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2021 Volume 37 Issue 5
2021, 37(5): 200502
doi: 10.3866/PKU.WHXB202005027
Abstract:
The acceleration of industrialization and the continuous upgradation of consumption structure has increased the atmospheric content of CO2 far beyond the past levels, leading to a serious global environmental problem. Photocatalytic reduction of CO2 is one of the most promising methods to solve the problem of rising atmospheric CO2 content. The core of this technology is to develop efficient, environment-friendly, and affordable photocatalysts. A photocatalyst is a semiconductor that can absorb photons from sunlight and produce electron-hole pairs to initiate a redox reaction. Owing to their low specific surface areas, significant electron-hole recombination, and less surface-active sites, bulk photocatalysts are not satisfactory. Ultrathin layered materials have shown great potential for photocatalytic CO2 reduction owing to their characteristics of large specific surface area, a large number of low-coordination surface atoms, short transfer distance from the inside to the catalyst surface, along with other advantages. Photoexcited electrons only need to cover a short distance to transfer to the nanowafer surface, and the speed of migrating electrons on the nanowafer surface is much higher than that in the layers or in the bulk catalyst. The ultrathin structure leads to significant coordinative unsaturation and even vacancy defects in the lattice structure of the atoms; while the former can be used as active sites for CO2 adsorption and reaction, the latter can improve the separation of the electron-hole pair. This review summarizes the latest developments in ultrathin layered photocatalysts for CO2 reduction. First, the photocatalytic reduction mechanism of CO2 is introduced briefly, and the factors governing product selectivity are explained. Second, the existing catalysts, such as g-C3N4, black phosphorus (BP), graphene oxide (GO), metal oxide, transition metal dichalcogenides (TMDCs), perovskite, BiOX (X = Cl, Br, I), layered double hydroxide (LDH), 2D-MOF, MXene, and two-dimensional honeycomb-like Ge―Si alloy compounds (gersiloxenes), are classified. In addition, the prevalent preparation methods are summarized, including mechanical stripping, gas stripping, liquid stripping, chemical etching, chemical vapor deposition (CVD), template method, self-assembly of surfactant, and the intermediate precursor method of lamellar Bi-oleate complex. Finally, we introduced the strategy of improving photocatalyst performance on the premise of maintaining its layered structure, including the factors of thickness adjustment, doping, structural defects, composite, etc. The future opportunities and challenges of ultrathin layered photocatalysts for the reduction of carbon dioxide have also been proposed. ![]()
The acceleration of industrialization and the continuous upgradation of consumption structure has increased the atmospheric content of CO2 far beyond the past levels, leading to a serious global environmental problem. Photocatalytic reduction of CO2 is one of the most promising methods to solve the problem of rising atmospheric CO2 content. The core of this technology is to develop efficient, environment-friendly, and affordable photocatalysts. A photocatalyst is a semiconductor that can absorb photons from sunlight and produce electron-hole pairs to initiate a redox reaction. Owing to their low specific surface areas, significant electron-hole recombination, and less surface-active sites, bulk photocatalysts are not satisfactory. Ultrathin layered materials have shown great potential for photocatalytic CO2 reduction owing to their characteristics of large specific surface area, a large number of low-coordination surface atoms, short transfer distance from the inside to the catalyst surface, along with other advantages. Photoexcited electrons only need to cover a short distance to transfer to the nanowafer surface, and the speed of migrating electrons on the nanowafer surface is much higher than that in the layers or in the bulk catalyst. The ultrathin structure leads to significant coordinative unsaturation and even vacancy defects in the lattice structure of the atoms; while the former can be used as active sites for CO2 adsorption and reaction, the latter can improve the separation of the electron-hole pair. This review summarizes the latest developments in ultrathin layered photocatalysts for CO2 reduction. First, the photocatalytic reduction mechanism of CO2 is introduced briefly, and the factors governing product selectivity are explained. Second, the existing catalysts, such as g-C3N4, black phosphorus (BP), graphene oxide (GO), metal oxide, transition metal dichalcogenides (TMDCs), perovskite, BiOX (X = Cl, Br, I), layered double hydroxide (LDH), 2D-MOF, MXene, and two-dimensional honeycomb-like Ge―Si alloy compounds (gersiloxenes), are classified. In addition, the prevalent preparation methods are summarized, including mechanical stripping, gas stripping, liquid stripping, chemical etching, chemical vapor deposition (CVD), template method, self-assembly of surfactant, and the intermediate precursor method of lamellar Bi-oleate complex. Finally, we introduced the strategy of improving photocatalyst performance on the premise of maintaining its layered structure, including the factors of thickness adjustment, doping, structural defects, composite, etc. The future opportunities and challenges of ultrathin layered photocatalysts for the reduction of carbon dioxide have also been proposed.
2021, 37(5): 200603
doi: 10.3866/PKU.WHXB202006034
Abstract:
Burning of fossil fuels increases CO2 concentration in the atmosphere, resulting in a series of climate- and environment-related concerns such as global warming, sea-level rise, and melting of glaciers. Therefore, utilization of renewable energy to reduce the CO2 concentration, in order to realize a sustainable development, is urgent. Capturing and utilizing CO2, a greenhouse gas, can not only address these concerns but also alleviate the current scenario of energy shortage. Thermal catalytic CO2 hydrogenation offers various pathways with high conversion efficiencies to produce fuels and industrial chemicals including CO, HCOOH, CH3OH, and CH4. However, CO2 is chemically inert due to the highly stable C＝O bond. Thus, harsh reaction conditions such as high temperature and pressure are required for CO2 hydrogenation.Electrocatalytic CO2 reduction using renewable electricity and water is a promising alternative to thermocatalysis. This technology can not only store and transport the intermittent solar or wind energy but can also use water as the proton source instead of H2, which is indispensable for thermal CO2 hydrogenation. Electrochemical CO2 reduction under ambient conditions is a proton-coupled electron transfer process. The key to promote the electrochemical reduction of CO2 is to develop highly selective and active catalysts with high stability. Among various CO2 electrocatalysts, copper-based catalysts have attracted significant attention and have been extensively investigated, since they exhibit good selectivity and efficiency for the reduction of CO2 to hydrocarbons and alcohols. A broad range of products, up to 16 different gases and liquids, can be obtained in the CO2 electroconversion on copper. Copper is the only metal that has a negative adsorption energy for *CO and a positive adsorption energy for *H. Thus, it has a unique property of generating > 2e− transfer products. However, selectivity of the target product is still low, especially for high value-added C2+ species (C2H4, C2H5OH, CH3COOH, CH3CHO, n-C3H7OH, etc.).The selectivity of various products on copper-based catalysts could be enhanced by surface engineering techniques such as tuning the morphologies, particle sizes, surface facets, strains levels, and atomic coordination. Electrolyte engineering could also aid in CO2 electroreduction. Therefore, improving the selectivity of C2+ products by modifying copper-based catalysts could be a hot research topic. In addition, C-C coupling is a key step in forming C2+ products, though the C2+ product formation pathway is complex, and the mechanisms are still unclear. Considering these, this paper mainly reviews the research progress in copper-based catalysts producing C2+ species in the last five years. It also discusses the possible reaction mechanisms and the factors that affect the product selectivities. In the end, further research directions are proposed. ![]()
Burning of fossil fuels increases CO2 concentration in the atmosphere, resulting in a series of climate- and environment-related concerns such as global warming, sea-level rise, and melting of glaciers. Therefore, utilization of renewable energy to reduce the CO2 concentration, in order to realize a sustainable development, is urgent. Capturing and utilizing CO2, a greenhouse gas, can not only address these concerns but also alleviate the current scenario of energy shortage. Thermal catalytic CO2 hydrogenation offers various pathways with high conversion efficiencies to produce fuels and industrial chemicals including CO, HCOOH, CH3OH, and CH4. However, CO2 is chemically inert due to the highly stable C＝O bond. Thus, harsh reaction conditions such as high temperature and pressure are required for CO2 hydrogenation.Electrocatalytic CO2 reduction using renewable electricity and water is a promising alternative to thermocatalysis. This technology can not only store and transport the intermittent solar or wind energy but can also use water as the proton source instead of H2, which is indispensable for thermal CO2 hydrogenation. Electrochemical CO2 reduction under ambient conditions is a proton-coupled electron transfer process. The key to promote the electrochemical reduction of CO2 is to develop highly selective and active catalysts with high stability. Among various CO2 electrocatalysts, copper-based catalysts have attracted significant attention and have been extensively investigated, since they exhibit good selectivity and efficiency for the reduction of CO2 to hydrocarbons and alcohols. A broad range of products, up to 16 different gases and liquids, can be obtained in the CO2 electroconversion on copper. Copper is the only metal that has a negative adsorption energy for *CO and a positive adsorption energy for *H. Thus, it has a unique property of generating > 2e− transfer products. However, selectivity of the target product is still low, especially for high value-added C2+ species (C2H4, C2H5OH, CH3COOH, CH3CHO, n-C3H7OH, etc.).The selectivity of various products on copper-based catalysts could be enhanced by surface engineering techniques such as tuning the morphologies, particle sizes, surface facets, strains levels, and atomic coordination. Electrolyte engineering could also aid in CO2 electroreduction. Therefore, improving the selectivity of C2+ products by modifying copper-based catalysts could be a hot research topic. In addition, C-C coupling is a key step in forming C2+ products, though the C2+ product formation pathway is complex, and the mechanisms are still unclear. Considering these, this paper mainly reviews the research progress in copper-based catalysts producing C2+ species in the last five years. It also discusses the possible reaction mechanisms and the factors that affect the product selectivities. In the end, further research directions are proposed.
