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2018 Volume 39 Issue 4
2018, 39(4):
Abstract:
2018, 39(4): 565-565
doi: 10.1016/S1872-2067(18)63062-1
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2018, 39(4): 566-582
doi: 10.1016/S1872-2067(17)62996-6
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This paper presents a detailed review of copper-based catalysts used in wide-ranging environmental remediation, including gas, liquid and solid phase pollutant elimination. Latest advances in the remarkable catalytic activity of copper-based catalysts, including bulk CuOx, supported CuOx, and solid solution CuOx-X are emphasized. The structure-activity relationships among the crystal structure, morphology, catalyst support, and catalytic performance in specific catalytic reactions for environmental remediation are discussed. Furthermore, current obstacles faced by Cu-based catalysts and potential strategies to address them have been proposed, which may aid the future research and development of highly efficient Cu-based non-precious metal catalysts.
This paper presents a detailed review of copper-based catalysts used in wide-ranging environmental remediation, including gas, liquid and solid phase pollutant elimination. Latest advances in the remarkable catalytic activity of copper-based catalysts, including bulk CuOx, supported CuOx, and solid solution CuOx-X are emphasized. The structure-activity relationships among the crystal structure, morphology, catalyst support, and catalytic performance in specific catalytic reactions for environmental remediation are discussed. Furthermore, current obstacles faced by Cu-based catalysts and potential strategies to address them have been proposed, which may aid the future research and development of highly efficient Cu-based non-precious metal catalysts.
2018, 39(4): 583-589
doi: 10.1016/S1872-2067(17)62989-9
Abstract:
The slow kinetics of oxygen reduction reaction (ORR) occurring at the cathode of a proton exchange membrane fuel cell require the presence of an electrocatalyst to reduce overpotential. MPt alloy nanocrystals (NCs) have been investigated over the last decade as efficient catalysts for ORR and recent studies have shown that structurally-ordered MPt NCs, i.e., intermetallic NCs (iNCs), are more active and exhibit enhanced stability compared with the corresponding randomly alloyed analogues. This mini-review highlights the recent progress in iNC catalyst development for ORR with emphasis on correlating the synthesis-structure-activity relationship. Perspectives and possible research directions to enhance MPt iNC catalytic performance are also proposed.
The slow kinetics of oxygen reduction reaction (ORR) occurring at the cathode of a proton exchange membrane fuel cell require the presence of an electrocatalyst to reduce overpotential. MPt alloy nanocrystals (NCs) have been investigated over the last decade as efficient catalysts for ORR and recent studies have shown that structurally-ordered MPt NCs, i.e., intermetallic NCs (iNCs), are more active and exhibit enhanced stability compared with the corresponding randomly alloyed analogues. This mini-review highlights the recent progress in iNC catalyst development for ORR with emphasis on correlating the synthesis-structure-activity relationship. Perspectives and possible research directions to enhance MPt iNC catalytic performance are also proposed.
2018, 39(4): 590-605
doi: 10.1016/S1872-2067(18)63052-9
Abstract:
Colloidal semiconductor nanocrystals have been proven to be promising candidates for applications in low-cost and high-performance photovoltaics, bioimaging, and photocatalysis due to their novel size-and shape-dependent properties. Among the colloidal systems, I-Ⅲ-VI semiconductor nanocrystals (NCs) have drawn much attention in the past few decades. Compared to binary NCs, ternary I-Ⅲ-VI NCs not only exhibit low toxicity, but also a high performance similar to that of binary NCs. In this review, we mainly focus on the synthesis, properties, and applications of I-Ⅲ-VI NCs. We summarize the major synthesis methods, analyze their photophysical and electronic properties, and highlight some of the latest applications of I-Ⅲ-VI NCs in solar cells, light-emitting diodes, bioimaging, and photocatalysis. Finally, based on the information reviewed, we highlight the existing problems and challenges.
Colloidal semiconductor nanocrystals have been proven to be promising candidates for applications in low-cost and high-performance photovoltaics, bioimaging, and photocatalysis due to their novel size-and shape-dependent properties. Among the colloidal systems, I-Ⅲ-VI semiconductor nanocrystals (NCs) have drawn much attention in the past few decades. Compared to binary NCs, ternary I-Ⅲ-VI NCs not only exhibit low toxicity, but also a high performance similar to that of binary NCs. In this review, we mainly focus on the synthesis, properties, and applications of I-Ⅲ-VI NCs. We summarize the major synthesis methods, analyze their photophysical and electronic properties, and highlight some of the latest applications of I-Ⅲ-VI NCs in solar cells, light-emitting diodes, bioimaging, and photocatalysis. Finally, based on the information reviewed, we highlight the existing problems and challenges.
2018, 39(4): 606-612
doi: 10.1016/S1872-2067(17)62939-5
Abstract:
Nanocatalysts consisting of three-dimensionally ordered macroporous (3DOM) TiO2-supported ultrafine Pd nanoparticles (Pd/3DOM-TiO2-GBMR) were readily fabricated by gas bubbling-assisted membrane reduction (GBMR) method. These catalysts had a well-defined and highly ordered macroporous nanostructure with an average pore size of 280 nm. In addition, ultrafine hemispherical Pd nanoparticles (NPs) with a mean particle size of 1.1 nm were found to be well dispersed over the surface of the 3DOM-TiO2 support and deposited on the inner walls of the material. The nanostructure of the 3DOM-TiO2 support ensured efficient contact between soot particles and the catalyst. The large interface area between the ultrafine Pd NPs and the TiO2 also increased the density of sites for O2 activation as a result of the strong metal (Pd)-support (TiO2) interaction (SMSI). A Pd/3DOM-TiO2-GBMR catalyst with ultrafine Pd NPs (1.1 nm) exhibited higher catalytic activity during diesel soot combustion compared with that obtained from a specimen having relatively large Pd NPs (5.0 nm). The T10, T50 and T90 values obtained from the former were 295, 370 and 415℃. Both the activity and nanostructure of the Pd/3DOM-TiO2-GBMR catalyst were stable over five replicate soot oxidation trials. These results suggest that nanocatalysts having a 3DOM structure together with ultrafine Pd NPs can decrease the amount of Pd required, and that this approach has potential practical applications in the catalytic combustion of diesel soot particles.
Nanocatalysts consisting of three-dimensionally ordered macroporous (3DOM) TiO2-supported ultrafine Pd nanoparticles (Pd/3DOM-TiO2-GBMR) were readily fabricated by gas bubbling-assisted membrane reduction (GBMR) method. These catalysts had a well-defined and highly ordered macroporous nanostructure with an average pore size of 280 nm. In addition, ultrafine hemispherical Pd nanoparticles (NPs) with a mean particle size of 1.1 nm were found to be well dispersed over the surface of the 3DOM-TiO2 support and deposited on the inner walls of the material. The nanostructure of the 3DOM-TiO2 support ensured efficient contact between soot particles and the catalyst. The large interface area between the ultrafine Pd NPs and the TiO2 also increased the density of sites for O2 activation as a result of the strong metal (Pd)-support (TiO2) interaction (SMSI). A Pd/3DOM-TiO2-GBMR catalyst with ultrafine Pd NPs (1.1 nm) exhibited higher catalytic activity during diesel soot combustion compared with that obtained from a specimen having relatively large Pd NPs (5.0 nm). The T10, T50 and T90 values obtained from the former were 295, 370 and 415℃. Both the activity and nanostructure of the Pd/3DOM-TiO2-GBMR catalyst were stable over five replicate soot oxidation trials. These results suggest that nanocatalysts having a 3DOM structure together with ultrafine Pd NPs can decrease the amount of Pd required, and that this approach has potential practical applications in the catalytic combustion of diesel soot particles.
2018, 39(4): 613-618
doi: 10.1016/S1872-2067(17)62987-5
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A bismuth vanadate (BiVO4) photoanode with a cocatalyst consisting of NiFe layered double-hydroxide (NiFe-LDH) nanoparticles was fabricated for photoelectrochemical (PEC) water splitting. NiFe-LDH nanoparticles, which can improve light-absorption capacities and facilitate efficient hole transfer to the surface, were deposited on the surface of the BiVO4 photoanode by a hydrothermal method. All the samples were characterized using X-ray diffraction, scanning electron microscopy, and diffuse-reflectance spectroscopy. Linear sweep voltammetry and current-time plots were used to investigate the PEC activity. The photocurrent response of NiFe-LDH/BiVO4 at 1.23 V vs the reversible hydrogen electrode was higher than those of Ni(OH)2/BiVO4, Fe(OH)2/BiVO4 and pure BiVO4 electrodes under visible-light illumination. NiFe-LDH/BiVO4 also gave a superior PEC hydrogen evolution performance. Furthermore, the stability of the NiFe-LDH/BiVO4 photoanode was excellent compared with that of the bare BiVO4 photoanode, and offers a novel method for solar-assisted water splitting.
