Authors:Pavel Čičmanec, Roman Bulánek, and Karel Frolich
of NO x by hydrocarbons. The detailed localization of Cu + ions was widely studied by the CO probe molecule adsorption using the FTIR, EXAFS, EPR, UV–Vis, microcalorimetry, and TPD techniques [ 15 – 27 ] assuming the presence of several
Authors:V. Rakić, V. Dondur, R. Hercigonja, and V. Andrić
The active sites of hydrogen-exchanged Y zeolite (HY) and dealuminated (HDY) zeolites are investigated by TPD of carbon monoxide.
Only the high temperature TPD spectra of CO (TM620-690C) were observed, meaning that CO molecules interact with very strong
acid sites. The amounts of CO bonded on these sites are small (less than 1 molecule per unit cell). The strong influence of
dealumination on the coverage degree is found. The calculated values for kinetic parameters indicate chemisorption of CO in
the investigated systems (Edes240 kJ mol-1, A1011 s-1).
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.
) series catalyst by H 2 -TPD, NH 3 /H 2 -TPD and thermodynamic calculations. The results have proved that hydrogen spillover on the catalyst surface is related to the hydrogen of H 3 PW 12 O 40 , and finally the equation H + + ( n − 1)H H n + ( n = 2
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.
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.
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.
Authors:Marylin Betancourt, Liliana Giraldo, Sonia Moreno, R. Molina, and J. Moreno
Total acidity for a series of modified clays obtained from a natural vermiculite is determined through temperature programmed
desorption (TPD) using ammonia as probe molecule. Results obtained for the acidity range from 15.1 to 68.5 meq/100 g.
Immersion enthalpies of the clays in benzene, water and aqueous solutions of NH3 0.058 M and NaHCO3 0.053 M are determined. The results obtained show that immersion enthalpies in benzene and water are between −6.26 and −25.6
J g−1 and −2.10 and 5.55 J g−1, respectively and are smaller than the values obtained for the immersion enthalpies in the solutions.
Immersion enthalpy values in NH3 solution are greater than the obtained using NaHCO3. Linear relations between the total acidity of the clays and the immersion enthalpies in the basic solutions are determined.
An interaction factor using ammonia is calculated since the relation between the immersion enthalpy in ammonia solution and
in water and it may be deduced that the relation with the total acidity is of second order tendency between them.