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2018 Volume 76 Issue 1
2018, 76(1): 9-21
doi: 10.6023/A17070320
Abstract:
Photonic crystals have been widely used in solar cells in recent years, owing to the characteristic photonic bandgap, "slow photon" effect and a series of unique light control performance. The introduction of photonic crystals can greatly optimize the propagation and distribution of light in solar cells. Photonic crystals can improve the performance of solar cells from five aspects:(1) Photonic crystals constructed as back mirrors to reduce light loss and increase absorption efficiency of solar cell. (2) The interaction between photons and sensitizers can be enhanced by the "slow photon effect" of the photonic crystal band gap, which enhances the excitation efficiency. (3) Photonic crystal can be used as a scattering layer, increasing the propagation path of light in the material, forming a resonance enhancement mode in the absorption layer, and improving the light absorption efficiency. (4) Photonic crystals have large specific surface area. Especially three-dimensional photonic crystals can provide excellent carrier for sensitizer, which can effectively increase the load and activity of sensitized molecules and improve the photoelectric conversion efficiency (5) Photonic crystals can be used to reduce the dependence of solar cells on the incident angle of sunlight. For example, when the incident light is tilted, the blue shift of the Bragg position results in more overlap with the dye absorption peak, generating a higher efficiency that partially compensates the reduced photon flux due to light inclination. However, photonic crystals in different locations of the solar cell will improve or inhibit photoelectric conversion efficiency. Therefore, the fully understanding of light manipulation of photonic crystals and their correctly application is the key to improve the photoelectric conversion efficiency. Here, the applications of different types of photonic crystals in silicon solar cells and sensitized solar cells are summarized, at the same time the possible problems are also analyzed and reviewed.
Photonic crystals have been widely used in solar cells in recent years, owing to the characteristic photonic bandgap, "slow photon" effect and a series of unique light control performance. The introduction of photonic crystals can greatly optimize the propagation and distribution of light in solar cells. Photonic crystals can improve the performance of solar cells from five aspects:(1) Photonic crystals constructed as back mirrors to reduce light loss and increase absorption efficiency of solar cell. (2) The interaction between photons and sensitizers can be enhanced by the "slow photon effect" of the photonic crystal band gap, which enhances the excitation efficiency. (3) Photonic crystal can be used as a scattering layer, increasing the propagation path of light in the material, forming a resonance enhancement mode in the absorption layer, and improving the light absorption efficiency. (4) Photonic crystals have large specific surface area. Especially three-dimensional photonic crystals can provide excellent carrier for sensitizer, which can effectively increase the load and activity of sensitized molecules and improve the photoelectric conversion efficiency (5) Photonic crystals can be used to reduce the dependence of solar cells on the incident angle of sunlight. For example, when the incident light is tilted, the blue shift of the Bragg position results in more overlap with the dye absorption peak, generating a higher efficiency that partially compensates the reduced photon flux due to light inclination. However, photonic crystals in different locations of the solar cell will improve or inhibit photoelectric conversion efficiency. Therefore, the fully understanding of light manipulation of photonic crystals and their correctly application is the key to improve the photoelectric conversion efficiency. Here, the applications of different types of photonic crystals in silicon solar cells and sensitized solar cells are summarized, at the same time the possible problems are also analyzed and reviewed.
2018, 76(1): 22-29
doi: 10.6023/A17070339
Abstract:
A series of Ni/C core-shell nano catalysts with abundant mesoporous and uniform size were prepared by Ni-MOF-74 pyrolysis. The Ni-MOF-74 was synthesized via hydrothermal method with nickel acetate and 2, 5-dihydroxyterephthalic acid (DHTA) as raw materials. The pyrolysis process was carried out in a tube furnace under Argon (Ar) atmosphere with a heating rate of 2℃/min. Completed pyrolytic product Ni/C can be obtained by extending the pyrolysis time (6 h) at 400℃ or increasing the pyrolysis temperature (≥ 500℃) based on the TG result. Moreover, the particle size of Ni/C varied with pyrolysis temperature from 3 nm (500℃) to 17 nm (800℃). The TEM images and Ar ion sputtering XPS indicated a core-shell structure of the pyrolysis product. Nickel species can be stable in the form of nickel (Ni0) due to the electronic properties regulating and confinement effect of the carbon shell. Moreover, the carbon shell greatly weaken the interaction between particles, which is favorable for the dispersion of the catalyst in the reaction system. H2-TPR results show that the interaction between nickel and amorphous carbon increases with the pyrolysis temperature, which is unfavorable to the interaction between Ni species and the reactant. The phenylacetylene (PA) hydrogenation reaction was carried out with 0.2 g catalyst, 10 mL of 1 mol/L ethanolic phenylacetylene solution and 1.0 MPa H2 in a 50 mL high-pressure autoclave under 50℃. Ni/C exhibits excellent catalytic activity and recyclability in phenylacetylene (PA) hydrogenation. In addition, we compared the activity of Ni/C with several reported catalyst system and found their activity increases in the order of Ni, NiSix, supported Ni2Si, Ni/C, Pd and Pt. With an activity of up to 0.833 mmol·min-1·gcat.-1 at 50℃ (Ni/C-400-6, Ni/C-500-2), Ni/C is the most promising transition metal catalyst that can be comparable with noble metal.
