Haoyang Sun

Mechanistic insight into the displacement of CH4 by CO2 in calcite slit nanopores: the effect of competitive adsorption

Grand Canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulation methods were used to investigate the adsorption and diffusion properties of CH4 and CO2 in calcite slit nanopores with a pore width of ∼22 A. It was found that, in contrast to CH4, CO2 molecules have a much higher capacity to be adsorbed onto calcite pore surfaces. The diffusion capacity of CO2 molecules is much less in comparison with that of CH4 molecules, which could be attributed to diverse interactions between CO2 gas molecules and calcite nanopore surfaces. An effective displacement process of residual adsorbed CH4 in calcite slit nanopores by CO2 was performed, and it was found that the displacement efficiency was enhanced with an increase in the bulk pressure. This work provides microscopic information about the adsorption and diffusion properties of CH4 and CO2 in calcite nanopores, and confirmed the feasibility of the displacement of adsorbed CH4 in calcite nanopores by CO2, with the purpose of providing useful guidance for enhancing the extraction of shale gas by injecting CO2.

Adsorption properties of CH4 and CO2 in quartz nanopores studied by molecular simulation

In this work, grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulation methods were used to study the adsorption properties of CH4 and CO2 as single components and binary mixtures in modeled quartz nanopores (d ∼ 2 nm), of which the surface was hydroxylated to different degrees. The variation of the adsorption and molecular diffusion characteristics of CH4 and CO2 as a function of temperature and pressure were determined, and the competitive adsorption of CH4 and CO2 was investigated. As single components, both the adsorption of CH4 and CO2 in the nanopore is described well by the Langmuir model, and the diffusion capacities of the gas molecules in a non-supercritical state are much larger than that in a supercritical state. It was found that there is a tight adsorption layer of CH4 with a thickness of 3–5 A in the nanopore, while CO2 molecules adsorb tightly as a whole phase, especially in the supercritical fluid state. In the binary mixed system, CO2 preferentially adsorbs to the nanopore surface compared to CH4 due to the strong interactions between the CO2 molecule and the hydrophilic groups on the pore surface. An obvious competitive adsorption of CO2 and CH4 occurs at certain temperature ranges (313–353 K) with increasing pressure. And the degree of surface hydroxylation has significant contributions to the adsorption selectivity of CO2 over CH4. This work provides microscopic information about adsorption properties of CH4 and CO2 in nanopores at the molecular level for the purpose of guidance towards the application of shale gas extraction by flowing CO2.

Gold nanorods coated by oxygen-deficient TiO2 as an advanced photocatalyst for hydrogen evolution

Gold nanorods coated by oxygen-deficient TiO2 are synthesized by slow hydrolysis followed with high-temperature annealing in a reducing atmosphere. This does not alter the morphology of the nanorods, but produces Ti3+ species and oxygen vacancies in the shell. These nanorods show superior photocatalytic ability in hydrogen generation. The enhanced performance may be attributed to the synergistic effect of Ti3+ species, oxygen vacancies and Au, which effectively enhance light absorption, reduce charge recombination and increase charge-transfer across the interface between the electrolyte and electrode.

Molecular insight into the micro-behaviors of CH4 and CO2 in montmorillonite slit-nanopores

Abstract The Grand Canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulation methods were used to investigate the adsorption and diffusion properties of CH4 and CO2 in montmorillonite slit-nanopores. It is found that, both CH4 and CO2 could adsorb closely onto the pore surface, while different adsorption states occur for CH4 and CO2, respectively, in montmorillonite slit-nanopores. Competitive adsorption of CO2 over CH4 exists in montmorillonite slit-nanopores, especially at the lower pressures, which is attributed to the different interaction intensity between the CH4–CO2 molecules and the pore surface. The diffusion coefficients of CH4 and CO2 both decrease with the enhanced pressures, while the CO2 has a relative weak diffusion coefficient comparing with CH4. A well displacement of the residual CH4 by CO2 in montmorillonite slit-nanopores was investigated, which is found that the displacement efficiency increases with the enhanced bulk pressures. It was determined that, the CO2 can be captured and reserved in the montmorillonite slit-nanopores during the displacement, and the sequestration amount of CO2 gets enhanced with the bulk pressure increasing. This study provides micro-behaviours of CH4 and CO2 in montmorillonite slit-nanopores, for the purpose to give out useful guidance for enhancing shale gas extraction by injecting CO2.

Understanding about how different foaming gases effect the interfacial array behaviors of surfactants and the foam properties

In this paper, the detailed behaviors of all the molecules, especially the interfacial array behaviors of surfactants and diffusion behaviors of gas molecules, in foam systems with different gases (N2, O2, and CO2) being used as foaming agents were investigated by combining molecular dynamics simulation and experimental approaches for the purpose of interpreting how the molecular behaviors effect the properties of the foam and find out the key factors which fundamentally determine the foam stability. Sodium dodecyl sulfate SDS was used as the foam stabilizer. The foam decay and the drainage process were determined by Foamscan. A texture analyzer (TA) was utilized to measure the stiffness and viscoelasticity of the foam films. The experimental results agreed very well with the simulation results by which how the different gas components affect the interfacial behaviors of surfactant molecules and thereby bring influence on foam properties was described.

