Login In
2023 Volume 39 Issue 9
2023, 39(9): 221003
doi: 10.3866/PKU.WHXB202210038
Abstract:
Carbonyl sulfide (COS) is commonly found in conventional fossil fuels, such as nature gas, oil-associated gas, and blast-furnace gas, and its untreated emission not only corrodes pipelines and poisons catalysts but will also inevitably pollute the environment and endanger human health. Catalytic hydrolysis is recognized as the most promising strategy to eliminate COS because it can be performed under mild reaction conditions with a high removal efficiency. Notably, alkali metals promote catalytic COS hydrolysis over Al2O3 owing to their electron donor properties, basicity, and electrostatic adsorption. However, despite the significant attraction of using potassium-promoted Al2O3 (K2CO3/Al2O3) as conventional catalysts for COS hydrolysis, the mechanism of COS hydrolysis over K2CO3/Al2O3 remains unclear and is controversial owing to the complex composition of the K species. In this study, commercial Al2O3 modified with potassium and sodium salts were synthesized using the wet impregnation method and characterized by various techniques. Based on the results of the activity measurements, the K2CO3-, K2C2O4-, NaHCO3-, Na2CO3-, and NaC2O4-modified catalysts had a positive effect on COS hydrolysis. Among them, the K2CO3/Al2O3 catalyst exhibited the highest COS conversion. Notably, the K2CO3/Al2O3 catalyst exhibited an excellent catalytic performance (~93%, 20 h), which is significantly better than that of pristine Al2O3 (~58%). Furthermore, this study provides strong evidence for the role of H2O during catalytic hydrolysis over K2CO3/Al2O3 using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and X-ray photoelectron spectroscopy (XPS). The in situ DRIFTS analysis revealed that hydrogen thiocarbonate formed as an intermediate during COS hydrolysis over K2CO3/Al2O3. Meanwhile, the XPS findings suggested that sulfates and elemental sulfur accumulated on the catalyst surface, which may have contributed to catalyst poisoning. Additionally, the effect of water vapor content in the reaction pathway of COS hydrolysis over K2CO3/Al2O3 was investigated. The presence of excess water resulted in a reduction in catalytic activity owing to competitive adsorption between H2O and COS molecules on the catalyst surface. The enhancement in the catalytic activity over K2CO3/Al2O3 may be attributed to the formation of HO-Al-O-K interfacial sites. More importantly, all the catalysts were used under industrially relevant conditions, which provides valuable theoretical guidance for practical applications in the future. Thus, this detailed mechanistic study reveals new insights into the roles of the interfacial K co-catalyst, which provides a new opportunity for the rational design of stable and efficient catalysts for COS hydrolysis. ![]()
Carbonyl sulfide (COS) is commonly found in conventional fossil fuels, such as nature gas, oil-associated gas, and blast-furnace gas, and its untreated emission not only corrodes pipelines and poisons catalysts but will also inevitably pollute the environment and endanger human health. Catalytic hydrolysis is recognized as the most promising strategy to eliminate COS because it can be performed under mild reaction conditions with a high removal efficiency. Notably, alkali metals promote catalytic COS hydrolysis over Al2O3 owing to their electron donor properties, basicity, and electrostatic adsorption. However, despite the significant attraction of using potassium-promoted Al2O3 (K2CO3/Al2O3) as conventional catalysts for COS hydrolysis, the mechanism of COS hydrolysis over K2CO3/Al2O3 remains unclear and is controversial owing to the complex composition of the K species. In this study, commercial Al2O3 modified with potassium and sodium salts were synthesized using the wet impregnation method and characterized by various techniques. Based on the results of the activity measurements, the K2CO3-, K2C2O4-, NaHCO3-, Na2CO3-, and NaC2O4-modified catalysts had a positive effect on COS hydrolysis. Among them, the K2CO3/Al2O3 catalyst exhibited the highest COS conversion. Notably, the K2CO3/Al2O3 catalyst exhibited an excellent catalytic performance (~93%, 20 h), which is significantly better than that of pristine Al2O3 (~58%). Furthermore, this study provides strong evidence for the role of H2O during catalytic hydrolysis over K2CO3/Al2O3 using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and X-ray photoelectron spectroscopy (XPS). The in situ DRIFTS analysis revealed that hydrogen thiocarbonate formed as an intermediate during COS hydrolysis over K2CO3/Al2O3. Meanwhile, the XPS findings suggested that sulfates and elemental sulfur accumulated on the catalyst surface, which may have contributed to catalyst poisoning. Additionally, the effect of water vapor content in the reaction pathway of COS hydrolysis over K2CO3/Al2O3 was investigated. The presence of excess water resulted in a reduction in catalytic activity owing to competitive adsorption between H2O and COS molecules on the catalyst surface. The enhancement in the catalytic activity over K2CO3/Al2O3 may be attributed to the formation of HO-Al-O-K interfacial sites. More importantly, all the catalysts were used under industrially relevant conditions, which provides valuable theoretical guidance for practical applications in the future. Thus, this detailed mechanistic study reveals new insights into the roles of the interfacial K co-catalyst, which provides a new opportunity for the rational design of stable and efficient catalysts for COS hydrolysis.
