Hydrogen Underground Storage in Silica-Clay Shales: Experimental and Density Functional Theory Investigation.

In the context of reducing the global emissions of greenhouse gases, hydrogen (H2) has become an attractive alternative to substitute the current fossil fuels. However, its properties, seasonal fluctuations, and the lack of extended energy stability made it extremely difficult to be economically and safely stored for a long term in recent years. Therefore, this paper investigated the potential of shale gas reservoirs (rich and low clay-rich silica minerals) to store hydrogen upon demand. Density functional theory molecular simulation was employed to explore hydrogen adsorption on the silica-kaolinite interface, and the physisorption of hydrogen on the shale surface is revealed. This is supported by low adsorption energies on different adsorption configurations (0.01 to -0.21 eV), and the lack of charge transfer showed by Bader charge analysis. Moreover, the experimental investigation was employed to consider the temperature (50-100 °C) and pressure (up to 20 bar) impact on hydrogen uptake on Midra shale, specifically palygorskite (100%), which is rich in silicate clay minerals (58.83% SiO2). The results showed that these formations do not chemically or physically maintain hydrogen; hence, hydrogen can be reversibly stored. The results highlight the potential of shale gas reservoirs to store hydrogen as no hydrogen is adsorbed on the shale surface, so there will be no hydrogen loss and no adverse effect on the shale's structural integrity, and it can be safely stored in shale reservoirs and recovered upon demand.

Ab Initio Molecular Dynamics Investigation of CH4/CO2 Adsorption on Calcite: Improving the Enhanced Gas Recovery Process.

Ab initio molecular dynamics simulations of CH4 and CO2 on the calcite (104) surface have been carried out for the molecular level analysis of CO2-enhanced gas recovery process (EGR). This process takes advantage of the stronger interaction of CO2 with the reservoir walls compared to CH4, therefore can improve the extraction of the latter, while at the same time sequestering the former underground. Pure and mixed gases were considered and the temperature effect on the systems behavior was analyzed. For pure gases, carbon dioxide shows great stability on the surface in the studied temperature range, while methane molecules start leaving the surface at 298 K. For gas mixtures, the reported results confirm that for low to medium concentrations, a temperature of 373 K could determine the best methane extraction efficiency, as CH4 interaction with the surface is quite weak and carbon dioxide binds strongly on the surface. On the other hand, when full coverage is achieved, the best efficiency is reached for the highest temperature. Finally, when considered a 2:2 gas layer, carbon dioxide tends to adsorb preferentially to the surface while methane keeps floating above it, thereby reducing its chance to be adsorbed back. These results reveal nanoscopic details for the design of suitable EGR processes.

Effect of surface morphology on methane interaction with calcite: a DFT study.

Natural gas, consisting primarily of methane, is found in carbonate reservoirs of which calcite is major component. However, the complexity and heterogeneity of carbonate reservoirs remain a major challenge in estimating ultimate recovery. Herein, density functional theory calculations are employed to study the effect of surface morphology on the adsorption of CH4 on the surface of CaCO3 (calcite). Among the 9 different surface symmetries considered, the strongest adsorption (and consequently the largest adsorption capacity) of methane is found for the 110 surface of the material. In fact, the adsorption capacity of this surface is more than an order of magnitude larger than the one for the 104 surface, which is the lowest energy surface for the calcite. The obtained results are explained by structural analysis and charge calculations. These findings can be useful for the estimation of the ultimate gas recovery taking into account heterogeneous porosity and permeability of the carbonate reservoirs.

Understanding and Tuning the Intrinsic Hydrophobicity of Rare-Earth Oxides: A DFT+U Study.

Rare-earth oxides (REOs) possess a remarkable intrinsic hydrophobicity, making them candidates for a myriad of applications. Although the superhydrophobicity of REOs has been explored experimentally, the atomistic details of the structure at the oxide-water interface are still not well understood. In this work, we report a density functional theory study of the interaction between water and CeO2, Nd2O3, and α-Al2O3 to explain their different wettability. The wetting of the metal oxide surface is controlled by geometric and electronic factors. While the electronic term is related to the acid-base properties of the surface layer, the geometric factor depends on the matching between adsorption sites and oxygen atoms from the hexagonal water network. For all the metal oxides considered here, water dissociation is confined to the first oxide-water layer. Hydroxyl groups on α-Al2O3 are responsible for the strong oxide-water interaction, and thus, both Al- and hydroxyl-terminated wet. On CeO2, the intrinsic hydrophobicity of the clean surface disappears when lattice hydroxyl groups (created by the reaction of water with oxygen vacancies) are present as they dominate the interaction and drive wetting. Therefore, hydroxyls may convert a intrinsic nonwetting surface into a wetting one. Finally, we also report that surface modifications, like cation substitution, do not change the acid-base character of the surface, and thus they show the same nonwetting properties as native CeO2 or Nd2O3.

