Dynamics of the Interaction Between Ceria and Platinum During Redox Processes

Electronic Structure of Heteronuclear Cerium-Platinum Clusters

Beyond the now well-known strong catalyst-support interactions reported for ceria-supported platinum catalysts, intermetallic Ce-Pt compounds exhibit fascinating properties such as heavy fermion behavior and magnetic instability. Small heterometallic Ce-Pt clusters, which can provide insights into the local features that govern bulk phenomena, have been less explored. Herein, the anion photoelectron spectra of three small mixed Ce-Pt clusters, Ce2OPt-, Ce2Pt-, and Ce3Pt-, are presented and interpreted with supporting density functional theory calculations. The calculations, which are readily reconciled with the experimental spectra, suggest the presence of numerous close-lying spin states, including states in which the Ce 4f electrons are ferromagnetically coupled or antiferromagnetically coupled. The Pt center is consistently in a nominal -2 charge state in all cluster neutrals and anions, giving the Ce-Pt bond ionic character. Ce-Pt bonds are stronger than Ce-Ce bonds, and the O atom in Ce2OPt- coordinates only with the Ce centers. The energy of the singly occupied Ce-local 4f orbitals relative to the Pt-local orbitals changes with cluster composition. Discussion of the results includes potential implications for Ce-rich intermetallic materials.

Preparation and stability of the hexagonal phase of samarium oxide on Ru(0001)

We have used low-energy electron microscopy (LEEM), micro-illumination low-energy electron diffraction (µLEED) supported by ab initio calculations, and X-ray absorption spectroscopy (XAS) to investigate in-situ and in real-time the structural properties of Sm₂O₃ deposits grown on Ru(0001), a rare-earth metal oxide model catalyst. Our results show that samarium oxide grows in a hexagonal A-Sm₂O₃ phase on Ru(0001), exhibiting a (0001) oriented-top facet and (113) side facets. Upon annealing, a structural transition from the hexagonal to cubic phase occurs, in which the Sm cations exhibit the +3 oxidation state. The unexpected initial growth in the A-Sm₂O₃ hexagonal phase and its gradual transition to a mixture with cubic C-Sm₂O₃ showcases the complexity of the system and the critical role of the substrate in the stabilization of the hexagonal phase, which was previously reported only at high pressures and temperatures for bulk samaria. Besides, these results highlight the potential interactions that Sm could have with other catalytic compounds with respect to the here gathered insights on the preparation conditions and the specific compounds with which it interacts.

Redox-mediated transformation of a Tb2O3(111) thin film from the cubic fluorite to bixbyite structure

We used temperature programmed desorption (TPD) and low energy electron diffraction (LEED) to investigate the isomeric structural transformation of a Tb2O3 thin film grown on Pt(111). We find that repeated oxidation and thermal reduction to 1000 K transforms an oxygen-deficient, cubic fluorite (CF) Tb2O3(111) thin film to the well-defined bixbyite, or c-Tb2O3(111) structure, whereas annealing the CF-Tb2O3(111) film in UHV is ineffective in causing this structural transformation. We estimate that the final stabilized film consists of about ten layers of c-Tb2O3(111) in the surface region plus about eight layers of CF-Tb2O3(111) located between the c-Tb2O3(111) and the Pt(111) substrate. Our measurements reveal the development of two distinct O2 TPD peaks during the CF to bixbyite transformation that arise from oxidation of c-Tb2O3 domains to the stoichiometrically-invariant ι-Tb7O12 and δ-Tb11O20 phases and demonstrate that the c-Tb2O3 phase oxidizes more facilely than 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.