2023, 39(9): 221200
doi: 10.3866/PKU.WHXB202212003
Abstract:
Currently, there is an urgent need to develop an efficient, non-toxic, and stable catalyst for the removal of NOx via selective catalytic reduction using NH3 (NH3-SCR) that is effective at low temperatures. Mn-based catalysts are particularly representative and have been widely studied. An investigation of the collaborative participation of Mn and Co can be of great importance for improving the catalytic activity and SO2 resistance of Mn-Co oxides with a spinel structure. Therefore, in this study, we prepared MnxCo3−xO4 spherical particles with high surface area using a co-precipitation method and investigated their ability to remove NOx via NH3-SCR. Mn-Co bimetal oxides mainly possess a spinel structure and undergo a tetragonal-to-cubic phase transformation with increasing Co-content. A high concentration of surface oxygen and strong effective electron transfer between the variable valence elements (Co3+ + Mn3+ ↔ Co2+ + Mn4+) improves the redox ability of typical MnxCo3−xO4 (x = 1.0, 1.5, 2.0) spinel catalysts. In addition, Mn-enrichment leads to more oxygen vacancies and abundant surface-active sites, which further promotes the SCR catalytic performance. The investigated MnxCo3−xO4 catalysts exhibit > 91% NOx conversion at 75 ℃, almost reaching 100% conversion with increasing reaction temperature. Notably, the NOx conversion rate remained above 80% during the test time of 15 h under 150 × 10−6 SO2 at 175 ℃. It was found that the coordination structure likely formed into a Cotet(CoMn)octO4 spinel structure in which Mn ions (Mn3+ and Mn4+, mainly in trivalent manganese) and partial Co ions are configured into octahedral sites. These species were identified as the activity descriptor for probably owing to their strong electronic transfer interactions that were directly correlated with SCR activity. Furthermore, the Cotet(CoMn)octO4 configuration was important for promoting low-temperature de-NOx activity and highly conducive to protecting Mn active sites from poisoning by SO2. The active sites in this particular spinel structure with the micro-coordination structure were effectively built and maintained to ensure the smooth circulation of electronic interactions in the core octahedron. The reaction of adsorbed NH3 and gaseous NO (or NO2) mainly occurred on the surface of Mn-Co spinel following the Eley-Rideal mechanism. Additionally, the NH4NO3 intermediate was likely first transformed into NH4NO2 and then to N2 with increasing reaction temperature. Herein, we successfully synthesized a spinel-structured Mn-Co oxide catalyst comprising a Mn-enriched surface of (MnCo)3O4−ƞ spinel oxides that exhibited high NH3-SCR catalytic activity and good resistance to SO2 poisoning. ![]()
Currently, there is an urgent need to develop an efficient, non-toxic, and stable catalyst for the removal of NOx via selective catalytic reduction using NH3 (NH3-SCR) that is effective at low temperatures. Mn-based catalysts are particularly representative and have been widely studied. An investigation of the collaborative participation of Mn and Co can be of great importance for improving the catalytic activity and SO2 resistance of Mn-Co oxides with a spinel structure. Therefore, in this study, we prepared MnxCo3−xO4 spherical particles with high surface area using a co-precipitation method and investigated their ability to remove NOx via NH3-SCR. Mn-Co bimetal oxides mainly possess a spinel structure and undergo a tetragonal-to-cubic phase transformation with increasing Co-content. A high concentration of surface oxygen and strong effective electron transfer between the variable valence elements (Co3+ + Mn3+ ↔ Co2+ + Mn4+) improves the redox ability of typical MnxCo3−xO4 (x = 1.0, 1.5, 2.0) spinel catalysts. In addition, Mn-enrichment leads to more oxygen vacancies and abundant surface-active sites, which further promotes the SCR catalytic performance. The investigated MnxCo3−xO4 catalysts exhibit > 91% NOx conversion at 75 ℃, almost reaching 100% conversion with increasing reaction temperature. Notably, the NOx conversion rate remained above 80% during the test time of 15 h under 150 × 10−6 SO2 at 175 ℃. It was found that the coordination structure likely formed into a Cotet(CoMn)octO4 spinel structure in which Mn ions (Mn3+ and Mn4+, mainly in trivalent manganese) and partial Co ions are configured into octahedral sites. These species were identified as the activity descriptor for probably owing to their strong electronic transfer interactions that were directly correlated with SCR activity. Furthermore, the Cotet(CoMn)octO4 configuration was important for promoting low-temperature de-NOx activity and highly conducive to protecting Mn active sites from poisoning by SO2. The active sites in this particular spinel structure with the micro-coordination structure were effectively built and maintained to ensure the smooth circulation of electronic interactions in the core octahedron. The reaction of adsorbed NH3 and gaseous NO (or NO2) mainly occurred on the surface of Mn-Co spinel following the Eley-Rideal mechanism. Additionally, the NH4NO3 intermediate was likely first transformed into NH4NO2 and then to N2 with increasing reaction temperature. Herein, we successfully synthesized a spinel-structured Mn-Co oxide catalyst comprising a Mn-enriched surface of (MnCo)3O4−ƞ spinel oxides that exhibited high NH3-SCR catalytic activity and good resistance to SO2 poisoning.
