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2023 Volume 39 Issue 4
2023, 39(4): 220503
doi: 10.3866/PKU.WHXB202205039
Abstract:
Visible-light-driven photocatalytic H2 evolution coupled with oxidative organic synthesis is attracting extensive attention owing to the environmental friendliness and sustainability of these processes, which can coproduce clean H2 fuel and high-value chemicals under mild conditions without requiring sacrificial agents. Semiconductor materials and metal-organic framework (MOF) materials have been widely used in photocatalys owing to their properties and advantages. In this work, we successfully synthesized a novel effective catalyst (named CdS/PFC-8) by electrostatic self-assembly. Among the components of the CdS/PFC-8 composite, PFC-8 was a nickel-based MOF. The CdS/PFC-8 composite, as a noble metal-free catalyst, enabled excellent photocatalytic H2 evolution and benzyl alcohol oxidation under visible light. A series of catalytic characterizations were performed with the CdS/PFC-8 composite. X-ray diffraction (XRD) and scanning electron microscopy (SEM) results demonstrated the successful synthesis of the CdS/PFC-8 composite. X-ray photoelectron spectroscopy (XPS) results demonstrated the existence of an interfacial interaction between CdS nanorods and PFC-8. The optoelectronic performance was characterized by ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS), photoluminescence (PL) spectroscopy, and electrochemical tests, and the results demonstrated the visible-light response and photocatalytic feasibility of the CdS/PFC-8 composite. The photocatalytic results obtained with different catalysts were compared. Under visible light, the CdS/PFC-8 composite enabled the generation of H2 with the selective oxidation of benzyl alcohol in a single reaction. It exhibited a remarkable H2 production rate of 3376 μmol∙g-1∙h-1; further, the benzaldehyde yield was 4120 μmol∙g-1∙h-1, which is higher than that of CdS alone. Photocatalytic cycling reactions were carried out to verify the activity and stability of the catalysts. The changes in the catalysts before and after the reaction were analyzed, and the results suggested that the nickel in PFC-8 could be used as a catalytically active site to improve the catalytic activity. In addition, the possible photocatalytic reaction mechanism was discussed. This work highlights the advantages of combining the functionalities of semiconductors and MOFs to enhance the photocatalytic activity.![]()
Visible-light-driven photocatalytic H2 evolution coupled with oxidative organic synthesis is attracting extensive attention owing to the environmental friendliness and sustainability of these processes, which can coproduce clean H2 fuel and high-value chemicals under mild conditions without requiring sacrificial agents. Semiconductor materials and metal-organic framework (MOF) materials have been widely used in photocatalys owing to their properties and advantages. In this work, we successfully synthesized a novel effective catalyst (named CdS/PFC-8) by electrostatic self-assembly. Among the components of the CdS/PFC-8 composite, PFC-8 was a nickel-based MOF. The CdS/PFC-8 composite, as a noble metal-free catalyst, enabled excellent photocatalytic H2 evolution and benzyl alcohol oxidation under visible light. A series of catalytic characterizations were performed with the CdS/PFC-8 composite. X-ray diffraction (XRD) and scanning electron microscopy (SEM) results demonstrated the successful synthesis of the CdS/PFC-8 composite. X-ray photoelectron spectroscopy (XPS) results demonstrated the existence of an interfacial interaction between CdS nanorods and PFC-8. The optoelectronic performance was characterized by ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS), photoluminescence (PL) spectroscopy, and electrochemical tests, and the results demonstrated the visible-light response and photocatalytic feasibility of the CdS/PFC-8 composite. The photocatalytic results obtained with different catalysts were compared. Under visible light, the CdS/PFC-8 composite enabled the generation of H2 with the selective oxidation of benzyl alcohol in a single reaction. It exhibited a remarkable H2 production rate of 3376 μmol∙g-1∙h-1; further, the benzaldehyde yield was 4120 μmol∙g-1∙h-1, which is higher than that of CdS alone. Photocatalytic cycling reactions were carried out to verify the activity and stability of the catalysts. The changes in the catalysts before and after the reaction were analyzed, and the results suggested that the nickel in PFC-8 could be used as a catalytically active site to improve the catalytic activity. In addition, the possible photocatalytic reaction mechanism was discussed. This work highlights the advantages of combining the functionalities of semiconductors and MOFs to enhance the photocatalytic activity.
