5 research outputs found
Hydrodeoxygenation of Phenol to Benzene and Cyclohexane on Rh(111) and Rh(211) Surfaces: Insights from Density Functional Theory
Herein
we describe the C–O cleavage of phenol and cyclohexanol
over Rh(111) and Rh(211) surfaces using density functional theory
calculations. Our analysis is complemented by a microkinetic model
of the reactions, which indicates that the C–O bond cleavage
of cyclohexanol is easier than that of phenol and that Rh(211) is
more active than Rh(111) for both reactions. This indicates that phenol
will react mainly following a pathway of initial hydrogenation to
cyclohexanol followed by hydrodeoxygenation to cyclohexane. We show
that there is a general relationship between the transition state
and the final state of both C–O cleavage reactions, and that
this relationship is the same for Rh(111) and Rh(211)
High-Pressure Single-Crystal Structures of 3D Lead-Halide Hybrid Perovskites and Pressure Effects on their Electronic and Optical Properties
We report the first high-pressure single-crystal structures of
hybrid perovskites. The crystalline semiconductors (MA)ÂPbX<sub>3</sub> (MA = CH<sub>3</sub>NH<sub>3</sub><sup>+</sup>, X = Br<sup>–</sup> or I<sup>–</sup>) afford us the rare opportunity of understanding
how compression modulates their structures and thereby their optoelectronic
properties. Using atomic coordinates obtained from high-pressure single-crystal
X-ray diffraction we track the perovskites’ precise structural
evolution upon compression. These structural changes correlate well
with pressure-dependent single-crystal photoluminescence (PL) spectra
and high-pressure bandgaps derived from density functional theory.
We further observe dramatic piezochromism where the solids become
lighter in color and then transition to opaque black with compression.
Indeed, electronic conductivity measurements of (MA)ÂPbI<sub>3</sub> obtained within a diamond-anvil cell show that the material’s
resistivity decreases by 3 orders of magnitude between 0 and 51 GPa.
The activation energy for conduction at 51 GPa is only 13.2(3) meV,
suggesting that the perovskite is approaching a metallic state. Furthermore,
the pressure response of mixed-halide perovskites shows new luminescent
states that emerge at elevated pressures. We recently reported that
the perovskites (MA)ÂPbÂ(Br<sub><i>x</i></sub>I<sub>1–<i>x</i></sub>)<sub>3</sub> (0.2 < <i>x</i> < 1)
reversibly form light-induced trap states, which pin their PL to a
low energy. This may explain the low voltages obtained from solar
cells employing these absorbers. Our high-pressure PL data indicate
that compression can mitigate this PL redshift and may afford higher
steady-state voltages from these absorbers. These studies show that
pressure can significantly alter the transport and thermodynamic properties
of these technologically important semiconductors
Factors Affecting the Electron Conductivity in Single Crystal Li<sub>7</sub>La<sub>3</sub>Zr<sub>2</sub>O<sub>12</sub> and Li<sub>7</sub>P<sub>3</sub>S<sub>11</sub>
One of the serious challenges in all solid-state Li ion
batteries
is neutral Li intrusion into the solid-state electrolyte that can
ultimately cause catastrophic failure. One possibility for this is
due to n-type electron conductivity that induces
the reaction Li+ + e– → Li0 at sites where the potential is less than the Li+/Li potential. This paper reports hybrid density functional theory
calculations of the electronic conductivity in two prototype single
crystalline solid-state electrolytes, cubic Li7La3Zr2O12 (c-LLZO) and Li7P3S11 (LPS). The formation energies of important point defects
that can affect electron conductivity are determined, and we find
that the mechanism of n-type electron conductivity
for both solid-state electrolytes is via “small” electron
polaron hopping, where the quotes signify that substantial Li ion
rearrangement is associated with the polaron formation and its migration.
In both electrolytes, the formation energies for the small polarons
at the Fermi energy are too high to generate measurable electron conductivity
at room temperature. For c-LLZO, the concentration of electron polarons
necessary to ensure charge neutrality from positively charged oxygen
vacancies formed in synthesis can be significantly higher. Hence,
the electron conductivity could be significant when measured with
ion-blocking metal electrodes, and we discuss how the synthesis conditions
could affect this magnitude. However, in the solid-state battery,
these polarons are replaced by negatively charged Li vacancies so
that the electron conductivity should remain minimal. For LPS single
crystals, the inherent minimal electron conductivity is independent
of synthesis conditions. We also show that the cost of forming Li0 in bulk c-LLZO is enormous due to strain effects so that
it could only potentially form at voids, grain boundaries, or around
vacancy defects which relax the lattice strain
Theoretical Investigations of the Electrochemical Reduction of CO on Single Metal Atoms Embedded in Graphene
Single transition
metal atoms embedded at single vacancies of graphene
provide a unique paradigm for catalytic reactions. We present a density
functional theory study of such systems for the electrochemical reduction
of CO. Theoretical investigations of CO electrochemical reduction
are particularly challenging in that electrochemical activation energies
are a necessary descriptor of activity. We determined the electrochemical
barriers for key proton–electron transfer steps using a state-of-the-art,
fully explicit solvent model of the electrochemical interface. The
accuracy of GGA-level functionals in describing these systems was
also benchmarked against hybrid methods. We find the first proton
transfer to form CHO from CO to be a critical step in C<sub>1</sub> product formation. On these single atom sites, the corresponding
barrier scales more favorably with the CO binding energy than for
211 and 111 transition metal surfaces, in the direction of improved
activity. Intermediates and transition states for the hydrogen evolution
reaction were found to be less stable than those on transition metals,
suggesting a higher selectivity for CO reduction. We present a rate
volcano for the production of methane from CO. We identify promising
candidates with high activity, stability, and selectivity for the
reduction of CO. This work highlights the potential of these systems
as improved electrocatalysts over pure transition metals for CO reduction