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2019 Volume 40 Issue 4
2019, 40(4):
Abstract:
2019, 40(4): 477-485
doi: S1872-2067(19)63281-X
Abstract:
The AEI cavity of SAPO-18 catalyst was modified with zinc cations with the conventional ion exchange procedure. The cavity modification effectively tunes the product selectivity, and shifts the products from mainly propylene to comparable production of ethylene and propylene in methanol to olefin (MTO) reaction. The incorporation of zinc ions and the generation of bicyclic aromatic species in the AEI cavity of SAPO-18 catalysts introduce additional diffusion hindrance that exert greater influence on the relatively bulky products (e.g. propylene and higher olefins), which increase the selectivity to small-sized products (e.g. ethylene). It appears that the incorporated zinc cations facilitate the generation of lower methylbenzenes which promote the generation of ethylene. The cavity modification via incorporating zinc ions effectively tunes the product selectivity over SAPO molecular sieves with relatively larger cavity, which provides a novel strategy to develop the potential alternative to SAPO-34 catalysts for industrial MTO reaction.
The AEI cavity of SAPO-18 catalyst was modified with zinc cations with the conventional ion exchange procedure. The cavity modification effectively tunes the product selectivity, and shifts the products from mainly propylene to comparable production of ethylene and propylene in methanol to olefin (MTO) reaction. The incorporation of zinc ions and the generation of bicyclic aromatic species in the AEI cavity of SAPO-18 catalysts introduce additional diffusion hindrance that exert greater influence on the relatively bulky products (e.g. propylene and higher olefins), which increase the selectivity to small-sized products (e.g. ethylene). It appears that the incorporated zinc cations facilitate the generation of lower methylbenzenes which promote the generation of ethylene. The cavity modification via incorporating zinc ions effectively tunes the product selectivity over SAPO molecular sieves with relatively larger cavity, which provides a novel strategy to develop the potential alternative to SAPO-34 catalysts for industrial MTO reaction.
2019, 40(4): 486-494
doi: S1872-2067(19)63311-5
Abstract:
Photocatalytic Z-scheme water splitting is considered as a promising approach to produce solar hydrogen. However, the forward hydrogen production reaction is often impeded by backward reactions. In the present study, in a photosystem Ⅱ-integrated hybrid Z-scheme water splitting system, the backward hydrogen oxidation reaction was significantly suppressed by loading a PtCrOx cocatalyst on a ZrO2/TaON photocatalyst. Due to the weak chemisorption and activation of molecular hydrogen on PtCrOx, where Pt is stabilized in the oxidized forms, PtⅡ and PtIV, hydrogen oxidation is inhibited. However, it is remarkably well-catalyzed by the metallic Pt cocatalyst, thereby rapidly consuming the produced hydrogen. This work describes an approach to inhibit the backward reaction in the photosystem Ⅱ-integrated hybrid Z-scheme water splitting system using Fe(CN)63-/Fe(CN)64- redox couple as an electron shuttle.
Photocatalytic Z-scheme water splitting is considered as a promising approach to produce solar hydrogen. However, the forward hydrogen production reaction is often impeded by backward reactions. In the present study, in a photosystem Ⅱ-integrated hybrid Z-scheme water splitting system, the backward hydrogen oxidation reaction was significantly suppressed by loading a PtCrOx cocatalyst on a ZrO2/TaON photocatalyst. Due to the weak chemisorption and activation of molecular hydrogen on PtCrOx, where Pt is stabilized in the oxidized forms, PtⅡ and PtIV, hydrogen oxidation is inhibited. However, it is remarkably well-catalyzed by the metallic Pt cocatalyst, thereby rapidly consuming the produced hydrogen. This work describes an approach to inhibit the backward reaction in the photosystem Ⅱ-integrated hybrid Z-scheme water splitting system using Fe(CN)63-/Fe(CN)64- redox couple as an electron shuttle.
2019, 40(4): 495-503
doi: S1872-2067(19)63289-4
Abstract:
CO methanation on Ni/CeO2 has recently received increasing attention. However, the low-temperature activity and carbon resistance of Ni/CeO2 still need to be improved. In this study, plasma decomposition of nickel nitrate was performed at ca. 150℃ and atmospheric pressure. This was followed by hydrogen reduction at 500℃ in the absence of plasma, and a highly dispersed Ni/CeO2 catalyst was obtained with improved CO adsorption and enhanced metal-support interaction. The plasma-decomposed catalyst showed significantly improved low-temperature activity with high methane selectivity (up to 100%) and enhanced carbon resistance for CO methanation. For example, at 250℃, the plasma-decomposed catalyst showed a CO conversion of 96.8% with high methane selectivity (almost 100%), whereas the CO conversion was only 14.7% for a thermally decomposed catalyst.
CO methanation on Ni/CeO2 has recently received increasing attention. However, the low-temperature activity and carbon resistance of Ni/CeO2 still need to be improved. In this study, plasma decomposition of nickel nitrate was performed at ca. 150℃ and atmospheric pressure. This was followed by hydrogen reduction at 500℃ in the absence of plasma, and a highly dispersed Ni/CeO2 catalyst was obtained with improved CO adsorption and enhanced metal-support interaction. The plasma-decomposed catalyst showed significantly improved low-temperature activity with high methane selectivity (up to 100%) and enhanced carbon resistance for CO methanation. For example, at 250℃, the plasma-decomposed catalyst showed a CO conversion of 96.8% with high methane selectivity (almost 100%), whereas the CO conversion was only 14.7% for a thermally decomposed catalyst.
2019, 40(4): 504-514
doi: S1872-2067(19)63304-8
Abstract:
To accelerate the kinetics of the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells, ultrafine Pt nanoparticles modified with trace amounts of cobalt were fabricated and decorated on carbon black through a strategy involving modified glycol reduction and chemical etching. The obtained Pt36Co/C catalyst exhibits a much larger electrochemical surface area (ECSA) and an improved ORR electrocatalytic activity compared to commercial Pt/C. Moreover, an electrode prepared with Pt36Co/C was further evaluated under H2-air single cell test conditions, and exhibited a maximum specific power density of 10.27 W mgPt-1, which is 1.61 times higher than that of a conventional Pt/C electrode and also competitive with most state-of-the-art Pt-based architectures. In addition, the changes in ECSA, power density, and reacting resistance during the accelerated degradation process further demonstrate the enhanced durability of the Pt36Co/C electrode. The superior performance observed in this work can be attributed to the synergy between the ultrasmall size and homogeneous distribution of catalyst nanoparticles, bimetallic ligand and electronic effects, and the dissolution of unstable Co with the rearrangement of surface structure brought about by acid etching. Furthermore, the accessible raw materials and simplified operating procedures involved in the fabrication process would result in great cost-effectiveness for practical applications of PEMFCs.
To accelerate the kinetics of the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells, ultrafine Pt nanoparticles modified with trace amounts of cobalt were fabricated and decorated on carbon black through a strategy involving modified glycol reduction and chemical etching. The obtained Pt36Co/C catalyst exhibits a much larger electrochemical surface area (ECSA) and an improved ORR electrocatalytic activity compared to commercial Pt/C. Moreover, an electrode prepared with Pt36Co/C was further evaluated under H2-air single cell test conditions, and exhibited a maximum specific power density of 10.27 W mgPt-1, which is 1.61 times higher than that of a conventional Pt/C electrode and also competitive with most state-of-the-art Pt-based architectures. In addition, the changes in ECSA, power density, and reacting resistance during the accelerated degradation process further demonstrate the enhanced durability of the Pt36Co/C electrode. The superior performance observed in this work can be attributed to the synergy between the ultrasmall size and homogeneous distribution of catalyst nanoparticles, bimetallic ligand and electronic effects, and the dissolution of unstable Co with the rearrangement of surface structure brought about by acid etching. Furthermore, the accessible raw materials and simplified operating procedures involved in the fabrication process would result in great cost-effectiveness for practical applications of PEMFCs.
2019, 40(4): 515-522
doi: S1872-2067(19)63318-8
Abstract:
Ethanol conversion to high-value-added products has attracted considerable attention in both academic research and industrial fields. In this study, we synthesized a series of tunable acid-base bifunctional Zn-Zr-Al metal oxides (represented as Zn2ZrxAl-MMO) in light of the structural topotactic transformation of Zn2ZrxAl-hydrotalcite precursors (Zn2ZrxAl-LDH). The resulting Zn2ZrxAl-MMO catalysts were employed in the conversion of ethanol to diethyl carbonate. The Zr4+ ion content of the LDH precursor plays a key role in modulating the acid-base properties and determining catalytic performance:the Zn2Zr0.1Al-MMO sample exhibits the optimal catalytic behavior with a diethyl carbonate (DEC) yield of 42.1%, which is the highest reported for metal oxide catalysts. Structure-property correlation investigations revealed that the synergic catalysis between medium-strong basic sites and weak acid sites plays a predominant role in the catalytic behavior. Furthermore, in situ Fourier transform infrared measurements showed that the weak acidic site promotes activation adsorption of the reactant (urea) and the intermediate product (ethyl carbamate), while the medium-strong basic site accelerates ethanol activation. Moreover, the Zn2Zr0.1Al-MMO catalyst has the advantages of cost effectiveness, good stability, and reusability. Therefore, the acid-base bifunctional catalysts developed in this work can be employed as promising candidates in acid-base catalytic reactions such as ethanol conversion.
Ethanol conversion to high-value-added products has attracted considerable attention in both academic research and industrial fields. In this study, we synthesized a series of tunable acid-base bifunctional Zn-Zr-Al metal oxides (represented as Zn2ZrxAl-MMO) in light of the structural topotactic transformation of Zn2ZrxAl-hydrotalcite precursors (Zn2ZrxAl-LDH). The resulting Zn2ZrxAl-MMO catalysts were employed in the conversion of ethanol to diethyl carbonate. The Zr4+ ion content of the LDH precursor plays a key role in modulating the acid-base properties and determining catalytic performance:the Zn2Zr0.1Al-MMO sample exhibits the optimal catalytic behavior with a diethyl carbonate (DEC) yield of 42.1%, which is the highest reported for metal oxide catalysts. Structure-property correlation investigations revealed that the synergic catalysis between medium-strong basic sites and weak acid sites plays a predominant role in the catalytic behavior. Furthermore, in situ Fourier transform infrared measurements showed that the weak acidic site promotes activation adsorption of the reactant (urea) and the intermediate product (ethyl carbamate), while the medium-strong basic site accelerates ethanol activation. Moreover, the Zn2Zr0.1Al-MMO catalyst has the advantages of cost effectiveness, good stability, and reusability. Therefore, the acid-base bifunctional catalysts developed in this work can be employed as promising candidates in acid-base catalytic reactions such as ethanol conversion.
2019, 40(4): 523-533
doi: S1872-2067(19)63314-0
Abstract:
Hydrogen peroxide (H2O2) is a very useful chemical reagent, but the current industrial methods for its production suffer from serious energy consumption problems. Using high-activity and high-selectivity catalysts to electrocatalyze the oxygen reduction reaction (ORR) through a two-electron (2e-) pathway is a very promising route to produce H2O2. In this work, we obtained partially oxidized multi-walled carbon nanotubes (MWCNTs) with controlled structure and composition by oxidation with concentrated sulfate and potassium permanganate at 40℃ for 1 h (O-CNTs-40-1). The outer layers of O-CNTs-40-1 are damaged with defects and oxygen-containing functional groups, while the inner layers are maintained intact. The optimized structure and composition of the partially oxidized MWCNTs ensure that O-CNTs-40-1 possesses both a sufficient number of catalytic sites and good conductivity. The results of rotating ring disk electrode measurements reveal that, among all oxidized MWCNTs, O-CNTs-40-1 shows the greatest improvement in hydrogen peroxide selectivity (from~30% to~50%) and electron transfer number (from~3.4 to~3.0) compared to those of the raw MWCNTs. The results of electrochemical impedance spectroscopy measurements indicate that both the charge-transfer and intrinsic resistances of O-CNTs-40-1 are lower than those of the raw MWCNTs and of the other oxidized MWCNTs. Finally, direct tests of the H2O2 production confirm the greatly improved catalytic activity of O-CNTs-40-1 relative to that of the raw MWCNTs.
Hydrogen peroxide (H2O2) is a very useful chemical reagent, but the current industrial methods for its production suffer from serious energy consumption problems. Using high-activity and high-selectivity catalysts to electrocatalyze the oxygen reduction reaction (ORR) through a two-electron (2e-) pathway is a very promising route to produce H2O2. In this work, we obtained partially oxidized multi-walled carbon nanotubes (MWCNTs) with controlled structure and composition by oxidation with concentrated sulfate and potassium permanganate at 40℃ for 1 h (O-CNTs-40-1). The outer layers of O-CNTs-40-1 are damaged with defects and oxygen-containing functional groups, while the inner layers are maintained intact. The optimized structure and composition of the partially oxidized MWCNTs ensure that O-CNTs-40-1 possesses both a sufficient number of catalytic sites and good conductivity. The results of rotating ring disk electrode measurements reveal that, among all oxidized MWCNTs, O-CNTs-40-1 shows the greatest improvement in hydrogen peroxide selectivity (from~30% to~50%) and electron transfer number (from~3.4 to~3.0) compared to those of the raw MWCNTs. The results of electrochemical impedance spectroscopy measurements indicate that both the charge-transfer and intrinsic resistances of O-CNTs-40-1 are lower than those of the raw MWCNTs and of the other oxidized MWCNTs. Finally, direct tests of the H2O2 production confirm the greatly improved catalytic activity of O-CNTs-40-1 relative to that of the raw MWCNTs.
