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In recent years, the attention of scientists has focused on the use of zeolites as catalysts supports due to their thermal and acid stability and moisture resistance. They have high specific surface area which is a prerequisite for the creation of active catalytic systems. One of the best adsorbents and supports for catalytic systems used for the degradation of VOCs are zeolite aluminosilicates, thanks to their adjustable surface properties, their controllable hydrophobicity and the ability to deposit metals and metal oxides. The starting ZSM-5 materials with different Si/Al were used for the present study. Cobalt was incorporated from Co(CH3OO)2.4H2O by incipient wetness impregnation technique followed by drying and calcination in air 2 h at 450 ºC. The obtained materials were characterized by XRD, N2 adsorption-desorption, TPR, TPO and XPS. The formation of different cobalt oxide species would be suggested from TPR data: extra framework Co3O4; CoO strongly interacting with zeolite framework and framework cobalt or some other types of cobalt silicate. XPS data show that Co2+ predominate on the surface. The catalysts performance has been evaluated by tests on combustion of propane. In order to extend further the analysis of the different behaviour of the studied catalyst samples, an investigation on the kinetics and mechanism of the reaction has been carried out. The kinetic parameters were calculated by applying the method, consisting of a direct integration of the reaction rate based on data from light-off curves. As a first step a power law kinetic model has been fitted to the experimental data. In order to account for the inhibiting effect of the water vapour (both produced by the reaction or additionally added to the gas feed), the mechanistic models are modified, thus containing an additional term in the denominator, in order to take into account, the adsorption of water. Based on the values of the observed reaction orders, established by power – law kinetics model, the following mechanistic models are proposed for checking their consistence with the obtained experimental results: Mars−van Krevelen, water molecules compete with the propane molecules for the oxidized and reduced adsorption sites; effect of slow desorption of the products is included; Langmuir−Hinshelwood, adsorption of propane and oxygen on different types of sites, dissociative adsorption of oxygen, water molecules compete with the methane and oxygen molecules for the corresponding type of adsorption sites; Eley−Rideal mechanism, propane molecules are impinging directly from gas phase, water molecules compete with the oxygen molecules for one and the same type of adsorption sites. It was concluded that the high catalytic activity of cobalt modified zeolites is explained by the high reducibility as result of lower interaction of the cobalt oxide withthe supports. The observed insignificant changes in the zeolite structure, morphology, oxide particle size and oxide phases after reaction reveal that the zeolite structure can prevents the agglomeration of the oxide particles, thus giving new opportunities for development of stable catalyst for prevention of greenhouse gas emissions. |