2021, 37(5): 200705
doi: 10.3866/PKU.WHXB202007052
Abstract:
The efficient utilization of the greenhouse gas CO2 as a C1 feedstock can effectively reduce its emission and create economic value. Hence, the efficient chemical conversion of CO2 has been receiving intense attention. Due to the extremely low energy level of the CO2 molecule, the high energy barrier is the primary challenge for the chemical conversion of CO2. The chemical conversion of CO2 is mainly carried out through non-reductive transformation in industrial. Yet, the new route of chemical synthesis based on CO2 reductive transformation is an interesting topic to expand its resource utilization. In this context, homogeneous reductive carbonylation is a hot topic for the utilization of CO2 via reductive transformation. In this process, the metal hydride intermediate derived from the activation of the hydrogen source is crucial to the CO2 reduction. Hydrogen, a clean source with high atom economy, can be used as a reducing agent for the reductive conversion of inert CO2 through carbonylation, to construct C―O, C―N, and C―C bonds and to synthesize aldehyde/alcohol, carboxylic acid, ester, amide, and other chemicals. These expand the scope of CO2 high-value utilization and show great potential application in terms of resource utilization and environmental protection. This CO2 utilization process is thought to involve cascading catalytic reactions of CO2 reduction and carbonylation. The catalytic systems require the corresponding catalysts to efficiently promote each step and effectively inhibit undesired side reactions. Recently, considerable progress has been made in the homogeneous reductive carbonylation of CO2 with H2. However, this kind of reaction is mostly of the cascade type, and hence, requires harsh conditions and noble metal catalysts. The chemoselectivity is low because of the multiple competing reactions. In addition, due to the steric hindrance and electronic effects of the substrate, there are limitations on the types of substrates that can be employed. With the development of new characterization techniques and theoretical calculations, some progress has been made in revealing the reaction mechanism and in the activation of the carbon-oxygen bonds of CO2. Therefore, there is an urgent need to develop a more efficient catalytic system that requires mild conditions for reductive carbonylation. In this review, we provide an overview of the groundbreaking studies and the recent breakthroughs that have demonstrated the potential of metal catalysts to utilize the combination of CO2 and H2 as a C1 synthon, including olefin carbonylation, amine carbonylation, and alcohol/ether carbonylation, while highlighting the effect of different types of metal catalysts on the reaction. We conclude with a perspective on the future prospects of the homogeneous reductive carbonylation of CO2 with H2, providing readers a snapshot of this rapidly evolving field. ![]()
The efficient utilization of the greenhouse gas CO2 as a C1 feedstock can effectively reduce its emission and create economic value. Hence, the efficient chemical conversion of CO2 has been receiving intense attention. Due to the extremely low energy level of the CO2 molecule, the high energy barrier is the primary challenge for the chemical conversion of CO2. The chemical conversion of CO2 is mainly carried out through non-reductive transformation in industrial. Yet, the new route of chemical synthesis based on CO2 reductive transformation is an interesting topic to expand its resource utilization. In this context, homogeneous reductive carbonylation is a hot topic for the utilization of CO2 via reductive transformation. In this process, the metal hydride intermediate derived from the activation of the hydrogen source is crucial to the CO2 reduction. Hydrogen, a clean source with high atom economy, can be used as a reducing agent for the reductive conversion of inert CO2 through carbonylation, to construct C―O, C―N, and C―C bonds and to synthesize aldehyde/alcohol, carboxylic acid, ester, amide, and other chemicals. These expand the scope of CO2 high-value utilization and show great potential application in terms of resource utilization and environmental protection. This CO2 utilization process is thought to involve cascading catalytic reactions of CO2 reduction and carbonylation. The catalytic systems require the corresponding catalysts to efficiently promote each step and effectively inhibit undesired side reactions. Recently, considerable progress has been made in the homogeneous reductive carbonylation of CO2 with H2. However, this kind of reaction is mostly of the cascade type, and hence, requires harsh conditions and noble metal catalysts. The chemoselectivity is low because of the multiple competing reactions. In addition, due to the steric hindrance and electronic effects of the substrate, there are limitations on the types of substrates that can be employed. With the development of new characterization techniques and theoretical calculations, some progress has been made in revealing the reaction mechanism and in the activation of the carbon-oxygen bonds of CO2. Therefore, there is an urgent need to develop a more efficient catalytic system that requires mild conditions for reductive carbonylation. In this review, we provide an overview of the groundbreaking studies and the recent breakthroughs that have demonstrated the potential of metal catalysts to utilize the combination of CO2 and H2 as a C1 synthon, including olefin carbonylation, amine carbonylation, and alcohol/ether carbonylation, while highlighting the effect of different types of metal catalysts on the reaction. We conclude with a perspective on the future prospects of the homogeneous reductive carbonylation of CO2 with H2, providing readers a snapshot of this rapidly evolving field.
2021, 37(5): 200804
doi: 10.3866/PKU.WHXB202008043
Abstract:
Carbon dioxide (CO2) is one of the main greenhouse gases in the atmosphere. The conversion of CO2 into solar fuels (CO, HCOOH, CH4, CH3OH, etc.) using artificial photosynthetic systems is an ideal way to utilize CO2 as a resource and reduce CO2 emissions. A typical artificial photosynthetic system is composed of three key components: a photosensitizer (PS) to harvest visible light, a catalyst (C) to catalyze CO2 or protons into carbon-based fuels or H2, respectively, and a sacrificial electron donor (SED) to consume the holes generated in the PS. In most cases, the PS and catalyst are two different components of a system. However, some components that possess both light harvesting and redox catalysis functionalities, e.g., nano-semiconductors, are referred to as photocatalysts. During photocatalysis, the PS is typically excited by photons to generate excited electrons. The excited electrons in the PS are transferred to the catalyst to generate a reduced catalyst. The reduced catalyst is used as an active intermediate to perform CO2 binding and transformation. The PS can be recovered through a reaction with the SED. Nano-semiconductors have been used as photosensitizers and/or photocatalysts in photocatalytic CO2 reduction systems owing to their excellent photophysical and photochemical properties and photostability. CdS and CdSe nano-semiconductors, such as quantum dots, nanorods, and nanosheets, have been widely used in the construction of photocatalytic CO2 reduction systems. Systems based on CdS or CdSe nano-semiconductors can be classified into three categories. The first category is systems based on CdS or CdSe photocatalysts. In these systems, CdS or CdSe nano-semiconductors function as photocatalysts to catalyze CO2 reduction without a co-catalyst under visible-light irradiation. The CO2 reduction reaction occurs at the surface of the CdS or CdSe nano-semiconductors. The second category is systems based on CdS or CdSe composite photocatalysts. CdS or CdSe nano-semiconductors are combined with functional materials, such as reduced graphene oxide or TiO2, to prepare composite photocatalysts. These composite photocatalysts are expected to improve the lifetime of the charge separation state and inhibit the photocorrosion of the nano-semiconductors during photocatalysis. The third category is hybrid systems containing a CdS nano-semiconductor and molecular catalysts, such as nickel and cobalt complexes and iron porphyrin. In these hybrid systems, CdS functions as a photosensitizer and the CO2 reduction reaction occurs at the molecular catalyst. This review article introduces the construction of artificial photosynthetic systems and the photocatalytic mechanism of nano-semiconductors, and summarizes the representative works in the three aforementioned categories of systems. Finally, the challenges of nano-semiconductors for photocatalytic CO2 reduction are discussed. ![]()
Carbon dioxide (CO2) is one of the main greenhouse gases in the atmosphere. The conversion of CO2 into solar fuels (CO, HCOOH, CH4, CH3OH, etc.) using artificial photosynthetic systems is an ideal way to utilize CO2 as a resource and reduce CO2 emissions. A typical artificial photosynthetic system is composed of three key components: a photosensitizer (PS) to harvest visible light, a catalyst (C) to catalyze CO2 or protons into carbon-based fuels or H2, respectively, and a sacrificial electron donor (SED) to consume the holes generated in the PS. In most cases, the PS and catalyst are two different components of a system. However, some components that possess both light harvesting and redox catalysis functionalities, e.g., nano-semiconductors, are referred to as photocatalysts. During photocatalysis, the PS is typically excited by photons to generate excited electrons. The excited electrons in the PS are transferred to the catalyst to generate a reduced catalyst. The reduced catalyst is used as an active intermediate to perform CO2 binding and transformation. The PS can be recovered through a reaction with the SED. Nano-semiconductors have been used as photosensitizers and/or photocatalysts in photocatalytic CO2 reduction systems owing to their excellent photophysical and photochemical properties and photostability. CdS and CdSe nano-semiconductors, such as quantum dots, nanorods, and nanosheets, have been widely used in the construction of photocatalytic CO2 reduction systems. Systems based on CdS or CdSe nano-semiconductors can be classified into three categories. The first category is systems based on CdS or CdSe photocatalysts. In these systems, CdS or CdSe nano-semiconductors function as photocatalysts to catalyze CO2 reduction without a co-catalyst under visible-light irradiation. The CO2 reduction reaction occurs at the surface of the CdS or CdSe nano-semiconductors. The second category is systems based on CdS or CdSe composite photocatalysts. CdS or CdSe nano-semiconductors are combined with functional materials, such as reduced graphene oxide or TiO2, to prepare composite photocatalysts. These composite photocatalysts are expected to improve the lifetime of the charge separation state and inhibit the photocorrosion of the nano-semiconductors during photocatalysis. The third category is hybrid systems containing a CdS nano-semiconductor and molecular catalysts, such as nickel and cobalt complexes and iron porphyrin. In these hybrid systems, CdS functions as a photosensitizer and the CO2 reduction reaction occurs at the molecular catalyst. This review article introduces the construction of artificial photosynthetic systems and the photocatalytic mechanism of nano-semiconductors, and summarizes the representative works in the three aforementioned categories of systems. Finally, the challenges of nano-semiconductors for photocatalytic CO2 reduction are discussed.
