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2024 Volume 40 Issue 4
2024, 40(4): 230400
doi: 10.3866/PKU.WHXB202304003
Abstract:
CO2 hydrogenation is critical to producing high-value-added carbon-based chemicals and fuels to achieving both hydrogen energy storage and CO2 utilization. Examples of CO2 hydrogenation include methanation (Sabatier process) to produce methane, reverse water-gas shift reaction (RWGS) to generate CO, methanol synthesis for the methanol economy, and CO2 direct Fischer-Tropsch (CO2-FT) reaction to produce olefins. The precious metal catalysts used in these reactions are efficient but too expensive to be used on a large scale. Further, although some non-precious metal catalysts can be used for these hydrogenation reactions, they suffer from deactivation during long-term processes. Over the past few decades, molybdenum carbides, which are transition metal carbides (TMCs), have attracted significant attention owing to their low cost and similar catalytic performance to precious metal catalysts in CO2 hydrogenation reactions. Recently, two-dimensional molybdenum carbide MXenes have shown impressive activity in CO2 hydrogenation reactions. Owing to the presence of carbon, the MXene lattice is expanded; this leads to an increase in valence electrons and endows the two-dimensional molybdenum carbide-based catalyst with different properties than metallic Mo. The two-dimensional molybdenum carbide-based materials can be prepared by temperature-programmed carburization, selective etching, mechanical alloying synthesis, chemical vapor deposition, in situ thermal carburization, and solution-phase synthesis methods. Thus far, a host of studies have been performed on CO2 conversion with molybdenum carbide-based materials, which show promising activity during CO2 conversion and selectivity towards target products. Both α-MoC1−x and β-MoCy have shown outstanding thermocatalytic activity, product selectivity, and reaction stability during CO2 hydrogenation to CO at 300-600 ℃. In addition, molybdenum carbide-based materials were also found to be an interesting catalyst for direct CO2 Fischer-Tropsch synthesis. The application potential of the molybdenum carbide-based materials could be further tuned by changing the C/Mo ratio in the bulk molybdenum carbide, strengthening the interactions between molybdenum carbide and the supported metal, and tailoring the interface structure of the materials. However, the thermal catalytic CO2 conversion based on molybdenum carbide-based materials is still in its infancy. This paper summarizes the progress toward molybdenum carbide catalysis of CO2 hydrogenation process for producing high-value-added chemicals and fuels. Furthermore, the challenges and opportunities for molybdenum carbide materials as catalysts for CO2 hydrogenation are discussed to provide insights for future development in this emerging field.
CO2 hydrogenation is critical to producing high-value-added carbon-based chemicals and fuels to achieving both hydrogen energy storage and CO2 utilization. Examples of CO2 hydrogenation include methanation (Sabatier process) to produce methane, reverse water-gas shift reaction (RWGS) to generate CO, methanol synthesis for the methanol economy, and CO2 direct Fischer-Tropsch (CO2-FT) reaction to produce olefins. The precious metal catalysts used in these reactions are efficient but too expensive to be used on a large scale. Further, although some non-precious metal catalysts can be used for these hydrogenation reactions, they suffer from deactivation during long-term processes. Over the past few decades, molybdenum carbides, which are transition metal carbides (TMCs), have attracted significant attention owing to their low cost and similar catalytic performance to precious metal catalysts in CO2 hydrogenation reactions. Recently, two-dimensional molybdenum carbide MXenes have shown impressive activity in CO2 hydrogenation reactions. Owing to the presence of carbon, the MXene lattice is expanded; this leads to an increase in valence electrons and endows the two-dimensional molybdenum carbide-based catalyst with different properties than metallic Mo. The two-dimensional molybdenum carbide-based materials can be prepared by temperature-programmed carburization, selective etching, mechanical alloying synthesis, chemical vapor deposition, in situ thermal carburization, and solution-phase synthesis methods. Thus far, a host of studies have been performed on CO2 conversion with molybdenum carbide-based materials, which show promising activity during CO2 conversion and selectivity towards target products. Both α-MoC1−x and β-MoCy have shown outstanding thermocatalytic activity, product selectivity, and reaction stability during CO2 hydrogenation to CO at 300-600 ℃. In addition, molybdenum carbide-based materials were also found to be an interesting catalyst for direct CO2 Fischer-Tropsch synthesis. The application potential of the molybdenum carbide-based materials could be further tuned by changing the C/Mo ratio in the bulk molybdenum carbide, strengthening the interactions between molybdenum carbide and the supported metal, and tailoring the interface structure of the materials. However, the thermal catalytic CO2 conversion based on molybdenum carbide-based materials is still in its infancy. This paper summarizes the progress toward molybdenum carbide catalysis of CO2 hydrogenation process for producing high-value-added chemicals and fuels. Furthermore, the challenges and opportunities for molybdenum carbide materials as catalysts for CO2 hydrogenation are discussed to provide insights for future development in this emerging field.
2024, 40(4): 230403
doi: 10.3866/PKU.WHXB202304037
Abstract:
Since 2004, poly(N-isopropylacrylamide) (PNIPAm) cross-linked thermo-responsive nanofibers mats have emerged as a responsive material with a phase transition temperature that can be easily controlled. These mats overcome the limitations, such as a high production cost and slow response rate, of huge traditional PNIPAm hydrogels. They also overcome the poor water resistance of PNIPAm non-cross-linked thermo-responsive nanofibers and, thus, have been widely studied. In 2017, continuous PNIPAm thermo-responsive nanofibers in pure aqueous solvents without beads were fabricated, which began the ecological and water-based era of uniform PNIPAm nanofiber production. In this review, we comprehensively analyzed the effects of physical and chemical cross-linking reaction types, cross-linking degree, cross-linking time, and cross-linking molecular weight on the morphological stability and response behavior of PNIPAm thermo-responsive nanofibers mats, providing theoretical support for their future cross-linking treatment. Because of their high specific surface area and porosity, PNIPAm thermo-responsive nanofibers mats are vulnerable to solvent erosion before cross-linking, which damage their morphology and reduce response rates and usage times. Increased water resistance and can be utilized repeatedly, by introducing cross-linking groups to these mats, such as in drug release, cell culture, drivers, and smart switches. Chemical cross-linking are more stable than physical cross-linking and can be divided into crosslinkers, chemical reactive cross-linking, and other cross-linking. The cross-linking networks produced by a cross-linking agent are more robust; however, the resulting nanofibers mats are not applicable to the human body owing to the small, non-degradable harmful molecules, such as formaldehyde and glutaraldehyde (GA). Random 3D networks generated by physical cross-linking are easier to break but relatively safe and pollution-free. The morphological stability and response behavior of PNIPAm thermo-responsive nanofibers mats are affected by the cross-linking. The cross-linking agent content and the cross-linking time are positively correlated with the morphological stability of PNIPAm thermo-responsive nanofibers mats. This is conducive to multiple recycling but has little effect on the response rate. Greener and more reliable cross-linking methods should be investigated to realize and expand the practical applications of PNIPAm thermo-responsive nanofibers mats, with increasing focus on the effect of cross-linking on the mechanical properties of the mats. We hope this review will result in ideas for improving the development and application of PNIPAm thermo-responsive nanofibers mats.
