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2017 Volume 38 Issue 7
2017, 38(7):
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2017, 38(7): 1101-1101
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2017, 38(7): 1102-1107
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2017, 38(7): 1108-1126
doi: 10.1016/S1872-2067(17)62852-3
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Metal-organic-framework (MOF)-based materials with novel physicochemical properties have emerged as promising catalysts for various hydrogenation reactions. In addition to metal clusters and multifunctional organic ligands, MOF-based catalysts can incorporate other functional species, and thus provide various active sites for hydrogenation processes. The structural properties of the catalysts play significant roles in enhancing the interactions among the reactants, products, and catalytic sites, which can be rationally designed. Because of the synergistic effects between the ac-tive sites and the structural properties, MOF-based catalysts can achieve higher activities and selec-tivities in hydrogenation reactions than can be obtained using traditional heterogeneous catalysts. This review provides an overview of recent developments in MOF-based catalysts in the hydro-genation of alkenes, alkynes, nitroarenes, cinnamaldehyde, furfural, benzene, and other compounds. Strategies for improving the catalytic performances of MOF-based catalysts are discussed as well as the different active sites and structural properties of the catalysts.
Metal-organic-framework (MOF)-based materials with novel physicochemical properties have emerged as promising catalysts for various hydrogenation reactions. In addition to metal clusters and multifunctional organic ligands, MOF-based catalysts can incorporate other functional species, and thus provide various active sites for hydrogenation processes. The structural properties of the catalysts play significant roles in enhancing the interactions among the reactants, products, and catalytic sites, which can be rationally designed. Because of the synergistic effects between the ac-tive sites and the structural properties, MOF-based catalysts can achieve higher activities and selec-tivities in hydrogenation reactions than can be obtained using traditional heterogeneous catalysts. This review provides an overview of recent developments in MOF-based catalysts in the hydro-genation of alkenes, alkynes, nitroarenes, cinnamaldehyde, furfural, benzene, and other compounds. Strategies for improving the catalytic performances of MOF-based catalysts are discussed as well as the different active sites and structural properties of the catalysts.
2017, 38(7): 1127-1137
doi: 10.1016/S1872-2067(17)62862-6
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Promoter-modified Ni-based catalysts were synthesized by an incipient-wetness impregnation method using 3D-mesoporous KIT-6 as a support modified by ethylene glycol, and evaluated for the catalytic production of synthetic natural gas (SNG) from CO methanation. Characterization results suggested that the addition of promoter species could remarkably improve the low-temperature catalytic activity for CO methanation, which was due to a large dispersion of Ni nanoparticles, an enhanced interaction between metal and support as well as a confinement effect of 3D-mesopores. Among all catalysts, Ni-V/KIT-6 possessed the best catalytic performance, which was ascribed to the largest H2 uptake of 177.6 μmol/g and Ni dispersion of 26.5%, an intimate interaction with the support from the formation of Si-O-V linkage and an enhanced confinement effect of 3D-mesopores to effectively prevent the growth of Ni nanoparticles and carbon filaments. In consequence, Ni-V/KIT-6 displayed excellent catalytic performance as well as high catalytic stability, which can be regarded as a promising candidate for CO methanation.
Promoter-modified Ni-based catalysts were synthesized by an incipient-wetness impregnation method using 3D-mesoporous KIT-6 as a support modified by ethylene glycol, and evaluated for the catalytic production of synthetic natural gas (SNG) from CO methanation. Characterization results suggested that the addition of promoter species could remarkably improve the low-temperature catalytic activity for CO methanation, which was due to a large dispersion of Ni nanoparticles, an enhanced interaction between metal and support as well as a confinement effect of 3D-mesopores. Among all catalysts, Ni-V/KIT-6 possessed the best catalytic performance, which was ascribed to the largest H2 uptake of 177.6 μmol/g and Ni dispersion of 26.5%, an intimate interaction with the support from the formation of Si-O-V linkage and an enhanced confinement effect of 3D-mesopores to effectively prevent the growth of Ni nanoparticles and carbon filaments. In consequence, Ni-V/KIT-6 displayed excellent catalytic performance as well as high catalytic stability, which can be regarded as a promising candidate for CO methanation.
2017, 38(7): 1138-1147
doi: 10.1016/S1872-2067(17)62843-2
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Density functional theory calculations corrected by on-site Coulomb interactions were carried out to study the structures of polar CeO2(100) surfaces as well as activities during catalytic CO oxida-tion. The stabilities of various CeO2(100) termination structures are discussed, and calculated ener-getics are presented. The most stable CeO2(100) surface was obtained by removing half the outer-most full layer of oxygen and the surface stability was found to decrease as the exposed oxygen concentration was increased. Assessing the reaction pathways leading to different final products during CO oxidation over the most stable CeO2(100) surface, we determined that the formation of carbonate species competed with CO2 desorption. However, during CO oxidation on the less stable CeO2(100) surfaces having more exposed oxygen, the CO is evidently able to react with surface oxygen, leading to CO2 formation and desorption. The calculation results and electronic analyses reported herein also indicate that the characteristic Ce 4ƒ orbitals are directly involved in deter-mining the surface stabilities and reactivities.
Density functional theory calculations corrected by on-site Coulomb interactions were carried out to study the structures of polar CeO2(100) surfaces as well as activities during catalytic CO oxida-tion. The stabilities of various CeO2(100) termination structures are discussed, and calculated ener-getics are presented. The most stable CeO2(100) surface was obtained by removing half the outer-most full layer of oxygen and the surface stability was found to decrease as the exposed oxygen concentration was increased. Assessing the reaction pathways leading to different final products during CO oxidation over the most stable CeO2(100) surface, we determined that the formation of carbonate species competed with CO2 desorption. However, during CO oxidation on the less stable CeO2(100) surfaces having more exposed oxygen, the CO is evidently able to react with surface oxygen, leading to CO2 formation and desorption. The calculation results and electronic analyses reported herein also indicate that the characteristic Ce 4ƒ orbitals are directly involved in deter-mining the surface stabilities and reactivities.
2017, 38(7): 1148-1154
doi: 10.1016/S1872-2067(17)62841-9
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From both fundamental and practical perspectives, the production of chemicals from biomass re-sources using high-efficiency non-precious metal catalysts is important. However, many processes require addition of stoichiometric or excess quantities of base, which leads to high energy consump-tion, leaching problems, and side reactions. In this study, we investigated the high-efficiency oxida-tive esterification of furfural to methylfuroate by molecular oxygen with a Co-N-C/MgO catalyst. The catalyst was prepared by direct pyrolysis of a cobalt(Ⅱ) phenanthroline complex on MgO at 800℃ under N2 atmosphere. From furfural, 93.0% conversion and 98.5% selectivity toward methylfuroate were achieved under 0.5 MPa O2 with reaction at 100℃ for 12 h without a basic additive. The con-version and selectivity were much higher than those obtained with cobalt catalysts produced by pyrolysis of a cobalt(Ⅱ) phenanthroline complex on activated carbon or typical basic supports, in-cluding NaX, NaY, and CaO. X-ray photoelectron spectroscopy, X-ray diffraction, transmission elec-tron microscopy, and experimental results revealed that the high efficiency of Co-N-C/MgO for pro-duction of methylfuroate was closely related to the cobalt-nitrogen-doped carbon species and its catalytic ability in hydrogen abstraction. In contrast, Co-N-C(HCl) that synthesized by removing MgO with HCl from Co-N-C/MgO, as the catalyst produced mainly an acetal as a condensation prod-uct, and chloride ions had a negative effect on the oxidative esterification. Although the catalytic performance of the cobalt-nitrogen-doped carbon species was greatly affected by HCl treatment, it could be recovered to a great extent by addition of MgO. Moreover, changes in the oxygen pressure hardly affected the oxidative esterification of furfural with Co-N-C/MgO. This study not only pro-vides an effective approach to prepare methylfuroate, but also for designing high-performance non-precious metal catalysts for the oxidative esterification of biomass-derived compounds.
From both fundamental and practical perspectives, the production of chemicals from biomass re-sources using high-efficiency non-precious metal catalysts is important. However, many processes require addition of stoichiometric or excess quantities of base, which leads to high energy consump-tion, leaching problems, and side reactions. In this study, we investigated the high-efficiency oxida-tive esterification of furfural to methylfuroate by molecular oxygen with a Co-N-C/MgO catalyst. The catalyst was prepared by direct pyrolysis of a cobalt(Ⅱ) phenanthroline complex on MgO at 800℃ under N2 atmosphere. From furfural, 93.0% conversion and 98.5% selectivity toward methylfuroate were achieved under 0.5 MPa O2 with reaction at 100℃ for 12 h without a basic additive. The con-version and selectivity were much higher than those obtained with cobalt catalysts produced by pyrolysis of a cobalt(Ⅱ) phenanthroline complex on activated carbon or typical basic supports, in-cluding NaX, NaY, and CaO. X-ray photoelectron spectroscopy, X-ray diffraction, transmission elec-tron microscopy, and experimental results revealed that the high efficiency of Co-N-C/MgO for pro-duction of methylfuroate was closely related to the cobalt-nitrogen-doped carbon species and its catalytic ability in hydrogen abstraction. In contrast, Co-N-C(HCl) that synthesized by removing MgO with HCl from Co-N-C/MgO, as the catalyst produced mainly an acetal as a condensation prod-uct, and chloride ions had a negative effect on the oxidative esterification. Although the catalytic performance of the cobalt-nitrogen-doped carbon species was greatly affected by HCl treatment, it could be recovered to a great extent by addition of MgO. Moreover, changes in the oxygen pressure hardly affected the oxidative esterification of furfural with Co-N-C/MgO. This study not only pro-vides an effective approach to prepare methylfuroate, but also for designing high-performance non-precious metal catalysts for the oxidative esterification of biomass-derived compounds.
2017, 38(7): 1155-1165
doi: 10.1016/S1872-2067(17)62848-1
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The catalytic wet air oxidation of aniline over Ru catalysts supported on modified TiO2 (TiO2, Ti0.9Ce0.1O2, Ti0.9Zr0.1O2) is investigated. A series of characterization techniques are conducted to determine the relationship between the physico-chemical properties and the catalytic performance. As a result of the good metal dispersion and large number of surface oxygen species, the Ru/Ti0.9Zr0.1O2 catalyst presents the best catalytic activity among the tested samples. The effects of the operating conditions on the reaction are investigated and the optimal reaction conditions are determined. Based on the relationship between the by-products concentration and the reaction time, the reaction path for the catalytic oxidation of aniline is established. Carbonaceous deposits on the surface of the support are known to be the main reason for catalyst deactivation. The catalysts maintain a constant activity even after three consecutive cycles.
The catalytic wet air oxidation of aniline over Ru catalysts supported on modified TiO2 (TiO2, Ti0.9Ce0.1O2, Ti0.9Zr0.1O2) is investigated. A series of characterization techniques are conducted to determine the relationship between the physico-chemical properties and the catalytic performance. As a result of the good metal dispersion and large number of surface oxygen species, the Ru/Ti0.9Zr0.1O2 catalyst presents the best catalytic activity among the tested samples. The effects of the operating conditions on the reaction are investigated and the optimal reaction conditions are determined. Based on the relationship between the by-products concentration and the reaction time, the reaction path for the catalytic oxidation of aniline is established. Carbonaceous deposits on the surface of the support are known to be the main reason for catalyst deactivation. The catalysts maintain a constant activity even after three consecutive cycles.
2017, 38(7): 1166-1173
doi: 10.1016/S1872-2067(17)62844-4
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Ni/Al2O3 catalysts were derived from spinel NiAl2O4 with different Ni content ((2.5, 5 and 7.5) wt%). The catalysts were obtained by H2 reduction and were investigated for the low-temperature hydro-genation of maleic anhydride (MA) to produce succinic anhydride (SA). The characterization results showed that Ni0 active sites were mainly derived during the H2 reduction from spinel NiAl2O4. Among the catalysts studied, employing the optimum preparation and reaction conditions with Ni(5%)/Al2O3 yielded the highest catalytic performance. A near-100% conversion of MA and~90% selectivity to SA were achieved at 120℃ and 0.5 MPa of H2 with a weighted hourly space velocity (MA) of 2 h-1.
Ni/Al2O3 catalysts were derived from spinel NiAl2O4 with different Ni content ((2.5, 5 and 7.5) wt%). The catalysts were obtained by H2 reduction and were investigated for the low-temperature hydro-genation of maleic anhydride (MA) to produce succinic anhydride (SA). The characterization results showed that Ni0 active sites were mainly derived during the H2 reduction from spinel NiAl2O4. Among the catalysts studied, employing the optimum preparation and reaction conditions with Ni(5%)/Al2O3 yielded the highest catalytic performance. A near-100% conversion of MA and~90% selectivity to SA were achieved at 120℃ and 0.5 MPa of H2 with a weighted hourly space velocity (MA) of 2 h-1.
2017, 38(7): 1174-1183
doi: 10.1016/S1872-2067(17)62849-3
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The semimetal Bi has received increasing interest as an alternative to noble metals for use in plas-monic photocatalysis. To enhance the photocatalytic efficiency of metallic Bi, Bi microspheres modi-fied by SiO2 nanoparticles were fabricated by a facile method. Bi-O-Si bonds were formed between Bi and SiO2, and acted as a transportation channel for hot electrons. The SiO2@Bi microspheres exhibited an enhanced plasmon-mediated photocatalytic activity for the removal of NO in air under 280 nm light irradiation, as a result of the enlarged specific surface areas and the promotion of elec-tron transfer via the Bi-O-Si bonds. The reaction mechanism of photocatalytic oxidation of NO by SiO2@Bi was revealed with electron spin resonance and in situ diffuse reflectance infrared Fourier transform spectroscopy experiments, and involved the chain reaction NO → NO2 → NO3- with ·OH and ·O2- radicals as the main reactive species. The present work could provide new insights into the in-depth mechanistic understanding of Bi plasmonic photocatalysis and the design of high-performance Bi-based photocatalysts.
The semimetal Bi has received increasing interest as an alternative to noble metals for use in plas-monic photocatalysis. To enhance the photocatalytic efficiency of metallic Bi, Bi microspheres modi-fied by SiO2 nanoparticles were fabricated by a facile method. Bi-O-Si bonds were formed between Bi and SiO2, and acted as a transportation channel for hot electrons. The SiO2@Bi microspheres exhibited an enhanced plasmon-mediated photocatalytic activity for the removal of NO in air under 280 nm light irradiation, as a result of the enlarged specific surface areas and the promotion of elec-tron transfer via the Bi-O-Si bonds. The reaction mechanism of photocatalytic oxidation of NO by SiO2@Bi was revealed with electron spin resonance and in situ diffuse reflectance infrared Fourier transform spectroscopy experiments, and involved the chain reaction NO → NO2 → NO3- with ·OH and ·O2- radicals as the main reactive species. The present work could provide new insights into the in-depth mechanistic understanding of Bi plasmonic photocatalysis and the design of high-performance Bi-based photocatalysts.
2017, 38(7): 1184-1195
doi: 10.1016/S1872-2067(17)62855-9
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We report a facile and modified sol-gel approach to synthesize brown TiO2 nanoparticles at low temperature (100-600℃). The TiO2 nanoparticles dried at 180 ℃ (TiO2-180℃) possessed a small particle size (5.0 nm), large specific surface area (213.45 m2/g), and efficient response to broadband light over the entire ultraviolet-visible spectrum with a narrow band gap of 1.84 eV. In addition, TiO2-180℃ exhibited the optimal reaction rate constant for the degradation of methylene blue (0.08287 mg/(L·min)), which is six times higher than that of the mixed rutile/anatase phase TiO2 photocatalytic standard P25 (0.01342 mg/(L·min)). Furthermore, cycling photodegradation ex-periments confirmed the stability and reusability of this catalyst. The unique physicochemical properties resulting from the low-temperature preparation of TiO2-180℃, including its broadband visible absorption associated with a high concentration of oxygen vacancies, large surface area, and enriched surface -OH/H2O may be responsible for this excellent photocatalytic performance. The use of as-prepared TiO2-180℃ for practical applications is expected after further optimization.
We report a facile and modified sol-gel approach to synthesize brown TiO2 nanoparticles at low temperature (100-600℃). The TiO2 nanoparticles dried at 180 ℃ (TiO2-180℃) possessed a small particle size (5.0 nm), large specific surface area (213.45 m2/g), and efficient response to broadband light over the entire ultraviolet-visible spectrum with a narrow band gap of 1.84 eV. In addition, TiO2-180℃ exhibited the optimal reaction rate constant for the degradation of methylene blue (0.08287 mg/(L·min)), which is six times higher than that of the mixed rutile/anatase phase TiO2 photocatalytic standard P25 (0.01342 mg/(L·min)). Furthermore, cycling photodegradation ex-periments confirmed the stability and reusability of this catalyst. The unique physicochemical properties resulting from the low-temperature preparation of TiO2-180℃, including its broadband visible absorption associated with a high concentration of oxygen vacancies, large surface area, and enriched surface -OH/H2O may be responsible for this excellent photocatalytic performance. The use of as-prepared TiO2-180℃ for practical applications is expected after further optimization.
2017, 38(7): 1196-1206
doi: 10.1016/S1872-2067(17)62840-7
Abstract:
Core-shell nanostructures have been widely investigated to improve the electrocatalytic perfor-mance of platinum. However, organic precursors, surfactants or high temperature are usually nec-essary during the preparation procedure. Unfortunately, these requirements limit the application of these methods on a large scale. Herein, a Pdcore@Ptshell nanostructure was fabricated through the reduction of K2PtCl4 by dissociated hydrogen at room temperature without the assistance of either a surfactant or a high-boiling point solvent. The shell thickness of this nanostructure was successfully controlled by varying the amount of K2PtCl4; core-shell nanoparticles with a shell thickness of 0.45, 0.75 and 0.90 nm were obtained, as determined by TEM. The remarkable crystallinity and epitaxial growth of the Pdcore@Ptshell nanostructure were revealed by HRTEM and EDS. According to ICP and XPS, surface segregation of Pt was established. The impressive ORR performance was attributed to the weak adsorption strength of the OHads species, which resulted from the electron transfer impact between the Pdcore and Ptshell. The facile and clean preparation method can be used to prepare other core-shell nanostructures under a mild atmosphere.
Core-shell nanostructures have been widely investigated to improve the electrocatalytic perfor-mance of platinum. However, organic precursors, surfactants or high temperature are usually nec-essary during the preparation procedure. Unfortunately, these requirements limit the application of these methods on a large scale. Herein, a Pdcore@Ptshell nanostructure was fabricated through the reduction of K2PtCl4 by dissociated hydrogen at room temperature without the assistance of either a surfactant or a high-boiling point solvent. The shell thickness of this nanostructure was successfully controlled by varying the amount of K2PtCl4; core-shell nanoparticles with a shell thickness of 0.45, 0.75 and 0.90 nm were obtained, as determined by TEM. The remarkable crystallinity and epitaxial growth of the Pdcore@Ptshell nanostructure were revealed by HRTEM and EDS. According to ICP and XPS, surface segregation of Pt was established. The impressive ORR performance was attributed to the weak adsorption strength of the OHads species, which resulted from the electron transfer impact between the Pdcore and Ptshell. The facile and clean preparation method can be used to prepare other core-shell nanostructures under a mild atmosphere.
2017, 38(7): 1207-1215
doi: 10.1016/S1872-2067(17)62853-5
Abstract:
Zeolite Beta containing ultra-small CoO particles was synthesized from a one-step hydrothermal process. The synthesis route involves the pre-mixture of hydrofluoric acid with tetraethylammo-nium hydroxide (in a molar ratio of 1.3-1.5:1) before the addition of a silicon and cobalt source. Investigations by scanning electron microscopy, X-ray diffraction, UV-Vis spectroscopy, X-ray pho-toelectron spectroscopy, H2-temperature-programmed reduction and transmission electron mi-croscopy confirm the presence of ultra-small CoO particles in the zeolite Beta structure. The ul-tra-small CoO particles in zeolite Beta present high stability against both oxidation and reduction atmospheres at high temperatures. The catalytic performance of the CoO-containing zeolite Beta catalysts was compared with other Co-containing zeolites by evaluating ethylbenzene oxidation reactivity. The CoO-containing zeolite Beta exhibits high ethylbenzene conversion and high selectiv-ity to acetophenone and 1-phenylethanol. The high activity of this catalyst system can be attributed to the high dispersion of the ultra-small CoO particles in the Beta structure.
