125 research outputs found
The energetics of water on oxide surfaces by quantum Monte Carlo
Density functional theory (DFT) is widely used in surface science, but gives
poor accuracy for oxide surface processes, while high-level quantum chemistry
methods are hard to apply without losing basis-set quality. We argue that
quantum Monte Carlo techniques allow these difficulties to be overcome, and we
present diffusion Monte Carlo results for the formation energy of the MgO(001)
surface and the adsorption energy of HO on this surface, using periodic
slab geometry. The results agree well with experiment. We note other oxide
surface problems where these techniques could yield immediate progress.Comment: 5 pages, 2 figure
Ab initio statistical mechanics of surface adsorption and desorption: II. Nuclear quantum effects
We show how the path-integral formulation of quantum statistical mechanics
can be used to construct practical {\em ab initio} techniques for computing the
chemical potential of molecules adsorbed on surfaces, with full inclusion of
quantum nuclear effects. The techniques we describe are based on the
computation of the potential of mean force on a chosen molecule, and generalise
the techniques developed recently for classical nuclei. We present practical
calculations based on density functional theory with a generalised-gradient
exchange-correlation functional for the case of HO on the MgO~(001) surface
at low coverage. We note that the very high vibrational frequencies of the
HO molecule would normally require very large numbers of time slices
(beads) in path-integral calculations, but we show that this requirement can be
dramatically reduced by employing the idea of thermodynamic integration with
respect to the number of beads. The validity and correctness of our
path-integral calculations on the HO/MgO~(001) system are demonstrated by
supporting calculations on a set of simple model systems for which quantum
contributions to the free energy are known exactly from analytic arguments.Comment: 11 pages, including 2 figure
Ab-initio chemical potentials of solid and liquid solutions and the chemistry of the Earth's core
A general set of methods is presented for calculating chemical potentials in
solid and liquid mixtures using {\em ab initio} techniques based on density
functional theory (DFT). The methods are designed to give an {\em ab initio}
approach to treating chemical equilibrium between coexisting solid and liquid
solutions, and particularly the partitioning ratio of solutes between such
solutions. For the liquid phase, the methods are based on the general technique
of thermodynamic integration, applied to calculate the change of free energy
associated with the continuous interconversion of solvent and solute atoms, the
required thermal averages being computed by DFT molecular dynamics simulation.
For the solid phase, free energies and hence chemical potentials are obtained
using DFT calculation of vibrational frequencies of systems containing
substitutional solute atoms, with anharmonic contributions calculated, where
needed, by thermodynamic integration. The practical use of the methods is
illustrated by applying them to study chemical equilibrium between the outer
liquid and inner solid parts of the Earth's core, modelled as solutions of S,
Si and O in Fe. The calculations place strong constraints on the chemical
composition of the core, and allow an estimate of the temperature at the
inner-core/outer-core boundary.Comment: 19 pages, two figure
Complementary approaches to the ab initio calculation of melting properties
Several research groups have recently reported {\em ab initio} calculations
of the melting properties of metals based on density functional theory, but
there have been unexpectedly large disagreements between results obtained by
different approaches. We analyze the relations between the two main approaches,
based on calculation of the free energies of solid and liquid and on direct
simulation of the two coexisting phases. Although both approaches rely on the
use of classical reference systems consisting of parameterized empirical
interaction models, we point out that in the free energy approach the final
results are independent of the reference system, whereas in the current form of
the coexistence approach they depend on it. We present a scheme for correcting
the predictions of the coexistence approach for differences between the
reference and {\em ab initio} systems. To illustrate the practical operation of
the scheme, we present calculations of the high-pressure melting properties of
iron using the corrected coexistence approach, which agree closely with earlier
results from the free energy approach. A quantitative assessment is also given
of finite-size errors, which we show can be reduced to a negligible size.Comment: 14 pages, two figure
Growth of Dome-Shaped Carbon Nanoislands on Ir(111): The Intermediate between Carbidic Clusters and Quasi-Free-Standing Graphene (vol 103, 166101, 2009)
Melting curve of face-centered-cubic nickel from first-principles calculations
