Ceria (CeO 2 ) has recently been found to catalyze the selective hydrogenation of alkynes, which has stimulated much discussion on the catalytic mechanism on various facets of reducible oxides. In this work, H 2 dissociation and acetylene hydrogenation on bare and Ni doped CeO 2 (110) surfaces are investigated using density functional theory (DFT). Similar to that on the CeO 2 (111) surface, our results suggest that catalysis is facilitated by frustrated Lewis pairs (FLPs) formed by oxygen vacancies (O v s) on the oxide surfaces. On bare CeO 2 (110) with a single O v (CeO 2 (110)–O v ), two surface Ce cations with one non-adjacent O anion are shown to form (Ce 3+ –Ce 4+ )/O quasi-FLPs, while for the Ni doped CeO 2 (110) surface with one (Ni–CeO 2 (110)–O v ) or two (Ni–CeO 2 (110)–2O v ) O v s, one Ce and a non-adjacent O counterions are found to form a mono-Ce/O FLP. DFT calculations indicate that Ce/O FLPs facilitate the H 2 dissociation via a heterolytic mechanism, while the resulting surface O–H and Ce–H species catalyze the subsequent acetylene hydrogenation. With CeO 2 (110)–O v and Ni–CeO 2 (110)–2O v , our DFT calculations suggest that the first hydrogenation step is the rate-determining step with a barrier of 0.43 and 0.40 eV, respectively. For Ni–CeO 2 (110)–O v , the reaction is shown to be controlled by the H 2 dissociation with a barrier of 0.41 eV. These barriers are significantly lower than that (about 0.7 eV) on CeO 2 (111), explaining the experimentally observed higher catalytic efficiency of the (110) facet of ceria. The change of the rate-determining step is attributed to the different electronic properties of Ce in the Ce/O FLPs – the Ce f states closer to the Fermi level not only facilitate the heterolytic dissociation of H 2 but also lead to a higher barrier of acetylene hydrogenation.
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Unexpected trends in the enhanced Ce 3+ surface concentration in ceria–zirconia catalyst materials
Despite the immense importance of ceria–zirconia solid solutions in heterogeneous catalysis, and the growing consensus that catalytic activity correlates with the concentration of reduced Ce 3+ species and accompanying oxygen vacancies, the extent of reduction at the surfaces of these materials, where catalysis occurs, is unknown. Using angle-resolved X-ray Absorption Near Edge Spectroscopy (XANES), we quantify under technologically relevant conditions the Ce 3+ concentration in the surface (2–3 nm) and bulk regions of ceria–zirconia films grown on single crystal yttria-stabilized zirconia, YSZ (001). In all circumstances, we observe substantial Ce 3+ enrichment at the surface relative to the bulk. Surprisingly, the degree of enhancement is highest in the absence of Zr. This behavior stands in direct contrast to that of the bulk in which the Ce 3+ concentration monotonically increases with increasing Zr content. These results suggest that while Zr enhances the oxygen storage capacity in ceria, undoped ceria may have higher surface catalytic activity. They further urge caution in the use of bulk properties as surrogate descriptors for surface characteristics and hence catalytic activity.
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- Award ID(s):
- 1505103
- NSF-PAR ID:
- 10235096
- Date Published:
- Journal Name:
- Journal of Materials Chemistry A
- Volume:
- 8
- Issue:
- 19
- ISSN:
- 2050-7488
- Page Range / eLocation ID:
- 9850 to 9858
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D0TA02762F
