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2018 Volume 34 Issue 6
2018, 34(6): 551-552
doi: 10.3866/PKU.WHXB201710172
Abstract:
2018, 34(6): 553-554
doi: 10.3866/PKU.WHXB201710301
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2018, 34(6): 555-556
doi: 10.3866/PKU.WHXB201710302
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2018, 34(6): 557-558
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2018, 34(6): 559-560
doi: 10.3866/PKU.WHXB201711133
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2018, 34(6): 561-562
doi: 10.3866/PKU.WHXB201712182
Abstract:
2018, 34(6): 563-566
doi: 10.3866/PKU.WHXB201802282
Abstract:
Chemical concepts such as structure, bonding, reactivity, etc. have been widely used in the literature and text books to appreciate molecular properties and chemical transformations. Even though modern theoretical and computational chemistry is well established from the perspective of accuracy and complexity, how to quantify these concepts is a still unresolved task. Conceptual density functional theory and its related recent developments provide unique opportunities to tackle this problem. In this Special Issue, 27 contributions from top investigators over the world are collected to highlight the state-of-art research on this topic, which not only showcases the status of where we are now but also unveils a number to possible future directions to be pursued.
Chemical concepts such as structure, bonding, reactivity, etc. have been widely used in the literature and text books to appreciate molecular properties and chemical transformations. Even though modern theoretical and computational chemistry is well established from the perspective of accuracy and complexity, how to quantify these concepts is a still unresolved task. Conceptual density functional theory and its related recent developments provide unique opportunities to tackle this problem. In this Special Issue, 27 contributions from top investigators over the world are collected to highlight the state-of-art research on this topic, which not only showcases the status of where we are now but also unveils a number to possible future directions to be pursued.
2018, 34(6): 567-580
doi: 10.3866/PKU.WHXB201801261
Abstract:
In this perspective, we review the chemical information encoded in electron density and other ingredients used in semilocal functionals. This information is usually looked at from the functional point of view: the exchange density or the enhancement factor are discussed in terms of the reduced density gradient. However, what parts of a molecule do these 3D functions represent? We look at these quantities in real space, aiming to understand the electronic structure information they encode and provide an insight from the quantum chemical topology (QCT). Generalized gradient approximations (GGAs) provide information about the presence of chemical interactions, whereas meta-GGAs can differentiate between the different bonding types. By merging these two techniques, we show new insight into the failures of semilocal functionals owing to three main errors: fractional charges, fractional spins, and non-covalent interactions. We build on simple models. We also analyze the delocalization error in hydrogen chains, showing the ability of QCT to reveal the delocalization error introduced by semilocal functionals. Then, we show how the analysis of localization can help understand the fractional spin error in alkali atoms, and how it can be used to correct it. Finally, we show that the poor description of GGAs of isodesmic reactions in alkanes is due to 1, 3-interactions.
In this perspective, we review the chemical information encoded in electron density and other ingredients used in semilocal functionals. This information is usually looked at from the functional point of view: the exchange density or the enhancement factor are discussed in terms of the reduced density gradient. However, what parts of a molecule do these 3D functions represent? We look at these quantities in real space, aiming to understand the electronic structure information they encode and provide an insight from the quantum chemical topology (QCT). Generalized gradient approximations (GGAs) provide information about the presence of chemical interactions, whereas meta-GGAs can differentiate between the different bonding types. By merging these two techniques, we show new insight into the failures of semilocal functionals owing to three main errors: fractional charges, fractional spins, and non-covalent interactions. We build on simple models. We also analyze the delocalization error in hydrogen chains, showing the ability of QCT to reveal the delocalization error introduced by semilocal functionals. Then, we show how the analysis of localization can help understand the fractional spin error in alkali atoms, and how it can be used to correct it. Finally, we show that the poor description of GGAs of isodesmic reactions in alkanes is due to 1, 3-interactions.
2018, 34(6): 581-597
doi: 10.3866/PKU.WHXB201711222
Abstract:
With solar, wind, and other types of renewable energy incorporated into electrical grids and with the construction of smart grids, energy storage technology has become essential to optimize energy utilization. Due primarily to its abundance and low cost, aqueous rechargeable sodium-ion batteries (ARSBs) have received increasing attention in the field of electrochemical energy storage technology, and represent a promising alternative to energy storage in future power grids. However, because of the limitations of the thermodynamics of electrochemical processes in water, reactions in aqueous solution are more complicated compared to an organic system. Many parameters must be taken into account in an aqueous system, such as electrolyte concentration, dissolved oxygen content, and pH. As a result, it is challenging to select an appropriate electrode material, whose capacity, electrochemical potential, adaptability, and even catalytic effect may seriously affect the battery performance and hamper its application. Therefore, the development of advanced electrode materials, which can suppress side reactions of the battery and have good electrochemical performance, has become the focus of ARSB research. This paper briefly discusses the characteristics of ARSBs and summarizes the latest research progress in the development of electrode materials, including oxides, polyanionic compounds, Prussian blue analogues, and organics. This review also discusses the challenges remaining in the development of ARSBs, and suggests several ways to solve them, such as by using multivalent ions, hybridized electrolytes, etc., and speculates about future research directions. The studies and concepts discussed herein will advance the development of ARSBs and promote the optimization of energy utilization.
With solar, wind, and other types of renewable energy incorporated into electrical grids and with the construction of smart grids, energy storage technology has become essential to optimize energy utilization. Due primarily to its abundance and low cost, aqueous rechargeable sodium-ion batteries (ARSBs) have received increasing attention in the field of electrochemical energy storage technology, and represent a promising alternative to energy storage in future power grids. However, because of the limitations of the thermodynamics of electrochemical processes in water, reactions in aqueous solution are more complicated compared to an organic system. Many parameters must be taken into account in an aqueous system, such as electrolyte concentration, dissolved oxygen content, and pH. As a result, it is challenging to select an appropriate electrode material, whose capacity, electrochemical potential, adaptability, and even catalytic effect may seriously affect the battery performance and hamper its application. Therefore, the development of advanced electrode materials, which can suppress side reactions of the battery and have good electrochemical performance, has become the focus of ARSB research. This paper briefly discusses the characteristics of ARSBs and summarizes the latest research progress in the development of electrode materials, including oxides, polyanionic compounds, Prussian blue analogues, and organics. This review also discusses the challenges remaining in the development of ARSBs, and suggests several ways to solve them, such as by using multivalent ions, hybridized electrolytes, etc., and speculates about future research directions. The studies and concepts discussed herein will advance the development of ARSBs and promote the optimization of energy utilization.
