CeO2nanowires with high aspect ratio and excellent catalytic activity for selective oxidation of styrene by molecular oxygen

Abstract

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.