Gulyaev R V, Kardash T Yu, Malykhin S E, Stonkus O A, Ivanova A S, Boronin A I
Boreskov Institute of Catalysis SB RAS, Novosibirsk 630090, Russia.
Phys Chem Chem Phys. 2014 Jul 14;16(26):13523-39. doi: 10.1039/c4cp01033g.
PdxCe1-xO2-x-δ solid solutions, which are highly efficient catalysts for the low-temperature oxidation of carbon monoxide, were examined using a set of structural (XRD-PDF, HRTEM, XRD) and spectral (XPS, Raman spectroscopy) methods in combination with quantum-chemical calculations. A comparison of the experimental results and pair distribution function (PDF) modeling data enabled reliable verification of the model of non-isomorphic substitution of Ce(4+) ions by Pd(2+) ions in PdxCe1-xO2-x-δ solid solutions. Palladium ions were shown to be in a near square planar environment with C4v symmetry, which is typical for Pd(2+) ions. Such a near square planar environment was revealed by Raman spectroscopy due to the appearance of the band at ω = 187 cm(-1), which corresponds to the A1 vibrational mode of Pd(2+) ions in [PdO4] subunits. The binding energy of Pd3d5/2 (Eb(Pd3d5/2)) for the Pd(2+) ion in the CeO2 lattice is 1 eV higher than that of Eb(Pd3d5/2) for PdO oxide due to a decrease in the Pd-O distances and the formation of more ionic bonds because of the displacement of Pd(2+) ions with respect to the position of Ce(4+) ions in the fluorite structure. Five structural models of solid solutions are considered in this work. As demonstrated by the DFT calculations, the most realistic model is based on the displacement of palladium ions leading to a near square planar PdO4 environment, which includes water molecules stabilizing the region of anion vacancies in their dissociated state as two hydroxyl groups. The introduction of water molecules in the composition of the PdxCe1-xO2-x-δ solution leads to a decrease in the formation energy and to additional stabilization of palladium in the CeO2 matrix. The formation of PdxCe1-xO2-x-δ solid solutions is accompanied by the dispersing effect caused by distortions of the fluorite structure induced by Pd(2+) ions. The coprecipitation method, which allows Pd(2+) ions to be introduced at the stage of fluorite structure formation, was demonstrated to be the optimal method for the synthesis of a homogeneous PdxCe1-xO2-x-δ solid solution.
PdxCe1-xO2-x-δ固溶体是一氧化碳低温氧化的高效催化剂，采用一系列结构（XRD-PDF、HRTEM、XRD）和光谱（XPS、拉曼光谱）方法，并结合量子化学计算对其进行了研究。实验结果与对分布函数（PDF）建模数据的比较，使得对PdxCe1-xO2-x-δ固溶体中Ce(4+)离子被Pd(2+)离子非同构取代模型的可靠验证成为可能。钯离子显示处于具有C4v对称性的近方形平面环境中，这对于Pd(2+)离子来说是典型的。由于在ω = 187 cm(-1)处出现了谱带，拉曼光谱揭示了这样的近方形平面环境，该谱带对应于[PdO4]亚基中Pd(2+)离子的A1振动模式。由于Pd-O距离的减小以及由于Pd(2+)离子相对于萤石结构中Ce(4+)离子位置的位移而形成更多离子键，CeO2晶格中Pd(2+)离子的Pd3d5/2结合能（Eb(Pd3d5/2)）比PdO氧化物的Eb(Pd3d5/2)高1 eV。本工作考虑了固溶体的五种结构模型。正如DFT计算所表明的，最现实的模型基于钯离子的位移，导致形成近方形平面的PdO4环境，其中包括以两个羟基的解离状态稳定阴离子空位区域的水分子。在PdxCe1-xO2-x-δ溶液组成中引入水分子会导致形成能降低，并使钯在CeO2基体中得到额外稳定。PdxCe
