71 research outputs found
Surface oxygen dynamics and oxidation on cobalt spinel surface probed by isotopic exchange and accounted for by DFT molecular modeling : facile interfacial oxygen atoms flipping through transient peroxy intermediate
Isotopic evidence for the tangled mechanism of the CO-PROX reaction over mixed and bare cobalt spinel catalysts
Adsorption of and on cobalt spinel nanocubes and interfacial dynamics of the resultant adspecies (x = 1, 2, and 3) : DFT, atomistic thermodynamic, IR, and isotopic exchange study
Periodic DFT+D molecular modeling of the interface : electronic structure, chemical bonding, adhesion and strain
Electronic structure, bonding characteristics, adhesion, and stress energy of the Zn-MOF-5(100)/(110) rutile interface were modeled by using periodic DFT+D calculations, corroborated by simulation of high resolution transmission electron microscopy (HR-TEM) images. Adjustment of the flexible metal–organic framework (MOF) moiety to the rigid rutile substrate was achieved within a supercell comprised of (1 × 1) Zn-MOF-5 and (4 × 9) TiO2 units. It was shown that binding of the Zn-MOF-5 layer takes place via bidentate 1,4-benzenedicarboxylate (BDC)–titania bridges. A coherent interface can be formed with the minimal periodicity along the [11̅0] direction defined by nine Ti5c adsorption sites (9 × 2.96 Å = 26.64 Å) and two consecutive linkers of the Zn-MOF-5 chain (2 × 12.94 Å = 25.88 Å). The MOF part is tuned to the oxide substrate by tilting the BDC linkers by 10° and twisting around their long axis by 34°. The resultant lattice strain of the Zn-MOF-5 layer was equal to ε[001] = 0.31% and ε[11̅0] = 2.86%, and the associated stress energy to σtotal = 4.8 eV. Pronounced adhesion energy of the Zn-MOF-5 layer deposited on the rutile surface (−0.33 eV/nm2) stems from the sizable dispersion (−0.39 eV/nm2) contribution, counterbalancing the unfavorable lattice strain and bonds distortion components. The calculated density of states structure of the Zn-MOF-5(100)/(110)TiO2 interface revealed that it can be described as an electronically coupled, staggered (Type II) charge injection system, where a photoinduced electron may be directly transferred from the Zn-MOF-5 moiety to the conduction band of the titania substrate
Mono- and diatomic reactive oxygen species produced upon interaction with the (111) facet of cobalt spinel at various conditions : molecular DFT and atomistic thermodynamic investigations
Periodic spin unrestricted
DFT + U calculations
joined with atomistic thermodynamics were used to study the location,
structure, and stability along with the electronic and magnetic properties
of various surface oxygen species and oxygen vacancies, produced under
different thermodynamic conditions on the (111) surface of the cobalt
spinel nano-octahedra. The density of state alignment diagrams between
dioxygen and cobalt centers were used to rationalize speciation of
the surface oxygen varieties into diatomic superoxo (μ-CoO3c–O2–CoT3c, η2-O2–CoO3c) or intrafacial peroxo ([O–Osurf.]2–) and monoatomic metal–oxo (CoT3c–O, CoO3c–O) entities.
It was shown that the surface cobalt cations work in tandem constituting
dual CoO3c–CoT3c sites for O2 adsorption, where the CoO3c dxz(β) and dyz(β) states act as spin-polarized electron
donor centers, producing the most stable bridging μ-CoO3c–O2–CoT3c adducts (ΔEa = −1.86 eV).
The single site mono- and bidentate binding modes η1-O2–CoO3c (ΔEa = −1.66 eV) and η2-O2–CoO3c (ΔEa = −1.12 eV) are less stable. These
results imply that the most probable pathway of dioxygen activation
involves an η1-O2–CoO3c → η2-O2–CoO3c → μ-CoO3c–O2–CoT3c sequence.
Subsequent dissociation of the O–O bond in the bridging peroxide
moiety leads to the formation of ferromagnetically coupled [↑CoO3cIVO↑]3+ and [↑↑↑CoT3cIIO↑]+ species,
characterized by a large difference of their stability (ΔEa = −1.24 and −0.43 eV respectively).
