Li W Q, Srinivasan S Goverapet, Salahub D R, Heine T
Department of Chemistry, Centre for Molecular Simulation and Institute for Quantum Science and Technology, University of Calgary, AB T2N 1N4, Canada.
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.
Phys Chem Chem Phys. 2016 Apr 28;18(16):11139-49. doi: 10.1039/c6cp00738d.
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.
我们报告了密度泛函理论（DFT）对镍吸附质和替代掺杂剂与二氧化铈（110）和（100）表面相互作用的计算，以解释镍/二氧化铈催化剂活性的起源。我们的结果表明，在（110）表面上的镍吸附原子在覆盖度达到0.25 ML之前，更倾向于吸附在二重桥位而非空心位上，并且发现镍吸附质在（100）表面上最稳定的位置是镍原子与两个表面氧原子配位的桥位。发现（110）表面上镍吸附原子的Ni(+)氧化态比二重桥位上的Ni(2+)态更有利，而在（100）表面上，镍吸附原子更倾向于其Ni(2+)氧化态而非Ni(+)氧化态。随着覆盖度的增加，发现（110）表面上镍吸附原子的结合能从0.083 ML覆盖度下的 -0.45 eV降低到0.25 ML覆盖度下的 -0.32 eV。镍吸附原子氧化为Ni(+)会将二氧化铈表面上的一个Ce(4+)离子还原为Ce(3+)，Ce(3+)离子更倾向于在最近邻位置紧邻Ni(+)离子。（100）表面上的Ce(3+)离子也倾向于留在吸附的镍原子附近，而它们在（111）表面上则倾向于远离镍吸附原子。当用镍原子替代铈原子时，未观察到Ce(4+)离子的还原。两个镍取代基更倾向于分布在（110）表面上相邻的金属离子位点上。（110）表面上的镍吸附质和取代基均被发现会引起显著的结构畸变。与纯二氧化铈（110）和（100）表面相比，我们表明镍吸附质增加了产生氧空位所需的能量，而镍掺杂剂则降低了该能量。虽然（110）表面上的多个掺杂剂确实降低了空位形成能，但与单个掺杂剂相比，降低程度较小，这表明存在一个最佳掺杂水平以获得增强的催化活性。
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