CeO2 nanowires with high aspect ratio and excellent catalytic activity for selective oxidation of styrene by molecular oxygen

CeO2 is a most promising oxidation catalyst and its superior oxidation performance is highly dependent on the extent of its Ce4+/Ce3+ redox cycle, shape, surface area and surface structure. Herein, a simple, efficient and aqueous solution based hydrothermal synthetic route for uniform CeO2 nanowires (NWs), with high aspect ratio and surface area, using an aqueous solution of cerium ammonium carbonate complex as precursor and poly(ethylene glycol) (PEG) as structure directing agent, is described. Cobalt incorporated CeO2 NW (Co-CeO2) were also synthesized by impregnation followed by calcination. Structural and morphological characterization by XRD, SEM and TEM showed that synthesized CeO2 NWs are of cubic fluorite crystal structure, with approximately 7 +/- 2 nm width and several micrometers in length, bundled, grown through the (110) surface keeping the active (100) surface exposed. XPS and TPD analysis revealed the presence of both Ce3+ and Ce4+ with higher amount of Ce3+ as well as Co2+ and Co3+ species. The amount of PEG is crucial for the synthesis of uniform CeO2 wires and other varying shapes. A probable formation mechanism of wires through the (110) surface is proposed. Synthesized CeO2 shapes were employed as catalyst for selective oxidation of styrene to styrene oxide using molecular oxygen as oxidant. Shape selective catalytic studies revealed that the synthesized Co-CeO2 NWs showed excellent catalytic activity. Kinetic study revealed that the oxidation reaction followed the Langmuir-Hinshelwood model. The synthesized CeO2 NW catalysts are recyclable with no significant loss in catalytic activity in subsequent cycles.