Abstract
The mechanism of the total oxidation of propane over alumina supported CuO, CeO 2, and CuO–CeO 2 is studied by means of Temporal Analysis of Products in the temperature range 623–873 K. A reaction scheme is proposed for the total oxidation of propane with O 2, as well as for the separate reduction and oxidation steps. For the reduction of the catalyst with propane, four elementary steps are considered as kinetically significant: (1) reversible associative propane sorption, (2) irreversible dissociative propane adsorption involving a methylene C–H bond breaking and ultimately forming CO 2 *,s, (3) desorption of CO 2 *,s to CO 2, and (4) dissociation of CO 2 *,s to CO *,s and O *,s, and recombination of CO *,s and O *,s to CO 2 *,s. For the oxidation of the catalyst with O 2, two elementary steps are considered as kinetically significant: (1) reversible dissociative adsorption of O 2 on two reduced active sites and (2) diffusion of lattice O atoms from the surface to the bulk and vice versa. An adequate description of the full mechanism, i.e., in the presence of propane and O 2, can only be obtained by considering additional steps, which distinguish between lattice oxygen atoms at the surface, O *,s, and weakly bound oxygen atoms, O weak. Apart from estimating the different kinetic parameters, a new approach to determine the initial concentration of reduced active sites is presented. CuO–CeO 2/γ-Al 2O 3 is a more efficient total oxidation catalyst than the corresponding single metal oxides. The activation energy for the first C–H bond activation in propane over the CuO–CeO 2/γ-Al 2O 3 amounts to 62 kJ mol −1 and is significantly lower than the activation energies over the single metal oxides, i.e., 95 kJ mol −1 for CuO/θ-Al 2O 3 and 126 kJ mol −1 for CeO 2/γ-Al 2O 3. The redox activity of CuO–CeO 2/γ-Al 2O 3 is created by the ability to reduce and re-oxidize both CuO and CeO 2, which is enhanced by a strong interaction between these phases.
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