Adsorption calorimetry Microcalorimetry

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]). 

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]. 

A system that links microcalorimetry to the volumetric measurement of quantities of adsorbed reactants makes it possible to study gas-solid interactions and catalyhc reactions. This system works under stahc vacuum. The admission of gases into the calorimeter can be performed either in a discontinuous way (by successive doses) by means of a valve, or in a continuous manner by means of a capillary. The classical technique of adsorption calorimetry by doses is the most appropriate way to measure the energy of interaction between the adsorbed species and the catalyst. If the surface can be a priori considered as heterogeneous, the heat of adsorption, the amount adsorbed and the kinetics of adsorption must be measured for very small successive doses of the adsorbate so as to obtain accu-... 

Microcalorimetry is an extremely sensitive technique that determines the heat emitted or adsorbed by a sample in a variety of processes. Microcalorimetry can be used to characterize pharmaceutical solids to obtain heats of solution, heats of crystallization, heats of reaction, heats of dilution, and heats of adsorption. Isothermal microcalorimetry has been used to investigate drug-excipient compatibility [82]. Pikal and co-workers have used isothermal microcalorimetry to investigate the enthalpy of relaxation in amorphous material [83]. Isothermal microcalorimetry is useful in determining even small amounts of amorphous content in a sample [84]. Solution calorimetry has also been used to quantitate the crystallinity of a sample [85]. Other aspects of isothermal microcalorimetry may be obtained from a review by Buckton [86]. 

An extensive study of the acidity and basicity of minerals by flow adsorption calorimetry over the past decade has been carried out by F. Fowkes and his group [25, 29]. His conclusion was that flow adsorption microcalorimetry was the only method that can correctly evaluate the distribution and strength of acid and base sites in solids, and predict the effect of these parameters on their catalytic and adhesive properties. This was fully supported by later work done by D. P. Ashton and D. Briggs [30]. 

In this chapter, a brief summary of studies that made use of calorimetry to characterize compounds comprising group IIIA elements (zeolites, nitrides, and oxides catalysts) was presented. It was demonstrated that adsorption microcalorimetry can be used as an efficient technique to characterize the acid-base strength of different types of materials and to provide information consistent with the catalytic data. 

It was proven that microcalorimetry technique is quite well developed and very useful in providing information on the strength and distribution of acidic and basic sites of catalysts. When interpreting calorimetric data, caution needs to be exercised. In general, one must be careful to determine if the experiments are conducted under such conditions that equilibration between the probe molecules and the adsorption sites can be attained. By itself, calorimetry only provides heats of interaction. It does not provide any information about the molecular nature of the species involved. Therefore, other complementary techniques should be used to help interpreting the calorimetric data. For example, IR spectroscopy needs to be used to determine whether a basic probe molecule adsorbs on a Brpnsted or Lewis acid site. 

An alternative method is flow adsorption microcalorimetry, which involves the use of a carrier gas passing continuously through the adsorption cell. The catalyst is placed on a glass frit in a gas circulation cell in the calorimeter. In order to determine the amounts of gas adsorbed, flow calorimetry must be used in combination with another technique, most frequently TG, MS or GC [8, 18]. 

The porous structure of active carbons can be characterized by various techniques adsorption of gases (Ni, Ar, Kr, CO ) [5.39] or vapors (benzene, water) [5,39] by static (volumetric or gravimetric) or dynamic methods [39] adsorption from liquid solutions of solutes with a limited solubility and of solutes that are completely miscible with the solvent in all proportions [39] gas chromatography [40] immersion calorimetry [3,41J flow microcalorimetry [42] temperature-programmed desorption [43] mercury porosimetry [36,41] transmission electron microscopy (TEM) [44] and scanning electron microscopy (SEM) [44] small-angle x-ray scattering (SAXS) [44] x-ray diffraction (XRD) [44]. 

The determination of adsorption thermodynamic quantities such as adsorption heats can now be performed through direct or indirect methods with a great degree of accuracy. The foundations of gas—solid interface calorimetry have been well established by combining adsorption microcalorimetry with adsorption in quasi-equilibrium. The experimental results reported so far, obtained from different calorimetries, concur with the values calculated from adsorption isotherms. 

Direct determination of adsorption enthalpies (or more precisely displacement enthalpies as indicated earfier) by titration microcalorimetry, which is the main form of calorimetry used in adsorption from solution. 

This type of microcalorimetry has been referred to as diathermal calorimetry and has been adopted by most adsorption calorimeters of the Tian-Calvet type [21]. 

Chapter 9, by Kiraly (Hungary), attempts to clarify the adsorption of surfactants at solid/solution interfaces by calorimetric methods. The author addresses questions related to the composition and structure of the adsorption layer, the mechanism of the adsorption, the kinetics, the thermodynamics driving forces, the nature of the solid surface and of the surfactant (ionic, nonionic, HLB, CMC), experimental conditions, etc. He describes the calorimetric methods used, to elucidate the description of thermodynamic properties of surfactants at the boundary of solid-liquid interfaces. Isotherm power-compensation calorimetry is an essential method for such measurements. Isoperibolic heat-flux calorimetry is described for the evaluation of adsorption kinetics, DSC is used for the evaluation of enthalpy measurements, and immersion microcalorimetry is recommended for the detection of enthalpic interaction between a bare surface and a solution. Batch sorption, titration sorption, and flow sorption microcalorimetry are also discussed. 

R. Denoyel, F. Rouquerol, J. Rouquerol, Interest and requirements of liquid-flow microcalorimetry in the study of adsorption from solution in the scope of tertiary oil recovery, in Adsorption from Solution, ed. by C. Rochester (Academic Press, London, 1982), pp. 1-10 G.W. Woodbury Jr, L.A. NoR, Heats of adsorption from flow calorimetry relationships between heats measured by different methods. CoUoids Surf. 28, 233-245 (1987). doi 10. 1016/0166- 6622(87)80187-7... 

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