Since 2004, poly(N-isopropylacrylamide) (PNIPAm) cross-linked thermo-responsive nanofibers mats have emerged as a responsive material with a phase transition temperature that can be easily controlled. These mats overcome the limitations, such as a high production cost and slow response rate, of huge traditional PNIPAm hydrogels. They also overcome the poor water resistance of PNIPAm non-cross-linked thermo-responsive nanofibers and, thus, have been widely studied. In 2017, continuous PNIPAm thermo-responsive nanofibers in pure aqueous solvents without beads were fabricated, which began the ecological and water-based era of uniform PNIPAm nanofiber production. In this review, we comprehensively analyzed the effects of physical and chemical cross-linking reaction types, cross-linking degree, cross-linking time, and cross-linking molecular weight on the morphological stability and response behavior of PNIPAm thermo-responsive nanofibers mats, providing theoretical support for their future cross-linking treatment. Because of their high specific surface area and porosity, PNIPAm thermo-responsive nanofibers mats are vulnerable to solvent erosion before cross-linking, which damage their morphology and reduce response rates and usage times. Increased water resistance and can be utilized repeatedly, by introducing cross-linking groups to these mats, such as in drug release, cell culture, drivers, and smart switches. Chemical cross-linking are more stable than physical cross-linking and can be divided into crosslinkers, chemical reactive cross-linking, and other cross-linking. The cross-linking networks produced by a cross-linking agent are more robust; however, the resulting nanofibers mats are not applicable to the human body owing to the small, non-degradable harmful molecules, such as formaldehyde and glutaraldehyde (GA). Random 3D networks generated by physical cross-linking are easier to break but relatively safe and pollution-free. The morphological stability and response behavior of PNIPAm thermo-responsive nanofibers mats are affected by the cross-linking. The cross-linking agent content and the cross-linking time are positively correlated with the morphological stability of PNIPAm thermo-responsive nanofibers mats. This is conducive to multiple recycling but has little effect on the response rate. Greener and more reliable cross-linking methods should be investigated to realize and expand the practical applications of PNIPAm thermo-responsive nanofibers mats, with increasing focus on the effect of cross-linking on the mechanical properties of the mats. We hope this review will result in ideas for improving the development and application of PNIPAm thermo-responsive nanofibers mats.
2024, 40(4): 230500
doi: 10.3866/PKU.WHXB202305005
Abstract:
Converting CO2 into valuable carbon products can effectively address the current energy crisis and environmental issues. Electrocatalytic CO2 reduction (ECR), powered by sustainable electricity, is an ideal approach to reduce carbon emissions and promote the carbon cycle. Electrocatalytic CO2 reduction, powered by sustainable electricity, is an ideal approach to reduce carbon emissions and promote the carbon cycle. However, CO2 is a thermodynamically inert molecule, making it challenging to obtain the desired products through ECR. Additionally, ECR involves a complex process of multi-electron and proton transfer, requiring different amounts of electrons and protons to gradually form various reduction products. This complexity highlights the urgent need to develop advanced catalysts to overcome the slow reaction kinetics and intricate coupling pathways associated with ECR. Single-atom catalysts (SACs) have emerged as a cutting-edge frontier in heterogeneous catalysis and find extensive application in ECR due to their high atom utilization, excellent activity, and selectivity. SACs defy the traditional design concept of nanoparticle catalysts and exhibit catalytic activity at the atomic level, maximizing their efficiency. Another advantage of SACs lies in their ability to tune the electronic structure of the active central atom through ligand atoms. However, while SACs provide separate metal active sites with no crosstalk between adjacent metal atoms, they do form strong chemical bonding interactions with the support. Currently, SACs for ECR still face challenges such as low selectivity and the goal of achieving high-value product generation. Therefore, optimizing the performance of SACs is of paramount importance. Considering the extensive exploration and application of SACs in the field of ECR, this review aims to summarize the research progress in SAC applications for ECR. It also addresses the challenges and prospects associated with SACs in ECR applications. Specifically, the review covers: (1) the introduction of the ECR reaction mechanism, (2) common preparation strategies for SACs, and (3) the application of SACs in novel devices based on Zn-CO2 batteries. Finally, the review discusses the challenges and opportunities that SACs present in the context of ECR.
Converting CO2 into valuable carbon products can effectively address the current energy crisis and environmental issues. Electrocatalytic CO2 reduction (ECR), powered by sustainable electricity, is an ideal approach to reduce carbon emissions and promote the carbon cycle. Electrocatalytic CO2 reduction, powered by sustainable electricity, is an ideal approach to reduce carbon emissions and promote the carbon cycle. However, CO2 is a thermodynamically inert molecule, making it challenging to obtain the desired products through ECR. Additionally, ECR involves a complex process of multi-electron and proton transfer, requiring different amounts of electrons and protons to gradually form various reduction products. This complexity highlights the urgent need to develop advanced catalysts to overcome the slow reaction kinetics and intricate coupling pathways associated with ECR. Single-atom catalysts (SACs) have emerged as a cutting-edge frontier in heterogeneous catalysis and find extensive application in ECR due to their high atom utilization, excellent activity, and selectivity. SACs defy the traditional design concept of nanoparticle catalysts and exhibit catalytic activity at the atomic level, maximizing their efficiency. Another advantage of SACs lies in their ability to tune the electronic structure of the active central atom through ligand atoms. However, while SACs provide separate metal active sites with no crosstalk between adjacent metal atoms, they do form strong chemical bonding interactions with the support. Currently, SACs for ECR still face challenges such as low selectivity and the goal of achieving high-value product generation. Therefore, optimizing the performance of SACs is of paramount importance. Considering the extensive exploration and application of SACs in the field of ECR, this review aims to summarize the research progress in SAC applications for ECR. It also addresses the challenges and prospects associated with SACs in ECR applications. Specifically, the review covers: (1) the introduction of the ECR reaction mechanism, (2) common preparation strategies for SACs, and (3) the application of SACs in novel devices based on Zn-CO2 batteries. Finally, the review discusses the challenges and opportunities that SACs present in the context of ECR.
2024, 40(4): 230501
doi: 10.3866/PKU.WHXB202305019
Abstract:
Hydrogen peroxide (H2O2) is an environmentally friendly oxidant that has been widely used in water treatment, medical disinfection, chemical synthesis, and other industrial applications. However, traditional methods used to produce H2O2 consume significant amounts of energy and generate hazardous by-products, which limit their scope. On-site and on-demand electrocatalytic two-electron water oxidation chemistry is an attractive option for directly producing H2O2 from water; it also avoids the hazardous anthraquinone method, has fewer transportation costs and risks, and is integratable with renewable electricity. Despite these advantages, the two-electron water oxidation reaction (2e– WOR) still suffers from poor selectivity and activity due to a lack of mechanistic, material-design, and reactor-engineering understanding. This study summarizes recent advances in H2O2 electrosynthesis technology using the 2e– WOR. The catalytic 2e– WOR mechanism is first introduced with a focus on selectivity, activity, and stability. This reaction involves the electrocatalytic oxidation of water to produce H2O2, which can be further oxidized to O2. Selectivity is influenced by a variety of factors, including the electrocatalyst, pH, and electrolyte. Various quantitative H2O2 methods are discussed along with in situ characterization studies into the 2e– WOR aimed at better understanding the reaction process. Such methods include in situ Fourier-transform infrared spectroscopy and in situ Raman spectroscopy. Researchers are able to identify reaction intermediates and understand reaction mechanisms better using these techniques, thereby providing guidance for the design of more efficient electrocatalysts. In turn, various strategies for preparing high-performance electrocatalysts are summarized, including defect, doping, facet, and interfacial engineering methods. Mechanism-guided multiscale materials engineering can improve the activities and selectivities of electrocatalysts, thereby increasing H2O2 yields. In addition, device-level engineering, especially in relation to reactor and system innovations, is emphasized, which is important for improving reaction efficiency and reducing the cost of the 2e– WOR. Finally, current challenges and future opportunities in the 2e– WOR H2O2 electrosynthesis field are discussed. More effort directed at improving reaction selectivity, activity, and durability is required, along with exploring suitable application scenarios. The 2e– WOR is expected to become a more sustainable and efficient method for producing H2O2 facilitated by continuing progress in the materials science and electrochemical technology fields.
