Influence of the surface adsorption-desorption processes on the ignition curves of volatile organic compounds (VOCs) complete oxidation over supported catalysts

The simplicity to obtain ignition curves has resulted in heir extended use for the measurement of the catalysts activity in the complete oxidation of VOCs, However, we have found that this method gives incorrect results for some systems, such as conversions greater than 100% or disagreements between the conversion values calculated through the concentration change of the reactants and the products, which can lead to misunderstandings. Toluene oxidation over reduced Pd/Al2O3 and acetone oxidation over supported Mn2O3 have presented those features. The study of these systems has been carried out performing DRIFTS spectra under reaction conditions and TPD experiments. The effect of the start-up procedure of the complete oxidation reactions has been considered as well. After discarding the formation of partial oxidation products and the hydrogen retention on Pd as possible causes, the adsorption-desorption processes of VOCs, surface intermediates and CO2 were considered. It can be concluded that when the activity of the active phase supported on the Al2O3 is high enough to allow the coupling at relatively low temperature of combustion with the adsorption and desorption processes of toluene or acetone and CO2, a sort of chain reaction occurs leading to CO2 peaks. (C) 2000 Elsevier Science B.V. All rights reserved.