Using first-principles density functional theory, we study the effect of particle size and surface structure on the chemisorption energy of OH and O on nanoparticles of Pt. We find that the chemisorption energies of O and OH are strongly affected by the size and structure of the Pt particle varying by up to 1.0 eV at different adsorption sites and particle sizes.

It is well-known that the amphiphilic solutes are surface-active and can accumulate at the oil-water interface. Here, we have investigated the water and a light-oil model interface by using molecular dynamic simulations. It was found that aromatics concentrated in the interfacial region, whereas the other hydrocarbons were uniformly distributed throughout the oil phase. Similar to previous studies, such concentrations were not observed at pure aromatics-water interfaces. We show that the self-accumulation of aromatics at the oil-water interface is driven by differences in the interfacial tension, which is lower for aromatics-water than between the others. The weak hydrogen bonding between the aromatic rings and the water protons provides the mechanism for lowering the interfacial tension.

No abstract

Controlling the selectivity of CO2 hydrogenation catalysts is a fundamental challenge. In this study, the selectivity of supported Ni catalysts prepared by the traditional impregnation method was found to change after a first CO2 hydrogenation reaction cycle from 100 to 800 °C. The usually high CH4 formation was suppressed leading to full selectivity toward CO. This behavior was also observed after the catalyst was treated under methane or propane atmospheres at elevated temperatures. In situ spectroscopic studies revealed that the accumulation of carbon species on the catalyst surface at high temperatures leads to a nickel carbide-like phase. The catalyst regains its high selectivity to CH4 production after carbon depletion from the surface of the Ni particles by oxidation. However, the selectivity readily shifts back toward CO formation after exposing the catalysts to a new temperature-programmed CO2 hydrogenation cycle. The fraction of weakly adsorbed CO species increases on the carbide-like surface when compared to a clean nickel surface, explaining the higher selectivity to CO. This easy protocol of changing the surface of a common Ni catalyst to gain selectivity represents an important step for the commercial use of CO2 hydrogenation to CO processes toward high-added-value products.

In our paper, we study the interface wettability, diffusivity, and molecular orientation between crude oil and different fluids for applications in improved oil recovery (IOR) processes through atomistic molecular dynamics (MD). The salt concentration, temperature, and pressure effects on the physical chemistry properties of different interfaces between IOR agents [brine (H(2)O + % NaCl), CO(2), N(2), and CH(4)] and crude oil have been determined. From the interfacial density profiles, an accumulation of aromatic molecules near the interface has been observed. In the case of brine interfaced with crude oil, our calculations indicate an increase in the interfacial tension with increasing pressure and salt concentration, which favors oil displacement. On the other hand, with the other fluids studied (CO(2), N(2), and CH(4)), the interfacial tension decreases with increasing pressure and temperature. With interfacial tension reduction, an increase in fluid diffusivity in the oil phase is observed. We also studied the molecular orientation properties of the hydrocarbon and fluids molecules in the interface region. We perceived that the molecular orientation could be affected by changes in the interfacial tension and diffusivity of the molecules in the interface region with the increased pressure and temperature: pressure (increasing) → interfacial tension (decreasing) → diffusion (increasing) → molecular ordering. From a molecular point of view, the combination of low interfacial tension and high diffusion of molecules in the oil phase gives the CO(2) molecules unique properties as an IOR fluid compared with other fluids studied here.

The structural, electronic, and thermodynamic properties of ammonia-borane complexes with varying amounts of hydrogen have been characterized by first principles calculations within density functional theory. The calculated structural parameters and thermodynamic functions (free energy, enthalpy and entropy) were found to be in good agreement with experimental and quantum chemistry data for the crystals, dimers, and molecules. The authors find that zero-point energies change several H2 release reactions from endothermic to exothermic. Both the ammonia-borane polymeric and borazine-cyclotriborazane cycles show a strong exothermic decomposition character (approximately -10 kcal/mol), implying that rehydrogenation may be difficult to moderate H2 pressures. Hydrogen bonding in these systems has been characterized and they find the N-H bond to be more covalent than the more ionic B-H bond.

For any possible application of hard nanoparticles as an improved oil recovery agent, it is necessary to have a stable wettability modifier with improved mobility properties under broader oil reservoir conditions. Typical oil reservoir conditions with high temperature, pH and electrolytes concentration modifies the way that nanoparticles interact with the solution. In this work, the wettability and fluid diffusivity at the molecular level were studied through Molecular Dynamics simulation. We have studied the stability and mobility of functionalized (hydroxylated, PEG and sulfonic acid) silica nanoparticles for enhanced oil recovery applications, particularly at high salt concentration and high temperature. In order to stabilize the interfacial energy in the nanoparticle-brine interface, ions tend to modify the transport properties of the nanoparticle itself. We have observed an increasing in the diffusion coefficients for nanoparticles with increasing salt concentration at 300K and 0.1 MPa pressure. Our calculations also indicate that adsorption properties and salt solutions greatly influence the interfacial tension in an order of CaCl2 > NaCl. This effect was found to be due to the difference in distribution of ions in solution, which modifies the hydration and electrostatic potential of those ions near the nanoparticle. The brine/oil interfacial tension variation due to functionalized silica nanoparticles was also determined as a function of the terminal group hydrophobicity at 1% salt concentration (CaCl2 and NaCl), 300K and 0.1 MPa pressure.

Contrary to ordinary solids, which are normally known to harden by compression, the compressibility of SiO 2 ͑silica͒ glass has a maximum at about 2-4 GPa and its mechanical strength shows a minimum around 10 GPa. At this pressure, the compression of silica glass undergoes a change from purely elastic to plastic, and samples recovered from above 10 GPa are found to be permanently densified. Using an improved, ab initio parametrized interatomic potential for SiO 2 we provide here a unified picture of the compression mechanisms based on the pressure-induced appearance of unquenchable fivefold defects. By means of molecular-dynamic simulations we find them to be responsible for the reduction of the mechanical strength and for permanent densification. We also find that the compressibility maximum does not require changes of the tetrahedral network topology.