Hydrogen peroxide (H2O2) is an environmentally friendly oxidant that has been widely used in water treatment, medical disinfection, chemical synthesis, and other industrial applications. However, traditional methods used to produce H2O2 consume significant amounts of energy and generate hazardous by-products, which limit their scope. On-site and on-demand electrocatalytic two-electron water oxidation chemistry is an attractive option for directly producing H2O2 from water; it also avoids the hazardous anthraquinone method, has fewer transportation costs and risks, and is integratable with renewable electricity. Despite these advantages, the two-electron water oxidation reaction (2e– WOR) still suffers from poor selectivity and activity due to a lack of mechanistic, material-design, and reactor-engineering understanding. This study summarizes recent advances in H2O2 electrosynthesis technology using the 2e– WOR. The catalytic 2e– WOR mechanism is first introduced with a focus on selectivity, activity, and stability. This reaction involves the electrocatalytic oxidation of water to produce H2O2, which can be further oxidized to O2. Selectivity is influenced by a variety of factors, including the electrocatalyst, pH, and electrolyte. Various quantitative H2O2 methods are discussed along with in situ characterization studies into the 2e– WOR aimed at better understanding the reaction process. Such methods include in situ Fourier-transform infrared spectroscopy and in situ Raman spectroscopy. Researchers are able to identify reaction intermediates and understand reaction mechanisms better using these techniques, thereby providing guidance for the design of more efficient electrocatalysts. In turn, various strategies for preparing high-performance electrocatalysts are summarized, including defect, doping, facet, and interfacial engineering methods. Mechanism-guided multiscale materials engineering can improve the activities and selectivities of electrocatalysts, thereby increasing H2O2 yields. In addition, device-level engineering, especially in relation to reactor and system innovations, is emphasized, which is important for improving reaction efficiency and reducing the cost of the 2e– WOR. Finally, current challenges and future opportunities in the 2e– WOR H2O2 electrosynthesis field are discussed. More effort directed at improving reaction selectivity, activity, and durability is required, along with exploring suitable application scenarios. The 2e– WOR is expected to become a more sustainable and efficient method for producing H2O2 facilitated by continuing progress in the materials science and electrochemical technology fields.
2024, 40(4): 230504
doi: 10.3866/PKU.WHXB202305048
Abstract:
With the rapid growth of the economy, environmental and energy issues have become increasingly prominent. Solar energy, as a renewable and environmentally friendly energy source, has attracted the attention of many researchers. Maximizing the utilization of solar energy resources has become a hot research topic in the future. It is well known that photocatalytic technology can convert solar energy into chemical or electric energy, offering a solution to environmental pollution. Therefore, semiconductor photocatalytic technology has been recognized as one of the most eco-friendly approaches for addressing energy crises and environmental pollution. Bismuth-based semiconductor materials, have gained popularity in the field of photocatalysis due to their suitable band structure, wide range of variations, non-toxicity, and low cost. However, pure Bi-based photocatalysts suffer from high recombination efficiency of photoexcited electron-hole pairs, poor quantum yield and limited light absorption ability, resulting in low photocatalytic performance. To overcome these limitations, various strategies such as metal or nonmetal doping, metal deposition, construction of heterojunctions and inducing defect generation have been employed to enhance their photocatalytic performance. Among these strategies, element doping or metal deposition is considered an effective approach for adjusting the band structure and physicochemical properties of bismuth-based materials. This extends the range of light response and improves photocatalytic performance. This mini review aims to summarize the recent research progress in Bi-based materials modified by metal doping, nonmetal doping, metal and nonmetal co-doping, and metal deposition. It also explores their applications in different fields, including photocatalytic removal of pollutants and heavy metal ions, nitrogen reduction, carbon dioxide reduction, and photocatalytic antibacterial, etc. Regarding metal doping, we classify it into three categories: alkali or alkaline earth metals doping, transition metal doping, and rare earth metal doping, and provide a detailed introduction to the advantages and disadvantages of each type of doping. Nonmetallic doping is categorized into halogen and non-halogen doping, with a focus on the impact of nonmetallic doping on bismuth-based materials. Furthermore, we present a vertical comparison of the advantages of each element vertically. Co-doping, which combines the advantages of both metal and nonmetal elements, is briefly outlined in terms of recent research progress. In the case of metal deposition, we mainly discuss the impact on Bi based materials from two aspects: the Schottky barrier and the localized surface plasmon resonance (LSPR) effect. Finally, we also discuss the current challenges and prospects faced by metal or nonmetal modified Bi-based photocatalysts.
With the rapid growth of the economy, environmental and energy issues have become increasingly prominent. Solar energy, as a renewable and environmentally friendly energy source, has attracted the attention of many researchers. Maximizing the utilization of solar energy resources has become a hot research topic in the future. It is well known that photocatalytic technology can convert solar energy into chemical or electric energy, offering a solution to environmental pollution. Therefore, semiconductor photocatalytic technology has been recognized as one of the most eco-friendly approaches for addressing energy crises and environmental pollution. Bismuth-based semiconductor materials, have gained popularity in the field of photocatalysis due to their suitable band structure, wide range of variations, non-toxicity, and low cost. However, pure Bi-based photocatalysts suffer from high recombination efficiency of photoexcited electron-hole pairs, poor quantum yield and limited light absorption ability, resulting in low photocatalytic performance. To overcome these limitations, various strategies such as metal or nonmetal doping, metal deposition, construction of heterojunctions and inducing defect generation have been employed to enhance their photocatalytic performance. Among these strategies, element doping or metal deposition is considered an effective approach for adjusting the band structure and physicochemical properties of bismuth-based materials. This extends the range of light response and improves photocatalytic performance. This mini review aims to summarize the recent research progress in Bi-based materials modified by metal doping, nonmetal doping, metal and nonmetal co-doping, and metal deposition. It also explores their applications in different fields, including photocatalytic removal of pollutants and heavy metal ions, nitrogen reduction, carbon dioxide reduction, and photocatalytic antibacterial, etc. Regarding metal doping, we classify it into three categories: alkali or alkaline earth metals doping, transition metal doping, and rare earth metal doping, and provide a detailed introduction to the advantages and disadvantages of each type of doping. Nonmetallic doping is categorized into halogen and non-halogen doping, with a focus on the impact of nonmetallic doping on bismuth-based materials. Furthermore, we present a vertical comparison of the advantages of each element vertically. Co-doping, which combines the advantages of both metal and nonmetal elements, is briefly outlined in terms of recent research progress. In the case of metal deposition, we mainly discuss the impact on Bi based materials from two aspects: the Schottky barrier and the localized surface plasmon resonance (LSPR) effect. Finally, we also discuss the current challenges and prospects faced by metal or nonmetal modified Bi-based photocatalysts.
2024, 40(4): 230600
doi: 10.3866/PKU.WHXB202306003
Abstract:
Transforming the current structure of energy production and consumption, which currently excessively relies on fossil fuels, into a more efficient utilization of renewable energy, is an effective solution for addressing the energy crisis and achieving carbon neutrality. Biomass represents one of the most promising sources of renewable energy, capable of replacing fossil fuels and yielding valuable organic compounds. In recent years, the vigorous utilization of biomass energy sources has become an inevitable trend. The conventional thermochemical catalysis method used for biomass conversion often requires harsh conditions, such as high temperatures and pressures, and even external sources of hydrogen or oxygen. In comparison, the electrocatalytic conversion of organic molecules derived from biomass offers a greener and more efficient strategy for producing high-value chemicals under relatively mild conditions. Particularly, the cleavage of carbon chains through C―C bond cleavage is crucial in transforming biomass-derived molecules into short-chain chemicals of high value. Numerous studies have demonstrated that transition metal (TM) electrocatalysts play a critical role in the C―C bond cleavage of organic compounds, owing to their rich 3d electron structure and unique eg orbitals that enhance the covalence of transition metal-oxygen bonds. Moreover, the coordination environments and electronic structures of TM electrocatalysts can influence the selectivity of the products. Undoubtedly, well-defined active sites and reaction pathways facilitate a comprehensive understanding of the structure-activity relationship between catalyst structure and reaction activity. However, the electrocatalytic cleavage of C―C bonds for biomass upgrading on TM electrocatalysts is still in its initial stages, and the reaction mechanism and catalytic processes remain unclear. Therefore, there is a need to systematically comprehend the role of electrocatalysts at the atomic level during the C―C bond cleavage process. This review begins by providing an overview of the extensively studied TM electrocatalysts that mediate C―C bond cleavage reactions of organic molecules derived from biomass, including glycerol, cyclohexanol, lignin, and furfural. Several representative examples and corresponding reaction pathways are presented. Subsequently, we systematically review the reaction mechanisms underlying the catalytic C―C bond cleavage by transition metal compounds, elucidate interfacial behaviors, and establish a structure-activity relationship between the structure of TM electrocatalysts and cleavage reaction activity. Finally, we provide a brief summary of the content covered and highlight the challenges and prospects in exploring C―C bond cleavage on TM electrocatalysts. It is anticipated that this work will serve as a guide for the controlled conversion of biomass and the rational design of TM electrocatalysts for C―C bond cleavage.
