Display omitted
•Spatially separated CoO and Au are selectively loaded on the surface of the TiO2.•The optimal photocatalytic activity of the sample is 60 times higher than that of the parent ...TiO2.•The synergetic effect of the separated dual-cocatalysts contributes to the excellent activity.•The active intermediate is the adsorbed CO2− during the photocatalytic process.•H2O vapor participates in the reaction as electron donator and hole scavenger.
Photocatalytic reduction of CO2 to solar fuels has been considered as a promising route to solve the energy crisis and environmental issues. Herein, the spatially separated CoO and Au dual cocatalysts are used as the hole and electron collectors, which are loaded on the internal and external surface of TiO2 hollow sphere (THS), respectively (denoted as Aux@THS@CoO). It is found that the Au nanoparticle and Co species are homogenous deposited on the surface of THS without doping. DRS results show that the photoabsorption performances of THS are enhanced obviously in the visible light region. XPS analysis reveals that an internal electric field is constructed, which could promote the separation of photoinduced charge carriers. Subsequently, Au2.0@THS@CoO displays the highest photocurrent density compared with the counterparts. Furthermore, the results of CO2 adsorption, ESR spectra and in-situ FTIR spectra show the high adsorption capacity of CO2 on the sample and the chemisorption of CO2 on the oxygen defects of THS conversed into the active intermediate CO2−. As a result, Au2.0@THS@CoO presents a remarkably enhanced photocatalytic activity for the reduction of CO2 with H2O in CH4. The optimal activity of the catalyst is 13.3 μmol h−1 g−1, which is 60 times higher than that of THS. In addition, in-situ FTIR spectra also suggest that H2O participates in the reaction as electron donator and hole scavenger during the photocatalysis process. Finally, a possible photocatalytic process has also been proposed.
The freezing of frost susceptible soils is a dynamic hydro-thermal–mechanical (THM) interacting process. One-side freezing experiments of saturated soil in an open system with no-pressure water ...supplement are carried out. In these experiments, we analyzed the influence of temperature gradients, overburden pressures and cooling temperatures on moisture migration and frost heave. Based on these experiments, a mathematical model of frost heave is proposed with the variables of temperature, porosity and displacement, in which Clapeyron equation is employed as the phase equilibrium condition of water and ice in soil. Ice lens is one of the major aspects of the frost heave for frost susceptible soils. According to the mechanical and physical characteristics, a comprehensive criterion for the formation and end of new ice lens is presented. To solve the nonlinear equations, the finite element algorithm is applied to solve the general form of governing equations. Finally, numerical simulations are implemented with the assistance of COMSOL. Validation of the model is illustrated by comparisons between the simulation and experimental results. From this study, it is found that, (1) cooling temperature is the necessary condition for moisture migration and frost heave since pore water phases into ice under cooling temperature. Then negative pore water pressure occurs in soil. Pore water pressure gradient is the direct driving force for water migration in saturated soil. However, temperature gradient and overburden pressure have important influence on the pore water pressure gradient. The response of soil sample to the variation of water content lags behind the response to the change of temperature. (2) Discontinuously distributed ice lenses form near the cold front when the accumulated water phases to ice. Temperature gradient, overburden pressure and cooling temperature are key factors to determine the frost heave and moisture migration. (3) Freezing and migration of unfrozen water cause the change of porosity in soil. Ice lens will block the migration of unfrozen water. The discrete ice lenses result in the oscillation in the distributions of water content. (4) Unsaturated phenomenon occurs in the unfrozen zone under the frozen fringe which might be related to the suction effect of large negative pressure in this zone.
Highly crystalline graphitic carbon nitride (g‐C3N4) with decreased structural imperfections benefits from the suppression of electron–hole recombination, which enhances its hydrogen generation ...activity. However, producing such g‐C3N4 materials by conventional heating in an electric furnace has proven challenging. Herein, we report on the synthesis of high‐quality g‐C3N4 with reduced structural defects by judiciously combining the implementation of melamine–cyanuric acid (MCA) supramolecular aggregates and microwave‐assisted thermolysis. The g‐C3N4 material produced after optimizing the microwave reaction time can effectively generate H2 under visible‐light irradiation. The highest H2 evolution rate achieved was 40.5 μmol h−1, which is two times higher than that of a g‐C3N4 sample prepared by thermal polycondensation of the same supramolecular aggregates in an electric furnace. The microwave‐assisted thermolysis strategy is simple, rapid, and robust, thereby providing a promising route for the synthesis of high‐efficiency g‐C3N4 photocatalysts.
