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2019 Volume 35 Issue 2
2019, 35(2): 129-130
doi: 10.3866/PKU.WHXB201712181
Abstract:
2019, 35(2): 131-132
doi: 10.3866/PKU.WHXB201712191
Abstract:
2019, 35(2): 133-134
doi: 10.3866/PKU.WHXB201801172
Abstract:
2019, 35(2): 135-136
doi: 10.3866/PKU.WHXB201806111
Abstract:
2019, 35(2): 137-138
doi: 10.3866/PKU.WHXB201806112
Abstract:
2019, 35(2): 139-144
doi: 10.3866/PKU.WHXB201805111
Abstract:
The structure of low-temperature α-Cu2Se, which is of great importance for understanding the mechanism of the significant increase in thermoelectric performance during the α-β phase transition of Cu2Se, has still not been fully solved. Because it is restricted by the quality of polycrystal and powder specimens and the accuracy of characterization methods such as the conventional transmission electron microscopy (TEM) and X-ray diffraction (XRD), direct observation with atomic-scale resolution to reveal the structural details has not been realized, although electron diffraction and high-resolution transmission electron microscopy (HRTEM) studies have indicated the complexity of the α-Cu2Se layered structure. Owing to developments in the focused ion beam (FIB) milling preparation method, high-quality single crystalline specimens with specific crystallographic orientations can be prepared to ensure that atomic-resolution images along a specific orientation can be acquired. Furthermore, the developments in aberration correction technology in TEM and scanning transmission electron microscopy (STEM) allow us to observe the subtle details of structural variation and evolution. Herein, we report, for the first time, the atomic-resolution high-angle annular dark field (HAADF) images acquired along the\begin{document}$ {{\left[ \bar{1}\bar{1}2 \right]}_{\text{c}}}$\end{document} \begin{document}$ {{\left[ \bar{1}\bar{1}2 \right]}_{\text{c}}}$\end{document} \begin{document}$ <\bar{1}10{{>}_{\text{c}}} $\end{document} \begin{document}${{\left[ \bar{1}01 \right]}_{\text{c}}} $\end{document}
The structure of low-temperature α-Cu2Se, which is of great importance for understanding the mechanism of the significant increase in thermoelectric performance during the α-β phase transition of Cu2Se, has still not been fully solved. Because it is restricted by the quality of polycrystal and powder specimens and the accuracy of characterization methods such as the conventional transmission electron microscopy (TEM) and X-ray diffraction (XRD), direct observation with atomic-scale resolution to reveal the structural details has not been realized, although electron diffraction and high-resolution transmission electron microscopy (HRTEM) studies have indicated the complexity of the α-Cu2Se layered structure. Owing to developments in the focused ion beam (FIB) milling preparation method, high-quality single crystalline specimens with specific crystallographic orientations can be prepared to ensure that atomic-resolution images along a specific orientation can be acquired. Furthermore, the developments in aberration correction technology in TEM and scanning transmission electron microscopy (STEM) allow us to observe the subtle details of structural variation and evolution. Herein, we report, for the first time, the atomic-resolution high-angle annular dark field (HAADF) images acquired along the
2019, 35(2): 145-157
doi: 10.3866/PKU.WHXB201803281
Abstract:
Over the past decade, significant progress has been made in theoretical and experimental research in the field of chemical reaction dynamics, moving from triatomic reactions to larger polyatomic reactions. This has challenged the theoretical and computational approaches to polyatomic reaction dynamics in two major areas: the potential energy surface and the dynamics. Highly accurate potential energy surfaces are essential for achieving accurate dynamical information in quantum dynamics calculations. The increased number of degrees of freedom in larger systems poses a significant challenge to the accurate construction of potential energy surfaces. Recently, there has been substantial progress in the development of potential energy surfaces for polyatomic reactive systems. In this article, we review the recent developments made by our group in constructing highly accurately fitted potential energy surfaces for polyatomic reactive systems, based on a neural network approach. A key advantage of the neural network approach is its more faithful representation of the ab initio points. We recently proposed a systematic procedure, based on neural network fitting, for the construction of accurate potential energy surfaces with very small root mean square errors. Based on the neural network approach, we successfully developed potential energy surfaces for polyatomic reactions in the gas phase, including the reactive systems OH3, HOCO, and CH5, and the dissociation of gas-phase molecules on metal surfaces, such as H2O on the Cu(111) surface. These potential energy surfaces were fitted to an unprecedented level of accuracy, representing the most accurate potential energy surfaces calculated for these systems, and were rigorously tested using quantum dynamics calculations. The quantum dynamics calculations based on these potential energy surfaces produce accurate results, which are in good agreement with experiments. We have also proposed a new method for developing permutationally invariant potential energy surfaces, named fundamental-invariant neural networks. Mathematically, fundamental invariants are used to finitely generate the permutation-invariant polynomial ring; thus, fundamental-invariant neural networks can approximate any function to arbitrary accuracy. The use of fundamental invariants minimizes the size of the input permutation-invariant polynomials, which reduces the evaluation time for potential energy calculations, especially for polyatomic systems. Potential energy surfaces for OH3 and CH4 were constructed using fundamental-invariant neural networks, with their accuracies confirmed by full-dimensional quantum dynamics and bound-state calculations. These developments in the construction of highly accurate potential energy surfaces are expected to extend the theoretical study of reaction dynamics to larger and more complex systems.
Over the past decade, significant progress has been made in theoretical and experimental research in the field of chemical reaction dynamics, moving from triatomic reactions to larger polyatomic reactions. This has challenged the theoretical and computational approaches to polyatomic reaction dynamics in two major areas: the potential energy surface and the dynamics. Highly accurate potential energy surfaces are essential for achieving accurate dynamical information in quantum dynamics calculations. The increased number of degrees of freedom in larger systems poses a significant challenge to the accurate construction of potential energy surfaces. Recently, there has been substantial progress in the development of potential energy surfaces for polyatomic reactive systems. In this article, we review the recent developments made by our group in constructing highly accurately fitted potential energy surfaces for polyatomic reactive systems, based on a neural network approach. A key advantage of the neural network approach is its more faithful representation of the ab initio points. We recently proposed a systematic procedure, based on neural network fitting, for the construction of accurate potential energy surfaces with very small root mean square errors. Based on the neural network approach, we successfully developed potential energy surfaces for polyatomic reactions in the gas phase, including the reactive systems OH3, HOCO, and CH5, and the dissociation of gas-phase molecules on metal surfaces, such as H2O on the Cu(111) surface. These potential energy surfaces were fitted to an unprecedented level of accuracy, representing the most accurate potential energy surfaces calculated for these systems, and were rigorously tested using quantum dynamics calculations. The quantum dynamics calculations based on these potential energy surfaces produce accurate results, which are in good agreement with experiments. We have also proposed a new method for developing permutationally invariant potential energy surfaces, named fundamental-invariant neural networks. Mathematically, fundamental invariants are used to finitely generate the permutation-invariant polynomial ring; thus, fundamental-invariant neural networks can approximate any function to arbitrary accuracy. The use of fundamental invariants minimizes the size of the input permutation-invariant polynomials, which reduces the evaluation time for potential energy calculations, especially for polyatomic systems. Potential energy surfaces for OH3 and CH4 were constructed using fundamental-invariant neural networks, with their accuracies confirmed by full-dimensional quantum dynamics and bound-state calculations. These developments in the construction of highly accurate potential energy surfaces are expected to extend the theoretical study of reaction dynamics to larger and more complex systems.
