58 research outputs found
Ammonia Synthesis on Proton-enriched Palladium Substrate
Production of Ammonia is one of the most important in the chemical industry; most of
the Ammonia produced is then converted to other Nitrogen-containing molecules, such as
anhydrous ammonium nitrate or urea, which are typically used for inhorganic fertilizers.
The standard industrial reaction for the production of Ammonia is the Haber-Bosch Process
(sec. 1.1); an high-pressure and high-temperature process known and developed since the
early twentieth century [31, 11]. The Haber-Bosch process produces Ammonia directly from
its components, Nitrogen and Hydrogen, supplied in gas form. Although successful, this
industrial process, consumes a large amount of energy which, given the huge production
volume, employs a significant percentage of total world energy production and natural gas
consuption [70].
Because of its economical importance, improving the Ammonia production process is still
an open challenge and a rich field of research. A search in related journal archives for
papers regarding Ammonia production typically returns hundreds or thousands of results;
nevertheless there is still room for improvement and better understanding of the reaction.
A part of the studies have been focused on improving the Haber-Bosch process itself; e.g.
replacing the original Iron catalyst with more efficient metal alloys. However other studies
have tried to find novel reaction mechanisms, that could possible overcome the high-pressure
and high-temperature requirements. Some studies have been based on certain biological
process, occuring in bacteria, that can produce Ammonia at room temperature and pressure
[77, 68, 24]. In the biological process the energy required by the reaction is not supplied thermally but by reduction of adenosine triphosphate (ATP) molecules, which makes them
dicult to reproduce in vitro.
Another possible approach is to supply the required energy electrochemically, by applying
an external electric eld to a solid-state catalyst, or having a certain amount of current
circulating through it, it is possible to transfer to the reactants the necessary amount of
energy. The electrical source can also act in an indirect way, modifying the nano-scale
properties of the catalyst in a way that improves its properties; this eect is called non-
Faradayc Electrochemical Modification of Catalytic Reaction (NEMCA) [56, 80].
In 1998 Stoukides and coworkers [50, 49] (sec. 1.2)demonstrated the possibility of producing
Ammonia at atmospheric pressure by supplying the required amount of Hydrogen via a
proton-conducting perovskite. The perovskite, in the form of a little brick or a pipe, is
coated on two sides with a Palladium paste, obtaining two catalytic surfaces separated by
a proton conductor. The two catalysts are then connected through an electrical circuit and
a certain bias is applied, making them act as an anode and a cathode. When the anode
is exposed to molecular Hydrogen, H2 dissociates spontaneously. Because of the bias, the
Hydrogen atoms are stripped of their electron and forced to cross the proton conductor; the
electron will instead travel through the external electrical circuit; recombination of protons
and hydrogens happens in the cathode. The cathode itself is exposed to Nitrogen gas.
Combining this electrochemical mechanism with high temperature (between 500 C and
750 C) the author were able to induce a steady
ux of Ammonia, with a high conversion
efficiency. They also proposed the presence of a NEMCA effect, although weak. In the
present work we will re-examine the original experiment and tackle its characteristic and
inner mechanics at the nano-scale level. We will use ab-initio methods (chap. 2) to construct
a model of the catalytic process and simulate its intermediate steps. In our study we have
used the computational tools provided by the Quantum-espresso distribution [28]. In
particular we will use Density Functional Theory (sec. 2.2) and the Projector-Augmented
Wave method (PAW, sec. 2.3) to reproduce the electronic structure of the system; the
Born-Oppenheimer approximation, together with the Hellmann-Feynman theorem, will be implicitly used to optimize the nanoscopic structure, and find intermediate steps of the
reactions. We will also used the Transition State Theory (sec. 3.3) and the Nudged-
Elastic Band method (sec. 3.4) to estimate the energy barriers involved in the reaction and,
consequently, the reaction rate.
In chapter 4 we will examine the catalyst structure in detail; in particular we will focus
on the effect of active Hydrogen pumping by mean of the applied cell potential. In section
4.1 we will see how a cell potential of realistic amplitude can force a very large amount
of Hydrogen in the Palladium bulk. The resulting system, called Palladium hydride, can
undergo a phase transition that changes its unit-cell volume up to 10%; we will see the
details of this phase change and the possibly resulting structures.
In section 4.2 we will move our focus to the catalyst surface. We will tackle the problem
of determining the adsorbed Hydrogen population, in the case of normal Palaldium and
Palladium hydride. This complex problem involves a three-phase equilibrium, where the
chemical potentials of Hydrogen in the gas and Hydrogen adsorbed in the bulk or on the
surface have to be equal. In order to estimate the chemical potential we will use a Monte
Carlo simulation built on top of a simplified model where the total energy is computed as
a sum of adsorption on-site energies and neighbour-site interactions.
Finally, in chapter 5, we will tackle the core of the problem, trying to find a suitable reaction
path for the Ammonia production. We will examine the possibility of Nitrogen dissociative
adsorption as well as the possiblity of Nitrogen hydrogenation prior to its dissociation. The
former will be easily proved impossible, at the experimental conditions, so we will focus
on the subsequent hydrogenations of the N2 molecules. We will examine the process up to
the final breaking of the N{N bond, where the formation of Ammonia can proceed without
further barriers. An order of magnitude estimate of the Ammonia production in the system
will be made and found to be compatible with the experimental findings
Anharmonic properties from a generalized third order ab~initio approach: theory and applications to graphite and graphene
We have implemented a generic method, based on the 2n+1 theorem within
density functional perturbation theory, to calculate the anharmonic scattering
coefficients among three phonons with arbitrary wavevectors. The method is used
to study the phonon broadening in graphite and graphene mono- and bi-layer. The
broadening of the high-energy optical branches is highly nonuniform and
presents a series of sudden steps and spikes. At finite temperature, the two
linearly dispersive acoustic branches TA and LA of graphene have nonzero
broadening for small wavevectors. The broadening in graphite and bi-layer
graphene is, overall, very similar to the graphene one, the most remarkable
feature being the broadening of the quasi acoustical ZO' branch. Finally, we
study the intrinsic anharmonic contribution to the thermal conductivity of the
three systems, within the single mode relaxation time approximation. We find
the conductance to be in good agreement with experimental data for the
out-of-plane direction but to underestimate it by a factor 2 in-plane
Ab initio variational approach for evaluating lattice thermal conductivity
We present a first-principles theoretical approach for evaluating the lattice
thermal conductivity based on the exact solution of the Boltzmann transport
equation. We use the variational principle and the conjugate gradient scheme,
which provide us with an algorithm faster than the one previously used in
literature and able to always converge to the exact solution. Three-phonon
normal and umklapp collision, isotope scattering and border effects are
rigorously treated in the calculation. Good agreement with experimental data
for diamond is found. Moreover we show that by growing more enriched diamond
samples it is possible to achieve values of thermal conductivity up to three
times larger than the commonly observed in isotopically enriched diamond
samples with 99.93% C12 and 0.07 C13
Second order structural phase transitions, free energy curvature, and temperature-dependent anharmonic phonons in the self-consistent harmonic approximation: theory and stochastic implementation
The self-consistent harmonic approximation is an effective harmonic theory to
calculate the free energy of systems with strongly anharmonic atomic
vibrations, and its stochastic implementation has proved to be an efficient
method to study, from first-principles, the anharmonic properties of solids.
The free energy as a function of average atomic positions (centroids) can be
used to study quantum or thermal lattice instability. In particular the
centroids are order parameters in second-order structural phase transitions
such as, e.g., charge-density-waves or ferroelectric instabilities. According
to Landau's theory, the knowledge of the second derivative of the free energy
(i.e. the curvature) with respect to the centroids in a high-symmetry
configuration allows the identification of the phase-transition and of the
instability modes. In this work we derive the exact analytic formula for the
second derivative of the free energy in the self-consistent harmonic
approximation for a generic atomic configuration. The analytic derivative is
expressed in terms of the atomic displacements and forces in a form that can be
evaluated by a stochastic technique using importance sampling. Our approach is
particularly suitable for applications based on first-principles
density-functional-theory calculations, where the forces on atoms can be
obtained with a negligible computational effort compared to total energy
determination. Finally we propose a dynamical extension of the theory to
calculate spectral properties of strongly anharmonic phonons, as probed by
inelastic scattering processes. We illustrate our method with a numerical
application on a toy model that mimics the ferroelectric transition in
rock-salt crystals such as SnTe or GeTe
Anharmonic phonon spectra of PbTe and SnTe in the self-consistent harmonic approximation
At room temperature, PbTe and SnTe are efficient thermoelectrics with a cubic
structure. At low temperature, SnTe undergoes a ferroelectric transition with a
critical temperature strongly dependent on the hole concentration, while PbTe
is an incipient ferroelectric. By using the stochastic self-consistent harmonic
approximation, we investigate the anharmonic phonon spectra and the occurrence
of a ferroelectric transition in both systems. We find that vibrational spectra
strongly depends on the approximation used for the exchange-correlation kernel
in density functional theory. If gradient corrections and the theoretical
volume are employed, then the calculation of the free energy Hessian leads to
phonon spectra in good agreement with experimental data for both systems. In
PbTe, we reproduce the transverse optical mode phonon satellite detected in
inelastic neutron scattering and the crossing between the transverse optical
and the longitudinal acoustic modes along the X direction. In the case
of SnTe, we describe the occurrence of a ferroelectric transition from the high
temperature Fmm structure to the low temperature R3m one.Comment: 12 pages, 15 Picture
First-principles calculations of phonon frequencies, lifetimes and spectral functions from weak to strong anharmonicity: the example of palladium hydrides
The variational stochastic self-consistent harmonic approximation is combined
with the calculation of third-order anharmonic coefficients within
density-functional perturbation theory and the "" theorem to calculate
anharmonic properties of crystals. It is demonstrated that in the perturbative
limit the combination of these two methods yields the perturbative phonon
linewidth and frequency shift in a very efficient way, avoiding the explicit
calculation of fourth-order anharmonic coefficients. Moreover, it also allows
calculating phonon lifetimes and inelastic neutron scattering spectra in solids
where the harmonic approximation breaks down and a non-perturbative approach is
required to deal with anharmonicity. To validate our approach, we calculate the
anharmonic phonon linewidth in the strongly anharmonic palladium hydrides. We
show that due to the large anharmonicity of hydrogen optical modes the
inelastic neutron scattering spectra are not characterized by a Lorentzian
line-shape, but by a complex structure including satellite peaks
Phonon anomalies and lattice dynamics in superconducting oxychlorides CaCuOCl
We present a comprehensive study of the phonon dispersion in an underdoped,
superconducting CaCuOCl crystal. We interpret the results using
lattice dynamical calculations based on a shell model, and we compare the
results, to other hole-doped cuprates, in particular to the ones isomorphic to
LaSrCuO (LSCO). We found that an anomalous dip in the Cu-O bond
stretching dispersion develops in oxychlorides with a simultaneous marked
broadening of the mode. The broadening is maximum at that corresponds to the charge-modulations propagation vector. Our analysis
also suggests that screening effects in calculations may cause an apparent
cosine-shaped bending of the Cu-O bond-stretching dispersion along both the
( 0 0) and ( 0) directions, that is not observed on the data close to
optimal doping. This observation suggests that the discrepancy between
experimental data and \textit{ab-initio} calculations on this mode originates
from an overestimation of the doping effects on the mode
Phonon Collapse and Second-Order Phase Transition in Thermoelectric SnSe
Since 2014 the layered semiconductor SnSe in the high-temperature Cmcm phase is known to be the most efficient intrinsic thermoelectric material. Making use of first-principles calculations we show that its vibrational and thermal transport properties are determined by huge nonperturbative anharmonic effects. We show that the transition from the Cmcm phase to the low-symmetry Pnma is a second-order phase transition driven by the collapse of a zone border phonon, whose frequency vanishes at the transition temperature. Our calculations show that the spectral function of the in-plane vibrational modes are strongly anomalous with shoulders and double-peak structures. We calculate the lattice thermal conductivity obtaining good agreement with experiments only when nonperturbative anharmonic scattering is included. Our results suggest that the good thermoelectric efficiency of SnSe is strongly affected by the nonperturbative anharmonicity
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