2018, 34(6): 598-617
doi: 10.3866/PKU.WHXB201711231
Abstract:
Vitamin E compounds are biologically active and are frequently used as antioxidants. The demand for Vitamin E compounds has increased significantly in recent years, and at present, more than 80% of the market demand for Vitamin E is fulfilled by its synthetic counterparts. Therefore, it is imperative to increase the production of Vitamin E. Vitamin E compounds contain tocopherol and tocotrienol derivatives, and α-tocopherol, which dominates the sound, is the most biologically active. This review covers the methods of preparation of α-tocopherol, focusing on the synthesis routes, chemical reactions, and corresponding catalysts. The synthesis of Vitamin E, including preparation of 2, 3, 5-trimethylhydroquinone (TMHQ), preparation of isophytol, and condensation of TMHQ and isophytol are discussed in detail. The disadvantages and issues related to the preparation methods are also included. In general, the preparation of TMHQ comprises three steps: (1) methylation of m-cresol to 2, 3, 6-trimethylphenol, (2) oxidation of 2, 3, 6-trimethylphenol to 2, 3, 5-trimethylbenzoquione (TMBQ), and (3) hydrogenation of TMBQ to TMHQ. Recently, a novel and attractive method using isophorone, which can be produced by self-condensation of acetone, as a source for synthesizing TMHQ has been developed. Among these procedures, it is important to attain high selectivity in the oxidative reactions, including oxidation of 2, 3, 6-trimethylphenol and isophorone (α-isophorone or β-isophorone), and to replace H2O2, a common oxidant, by oxygen or air. One of the methods of preparation of isophytol using citral as a source has been abandoned because of shortage of oil of litsea cubeba, which is a natural source of citral. Linalool, produced from 6-methyl-5-hepten-2-one, is a key intermediate in the main process of preparation of isophytol. Both BASF SE and Roche have developed effective methods for the preparation of 6-methyl-5-hepten-2-one, respectively. Semi-hydrogenation of alkynols plays a key role in the whole process. The selectivity, especially at high conversion is directly related to the profit; therefore, it is of great importance for industries. The condensation of TMHQ and isophytol is essentially a Friedel-Crafts alkylation reaction catalyzed by acids. Similar reactions include methylation of m-cresol. Bronsted acids are usually effective for these reactions; however, it is difficult to recover these catalysts from the homogeneous systems. Therefore solid acid has a great potential in this area and it is also a promising topic to reduce the loss of acid sites when using acid-immobilized catalysts. The supply of various sources of the reactants and the local policy need to be considered while choosing an appropriate method for the preparation of Vitamin E.
Vitamin E compounds are biologically active and are frequently used as antioxidants. The demand for Vitamin E compounds has increased significantly in recent years, and at present, more than 80% of the market demand for Vitamin E is fulfilled by its synthetic counterparts. Therefore, it is imperative to increase the production of Vitamin E. Vitamin E compounds contain tocopherol and tocotrienol derivatives, and α-tocopherol, which dominates the sound, is the most biologically active. This review covers the methods of preparation of α-tocopherol, focusing on the synthesis routes, chemical reactions, and corresponding catalysts. The synthesis of Vitamin E, including preparation of 2, 3, 5-trimethylhydroquinone (TMHQ), preparation of isophytol, and condensation of TMHQ and isophytol are discussed in detail. The disadvantages and issues related to the preparation methods are also included. In general, the preparation of TMHQ comprises three steps: (1) methylation of m-cresol to 2, 3, 6-trimethylphenol, (2) oxidation of 2, 3, 6-trimethylphenol to 2, 3, 5-trimethylbenzoquione (TMBQ), and (3) hydrogenation of TMBQ to TMHQ. Recently, a novel and attractive method using isophorone, which can be produced by self-condensation of acetone, as a source for synthesizing TMHQ has been developed. Among these procedures, it is important to attain high selectivity in the oxidative reactions, including oxidation of 2, 3, 6-trimethylphenol and isophorone (α-isophorone or β-isophorone), and to replace H2O2, a common oxidant, by oxygen or air. One of the methods of preparation of isophytol using citral as a source has been abandoned because of shortage of oil of litsea cubeba, which is a natural source of citral. Linalool, produced from 6-methyl-5-hepten-2-one, is a key intermediate in the main process of preparation of isophytol. Both BASF SE and Roche have developed effective methods for the preparation of 6-methyl-5-hepten-2-one, respectively. Semi-hydrogenation of alkynols plays a key role in the whole process. The selectivity, especially at high conversion is directly related to the profit; therefore, it is of great importance for industries. The condensation of TMHQ and isophytol is essentially a Friedel-Crafts alkylation reaction catalyzed by acids. Similar reactions include methylation of m-cresol. Bronsted acids are usually effective for these reactions; however, it is difficult to recover these catalysts from the homogeneous systems. Therefore solid acid has a great potential in this area and it is also a promising topic to reduce the loss of acid sites when using acid-immobilized catalysts. The supply of various sources of the reactants and the local policy need to be considered while choosing an appropriate method for the preparation of Vitamin E.
2018, 34(6): 618-624
doi: 10.3866/PKU.WHXB201710252
Abstract:
Methyl pentanoate (MPE, C6H12O2) is one of the intermediate produced species during the combustion of biodiesel and long-chain methyl esters. Until now, experimental results for MPE ignition are not available in literature, hence, a study on the ignition characteristics of MPE is necessary. In the present work, the ignition delay times for the gas phase of MPE/air and MPE/4%O2/Ar were measured behind reflected shock waves. Experimental conditions included temperatures of 1050–1350 K, pressures of 1.5 × 105 and 16 × 105 Pa, and equivalence ratios of 0.5, 1, and 2 for MPE/air, and temperatures of 1210–1410 K, pressures of 3.5 × 105 and 7 × 105 Pa, and equivalence ratios of 0.75 and 1.25 for MPE/4%O2/Ar. Ignition delay time was determined using the arrival of a reflected shock wave and the excited CH radical emission at the observation location, which is at a distance of 15 mm from the shock tube end wall. The obtained results indicate that as temperature or pressure increases, the ignition delay time of MPE/air and MPE/4%O2/Ar mixtures decreases definitely. However, the effect of the equivalence ratio on the ignition delay time of MPE/air is different at high and low pressures, (16 × 105 Pa: τign = 5.43 × 10−6Ф−0.411exp(1.73 × 102/RT), 1.5 × 105 Pa: τign = 7.58 × 10−7Ф0.193exp(2.11 × 102/RT). In addition, an ignition delay correlation, i.e., τign = 2.80 × 10−5(10−5P)−0.446±0.032Ф0.246±0.044exp((1.88 ± 0.03) × 102/RT)), is developed for MPE/4%O2/Ar in a pressure range of 3.5 × 105–7 × 105 Pa. This correlation shows that ignition delay depends on temperature, pressure, and the equivalence ratio, and it is useful for scaling experimental data to certain conditions for comparison purposes. In addition, the ignition delay times of MPE/air are considerably lower than that of MPE/4%O2/Ar because the fuel concentration of MPE/air is significantly higher than that of MPE/4%O2/Ar in the present ignition conditions. The ignition delay time behaviors of MPE were compared with those of other long-chain methyl esters, and results indicate that the ignition delay times of MPE are longer than those of long-chain methyl esters at lower temperatures (under 1200 K for MPE/air, under 1280 K for MPE/4%O2/Ar). The two available chemical kinetic mechanisms of MPE cannot predict current measured data accurately, a further refinement of the mechanisms of MPE ignition is required. Sensitivity analyses indicate that the chain-branching reaction, H + O2 = O + OH, plays the most promoting role in the high temperature ignition of MPE. To the best of the authors’ knowledge, this is the first study to report the high temperature experimental ignition delay data of MPE. The results of this study are useful for understanding the ignition characteristics of MPE and for providing measured data to refine the chemical kinetic mechanism of MPE.
