Synthesis and Catalytic Activity of Nanostructured Cerium Oxide

Authors

Neil J. Lawrence, University of Nebraska-LincolnFollow

Date of this Version

Fall 12-1-2010

Document Type

Article

Comments

A THESIS Presented to the Faculty of The Graduate College of the University of Nebraska In Partial Fulfillment of the Requirements for The Degree of Master of Science, Major: Chemistry, Under the supervision of Professor Chin Li Cheung. Lincoln, Nebraska: December 2010
Copyright 2010 Neil J. Lawrence

Abstract

Cerium oxide (ceria, CeO_2-x where x is 0 to 0.5) has been one of the most widely used heterogeneous catalysts particularly in three way catalytic converters. Most of the catalytic traits can be attributed to two properties of ceria: first, the high mobility and storage capacity of oxygen within the lattice; second, the ease with which cerium changes between Ce³⁺ and Ce⁴⁺ states. These properties, combined with the abundance of cerium on earth, make ceria a low-cost highly effective alternative to noble metal catalysts. Recent research has been focused on the nanoscale properties of ceria.

The effect on the catalytic activity of cerium oxide caused by varying the density of oxygen vacancy defects (OVD) has not been previously studied experimentally. This is due to the perceived inability to engineer stable defects not attributed to the presence of dopant atoms. It was found that the number of stable OVDs on cerium oxide nanoparticles and nanotubes can be increased with annealing at elevated temperatures under low pressure. The oxidative catalytic activity of these nanostructured catalysts was evaluated. Samples with higher densities of OVD were found to have much lower light-off temperatures when compared to that of their bulk counterpart. The chemical equilibrium reactions on the catalysts surface under low pressure were hypothesized to explain the unusual increase in the OVD density of the reported cerium oxide nanostructured catalysts.

Cerium oxide is well known to exfoliate from the surface of cerium metal in the same way that rust exfoliates from iron or steel. A two-step process to fabricate nanoporous ceria membranes via anodization and subsequent calcinations is reported. These membranes have the potential to be used in solid oxide fuel cells and solid-state oxygen sensors. Cerium metal foil was first anodized into adherent porous cerium hydroxide film, followed by calcination for conversion into ceria membranes. These membranes are composed of ribbon-like structures that form the backbone of the porous framework. A proposed anodization model for the growth of the nanoribbons is discussed.

Adviser Chin Li Cheung