Data_Sheet_1_Efficient Calculation of the Negative Thermal Expansion in ZrW2O8.ZIP

<p>We present a study of the origin of the negative thermal expansion (NTE) on ZrW₂O₈ by combining an efficient approach for computing the dynamical matrix with the Lanczos algorithm for generating the phonon density of states in the quasi-harmonic approximation. The simulations show that the NTE arises primarily from the motion of the O-sublattice, and in particular, from the transverse motion of the O atoms in the W–O and W–O–Zr bonds. In the low frequency range these combine to keep the WO₄ tetrahedra rigid and induce internal distortions in the ZrO₆ octahedra. The force constants associated with these distortions become stronger with expansion, resulting in negative Grüneisen parameters and NTE from the low frequency modes that dominate the positive contributions from the high frequency modes. This leads us to propose an anharmonic, two-frequency Einstein model that quantitatively captures the NTE behavior.</p

Anomalous Structural Disorder in Supported Pt Nanoparticles

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

Molecular Dynamics Simulations of Supported Pt Nanoparticles with a Hybrid Sutton–Chen Potential

Understanding the physical and chemical behavior of supported nanoscale catalysts is of fundamental and technological importance. However, their behavior remains poorly understood, in part due to their complex, dynamical structure and the nature of interactions at the nanoscale. We found previously that real-time ab initio finite temperature DFT simulations provide fundamental insights into the dynamic and electronic structure of nanoparticles. Unfortunately, such first-principles calculations are very computationally intensive. To make such simulations more feasible, we have developed a hybrid version of the classical Sutton–Chen model potential which is orders of magnitude more efficient. This potential is parametrized to previous DFT/MD simulations and accounts for many-body effects induced by the support. The model is applied to Pt_10,20 nanoparticles supported on a model γ-Al₂O₃ surface. In addition to the thermal variation of the internal structure, the model also predicts diffusion coefficients and bond-breaking rates. The simulations reveal size-dependent dynamical changes with increasing temperature, as the clusters go from a “frozen” state attached to the support, to a “liquid” state where they are free to diffuse. These changes provide a rationale for the observed negative thermal expansion. Implications for nanoscale catalysis are briefly discussed

Cation Incorporation into Copper Oxide Lattice at Highly Oxidizing Potentials

Electrolyte cations can have significant effects on the kinetics and selectivity of electrocatalytic reactions. We show an atypical mechanism through which electrolyte cations can impact electrocatalyst performancedirect incorporation of the cation into the oxide electrocatalyst lattice. We investigate the transformations of copper electrodes in alkaline electrochemistry through operando X-ray absorption spectroscopy in KOH and Ba­(OH)2 electrolytes. In KOH electrolytes, both the near-edge structure and extended fine-structure agree with previous studies; however, the X-ray absorption spectra vary greatly in Ba­(OH)2 electrolytes. Through a combination of electronic structure modeling, near-edge simulation, and postreaction characterization, we propose that Ba2+ cations are directly incorporated into the lattice and form an ordered BaCuO2 phase at potentials more oxidizing than 200 mV vs the normal hydrogen electrode (NHE). BaCuO2 formation is followed by further oxidation to a bulk Cu3+-like BaxCuyOz phase at 900 mV vs NHE. Additionally, during reduction in Ba­(OH)2 electrolyte, we find both Cu–O bonds and Cu–Ba scattering persist at potentials as low as −400 mV vs NHE. To our knowledge, this is the first evidence for direct oxidative incorporation of an electrolyte cation into the bulk lattice to form a mixed oxide electrode. The oxidative incorporation of electrolyte cations to form mixed oxides could open a new route for the in situ formation of active and selective oxidation electrocatalysts