2023, 39(4): 220600
doi: 10.3866/PKU.WHXB202206006
Abstract:
Tungsten carbide (WC) is commonly used as a photocatalytic material for hydrogen production via water reduction. However, it is often combined with an effective photoabsorber to provide sufficient photoactivity. This is attributed to the narrow band gap of WC, which leads to an inadequate redox capability for water reduction. Notably, this limitation was overcome using a novel solid-liquid photocatalytic system that compliments bare WC photocatalysts with liquid-phase photosensitizing erythrosine B (ErB). The proposed concept eliminates the need to couple WC with photoabsorbing semiconductors, which often requires tedious procedures for the proper functionalization of photocatalytic composites. The experimental results indicated significant hydrogen production from the proposed solid-liquid photocatalytic system under irradiation with visible light (λ = 520 nm); however, only in the presence of triethanolamine (TEOA) as a sacrificial reagent. Evidently, a blank experiment with only WC and ErB under typical photoreaction conditions exhibited nearly zero photoactivity and the production of H2 was undetected. Similarly, nonactivity was observed for the photoreaction in the presence of ErB or WC in the irradiated TEOA solution. These blank experiments confirmed the significance of all three components, namely WC, ErB, and TEOA, which functioned as the photocatalyst, photoabsorber, and sacrificial reagent, respectively, for suitable H2 production in the proposed system. The effects of three critical parameters, such as pH, ErB concentration, and WC concentration, were systematically investigated. The optimum pH for H2 production was 8, with a slight variation to more basic or acidic conditions reducing the photoactivity of the system. At pH < 8, part of TEOA undergoes partial protonation, thereby losing its activity as a sacrificial reagent in the photocatalytic system. As the pH increased to > 8, the low proton concentration in the reaction medium perturbed the thermodynamic drive, leading to suppressed H2 production. The optimum ErB concentration was 1 mmol·L-1, and decreasing or increasing the ErB concentration from the optimal point was detrimental to H2 production. The diluted system (ErB concentration < 1 mmol·L-1) provided insufficient sensitizing agents, whereas the concentrated system (> 1 mmol·L-1 ErB) induced significant scattering effects that prevent light from penetrating into the reactive liquid phase. Conversely, the WC concentration exhibited a positive correlation with H2 production in a steady manner, and the highest H2 production measured by the system was at a WC concentration of 12 mmol·L-1. Under optimum conditions, 66 μmol∙h-1 of H2 was successfully produced, with a slightly higher apparent quantum efficiency (AQE) of 6.6% at 520 nm, which was attributed to the synergism of ErB-TEOA-WC in the proposed system. The photoelectrochemical evaluation confirmed the positive interactions between ErB, TEOA, and WC, which caused reduced impedance while improving charge utilization in the system. Consequently, an excellent H2 turnover number (TON) of 15 was achieved with negligible activity decay for at least 20 h of reaction. Density functional theory (DFT) calculations confirmed the major roles of W- and C-vacant sites in H2 production, which were attributed to their enhanced product desorption that facilitates high turnover rates during photoreactions. In conclusion, the proposed novel liquid-solid photocatalytic WC/ErB/TEOA system provides more facile photo-derived H2 energy from water, which circumvents the tedious photoabsorber coupling of metal carbide photocatalysts.![]()
Tungsten carbide (WC) is commonly used as a photocatalytic material for hydrogen production via water reduction. However, it is often combined with an effective photoabsorber to provide sufficient photoactivity. This is attributed to the narrow band gap of WC, which leads to an inadequate redox capability for water reduction. Notably, this limitation was overcome using a novel solid-liquid photocatalytic system that compliments bare WC photocatalysts with liquid-phase photosensitizing erythrosine B (ErB). The proposed concept eliminates the need to couple WC with photoabsorbing semiconductors, which often requires tedious procedures for the proper functionalization of photocatalytic composites. The experimental results indicated significant hydrogen production from the proposed solid-liquid photocatalytic system under irradiation with visible light (λ = 520 nm); however, only in the presence of triethanolamine (TEOA) as a sacrificial reagent. Evidently, a blank experiment with only WC and ErB under typical photoreaction conditions exhibited nearly zero photoactivity and the production of H2 was undetected. Similarly, nonactivity was observed for the photoreaction in the presence of ErB or WC in the irradiated TEOA solution. These blank experiments confirmed the significance of all three components, namely WC, ErB, and TEOA, which functioned as the photocatalyst, photoabsorber, and sacrificial reagent, respectively, for suitable H2 production in the proposed system. The effects of three critical parameters, such as pH, ErB concentration, and WC concentration, were systematically investigated. The optimum pH for H2 production was 8, with a slight variation to more basic or acidic conditions reducing the photoactivity of the system. At pH < 8, part of TEOA undergoes partial protonation, thereby losing its activity as a sacrificial reagent in the photocatalytic system. As the pH increased to > 8, the low proton concentration in the reaction medium perturbed the thermodynamic drive, leading to suppressed H2 production. The optimum ErB concentration was 1 mmol·L-1, and decreasing or increasing the ErB concentration from the optimal point was detrimental to H2 production. The diluted system (ErB concentration < 1 mmol·L-1) provided insufficient sensitizing agents, whereas the concentrated system (> 1 mmol·L-1 ErB) induced significant scattering effects that prevent light from penetrating into the reactive liquid phase. Conversely, the WC concentration exhibited a positive correlation with H2 production in a steady manner, and the highest H2 production measured by the system was at a WC concentration of 12 mmol·L-1. Under optimum conditions, 66 μmol∙h-1 of H2 was successfully produced, with a slightly higher apparent quantum efficiency (AQE) of 6.6% at 520 nm, which was attributed to the synergism of ErB-TEOA-WC in the proposed system. The photoelectrochemical evaluation confirmed the positive interactions between ErB, TEOA, and WC, which caused reduced impedance while improving charge utilization in the system. Consequently, an excellent H2 turnover number (TON) of 15 was achieved with negligible activity decay for at least 20 h of reaction. Density functional theory (DFT) calculations confirmed the major roles of W- and C-vacant sites in H2 production, which were attributed to their enhanced product desorption that facilitates high turnover rates during photoreactions. In conclusion, the proposed novel liquid-solid photocatalytic WC/ErB/TEOA system provides more facile photo-derived H2 energy from water, which circumvents the tedious photoabsorber coupling of metal carbide photocatalysts.
