78.1-89.2 and 83.5-89.2°K. The heat-coverage curve for nitrogen adsorption on polycrystalline chromic oxide at 90°K. has been calculated from the calorimetric and adsorption data of Beebe and Dowden. The experimental errors are indicated as in Fig. 31. [After Rhodin, J. Am. Chem. Soc. 72, 5641 (1950).]... 

Heats of Adsorption of Nitrogen at —195°. The results of the calorimetric work for nitrogen on bone mineral at —195° are represented in Figure 2, where the differential heats as measured for successive small increments are plotted against the coverage expressed as V/Vm. The Vm values were determined as described above by BET treatment of the nitrogen isotherms. Curves a, b, and c of Figure 2 represent, respectively, the heat for the bare, water-covered, and methanol-covered surface. 

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). 

Adsorption isotherms for the system BaS04-H20 at three temperatures have been obtained. Thermodynamic study of these data reveals that part of the free energy decrease in the adsorption process involves changes in the partial molal free energy of the adsorbent. From the three isotherms differential and integral heats of adsorption were derived and compared with new calorimetric determinations of the same thermodynamic functions. In both kinds of measurements exactly the same system and exactly the same materials were used. 

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