2023, 39(9): 221202
doi: 10.3866/PKU.WHXB202212025
Abstract:
The oxygen evolution reaction (OER) is considered the rate-limiting step in electrochemical water splitting, and has been widely used to solve energy and environmental issues. Perovskite oxides (ABO3) exhibit good OER activity, owing to their tunable electronic structures and highly flexible elemental compositions. However, the preparation of perovskite oxides usually requires long exposure to high temperatures, resulting in metal agglomeration and undesirable effects on intrinsic activity. Vapor-phase microwave technology can significantly reduce the duration of heat treatment and subsequently reduce the associated carbon emissions. This technology not only addresses the growing demand for carbon-neutral processes but also enables increased control of the synthesis to avoid undesirable agglomeration of the product. In this study, a 2D porous La0.2Sr0.8CoO3 perovskite was rapidly prepared using a microwave shock method. The rapid entropy increase associated with the microwave process can effectively expose abundant active sites in the La0.2Sr0.8CoO3 structure. Furthermore, the high-energy microwave shock process can precisely introduce Sr2+ into the lattice of LaCoO3, increasing the number of oxygen vacancies by increasing the oxidation state of Co. The oxygen vacancies introduced by replacing La with Sr can considerably improve the intrinsic catalytic activity of the material. For the OER in alkaline electrolytes, the prepared La0.2Sr0.8CoO3 catalyst displayed an excellent overpotential of 360 mV at 10 mA·cm−2 and a Tafel slope of 76.6 mV·dec−1. After a long-term cycle test of 30000 s, 97% of the initial current density was maintained. This study presents a facile and rapid strategy for the synthesis of highly active 2D perovskites. ![]()
The oxygen evolution reaction (OER) is considered the rate-limiting step in electrochemical water splitting, and has been widely used to solve energy and environmental issues. Perovskite oxides (ABO3) exhibit good OER activity, owing to their tunable electronic structures and highly flexible elemental compositions. However, the preparation of perovskite oxides usually requires long exposure to high temperatures, resulting in metal agglomeration and undesirable effects on intrinsic activity. Vapor-phase microwave technology can significantly reduce the duration of heat treatment and subsequently reduce the associated carbon emissions. This technology not only addresses the growing demand for carbon-neutral processes but also enables increased control of the synthesis to avoid undesirable agglomeration of the product. In this study, a 2D porous La0.2Sr0.8CoO3 perovskite was rapidly prepared using a microwave shock method. The rapid entropy increase associated with the microwave process can effectively expose abundant active sites in the La0.2Sr0.8CoO3 structure. Furthermore, the high-energy microwave shock process can precisely introduce Sr2+ into the lattice of LaCoO3, increasing the number of oxygen vacancies by increasing the oxidation state of Co. The oxygen vacancies introduced by replacing La with Sr can considerably improve the intrinsic catalytic activity of the material. For the OER in alkaline electrolytes, the prepared La0.2Sr0.8CoO3 catalyst displayed an excellent overpotential of 360 mV at 10 mA·cm−2 and a Tafel slope of 76.6 mV·dec−1. After a long-term cycle test of 30000 s, 97% of the initial current density was maintained. This study presents a facile and rapid strategy for the synthesis of highly active 2D perovskites.
2023, 39(9): 230100
doi: 10.3866/PKU.WHXB202301005
Abstract:
Hydrogen energy is a potential energy storage carrier due to its advantages of cleanliness, high efficiency and renewability. Electrocatalytic water splitting is an ideal method to generate hydrogen, and the slow kinetics of water oxidation, namely, oxygen evolution reaction (OER), greatly restricts its practical application. To reduce the energy consumption required for OER, methanol oxidation reaction (MOR) with a much lower theoretical potential is very promising to replace OER to assist hydrogen generation. The theoretical potential of MOR is only 0.016 V vs. SHE (standard hydrogen electrode), which is much lower than that of OER (1.23 V), and the energy-saving can be about 60% compared to that of traditional water electrolysis. Therefore, using MOR instead of OER to realize methanol electrolysis for hydrogen production is an effective way to reduce energy consumption. An efficient bifunctional catalyst is very important for green hydrogen generation from overall methanol electrolysis. Currently, Pt-based materials are still the best catalyst for hydrogen evolution reaction (HER) and MOR, while they are more challenging in the MOR as they are prone to intermediates poisoning during the catalytic reactions. The introduction of transition metal-based promoters such as phosphides is an effective strategy to promote the catalytic ability for methanol oxidation. Herein, ultrafine Pt nanoparticles with an average particle size of 2.53 nm evenly grown on MoP-NC nanosphere (Pt/MoP-NC) were demonstrated as an efficient electrocatalyst for methanol electrolysis towards hydrogen generation. The introduction of MoP-NC nanospheres support not only restricts the aggregation of Pt, but also improves the catalytic performance and anti-poisoning ability. Specifically, Pt/MoP-NC catalyst exhibited high methanol oxidation performance with a peak current density of 90.7 mA∙cm−2, which was 3.2 times higher than that of commercial Pt/C catalysts, and good hydrogen evolution reaction performance with a low overpotential of 30 mV to offer 10 mA∙cm−2 in an acid medium, which was comparable to commercial Pt/C. The assembled Pt/MoP-NC||Pt/MoP-NC electrolyzer showed a cell voltage of 0.67 V at 10 mA∙cm−2, ca. 1.02 V less than that of the overall water splitting system (1.69 V). The high catalytic ability of Pt/MoP-NC originated from the electronic effect between noble metal active center Pt and the adjacent MoP-NC support with a unique layered porous spherical structure. The partial electron transfer from MoP to Pt can lower the d-energy band center of Pt, which weakened the binding energy between Pt and adsorbed toxic intermediates. In addition, the oxophilic MoP-NC nanospheres can activate water to provide more hydroxyl species and facilitate the oxidative removal of CO intermediates adsorbed on the Pt active sites. The current work might inspire the design and preparation of novel catalyst platforms for methanol electrolysis in hydrogen generation. ![]()
Hydrogen energy is a potential energy storage carrier due to its advantages of cleanliness, high efficiency and renewability. Electrocatalytic water splitting is an ideal method to generate hydrogen, and the slow kinetics of water oxidation, namely, oxygen evolution reaction (OER), greatly restricts its practical application. To reduce the energy consumption required for OER, methanol oxidation reaction (MOR) with a much lower theoretical potential is very promising to replace OER to assist hydrogen generation. The theoretical potential of MOR is only 0.016 V vs. SHE (standard hydrogen electrode), which is much lower than that of OER (1.23 V), and the energy-saving can be about 60% compared to that of traditional water electrolysis. Therefore, using MOR instead of OER to realize methanol electrolysis for hydrogen production is an effective way to reduce energy consumption. An efficient bifunctional catalyst is very important for green hydrogen generation from overall methanol electrolysis. Currently, Pt-based materials are still the best catalyst for hydrogen evolution reaction (HER) and MOR, while they are more challenging in the MOR as they are prone to intermediates poisoning during the catalytic reactions. The introduction of transition metal-based promoters such as phosphides is an effective strategy to promote the catalytic ability for methanol oxidation. Herein, ultrafine Pt nanoparticles with an average particle size of 2.53 nm evenly grown on MoP-NC nanosphere (Pt/MoP-NC) were demonstrated as an efficient electrocatalyst for methanol electrolysis towards hydrogen generation. The introduction of MoP-NC nanospheres support not only restricts the aggregation of Pt, but also improves the catalytic performance and anti-poisoning ability. Specifically, Pt/MoP-NC catalyst exhibited high methanol oxidation performance with a peak current density of 90.7 mA∙cm−2, which was 3.2 times higher than that of commercial Pt/C catalysts, and good hydrogen evolution reaction performance with a low overpotential of 30 mV to offer 10 mA∙cm−2 in an acid medium, which was comparable to commercial Pt/C. The assembled Pt/MoP-NC||Pt/MoP-NC electrolyzer showed a cell voltage of 0.67 V at 10 mA∙cm−2, ca. 1.02 V less than that of the overall water splitting system (1.69 V). The high catalytic ability of Pt/MoP-NC originated from the electronic effect between noble metal active center Pt and the adjacent MoP-NC support with a unique layered porous spherical structure. The partial electron transfer from MoP to Pt can lower the d-energy band center of Pt, which weakened the binding energy between Pt and adsorbed toxic intermediates. In addition, the oxophilic MoP-NC nanospheres can activate water to provide more hydroxyl species and facilitate the oxidative removal of CO intermediates adsorbed on the Pt active sites. The current work might inspire the design and preparation of novel catalyst platforms for methanol electrolysis in hydrogen generation.