Highly crystalline graphitic carbon nitride (g‐C3N4) was synthesized within 16 min by microwave‐assisted thermolysis of melamine–cyanuric acid supramolecular aggregates. The H2 generation rate achieved with the as‐obtained g‐C3N4 material is two times higher than that of a sample prepared by heating the same supramolecular aggregates in a furnace at 540 °C for 2 h.
•We present a simplified method for measuring the permeability of highly porous media.•The permeability characteristics of the single-size aggregate columns were tested.•Permeability increases ...linearly with aggregate diameter in a log-log graph.
Fluid flow through highly porous media is important in a variety of science and technology fields, including hydrology, chemical engineering, convections in porous media, and others. While many methods have been available to measure the permeability of tight solid materials, such as concrete and rock, the technique for measuring the permeability of highly porous media is limited (such as gravel, aggregated soils, and crushed rock). This study proposes a new simplified method for measuring the permeability of highly porous media with a permeability of 10−8–10−4 m2, using a Venturi tube to gauge the gas flowing rate through the sample. Using crushed rocks and glass beads as the test media, we measure the permeability and inertial resistance factor of six types of single-size aggregate columns. We compare the testing results with the published permeability and inertial resistance factor of crushed rock and of glass beads. We found that in a log-log graph, the permeability and inertial resistance factor of a single-size aggregate heap increases linearly with the mean diameter of the aggregate. We speculate that the proposed simplified method is suitable to efficiently test the permeability and inertial resistance factor of a variety of porous media with an intrinsic permeability of 10−8–10−4 m2.
•Water-vapor migration process and deformation of the unsaturated silty soil were studied.•Initial water content controls the water and vapor migration intensity of unsaturated soils.•A coupled ...heat-water-vapor-mechanics model was proposed.
Freezing of unsaturated soils jointly involves heat transfer, water-vapor migration, water-ice and water-vapor phase changes and deformation. To understand this freezing, we conducted a series of one-dimensional freezing tests to study the influences of the initial water content and cooling temperature on the water-vapor migration and deformation of unsaturated soils. Results show that the initial water content determines the water and vapor migration intensity and deformation of unsaturated soils. Vapor migrates prominently and frost heave develops slowly when the initial water content is lower than a critical value. Above this value, the liquid water migration dominates, and frost heave develops rapidly. It is also found that the cooling temperature dominates water and vapor fluxes in such a manner that a higher cooling temperature leads to faster migrations of water and vapor in unsaturated soil. We further analyzed the coupled heat-water-vapor-mechanics process of unsaturated soils and proposed a coupled heat-water-vapor-mechanics model. The simulated temperatures, volumetric water content, and displacement agree well with those measured data, validating that the model can describe effectively the coupled heat-water-vapor-mechanics process in freezing unsaturated soils.
•An unfrozen water content model is developed based on the pore size distribution.•The content of micro-pore has a significant effect on the three parameters.•The model could simulate the change of ...unfrozen water content in freezing soils.•An expression is developed for calculating the thickness of unfrozen water films.
A clear fundamental understanding of the soil freezing characteristic curve is crucial for studying soil freezing behavior. In this paper, based on the assumption that the shape of the soil freezing characteristic curve is mainly dependent upon the pore-size distribution of the soil, a mathematic model for estimating the soil freezing characteristic curve is proposed. The formula has the form of an integrated frequency distribution curve, which is verified by previous researches (seven representative soil samples and six representative mineral compositions, a number of special mineral particles and soils, and unsaturated soils). By nonlinear curve fitting, the correlation coefficients are generally larger than 0.95. The proposed model is more convenient than the original empirical formulas in numerical modeling, and it can overcome the shortcoming that the original empirical formulas are not derivative at the temperatures near the freezing point. In addition, the proposed model directly expresses the relationship between residual unfrozen water content and temperature under extremely low temperature conditions. Of course, the new model and results in this study may provide a reference for the research on basic physical properties of freezing soils, and the related numerical modeling in cold regions engineering.
•A coupled kinetics models for crystal growth is established.•A simple expression of macroscopic crystallization stress (MCS) is presented.•The proposed model is used to explain successfully the ...deformation of saline frozen soil.
