Structural properties and reduction behavior of novel nanostructured pd/gadolinia-doped ceria catalysts with tubular morphology

Abstract

In this work, nanostructured 1 wt % Pd/Gd2O3-CeO 2 (Pd/GDC) mixed oxide tubes with 90 mol % CeO2 were synthesized following a very simple, high yield procedure, and their properties were characterized by synchrotron radiation XRD, by X-ray absorption near-edge spectroscopy (XANES), and by scanning and high resolution electron microscopy (SEM and HRTEM). In situ XANES experiments were carried out under reducing conditions to investigate the reduction behavior of these novel tubular materials. The nanostructured mixed oxide tubes were found to have a cubic crystal phase (Fm3m space group) and large specific surface area (∼83 m 2 g-1). The tube walls were composed of nanoparticles with an average crystallite size of about 8 nm. The SEM and HRTEM results showed that individual tubes were composed of a curved sheet of these nanoparticles. Elemental analysis showed that Ce/Gd/Pd ratios appeared to be constant across space, suggesting compositional homogeneity in the samples. XANES results indicated that the extent of reduction of these materials was small. Nevertheless, the Ce3+ state was detected. These results suggest that Pd cations, most likely Pd2+, formed a Pd-Ce-Gd oxide solid solution and that the Pd2+ was stabilized against reduction in this phase. However, incorporation of the Pd (1 wt %) into the crystal lattice of the nanostructured tubes also appeared to destabilize Ce4+, favoring reduction to Ce3+ and giving rise to a significant increase in the reducibility of this material. Preliminary catalytic studies over Pd/GDC samples exhibited an improved catalytic activity toward CH4 oxidation compared to pure GDC (either nanotubes or nanopowders).