Transforming the current structure of energy production and consumption, which currently excessively relies on fossil fuels, into a more efficient utilization of renewable energy, is an effective solution for addressing the energy crisis and achieving carbon neutrality. Biomass represents one of the most promising sources of renewable energy, capable of replacing fossil fuels and yielding valuable organic compounds. In recent years, the vigorous utilization of biomass energy sources has become an inevitable trend. The conventional thermochemical catalysis method used for biomass conversion often requires harsh conditions, such as high temperatures and pressures, and even external sources of hydrogen or oxygen. In comparison, the electrocatalytic conversion of organic molecules derived from biomass offers a greener and more efficient strategy for producing high-value chemicals under relatively mild conditions. Particularly, the cleavage of carbon chains through C―C bond cleavage is crucial in transforming biomass-derived molecules into short-chain chemicals of high value. Numerous studies have demonstrated that transition metal (TM) electrocatalysts play a critical role in the C―C bond cleavage of organic compounds, owing to their rich 3d electron structure and unique eg orbitals that enhance the covalence of transition metal-oxygen bonds. Moreover, the coordination environments and electronic structures of TM electrocatalysts can influence the selectivity of the products. Undoubtedly, well-defined active sites and reaction pathways facilitate a comprehensive understanding of the structure-activity relationship between catalyst structure and reaction activity. However, the electrocatalytic cleavage of C―C bonds for biomass upgrading on TM electrocatalysts is still in its initial stages, and the reaction mechanism and catalytic processes remain unclear. Therefore, there is a need to systematically comprehend the role of electrocatalysts at the atomic level during the C―C bond cleavage process. This review begins by providing an overview of the extensively studied TM electrocatalysts that mediate C―C bond cleavage reactions of organic molecules derived from biomass, including glycerol, cyclohexanol, lignin, and furfural. Several representative examples and corresponding reaction pathways are presented. Subsequently, we systematically review the reaction mechanisms underlying the catalytic C―C bond cleavage by transition metal compounds, elucidate interfacial behaviors, and establish a structure-activity relationship between the structure of TM electrocatalysts and cleavage reaction activity. Finally, we provide a brief summary of the content covered and highlight the challenges and prospects in exploring C―C bond cleavage on TM electrocatalysts. It is anticipated that this work will serve as a guide for the controlled conversion of biomass and the rational design of TM electrocatalysts for C―C bond cleavage.
2024, 40(4): 230604
doi: 10.3866/PKU.WHXB202306040
Abstract:
With the increasing use of fossil energy sources, the concentration of CO2 in the atmosphere is rising, leading to environmental challenges. However, the conversion of CO2 into high value-added chemicals through catalysis presents an opportunity to address these issues and create a new pathway for fuel synthesis, ultimately helping to reduce CO2 emissions and achieve carbon neutrality. Among various methods, the CO2 electroreduction reaction (CO2RR) using renewable clean energy has garnered significant attention due to its mild reaction conditions, controlled reactions progress, environmental friendliness, and numerous value-added products it can yield. In this context, imidazolium-based materials and their derivatives have emerged as promising candidates for CO2RR. These materials have a strong affinity for CO2 and find applications as both electrolytes and electrocatalysts in CO2RR systems. So one of their key advantages, especially Im-ILs, is their ability to enrich CO2 in catalytic systems, effectively preventing the undesired hydrogen evolution reaction (HER) and enhancing the selectivity towards CO2RR products. Understanding the interaction mechanism between imidazolium-based ionic liquids (Im-ILs) and CO2 molecules under electrocatalytic conditions is crucial for gaining deeper insights into why the addition of Im-ILs can improve CO2RR performance from a molecular perspective. Furthermore, Im-ILs can serve as both surface modifier groups and trapping agents in heterogeneous electrocatalysts, which can significantly alter the surface environment and hydrophobicity of the catalysts, leading to improved CO2RR. Notably, the imidazolium groups present in Lehn-type and metal-porphyrin molecular catalysts have been found to have an impact on the performance of these catalysts in CO2RR. Lastly, N-heterocyclic carbene (NHC)-based electrocatalysts, as one of the active forms of imidazolium interaction with CO2, have demonstrated exceptional performance. When introduced into porous heterogeneous catalysts and molecular catalysts, NHC-based electrocatalysts stabilize metal nanoparticles and enhance the ability to capture CO2, thus promoting CO2RR activity. In summary, the utilization of imidazolium-based materials in CO2RR holds great promise for advancing the field of CO2 conversion and achieving more sustainable and efficient processes for high-value chemical synthesis.
With the increasing use of fossil energy sources, the concentration of CO2 in the atmosphere is rising, leading to environmental challenges. However, the conversion of CO2 into high value-added chemicals through catalysis presents an opportunity to address these issues and create a new pathway for fuel synthesis, ultimately helping to reduce CO2 emissions and achieve carbon neutrality. Among various methods, the CO2 electroreduction reaction (CO2RR) using renewable clean energy has garnered significant attention due to its mild reaction conditions, controlled reactions progress, environmental friendliness, and numerous value-added products it can yield. In this context, imidazolium-based materials and their derivatives have emerged as promising candidates for CO2RR. These materials have a strong affinity for CO2 and find applications as both electrolytes and electrocatalysts in CO2RR systems. So one of their key advantages, especially Im-ILs, is their ability to enrich CO2 in catalytic systems, effectively preventing the undesired hydrogen evolution reaction (HER) and enhancing the selectivity towards CO2RR products. Understanding the interaction mechanism between imidazolium-based ionic liquids (Im-ILs) and CO2 molecules under electrocatalytic conditions is crucial for gaining deeper insights into why the addition of Im-ILs can improve CO2RR performance from a molecular perspective. Furthermore, Im-ILs can serve as both surface modifier groups and trapping agents in heterogeneous electrocatalysts, which can significantly alter the surface environment and hydrophobicity of the catalysts, leading to improved CO2RR. Notably, the imidazolium groups present in Lehn-type and metal-porphyrin molecular catalysts have been found to have an impact on the performance of these catalysts in CO2RR. Lastly, N-heterocyclic carbene (NHC)-based electrocatalysts, as one of the active forms of imidazolium interaction with CO2, have demonstrated exceptional performance. When introduced into porous heterogeneous catalysts and molecular catalysts, NHC-based electrocatalysts stabilize metal nanoparticles and enhance the ability to capture CO2, thus promoting CO2RR activity. In summary, the utilization of imidazolium-based materials in CO2RR holds great promise for advancing the field of CO2 conversion and achieving more sustainable and efficient processes for high-value chemical synthesis.
2024, 40(4): 230700
doi: 10.3866/PKU.WHXB202307004
Abstract:
Carbon dioxide (CO2) serves as one of the major greenhouse gases in the atmosphere. However, it is also abundant, non-toxic, and renewable, making it a valuable one-carbon source. Therefore, converting CO2 into valuable chemicals holds immense significance as an effective approach towards achieving carbon neutrality. Nevertheless, due to CO2’s thermodynamic stability and kinetic inertness, its activation and conversion present considerable challenges. Organic carbamates, both cyclic and acyclic, represent a crucial class of bioactive compounds found in various natural products, agricultural chemicals, and pharmaceutically relevant molecules. They are also widely used as essential intermediates in organic synthesis. Unfortunately, traditional methods for preparing organic carbamates often rely on highly toxic phosgene and its derivatives as raw materials, posing serious environmental and safety concerns and limiting practical applications. From a cost-effective and sustainable standpoint, substituting CO2 for phosgene in the synthesis of organic carbamates is highly appealing. In recent decades, numerous new reactions, particularly multicomponent reactions involving CO2 and amines, have emerged, providing efficient methods for constructing diverse and valuable carbamates. Some of these reactions can be conducted under transition-metal-free conditions, utilizing organic and inorganic bases, ionic liquids, or small organic molecules as catalysts or promoters. However, in certain cases, transition-metal catalysts, such as those based on copper, palladium, or silver, are required, especially when the reactions involve activating unsaturated hydrocarbons like alkenes and alkynes. Mechanistically, most of these methods involve in situ generation of nucleophilic CO2-amine adducts, such as carbamate salts or carbamic acids, which then react with other electrophiles or coupling partners to yield the desired carbamates. Notably, recent advancements have led to the successful development of several elegant methods for synthesizing specific types of carbamates using electrocatalysis or photocatalysis, which are not achievable through conventional thermal catalysis. This review comprehensively summarizes the recent progress in the synthesis of organic carbamates using CO2 and amines under various catalytic conditions, including transition metal-free conditions, transition metal-catalysis, electrocatalysis, and photocatalysis. Additionally, the review discusses the challenges and future prospects associated with converting CO2 into organic carbamates.
