Adsorption microcalorimetry interaction measurement

Measurement of heat of adsorption by means of microcalorimetry has been used extensively in heterogeneous catalysis to gain more insight into the strength of gas-surface interactions and the catalytic properties of solid surfaces [61-65]. Microcalorimetry coupled with volumetry is undoubtedly the most reliable method, for two main reasons (i) the expected physical quantities (the heat evolved and the amount of adsorbed substance) are directly measured (ii) no hypotheses on the actual equilibrium of the system are needed. Moreover, besides the provided heat effects, adsorption microcalorimetry can contribute in the study of all phenomena, which can be involved in one catalyzed process (activation/deactivation of the catalyst, coke production, pore blocking, sintering, and adsorption of poisons in the feed gases) [66]. 

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-... 

Rouquerol et al. (11, 12) have recently described the experimental determination of entropies of adsorption by applying thermodynamic principles to reversible gas-solid interactions. Theoretically, the entropy change associated with the adsorption process can only be measured in the case of reversible heat exchange. The authors showed how isothermal adsorption microcalorimetry can be used to obtain directly and continuously the integral entropy of adsorption by a slow and constant introduction of adsorbate under quasi-equilibrium conditions (11) or by discontinuous introduction of the adsorbate in an open system (12). 

JSnchen et al. [64] have reported that the heats of adsorption of acetonitrile on mesoporous (MCM-41) and microporous (FAU and MFI) molecular sieves are mainly influenced by a specific interaction with the acidic sites, while the adsorption heats of a non-polar molecule like w-hexane are determined by the pore size or density of those materials. However, a pore-size effect, affecting the heats of acetonitrile adsorption on acidic molecular sieves, has to be taken into account when employing those heats as a measurement of acidic strength. The contribution of the pore-size governed dispersion interaction in mesoporous MCM-41 is about 15 kJ mof less than that in the narrow channels of MFI. The adsorption of molecules of different sizes (toluene, xylenes, etc.), and the consecutive adsorption of these same molecules, studied by adsorption microcalorimetry together with reaction tests, can provide useful indications of the pore geometry and reactant accessibility of new zeolitic materials such as MCM-22 [65] or ZSM-11, SSZ-24, ZSM-12, H-M and CIT-1 [66]. 

Adsorption microcalorimetry is the measure of the heat of adsorption evolved when dosing measured small amounts of a vapor probe on a surface. Cardona-Martinez and Dumesic [36] summarized the results obtained for oxides, zeolite, and metal catalysts before 1992. Summaries of the application of these techniques to gas-solid interactions and heterogeneous catalysis have been published recently [37-39]. As done by Auroux and Gervasini [40] for a number of binary metal oxides, calorimetric studies of the acidity and basicity are mostly performed using ammonia as an acidity probe and carbon dioxide as a basicity probe [41]. 

In Fig. 1.21a, the differential heats of adsorption of CO on H—BEA zeolite and on MFI-Silicalite are reported as a function of the adsorbed amounts. Volumetric isotherms are illustrated in the figure inset. In both cases the adsorption was fully reversible upon evacuation of the CO pressure, as typical of both physical and weak, associative chemical adsorption. For H-BEA a constant heat plateau at 60kJ mol was measured. This value is typical of a specific interaction of CO with coordinative unsaturated Al(III) atoms, as it was confirmed by combining adsorption microcalorimetry and molecular modeling [73, 74, 78, 89] Note that the heat value was close to the heat of adsorption of CO at cus Al(III) sites on transition catalytic alumina, a typical Lewis acidic oxide [55, 73], Once saturated the Al(III) defects, the heat of adsorption started decreasing down to values typical of the H-bonding interaction of CO with the Br0nsted acidic sites (- 30 kJ mol , as reported by Savitz et al. [93]) and with polar defects, either confined in the zeolite nanopores or at the external surface. 

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. 

The measurement of heats of adsorption by means of microcalorimetry has been used extensively in heterogeneous catalysis in the past few decades to gain more insight into the nature of gas-surface interactions and the catalytic properties of solid surfaces. Specific attention will be focused on group IIIA containing samples in this section. 

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. 

Auroux et al. [109] compared the activity of substituted ZSM-5 zeolites for the cracking of propane, n-butane, and isobutane with the reactivity of the C - C and C - H bonds. The heats of adsorption of the alkanes on the H-ZSM-5 samples, measured by microcalorimetry, decreased with the size of the alkane, which was related to the electron density at the C - C or C - H bonds. A trend to a lower overall cracking activity with increasing basidty of the alkane C - C bond was observed. Less acidic hydroxyl groups tend to interact selectively with the C - H bonds rather than the C - C bonds of the alkane and, therefore, increase the selectivity of the catalysts to dehydrogenation. 

Microcalorimetry measurements are combined with Grand Canonical Monte Carlo simulations in order to understand more deeply the interactions between methane and two types of faujasite systems. The modelling study, based on newly derived force fields for describing the adsorbate/adsorbate and adsoibate/adsorbent interactions, provide isotherms and evolutions of the differential enthalpy of adsorption as a fimction of coverage for DAY and NaX which are in very good accordance with those obtained experimentally. The influence of the location of the extra-framework cations within the supercages on these thermodynamics properties is also pointed out. Furthermore, the microscopic mechanisms of CH4 adsorption is then carefully analysed in each faujasite system which are consistent with the trend observed for the differential enthalpies of adsorption. 

For such ambitious applications, it is first necessary to understand more deeply the interactions between methane and the microporous adsorbent surface. The enthalpy of CH4 adsorption has been evaluated in various zeolite systems by using isosteric methods via the Clapeyron equation or performed using microcalorimetry measurements which allow direct... 

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