Heat-flow adsorption microcalorimetry

Heat-flow adsorption microcalorimetry. The most important type of isothermal calorimeter in current use is that based on the principle of the heat flowmeter, which was first applied by Tian (1923) and improved by Calvet (Calvet and Prat, 1958,... 

Flow adsorption microcalorimetry has been used to measure the heats of adsorption of ammonia in a nitrogen carrier on the H and Na forms of a Y zeolite [21]. The calorimeter was linked to a thermal conductivity detector in which the rates of adsorption and desorption and the associated rates of heat evolution or absorption were measured simultaneously at atmospheric pressure. The authors found that, as surface coverage increased, the sites covered first were not necessarily those with the highest molar heats of adsorption. 

Groszek [56, 57] studied the adsorption of simple gases (COj, CH4, SOj, O2, He, and Nj) on microporous carbons using flow adsorption microcalorimetry. Shen and Biilow [58] demonstrated that the isosteric adsorption technique (Eqns (3.11) or (3.31)) is a useful and effective tool to obtain highly accurate thermodynamic data for microporous adsorption systems like the heat of adsorption given by Eqn (3.47). They studied the adsorption of CO2 and N2—O2 mixtures on a super-activated, almost entirely microporous, carbon (M-30, from Osaka Gas) and three faujasite-type zeolites. They also estimated the energetic heterogeneity of the solids due to specific interactions between the adsorbate and the solid. 

In many cases the increases in surface areas of carbons by oxidation or by comminution were accompanied by the creation of microporosity which influenced the correlation between the heats of adsorption and surface area estimates as shown below in section 3.4. For non-porous purely hydrophobic solids there is an excellent agreement between the determinations of surface areas by flow adsorption microcalorimetry and the BETfN i) method as can be seen for Graphon in Table 2. 

In relatively simple cases of non-admission into the pores that are too small for the adsorptive, flow adsorption microcalorimetry can produce striking results as exemplified by the heats of adsorptions of normal and tertiary butyl alcohols shown in Figure 18 indicating a major reduction in adsorptive accessibility due to a... 

One of the first applications of flow adsorption microcalorimetry has been the study of interactions of n-heptane solutions of long chain alcohols, carboxylic acids, and amines with metal surfaces and metal oxides. Strong correlation was found between the heats of adsorption of their long chain compounds and anti-wear action. Later work was extended to the study of adsorption on freshly formed metal surfaces, which behaved quite differently from the metal oxides [46]. Freshly formed... 

The adsorption of at least one reactant is the first step of the mechanism of any catalytic reaction. This step is followed by surface interactions between adsorbed species or between a gaseous reactant and adsorbed species. In many cases, these interactions may be detected by the successive adsorptions of the reactants in different sequences. Heat-flow microcalorimetry can be used with profit for such studies (19). 

Heat-flow microcalorimetry may be used, therefore, not only to detect, by means of adsorption sequences, the different surface interactions between reactants which constitute, in favorable cases, the steps of probable reaction mechanisms, but also to determine the rates of these surface processes. The comparison of the adsorption or interaction rates, deduced from the thermograms recorded during an adsorption sequence, is particularly reliable, because the arrangement of the calorimetric cells remains unchanged during all the steps of the sequence. Moreover, it should be remembered that the curves on Fig. 28 represent the adsorption or interaction rates on a very small fraction of the catalyst surface which is, very probably, active during the catalytic reaction (Table VI). It is for these... 

P. C. Gravelle reviews Heat-Flow Microcalorimetry and shows its applications to the study of adsorption and heterogeneous catalysis. 

Chemical composition was determined by elemental analysis, by means of a Varian Liberty 200 ICP spectrometer. X-ray powder diffraction (XRD) patterns were collected on a Philips PW 1820 powder diffractometer, using the Ni-filtered C Ka radiation (A, = 1.5406 A). BET surface area and pore size distribution were determined from N2 adsorption isotherms at 77 K (Thermofinnigan Sorptomatic 1990 apparatus, sample out gassing at 573 K for 24 h). Surface acidity was analysed by microcalorimetry at 353 K, using NH3 as probe molecule. Calorimetric runs were performed in a Tian-Calvet heat flow calorimeter (Setaram). Main physico-chemical properties and the total acidity of the catalysts are reported in Table 1. 