2019, 35(2): 158-166
doi: 10.3866/PKU.WHXB201802272
Abstract:
In modern advanced internal combustion engines such as homogeneous compression ignition engine (HCCI) and reactivity controlled compression ignition engine (RCCI), turbulence/chemistry interactions have a dramatic influence on the combustion efficiency. In particular, the low-temperature fuel chemistry and two-stage ignition of large hydrocarbon fuels can significantly affect the turbulent flame regimes and propagation. The turbulent flame propagation and flame structure of a turbulent premixed n-heptane/air flame is simulated in a slot, i.e., reactor-assisted turbulent slot (RATS) burner. In the center, a premixed n-heptane/air gas mixture flows out from the burner, exiting into the surrounding atmosphere. In order to maintain a high Reynolds number for the flame, a pilot flame consisting of stoichiometric methane/air is applied. The GRI3.0 mechanism for methane/air mixture is adopted. A three-dimensional (3D) numerical simulation model is established based on OpenFOAM reactingFoam solver. A reduced kinetic mechanism of n-heptane consisting of 44 species and 112 reactions is employed, which is validated against the detailed mechanism with regard to the ignition delay time over a wide range of the initial temperature, equivalence ratio, and pressure. Then, the effects of the reactant temperature (ranging from 450 to 700 K), inlet velocity (6 m·s−1 and 10 m·s−1), and pre-flame flow residence time (100 ms and 60 ms) on the turbulent flame combustion of the n-heptane/air mixture with an equivalent ratio of 0.6, are investigated by performing 3D simulations. Twelve cases are considered and analyzed based on the flow residence time and ignition delay time. The 2D span-wise temperature contour is used to show that when the ignition Da number and fuel reactivity increase, the flame temperature increases and the flame height decreases, indicating a stronger turbulent burning velocity (ST). The results coincide well with experiment results and indicate that the extent of fuel oxidation is affected by the reactant temperature and inlet velocity during the low-temperature ignition stage, since the ratio between the ignition delay time and flow residence time plays an important role. Moreover, a quantitative analysis is performed on the flame front. The intermediate species CH is used to mark the thin reaction zone, and the turbulent burning velocity is obtained. Two branches of turbulent burning velocities verified the upper and lower limits and are consistent with the fitting correlation of ST/SL from a previous semi-implicit expression by Won et al. (2014), in which SL is laminar burning velocity. In the upper limit, the fuel decomposes and produces a large amount of intermediate species like CH2O in the pre-heat zone, which subsequently increases the turbulent burning velocity. While in the lower limit in which the flow residence time is shorter than the first ignition delay time, it shows a smaller turbulent burning velocity and a thin reaction zone, which is in chemically-frozen-flow regime. A transitional regime between the low- to high-temperature ignition regimes is also identified, where the ignition delay time is comparable with the heated flow residence time before the flame was produced. With an increase in the reactant temperature, the turbulent flame gradually changes from the chemically frozen flow regime to the low-temperature ignition regime. When the temperature is higher than a certain value, the low-temperature ignition will not happen again and the flame will be classified as being in the high-temperature ignition regime.
In modern advanced internal combustion engines such as homogeneous compression ignition engine (HCCI) and reactivity controlled compression ignition engine (RCCI), turbulence/chemistry interactions have a dramatic influence on the combustion efficiency. In particular, the low-temperature fuel chemistry and two-stage ignition of large hydrocarbon fuels can significantly affect the turbulent flame regimes and propagation. The turbulent flame propagation and flame structure of a turbulent premixed n-heptane/air flame is simulated in a slot, i.e., reactor-assisted turbulent slot (RATS) burner. In the center, a premixed n-heptane/air gas mixture flows out from the burner, exiting into the surrounding atmosphere. In order to maintain a high Reynolds number for the flame, a pilot flame consisting of stoichiometric methane/air is applied. The GRI3.0 mechanism for methane/air mixture is adopted. A three-dimensional (3D) numerical simulation model is established based on OpenFOAM reactingFoam solver. A reduced kinetic mechanism of n-heptane consisting of 44 species and 112 reactions is employed, which is validated against the detailed mechanism with regard to the ignition delay time over a wide range of the initial temperature, equivalence ratio, and pressure. Then, the effects of the reactant temperature (ranging from 450 to 700 K), inlet velocity (6 m·s−1 and 10 m·s−1), and pre-flame flow residence time (100 ms and 60 ms) on the turbulent flame combustion of the n-heptane/air mixture with an equivalent ratio of 0.6, are investigated by performing 3D simulations. Twelve cases are considered and analyzed based on the flow residence time and ignition delay time. The 2D span-wise temperature contour is used to show that when the ignition Da number and fuel reactivity increase, the flame temperature increases and the flame height decreases, indicating a stronger turbulent burning velocity (ST). The results coincide well with experiment results and indicate that the extent of fuel oxidation is affected by the reactant temperature and inlet velocity during the low-temperature ignition stage, since the ratio between the ignition delay time and flow residence time plays an important role. Moreover, a quantitative analysis is performed on the flame front. The intermediate species CH is used to mark the thin reaction zone, and the turbulent burning velocity is obtained. Two branches of turbulent burning velocities verified the upper and lower limits and are consistent with the fitting correlation of ST/SL from a previous semi-implicit expression by Won et al. (2014), in which SL is laminar burning velocity. In the upper limit, the fuel decomposes and produces a large amount of intermediate species like CH2O in the pre-heat zone, which subsequently increases the turbulent burning velocity. While in the lower limit in which the flow residence time is shorter than the first ignition delay time, it shows a smaller turbulent burning velocity and a thin reaction zone, which is in chemically-frozen-flow regime. A transitional regime between the low- to high-temperature ignition regimes is also identified, where the ignition delay time is comparable with the heated flow residence time before the flame was produced. With an increase in the reactant temperature, the turbulent flame gradually changes from the chemically frozen flow regime to the low-temperature ignition regime. When the temperature is higher than a certain value, the low-temperature ignition will not happen again and the flame will be classified as being in the high-temperature ignition regime.