Effects of Surface Composition on the Microbehaviors of CH4 and CO2 in Slit-Nanopores: A Simulation Exploration

Molecular dynamics simulation studies were employed to investigate the microscopic behaviors of CH4 and CO2 molecules in slit-nanopores (SNPs) with various surfaces and different compositions. Three kinds of SNPs were constructed by a pair-wise combination of graphene, silica, and the calcite surface. The grand canonical Monte Carlo and molecular dynamics simulation methods were used to investigate the adsorption and self-diffusion of the gases in the nanopores. It is found that in all three cases, the CH4 molecules prefer to adsorb onto the graphene surface, whereas the CO2 molecules prefer to adsorb onto the calcite surface. The adsorption intensity of gases adsorbed onto various surfaces, the adsorption distances, along with the details of adsorption orientations of CH4 and CO2 molecules on various surfaces are calculated. The surface characteristics, such as surface roughness and charge distribution, are analyzed to help understand the microscopic adsorption behaviors of the gases on the specific surface. It was found that competitive adsorptions of CO2 over CH4 broadly occurred, especially in the SNPs containing calcite, because of the strong adsorption interactions between the CO2 molecules and the calcite surface. This work provides the microbehaviors of CH4 and CO2 in SNPs with various surfaces in different compositions to provide useful guidance for better understanding about the microstate of gases in complex nanoporous shale formation and to give out useful guidance for enhancing shale gas recovery by injecting CO2.

Microcosmic understanding on thickening capability of copolymers in supercritical carbon dioxide: the key role of π–π stacking

In this study, styrene/heptadecafluorodecyl acrylate (St–HFDA) copolymers of different compositions were synthetized for the purpose of thickening supercritical carbon dioxide (SC-CO2). The cloud point pressures of the copolymer–CO2 mixtures and the thickening effects of these copolymers for SC-CO2 were measured. Molecular dynamics (MD) simulations were used to evaluate the intermolecular interactions and microstructures of polymer–CO2 systems, the copolymer–CO2 interaction energy, cohesive energy density (CED), solubility parameter, equilibrium conformations and radial distribution functions (RDFs) were obtained, which provided useful information for microscopic understanding on the thickening capability of copolymers in SC-CO2. It was found that all the synthesized St–HFDA copolymers induced greater viscosity enhancements of SC-CO2 compared to poly(Heptadecafluorodecyl acrylate) (PHFDA), and π–π stacking of the Styrene (St) groups played a key role in thickening SC-CO2. On one hand, the introduction of the St groups into PHFDA weakened the CO2-philicity of the polymers by reducing the polymer–CO2 interaction and increasing polymer–polymer interactions, resulting in higher cloud point pressure in SC-CO2 compared to PHFDA. On the other hand, the increase of the polymer–polymer interaction via π–π stacking provided an associative force to thicken SC-CO2. The subtle relationship between the copolymer composition and thickening abilities of the copolymers in SC-CO2 were evaluated and the optimum styrene molar ratio was determined. It can be concluded that the content of the CO2-philic HFDA groups and the CO2-phobic St groups in the copolymers should be optimized to achieve the balance between the solubility and the thickening capability.

Study of the molecular array behaviours and interfacial activities of green surfactant alkyl polyglycoside and the mixed systems with other surfactants on oil–water interface

Abstract The widely performance of surfactants is closely related to their interfacial activity, which is essentially determined by the molecular array behaviours at the interface, of which the studies are significance for clearly understanding their structure-performance relationships. In this paper, the detailed molecular array behaviours of green surfactant alkyl polyglycoside (APG) and the mixed systems with other types of surfactants on oil/water interface have been studied using molecular dynamics simulations, and the key theoretical principle was confirmed by quantum chemistry calculations. It was found that the hydrophilic maltose ring head groups of decyl polyglycoside (C10-APG) are prone to lie flatly at the oil–water interface, the steric hindrance results in the low interfacial density, which critically determines the limit of the interfacial activity. The interfacial adsorption behaviours of the binary mixtures of C10-APG and SDS or DATB and the ternary mixtures of C10-APG, SDS and DATB were studied in detail, how the efficient synergism effect could be achieved for the mixture to get super high interfacial activity was discussed. This study provides a strategy to reveal how the molecular interfacial behaviours determine the key interfacial characteristics of the novel surfactants, which might provide help to promote their applications.

Thickening Supercritical CO2 with π-Stacked Co-Polymers: Molecular Insights into the Role of Intermolecular Interaction

Vinyl Benzoate/Heptadecafluorodecyl acrylate (VBe/HFDA) co-polymers were synthesized and characterized as thickening agents for supercritical carbon dioxide (SC-CO2). The solubility and thickening capability of the co-polymer samples in SC-CO2 were evaluated by measuring cloud point pressure and relative viscosity. The molecular dynamics (MD) simulation for all atoms was employed to simulate the microscopic molecular behavior and the intermolecular interaction of co-polymer–CO2 systems. We found that the introduction of VBe group decreased the polymer–CO2 interaction and increased the polymer–polymer interaction, leading to a reduction in solubility of the co-polymers in SC-CO2. However, the co-polymer could generate more effective inter-chain interaction and generate more viscosity enhancement compared to the Poly(Heptadecafluorodecyl) (PHFDA) homopolymer due to the driving force provided by π-π stacking of the VBe groups. The optimum molar ratio value for VBe in co-polymers for the viscosity enhancement of SC-CO2 was found to be 0.33 in this work. The P(HFDA0.67-co-VBe0.33) was able to enhance the viscosity of SC-CO2 by 438 times at 5 wt. %. Less VBe content would result in a lack of intermolecular interaction, although excessive VBe content would generate more intramolecular π-π stacking and less intermolecular π-π stacking. Both conditions reduce the thickening capability of the P(HFDA-co-VBe) co-polymer. This work presented the relationship between structure and performance of the co-polymers in SC-CO2 by combining experiment and molecular simulations.