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On the equilibrium nature of thermodesorption processes. TPD-NH3 studies of surface acidity of Ni/MgO-Al2O3 catalysts.
MedLine Citation:
PMID:  16831004     Owner:  NLM     Status:  PubMed-not-MEDLINE    
This article deals with a quantitative analysis of thermodesorption spectra of ammonia: a technique commonly applied to study the surface acidity of solids. The method used for determination of adsorption energy distributions of ammonia is the same as that published recently for the case of hydrogen thermodesorption (Panczyk, T.; et al. Langmuir 2005, 21, 7311). The developed theoretical expression describing the thermodesorption process is based on the statistical rate theory (SRT) and its analysis leads to the conclusion that majority of thermodesorption processes, carried out under flow conditions, are in fact quasi-equilibrium ones. Similar conclusion has already been drawn by some authors applying the classical absolute rate theory (ART) for analysis of thermodesorption data. This conclusion has important practical consequences. Namely, it greatly simplifies the quantitative analysis of thermodesorption processes since there is no need to use any kinetic approaches to that purpose. The quantitative analysis of thermodesorption spectra can thus be based on commonly accepted relations following from equilibrium thermodynamics. It is worth noting that in quasiequilibrium conditions either the SRT or the ART lead to this same expression with only a slightly different meaning of some constants. Thus, in quasiequilibrium conditions there is no need to decide which theoretical approach should be applied. As an illustration, the ammonia thermodesorption spectra from the modified nickel catalysts are analyzed. The catalysts were prepared by the coprecipitation method and differ by the amount of MgO and NiO, whereas the amount of Al(2)O(3) is constant and equals 30%. It was stated that the presence of MgO reduces the number of acid centers corresponding to high values of ammonia adsorption energy.
Tomasz Panczyk; Wojciech Gac; Monika Panczyk; Tadeusz Borowiecki; Wladyslaw Rudzinski
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Publication Detail:
Type:  Journal Article    
Journal Detail:
Title:  Langmuir : the ACS journal of surfaces and colloids     Volume:  22     ISSN:  0743-7463     ISO Abbreviation:  Langmuir     Publication Date:  2006 Jul 
Date Detail:
Created Date:  2006-07-11     Completed Date:  2007-08-16     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  9882736     Medline TA:  Langmuir     Country:  United States    
Other Details:
Languages:  eng     Pagination:  6613-21     Citation Subset:  -    
Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, ul. Niezapominajek 8, 30239 Krakow, Poland.
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