2019, 35(2): 182-192
doi: 10.3866/PKU.WHXB201801264
Abstract:
The ignition characteristics of fuels and the release of energy in combustion engines are of crucial importance to engine design and improvement. To improve the fuel combustion efficiency and to reduce the associated pollutant emission, it is necessary to develop reliable high-precision reaction mechanisms for simulating combustion. Consequently, we need to comprehensively understand the combustion mechanisms of hydrocarbon fuels, and to explore their complicated chemical reaction networks. In order to construct combustion mechanisms that can be applied to conditions over a wide temperature range, wide pressure range, and for different equivalent ratios, two detailed mechanisms for the combustion of large hydrocarbons were developed based on ReaxGen, an automatic generation program for combustion and pyrolysis mechanisms developed by LI Xiangyuan et al. Using this program, one mechanism for n-decane combustion was developed, containing 1499 species and 5713 reactions, and another was developed for n-undecane combustion, containing 1843 species and 6993 reactions. All the detailed mechanisms of the alkanes consisted of two parts, a validated core mechanism and a sub-mechanism produced by ReaxGen which worked mainly based on the rules of the reaction class. The major classes of elementary reactions considered in our detailed mechanisms for n-decane and n-undecane combustion included 10 kinds of high-temperature combustion reactions and 19 kinds of low-temperature combustion reactions. To verify the rationality and reliability of the mechanisms, ignition delay times in shock tubes and the concentration profiles of important species in a jet-stirred reactor were obtained using CHEMKIN software. The obtained calculated data were compared with the experimental data and the results of similar mechanisms at home and abroad. It was shown that the numerically predicted results of our new mechanisms were in good agreement with available experimental data in the literature. Our newly developed n-decane and n-undecane combustion mechanisms are useful for completing the combustion model of aviation kerosene. Furthermore, considering the complexity of the detailed mechanisms, the large amount of calculation and the long time required for mechanism analysis, mechanism simplification was carried out. The sampling points required for mechanism reduction were taken from simulation results near the ignition delay time with pressures ranging from 1.0 × 105 Pa to 1.0 × 106 Pa, equivalence ratios ranging from 0.5 to 2.0, and initial temperatures ranging from 600 K to 1400 K. The species n-C10H22, N2, and O2 were selected as the initial important species for the n-decane combustion mechanism and the species n-C11H24, N2, and O2 were selected as the initial important species for the n-undecane combustion mechanism. The predicted results of ignition delay time from the simplified mechanism for n-decane combustion (including 709 species and 2793 reactions) and simplified mechanism for n-undecane combustion (including 820 species and 3115 reactions) generated by the reduction method of Directed Relation Graph with Error Propagation (DRGEP) agreed well with the detailed mechanisms. Finally, sensitivity analysis for the ignition delay time was carried out to identify reactions that affected ignition delay times at specific temperatures, pressures and equivalence ratios. The results indicate that these mechanisms are reliable for describing the auto-ignition characteristics of n-decane and n-undecane. These mechanisms would also be helpful in computational fluid dynamics (CFD) for engine design.
The ignition characteristics of fuels and the release of energy in combustion engines are of crucial importance to engine design and improvement. To improve the fuel combustion efficiency and to reduce the associated pollutant emission, it is necessary to develop reliable high-precision reaction mechanisms for simulating combustion. Consequently, we need to comprehensively understand the combustion mechanisms of hydrocarbon fuels, and to explore their complicated chemical reaction networks. In order to construct combustion mechanisms that can be applied to conditions over a wide temperature range, wide pressure range, and for different equivalent ratios, two detailed mechanisms for the combustion of large hydrocarbons were developed based on ReaxGen, an automatic generation program for combustion and pyrolysis mechanisms developed by LI Xiangyuan et al. Using this program, one mechanism for n-decane combustion was developed, containing 1499 species and 5713 reactions, and another was developed for n-undecane combustion, containing 1843 species and 6993 reactions. All the detailed mechanisms of the alkanes consisted of two parts, a validated core mechanism and a sub-mechanism produced by ReaxGen which worked mainly based on the rules of the reaction class. The major classes of elementary reactions considered in our detailed mechanisms for n-decane and n-undecane combustion included 10 kinds of high-temperature combustion reactions and 19 kinds of low-temperature combustion reactions. To verify the rationality and reliability of the mechanisms, ignition delay times in shock tubes and the concentration profiles of important species in a jet-stirred reactor were obtained using CHEMKIN software. The obtained calculated data were compared with the experimental data and the results of similar mechanisms at home and abroad. It was shown that the numerically predicted results of our new mechanisms were in good agreement with available experimental data in the literature. Our newly developed n-decane and n-undecane combustion mechanisms are useful for completing the combustion model of aviation kerosene. Furthermore, considering the complexity of the detailed mechanisms, the large amount of calculation and the long time required for mechanism analysis, mechanism simplification was carried out. The sampling points required for mechanism reduction were taken from simulation results near the ignition delay time with pressures ranging from 1.0 × 105 Pa to 1.0 × 106 Pa, equivalence ratios ranging from 0.5 to 2.0, and initial temperatures ranging from 600 K to 1400 K. The species n-C10H22, N2, and O2 were selected as the initial important species for the n-decane combustion mechanism and the species n-C11H24, N2, and O2 were selected as the initial important species for the n-undecane combustion mechanism. The predicted results of ignition delay time from the simplified mechanism for n-decane combustion (including 709 species and 2793 reactions) and simplified mechanism for n-undecane combustion (including 820 species and 3115 reactions) generated by the reduction method of Directed Relation Graph with Error Propagation (DRGEP) agreed well with the detailed mechanisms. Finally, sensitivity analysis for the ignition delay time was carried out to identify reactions that affected ignition delay times at specific temperatures, pressures and equivalence ratios. The results indicate that these mechanisms are reliable for describing the auto-ignition characteristics of n-decane and n-undecane. These mechanisms would also be helpful in computational fluid dynamics (CFD) for engine design.
2019, 35(2): 193-199
doi: 10.3866/PKU.WHXB201801241
Abstract:
As a potential substitute for commercial lithium ion batteries (LIBs), sodium ion batteries (NIBs) have attracted increasing interest during the last decade. However, compared to the LIBs, the sluggish kinetics of sodium ion diffusion in NIBs due to its larger ionic radius results in deteriorated electrochemical performances, which hinders the future development and application of NIBs. Therefore, exploring anode materials that exhibit a novel kinetic mechanism is desired. Recently, extremely rapid kinetics has been realized by introducing the pseudocapacitance effect into battery systems; this effect generally refers to faradaic charge-transfer reactions, including surface or near-surface redox reactions, and fast bulk ion intercalation. To obtain a pseudocapacitance effect in battery systems, the critical step involves the rational design of a two-dimensional structure with a high conductivity. In this regard, the bimetallic sulfide thiospinel NiCo2S4 stands out by virtue of its high conductivity (1.25 × 106 S·m-1) at room temperature, which is at least two orders of magnitude higher than that of the oxide counterpart (NiCo2O4). Herein, NiCo2S4 hexagonal nanosheets with a large lateral dimension of ~2 μm and thickness ~30 nm have been successfully synthesized through coprecipitation followed by a vapor sulfidation method. As the anode material in NIBs, the NiCo2S4 nanosheets deliver a reversible capacity of 387 mAh·g-1 after 60 cycles at a current density of 1000 mA·g-1. Additionally, the NiCo2S4 nanosheets exhibit high reversible capacities of 542, 398, 347, 300, and 217 mAh·g-1 at the current densities 200, 400, 800, 1000, and 2000 mA·g-1, respectively. Ex situ X-ray diffraction analysis has been employed to reveal that the sodium ion storage process is a result of a combined Na+ intercalation and conversion reaction between Na+ and NiCo2S4. Further quantitative analysis of the kinetics has verified the extrinsic pseudocapacitance mechanism of the Na+ storage process, in which the capacitive contribution enlarges as the current density increases. The observed capacitive contribution of NiCo2S4 electrode is as high as 71% at a scan rate of 0.4 mV·s-1. This is closely attributed to the modified thin-sheet structure of NiCo2S4 and hybridization with graphene that account for the superior high-rate performance with long-term cyclability. These intriguing results shed light on a new strategy for the structural design of electrode materials for advanced NIBs. Moreover, this vapor transformation route can be extended to the preparation of other transition metal disulfides with high electrochemical activities, such as FeCo2S4, ZnCo2S4, CuCo2S4, etc.
