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Ethan Lawrence1 Barnaby Levin1 Shery Chang1 Tara Boland1 Peter Crozier1

1, Arizona State University, Tempe, Arizona, United States

CeO2 (ceria) and ceria-based materials have many applications in heterogeneous catalysis, largely due to their oxygen exchange properties [1]. Under reducing conditions, oxygen vacancies form on ceria surfaces accompanied by a subsequent reduction of neighboring Ce4+ cations to Ce3+ [2]. Moreover, a lattice expansion (~0.1-0.2 Å) occurs once oxygen vacancies are created on ceria surfaces [3]. The relative ease with which oxygen vacancies are created/annihilated on CeO2 surfaces is strongly surface and size dependent and is an important catalytic property of ceria nanoparticles [4]. In situ environmental transmission electron microscopy (ETEM) can be used to observe dynamical processes that occur on ceria surfaces in reducing and oxidizing environments. Thus, atomic-level studies of CeO2 surfaces may enable the reaction pathways and active sites to be determined for oxygen exchange reactions, a fundamental outstanding problem in heterogeneous catalysis.

An aberration-corrected FEI Titan ETEM equipped with a Gatan K2 IS direct detection camera (with high detection quantum efficiency and capable of up to 1600 fps) was used to image CeO2 nanocubes. The image corrector of the microscope was tuned to an optimum negative Cs condition to enhance contrast from weakly scattering oxygen atomic columns – enabling simultaneous imaging of Ce and O atomic column positions. Imaging of (111), (110), and (100) CeO2 surfaces was done in reducing (pO2 < 10-7 Torr) and oxidizing (pO2 = 0.5 Torr) environments. Images were acquired at 40 fps with 1 sec. exposures to capture dynamic motion of CeO2 surfaces between individual frames. Under reducing conditions, large displacements of Ce atomic column positions were observed on (110) and (100) surfaces and at step edge sites of (111) surfaces; atomic displacements were likely caused by creation/annihilation of oxygen vacancies. Ce atomic motion was diminished on all surfaces during oxidizing conditions. Molecular dynamics (MD) and density functional theory (DFT) calculations will be used to relate relaxed surface structures and estimated oxygen vacancy concentrations to experimental data. Furthermore, Ce atomic displacements will be correlated to surface-dependent oxygen vacancy formation energies [5].


References:
[1] Gorte, R.J., AIChE Journal 56 (2010) p. 1126-1135.
[2] Skorodumova, N.V., et al, Physical Review Letters 89 (2002) p. 166601.
[3] Marrocchelli, D., et al, Advanced Functional Materials 22 (2012) p. 1958-1965.
[4] Trovarelli, A. and Llorca, J., ACS Catalysis 7 (2017) p. 4716-4735.
[5] We gratefully acknowledge support of NSF grant DMR-1308085, the use of ASU’s John M. Cowley Center for High Resolution Electron Microscopy and use of the K2 IS camera courtesy of Gatan.

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