Carbon dioxide (CO2) serves as one of the major greenhouse gases in the atmosphere. However, it is also abundant, non-toxic, and renewable, making it a valuable one-carbon source. Therefore, converting CO2 into valuable chemicals holds immense significance as an effective approach towards achieving carbon neutrality. Nevertheless, due to CO2’s thermodynamic stability and kinetic inertness, its activation and conversion present considerable challenges. Organic carbamates, both cyclic and acyclic, represent a crucial class of bioactive compounds found in various natural products, agricultural chemicals, and pharmaceutically relevant molecules. They are also widely used as essential intermediates in organic synthesis. Unfortunately, traditional methods for preparing organic carbamates often rely on highly toxic phosgene and its derivatives as raw materials, posing serious environmental and safety concerns and limiting practical applications. From a cost-effective and sustainable standpoint, substituting CO2 for phosgene in the synthesis of organic carbamates is highly appealing. In recent decades, numerous new reactions, particularly multicomponent reactions involving CO2 and amines, have emerged, providing efficient methods for constructing diverse and valuable carbamates. Some of these reactions can be conducted under transition-metal-free conditions, utilizing organic and inorganic bases, ionic liquids, or small organic molecules as catalysts or promoters. However, in certain cases, transition-metal catalysts, such as those based on copper, palladium, or silver, are required, especially when the reactions involve activating unsaturated hydrocarbons like alkenes and alkynes. Mechanistically, most of these methods involve in situ generation of nucleophilic CO2-amine adducts, such as carbamate salts or carbamic acids, which then react with other electrophiles or coupling partners to yield the desired carbamates. Notably, recent advancements have led to the successful development of several elegant methods for synthesizing specific types of carbamates using electrocatalysis or photocatalysis, which are not achievable through conventional thermal catalysis. This review comprehensively summarizes the recent progress in the synthesis of organic carbamates using CO2 and amines under various catalytic conditions, including transition metal-free conditions, transition metal-catalysis, electrocatalysis, and photocatalysis. Additionally, the review discusses the challenges and future prospects associated with converting CO2 into organic carbamates.
2024, 40(4): 230402
doi: 10.3866/PKU.WHXB202304020
Abstract:
Mixed solutions of polyvinylpyrrolidone (PVP) and sodium dodecyl sulfate (SDS) were investigated by capillary electrophoresis, together with other technologies, to confirm the formation of PVP-bound SDS micelles through cation-bridging association, which shows the nature of polyanions and the ability to exchange Na+ of SDS with environmental cations. Because additional water-soluble precursors of nanomaterials are capable of entering the cation-bridging layer of the PVP-SDS pseudo-polyanions, we speculate that the PVP chain and PVP-bound SDS micelles located on both sides of the precursor may act as a two-stage soft template to synthesize nanomaterials with unique morphologies. In this study, PVP-SDS pseudo-polyanions were used as soft templates to promote the growth of gold particles into gold nanoflowers (AuNFs). Tensiometry, conductometry, capillary electrophoresis, and zeta potential measurements confirmed the formation of the new PVP-SDS-HAuCl4 pseudo-polyanions. Transmission electron microscopy, X-ray diffraction, and UV-Vis spectroscopy analyses showed that the AuNFs synthesized in the mixed solution of PVP (50 g·L−1)-SDS (2 mmol·L−1)-HAuCl4 (0.25 mmol·L−1) possessed a face-centered cubic structure with abundant {111} crystal plane, demonstrating an average equivalent diameter of 108 nm with rich nano-protrusion of approximately 16.5 nm on the surface of AuNFs. The mechanistic study shows that PVP mainly acts as an in situ reductant for HAuCl4 in PVP-bound SDS micelles that simultaneously act as the primary template to govern the size of the primary gold crystals. In addition to continuously reducing HAuCl4, PVP functions as a secondary template, leading to primary gold crystals in a finite space linked by the PVP chain through preferential adsorption, stacked, and grown into mature AuNFs. Finally, a lower HAuCl4 concentration and an adequate reduction period are favorable in the aforementioned process dominated by the soft template rather than by the crystal growth rule of gold particles; thus, the reduction rate of HAuCl4 and nucleation-growth competition of gold particles can be regulated. Therefore, the optimal combination of low concentrations of SDS, PVP, and HAuCl4, together with an appropriate reduction period, would result in synergism between the reduction rate of HAuCl4, crystal growth rule of gold particles, and stacking degree of the primary gold crystals. AuNFs show strong surface-enhanced Raman scattering (SERS) activity for the Raman probe molecule of rhodamine 6G, strongly depending on the nano-protrusion morphology of AuNFs. The highest SERS enhancement factor can reach 6.71×107, which is superior to the reported level of the similar AuNFs (106). Because the particle sizes and morphologies of AuNFs can be precisely regulated, this strategy is a facile aqueous one-pot method for the synthesis of nanomaterials under normal temperature and pressure, which eliminates carrier requirements or adsorption interference from cationic surfactants, and has the potential to further enhance the SERS activity of the nanomaterials.
Mixed solutions of polyvinylpyrrolidone (PVP) and sodium dodecyl sulfate (SDS) were investigated by capillary electrophoresis, together with other technologies, to confirm the formation of PVP-bound SDS micelles through cation-bridging association, which shows the nature of polyanions and the ability to exchange Na+ of SDS with environmental cations. Because additional water-soluble precursors of nanomaterials are capable of entering the cation-bridging layer of the PVP-SDS pseudo-polyanions, we speculate that the PVP chain and PVP-bound SDS micelles located on both sides of the precursor may act as a two-stage soft template to synthesize nanomaterials with unique morphologies. In this study, PVP-SDS pseudo-polyanions were used as soft templates to promote the growth of gold particles into gold nanoflowers (AuNFs). Tensiometry, conductometry, capillary electrophoresis, and zeta potential measurements confirmed the formation of the new PVP-SDS-HAuCl4 pseudo-polyanions. Transmission electron microscopy, X-ray diffraction, and UV-Vis spectroscopy analyses showed that the AuNFs synthesized in the mixed solution of PVP (50 g·L−1)-SDS (2 mmol·L−1)-HAuCl4 (0.25 mmol·L−1) possessed a face-centered cubic structure with abundant {111} crystal plane, demonstrating an average equivalent diameter of 108 nm with rich nano-protrusion of approximately 16.5 nm on the surface of AuNFs. The mechanistic study shows that PVP mainly acts as an in situ reductant for HAuCl4 in PVP-bound SDS micelles that simultaneously act as the primary template to govern the size of the primary gold crystals. In addition to continuously reducing HAuCl4, PVP functions as a secondary template, leading to primary gold crystals in a finite space linked by the PVP chain through preferential adsorption, stacked, and grown into mature AuNFs. Finally, a lower HAuCl4 concentration and an adequate reduction period are favorable in the aforementioned process dominated by the soft template rather than by the crystal growth rule of gold particles; thus, the reduction rate of HAuCl4 and nucleation-growth competition of gold particles can be regulated. Therefore, the optimal combination of low concentrations of SDS, PVP, and HAuCl4, together with an appropriate reduction period, would result in synergism between the reduction rate of HAuCl4, crystal growth rule of gold particles, and stacking degree of the primary gold crystals. AuNFs show strong surface-enhanced Raman scattering (SERS) activity for the Raman probe molecule of rhodamine 6G, strongly depending on the nano-protrusion morphology of AuNFs. The highest SERS enhancement factor can reach 6.71×107, which is superior to the reported level of the similar AuNFs (106). Because the particle sizes and morphologies of AuNFs can be precisely regulated, this strategy is a facile aqueous one-pot method for the synthesis of nanomaterials under normal temperature and pressure, which eliminates carrier requirements or adsorption interference from cationic surfactants, and has the potential to further enhance the SERS activity of the nanomaterials.