2021, 37(5): 200806
doi: 10.3866/PKU.WHXB202008068
Abstract:
Nowadays, more than 85% of the energy is generated by fossil fuels. The excessive utilization of finite fossil fuels has resulted in the crises of energy shortage and global warming caused by greenhouse gas emissions. Researchers have conceived several means for trying to solve these problems, among which the sunlight-driven CO2 reduction is viewed as a sustainable process that utilizes CO2 as the raw material to produce chemical fuels, including CO, formate, and CH4; this method not only realizes the conversion and storage of intermittent solar energy, but also decreases the CO2 concentration in the atmosphere and alleviates global warming. However, photochemical CO2 reduction usually undergoes a sluggish process due to the inertness of CO2. Moreover, the selectivity of the CO2 reduction reaction is also challenged by the hydrogen evolution reaction, which exhibits faster reaction kinetics. In this context, the rational design and synthesis of efficient and selective catalysts for photochemical CO2 reduction are major challenges. Recently, non-noble metal Co(Ⅱ) complexes as molecular catalysts have shown excellent catalytic performances in photocatalytic CO2 reduction. During the past several decades, significant progress has been achieved in improving the applicability of Co(Ⅱ) complexes for photocatalytic CO2 reduction. In this review, we systematically report the latest research progress on the use of Co(Ⅱ) complexes in photocatalytic CO2 reduction. To describe the progress of this research, we characterized the Co(Ⅱ)-based molecular catalysts into four categories according to ligand types, namely: (1) macrocyclic ligands, (2) polypyridine ligands, (3) porphyrin and porphyrin-like ligands, and (4) nonplanar N4 ligands. The progress of the research on the heterogeneity of Co(Ⅱ) molecular complexes used for photochemical CO2 reduction was also introduced and discussed. Furthermore, the effects of catalyst structures on catalytic efficiency, selectivity, and stability were particularly summarized and discussed, with the aim of revealing and building the relationship between catalyst structures and catalytic performances to guide the future design and synthesis of Co(Ⅱ) molecular complexes with excellent catalytic performance. Finally, the current challenges and problems in photocatalytic CO2 reduction were summarized, and several suggestions for designing efficient Co(Ⅱ)-based molecular catalysts for photocatalytic CO2 reduction were put forward. The trends for the future development of Co(Ⅱ) complex molecular catalysts were also investigated with regard to photocatalytic CO2 reduction. ![]()
Nowadays, more than 85% of the energy is generated by fossil fuels. The excessive utilization of finite fossil fuels has resulted in the crises of energy shortage and global warming caused by greenhouse gas emissions. Researchers have conceived several means for trying to solve these problems, among which the sunlight-driven CO2 reduction is viewed as a sustainable process that utilizes CO2 as the raw material to produce chemical fuels, including CO, formate, and CH4; this method not only realizes the conversion and storage of intermittent solar energy, but also decreases the CO2 concentration in the atmosphere and alleviates global warming. However, photochemical CO2 reduction usually undergoes a sluggish process due to the inertness of CO2. Moreover, the selectivity of the CO2 reduction reaction is also challenged by the hydrogen evolution reaction, which exhibits faster reaction kinetics. In this context, the rational design and synthesis of efficient and selective catalysts for photochemical CO2 reduction are major challenges. Recently, non-noble metal Co(Ⅱ) complexes as molecular catalysts have shown excellent catalytic performances in photocatalytic CO2 reduction. During the past several decades, significant progress has been achieved in improving the applicability of Co(Ⅱ) complexes for photocatalytic CO2 reduction. In this review, we systematically report the latest research progress on the use of Co(Ⅱ) complexes in photocatalytic CO2 reduction. To describe the progress of this research, we characterized the Co(Ⅱ)-based molecular catalysts into four categories according to ligand types, namely: (1) macrocyclic ligands, (2) polypyridine ligands, (3) porphyrin and porphyrin-like ligands, and (4) nonplanar N4 ligands. The progress of the research on the heterogeneity of Co(Ⅱ) molecular complexes used for photochemical CO2 reduction was also introduced and discussed. Furthermore, the effects of catalyst structures on catalytic efficiency, selectivity, and stability were particularly summarized and discussed, with the aim of revealing and building the relationship between catalyst structures and catalytic performances to guide the future design and synthesis of Co(Ⅱ) molecular complexes with excellent catalytic performance. Finally, the current challenges and problems in photocatalytic CO2 reduction were summarized, and several suggestions for designing efficient Co(Ⅱ)-based molecular catalysts for photocatalytic CO2 reduction were put forward. The trends for the future development of Co(Ⅱ) complex molecular catalysts were also investigated with regard to photocatalytic CO2 reduction.