2023, 39(4): 220603
doi: 10.3866/PKU.WHXB202206034
Abstract:
Owing to its renewability, abundance, and low environmental impact, biomass is considered to be a viable eco-friendly fuel. Various biofuel-fired power plants have been built worldwide to reduce carbon emissions. Potassium (K) is a typical impurity in the flue gas from biofuel combustion that can deactivate the catalyst used in the selective catalytic reduction of NOx by ammonia (NH3-SCR). CuSO4/TiO2, with excellent sulfur dioxide tolerance, is thought to be a promising vanadium-free catalyst for NH3-SCR; however, the influence of K on the CuSO4/TiO2 catalyst is still unknown. Therefore, in this study, the effect of K on the NH3-SCR performance of CuSO4/TiO2 were investigated and compared with the effect on the performance of the commercial V2O5-WO3/TiO2 (VWTi) catalyst. K-poisoned catalysts were prepared via wet impregnation using potassium acetate as the K source. Nitrogen (N2) adsorption-desorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), NH3-temperature programmed desorption (NH3-TPD), H2-temperature programmed reduction (H2-TPR), and in situ diffuse reflectance infrared Fourier-transform spectroscopy (in situ DRIFTS) were used to characterize the prepared catalysts. The NOx conversion over CuSO4/TiO2 with 1.0% (w) K was 92.1% (at 350 ℃), which was higher than the conversion (75.1%) achieved over the commercial VWTi catalyst with the same K content. The XRD, XPS, and H2-TPR results suggested that K reacted with the CuSO4 in the CuSO4/TiO2 catalyst to form CuO and K2SO4. The presence of CuO enhanced the oxidation of NH3 to N2O, NO, and NO2 during NH3-SCR, thereby decreasing the NOx conversion and N2 selectivity over CuSO4/TiO2. Moreover, based on the results from NH3-TPD and in situ DRIFTS of NH3 adsorption, it can be concluded that the Brønsted acid sites (S-OH) were poisoned by K, which restrained the adsorption of NH3 on CuSO4/TiO2. Additionally, the high K content altered the pore structure of the catalyst, leading to a decrease in the specific surface area. However, according to the in situ DRIFTS results, NH3-SCR over K-poisoned CuSO4/TiO2 still followed the Eley-Rideal mechanism: First, NH3 was adsorbed on the Lewis and Brønsted acid sites of the catalyst, and then gaseous NO and O2 reacted with the adsorbed NH3/NH4+ on the acid sites, resulting in the formation of N2 and H2O. Notably, the abundance of acid sites and surface-adsorbed oxygen species on CuSO4/TiO2 could be the main reason for its higher resistance to K-poisoning. In conclusion, our current findings suggested that CuSO4/TiO2 might be a suitable NH3-SCR catalyst for use in the flue gas streams from biofuel-fired power plants.![]()
Owing to its renewability, abundance, and low environmental impact, biomass is considered to be a viable eco-friendly fuel. Various biofuel-fired power plants have been built worldwide to reduce carbon emissions. Potassium (K) is a typical impurity in the flue gas from biofuel combustion that can deactivate the catalyst used in the selective catalytic reduction of NOx by ammonia (NH3-SCR). CuSO4/TiO2, with excellent sulfur dioxide tolerance, is thought to be a promising vanadium-free catalyst for NH3-SCR; however, the influence of K on the CuSO4/TiO2 catalyst is still unknown. Therefore, in this study, the effect of K on the NH3-SCR performance of CuSO4/TiO2 were investigated and compared with the effect on the performance of the commercial V2O5-WO3/TiO2 (VWTi) catalyst. K-poisoned catalysts were prepared via wet impregnation using potassium acetate as the K source. Nitrogen (N2) adsorption-desorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), NH3-temperature programmed desorption (NH3-TPD), H2-temperature programmed reduction (H2-TPR), and in situ diffuse reflectance infrared Fourier-transform spectroscopy (in situ DRIFTS) were used to characterize the prepared catalysts. The NOx conversion over CuSO4/TiO2 with 1.0% (w) K was 92.1% (at 350 ℃), which was higher than the conversion (75.1%) achieved over the commercial VWTi catalyst with the same K content. The XRD, XPS, and H2-TPR results suggested that K reacted with the CuSO4 in the CuSO4/TiO2 catalyst to form CuO and K2SO4. The presence of CuO enhanced the oxidation of NH3 to N2O, NO, and NO2 during NH3-SCR, thereby decreasing the NOx conversion and N2 selectivity over CuSO4/TiO2. Moreover, based on the results from NH3-TPD and in situ DRIFTS of NH3 adsorption, it can be concluded that the Brønsted acid sites (S-OH) were poisoned by K, which restrained the adsorption of NH3 on CuSO4/TiO2. Additionally, the high K content altered the pore structure of the catalyst, leading to a decrease in the specific surface area. However, according to the in situ DRIFTS results, NH3-SCR over K-poisoned CuSO4/TiO2 still followed the Eley-Rideal mechanism: First, NH3 was adsorbed on the Lewis and Brønsted acid sites of the catalyst, and then gaseous NO and O2 reacted with the adsorbed NH3/NH4+ on the acid sites, resulting in the formation of N2 and H2O. Notably, the abundance of acid sites and surface-adsorbed oxygen species on CuSO4/TiO2 could be the main reason for its higher resistance to K-poisoning. In conclusion, our current findings suggested that CuSO4/TiO2 might be a suitable NH3-SCR catalyst for use in the flue gas streams from biofuel-fired power plants.