2024, 40(4): 230402
doi: 10.3866/PKU.WHXB202304024
Abstract:
Traditional ink printing is convenient, but the excessive use of ink in this process has harmed both human health and the environment. Inkless printing technologies are mainly thermal and photosensitive in nature; however, they are susceptible to environmental impact, resulting in unstable printing and easy fading. In recent years, laser ablation printing technology has received considerable attention. However, the energy of the laser acting on the paper surface is affected by the surface roughness and tightness of the paper, which makes it difficult to obtain a uniform printing effect. Therefore, a complex and expensive paper-laser monitoring feedback system is required for modulating the laser parameters in real time to obtain uniform printing; however, such a system is not conducive to the popularization of laser ablation printing technology. To address this issue, a laser-induced inkless printing method, combined with micro-zone processing, is proposed. The associated high-energy characteristics and the thermal effect of laser are examined in this study. A multifunctional paper with an “organic-inorganic-organic” sandwich structure is constructed by vacuum filtration combined with thermocompression, using hydroxyapatite as the thermal insulating layer and wood fiber as the carbonized layer. The obtained functional papers display high flexibility, high tensile strength, and excellent flame resistance. Laser-induced inkless printing is realized via laser under-focusing or laser focusing onto hydroxyapatite sandwich paper. When the laser is irradiated on the surface of the functional paper, the surface-located cellulose fibers are carbonized due to the photothermal radiation effect of the laser, leaving graphitized carbon on the surface of hydroxyapatite layer. Notably, the hydroxyapatite in the interlayer of the functional paper is an outstanding thermal insulation material, which can effectively prevent further transfer of laser energy. Thus, owing to the sandwich structure of the multifunctional paper, laser inkless printing of both words and patterns is realized. This innovative approach offers a sustainable solution to traditional ink printing, while also providing a new avenue for micro-zone processing. The printed text or patterns can be preserved in harsh environments for long periods to be used for the storage of archival data. Meanwhile, the laser inkless printing method provides a new kind of touch reading material for acquired blindness, which avoids long-term contact with toxic printing ink. This study demonstrates that the hydroxyapatite sandwich paper is critical to realizing laser inkless printing without using expensive monitoring feedback systems, thus significantly reducing printing costs and effectively mitigating environmental pollution, marking it as an ideal technology for widespread adoption.
Traditional ink printing is convenient, but the excessive use of ink in this process has harmed both human health and the environment. Inkless printing technologies are mainly thermal and photosensitive in nature; however, they are susceptible to environmental impact, resulting in unstable printing and easy fading. In recent years, laser ablation printing technology has received considerable attention. However, the energy of the laser acting on the paper surface is affected by the surface roughness and tightness of the paper, which makes it difficult to obtain a uniform printing effect. Therefore, a complex and expensive paper-laser monitoring feedback system is required for modulating the laser parameters in real time to obtain uniform printing; however, such a system is not conducive to the popularization of laser ablation printing technology. To address this issue, a laser-induced inkless printing method, combined with micro-zone processing, is proposed. The associated high-energy characteristics and the thermal effect of laser are examined in this study. A multifunctional paper with an “organic-inorganic-organic” sandwich structure is constructed by vacuum filtration combined with thermocompression, using hydroxyapatite as the thermal insulating layer and wood fiber as the carbonized layer. The obtained functional papers display high flexibility, high tensile strength, and excellent flame resistance. Laser-induced inkless printing is realized via laser under-focusing or laser focusing onto hydroxyapatite sandwich paper. When the laser is irradiated on the surface of the functional paper, the surface-located cellulose fibers are carbonized due to the photothermal radiation effect of the laser, leaving graphitized carbon on the surface of hydroxyapatite layer. Notably, the hydroxyapatite in the interlayer of the functional paper is an outstanding thermal insulation material, which can effectively prevent further transfer of laser energy. Thus, owing to the sandwich structure of the multifunctional paper, laser inkless printing of both words and patterns is realized. This innovative approach offers a sustainable solution to traditional ink printing, while also providing a new avenue for micro-zone processing. The printed text or patterns can be preserved in harsh environments for long periods to be used for the storage of archival data. Meanwhile, the laser inkless printing method provides a new kind of touch reading material for acquired blindness, which avoids long-term contact with toxic printing ink. This study demonstrates that the hydroxyapatite sandwich paper is critical to realizing laser inkless printing without using expensive monitoring feedback systems, thus significantly reducing printing costs and effectively mitigating environmental pollution, marking it as an ideal technology for widespread adoption.
2024, 40(4): 230402
doi: 10.3866/PKU.WHXB202304029
Abstract:
Lead ions (Pb2+) are among the most prevalent toxic heavy-metal pollutants in daily human life, particularly in children and pregnant women. Although atomic absorption spectroscopy is the most commonly used method owing to its accuracy and reliability, it requires complex sample preparation and expensive equipment. Therefore, efficient detection of Pb2+ is currently the focus of optical sensing research. In this study, we develop a reflective fiber-optic interferometric sensor to detect trace levels of lead ions. The sensor is composed of a single-mode fiber, no-core fiber (NCF), and thin-core fiber (TCF). When light from the broadband light source is transmitted to the sensor via ports 1 and 2 of the fiber optic circulator, the light diverges and propagates forward in the NCF. Owing to the fiber-core mismatch of different optical fibers, the beams can excite the core and cladding modes in the TCF. When the beams are reflected back into the NCF, the core and cladding modes can effectively interfere in the NCF due to their optical path differences. Subsequently, the light signal is recorded by an optical spectrum analyzer through port 3 of the circulator. The TCF’s cladding is partially etched and coated with a functionalized hydrogel-sensing film made of 2-hydroxyethyl methacrylate (2-HEMA) as the recognition monomer. The oxygen atoms in the 2-HEMA are specifically matched with Pb2+ to form “-O-Pb-O-” cross-linked structures. Therefore, the absorption of Pb2+ by the hydrogel can change the effective refractive index of a new cladding of the TCF, formed by the sensing film and the TCF’s original cladding, thereby the Pb2+ concentration is detected by the change of the optical signal. Owing to the trace levels of the detected Pb2+ in aqueous solutions (in the ppt range), we employ an equation system to eliminate temperature interference and ensure accurate detection results under environmental temperature fluctuations. Additionally, for the same sensing length, the concentration sensitivity of fiber-optic sensors with reflective structures is twice that of the transmission structures, and the reflective structure is convenient for real-time remote detection. The experimental results show that the optimal sensitivity of the sensor is 1.926×109 nm∙mol−1∙L, and its detection limit can reach 2.0×10−11 mol∙L−1 (4.14 ppt, 1 ng∙L−1 = 1 ppt), which is far lower than the standard (10 ppb, 1 μg∙L−1 = 1 ppb) set by the World Health Organization. Moreover, the sensor exhibits good stability, specificity, and a wide detection range. Consequently, the designed reflective fiber optic sensor can provide broad prospects for environmental and human health monitoring.
Lead ions (Pb2+) are among the most prevalent toxic heavy-metal pollutants in daily human life, particularly in children and pregnant women. Although atomic absorption spectroscopy is the most commonly used method owing to its accuracy and reliability, it requires complex sample preparation and expensive equipment. Therefore, efficient detection of Pb2+ is currently the focus of optical sensing research. In this study, we develop a reflective fiber-optic interferometric sensor to detect trace levels of lead ions. The sensor is composed of a single-mode fiber, no-core fiber (NCF), and thin-core fiber (TCF). When light from the broadband light source is transmitted to the sensor via ports 1 and 2 of the fiber optic circulator, the light diverges and propagates forward in the NCF. Owing to the fiber-core mismatch of different optical fibers, the beams can excite the core and cladding modes in the TCF. When the beams are reflected back into the NCF, the core and cladding modes can effectively interfere in the NCF due to their optical path differences. Subsequently, the light signal is recorded by an optical spectrum analyzer through port 3 of the circulator. The TCF’s cladding is partially etched and coated with a functionalized hydrogel-sensing film made of 2-hydroxyethyl methacrylate (2-HEMA) as the recognition monomer. The oxygen atoms in the 2-HEMA are specifically matched with Pb2+ to form “-O-Pb-O-” cross-linked structures. Therefore, the absorption of Pb2+ by the hydrogel can change the effective refractive index of a new cladding of the TCF, formed by the sensing film and the TCF’s original cladding, thereby the Pb2+ concentration is detected by the change of the optical signal. Owing to the trace levels of the detected Pb2+ in aqueous solutions (in the ppt range), we employ an equation system to eliminate temperature interference and ensure accurate detection results under environmental temperature fluctuations. Additionally, for the same sensing length, the concentration sensitivity of fiber-optic sensors with reflective structures is twice that of the transmission structures, and the reflective structure is convenient for real-time remote detection. The experimental results show that the optimal sensitivity of the sensor is 1.926×109 nm∙mol−1∙L, and its detection limit can reach 2.0×10−11 mol∙L−1 (4.14 ppt, 1 ng∙L−1 = 1 ppt), which is far lower than the standard (10 ppb, 1 μg∙L−1 = 1 ppb) set by the World Health Organization. Moreover, the sensor exhibits good stability, specificity, and a wide detection range. Consequently, the designed reflective fiber optic sensor can provide broad prospects for environmental and human health monitoring.