As a potential substitute for commercial lithium ion batteries (LIBs), sodium ion batteries (NIBs) have attracted increasing interest during the last decade. However, compared to the LIBs, the sluggish kinetics of sodium ion diffusion in NIBs due to its larger ionic radius results in deteriorated electrochemical performances, which hinders the future development and application of NIBs. Therefore, exploring anode materials that exhibit a novel kinetic mechanism is desired. Recently, extremely rapid kinetics has been realized by introducing the pseudocapacitance effect into battery systems; this effect generally refers to faradaic charge-transfer reactions, including surface or near-surface redox reactions, and fast bulk ion intercalation. To obtain a pseudocapacitance effect in battery systems, the critical step involves the rational design of a two-dimensional structure with a high conductivity. In this regard, the bimetallic sulfide thiospinel NiCo2S4 stands out by virtue of its high conductivity (1.25 × 106 S·m-1) at room temperature, which is at least two orders of magnitude higher than that of the oxide counterpart (NiCo2O4). Herein, NiCo2S4 hexagonal nanosheets with a large lateral dimension of ~2 μm and thickness ~30 nm have been successfully synthesized through coprecipitation followed by a vapor sulfidation method. As the anode material in NIBs, the NiCo2S4 nanosheets deliver a reversible capacity of 387 mAh·g-1 after 60 cycles at a current density of 1000 mA·g-1. Additionally, the NiCo2S4 nanosheets exhibit high reversible capacities of 542, 398, 347, 300, and 217 mAh·g-1 at the current densities 200, 400, 800, 1000, and 2000 mA·g-1, respectively. Ex situ X-ray diffraction analysis has been employed to reveal that the sodium ion storage process is a result of a combined Na+ intercalation and conversion reaction between Na+ and NiCo2S4. Further quantitative analysis of the kinetics has verified the extrinsic pseudocapacitance mechanism of the Na+ storage process, in which the capacitive contribution enlarges as the current density increases. The observed capacitive contribution of NiCo2S4 electrode is as high as 71% at a scan rate of 0.4 mV·s-1. This is closely attributed to the modified thin-sheet structure of NiCo2S4 and hybridization with graphene that account for the superior high-rate performance with long-term cyclability. These intriguing results shed light on a new strategy for the structural design of electrode materials for advanced NIBs. Moreover, this vapor transformation route can be extended to the preparation of other transition metal disulfides with high electrochemical activities, such as FeCo2S4, ZnCo2S4, CuCo2S4, etc.
2019, 35(2): 200-207
doi: 10.3866/PKU.WHXB201803083
Abstract:
Electric double-layer capacitors (EDLCs) are advanced electrochemical devices that have attracted tremendous attention because of their high power density, ultra-fast charging/discharging rate, and superior lifespan. A major challenge is how to further improve their energy density. At present, a large number of research efforts are primarily focusing on engineering the morphology and microstructure of electrodes to achieve better performance, for example, enlarging the specific surface area and designing the pore size. More importantly, wettability plays a crucial role in maximizing the effective utilization and accessibility of electrode materials. However, its primary mechanisms/phenomena are still partially resolved. Here, we explore the effects of wettability on the charging dynamics of EDLCs using molecular dynamics (MD) simulations. Typically, hydrophobic graphene (GP) and hydrophilic copper (Cu) are employed as the electrode materials. Differential capacitances (CD) as a function of electrode potentials (ϕ) are computed by means of Poisson and Gaussian equation calculations. Simulation results show that during the charging process of EDLCs, the differential capacitances of hydrophobic GP are insensitive to the electrode potentials. However, superhydrophilic Cu electrode exhibits an asymmetric U-shaped CD–ϕ curve, in which the capacitance at the negative polarization can be ~5.77 times greater than that of the positive counterpart. Such an unusual behavior is obviously different with the conventional Gouy-Chapman-Stern theory (i.e., symmetric U-shaped), room temperature ionic liquids (i.e., camel-, or bell-shaped), and hydrophobic counterpart, which is closely correlated with the free energy barrier distributions. Compared with the positive polarization or hydrophobic case, the energy barriers near the negative hydrophilic electrodes are remarkably suppressed, which benefits ion populations at the interface and enables the convenient orientation or distribution of ions to shield the external electric fields from electrodes, thereby yielding higher differential capacitances. With differentiating the ion charge density, the as-obtained CD–ϕ curves are well resembled, quantitatively establishing the correlations between EDL microstructures and differential capacitances. Besides, we also point out that enhancing the wettability could significantly decrease the EDL thickness from ~1.0 nm (hydrophobic) to ~0.5 nm (hydrophilic). In the end, we demonstrate that wetting property also impacts a prominent role in the charge storage behavior of EDLCs, transforming the charging mechanism dominated by counter-ion adsorption and ion exchange (hydrophobic) to pure counter-ion adsorption (hydrophilic). The as-obtained insights highlight the significance of wettability in regulating charging dynamics and mechanisms, providing useful guidelines for precisely controlling the wetting property of electrode materials for advanced charge storage of EDLCs.
Electric double-layer capacitors (EDLCs) are advanced electrochemical devices that have attracted tremendous attention because of their high power density, ultra-fast charging/discharging rate, and superior lifespan. A major challenge is how to further improve their energy density. At present, a large number of research efforts are primarily focusing on engineering the morphology and microstructure of electrodes to achieve better performance, for example, enlarging the specific surface area and designing the pore size. More importantly, wettability plays a crucial role in maximizing the effective utilization and accessibility of electrode materials. However, its primary mechanisms/phenomena are still partially resolved. Here, we explore the effects of wettability on the charging dynamics of EDLCs using molecular dynamics (MD) simulations. Typically, hydrophobic graphene (GP) and hydrophilic copper (Cu) are employed as the electrode materials. Differential capacitances (CD) as a function of electrode potentials (ϕ) are computed by means of Poisson and Gaussian equation calculations. Simulation results show that during the charging process of EDLCs, the differential capacitances of hydrophobic GP are insensitive to the electrode potentials. However, superhydrophilic Cu electrode exhibits an asymmetric U-shaped CD–ϕ curve, in which the capacitance at the negative polarization can be ~5.77 times greater than that of the positive counterpart. Such an unusual behavior is obviously different with the conventional Gouy-Chapman-Stern theory (i.e., symmetric U-shaped), room temperature ionic liquids (i.e., camel-, or bell-shaped), and hydrophobic counterpart, which is closely correlated with the free energy barrier distributions. Compared with the positive polarization or hydrophobic case, the energy barriers near the negative hydrophilic electrodes are remarkably suppressed, which benefits ion populations at the interface and enables the convenient orientation or distribution of ions to shield the external electric fields from electrodes, thereby yielding higher differential capacitances. With differentiating the ion charge density, the as-obtained CD–ϕ curves are well resembled, quantitatively establishing the correlations between EDL microstructures and differential capacitances. Besides, we also point out that enhancing the wettability could significantly decrease the EDL thickness from ~1.0 nm (hydrophobic) to ~0.5 nm (hydrophilic). In the end, we demonstrate that wetting property also impacts a prominent role in the charge storage behavior of EDLCs, transforming the charging mechanism dominated by counter-ion adsorption and ion exchange (hydrophobic) to pure counter-ion adsorption (hydrophilic). The as-obtained insights highlight the significance of wettability in regulating charging dynamics and mechanisms, providing useful guidelines for precisely controlling the wetting property of electrode materials for advanced charge storage of EDLCs.