2023, 39(4): 220704
doi: 10.3866/PKU.WHXB202207045
Abstract:
The photocatalytic reduction of water to hydrogen (H2) over semiconductors potentially offers an economic way to alleviate the global energy crisis and environmental pollution. Optimal modulation of charge-carrier kinetics is of great importance for enhancing the photocatalytic activity of semiconductors for reducing water to green H2. The design and manufacture of semiconductor-based heterostructure systems have emerged as promising tactics for modulating charge-carrier kinetics based on sensitization either via the semiconductor heterojunction effect or localized surface plasmon resonance. However, the cascade modulation of charge-carrier kinetics is still difficult to achieve through rationally coupling the abovementioned sensitization processes in well-designed heterostructures for highly-efficient photocatalytic H2 generation. In this study, we developed a novel quaternary hetero-component nanofibers (HNFs) system by assembling plasmonic Ag nanoparticles (NPs) and two different semiconductors of Ag2S NPs and g-C3N4 nanosheets (NSs) into the electrospun TiO2 nanofibers (NFs) via in situ oxidation (for g-C3N4 exfoliation and Ag2S) and reduction (for Ag) reactions. By combining time-resolved photoluminescence spectroscopy, three-dimensional finite-difference-time-domain simulation, and control experiments, we found that the overlapping absorption peak of plasmonic Ag NPs and g-C3N4 NSs could induce plasmonic resonant energy transfer from the Ag NPs to the neighboring g-C3N4, thereby improving the generation of photoinduced charge carriers of g-C3N4 in the quaternary HNFs system. Simultaneously, plasmonic hot electrons could be generated on the Ag NPs and transferred to the near-by hetero-components of TiO2, g-C3N4, and Ag2S, to boost the generation and separation of photoinduced charge carriers in the system. Furthermore, the energy band structure at the g-C3N4/TiO2 hetero-interface belongs to the "type II" heterojunction, while the energy band structure at the TiO2/Ag2S hetero-interface can be classified as a "type I" heterojunction. This way, the successive "energy band step" could be constructed at the g-C3N4/TiO2/Ag2S hetero-interface, resulting in improved separation and migration of photoinduced charge carriers through the transfer of photoinduced electrons from g-C3N4 to Ag2S across TiO2. Thus, the plasmonic resonant energy transfer, hot electron transfer, and successive energy-band-step-induced charge separation processes were integrated into the as-synthesized quaternary Ag/Ag2S/g-C3N4/TiO2 HNFs system, thereby achieving the effective cascade modulation of the generation, separation, and migration of photoinduced charge carriers. As such, the photocatalytic H2-generation rate of the optimal Ag/Ag2S/g-C3N4/TiO2 HNFs system was higher than that of the mechanically mixed TiO2 NFs, g-C3N4 NSs, Ag NPs, and Ag2S NPs, with the same amounts as the optimal Ag/Ag2S/g-C3N4/TiO2 HNFs photocatalyst, by approximately 9-fold under simulated sunlight irradiation. This interesting cascade modulation of charge-carrier kinetics might open new avenues for the development of highly active semiconductor-based heterostructure system for solar-to-fuels conversion.![]()
The photocatalytic reduction of water to hydrogen (H2) over semiconductors potentially offers an economic way to alleviate the global energy crisis and environmental pollution. Optimal modulation of charge-carrier kinetics is of great importance for enhancing the photocatalytic activity of semiconductors for reducing water to green H2. The design and manufacture of semiconductor-based heterostructure systems have emerged as promising tactics for modulating charge-carrier kinetics based on sensitization either via the semiconductor heterojunction effect or localized surface plasmon resonance. However, the cascade modulation of charge-carrier kinetics is still difficult to achieve through rationally coupling the abovementioned sensitization processes in well-designed heterostructures for highly-efficient photocatalytic H2 generation. In this study, we developed a novel quaternary hetero-component nanofibers (HNFs) system