2021, 37(5): 200909
doi: 10.3866/PKU.WHXB202009098
Abstract:
Ever-increasing energy demands due to rapid industrialization and urban population growth have drastically reduced petroleum reserves and increased greenhouse-gas production, and the latter has consequently contributed to climate change and environmental damage. Therefore, it is highly desirable to produce fuels and chemicals from non-petroleum feedstocks and to reduce the atmospheric concentrations of greenhouse gases. One solution has involved using carbon dioxide (CO2), a main greenhouse gas, as a C1 feedstock for producing industrial fuels and chemicals. However, this requires high energy input from reductants or reactants with relatively high free energy (e.g., H2 gas) because CO2 is a highly oxidized, thermodynamically stable form of carbon. H2 can be generated through water photolysis, making it an ideal reductant for hydrogenating CO2 to CO. In situ generation of CO such as this has been developed for various carbonylation reactions that produce high value-added chemicals and avoid deriving CO from fossil fuels. This is beneficial because CO is toxic, and when extracted from fossil fuels it requires tedious separation and transportation. This combination of CO2 and H2 allows for functional molecules to be synthesized as entries into the chemical industry value chain and would generate a carbon footprint much lower than that of conventional petrochemical pathways. Based on this, CO2/H2 carbonylations using homogeneous transition metal-based catalysts have attracted increasing attention. Through this process, alkenes have been converted to alcohols, carboxylic acids, amines, and aldehydes. Heterogeneous catalysis has also provided an innovative approach for the carbonylation of alkenes with CO2/H2. Based on these alkene carbonylations, the scope of CO2/H2 carbonylations has been expanded to include aryl halides, methanol, and methanol derivatives, which give the corresponding aryl aldehyde, acetic acid, and ethanol products. These carbonylations revealed indirect CO2-HCOOH-CO pathways and direct CO2 insertion pathways. The use of this process is ever-increasing and has expanded the scope of CO2 utilization to produce novel, high value-added or bulk chemicals, and has promoted sustainable chemistry. This review summarizes the recent advances in transition-metal-catalyzed carbonylations with CO2/H2 and discusses the perspectives and challenges of further research. ![]()
Ever-increasing energy demands due to rapid industrialization and urban population growth have drastically reduced petroleum reserves and increased greenhouse-gas production, and the latter has consequently contributed to climate change and environmental damage. Therefore, it is highly desirable to produce fuels and chemicals from non-petroleum feedstocks and to reduce the atmospheric concentrations of greenhouse gases. One solution has involved using carbon dioxide (CO2), a main greenhouse gas, as a C1 feedstock for producing industrial fuels and chemicals. However, this requires high energy input from reductants or reactants with relatively high free energy (e.g., H2 gas) because CO2 is a highly oxidized, thermodynamically stable form of carbon. H2 can be generated through water photolysis, making it an ideal reductant for hydrogenating CO2 to CO. In situ generation of CO such as this has been developed for various carbonylation reactions that produce high value-added chemicals and avoid deriving CO from fossil fuels. This is beneficial because CO is toxic, and when extracted from fossil fuels it requires tedious separation and transportation. This combination of CO2 and H2 allows for functional molecules to be synthesized as entries into the chemical industry value chain and would generate a carbon footprint much lower than that of conventional petrochemical pathways. Based on this, CO2/H2 carbonylations using homogeneous transition metal-based catalysts have attracted increasing attention. Through this process, alkenes have been converted to alcohols, carboxylic acids, amines, and aldehydes. Heterogeneous catalysis has also provided an innovative approach for the carbonylation of alkenes with CO2/H2. Based on these alkene carbonylations, the scope of CO2/H2 carbonylations has been expanded to include aryl halides, methanol, and methanol derivatives, which give the corresponding aryl aldehyde, acetic acid, and ethanol products. These carbonylations revealed indirect CO2-HCOOH-CO pathways and direct CO2 insertion pathways. The use of this process is ever-increasing and has expanded the scope of CO2 utilization to produce novel, high value-added or bulk chemicals, and has promoted sustainable chemistry. This review summarizes the recent advances in transition-metal-catalyzed carbonylations with CO2/H2 and discusses the perspectives and challenges of further research.
2021, 37(5): 201002
doi: 10.3866/PKU.WHXB202010022
Abstract:
The efficient utilization of carbon dioxide (CO2) as a C1 feedstock is of great significance for green and sustainable development. Therefore, the efficient chemical conversion of CO2 into value-added products has recently attracted a lot of research attention in recent years. The transformation of CO2 generally requires high-energy substrates, specific catalysts, and harsh reaction conditions due to its high thermodynamic stability and kinetic inertness. Consequently, several efforts have been dedicated toward the development of high-performance catalysts and new reaction routes for CO2 conversion over the last few decades. To date, many routes of convert CO2 into value-added chemicals have been proposed, together with the development of heterogeneous and homogeneous catalysts. Among the advanced catalysts reported to date, ionic liquids (ILs) have been widely investigated and show great potential for the efficient, selective, and economical conversion of CO2 into highly valuable products under mild conditions, even under ambient conditions. Some task-specific ILs have been designed with unique functional groups (e.g., —OH, —SO3H, —NH2, —COOH, and —C≡N), which can act as the solvent, absorbent, activating agent, catalyst, or cocatalyst to realize the transformation of CO2 under metal-free and mild conditions. In addition, a variety of catalytic systems composed of ILs and metal catalysts have also been reported for the transformation of CO2, in which the combination of the IL and metal catalyst is responsible for CO2 conversion with high efficiency. In this review article, we summarize the recent advances in IL-mediated CO2 transformation into chemicals prepared via C—O, C—N, C—S, C—H, and C—C bond forming processes. ILs that can chemically capture CO2 with high capacity are first introduced, which can activate CO2 via the formation of IL-based carbonates or carbamates, thus realizing the transformation of CO2 under metal-free and mild conditions. Recent progress in IL-mediated CO2 transformations to form carbonates and various kinds of N- and S-containing compounds (e.g., oxazolidinones, ureas, benzimidazolones, formamides, methylamines, benzothiazoles, and other chemicals) as well as CO2 hydrogenation to give formic acid, methane, acetic acid, low-carbon alcohols, and hydrocarbons has been summarized in this review with a focus on the reaction routes, catalytic systems, and reaction mechanism. In these reactions, ILs can simultaneously activate the substrate via strong H-bonding in addition to activating CO2, and the cooperative effects among the ionic and molecular species and metal catalysts accomplish the reactions of CO2 with various kinds of substrates to afford a wide range of value-added chemicals. Finally, the shortcomings and perspectives of ILs are discussed. In short, IL-mediated CO2 transformations provide green and effective routes for the synthesis of high-value chemicals, which may have great potential for a wide range of applications. ![]()
The efficient utilization of carbon dioxide (CO2) as a C1 feedstock is of great significance for green and sustainable development. Therefore, the efficient chemical conversion of CO2 into value-added products has recently attracted a lot of research attention in recent years. The transformation of CO2 generally requires high-energy substrates, specific catalysts, and harsh reaction conditions due to its high thermodynamic stability and kinetic inertness. Consequently, several efforts have been dedicated toward the development of high-performance catalysts and new reaction routes for CO2 conversion over the last few decades. To date, many routes of convert CO2 into value-added chemicals have been proposed, together with the development of heterogeneous and homogeneous catalysts. Among the advanced catalysts reported to date, ionic liquids (ILs) have been widely investigated and show great potential for the efficient, selective, and economical conversion of CO2 into highly valuable products under mild conditions, even under ambient conditions. Some task-specific ILs have been designed with unique functional groups (e.g., —OH, —SO3H, —NH2, —COOH, and —C≡N), which can act as the solvent, absorbent, activating agent, catalyst, or cocatalyst to realize the transformation of CO2 under metal-free and mild conditions. In addition, a variety of catalytic systems composed of ILs and metal catalysts have also been reported for the transformation of CO2, in which the combination of the IL and metal catalyst is responsible for CO2 conversion with high efficiency. In this review article, we summarize the recent advances in IL-mediated CO2 transformation into chemicals prepared via C—O, C—N, C—S, C—H, and C—C bond forming processes. ILs that can chemically capture CO2 with high capacity are first introduced, which can activate CO2 via the formation of IL-based carbonates or carbamates, thus realizing the transformation of CO2 under metal-free and mild conditions. Recent progress in IL-mediated CO2 transformations to form carbonates and various kinds of N- and S-containing compounds (e.g., oxazolidinones, ureas, benzimidazolones, formamides, methylamines, benzothiazoles, and other chemicals) as well as CO2 hydrogenation to give formic acid, methane, acetic acid, low-carbon alcohols, and hydrocarbons has been summarized in this review with a focus on the reaction routes, catalytic systems, and reaction mechanism. In these reactions, ILs can simultaneously activate the substrate via strong H-bonding in addition to activating CO2, and the cooperative effects among the ionic and molecular species and metal catalysts accomplish the reactions of CO2 with various kinds of substrates to afford a wide range of value-added chemicals. Finally, the shortcomings and perspectives of ILs are discussed. In short, IL-mediated CO2 transformations provide green and effective routes for the synthesis of high-value chemicals, which may have great potential for a wide range of applications.