A series of Ni/C core-shell nano catalysts with abundant mesoporous and uniform size were prepared by Ni-MOF-74 pyrolysis. The Ni-MOF-74 was synthesized via hydrothermal method with nickel acetate and 2, 5-dihydroxyterephthalic acid (DHTA) as raw materials. The pyrolysis process was carried out in a tube furnace under Argon (Ar) atmosphere with a heating rate of 2℃/min. Completed pyrolytic product Ni/C can be obtained by extending the pyrolysis time (6 h) at 400℃ or increasing the pyrolysis temperature (≥ 500℃) based on the TG result. Moreover, the particle size of Ni/C varied with pyrolysis temperature from 3 nm (500℃) to 17 nm (800℃). The TEM images and Ar ion sputtering XPS indicated a core-shell structure of the pyrolysis product. Nickel species can be stable in the form of nickel (Ni0) due to the electronic properties regulating and confinement effect of the carbon shell. Moreover, the carbon shell greatly weaken the interaction between particles, which is favorable for the dispersion of the catalyst in the reaction system. H2-TPR results show that the interaction between nickel and amorphous carbon increases with the pyrolysis temperature, which is unfavorable to the interaction between Ni species and the reactant. The phenylacetylene (PA) hydrogenation reaction was carried out with 0.2 g catalyst, 10 mL of 1 mol/L ethanolic phenylacetylene solution and 1.0 MPa H2 in a 50 mL high-pressure autoclave under 50℃. Ni/C exhibits excellent catalytic activity and recyclability in phenylacetylene (PA) hydrogenation. In addition, we compared the activity of Ni/C with several reported catalyst system and found their activity increases in the order of Ni, NiSix, supported Ni2Si, Ni/C, Pd and Pt. With an activity of up to 0.833 mmol·min-1·gcat.-1 at 50℃ (Ni/C-400-6, Ni/C-500-2), Ni/C is the most promising transition metal catalyst that can be comparable with noble metal.
2018, 76(1): 30-34
doi: 10.6023/A17060279
Abstract:
Among currently reported anodic nano-alloy electrocatlysts for direct alkaline ethanol fuel cells (DAEFCs), the mass fraction (w) of co-catalysts is generally larger than 20%. This could increase the thickness of the catalyst layer in Membrane Electrode Assembly (MEA), which not only decreases the discharge voltage of fuel cells, also reduces the utilization of the noble metals such as Pt and Pd. To solve this problem, here we synthesized a series of Pd-Ni-P alloy electrocatalysts with ultra-low doping amount of Ni and P, using ca. 1.5 mg NaH2PO2 as reducing agent. To obtain different doping amount of Ni and P, the pH value of the synthetic solution was adjusted from 8 to 12 by 0.1 mol/L NaOH. Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) results showed that the mass fraction of Ni and P were low to 0.2% and 0.05%, respectively, when the pH value of the synthetic solution is 10. Transmission Electron Microscopy (TEM) images showed that nanoparticles were distributed evenly on the carbon base, and their mean particle sizes increased from ca. 3.78 nm to ca. 5.4 nm with alkalinity of synthetic solutions increasing. Cyclic Voltammograms in 0.5 mol/L CH3CH2OH+1 mol/L NaOH solution revealed that the catalyst obtained under the pH 10 synthetic solution (hereafter denoted as Pd-Ni-P/C-pH10) gave a highest apparent current density of ca. 2466 mA•mg-1 Pd, nearly 2.7 times in respect of that of the commercial Pd/C catalyst (JM). Meanwhile, the durability of Pd-Ni-P/C-pH10 for ethanol oxidation was improved by ca. 2.8 times compared to commercial catalyst. Relative to pure Pd, the binding energy of Pd 3d5/2 of as-prepared catalysts all positively shifted, suggesting an obvious electronic interaction between Pd, Ni and P component in as-prepared catalysts. This interaction could led to a shift of the d-band center of Pd component, which may play a pivotal and dominated role in improving the catalytic performance for the ethanol electrooxidation in alkaline media.