by assembling plasmonic Ag nanoparticles (NPs) and two different semiconductors of Ag2S NPs and g-C3N4 nanosheets (NSs) into the electrospun TiO2 nanofibers (NFs) via in situ oxidation (for g-C3N4 exfoliation and Ag2S) and reduction (for Ag) reactions. By combining time-resolved photoluminescence spectroscopy, three-dimensional finite-difference-time-domain simulation, and control experiments, we found that the overlapping absorption peak of plasmonic Ag NPs and g-C3N4 NSs could induce plasmonic resonant energy transfer from the Ag NPs to the neighboring g-C3N4, thereby improving the generation of photoinduced charge carriers of g-C3N4 in the quaternary HNFs system. Simultaneously, plasmonic hot electrons could be generated on the Ag NPs and transferred to the near-by hetero-components of TiO2, g-C3N4, and Ag2S, to boost the generation and separation of photoinduced charge carriers in the system. Furthermore, the energy band structure at the g-C3N4/TiO2 hetero-interface belongs to the "type II" heterojunction, while the energy band structure at the TiO2/Ag2S hetero-interface can be classified as a "type I" heterojunction. This way, the successive "energy band step" could be constructed at the g-C3N4/TiO2/Ag2S hetero-interface, resulting in improved separation and migration of photoinduced charge carriers through the transfer of photoinduced electrons from g-C3N4 to Ag2S across TiO2. Thus, the plasmonic resonant energy transfer, hot electron transfer, and successive energy-band-step-induced charge separation processes were integrated into the as-synthesized quaternary Ag/Ag2S/g-C3N4/TiO2 HNFs system, thereby achieving the effective cascade modulation of the generation, separation, and migration of photoinduced charge carriers. As such, the photocatalytic H2-generation rate of the optimal Ag/Ag2S/g-C3N4/TiO2 HNFs system was higher than that of the mechanically mixed TiO2 NFs, g-C3N4 NSs, Ag NPs, and Ag2S NPs, with the same amounts as the optimal Ag/Ag2S/g-C3N4/TiO2 HNFs photocatalyst, by approximately 9-fold under simulated sunlight irradiation. This interesting cascade modulation of charge-carrier kinetics might open new avenues for the development of highly active semiconductor-based heterostructure system for solar-to-fuels conversion.
2023, 39(4): 221102
doi: 10.3866/PKU.WHXB202211029
Abstract:
Supported metal nanocatalysts are promising candidates for heterogenous photocatalysis because the metal nanoparticles (e.g., Au, Pt, or Pd) loaded on the semiconductor surface not only act as a reductive cocatalyst, which accelerates the kinetics of reactions such as H+ reduction, but also trap the photoelectrons, which allows charge separation. Owing to these unique benefits, supported metal photocatalysts have been extensively studied for green H2 production at the reductive side integrated with organic selective oxidation at the oxidative side in a closed photocatalytic redox cycle. Imines and their derivatives are important chemicals in the industrial production of functional polymers, agrochemicals, and pharmaceuticals. Recently, imines have been successfully produced via the photocatalytic dehydrogenative coupling of amines over supported metal nanocatalysts. However, owing to the strong adsorption of H atoms and imines on the metal surface, the produced imines are converted to secondary amines via a self-hydrogenation process, thus greatly decreasing the selectivity toward the desired imines. Herein, we demonstrate that the construction of an ultrathin carbon layer on a Pd/TiO2 photocatalyst (Pd/TiO2@C) via the thermal annealing of self-assembled polydopamine layers is a simple yet effective strategy to address this issue. Temperature-programmed reduction of hydro-oxygen titration and cyclic voltammetry curves for Pd/TiO2vs. Pd/TiO2@C indicate that the conformable coating of the carbon layer on the catalyst surface facilitates kinetic control of H atom adsorption on the supported Pd nanoparticles. Furthermore, in situ Fourier-transform infrared spectroscopy demonstrates that the conformably coated ultrathin carbon layer also decreases the adsorption of substrate molecules such as N-benzylidenebenzylamine on the catalyst surface, which weakens their interaction with the supported Pd nanoparticles. Thus, the construction of an ultrathin conformable carbon coating on Pd/TiO2 is a facile strategy to kinetically control the adsorption behavior of H atoms and imines on the Pd surface during photocatalytic redox reactions, which can suppress the excessive hydrogenation of imines toward selectivity improvement. In addition, owing to the strong electronic interaction between the Pd nanoparticles and the carbon layer, the encapsulated Pd nanoparticles retain their unique catalytic properties toward the H2 evolution reaction. As a result, Pd/TiO2@C with an optimized carbon layer thickness facilitates improved photocatalytic synthesis of imines, with conversion and selectivity as high as 95% and 99%, respectively. This study provides an effective strategy to develop high-performance supported metal nanocatalysts for integrated photocatalytic systems to produce H2 and valuable organic chemicals.![]()