2019, 35(2): 208-214
doi: 10.3866/PKU.WHXB201802121
Abstract:
The molten salt CO2 capture and electrochemical transformation (MSCC-ET) process is a potentially efficient method for CO2 utilization, which can convert CO2 into value-added carbon and oxygen with a current density of 100–1000 mA cm-2. The electrolytic carbon (EC) prepared through the MSCC-ET process is highly electrically conductive and forms flexible microstructures. These structures show excellent adsorption ability towards environmental pollutants and high energy storage capacity when used in supercapacitors. Although the morphology, structure, and application of EC prepared under different electrolysis conditions have been previously reported, their intrinsic electrochemical properties have not yet been elucidated. Powder microelectrodes (PMEs) are useful for studying the electrochemical kinetics of various powdery materials. In this study, we systematically investigated the electrochemical properties of ECs obtained using molten Li2CO3-Na2CO3-K2CO3 under different temperature and electrolysis voltage conditions by cyclic voltammetry (CV) with a carbon powder microelectrode in 10 mmol L-1 Na2SO4. The electrochemical behavior of the EC obtained at 450 ℃ and a cell voltage of 4.5 V (450 ℃-4.5 V-EC) differs significantly from that of other carbon materials, i.e., multi-walled carbon nanotubes, graphene, graphite, and acetylene black. In addition to a much larger charging-discharging capacity, unusual hysteresis of the charge/discharge current response of ECs in the negative potential region (-0.6 to -0.2 V vs SCE) was observed. This phenomenon was eliminated by annealing the material under Ar at 550 ℃, demonstrating that the unique electrochemical behavior of ECs is closely related to the oxygen-containing groups on its surface. Furthermore, CVs of EC-PME were compared in solutions with different pH, Na2SO4 concentrations, and other ions. The pH of the solution did not affect the CVs, excluding a redox mechanism involving the surface functional groups. Hysteresis was weakened by a certain degree at slower potential sweep speeds (< 10 mV s-1) or in higher concentrations of electrolyte (100 mmol L-1 Na2SO4). The onset potential for discharging was negatively shifted in electrolytes with a larger cation ((NH4)2SO4) and was unaffected by larger anions (Na2S2O8). This indicates that the hysteresis is more likely related to the specific adsorption of cations, caused by the unique surface properties of EC. It should be noted that the specific surface area and oxygen concentration of EC can be adjusted by the electrolysis temperature and cell voltage. Generally, the Brunauer–Emmett–Teller (BET) specific surface area and oxygen content decrease with increasing temperature and the BET-area increases with increasing cell voltage. The CVs of ECs prepared at different cell voltages were similar, but the adsorption capacity decreased for those prepared at higher temperatures (550 and 650 ℃). Interestingly, the specific capacitance of the ECs is much higher at negative potentials (-0.6 to 0 V vs. SCE) than that at positive potentials (0 to 0.6 V vs. SCE). Therefore, it is anticipated that a better capacitance performance can be achieved when the ECs are used as a negative electrode material in supercapacitors. ![]()
The molten salt CO2 capture and electrochemical transformation (MSCC-ET) process is a potentially efficient method for CO2 utilization, which can convert CO2 into value-added carbon and oxygen with a current density of 100–1000 mA cm-2. The electrolytic carbon (EC) prepared through the MSCC-ET process is highly electrically conductive and forms flexible microstructures. These structures show excellent adsorption ability towards environmental pollutants and high energy storage capacity when used in supercapacitors. Although the morphology, structure, and application of EC prepared under different electrolysis conditions have been previously reported, their intrinsic electrochemical properties have not yet been elucidated. Powder microelectrodes (PMEs) are useful for studying the electrochemical kinetics of various powdery materials. In this study, we systematically investigated the electrochemical properties of ECs obtained using molten Li2CO3-Na2CO3-K2CO3 under different temperature and electrolysis voltage conditions by cyclic voltammetry (CV) with a carbon powder microelectrode in 10 mmol L-1 Na2SO4. The electrochemical behavior of the EC obtained at 450 ℃ and a cell voltage of 4.5 V (450 ℃-4.5 V-EC) differs significantly from that of other carbon materials, i.e., multi-walled carbon nanotubes, graphene, graphite, and acetylene black. In addition to a much larger charging-discharging capacity, unusual hysteresis of the charge/discharge current response of ECs in the negative potential region (-0.6 to -0.2 V vs SCE) was observed. This phenomenon was eliminated by annealing the material under Ar at 550 ℃, demonstrating that the unique electrochemical behavior of ECs is closely related to the oxygen-containing groups on its surface. Furthermore, CVs of EC-PME were compared in solutions with different pH, Na2SO4 concentrations, and other ions. The pH of the solution did not affect the CVs, excluding a redox mechanism involving the surface functional groups. Hysteresis was weakened by a certain degree at slower potential sweep speeds (< 10 mV s-1) or in higher concentrations of electrolyte (100 mmol L-1 Na2SO4). The onset potential for discharging was negatively shifted in electrolytes with a larger cation ((NH4)2SO4) and was unaffected by larger anions (Na2S2O8). This indicates that the hysteresis is more likely related to the specific adsorption of cations, caused by the unique surface properties of EC. It should be noted that the specific surface area and oxygen concentration of EC can be adjusted by the electrolysis temperature and cell voltage. Generally, the Brunauer–Emmett–Teller (BET) specific surface area and oxygen content decrease with increasing temperature and the BET-area increases with increasing cell voltage. The CVs of ECs prepared at different cell voltages were similar, but the adsorption capacity decreased for those prepared at higher temperatures (550 and 650 ℃). Interestingly, the specific capacitance of the ECs is much higher at negative potentials (-0.6 to 0 V vs. SCE) than that at positive potentials (0 to 0.6 V vs. SCE). Therefore, it is anticipated that a better capacitance performance can be achieved when the ECs are used as a negative electrode material in supercapacitors.