Methyl pentanoate (MPE, C6H12O2) is one of the intermediate produced species during the combustion of biodiesel and long-chain methyl esters. Until now, experimental results for MPE ignition are not available in literature, hence, a study on the ignition characteristics of MPE is necessary. In the present work, the ignition delay times for the gas phase of MPE/air and MPE/4%O2/Ar were measured behind reflected shock waves. Experimental conditions included temperatures of 1050–1350 K, pressures of 1.5 × 105 and 16 × 105 Pa, and equivalence ratios of 0.5, 1, and 2 for MPE/air, and temperatures of 1210–1410 K, pressures of 3.5 × 105 and 7 × 105 Pa, and equivalence ratios of 0.75 and 1.25 for MPE/4%O2/Ar. Ignition delay time was determined using the arrival of a reflected shock wave and the excited CH radical emission at the observation location, which is at a distance of 15 mm from the shock tube end wall. The obtained results indicate that as temperature or pressure increases, the ignition delay time of MPE/air and MPE/4%O2/Ar mixtures decreases definitely. However, the effect of the equivalence ratio on the ignition delay time of MPE/air is different at high and low pressures, (16 × 105 Pa: τign = 5.43 × 10−6Ф−0.411exp(1.73 × 102/RT), 1.5 × 105 Pa: τign = 7.58 × 10−7Ф0.193exp(2.11 × 102/RT). In addition, an ignition delay correlation, i.e., τign = 2.80 × 10−5(10−5P)−0.446±0.032Ф0.246±0.044exp((1.88 ± 0.03) × 102/RT)), is developed for MPE/4%O2/Ar in a pressure range of 3.5 × 105–7 × 105 Pa. This correlation shows that ignition delay depends on temperature, pressure, and the equivalence ratio, and it is useful for scaling experimental data to certain conditions for comparison purposes. In addition, the ignition delay times of MPE/air are considerably lower than that of MPE/4%O2/Ar because the fuel concentration of MPE/air is significantly higher than that of MPE/4%O2/Ar in the present ignition conditions. The ignition delay time behaviors of MPE were compared with those of other long-chain methyl esters, and results indicate that the ignition delay times of MPE are longer than those of long-chain methyl esters at lower temperatures (under 1200 K for MPE/air, under 1280 K for MPE/4%O2/Ar). The two available chemical kinetic mechanisms of MPE cannot predict current measured data accurately, a further refinement of the mechanisms of MPE ignition is required. Sensitivity analyses indicate that the chain-branching reaction, H + O2 = O + OH, plays the most promoting role in the high temperature ignition of MPE. To the best of the authors’ knowledge, this is the first study to report the high temperature experimental ignition delay data of MPE. The results of this study are useful for understanding the ignition characteristics of MPE and for providing measured data to refine the chemical kinetic mechanism of MPE.
2018, 34(6): 625-630
doi: 10.3866/PKU.WHXB201711071
Abstract:
By extending the Levy wavefunction constrained search to Fock Space, one can define a wavefunction constrained search for electron densities in systems having noninteger number of electrons. For pure-state v-representable densities, the results are equivalent to what one would obtain with the zero-temperature grand canonical ensemble. In other cases, the wavefunction constrained search in Fock space presents an upper bound to the grand canonical ensemble functional. One advantage of the Fock-space wavefunction constrained search functional over the zero-temperature grand-canonical ensemble constrained search functional is that certain specific excited states (i.e., those that are not ground-state v-representable) are the stationary points of the Fock-space functional. However, a potential disadvantage of the Fock-space constrained search functional is that it is not convex.
By extending the Levy wavefunction constrained search to Fock Space, one can define a wavefunction constrained search for electron densities in systems having noninteger number of electrons. For pure-state v-representable densities, the results are equivalent to what one would obtain with the zero-temperature grand canonical ensemble. In other cases, the wavefunction constrained search in Fock space presents an upper bound to the grand canonical ensemble functional. One advantage of the Fock-space wavefunction constrained search functional over the zero-temperature grand-canonical ensemble constrained search functional is that certain specific excited states (i.e., those that are not ground-state v-representable) are the stationary points of the Fock-space functional. However, a potential disadvantage of the Fock-space constrained search functional is that it is not convex.
2018, 34(6): 631-638
doi: 10.3866/PKU.WHXB201710201
Abstract:
Chemical reactivity towards electron transfer is captured by the Fukui function. However, this is not well defined when the system or its ions have degenerate or pseudo-degenerate ground states. In such a case, the first-order chemical response is not independent of the perturbation and the correct response has to be computed using the mathematical formalism of perturbation theory for degenerate states. Spatial pseudo-degeneracy is ubiquitous in nanostructures with high symmetry and totally extended systems. Given the size of these systems, using degenerate-state perturbation theory is impractical because it requires the calculation of many excited states. Here we present an alternative to compute the chemical response of extended systems using models of local softness in terms of the local density of states. The local softness is approximately equal to the density of states at the Fermi level. However, such approximation leaves out the contribution of inner states. In order to include and weight the contribution of the states around the Fermi level, a model inspired by the long-range behavior of the local softness is presented. Single wall capped carbon nanotubes (SWCCNT) illustrate the limitation of the frontier orbital theory in extended systems. Thus, we have used a C360 SWCCNT to test the proposed model and how it compares with available models based on the local density of states. Interestingly, a simple Hückel approximation captures the main features of chemical response of these systems. Our results suggest that density-of-states models of the softness along simple tight binding Hamiltonians could be used to explore the chemical reactivity of more complex system, such a surfaces and nanoparticles.
Chemical reactivity towards electron transfer is captured by the Fukui function. However, this is not well defined when the system or its ions have degenerate or pseudo-degenerate ground states. In such a case, the first-order chemical response is not independent of the perturbation and the correct response has to be computed using the mathematical formalism of perturbation theory for degenerate states. Spatial pseudo-degeneracy is ubiquitous in nanostructures with high symmetry and totally extended systems. Given the size of these systems, using degenerate-state perturbation theory is impractical because it requires the calculation of many excited states. Here we present an alternative to compute the chemical response of extended systems using models of local softness in terms of the local density of states. The local softness is approximately equal to the density of states at the Fermi level. However, such approximation leaves out the contribution of inner states. In order to include and weight the contribution of the states around the Fermi level, a model inspired by the long-range behavior of the local softness is presented. Single wall capped carbon nanotubes (SWCCNT) illustrate the limitation of the frontier orbital theory in extended systems. Thus, we have used a C360 SWCCNT to test the proposed model and how it compares with available models based on the local density of states. Interestingly, a simple Hückel approximation captures the main features of chemical response of these systems. Our results suggest that density-of-states models of the softness along simple tight binding Hamiltonians could be used to explore the chemical reactivity of more complex system, such a surfaces and nanoparticles.
2018, 34(6): 639-649
doi: 10.3866/PKU.WHXB201710231
Abstract:
Although a large variety of aromatic systems have been unveiled in the literature, justifying their origin of stability and understanding their nature of aromaticity is still an unaccomplished task. In this work, using tools recently developed by us within the density functional reactivity theory framework, where we employ simple density functionals to quantify molecular structural and reactivity properties, we examine the aromaticity concept from a different perspective. Using six quantities from the information-theoretic approach, namely, the Shannon entropy, Fisher information, Ghosh-Berkowitz-Parr entropy, Onicescu information energy, information gain, and relative Rényi entropy, and four aromaticity descriptors, namely, the aromatic stabilization energy (ASE) index, the harmonic oscillator model of aromaticity (HOMA) index, the aromatic fluctuation (FLU) index, and the nucleus-independent chemical shift (NICS) index, we systematically examined the correlations between substituted fulvene derivatives fused with one, two, and three benzene rings. Among the 14 benzofulvene derivatives studied in this work, there were seven single-fused, four double-fused, and three triple-fused benzofulvene derivatives. Our results show that the aromaticity indexes are often well correlated with one another. The same is true for information-theoretic quantities. Moreover, these correlations are valid across all series of benzofulvene derivatives with different ring structures. The cross-correlations between information-theoretic quantities and aromaticity indexes were usually strong. However, two completely opposite patterns were observed; as a consequence, these correlations are not valid across all series of benzofulvene derivatives. The nature of these correlations depends on the nature of the ring structure. The two groups of systems, each obeying the same cross-correlation patterns, have a total number of 4n + 2 and 4n π electrons, respectively, which are in agreement with Hückel's rule of aromaticity and antiaromaticity. Compared with the results obtained for systems without a benzene fused ring, the correlation patterns of these quantities were always found to be the same, both with and without fused benzene rings. This suggests that, despite benzene's aromaticity, its fusion with a fulvene moiety does not modify the aromaticity and antiaromaticity of the fulvene ring. These results confirm that the fusion of benzene rings with a fulvene moiety has no influence on the aromatic nature of the fulvene moiety. Thus, the aromaticity and antiaromaticity of benzene-fused fulvene derivatives are solely determined by the fulvene moiety. These results should provide a new understanding of the origin and nature of aromaticity and antiaromaticity.