First principles thermodynamic modeling revealed that in typical catalytic
pressures of dioxygen (p O2/p° ∼ 0.01 ÷ 1), the most stable
bridging μ-CoO3c–O2–CoT3c species persist on the surface below 375 °C,
whereas above this temperature the surface is covered with monoatomic
species (CoO3c–O stable up to 475 °C).
At T > 475 °C, bare and then oxygen vacancy
bearing (T > 575 °C) surfaces are thermodynamically
preferred. The cobalt–oxo species may diffuse across the surface
with the involvement of the thermodynamically metastable O2O,1T–O and O3O–O peroxy species. The obtained
results provide a convenient conceptual background for the interpretation
of the redox catalytic and electrocatalytic processes over cobalt
spinel with dioxygen participation
Catalytic performance of mixed spinels obtained by combustion synthesis for preferential carbon monoxide oxidation (CO-PROX) : insights into the factors controlling catalyst selectivity and activity
Copper ionic pairs as possible active sites in decomposition on catalysts
Synthesized
by
impregnation
and
coprecipitation
methods,
ceria-supported
copper
samples
were
tested
in
catalytic
decomposition
of
N
2
O
(deN
2
O)
under
dry
and
wet
conditions.
Basing
on
the
structural
(XRD,
SEM),
spectroscopic
(EPR,
RS)
and
catalytic
characterization,
supported
by
DFT
calculations,
the
role
of
mono-
and
dimeric
copper
as
active
centers
in
the
deN
2
O
reaction
was
evaluated.
Particular
attention
was
paid
to
elucidation
of
the
structure
and
localization
of
the
Cu
2+
Cu
2+
pairs
within
the
CeO
2
matrix,
their
stability
and
chemical
conditions
of
their
formation
Cobalt Spinel at Various Redox Conditions: DFT+U Investigations into the Structure and Surface Thermodynamics of the (100) Facet
Periodic spin unrestricted DFT-PW91+U
calculations together with ab initio thermodynamic modeling were used
to study the structure, defects, and stability of different terminations
of the (100) surface of cobalt spinel under various redox conditions
imposed by different oxygen partial pressure and temperature. Three
terminations containing under-stoichiometric (100)-<b>O</b>,
stoichiometric (100)-<b>S</b>, and overstoichiometric (100)-<b>R</b> amount of cobalt ions were analyzed, and their atomic and
defect structure, reconstruction, and stability were elucidated. For
the most stable (100)-<b>S</b> and (100)-<b>O</b> facets,
formation of cationic and anionic vacancies was examined, and a surface
redox state diagram of possible spinel (100) terminations in the stoichiometry
range from Co<sub>2.75</sub>O<sub>4</sub> to Co<sub>3</sub>O<sub>3.75</sub> was constructed and discussed in detail. The results revealed that
the bare (100)-<b>S</b> surface is the most stable at temperatures
and pressures of typical catalytic processes (<i>T</i> ∼
200 °C to ∼500 °C, <i>p</i><sub>O2</sub>/<i>p</i>° ∼ 0.001 to ∼1). In more reducing
conditions (<i>T</i> > 600 °C and <i>p</i><sub>O2</sub>/<i>p</i>° < 0.0001), the (100)-<b>S</b> facet is readily reduced by formation of oxygen vacancies,
whereas in the oxidizing conditions (<i>T</i> < 200 °C
and <i>p</i><sub>O2</sub>/<i>p</i>° >
10), coexistence of (100)-<b>S</b> and (100)-<b>O</b> terminations
was revealed. Formation of the oxygen vacancies involves reduction
of the octahedral trivalent cobalt and is accompanied by migration
of the divalent tetrahedral cobalt into empty, interstitial octahedral
positions. It was also found that the constituent octahedral Co cation
proximal to the interstitial cobalt adopts a low spin configuration
in contrast to the distal one that preserves its surface high spin
state. In the case of the Co depleted surfaces, the octahedral vacancies
are thermodynamically disfavored with respect to the tetrahedral ones
in the whole range of the examined <i>T</i> and <i>p</i><sub>O2</sub> values. The obtained theoretical results,
supported by TPD-O<sub>2</sub> and TG experiments, show that the octahedral
cobalt ions are directly involved in the redox processes of Co<sub>3</sub>O<sub>4</sub>
Decomposition of N_2O over the surface of cobalt spinel : A DFT account of reactivity experiments
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