Table II. Heats of Adsorption of OLOA 1200 on Carbon Black By Flow Microcalorimetry (40°C.)...

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]). 

In the adsorption microcalorimetry technique, the sample is kept at a constant temperature, while a probe molecule adsorbs onto its surface, and a heat-flow detector emits a signal proportional to the amount of heat transferred per unit time. 

In flow microcalorimetry a small sample is put into the cell of the calorimeter and the probe molecule passes through it in an appropriate solvent. Adsorption of the probe results in an increase in temperature and integration of the area under the signal gives the heat of adsorption [70]. This quantity can be used for the calculation of the reversible work of adhesion according to Eq. 13. The capabilities of the technique can be further increased if a HPLC detector is attached to... 

In a standard set-up for heat-flow gas adsorption microcalorimetry, the adsorbent bulb, (which is located in the thermopile (Figure 3.15)), is connected to a device to allow the simultaneous determination of the adsorption isotherm by one of the techniques listed in the previous sections. The simultaneous determination of the amount adsorbed and corresponding heat evolution allows one to assess the energy of adsorption (for the exact procedure, see Section 3.3.3) provided the following points are... 

Water adsorbed on the surface of pharmaceutical or food powders, even at low levels, has a significant impact on the behavior of such powders during processing, and ultimately on the quality and performance of the final product. Bakri (1993) reported the application of heat conduction microcalorimetry to study the adsorption of water vapor onto solid pharmaceuticals. The high sensitivity of the calorimeter permits detection, even at low humidities, of heat flow as a result of interaction of solvent vapor with the solid. The heat flow (Pj ) can be described by ... 

The acid-base properties of the samples were investigated using adsorption of appropriate probe molecules, namely ammonia and sulfur dioxide, monitored by microcalorimetry. The microcalorimetric studies were performed at 353 K for sulfur dioxide adsorption and at 423 K for ammonia adsorption in a heat flow calorimeter of Tian-Calvet type (Setaram C80), linked to a conventional volumetric apparatus. Before each experiment the samples were outgassed overnight at 673 K. 

Table 2 shows the heats of adsorption of benzene, cyclohexene and water on the catalysts at room temperature by flow microcalorimetry. n-Hexane for the two organic compounds and methyl ethyl ketone for water were used as the liquid carriers in measuring the heats. The heats of adsorption of water are overwhelmingly bigger than those of benzene and cyclohexene, namely, are about twenty times for the sol-gel catalysts and about ten times for the impregnation catalysts. 

Kwon, K.C., Determination of heats of adsorption on activated carbons by flow microcalorimetry. Gas Sep. Purif, 3(1), 13-18 (1989). 

Rossi, P.F., Adsorption heat-flow microcalorimetry applied to coal surlace properties. Adsorpt. Sci. Technol., 9(3), 148-189(1993). 

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... 

In the case of water pollution, the estimation of adsorption affinity of potential solid adsorbent toward the specific pollutant can be done using the so-called liquid microcalorimetry. The instruments used for this purpose are differential heat flow microcalorimeters modified to allow continuous stirring of liquid samples. The adsorbate is added to both sample and reference cells simultaneously using a programmable twin syringe pump, linked to the calorimeter. The heat evolved as a result of adsorption can be obtained by integration of the area under the calorimeter signal, for each particular injection (dose). The output of typical microcalorimetric experiment of this type is shown in Fig. 10.9. 

Adsorption microcalorimetry of N2 and Ar at 77K was carried out with an equipment described by Rouquerol (ref. 8) and which associates quasi equilibrium adsorption volumetry with isothermal low temperature microcalorimetry (using Tian Calvet heat flow-meters) so that two curves are continuously recorded (heat flow and quasi equilibrium pressure) as a function of the amount of gas introduced into the systems. Continuous plots of the adsorption isotherm and of the derivative enthalpy of adsorption h vs surface coverage may easily be doived (refs. 4,7). 

---