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Ni Nanoparticles on CeO2 (111): Energetics, Electron Transfer and Structure by Ni Adsorption Calorimetry, Spectroscopies and DFT

preprint
submitted on 26.03.2020 and posted on 27.03.2020 by Zhongtian Mao, Pablo Lustemberg, John R. Rumptz, M. V. Ganduglia-Pirovano, Charles T. Campbell
The morphology, interfacial bonding energetics and charge transfer of Ni clusters and nanoparticles on slightly-reduced CeO2-x (111) surfaces at 100 to 300 K have been studied using single crystal adsorption calorimetry (SCAC), low-energy ion scattering spectroscopy (LEIS), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and density functional theory (DFT). The initial heat of adsorption of Ni vapor decreased with the extent of pre-reduction (x) of the CeO2-x (111), showing that stoichiometric ceria adsorbs Ni more strongly than oxygen vacancies. On CeO1.95 (111) at 300 K, the heat dropped quickly with coverage in the first 0.1 ML, attributed to nucleation of Ni clusters on stoichiometric steps, followed by the Ni particles spreading onto less favorable terrace sites. At 100 K, the clusters nucleate on terraces due
to slower diffusion. Adsorbed Ni monomers are in the +2 oxidation state, and they bind by ~45 kJ/mol more strongly to step sites than terraces. The measured heat of adsorption versus average particle size on terraces is favorably compared to DFT calculations. The Ce 3d XPS lineshape
showed an increase in Ce3+/Ce4+ ratio with Ni coverage, providing the number of electrons donated to the ceria per Ni atom. The charge transferred per Ni is initially large but strongly decreases with increasing cluster size for both experiments and DFT, and shows large differences between clusters at steps versus terraces. This charge is localized on the interfacial Ni and Ce atoms in their atomic layers closest to the interface. This knowledge is crucial to understanding the nature of the active sites on the surface of Ni-CeO2 catalysts for which metal-oxide interactions play a very important role in the activation of O−H and C−H bonds. The changes in these interactions with Ni particle size (metal loading) and the extent of reduction of the ceria help to explain how previously reported catalytic activity and selectivity change with these same structural details.

Funding

This project also received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 832121

The authors (C.T.C. and Z.M.) acknowledge the Department of Energy, Office of Basic Energy Sciences, Chemical Sciences Division grant number DE-FG02-96ER14630, for support of this work.

M.V.G.P. thanks the support by the MINECO and MICINN-Spain (CTQ2015-71823-R and RTI2018-101604-B-I00, respectively)

Computer time provided by the BIFI-ZCAM, the RES (Red Española de Supercomputación) resources at MareNostrum 4 (BSC, Barcelona) and Altamira (IFCA, Cantabria) nodes, the Piluso node within SNCAD (Sistema Nacional de Computación de Alto Desempeño, Argentina), is acknowledged. Computer time provided by the DECI resources at Finis Terrae II based in Spain at CESGA, with the support from PRACE aislb, is also acknowledge.

History

Email Address of Submitting Author

lustemberg@ifir-conicet.gov.ar

Institution

Instituto de Catálisis y Petroleoquímica (ICP-CSIC), C/Marie Curie 2, 28049 Madrid

Country

Spain

ORCID For Submitting Author

0000-0003-4058-4023

Declaration of Conflict of Interest

No conflict of interest

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