Supported metal nanocatalysts are promising candidates for heterogenous photocatalysis because the metal nanoparticles (e.g., Au, Pt, or Pd) loaded on the semiconductor surface not only act as a reductive cocatalyst, which accelerates the kinetics of reactions such as H+ reduction, but also trap the photoelectrons, which allows charge separation. Owing to these unique benefits, supported metal photocatalysts have been extensively studied for green H2 production at the reductive side integrated with organic selective oxidation at the oxidative side in a closed photocatalytic redox cycle. Imines and their derivatives are important chemicals in the industrial production of functional polymers, agrochemicals, and pharmaceuticals. Recently, imines have been successfully produced via the photocatalytic dehydrogenative coupling of amines over supported metal nanocatalysts. However, owing to the strong adsorption of H atoms and imines on the metal surface, the produced imines are converted to secondary amines via a self-hydrogenation process, thus greatly decreasing the selectivity toward the desired imines. Herein, we demonstrate that the construction of an ultrathin carbon layer on a Pd/TiO2 photocatalyst (Pd/TiO2@C) via the thermal annealing of self-assembled polydopamine layers is a simple yet effective strategy to address this issue. Temperature-programmed reduction of hydro-oxygen titration and cyclic voltammetry curves for Pd/TiO2vs. Pd/TiO2@C indicate that the conformable coating of the carbon layer on the catalyst surface facilitates kinetic control of H atom adsorption on the supported Pd nanoparticles. Furthermore, in situ Fourier-transform infrared spectroscopy demonstrates that the conformably coated ultrathin carbon layer also decreases the adsorption of substrate molecules such as N-benzylidenebenzylamine on the catalyst surface, which weakens their interaction with the supported Pd nanoparticles. Thus, the construction of an ultrathin conformable carbon coating on Pd/TiO2 is a facile strategy to kinetically control the adsorption behavior of H atoms and imines on the Pd surface during photocatalytic redox reactions, which can suppress the excessive hydrogenation of imines toward selectivity improvement. In addition, owing to the strong electronic interaction between the Pd nanoparticles and the carbon layer, the encapsulated Pd nanoparticles retain their unique catalytic properties toward the H2 evolution reaction. As a result, Pd/TiO2@C with an optimized carbon layer thickness facilitates improved photocatalytic synthesis of imines, with conversion and selectivity as high as 95% and 99%, respectively. This study provides an effective strategy to develop high-performance supported metal nanocatalysts for integrated photocatalytic systems to produce H2 and valuable organic chemicals.
2023, 39(4): 230102
doi: 10.3866/PKU.WHXB202301023
Abstract:
NiMo(O) catalysts show extremely low overpotential at high current density for the electrocatalytic hydrogen evolution reaction (HER). However, the real reason for the remarkable electrocatalytic activity is unclear. A new perspective for revealing the relation between the phase structures of the electrocatalysts and their electrocatalytic HER performance provides a deep insight into the nature of the HER. Herein, the dehydration and oxygenation of as-synthesized nickel molybdate hydrate (NiMoO4·nH2O) are discussed and confirmed to be critical for evolving the catalytic phase structures in the subsequent reduction treatment. The typical phase evolution processes of the electrocatalysts were investigated using thermogravimetric (TG) analysis and H2 temperature-programmed reduction (H2-TPR). The crystalline phases were identified through X-ray diffraction (XRD), Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM) analyses. The phases of the electrocatalysts during the electrochemical tests were confirmed by in situ electrochemical XRD characterization. Three typical crystalline phases, MoNi4, β-NiMoO4, and α-NiMoO4, corresponding to significantly different HER activities, were proposed. The β-NiMoO4 dominant electrocatalyst (NiMoO4-400air-H2) exhibited the worst performance for alkaline water reduction, and an improvement was observed for the α-NiMoO4 electrocatalyst (NiMoO4-500air-H2). The NiMoO4-300air-H2 electrode derived from NiMoO4·(n−x)H2O exhibited the most active