Adsorption microcalorimetry pressure

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. 

The aim of this paper is therefore to check the consistency of three basic techniques (adsorption manometry, adsorption gravimetry and adsorption microcalorimetry) in the 0-50 bar pressure range, using a standard NaX zeolite adsorbent and selecting three adsorbable gases, namely ... 

Flow adsorption microcalorimetry has now been developed to a point at which it provides accurate and reliable adsorption and desorption data for events occurring at solid-liquid and solid-gas interfaces within a wide range of temperatures, pressures, and/or solution concentrations. 

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. 

Li and Na zeolites presented much higher heats of NH3 adsorption and greater coverage at the same pressure in comparison with the other zeolites. The acid-base properties of alkali-metal ion exchanged X and Y zeolites have also been investigated by ammonia and sulphur dioxide adsorption microcalorimetry, in parallel with the study of a catalytic reaction, viz. 4-methylpentan-2-ol conversion [111]. 

Outgassing for adsorption microcalorimetry and thermal analysis was carried out by controlled transformation rate thermal analysis (CTRTA) (ref. 6). Its interest is both the rather high resolution achieved on the thermal analysis curves (ref. 7) and the possibility of carrying out the experiment directly with the sample bulbs needed for adsorption microcalorimetry. The experimental conditions selected were a sample mass of about 0.260 g, a residual pressure of 2 Pa over the sample and a dehydration rate of 2.77 mg/h. 

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). 

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... 

For strongly chemisorbed species it can be difficult to obtain equilibrium uptakes below saturation at usual temperatures and measurable pressures (the adsorption isotherms rise very steeply with pressure to the monolayer coverage). Furthermore, chemisorption may be thermally activated, resulting in very long equilibration times. For these reasons other approaches are required to measure the thermodynamics of chemisorption. The two main ways are microcalorimetry and thermal desorption. 

An experiment of adsorption from the gas-phase, performed in microcalorimeter coupled with volumetric line can give a profile of Qdi/ versus the amount adsorbed, integral heats of adsorption, adsorption isotherms (adsorbed amounts vs. equilibrium pressure) and irreversibly absorbed amount of a chemisorbed gas the same stands for the adsorption from the liquid-phase, where the adsorbate (titrant) is added to both sample and reference ceUs simultaneously. The profile of differential heats versus the uptake of probe gives the data concCTning the amount, strength and distribution of the active sites. Besides, the values of initial heats of adsorption characterize the strongest sites active in adsorption process. For the sake of acidic/basic characterization of solids surface, the most commonly used gas-phase probes are ammonia, pyridine or some amines for the interaction with acidic sites. SO2 and CO2 are the probes used to notice and characterize the basic sites. In microporous solids, the accessibility of active sites is not the same for the molecules of different sizes. Therefore, many different probes can be applied to study acidity or basicity of same solid materials this approach brings additional information. For example, acidity of zeolites can be characterized by adsorption of ammonia, but also by adsorption of pyridine (from the gas phase) and aniline (from the liquid phase) [20-22], Liquid microcalorimetry can be also used for the determination of acidic character of solid adsorbent the common liquid-phase probe is aniline dissolved in n-decane [40]. 

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