The melting curve of Ni up to 100 GPa has been calculated using first-principles methods based on density functional theory (DFT). We used two complementary approaches: (i) coexistence simulations with a reference system and then free-energy corrections between DFT and the reference system, and (ii) direct DFT coexistence using simulation cells including 1000 atoms. The calculated zero pressure melting temperature is slightly underestimated at 1637±10 K (experimental value is 1728 K), and at high pressure is significantly higher than recent measurements in diamond-anvil cell experiments [ Phys. Rev. B 87 054108 (2013)]. The zero pressure DFT melting slope is calculated to be 30±2 K, in good agreement with the experimental value of 28 K
An Old but Lively Nanomaterial: Exploiting Carbon Black for the Synthesis of Advanced Materials
Carbon black (CB) is an old-concept but versatile carbonaceous material prone to be structurally and chemically modified under quite mild wet conditions. Recently, we exploited the potentiality of CB for the production of a highly varied array of advanced materials with applications in energetics, water remediation and sensoristic. The proposed approaches are devised to meet specific needs: low production costs, scalable synthetic approaches, flexibility i.e. easy tuning of chemico-physical properties of the carbon-based advanced materials. Two main approaches have been exploited: modification of CB at the surface and highly CB de-structuration. The former approach allows obtaining highly homogenous CB-modified nanoparticles (around 160 nm) with tunable surface properties (hydrophilicity, typology of functional groups and surface charge density, pore size distribution), supports for ionic liquid (SILP) and composites (carbon-iron oxide). The latter approach exploiting a top-down demolition of CB produces a highly versatile graphene related material (GRM), made up by stacked short graphene-like layers (GL) particularly suitable for advanced composites synthesis and ultrathin carbon-based films production
Eumelanin Graphene-Like Integration: The Impact on Physical Properties and Electrical Conductivity
The recent development of eumelanin pigment-based blends integrating "classical" organic conducting materials is expanding the scope of eumelanin in bioelectronics. Beyond the achievement of high conductivity level, another major goal lays in the knowledge and feasible control of structure/properties relationship. We systematically investigated different hybrid materials prepared by in situ polymerization of the eumelanin precursor 5,6-dihydroxyindole (DHI) in presence of various amounts of graphene-like layers. Spectroscopic studies performed by solid state nuclear magnetic resonance (ss-NMR), x-ray photoemission, and absorption spectroscopies gave a strong indication of the direct impact that the integration of graphene-like layers into the nascent polymerized DHI-based eumelanin has on the structural organization of the pigment itself, while infrared, and photoemission spectroscopies indicated the occurrence of negligible changes as concerns the chemical units. A tighter packing of the constituent units could represent a strong factor responsible for the observed improved electrical conductivity of the hybrid materials, and could be possible exploited as a tool for electrical conductivity tuning
Inherent Metal Elements in Biomass Pyrolysis: A Review
One of the main drawbacks of using biomass as pyrolysis feedstock consists of the huge variability of the different biomass resources which undermines the viability of downstream processes. Inherent inorganic elements greatly contribute to enhance the compositional variability issues due to their catalytic effect (especially alkali and alkaline earth metals (AAEMs)) and the technical problems arising due to their presence. Due to the different pretreatments adopted in the experimental investigations as well as the different reactor configurations and experimental conditions, some mechanisms involving interactions between these elements and the biomass organic fraction during pyrolysis are still debated. This is the reason why predicting the results of these interactions by adapting the existing kinetic models of pyrolysis is still challenging. In this work, the most prominent experimental works of the last 10 years dealing with the catalytic effects of biomass inherent metals on the pyrolysis process are reviewed. Reaction pathways, products distributions and characteristics, and impacts on the products utilization are discussed with a focus on AAEMs and on potential toxic metallic elements in hyperaccumulator plants. The literature findings are discussed in relation to the applied laboratory procedures controlling the concentration of inherent inorganic elements, their capability of preserving the chemical integrity of the main organic components, and the ability of resembling the inherent inorganic elements in the raw biomass. The goal is to reveal possible experimental inconsistencies and to provide a clear scheme of the reaction pathways altered by the presence of inherent inorganics. This analysis paves the way for the examination of the proposed modifications of the existing models aiming at capturing the effect of inorganics on pyrolysis kinetics. Finally, the most relevant shortcomings and bottlenecks in existing experimental and modeling approaches are analyzed and directions for further studies are suggested
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