Preparation and structural characterization of ZnO and CeO2 nanocomposite powders as ‘active catalytic supports’

Cerium(IV) and zinc(II) oxide nanocomposite powders (Zn-to-Ce atomic ratio between 0.5 and 2) as well as the pure oxides were prepared by co-precipitation of cerium(III) and zinc(II) cations with oxalate (OC) or carbonate anions (CC), as active catalytic supports for the steam reforming of methanol (SRM) reaction. After the study of the decomposition of the precipitates in oxidizing atmosphere by thermogravimetry, differential scanning calorimetry, temperature-programmed oxidation (using infrared and mass spectrometry) and X-ray diffraction, a global reaction pathway for the decomposition of each type of precipitates was proposed. The nanostructural characterization of the calcined composites at 723 K was followed by refined XRD analysis. It was possible to conclude that Zn(II) was not incorporated into the ceria lattice in any case. The size of the CeO2 crystals was rather constant (~ 10–15 nm) in all the ZnO–CeO2 nanocomposite powders, just as in the pure ceria (13–14 nm). Instead, the crystalline domains of ZnO were smaller in the nanocomposites than in pure zinc oxide (31–78 vs. 103–118 nm, respectively), and even smaller in the CC than in the OC materials (31–38 vs. 57–78 nm for CC vs. OC, respectively). Upon the incorporation of palladium to the oxides, to provide the metallic function on the catalysts, the carbonate-derived composites showed a better catalytic performance for the SRM reaction. Said improvement (in terms of percent selectivity to CO2) could be attributed to the improved dispersion of zinc oxide achieved on the CC nanocomposites.