Paper SS-ThM11
Synthesis and Characterization of Cerium Oxide Nanoparticles with Well-defined Crystallographic Terminations

Thursday, October 21, 2010, 11:20 am, Room Picuris

Through hydrothermal synthesis techniques it is possible to create oxide nanoparticles with highly uniform dimensions and shapes and therefore with crystallographically well-defined terminations. These mono-faceted nanocrystals provide an opportunity to examine structure dependence in surface chemistry with the advantage of using methods appropriate to high surface area materials. We have synthesized cubic, octahedral and rod-shaped nanoparticles of CeO₂ that are terminated exclusively in (100), (111) or a mixture of (100) + (110) crystallographic terminations, respectively. These have been used to compare reducibility, stability and adsorption of water, oxygen and alcohol with the goal of characterizing and understanding the effect of surface structure upon the reaction of these molecules with the surface or with other adsorbates. Multi-wavelength Raman and FTIR in the DRIFTS mode are performed in controlled ambient pressure gas streams, and are combined with pulsed / switched chemisorption and temperature programmed techniques. UV Raman reveals that the CeO₂ nanocrystals contain stable bulk-like Frenkel defects, identified by a characteristic Raman feature, and these are more prevalent in the rod-like particles with (110) + (100) termination. Reduction of the nanoparticles in hydrogen at 673 K leads to oxygen vacancies that are identified by a Raman feature near 560 cm^-1 and the rods are more easily reduced while the octahedral are the least. Oxygen vacancies are immediately removed by O₂ at room temperature on all ceria polymorphs. Upon adsorption of isotopically labeled O₂ at 80 K on reduced surfaces, peroxide and superoxide species are observed and these dissociate or desorb as the temperature is increased. Clustered and isolated peroxides are distinguished by their Raman frequency and their relative ratio depends upon degree of reduction and surface structure. Adsorption of D₂O and H2O on each of the surfaces was probed by DRIFTS at room temperature following dehydroxylation at 673 K. Following a pulse of water, OD (OH) features are observed with characteristic IR frequencies. These results are compared to “single crystal” RAIRS studies on CeO₂(111) oriented films and with previously published IR studies of highly dispersed, polycrystalline CeO₂ crystals of undefined structure terminations. The features observed agree with previous work but their prevalence differ between the faceted nanocrystals and the differences allow structural assignments of the IR features. The results lead to a re-assignment of the IR features.