2024, 40(4): 230405
doi: 10.3866/PKU.WHXB202304050
Abstract:
Environmental problems have become more and more serious with the continuous development of industrialized society. Especially, the problem of industrial wastewater has been a hot research issue in the field of catalytic degradation. Coupling photocatalysis and advanced oxidation processes (AOPs) is considered to be an efficient organic pollutant degradation technology due to its high efficiency, non-selectivity, and mild treatment conditions. In this article, the authors focused on the synthesis and characterization of Bismuth tungstate (Bi2WO6) nanoflowers, which were prepared using a straightforward hydrothermal method in the presence of cetyltrimethylammonium bromide (CTAB) surfactant. To investigate the micro-morphology, crystal phase, surface chemical element states, and optical characteristics of the Bi2WO6 nanoflowers, various methods such as X-ray diffraction (XRD), Fourier transform infrared (FTIR), scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), and diffuse reflection spectroscopy (DRS) were used. The catalytic performance of the Bi2WO6 nanoflowers was then investigated for degrading organic pollutants under different catalytic systems. The removal efficiency of Rhodamine B (RhB) was up to 96.39% within 40 min under vis/potassium monopersulfate triple salt (PMS)/ Bi2WO6 system, which is obviously superior to that in both PMS/ Bi2WO6 (38.77% in 40 min) and vis/Bi2WO6 (31.82% in 40 min) systems, indicating that synergistic effects between visible-light irradiation and PMS accelerated the catalytic activity of Bi2WO6 on the RhB degradation. The researchers also investigated the effect of ambient conditions on the catalytic performance of the systems, such as catalyst dosage, PMS concentration, pH value, and ion concentration. Interestingly, the vis/PMS/Bi2WO6 system demonstrated high removal efficiency (up to 90%) despite changes in these parameters. However, the catalytic degradation rate (k) was influenced by these parameters in this system. Conversely, the environmental parameters have obvious influence on the catalytic degradation rate (k) under vis/PMS/ Bi2WO6 system. The results showed that when the catalyst dosage and PMS concentration increased, so did the K value. On the other hand, the K value increased firstly and then decreased with the rise of pH value in the catalytic system. And the catalytic degradation rate reached its maximum value (0.1502 min−1) at pH = 7 in the catalytic system. Interestingly, the presence of Cl− in the system would be beneficial for promoting the catalytic degradation efficiency. Conversely, the existence of CO32− in the system would obviously inhibit the catalytic degradation efficiency. The result of the cycling experiments also verified that the catalyst possessed excellent stability for the degradation of organic dyes. Furthermore, the researchers conducted quenching experiments and EPR (electron paramagnetic resonance) tests, which revealed the crucial roles of superoxide radicals (•O− 2) and singlet oxygen (1O2) in the degradation of organic pollutants. Overall, the excellent catalytic activity of Bi2WO6 in the vis/PMS synergistic catalytic system was attributed to its outstanding visible-light-response photocatalysis activity and the superior ability of bismuth ions in activating PMS.
Environmental problems have become more and more serious with the continuous development of industrialized society. Especially, the problem of industrial wastewater has been a hot research issue in the field of catalytic degradation. Coupling photocatalysis and advanced oxidation processes (AOPs) is considered to be an efficient organic pollutant degradation technology due to its high efficiency, non-selectivity, and mild treatment conditions. In this article, the authors focused on the synthesis and characterization of Bismuth tungstate (Bi2WO6) nanoflowers, which were prepared using a straightforward hydrothermal method in the presence of cetyltrimethylammonium bromide (CTAB) surfactant. To investigate the micro-morphology, crystal phase, surface chemical element states, and optical characteristics of the Bi2WO6 nanoflowers, various methods such as X-ray diffraction (XRD), Fourier transform infrared (FTIR), scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), and diffuse reflection spectroscopy (DRS) were used. The catalytic performance of the Bi2WO6 nanoflowers was then investigated for degrading organic pollutants under different catalytic systems. The removal efficiency of Rhodamine B (RhB) was up to 96.39% within 40 min under vis/potassium monopersulfate triple salt (PMS)/ Bi2WO6 system, which is obviously superior to that in both PMS/ Bi2WO6 (38.77% in 40 min) and vis/Bi2WO6 (31.82% in 40 min) systems, indicating that synergistic effects between visible-light irradiation and PMS accelerated the catalytic activity of Bi2WO6 on the RhB degradation. The researchers also investigated the effect of ambient conditions on the catalytic performance of the systems, such as catalyst dosage, PMS concentration, pH value, and ion concentration. Interestingly, the vis/PMS/Bi2WO6 system demonstrated high removal efficiency (up to 90%) despite changes in these parameters. However, the catalytic degradation rate (k) was influenced by these parameters in this system. Conversely, the environmental parameters have obvious influence on the catalytic degradation rate (k) under vis/PMS/ Bi2WO6 system. The results showed that when the catalyst dosage and PMS concentration increased, so did the K value. On the other hand, the K value increased firstly and then decreased with the rise of pH value in the catalytic system. And the catalytic degradation rate reached its maximum value (0.1502 min−1) at pH = 7 in the catalytic system. Interestingly, the presence of Cl− in the system would be beneficial for promoting the catalytic degradation efficiency. Conversely, the existence of CO32− in the system would obviously inhibit the catalytic degradation efficiency. The result of the cycling experiments also verified that the catalyst possessed excellent stability for the degradation of organic dyes. Furthermore, the researchers conducted quenching experiments and EPR (electron paramagnetic resonance) tests, which revealed the crucial roles of superoxide radicals (•O− 2) and singlet oxygen (1O2) in the degradation of organic pollutants. Overall, the excellent catalytic activity of Bi2WO6 in the vis/PMS synergistic catalytic system was attributed to its outstanding visible-light-response photocatalysis activity and the superior ability of bismuth ions in activating PMS.
2024, 40(4): 230502
doi: 10.3866/PKU.WHXB202305026
Abstract:
The electrocatalytic carbon dioxide (CO2) reduction has gained recognition as an outstanding approach for transforming CO2 into renewable energy products. To accomplish this reduction reaction, the development of efficient electrocatalysts is required. Nickel-based electrocatalysts have been extensively investigated for CO2 reduction; however, nickel nanoparticles (NiNPs) have demonstrated limited catalytic performance. In this study, NiNPs implanted in N-doped porous carbon (NiNPs-NC) were prepared by thermal treatment of nickel metal-organic framework, urea, and carbon black under an N2 atmosphere. The NiNPs-NC exhibited high catalytic performance for the electroreduction of CO2 to CO in both H-type and flow cells. In the H-type cell, the CO faradaic efficiencies (FEs) of NiNPs-NC exceeded 90% in the potential window from −0.67 to −1.07 V vs. reversible hydrogen electrode (RHE), reaching a maximum CO FE of approximately 100% at −0.87 V vs. RHE. In the flow cell, the CO selectivities of NiNPs-NC exceeded 95% in the potential window from −0.50 to −0.70 V vs. RHE. The fast charge transfer, as demonstrated by electrochemical impedance spectroscopy and Tafel slope, can be attributed to the high catalytic activity of NiNPs-NC. This study provides a simple method to develop highly efficient catalysts for electrocatalytic CO2 reduction.