Although a large variety of aromatic systems have been unveiled in the literature, justifying their origin of stability and understanding their nature of aromaticity is still an unaccomplished task. In this work, using tools recently developed by us within the density functional reactivity theory framework, where we employ simple density functionals to quantify molecular structural and reactivity properties, we examine the aromaticity concept from a different perspective. Using six quantities from the information-theoretic approach, namely, the Shannon entropy, Fisher information, Ghosh-Berkowitz-Parr entropy, Onicescu information energy, information gain, and relative Rényi entropy, and four aromaticity descriptors, namely, the aromatic stabilization energy (ASE) index, the harmonic oscillator model of aromaticity (HOMA) index, the aromatic fluctuation (FLU) index, and the nucleus-independent chemical shift (NICS) index, we systematically examined the correlations between substituted fulvene derivatives fused with one, two, and three benzene rings. Among the 14 benzofulvene derivatives studied in this work, there were seven single-fused, four double-fused, and three triple-fused benzofulvene derivatives. Our results show that the aromaticity indexes are often well correlated with one another. The same is true for information-theoretic quantities. Moreover, these correlations are valid across all series of benzofulvene derivatives with different ring structures. The cross-correlations between information-theoretic quantities and aromaticity indexes were usually strong. However, two completely opposite patterns were observed; as a consequence, these correlations are not valid across all series of benzofulvene derivatives. The nature of these correlations depends on the nature of the ring structure. The two groups of systems, each obeying the same cross-correlation patterns, have a total number of 4n + 2 and 4n π electrons, respectively, which are in agreement with Hückel's rule of aromaticity and antiaromaticity. Compared with the results obtained for systems without a benzene fused ring, the correlation patterns of these quantities were always found to be the same, both with and without fused benzene rings. This suggests that, despite benzene's aromaticity, its fusion with a fulvene moiety does not modify the aromaticity and antiaromaticity of the fulvene ring. These results confirm that the fusion of benzene rings with a fulvene moiety has no influence on the aromatic nature of the fulvene moiety. Thus, the aromaticity and antiaromaticity of benzene-fused fulvene derivatives are solely determined by the fulvene moiety. These results should provide a new understanding of the origin and nature of aromaticity and antiaromaticity.
2018, 34(6): 650-655
doi: 10.3866/PKU.WHXB201710251
Abstract:
In this work it is shown that the kinetic energy and the exchange-correlation energy are mutual dependent on each other. This aspect is first derived in an orbital-free context. It is shown that the total Fermi potential depends on the density only, the individual parts, the Pauli kinetic energy and the exchange-correlation energy, however, are orbital dependent and as such mutually influence each other. The numerical investigation is performed for the orbital-based non-interacting Kohn-Sham system in order to avoid additional effects due to further approximations of the kinetic energy. The numerical influence of the exchange-correlation functional on the non-interacting kinetic energy is shown to be of the order of a few Hartrees. For chemical purposes, however, the energetic performance as a function of the nuclear coordinates is much more important than total energies. Therefore, the effect on the bond dissociation curve was studied exemplarily for the carbon monoxide. The data reveals that, the mutual influence between the exchange-correlation functional and the kinetic energy has a significant influence on bond dissociation energies and bond distances. Therefore, the effect of the exchange-correlation treatment must be considered in the design of orbital-free density functional approximations for the kinetic energy.
In this work it is shown that the kinetic energy and the exchange-correlation energy are mutual dependent on each other. This aspect is first derived in an orbital-free context. It is shown that the total Fermi potential depends on the density only, the individual parts, the Pauli kinetic energy and the exchange-correlation energy, however, are orbital dependent and as such mutually influence each other. The numerical investigation is performed for the orbital-based non-interacting Kohn-Sham system in order to avoid additional effects due to further approximations of the kinetic energy. The numerical influence of the exchange-correlation functional on the non-interacting kinetic energy is shown to be of the order of a few Hartrees. For chemical purposes, however, the energetic performance as a function of the nuclear coordinates is much more important than total energies. Therefore, the effect on the bond dissociation curve was studied exemplarily for the carbon monoxide. The data reveals that, the mutual influence between the exchange-correlation functional and the kinetic energy has a significant influence on bond dissociation energies and bond distances. Therefore, the effect of the exchange-correlation treatment must be considered in the design of orbital-free density functional approximations for the kinetic energy.
2018, 34(6): 656-661
doi: 10.3866/PKU.WHXB201801101
Abstract:
The kernel energy method (KEM) has been shown to provide fast and accurate molecular energy calculations for molecules at their equilibrium geometries. KEM breaks a molecule into smaller subsets, called kernels, for the purposes of calculation. The results from the kernels are summed according to an expression characteristic of KEM to obtain the full molecule energy. A generalization of the kernel expansion to density matrices provides the full molecule density matrix and orbitals. In this study, the kernel expansion for the density matrix is examined in the context of density functional theory (DFT) Kohn-Sham (KS) calculations. A kernel expansion for the one-body density matrix analogous to the kernel expansion for energy is defined, and is then converted into a normalized projector by using the Clinton algorithm. Such normalized projectors are factorizable into linear combination of atomic orbitals (LCAO) matrices that deliver full-molecule Kohn-Sham molecular orbitals in the atomic orbital basis. Both straightforward KEM energies and energies from a normalized, idempotent density matrix obtained from a density matrix kernel expansion to which the Clinton algorithm has been applied are compared to reference energies obtained from calculations on the full system without any kernel expansion. Calculations were performed both for a simple proof-of-concept system consisting of three atoms in a linear configuration and for a water cluster consisting of twelve water molecules. In the case of the proof-of-concept system, calculations were performed using the STO-3G and 6-31G(d, p) bases over a range of atomic separations, some very far from equilibrium. The water cluster was calculated in the 6-31G(d, p) basis at an equilibrium geometry. The normalized projector density energies are more accurate than the straightforward KEM energy results in nearly all cases. In the case of the water cluster, the energy of the normalized projector is approximately four times more accurate than the straightforward KEM energy result. The KS density matrices of this study are applicable to quantum crystallography.