Adsorption of small mono- and poly-alcohols on rutile TiO2: a density functional theory study.

We have studied by means of density functional theory including dispersion contributions, the interaction of small chain alcohols with up to four carbons and three hydroxyl groups on the TiO2(110) rutile surface with different reduction degrees. Adsorption takes place through an acid-base interaction that can lead to both molecular and dissociated species. The latter are energetically preferred. Bulk reduction does not apport significant change neither in the structure nor in the adsorption energies, because the electrons are delocalized to a great extent. If vacancies are present at the surface these are the best adsorption sites for primary and secondary monoalcohols. Tertiary or poly-alcohols prefer the Ticus channels, but the reasons for the site preference are different. In the case of bulky alcohols, steric hindrance is the main adsorption-controlling factor, while templating effects of the basic (oxygen) sites on the surface are the key parameters to understand the adsorption of poly-alcohols. Vicinal polyalcohols behave even in a more complex way, for that they prefer the vacancy position only when dissociated, otherwise they stay in the Ticus channel. Our results warn about the use of small surrogates to investigate the chemistry of large alcohols as the adsorption patterns are not only quantitatively but also qualitatively wrong.

Costless Derivation of Dispersion Coefficients for Metal Surfaces.

Many common density functional theory methods used in the study of adsorption on metals lack dispersion contributions. Formulations like the random phase approximations would mitigate this error, but they are computationally too expensive. Therefore, semiempiric treatments based on dispersion coefficients turn out to be a practical solution. However, the parameters derived for atoms and molecules are not easily transferable to solids. In the case of metals, they cause severe overbinding as screening is not properly taken into consideration. Alternative ways to determine these parameters for metal surfaces have been put forward, but they are complex and not very flexible when employed to address low-coordinated atoms or alloys. In this work, we present a self-consistent, fast, and costless tool to obtain the dispersion coefficients for metals and alloys for pristine and defective surfaces. Binding energies computed with these parameters are compared to both the experimental and theoretical values in the literature thus demonstrating the validity of our approach.

On the properties of binary rutile MO2 compounds, M = Ir, Ru, Sn, and Ti: a DFT study.

We have studied the properties of bulk and different surfaces of rutile oxides, IrO2, RuO2, SnO2, and TiO2, and their binary compounds by means of density functional theory. As mixtures are employed in many applications, we have investigated the solubility, segregation, and overlayer formation of one of these oxides on a second metal from the series, as these aspects are critical for the chemical and electrochemical performances. Our results show that the bulk solubility is possible for several combinations. The electronic structure analysis indicates the activation of Ir states in Ir(x)Ti(1-x)O2 mixtures when compared to the parent IrO2 compound or the reduction in the band gap of TiO2 when Sn impurities are present. Segregation and oxygen-induced segregation of the second metal for the most common surfaces show a great extent of possibilities ranging from strong segregation to antisegregation, which depends on the oxygen ambient. The interaction of guest rutile overlayers on hosts is favourable and a wide range of growth properties (from multilayer formation to tridimensional particles) can be observed. Finally, a careful comparison with experimental information is presented, and for those cases where no data is available, the computed database can be used as a guideline by experimentalists.

Analysis of infrared spectra of ß-hairpin peptides as derived from molecular dynamics simulations.

Infrared temperature-dependent spectroscopy is a well-known tool to characterize folding/unfolding transitions in peptides and proteins, assuming that the higher the temperature, the higher the unfolded population. The infrared spectra at different temperatures of two ß-hairpin peptides (gramicidin S analogues GS6 and GS10) are here reconstructed by means of molecular dynamics (MD) simulations and a theoretical-computational method based on the perturbed matrix method. The calculated temperature-dependent spectra result in good agreement with the experimental available spectra. The same methodology has been then used to reconstruct the spectra corresponding to the pure unfolded and folded states, as defined from the MD simulations, in order to better understand the temperature-dependent spectra and to help the interpretation of the experimental spectra. For example, our results show that in the case of the GS6 peptide the analysis of the temperature-dependent spectra cannot be used to investigate the folding/unfolding kinetics within the usual assumption that the higher the temperature, the higher the probability of the unfolded state.