Journal of Thermal Analysis and Calorimetry
Volume/Issue:
Volume 53: Issue 1
Heterogeneity of OH Groups in H-Mordenites TPD and IR studies of ammonia desorption
Authors:
B. Hunger University of Leipzig Institute of Physical and Theoretical Chemistry D-04103 Leipzig Germany D-04103 Leipzig Germany

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J. Datka Jagiellonian University Faculty of Chemistry Ingardena 3 30-060 Cracow Poland Ingardena 3 30-060 Cracow Poland

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Pages:
217–225
Online Publication Date:
03 Nov 2004
Publication Date:
01 May 1998
Article Category:
Research Article
DOI:
https://doi.org/10.1023/a:1010144410870
Keywords:
desorption energy distribution functions; heterogeneity of Si-OH-Al groups in HNa-mordenites; infrared spectroscopic studies of ammonia desorption; temperature-programmed desorption (TPD) of ammonia
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Abstract  

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.

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Cover Journal of Thermal Analysis and Calorimetry
Journal of Thermal Analysis and Calorimetry
Print ISSN:
1388-6150
Online ISSN:
1572-8943

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Journal of Thermal Analysis and Calorimetry
Language English
Size A4
Year of
Foundation		 1969
Volumes
per Year		 1
Issues
per Year		 24
Founder Akadémiai Kiadó
Founder's
Address		 H-1117 Budapest, Hungary 1516 Budapest, PO Box 245.
Publisher Akadémiai Kiadó
Springer Nature Switzerland AG
Publisher's
Address		 H-1117 Budapest, Hungary 1516 Budapest, PO Box 245.
CH-6330 Cham, Switzerland Gewerbestrasse 11.
Responsible
Publisher		 Chief Executive Officer, Akadémiai Kiadó
ISSN 1388-6150 (Print)
ISSN 1588-2926 (Online)

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