Tatyana Kuznetsova

Multiscale approach to CO2 hydrate formation in aqueous solution: Phase field theory and molecular dynamics. Nucleation and growth

A phase field theory with model parameters evaluated from atomistic simulations/experiments is applied to predict the nucleation and growth rates of solid CO(2) hydrate in aqueous solutions under conditions typical to underwater natural gas hydrate reservoirs. It is shown that under practical conditions a homogeneous nucleation of the hydrate phase can be ruled out. The growth rate of CO(2) hydrate dendrites has been determined from phase field simulations as a function of composition while using a physical interface thickness (0.85+/-0.07 nm) evaluated from molecular dynamics simulations. The growth rate extrapolated to realistic supersaturations is about three orders of magnitude larger than the respective experimental observation. A possible origin of the discrepancy is discussed. It is suggested that a kinetic barrier reflecting the difficulties in building the complex crystal structure is the most probable source of the deviations.

Pt-Supported Nanocrystalline Ceria-Zirconia Doped with La, Pr or Gd: Factors Controlling Syngas Generation in Partial Oxidation/Autothermal Reforming of Methane or Oxygenates

Nanocrystalline CeO2-ZrO2 (Ce:Zr 1:1) samples doped with La, Pr or Gd cations (containing up to 30 at.%) were prepared via the Pechini route. Pt (1.4 wt.%) was supported via impregnation with H2PtCl6 solution followed by drying and calcination. The samples’ surface features were studied by SIMS and FTIRS of adsorbed CO. The oxygen mobility was characterized by the dynamic oxygen isotope exchange and H2 TPR. Catalytic activity was studied in the flow installation using diluted feeds (0.7% CH4 +0.5% O2 or 1% C3H6O + 0.5% O2 +0.5% H2O in He). In the selective oxidation of methane (POM), the catalytic activity correlates with Pt dispersion controlled by the oxidized sample’s ability to stabilize Pt2+ cations as precursors of small reactive Pt clusters formed under reaction conditions. This is favoured by a larger doping cation (La) and a developed network of nanodomain boundaries. At comparable Pt dispersion, the highest performance was demonstrated by a La-doped system, which correlates with the highest surface/near-surface oxygen mobility controlled by the strength of Ce-O bonds in the surface layer. In the autothermal reforming of acetone, the activity trends differ from those in POM because of the more prominent role of the oxygen mobility required to prevent surface coking.

Thermodynamic properties and phase transtions in the H2O/CO2/CH4 system

The availability of free energy densities as functions of temperature, pressure and the composition of all components is required for the development of a three-component phase field theory for hydrate phase transitions. We have broadened the extended adsorption theory due to Kvamme and Tanaka (J. Phys. Chem., 1995, 99, 7114) through derivation of the free energy density surface in case of CO2 and CH4 hydrates. A combined free energy surface for the liquid phases has been obtained from a SRK equation of state and solubility measurements outside hydrate stability. The full thermodynamic model is shown to predict water–hydrate equilibrium properties in agreement with experiments. Molecular dynamics simulations of hydrates in contact with water at 200 bar and various temperatures allowed us to estimate hard-to-establish properties needed as input parameters for the practical applications of proposed theories. The 5–95 confidence interval for the interface thickness for the methane hydrate/liquid water is estimated to 8.54 A. With the additional information on the interface free energy, the phase field theory will contain no adjustable parameters. We provide a demonstration of how this theory can be applied to model the kinetics of hydrate phase transitions. The growth of hydrate from aqueous solution was found to be rate limited by mass transport, with the concentration of solute close to the hydrate approaching the value characterizing the equilibrium between the hydrate and the aqueous solution. The depth of the interface was estimated by means of the phase field analysis; its value is close to the interface thickness yielded by molecular simulations. The variation range of the concentration field was estimated to approximately 1/3 of the range of the phase field.

Thermodynamic properties and interfacial tension of a model water–carbon dioxide system

We performed molecular dynamics (MD) simulations of liquid–liquid and liquid–vapor interfaces between bulk water and carbon dioxide. Interfacial systems, constructed from periodically replicated slabs, were studied at different pressures and temperatures by means of npT and nVT MD. Constant-pressure runs of 3, 1.2, and 0.3 nanoseconds were used to estimate the water–CO2 interfacial tension at 284.5 K and 298 K for a liquid–liquid system comprising 108 SPC water molecules and 108 three-site CO2 molecules. The liquid–vapor interface was studied under nVT conditions using 108 water molecules and 32 CO2 molecules. Interfacial tension was obtained from the difference between pressure components normal and tangential to the interface. The results showed a surprisingly (CO2 potential used has been never optimized for water–CO2 interaction) good agreement with experimental data; our model system also reproduced the pressure–temperature relationship of the interfacial tension. A second liquid–liquid system of 256 SPC water and 108 CO2 molecules was tested for temperature persistence of the interface at higher pressures (100 atm and 300 atm). The results of the simulation prove the feasibility of using the model system to predict the key properties of liquid–liquid water–carbon dioxide interface under widely varying conditions, including those relevant for deep-sea disposal of carbon dioxide.

Ceria-Zirconia Nanoparticles Doped with La or Gd: Effect of the Doping Cation on the Real Structure

The real structure of nanocrystalline CeO2-ZrO2 (Ce:Zr=1:1) systems prepared via the polymerized polyester precursor (Pechini) route and doped with La3+ or Gd3+ cations, up to 30 at.%, was studied by X-ray powder diffraction, EXAFS and Raman spectroscopy and the surface features characterized by XPS and SIMS. Undoped CeO2-ZrO2 system revealed nanoscale heterogeneity, perhaps due to the co-existence of Zr- or Ce-enriched domains. With large La3+ dopant the system remains bi-phasic within the studied ranges of composition, incorporation of the smaller Gd3+ cation stabilizes the single-phase solid solution. For both systems, the increase of dopant content was accompanied by a decline of domain size and an increase of the average lattice parameter of fluorite-like phases. Depletion of the surface layer by smaller Zr4+ cations was observed, while the surface content of a doping cation is either, close to that in the bulk (La) or below it (Gd). Such a spatial distribution of components results in some ordering of cations within the lattice. It is reflected in different modes of rearrangement of oxygen coordination polyhedra with the Gd or La content (distances and coordination numbers by EXAFS), and specificity of XRD patterns not conforming to a simple model with statistical distribution of oxygen vacancies.