2019, 40(4): 534-542
doi: S1872-2067(19)63319-X
Abstract:
Glycerol is a by-product of biodiesel production and is an important readily available platform chemical. Valorization of glycerol into value-added chemicals has gained immense attention. Herein, we carried out the conversion of glycerol to formic acid and glycolic acid using H2O2 as an oxidant and metal (Ⅲ) triflate-based catalytic systems. Aluminum(Ⅲ) triflate was found to be the most efficient catalyst for the selective oxidation of glycerol to formic acid. A correlation between the catalytic activity of the metal cations and their hydrolysis constants (Kh) and water exchange rate constants was observed. At 70℃, a formic acid yield of up to 72% could be attained within 12 h. The catalyst could be recycled at least five times with a high conversion rate, and hence can also be used for the selective oxidation of other biomass platform molecules. Reaction kinetics and 1H NMR studies showed that the oxidation of glycerol (to formic acid) involved glycerol hydrolysis pathways with glyceric acid and glycolic acid as the main intermediate products. Both the[Al(OH)x]n+ Lewis acid species and CF3SO3H Brønsted acid, which were generated by the in-situ hydrolysis of Al(OTf)3, were responsible for glycerol conversion. The easy availability, high efficiency, and good recyclability of Al(OTf)3 render it suitable for the selective oxidation of glycerol to high value-added products.
Glycerol is a by-product of biodiesel production and is an important readily available platform chemical. Valorization of glycerol into value-added chemicals has gained immense attention. Herein, we carried out the conversion of glycerol to formic acid and glycolic acid using H2O2 as an oxidant and metal (Ⅲ) triflate-based catalytic systems. Aluminum(Ⅲ) triflate was found to be the most efficient catalyst for the selective oxidation of glycerol to formic acid. A correlation between the catalytic activity of the metal cations and their hydrolysis constants (Kh) and water exchange rate constants was observed. At 70℃, a formic acid yield of up to 72% could be attained within 12 h. The catalyst could be recycled at least five times with a high conversion rate, and hence can also be used for the selective oxidation of other biomass platform molecules. Reaction kinetics and 1H NMR studies showed that the oxidation of glycerol (to formic acid) involved glycerol hydrolysis pathways with glyceric acid and glycolic acid as the main intermediate products. Both the[Al(OH)x]n+ Lewis acid species and CF3SO3H Brønsted acid, which were generated by the in-situ hydrolysis of Al(OTf)3, were responsible for glycerol conversion. The easy availability, high efficiency, and good recyclability of Al(OTf)3 render it suitable for the selective oxidation of glycerol to high value-added products.
2019, 40(4): 543-552
doi: S1872-2067(19)63292-4
Abstract:
Pt/Eu2O3-CeO2 materials with different Eu concentrations were prepared and applied to toluene destruction, and the remarkable promotion impact of EuOx on Pt/CeO2 can be observed. The characterization results reveal that the presence of EuOx significantly enhances the redox property, lattice O concentration, and Ce3+ ratio of the Pt/CeO2 material, which facilitates the dispersion and activity of Pt active sites and thus accelerates the decomposition process of toluene. Among all catalysts, a sample with an Eu content of 2.5 at.% (Pt/EC-2.5) possesses the best catalytic activity with 0.09 vol% of toluene completely destructed at 200℃ under a relatively high GHSV of 50000 h-1. The possible reaction pathway and mechanism of toluene combustion over Pt/Eu2O3-CeO2 samples are presented according to in-situ DRIFTS, which confirms that the toluene oxidation process obeys the Mars-van Krevelen mechanism with aldehydes and ketones as primary organic intermediates.