A bismuth vanadate (BiVO4) photoanode with a cocatalyst consisting of NiFe layered double-hydroxide (NiFe-LDH) nanoparticles was fabricated for photoelectrochemical (PEC) water splitting. NiFe-LDH nanoparticles, which can improve light-absorption capacities and facilitate efficient hole transfer to the surface, were deposited on the surface of the BiVO4 photoanode by a hydrothermal method. All the samples were characterized using X-ray diffraction, scanning electron microscopy, and diffuse-reflectance spectroscopy. Linear sweep voltammetry and current-time plots were used to investigate the PEC activity. The photocurrent response of NiFe-LDH/BiVO4 at 1.23 V vs the reversible hydrogen electrode was higher than those of Ni(OH)2/BiVO4, Fe(OH)2/BiVO4 and pure BiVO4 electrodes under visible-light illumination. NiFe-LDH/BiVO4 also gave a superior PEC hydrogen evolution performance. Furthermore, the stability of the NiFe-LDH/BiVO4 photoanode was excellent compared with that of the bare BiVO4 photoanode, and offers a novel method for solar-assisted water splitting.
2018, 39(4): 619-629
doi: 10.1016/S1872-2067(18)63029-3
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Both MnOx and g-C3N4 have been proved to be active in the catalytic oxidation of NO, and their individual mechanisms for catalytic NO conversion have also been investigated. However, the mechanism of photo-thermal catalysis of the MnOx/g-C3N4composite remains unresolved. In this paper, MnOx/g-C3N4 catalysts with different molar ratios were synthesized by the precipitation approach at room temperature. The as-prepared catalysts exhibit excellent synergistic photo-thermal catalytic performance towards the purification of NO in air. The MnOx/g-C3N4 catalysts contain MnOx with different valence states on the surface of g-C3N4. The thermal catalytic reaction for NO oxidation on MnOx and the photo-thermal catalytic reaction on 1:5 MnOx/g-C3N4 were investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS). The results show that light exerted a weak effect on NO oxidation over MnOx,and it exerted a positive synergistic effect on NO conversion over 1:5 MnOx/g-C3N4. A synergistic photo-thermal catalytic cycle of NO oxidation on MnOx/g-C3N4 is proposed. Specifically, photo-generated electrons (e-) are transferred to MnOx and participate in the synergistic photo-thermal reduction cycle (Mn4+→Mn3+→Mn2+). The reverse cycle (Mn2+→Mn3+→Mn4+) can regenerate the active oxygen vacancy sites and inject electrons into the g-C3N4 hole (h+). The active oxygen (O-) was generated in the redox cycles among manganese species (Mn4+/Mn3+/Mn2+) and could oxidize the intermediates (NOH and N2O2-) to final products (NO2- and NO3-). This paper can provide insightful guidance for the development of better catalysts for NOx purification.
Both MnOx and g-C3N4 have been proved to be active in the catalytic oxidation of NO, and their individual mechanisms for catalytic NO conversion have also been investigated. However, the mechanism of photo-thermal catalysis of the MnOx/g-C3N4composite remains unresolved. In this paper, MnOx/g-C3N4 catalysts with different molar ratios were synthesized by the precipitation approach at room temperature. The as-prepared catalysts exhibit excellent synergistic photo-thermal catalytic performance towards the purification of NO in air. The MnOx/g-C3N4 catalysts contain MnOx with different valence states on the surface of g-C3N4. The thermal catalytic reaction for NO oxidation on MnOx and the photo-thermal catalytic reaction on 1:5 MnOx/g-C3N4 were investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS). The results show that light exerted a weak effect on NO oxidation over MnOx,and it exerted a positive synergistic effect on NO conversion over 1:5 MnOx/g-C3N4. A synergistic photo-thermal catalytic cycle of NO oxidation on MnOx/g-C3N4 is proposed. Specifically, photo-generated electrons (e-) are transferred to MnOx and participate in the synergistic photo-thermal reduction cycle (Mn4+→Mn3+→Mn2+). The reverse cycle (Mn2+→Mn3+→Mn4+) can regenerate the active oxygen vacancy sites and inject electrons into the g-C3N4 hole (h+). The active oxygen (O-) was generated in the redox cycles among manganese species (Mn4+/Mn3+/Mn2+) and could oxidize the intermediates (NOH and N2O2-) to final products (NO2- and NO3-). This paper can provide insightful guidance for the development of better catalysts for NOx purification.
2018, 39(4): 630-638
doi: 10.1016/S1872-2067(18)63036-0
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KIT-6 mesoporous silica aged at 40, 100, and 150℃ were used as hard templates to prepare different mesoporous MnO2 catalysts, marked as Mn-40, Mn-100, and Mn-150, respectively. The catalytic activities of these catalysts and the effect of pore sizes on ethanol catalytic oxidation were investigated. Mn-40, Mn-100, and Mn-150 have triple, double, and single pore systems, respectively. On decreasing the aging temperature of KIT-6, the pore sizes of KIT-6 decrease and that of mesoporous MnO2 catalysts increase. The pore sizes and catalytic activities increase in the order:Mn-40 > Mn-100 > Mn-150. Mn-40 catalyst has a higher TOF (0.11 s-1 at 120℃) and the best catalytic activity for ethanol oxidation because of a bigger pore size with three pore systems with maximum distribution at 1.9, 3.4, and 6.6 nm, decrease in symmetry and degree of order, more surface lattice oxygen species, oxygen vacancies resulting from more Mn3+ ions, and better low-temperature reducibility.
KIT-6 mesoporous silica aged at 40, 100, and 150℃ were used as hard templates to prepare different mesoporous MnO2 catalysts, marked as Mn-40, Mn-100, and Mn-150, respectively. The catalytic activities of these catalysts and the effect of pore sizes on ethanol catalytic oxidation were investigated. Mn-40, Mn-100, and Mn-150 have triple, double, and single pore systems, respectively. On decreasing the aging temperature of KIT-6, the pore sizes of KIT-6 decrease and that of mesoporous MnO2 catalysts increase. The pore sizes and catalytic activities increase in the order:Mn-40 > Mn-100 > Mn-150. Mn-40 catalyst has a higher TOF (0.11 s-1 at 120℃) and the best catalytic activity for ethanol oxidation because of a bigger pore size with three pore systems with maximum distribution at 1.9, 3.4, and 6.6 nm, decrease in symmetry and degree of order, more surface lattice oxygen species, oxygen vacancies resulting from more Mn3+ ions, and better low-temperature reducibility.
2018, 39(4): 639-645
doi: 10.1016/S1872-2067(17)62980-2
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TiO2 mesocrystals can considerably enhance charge separation owing to their oriented superstructures, with fewer internal defects and porous properties providing more active sites. In this work, we prepared TiO2 mesocrystal films by a direct annealing method. The morphology and crystal phase of the film were controlled by adjusting the ratio of NH4F and the calcination temperature. Moreover, we found that Au nanoparticles loaded on a TiO2 mesocrystal film enabled highly efficient visible light photocatalytic properties. The photocatalytic activities were studied by hydrogen generation and photoreduction of Cr(VI). This work represents a considerable advance in the development and application of the TiO2 mesocrystals.
TiO2 mesocrystals can considerably enhance charge separation owing to their oriented superstructures, with fewer internal defects and porous properties providing more active sites. In this work, we prepared TiO2 mesocrystal films by a direct annealing method. The morphology and crystal phase of the film were controlled by adjusting the ratio of NH4F and the calcination temperature. Moreover, we found that Au nanoparticles loaded on a TiO2 mesocrystal film enabled highly efficient visible light photocatalytic properties. The photocatalytic activities were studied by hydrogen generation and photoreduction of Cr(VI). This work represents a considerable advance in the development and application of the TiO2 mesocrystals.
2018, 39(4): 646-653
doi: 10.1016/S1872-2067(17)62974-7
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Photocatalysis is considered a promising technique for removal of pollutants from indoor air. However, the low selectivity and limited recyclability of photocatalysts in powder form currently limit their practical application. In this work, we reported the successful preparation of a monolithic tungsten oxide (WO3)/graphene oxide (GO) aerogel photocatalyst through a cost-effective freeze-drying method. GO not only acts as a macroscopic support, but also increases the catalyst surface area from 46 to 57 m2/g, enhances the light absorption in the visible-light region, and raises the separation efficiency of photogenerated electron-hole pairs. The Obtained WO3/GO aerogel exhibited an outstanding visible-light photocatalytic degradation rate of nitric oxide of 51%, which was 3.3 times that of pristine WO3 powder. In addition, the aerogel displayed excellent selectivity, with a generation fraction of toxic nitrogen dioxide of as low as 0.5%. This work presents a facile synthesis route to fabricate a monolithic WO3/GO aerogel photocatalyst with great promise for air purification.