CATION/ANION MODIFIED CERIA-ZIRCONIA SOLID SOLUTIONS PROMOTED BY Pt AS CATALYSTS OF METHANE OXIDATION INTO SYNGAS BY WATER IN REVERSIBLE REDOX CYCLES

Ca and/or F-modified fluorite-like Ce-Zr-mixed oxides have been prepared by Pechinis method. The bulk structure of samples was characterized by XRD, EXAFS and FTIRS of the lattice modes. The surface properties were studied by SIMS and FTIRS of adsorbed CO and surface hydroxyls. The specific reactivity of the surface oxygen, its amount, coefficients of bulk and near-surface diffusion, as dependent upon the sample composition and temperature, were estimated using sample reduction by CO in the pulse/flow mode. Insertion of fluorine into the lattice results in decreasing the degree of oxygen polyhedra distortion, thus decreasing the amount of reactive oxygen and diffusion coefficients. Calcium and Pt addition counteracts this effect. At 500oC for Pt-supported Ce-Zr-O samples including those modified by Ca and F, the lattice oxygen is easily removed by methane generating CO and hydrogen with high selectivity. Reoxidation of reduced samples by water or carbon dioxide at the same temperature restores the oxygen capacity producing more hydrogen or carbon monoxide.

GRAND CANONICAL MOLECULAR DYNAMICS FOR TIP4P WATER SYSTEMS

An algorithm was developed enabling implementation of a Nose—Hoover thermostat within the framework of grand canonical molecular dynamics [Lynch, C. G. and Pettitt, B. M., 1997, J. chem. Phys., 107, 8594]. The proposed algorithm could readily be extended to mixtures of molecular species with different chemical potentials as shown in the paper. This algorithm was first applied to simulate a μVT ensemble of TIP4P water molecules at 298 K by means of a system comprising a number of full particles and a single scaled (fractional) particle, with the scaling factor considered as a dynamic variable in its own right and chemical potential a pre-set parameter. Our finding showed that the scheme with a single fractional particle tended to freeze in metastable states as well as failed to reproduce either the real-life (−24.05 kJmol−1) or the model-specific chemical potential of water (−23.0kJ mol−1). In order to overcome the inadequacy of a single fractional particle as a chemical potential ‘probe’ the treatment of ...

Atomistic computer simulations for thermodynamic properties of carbon dioxide at low temperatures

Abstract Investigation into the volumetric and energetic properties of two atomistic models mimicking carbon dioxide geometry and quadrupole moment covered the liquid–vapor coexistence curve. Thermodynamic integration over a polynomial path was used to calculate free energy. Computational results showed that the model using GROMOS Lennard–Jones parameters was unsuitable for bulk or interface CO 2 simulations. On the other hand, the model with potential fitted to reproduce only the correct density–pressure relationship in the supercritical region proved to yield the correct enthalpy of vaporization and free energy of liquid CO 2 in the low temperature region. NPT molecular dynamics was used to estimate the water–CO 2 interfacial tension and solubilities at 276 K for a liquid–liquid system at 100 and 300 atm.

Phase Field Approaches to the Kinetic Modeling of Hydrate Phase Transitions

A phase field theory (PFT) with model parameters evaluated from atomistic simulations and experiments is applied for describing the nucleation and growth and the dissolution of CO2 hydrate in aqueous solutions under conditions typical to underwater natural-gas-hydrate reservoirs. We show that the size of the critical fluctuations (nuclei) is comparable to the interface thickness, and thus the PFT predicts a considerably lower nucleation barrier height and higher nucleation rate than the classical approach that relies on a sharp interface. The growth rates of CO2 hydrate corresponding to different growth geometries (planar, circular, and dendritic) have been determined. The predicted growth rates are consistent with experiments performed under similar conditions. An alternative phase approach, based on cellular automata, has also been formulated and applied to the same model systems. Time dependence for this approach is derived by relating the diffusivity to the interface thickness. For small times, the two approaches appear to give similar results but deviate significantly for larger time scales. Dissolution rates of the hydrate phase have been studied as a function of CO2 concentration in the aqueous solution. On the basis of a simple model of foreign particles, qualitative simulations were performed to describe hydrate formation in porous media. The Avrami-Kolmogorov exponent evaluated from these simulations varies substantially with the volume fraction occupied by the foreign particles.

Viabilty of atomistic potentials for thermodynamic properties of carbon dioxide at low temperatures.

Investigation into volumetric and energetic properties of several atomistic models mimicking carbon dioxide geometry and quadrupole momentum covered the liquid–vapor coexistence curve. Thermodynamic integration over a polynomial and an exponential‐polynomial path was used to calculate free energy. Computational results showed that model using GROMOS Lennard–Jones parameters was unsuitable for bulk CO2 simulations. On the other hand, model with potential fitted to reproduce only correct density–pressure relationship in the supercritical region proved to yield correct enthalpy of vaporization and free energy of liquid CO2 in the low‐temperature region. Except for molar volume at the upper part of the vapor–liquid equilibrium line, the bulk properties of exp‐6‐1 parametrization of ab initio CO2 potential were in a close agreement with the experimental results.