1,242 research outputs found
Mechanism for Degradation of Nafion in PEM Fuel Cells from Quantum Mechanics Calculations
We report results of quantum mechanics (QM) mechanistic studies of Nafion membrane degradation in a polymer electrolyte membrane (PEM) fuel cell. Experiments suggest that Nafion degradation is caused by generation of trace radical species (such as OH^●, H^●) only when in the presence of H_2, O_2, and Pt. We use density functional theory (DFT) to construct the potential energy surfaces for various plausible reactions involving intermediates that might be formed when Nafion is exposed to H_2 (or H^+) and O_2 in the presence of the Pt catalyst. We find a barrier of 0.53 eV for OH radical formation from HOOH chemisorbed on Pt(111) and of 0.76 eV from chemisorbed OOH_(ad), suggesting that OH might be present during the ORR, particularly when the fuel cell is turned on and off. Based on the QM, we propose two chemical mechanisms for OH radical attack on the Nafion polymer: (1) OH attack on the S–C bond to form H_2SO_4 plus a carbon radical (barrier: 0.96 eV) followed by decomposition of the carbon radical to form an epoxide (barrier: 1.40 eV). (2) OH attack on H_2 crossover gas to form hydrogen radical (barrier: 0.04 eV), which subsequently attacks a C–F bond to form HF plus carbon radicals (barrier as low as 1.00 eV). This carbon radical can then decompose to form a ketone plus a carbon radical with a barrier of 0.86 eV. The products (HF, OCF_2, SCF_2) of these proposed mechanisms have all been observed by F NMR in the fuel cell exit gases along with the decrease in pH expected from our mechanism
Lancifodilactone G : insights about an unusually stable enol
From quantum mechanics calculations we confirm that the
naturally occurring enol lancifodilactone G is stable over the keto form (by 2.6 kcal/mol in water), the only known stable aliphatic enol (devoid of conjugated or bulky aromatics and lacking a 1,3-diketone structural motif known to stabilize enols). We determine architectural elements responsible for the enol stabilization and find a mechanism for keto-enol conversion in solution. In addition, we correct previously reported computational results that were performed on the misinterpreted structure demonstrating that the enol form of this natural product is more stable than previously thought
Intramolecular Torque, an Indicator of the Internal Rotation Direction of Rotor Molecules and Similar Systems
Torque is ubiquitous in many molecular systems, including collisions,
chemical reactions, vibrations, electronic excitations and especially rotor
molecules. We present a straightforward theoretical method based on forces
acting on atoms and obtained from atomistic quantum mechanics calculations, to
quickly and qualitatively determine whether a molecule or sub-unit thereof has
a tendency to rotation and, if so, around which axis and in which sense:
clockwise or counterclockwise. The method also indicates which atoms, if any,
are predominant in causing the rotation. Our computational approach can in
general efficiently provide insights into the rotational ability of many
molecules and help to theoretically screen or modify them in advance of
experiments or before analyzing their rotational behavior in more detail with
more extensive computations guided by the results from the torque approach. As
an example, we demonstrate the effectiveness of the approach using a specific
light-driven molecular rotary motor which was successfully synthesized and
analyzed in prior experiments and simulations.Comment: 11 pages, 4 figures, 1 SI fil
Elinvar effect in Ti simulated by on-the-fly trained moment tensor potential
A combination of quantum mechanics calculations with machine learning (ML)
techniques can lead to a paradigm shift in our ability to predict materials
properties from first principles. Here we show that on-the-fly training of an
interatomic potential described through moment tensors provides the same
accuracy as state-of-the-art {\it ab inito} molecular dynamics in predicting
high-temperature elastic properties of materials with two orders of magnitude
less computational effort. Using the technique, we investigate high-temperature
bcc phase of titanium and predict very weak, Elinvar, temperature dependence of
its elastic moduli, similar to the behavior of the so-called GUM Ti-based
alloys [T. Sato {\ it et al.}, Science {\bf 300}, 464 (2003)]. Given the fact
that GUM alloys have complex chemical compositions and operate at room
temperature, Elinvar properties of elemental bcc-Ti observed in the wide
temperature interval 1100--1700 K is unique.Comment: 15 pages, 4 figure
Oxygen Hydration Mechanism for the Oxygen Reduction Reaction at Pt and Pd Fuel Cell Catalysts
We report the reaction pathways and barriers for the oxygen reduction reaction (ORR) on platinum, both for gas phase and in solution, based on quantum mechanics calculations (PBE-DFT) on semi-infinite slabs. We find a new mechanism in solution: O_2 → 2O_(ad) (E_(act) = 0.00 eV), O_(ad) + H_2O_(ad) → 2OH_(ad) (E_(act) = 0.50 eV), OH_(ad) + H_(ad) → H_2O_(ad) (E_(act) = 0.24 eV), in which OH_(ad) is formed by the hydration of surface O_(ad). For the gas phase (hydrophilic phase of Nafion), we find that the favored step for activation of the O_2 is H_(ad) + O_(2ad) → HOO_(ad) (E_(act) = 0.30 eV) → HO_(ad) + O_(ad) (E_(act) = 0.12 eV) followed by O_(ad) + H_2O_(ad) → 2OH_(ad) (E_(act) = 0.23 eV), OH_(ad) + H_(ad) → H_2O_(ad) (E_(act) = 0.14 eV). This suggests that to improve the efficiency of ORR catalysts, we should focus on decreasing the barrier for Oad hydration while providing hydrophobic conditions for the OH and H_2O formation steps
RQM description of PS meson form factors, constraints from space-time translations, and underlying dynamics
The role of Poincar\'e covariant space-time translations is investigated in
the case of the pseudoscalar-meson charge form factors. It is shown that this
role extends beyond the standard energy-momentum conservation, which is
accounted for in all relativistic quantum mechanics calculations. It implies
constraints that have been largely ignored until now but should be fulfilled to
ensure the full Poincar\'e covariance. The violation of these constraints,
which is more or less important depending on the form of relativistic quantum
mechanics that is employed, points to the validity of using a single-particle
current, which is generally assumed in calculations of form factors. In short,
these constraints concern the relation of the momentum transferred to the
constituents to the one transferred to the system. How to account for the
related constraints, as well as restoring the equivalence of different
relativistic quantum mechanics approaches in estimating form factors, is
discussed. Some conclusions relative to the underlying dynamics are given in
the pion case.Comment: 37 pages, 13 figures; figures completed for notations, revised text
with better emphasis on differences with previous works; accepted for
publication in EPJ
Structures, Energetics, and Reaction Barriers for CH_x Bound to the Nickel (111) Surface
To provide a basis for understanding and improving such reactive processes on nickel surfaces as the catalytic
steam reforming of hydrocarbons, the decomposition of hydrocarbons at fuel cell anodes, and the growth of
carbon nanotubes, we report quantum mechanics calculations (PBE flavor of density functional theory, DFT)
of the structures, binding energies, and reaction barriers for all CH_x species on the Ni(111) surface using
periodically infinite slabs. We find that all CH_x species prefer binding to μ3 (3-fold) sites leading to bond
energies ranging from 42.7 kcal/mol for CH_3 to 148.9 kcal/mol for CH (the number of Ni-C bonds is not
well-defined). We find reaction barriers of 18.3 kcal/mol for CH_(3,ad) → CH_(2,ad) + H_(ad) (with ΔE = +1.3 kcal/
mol), 8.2 kcal/mol for CH_(2,ad) → CH_(ad) + H_(ad) (with ΔE = -10.2 kcal/mol) and 32.3 kcal/mol for CH_(ad) → C_(ad)
+ H_(ad) (with ΔE = 11.6 kcal/mol). Thus, CH_(ad) is the stable form of CH_x on the surface. These results are in
good agreement with the experimental data for the thermodynamic stability of small hydrocarbon species
following dissociation of methane on Ni(111) and with the intermediates isolated during the reverse methanation
process
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