Kinetic analysis of temperature-programmed reactions

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.