2019, 35(2): 230-240
doi: 10.3866/PKU.WHXB201711281
Abstract:
The rational design of naphthalimide derivatives, which can target specific DNA sequences and secondary structural DNA, is important for developing potential anticancer drugs. In this work, the naphthalimide-imidazole conjugate (3) and its alkylated derivatives (4a–c) were synthesized, and characterized by 1H NMR, 13C NMR, and mass spectrometry (MS). The interactions of these compounds with calf thymus DNA (CT DNA) and G-quadruplex DNA were investigated by UV-Vis spectroscopy, fluorescence spectroscopy, circular dichroism, and fluorescence resonance energy transfer (FRET). The studies revealed that the naphthalimides with imidazolium displayed higher affinity towards CT DNA than those with the imidazole moiety, suggesting that the electrostatic interaction plays an important role in the interactions between the naphthalimide and the DNA duplex. All of the obtained naphthalimide derivatives possessed high affinity (Ka > 4 × 106 L·mol-1) towards the telomeric G-quadruplex, and exhibited more than 30-fold selectivity for the quadruplex versus CT DNA. The viscosity of CT DNA increased upon addition of the naphthalimides, suggesting that the latter could bind to the former via a classical intercalation mode. FRET results indicated that the compounds 3 and 4a–c stabilized the structure of the telomeric G-quadruplex by increasing its melting temperature by 5.8, 10.7, 8.4, and 7.8 ℃, respectively. CD spectral results suggested that the telomeric G-quadruplex maintained a mixture of antiparallel and parallel conformation in the presence of the naphthalimide derivatives (3 and 4a–c) in a buffer containing K+. The fluorescence intensity of the naphthalimide derivatives 3 and 4a, b with octylimidazolium was significantly enhanced upon interaction with the G-quadruplex, which could be attributed to the immersion of naphthalimide moieties in the hydrophobic region of the G-quadruplex. However, the fluorescence of compound 4c with hexadecylimidazolium increased only slightly upon addition of the G-quadruplex. Molecular docking studies indicated that the naphthalimide derivatives were associated with the loop and groove of the human telomeric G-quadruplex via hydrophobic interactions. A hydrogen bond was formed between the imidazole group in compound 3 and the guanine residue DG16. The phosphate group from the G-quadruplex backbone pointed to the imidazolium moiety of 4a–c, suggesting that the electrostatic interactions also played an important role. Being fluorescent, the cellular localization of 3 and 4a–c could be conveniently tracked by fluorescence imaging. The results showed that compounds 4a–c, which contained the imidazolium moiety, were mainly localized in the nucleus after 4.0 h of incubation, while compound 3 with the imidazole moiety was partially localized in the nucleus. The enhancement of the nuclear localization of 4a–c may be attributed to the positive charge in 4a–c and their higher DNA affinity. Based on the MTT assay results, it was concluded that compounds 4a–c displayed much stronger cytotoxic activity against breast cancer cells than 3. Furthermore, compounds 4a and 4b selectively inhibited the A549 cells over normal human lung fibroblast MRC-5 cells, with high anticancer activity. These results indicated that the G-quadruplex binding affinity and anticancer activity of naphthalimide could be modulated by conjugation with the imidazole moiety. ![]()
The rational design of naphthalimide derivatives, which can target specific DNA sequences and secondary structural DNA, is important for developing potential anticancer drugs. In this work, the naphthalimide-imidazole conjugate (3) and its alkylated derivatives (4a–c) were synthesized, and characterized by 1H NMR, 13C NMR, and mass spectrometry (MS). The interactions of these compounds with calf thymus DNA (CT DNA) and G-quadruplex DNA were investigated by UV-Vis spectroscopy, fluorescence spectroscopy, circular dichroism, and fluorescence resonance energy transfer (FRET). The studies revealed that the naphthalimides with imidazolium displayed higher affinity towards CT DNA than those with the imidazole moiety, suggesting that the electrostatic interaction plays an important role in the interactions between the naphthalimide and the DNA duplex. All of the obtained naphthalimide derivatives possessed high affinity (Ka > 4 × 106 L·mol-1) towards the telomeric G-quadruplex, and exhibited more than 30-fold selectivity for the quadruplex versus CT DNA. The viscosity of CT DNA increased upon addition of the naphthalimides, suggesting that the latter could bind to the former via a classical intercalation mode. FRET results indicated that the compounds 3 and 4a–c stabilized the structure of the telomeric G-quadruplex by increasing its melting temperature by 5.8, 10.7, 8.4, and 7.8 ℃, respectively. CD spectral results suggested that the telomeric G-quadruplex maintained a mixture of antiparallel and parallel conformation in the presence of the naphthalimide derivatives (3 and 4a–c) in a buffer containing K+. The fluorescence intensity of the naphthalimide derivatives 3 and 4a, b with octylimidazolium was significantly enhanced upon interaction with the G-quadruplex, which could be attributed to the immersion of naphthalimide moieties in the hydrophobic region of the G-quadruplex. However, the fluorescence of compound 4c with hexadecylimidazolium increased only slightly upon addition of the G-quadruplex. Molecular docking studies indicated that the naphthalimide derivatives were associated with the loop and groove of the human telomeric G-quadruplex via hydrophobic interactions. A hydrogen bond was formed between the imidazole group in compound 3 and the guanine residue DG16. The phosphate group from the G-quadruplex backbone pointed to the imidazolium moiety of 4a–c, suggesting that the electrostatic interactions also played an important role. Being fluorescent, the cellular localization of 3 and 4a–c could be conveniently tracked by fluorescence imaging. The results showed that compounds 4a–c, which contained the imidazolium moiety, were mainly localized in the nucleus after 4.0 h of incubation, while compound 3 with the imidazole moiety was partially localized in the nucleus. The enhancement of the nuclear localization of 4a–c may be attributed to the positive charge in 4a–c and their higher DNA affinity. Based on the MTT assay results, it was concluded that compounds 4a–c displayed much stronger cytotoxic activity against breast cancer cells than 3. Furthermore, compounds 4a and 4b selectively inhibited the A549 cells over normal human lung fibroblast MRC-5 cells, with high anticancer activity. These results indicated that the G-quadruplex binding affinity and anticancer activity of naphthalimide could be modulated by conjugation with the imidazole moiety.
2019, 35(2): 167-181
doi: 10.3866/PKU.WHXB201803022
Abstract:
Sensitivity analysis is an important tool in model validation and evaluation that has been employed extensively in the analysis of chemical kinetic models of combustion processes. The input parameters of a chemical kinetic model are always associated with some uncertainties, and the effects of these uncertainties on the predicted combustion properties can be determined through sensitivity analysis. In this work, first- and second-order global and local sensitivity coefficients of ignition delay time with respect to the scaling factor for reaction rate constants in chemical kinetic mechanisms for combustion of H2, methane, n-butane, and n-heptane are examined. In the sensitivity analysis performed here, the output of the model is taken to be natural logarithm of ignition delay time and the input parameters are the natural logarithms of the factors that scale the reaction rate constants. The output of the model is expressed as a polynomial function of the input parameters, with up to coupling between two input parameters in the present sensitivity analysis. This polynomial function is determined by varying one or two input parameters, and allows the determination of both local and global sensitivity coefficients. The order of the polynomial function in the present work is four, and the factor that scales the reaction rate constant is in the range from 1/e to e, where e is the base of the natural logarithm. A relatively small number of sample runs are required in this approach compared to the global sensitivity analysis based on the highly dimensional model representation method, which utilizes random sampling of input (RS-HDMR). In RS-HDMR, sensitivity coefficients are determined only for the rate constants of a limited number of reactions; the present approach, by contrast, affords sensitivity coefficients for a larger number of reactions. Reactions and reaction pairs with the largest sensitivity coefficients are listed for ignition delay times of four typical fuels. Global sensitivity coefficients are always positive, while local sensitivity coefficients can be either positive or negative. A negative local sensitivity coefficient indicates that the reaction promotes ignition, while a positive local sensitivity coefficient suggests that the reaction actually suppresses ignition. Our results show that important reactions or reaction pairs identified by global sensitivity analysis are usually rather similar to those based on local sensitivity analysis. This finding can probably be attributed to the fact that the values of input parameters are within a rather small range in the sensitivity analysis, and nonlinear effects for such a small range of parameters are negligible. It is possible to determine global sensitivity coefficients by varying the input parameters over a larger range using the present approach. Such analysis shows that correlation effects between an important reaction and a minor reaction can have relatively sizable second-order sensitivity coefficient in some cases. On the other hand, first-order global sensitivity coefficients in the present approach will be affected by coupling between two reactions, and some results of the first-order global sensitivity analysis will be different from those determined by local sensitivity analysis or global sensitivity analysis under conditions where the correlation effects of two reactions are neglected. The present sensitivity analysis approach provides valuable information on important reactions as well as correlated effects of two reactions on the combustion characteristics of a chemical kinetic mechanism. In addition, the analysis can also be employed to aid global sensitivity analysis using RS-HDMR, where global sensitivity coefficients are determined more reliably.