2021, 37(5): 201004
doi: 10.3866/PKU.WHXB202010040
Abstract:
Converting CO2 into value-added products via sustainable energy, such as electrical energy, has several advantages. First, it is one of the most promising routes to close the carbon loop and plays a crucial role in significantly reducing the CO2 concentration in the atmosphere. Second, it can utilize CO2 as a valuable industry reactant that can store energy by converting electrical energy to chemical energy. Although the CO2 reduction reaction has been studied for more than three decades, the sluggish kinetics remain a bottleneck, which requires a highly efficient catalyst. However, none of the reported catalysts meets the requirements for any practical application due to low activity and poor selectivity. To rationally design a more efficient CO2 reduction catalyst, understanding the reaction mechanism is crucial. Although it is challenging to experimentally capture and characterize the reactive intermediates, atomic modeling serves as an alternative for providing an understanding of the elementary reactions on a microscale. Significant progress has been made in understanding the reaction mechanism using multiscale simulations. In this study, important progress in revealing the reaction mechanism of CO2 reduction using computational simulation in recent years is summarized. First, the advances in simulation methods for electrochemical reactions are introduced, and the advantages and disadvantages of various methods are compared. Second, the detailed reaction mechanism of CO2 reduction to various major products, such as CO, CH4, and C2H4, and minor products, such as ethanol and acetate, are disused. Different results obtained from different approximations are compared, while a mechanism that can better explain the existing experimental results is recommended. Third, the operando technique, such as ambient pressure X-ray photoelectron spectroscopy, is disused. The operando analysis results are direct evidence to validate the theoretically proposed reaction pathway. In turn, the theoretical predictions can help resolve the experimental spectrum, which is usually too complex to refer to a reference system. The combination of theory and operando experiments should be one of the most promising directions in determining the reaction mechanism. Fourth, novel synthesis strategies are discussed. These new ideas are beneficial for simplifying the synthesis process or increasing the diversity of products. Finally, the recent progress in the application of machine learning to big data for CO2 reduction is discussed. These new powerful tools may play a crucial role in reaction mechanism studies. Overall, in the study of electrochemical reaction mechanism, theoretical simulation can provide the reaction details and energy information of elementary reactions at the atomic level. Therefore, in the study of electrochemical reaction mechanism of carbon dioxide, the microscopic mechanism that the experiment cannot provide is supplemented. On the one hand, it explains the existing experimental phenomena; however, on the other hand, it provides new insights for the study of reaction mechanism. On this basis, the use of new research paradigms, such as high-throughput computing and machine learning, provides new ideas for a rational design for accelerating material development. ![]()
Converting CO2 into value-added products via sustainable energy, such as electrical energy, has several advantages. First, it is one of the most promising routes to close the carbon loop and plays a crucial role in significantly reducing the CO2 concentration in the atmosphere. Second, it can utilize CO2 as a valuable industry reactant that can store energy by converting electrical energy to chemical energy. Although the CO2 reduction reaction has been studied for more than three decades, the sluggish kinetics remain a bottleneck, which requires a highly efficient catalyst. However, none of the reported catalysts meets the requirements for any practical application due to low activity and poor selectivity. To rationally design a more efficient CO2 reduction catalyst, understanding the reaction mechanism is crucial. Although it is challenging to experimentally capture and characterize the reactive intermediates, atomic modeling serves as an alternative for providing an understanding of the elementary reactions on a microscale. Significant progress has been made in understanding the reaction mechanism using multiscale simulations. In this study, important progress in revealing the reaction mechanism of CO2 reduction using computational simulation in recent years is summarized. First, the advances in simulation methods for electrochemical reactions are introduced, and the advantages and disadvantages of various methods are compared. Second, the detailed reaction mechanism of CO2 reduction to various major products, such as CO, CH4, and C2H4, and minor products, such as ethanol and acetate, are disused. Different results obtained from different approximations are compared, while a mechanism that can better explain the existing experimental results is recommended. Third, the operando technique, such as ambient pressure X-ray photoelectron spectroscopy, is disused. The operando analysis results are direct evidence to validate the theoretically proposed reaction pathway. In turn, the theoretical predictions can help resolve the experimental spectrum, which is usually too complex to refer to a reference system. The combination of theory and operando experiments should be one of the most promising directions in determining the reaction mechanism. Fourth, novel synthesis strategies are discussed. These new ideas are beneficial for simplifying the synthesis process or increasing the diversity of products. Finally, the recent progress in the application of machine learning to big data for CO2 reduction is discussed. These new powerful tools may play a crucial role in reaction mechanism studies. Overall, in the study of electrochemical reaction mechanism, theoretical simulation can provide the reaction details and energy information of elementary reactions at the atomic level. Therefore, in the study of electrochemical reaction mechanism of carbon dioxide, the microscopic mechanism that the experiment cannot provide is supplemented. On the one hand, it explains the existing experimental phenomena; however, on the other hand, it provides new insights for the study of reaction mechanism. On this basis, the use of new research paradigms, such as high-throughput computing and machine learning, provides new ideas for a rational design for accelerating material development.
2021, 37(5): 200608
doi: 10.3866/PKU.WHXB202006080
Abstract:
Industrial revolution has led to increased combustion of fossil fuels. Consequently, large amounts of CO2 are emitted to the atmosphere, throwing the carbon cycle out of balance. Currently, the most effective method to reduce the CO2 concentration is direct CO2 capture from the atmosphere and pumping of the captured CO2 deep underground or into the mid-ocean. The transformation of CO2 into high-value chemicals is an attractive yet challenging task. In recent years, there has been much interest in the development of CO2 utilization technologies based on electrochemical CO2 reduction, photochemical CO2 reduction, and thermal CO2 reduction, and CO2 valorization has emerged as a hot research topic. In electrochemical CO2 reduction, the cathodic reaction is the reduction of CO2 to value-added chemicals. The anodic reaction should be the oxygen evolution reaction, and water is the only renewable and scalable source of electrons and protons in this reaction. There is a plethora of research on the use of various metals to catalyze this reaction. Among these, Cu-based materials have been demonstrated to show unique catalytic activity and stability for the electrochemical conversion of CO2 to valuable fuels and chemicals. Moreover, the solar-driven conversion of CO2 into value-added chemical fuels has attracted great attention, and much effort is being devoted to develop novel catalysts for the photoreduction of CO2, especially by mimicking the natural photosynthetic process. The key step in the photocatalytic process is the efficient generation of electron-hole pairs and separation of these charge carriers. The efficient separation of photoinduced charge carriers plays a crucial role in the final catalytic activity. Compared with CO2 reduction via electrocatalysis and photocatalysis, thermal reduction is more attractive because of its potential large-scale application in the industry. Heterogeneous nanomaterials show excellent activity in the electrocatalytic, photocatalytic, and thermal catalytic conversion of CO2. However, nanostructured materials have drawbacks on the investigation of the intrinsic activity of the active sites. In recent years, single-site catalysts have become popular because they allow for maximum utilization of the metal centers, show specific catalytic performance, and facilitate easy elucidation of the catalytic mechanism at the molecular level. Accordingly, numerous single-site catalysts were developed for CO2 reduction to produce value-added chemicals such as CO, CH4, CH3OH, formate, and C2+ products. Value-added chemicals have also been synthesized with the aid of amines and epoxides. This review summarizes recent state-of-the-art single-site catalysts and their application as heterogeneous catalysts for the electroreduction, photoreduction, and thermal reduction of CO2. In the discussion, we will highlight the structure-activity relationships for the catalytic conversion of CO2 with single-site catalysts. ![]()
Industrial revolution has led to increased combustion of fossil fuels. Consequently, large amounts of CO2 are emitted to the atmosphere, throwing the carbon cycle out of balance. Currently, the most effective method to reduce the CO2 concentration is direct CO2 capture from the atmosphere and pumping of the captured CO2 deep underground or into the mid-ocean. The transformation of CO2 into high-value chemicals is an attractive yet challenging task. In recent years, there has been much interest in the development of CO2 utilization technologies based on electrochemical CO2 reduction, photochemical CO2 reduction, and thermal CO2 reduction, and CO2 valorization has emerged as a hot research topic. In electrochemical CO2 reduction, the cathodic reaction is the reduction of CO2 to value-added chemicals. The anodic reaction should be the oxygen evolution reaction, and water is the only renewable and scalable source of electrons and protons in this reaction. There is a plethora of research on the use of various metals to catalyze this reaction. Among these, Cu-based materials have been demonstrated to show unique catalytic activity and stability for the electrochemical conversion of CO2 to valuable fuels and chemicals. Moreover, the solar-driven conversion of CO2 into value-added chemical fuels has attracted great attention, and much effort is being devoted to develop novel catalysts for the photoreduction of CO2, especially by mimicking the natural photosynthetic process. The key step in the photocatalytic process is the efficient generation of electron-hole pairs and separation of these charge carriers. The efficient separation of photoinduced charge carriers plays a crucial role in the final catalytic activity. Compared with CO2 reduction via electrocatalysis and photocatalysis, thermal reduction is more attractive because of its potential large-scale application in the industry. Heterogeneous nanomaterials show excellent activity in the electrocatalytic, photocatalytic, and thermal catalytic conversion of CO2. However, nanostructured materials have drawbacks on the investigation of the intrinsic activity of the active sites. In recent years, single-site catalysts have become popular because they allow for maximum utilization of the metal centers, show specific catalytic performance, and facilitate easy elucidation of the catalytic mechanism at the molecular level. Accordingly, numerous single-site catalysts were developed for CO2 reduction to produce value-added chemicals such as CO, CH4, CH3OH, formate, and C2+ products. Value-added chemicals have also been synthesized with the aid of amines and epoxides. This review summarizes recent state-of-the-art single-site catalysts and their application as heterogeneous catalysts for the electroreduction, photoreduction, and thermal reduction of CO2. In the discussion, we will highlight the structure-activity relationships for the catalytic conversion of CO2 with single-site catalysts.
