The Polarizable Charge Equilibration Model for Transition-Metal Elements

The polarizable charge equilibration (PQEq) method was developed to provide a simple but accurate description of the electrostatic interactions and polarization effects in materials. Previously, we optimized four parameters per element for the main group elements. Here, we extend this optimization to the 24 d-block transition-metal (TM) elements, columns 4–11 of the periodic table including Ti–Cu, Zr–Ag, and Hf–Au. We validate the PQEq description for these elements by comparing to interaction energies computed by quantum mechanics (QM). Because many materials applications involving TM are for oxides and other compounds that formally oxidize the metal, we consider a variety of oxidation states in 24 different molecular clusters. In each case, we compare interaction energies and induced fields from QM and PQEq along various directions. We find that the original χ and J parameters (electronegativity and hardness) related to the ionization of the atom remain valid; however, we find that the atomic radius parameter needs to be close to the experimental ionic radii of the transition metals. This leads to a much higher spring constant to describe the atomic polarizability. We find that these optimized parameters for PQEq provide accurate interaction energies compared to QM with charge distributions that depend in a reasonable way on the coordination number and oxidation states of the transition metals. We expect that this description of the electrostatic interactions for TM will be useful in molecular dynamics simulations of inorganic and organometallic materials

Artificial Intelligence and QM/MM with a Polarizable Reactive Force Field for Next-Generation Electrocatalysts

To develop new generations of electrocatalysts, we need the accuracy of full explicit solvent quantum mechanics (QM) for practical-sized nanoparticles and catalysts. To do this, we start with the RexPoN reactive force field that provides higher accuracy than density functional theory (DFT) for water and combine it with QM to accurately include long-range interactions and polarization effects to enable reactive simulations with QM accuracy in the presence of explicit solvent. We apply this RexPoN-embedded QM (ReQM) to reactive simulations of electrocatalysis, demonstrating that ReQM accurately replaces DFT water for computing the Raman frequencies of reaction intermediates during CO₂ reduction to ethylene. Then, we illustrate the power of this approach by combining with machine learning to predict the performance of about 10,000 surface sites and identify the active sites of solvated gold (Au) nanoparticles and dealloyed Au surfaces. This provides an accurate but practical way to design high-performance electrocatalysts

The Polarizable Charge Equilibration Model for Transition-Metal Elements

The polarizable charge equilibration (PQEq) method was developed to provide a simple but accurate description of the electrostatic interactions and polarization effects in materials. Previously, we optimized four parameters per element for the main group elements. Here, we extend this optimization to the 24 d-block transition-metal (TM) elements, columns 4–11 of the periodic table including Ti–Cu, Zr–Ag, and Hf–Au. We validate the PQEq description for these elements by comparing to interaction energies computed by quantum mechanics (QM). Because many materials applications involving TM are for oxides and other compounds that formally oxidize the metal, we consider a variety of oxidation states in 24 different molecular clusters. In each case, we compare interaction energies and induced fields from QM and PQEq along various directions. We find that the original χ and J parameters (electronegativity and hardness) related to the ionization of the atom remain valid; however, we find that the atomic radius parameter needs to be close to the experimental ionic radii of the transition metals. This leads to a much higher spring constant to describe the atomic polarizability. We find that these optimized parameters for PQEq provide accurate interaction energies compared to QM with charge distributions that depend in a reasonable way on the coordination number and oxidation states of the transition metals. We expect that this description of the electrostatic interactions for TM will be useful in molecular dynamics simulations of inorganic and organometallic materials

Highly active and stable stepped Cu surface for enhanced electrochemical CO₂ reduction to C₂H₄

Electrochemical CO₂ reduction to value-added chemical feedstocks is of considerable interest for renewable energy storage and renewable source generation while mitigating CO₂ emissions from human activity. Copper represents an effective catalyst in reducing CO₂ to hydrocarbons or oxygenates, but it is often plagued by a low product selectivity and limited long-term stability. Here we report that copper nanowires with rich surface steps exhibit a remarkably high Faradaic efficiency for C₂H₄ that can be maintained for over 200 hours. Computational studies reveal that these steps are thermodynamically favoured compared with Cu(100) surface under the operating conditions and the stepped surface favours C₂ products by suppressing the C₁ pathway and hydrogen production

