On the examples of the temperature programmed desorption of water from a NaA and a 0.45 NiNaX zeolite, it is shown that, through the use of non-linear temperature programmes with increasing heating rate, the individual desorption steps at higher temperatures of a complex desorption process are better recognizable, or can be distinguished almost completely from desorption at lower temperatures. This type of temperature processing therefore offers a suitable means of improving the finding on complex desorption processes relating to porous catalysts and other systems.
TPD studies on the kinetics of deammoniation of an NH4NaY zeolite showed that the use of a hyperbolic temperature programme led to kinetic parameters agreeing with those resulting from a linear heating process. Because of the progressive increase of the heating rate in the case of hyperbolic heating schedules, the parameters can be considered as independent of the heating rate within certain limits. The better resolution of complex desorption spectra with hyperbolic programmes is an additional reason for their use.
Curves of the temperature-programmed desorption (TPD) of ammonia from zeolites were evaluated with several kinetic models. An approximately linear correlation was found between the activation energy of desorption or the heat of adsorption of H zeolites with various Si/Al ratios and the intermediate electronegativity of the zeolites, the latter representing a measure of the acid strength.
We have investigated the temperature-programmed desorption (TPD) of ammonia during the activation of NH4Na-mordenites of different
exchange degrees. Using a regularization method, desorption energy distribution functions have been calculated. The obtained
results indicate the heterogeneity of the bridging Si-OH-Al groups in HNa-mordenites. This was concluded from the width of
the distribution functions and from the presence of submaxima. For HNa-mordenites of exchange degrees below 50%, containing
only hydroxyls in the broad channels, two distinct submaxima are present, thus suggesting the presence of at least two kinds
of bridging hydroxyls of various acid strengths. In HNa-mordenites of exchange degrees above 50%, the hydroxyls appear in
narrow channels and the distribution of ammonia desorption energy broadens on the side of higher energies. This may be related
to a strong stabilization of ammonium ions inside narrow channels. The maximum concentrations of hydroxyls of desorption energies
between 95 and 135 kJ mol-1 and between 135 and 165 kJ mol-1 calculated from TPD data were 3.9 and 3.3 OH per unit cell (u.c.). These values agree well with our previous IR results of
concentrations of hydroxyls in broad and in narrow channels (3.7 and 2.8 OH per u.c.). The TPD data obtained for the heterogeneity
of OH groups in HNa-mordenites are in accordance with the IR data concerning ammonia desorption. The IR band of OH groups
restoring upon saturation of all the hydroxyls with ammonia and subsequent step-by-step desorption at increasing temperatures
shifts to lower frequencies indicating that there are hydroxyls of various acid strengths and the less acidic hydroxyls restore
first at lower desorption temperatures.
By means of model calculations it could be shown for an irreversible surface reaction of 1st order that the determination of the activation energy of the desorption of the reactant or, respectively, of the surface reaction is possible by application of the method of variation of the heating rate to the desorption curve of the reactant, according to circumstances whether the ratio of the activation energy of the surface reaction and of the desorption of the reactant is greater or smaller than one.
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.
On the basis of calculations using a simple model of the energetic heterogeneity of a solid surface (assuming linear dependence
of activation energy of desorption of the reactant on the degree of coverage), it is shown that both the degree of conversion
and the course of desorption of the reactants are strongly influenced by the degree of heterogeneity assuming non-isothermal
conditions. In contrast to a homogeneous solid surface, the degree of conversion for a heterogeneous surface depends strongly
on the initial coverage of a catalyst by reactant. Possibilities for kinetic evaluation are indicated from the modelling calculations.
Through simulation of TPD model curves for desorption from adsorption sites of different strength could be shown that the use of non-linear temperature programmes influences the complex desorption process in such a manner that the appearing desorption maxima are better visible.
Authors:B. Hunger, J. Hoffmann, O. Heitzsch, and M. Hunger
The temperature-programmed desorption (tpd) of the amount of ammonia which is preadsorbed at about 373 K at HZSM-5 zeolites yields a complex desorption curve consisting of two overlapped peaks (Β andγ peak). Parts of the ammonia desorbed can be attributed to SiOHAl groups considering also1H-MAS NMR measurements.
Authors:Rita Sattler, W.-D. Einicke, and B. Hunger
Temperature-programmed desorption (TPD) of water was applied to characterize short-time dealuminated HZSM-5 zeolites. Using
a regularization method, distribution functions of the effective desorption energy of water were calculated. The results clearly
show that during dealumination a new adsorption site is formed which can be attributed to non-framework or transient aluminium
species. The highest concentration of these sites was observed for a dealumination time of 25-30 min. NO adsorption studies
support this result. Furthermore, it could be concluded that the heterogeneity and the average acid strength of the remaining
Si-OH-Al groups of the dealuminated samples do not change compared to the Si-OH-Al groups of the parent HZSM-5 zeolite.