2021, 37(5): 200902
doi: 10.3866/PKU.WHXB202009021
Abstract:
The electrocatalytic CO2 reduction reaction (CO2RR) driven by renewable energy is an efficient approach to achieve the conversion and utilization of CO2. In this context, CO2RR has become an emerging research focus in the field of electrocatalysis over the past decade. While a large number of nanostructured catalysts have been developed to accelerate CO2RR, the tradeoff between activity and selectivity usually renders the overall electrocatalytic performance very poor. Beyond catalyst design, rationally designing electrolyzers is also of substantial importance for improving the CO2RR performance and achieving its scale-up for practical applications. To a large extent, the electrolyzer configuration determines the local reaction environment near an electrode by affecting the process conditions, thereby resulting in remarkably different electrocatalytic performances. To be techno-economically viable, the performance of CO2 electrolyzers is expected to be at least comparable to that of the current state-of-the-art proton exchange membrane (PEM) water electrolyzers, with regard to their activity, selectivity, and stability. Researchers have made great progress in the development of CO2 electrolyzers over the past few years, but they are also facing many issues and challenges. This review aims to provide an in-depth analysis of the research progress and status of current CO2 electrolyzers including H-cell, flow-cell, and membrane electrode assembly cell (MEA-cell) electrolyzers. Herein, operation at industrial current densities (> 200 mA∙cm−2) is set as a basis when these electrolyzers are discussed and compared in terms of the four main figures of merit (current density, Faradic efficiency, energy efficiency and stability) that describe the CO2RR performance of an electrolyzer. The advantages and drawbacks of each electrolyzer are discussed and highlighted with emphasis on the key achievements reported to date. Compared to conventional H-cell electrolyzers that work well in mechanistic studies, the newly developed electrolyzers using gas diffusion electrodes, both flow-cell and MEA-cell electrolyzers, are able to break the limitation of CO2 solubility in water and acquire industrial current densities. Although flow-cell electrolyzers have achieved current densities exceeding 1 A∙cm−2, they suffer from low energy efficiencies because of the significant iR drop and poor stability owing to the use of alkaline electrolytes. These issues can be overcome in the case of zero-gap MEA-cell electrolyzers with ion exchange membranes being as solid electrolytes. The anion exchange membrane (AEM)-based CO2 electrolyzers are at the center of the current research, as they demonstrate promising activity and selectivity toward specific CO2RR products and exhibit excellent stability for over thousands of hours in few cases. Meanwhile, the crossover of CO2 and liquid products from the cathode to the anode through the membrane tends to lower the utilization efficiency of the CO2 supplied to the AEM electrolyzers. MEA-cell electrolyzers using cation exchange membranes and bipolar membranes have also been explored; however, neither of them have shown satisfactory CO2RR performance. The development of new polymer electrolyte membranes and ionomers would help address these problems. While issues and challenges still exist, MEA-cell electrolyzers hold the greatest promise for practical applications. As concluding remarks, research strategies and opportunities for the future have been proposed to accelerate the development of CO2RR technology for practical applications and to deepen the mechanistic understanding behind improved performance. This review provides new insights into rational electrolyzer design and guidelines for researchers in this field. ![]()
The electrocatalytic CO2 reduction reaction (CO2RR) driven by renewable energy is an efficient approach to achieve the conversion and utilization of CO2. In this context, CO2RR has become an emerging research focus in the field of electrocatalysis over the past decade. While a large number of nanostructured catalysts have been developed to accelerate CO2RR, the tradeoff between activity and selectivity usually renders the overall electrocatalytic performance very poor. Beyond catalyst design, rationally designing electrolyzers is also of substantial importance for improving the CO2RR performance and achieving its scale-up for practical applications. To a large extent, the electrolyzer configuration determines the local reaction environment near an electrode by affecting the process conditions, thereby resulting in remarkably different electrocatalytic performances. To be techno-economically viable, the performance of CO2 electrolyzers is expected to be at least comparable to that of the current state-of-the-art proton exchange membrane (PEM) water electrolyzers, with regard to their activity, selectivity, and stability. Researchers have made great progress in the development of CO2 electrolyzers over the past few years, but they are also facing many issues and challenges. This review aims to provide an in-depth analysis of the research progress and status of current CO2 electrolyzers including H-cell, flow-cell, and membrane electrode assembly cell (MEA-cell) electrolyzers. Herein, operation at industrial current densities (> 200 mA∙cm−2) is set as a basis when these electrolyzers are discussed and compared in terms of the four main figures of merit (current density, Faradic efficiency, energy efficiency and stability) that describe the CO2RR performance of an electrolyzer. The advantages and drawbacks of each electrolyzer are discussed and highlighted with emphasis on the key achievements reported to date. Compared to conventional H-cell electrolyzers that work well in mechanistic studies, the newly developed electrolyzers using gas diffusion electrodes, both flow-cell and MEA-cell electrolyzers, are able to break the limitation of CO2 solubility in water and acquire industrial current densities. Although flow-cell electrolyzers have achieved current densities exceeding 1 A∙cm−2, they suffer from low energy efficiencies because of the significant iR drop and poor stability owing to the use of alkaline electrolytes. These issues can be overcome in the case of zero-gap MEA-cell electrolyzers with ion exchange membranes being as solid electrolytes. The anion exchange membrane (AEM)-based CO2 electrolyzers are at the center of the current research, as they demonstrate promising activity and selectivity toward specific CO2RR products and exhibit excellent stability for over thousands of hours in few cases. Meanwhile, the crossover of CO2 and liquid products from the cathode to the anode through the membrane tends to lower the utilization efficiency of the CO2 supplied to the AEM electrolyzers. MEA-cell electrolyzers using cation exchange membranes and bipolar membranes have also been explored; however, neither of them have shown satisfactory CO2RR performance. The development of new polymer electrolyte membranes and ionomers would help address these problems. While issues and challenges still exist, MEA-cell electrolyzers hold the greatest promise for practical applications. As concluding remarks, research strategies and opportunities for the future have been proposed to accelerate the development of CO2RR technology for practical applications and to deepen the mechanistic understanding behind improved performance. This review provides new insights into rational electrolyzer design and guidelines for researchers in this field.
