Adsorption microcalorimetry probe molecules

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. 

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]. 

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. 

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. 

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 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... 

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 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]. 

The methods for measuring the acidity of nanoporous aluminosilicates such as MCM41 have been reviewed by Zheng et al. [243], including microcalorimetry measurements of probe molecules adsorption. 

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. 

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 adsorption microcalorimetry, surface equilibration depends not only on the chosen probe molecule but also on the adsorption temperature. It is worth mentioning that the literature contains some controversial articles on this subject [29]. 

Appropriate probe molecules to be selected for adsorption microcalorimetry should be stable with time and with temperature. Furthermore, in the case of... 

The effect of adsorption temperature on metals or supported metals on the mobility of adsorbed probe molecules has not received as much attention as on metal oxides. Gelin and co-workers (96) used adsorption microcalorimetry at 296 and 423 K and IR spectroscopy to study the adsorption of CO on Ir supported on NaY zeolite reduced from 383 to 923 K and on Ir supported on silica. At 296 K it was observed that for intermediate coverages (6 > 0.3) the kinetics of adsorption changed, with the thermograms displaying long tails... 

The acid-base properties of the decationated HY zeolites have been extensively studied with adsorption microcalorimetry. Tables II and III present a summary of calorimetric studies of the adsorption of ammonia and other probe molecules on HY zeolites with different Si/AI ratios, preparation methods, pretreatments, adsorption temperatures, and sodium contents. The large variety of conditions used in these studies complicates the comparison of the materials. For example, the initial differential heat of ammonia adsorption at... 

The effective pore diameter of Y zeolite is determined by the kind of cation that balances the negative charge on the structure. Table IV shows micro-calorimetric measurements of different probe molecules adsorbed on cation-exchanged Y zeolite. Adsorption microcalorimetry has also proved to be a useful technique to study cation migration in zeolites 152). Specifically, repeated adsorption-desorption calorimetric measurements increased the heat of CO adsorption on a Cu-exchanged Y zeolite, indicating that Cu " cations were migrating from inaccessible sites for CO to accessible sites. Previously it had been shown that addition of Cu to NaY increased the differential heat of CO adsorption on these materials. 

For most effective utilization in heterogeneous catalysis research, adsorption microcalorimetry must be used in combination with other techniques which probe the nature of the surface-adsorbed species. In the case of acidity studies, for example, IR spectroscopy is needed to identify which regions of the acid strength distribution correspond to Lewis verus Brpnsted acid sites. As the application of adsorption microcalorimetry in heterogeneous catalysis evolves from studies involving primarily probe molecules to studies involving more reactive molecules, it will become even more important to combine these calorimetric studies with surface spectroscopic investigations. 

In the present paper, we report some characterizations of TS-1 samples with different Ti contents by adsorption microcalorimetry of various probe molecules. The presence of acid sites was also confirmed by infrared spectroscopy. 

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... 

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. 

Microcalorimetric NH3 adsorption is one of the powerful techniques for energetic characterization of solid surfaces and provides a direct and accurate method for the quantitative determination of the number of acid sites of different strengths. Microcalorimetry invplves the measurement of differential heats evolved updn adsorption of smeill quantities (micromoles) of basic probe molecules on to the catalysts. Such measurement yields information about the acid strength distribution i.e., the number of sites having the particular heat of adsorption for the basic probe molecule. 

In the present work the behaviour of zirconia samples doped with oxides of alkali metals and alkaline-earth metals was investigated, in order to better understand the role of both the nature and the amount of the doping cation. Li-, K-, Ca-, and Ba-doped zirconia samples were prepared. Their surface acid-base properties were assessed by means of adsorption microcalorimetry, using ammonia and carbon dioxide as probe molecules. Their catalytic activity for the 4-methylpentan-2-ol dehydration was tested in a flow microreactor. 

The present paper highlights the influence of molecular sized micropores on the ordering of the adsorbed phase within AIPO4-11. A range of simple probe molecules was used including Ar, Kr, CH4, O2, N2 and CO. Their adsorption properties were studied by adsorption microcalorimetry at 77 K and 87 K as well as by neutron scattering measurements in the temperature range from 20 to 100 K. 

It is often difficult to determine the nature of the adsorbed species, or even to distinguish between the different kinds of adsorbed species from the calorimetric data. In many cases this technique fails to distinguish between cations md protonic sites due to the insufficient selectivity of the adsorption. For example, the differential heats of NH3 adsorption on strong Lewis centres and strong Brdnsted sites are relatively close to each other. This can make it difficult in some cases to discriminate Lewis and Bronsted sites solely by adsorption microcalorimetry of basic probe molecules if no complementary techniques are used. Because no exact information can be obtained regarding the nature of the acid centres from the calorimetric measurements, suitable IR, MAS NMR, and/or XPS [36] investigations are necessary to identify these sites. However,... 

In the determination of acidity by microcalorimetry, several factors play an important role, such as the adsorption temperature, the pretreatment temperature and the choice of the probe molecule. O er factors, more specific to zeolites, are the topology, the Si/Al ratio, the chemical composition, and the modifications to which the samples have been subjected. The shape of the differential heat curves versus coverage demonstrates quite explicitly that the number, the reactivity and the distribution of surface sites are significantly modified when the composition, or the pretreatment, of the samples are changed. However, the two main factors are the zeolite structure and the fi amework aluminum content. Most of the studies described herein are summarized as reference tables in a review by Cardona-Martinez et al. [6] or fiilly detailed in [4]. 

The most promising approach to this problem is the use of suitable probe molecules for the quantitative characterization of site density and strength by means of adsorption microcalorimetry. The best-known appUcations of chemisorption involve the use of bases such as NH3 or pyridine to probe the acidity of zeoUtes. Moreover, it is well known that adsorption influences all phenomena depending on surface properties, since it constitutes the primary step for every catalytic reaction involving sohd catalysts. Adsorption is generally exothermic (AH < 0) and the heat evolved is called heat of adsorption. This heat is related to the ability of the sites to interact with the probe molecule, i.e. to their basic or acidic character. 

Another interesting comparison of various thermal analysis techniques, namely adsorption microcalorimetry, thermoprogrammed desorption, and thermoprogrammed reaction using constant rate thermal analysis (CRTA), has been performed by Fesenko et al. in order to study the reactivity of zeolites in terms of the adsorption or desorption of base probe molecules [24]. As an example, CRTA was applied to the desorption of isopropylamine from Na-Y zeolite and its acidic form HY. 

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