The kernel energy method (KEM) has been shown to provide fast and accurate molecular energy calculations for molecules at their equilibrium geometries. KEM breaks a molecule into smaller subsets, called kernels, for the purposes of calculation. The results from the kernels are summed according to an expression characteristic of KEM to obtain the full molecule energy. A generalization of the kernel expansion to density matrices provides the full molecule density matrix and orbitals. In this study, the kernel expansion for the density matrix is examined in the context of density functional theory (DFT) Kohn-Sham (KS) calculations. A kernel expansion for the one-body density matrix analogous to the kernel expansion for energy is defined, and is then converted into a normalized projector by using the Clinton algorithm. Such normalized projectors are factorizable into linear combination of atomic orbitals (LCAO) matrices that deliver full-molecule Kohn-Sham molecular orbitals in the atomic orbital basis. Both straightforward KEM energies and energies from a normalized, idempotent density matrix obtained from a density matrix kernel expansion to which the Clinton algorithm has been applied are compared to reference energies obtained from calculations on the full system without any kernel expansion. Calculations were performed both for a simple proof-of-concept system consisting of three atoms in a linear configuration and for a water cluster consisting of twelve water molecules. In the case of the proof-of-concept system, calculations were performed using the STO-3G and 6-31G(d, p) bases over a range of atomic separations, some very far from equilibrium. The water cluster was calculated in the 6-31G(d, p) basis at an equilibrium geometry. The normalized projector density energies are more accurate than the straightforward KEM energy results in nearly all cases. In the case of the water cluster, the energy of the normalized projector is approximately four times more accurate than the straightforward KEM energy result. The KS density matrices of this study are applicable to quantum crystallography.
2018, 34(6): 662-674
doi: 10.3866/PKU.WHXB201711021
Abstract:
Quantitative correlation of several theoretical electrophilicity measures over different families of organic compounds are examined relative to the experimental values of Mayr et al. Notably, the ability to predict these values accurately will help to elucidate the reactivity and selectivity trends observed in charge-transfer reactions. A crucial advantage of this theoretical approach is that it provides this information without the need of experiments, which are often demanding and time-consuming. Here, two different types of electrophilicity measures were analyzed. First, models derived from conceptual density functional theory (c-DFT), including Parr's original proposal and further generalizations of this index, are investigated. For instance, the approaches of Gázquez et al. and Chamorro et al. are considered, whereby it is possible to distinguish between processes in which a molecule gains or loses electrons. Further, we also explored two novel electrophilicity definitions. On one hand, the potential of environmental perturbations to affect electron incorporation into a system is analyzed in terms of recent developments in c-DFT. These studies highlight the importance of considering the molecular surroundings when a consistent description of chemical reactivity is needed. On the other hand, we test a new definition of electrophilicity that is free from inconsistencies (so-called thermodynamic electrophilicity). This approach is based on Parr's pioneering insights, though it corrects issues present in the standard working expression for the calculation of electrophilicity. Additionally, we use machine-learning tools (i.e., symbolic regression) to identify the models that best fit the experimental values. In this way, the best possible description of the electrophilicity values in terms of different electronic structure quantities is obtained. Overall, this straightforward approach enables one to obtain good correlations between the theoretical and experimental quantities by using the simple, yet powerful, interpretative advantage of c-DFT methods. In general, we observed that the correlations found at the HF/6-31G(d) level of theory are of semi-quantitative value. To obtain more accurate results, we showed that working with families of compounds with similar functional groups is indispensable.
Quantitative correlation of several theoretical electrophilicity measures over different families of organic compounds are examined relative to the experimental values of Mayr et al. Notably, the ability to predict these values accurately will help to elucidate the reactivity and selectivity trends observed in charge-transfer reactions. A crucial advantage of this theoretical approach is that it provides this information without the need of experiments, which are often demanding and time-consuming. Here, two different types of electrophilicity measures were analyzed. First, models derived from conceptual density functional theory (c-DFT), including Parr's original proposal and further generalizations of this index, are investigated. For instance, the approaches of Gázquez et al. and Chamorro et al. are considered, whereby it is possible to distinguish between processes in which a molecule gains or loses electrons. Further, we also explored two novel electrophilicity definitions. On one hand, the potential of environmental perturbations to affect electron incorporation into a system is analyzed in terms of recent developments in c-DFT. These studies highlight the importance of considering the molecular surroundings when a consistent description of chemical reactivity is needed. On the other hand, we test a new definition of electrophilicity that is free from inconsistencies (so-called thermodynamic electrophilicity). This approach is based on Parr's pioneering insights, though it corrects issues present in the standard working expression for the calculation of electrophilicity. Additionally, we use machine-learning tools (i.e., symbolic regression) to identify the models that best fit the experimental values. In this way, the best possible description of the electrophilicity values in terms of different electronic structure quantities is obtained. Overall, this straightforward approach enables one to obtain good correlations between the theoretical and experimental quantities by using the simple, yet powerful, interpretative advantage of c-DFT methods. In general, we observed that the correlations found at the HF/6-31G(d) level of theory are of semi-quantitative value. To obtain more accurate results, we showed that working with families of compounds with similar functional groups is indispensable.
2018, 34(6): 675-682
doi: 10.3866/PKU.WHXB201801021
Abstract:
The addition of electrons to form gas-phase multiply charged anions (MCAs) normally requires sophisticated experiments or calculations.In this work, the factors stabilizing the MCAs, the maximum electron uptake of gas-phase molecules, X, and the electronic stability of MCAs XQ-, are discussed. The drawbacks encountered when applying computational and/or conceptual density functional theory (DFT) to MCAs are highlighted. We develop and test a different model based on the valence-state concept. As in DFT, the electronic energy, E(N, vex), is a continuous function of the average electron number, N, and the external potential, vex, of the nuclei. The valence-state-parabola is a second-order polynomial that allows extending E(N, vex) to dianions and higher MCAs. The model expresses the maximum electron acceptance, Qmax, and the higher electron affinities, AQ, as simple functions of the first electron affinity, A1, and the ionization energy, I, of the "ancestor" system. Thus, the maximum electron acceptance is Qmax, calc = 1 + 12A1/7(I -A1). The ground-state parabola model of the conceptual DFT yields approximately half of this value, and it is termed Qmax, GS =\begin{document}${}^{1}\!\!\diagup\!\!{}_{2}\; $\end{document}
The addition of electrons to form gas-phase multiply charged anions (MCAs) normally requires sophisticated experiments or calculations.In this work, the factors stabilizing the MCAs, the maximum electron uptake of gas-phase molecules, X, and the electronic stability of MCAs XQ-, are discussed. The drawbacks encountered when applying computational and/or conceptual density functional theory (DFT) to MCAs are highlighted. We develop and test a different model based on the valence-state concept. As in DFT, the electronic energy, E(N, vex), is a continuous function of the average electron number, N, and the external potential, vex, of the nuclei. The valence-state-parabola is a second-order polynomial that allows extending E(N, vex) to dianions and higher MCAs. The model expresses the maximum electron acceptance, Qmax, and the higher electron affinities, AQ, as simple functions of the first electron affinity, A1, and the ionization energy, I, of the "ancestor" system. Thus, the maximum electron acceptance is Qmax, calc = 1 + 12A1/7(I -A1). The ground-state parabola model of the conceptual DFT yields approximately half of this value, and it is termed Qmax, GS =
2018, 34(6): 683-391
doi: 10.3866/PKU.WHXB201801031
Abstract:
A new definition of the dual descriptor, namely, the thermodynamic dual descriptor, is developed within the grand canonical potential formalism. This new definition is formulated to describe the same physical phenomenon as the original definition proposed by Morell, Grand, and Toro-Labbé (J. Phys. Chem. A 2005, 109, 205), which is characterized by a second-order response of the electron density towards an electron flux. To formulate the new definition, we performed two successive partial derivatives of the average electron density, one with respect to the average number of electrons, and the other with respect to the chemical potential of the electron reservoir. When the derivative is expressed in terms of the three-state ensemble model, in the regime of low temperatures up to temperatures of chemical interest, one finds that the thermodynamic dual descriptor can be expressed as ∆fT(r) = (β/2)C[f+(r)-f-(r)], where β = 1/kBT, C is a global quantity that depends on the temperature and global electronic properties of the molecule (the first ionization potential and the electron affinity), C = 1 for systems with zero fractional charge, and C = Cω > 0 (albeit very close to zero) for systems with nonzero fractional charge, , and f+(r) and f-(r) are the nucleophilic and electrophilic Fukui functions, respectively. The quantity within the square brackets is the original definition of the dual descriptor. As the local terms (the ones containing regioselectivity information) are equal to those of the dual descriptor, ∆fT(r) has the same regioselectivity information, multiplied by the global quantity (β/2)C. This implies that the regioselectivity information contained in the original dual descriptor is preserved at all temperatures different from zero, and for any value of C > 0. One of the most important features of this new definition is that it avoids the undesired Dirac delta behavior observed when the second order partial derivative of the average density is taken with respect to the average number of electrons, using the exact density dependence of the average number of electrons.
A new definition of the dual descriptor, namely, the thermodynamic dual descriptor, is developed within the grand canonical potential formalism. This new definition is formulated to describe the same physical phenomenon as the original definition proposed by Morell, Grand, and Toro-Labbé (J. Phys. Chem. A 2005, 109, 205), which is characterized by a second-order response of the electron density towards an electron flux. To formulate the new definition, we performed two successive partial derivatives of the average electron density, one with respect to the average number of electrons, and the other with respect to the chemical potential of the electron reservoir. When the derivative is expressed in terms of the three-state ensemble model, in the regime of low temperatures up to temperatures of chemical interest, one finds that the thermodynamic dual descriptor can be expressed as ∆fT(r) = (β/2)C[f+(r)-f-(r)], where β = 1/kBT, C is a global quantity that depends on the temperature and global electronic properties of the molecule (the first ionization potential and the electron affinity), C = 1 for systems with zero fractional charge, and C = Cω > 0 (albeit very close to zero) for systems with nonzero fractional charge, , and f+(r) and f-(r) are the nucleophilic and electrophilic Fukui functions, respectively. The quantity within the square brackets is the original definition of the dual descriptor. As the local terms (the ones containing regioselectivity information) are equal to those of the dual descriptor, ∆fT(r) has the same regioselectivity information, multiplied by the global quantity (β/2)C. This implies that the regioselectivity information contained in the original dual descriptor is preserved at all temperatures different from zero, and for any value of C > 0. One of the most important features of this new definition is that it avoids the undesired Dirac delta behavior observed when the second order partial derivative of the average density is taken with respect to the average number of electrons, using the exact density dependence of the average number of electrons.
2018, 34(6): 692-698
doi: 10.3866/PKU.WHXB201801121
Abstract:
A global and local charge transfer partitioning model, based on the cornerstone theory developed by Robert G. Parr and Robert G. Pearson, which introduces two charge transfer channels (one for accepting electrons (electrophilic) and another for donating (nucleophilic)), is applied to the reaction of a set of indoles with 4, 6-dinitrobenzofuroxan. The global analysis indicates that the prevalent electron transfer mechanism in the reaction is a nucleophilic one on the indoles, i.e., the indoles under consideration transfer electrons to 4, 6-dinitrobenzofuroxan. Evaluating the reactivity descriptors with exchange-correlation functionals including exact exchange (global hybrids) yields slightly better correlations than those obtained with generalized gradient-approximated functionals; however, the trends are preserved. Comparing the trend obtained with the number of electrons donated by the indoles, and predicted by the partitioning model, with that observed experimentally based on the measured rate constants, we propose that the number of electrons transferred through this channel can be used as a nucleophilicity scale to order the reactivity of indoles towards 4, 6-dinitrobenzofuroxan. This approach to obtain reactivity scales has the advantage of depending on the intrinsic properties of the two reacting species; therefore, it opens the possibility that the same group of molecules may show different reactivity trends depending on the species with which they are reacting. The local model allows systematic incorporation of the reactive atoms based on the their decreasing condensed Fukui functions, and the correlations obtained by increasing the number of reactive atoms participating in the local analysis of the transferred nucleophilic charge improve, reaching an optimal correlation, which in the present case indicates keeping three atoms from the indoles and two from 4, 6-dinitrobenzofuroxan. The atoms selected by this procedure provide valuable information about the local reactivity of the indoles. We further show that this information about the most reactive atoms on each reactant, combined with the spatial distribution of the nucleophilic and electrophilic Fukui functions of both reactants, allows one to propose non-trivial candidates of starting geometries for the search of the transition state structures present in these reactions.
A global and local charge transfer partitioning model, based on the cornerstone theory developed by Robert G. Parr and Robert G. Pearson, which introduces two charge transfer channels (one for accepting electrons (electrophilic) and another for donating (nucleophilic)), is applied to the reaction of a set of indoles with 4, 6-dinitrobenzofuroxan. The global analysis indicates that the prevalent electron transfer mechanism in the reaction is a nucleophilic one on the indoles, i.e., the indoles under consideration transfer electrons to 4, 6-dinitrobenzofuroxan. Evaluating the reactivity descriptors with exchange-correlation functionals including exact exchange (global hybrids) yields slightly better correlations than those obtained with generalized gradient-approximated functionals; however, the trends are preserved. Comparing the trend obtained with the number of electrons donated by the indoles, and predicted by the partitioning model, with that observed experimentally based on the measured rate constants, we propose that the number of electrons transferred through this channel can be used as a nucleophilicity scale to order the reactivity of indoles towards 4, 6-dinitrobenzofuroxan. This approach to obtain reactivity scales has the advantage of depending on the intrinsic properties of the two reacting species; therefore, it opens the possibility that the same group of molecules may show different reactivity trends depending on the species with which they are reacting. The local model allows systematic incorporation of the reactive atoms based on the their decreasing condensed Fukui functions, and the correlations obtained by increasing the number of reactive atoms participating in the local analysis of the transferred nucleophilic charge improve, reaching an optimal correlation, which in the present case indicates keeping three atoms from the indoles and two from 4, 6-dinitrobenzofuroxan. The atoms selected by this procedure provide valuable information about the local reactivity of the indoles. We further show that this information about the most reactive atoms on each reactant, combined with the spatial distribution of the nucleophilic and electrophilic Fukui functions of both reactants, allows one to propose non-trivial candidates of starting geometries for the search of the transition state structures present in these reactions.
2018, 34(6): 699-707
doi: 10.3866/PKU.WHXB201711221
Abstract:
In view of its use as reactivity theory, Conceptual Density Functional Theory (DFT), introduced by Parr et al., has mainly concentrated up to now on the E = E[N, v] functional. However, different ensemble representations can be used involving other variables also, such as ρ and µ. In this study, these different ensemble representations (E, Ω, F, and R) are briefly reviewed. Particular attention is then given to the corresponding second-order (functional) derivatives, and their analogies with the second-order derivatives of thermodynamic state functions U, F, H, and G, which are related to each other via Legendre transformations, just as the DFT functionals (Nalewajski and Parr, 1982). Starting from an analysis of the convexity/concavity of the DFT functionals, for which explicit proofs are discussed for some cases, the positive/negative definiteness of the associated kernels is derived and a detailed comparison is made with the thermodynamic derivatives.The stability conditions in thermodynamics are similar in structure to the convexity/concavity conditions for the DFT functionals. Thus, the DFT functionals are scrutinized based on the convexity/concavity of their two variables, to yield the possibility of establishing a relationship between the three second-order reactivity descriptors derived from the considered functional. Considering two ensemble representations, F and Ω, F is eliminated as it has two dependent (extensive) variables, N and ρ. For Ω, on the other hand, which is concave for both of its intensive variables (µ and υ), an inequality is derived from its three second-order (functional) derivatives: the global softness, the local softness, and the softness kernel. Combined with the negative value of the diagonal element of the linear response function, this inequality is shown to be compatible with the Berkowitz-Parr relationship, which relates the functional derivatives of ρ with υ, at constant N and µ. This was recently at stake upon quantifying Kohn's Nearsightedness of Electronic Matter. The analogy of the resulting inequality and the thermodynamic inequality for the G derivatives is highlighted. Potential research paths for this study are briefly addressed; the analogies between finite-temperature DFT response functions and their thermodynamic counterparts and the quest for analogous relationships, as derived in this paper, for DFT functionals that are analogues of entropy-dimensioned thermodynamic functions such as the Massieu function.