Photocatalysis is considered a promising technique for removal of pollutants from indoor air. However, the low selectivity and limited recyclability of photocatalysts in powder form currently limit their practical application. In this work, we reported the successful preparation of a monolithic tungsten oxide (WO3)/graphene oxide (GO) aerogel photocatalyst through a cost-effective freeze-drying method. GO not only acts as a macroscopic support, but also increases the catalyst surface area from 46 to 57 m2/g, enhances the light absorption in the visible-light region, and raises the separation efficiency of photogenerated electron-hole pairs. The Obtained WO3/GO aerogel exhibited an outstanding visible-light photocatalytic degradation rate of nitric oxide of 51%, which was 3.3 times that of pristine WO3 powder. In addition, the aerogel displayed excellent selectivity, with a generation fraction of toxic nitrogen dioxide of as low as 0.5%. This work presents a facile synthesis route to fabricate a monolithic WO3/GO aerogel photocatalyst with great promise for air purification.
Formation of BiOI/g-C3N4 nanosheet composites with high visible-light-driven photocatalytic activity
2018, 39(4): 654-663
doi: 10.1016/S1872-2067(17)62927-9
Abstract:
Constructing binary heterojunctions is an important strategy to improve the photocatalytic performance of graphitic carbon nitride (g-C3N4). In this paper, a novel g-C3N4 nanosheet-based composite was constructed via in situ growth of bismuth oxyiodide (BiOI) nanoplates on the surface of g-C3N4 nanosheets. The crystal phase, microstructure, optical absorption and textural properties of the synthesized photocatalysts were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy (DRS), and nitrogen adsorption-desorption isotherm measurements. The BiOI/g-C3N4 nanosheet composite showed high activity and recyclability for the photodegradation of the target pollutant rhodamine B (RhB). The conversion of RhB (20 mg L-1) by the photocatalyst was nearly 100% after 50 min under visible-light irradiation. The high photoactivity of the BiOI/g-C3N4 nanosheet composite can be attributed to the enhanced visible-light absorption of the g-C3N4 nanosheets sensitized by BiOI nanoplates as well as the high charge separation efficiency obtained by the establishment of an internal electric field between the n-type g-C3N4 and p-type BiOI. Based on the characterization and experimental results, a double-transfer mechanism of the photoinduced electrons in the BiOI/g-C3N4 nanosheet composite was proposed to explain its activity. This work represents a new strategy to understand and realize the design and synthesis of g-C3N4 nanosheet-based heterojunctions that display highly efficient charge separation and transfer.
Constructing binary heterojunctions is an important strategy to improve the photocatalytic performance of graphitic carbon nitride (g-C3N4). In this paper, a novel g-C3N4 nanosheet-based composite was constructed via in situ growth of bismuth oxyiodide (BiOI) nanoplates on the surface of g-C3N4 nanosheets. The crystal phase, microstructure, optical absorption and textural properties of the synthesized photocatalysts were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy (DRS), and nitrogen adsorption-desorption isotherm measurements. The BiOI/g-C3N4 nanosheet composite showed high activity and recyclability for the photodegradation of the target pollutant rhodamine B (RhB). The conversion of RhB (20 mg L-1) by the photocatalyst was nearly 100% after 50 min under visible-light irradiation. The high photoactivity of the BiOI/g-C3N4 nanosheet composite can be attributed to the enhanced visible-light absorption of the g-C3N4 nanosheets sensitized by BiOI nanoplates as well as the high charge separation efficiency obtained by the establishment of an internal electric field between the n-type g-C3N4 and p-type BiOI. Based on the characterization and experimental results, a double-transfer mechanism of the photoinduced electrons in the BiOI/g-C3N4 nanosheet composite was proposed to explain its activity. This work represents a new strategy to understand and realize the design and synthesis of g-C3N4 nanosheet-based heterojunctions that display highly efficient charge separation and transfer.
2018, 39(4): 664-672
doi: 10.1016/S1872-2067(17)62988-7
Abstract:
Here, we report cobalt nanoparticles encapsulated in nitrogen-doped carbon (Co@NC) that exhibit excellent catalytic activity and chemoselectivity for room-temperature hydrogenation of nitroarenes. Co@NC was synthesized by pyrolyzing a mixture of a cobalt salt, an inexpensive organic molecule, and carbon nitride. Using the Co@NC catalyst, a turnover frequency of~12.3 h-1 and selectivity for 4-aminophenol of >99.9% were achieved for hydrogenation of 4-nitrophenol at room temperature and 10 bar H2 pressure. The excellent catalytic performance can be attributed to the cooperative effect of hydrogen activation by electron-deficient Co nanoparticles and energetically preferred adsorption of the nitro group of nitroarenes to electron-rich N-doped carbon. In addition, there is electron transfer from the Co nanoparticles to N-doped carbon, which further enhances the functionality of the metal center and carbon support. The catalyst also exhibits stable recycling performance and high activity for nitroaromatics with various substituents.
Here, we report cobalt nanoparticles encapsulated in nitrogen-doped carbon (Co@NC) that exhibit excellent catalytic activity and chemoselectivity for room-temperature hydrogenation of nitroarenes. Co@NC was synthesized by pyrolyzing a mixture of a cobalt salt, an inexpensive organic molecule, and carbon nitride. Using the Co@NC catalyst, a turnover frequency of~12.3 h-1 and selectivity for 4-aminophenol of >99.9% were achieved for hydrogenation of 4-nitrophenol at room temperature and 10 bar H2 pressure. The excellent catalytic performance can be attributed to the cooperative effect of hydrogen activation by electron-deficient Co nanoparticles and energetically preferred adsorption of the nitro group of nitroarenes to electron-rich N-doped carbon. In addition, there is electron transfer from the Co nanoparticles to N-doped carbon, which further enhances the functionality of the metal center and carbon support. The catalyst also exhibits stable recycling performance and high activity for nitroaromatics with various substituents.
2018, 39(4): 673-681
doi: 10.1016/S1872-2067(18)63031-1
Abstract:
TiO2-supported Pd-Sb bimetallic catalysts were prepared and evaluated for the direct synthesis of H2O2 at ambient pressure. The addition of Sb to Pd significantly enhanced catalytic performance, and a Pd50Sb catalyst showed the greatest selectivity of up to 73%. Sb promoted the dispersion of Pd on TiO2, as evidenced by transmission electron microscopy and X-ray diffraction. X-ray photoelectron spectroscopy indicated that the oxidation of Pd was suppressed by Sb. In addition, Sb2O3 layers were formed and partially wrapped the surfaces of Pd catalysts, thus suppressing the activation of H2 and subsequent hydrogenation of H2O2. In situ diffuse reflection infrared Fourier transform spectroscopy for CO adsorption suggested that Sb homogenously located on the surface of Pd-Sb catalysts and isolated contiguous Pd sites, resulting in the rise of the ratio of Pd monomer sites that are favorable for H2O2 formation. As a result, the Sb modified Pd surfaces significantly enhanced the non-dissociative activation of O2 and H2O2 selectivity.
TiO2-supported Pd-Sb bimetallic catalysts were prepared and evaluated for the direct synthesis of H2O2 at ambient pressure. The addition of Sb to Pd significantly enhanced catalytic performance, and a Pd50Sb catalyst showed the greatest selectivity of up to 73%. Sb promoted the dispersion of Pd on TiO2, as evidenced by transmission electron microscopy and X-ray diffraction. X-ray photoelectron spectroscopy indicated that the oxidation of Pd was suppressed by Sb. In addition, Sb2O3 layers were formed and partially wrapped the surfaces of Pd catalysts, thus suppressing the activation of H2 and subsequent hydrogenation of H2O2. In situ diffuse reflection infrared Fourier transform spectroscopy for CO adsorption suggested that Sb homogenously located on the surface of Pd-Sb catalysts and isolated contiguous Pd sites, resulting in the rise of the ratio of Pd monomer sites that are favorable for H2O2 formation. As a result, the Sb modified Pd surfaces significantly enhanced the non-dissociative activation of O2 and H2O2 selectivity.
2018, 39(4): 682-692
doi: 10.1016/S1872-2067(17)62975-9
Abstract:
A highly active photocatalyst CeO2/NaNbO3 is fabricated by a simple and facile hydrothermal method. The obtained photocatalyst composites are characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy and ultraviolet-visible diffuse reflectance spectroscopy. The photocatalytic activity of the obtained samples is demonstrated by the photocatalytic degradation of the colorless antibiotic agent ciprofloxacin and the dye rhodamine B. The results reveal that CeO2/NaNbO3 composites exhibit a higher photocatalytic property than pure NaNbO3 under both UV and visible light irradiation. Furthermore, the optimum mass ratio of CeO2 in the CeO2/NaNbO3 composites is 2.0 wt%. The improved photocatalytic activity is attributed to the higher separation rate of the photo-induced electrons and holes, and the higher migration rate of the photogenerated charge in the interfacial region. Furthermore, the photoluminescence pectra, photocurrent, electrochemical impedance spectroscopy and trapping experiment are applied to demonstrate the photocatalytic reaction mechanism of the as-prepared samples. The result of the trapping experiment indicates that·OH radicals,·O2- radicals and holes are all involved in the photocatalytic degradation process of RhB. Furthermore, a possible mechanism for the enhancement of the photocatalytic activity is also proposed.