The electrocatalytic carbon dioxide (CO2) reduction has gained recognition as an outstanding approach for transforming CO2 into renewable energy products. To accomplish this reduction reaction, the development of efficient electrocatalysts is required. Nickel-based electrocatalysts have been extensively investigated for CO2 reduction; however, nickel nanoparticles (NiNPs) have demonstrated limited catalytic performance. In this study, NiNPs implanted in N-doped porous carbon (NiNPs-NC) were prepared by thermal treatment of nickel metal-organic framework, urea, and carbon black under an N2 atmosphere. The NiNPs-NC exhibited high catalytic performance for the electroreduction of CO2 to CO in both H-type and flow cells. In the H-type cell, the CO faradaic efficiencies (FEs) of NiNPs-NC exceeded 90% in the potential window from −0.67 to −1.07 V vs. reversible hydrogen electrode (RHE), reaching a maximum CO FE of approximately 100% at −0.87 V vs. RHE. In the flow cell, the CO selectivities of NiNPs-NC exceeded 95% in the potential window from −0.50 to −0.70 V vs. RHE. The fast charge transfer, as demonstrated by electrochemical impedance spectroscopy and Tafel slope, can be attributed to the high catalytic activity of NiNPs-NC. This study provides a simple method to develop highly efficient catalysts for electrocatalytic CO2 reduction.
2024, 40(4): 230604
doi: 10.3866/PKU.WHXB202306046
Abstract:
Efficiently converting CO2 and H2O into value-added chemicals using solar energy is a viable approach to address global warming and the energy crisis. However, achieving artificial photocatalytic CO2 reduction using H2O as the reductant poses challenges is due to the difficulty in efficient cooperation among multiple functional moieties. Metal-organic frameworks (MOFs) are promising candidates for overall CO2 photoreduction due to their large surface area, diverse active sites, and excellent tailorability. In this study, we designed a metal-organic framework photocatalyst, named PCN-224(Zn)-Bpy(Ru), by integrating photoactive Zn(II)-porphyrin and Ru(II)-bipyridyl moieties. In comparison, two isostructural MOFs just with either Zn(II)-porphyrin or Ru(II)-bipyridyl moiety, namely PCN-224-Bpy(Ru) and PCN-224(Zn)-Bpy were also synthesized. As a result, PCN-224(Zn)-Bpy(Ru) exhibited the highest photocatalytic conversion rate of CO2 to CO, with a production rate of 7.6 µmol·g−1·h−1 in a mixed solvent of CH3CN and H2O, without the need for co-catalysts, photosensitizers, or sacrificial agents. Mass spectrometer analysis detected the signals of 13CO (m/z = 29), 13C18O (m/z = 31), 16O18O (m/z = 34), and 18O2 (m/z = 36), confirming that CO2 and H2O acted as the carbon and oxygen sources for CO and O2, respectively, thereby confirming the coupling of photocatalytic CO2 reduction with H2O oxidation. In contrast, using PCN-224-Bpy(Ru) or PCN-224(Zn)-Bpy as catalysts under the same conditions resulted in significantly lower CO production rates of only 1.5 and 0 µmol·g−1·h−1, respectively. Mechanistic studies revealed that the lowest unoccupied molecular orbital (LUMO) potential of PCN-224(Zn)-Bpy(Ru) is more negative than the redox potentials of CO2/CO, and the highest occupied molecular orbital (HOMO) potential is more positive than that of H2O/O2, satisfying the thermodynamic requirements for overall photocatalytic CO2 reduction. In comparison, the HOMO potential of PCN-224(Zn)-Bpy without Ru(II)-bipyridyl moieties is less positive than that of H2O/O2, indicating that the Ru(II)-bipyridyl moiety is thermodynamically necessary for CO2 reduction coupled with H2O oxidation. Additionally, photoluminescence spectroscopy revealed that the fluorescence of PCN-224(Zn)-Bpy(Ru) was almost completely quenched, and a longer average photoluminescence lifetime compared to PCN-224(Zn)-Bpy and PCN-224-Bpy(Ru) was observed. These suggest a low recombination rate of photogenerated carriers in PCN-224(Zn)-Bpy(Ru), which also supported by the higher photocurrent observed in PCN-224(Zn)-Bpy(Ru) compared to PCN-224(Zn)-Bpy and PCN-224-Bpy(Ru). In summary, the integrated Zn(II)-porphyrin and Ru(II)-bipyridyl moieties in PCN-224(Zn)-Bpy(Ru) play important roles of a photosensitizer and CO2 reduction as well as H2O oxidation sites, and their efficient cooperation optimizes the band structure, thereby facilitating the coupling of CO2 reduction with H2O oxidation and resulting in high-performance artificial photocatalytic CO2 reduction.
Efficiently converting CO2 and H2O into value-added chemicals using solar energy is a viable approach to address global warming and the energy crisis. However, achieving artificial photocatalytic CO2 reduction using H2O as the reductant poses challenges is due to the difficulty in efficient cooperation among multiple functional moieties. Metal-organic frameworks (MOFs) are promising candidates for overall CO2 photoreduction due to their large surface area, diverse active sites, and excellent tailorability. In this study, we designed a metal-organic framework photocatalyst, named PCN-224(Zn)-Bpy(Ru), by integrating photoactive Zn(II)-porphyrin and Ru(II)-bipyridyl moieties. In comparison, two isostructural MOFs just with either Zn(II)-porphyrin or Ru(II)-bipyridyl moiety, namely PCN-224-Bpy(Ru) and PCN-224(Zn)-Bpy were also synthesized. As a result, PCN-224(Zn)-Bpy(Ru) exhibited the highest photocatalytic conversion rate of CO2 to CO, with a production rate of 7.6 µmol·g−1·h−1 in a mixed solvent of CH3CN and H2O, without the need for co-catalysts, photosensitizers, or sacrificial agents. Mass spectrometer analysis detected the signals of 13CO (m/z = 29), 13C18O (m/z = 31), 16O18O (m/z = 34), and 18O2 (m/z = 36), confirming that CO2 and H2O acted as the carbon and oxygen sources for CO and O2, respectively, thereby confirming the coupling of photocatalytic CO2 reduction with H2O oxidation. In contrast, using PCN-224-Bpy(Ru) or PCN-224(Zn)-Bpy as catalysts under the same conditions resulted in significantly lower CO production rates of only 1.5 and 0 µmol·g−1·h−1, respectively. Mechanistic studies revealed that the lowest unoccupied molecular orbital (LUMO) potential of PCN-224(Zn)-Bpy(Ru) is more negative than the redox potentials of CO2/CO, and the highest occupied molecular orbital (HOMO) potential is more positive than that of H2O/O2, satisfying the thermodynamic requirements for overall photocatalytic CO2 reduction. In comparison, the HOMO potential of PCN-224(Zn)-Bpy without Ru(II)-bipyridyl moieties is less positive than that of H2O/O2, indicating that the Ru(II)-bipyridyl moiety is thermodynamically necessary for CO2 reduction coupled with H2O oxidation. Additionally, photoluminescence spectroscopy revealed that the fluorescence of PCN-224(Zn)-Bpy(Ru) was almost completely quenched, and a longer average photoluminescence lifetime compared to PCN-224(Zn)-Bpy and PCN-224-Bpy(Ru) was observed. These suggest a low recombination rate of photogenerated carriers in PCN-224(Zn)-Bpy(Ru), which also supported by the higher photocurrent observed in PCN-224(Zn)-Bpy(Ru) compared to PCN-224(Zn)-Bpy and PCN-224-Bpy(Ru). In summary, the integrated Zn(II)-porphyrin and Ru(II)-bipyridyl moieties in PCN-224(Zn)-Bpy(Ru) play important roles of a photosensitizer and CO2 reduction as well as H2O oxidation sites, and their efficient cooperation optimizes the band structure, thereby facilitating the coupling of CO2 reduction with H2O oxidation and resulting in high-performance artificial photocatalytic CO2 reduction.
2024, 40(4): 230702
doi: 10.3866/PKU.WHXB202307022
Abstract:
