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2020 Volume 41 Issue 12
2020, 41(12):
Abstract:
2020, 41(12): 1791-1811
doi: 10.1016/S1872-2067(20)63652-X
Abstract:
Among the various types of heterogeneous catalysts, supported metal nanocatalysts (SMNCs) have attracted widespread interest in chemistry and materials science, due to their advantageous features, such as high efficiency, stability, and reusability for catalytic reactions. However, to obtain well-defined SMNCs and inhibit nanoparticle aggregation, traditional approaches generally involve numerous organic reagents, complex steps, and specialized equipment, thus hindering the practical and large-scale synthesis of SMNCs. In this review, we summarize green and sustainable synthetic methodologies for the assembly of SMNCs, including low temperature pyrolysis and solid-state, surfactant- and reductant-free, and ionic liquid assisted syntheses. The conventional application of SMNCs for electrochemical hydrogen evolution and the corresponding achievements are subsequently discussed. Finally, future perspectives toward the sustainable production of SMNCs are presented.
Among the various types of heterogeneous catalysts, supported metal nanocatalysts (SMNCs) have attracted widespread interest in chemistry and materials science, due to their advantageous features, such as high efficiency, stability, and reusability for catalytic reactions. However, to obtain well-defined SMNCs and inhibit nanoparticle aggregation, traditional approaches generally involve numerous organic reagents, complex steps, and specialized equipment, thus hindering the practical and large-scale synthesis of SMNCs. In this review, we summarize green and sustainable synthetic methodologies for the assembly of SMNCs, including low temperature pyrolysis and solid-state, surfactant- and reductant-free, and ionic liquid assisted syntheses. The conventional application of SMNCs for electrochemical hydrogen evolution and the corresponding achievements are subsequently discussed. Finally, future perspectives toward the sustainable production of SMNCs are presented.
2020, 41(12): 1812-1817
doi: 10.1016/S1872-2067(20)63651-8
Abstract:
Selective aerobic oxidation of alcohols under mild conditions is of great importance yet challenging, with the activation of molecular oxygen (O2) as a crucial capability of the catalysts. Herein, we demonstrate that an Al2O3-supported Pd single-atom catalyst leads to higher activity and selectivity compared to Pd nanoparticles for the oxidation of cinnamyl alcohol. The Al2O3 support used in this study is rich in coordinately unsaturated Al3+ sites, which are apt for binding to Pd atoms through oxygen bridges and present a distinct metal-support interaction (MSI). The suitable MSI then leads to a unique electronic characteristic of the Pd single atoms, which can be confirmed via X-ray photoelectron spectroscopy, normalized X-ray absorption near-edge structure, and diffuse reflectance Fourier transform infrared spectroscopy. Moreover, this unique electronic state is proposed to be responsible for its high catalytic activity. With the help of in-situ UV-vis spectra and electron spin resonance spectra, a specific alcohol oxidation route with O2 activation mechanism is then identified. Active oxygen species behaving chemically like singlet-O2 are generated from the interaction of O2 with Pd1/Al2O3, and then oxidize the partially dehydrogenated intermediates produced by the adsorbed allylic alcohols and Pd atoms to the desired alkenyl aldehyde. This work provides a promising path for the design and development of high-activity catalysts for aerobic oxidation reactions.
Selective aerobic oxidation of alcohols under mild conditions is of great importance yet challenging, with the activation of molecular oxygen (O2) as a crucial capability of the catalysts. Herein, we demonstrate that an Al2O3-supported Pd single-atom catalyst leads to higher activity and selectivity compared to Pd nanoparticles for the oxidation of cinnamyl alcohol. The Al2O3 support used in this study is rich in coordinately unsaturated Al3+ sites, which are apt for binding to Pd atoms through oxygen bridges and present a distinct metal-support interaction (MSI). The suitable MSI then leads to a unique electronic characteristic of the Pd single atoms, which can be confirmed via X-ray photoelectron spectroscopy, normalized X-ray absorption near-edge structure, and diffuse reflectance Fourier transform infrared spectroscopy. Moreover, this unique electronic state is proposed to be responsible for its high catalytic activity. With the help of in-situ UV-vis spectra and electron spin resonance spectra, a specific alcohol oxidation route with O2 activation mechanism is then identified. Active oxygen species behaving chemically like singlet-O2 are generated from the interaction of O2 with Pd1/Al2O3, and then oxidize the partially dehydrogenated intermediates produced by the adsorbed allylic alcohols and Pd atoms to the desired alkenyl aldehyde. This work provides a promising path for the design and development of high-activity catalysts for aerobic oxidation reactions.
2020, 41(12): 1818-1825
doi: 10.1016/S1872-2067(20)63624-5
Abstract:
Spinel-type manganese-cobalt oxides have been regarded as important class of electrocatalysts for oxygen reduction reaction (ORR). However, they are usually synthesized through oxidation-precipitation under aqueous ammonia and then crystallization at high temperature (150-180 ℃), which not only increases the energy consumption but also induces the growth of particles that is unfavorable for ORR. Herein, through a facile precipitation-dehydration method, ultrasmall spinel manganese-cobalt oxide nanoparticles (~5 nm) homogeneously dispersed on conductive carbon black (MnxCo3-xO4/C) were fabricated at low temperature (60 ℃). And the bimetallic composite oxide (Mn1.5Co1.5O4/C) with cubic spinel structure and high Mn content exhibits remarkable enhancement of ORR activity and stability compared with single metal oxide (both Mn3O4/C and Co3O4/C). The essential reason for the enhancement of activity can be attributed to the presence of the mixed Mn3+ and Mn4+ cations in Mn1.5Co1.5O4/C. Moreover, the ORR activity of Mn1.5Co1.5O4/C is comparable to that of commercial 20 wt% Pt/C, and the relative current density only decreases 1.4% after 12 h test, exceeding that of Pt/C and most reported manganese-cobalt oxide electrocatalysts.
Spinel-type manganese-cobalt oxides have been regarded as important class of electrocatalysts for oxygen reduction reaction (ORR). However, they are usually synthesized through oxidation-precipitation under aqueous ammonia and then crystallization at high temperature (150-180 ℃), which not only increases the energy consumption but also induces the growth of particles that is unfavorable for ORR. Herein, through a facile precipitation-dehydration method, ultrasmall spinel manganese-cobalt oxide nanoparticles (~5 nm) homogeneously dispersed on conductive carbon black (MnxCo3-xO4/C) were fabricated at low temperature (60 ℃). And the bimetallic composite oxide (Mn1.5Co1.5O4/C) with cubic spinel structure and high Mn content exhibits remarkable enhancement of ORR activity and stability compared with single metal oxide (both Mn3O4/C and Co3O4/C). The essential reason for the enhancement of activity can be attributed to the presence of the mixed Mn3+ and Mn4+ cations in Mn1.5Co1.5O4/C. Moreover, the ORR activity of Mn1.5Co1.5O4/C is comparable to that of commercial 20 wt% Pt/C, and the relative current density only decreases 1.4% after 12 h test, exceeding that of Pt/C and most reported manganese-cobalt oxide electrocatalysts.
2020, 41(12): 1826-1836
doi: 10.1016/S1872-2067(20)63646-4
Abstract:
Cobalt-based oxides, with high abundance, good stability and excellent catalytic performance, are regarded as promising photocatalysts for artificial photosynthetic systems to alleviate foreseeable energy shortages and global warming. Herein, for the first time, a series of novel spongy porous CDs@CoOx materials were synthesized to act as an efficient and stable bifunctional photocatalyst for water oxidation and CO2 reduction. Notably, the preparation temperatures visibly influence the morphologies and photocatalytic performances of the CDs@CoOx. Under the optimal conditions, a maximum O2 yield of 40.4% and pretty apparent quantum efficiency (AQE) of 58.6% at 460 nm were obtained over CDs@CoOx-300 for water oxidation. Similarly, the optimized sample CDs@CoOx-300 manifests significant enhancement on the CO2-to-CO conversion with a high selectivity of 89.3% and CO generation rate of 8.1 μmol/h, which is superior to most previous cobalt-based catalysts for CO2 reduction. The composite CDs@CoOx-300 not only exposes more active sites but also facilitates electron transport, which results in excellent photocatalytic activity. In addition, the boosted photocatalytic behavior is attributed to the synergistic effect between CoOx and CDs, which was verified by the photocatalytic activity control experiments and electrochemical characterization. The work offers a novel strategy to fabricate a high performance bifunctional photocatalyst for water oxidation and CO2 reduction.
Cobalt-based oxides, with high abundance, good stability and excellent catalytic performance, are regarded as promising photocatalysts for artificial photosynthetic systems to alleviate foreseeable energy shortages and global warming. Herein, for the first time, a series of novel spongy porous CDs@CoOx materials were synthesized to act as an efficient and stable bifunctional photocatalyst for water oxidation and CO2 reduction. Notably, the preparation temperatures visibly influence the morphologies and photocatalytic performances of the CDs@CoOx. Under the optimal conditions, a maximum O2 yield of 40.4% and pretty apparent quantum efficiency (AQE) of 58.6% at 460 nm were obtained over CDs@CoOx-300 for water oxidation. Similarly, the optimized sample CDs@CoOx-300 manifests significant enhancement on the CO2-to-CO conversion with a high selectivity of 89.3% and CO generation rate of 8.1 μmol/h, which is superior to most previous cobalt-based catalysts for CO2 reduction. The composite CDs@CoOx-300 not only exposes more active sites but also facilitates electron transport, which results in excellent photocatalytic activity. In addition, the boosted photocatalytic behavior is attributed to the synergistic effect between CoOx and CDs, which was verified by the photocatalytic activity control experiments and electrochemical characterization. The work offers a novel strategy to fabricate a high performance bifunctional photocatalyst for water oxidation and CO2 reduction.
2020, 41(12): 1837-1845
doi: 10.1016/S1872-2067(20)63654-3
Abstract:
Ordered macroporous materials with rapid mass transport and enhanced active site accessibility are essential for achieving improved catalytic activity. In this study, boron phosphate crystals with a three-dimensionally interconnected ordered macroporous structure and a robust framework were fabricated and used as stable and selective catalysts in the oxidative dehydrogenation (ODH) of propane. Due to the improved mass diffusion and higher number of exposed active sites in the ordered macroporous structure, the catalyst exhibited a remarkable olefin productivity of ~16 golefin gcat-1 h-1, which is up to 2-100 times higher than that of ODH catalysts reported to date. The selectivity for olefins was 91.5% (propene: 82.5%, ethene:9.0%) at 515 ℃, with a propane conversion of 14.3%. At the same time, the selectivity for the unwanted deep-oxidized CO2 product remained less than 1.0%. The tri-coordinated surface boron species were identified as the active catalytic sites for the ODH of propane. This study provides a route for preparing a new type of metal-free catalyst with stable structure against oxidation and remarkable catalytic activity, which may represent a potential candidate to promote the industrialization of the ODH process.
Ordered macroporous materials with rapid mass transport and enhanced active site accessibility are essential for achieving improved catalytic activity. In this study, boron phosphate crystals with a three-dimensionally interconnected ordered macroporous structure and a robust framework were fabricated and used as stable and selective catalysts in the oxidative dehydrogenation (ODH) of propane. Due to the improved mass diffusion and higher number of exposed active sites in the ordered macroporous structure, the catalyst exhibited a remarkable olefin productivity of ~16 golefin gcat-1 h-1, which is up to 2-100 times higher than that of ODH catalysts reported to date. The selectivity for olefins was 91.5% (propene: 82.5%, ethene:9.0%) at 515 ℃, with a propane conversion of 14.3%. At the same time, the selectivity for the unwanted deep-oxidized CO2 product remained less than 1.0%. The tri-coordinated surface boron species were identified as the active catalytic sites for the ODH of propane. This study provides a route for preparing a new type of metal-free catalyst with stable structure against oxidation and remarkable catalytic activity, which may represent a potential candidate to promote the industrialization of the ODH process.
2020, 41(12): 1846-1854
doi: 10.1016/S1872-2067(20)63635-X
Abstract:
A mechanochemical redox reaction between KMnO4 and CoCl2 was developed to obtain a CoxMn1-xOy catalyst with a specific surface area of 479 m2 g-1, which was higher than that obtained using a co-precipitation (CP) method (34 m2 g-1), sol-gel (SG) method (72 m2 g-1), or solution redox process (131 m2 g-1). During catalytic combustion, this CoxMn1-xOy catalyst exhibited better activity (T100 for propylene= ~200 ℃) than the control catalysts obtained using the SG (325 ℃) or CP (450 ℃) methods. The mechanical action, mainly in the form of kinetic energy and frictional heating, may generate a high degree of interstitial porosity, while the redox reaction could contribute to good dispersion of cobalt and manganese species. Moreover, the as-prepared CoxMn1-xOy catalyst worked well in the presence of water vapor (H2O 4.2%, >60 h) or SO2 (100 ppm) and at high temperature (400 ℃, >60 h). The structure MnO2·(CoOOH)2.93 was suggested for the current CoxMn1-xOy catalyst. This catalyst could be extended to the total oxidation of other typical hydrocarbons (T90=150 ℃ for ethanol, T90 =225 ℃ for acetone, T90=250 ℃ for toluene, T90 =120 ℃ for CO, and T90=540 ℃ for CH4). Scale-up of the synthesis of CoxMn1-xOy catalyst (1 kg) can be achieved via ball milling, which may provide a potential strategy for real world catalysis.
A mechanochemical redox reaction between KMnO4 and CoCl2 was developed to obtain a CoxMn1-xOy catalyst with a specific surface area of 479 m2 g-1, which was higher than that obtained using a co-precipitation (CP) method (34 m2 g-1), sol-gel (SG) method (72 m2 g-1), or solution redox process (131 m2 g-1). During catalytic combustion, this CoxMn1-xOy catalyst exhibited better activity (T100 for propylene= ~200 ℃) than the control catalysts obtained using the SG (325 ℃) or CP (450 ℃) methods. The mechanical action, mainly in the form of kinetic energy and frictional heating, may generate a high degree of interstitial porosity, while the redox reaction could contribute to good dispersion of cobalt and manganese species. Moreover, the as-prepared CoxMn1-xOy catalyst worked well in the presence of water vapor (H2O 4.2%, >60 h) or SO2 (100 ppm) and at high temperature (400 ℃, >60 h). The structure MnO2·(CoOOH)2.93 was suggested for the current CoxMn1-xOy catalyst. This catalyst could be extended to the total oxidation of other typical hydrocarbons (T90=150 ℃ for ethanol, T90 =225 ℃ for acetone, T90=250 ℃ for toluene, T90 =120 ℃ for CO, and T90=540 ℃ for CH4). Scale-up of the synthesis of CoxMn1-xOy catalyst (1 kg) can be achieved via ball milling, which may provide a potential strategy for real world catalysis.
2020, 41(12): 1855-1863
doi: 10.1016/S1872-2067(20)63638-5
Abstract:
Cation substitution in spinel cobaltites (e.g., ACo2O4, in which A=Mn, Fe, Co, Ni, Cu, or Zn) is a promising strategy to precisely modulate their electronic structure/properties and thus improve the corresponding electrochemical performance for water splitting. However, the fundamental principles and mechanisms are not fully understood. This research aims to systematically investigate the effects of cation substitution in spinel cobaltites derived from mixed-metal-organic frameworks on the oxygen evolution reaction (OER). Among the obtained ACo2O4 catalysts, FeCo2O4 showed excellent OER performance with a current density of 10 mA·cm-2 at an overpotential of 164 mV in alkaline media. Both theoretical calculations and experimental results demonstrate that the Fe substitution in the crystal lattice of ACo2O4 can significantly accelerate charge transfer, thereby achieving enhanced electrochemical properties. The crystal field of spinel ACo2O4, which determines the valence states of cations A, is identified as the key factor to dictate the OER performance of these spinel cobaltites.
Cation substitution in spinel cobaltites (e.g., ACo2O4, in which A=Mn, Fe, Co, Ni, Cu, or Zn) is a promising strategy to precisely modulate their electronic structure/properties and thus improve the corresponding electrochemical performance for water splitting. However, the fundamental principles and mechanisms are not fully understood. This research aims to systematically investigate the effects of cation substitution in spinel cobaltites derived from mixed-metal-organic frameworks on the oxygen evolution reaction (OER). Among the obtained ACo2O4 catalysts, FeCo2O4 showed excellent OER performance with a current density of 10 mA·cm-2 at an overpotential of 164 mV in alkaline media. Both theoretical calculations and experimental results demonstrate that the Fe substitution in the crystal lattice of ACo2O4 can significantly accelerate charge transfer, thereby achieving enhanced electrochemical properties. The crystal field of spinel ACo2O4, which determines the valence states of cations A, is identified as the key factor to dictate the OER performance of these spinel cobaltites.