phase (MoNi4) and the best electrocatalytic HER performance. Moreover, the intrinsic electrocatalytic HER performance obtained from the electrochemical active surface area (ECSA) normalized activities exhibits the same tendency as the geometrically normalized ones. Varied adsorption capacities of the H2O, OH, and H intermediate species for water reduction on these typical phases are assumed to be responsible for the significantly different HER performance of the NiMoO4-(T)air-H2 electrodes through density functional theory analysis. Poor adsorption of H, OH radicals, and H2O on β-NiMoO4 impedes the water dissociation process, which may be the reason that it exhibits the worst electrocatalytic hydrogen evolution activity. Optimized adsorption abilities of H, OH, and H2O on α-NiMoO4 benefit the water reduction kinetics, leading to an enhanced electrocatalytic HER performance. MoNi4 forms the strongest interactions with H2O, H, and OH species, contributing to the best electrocatalytic hydrogen evolution activity. Further analysis of the energy barrier of the water-splitting reaction shows that these three crystalline phases exhibit different water dissociation ability, which is attributed to their varied adsorption capacities of the intermediate species for water reduction. Among them, MoNi4 and β-NiMoO4 exhibit the lowest and highest water dissociation barriers, respectively, in line with their electrocatalytic hydrogen evolution activities. The phase-dependent HER activity identified in this work can provide guidelines for rationally designing and adjusting the structures of active NiMo(O) electrocatalysts.![]()
NiMo(O) catalysts show extremely low overpotential at high current density for the electrocatalytic hydrogen evolution reaction (HER). However, the real reason for the remarkable electrocatalytic activity is unclear. A new perspective for revealing the relation between the phase structures of the electrocatalysts and their electrocatalytic HER performance provides a deep insight into the nature of the HER. Herein, the dehydration and oxygenation of as-synthesized nickel molybdate hydrate (NiMoO4·nH2O) are discussed and confirmed to be critical for evolving the catalytic phase structures in the subsequent reduction treatment. The typical phase evolution processes of the electrocatalysts were investigated using thermogravimetric (TG) analysis and H2 temperature-programmed reduction (H2-TPR). The crystalline phases were identified through X-ray diffraction (XRD), Raman spectroscopy, and high-resolution transmission electron microscopy (HRTEM) analyses. The phases of the electrocatalysts during the electrochemical tests were confirmed by in situ electrochemical XRD characterization. Three typical crystalline phases, MoNi4, β-NiMoO4, and α-NiMoO4, corresponding to significantly different HER activities, were proposed. The β-NiMoO4 dominant electrocatalyst (NiMoO4-400air-H2) exhibited the worst performance for alkaline water reduction, and an improvement was observed for the α-NiMoO4 electrocatalyst (NiMoO4-500air-H2). The NiMoO4-300air-H2 electrode derived from NiMoO4·(n−x)H2O exhibited the most active phase (MoNi4) and the best electrocatalytic HER performance. Moreover, the intrinsic electrocatalytic HER performance obtained from the electrochemical active surface area (ECSA) normalized activities exhibits the same tendency as the geometrically normalized ones. Varied adsorption capacities of the H2O, OH, and H intermediate species for water reduction on these typical phases are assumed to be responsible for the significantly different HER performance of the NiMoO4-(T)air-H2 electrodes through density functional theory analysis. Poor adsorption of H, OH radicals, and H2O on β-NiMoO4 impedes the water dissociation process, which may be the reason that it exhibits the worst electrocatalytic hydrogen evolution activity. Optimized adsorption abilities of H, OH, and H2O on α-NiMoO4 benefit the water reduction kinetics, leading to an enhanced electrocatalytic HER performance. MoNi4 forms the strongest interactions with H2O, H, and OH species, contributing to the best electrocatalytic hydrogen evolution activity. Further analysis of the energy barrier of the water-splitting reaction shows that these three crystalline phases exhibit different water dissociation ability, which is attributed to their varied adsorption capacities of the intermediate species for water reduction. Among them, MoNi4 and β-NiMoO4 exhibit the lowest and highest water dissociation barriers, respectively, in line with their electrocatalytic hydrogen evolution activities. The phase-dependent HER activity identified in this work can provide guidelines for rationally designing and adjusting the structures of active NiMo(O) electrocatalysts.
2023, 39(4): 220803
doi: 10.3866/PKU.WHXB202208033
Abstract:
The current global population and economy depends on fossil fuel consumption; however, the uncontrolled exploitation of fossil fuels has caused a series of energy crises and environmental problems, such as energy exhaustion, annual temperature rise, climate deterioration, and ocean acidification, which have already threatened the sustainable development of all living organisms. Therefore, finding renewable and reliable energy sources as well as reducing carbon dioxide (CO2) emissions have become the key focus in recent years. During the electrocatalytic CO2 reduction reaction (CO2RR) under relatively mild conditions, CO2 is converted into valuable products, such as C1, C2, and C2+ hydrocarbons, which is an effective strategy towards realizing "carbon neutrality". Electrocatalytic CO2RR is complex as it involves multiple electron/proton transfer processes. The reaction mechanism is also complex and involves many intermediates, which ultimately affects product selectivity. The large-scale application of the CO2RR requires the development of cheap and efficient electrocatalysts. Atomically dispersed metal and nitrogen co-doped carbon (M-N-C) materials, with large surface areas, 100% atomic availability, unsaturated coordination, and relatively uniform active sites, are promising catalysts for the CO2RR. M-N-C materials also have adjustable properties. For example, tuning the coordination environment of the central metal ions changes the electronic properties and atomic structures of the metal ions, which provides a new way for designing catalysts with high CO2RR performances. Therefore, it is of great significance to investigate the effect of regulating the electronic structure of M-N-C materials at the atomic level on catalytic activity and selectivity during the CO2RR. Additionally, the reduction potentials of the half reactions of most CO2RR products are within ±0.2 V of the hydrogen evolution reaction (HER), and most catalysts that bind CO2 are rich in electrons and active for the HER. Therefore, it is also necessary to design catalysts that can kinetically inhibit the competitive HER during the CO2RR. In this review, we discuss the synthesis methods of M-N-C materials, the reaction pathways of CO2 reduction to C1, C2, and C2+ hydrocarbons, and the main factors affecting the CO2RR. Specifically, three strategies for regulating the electronic structures and geometric configurations of M-N-C materials are systematically reviewed, namely, the modification of the carbon base surface of M-N-C materials, selection of appropriate central metal ions, and regulation of the coordination environment of the central metal ions. The effects of different active sites on the selectivity towards various products during the catalytic CO2RR are also discussed in detail. Finally, we highlight the current challenges and future development directions of M-N-C materials for the electrocatalytic CO2RR.![]()
The current global population and economy depends on fossil fuel consumption; however, the uncontrolled exploitation of fossil fuels has caused a series of energy crises and environmental problems, such as energy exhaustion, annual temperature rise, climate deterioration, and ocean acidification, which have already threatened the sustainable development of all living organisms. Therefore, finding renewable and reliable energy sources as well as reducing carbon dioxide (CO2) emissions have become the key focus in recent years. During the electrocatalytic CO2 reduction reaction (CO2RR) under relatively mild conditions, CO2 is converted into valuable products, such as C1, C2, and C2+ hydrocarbons, which is an effective strategy towards realizing "carbon neutrality". Electrocatalytic CO2RR is complex as it involves multiple electron/proton transfer processes. The reaction mechanism is also complex and involves many intermediates, which ultimately affects product selectivity. The large-scale application of the CO2RR requires the development of cheap and efficient electrocatalysts. Atomically dispersed metal and nitrogen co-doped carbon (M-N-C) materials, with large surface areas, 100% atomic availability, unsaturated coordination, and relatively uniform active sites, are promising catalysts for the CO2RR. M-N-C materials also have adjustable properties. For example, tuning the coordination environment of the central metal ions changes the electronic properties and atomic structures of the metal ions, which provides a new way for designing catalysts with high CO2RR performances. Therefore, it is of great significance to investigate the effect of regulating the electronic structure of M-N-C materials at the atomic level on catalytic activity and selectivity during the CO2RR. Additionally, the reduction potentials of the half reactions of most CO2RR products are within ±0.2 V of the hydrogen evolution reaction (HER), and most catalysts that bind CO2 are rich in electrons and active for the HER. Therefore, it is also necessary to design catalysts that can kinetically inhibit the competitive HER during the CO2RR. In this review, we discuss the synthesis methods of M-N-C materials, the reaction pathways of CO2 reduction to C1, C2, and C2+ hydrocarbons, and the main factors affecting the CO2RR. Specifically, three strategies for regulating the electronic structures and geometric configurations of M-N-C materials are systematically reviewed, namely, the modification of the carbon base surface of M-N-C materials, selection of appropriate central metal ions, and regulation of the coordination environment of the central metal ions. The effects of different active sites on the selectivity towards various products during the catalytic CO2RR are also discussed in detail. Finally, we highlight the current challenges and future development directions of M-N-C materials for the electrocatalytic CO2RR.
2023, 39(4): 221002
doi: 10.3866/PKU.WHXB202210025
Abstract:
Electrocatalytic reactions and processes are expected to be major drivers in society's shift toward renewable energy and chemicals. Electrocatalytic kinetic analysis is an accessible and informative technique to interrogate reaction mechanisms and establish structure–activity relationships. In this tutorial, we discuss general procedures, implicit assumptions, and potential pitfalls when conducting the Tafel analysis in the context of three widely investigated electrocatalytic reactions, i.e., the electrochemical CO2, CO, and O2 reduction reactions. Basic concepts and relations among key thermodynamic and kinetic variables are also covered to help interpret the activation parameters of electrochemical reactions.![]()
Electrocatalytic reactions and processes are expected to be major drivers in society's shift toward renewable energy and chemicals. Electrocatalytic kinetic analysis is an accessible and informative technique to interrogate reaction mechanisms and establish structure–activity relationships. In this tutorial, we discuss general procedures, implicit assumptions, and potential pitfalls when conducting the Tafel analysis in the context of three widely investigated electrocatalytic reactions, i.e., the electrochemical CO2, CO, and O2 reduction reactions. Basic concepts and relations among key thermodynamic and kinetic variables are also covered to help interpret the activation parameters of electrochemical reactions.
