Adsorption microcalorimetry types

FIGURE 13.6 Types of generalized thermograms obtained in isothermal adsorption microcalorimetry. (From Solinas, V. and Ferino, 1., Catal. Today, 41, 179-89, 1998 and Andersen, P. J. and Kung, H. H., Catalysis, 11, 441-66, 1994. With permission.)... 

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]. 

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. 

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,... 

The passivation by oxygen of a commercial ammonia synthesis catalyst was studied with adsorption microcalorimetry by Tsarev and co-workers (240). Two types of adsorbed oxygen at 293 K were found to participate in the formation of a passivating layer. One type was characterized by differential heats of adsorption near 420 kJ mol" that were close to the heat of iron oxidation and which were independent of surface coverage for several mono-layers. The other form was obtained after a large dose, sufficient for coverage of the entire metal surface with a molecular monolayer. Subsequent adsorp-... 

Chemisorption of gaseous bases, e.g., ammonia or pyridine, followed by adsorption microcalorimetry, FTIR, and/or TPD, can determine the concentration, strength, and type of surface acid sites. 

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. 

Abstract We review the use of adsorption microcalorimetry for the determination of the surface acidity and basicity of various types of zeolites and related materials, as well as the relationship between the information gathered by this technique and catalytic activity. 

A sample of mordenite (98% degree of ammonium-ion exchange) deam-moniated at various temperatures from 693 K to 923 K was studied at 573 K by NH3 adsorption microcalorimetry by Bankos et al. [203]. On increasing the pretreatment temperature, the number of acid sites passed through a maximum at 753 K as a result of simultaneous decationation and dehydroxylation. The heat of adsorption of NH3 on Bronsted acid sites formed by decationation was 110 -160 kJ mol During dehydroxylation, two types of Lewis sites were formed, characterized by heats of NH3 adsorption of 170-185kjmol and 95-100 kJ mol respectively, and on which dissociative chemisorption of ammonia was evidenced by IR [101]. 

Crystalline MFI-type indosilicates containing indium ions in framework positions were hydrothermally synthesized by Vorbeck et al. [246] and characterized by adsorption microcalorimetry of NH3 at 423 K (after activation of the sample at 670 K). They exhibited rather weak acidity attributed to Br5n-sted sites. However, the number of acid sites with a sorption heat between 80 and 110 kj mol was significantly higher than for silicalite [246]. 

This survey has demonstrated that adsorption microcalorimetry may be used successfully in order to probe add site strengths, to obtain the relative populations of BrOnsted and Lewis acid sites, and to determine their number quite exactly. However, a closer correlation between site strength and site type (preferably by combining microcalorimetry with IR spectroscopy) would result in a clearer description of the acidity of zeolites. 

In these literature reports [43-58], the data obtained by adsorption microcalorimetry are considered together with those obtained from complementary techniques (i.e. infrared spectroscopy, temperature programmed desorption, X-ray photoelectron spectroscopy) in order to elucidate the influence of loading and dispersion of Pt, type of support material and the reduction temperature, on energetics and mechanism of CO adsorption on supported Pt catalysts for a better understanding of their catalytic performances. 

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]). 

The adsorption up to 50 bars was carried out by means of a Tian-Calvet type isothermal microcalorimeter built in the former CNRS Centre for Thermodynamics and Microcalorimetry. For these experiments, around 2 g of sample was used which were outgassed by Controlled Rate Thermal Analysis (CRTA) [7]. The experiments were carried out at 30°C (303 K). Approximately 6 hours is required after introduction of the sample cell into the thermopile for the system to be within 1/100 of a degree Celsius. At this point the baseline recording is taken for 20 minutes. After this thermal equilibrium was attained, a point by point adsorptive dosing procedure was used. Equilibrium was considered attained when the thermal flow measured on adsorption by the calorimeter returned to the base line. For each point the thermal flow and the equilibrium pressure (by means of a 0-70 bar MKS pressure transdueer providing a sensitivity of 0.5% of the measured value) were recorded. The area under the peak in the thermal flow, Q eas, is measured to determine the pseudo-differential... 

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. 

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]. 

A good description of the ammonia storage and desorption is critical in order to describe transient features of the SCR system, and usually a Temkin type of kinetics is used that considers the adsorbate-adsorbate interactions. The parameters for these reactions are usually fitted to TPD experiments, but also microcalorimetry studies are presented. The most common approach is to consider one ammonia adsorption site, but more detailed kinetic models use several adsorption sites. Ammonia oxidation is a reaction occurring at high temperatures, which unfortunately decreases the selectivity of the NOx reduction in SCR. It is therefore crucial to include this reaction in kinetic models for this system. 

Silicalite is a microporous crystalline silica, i.e., an aluminium-free zeolite, belonging to the MFI-type structure and being the Si/Al oo limit of the ZSM-5 zeohte. The nature, population, and acidic properties of the hy-droxylated species (hydroxyl nests) present in the nanocavities of variously prepared defective silicahtes have been characterized by adsorption of NH3 at room temperature, monitored through the combined use of microcalorimetry and IR spectroscopy [196]. It was foimd that a perfect sihcahte sample eidiibits a very low activity towards NH3, confirming the almost complete absence of defects. The energetics of the interaction indicated that the (mild) acidity of silanols increases as far as the extension of the silanol patches increases. 

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. 

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