In view of its use as reactivity theory, Conceptual Density Functional Theory (DFT), introduced by Parr et al., has mainly concentrated up to now on the E = E[N, v] functional. However, different ensemble representations can be used involving other variables also, such as ρ and µ. In this study, these different ensemble representations (E, Ω, F, and R) are briefly reviewed. Particular attention is then given to the corresponding second-order (functional) derivatives, and their analogies with the second-order derivatives of thermodynamic state functions U, F, H, and G, which are related to each other via Legendre transformations, just as the DFT functionals (Nalewajski and Parr, 1982). Starting from an analysis of the convexity/concavity of the DFT functionals, for which explicit proofs are discussed for some cases, the positive/negative definiteness of the associated kernels is derived and a detailed comparison is made with the thermodynamic derivatives.The stability conditions in thermodynamics are similar in structure to the convexity/concavity conditions for the DFT functionals. Thus, the DFT functionals are scrutinized based on the convexity/concavity of their two variables, to yield the possibility of establishing a relationship between the three second-order reactivity descriptors derived from the considered functional. Considering two ensemble representations, F and Ω, F is eliminated as it has two dependent (extensive) variables, N and ρ. For Ω, on the other hand, which is concave for both of its intensive variables (µ and υ), an inequality is derived from its three second-order (functional) derivatives: the global softness, the local softness, and the softness kernel. Combined with the negative value of the diagonal element of the linear response function, this inequality is shown to be compatible with the Berkowitz-Parr relationship, which relates the functional derivatives of ρ with υ, at constant N and µ. This was recently at stake upon quantifying Kohn's Nearsightedness of Electronic Matter. The analogy of the resulting inequality and the thermodynamic inequality for the G derivatives is highlighted. Potential research paths for this study are briefly addressed; the analogies between finite-temperature DFT response functions and their thermodynamic counterparts and the quest for analogous relationships, as derived in this paper, for DFT functionals that are analogues of entropy-dimensioned thermodynamic functions such as the Massieu function.
2018, 34(6): 708-718
doi: 10.3866/PKU.WHXB201710162
Abstract:
Carbon quantum dots (CQDs) are emerging as the new-generation light absorber for solar energy conversion. However, the low photosensitization efficiency of CQDs is one of the current bottlenecks impeding their large-scale practical applications in photocatalysis. Therefore, developing a facile approach for the engineering and functionalization of CQDs-based composites to improve the photosensitization efficiency of CQDs is highly desirable. On account of the abundant functional groups, especially oxygen-containing functional groups such as carbonyl, carboxyl, and hydroxyl present on their surface, CQDs can be readily combined with various organic molecules or polymers as a surface passivation component to reduce the nonradioactive surface recombination of photo-generated charge carriers, thus enabling the CQDs to exhibit strong photoluminescence in the visible and near-infrared spectral regions. Consequently, polymer passivation has been demonstrated as an ideal strategy to make it accessible for improving the sensitization efficiency of CQDs in photocatalytic applications. Branched polyethylenimine (BPEI) is one of polymers that contains a high density of amine groups and exhibits high electron mobility, which can be used as an electron injection material at the interface of nanomaterials. Besides, the BPEI polymer with amino groups exhibiting positive charge has been utilized for designing heterogeneous catalysts by an electrostatic self-assembly strategy. Therefore, BPEI is expected to modify the surface of inorganic oxides semiconductor to enhance the photosensitization efficiency of CQDs under visible light. However, to date, the study in this regard has been still unavailable. In this work, we developed a facile approach to engineer well-distributed CQDs via electrostatic interaction on BPEI passivated TiO2 composites (BTC) as photocatalysts. The BTC composites with an optimal loading of 5% (w, mass fraction) CQDs outperformed the TiO2/CQDs (TC) composite and referential BPEI/SiO2/CQDs (BSC) composites for the photoreduction of 4-nitroaniline under visible light irradiation. The structure of the fabricated BTC composites was systematically investigated by the combined use of structural and spectral characterizations, demonstrating that the photosensitizer CQDs contacted well with the BPEI modified TiO2 nanoparticles. The comparison characterizations revealed that BPEI facilitated the dissociation and transfer of excitons as an electron transfer channel. The as-prepared BTC composites benefited from the favorable interfacial contact and effective transfer of photo-generated charge carriers, and thus manifested superior photocatalytic activity to the TC composite. It is expected that this strategy would be extended to other wide band gap semiconductor photocatalyst systems and open up new possibilities in designing efficient CQDs-based semiconductor artificial light harvesting systems by interfacial optimization.
Carbon quantum dots (CQDs) are emerging as the new-generation light absorber for solar energy conversion. However, the low photosensitization efficiency of CQDs is one of the current bottlenecks impeding their large-scale practical applications in photocatalysis. Therefore, developing a facile approach for the engineering and functionalization of CQDs-based composites to improve the photosensitization efficiency of CQDs is highly desirable. On account of the abundant functional groups, especially oxygen-containing functional groups such as carbonyl, carboxyl, and hydroxyl present on their surface, CQDs can be readily combined with various organic molecules or polymers as a surface passivation component to reduce the nonradioactive surface recombination of photo-generated charge carriers, thus enabling the CQDs to exhibit strong photoluminescence in the visible and near-infrared spectral regions. Consequently, polymer passivation has been demonstrated as an ideal strategy to make it accessible for improving the sensitization efficiency of CQDs in photocatalytic applications. Branched polyethylenimine (BPEI) is one of polymers that contains a high density of amine groups and exhibits high electron mobility, which can be used as an electron injection material at the interface of nanomaterials. Besides, the BPEI polymer with amino groups exhibiting positive charge has been utilized for designing heterogeneous catalysts by an electrostatic self-assembly strategy. Therefore, BPEI is expected to modify the surface of inorganic oxides semiconductor to enhance the photosensitization efficiency of CQDs under visible light. However, to date, the study in this regard has been still unavailable. In this work, we developed a facile approach to engineer well-distributed CQDs via electrostatic interaction on BPEI passivated TiO2 composites (BTC) as photocatalysts. The BTC composites with an optimal loading of 5% (w, mass fraction) CQDs outperformed the TiO2/CQDs (TC) composite and referential BPEI/SiO2/CQDs (BSC) composites for the photoreduction of 4-nitroaniline under visible light irradiation. The structure of the fabricated BTC composites was systematically investigated by the combined use of structural and spectral characterizations, demonstrating that the photosensitizer CQDs contacted well with the BPEI modified TiO2 nanoparticles. The comparison characterizations revealed that BPEI facilitated the dissociation and transfer of excitons as an electron transfer channel. The as-prepared BTC composites benefited from the favorable interfacial contact and effective transfer of photo-generated charge carriers, and thus manifested superior photocatalytic activity to the TC composite. It is expected that this strategy would be extended to other wide band gap semiconductor photocatalyst systems and open up new possibilities in designing efficient CQDs-based semiconductor artificial light harvesting systems by interfacial optimization.