Among currently reported anodic nano-alloy electrocatlysts for direct alkaline ethanol fuel cells (DAEFCs), the mass fraction (w) of co-catalysts is generally larger than 20%. This could increase the thickness of the catalyst layer in Membrane Electrode Assembly (MEA), which not only decreases the discharge voltage of fuel cells, also reduces the utilization of the noble metals such as Pt and Pd. To solve this problem, here we synthesized a series of Pd-Ni-P alloy electrocatalysts with ultra-low doping amount of Ni and P, using ca. 1.5 mg NaH2PO2 as reducing agent. To obtain different doping amount of Ni and P, the pH value of the synthetic solution was adjusted from 8 to 12 by 0.1 mol/L NaOH. Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) results showed that the mass fraction of Ni and P were low to 0.2% and 0.05%, respectively, when the pH value of the synthetic solution is 10. Transmission Electron Microscopy (TEM) images showed that nanoparticles were distributed evenly on the carbon base, and their mean particle sizes increased from ca. 3.78 nm to ca. 5.4 nm with alkalinity of synthetic solutions increasing. Cyclic Voltammograms in 0.5 mol/L CH3CH2OH+1 mol/L NaOH solution revealed that the catalyst obtained under the pH 10 synthetic solution (hereafter denoted as Pd-Ni-P/C-pH10) gave a highest apparent current density of ca. 2466 mA•mg-1 Pd, nearly 2.7 times in respect of that of the commercial Pd/C catalyst (JM). Meanwhile, the durability of Pd-Ni-P/C-pH10 for ethanol oxidation was improved by ca. 2.8 times compared to commercial catalyst. Relative to pure Pd, the binding energy of Pd 3d5/2 of as-prepared catalysts all positively shifted, suggesting an obvious electronic interaction between Pd, Ni and P component in as-prepared catalysts. This interaction could led to a shift of the d-band center of Pd component, which may play a pivotal and dominated role in improving the catalytic performance for the ethanol electrooxidation in alkaline media.
2018, 76(1): 35-42
doi: 10.6023/A17070336
Abstract:
Hypoxia is a hallmark of tumor. Based on this feature, a hypoxia-responsive drug delivery system combined with tumor-targeting was developed. HA-NI conjugates were prepared by grafting the carboxyl group of hyaluronic acid (HA) with an amine group of nitroimidazole (NI) derivative in the presence of 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) and N-hydroxysuccinimide (NHS). The structure of HA-NI conjugates was confirmed by FT-IR and 1H NMR, the degree of substitution (DS) of NI derivative was also calculated based on 1H NMR. Amphiphilic HA-NI conjugates could self-assemble into micelles by ultrasonic method. The size and morphology of micelles were characterized by dynamic light scattering (DLS), atomic force microscope (AFM) and transmission electron microscopy (TEM), the stability of micelles was also investigated by DLS. It was found the size of micelles was in the range of 80~220 nm while the DS decreased from 6.5% to 3.6%. Doxorubicin (DOX) was encapsulated in micelles, and DOX-loaded micelles had smaller sizes compared with blank micelles. Drug-loading (DL) and entrapment efficiency (EE) were obtained by UV-Vis analysis and increased with the increasing DS. Under hypoxic condition, micelles became bigger and size distribution of micelles became wider, it was clear to observe the destruction of micelles by AFM and TEM. UV spectrum revealed the characteristic peak belonging to NI at 325 nm disappeared and Zeta potential increased from -30.6±0.4 mV to -24.9±0.5 mV 6 h later. In vitro drug release studies demonstrated that DOX was released from HA-NI micelles in a hypoxia-dependent manner:micelles were sufficiently stable at normoxic condition while accomplished a rapid drug release under hypoxic condition.
