Adsorption, calorimetric measurements

The adsorption calorimetric measurements were carried out at 423 K on a SETARAM microcalorimeter of calvet-type connected with a standard volumetric adsorption apparatus. The pressure measurements were made using a MKS Baratron membrane manometer. Prior to the ammonia adsorption, the samples (900 mg) were carefully calcined in high vacuum at 673 K for 15 h. 

Today there are many experimental techniques and instruments at hand to perform measurements of this type both for characterization of the sorbent material, cp. for example [2.25], but also for industrial purposes [2.26]. A good overview of adsorption calorimetric measurement methods is given in [2.2]. 

The adsorption properties of titanium silicalites-1 synthesized via two different routes, viz. in the presence or the absence of sodium in the precursor gel, have been compared by Auroux et al. [248]. Adsorption calorimetric measurements of a basic probe (NH3) and an acidic probe (SO2) showed that these solids were very acidic compared to a silicalite-1 sample. The presence of Na in the different samples decreased the number and the strength of the acid sites. The modification strongly depended on the synthesis procedure [248]. 

The acidic properties of cobalt and silicon-substituted AlPO-5, -11, and -44 have been characterized by JSnchen et al. [111,276] by adsorption calorimetry of acetonitrile at 303 K, after activation at 720 K. Adsorption calorimetric measurements indicated that the adsorption potential of the samples for acetonitrile was enhanced upon cobalt incorporation. The heat curves exhibited at least two steps indicating the existence of acid sites of different strengths. The heats of adsorption indicated the formation of strong acid sites, due to the cobalt incorporation, as well as the presence of weaker acid sites, probably terminal P - OH groups. 

A different and unexpected behavior was observed for the Pt/C catalyst provided by Johnson Matthey showing that the results obtained by direct adsorption calorimetric measurements provide accurate values of the heats of CO adsorption, and that they are directly related with the catalyst history (carbon used as support, the method of preparation). The calorimetric results obtained for this sample are given in Fig. 12.9 and Table 12.3. 

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. 

Gorman-Lewis D, Fein JB, Jensen MP (2006) Enthalpies and entropies of proton and cadmium adsorption onto Bacillus subtilis bacterial cells from calorimetric measurements. Geochim Cosmochim Acta 70 4862-4873... 

Hysteresis is observed not only in the sorption isotherms but also in calorimetric measurements of heat of wetting at different moisture contents, and it is thus a combined entropy and enthalpy phenomenon. A reliable explanation for this effect is not currently available, but there is speculation that it is due to the stresses which are induced as the cellulose swells. Since the swelling of cellulose is not completely reversible, mechanical recovery is incomplete and hysteresis will therefore be present both in the internal stress-strain curve of the sample, and also in the water adsorption isotherm. 

Despite the importance of mixtures containing steam as a component there is a shortage of thermodynamic data for such systems. At low densities the solubility of water in compressed gases has been used (J, 2 to obtain cross term second virial coefficients Bj2- At high densities the phase boundaries of several water + hydrocarbon systems have been determined (3,4). Data which would be of greatest value, pVT measurements, do not exist. Adsorption on the walls of a pVT apparatus causes such large errors that it has been a difficult task to determine the equation of state of pure steam, particularly at low densities. Flow calorimetric measurements, which are free from adsorption errors, offer an alternative route to thermodynamic information. Flow calorimetric measurements of the isothermal enthalpy-pressure coefficient pressure yield the quantity 4>c = B - TdB/dT where B is the second virial coefficient. From values of obtain values of B without recourse to pVT measurements. 

A solid surface interacts with its surrounding molecules (in the gas or liquid phase) in varying degrees. For example, if a solid is immersed in a liquid, the interaction between the two bodies will be of interest. The interaction of a substance with a solid surface can be studied by measuring the heat of adsorption (besides other methods). The information one needs is whether the process is exothermic (heat is produced) or endothermic (heat is absorbed). This leads to the understanding of the mechanism of adsorption and helps in the application and design of the system. Calorimetric measurements have provided much useful information. When a solid is immersed in a liquid (Figure 5.10), in most cases there is a liberation of heat ... 

Heats of adsorption are usually determined in two ways either by direct calorimetric determination at a chosen temperature, or by calculating the isosteric heats from adsorption isotherms measured at different temperatures and using the Clausius-Clapeyron equation. Thus, isosteric heats of adsorption are calculated from the... 

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.)... 

Heats of adsorption of ammonia were measured with a twin-conduction-type microcalorimeter equipped with a volumetric vacuum line. The details and procedures have been described previously [6-8], Prior to calorimetric measurements, samples were activated by calcination under 1 mPa pressure on increasing the temperature at a rate of 3 K min and at the final temperature, in general 723 K, for 10 h. Adsorption of ammonia was carried out at 473, 573 and 623 K. The Si-MAS-NMR spectra were taken using a JEOL GX-270. 

The simplest case to study is hydrogen. Earlier papers, where heats of adsorption were measured calorimetrically (59, 60), reported a decrease of the heat of adsorption when Ni was alloyed with Cu. The qualitatively same result was obtained later in a very detailed study by Prinsloo and Gravelle... 

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. 

These observations suggest that the heterogeneous effect in the S02-modenite system represents an extreme case, so much so that chemisorp-tive bonds may be stipulated (probably between the S02 and the cations). These effects would, of course, involve energy emission and show up in the calorimetric measurements. However, the specificity of the adsorption would tend to show a relatively temperature-insensitive isotherm in the low-pressure region, thus rendering the isosteric techniques of obtaining heats of adsorption/chemisorption ineffective. 

At the plateau region direct calorimetric measurements of AadsH (adsorption enthalpy) showed that for a number of protein-surface combinations there is a part of the pH range where AadsH > O. Since AadsG < O for the process to occur, TAadsS... 

However, the heats of adsorption indicated by Fig. 24 are even more strongly in conflict with the isosteric heats than those derived from Fig. 23, and although the reality of the isosteric values is in some doubt, the discrepancy appears too great to consider model II as realistic. The data of Fig. 24 could be made to agree with the calorimetric data if it is assumed according to Beeck (6) that the calorimetric measurements at low cover-... 

The temperature coefficient for both the pyridine and triethylamine adsorption was small, and will need checking by calorimetric measurements. 

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. 

At the beginning, the electric double layer at the solid-aqueous electrolyte solution interface was characterized by the measurements of the electrokinetic potential and stability of dispersed systems. Later, the investigations were supported by potentiometric titration of the suspension, adsorption and calorimetric measurements [2]. Now, much valuable information on the mechanism of the ion adsorption can be obtained by advanced spectroscopic methods (especially infrared ATR and diffuse spectroscopy) [3], Mosbauer spectroscopy [4] and X-ray spectroscopy [5]. Some data concerning the interface potential were obtained with MOSFET [6], and AFM [7]. An enthalpy of the reaction of the metal oxide-solution systems can be obtained by... 

D. Calorimetric Measurements of the Adsorption Effects on the Metal Oxide-Electrolyte Solution Interface... 

In Table 1 are assembled the heats of adsorption for various absorbate systems on different substrates determined via isosteric heat measurement as well as calorimetric measurements. In some cases the heat of adsorption for one system has been measured using different techniques. This allows an estimation of the error involved in using those values based on different measurements. It is clear that the value for CO adsorption on Fe, for example, is considerably higher than values for other CO-adsorbate systems. In order to judge this, it must be understood that at room temperature CO partly dissociates on polycrystallinc Fe which contributes to the observed calorimetric value. This is a useful reminder that consecutive processes have to be considered in calorimetric measurements. 

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