2018, 34(6): 719-730
doi: 10.3866/PKU.WHXB201712011
Abstract:
A series of MnOx-CeO2 with different Mn contents was prepared using CeBTC-MOF as the sacrificial template. These constituted a new kind of porous crystalline materials assembled by cerium as metal ions and 1, 3, 5-benzenetricarboxylic acid as organic ligands. The composite oxides exhibited good redox properties and were tested as catalysts in the oxidation of toluene. To obtain insight into the structure-activity relationship of the catalysts, the samples were characterized using powder X-ray diffraction (XRD), nitrogen adsorption-desorption, thermogravimetric analysis (TG), elemental analysis (EA), inductively coupled plasma-optical emission spectrometry (ICP-OES), scanning electron microscopy (SEM), transmission electron microscopy (TEM), H2 temperature-programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy (Raman), and UV-Vis diffuse reflectance spectroscopy. Studies of the CeBTC-MOF template showed that the metal-organic framework could be completely decomposed at a calcination temperature of 300 ℃. Therefore, CeBTC-MOF decomposed and generated CO2 and H2O during the calcination process. The gas molecule spilled out from the structure to form the interior void space. The spilling out could be controlled by varying the calcination temperature. This regulated the quantity and size of the interior void, which in turn made the surface area controllable. The secondary building unit of CeBTC-MOF was oxidized to nano-sized crystalline particles, which exhibited outstanding interfacial contact. SEM and TEM results showed that the composite oxides prepared by pyrolysis of the CeBTC-MOF template exhibited rod-shaped nanocrystalline particles. While introducing Mn into MOF, part of Mn entered the ceria lattice to form solid solution and the remaining Mn was dispersed on CeO2 surface. The elemental mappings revealed a well-proportioned distribution of Mn, which confirmed the successful formation of bimetallic metal oxides using the MOF-template method. All the samples exhibited sizes and shapes similar to their parent MOFs. As for catalytic activity, all the composite oxides showed better performances than pure CeO2 for catalytic oxidation of toluene. This could be attributed to higher concentration of oxygen vacancies, which was characterized by Raman spectroscopy. In addition, the XPS results indicated that Mn4+/(Mn2++Mn3+), Ce4+/Ce3+, Olatt (lattice oxygen), and Osur(surface oxygen) all participated in the redox process during catalytic oxidation of toluene, which helped elucidate the mechanism at a micro level.Interestingly, the catalytic activity did not improve further when the Mn content of the composite oxides reached 5%. This could be ascribed to two different states of the dispersed Mn: monolayer dispersion state and crystalline phase. The strong interaction between ceria oxides and dispersed Mn species played an important role in affecting catalytic activity. The results showed the presence of a monolayer dispersion threshold (6.2%), confirmed by XPS characterization, which was in accordance with all the characterization results; it was proved that this threshold had a significant impact on the catalytic activity. When the dispersed Mn content was lower than the monolayer dispersion threshold, Mn reacted with the surface CeO2 in the form of an incorporation model, leading to charge transfer and higher concentration of oxygen vacancies, which in turn effectively promoted the catalytic performance. When the dispersed Mn content exceeded the monolayer dispersion threshold, Mn3O4 was formed on the CeO2 surface; this disrupted the promotion of catalytic activity, which explains the same catalytic activity of all the samples (5% MnOx-CeO2, 8% MnOx-CeO2, and 10% MnOx-CeO2).This successful formation of bimetallic metal oxides using CeBTC-MOF template indicated that composite oxide synthesis was feasible using the MOF template method. To obtain high catalyst performance of these composite oxides, it was important to control the metal content at the level of the monolayer dispersion threshold.
A series of MnOx-CeO2 with different Mn contents was prepared using CeBTC-MOF as the sacrificial template. These constituted a new kind of porous crystalline materials assembled by cerium as metal ions and 1, 3, 5-benzenetricarboxylic acid as organic ligands. The composite oxides exhibited good redox properties and were tested as catalysts in the oxidation of toluene. To obtain insight into the structure-activity relationship of the catalysts, the samples were characterized using powder X-ray diffraction (XRD), nitrogen adsorption-desorption, thermogravimetric analysis (TG), elemental analysis (EA), inductively coupled plasma-optical emission spectrometry (ICP-OES), scanning electron microscopy (SEM), transmission electron microscopy (TEM), H2 temperature-programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy (Raman), and UV-Vis diffuse reflectance spectroscopy. Studies of the CeBTC-MOF template showed that the metal-organic framework could be completely decomposed at a calcination temperature of 300 ℃. Therefore, CeBTC-MOF decomposed and generated CO2 and H2O during the calcination process. The gas molecule spilled out from the structure to form the interior void space. The spilling out could be controlled by varying the calcination temperature. This regulated the quantity and size of the interior void, which in turn made the surface area controllable. The secondary building unit of CeBTC-MOF was oxidized to nano-sized crystalline particles, which exhibited outstanding interfacial contact. SEM and TEM results showed that the composite oxides prepared by pyrolysis of the CeBTC-MOF template exhibited rod-shaped nanocrystalline particles. While introducing Mn into MOF, part of Mn entered the ceria lattice to form solid solution and the remaining Mn was dispersed on CeO2 surface. The elemental mappings revealed a well-proportioned distribution of Mn, which confirmed the successful formation of bimetallic metal oxides using the MOF-template method. All the samples exhibited sizes and shapes similar to their parent MOFs. As for catalytic activity, all the composite oxides showed better performances than pure CeO2 for catalytic oxidation of toluene. This could be attributed to higher concentration of oxygen vacancies, which was characterized by Raman spectroscopy. In addition, the XPS results indicated that Mn4+/(Mn2++Mn3+), Ce4+/Ce3+, Olatt (lattice oxygen), and Osur(surface oxygen) all participated in the redox process during catalytic oxidation of toluene, which helped elucidate the mechanism at a micro level.Interestingly, the catalytic activity did not improve further when the Mn content of the composite oxides reached 5%. This could be ascribed to two different states of the dispersed Mn: monolayer dispersion state and crystalline phase. The strong interaction between ceria oxides and dispersed Mn species played an important role in affecting catalytic activity. The results showed the presence of a monolayer dispersion threshold (6.2%), confirmed by XPS characterization, which was in accordance with all the characterization results; it was proved that this threshold had a significant impact on the catalytic activity. When the dispersed Mn content was lower than the monolayer dispersion threshold, Mn reacted with the surface CeO2 in the form of an incorporation model, leading to charge transfer and higher concentration of oxygen vacancies, which in turn effectively promoted the catalytic performance. When the dispersed Mn content exceeded the monolayer dispersion threshold, Mn3O4 was formed on the CeO2 surface; this disrupted the promotion of catalytic activity, which explains the same catalytic activity of all the samples (5% MnOx-CeO2, 8% MnOx-CeO2, and 10% MnOx-CeO2).This successful formation of bimetallic metal oxides using CeBTC-MOF template indicated that composite oxide synthesis was feasible using the MOF template method. To obtain high catalyst performance of these composite oxides, it was important to control the metal content at the level of the monolayer dispersion threshold.