2023, 39(9): 221201
doi: 10.3866/PKU.WHXB202212014
Abstract:
Emerging as one of the youngest members in the family of porous materials, metal aerogels (MAs) are a new class of aerogels entirely built of nanostructured metals such as gold, silver, palladium, platinum, ruthenium, rhodium, osmium, copper, and nickel. They are typically fabricated via a sol-gel process coupled with special drying techniques (e.g., supercritical drying and freeze-drying). Combining the unique physicochemical properties of various nanostructured metals with the structural attributes of aerogels, MAs mark rapid mass transfer channels, highly conductive three-dimensional (3D) networks, and optical and magnetic properties. In this regard, MAs outperform conventional materials in various territories such as electrocatalysis, enzyme-like catalysis, surface-enhanced Raman scattering, diverse sensing, and actuators. Additionally, a substantial number of metal elements can offer vast opportunities for creating numerous MAs featuring desired properties, which is critical for a deep exploration and releases the full potential of aerogels. Consequently, MAs have received wide attention since their discovery in 2009.However, compared with conventional aerogels, MAs only appeared around a decade ago. A short research history challenges their fundamental studies, including controlled synthesis and structure-performance investigations, thereby retarding on-target materials design for practical applications. Currently, the majority of studies are restricted to MAs based on noble metals. This fact is ascribed to both their intrinsically high catalytic activity and simple fabrication due to the relatively high redox potential. In contrast, reports on low-cost non-noble metal aerogels are largely constrained, not to mention controlled synthesis as well as practical applications. As a result, the compositional and structural diversity of MAs is highly limited. Furthermore, the scope of the application of MAs is still constrained and is mostly restricted to electrocatalysis. Though certain remarkable MA-based electrocatalysts have been reported in the last few years, their limited composition and structure have retarded the systematic investigations into correlations between the material parameters and electrocatalytic properties. This discourages on-demand material design and performance optimization. Hence, fundamental studies and application attempts need to be strengthened to allow further development of this new material system. On this occasion, it is essential to thoroughly summarize the knowledge and design principles of MAs that have been presented in the past ten years. This study comprehensively introduces the state-of-the-art research progress in this field, which includes synthesis strategies, potential gelation mechanisms, and diverse applications of MAs. Additionally, the challenges and opportunities presented by MAs will be drawn from aspects of synthesis and applications. This review expects to attract widespread scientists from different fields (e.g., chemistry, physics, materials science, and life science) to join the area of MAs, thus jointly promoting the development of this young and promising field. ![]()
Emerging as one of the youngest members in the family of porous materials, metal aerogels (MAs) are a new class of aerogels entirely built of nanostructured metals such as gold, silver, palladium, platinum, ruthenium, rhodium, osmium, copper, and nickel. They are typically fabricated via a sol-gel process coupled with special drying techniques (e.g., supercritical drying and freeze-drying). Combining the unique physicochemical properties of various nanostructured metals with the structural attributes of aerogels, MAs mark rapid mass transfer channels, highly conductive three-dimensional (3D) networks, and optical and magnetic properties. In this regard, MAs outperform conventional materials in various territories such as electrocatalysis, enzyme-like catalysis, surface-enhanced Raman scattering, diverse sensing, and actuators. Additionally, a substantial number of metal elements can offer vast opportunities for creating numerous MAs featuring desired properties, which is critical for a deep exploration and releases the full potential of aerogels. Consequently, MAs have received wide attention since their discovery in 2009.However, compared with conventional aerogels, MAs only appeared around a decade ago. A short research history challenges their fundamental studies, including controlled synthesis and structure-performance investigations, thereby retarding on-target materials design for practical applications. Currently, the majority of studies are restricted to MAs based on noble metals. This fact is ascribed to both their intrinsically high catalytic activity and simple fabrication due to the relatively high redox potential. In contrast, reports on low-cost non-noble metal aerogels are largely constrained, not to mention controlled synthesis as well as practical applications. As a result, the compositional and structural diversity of MAs is highly limited. Furthermore, the scope of the application of MAs is still constrained and is mostly restricted to electrocatalysis. Though certain remarkable MA-based electrocatalysts have been reported in the last few years, their limited composition and structure have retarded the systematic investigations into correlations between the material parameters and electrocatalytic properties. This discourages on-demand material design and performance optimization. Hence, fundamental studies and application attempts need to be strengthened to allow further development of this new material system. On this occasion, it is essential to thoroughly summarize the knowledge and design principles of MAs that have been presented in the past ten years. This study comprehensively introduces the state-of-the-art research progress in this field, which includes synthesis strategies, potential gelation mechanisms, and diverse applications of MAs. Additionally, the challenges and opportunities presented by MAs will be drawn from aspects of synthesis and applications. This review expects to attract widespread scientists from different fields (e.g., chemistry, physics, materials science, and life science) to join the area of MAs, thus jointly promoting the development of this young and promising field.