2021, 37(5): 200708
doi: 10.3866/PKU.WHXB202007089
Abstract:
Cu/ZrO2 catalysts have demonstrated effective in hydrogenation of CO2 to methanol, during which the Cu-ZrO2 interface plays a key role. Thus, maximizing the number of Cu-ZrO2 interface active sites is an effective strategy to develop ideal catalysts. This can be achieved by controlling the active metal size and employing porous supports. Metal-organic frameworks (MOFs) are valid candidates because of their rich, open-framework structures and tunable compositions. UiO-66 is a rigid metal-organic skeleton material with excellent hydrothermal and chemical stability that comprises Zr as the metal center and terephthalic acid (H2BDC) as the organic ligand. Herein, porous UiO-66 was chosen as the ZrO2 precursor, which can confine Cu nanoparticles within its pores/defects. As a result, we constructed a Cu-ZrO2 nanocomposite catalyst with high activity for CO2 hydrogenation to methanol. Many active interfaces could form when the catalysts were calcined at a moderate temperature, and the active interface was optimized by adjusting the calcination temperature and active metal size. Furthermore, the Cu-ZrO2 interface remained after CO2 hydrogenation to methanol, as confirmed by transmission electron microscopy (TEM), demonstrating the stability of the active interface. The catalyst structure and hydrogenation activity were influenced by the content of the active component and the calcination temperature; therefore, these parameters were explored to obtain an optimized catalyst. At 280 ℃ and 4.5 MPa, the optimized CZ-0.5-400 catalyst gave the highest methanol turnover frequency (TOF) of 13.4 h-1 with a methanol space-time yield (STY) of 587.8 g·kg-1·h-1 (calculated per kilogram of catalyst, the same below), a CO2 conversion of 12.6%, and a methanol selectivity of 62.4%. In situ diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS) of CO adsorption over the optimized catalyst revealed a predominant, unreducible Cu+ species that was also identified by X-ray photoelectron spectroscopy (XPS). The favorable activity observed was due to this abundant Cu+ species coming from the Cu+-ZrO2 interface that served as the methanol synthesis active center and acted as a bridge for transporting hydrogen from the active Cu species to ZrO2. In addition, the oxygen vacancies of ZrO2 promoted the adsorption and activation of CO2. These vacancies and Cu+ trapped in the ZrO2 lattice are the active sites for methanol synthesis from CO2 hydrogenation. The X-ray diffraction (XRD) patterns of the catalyst before and after reaction revealed the stability of its structure, which was further verified by time-on-stream (TOS) tests. Furthermore, in situ DRIFTS and temperature-programmed surface reaction-mass spectroscopy (TPSR-MS) revealed the reaction mechanism of CO2 hydrogenation to methanol, which followed an HCOO-intermediated pathway. ![]()
Cu/ZrO2 catalysts have demonstrated effective in hydrogenation of CO2 to methanol, during which the Cu-ZrO2 interface plays a key role. Thus, maximizing the number of Cu-ZrO2 interface active sites is an effective strategy to develop ideal catalysts. This can be achieved by controlling the active metal size and employing porous supports. Metal-organic frameworks (MOFs) are valid candidates because of their rich, open-framework structures and tunable compositions. UiO-66 is a rigid metal-organic skeleton material with excellent hydrothermal and chemical stability that comprises Zr as the metal center and terephthalic acid (H2BDC) as the organic ligand. Herein, porous UiO-66 was chosen as the ZrO2 precursor, which can confine Cu nanoparticles within its pores/defects. As a result, we constructed a Cu-ZrO2 nanocomposite catalyst with high activity for CO2 hydrogenation to methanol. Many active interfaces could form when the catalysts were calcined at a moderate temperature, and the active interface was optimized by adjusting the calcination temperature and active metal size. Furthermore, the Cu-ZrO2 interface remained after CO2 hydrogenation to methanol, as confirmed by transmission electron microscopy (TEM), demonstrating the stability of the active interface. The catalyst structure and hydrogenation activity were influenced by the content of the active component and the calcination temperature; therefore, these parameters were explored to obtain an optimized catalyst. At 280 ℃ and 4.5 MPa, the optimized CZ-0.5-400 catalyst gave the highest methanol turnover frequency (TOF) of 13.4 h-1 with a methanol space-time yield (STY) of 587.8 g·kg-1·h-1 (calculated per kilogram of catalyst, the same below), a CO2 conversion of 12.6%, and a methanol selectivity of 62.4%. In situ diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS) of CO adsorption over the optimized catalyst revealed a predominant, unreducible Cu+ species that was also identified by X-ray photoelectron spectroscopy (XPS). The favorable activity observed was due to this abundant Cu+ species coming from the Cu+-ZrO2 interface that served as the methanol synthesis active center and acted as a bridge for transporting hydrogen from the active Cu species to ZrO2. In addition, the oxygen vacancies of ZrO2 promoted the adsorption and activation of CO2. These vacancies and Cu+ trapped in the ZrO2 lattice are the active sites for methanol synthesis from CO2 hydrogenation. The X-ray diffraction (XRD) patterns of the catalyst before and after reaction revealed the stability of its structure, which was further verified by time-on-stream (TOS) tests. Furthermore, in situ DRIFTS and temperature-programmed surface reaction-mass spectroscopy (TPSR-MS) revealed the reaction mechanism of CO2 hydrogenation to methanol, which followed an HCOO-intermediated pathway.
2021, 37(5): 200806
doi: 10.3866/PKU.WHXB202008066
Abstract:
Establishing a reliable method to predict the global mean temperature (Te) is of great importance because CO2 reduction activities require political and global cooperation and significant financial resources. The current climate models all seem to predict that the earth's temperature will continue to increase, mainly based on the assumption that CO2 emissions cannot be lowered significantly in the foreseeable future. Given the earth's multifactor climate system, attributing atmospheric CO2 as the only cause for the observed temperature anomaly is most likely an oversimplification; the presence of water (H2O) in the atmosphere should at least be considered. As such, Te is determined by atmospheric water content controlled by solar activity, along with anthropogenic CO2 activities. It is possible that the anthropogenic CO2 activities can be reduced in the future. Based on temperature measurements and thermodynamic data, a new model for predicting Te has been developed. Using this model, past, current, and future CO2 and H2O data can be analyzed and the associated Te calculated. This new, esoteric approach is more accurate than various other models, but has not been reported in the open literature. According to this model, by 2050, Te may increase to 15.5 ℃ under "business-as-usual" emissions. By applying a reasonable green technology activity scenario, Te may be reduced to approximately 14.2 ℃. To achieve CO2 reductions, the scenario described herein predicts a CO2 reduction potential of 513 gigatons in 30 years. This proposed scenario includes various CO2 reduction activities, carbon capturing technology, mineralization, and bio-char production; the most important CO2 reductions by 2050 are expected to be achieved mainly in the electricity, agriculture, and transportation sectors. Other more aggressive and plausible drawdown scenarios have been analyzed as well, yielding CO2 reduction potentials of 1051 and 1747 gigatons, respectively, in 30 years, but they may reduce global food production. It is emphasized that the causes and predictions of the global warming trend should be regarded as open scientific questions because several details concerning the physical processes associated with global warming remain uncertain. For example, the role of solar activities coupled with Milankovitch cycles are not yet fully understood. In addition, other factors, such as ocean CO2 uptake and volcanic activity, may not be negligible. ![]()
Establishing a reliable method to predict the global mean temperature (Te) is of great importance because CO2 reduction activities require political and global cooperation and significant financial resources. The current climate models all seem to predict that the earth's temperature will continue to increase, mainly based on the assumption that CO2 emissions cannot be lowered significantly in the foreseeable future. Given the earth's multifactor climate system, attributing atmospheric CO2 as the only cause for the observed temperature anomaly is most likely an oversimplification; the presence of water (H2O) in the atmosphere should at least be considered. As such, Te is determined by atmospheric water content controlled by solar activity, along with anthropogenic CO2 activities. It is possible that the anthropogenic CO2 activities can be reduced in the future. Based on temperature measurements and thermodynamic data, a new model for predicting Te has been developed. Using this model, past, current, and future CO2 and H2O data can be analyzed and the associated Te calculated. This new, esoteric approach is more accurate than various other models, but has not been reported in the open literature. According to this model, by 2050, Te may increase to 15.5 ℃ under "business-as-usual" emissions. By applying a reasonable green technology activity scenario, Te may be reduced to approximately 14.2 ℃. To achieve CO2 reductions, the scenario described herein predicts a CO2 reduction potential of 513 gigatons in 30 years. This proposed scenario includes various CO2 reduction activities, carbon capturing technology, mineralization, and bio-char production; the most important CO2 reductions by 2050 are expected to be achieved mainly in the electricity, agriculture, and transportation sectors. Other more aggressive and plausible drawdown scenarios have been analyzed as well, yielding CO2 reduction potentials of 1051 and 1747 gigatons, respectively, in 30 years, but they may reduce global food production. It is emphasized that the causes and predictions of the global warming trend should be regarded as open scientific questions because several details concerning the physical processes associated with global warming remain uncertain. For example, the role of solar activities coupled with Milankovitch cycles are not yet fully understood. In addition, other factors, such as ocean CO2 uptake and volcanic activity, may not be negligible.
2021, 37(5): 200902
doi: 10.3866/PKU.WHXB202009023
Abstract:
Fossil fuels are expected to be the major source of energy for the next few decades. However, combustion of nonrenewable resources leads to the release of large quantities of CO2, the primary greenhouse gas. Notably, the concentration of CO2 in the atmosphere is increasing annually at an astounding rate. Electrochemical CO2 reduction (ECR) to value-added fuels and chemicals using electricity from intermittent renewable energy sources is a carbon-neutral method to alleviate anthropogenic CO2 emissions. Despite the steady progress in the selective generation of C1 products (CO and formic acid), the production of multi-carbon species still suffers from low selectivity and efficiency. As an ECR product, ethylene (C2H4) has a higher energy density than do C1 species and is an important industrial feedstock in high demand. However, the conversion of CO2 to C2H4 is plagued by low productivity and large overpotential, in addition to the severe competing hydrogen evolution reaction (HER) during the ECR. To address these issues, the design and development of advanced electrocatalysts are critical. Here, we demonstrate fine-tuning of ECR to C2H4 by taking advantage of the prominent interaction of Cu with shape-controlled CeO2 nanocrystals, that is, cubes, rods, and octahedra predominantly covered with (100), (110), and (111) surfaces, respectively. We found that the selectivity and activity of the ECR depended strongly on the exposed crystal facets of CeO2. The overall ECR Faradaic efficiency (FE) over Cu/CeO2(110) (FE ≈ 56.7%) surpassed that of both Cu/CeO2(100) (FE ≈ 51.5%) and Cu/CeO2(111) (FE ≈ 48.4%) in 0.1 mol·L-1 KHCO3 solutions with an H-type cell. This was in stark contrast to the exclusive occurrence of the HER over pure carbon paper, CeO2(100), CeO2(110), and CeO2(111). In particular, the FE toward C2H4 formation and the partial current density increased in the sequence Cu/CeO2(111) < Cu/CeO2(100) < Cu/CeO2(110) within applied bias potentials from -1.00 to -1.15 V (vs. the reversible hydrogen electrode), reaching 39.1% over Cu/CeO2(110) at a mild overpotential (1.13 V). The corresponding values for Cu/CeO2(100) and Cu/CeO2(111) were FEC2H4 ≈ 31.8% and FEC2H4 ≈ 29.6%, respectively. The C2H4 selectivity was comparable to that of many reported Cu-based electrocatalysts at similar overpotentials. Furthermore, the FE for C2H4 remained stable even after 6 h of continuous electrolysis. The superior ECR activity of Cu/CeO2(110) to yield C2H4 was attributed to the metastable (110) surface, which not only promoted the effective adsorption of CO2 but also remarkably stabilized Cu+, thereby boosting the ECR to produce C2H4. This work offers an alternative strategy to enhance the ECR efficiency by crystal facet engineering. ![]()
Fossil fuels are expected to be the major source of energy for the next few decades. However, combustion of nonrenewable resources leads to the release of large quantities of CO2, the primary greenhouse gas. Notably, the concentration of CO2 in the atmosphere is increasing annually at an astounding rate. Electrochemical CO2 reduction (ECR) to value-added fuels and chemicals using electricity from intermittent renewable energy sources is a carbon-neutral method to alleviate anthropogenic CO2 emissions. Despite the steady progress in the selective generation of C1 products (CO and formic acid), the production of multi-carbon species still suffers from low selectivity and efficiency. As an ECR product, ethylene (C2H4) has a higher energy density than do C1 species and is an important industrial feedstock in high demand. However, the conversion of CO2 to C2H4 is plagued by low productivity and large overpotential, in addition to the severe competing hydrogen evolution reaction (HER) during the ECR. To address these issues, the design and development of advanced electrocatalysts are critical. Here, we demonstrate fine-tuning of ECR to C2H4 by taking advantage of the prominent interaction of Cu with shape-controlled CeO2 nanocrystals, that is, cubes, rods, and octahedra predominantly covered with (100), (110), and (111) surfaces, respectively. We found that the selectivity and activity of the ECR depended strongly on the exposed crystal facets of CeO2. The overall ECR Faradaic efficiency (FE) over Cu/CeO2(110) (FE ≈ 56.7%) surpassed that of both Cu/CeO2(100) (FE ≈ 51.5%) and Cu/CeO2(111) (FE ≈ 48.4%) in 0.1 mol·L-1 KHCO3 solutions with an H-type cell. This was in stark contrast to the exclusive occurrence of the HER over pure carbon paper, CeO2(100), CeO2(110), and CeO2(111). In particular, the FE toward C2H4 formation and the partial current density increased in the sequence Cu/CeO2(111) < Cu/CeO2(100) < Cu/CeO2(110) within applied bias potentials from -1.00 to -1.15 V (vs. the reversible hydrogen electrode), reaching 39.1% over Cu/CeO2(110) at a mild overpotential (1.13 V). The corresponding values for Cu/CeO2(100) and Cu/CeO2(111) were FEC2H4 ≈ 31.8% and FEC2H4 ≈ 29.6%, respectively. The C2H4 selectivity was comparable to that of many reported Cu-based electrocatalysts at similar overpotentials. Furthermore, the FE for C2H4 remained stable even after 6 h of continuous electrolysis. The superior ECR activity of Cu/CeO2(110) to yield C2H4 was attributed to the metastable (110) surface, which not only promoted the effective adsorption of CO2 but also remarkably stabilized Cu+, thereby boosting the ECR to produce C2H4. This work offers an alternative strategy to enhance the ECR efficiency by crystal facet engineering.
2021, 37(5): 200910
doi: 10.3866/PKU.WHXB202009101
Abstract:
Using renewable green hydrogen and carbon dioxide (CO2) to produce methanol is one of the fundamental ways to reduce CO2 emissions in the future, and research and development related to catalysts for efficient and stable methanol synthesis is one of the key factors in determining the entire synthesis process. Metal nanoparticles stabilized on a support are frequently employed to catalyze the methanol synthesis reaction. Metal-support interactions (MSIs) in these supported catalysts can play a significant role in catalysis. Tuning the MSI is an effective strategy to modulate the activity, selectivity, and stability of heterogeneous catalysts. Numerous studies have been conducted on this topic; however, a systematic understanding of the role of various strengths of MSI is lacking. Herein, three Cu/ZnO-SiO2 catalysts with different strengths of MSI, namely, normal precipitation Cu/ZnO-SiO2 (Nor-CZS), co-precipitation Cu/ZnO-SiO2 (Co-CZS), and reverse precipitation Cu/ZnO-SiO2 (Re-CZS), were successfully prepared to determine the role of such interactions in the hydrogenation of CO2 to methanol. The results of temperature-programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS) characterization illustrated that the MSI of the catalysts was considerably affected by the precipitation sequence. Fourier transform infrared reflection spectroscopy (FT-IR) results indicated that the Cu species existed as CuO in all cases and that copper phyllosilicate was absent (except for strong Cu-SiO2 interaction). Transmission electron microscopy (TEM), X-ray diffraction (XRD), and N2O chemical titration results revealed that strong interactions between the Cu and Zn species would promote the dispersion of Cu species, thereby leading to a higher CO2 conversion rate and improved catalytic stability. As expected, the Re-CZS catalyst exhibited the highest activity with 12.4% CO2 conversion, followed by the Co-CZS catalyst (12.1%), and the Nor-CZS catalyst (9.8%). After the same reaction time, the normalized CO2 conversion of the three catalysts decreased in the following order: Re-CZS (75%) > Co-CZS (70%) > Nor-CZS (65%). Notably, the methanol selectivity of the Re-CZS catalyst was found to level off after a prolonged period, in contrast to that of Co-CZS and Nor-CZS. Investigation of the structural evolution of the catalyst with time on stream revealed that the high methanol selectivity of the catalyst was caused by the reconstruction of the catalyst, which was induced by the strong MSI between the Cu and Zn species, and the migration of ZnO onto Cu species, which caused an enlargement of the Cu/ZnO interface. This work offers an alternative strategy for the rational and optimized design of efficient catalysts. ![]()
Using renewable green hydrogen and carbon dioxide (CO2) to produce methanol is one of the fundamental ways to reduce CO2 emissions in the future, and research and development related to catalysts for efficient and stable methanol synthesis is one of the key factors in determining the entire synthesis process. Metal nanoparticles stabilized on a support are frequently employed to catalyze the methanol synthesis reaction. Metal-support interactions (MSIs) in these supported catalysts can play a significant role in catalysis. Tuning the MSI is an effective strategy to modulate the activity, selectivity, and stability of heterogeneous catalysts. Numerous studies have been conducted on this topic; however, a systematic understanding of the role of various strengths of MSI is lacking. Herein, three Cu/ZnO-SiO2 catalysts with different strengths of MSI, namely, normal precipitation Cu/ZnO-SiO2 (Nor-CZS), co-precipitation Cu/ZnO-SiO2 (Co-CZS), and reverse precipitation Cu/ZnO-SiO2 (Re-CZS), were successfully prepared to determine the role of such interactions in the hydrogenation of CO2 to methanol. The results of temperature-programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS) characterization illustrated that the MSI of the catalysts was considerably affected by the precipitation sequence. Fourier transform infrared reflection spectroscopy (FT-IR) results indicated that the Cu species existed as CuO in all cases and that copper phyllosilicate was absent (except for strong Cu-SiO2 interaction). Transmission electron microscopy (TEM), X-ray diffraction (XRD), and N2O chemical titration results revealed that strong interactions between the Cu and Zn species would promote the dispersion of Cu species, thereby leading to a higher CO2 conversion rate and improved catalytic stability. As expected, the Re-CZS catalyst exhibited the highest activity with 12.4% CO2 conversion, followed by the Co-CZS catalyst (12.1%), and the Nor-CZS catalyst (9.8%). After the same reaction time, the normalized CO2 conversion of the three catalysts decreased in the following order: Re-CZS (75%) > Co-CZS (70%) > Nor-CZS (65%). Notably, the methanol selectivity of the Re-CZS catalyst was found to level off after a prolonged period, in contrast to that of Co-CZS and Nor-CZS. Investigation of the structural evolution of the catalyst with time on stream revealed that the high methanol selectivity of the catalyst was caused by the reconstruction of the catalyst, which was induced by the strong MSI between the Cu and Zn species, and the migration of ZnO onto Cu species, which caused an enlargement of the Cu/ZnO interface. This work offers an alternative strategy for the rational and optimized design of efficient catalysts.
2021, 37(5): 201002
doi: 10.3866/PKU.WHXB202010020
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Abstract:
