Zinc doping of monoclinic ZrO₂ oxides to improve their redox properties as catalysts

Full text: A simple technique was used to dope monoclinic ZrO₂ oxides to form mixed oxides type Zn_y Zr_1-yO_1-x with 0.0, 0.4, 0.6 and 0.7 at% of Zn. The mixed oxide was used to enhance the redox properties of m-ZrO₂ as catalyst. The oxides were analyzed by X-ray diffraction (XRD), nitrogen physisorption (BET), H₂ temperature programmed reduction (H₂-TPR), infra-red spectroscopy (FTIR), electron paramagnetic resonance (EPR), CO₂ and NH₃ temperature programmed desorption (CO₂-TPD, NH₃-TPD). The oxides were also tested in the Reverse Water Gas Shift reaction (RGWS), which depends on the redox properties of the catalyst. Rietveld refinement of XRD reveals that the systems contain mainly monoclinic phase of zirconia (m-ZrO₂) with small amounts of tetragonal phase (t-ZrO₂) present even in the raw zirconia. The crystallite size of the particles was approximately 9 nm (LVol-IB) while the crystal cell volume decreases as the Zn concentration is increased. BET surface area of the samples remains constant after doping, approximately 90 cm2g^-1; H₂-TPR shows an increase of hydrogen consumption, i.e. reducibility, as at% of Zn increase in the samples; FTIR-ethanol desorption shows a reducibility improvement of m-ZrO₂ when Zn is added. An increase of vacancies concentration was confirmed by EPR; CO₂-TPD and NH₃- TPD show small changes in basicity and acidity after doping, respectively. The tests of the catalysts in the RWGS reaction show that the reagent consumption rate, r_CO2, greatly increases with the increasing of Zn in the oxides. Due to Zn ionic radii and its oxidation state, EPR and XRD refinement suggest that Zn atoms may have substitutionally diffused into m-ZrO₂ lattice, creating oxygen vacancies, which promote oxygen mobility to the surface and, thus, enhance its redox properties. (author). (author)