Showing papers by "Vladimir Galvita published in 2012"

Structural and Kinetic Study of the Reduction of CuO–CeO2/Al2O3 by Time-Resolved X-ray Diffraction

Abstract: The crystallographic structure of (11 wt.%)CuO–(6 wt.%)CeO2/γ-Al2O3 has been studied and compared with (11 wt.%)CuO/γ-Al2O3 under reducing conditions, using time-resolved in situ X-ray diffraction in the temperature range 25–800 °C. In CuO–CeO2/Al2O3, H2-TPR reduces the CuO phase to Cu, while in C3H8-TPR reduction follows a two-step pathway via Cu2O. A thermal treatment in He also induces reduction for CuO, albeit at higher temperature. In addition to CuO reduction, the CeO2 promoter in CuO–CeO2/Al2O3 is also partially reduced, without crystallographic transition, regardless of the atmosphere and at similar temperature where reduction of CuO occurs. Supported CuO as in CuO–CeO2/Al2O3 or CuO/Al2O3, is more readily reduced by thermal treatment in He than unsupported CuO and Cu2O. Moreover, the addition of CeO2 to the CuO–CeO2/Al2O3 catalyst allows for enhanced reducibility of CuO, compared to CuO/Al2O3. The CuO phase in CuO–CeO2/Al2O3 is reduced to Cu2O and partly to Cu at 700 °C and mainly to Cu at 800 °C in He flow. The thermal reduction of CuO–CeO2/Al2O3 requires an apparent activation energy of 216 kJ/mol. An isothermal reduction treatment at 800 oC in He reduces CuO–CeO2/Al2O3, as demonstrated by time-resolved in situ X-ray diffraction. Supported CuO are more easily reduced by thermal treatment compared to unsupported CuO and Cu2O. The CuO phase in CuO–CeO2/Al2O3 is reduced to Cu2O and partly to Cu at 700 °C and mainly to Cu at 800 °C in He flow (see figure)

Microkinetics for toluene total oxidation over CuO-CeO2/Al2O3

Abstract: Abstract Total oxidation of toluene over a CuO–CeO2/Al2O3 catalyst was studied by means of Temporal Analysis of Products (TAP) at temperatures of 723–873 K in the absence and presence of oxygen (molar toluene:oxygen ratio = 1:9) at degrees of reduction of the catalyst up to 0.42. A single set of kinetic parameters corresponding to the steps of the detailed mechanism can describe the experimental data. A detailed mechanism with oxidation occurring on Ce3+ sites and reduction on Cu2+ sites formed the basis of a microkinetic model. The interaction of reactants and products with the catalyst support was taken into account. A distinction was made between O atoms at the surface of the catalytically active phase and those in the bulk. Transport of the latter to the surface was explicitly accounted for in the model. The abstraction of hydrogen atoms leading to the formation of water is the fastest process. The formation of carbon dioxide occurs through three kinetically significant steps. The potentially slowest step in the whole process was found to be the destruction of the aromatic ring. A linear dependency of the activation energies of the processes which include transport of oxygen from the bulk to the catalyst surface, on the degree of reduction of the catalyst was found. Analysis of the dependency of the catalytic behavior on the catalyst descriptors provided information about the optimum catalyst composition and fraction of the active component exposed, which can be used for catalyst optimization.