A highly active photocatalyst CeO2/NaNbO3 is fabricated by a simple and facile hydrothermal method. The obtained photocatalyst composites are characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy and ultraviolet-visible diffuse reflectance spectroscopy. The photocatalytic activity of the obtained samples is demonstrated by the photocatalytic degradation of the colorless antibiotic agent ciprofloxacin and the dye rhodamine B. The results reveal that CeO2/NaNbO3 composites exhibit a higher photocatalytic property than pure NaNbO3 under both UV and visible light irradiation. Furthermore, the optimum mass ratio of CeO2 in the CeO2/NaNbO3 composites is 2.0 wt%. The improved photocatalytic activity is attributed to the higher separation rate of the photo-induced electrons and holes, and the higher migration rate of the photogenerated charge in the interfacial region. Furthermore, the photoluminescence pectra, photocurrent, electrochemical impedance spectroscopy and trapping experiment are applied to demonstrate the photocatalytic reaction mechanism of the as-prepared samples. The result of the trapping experiment indicates that·OH radicals,·O2- radicals and holes are all involved in the photocatalytic degradation process of RhB. Furthermore, a possible mechanism for the enhancement of the photocatalytic activity is also proposed.
2018, 39(4): 693-700
doi: 10.1016/S1872-2067(17)62937-1
Abstract:
We report a one-pot surfactant-free wet-chemical reduction approach to the synthesis of palladium/titanium nitride (Pd/TiN) and Pd/carbon (Pd/C) composites, in which~5 nm Pd NPs were uniformly dispersed on TiN or C. In terms of catalytic performance, Pd/TiN showed enhanced efficiency and stability compared with those of Pd/C and bare TiN in the electrocatalytic hydrodechlorination (EHDC) reaction of 2,4-dichlorophenol (2,4-DCP) in aqueous solution. The superior performance of Pd/TiN arises from the promotion effect of TiN. Strong metal-support interactions modified the electronic structure of Pd, which optimized generation of Hads* and 2,4-DCP adsorption/activation. The cathode potential plays a vital role in controlling the EHDC efficiency and the product distribution. A working potential of -0.80 V was shown to be optimal for achieving the highest EHDC efficiency and maximizing conversion of 2,4-DCP to phenol (P). Our studies of the reaction pathway show that EHDC of 2,4-DCP on Pd/TiN proceeded by 2,4-DCP→p-chlorophenol (p-CP), o-chlorophenol (o-CP)→P; however, Pd/TiN presented little selectivity for cleavage of p-C-Cl vs o-C-Cl. This work presents a new approach to enhancing Pd performance towards EHDC through the effects of a support. The strategy demonstrated here could also be extended to design highly efficient catalysts for other hydrogenation reactions.
We report a one-pot surfactant-free wet-chemical reduction approach to the synthesis of palladium/titanium nitride (Pd/TiN) and Pd/carbon (Pd/C) composites, in which~5 nm Pd NPs were uniformly dispersed on TiN or C. In terms of catalytic performance, Pd/TiN showed enhanced efficiency and stability compared with those of Pd/C and bare TiN in the electrocatalytic hydrodechlorination (EHDC) reaction of 2,4-dichlorophenol (2,4-DCP) in aqueous solution. The superior performance of Pd/TiN arises from the promotion effect of TiN. Strong metal-support interactions modified the electronic structure of Pd, which optimized generation of Hads* and 2,4-DCP adsorption/activation. The cathode potential plays a vital role in controlling the EHDC efficiency and the product distribution. A working potential of -0.80 V was shown to be optimal for achieving the highest EHDC efficiency and maximizing conversion of 2,4-DCP to phenol (P). Our studies of the reaction pathway show that EHDC of 2,4-DCP on Pd/TiN proceeded by 2,4-DCP→p-chlorophenol (p-CP), o-chlorophenol (o-CP)→P; however, Pd/TiN presented little selectivity for cleavage of p-C-Cl vs o-C-Cl. This work presents a new approach to enhancing Pd performance towards EHDC through the effects of a support. The strategy demonstrated here could also be extended to design highly efficient catalysts for other hydrogenation reactions.
2018, 39(4): 701-709
doi: 10.1016/S1872-2067(17)62916-4
Abstract:
A synergistic UV/TiO2/Fenton (PCF) process is investigated for the degradation of ibuprofen (IBP) at circumneutral pH. The IBP decay in the PCF process is much faster than that with the conventional UV, UV/H2O2, Fenton, photo-Fenton, and photocatalysis processes. The kinetics analysis showed that the IBP decay follows a two-stage pseudo-first order profile, that is, a fast IBP decay (k1) followed by a slow decay (k2). The effects of various parameters, including initial pH level, dosage of Fenton's reagent and TiO2, wavelength of UV irradiation, and initial IBP concentration, are evaluated. The optimum pH level,[Fe2+]0,[Fe2+]0/[H2O2]0 molar ratio, and[TiO2]0 are determined to be approximately 4.22, 0.20 mmol/L, 1/40, and 1.0 g/L, respectively. The IBP decay at circumneutral pH (i.e., 6.0-8.0 for wastewater) shows the same IBP decay efficiency as that at the optimum pH of 4.22 after 30 min, which suggests that the PCF process is applicable for the treatment of wastewater in the circumneutral pH range. The lnk1 and lnk2 are observed to be linearly correlated to 1/pH0,[IBP]0,[H2O2]0,[H2O2]0/[Fe2+]0 and ln[TiO2]0. Mathematical models are therefore derived to predict the IBP decay.
A synergistic UV/TiO2/Fenton (PCF) process is investigated for the degradation of ibuprofen (IBP) at circumneutral pH. The IBP decay in the PCF process is much faster than that with the conventional UV, UV/H2O2, Fenton, photo-Fenton, and photocatalysis processes. The kinetics analysis showed that the IBP decay follows a two-stage pseudo-first order profile, that is, a fast IBP decay (k1) followed by a slow decay (k2). The effects of various parameters, including initial pH level, dosage of Fenton's reagent and TiO2, wavelength of UV irradiation, and initial IBP concentration, are evaluated. The optimum pH level,[Fe2+]0,[Fe2+]0/[H2O2]0 molar ratio, and[TiO2]0 are determined to be approximately 4.22, 0.20 mmol/L, 1/40, and 1.0 g/L, respectively. The IBP decay at circumneutral pH (i.e., 6.0-8.0 for wastewater) shows the same IBP decay efficiency as that at the optimum pH of 4.22 after 30 min, which suggests that the PCF process is applicable for the treatment of wastewater in the circumneutral pH range. The lnk1 and lnk2 are observed to be linearly correlated to 1/pH0,[IBP]0,[H2O2]0,[H2O2]0/[Fe2+]0 and ln[TiO2]0. Mathematical models are therefore derived to predict the IBP decay.
2018, 39(4): 710-717
doi: 10.1016/S1872-2067(18)63011-6
Abstract:
Alkali and alkaline-earth metals from fly ash have a significant deactivation effect on catalysts used for selective catalytic reduction of NOx by NH3 (NH3-SCR). Bromides are considered effective additives to improve Hg0 oxidation on SCR catalysts. In this work, the effects of different bromides (NH4Br, NaBr, KBr, and CaBr2) on a commercial V2O5-WO3/TiO2 catalyst were studied. NOx conversion decreased significantly over the KBr-poisoned catalyst (denoted as L-KBr), while that over NaBr-and CaBr2-poisoned catalysts (denoted as L-NaBr and L-CaBr, respectivity) decreased to a lesser extent compared with the fresh sample. Poor N2 selectivity was observed over L-NaBr, L-KBr and L-CaBr catalysts. The decrease in the ratio of chemisorbed oxygen to total surface oxygen (Oα/(Oα + Oβ + Ow)), reducibility and surface acidity might contribute to the poor activity and N2 selectivity over L-KBr catalyst. The increased Oαratio was conducive to the enhanced reducibility of L-CaBr. Combined with enhanced surface acidity, this might offset the negative effect of the loss of active sites by CaBr2 covering. The overoxidation of NH3 and poor N2 selectivity in NH3 oxidation should retard the SCR activity at high temperatures over L-CaBr catalyst. The increased basicity might contribute to increased NOx adsorption on L-KBr and L-CaBr catalysts. A correlation between the acid-basic and redox properties of bromide-poisoned catalysts and their catalytic properties is established.