Pt/Eu2O3-CeO2 materials with different Eu concentrations were prepared and applied to toluene destruction, and the remarkable promotion impact of EuOx on Pt/CeO2 can be observed. The characterization results reveal that the presence of EuOx significantly enhances the redox property, lattice O concentration, and Ce3+ ratio of the Pt/CeO2 material, which facilitates the dispersion and activity of Pt active sites and thus accelerates the decomposition process of toluene. Among all catalysts, a sample with an Eu content of 2.5 at.% (Pt/EC-2.5) possesses the best catalytic activity with 0.09 vol% of toluene completely destructed at 200℃ under a relatively high GHSV of 50000 h-1. The possible reaction pathway and mechanism of toluene combustion over Pt/Eu2O3-CeO2 samples are presented according to in-situ DRIFTS, which confirms that the toluene oxidation process obeys the Mars-van Krevelen mechanism with aldehydes and ketones as primary organic intermediates.
2019, 40(4): 553-566
doi: S1872-2067(19)63291-2
Abstract:
A series of metal-organic frameworks MOF-808-X (6-connected) were synthesized by regulating the ZrOCl2·8H2O/1,3,5-benzenetricarboxylic acid (BTC) molar ratio (X) and tested for the direct synthesis of dimethyl carbonate (DMC) from CO2 and CH3OH with 1,1,1-trimethoxymethane (TMM) as a dehydrating agent. The effect of the ZrOCl2·8H2O/BTC molar ratio on the physicochemical properties and catalytic performance of MOF-808-X was investigated. Results showed that a proper ZrOCl2·8H2O/BTC molar ratio during MOF-808-X synthesis was fairly important to reduce the redundant BTC or zirconium clusters trapped in the micropores of MOF-808-X. MOF-808-4, with almost no redundant BTC or zirconium clusters trapped in the micropores, exhibited the largest surface area, micropore size, and the number of acidic-basic sites, and consequently showed the best activity among all MOF-808-X, with the highest DMC yield of 21.5% under the optimal reaction conditions. Moreover, benefiting from the larger micropore size, MOF-808-4 outperformed our previously reported UiO-66-24 (12-connected), which had even more acidic-basic sites and larger surface area than MOF-808-4, mainly because the larger micropore size of MOF-808-4 provided higher accessibility for the reactant to the active sites located in the micropores. Furthermore, a possible reaction mechanism over MOF-808-4 was proposed based on the in situ FT-IR results. The effects of different reaction parameters on DMC formation and the reusability of MOF-808-X were also studied.
A series of metal-organic frameworks MOF-808-X (6-connected) were synthesized by regulating the ZrOCl2·8H2O/1,3,5-benzenetricarboxylic acid (BTC) molar ratio (X) and tested for the direct synthesis of dimethyl carbonate (DMC) from CO2 and CH3OH with 1,1,1-trimethoxymethane (TMM) as a dehydrating agent. The effect of the ZrOCl2·8H2O/BTC molar ratio on the physicochemical properties and catalytic performance of MOF-808-X was investigated. Results showed that a proper ZrOCl2·8H2O/BTC molar ratio during MOF-808-X synthesis was fairly important to reduce the redundant BTC or zirconium clusters trapped in the micropores of MOF-808-X. MOF-808-4, with almost no redundant BTC or zirconium clusters trapped in the micropores, exhibited the largest surface area, micropore size, and the number of acidic-basic sites, and consequently showed the best activity among all MOF-808-X, with the highest DMC yield of 21.5% under the optimal reaction conditions. Moreover, benefiting from the larger micropore size, MOF-808-4 outperformed our previously reported UiO-66-24 (12-connected), which had even more acidic-basic sites and larger surface area than MOF-808-4, mainly because the larger micropore size of MOF-808-4 provided higher accessibility for the reactant to the active sites located in the micropores. Furthermore, a possible reaction mechanism over MOF-808-4 was proposed based on the in situ FT-IR results. The effects of different reaction parameters on DMC formation and the reusability of MOF-808-X were also studied.
2019, 40(4): 567-579
doi: S1872-2067(19)63302-4
Abstract:
Ni-Re/SiO2 catalysts with controllable Ni particle sizes (4.5-18.0 nm) were synthesized to investigate the effects of the particle size on the amination of monoethanolamine (MEA). The catalysts were characterized by various techniques and evaluated for the amination reaction in a trickle bed reactor at 170℃, 8.0 MPa, and 0.5 h-1 liquid hourly space velocity of MEA (LHSVMEA) in NH3/H2 atmosphere. The Ni-Re/SiO2 catalyst with the lowest Ni particle size (4.5 nm) exhibited the highest yield (66.4%) of the desired amines (ethylenediamine (EDA) and piperazine (PIP)). The results of the analysis show that the turnover frequency of MEA increased slightly (from 193 to 253 h-1) as the Ni particle sizes of the Ni-Re/SiO2 catalysts increased from 4.5 to 18.0 nm. Moreover, the product distribution could be adjusted by varying the Ni particle size. The ratio of primary to secondary amines increased from 1.0 to 2.0 upon increasing the Ni particle size from 4.5 to 18.0 nm. Further analyses reveal that the Ni particle size influenced the electronic properties of surface Ni, which in turn affected the adsorption of MEA and the reaction pathway of MEA amination. Compared to those of small Ni particles, large particles possessed a higher proportion of high-coordinated terrace Ni sites and a higher surface electron density, which favored the amination of MEA and NH3 to form EDA.
Ni-Re/SiO2 catalysts with controllable Ni particle sizes (4.5-18.0 nm) were synthesized to investigate the effects of the particle size on the amination of monoethanolamine (MEA). The catalysts were characterized by various techniques and evaluated for the amination reaction in a trickle bed reactor at 170℃, 8.0 MPa, and 0.5 h-1 liquid hourly space velocity of MEA (LHSVMEA) in NH3/H2 atmosphere. The Ni-Re/SiO2 catalyst with the lowest Ni particle size (4.5 nm) exhibited the highest yield (66.4%) of the desired amines (ethylenediamine (EDA) and piperazine (PIP)). The results of the analysis show that the turnover frequency of MEA increased slightly (from 193 to 253 h-1) as the Ni particle sizes of the Ni-Re/SiO2 catalysts increased from 4.5 to 18.0 nm. Moreover, the product distribution could be adjusted by varying the Ni particle size. The ratio of primary to secondary amines increased from 1.0 to 2.0 upon increasing the Ni particle size from 4.5 to 18.0 nm. Further analyses reveal that the Ni particle size influenced the electronic properties of surface Ni, which in turn affected the adsorption of MEA and the reaction pathway of MEA amination. Compared to those of small Ni particles, large particles possessed a higher proportion of high-coordinated terrace Ni sites and a higher surface electron density, which favored the amination of MEA and NH3 to form EDA.
2019, 40(4): 580-589
doi: S1872-2067(19)63296-1
Abstract:
In this work, samples consisting of BiVO4 with exposed (040) facets coupled with Bi2S3 (Bi2S3/BiVO4) were prepared through a one-pot hydrothermal method, using ethylenediaminetetraacetic acid as directing agent and L-cysteine as sulfur source and soft template. X-ray diffraction, field emission scanning electron microscopy, and high-resolution transmission electron microscopy measurements indicated that the Bi2S3 content had a significant influence on the growth of (040) and (121) facets as well as on the morphology of the Bi2S3/BiVO4 samples. When the Bi2S3 content reached 1 mmol, the Bi2S3/BiVO4 samples exhibited a peony-like morphology. The results of transient photocurrent tests and electrochemical impedance spectroscopy measurements confirmed that a more effective charge separation and a faster interfacial charge transfer occurred in Bi2S3/BiVO4 than BiVO4. The enhanced photocatalytic activity of the Bi2S3/BiVO4 samples could be attributed to the improved absorption capability in the visible light region and the enhanced electron-hole pair separation efficiency due to the formation of the Bi2S3/BiVO4 heterostructure. In addition, the Bi2S3/BiVO4 samples showed relative stability and reusability. The simple method presented in this work could be used to fabricate composite photocatalysts with high activity for different applications, such as photocatalytic degradation of organic pollutants, photocatalytic splitting of water, and photocatalytic reduction of carbon dioxide.
In this work, samples consisting of BiVO4 with exposed (040) facets coupled with Bi2S3 (Bi2S3/BiVO4) were prepared through a one-pot hydrothermal method, using ethylenediaminetetraacetic acid as directing agent and L-cysteine as sulfur source and soft template. X-ray diffraction, field emission scanning electron microscopy, and high-resolution transmission electron microscopy measurements indicated that the Bi2S3 content had a significant influence on the growth of (040) and (121) facets as well as on the morphology of the Bi2S3/BiVO4 samples. When the Bi2S3 content reached 1 mmol, the Bi2S3/BiVO4 samples exhibited a peony-like morphology. The results of transient photocurrent tests and electrochemical impedance spectroscopy measurements confirmed that a more effective charge separation and a faster interfacial charge transfer occurred in Bi2S3/BiVO4 than BiVO4. The enhanced photocatalytic activity of the Bi2S3/BiVO4 samples could be attributed to the improved absorption capability in the visible light region and the enhanced electron-hole pair separation efficiency due to the formation of the Bi2S3/BiVO4 heterostructure. In addition, the Bi2S3/BiVO4 samples showed relative stability and reusability. The simple method presented in this work could be used to fabricate composite photocatalysts with high activity for different applications, such as photocatalytic degradation of organic pollutants, photocatalytic splitting of water, and photocatalytic reduction of carbon dioxide.
2019, 40(4): 590-599
doi: S1872-2067(19)63312-7
Abstract:
Contaminants (K, Na, Ca, and Mg) were introduced into Cu-SAPO-18 via incipient wetness impregnation to investigate their effect on the selective catalytic reduction of NOx with NH3 (NH3-SCR) over Cu-SAPO-18. After the introduction of contaminants into Cu-SAPO-18, the quantity of acidic sites and Cu2+ species in catalyst decreases owing to the replacement of H+ and Cu2+ by K+, Na+, Ca2+, and Mg2+. Furthermore, the loss of isolated Cu2+ induces the generation of CuO and CuAl2O4-like phases, which causes further loss in the Brunauer-Emmett-Teller surface area of the catalyst. Consequently, the deNOx performance of the contaminated Cu-SAPO-18 catalysts drops. Such decline in NH3-SCR performance becomes more pronounced by increasing the contaminant contents from 0.5 to 1.0 mmol/gcatal. In addition, the deactivation influence of the contaminants on Cu-SAPO-18 is presented in the order of K > Na > Ca > Mg, which is consistent with the order of reduction of acidic sites. To a certain degree, the effect of the acidic sites on the deactivation of Cu-SAPO-18 might be more significant than that of isolated Cu2+ and the catalyst framework. Moreover, kinetic analysis of NH3-SCR was conducted, and the results indicate that there is no influence of contaminants on the NH3-SCR mechanism.
Contaminants (K, Na, Ca, and Mg) were introduced into Cu-SAPO-18 via incipient wetness impregnation to investigate their effect on the selective catalytic reduction of NOx with NH3 (NH3-SCR) over Cu-SAPO-18. After the introduction of contaminants into Cu-SAPO-18, the quantity of acidic sites and Cu2+ species in catalyst decreases owing to the replacement of H+ and Cu2+ by K+, Na+, Ca2+, and Mg2+. Furthermore, the loss of isolated Cu2+ induces the generation of CuO and CuAl2O4-like phases, which causes further loss in the Brunauer-Emmett-Teller surface area of the catalyst. Consequently, the deNOx performance of the contaminated Cu-SAPO-18 catalysts drops. Such decline in NH3-SCR performance becomes more pronounced by increasing the contaminant contents from 0.5 to 1.0 mmol/gcatal. In addition, the deactivation influence of the contaminants on Cu-SAPO-18 is presented in the order of K > Na > Ca > Mg, which is consistent with the order of reduction of acidic sites. To a certain degree, the effect of the acidic sites on the deactivation of Cu-SAPO-18 might be more significant than that of isolated Cu2+ and the catalyst framework. Moreover, kinetic analysis of NH3-SCR was conducted, and the results indicate that there is no influence of contaminants on the NH3-SCR mechanism.
2019, 40(4): 600-608
doi: S1872-2067(19)63295-X
Abstract:
Heterogeneous gold nanocatalysts have both inspired researchers with their unique catalytic performance and frustrated them due to the contradictions observed in their activities and stabilities. A recent breakthrough has shown that gold nanoparticles (NPs) can retain their catalytically active size over a MgGa2O4 spinel support upon sintering at high temperatures. Herein, we report the catalytic activity of anti-sintering Au₲MgGa2O4 for use in water gas shift reaction (WGSR) and catalytic combustion reactions, and the promoting effect of ceria. Upon adding ceria to 800℃-aged Au₲MgGa2O4, the CO conversion in the WGSR was increased from~1.5% to~34.0% at 450℃, and the "light-off" temperatures (T50) for methane combustion and CO oxidation were decreased by~80 and~100℃, respectively. Characterizations using XRD, HAADF-STEM, EDS mapping, H2-TPR, XPS, and DRIFTs confirmed the proximate contact of Au with ceria and their significant synergistic effect, which thereby combined the benefits of ceria toward the dissociation of H2O or O2 and the Au NPs toward activating CO or CH4. These results show that this stepwise stabilization-activation strategy is efficient for rationally constructing stable and active gold nanocatalysts, which may open up possibilities for the wide application of gold nanocatalysts at elevated temperatures.
Heterogeneous gold nanocatalysts have both inspired researchers with their unique catalytic performance and frustrated them due to the contradictions observed in their activities and stabilities. A recent breakthrough has shown that gold nanoparticles (NPs) can retain their catalytically active size over a MgGa2O4 spinel support upon sintering at high temperatures. Herein, we report the catalytic activity of anti-sintering Au₲MgGa2O4 for use in water gas shift reaction (WGSR) and catalytic combustion reactions, and the promoting effect of ceria. Upon adding ceria to 800℃-aged Au₲MgGa2O4, the CO conversion in the WGSR was increased from~1.5% to~34.0% at 450℃, and the "light-off" temperatures (T50) for methane combustion and CO oxidation were decreased by~80 and~100℃, respectively. Characterizations using XRD, HAADF-STEM, EDS mapping, H2-TPR, XPS, and DRIFTs confirmed the proximate contact of Au with ceria and their significant synergistic effect, which thereby combined the benefits of ceria toward the dissociation of H2O or O2 and the Au NPs toward activating CO or CH4. These results show that this stepwise stabilization-activation strategy is efficient for rationally constructing stable and active gold nanocatalysts, which may open up possibilities for the wide application of gold nanocatalysts at elevated temperatures.
2019, 40(4): 609-617
doi: S1872-2067(19)63317-6
Abstract:
Efficient conversion of lignin to aromatic hydrocarbons via depolymerization and subsequent hydrodeoxygenation is important. Previously, we found that NbOx species played a key role in the activation and cleavage of C-O bonds in lignin and its model compounds. In this study, commercial niobic acid (HY-340), niobium phosphate (NbPO-CBMM) and lab-made layered niobium oxide (Nb2O5-Layer) were chosen as supports to study the effect of Brönsted and Lewis acids on the activation of C-O bonds in lignin conversion. A variety of Ru-loaded, Nb-based catalysts with different Ru particle sizes were prepared and applied to the conversion of p-cresol. The results show that all the Ru/Nb-based catalysts produce high mole yields of C7-C9 hydrocarbons (82.3%-99.1%). What's more, Ru/Nb2O5-Layer affords the best mole yield of C7-C9 hydrocarbons and selectivity for C7-C9 aromatic hydrocarbons, of up to 99.1% and 88.0%, respectively. Moreover, it was found that Lewis acid sites play important roles in the depolymerization of enzymatic lignin into phenolic monomers and the cleavage of the C-O bond of phenols. Additionally, the electronic state and particle size of Ru are significant factors which influence the selectivity for aromatic hydrocarbons. A partial positive charge on the metallic Ru surface and a smaller Ru particle size are beneficial in improving the selectivity for aromatic hydrocarbons.
Efficient conversion of lignin to aromatic hydrocarbons via depolymerization and subsequent hydrodeoxygenation is important. Previously, we found that NbOx species played a key role in the activation and cleavage of C-O bonds in lignin and its model compounds. In this study, commercial niobic acid (HY-340), niobium phosphate (NbPO-CBMM) and lab-made layered niobium oxide (Nb2O5-Layer) were chosen as supports to study the effect of Brönsted and Lewis acids on the activation of C-O bonds in lignin conversion. A variety of Ru-loaded, Nb-based catalysts with different Ru particle sizes were prepared and applied to the conversion of p-cresol. The results show that all the Ru/Nb-based catalysts produce high mole yields of C7-C9 hydrocarbons (82.3%-99.1%). What's more, Ru/Nb2O5-Layer affords the best mole yield of C7-C9 hydrocarbons and selectivity for C7-C9 aromatic hydrocarbons, of up to 99.1% and 88.0%, respectively. Moreover, it was found that Lewis acid sites play important roles in the depolymerization of enzymatic lignin into phenolic monomers and the cleavage of the C-O bond of phenols. Additionally, the electronic state and particle size of Ru are significant factors which influence the selectivity for aromatic hydrocarbons. A partial positive charge on the metallic Ru surface and a smaller Ru particle size are beneficial in improving the selectivity for aromatic hydrocarbons.
