Abstract In this work, we employ density functional theory simulations to investigate possible spin polarization of CeO 2 -(111) surface and its impact on the interactions between a ceria support and Pt nanoparticles. With a Gaussian type orbital basis, our simulations suggest that the CeO 2 -(111) surface exhibits a robust surface spin polarization due to the internal charge transfer between atomic Ce and O layers. In turn, it can lower the surface oxygen vacancy formation energy and enhance the oxide reducibility. We show that the inclusion of spin polarization can significantly reduce the major activation barrier in the proposed reaction pathway of CO oxidation on ceria-supported Pt nanoparticles. For metal-support interactions, surface spin polarization enhances the bonding between Pt nanoparticles and ceria surface oxygen, while CO adsorption on Pt nanoparticles weakens the interfacial interaction regardless of spin polarization. However, the stable surface spin polarization can only be found in the simulations based on the Gaussian type orbital basis. Given the potential importance in the design of future high-performance catalysts, our present study suggests a pressing need to examine the surface ferromagnetism of transition metal oxides in both experiment and theory.
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Acetylene hydrogenation catalyzed by bare and Ni doped CeO 2 (110): the role of frustrated Lewis pairs
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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- Award ID(s):
- 1951328
- PAR ID:
- 10326067
- Date Published:
- Journal Name:
- Physical Chemistry Chemical Physics
- Volume:
- 24
- Issue:
- 18
- ISSN:
- 1463-9076
- Page Range / eLocation ID:
- 11295 to 11304
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Deoxydehydration (DODH) is an emerging biomass deoxygenation process whereby vicinal OH groups are removed. Based on DFT calculations and microkinetic modeling, we seek to understand the mechanism of the Re-catalyzed deoxydehydration supported on CeO 2 (111). In addition, we aim at understanding the promotional effect of Pd in a heterogeneous ReO x –Pd/CeO 2 DODH catalyst system. We disentangle the contribution of the oxide support, the oxide-supported single ReO x species, and a co-adsorbed Pd promoter that has no direct interaction with the Re species. In the absence of a nearby Pd cluster, a Re site is able to reduce subsurface Ce-ions of a hydroxylated CeO 2 (111) surface, leading to a catalytically active Re +6 species. The effect of Pd is twofold: (i) Pd catalyzes the hydrogen dissociation and spillover onto CeO 2 , which is an indispensable process for the regeneration of the Re catalyst, and (ii) Pd adsorbed in close proximity to Re on CeO 2 (111) facilitates the oxidation of Re to a +7 oxidation state, which leads to an even more active Re species than the Re +6 site present in the absence of Pd. The latter promotional effect of Pd (and change in oxidation state of Re) disappears with increasing Pd–Re distance and in the presence of oxygen defects on the ceria support. Under these conditions, the ReO x –Pd/CeO 2 catalyst system exhibits appreciable activity consistent with recent experiments. The established mechanism and role of various species in the catalyst system help to better understand the deoxydehydration catalysis. Also, the importance of the Re oxidation state and the identified oxidation state modification mechanisms suggest a new pathway for tuning the properties of metal-oxide supported catalysts.more » « less
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Abstract Oxide supports with well‐defined shapes enable investigations on the effects of surface structure on metal–support interactions and correlations to catalytic activity and selectivity. Here, a modified atomic layer deposition technique was developed to achieve ultra‐low loadings (8–16 ppm) of Pt on shaped ceria nanocrystals. Using octahedra and cubes, which expose exclusively (111) and (100) surfaces, respectively, the effect of CeO2surface facet on Pt‐CeO2interactions under reducing conditions was revealed. Strong electronic interactions result in electron‐deficient Pt species on CeO2(111) after reduction, which increased the stability of the atomically dispersed Pt. This afforded significantly higher NMR signal enhancement in parahydrogen‐induced polarization experiments compared with the electron‐rich platinum on CeO2(100), and a factor of two higher pairwise selectivity (6.1 %) in the hydrogenation of propene than any previously reported monometallic heterogeneous Pt catalyst.
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Abstract Oxide supports with well‐defined shapes enable investigations on the effects of surface structure on metal–support interactions and correlations to catalytic activity and selectivity. Here, a modified atomic layer deposition technique was developed to achieve ultra‐low loadings (8–16 ppm) of Pt on shaped ceria nanocrystals. Using octahedra and cubes, which expose exclusively (111) and (100) surfaces, respectively, the effect of CeO2surface facet on Pt‐CeO2interactions under reducing conditions was revealed. Strong electronic interactions result in electron‐deficient Pt species on CeO2(111) after reduction, which increased the stability of the atomically dispersed Pt. This afforded significantly higher NMR signal enhancement in parahydrogen‐induced polarization experiments compared with the electron‐rich platinum on CeO2(100), and a factor of two higher pairwise selectivity (6.1 %) in the hydrogenation of propene than any previously reported monometallic heterogeneous Pt catalyst.
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D2CP00925K