Alkali and alkaline-earth metals from fly ash have a significant deactivation effect on catalysts used for selective catalytic reduction of NOx by NH3 (NH3-SCR). Bromides are considered effective additives to improve Hg0 oxidation on SCR catalysts. In this work, the effects of different bromides (NH4Br, NaBr, KBr, and CaBr2) on a commercial V2O5-WO3/TiO2 catalyst were studied. NOx conversion decreased significantly over the KBr-poisoned catalyst (denoted as L-KBr), while that over NaBr-and CaBr2-poisoned catalysts (denoted as L-NaBr and L-CaBr, respectivity) decreased to a lesser extent compared with the fresh sample. Poor N2 selectivity was observed over L-NaBr, L-KBr and L-CaBr catalysts. The decrease in the ratio of chemisorbed oxygen to total surface oxygen (Oα/(Oα + Oβ + Ow)), reducibility and surface acidity might contribute to the poor activity and N2 selectivity over L-KBr catalyst. The increased Oαratio was conducive to the enhanced reducibility of L-CaBr. Combined with enhanced surface acidity, this might offset the negative effect of the loss of active sites by CaBr2 covering. The overoxidation of NH3 and poor N2 selectivity in NH3 oxidation should retard the SCR activity at high temperatures over L-CaBr catalyst. The increased basicity might contribute to increased NOx adsorption on L-KBr and L-CaBr catalysts. A correlation between the acid-basic and redox properties of bromide-poisoned catalysts and their catalytic properties is established.
2018, 39(4): 718-727
doi: 10.1016/S1872-2067(17)62913-9
Abstract:
As a two dimensional (2D) visible-light-responsive semiconductor photocatalyst, the photoreactivity of Bi2WO6 is not high enough for practical application owing to its limited response to visible light and rapid recombination of photogenerated electron-hole pairs. In this paper, 2D core-shell structured Bi2WO6@Bi2S3 nanoplates were prepared by calcination of a mixture of Bi2WO6 (1.3 g) and a certain amount of Na2S·9H2O (0-3.0 g) at 350℃ for 2 h. The reactivity of the resulting photocatalyst materials was evaluated by photocatalytic degradation of Brilliant Red X-3B (X3B), an anionic dye, under visible light irradiation (λ > 420 nm). As the amount of Na2S·9H2O was increased from 0 to 1.5 g, the degradation rate constant of X3B sharply increased from 0.40×10-3 to 6.6×10-3 min-1. The enhanced photocatalytic activity of Bi2WO6@Bi2S3 was attributed to the photosensitization of Bi2S3, which greatly extended the light-responsive range from the visible to the NIR, and the formation of a heterojunction, which retarded the recombination rate of photogenerated electron-hole pairs. However, further increases in the amount of Na2S·9H2O (from 1.5 to 3.0 g) resulted in a decrease of the photocatalytic activity of the Bi2WO6@Bi2S3 nanoplates owing to the formation of a photo-inactive NaBiS2 layer covering the Bi2WO6 surface.
As a two dimensional (2D) visible-light-responsive semiconductor photocatalyst, the photoreactivity of Bi2WO6 is not high enough for practical application owing to its limited response to visible light and rapid recombination of photogenerated electron-hole pairs. In this paper, 2D core-shell structured Bi2WO6@Bi2S3 nanoplates were prepared by calcination of a mixture of Bi2WO6 (1.3 g) and a certain amount of Na2S·9H2O (0-3.0 g) at 350℃ for 2 h. The reactivity of the resulting photocatalyst materials was evaluated by photocatalytic degradation of Brilliant Red X-3B (X3B), an anionic dye, under visible light irradiation (λ > 420 nm). As the amount of Na2S·9H2O was increased from 0 to 1.5 g, the degradation rate constant of X3B sharply increased from 0.40×10-3 to 6.6×10-3 min-1. The enhanced photocatalytic activity of Bi2WO6@Bi2S3 was attributed to the photosensitization of Bi2S3, which greatly extended the light-responsive range from the visible to the NIR, and the formation of a heterojunction, which retarded the recombination rate of photogenerated electron-hole pairs. However, further increases in the amount of Na2S·9H2O (from 1.5 to 3.0 g) resulted in a decrease of the photocatalytic activity of the Bi2WO6@Bi2S3 nanoplates owing to the formation of a photo-inactive NaBiS2 layer covering the Bi2WO6 surface.
2018, 39(4): 728-735
doi: 10.1016/S1872-2067(17)63008-0
Abstract:
In the present study, we synthesized CeO2 catalysts doped with various transition metals (M=Co, Fe, or Cu) using a supercritical water hydrothermal route, which led to the incorporation of the metal ions in the CeO2 lattice, forming solid solutions. The catalysts were then used for the selective catalytic reduction (SCR) of NO by CO. The Cu-doped catalyst exhibited the highest SCR activity; it had a T50 (i.e., 50% NO conversion) of only 83℃ and a T90 (i.e., 90% NO conversion) of 126℃. Such an activity was also higher than in many state-of-the-art catalysts. In situ diffuse reflectance Fourier transform infrared spectroscopy suggested that the MOx-CeO2 catalysts (M=Co and Fe) mainly followed an Eley-Rideal reaction mechanism for CO-SCR. In contrast, a Langmuir-Hinshelwood SCR reaction mechanism occurred in CuO-CeO2 owing to the presence of Cu+ species, which ensured effective adsorption of CO. This explains why CuO-CeO2 exhibited the highest activity with regard to the SCR of NO by CO.
In the present study, we synthesized CeO2 catalysts doped with various transition metals (M=Co, Fe, or Cu) using a supercritical water hydrothermal route, which led to the incorporation of the metal ions in the CeO2 lattice, forming solid solutions. The catalysts were then used for the selective catalytic reduction (SCR) of NO by CO. The Cu-doped catalyst exhibited the highest SCR activity; it had a T50 (i.e., 50% NO conversion) of only 83℃ and a T90 (i.e., 90% NO conversion) of 126℃. Such an activity was also higher than in many state-of-the-art catalysts. In situ diffuse reflectance Fourier transform infrared spectroscopy suggested that the MOx-CeO2 catalysts (M=Co and Fe) mainly followed an Eley-Rideal reaction mechanism for CO-SCR. In contrast, a Langmuir-Hinshelwood SCR reaction mechanism occurred in CuO-CeO2 owing to the presence of Cu+ species, which ensured effective adsorption of CO. This explains why CuO-CeO2 exhibited the highest activity with regard to the SCR of NO by CO.
2018, 39(4): 736-746
doi: 10.1016/S1872-2067(18)63039-6
Abstract:
Hierarchical TiO2 hollow nanoboxes (TiO2-HNBs) assembled from TiO2 nanosheets (TiO2-NSs) show improved photoreactivity when compared with the building blocks of discrete TiO2-NSs. However, TiO2-HNBs can only be excited by ultraviolet light. In this paper, visible-light-responsive N and S co-doped TiO2-HNBs were prepared by calcining the mixture of cubic TiOF2 and methionine (C5H11NO2S), a N-and S-containing biomacromolecule. The effect of calcination temperature on the structure and performance of the TiO2-HNBs was systematically studied. It was found that methionine can prevent TiOF2-to-anatase TiO2 phase transformation. Both N and S elements are doped into the lattice of TiO2-HNBs when the mixture of TiOF2 and methionine undergoes calcination at 400℃, which is responsible for the visible-light response. When compared with that of pure 400℃-calcined TiO2-HNBs (T400), the photoreactivity of 400℃-calcined methionine-modified TiO2-HNBs (TM400) improves 1.53 times in photocatalytic degradation of rhodamine-B dye under visible irradiation (λ > 420 nm). The enhanced visible photoreactivity of methionine-modified TiO2-HNBs is also confirmed by photocatalytic oxidation of NO. The successful doping of N and S elements into the lattice of TiO2-HNBs, resulting in the improved light-harvesting ability and efficient separation of photo-generated electron-hole pairs, is responsible for the enhanced visible photocatalytic activity of methionine-modified TiO2-HNBs. The photoreactivity of methionine modified TiO2-HNBs remains nearly unchanged even after being recycled five times, indicating its promising use in practical applications.
Hierarchical TiO2 hollow nanoboxes (TiO2-HNBs) assembled from TiO2 nanosheets (TiO2-NSs) show improved photoreactivity when compared with the building blocks of discrete TiO2-NSs. However, TiO2-HNBs can only be excited by ultraviolet light. In this paper, visible-light-responsive N and S co-doped TiO2-HNBs were prepared by calcining the mixture of cubic TiOF2 and methionine (C5H11NO2S), a N-and S-containing biomacromolecule. The effect of calcination temperature on the structure and performance of the TiO2-HNBs was systematically studied. It was found that methionine can prevent TiOF2-to-anatase TiO2 phase transformation. Both N and S elements are doped into the lattice of TiO2-HNBs when the mixture of TiOF2 and methionine undergoes calcination at 400℃, which is responsible for the visible-light response. When compared with that of pure 400℃-calcined TiO2-HNBs (T400), the photoreactivity of 400℃-calcined methionine-modified TiO2-HNBs (TM400) improves 1.53 times in photocatalytic degradation of rhodamine-B dye under visible irradiation (λ > 420 nm). The enhanced visible photoreactivity of methionine-modified TiO2-HNBs is also confirmed by photocatalytic oxidation of NO. The successful doping of N and S elements into the lattice of TiO2-HNBs, resulting in the improved light-harvesting ability and efficient separation of photo-generated electron-hole pairs, is responsible for the enhanced visible photocatalytic activity of methionine-modified TiO2-HNBs. The photoreactivity of methionine modified TiO2-HNBs remains nearly unchanged even after being recycled five times, indicating its promising use in practical applications.
