9 research outputs found
Adsorption and dissociation of molecular oxygen on the (0001) surface of double hexagonal close packed americium
In our continuing attempts to understand theoretically various surface
properties such as corrosion and potential catalytic activity of actinide
surfaces in the presence of environmental gases, we report here the first ab
initio study of molecular adsorption on the double hexagonal packed (dhcp)
americium (0001) surface. Dissociative adsorption is found to be energetically
more favorable compared to molecular adsorption. The most stable configuration
corresponds to a horizontal approach molecular dissociation with the oxygen
atoms occupying neighboring h3 sites, with chemisorption energies at the NSOC
and SOC theoretical levels being 9.395 eV and 9.886 eV, respectively. The
corresponding distances of the oxygen molecule from the surface and
oxygen-oxygen distance were found to be 0.953 Ang. and 3.731 Ang.,
respectively. Overall our calculations indicate that chemisorption energies in
cases with SOC are slightly more stable than the cases with NSOC in the
0.089-0.493 eV range. The work functions and net magnetic moments respectively
increased and decreased in all cases compared with the corresponding quantities
of the bare dhcp Am (0001) surface. The adsorbate-substrate interactions have
been analyzed in detail using the partial charges inside the muffin-tin
spheres, difference charge density distributions, and the local density of
states. The effects, if any, of chemisorption on the Am 5f electron
localization-delocalization characteristics in the vicinity of the Fermi level
are also discussed.Comment: 6 tables, 10 figure
Exploring the Ring-Opening Pathways in the Reaction of Morpholinyl Radicals with Oxygen Molecule
Quantum chemistry calculations using hybrid density functional
theory and the coupled-cluster method have been performed to investigate
the ring-opening pathways in the oxidation of morpholine (1-oxa-4-aza-cyclohexane).
Hydrogen abstraction can form two different carbon-centered radicals,
morpholin-2-yl or morpholin-3-yl, or the nitrogen-centered radical,
morpholin-4-yl, none of which are found to have low-energy pathways
to ring-opening. Extensive exploration of multiple reaction pathways
following molecular oxygen addition to these three radicals revealed
two competitive low energy pathways to ring-opening. Addition of O<sub>2</sub> to either carbon-centered radical, followed by a 1,4-H shifting
mechanism can yield a long-lived cyclic epoxy intermediate, susceptible
to ring-opening, following further radical attack. In particular,
the second pathway begins with O<sub>2</sub> attack on morpholin-2-yl,
followed by a 1,5-H shift and a unimolecular ring-opening without
having to overcome a high barrier, releasing a significant amount
of heat in the overall ring-opening reaction. The calculations provide
valuable context for the development of mechanisms for the low temperature
combustion chemistry of nitrogen and oxygen-containing fuels
Atomic-Scale Structure and Stability of the Low-Index Surfaces of Pyrochlore Oxides
The
multifunctional properties of complex ternary oxides such as
pyrochlores are often influenced by surface structure. Optimizing
the surface-driven attributes of these materials necessitates a detailed
understanding of the structure and chemical composition of those surfaces.
Here we report atomistic simulations elucidating the diverse atomic-scale
structures of a set of low-index surfaces [(100), (110), (111), and
(112)] in pyrochlore compounds as a function of both A and B cation
chemistry. In pyrochlores, the low-index facets are all dipolar, requiring
the introduction of surface defects to eliminate the surface dipole.
We find that, due to the corresponding higher coordination of the
surface cations, the (110) facet is the most energetically stable
in all of the compounds considered, an interesting contrast to fluorite,
in which the (111) surface is most stable. We also reveal a correlation
between the surface energy and the energy to disorder the pyrochlore
as a function of B cation chemistry, implying a similar physical origin
for the two phenomena. Further, we find that surface rumpling is common
across all pyrochlore compounds. An even more interesting feature
emerging at these surfaces is the formation of extended structural
defects such as steps and trenches, which are found to be stable after
high-temperature annealing. As the formation of these features is
a consequence of surface defects introduced to eliminate the surface
dipole, we propose that the superior surface properties of materials
of pyrochlores are due to these extended structural features, which
are a direct consequence of the inherent dipole at the surfaces