2023, 39(9): 221203
doi: 10.3866/PKU.WHXB202212038
Abstract:
To achieve the stated goal of carbon neutrality, solar energy is regarded as the most promising alternative to traditional fossil fuels as a sustainable and clean resource. The key prerequisite for improving the efficiency of solar conversion is to maximize solar energy utilization. As a promising technology, photothermal catalysis can harness full-spectrum sunlight to activate photocatalysis and thermocatalysis through hot carrier generation and local heating. These synergistic catalytic effects driven by both light and heat can overcome the challenges associated with the low catalytic efficiency of photocatalysis and high energy consumption of thermocatalysis as well as modulate the reaction pathways to achieve desirable activity and selectivity. To achieve outstanding catalytic performance, photothermal materials should meet the requirements for sufficient electron-hole separation, efficient solar thermal generation, and abundant exposed active sites. Common fabrication strategies are based on the integration of materials with photo-active and photothermal conversion capabilities that often suffer from buried active sites, high temperature-induced deactivation, and complicated synthetic procedures. Single-atom catalysts (SACs) with isolated single atoms uniformly dispersed on a solid surface are advantageous for 100% atomic utilization and excellent catalytic activity. Therefore, these materials have received increasing attention for a wide range of applications. Many SAC substrates are endowed with hot carrier generation and photothermal conversion abilities under illumination. The strong chemical interaction between metal atoms and supports or surface lattice reconstruction can also prevent catalyst sintering even in long-term high-temperature environments. These unique features make SACs highly suitable for photothermal catalytic processes. Therefore, it is important to summarize recent advances in this field and provide in-depth insights into SACs-based photothermal catalysis to accelerate solar conversion technology development. Herein, the fundamental mechanisms and characteristics of photocatalysis, thermocatalysis, and photothermal catalysis are introduced and three photothermal catalysis modes categorized by driving force (including photo-driven thermocatalysis, thermal-assisted photocatalysis, and photo-thermal co-catalysis) are described and compared along with representative examples. The photothermal properties of SACs supported by carbon, semiconductors, and plasmonic materials are reviewed and pioneering studies for different applications are discussed in detail. Finally, the challenges and future research directions are proposed. This review aims to give a comprehensive understanding of photothermal catalytic processes driven by solar energy based on SACs and provide accessible guidelines for future development to achieve carbon neutrality targets. ![]()
To achieve the stated goal of carbon neutrality, solar energy is regarded as the most promising alternative to traditional fossil fuels as a sustainable and clean resource. The key prerequisite for improving the efficiency of solar conversion is to maximize solar energy utilization. As a promising technology, photothermal catalysis can harness full-spectrum sunlight to activate photocatalysis and thermocatalysis through hot carrier generation and local heating. These synergistic catalytic effects driven by both light and heat can overcome the challenges associated with the low catalytic efficiency of photocatalysis and high energy consumption of thermocatalysis as well as modulate the reaction pathways to achieve desirable activity and selectivity. To achieve outstanding catalytic performance, photothermal materials should meet the requirements for sufficient electron-hole separation, efficient solar thermal generation, and abundant exposed active sites. Common fabrication strategies are based on the integration of materials with photo-active and photothermal conversion capabilities that often suffer from buried active sites, high temperature-induced deactivation, and complicated synthetic procedures. Single-atom catalysts (SACs) with isolated single atoms uniformly dispersed on a solid surface are advantageous for 100% atomic utilization and excellent catalytic activity. Therefore, these materials have received increasing attention for a wide range of applications. Many SAC substrates are endowed with hot carrier generation and photothermal conversion abilities under illumination. The strong chemical interaction between metal atoms and supports or surface lattice reconstruction can also prevent catalyst sintering even in long-term high-temperature environments. These unique features make SACs highly suitable for photothermal catalytic processes. Therefore, it is important to summarize recent advances in this field and provide in-depth insights into SACs-based photothermal catalysis to accelerate solar conversion technology development. Herein, the fundamental mechanisms and characteristics of photocatalysis, thermocatalysis, and photothermal catalysis are introduced and three photothermal catalysis modes categorized by driving force (including photo-driven thermocatalysis, thermal-assisted photocatalysis, and photo-thermal co-catalysis) are described and compared along with representative examples. The photothermal properties of SACs supported by carbon, semiconductors, and plasmonic materials are reviewed and pioneering studies for different applications are discussed in detail. Finally, the challenges and future research directions are proposed. This review aims to give a comprehensive understanding of photothermal catalytic processes driven by solar energy based on SACs and provide accessible guidelines for future development to achieve carbon neutrality targets.