2018, 39(4): 747-759
doi: 10.1016/S1872-2067(18)63038-4
Abstract:
Lattice-doping and surface decoration are prospective routes to improve the visible-light photocatalytic ability of TiO2, but the two techniques are difficult to combine into one preparation process because they are usually conducted under different conditions, which limits the efficiency of TiO2 modification. In this study, TiO2 was successfully modified by simultaneous lattice-doping and surface decoration, and the visible-light photocatalytic capacity was largely improved. Upon comparing the method reported here with previous ones, the most significant difference is that Fe(Ⅱ)-phenanthroline was first used as the co-precursor of the introduced elements of C, N, and Fe. These three elements were simultaneously introduced to TiO2 at high levels by this co-precursor method. The as-synthesized photocatalysts were systemically investigated and analyzed by several characterization methods such as XRD, FT-IR, XPS, Raman spectroscopy, EPR, UV-Vis DRS, photoluminescence spectra, photocurrent, electrochemical impedance spectra, TEM, and HRTEM. The photocatalytic degradation of 4-NP under visible-light irradiation was used to evaluate the photocatalytic activity of the photocatalysts. Based on the experimental data, a probable mechanism for the photocatalytic degradation by the photocatalysts is proposed. This is a novel method of using one source to simultaneously introduce metal and non-metal elements to TiO2 at high levels, which may provide a new way to prepare highly effective TiO2 photocatalysts.
Lattice-doping and surface decoration are prospective routes to improve the visible-light photocatalytic ability of TiO2, but the two techniques are difficult to combine into one preparation process because they are usually conducted under different conditions, which limits the efficiency of TiO2 modification. In this study, TiO2 was successfully modified by simultaneous lattice-doping and surface decoration, and the visible-light photocatalytic capacity was largely improved. Upon comparing the method reported here with previous ones, the most significant difference is that Fe(Ⅱ)-phenanthroline was first used as the co-precursor of the introduced elements of C, N, and Fe. These three elements were simultaneously introduced to TiO2 at high levels by this co-precursor method. The as-synthesized photocatalysts were systemically investigated and analyzed by several characterization methods such as XRD, FT-IR, XPS, Raman spectroscopy, EPR, UV-Vis DRS, photoluminescence spectra, photocurrent, electrochemical impedance spectra, TEM, and HRTEM. The photocatalytic degradation of 4-NP under visible-light irradiation was used to evaluate the photocatalytic activity of the photocatalysts. Based on the experimental data, a probable mechanism for the photocatalytic degradation by the photocatalysts is proposed. This is a novel method of using one source to simultaneously introduce metal and non-metal elements to TiO2 at high levels, which may provide a new way to prepare highly effective TiO2 photocatalysts.
2018, 39(4): 760-770
doi: 10.1016/S1872-2067(17)62978-4
Abstract:
Gold (Au) plasmonic nanoparticles were grown evenly on monolayer graphitic carbon nitride (g-C3N4) nanosheets via a facile oil-bath method. The photocatalytic activity of the Au/monolayer g-C3N4 composites under visible light was evaluated by photocatalytic hydrogen evolution and environmental treatment. All of the Au/monolayer g-C3N4 composites showed better photocatalytic performance than that of monolayer g-C3N4 and the 1% Au/monolayer g-C3N4 composite displayed the highest photocatalytic hydrogen evolution rate of the samples. The remarkable photocatalytic activity was attributed largely to the successful introduction of Au plasmonic nanoparticles, which led to the surface plasmon resonance (SPR) effect. The SPR effect enhanced the efficiency of light harvesting and induced an efficient hot electron transfer process. The hot electrons were injected from the Au plasmonic nanoparticles into the conduction band of monolayer g-C3N4. Thus, the Au/monolayer g-C3N4 composites possessed higher migration and separation efficiencies and lower recombination probability of photogenerated electron-hole pairs than those of monolayer g-C3N4. A photocatalytic mechanism for the composites was also proposed.
Gold (Au) plasmonic nanoparticles were grown evenly on monolayer graphitic carbon nitride (g-C3N4) nanosheets via a facile oil-bath method. The photocatalytic activity of the Au/monolayer g-C3N4 composites under visible light was evaluated by photocatalytic hydrogen evolution and environmental treatment. All of the Au/monolayer g-C3N4 composites showed better photocatalytic performance than that of monolayer g-C3N4 and the 1% Au/monolayer g-C3N4 composite displayed the highest photocatalytic hydrogen evolution rate of the samples. The remarkable photocatalytic activity was attributed largely to the successful introduction of Au plasmonic nanoparticles, which led to the surface plasmon resonance (SPR) effect. The SPR effect enhanced the efficiency of light harvesting and induced an efficient hot electron transfer process. The hot electrons were injected from the Au plasmonic nanoparticles into the conduction band of monolayer g-C3N4. Thus, the Au/monolayer g-C3N4 composites possessed higher migration and separation efficiencies and lower recombination probability of photogenerated electron-hole pairs than those of monolayer g-C3N4. A photocatalytic mechanism for the composites was also proposed.
2018, 39(4): 771-778
doi: 10.1016/S1872-2067(18)63034-7
Abstract:
A sample of sulfated anatase TiO2 with high-energy (001) facets (TiO2-001) was prepared by a simple one-step hydrothermal route using SO42- as a morphology-controlling agent. After doping ceria, Ce/TiO2-001 was used as the catalyst for selective catalytic reduction (SCR) of NO with NH3. Compared with Ce/P25 (Degussa P25 TiO2) and Ce/P25-S (sulfated P25) catalysts, Ce/TiO2-001 was more suitable for medium-and high-temperature SCR of NO due to the high surface area, sulfation, and the excellent properties of the active-energy (001) facets. All of these facilitated the generation of abundant acidity, chemisorbed oxygen, and activated NOx-adsorption species, which were the important factors for the SCR reaction.
A sample of sulfated anatase TiO2 with high-energy (001) facets (TiO2-001) was prepared by a simple one-step hydrothermal route using SO42- as a morphology-controlling agent. After doping ceria, Ce/TiO2-001 was used as the catalyst for selective catalytic reduction (SCR) of NO with NH3. Compared with Ce/P25 (Degussa P25 TiO2) and Ce/P25-S (sulfated P25) catalysts, Ce/TiO2-001 was more suitable for medium-and high-temperature SCR of NO due to the high surface area, sulfation, and the excellent properties of the active-energy (001) facets. All of these facilitated the generation of abundant acidity, chemisorbed oxygen, and activated NOx-adsorption species, which were the important factors for the SCR reaction.
2018, 39(4): 779-789
doi: 10.1016/S1872-2067(18)63056-6
Abstract:
In this work, a series of BiOBr nanoplates with oxygen vacancies (OVs) were synthesized by a solvothermal method using a water/ethylene glycol solution. The number of OVs and facets of BiOBr were tuned by changing the water/ethylene glycol ratio. Although the role of OVs in photocatalysis has been investigated, the underlying mechanisms of charge transfer and reactant activation remain unknown. To unravel the effect of OVs on the reactant activation and photocatalytic NO oxidation process, in situ diffuse reflectance infrared Fourier transform spectroscopy, so-called DRIFTS, and theoretical calculations were performed and their results combined. The photocatalytic efficiency of the as-prepared BiOBr was significantly increased by increasing the amount of OVs. The oxygen vacancies had several effects on the photocatalysts, including the introduction of intermediate energy levels that enhanced light absorption, promoted electron transfer, acted as active sites for catalytic reaction and the activation of oxygen molecules, and facilitated the conversion of the intermediate products to the final product, thus increasing the overall visible light photocatalysis efficiency. The present work provides new insights into the understanding of the role of OVs in photocatalysts and the mechanism of photocatalytic NO oxidation.
In this work, a series of BiOBr nanoplates with oxygen vacancies (OVs) were synthesized by a solvothermal method using a water/ethylene glycol solution. The number of OVs and facets of BiOBr were tuned by changing the water/ethylene glycol ratio. Although the role of OVs in photocatalysis has been investigated, the underlying mechanisms of charge transfer and reactant activation remain unknown. To unravel the effect of OVs on the reactant activation and photocatalytic NO oxidation process, in situ diffuse reflectance infrared Fourier transform spectroscopy, so-called DRIFTS, and theoretical calculations were performed and their results combined. The photocatalytic efficiency of the as-prepared BiOBr was significantly increased by increasing the amount of OVs. The oxygen vacancies had several effects on the photocatalysts, including the introduction of intermediate energy levels that enhanced light absorption, promoted electron transfer, acted as active sites for catalytic reaction and the activation of oxygen molecules, and facilitated the conversion of the intermediate products to the final product, thus increasing the overall visible light photocatalysis efficiency. The present work provides new insights into the understanding of the role of OVs in photocatalysts and the mechanism of photocatalytic NO oxidation.