2020, 41(12): 1864-1872
doi: 10.1016/S1872-2067(20)63653-1
Abstract:
The solubility of ammonium tungstate in a special hydrothermal condition is exploited to synthesize uniform microspheres of Ce-Cu-W-O oxides. Compared to their W-undoped counterparts, they possess more Ce3+ and oxygen vacancies, thereby promoting oxygen mobility. The formed rich WO3 surface can effectively provide acid sites, which is helpful for adsorption of vinyl chloride and interrupting the C-Cl bond. In addition, the presence of WO3 induces the formation of finer CuO nanoparticles with respect to the traditional coprecipitation method, thereby resulting in a better reducibility. Benefiting from both the enhanced acidity and reducibility, the Ce-Cu-W-O microspheres deliver excellent low-temperature vinyl chloride oxidation activity (a reaction rate of 2.01×10-7 mol/(gcat·s) at 250 ℃) and high HCl selectivity. Moreover, subtle deactivation occurs after the three cycling activity tests, and a stable vinyl chloride conversion as well as mineralization are observed during the 72-h durability test at 300 ℃, which demonstrates good thermal stability. Our strategy can provide new insights into the design and synthesis of metal oxides for catalytic oxidation of chlorinated volatile organic compounds.
The solubility of ammonium tungstate in a special hydrothermal condition is exploited to synthesize uniform microspheres of Ce-Cu-W-O oxides. Compared to their W-undoped counterparts, they possess more Ce3+ and oxygen vacancies, thereby promoting oxygen mobility. The formed rich WO3 surface can effectively provide acid sites, which is helpful for adsorption of vinyl chloride and interrupting the C-Cl bond. In addition, the presence of WO3 induces the formation of finer CuO nanoparticles with respect to the traditional coprecipitation method, thereby resulting in a better reducibility. Benefiting from both the enhanced acidity and reducibility, the Ce-Cu-W-O microspheres deliver excellent low-temperature vinyl chloride oxidation activity (a reaction rate of 2.01×10-7 mol/(gcat·s) at 250 ℃) and high HCl selectivity. Moreover, subtle deactivation occurs after the three cycling activity tests, and a stable vinyl chloride conversion as well as mineralization are observed during the 72-h durability test at 300 ℃, which demonstrates good thermal stability. Our strategy can provide new insights into the design and synthesis of metal oxides for catalytic oxidation of chlorinated volatile organic compounds.
2020, 41(12): 1873-1883
doi: 10.1016/S1872-2067(20)63641-5
Abstract:
Herein, a bottom-down design is presented to successfully fabricate ZIF-derived Co3O4, grown in situ on a one-dimensional (1D) α-MnO2 material, denoted as α-MnO2@Co3O4. The synergistic effect derived from the coupled interface constructed between α-MnO2 and Co3O4 is responsible for the enhanced catalytic activity. The resultant α-MnO2@Co3O4 catalyst exhibits excellent catalytic activity at a T90% (temperature required to achieve a toluene conversion of 90%) of approximately 229 ℃, which is 47 and 28 ℃ lower than those of the pure α-MnO2 nanowire and Co3O4-b obtained via pyrolysis of ZIF-67, respectively. This activity is attributed to the increase in the number of surface-adsorbed oxygen species, which accelerate the oxygen mobility and enhance the redox pairs of Mn4+/Mn3+ and Co2+/Co3+. Moreover, the result of in situ diffuse reflectance infrared Fourier transform spectroscopy suggests that the gaseous oxygen could be more easily activated to adsorbed oxygen species on the surface of α-MnO2@Co3O4 than on that of α-MnO2. The catalytic reaction route of toluene oxidation over the α-MnO2@Co3O4 catalyst is as follows:toluene → benzoate species → alkanes containing oxygen functional group → CO2 and H2O. In addition, the α-MnO2@Co3O4 catalyst shows excellent stability and good water resistance for toluene oxidation. Furthermore, the preparation method can be extended to other 1D MnO2 materials. A new strategy for the development of high-performance catalysts of practical significance is provided.
Herein, a bottom-down design is presented to successfully fabricate ZIF-derived Co3O4, grown in situ on a one-dimensional (1D) α-MnO2 material, denoted as α-MnO2@Co3O4. The synergistic effect derived from the coupled interface constructed between α-MnO2 and Co3O4 is responsible for the enhanced catalytic activity. The resultant α-MnO2@Co3O4 catalyst exhibits excellent catalytic activity at a T90% (temperature required to achieve a toluene conversion of 90%) of approximately 229 ℃, which is 47 and 28 ℃ lower than those of the pure α-MnO2 nanowire and Co3O4-b obtained via pyrolysis of ZIF-67, respectively. This activity is attributed to the increase in the number of surface-adsorbed oxygen species, which accelerate the oxygen mobility and enhance the redox pairs of Mn4+/Mn3+ and Co2+/Co3+. Moreover, the result of in situ diffuse reflectance infrared Fourier transform spectroscopy suggests that the gaseous oxygen could be more easily activated to adsorbed oxygen species on the surface of α-MnO2@Co3O4 than on that of α-MnO2. The catalytic reaction route of toluene oxidation over the α-MnO2@Co3O4 catalyst is as follows:toluene → benzoate species → alkanes containing oxygen functional group → CO2 and H2O. In addition, the α-MnO2@Co3O4 catalyst shows excellent stability and good water resistance for toluene oxidation. Furthermore, the preparation method can be extended to other 1D MnO2 materials. A new strategy for the development of high-performance catalysts of practical significance is provided.
2020, 41(12): 1884-1893
doi: 10.1016/S1872-2067(20)63637-3
Abstract:
Designing low-cost and high-performance photoelectrodes with improved light harvesting and charge separation rates is significant in photoelectrochemical water splitting. Here, a novel TiO2/Cu2O/Al/Al2O3 photoelectrode is manufactured by depositing plasmonic nanoparticles of the non-noble metal Al on the surface of a TiO2/Cu2O core/shell heterojunction for the first time. The Al nanoparticles, which exhibit a surface plasmon resonance (SPR) effect and are substantially less expensive than noble metals such as Au and Ag, generate hot electron-hole pairs and amplify the electromagnetic field at the interface under illumination. The as-prepared TiO2/Cu2O/Al/Al2O3 photoelectrodes have an extended absorption range and enhanced carrier separation and transfer. Their photocurrent density of 4.52 mA·cm-2 at 1.23 V vs. RHE represents an 1.84-fold improvement over that of TiO2/Cu2O. Specifically, the ultrathin Al2O3 passivation layer spontaneously generated on the surface of Al in air could act as a protective layer to significantly increase its stability. In this work, the synergistic effect of the heterojunctions and the SPR effect of the non-noble metal Al significantly improve the photoelectrode performance, providing a novel concept for the design of electrodes with good properties and high practicability.
Designing low-cost and high-performance photoelectrodes with improved light harvesting and charge separation rates is significant in photoelectrochemical water splitting. Here, a novel TiO2/Cu2O/Al/Al2O3 photoelectrode is manufactured by depositing plasmonic nanoparticles of the non-noble metal Al on the surface of a TiO2/Cu2O core/shell heterojunction for the first time. The Al nanoparticles, which exhibit a surface plasmon resonance (SPR) effect and are substantially less expensive than noble metals such as Au and Ag, generate hot electron-hole pairs and amplify the electromagnetic field at the interface under illumination. The as-prepared TiO2/Cu2O/Al/Al2O3 photoelectrodes have an extended absorption range and enhanced carrier separation and transfer. Their photocurrent density of 4.52 mA·cm-2 at 1.23 V vs. RHE represents an 1.84-fold improvement over that of TiO2/Cu2O. Specifically, the ultrathin Al2O3 passivation layer spontaneously generated on the surface of Al in air could act as a protective layer to significantly increase its stability. In this work, the synergistic effect of the heterojunctions and the SPR effect of the non-noble metal Al significantly improve the photoelectrode performance, providing a novel concept for the design of electrodes with good properties and high practicability.
2020, 41(12): 1894-1905
doi: 10.1016/S1872-2067(20)63620-8
Abstract:
Uniformly distributed single layer of ZIF67-derived C3N4 (ZIF67-C3N4) was synthesized and applied to the photocatalytic degradation of methylene blue (MB) under visible light. Results indicated that the obtained ZIF67-C3N4has a maximum specific surface area of 541.392 m2/g, which is much larger than that of raw C3N4 of 97.291 m2/g. The investigation of C3N4 amount involved in ZIF67-C3N4on the photoactivity revealed that 2.57 g ZIF67 with 0.3 g C3N4, which named ZIF67-C3N4(0.3) exhibited superior photocatalytic activities. More than 90% of MB at 10 mg/L was degraded within 70 min with the addition of 0.01 g ZIF67-C3N4(0.3), while this time required for raw C3N4 was over 140 min. The effects of pH of solution, initial concentration of MB and dosage of C3N4 in ZIF67-C3N4composites on the photocatalytic efficiency for MB degradation were also evaluated. Quenching experiments indicated that the photo-induced holes (h+) and superoxide radicals (O2-·) were mainly responsible for MB degradation. It is anticipated that the insertion of ZIF67 nanoparticles not only increases the adsorption capacity of C3N4 but also promotes the generation and migration of the photo-induced active species.
Uniformly distributed single layer of ZIF67-derived C3N4 (ZIF67-C3N4) was synthesized and applied to the photocatalytic degradation of methylene blue (MB) under visible light. Results indicated that the obtained ZIF67-C3N4has a maximum specific surface area of 541.392 m2/g, which is much larger than that of raw C3N4 of 97.291 m2/g. The investigation of C3N4 amount involved in ZIF67-C3N4on the photoactivity revealed that 2.57 g ZIF67 with 0.3 g C3N4, which named ZIF67-C3N4(0.3) exhibited superior photocatalytic activities. More than 90% of MB at 10 mg/L was degraded within 70 min with the addition of 0.01 g ZIF67-C3N4(0.3), while this time required for raw C3N4 was over 140 min. The effects of pH of solution, initial concentration of MB and dosage of C3N4 in ZIF67-C3N4composites on the photocatalytic efficiency for MB degradation were also evaluated. Quenching experiments indicated that the photo-induced holes (h+) and superoxide radicals (O2-·) were mainly responsible for MB degradation. It is anticipated that the insertion of ZIF67 nanoparticles not only increases the adsorption capacity of C3N4 but also promotes the generation and migration of the photo-induced active species.
2020, 41(12): 1906-1915
doi: 10.1016/S1872-2067(20)63627-0
Abstract:
Direct syngas conversion to light olefins on bifunctional oxide-zeolite (OX-ZEO) catalysts is of great interest to both academia and industry, but the role of oxygen vacancy (Vo) in metal oxides and whether the key intermediate in the reaction mechanism is ketene or methanol are still not well-understood. To address these two issues, we carry out a theoretical study of the syngas conversion on the typical reducible metal oxide, CeO2, using density functional theory calculations. Our results demonstrate that by forming frustrated Lewis pairs (FLPs), the VOs in CeO2 play a key role in the activation of H2 and CO. The activation of H2 on FLPs undergoes a heterolytic dissociative pathway with a tiny barrier of 0.01 eV, while CO is activated on FLPs by combining with the basic site (O atom) of FLPs to form CO22-. Four pathways for the conversion of syngas were explored on FLPs, two of which are prone to form ketene and the other two are inclined to produce methanol suggesting a compromise to resolve the debate about the key intermediates (ketene or methanol) in the experiments. Rate constant calculations showed that the route initiating with the coupling of two CO* into OCCO* and ending with the formation of ketene is the dominant pathway, with the neighboring FLPs playing an important role in this pathway. Overall, our study reveals the function of the surface FLPs in the activation of H2 and CO and the reaction mechanism for the production of ketene and methanol for the first time, providing novel insights into syngas conversion over OX-ZEO catalysts.
Direct syngas conversion to light olefins on bifunctional oxide-zeolite (OX-ZEO) catalysts is of great interest to both academia and industry, but the role of oxygen vacancy (Vo) in metal oxides and whether the key intermediate in the reaction mechanism is ketene or methanol are still not well-understood. To address these two issues, we carry out a theoretical study of the syngas conversion on the typical reducible metal oxide, CeO2, using density functional theory calculations. Our results demonstrate that by forming frustrated Lewis pairs (FLPs), the VOs in CeO2 play a key role in the activation of H2 and CO. The activation of H2 on FLPs undergoes a heterolytic dissociative pathway with a tiny barrier of 0.01 eV, while CO is activated on FLPs by combining with the basic site (O atom) of FLPs to form CO22-. Four pathways for the conversion of syngas were explored on FLPs, two of which are prone to form ketene and the other two are inclined to produce methanol suggesting a compromise to resolve the debate about the key intermediates (ketene or methanol) in the experiments. Rate constant calculations showed that the route initiating with the coupling of two CO* into OCCO* and ending with the formation of ketene is the dominant pathway, with the neighboring FLPs playing an important role in this pathway. Overall, our study reveals the function of the surface FLPs in the activation of H2 and CO and the reaction mechanism for the production of ketene and methanol for the first time, providing novel insights into syngas conversion over OX-ZEO catalysts.