Water, salt and heat transfer in porous media generally accompanied with phase change, causing serious damages to the porous material. In this paper, a kinetics model considering nucleation, molecular diffusion and crystal growth was established. Then, a simple expression of macroscopic crystallization stress (MCS), depending on the microscopic crystallization pressure and the amount of precipitated crystal, was presented. Finally, the model was calibrated through experimental study on the deformation behavior of soil combined with phase change during freezing. The experimental results indicate that the soil deformation is determined by MCS in essentially. The MCS destroys the pore structure resulting in soil deformation when the MCS exceeds the tensile strength of the soil material. A good agreement between the model predictions and the experimental results demonstrates the effectiveness of the proposed kinetics approach with phase change.
Carbon nanofibers/silver nanoparticles (CNFs/AgNPs) composite nanofibers were fabricated by two steps consisting of the preparation of the CNFs by electrospinning and the hydrothermal growth of the ...AgNPs on the CNFs. The as-prepared nanofibers were characterized by scanning electron microscopy, energy dispersive spectroscopy, transmission electron microscopy, X-ray diffraction, resonant Raman spectra, thermal gravimetric and differential thermal analysis, and X-ray photoelectron spectroscopy, respectively. The results indicated that not only were AgNPs (25-50 nm) successfully grown on the CNFs but also the AgNPs were distributed without aggregation on the CNFs. Further more, by adjusting the parameters in hydrothermal processing, the content of silver supported on the CNFs could be easily controlled. The catalytic activities of the CNFs/AgNPs composite nanofibers to the reduction of 4-nitrophenol (4-NP) with NaBH(4) were tracked by UV-visible spectroscopy. It was suggested that the CNFs/AgNPs composite nanofibers exhibited high catalytic activity in the reduction of 4-NP, which might be attributed to the high surface areas of AgNPs and synergistic effect on delivery of electrons between CNFs and AgNPs. And, the catalytic efficiency was enhanced with the increasing of the content of silver on the CNFs/AgNPs composite nanofibers. Notably, the CNFs/AgNPs composite nanofibers could be easily recycled due to their one-dimensional nanostructural property.
Ti3C2-MXene, which exhibits excellent electronic conductivity and optical properties, has been identified as a promising noble-metal-free co-catalyst for the development of efficient photocatalysts ...for environmental remediation. Herein, CeO2/Ti3C2-MXene hybrids were prepared by the in-situ growth of cube-like CeO2 using ultrathin Ti3C2-MXene nanosheets as a two-dimensional platform via a simple hydrothermal route. Upon exposure to solar light, the CeO2/Ti3C2-MXene hybrid with the optimal ratio of Ti3C2-MXene exhibited enhanced performance in photocatalytic tetracycline degradation and CO2 reduction, with activities 6.3 and 1.5 times greater than those of pristine CeO2, respectively. The improved activity of CeO2/Ti3C2-MXene was attributed to the Schottky junction induced by the built-in electric field between CeO2 and Ti3C2-MXene, which drives the photogenerated electrons from CeO2 to Ti3C2-MXene and expedites the segregation of the electrons and holes. This work may shed light on the careful design of novel Schottky junctions utilizing noble-metal free Ti3C2-MXene as co-catalysts for building efficient photocatalytic systems enacted in pollutant degradation and energy conversion.
•A soil freezing characteristic curve equation was given from the view of soil pore structure.•Parameters in the equation have clear physical meanings.•Numerical calculation was performed using the ...equation under freezing condition.
Unfrozen water in frozen soil is a key factor for water migration, frost heave and thaw settlement under freeze-thaw cycles. The relationship between unfrozen water content and temperature in frozen soil is known as soil freezing characteristic curve. The integral form for a soil freezing characteristic curve was derived from the perspective of soil pore structure in this study. According to the relationship between pore radius and freezing temperature, using existed test data, a distribution function was basically determined. Then, theoretical expression of the soil freezing characteristic curve was proposed. The calculated results by the theoretical expression were in good agreement with the test data. The parameters of the soil freezing characteristic curve have clear physical meanings, and the theoretical expression is continuous at the point of initial freezing temperature. In order to apply the proposed theoretical expression to coupled hydro-thermal-vapor transfer model, the equation was substituted into a coupled hydro-thermal-vapor transfer model, and the numerical simulations for frozen soils were carried out. The simulated results illustrated the reasonableness of the proposed equation.