2018, 39(4): 790-799
doi: 10.1016/S1872-2067(17)62982-6
Abstract:
The oxygen reduction reaction (ORR) is a vitally important process in fuel cells. The development of high-performance and low-cost ORR electrocatalysts with outstanding stability is essential for the commercialization of the electrochemical energy technology. Herein, we report a facile synthesis of cobalt (Co) and nitrogen (N) co-doped carbon nanotube@porous carbon (Co/N/CNT@PC-800) electrocatalyst through a one-step pyrolysis of waste paper, dicyandiamide, and cobalt(Ⅱ) acetylacetonate. The surface of the hierarchical porous carbon supported a large number of carbon nanotubes (CNTs), which were derived from dicyandiamide through the catalysis of Co. The addition of Co resulted in the formation of a hierarchical micro/mesoporous structure, which was beneficial for the exposure of active sites and rapid transportation of ORR-relevant species (O2, H+, OH-, and H2O). The doped N and Co formed more active sites to enhance the ORR activity of the electrocatalyst. The Co/N/CNT@PC-800 material exhibited optimal ORR performance with an onset potential of 0.005 V vs. Ag/AgCl and a half-wave potential of -0.173 V vs. Ag/AgCl. Meanwhile, the electrocatalyst showed an excellent methanol tolerance and a long-term operational durability than that of Pt/C, as well as a quasi-four-electron reaction pathway. The low-cost and simple synthesis approach makes the Co/N/CNT@PC-800 a prospective electrocatalyst for the ORR. Furthermore, this work provides an alternative approach for exploring the use of biomass-derived electrocatalysts for renewable energy applications.
The oxygen reduction reaction (ORR) is a vitally important process in fuel cells. The development of high-performance and low-cost ORR electrocatalysts with outstanding stability is essential for the commercialization of the electrochemical energy technology. Herein, we report a facile synthesis of cobalt (Co) and nitrogen (N) co-doped carbon nanotube@porous carbon (Co/N/CNT@PC-800) electrocatalyst through a one-step pyrolysis of waste paper, dicyandiamide, and cobalt(Ⅱ) acetylacetonate. The surface of the hierarchical porous carbon supported a large number of carbon nanotubes (CNTs), which were derived from dicyandiamide through the catalysis of Co. The addition of Co resulted in the formation of a hierarchical micro/mesoporous structure, which was beneficial for the exposure of active sites and rapid transportation of ORR-relevant species (O2, H+, OH-, and H2O). The doped N and Co formed more active sites to enhance the ORR activity of the electrocatalyst. The Co/N/CNT@PC-800 material exhibited optimal ORR performance with an onset potential of 0.005 V vs. Ag/AgCl and a half-wave potential of -0.173 V vs. Ag/AgCl. Meanwhile, the electrocatalyst showed an excellent methanol tolerance and a long-term operational durability than that of Pt/C, as well as a quasi-four-electron reaction pathway. The low-cost and simple synthesis approach makes the Co/N/CNT@PC-800 a prospective electrocatalyst for the ORR. Furthermore, this work provides an alternative approach for exploring the use of biomass-derived electrocatalysts for renewable energy applications.
2018, 39(4): 800-809
doi: 10.1016/S1872-2067(18)63013-X
Abstract:
Cu-Mn bimetal catalysts were prepared to remove nitrogen oxides (NOx) from diesel engine exhaust at low temperatures. At a Cu/Mn ratio of 3:2, the NOx conversions at 200℃ reached 65% and 90% on Cu-Mn/ZSM-5 and Cu-Mn/SAPO-34, respectively. After a hydrothermal treatment and reaction in the presence of C3H6, the activity of Cu-Mn/SAPO-34 was more stable than that of Cu-Mn/ZSM-5. No obvious variations in the crystal structure or dealumination were observed, whereas the physical structure was best maintained in Cu-Mn/SAPO-34. The atomic concentration of Cuon the surface of Cu-Mn/SAPO-34 was quite stable, and the consumption of octahedrally coordinated Cu2+ could be recovered. Conversely, the proportion of octahedrally coordinated Cu2+ on the surface of Cu-Mn/ZSM-5 significantly decreased. Therefore, besides the structure, the redox cycle between Cu+ and octahedrally coordinated Cu2+ played an important role in the stability of the catalysts.
Cu-Mn bimetal catalysts were prepared to remove nitrogen oxides (NOx) from diesel engine exhaust at low temperatures. At a Cu/Mn ratio of 3:2, the NOx conversions at 200℃ reached 65% and 90% on Cu-Mn/ZSM-5 and Cu-Mn/SAPO-34, respectively. After a hydrothermal treatment and reaction in the presence of C3H6, the activity of Cu-Mn/SAPO-34 was more stable than that of Cu-Mn/ZSM-5. No obvious variations in the crystal structure or dealumination were observed, whereas the physical structure was best maintained in Cu-Mn/SAPO-34. The atomic concentration of Cuon the surface of Cu-Mn/SAPO-34 was quite stable, and the consumption of octahedrally coordinated Cu2+ could be recovered. Conversely, the proportion of octahedrally coordinated Cu2+ on the surface of Cu-Mn/ZSM-5 significantly decreased. Therefore, besides the structure, the redox cycle between Cu+ and octahedrally coordinated Cu2+ played an important role in the stability of the catalysts.
2018, 39(4): 810-820
doi: 10.1016/S1872-2067(17)63004-3
Abstract:
The photocatalytic ability of ZnO is improved through the addition of flower-like Bi2WO6 to prepare a Bi2WO6/ZnO composite with visible light activity. The composite is characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy with UV-vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy and N2 adsorption-desorption isotherms. After modification, the band gap energy of Bi2WO6/ZnO is reduced from 3.2 eV for ZnO to 2.6 eV. Under visible light irradiation, the Bi2WO6/ZnO composite shows an excellent photocatalytic activity for degrading methylene blue (MB) and tetracycline. The photo-degradation efficiencies of (0.3:1) Bi2WO6/ZnO for MB and tetracycline are approximately 246 and 4500 times higher than those of bare ZnO, respectively, and correspondingly, the photo-degradation rates for the two pollutants are approximately 120 and 200 times higher than those with bare ZnO, respectively. Moreover, the photocatalyst of (0.3:1) Bi2WO6/ZnO exhibits a higher transient photocurrent density of approximately 4.5 μA compared with those of bare Bi2WO6 and ZnO nanoparticles. The successful recombination of Bi2WO6 and ZnO enhances the photocatalytic activity and reduces the band gap energy of ZnO, which can be attributed to the effective separation of electron-hole pairs. Active species trapping experiments display that[O2]- is the major species involved during photocatalysis rather than·OH and h+. This study provides insight into designing a meaningful visible-light-driven photocatalyst for environmental remediation.
The photocatalytic ability of ZnO is improved through the addition of flower-like Bi2WO6 to prepare a Bi2WO6/ZnO composite with visible light activity. The composite is characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy with UV-vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy and N2 adsorption-desorption isotherms. After modification, the band gap energy of Bi2WO6/ZnO is reduced from 3.2 eV for ZnO to 2.6 eV. Under visible light irradiation, the Bi2WO6/ZnO composite shows an excellent photocatalytic activity for degrading methylene blue (MB) and tetracycline. The photo-degradation efficiencies of (0.3:1) Bi2WO6/ZnO for MB and tetracycline are approximately 246 and 4500 times higher than those of bare ZnO, respectively, and correspondingly, the photo-degradation rates for the two pollutants are approximately 120 and 200 times higher than those with bare ZnO, respectively. Moreover, the photocatalyst of (0.3:1) Bi2WO6/ZnO exhibits a higher transient photocurrent density of approximately 4.5 μA compared with those of bare Bi2WO6 and ZnO nanoparticles. The successful recombination of Bi2WO6 and ZnO enhances the photocatalytic activity and reduces the band gap energy of ZnO, which can be attributed to the effective separation of electron-hole pairs. Active species trapping experiments display that[O2]- is the major species involved during photocatalysis rather than·OH and h+. This study provides insight into designing a meaningful visible-light-driven photocatalyst for environmental remediation.
2018, 39(4): 821-830
doi: 10.1016/S1872-2067(18)63059-1
Abstract:
In this study, a MnOx@TiO2 core-shell catalyst prepared by a two-step method was used for the low-temperature selective catalytic reduction of NOx with NH3. The catalyst exhibits high activity, high stability, and excellent N2 selectivity. Furthermore, it displays better SO2 and H2O tolerance than its MnOx, TiO2, and MnOx/TiO2 counterparts. The prepared catalyst was characterized systematically by transmission electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, Raman, BET, X-ray photoelectron spectroscopy, NH3 temperature-programmed desorption and H2 temperature-programmed reduction analyses. The optimized MnOx@TiO2 catalyst exhibits an obvious core-shell structure, where the TiO2 shell is evenly distributed over the MnOx nanorod core. The catalyst also presents abundant mesopores, Lewis-acid sites, and high redox capability, all of which enhance its catalytic performance. According to the XPS results, the decrease in the number of Mn4+ active centers after SO2 poisoning is significantly lower in MnOx@TiO2 than in MnOx/TiO2. The core-shell structure is hence able to protect the catalytic active sites from H2O and SO2 poisoning.