Sensitivity analysis is an important tool in model validation and evaluation that has been employed extensively in the analysis of chemical kinetic models of combustion processes. The input parameters of a chemical kinetic model are always associated with some uncertainties, and the effects of these uncertainties on the predicted combustion properties can be determined through sensitivity analysis. In this work, first- and second-order global and local sensitivity coefficients of ignition delay time with respect to the scaling factor for reaction rate constants in chemical kinetic mechanisms for combustion of H2, methane, n-butane, and n-heptane are examined. In the sensitivity analysis performed here, the output of the model is taken to be natural logarithm of ignition delay time and the input parameters are the natural logarithms of the factors that scale the reaction rate constants. The output of the model is expressed as a polynomial function of the input parameters, with up to coupling between two input parameters in the present sensitivity analysis. This polynomial function is determined by varying one or two input parameters, and allows the determination of both local and global sensitivity coefficients. The order of the polynomial function in the present work is four, and the factor that scales the reaction rate constant is in the range from 1/e to e, where e is the base of the natural logarithm. A relatively small number of sample runs are required in this approach compared to the global sensitivity analysis based on the highly dimensional model representation method, which utilizes random sampling of input (RS-HDMR). In RS-HDMR, sensitivity coefficients are determined only for the rate constants of a limited number of reactions; the present approach, by contrast, affords sensitivity coefficients for a larger number of reactions. Reactions and reaction pairs with the largest sensitivity coefficients are listed for ignition delay times of four typical fuels. Global sensitivity coefficients are always positive, while local sensitivity coefficients can be either positive or negative. A negative local sensitivity coefficient indicates that the reaction promotes ignition, while a positive local sensitivity coefficient suggests that the reaction actually suppresses ignition. Our results show that important reactions or reaction pairs identified by global sensitivity analysis are usually rather similar to those based on local sensitivity analysis. This finding can probably be attributed to the fact that the values of input parameters are within a rather small range in the sensitivity analysis, and nonlinear effects for such a small range of parameters are negligible. It is possible to determine global sensitivity coefficients by varying the input parameters over a larger range using the present approach. Such analysis shows that correlation effects between an important reaction and a minor reaction can have relatively sizable second-order sensitivity coefficient in some cases. On the other hand, first-order global sensitivity coefficients in the present approach will be affected by coupling between two reactions, and some results of the first-order global sensitivity analysis will be different from those determined by local sensitivity analysis or global sensitivity analysis under conditions where the correlation effects of two reactions are neglected. The present sensitivity analysis approach provides valuable information on important reactions as well as correlated effects of two reactions on the combustion characteristics of a chemical kinetic mechanism. In addition, the analysis can also be employed to aid global sensitivity analysis using RS-HDMR, where global sensitivity coefficients are determined more reliably.
2019, 35(2): 215-222
doi: 10.3866/PKU.WHXB201803061
Abstract:
The development of the photocatalytic production of hydrogen from water splitting has attracted immense attention in recent years. CdS is a potential photocatalyst with a visible light response, though it still suffers from a limited activity for hydrogen production due to the fast recombination of photo-induced electron/hole pairs and the low reaction rate of hydrogen evolution on the surface. Studies on the effect of CdS surface structure and properties on hydrogen production are still very limited. In this work, we prepared three CdS nanocrystals with different morphologies: long rod, short rod, and triangular plate. The prepared samples were well characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) specific surface area analysis, X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectroscopy, and UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS). From the results of TEM, XRD and XPS, we find that the three CdS nanocrystals with different morphologies were successfully synthesized. From the PL spectra, we conclude that the area of exposed nonpolar surface and degree of surface defects increase with an increase in aspect ratio. We also performed the photocatalytic hydrogen production reaction using the three CdS crystals. Long rod-like CdS (lr-CdS) exhibits the highest photocatalytic activity, with a hydrogen production rate of 482 μmol·h-1·g-1, which is 2.6 times that of short rod-like CdS (sr-CdS) (183 μmol·h-1·g-1) and 8.8 times that of triangular plate-like CdS (tp-CdS, 55 μmol h-1·g-1). It is found that lr-CdS shows a higher hydrogen production rate than sr-CdS and tp-CdS. We find that the hydrogen production rate is related to the degree of surface defects. Surface defects can trap the photo-induced electrons/holes, thus decreasing their probability of recombination. In addition, these defects can be used to anchor Pd particles to form a heterojunction structure that facilitates the separation of photo-induced charges. Therefore, we also compared three CdS/Pd nanocrystals synthesized with the three abovementioned morphologies with respect to hydrogen production. With 1% (w, mass fraction) Pd, the hydrogen production rate was greatly enhanced compared to all the CdS catalysts. Compared to the unpromoted CdS, the reaction rate is enhanced 43.1, 10.7 and 6.0 times over those of sr-CdS, lr-CdS and tp-CdS, respectively. Notably, the hydrogen production rate with short rod-like CdS/Pd reaches 7884 μmol·h-1·g-1, which can be favorably compared with the ever-increasing values reported in the literature. Hopefully, this work provides knowledge on the effect of crystal surface structure and properties on photocatalysis. ![]()
The development of the photocatalytic production of hydrogen from water splitting has attracted immense attention in recent years. CdS is a potential photocatalyst with a visible light response, though it still suffers from a limited activity for hydrogen production due to the fast recombination of photo-induced electron/hole pairs and the low reaction rate of hydrogen evolution on the surface. Studies on the effect of CdS surface structure and properties on hydrogen production are still very limited. In this work, we prepared three CdS nanocrystals with different morphologies: long rod, short rod, and triangular plate. The prepared samples were well characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) specific surface area analysis, X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectroscopy, and UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS). From the results of TEM, XRD and XPS, we find that the three CdS nanocrystals with different morphologies were successfully synthesized. From the PL spectra, we conclude that the area of exposed nonpolar surface and degree of surface defects increase with an increase in aspect ratio. We also performed the photocatalytic hydrogen production reaction using the three CdS crystals. Long rod-like CdS (lr-CdS) exhibits the highest photocatalytic activity, with a hydrogen production rate of 482 μmol·h-1·g-1, which is 2.6 times that of short rod-like CdS (sr-CdS) (183 μmol·h-1·g-1) and 8.8 times that of triangular plate-like CdS (tp-CdS, 55 μmol h-1·g-1). It is found that lr-CdS shows a higher hydrogen production rate than sr-CdS and tp-CdS. We find that the hydrogen production rate is related to the degree of surface defects. Surface defects can trap the photo-induced electrons/holes, thus decreasing their probability of recombination. In addition, these defects can be used to anchor Pd particles to form a heterojunction structure that facilitates the separation of photo-induced charges. Therefore, we also compared three CdS/Pd nanocrystals synthesized with the three abovementioned morphologies with respect to hydrogen production. With 1% (w, mass fraction) Pd, the hydrogen production rate was greatly enhanced compared to all the CdS catalysts. Compared to the unpromoted CdS, the reaction rate is enhanced 43.1, 10.7 and 6.0 times over those of sr-CdS, lr-CdS and tp-CdS, respectively. Notably, the hydrogen production rate with short rod-like CdS/Pd reaches 7884 μmol·h-1·g-1, which can be favorably compared with the ever-increasing values reported in the literature. Hopefully, this work provides knowledge on the effect of crystal surface structure and properties on photocatalysis.