Highly active and stable stepped Cu surface for enhanced electrochemical CO₂ reduction to C₂H₄

Electrochemical CO₂ reduction to value-added chemical feedstocks is of considerable interest for renewable energy storage and renewable source generation while mitigating CO₂ emissions from human activity. Copper represents an effective catalyst in reducing CO₂ to hydrocarbons or oxygenates, but it is often plagued by a low product selectivity and limited long-term stability. Here we report that copper nanowires with rich surface steps exhibit a remarkably high Faradaic efficiency for C₂H₄ that can be maintained for over 200 hours. Computational studies reveal that these steps are thermodynamically favoured compared with Cu(100) surface under the operating conditions and the stepped surface favours C₂ products by suppressing the C₁ pathway and hydrogen production

Understanding pH within a nanoscopic water pool

The nature of pH within a water pool that is too small to have a constant population of ions formed by water dissociation is a long-standing question. The breakdown of the conventional pH definition is due to the rare and intermittent presence of ion pairs (H^+ and OH^-) in these pools, leading to pH = -log_10 (0). To characterize water ion pair lifetimes and populations in such systems, we have performed a set of stochastic kinetics simulations of the water dissociation reaction in pools ranging from 10^3 to 10^10 waters. We extract a kinetically derived availability coefficient, α_(i>0), as a suitable parameter to quantify the intermittent presence of a number i of ion pairs during an observation period. In this way, pH in confinement is intrinsically connected with the transient ion pair lifetimes and the stochastics associated with their formation. We propose that α_(i>0) is equivalent to an activity coefficient, and use α_(i>0) along with the thermodynamic definition of pH to estimate an effective pH for a nanoscopic H_2 O pool. As the availability coefficient quickly converges to 1 for pool containing well over 5×10^9 waters, the effective pH converges to the pure water bulk pH of 7. The implications of the effective pH concept for confined aqueous environments are discussed

A stochastic description of pH within nanoscopic water pools

The nature of pH in a pure water pool that is too small to have a continuous population of ions formed by water dissociation is not settled. Here, we use stochastic kinetics simulations of the water dissociation reaction in pools ranging from 103 to 1016 waters to characterize water ion lifetimes and populations. An availability coefficient is defined to quantify the intermittent presence of ion pairs during an arbitrary observation period and used in a proposed definition for the effective pH of nanoscopic pools. The effective pH converges to the bulk value of 7 for water pools in the range of 1010 waters. The lifetimes of water ion pairs are found to increase with increasing water pool size due to the balance between their formation and recombination kinetics, with a maximum near the size where bulk-like characteristics begin to dominate and ion pairs are continuously present

3D LOCALIZATION FOR LAUNCH VEHICLE USING COMBINED TOA AND AOA

Generally, a ground telemetry station for launch vehicle (LV) has tracking function only; therefore, position measurements depend on radar. Time of arrival (TOA) and angle of arrival (AOA) are typical location techniques for emitting targets. In this paper, we propose a Combined TOA and AOA localization method for LV using two ground stations. When transmitter (Tx) time is not known, it is necessary to make virtual onboard timer for TOA estimation. The virtual onboard timer generates time stamps of streaming frame according to data rate. First station which is located in space center has no tracking function. But it can generate the virtual onboard timer. Second station has tracking function, so it generates AOA information. By solving sphere equation(s) of TOA from at least one station and a line equation of AOA, target position in three-dimensions (3D) can be obtained. We confirm the localization performance by means of comparison with an on-board GPS of a real launch mission.International Foundation for TelemeteringProceedings from the International Telemetering Conference are made available by the International Foundation for Telemetering and the University of Arizona Libraries. Visit http://www.telemetry.org/index.php/contact-us if you have questions about items in this collection