449 research outputs found
Photochemical CO2conversion on pristine and Mg-doped gallium nitride (GaN): A comprehensive DFT study based on a cluster model approach
The photochemical reduction of carbon dioxide (CO2) into methanol is very appealing since it requires sunlight as the only energy input. However, the development of highly selective and efficient photocatalysts is still very challenging. It has been reported that CO2 can be spontaneously activated on gallium nitride (GaN). Moreover, the photocatalytic activity for CO2 conversion into methanol can be drastically enhanced by incorporating a small amount of Mg dopant. In this work, density functional theory (DFT) based on a cluster model approach has been applied to further explore the photocatalytic activity of bare GaN towards CO2 adsorption and conversion. We extended the investigation of Mg-doping replacing one Ga atom with Mg on three different sites and evaluated the consequent effects on the band gaps and CO2 adsorption energies. Finally, we explore different routes leading to the production of methanol and evaluate the catalytic activity of bare GaN by applying the energetic span model (ESM) in order to identify the rate-determining states which are fundamental for suggesting modifications that can improve the photocatalytic activity of this promising material
Cyclic Carbonate Formation from Epoxides and CO2Catalyzed by Sustainable Alkali Halide-Glycol Complexes: A DFT Study to Elucidate Reaction Mechanism and Catalytic Activity
We provide a comprehensive DFT investigation of the mechanistic details of CO2 fixation into styrene oxide to form styrene carbonate, catalyzed by potassium iodide-tetraethylene glycol complex. A detailed view on the intermediate steps of the overall reaction clarifies the role of hydroxyl substances as co-catalysts for the alkali halide-catalyzed cycloaddition. The increase of iodide nucleophilicity in presence of tetraethylene glycol is examined and rationalized by NBO and Hirshfeld charge analysis, and bond distances. We explore how different alkali metal salts and glycols affect the catalytic performance. Our results provide important hints on the synthesis of cyclic carbonates from CO2 and epoxides promoted by alkali halides and glycol complexes, allowing the development of more efficient catalysts
Practical cluster models for a layered β-NiOOH material
Due to the high oxygen evolution reaction (OER) activity, stability, and abundance of NiOx materials, they are found to be promising catalysts, competitive with expensive metal oxides such as IrO2 and RuO2. From a theoretical point of view, studies reported in the literature so far are mostly based on density functional theory using periodic slab models for the bulk and surface of βNiOOH, one of the active NiOx phases. However, cluster models are a valid method to investigate many aspects about structure, charge carrier transport properties, and OER activity of β-NiOOH. Hence, here we present new cluster models for the surface of β-NiOOH, where the oxygen atoms are bonded to Mg effective core potentials (ECPs) mimicking neighboring atom cores. This cluster embedding procedure is superior to saturating the cluster with hydrogen atoms, and to using other atomic ECPs for β-NiOOH.We find that layered materials such as β-NiOOH are more vulnerable to geometrical rupture and therefore a cluster approach requires additional care in choosing the embedding approach. We evaluated the models by using them to calculate the energy required for water adsorption and deprotonation, which are essential ingredients for OER. Specifically, our results agree with previous slab models that the first deprotonation reaction step requires a large amount of energy. In addition, we find that water and hydroxyl groups have high adsorption energy and therefore the first deprotonation step is limiting the reaction efficiency
High-temperature expansions through order 24 for the two-dimensional classical XY model on the square lattice
The high-temperature expansion of the spin-spin correlation function of the
two-dimensional classical XY (planar rotator) model on the square lattice is
extended by three terms, from order 21 through order 24, and analyzed to
improve the estimates of the critical parameters.Comment: 7 pages, 2 figure
Dynamical Linked Cluster Expansions: A Novel Expansion Scheme for Point-Link-Point-Interactions
Dynamical linked cluster expansions are linked cluster expansions with
hopping parameter terms endowed with their own dynamics. This amounts to a
generalization from 2-point to point-link-point interactions. We develop an
associated graph theory with a generalized notion of connectivity and describe
an algorithmic generation of the new multiple-line graphs. We indicate physical
applications to spin glasses, partially annealed neural networks and SU(N)
gauge Higgs systems. In particular the new expansion technique provides the
possibility of avoiding the replica-trick in spin glasses. We consider
variational estimates for the SU(2) Higgs model of the electroweak phase
transition. The results for the transition line, obtained by dynamical linked
cluster expansions, agree quite well with corresponding high precision Monte
Carlo results.Comment: 41 pages, latex2e, 10 postscript figure
Updated tests of scaling and universality for the spin-spin correlations in the 2D and 3D spin-S Ising models using high-temperature expansions
We have extended, from order 12 through order 25, the high-temperature series
expansions (in zero magnetic field) for the spin-spin correlations of the
spin-S Ising models on the square, simple-cubic and body-centered-cubic
lattices. On the basis of this large set of data, we confirm accurately the
validity of the scaling and universality hypotheses by resuming several tests
which involve the correlation function, its moments and the exponential or the
second-moment correlation-lengths.Comment: 21 pages, 8 figure
Effect of boundaries on vacuum field fluctuations and radiation-mediated interactions between atoms
In this paper we discuss and review several aspects of the effect of boundary
conditions and structured environments on dispersion and resonance interactions
involving atoms or molecules, as well as on vacuum field fluctuations. We first
consider the case of a perfect mirror, which is free to move around an
equilibrium position and whose mechanical degrees of freedom are treated
quantum mechanically. We investigate how the quantum fluctuations of the
mirror's position affect vacuum field fluctuations for both a one-dimensional
scalar and electromagnetic field, showing that the effect is particularly
significant in the proximity of the moving mirror. This result can be also
relevant for possible gravitational effects, since the field energy density
couples to gravity. We stress that this interaction-induced modification of the
vacuum field fluctuations can be probed through the Casimir-Polder interaction
with a polarizable body, thus allowing to detect the effect of the mirror's
quantum position fluctuations. We then consider the effect of an environment
such as an isotropic photonic crystal or a metallic waveguide, on the resonance
interaction between two entangled identical atoms, one excited and the other in
the ground state. We discuss the strong dependence of the resonance interaction
with the relative position of the atomic transition frequency with the gap of
the photonic crystal in the former case, and with the cut-off frequency of
waveguide in the latter.Comment: 8 pages, 2 figures, Proceedings of the Eighth International Workshop
DICE 2016 Spacetime - Matter - Quantum Mechanic
A study of logarithmic corrections and universal amplitude ratios in the two-dimensional 4-state Potts model
Monte Carlo (MC) and series expansion (SE) data for the energy, specific
heat, magnetization and susceptibility of the two-dimensional 4-state Potts
model in the vicinity of the critical point are analysed. The role of
logarithmic corrections is discussed and an approach is proposed in order to
account numerically for these corrections in the determination of critical
amplitudes. Accurate estimates of universal amplitude ratios ,
, and are given, which arouse
new questions with respect to previous works
The RANLUX generator: resonances in a random walk test
Using a recently proposed directed random walk test, we systematically
investigate the popular random number generator RANLUX developed by Luescher
and implemented by James. We confirm the good quality of this generator with
the recommended luxury level. At a smaller luxury level (for instance equal to
1) resonances are observed in the random walk test. We also find that the
lagged Fibonacci and Subtract-with-Carry recipes exhibit similar failures in
the random walk test. A revised analysis of the corresponding dynamical systems
leads to the observation of resonances in the eigenvalues of Jacobi matrix.Comment: 18 pages with 14 figures, Essential addings in the Abstract onl
Extension to order of the high-temperature expansions for the spin-1/2 Ising model on the simple-cubic and the body-centered-cubic lattices
Using a renormalized linked-cluster-expansion method, we have extended to
order the high-temperature series for the susceptibility
and the second-moment correlation length of the spin-1/2 Ising models on
the sc and the bcc lattices. A study of these expansions yields updated direct
estimates of universal parameters, such as exponents and amplitude ratios,
which characterize the critical behavior of and . Our best
estimates for the inverse critical temperatures are
and . For the
susceptibility exponent we get and for the correlation
length exponent we get .
The ratio of the critical amplitudes of above and below the critical
temperature is estimated to be . The analogous ratio for
is estimated to be . For the correction-to-scaling
amplitude ratio we obtain .Comment: Misprints corrected, 8 pages, latex, no figure
- …