title: Ni on the CeO₂(110) and (100) surfaces: adsorption vs. substitution effects on the electronic and geometric structures and oxygen vacancies.
abstract: We report density functional theory (DFT) calculations of the interactions of both Ni adsorbate and substitutional dopant with the ceria (110) and (100) surfaces to explain the origin of the activity of Ni/ceria catalysts. Our results indicate that the Ni adatom on the (110) surface prefers to adsorb on a two-fold bridge site over a hollow site up to 0.25 ML coverage, and the most stable position of a Ni adsorbate on the (100) surface was found to be the bridge site where the Ni atom is coordinated to two surface O atoms. The Ni(+) oxidation state for the Ni adatom on the (110) surface was found to be more favorable than the Ni(2+) state on the two-fold bridge site while on the (100) surface, a Ni adatom prefers its Ni(2+) oxidation state over the Ni(+) oxidation state. With increasing coverage, the binding energy of a Ni adatom on the (110) surface was found to decrease from -0.45 eV at 0.083 ML coverage to -0.32 eV at 0.25 ML coverage. Oxidation of the Ni adatom to Ni(+) reduces one Ce(4+) ion on the ceria surface to Ce(3+) which preferred to be located next to the Ni(+) ion in the nearest neighbor location. The Ce(3+) ions on the (100) surface also prefer to stay in the vicinity of the adsorbed Ni atom, while they prefer to be located away from the Ni adatom on the (111) surface. No reduction of Ce(4+) ions was observed upon substitution of Ce atoms by Ni atoms. Two Ni substituents preferred to be distributed on adjacent metal ion sites on the (110) surface. Ni adsorbate and substituent on the (110) surface were both found to induce significant structural distortions. In comparison to the pure ceria (110) and (100) surfaces, we show that a Ni adsorbate increases the energy required to create an oxygen vacancy while a Ni dopant reduces it. While multiple dopants on the (110) surface do reduce the vacancy formation energy, the degree of reduction is smaller compared to a single dopant indicating the presence of an optimum level of doping to obtain enhanced catalytic activity.
authors.affiliation: Department of Chemistry, Centre for Molecular Simulation and Institute for Quantum Science and Technology, University of Calgary, AB T2N 1N4, Canada. dsalahub@ucalgary.ca and Department of Physics and Earth Science, Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany.
authors.forename: W Q
authors.initials: WQ
authors.lastname: Li
authors.affiliation: Department of Chemistry, Centre for Molecular Simulation and Institute for Quantum Science and Technology, University of Calgary, AB T2N 1N4, Canada. dsalahub@ucalgary.ca.
authors.forename: S Goverapet
authors.initials: SG
authors.lastname: Srinivasan
authors.affiliation: Department of Chemistry, Centre for Molecular Simulation and Institute for Quantum Science and Technology, University of Calgary, AB T2N 1N4, Canada. dsalahub@ucalgary.ca.
authors.forename: D R
authors.initials: DR
authors.lastname: Salahub
authors.affiliation: Department of Physics and Earth Science, Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany and Wilhelm-Ostwald-Institut für Physikalische und Theoretische Chemie, Universität Leipzig, Linnéstr. 2, 04103 Leipzig, Germany.
authors.forename: T
authors.initials: T
authors.lastname: Heine