Kinetic analysis of temperature-programmed reduction:: Behavior of a CrOx/Al2O3 catalyst

The kinetic modeling of hydrogen reduction was studied with the use of experimental data obtained by temperature-programmed method. The reduction of a CrOx/gamma -Al2O3 catalyst prepared by atomic layer deposition (ALD) was used as the test reaction. Reduction kinetic models reported in the literature were evaluated by model estimation techniques. It was found that an experimental data set with three different heating rates was needed to discriminate between the models. The Avrami-Erofeyev model, which assumes the reaction to proceed via nucleation and nuclei growth, was found to be promising, and it was investigated in detail. Problems often arise in the application of this model to temperature-programmed reduction kinetics: (1) the physical meaning of the model and the parameters are often poorly defined, (2) the temperature dependencies of the rates are not correctly included, and (3) various limiting cases of the model are customarily used without consideration of the underlying assumptions. These problems were considered in this work, and revisions were made in the model. The reduction kinetics of the catalyst was best described with the model assuming instantaneous nucleation and two-dimensional nuclei growth.