Hypoxia is a hallmark of tumor. Based on this feature, a hypoxia-responsive drug delivery system combined with tumor-targeting was developed. HA-NI conjugates were prepared by grafting the carboxyl group of hyaluronic acid (HA) with an amine group of nitroimidazole (NI) derivative in the presence of 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) and N-hydroxysuccinimide (NHS). The structure of HA-NI conjugates was confirmed by FT-IR and 1H NMR, the degree of substitution (DS) of NI derivative was also calculated based on 1H NMR. Amphiphilic HA-NI conjugates could self-assemble into micelles by ultrasonic method. The size and morphology of micelles were characterized by dynamic light scattering (DLS), atomic force microscope (AFM) and transmission electron microscopy (TEM), the stability of micelles was also investigated by DLS. It was found the size of micelles was in the range of 80~220 nm while the DS decreased from 6.5% to 3.6%. Doxorubicin (DOX) was encapsulated in micelles, and DOX-loaded micelles had smaller sizes compared with blank micelles. Drug-loading (DL) and entrapment efficiency (EE) were obtained by UV-Vis analysis and increased with the increasing DS. Under hypoxic condition, micelles became bigger and size distribution of micelles became wider, it was clear to observe the destruction of micelles by AFM and TEM. UV spectrum revealed the characteristic peak belonging to NI at 325 nm disappeared and Zeta potential increased from -30.6±0.4 mV to -24.9±0.5 mV 6 h later. In vitro drug release studies demonstrated that DOX was released from HA-NI micelles in a hypoxia-dependent manner:micelles were sufficiently stable at normoxic condition while accomplished a rapid drug release under hypoxic condition.
2018, 76(1): 43-48
doi: 10.6023/A17090428
Abstract:
Conventionally, red shift of QD photoluminescence (PL) could be achieved by growing QDs to larger sizes using hydrothermal method, which is usually a very slow process. We synthesized green fluorescent CdTe quantum dots with GSH as the ligand and proved the successful synthesis of (NH4)2MoS4 by UV-Vis, X-Ray Diffraction, Raman spectroscopy. In the process of research, we found that (NH4)2MoS4 can change the wavelength of CdTe quantum dots under the condition of heating or at room temperature. Redshift of emission wavelength can change with the different ratio between (NH4)2MoS4and CdTe QDs. In this study, we report a rapid and convenient method to achieve red-shift of CdTe QD PL via (NH4)2MoS4-guided QD self-assembly. We show that the emission wavelength of CdTe QDs underwent a red-shift of more than 100 nm for 15 min at 100℃ in the presence of (NH4)2MoS4. At the same time, we conduct a experiment, which have no red-shift in the absence of (NH4)2MoS4 for 15 min at 100℃. This illustrates that the (NH4)2MoS4 plays an important role in CdTe QDs self-assembly. The red-shift of QD PL was also observed at room temperature but relatively slower. The formation of QD assembly was confirmed by gel electrophoresis, transmission electron microscopy, and X-ray photoelectron spectroscopy. The result of gel electrophoresis and transmission electron microscopy directly shows the self-assembly morphology of CdTe QDs and the change of size and shape. Self-assembly entity was proved to contain Mo and Cd by the X-ray photoelectron spectroscopy, which confirmed the connection between (NH4)2MoS4and CdTe QDs. A control experiment was conducted by replacing (NH4)2MoS4with Na2S for QD assembly, in that case no apparent change of emission wavelength was observed. These results reveal that MoS42- within (NH4)2MoS4 is crucial for self-assembly of CdTe QDs. Accordingly, we propose a reasonable model of (NH4)2MoS4-guided CdTe QD self-assembly. In this model, we consider the connection between a (NH4)2MoS4 and two CdTe QDs in ideal condition. With the increasing ratio of (NH4)2MoS4, much more connection between (NH4)2MoS4 and CdTe will be obtained. Assembling entity morphology changed with different cross-linking way. The resulting QD assembly was further applied to cell imaging experiments, demonstrating their potentials in this field.
Conventionally, red shift of QD photoluminescence (PL) could be achieved by growing QDs to larger sizes using hydrothermal method, which is usually a very slow process. We synthesized green fluorescent CdTe quantum dots with GSH as the ligand and proved the successful synthesis of (NH4)2MoS4 by UV-Vis, X-Ray Diffraction, Raman spectroscopy. In the process of research, we found that (NH4)2MoS4 can change the wavelength of CdTe quantum dots under the condition of heating or at room temperature. Redshift of emission wavelength can change with the different ratio between (NH4)2MoS4and CdTe QDs. In this study, we report a rapid and convenient method to achieve red-shift of CdTe QD PL via (NH4)2MoS4-guided QD self-assembly. We show that the emission wavelength of CdTe QDs underwent a red-shift of more than 100 nm for 15 min at 100℃ in the presence of (NH4)2MoS4. At the same time, we conduct a experiment, which have no red-shift in the absence of (NH4)2MoS4 for 15 min at 100℃. This illustrates that the (NH4)2MoS4 plays an important role in CdTe QDs self-assembly. The red-shift of QD PL was also observed at room temperature but relatively slower. The formation of QD assembly was confirmed by gel electrophoresis, transmission electron microscopy, and X-ray photoelectron spectroscopy. The result of gel electrophoresis and transmission electron microscopy directly shows the self-assembly morphology of CdTe QDs and the change of size and shape. Self-assembly entity was proved to contain Mo and Cd by the X-ray photoelectron spectroscopy, which confirmed the connection between (NH4)2MoS4and CdTe QDs. A control experiment was conducted by replacing (NH4)2MoS4with Na2S for QD assembly, in that case no apparent change of emission wavelength was observed. These results reveal that MoS42- within (NH4)2MoS4 is crucial for self-assembly of CdTe QDs. Accordingly, we propose a reasonable model of (NH4)2MoS4-guided CdTe QD self-assembly. In this model, we consider the connection between a (NH4)2MoS4 and two CdTe QDs in ideal condition. With the increasing ratio of (NH4)2MoS4, much more connection between (NH4)2MoS4 and CdTe will be obtained. Assembling entity morphology changed with different cross-linking way. The resulting QD assembly was further applied to cell imaging experiments, demonstrating their potentials in this field.
2018, 76(1): 49-54
doi: 10.6023/A17090406
Abstract:
Halide perovskite (ABC3) solar cell has received a lot of attentions due to its excellent photoelectronic properties. It has been proven to be an effective way to modify halide perovskite materials' bandgap by replacing A or B ions with other equivalent ions. However, C ions have much fewer choices and are limited to halogen anions or pseudohalides anions. We designed a series of new cubic perovskite structures through substituting C anions by superhalogen clusters anions (BeX3-, MgX3-, BX4-, AlX4-, SiX5-, PX6-, X=F, Cl), and studied their structures and properties in first-principles way. Calculations were performed by using the Vienna ab initio simulation package (VASP) based on density functional theory. The DOS (Density of States) and bandgaps were calculated to analyze properties of the new perovskite structures. The results show that BeX3-, MgX3- (X=F, Cl) and SiCl5- could not remain its structure which means these three clusters are not superhalogen anions anymore after doping. The size and symmetry of superhalogen anions have influences on the structures of doped perovskites. The superhalogen anion whose symmetry is higher and size is closed to I- ion induces less distortions to doped perovskite structures. Comparing to the VBM (Valence Band Maximum) and CBM (Conduction Band Minimum) of CsPbI3, superhalogen anions substitutions could change the compositions of CBM and VBM and bandgaps. The bandgaps of superhalogen anions partial substitutions in halide perovskite become smaller compared to structures with superhalogen anions substituting completely. We demonstrate that the CsPb(PCl6)3, with a direct-bandgap of 1.58 eV located at M(0, 0.5, 0.5) point, could be a potential candidate material for solar cells. Its CBM mainly is dominated by Cl 3p states, P 3s states and Pb 6p states. The other doped perovskites with wide bandgaps may have potential applications in transistors or memristors. We hope that these results could provide theoretical guidance for synthesis of new perovskite materials for solar cells.
Halide perovskite (ABC3) solar cell has received a lot of attentions due to its excellent photoelectronic properties. It has been proven to be an effective way to modify halide perovskite materials' bandgap by replacing A or B ions with other equivalent ions. However, C ions have much fewer choices and are limited to halogen anions or pseudohalides anions. We designed a series of new cubic perovskite structures through substituting C anions by superhalogen clusters anions (BeX3-, MgX3-, BX4-, AlX4-, SiX5-, PX6-, X=F, Cl), and studied their structures and properties in first-principles way. Calculations were performed by using the Vienna ab initio simulation package (VASP) based on density functional theory. The DOS (Density of States) and bandgaps were calculated to analyze properties of the new perovskite structures. The results show that BeX3-, MgX3- (X=F, Cl) and SiCl5- could not remain its structure which means these three clusters are not superhalogen anions anymore after doping. The size and symmetry of superhalogen anions have influences on the structures of doped perovskites. The superhalogen anion whose symmetry is higher and size is closed to I- ion induces less distortions to doped perovskite structures. Comparing to the VBM (Valence Band Maximum) and CBM (Conduction Band Minimum) of CsPbI3, superhalogen anions substitutions could change the compositions of CBM and VBM and bandgaps. The bandgaps of superhalogen anions partial substitutions in halide perovskite become smaller compared to structures with superhalogen anions substituting completely. We demonstrate that the CsPb(PCl6)3, with a direct-bandgap of 1.58 eV located at M(0, 0.5, 0.5) point, could be a potential candidate material for solar cells. Its CBM mainly is dominated by Cl 3p states, P 3s states and Pb 6p states. The other doped perovskites with wide bandgaps may have potential applications in transistors or memristors. We hope that these results could provide theoretical guidance for synthesis of new perovskite materials for solar cells.
2018, 76(1): 55-61
doi: 10.6023/A17080357
Abstract:
Density functional theory (DFT) calculations at the B3LYP level, combining with the double-ζ valence polarized (DZVP) all-electron basis set as embeded in Gaussian 09 Program, were carried out to investigate the reaction mechanisms and substituent effect of Diels-Alder-alike reactions between 1-iodo-2-lithio-o-carborane and fulvenes. For maximum analogy with experimental conditions, all calculations were carried out in cyclohexane solution by using the IDSCRF solvent model, and all energies reported here had been adjusted adaptive to experimental temperature (353 K). According to presently obtained results, this reaction needs to go through a four-step process successively before the final carboranonorbornadiene products are formed. These four steps include forming carboryne intermediate by release of LiI fragment, interaction of carboryne with fulvenes, 1, 2-σ migration of carboranyl, and the cycloaddition process. Among all four steps mentioned above, the 1, 2-σ migration of carboranyl is predicted to be the rate-determining step (RDS), features an activation free energy barrier of 28.3 kcal·mol-1 under experimental temperature of 353 K. A half-life of 8.7 h converted from the RDS activation free energy barrier coincides well with corresponding 56% isolated yield of carboranonorbornadiene after reacted 8 h. The LiI fragment is found to be vital in stabilizing most stationary points and driving the reaction ahead. The reaction mechanisms change little when the 4-H substituents on diphenylfulvenes (denoted reaction a) is replaced by 4-Me groups (denoted reaction b), but the corresponding RDS activation free energy barrier increased by 2.8 kcal·mol-1 (from 28.3 to 31.1 kcal·mol-1), transferring to a decrease in reaction rate of ca. 50 times. The obvious slower reaction rate predicted in reaction b than in reaction a gives out correct trends with an experimental yield reduction of carboranonorbornadienes from 56% to 42%, and verifies the rationality of B3LYP results in these carboranyl-involved Diels-Alder-alike reactions. Natural bond orbital (NBO) analysis about corresponding reactants and stationary points shows similar electronic characteristics of this reaction with normal-electron-demand Diels-Alder (NEDDA) reactions, i.e., the fulvenes act as electron donator when react with carboryne intermediate.
Density functional theory (DFT) calculations at the B3LYP level, combining with the double-ζ valence polarized (DZVP) all-electron basis set as embeded in Gaussian 09 Program, were carried out to investigate the reaction mechanisms and substituent effect of Diels-Alder-alike reactions between 1-iodo-2-lithio-o-carborane and fulvenes. For maximum analogy with experimental conditions, all calculations were carried out in cyclohexane solution by using the IDSCRF solvent model, and all energies reported here had been adjusted adaptive to experimental temperature (353 K). According to presently obtained results, this reaction needs to go through a four-step process successively before the final carboranonorbornadiene products are formed. These four steps include forming carboryne intermediate by release of LiI fragment, interaction of carboryne with fulvenes, 1, 2-σ migration of carboranyl, and the cycloaddition process. Among all four steps mentioned above, the 1, 2-σ migration of carboranyl is predicted to be the rate-determining step (RDS), features an activation free energy barrier of 28.3 kcal·mol-1 under experimental temperature of 353 K. A half-life of 8.7 h converted from the RDS activation free energy barrier coincides well with corresponding 56% isolated yield of carboranonorbornadiene after reacted 8 h. The LiI fragment is found to be vital in stabilizing most stationary points and driving the reaction ahead. The reaction mechanisms change little when the 4-H substituents on diphenylfulvenes (denoted reaction a) is replaced by 4-Me groups (denoted reaction b), but the corresponding RDS activation free energy barrier increased by 2.8 kcal·mol-1 (from 28.3 to 31.1 kcal·mol-1), transferring to a decrease in reaction rate of ca. 50 times. The obvious slower reaction rate predicted in reaction b than in reaction a gives out correct trends with an experimental yield reduction of carboranonorbornadienes from 56% to 42%, and verifies the rationality of B3LYP results in these carboranyl-involved Diels-Alder-alike reactions. Natural bond orbital (NBO) analysis about corresponding reactants and stationary points shows similar electronic characteristics of this reaction with normal-electron-demand Diels-Alder (NEDDA) reactions, i.e., the fulvenes act as electron donator when react with carboryne intermediate.
2018, 76(1): 62-67
doi: 10.6023/A17080348
Abstract:
With the advent of increasingly stringent regulations on sulfur containing in oil products, desulfurization of crude oil has become an urgent task for petrochemical production. Molybdenum sulfide (MoS2) have been extensively studied as one of the most efficient hydrodesulfurization catalysts. Co-doped molybdenum sulfides are usually used in desulfurization processes of sulfur-containing compounds, in which the transition metal Co could promote its catalytic performance. Herein, density functional theory was employed to investigate the formation of coordinative unsaturated active sites (CUS) and the catalytic desulfurization process of methanethiol (CH3SH) at the Co-doped MoS2 triangular clusters. Results showed that Co was not the effective reaction site on hydrogenation process, and Mo and S atoms acted as the active sites of hydrogen dissociation during the formation of CUS sites, followed by the H2S generation and desorption. The charge population analyses showed that Co promoted the hydrogenation process indirectly. CH3SH prefered to be adsorbed at the TopCo site with a high adsorption energy of -1.44 eV. The charge population and frontier orbitial analyses illustrated that Co can alter the distribution of the surface atoms' charge and the LUMO orbital of CUS and showed the strong electrophile and thus strengthening the CH3SH adsorption. When CH3SH was adsorbed at the Co-doped MoS2 clusters, electrons transfered from CH3SH to the surface atoms of MoS2. In this work, three desulfurization pathways of CH3SH at the Co-doped MoS2 were investigated, namely, the C-S bond initial scission, the S-H bond initial scissions, and the C-S and S-H scissions simultaneously. The competitive route during the CH3SH desulfurization process started with the S-H and C-S bond scissions successively, followed by the methane formation in the terms of thermodynamics and kinetics, and the formation of methane was the rate-determining step with the energy barrier of 1.51 eV.
With the advent of increasingly stringent regulations on sulfur containing in oil products, desulfurization of crude oil has become an urgent task for petrochemical production. Molybdenum sulfide (MoS2) have been extensively studied as one of the most efficient hydrodesulfurization catalysts. Co-doped molybdenum sulfides are usually used in desulfurization processes of sulfur-containing compounds, in which the transition metal Co could promote its catalytic performance. Herein, density functional theory was employed to investigate the formation of coordinative unsaturated active sites (CUS) and the catalytic desulfurization process of methanethiol (CH3SH) at the Co-doped MoS2 triangular clusters. Results showed that Co was not the effective reaction site on hydrogenation process, and Mo and S atoms acted as the active sites of hydrogen dissociation during the formation of CUS sites, followed by the H2S generation and desorption. The charge population analyses showed that Co promoted the hydrogenation process indirectly. CH3SH prefered to be adsorbed at the TopCo site with a high adsorption energy of -1.44 eV. The charge population and frontier orbitial analyses illustrated that Co can alter the distribution of the surface atoms' charge and the LUMO orbital of CUS and showed the strong electrophile and thus strengthening the CH3SH adsorption. When CH3SH was adsorbed at the Co-doped MoS2 clusters, electrons transfered from CH3SH to the surface atoms of MoS2. In this work, three desulfurization pathways of CH3SH at the Co-doped MoS2 were investigated, namely, the C-S bond initial scission, the S-H bond initial scissions, and the C-S and S-H scissions simultaneously. The competitive route during the CH3SH desulfurization process started with the S-H and C-S bond scissions successively, followed by the methane formation in the terms of thermodynamics and kinetics, and the formation of methane was the rate-determining step with the energy barrier of 1.51 eV.