2019, 35(2): 223-229
doi: 10.3866/PKU.WHXB201802263
Abstract:
Catalytic hydrogenation of CO2 to methanol is an important chemical process owing to its contribution in alleviating the impacts of the greenhouse effect and in realizing the requirement for renewable energy sources. Owing to their excellent synergic functionalities and unique optoelectronic as well as catalytic properties, transition metal/ZnO (M/ZnO) nanocomposites have been widely used as catalysts for this reaction in recent years. Development of size-controlled synthesis of metal/oxide complexes is highly desirable. Further, because it is extremely difficult to achieve the strong-metal-support-interaction (SMSI) effect when the M/ZnO nanocomposites are prepared via physical methods, the use of chemical methods is more favorable for the fabrication of multi-component catalysts. However, because of the requirement for an extra H2 reduction step to obtain the active metallic phase (M) and surfactants to control the size of nanoparticles, most M/ZnO nanocomposites undergo two- or multi-step synthesis, which is disadvantageous for the stable catalytic performance of the M/ZnO nanocomposites. In this work, we demonstrate facile one-pot synthesis of M/ZnO (M = Pd, Au, Ag, and Cu) nanocomposites in refluxed ethylene glycol as a solvent, without using any surfactants. During the synthesis process, Pd and ZnO species can stabilize each other from further aggregation by reducing their individual surface energies, thereby achieving size control of particles. Besides, NaHCO3 serves as a size-control tool for Pd nanoparticles by adjusting the alkaline conditions. Ethylene glycol serves as a mild reducing agent and solvent owing to its capacity to reduce Pd ions to generate Pd crystals. The nucleation and growth of Pd particles are achieved by thermal reduction, while the ZnO nanocrystals are formed by thermal decomposition of Zn(OAc)2. X-ray diffraction patterns of the M/ZnO and ZnO were analyzed to study the phase of the nanocomposites, and the results show that no impurity phase was detected. Transmission electron microscopy (TEM) was used to study the morphology and structural properties. In addition, X-ray photoelectron spectroscopy analysis was performed to further confirm the formation of M/ZnO hybrid materials, and the results confirm SMSI between Pd and ZnO. Inductively coupled plasma mass spectrometry was used to check the actual elemental compositions, and the results show that the detected atomic ratios of Pd/Zn were consistent with the values in the theoretical recipe. To investigate the effects of the Pd/Zn molar ratios and the added amount of NaHCO3 on Pd size, the average sizes of Pd particles were calculated, and the results were confirmed by TEM observation. The Cu/ZnO/Al2O3 composite is a widely known catalyst for hydrogenation of CO2 to methanol, and other M/ZnO composites are also catalytic for this reaction. Therefore, different M/ZnO hybrids were further studied as catalysts for hydrogenation of CO2 to methanol, among which Pd/ZnO (1 : 9) demonstrated the best performance (30% CO2 conversion, 69% methanol selectivity, and 421.9 gmethanol·(kg catalyst·h)-1 at 240 ℃ and 5 MPa. The outstanding catalytic performance may be explained by the following two factors: first, Pd is a good catalyst for the dissociation of H2 to give active H atoms, and second, SMSI between Pd and ZnO favors the formation of surface oxygen vacancies on ZnO. Moreover, most M/ZnO composites exhibit excellent performance in methanol selectivity, especially the Au/ZnO catalyst, which has the highest methanol selectivity (82%) despite having the lowest CO2 conversion. Hopefully, this work would provide a simple route for synthesis of M/ZnO nanocomposites with clean surfaces for catalysis. ![]()
Catalytic hydrogenation of CO2 to methanol is an important chemical process owing to its contribution in alleviating the impacts of the greenhouse effect and in realizing the requirement for renewable energy sources. Owing to their excellent synergic functionalities and unique optoelectronic as well as catalytic properties, transition metal/ZnO (M/ZnO) nanocomposites have been widely used as catalysts for this reaction in recent years. Development of size-controlled synthesis of metal/oxide complexes is highly desirable. Further, because it is extremely difficult to achieve the strong-metal-support-interaction (SMSI) effect when the M/ZnO nanocomposites are prepared via physical methods, the use of chemical methods is more favorable for the fabrication of multi-component catalysts. However, because of the requirement for an extra H2 reduction step to obtain the active metallic phase (M) and surfactants to control the size of nanoparticles, most M/ZnO nanocomposites undergo two- or multi-step synthesis, which is disadvantageous for the stable catalytic performance of the M/ZnO nanocomposites. In this work, we demonstrate facile one-pot synthesis of M/ZnO (M = Pd, Au, Ag, and Cu) nanocomposites in refluxed ethylene glycol as a solvent, without using any surfactants. During the synthesis process, Pd and ZnO species can stabilize each other from further aggregation by reducing their individual surface energies, thereby achieving size control of particles. Besides, NaHCO3 serves as a size-control tool for Pd nanoparticles by adjusting the alkaline conditions. Ethylene glycol serves as a mild reducing agent and solvent owing to its capacity to reduce Pd ions to generate Pd crystals. The nucleation and growth of Pd particles are achieved by thermal reduction, while the ZnO nanocrystals are formed by thermal decomposition of Zn(OAc)2. X-ray diffraction patterns of the M/ZnO and ZnO were analyzed to study the phase of the nanocomposites, and the results show that no impurity phase was detected. Transmission electron microscopy (TEM) was used to study the morphology and structural properties. In addition, X-ray photoelectron spectroscopy analysis was performed to further confirm the formation of M/ZnO hybrid materials, and the results confirm SMSI between Pd and ZnO. Inductively coupled plasma mass spectrometry was used to check the actual elemental compositions, and the results show that the detected atomic ratios of Pd/Zn were consistent with the values in the theoretical recipe. To investigate the effects of the Pd/Zn molar ratios and the added amount of NaHCO3 on Pd size, the average sizes of Pd particles were calculated, and the results were confirmed by TEM observation. The Cu/ZnO/Al2O3 composite is a widely known catalyst for hydrogenation of CO2 to methanol, and other M/ZnO composites are also catalytic for this reaction. Therefore, different M/ZnO hybrids were further studied as catalysts for hydrogenation of CO2 to methanol, among which Pd/ZnO (1 : 9) demonstrated the best performance (30% CO2 conversion, 69% methanol selectivity, and 421.9 gmethanol·(kg catalyst·h)-1 at 240 ℃ and 5 MPa. The outstanding catalytic performance may be explained by the following two factors: first, Pd is a good catalyst for the dissociation of H2 to give active H atoms, and second, SMSI between Pd and ZnO favors the formation of surface oxygen vacancies on ZnO. Moreover, most M/ZnO composites exhibit excellent performance in methanol selectivity, especially the Au/ZnO catalyst, which has the highest methanol selectivity (82%) despite having the lowest CO2 conversion. Hopefully, this work would provide a simple route for synthesis of M/ZnO nanocomposites with clean surfaces for catalysis.
