| AVS 66th International Symposium & Exhibition | |
| Fundamental Discoveries in Heterogeneous Catalysis Focus Topic | Thursday Sessions |
| Session HC+SS+TL-ThA |
| Session: | Reaction Pathways and Addressing Challenges for Energy Production in the 21st Century & Heterogeneous Catalysis Graduate Student Award Presentation |
| Presenter: | Christopher Lee, University of Florida |
| Authors: | C.L. Lee, University of Florida J.F. Weaver, University of Florida |
| Correspondent: | Click to Email |
The terbium oxides, a member of the rare earth oxide family, exhibit favorable properties in selective oxidation catalysis due to the high mobility of oxygen stored and released within the lattice. Of particular note is the ease of structural rearrangement into highly stable, well-ordered intermediates between the Tb2O3 and TbO2 stoichiometries in addition to a continuum of nonstoichiometric states. As opposed to ceria, which stabilizes strongly in the CeO2 stoichiometry, thin film terbia is very stable in the Tb2O3 stoichiometry and can exist in an oxygen deficient cubic fluorite arrangement (CF-Tb2O3) as well as the bixbyite structure (c-Tb2O3).
We discovered a redox-mediated mechanism for the transformation of thin film CF-Tb2O3(111)/Pt(111) to c-Tb2O3(111)/Pt(111) in ultrahigh vacuum (UHV). Low energy electron diffraction (LEED) and temperature programmed desorption (TPD) shows that repeated oxidation and thermal reduction to 1000 K transforms an oxygen deficient cubic fluorite Tb2O3(111) thin film to the well-defined bixbyite, or c-Tb2O3(111) structure. In addition, TPD measurements show the development of several distinct O2 desorption peaks arising from the oxidation of c-Tb2O3 domains to the stoichiometrically-invariant ι-Tb7O12 and δ-Tb11O20 phases and demonstrates the more facile oxidation of c-Tb2O3 relative to CF-Tb2O3. We present evidence that nucleation and growth of c-Tb2O3 domains occurs at the buried TbOx/CF-Tb2O3 interface, and that conversion of the interfacial CF-Tb2O3 to bixbyite takes place mainly during thermal reduction of TbOx above ~900 K and causes newly-formed c-Tb2O3 to advance deeper into the film. The avoidance of low Tb oxidation states may facilitate the CF to bixbyite transformation via this redox-mechanism.
Further oxidation of a well-ordered c-Tb2O3 film provides evidence of the sequential phase stabilization of ι-Tb7O12, δ-Tb11O20, and α-TbO2-x stoichiometric structures along with lower temperature peaks corresponding with more weakly-bound surface oxygen. Oxidation at temperatures between 300-500 K reveals an apparent Arrhenius activation barrier of ~7.4 kJ/mol for the initial conversion of c-Tb2O3 to ι-Tb7O12. Furthermore, oxidation at 100 K creates an additional oxygen species stable at lower temperatures that has a much more pronounced effect on oxidation of the film surface over the bulk of the film . The ability to control the surface termination of the TbOx(111) thin films along with selectively creating surface bound oxygen species provides the structural basis necessary to clarify the partial oxidation mechanisms associated with terbia-based catalysis.