Microcalorimetry of adsorption

Instead of TPD, microcalorimetry of adsorption shows the heat evolved during the adsorption of probe molecules, usually ammonia, on acid sites [91-94]. This measurement can determine the distribution of adsorption enthalpies but cannot differentiate between adsorption on Lewis and Br0nsted acid sites. 

Experimentally, the acid strength has been estimated by various methods such as temperature programmed desorption (TPD) of probe molecules such as microcalorimetry of adsorption of base molecules such as... 

M. J. Blandamer, B. Briggs, P. M. Cullis, K. D. Mam, J. B. F. N. Engberts, J. Kevelam, Titration microcalorimetry of adsorption processes in aqueous systems - Interaction of sodium dodecylsulfate and sodium decylsulfate with poly(A-vinylpyrrohdone), J. Chem. Soc. Faraday Trans., 1998, 94, 259-266. 

Figure 7.5. Sticking coefficients along with differential heats of adsorption as measured by microcalorimetry for ethylene and acetylene on Rh(lOO). [Adapted from R. Kose, W.A. Brown and D.A. King, Chem. Rhys. Lett. 311 (1999) 109.]...

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. 

It is generally accepted that localization and coordination of monovalent Cu ions in different zeolites have significant influence on the catalytic activity. The localization and coordination of Cu ions was studied by means of adsorption of small probe molecules, in particular, carbon monoxide was used often due to its ability to form a stable mono-carbonyl complex with the Cu+ ion. The formation of this complex was investigated by the FTIR and by the microcalorimetry [1-3]. 

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

J.A. (1992) Applications of adsorption microcalorimetry to the study of heterogeneous catalysis. Adv. Catal.,... 

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]. 

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]. 

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. 

The pretreatment temperature is an important factor that influences the acidic/ basic properties of solids. For Brpnsted sites, the differential heat is the difference between the enthalpy of dissociation of the acidic hydroxyl and the enthalpy of protonation of the probe molecule. For Lewis sites, the differential heat of adsorption represents the energy associated with the transfer of electron density toward an electron-deficient, coordinatively unsaturated site, and probably an energy term related to the relaxation of the strained surface [147,182]. Increasing the pretreatment temperature modifies the surface acidity of the solids. The influence of the pretreatment temperature, between 300 and 800°C, on the surface acidity of a transition alumina has been studied by ammonia adsorption microcalorimetry [62]. The number and strength of the strong sites, which should be mainly Lewis sites, have been found to increase when the temperature increases. This behavior can be explained by the fact that the Lewis sites are not completely free and that their electron pair attracting capacity can be partially modified by different OH group environments. The different pretreatment temperatures used affected the whole spectrum of adsorption heats... 

Another study examined the acidity and basicity of bulk Ga203 by NH3 and SO2 adsorptions microcalorimetry performed at 150°C. As alumina, Ga203 is amphoteric, with heats higher than 100 kJ/mol for both NH3 and SO2 adsorption, respectively [186]. The amphoteric character of bulk gallium oxides and strong heterogeneity of the surface acidic and basic sites were proved also by Petre et al. [179] using microcalorimetry of pyridine adsorption at 150°C and CO2 adsorption at 30°C. 

The determination of acidity in FCCs from adsorption microcalorimetry of probe molecules was the object of a review article by Shen and Auroux [105], Adsorption microcalorimetry results obtained using anunonia as a probe molecule revealed that, as long as Lewis acid sites with strength greater than 100 kJ/mol are present and as long as these sites are available to gas oil, FCCs can retain their useful cracking activity and selectivity properties [221],... 

The effect of the Si/Al ratio of H-ZSM5 zeolite-based catalysts on surface acidity and on selectivity in the transformation of methanol into hydrocarbons has been studied using adsorption microcalorimetry of ammonia and tert-butylamine. The observed increase in light olefins selectivity and decrease in methanol conversion with increasing Si/Al ratio was explained by a decrease in total acidity [237]. 

The effect of temperature on ammonia adsorption by ZSM5 samples has been investigated by microcalorimetry, varying the adsorption temperature from 150 to 400°C [235]. The initial heats of adsorption were independent of temperature up to 300°C. When the adsorption temperature increased, there was a competition between the formation of ammonium ions on Brpnsted sites and their decomposition. The total number of titrated sites decreased with increasing adsorption temperature. It appeared that an adsorption temperature between 150 and 300°C is appropriate for these calorimetric experiments. 

The microcalorimetry of NH3 adsorption coupled with infrared spectroscopy was used to study the effect of the synthesis medium (OH or F ) on the nature and amount of acid sites present in Al,Si-MFl zeolites [103]. Both techniques revealed that H-MFl (F ) with Si/Al < 30 contained extra-framework aluminum species. Such species were responsible for the presence of Lewis acid sites and poisoning of the Brpnsted acidity. In contrast, MFl (F ) characterized by Si/Al > 30 presented the same behavior as H-MFl (OH ). 

The adsorption microcalorimetry has been also used to measure the heats of adsorption of ammonia and pyridine at 150°C on zeolites with variable offretite-erionite character [241]. The offretite sample (Si/Al = 3.9) exhibited only one population of sites with adsorption heats of NH3 near 155 kJ/mol. The presence of erionite domains in the crystals provoked the appearance of different acid site strengths and densities, as well as the presence of very strong acid sites attributed to the presence of extra-framework Al. In contrast, when the same adsorption experiments were repeated using pyridine, only crystals free from stacking faults, such as H-offretite, adsorbed this probe molecule. The presence of erionite domains in offretite drastically reduced pyridine adsorption. In crystals with erionite character, pyridine uptake could not be measured. Thus, it appears that chemisorption experiments with pyridine could serve as a diagnostic tool to quickly prove the existence of stacking faults in offretite-type crystals [241]. 

A combined microcalorimetry and adsorption stndy was nsed by Meziani et al. [49] to characterize the surface acidity of a series of MCM41 aluminosilicates (referred to as SiAl C , where x is the molar Si Al ratio and n the chain length of the surfactant template). With the exception of H-SiAl32Ci4 and SiAlgCi4, all samples were found to present low surface acidity. 

The acidic/basic properties of zeolites can be changed by introdnction of B, In, Ga elements into the crystal framework. For example, a coincorporation of alnminnm and boron in the zeolite lattice has revealed weak acidity for boron-associated sites [246] in boron-snbstitnted ZSM5 and ZSMll zeolites. Ammonia adsorption microcalorimetry gave initial heats of adsorption of abont 65 kJ/mol for H-B-ZSMll and showed that B-substituted pentasils have only very weak acidity [247]. Calcination at 800°C increased the heats of NH3 adsorption to about 170 kJ/mol by creation of strong Lewis acid sites as it can be seen in Figure 13.13. The lack of strong Brpnsted acid sites in H-B-ZSMll was confirmed by poor catalytic activity in methanol conversion and in toluene alkylation with methanol. 

Auroux et al. [251] used adsorption microcalorimetry of different alkanes to investigate Ga and Al substituted MFI zeolites used as catalysts in dehydrogenation and cracking reactions. 

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... 

The major advantage of protein adsorption studies on high surface area materials is that changes of some extensive properties which accompany the process of adsorption are large enough to be directly measured heat of adsorption through microcalorimetry 141), uptake or release of small ions by a combination of electrokinetic methods and titration 142), thickness of adsorbed layer or an increase of the volume fraction of solid phase by a hydrodynamic method like viscometry 143). Chromatographiclike analysis can also be applied to protein adsorption 144). 

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