On the structure dependence of CO oxidation over CeO 2 nanocrystals with well-defined surface planes

Zili Wu, Meijun Li, Steven H. Overbury

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Abstract

CO oxidation is a model reaction for probing the redox property of ceria-based catalysts. In this study, CO oxidation was investigated over ceria nanocrystals with defined surface planes (nanoshapes) including rods ({1 1 0} + {1 0 0}), cubes ({1 0 0}), and octahedra ({1 1 1}). To understand the strong dependence of CO oxidation observed on these different ceria nanoshapes, in situ techniques including infrared and Raman spectroscopy coupled with online mass spectrometer, 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 surface. CO-TPR, proceeding via several channels including CO removal of lattice oxygen, surface water-gas shift reaction, and CO disproportionation reaction, 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 dependence. It is suggested that surface oxygen vacancy formation energy, defect sites, and coordinatively unsaturated sites on ceria play a direct role in facilitating both CO interaction with ceria surface and the reactivity and mobility of lattice oxygen. The oxygen vacancy formation energy, nature and amount of the defect and low coordination sites are intrinsically affected by the surface planes of the ceria nanoshapes. Several reaction pathways for CO oxidation over the ceria nanoshapes are proposed, and certain types of carbonates, especially those associated with reduced ceria surface, are considered among the reaction intermediates to form CO 2, while the majority of carbonate species observed under CO oxidation condition are believed to be spectators.

Original languageEnglish
Pages (from-to)61-73
Number of pages13
JournalJournal of Catalysis
Volume285
Issue number1
DOIs
StatePublished - Jan 2012

Funding

This Research is sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, US Department of Energy. Part of the work including Raman and TEM/SEM was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory, by the Office of Basic Energy Science, US 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.

Keywords

  • CO oxidation
  • Ceria nanoshapes
  • Cubes
  • In situ spectroscopy
  • Octahedra
  • Reaction mechanism
  • Rods
  • Structure dependence

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