(222a) In Situ Spectroscopic Investigation of the Surface Dependence of CO Oxidation Over CeO2 Nanocrystals with Well-Defined Surface Planes

CO oxidation is a model reaction for probing the redox property of ceria-based catalysts. Despite the intensive experimental and theoretical studies of CO adsorption and oxidation on ceria catalysts, there is still a lack of spectroscopic insights of how CO interacts with different ceria surfaces and what the reaction mechanism is. In this study, CO oxidation was investigated over ceria nanocrystals with defined surface planes (nanoshapes) including rods ({110} + {100}), cubes ({100}), and octahedra ({111}). To understand the strong surface dependence of CO oxidation over these ceria nanoshapes, in situ techniques including infrared and Raman spectroscopy coupled with online mass spectrometry, and temperature-programmed reduction (TPR) were employed to reveal how CO interacts with the different ceria surfaces, while the mobility of ceria lattice oxygen was investigated via oxygen isotopic exchange experiment. CO adsorption at room temperature leads to strongly bonded carbonate species on the more reactive surfaces of rods and cubes but weakly bonded ones on the rather inert octahedra. Ceria reduction proceeds via several channels including CO reduction of lattice oxygen, surface water-gas shift reaction, and CO disproportionation reaction.  It reveals that the reducibility of these ceria nanoshapes is in line with their CO oxidation activity, i.e., rods > cubes > octahedra. The mobility of lattice oxygen also shows similar surface dependence. It is suggested that both the reducibility and mobility of ceria lattice oxygen are assisted by the presence of defect sites on ceria, whose nature and amount are intrinsically determined by the surface terminations of ceria nanocrystals. Several reaction pathways for CO oxidation over the ceria nanoshapes are proposed and certain carbonate species, especially those associated with reduced ceria surface, are considered among the reaction intermediates to form CO₂, although the majority carbonate species observed under CO oxidation condition are believed to be spectators. 

 ACKNOWLEDGMENT: This Research is sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy. Part of the work was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory, by the Office of Basic Energy Science, U. S. Department of Energy. The research was supported in part by the appointment for M.J. Li to the ORNL Postdoctoral Research Associates Program, administered jointly by ORNL and the Oak Ridge Associated Universities.