The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
Aalto

Kinetic Analysis of Temperature-Programmed Reactions

Jaana Kanervo

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Chemical Technology for public examination and debate in Auditorium KE 2 (Komppa Auditorium) at Helsinki University of Technology (Espoo, Finland) on the 16th of October, 2003, at 12 o'clock noon.

Overview in PDF format (ISBN 951-22-6747-0)   [1437 KB]
Dissertation is also available in print (ISBN 951-22-6746-2)

Abstract

Temperature-programmed desorption (TPD), reduction (TPR) and oxidation (TPO) are thermoanalytical techniques for characterising chemical interactions between gaseous reactants and solid substances. The data collected by these techniques are commonly interpreted on a qualitative basis or by utilising simple, approximate kinetic methods. However, temperature-programmed techniques can also be regarded as transient response techniques and the experimental data can be utilised for dynamic modelling. This work comprises case studies on kinetic analysis of TPR, TPD and TPO related to the characterisation of heterogeneous catalysts. The emphasis is on methodological aspects and on assessing the potential of temperature-programmed data as a source of kinetic information.

Kinetic analysis was applied to the TPR results for series of alumina-supported chromium oxide and vanadium oxide catalysts. Hydrogen was used as the reducing agent. Different kinetic models were tested against the experimental data and parameters were estimated. The chromium oxide and vanadium oxide contents of the catalysts were clearly reflected in the reduction behaviour and in the best-fit kinetic models and their parameters. The kinetic results suggested that reduction takes place via a topochemical mechanism, as growing domains, on both supported chromium and supported vanadium oxide catalysts with close to monolayer content.

The interaction of hydrogen with a commercial nickel catalyst was studied in TPD experiments under continuous flow and ambient pressure. A model to account for the heterogeneity in the chemisorption interaction and for the readsorption was formulated and tested against experimental data. The heterogeneity was described by introducing a sufficient number of different adsorption states. The rapid readsorption occurring during TPD was taken into account by describing the intrinsic dynamics of an adsorption state as a quasi-equilibrium adsorption/desorption between the gas phase and the surface. A model with two adsorption states of hydrogen was able to describe the experimental data with physically acceptable parameters in the temperature range of 323-673 K.

The regeneration kinetics of a deactivated cracking catalyst was investigated on the basis of the experimental evolution rates of carbon monoxide and carbon dioxide during TPO. Different kinetic models were tested and kinetic parameters were estimated. A power-law kinetic expression with orders unity and 0.6 for coke and oxygen, respectively, was capable of describing the experimental data.

In each case study, a phenomenological model was established and the kinetic parameters of the model were determined via nonlinear regression analysis in MATLAB® environment. The results demonstrate that common catalyst characterisation data on reduction, desorption and oxidation collected in the temperature-programmed mode can fruitfully be subjected to detailed kinetic analysis. Mechanism and parameter identifiability require diversity in the experimental data, which can be achieved, for example, by applying multiple heating rates in experiments. Kinetic analysis extends the interpretability of temperature-programmed reactions in catalyst characterisation and it is potentially useful for the elucidation of fundamental reaction mechanistic information and establishing kinetic models for engineering applications.

This thesis consists of an overview and of the following 5 publications:

  1. Kanervo J. M. and Krause A. O. I., 2001. Kinetic analysis of temperature-programmed reduction: behavior of a CrOx/Al2O3 catalyst. The Journal of Physical Chemistry B 105, pages 9778-9784.
  2. Kanervo J. M. and Krause A. O. I., 2002. Characterisation of supported chromium oxide catalysts by kinetic analysis of H2-TPR data. Journal of Catalysis 207, pages 57-65.
  3. Kanervo J. M., Harlin M. E., Krause A. O. I. and Bañares M. A., 2003. Characterisation of alumina-supported vanadium oxide catalysts by kinetic analysis of H2-TPR data. Catalysis Today 78, pages 171-180.
  4. Kanervo J. M., Reinikainen K. M. and Krause A. O. I., Kinetic analysis of temperature-programmed desorption. Applied Catalysis A: General, in press.
  5. Kanervo J. M., Krause A. O. I., Aittamaa J. R., Hagelberg P., Lipiäinen K. J. T., Eilos I. H., Hiltunen J. S. and Niemi V. M., 2001. Kinetics of the regeneration of a cracking catalyst derived from TPO measurements. Chemical Engineering Science 56, pages 1221-1227.

Keywords: kinetic analysis, temperature-programmed reactions, kinetic modelling, heterogeneous catalysts, temperature-programmed desorption, temperature-programmed reduction, temperature-programmed oxidation

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© 2003 Helsinki University of Technology

Last update 2011-05-26