Vila Fernando D, Rehr John J, Nuzzo Ralph G, Frenkel Anatoly I
Department of Physics, University of Washington , Seattle, Washington 98195, United States.
Department of Chemistry, University of Illinois , Urbana, Illinois 61801, United States.
J Phys Chem Lett. 2017 Jul 20;8(14):3284-3288. doi: 10.1021/acs.jpclett.7b01446. Epub 2017 Jul 6.
Supported Pt nanocatalysts generally exhibit anomalous behavior, including negative thermal expansion and large structural disorder. Finite temperature DFT/MD simulations reproduce these properties, showing that they are largely explained by a combination of thermal vibrations and low-frequency disorder. We show here that a full interpretation is more complex and that the DFT/MD mean-square relative displacements (MSRD) can be further separated into vibrational disorder, "dynamic structural disorder" (DSD), and long-time equilibrium fluctuations of the structure dubbed "anomalous structural disorder" (ASD). We find that the vibrational and DSD components behave normally, increasing linearly with temperature while the ASD decreases, reflecting the evolution of mean nanoparticle geometry. As a consequence the usual procedure of fitting the MSRD to normal vibrations plus temperature-independent static disorder results in unphysical bond strengths and Grüneisen parameters.
负载型铂纳米催化剂通常表现出异常行为，包括负热膨胀和较大的结构无序性。有限温度密度泛函理论/分子动力学（DFT/MD）模拟再现了这些性质，表明它们在很大程度上可由热振动和低频无序性的组合来解释。我们在此表明，完整的解释更为复杂，并且DFT/MD均方相对位移（MSRD）可进一步细分为振动无序、“动态结构无序”（DSD）以及被称为“异常结构无序”（ASD）的结构的长时间平衡涨落。我们发现，振动和DSD分量表现正常，随温度线性增加，而ASD则减小，这反映了平均纳米颗粒几何形状的演变。因此，将MSRD拟合为正常振动加上与温度无关的静态无序的常规程序会导致不合理的键强度和格林艾森参数。