Zeolite Beta containing ultra-small CoO particles was synthesized from a one-step hydrothermal process. The synthesis route involves the pre-mixture of hydrofluoric acid with tetraethylammo-nium hydroxide (in a molar ratio of 1.3-1.5:1) before the addition of a silicon and cobalt source. Investigations by scanning electron microscopy, X-ray diffraction, UV-Vis spectroscopy, X-ray pho-toelectron spectroscopy, H2-temperature-programmed reduction and transmission electron mi-croscopy confirm the presence of ultra-small CoO particles in the zeolite Beta structure. The ul-tra-small CoO particles in zeolite Beta present high stability against both oxidation and reduction atmospheres at high temperatures. The catalytic performance of the CoO-containing zeolite Beta catalysts was compared with other Co-containing zeolites by evaluating ethylbenzene oxidation reactivity. The CoO-containing zeolite Beta exhibits high ethylbenzene conversion and high selectiv-ity to acetophenone and 1-phenylethanol. The high activity of this catalyst system can be attributed to the high dispersion of the ultra-small CoO particles in the Beta structure.
2017, 38(7): 1216-1228
doi: 10.1016/S1872-2067(17)62854-7
Abstract:
Production of aromatics from lignin has attracted much attention. Because of the coexistence of C-O and C-C bonds and their complex combinations in the lignin macromolecular network, a plausible roadmap for developing a lignin catalytic decomposition process could be developed by exploring the transformation mechanisms of various model compounds. Herein, decomposition of a lignin model compound, 2-phenoxyacetophenone (2-PAP), was investigated over several ce-sium-exchanged polyoxometalate (Cs-POM) catalysts. Decomposition of 2-PAP can follow two dif-ferent mechanisms:an active hydrogen transfer mechanism or an oxonium cation mechanism. The mechanism for most reactions depends on the competition between the acidity and redox proper-ties of the catalysts. The catalysts of POMs perform the following functions:promoting active hy-drogen liberated from ethanol and causing formation of and then temporarily stabilizing oxonium cations from 2-PAP. The use of Cs-PMo, which with strong redox ability, enhances hydrogen libera-tion and promotes liberated hydrogen transfer to the reaction intermediates. As a consequence, complete conversion of 2-PAP (>99%) with excellent selectivities to the desired products (98.6% for phenol and 91.1% for acetophenone) can be achieved.
Production of aromatics from lignin has attracted much attention. Because of the coexistence of C-O and C-C bonds and their complex combinations in the lignin macromolecular network, a plausible roadmap for developing a lignin catalytic decomposition process could be developed by exploring the transformation mechanisms of various model compounds. Herein, decomposition of a lignin model compound, 2-phenoxyacetophenone (2-PAP), was investigated over several ce-sium-exchanged polyoxometalate (Cs-POM) catalysts. Decomposition of 2-PAP can follow two dif-ferent mechanisms:an active hydrogen transfer mechanism or an oxonium cation mechanism. The mechanism for most reactions depends on the competition between the acidity and redox proper-ties of the catalysts. The catalysts of POMs perform the following functions:promoting active hy-drogen liberated from ethanol and causing formation of and then temporarily stabilizing oxonium cations from 2-PAP. The use of Cs-PMo, which with strong redox ability, enhances hydrogen libera-tion and promotes liberated hydrogen transfer to the reaction intermediates. As a consequence, complete conversion of 2-PAP (>99%) with excellent selectivities to the desired products (98.6% for phenol and 91.1% for acetophenone) can be achieved.
2017, 38(7): 1229-1236
doi: 10.1016/S1872-2067(17)62857-2
Abstract:
Supported PtCu alloys have been broadly applied in heterogeneous catalysis and electrocatalysis owing to their excellent catalytic performance and high CO tolerance. It is important to analyze the outermost surface composition of the supported alloy nanoparticles to understand the nature of the catalytically active sites. In this paper, homogeneous face-centered cubic PtCu nanoparticles with a narrow particle size distribution were successfully fabricated and dispersed on a high-surface-area TiO2 powder support. The samples were oxidized and reduced in situ and then introduced into the ultrahigh vacuum chamber to measure the topmost surface composition by high-sensitivity low-energy ion scattering spectroscopy, and to determine the oxidation states of the elements by X-ray photoelectron spectroscopy. The surface composition and morphology, elemental distribu-tion, and oxidation states of the components were found to be significantly affected by the support and treatment conditions. The PtCu is de-alloyed upon oxidation with CuO wetting on the TiO2 sur-face and re-alloyed upon reduction. Phase diagrams of the surface composition and the bulk com-position were plotted and compared for the supported and unsupported materials.
Supported PtCu alloys have been broadly applied in heterogeneous catalysis and electrocatalysis owing to their excellent catalytic performance and high CO tolerance. It is important to analyze the outermost surface composition of the supported alloy nanoparticles to understand the nature of the catalytically active sites. In this paper, homogeneous face-centered cubic PtCu nanoparticles with a narrow particle size distribution were successfully fabricated and dispersed on a high-surface-area TiO2 powder support. The samples were oxidized and reduced in situ and then introduced into the ultrahigh vacuum chamber to measure the topmost surface composition by high-sensitivity low-energy ion scattering spectroscopy, and to determine the oxidation states of the elements by X-ray photoelectron spectroscopy. The surface composition and morphology, elemental distribu-tion, and oxidation states of the components were found to be significantly affected by the support and treatment conditions. The PtCu is de-alloyed upon oxidation with CuO wetting on the TiO2 sur-face and re-alloyed upon reduction. Phase diagrams of the surface composition and the bulk com-position were plotted and compared for the supported and unsupported materials.
2017, 38(7): 1237-1244
doi: 10.1016/S1872-2067(17)62859-6
Abstract:
Bifunctional catalysts that contain both metal and acidic functions have been widely used in re-newable biomass conversions. The bifunctionality closely depends on the distance between the metal and acid sites. However, the metal-acid proximity effect has rarely investigated in biomass conversions. In this work, we precisely deposited a porous Al2O3 overcoat onto a Pt/Al2O3 catalyst using atomic layer deposition to improve the proximity between the Pt metal and the alumina acid sites by increasing the area of the metal-acid interface. Diffuse reflectance infrared Fourier trans-form spectroscopy (DRIFTS) of pyridine chemisorption confirmed that the overall catalyst acidity did not change considerably after applying the alumina overcoat. In the aqueous-phase, hydrogen-olysis of glycerol was used to demonstrate that the alumina overcoat significantly improved the activity approximately 2.8-fold, as well as the selectivity to 1,2-propanediol (1,2-PD) at high conver-sions. DRIFTS measurements of CO chemisorption indicated that the Pt-alumina interface had greater area for alumina coated Pt/Al2O3 than for the uncoated analog. Moreover, we used the hy-drogenation of acetol, the key reaction intermediate in glycerol hydrogenolysis, as a control exper-iment to confirm that the observed activity improvement in the hydrogenolysis of glycerol could be attributed to the enhancement of the dehydration reaction step, which requires acidic function. In brief, our work provides solid evidence that close metal-acid proximity enhances bifunctionality, thus improving the catalytic activity.
Bifunctional catalysts that contain both metal and acidic functions have been widely used in re-newable biomass conversions. The bifunctionality closely depends on the distance between the metal and acid sites. However, the metal-acid proximity effect has rarely investigated in biomass conversions. In this work, we precisely deposited a porous Al2O3 overcoat onto a Pt/Al2O3 catalyst using atomic layer deposition to improve the proximity between the Pt metal and the alumina acid sites by increasing the area of the metal-acid interface. Diffuse reflectance infrared Fourier trans-form spectroscopy (DRIFTS) of pyridine chemisorption confirmed that the overall catalyst acidity did not change considerably after applying the alumina overcoat. In the aqueous-phase, hydrogen-olysis of glycerol was used to demonstrate that the alumina overcoat significantly improved the activity approximately 2.8-fold, as well as the selectivity to 1,2-propanediol (1,2-PD) at high conver-sions. DRIFTS measurements of CO chemisorption indicated that the Pt-alumina interface had greater area for alumina coated Pt/Al2O3 than for the uncoated analog. Moreover, we used the hy-drogenation of acetol, the key reaction intermediate in glycerol hydrogenolysis, as a control exper-iment to confirm that the observed activity improvement in the hydrogenolysis of glycerol could be attributed to the enhancement of the dehydration reaction step, which requires acidic function. In brief, our work provides solid evidence that close metal-acid proximity enhances bifunctionality, thus improving the catalytic activity.
2017, 38(7): 1245-1251
doi: 10.1016/S1872-2067(17)62827-4
Abstract:
A basic ionic liquid, namely 1,1'-(butane-1,4-diyl)bis(1,4-diazabicyclo[2.2.2]octan-1-ium) hydrox-ide, was prepared and characterized using Fourier-transform infrared spectroscopy, 1H nuclear magnetic resonance spectroscopy, and pH measurements. The ionic liquid was used for efficient promotion of the synthesis of pyrano[2,3-d]pyrimidinone and pyrido[2,3-d]pyrimidine derivatives at room temperature under grinding conditions. A simple procedure, short reaction time, high yields, non-column chromatographic separation, commercial availability of the starting materials, and recyclability of the catalyst are attractive features of this process.
A basic ionic liquid, namely 1,1'-(butane-1,4-diyl)bis(1,4-diazabicyclo[2.2.2]octan-1-ium) hydrox-ide, was prepared and characterized using Fourier-transform infrared spectroscopy, 1H nuclear magnetic resonance spectroscopy, and pH measurements. The ionic liquid was used for efficient promotion of the synthesis of pyrano[2,3-d]pyrimidinone and pyrido[2,3-d]pyrimidine derivatives at room temperature under grinding conditions. A simple procedure, short reaction time, high yields, non-column chromatographic separation, commercial availability of the starting materials, and recyclability of the catalyst are attractive features of this process.
2017, 38(7): 1252-1260
doi: 10.1016/S1872-2067(17)62833-X
Abstract:
Due to the advantages of high surface areas, large pore volumes and pore sizes, abundant nitrogen content that favored the metal-support interactions, N-doped ordered mesoporous carbons are regarded as a kind of fascinating and potential support for the synthesis of effective supported cat-alysts. Here, a N-doped ordered mesoporous carbon with a high N content (9.58 wt%), high surface area (417 m2/g), and three-dimensional cubic structure was synthesized successfully and used as an effective support for immobilizing Pt nanoparticles (NPs). The positive effects of nitrogen on the metal particle size enabled ultrasmall Pt NPs (about 1.0 ±0.5 nm) to be obtained. Moreover, most of the Pt NPs are homogeneously dispersed in the mesoporous channels. However, using the ordered mesoporous carbon without nitrogen as support, the particles were larger (4.4 ±1.7 nm) and many Pt NPs were distributed on the external surface, demonstrating the important role of the nitrogen species. The obtained N-doped ordered mesoporous material supported catalyst showed excellent catalytic activity (conversion 100%) and selectivity (>99%) in the hydrogenation of halogenated nitrobenzenes under mild conditions. These values are much higher than those achieved using a commercial Pt/C catalyst (conversion 89% and selectivity 90%). This outstanding catalytic perfor-mance can be attributed to the synergetic effects of the mesoporous structure, N-functionalized support, and stabilized ultrasmall Pt NPs. Moreover, such supported catalyst also showed excellent catalytic performance in the hydrogenation of other halogenated nitrobenzenes and nitroarenes. In addition, the stability of the multifunctional catalyst was excellent and it could be reused more than 10 times without significant losses of activity and selectivity. Our results conclusively show that a N-doped carbon support enable the formation of ultrafine metal NPs and improve the reaction ac-tivity and selectivity.
Due to the advantages of high surface areas, large pore volumes and pore sizes, abundant nitrogen content that favored the metal-support interactions, N-doped ordered mesoporous carbons are regarded as a kind of fascinating and potential support for the synthesis of effective supported cat-alysts. Here, a N-doped ordered mesoporous carbon with a high N content (9.58 wt%), high surface area (417 m2/g), and three-dimensional cubic structure was synthesized successfully and used as an effective support for immobilizing Pt nanoparticles (NPs). The positive effects of nitrogen on the metal particle size enabled ultrasmall Pt NPs (about 1.0 ±0.5 nm) to be obtained. Moreover, most of the Pt NPs are homogeneously dispersed in the mesoporous channels. However, using the ordered mesoporous carbon without nitrogen as support, the particles were larger (4.4 ±1.7 nm) and many Pt NPs were distributed on the external surface, demonstrating the important role of the nitrogen species. The obtained N-doped ordered mesoporous material supported catalyst showed excellent catalytic activity (conversion 100%) and selectivity (>99%) in the hydrogenation of halogenated nitrobenzenes under mild conditions. These values are much higher than those achieved using a commercial Pt/C catalyst (conversion 89% and selectivity 90%). This outstanding catalytic perfor-mance can be attributed to the synergetic effects of the mesoporous structure, N-functionalized support, and stabilized ultrasmall Pt NPs. Moreover, such supported catalyst also showed excellent catalytic performance in the hydrogenation of other halogenated nitrobenzenes and nitroarenes. In addition, the stability of the multifunctional catalyst was excellent and it could be reused more than 10 times without significant losses of activity and selectivity. Our results conclusively show that a N-doped carbon support enable the formation of ultrafine metal NPs and improve the reaction ac-tivity and selectivity.
2017, 38(7): 1261-1269
doi: 10.1016/S1872-2067(17)62838-9
Abstract:
Leached Pt-Fe and Pt-Co catalysts were prepared by acid leaching the reduced catalysts in acid solution. Oxidation treatments of leached catalysts produced the structure of metal oxides decorat-ing the surface of nanoparticles. The fully oxidized Fe2O3 and Co3O4 species on Pt nanoparticle sur-faces result in the low performance of the CO complete oxidation (COOX) reaction. In contrast, un-saturated FeO and CoO surface species can be formed during exposure to the CO preferential oxida-tion (CO-PROX) reaction with an excess of H2, leading to a high O2 activation ability and enhancing the CO-PROX activity. The FeOx surface structures can be transformed between these two states by varying the reactive gas environments, exhibiting oscillating activity in these two reactions. Con-versely, the CoO surface structure formed in the H2-rich atmosphere is stable when exposed to the COOX reaction and exhibits similar activity in these two reactions. It is hoped that this work may assist in understanding the important role of surface oxides in real reactions.
Leached Pt-Fe and Pt-Co catalysts were prepared by acid leaching the reduced catalysts in acid solution. Oxidation treatments of leached catalysts produced the structure of metal oxides decorat-ing the surface of nanoparticles. The fully oxidized Fe2O3 and Co3O4 species on Pt nanoparticle sur-faces result in the low performance of the CO complete oxidation (COOX) reaction. In contrast, un-saturated FeO and CoO surface species can be formed during exposure to the CO preferential oxida-tion (CO-PROX) reaction with an excess of H2, leading to a high O2 activation ability and enhancing the CO-PROX activity. The FeOx surface structures can be transformed between these two states by varying the reactive gas environments, exhibiting oscillating activity in these two reactions. Con-versely, the CoO surface structure formed in the H2-rich atmosphere is stable when exposed to the COOX reaction and exhibits similar activity in these two reactions. It is hoped that this work may assist in understanding the important role of surface oxides in real reactions.
2017, 38(7): 1270-1280
doi: 10.1016/S1872-2067(17)62860-2
Abstract:
Nitric oxide (NO) deep oxidation to dinitrogen pentoxide (N2O5) by ozone together with wet scrub-bing has become a promising technology for nitrogen-oxide (NOx) removal in industrial boilers. Catalysts were introduced to enhance the N2O5 formation rate with less ozone injection and leakage. A series of monometallic catalysts (manganese, cobalt, cerium, iron, copper, and chromium) as pre-pared by the sol-gel method were tested. The manganese oxides achieved an almost 80% conver-sion efficiency at an ozone (O3)/NO molar ratio of 2.0 in 0.12 s. The crystalline structure and porous parameters were determined. The thermodynamic reaction threshold of NO conversion to N2O5 is oxidation with an O3/NO molar ratio of 1.5. Spherical alumina was selected as the support to achieve the threshold, which was believed to improve the catalytic activity by increasing the surface area and the gas-solid contact time. Based on the manganese oxides, cerium, iron, chromium, cop-per, and cobalt were introduced as promoters. Cerium and iron improved the deep-oxidation effi-ciency compared with manganese/spherical alumina, with less than 50 mg/m3 of outlet NO + nitro-gen oxide, and less than 25 mg/m3 of residual ozone at an O3/NO molar ratio of 1.5. The other three metal oxides inhibited catalytic activity. X-ray diffraction, nitrogen adsorption, hydrogen tempera-ture-programmed reduction, and X-ray photoelectron spectroscopy results indicate that the cata-lytic activity is affected by the synergistic action of NOx oxidation and ozone decomposition.
Nitric oxide (NO) deep oxidation to dinitrogen pentoxide (N2O5) by ozone together with wet scrub-bing has become a promising technology for nitrogen-oxide (NOx) removal in industrial boilers. Catalysts were introduced to enhance the N2O5 formation rate with less ozone injection and leakage. A series of monometallic catalysts (manganese, cobalt, cerium, iron, copper, and chromium) as pre-pared by the sol-gel method were tested. The manganese oxides achieved an almost 80% conver-sion efficiency at an ozone (O3)/NO molar ratio of 2.0 in 0.12 s. The crystalline structure and porous parameters were determined. The thermodynamic reaction threshold of NO conversion to N2O5 is oxidation with an O3/NO molar ratio of 1.5. Spherical alumina was selected as the support to achieve the threshold, which was believed to improve the catalytic activity by increasing the surface area and the gas-solid contact time. Based on the manganese oxides, cerium, iron, chromium, cop-per, and cobalt were introduced as promoters. Cerium and iron improved the deep-oxidation effi-ciency compared with manganese/spherical alumina, with less than 50 mg/m3 of outlet NO + nitro-gen oxide, and less than 25 mg/m3 of residual ozone at an O3/NO molar ratio of 1.5. The other three metal oxides inhibited catalytic activity. X-ray diffraction, nitrogen adsorption, hydrogen tempera-ture-programmed reduction, and X-ray photoelectron spectroscopy results indicate that the cata-lytic activity is affected by the synergistic action of NOx oxidation and ozone decomposition.
2017, 38(7): 1281-1290
doi: 10.1016/S1872-2067(17)62846-8
Abstract:
The development of highly efficient catalysts for cathodes remains an important objective of fuel cell research. Here, we report Co3O4 nanoparticles assembled on a polypyrrole/graphene oxide electrocatalyst (Co3O4/Ppy/GO) as an efficient catalyst for the oxygen reduction reaction (ORR) in alkaline media. The catalyst was prepared via the hydrothermal reaction of Co2+ ions with Ppy-modified GO. The GO, Ppy/GO, and Co3O4/Ppy/GO were characterized using scanning electron microscopy, Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The incorporation of Ppy into GO nanosheets resulted in the formation of a nitrogen-modified GO po-rous structure, which acted as an efficient electron-transport network for the ORR. With further anchoring of Co3O4 on Ppy/GO, the as-prepared Co3O4/Ppy/GO exhibited excellent ORR activity and followed a four-electron route mechanism for the ORR in alkaline solution. An onset potential of -0.10 V vs. a saturated calomel electrode and a diffusion limiting current density of 2.30 mA/cm2 were achieved for the Co3O4/Ppy/GO catalyst heated at 800℃; these values are comparable to those for noble-metal-based Pt/C catalysts. Our work demonstrates that Co3O4/Ppy/GO is highly active for the ORR. Notably, the Ppy coupling effects between Co3O4 and GO provide a new route for the preparation of efficient non-precious electrocatalysts with hierarchical porous structures for fuel cell applications.
The development of highly efficient catalysts for cathodes remains an important objective of fuel cell research. Here, we report Co3O4 nanoparticles assembled on a polypyrrole/graphene oxide electrocatalyst (Co3O4/Ppy/GO) as an efficient catalyst for the oxygen reduction reaction (ORR) in alkaline media. The catalyst was prepared via the hydrothermal reaction of Co2+ ions with Ppy-modified GO. The GO, Ppy/GO, and Co3O4/Ppy/GO were characterized using scanning electron microscopy, Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The incorporation of Ppy into GO nanosheets resulted in the formation of a nitrogen-modified GO po-rous structure, which acted as an efficient electron-transport network for the ORR. With further anchoring of Co3O4 on Ppy/GO, the as-prepared Co3O4/Ppy/GO exhibited excellent ORR activity and followed a four-electron route mechanism for the ORR in alkaline solution. An onset potential of -0.10 V vs. a saturated calomel electrode and a diffusion limiting current density of 2.30 mA/cm2 were achieved for the Co3O4/Ppy/GO catalyst heated at 800℃; these values are comparable to those for noble-metal-based Pt/C catalysts. Our work demonstrates that Co3O4/Ppy/GO is highly active for the ORR. Notably, the Ppy coupling effects between Co3O4 and GO provide a new route for the preparation of efficient non-precious electrocatalysts with hierarchical porous structures for fuel cell applications.