2023, 39(9): 221206
doi: 10.3866/PKU.WHXB202212060
Abstract:
During the development of traditional industries, large amounts of greenhouse gases have been emitted due to the increasing consumption of fossil energy. CH4 and CO2 account for more than 98% of greenhouse gas emissions, and the conversion of CH4 and CO2 into high value-added chemicals has attracted extensive attention from both industry and academia. Dry reforming of methane (DRM) can co-convert CH4 and CO2 into syngas, which can be further converted into various value-added fuels and chemicals through Fischer-Tropsch synthesis. The dry reforming of methane into syngas by thermal catalysis provides an effective strategy for the consumption of both CH4 and CO2, which is beneficial for alleviating environmental problems such as global warming. However, a high-intensity energy input is needed at high temperatures owing to the thermodynamic limitations of the DRM reaction and catalyst instability caused by coke formation. Environmentally friendly photocatalytic technology can make the DRM reaction proceed under mild conditions. However, its development is greatly restricted owing to the low utilization rate of sunlight and low reaction conversion rate. Recently, photothermocatalysis has been widely used in various fields. Many studies have shown that under relatively mild conditions, photothermocatalysis of DRM can achieve promising catalytic performance and effectively convert solar energy into chemical energy. Photothermocatalysis can greatly increase the reaction rate of photocatalytic DRM without a high energy input. In addition, the introduction of light is beneficial for the thermal catalysis of DRM by reducing the reaction activation energy, inhibiting coke formation, and reversing the water-gas shift reaction. In this paper, the advantages and disadvantages of thermal catalysis, photocatalysis, and photothermal catalysis of DRM are first discussed. Then, recent research progress in photothermocatalysis of the DRM reaction, especially the application of different metal-based catalysts (Ni, Pt, Rh, Ru, and Co) is summarized. Localized surface plasmon resonance effects, types of carriers, elimination of coke formation, and suppression of the reverse water-gas shift reaction are briefly mentioned. Finally, the future challenges and new perspectives on the photothermocatalysis of DRM are highlighted, including high utilization of sunlight, catalyst long-term stability, reactor optimization, and the photothermocatalytic mechanism. ![]()
During the development of traditional industries, large amounts of greenhouse gases have been emitted due to the increasing consumption of fossil energy. CH4 and CO2 account for more than 98% of greenhouse gas emissions, and the conversion of CH4 and CO2 into high value-added chemicals has attracted extensive attention from both industry and academia. Dry reforming of methane (DRM) can co-convert CH4 and CO2 into syngas, which can be further converted into various value-added fuels and chemicals through Fischer-Tropsch synthesis. The dry reforming of methane into syngas by thermal catalysis provides an effective strategy for the consumption of both CH4 and CO2, which is beneficial for alleviating environmental problems such as global warming. However, a high-intensity energy input is needed at high temperatures owing to the thermodynamic limitations of the DRM reaction and catalyst instability caused by coke formation. Environmentally friendly photocatalytic technology can make the DRM reaction proceed under mild conditions. However, its development is greatly restricted owing to the low utilization rate of sunlight and low reaction conversion rate. Recently, photothermocatalysis has been widely used in various fields. Many studies have shown that under relatively mild conditions, photothermocatalysis of DRM can achieve promising catalytic performance and effectively convert solar energy into chemical energy. Photothermocatalysis can greatly increase the reaction rate of photocatalytic DRM without a high energy input. In addition, the introduction of light is beneficial for the thermal catalysis of DRM by reducing the reaction activation energy, inhibiting coke formation, and reversing the water-gas shift reaction. In this paper, the advantages and disadvantages of thermal catalysis, photocatalysis, and photothermal catalysis of DRM are first discussed. Then, recent research progress in photothermocatalysis of the DRM reaction, especially the application of different metal-based catalysts (Ni, Pt, Rh, Ru, and Co) is summarized. Localized surface plasmon resonance effects, types of carriers, elimination of coke formation, and suppression of the reverse water-gas shift reaction are briefly mentioned. Finally, the future challenges and new perspectives on the photothermocatalysis of DRM are highlighted, including high utilization of sunlight, catalyst long-term stability, reactor optimization, and the photothermocatalytic mechanism.
2023, 39(9): 221206
doi: 10.3866/PKU.WHXB202212065
Abstract:
Formic acid (FA) is an important chemical for the production of leathers, medicines, preservatives, rubbers, textiles, and other materials. FA is also used as H2 and CO carriers, and as a fuel in fuel cells. Although most commonly synthesized from fossil fuels, FA can also be obtained from more sustainable sources, such as biomass (e.g., straw, husk, and sawdust). Oxygen and air are affordable and easily available oxidants used for the oxidation of biomass to FA. Because solid biomass is not soluble in water or organic solvents, homogeneous catalysts are preferred for the catalytic oxidation of biomass to FA by O2 in water. It has been demonstrated that homogeneous catalysts, such as vanadium-containing heteropoly acids (HPA), HPA+H2SO4, NaVO3+H2SO4, HPA-containing ionic liquids, VOSO4, NaVO3-FeCl3+H2SO4, and FeCl3+H2SO4, can convert complex biomass substrates to FA with high atom economy using O2 as the oxidant. The reported biomass substrates include model compounds, cellulose, wood, straw, and corncobs. The reaction conditions were summarized to compare the biomass conversions and FA yields. Vanadium-containing catalysts had the highest FA yield at mild conditions (T ≤ 170 ºC and P(O2) ≤ 3 MPa). Both the reaction rate and FA yield were improved by adding H2SO4. This high conversion can be explained by an electron transfer and oxygen transfer (ET-OT) mechanism, where high-valence transition metals (V5+ or Fe3+) oxidize biomass to FA and are reduced to low-valence species (V4+ or Fe2+). The catalysts are then regenerated by O2. This reaction occurs through C2―C3 and/or C3―C4 bond cleavages via retro-aldol condensation, followed by continued C―C bond cleavages to form FA. Using isotope-labeled D-glucose as substrate, we determined that oxidation occurs via successive C1―C2 bond cleavages; a V5+ catalyst reacts with C1―C2 to form a five-membered ring complex, without C―H bond cleavages, followed by oxidation from another V5+ species to form FA. The oxidation of solid cellulose occurs through hydrolysis (hydrolysis of cellulose to monosaccharides, and deep hydrolysis of monosaccharides to levulinic acid) and oxidation (monosaccharides to FA and levulinic acid to acetic acid) reactions. The catalytic oxidation of monosaccharides and deep hydrolysis steps are competitive, and the reaction rate of the latter increases faster with increasing temperature. However, catalytic oxidation was favored by higher P(O2). The addition of methanol, ethanol and DMSO to the reaction system, and in situ extraction of FA were performed to inhibit CO2 formation. FA was separated by extraction and the catalyst system was reused. A continuous process for producing FA from molasses was established using a three-phase liquid-liquid-gas system with a reaction volume of 2 L. Finally, the limitations and future requirements of this oxidation reaction are discussed: (1) improving separation or in situ conversion of FA; (2) improving homogeneous catalysts for both biomass hydrolysis and catalytic oxidation to FA; (3) studying the impact of ash in biomass, particularly after catalyst reuse; and (4) understanding the mechanism through which organic solvents such as methanol inhibit CO2 formation. ![]()
Formic acid (FA) is an important chemical for the production of leathers, medicines, preservatives, rubbers, textiles, and other materials. FA is also used as H2 and CO carriers, and as a fuel in fuel cells. Although most commonly synthesized from fossil fuels, FA can also be obtained from more sustainable sources, such as biomass (e.g., straw, husk, and sawdust). Oxygen and air are affordable and easily available oxidants used for the oxidation of biomass to FA. Because solid biomass is not soluble in water or organic solvents, homogeneous catalysts are preferred for the catalytic oxidation of biomass to FA by O2 in water. It has been demonstrated that homogeneous catalysts, such as vanadium-containing heteropoly acids (HPA), HPA+H2SO4, NaVO3+H2SO4, HPA-containing ionic liquids, VOSO4, NaVO3-FeCl3+H2SO4, and FeCl3+H2SO4, can convert complex biomass substrates to FA with high atom economy using O2 as the oxidant. The reported biomass substrates include model compounds, cellulose, wood, straw, and corncobs. The reaction conditions were summarized to compare the biomass conversions and FA yields. Vanadium-containing catalysts had the highest FA yield at mild conditions (T ≤ 170 ºC and P(O2) ≤ 3 MPa). Both the reaction rate and FA yield were improved by adding H2SO4. This high conversion can be explained by an electron transfer and oxygen transfer (ET-OT) mechanism, where high-valence transition metals (V5+ or Fe3+) oxidize biomass to FA and are reduced to low-valence species (V4+ or Fe2+). The catalysts are then regenerated by O2. This reaction occurs through C2―C3 and/or C3―C4 bond cleavages via retro-aldol condensation, followed by continued C―C bond cleavages to form FA. Using isotope-labeled D-glucose as substrate, we determined that oxidation occurs via successive C1―C2 bond cleavages; a V5+ catalyst reacts with C1―C2 to form a five-membered ring complex, without C―H bond cleavages, followed by oxidation from another V5+ species to form FA. The oxidation of solid cellulose occurs through hydrolysis (hydrolysis of cellulose to monosaccharides, and deep hydrolysis of monosaccharides to levulinic acid) and oxidation (monosaccharides to FA and levulinic acid to acetic acid) reactions. The catalytic oxidation of monosaccharides and deep hydrolysis steps are competitive, and the reaction rate of the latter increases faster with increasing temperature. However, catalytic oxidation was favored by higher P(O2). The addition of methanol, ethanol and DMSO to the reaction system, and in situ extraction of FA were performed to inhibit CO2 formation. FA was separated by extraction and the catalyst system was reused. A continuous process for producing FA from molasses was established using a three-phase liquid-liquid-gas system with a reaction volume of 2 L. Finally, the limitations and future requirements of this oxidation reaction are discussed: (1) improving separation or in situ conversion of FA; (2) improving homogeneous catalysts for both biomass hydrolysis and catalytic oxidation to FA; (3) studying the impact of ash in biomass, particularly after catalyst reuse; and (4) understanding the mechanism through which organic solvents such as methanol inhibit CO2 formation.
2023, 39(9): 221202
doi: 10.3866/PKU.WHXB202212020
Abstract:
Past decades have witnessed the flourish of single atom catalysts (SACs) owing to their high atom-utilization efficiency and completely exposed active sites, which endows SACs with remarkably enhanced catalytic activities for various reactions. In the early development stage of SACs, researchers focus on the improvement of the catalytic performance of the catalysts, whereas the intrinsic catalytic reaction mechanism and the relationship between the electronic states of the metal sites and catalytic performance are usually ignored. Moreover, some sophisticated and complex structures, such as dual-atom SACs, heteroatomic doped SACs, SACs with precise second coordination shell, and other synergetic catalysts containing SACs, were fabricated recently. The insight into electronic metal-support interaction (EMSI) aids the understanding of the catalytic mechanism and thus serves as a guide for the fabrication of heterogeneous catalysts. Notably, the uniform active sites and characteristic local coordination configuration of SACs provide excellent platforms to study EMSI and bridge the gap between homogeneous and heterogeneous catalysts, which will contribute to the understanding of structure-performance relationships and enhance the development of SACs and rational design of heterogeneous catalysts. EMSI is especially important in heterogeneous catalysis. Through the rational design of the local coordination environment of SACs, the electronic structure of active sites can be accurately regulated, which will shift their d-band centers. This significantly alters the adsorption capability of intermediates and influences the final catalytic performance of the catalysts. With the development of advanced operando characterization techniques, the evolution of configuration, electronic properties, and local coordination environment could be revealed, thus providing researchers with a clear picture of the intrinsic mechanism of the catalytic system. In addition, with the aid of theoretical calculations, catalyst screening will be considerably more convenient, which will significantly reduce the number of aimless trials. After the optimal structure is determined, researchers should devise precise fabrication methods to realize the configuration. Herein, we initially introduce the basic principles and effects of EMSI. The stabilization, electronic property regulation, and electron transfer tunneling effects of EMSI are the foundation of SACs synthesis and catalytic mechanism elucidation, of which the former requires strong coordination stabilization energies while the latter focuses on the electronic state evolution of the active sites. Subsequently, EMSI applications in several important heterogeneous catalysis processes, such as selective hydrogenation, alcohol oxidation, water-gas shift reaction, and hydroformylation, are reviewed. Finally, the review discusses the challenges and future prospects for the future development of EMSI on SACs.![]()
Past decades have witnessed the flourish of single atom catalysts (SACs) owing to their high atom-utilization efficiency and completely exposed active sites, which endows SACs with remarkably enhanced catalytic activities for various reactions. In the early development stage of SACs, researchers focus on the improvement of the catalytic performance of the catalysts, whereas the intrinsic catalytic reaction mechanism and the relationship between the electronic states of the metal sites and catalytic performance are usually ignored. Moreover, some sophisticated and complex structures, such as dual-atom SACs, heteroatomic doped SACs, SACs with precise second coordination shell, and other synergetic catalysts containing SACs, were fabricated recently. The insight into electronic metal-support interaction (EMSI) aids the understanding of the catalytic mechanism and thus serves as a guide for the fabrication of heterogeneous catalysts. Notably, the uniform active sites and characteristic local coordination configuration of SACs provide excellent platforms to study EMSI and bridge the gap between homogeneous and heterogeneous catalysts, which will contribute to the understanding of structure-performance relationships and enhance the development of SACs and rational design of heterogeneous catalysts. EMSI is especially important in heterogeneous catalysis. Through the rational design of the local coordination environment of SACs, the electronic structure of active sites can be accurately regulated, which will shift their d-band centers. This significantly alters the adsorption capability of intermediates and influences the final catalytic performance of the catalysts. With the development of advanced operando characterization techniques, the evolution of configuration, electronic properties, and local coordination environment could be revealed, thus providing researchers with a clear picture of the intrinsic mechanism of the catalytic system. In addition, with the aid of theoretical calculations, catalyst screening will be considerably more convenient, which will significantly reduce the number of aimless trials. After the optimal structure is determined, researchers should devise precise fabrication methods to realize the configuration. Herein, we initially introduce the basic principles and effects of EMSI. The stabilization, electronic property regulation, and electron transfer tunneling effects of EMSI are the foundation of SACs synthesis and catalytic mechanism elucidation, of which the former requires strong coordination stabilization energies while the latter focuses on the electronic state evolution of the active sites. Subsequently, EMSI applications in several important heterogeneous catalysis processes, such as selective hydrogenation, alcohol oxidation, water-gas shift reaction, and hydroformylation, are reviewed. Finally, the review discusses the challenges and future prospects for the future development of EMSI on SACs.
2023, 39(9): 221205
doi: 10.3866/PKU.WHXB202212057
Abstract:
Increasing global energy consumption and the depletion of traditional energy sources pose severe challenges to environmental protection and energy supply security. Electrochemical decomposition of water is a green and promising technology and is also a key technology for efficient and sustainable energy production and storage by fuel cells and metal-air batteries. The electrocatalytic oxygen evolution reaction (OER), as the anode reaction for the electrolysis of water, requires large amounts of energy owing to multielectron participation, and to the breaking of O―H bonds and formation of O―O bonds. Many precious metal catalysts are expensive, and these are responsible for secondary environmental pollution, which is detrimental for the large-scale application of the OER. Therefore, it is necessary to develop a stable, clean, and efficient electrocatalyst to improve the efficiency of the OER. The application of covalent organic frameworks (COFs) to the electrocatalytic oxygen evolution reaction (OER) has received widespread attention. However, most metal-free covalent organic frameworks (COFs) have unsuitably poor conductivity for the OER. Herein, we report a 2D metal-free tetrathiafulvalene (TTF)-based COF, termed JUC-630. To improve the conductivity of COFs, we introduced TTF, which is a good electron donor, into the COF material. At the same time, compared with its analogue without TTF (Etta-Td COF), we found that JUC-630 has a large surface area, better crystallinity, and higher stability. Furthermore, we tested their OER performance in a 1 mol∙L−1 KOH solution, and the results show that JUC-630 has a higher current density than Etta-Td COF and TTF at the same potential. For example, at a current density of 10 mA∙cm−2, the overpotential of JUC-630 was 400 mV, which was significantly lower than that of Etta-Td COF (450 mV). This overpotential is comparable to or even better than those of the widely discussed carbon and graphene materials. The lower overpotential, Tafel slope values, and smaller electrochemical impedance illustrate that the introduction of TTF monomers into the COF material results in a significantly improved OER performance for JUC-630. This study proposes a strategy for the rational design of functional motifs that can greatly improve the OER catalytic activity of COF materials. Thus, the results should help to suggest new efficient approaches for the preparation of catalysts for energy conversion from water resources. ![]()
Increasing global energy consumption and the depletion of traditional energy sources pose severe challenges to environmental protection and energy supply security. Electrochemical decomposition of water is a green and promising technology and is also a key technology for efficient and sustainable energy production and storage by fuel cells and metal-air batteries. The electrocatalytic oxygen evolution reaction (OER), as the anode reaction for the electrolysis of water, requires large amounts of energy owing to multielectron participation, and to the breaking of O―H bonds and formation of O―O bonds. Many precious metal catalysts are expensive, and these are responsible for secondary environmental pollution, which is detrimental for the large-scale application of the OER. Therefore, it is necessary to develop a stable, clean, and efficient electrocatalyst to improve the efficiency of the OER. The application of covalent organic frameworks (COFs) to the electrocatalytic oxygen evolution reaction (OER) has received widespread attention. However, most metal-free covalent organic frameworks (COFs) have unsuitably poor conductivity for the OER. Herein, we report a 2D metal-free tetrathiafulvalene (TTF)-based COF, termed JUC-630. To improve the conductivity of COFs, we introduced TTF, which is a good electron donor, into the COF material. At the same time, compared with its analogue without TTF (Etta-Td COF), we found that JUC-630 has a large surface area, better crystallinity, and higher stability. Furthermore, we tested their OER performance in a 1 mol∙L−1 KOH solution, and the results show that JUC-630 has a higher current density than Etta-Td COF and TTF at the same potential. For example, at a current density of 10 mA∙cm−2, the overpotential of JUC-630 was 400 mV, which was significantly lower than that of Etta-Td COF (450 mV). This overpotential is comparable to or even better than those of the widely discussed carbon and graphene materials. The lower overpotential, Tafel slope values, and smaller electrochemical impedance illustrate that the introduction of TTF monomers into the COF material results in a significantly improved OER performance for JUC-630. This study proposes a strategy for the rational design of functional motifs that can greatly improve the OER catalytic activity of COF materials. Thus, the results should help to suggest new efficient approaches for the preparation of catalysts for energy conversion from water resources.
2023, 39(9): 230303
doi: 10.3866/PKU.WHXB202303032
Abstract:
