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Abstract  

The condensation approximation (CA) and numerical regularization procedure (RP) methods used to solve a Fredholm integral equation of the first kind describing the adsorption equilibria on a heterogeneous solid surface under isothermal conditions have been adopted in the present study to evaluate desorption energy distributions from temperature-programmed desorption (TPD) spectra. From comparisons of the computational results obtained by means of these methods on the basis of simulated TPD spectra, it follows that the CA gives stable solutions for wide desorption energy distributions and it can be used successfully for calculations from wide and clear resolved peaks in the TPD spectra. The use of the RP is more advantageous for acquisition of the distributions from closely related narrow peaks in the TPD spectra.

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Abstract  

We have investigated the interaction of water with Na+-ion exchanged zeolites of different structures (LTA, FAU, ERI, MOR and MFI) by means of temperature-programmed desorption (TPD). The non-isothermal desorption of water shows, depending on the zeolite type, differently structured desorption profiles. In every case the profiles have, however, two main ranges. Using a regularization method, desorption energy distribution functions have been calculated. The desorption energy distributions between 42–60 kJ mol−1, which can be attributed to a non-specific interaction of water, show two clearly distinguished energy ranges. The water desorption behaviour of this range correlates with the electronegativity of the zeolites and the average charge of the lattice oxygen atoms calculated by means of the electronegativity equalization method (EEM). The part of the desorption energy distributions in the range of 60–90 kJ mol−1, reflecting interactions of water with Na+ cations, shows two more or less pronounced maxima. In agreement with vibrational spectroscopic studies in the far infrared region, it may be concluded that all samples under study possess at least two different cation sites.

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Abstract  

We have investigated the interaction of a few 5-ring organic compounds (cyclopentane, cyclopentene, furan, 2-methylfuran, 2,5-dihydrofuran and tetrahydrofuran) with alkali-metal cation-exchanged faujasites (LSX, X and Y types) by means of temperature-programmed desorption (TPD). The desorption behavior at higher temperatures of all probe molecules on the sodium ion containing faujasites with different Si/Al ratios reflects the higher cation content of zeolites with greater aluminum content. Only the desorption profiles of tetrahydrofuran and 2,5-dihydrofuran show, depending on the kind of cation, additional desorption features at higher temperatures. Using a regularization method, desorption energy distribution functions for furan and tetrahydrofuran were calculated. The calculated desorption energy distributions clearly illustrate the very different adsorption behavior of furan and tetrahydrofuran which leads to large differences in the binding energies between the corresponding adsorption complexes.

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Abstract  

Uniformly carbon-covered alumina (CCA) was prepared via the carbonisation of sucrose highly dispersed on the alumina surface. Using special thermogravimetry and sorptometry methods physicochemical properties of carbon-covered alumina surfaces were investigated. A numerical and analytical procedure for the evaluation of total heterogeneous properties (desorption energy distribution and pore-size distribution functions) on the basis of liquid thermodesorption from the sample surfaces under the quasi-equilibrium conditions are presented. The desorption energy distribution was derived from the mass loss Q-TG and the differential mass loss Q-DTG curves of thermodesorption of pre-adsorbed polar and apolar liquid films. For the first time, the evaluation of the fractal dimensions of carbon-covered alumina using the sorptometry, thermogravimetry and AFM data is presented.

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Abstract  

Physico-chemical properties (adsorption capacity, desorption energy distribution and pore-size distribution functions) of nanomaterial surfaces from selected materials, based on sorptometric and liquid thermodesorption measurements under quasi-equilibrium conditions, are presented. The fractal dimensions of nanotubes using sorptometric and AFM data have been evaluated. Comparison of thermogravimetric and other data provide new information about the adsorption and pore structure of the studied materials. The fractal dimensions of nanomaterial surfaces using sorptometry are in good agreement with those from AFM.

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Journal of Thermal Analysis and Calorimetry
Authors: B. Hunger, M. v. Szombathely, J. Hoffmann, and P. Bräuer

Abstract  

Desorption energy distributions were calculated for temperature-programmed desorption (TPD) of ammonia from H zeolites of different type by means of regularization. This method does not require any limiting assumptions about the distribution function. It could be shown that the desorption energy distributions obtained are nearly independent of the experimental conditions and therefore they should represent a suitable measure for the distribution of the strength of acidic sites. The calculated desorption energy distributions for the ammonia desorption from the isolated bridging SiOHAl groups of H zeolites of different type significantly differ from each other in shape. The increase of the desorption energy of the main range of the distribution functions correlates well with the increase of the average acid strength of the SiOHAl groups with decreasing Al content of the zeolites.

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Abstract  

The paper presents physico-chemical properties of mixed adsorbents in the clinoptylolite (mordenite)/SiO2 system containing 30, 50, 80 mass% zeolite. Adsorption capacity towards polar (water, butanol) and non-polar (n-octane) substances as well as total surface heterogeneity (energetic and geometrical) were determined. Desorption energy distribution functions as well as fractal dimensions were also determined and compared with the low-temperature nitrogen adsorption data. Irregular shapes of the curves q=f(E d) as well as large values of volumetric fractal dimensions (D f~2.6) revealed heterogeneous properties of the zeolite/SiO2 system surfaces. Addition of zeolite increases total heterogeneity of the material.

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Abstract  

Using thermo-analytical and sorptometric methods physicochemical properties and especially surface heterogeneity of HgBa2Ca2Cu3O8+, (Hg-1223) was investigated. The desorption energy distribution was derived from mass loss Q-TG and differential mass loss Q-DTG curves of thermodesorption in quasi-isothermal conditions of pre-adsorbed n-octane and water vapour. It is shown that the superconducting Hg-1223 phase is highly sensitive to water vapours. The mechanism of water adsorption depends largely on the activation time. By water vapour saturation in a period of 90 min, physisorption takes place. Prolonged periods result in a chemical decomposition. From nitrogen ad- and desorption isotherms the fractal dimension of superconductors were calculated. A new approach is proposed to calculate fractal dimension from Q-TG curves.

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Abstract  

A simple method has been adopted to obtain an important information on the heterogeneous properties of materials studied, adsorbed liquid films and liquid-solid interactions. The method utilizes Q-TG mass loss and the first derivative Q-DTG mass loss curves with respect to temperature and time obtained during programmed liquid thermodesorption in quasi-isothermal conditions. The values of the adsorption capacity, total porosity of material, the value of active centers, desorption energy distribution and mesopore-size distribution functions obtained by this method are in good agreement with those estimated on the basis of independent methods. The theoretical and experimental results provided novel and unique data on the heterogeneity of solid surfaces, properties of liquid adsorbed films and thermal stability of the liquid/solid interfaces. This paper presents the more important results obtained so far in the studies the liquid/solid systems by means of the Paulik's classical and quasi-isothermal techniques.

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Summary Using temperature-programmed desorption (TPD), we have investigated the interaction of carbon dioxide with alkali-metal cation-exchanged faujasite type zeolites (LSX, X and Y). TPD in the temperature range between 300 and 500 K results in desorption profiles of different intensities depending on the kind of cation and the aluminium content of zeolites. For NaX the desorbed amount corresponds to about one percent of the saturation capacity at 298 K. In case of NaX and X type zeolites exchanged with Cs+ ions an additional desorption peak above 500 K could be observed. Taking into account desorption curves of different heating rates, desorption energy distribution functions were calculated by using an extended integral equation. Initial adsorbed CO2 could be assigned to carbonate species in different environments by DRIFT spectroscopy.

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