In this study, a MnOx@TiO2 core-shell catalyst prepared by a two-step method was used for the low-temperature selective catalytic reduction of NOx with NH3. The catalyst exhibits high activity, high stability, and excellent N2 selectivity. Furthermore, it displays better SO2 and H2O tolerance than its MnOx, TiO2, and MnOx/TiO2 counterparts. The prepared catalyst was characterized systematically by transmission electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, Raman, BET, X-ray photoelectron spectroscopy, NH3 temperature-programmed desorption and H2 temperature-programmed reduction analyses. The optimized MnOx@TiO2 catalyst exhibits an obvious core-shell structure, where the TiO2 shell is evenly distributed over the MnOx nanorod core. The catalyst also presents abundant mesopores, Lewis-acid sites, and high redox capability, all of which enhance its catalytic performance. According to the XPS results, the decrease in the number of Mn4+ active centers after SO2 poisoning is significantly lower in MnOx@TiO2 than in MnOx/TiO2. The core-shell structure is hence able to protect the catalytic active sites from H2O and SO2 poisoning.
2018, 39(4): 831-840
doi: 10.1016/S1872-2067(17)62997-8
Abstract:
Polypyrrole-modified graphitic carbon nitride composites (PPy/g-C3N4) are fabricated using an in-situ polymerization method to improve the visible light photocatalytic activity of g-C3N4. The PPy/g-C3N4 is applied to the photocatalytic degradation of methylene blue (MB) under visible light irradiation. Various characterization techniques are employed to investigate the relationship between the structural properties and photoactivities of the as-prepared composites. Results show that the specific surface area of the PPy/g-C3N4 composites increases upon assembly of the amorphous PPy nanoparticles on the g-C3N4 surface. Owing to the strong conductivity, the PPy can be used as a transition channel for electrons to move onto the g-C3N4 surface, thus inhibiting the recombination of photogenerated carriers of g-C3N4 and improving the photocatalytic performance. The elevated light adsorption of PPy/g-C3N4 composites is attributed to the strong absorption coefficient of PPy. The composite containing 0.75 wt% PPy exhibits a photocatalytic efficiency that is 3 times higher than that of g-C3N4in 2 h. Moreover, the degradation kinetics follow a pseudo-first-order model. A detailed photocatalytic mechanism is proposed with·OH and·O2- radicals as the main reactive species. The present work provides new insights into the mechanistic understanding of PPy in PPy/g-C3N4 composites for environmental applications.
Polypyrrole-modified graphitic carbon nitride composites (PPy/g-C3N4) are fabricated using an in-situ polymerization method to improve the visible light photocatalytic activity of g-C3N4. The PPy/g-C3N4 is applied to the photocatalytic degradation of methylene blue (MB) under visible light irradiation. Various characterization techniques are employed to investigate the relationship between the structural properties and photoactivities of the as-prepared composites. Results show that the specific surface area of the PPy/g-C3N4 composites increases upon assembly of the amorphous PPy nanoparticles on the g-C3N4 surface. Owing to the strong conductivity, the PPy can be used as a transition channel for electrons to move onto the g-C3N4 surface, thus inhibiting the recombination of photogenerated carriers of g-C3N4 and improving the photocatalytic performance. The elevated light adsorption of PPy/g-C3N4 composites is attributed to the strong absorption coefficient of PPy. The composite containing 0.75 wt% PPy exhibits a photocatalytic efficiency that is 3 times higher than that of g-C3N4in 2 h. Moreover, the degradation kinetics follow a pseudo-first-order model. A detailed photocatalytic mechanism is proposed with·OH and·O2- radicals as the main reactive species. The present work provides new insights into the mechanistic understanding of PPy in PPy/g-C3N4 composites for environmental applications.
2018, 39(4): 841-848
doi: 10.1016/S1872-2067(17)62972-3
Abstract:
Zero-dimensional carbon dots (0D C-dots) and one-dimensional sulfide cadmium nanowires (1D CdS NWs) were prepared by microwave and solvothermal methods, respectively. A series of heterogeneous photocatalysts that consisted of 1D CdS NWs that were modified with 0D C-dots (C-dots/CdS NWs) were synthesized using chemical deposition methods. The mass fraction of C-dots to CdS NWs in these photocatalysts was varied. The photocatalysts were characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and ultraviolet-visible spectroscopy. Their photocatalytic performance for the spitting of water and the degradation of rhodamine B (RhB) under visible light irradiation were investigated. The photocatalytic performance of the C-dots/CdS NWs was enhanced when compared with that of the pure CdS NWs, with the 0.4% C-dots/CdS NWs exhibiting the highest photocatalytic activity for the splitting of water and the degradation of RhB. The enhanced photocatalytic activity was attributed to a higher carrier density because of the heterojunction between the C-dots and CdS NWs. This heterojunction improved the electronic transmission capacity and promoted efficient separation of photogenerated electrons and holes.
Zero-dimensional carbon dots (0D C-dots) and one-dimensional sulfide cadmium nanowires (1D CdS NWs) were prepared by microwave and solvothermal methods, respectively. A series of heterogeneous photocatalysts that consisted of 1D CdS NWs that were modified with 0D C-dots (C-dots/CdS NWs) were synthesized using chemical deposition methods. The mass fraction of C-dots to CdS NWs in these photocatalysts was varied. The photocatalysts were characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and ultraviolet-visible spectroscopy. Their photocatalytic performance for the spitting of water and the degradation of rhodamine B (RhB) under visible light irradiation were investigated. The photocatalytic performance of the C-dots/CdS NWs was enhanced when compared with that of the pure CdS NWs, with the 0.4% C-dots/CdS NWs exhibiting the highest photocatalytic activity for the splitting of water and the degradation of RhB. The enhanced photocatalytic activity was attributed to a higher carrier density because of the heterojunction between the C-dots and CdS NWs. This heterojunction improved the electronic transmission capacity and promoted efficient separation of photogenerated electrons and holes.
2018, 39(4): 849-856
doi: 10.1016/S1872-2067(17)62950-4
Abstract:
MCM-41 was synthesized by a soft template technique. The specific surface area and pore volume of the MCM-41 were 805.9 m2/g and 0.795 cm3/g, respectively. MCM-41-supported manganese and cobalt oxide catalysts were prepared by an impregnation method. The energy dispersive X-ray spectroscopy clearly confirmed the existence of Mn, Co, and O, which indicated the successful loading of the active components on the surface of MCM-41. The structure and function of the catalysts were changed by modulating the molar ratio of manganese to cobalt. The 10%MnCo(6:1)/MCM-41 (Mn/Co molar ratio is 6:1) catalyst displayed the best catalytic activity according to the activity evaluation experiments, and chlorobenzene (1000 ppm) was totally decomposed at 270℃. The high activity correlated with a high dispersion of the oxides and was attributed to the exposure of more active sites, which was demonstrated by X-ray diffraction and high-resolution transmission electron microscopy. The strong interactions between MnO2, Co3O4, MnCoOx, and MCM-41 indicated that cobalt promoted the redox cycles of the manganese system. The bimetal-oxide-based catalyst showed better catalytic activity than that of the single metal oxide catalysts, which was further confirmed by H2 temperature-programmed reduction. Chlorobenzene temperature-programmed desorption results showed that 10%MnCo(6:1)/MCM-41 had higher adsorption strength for chlorobenzene than that of single metal catalysts. And stronger adsorption was beneficial for combustion of chlorobenzene. Furthermore, 10%MnCo(6:1)/MCM-41 was not deactivated during a continuous reaction for 1000 h at 260℃ and displayed good resistance to water and benzene, which indicated that the catalyst could be used in a wide range of applications.
MCM-41 was synthesized by a soft template technique. The specific surface area and pore volume of the MCM-41 were 805.9 m2/g and 0.795 cm3/g, respectively. MCM-41-supported manganese and cobalt oxide catalysts were prepared by an impregnation method. The energy dispersive X-ray spectroscopy clearly confirmed the existence of Mn, Co, and O, which indicated the successful loading of the active components on the surface of MCM-41. The structure and function of the catalysts were changed by modulating the molar ratio of manganese to cobalt. The 10%MnCo(6:1)/MCM-41 (Mn/Co molar ratio is 6:1) catalyst displayed the best catalytic activity according to the activity evaluation experiments, and chlorobenzene (1000 ppm) was totally decomposed at 270℃. The high activity correlated with a high dispersion of the oxides and was attributed to the exposure of more active sites, which was demonstrated by X-ray diffraction and high-resolution transmission electron microscopy. The strong interactions between MnO2, Co3O4, MnCoOx, and MCM-41 indicated that cobalt promoted the redox cycles of the manganese system. The bimetal-oxide-based catalyst showed better catalytic activity than that of the single metal oxide catalysts, which was further confirmed by H2 temperature-programmed reduction. Chlorobenzene temperature-programmed desorption results showed that 10%MnCo(6:1)/MCM-41 had higher adsorption strength for chlorobenzene than that of single metal catalysts. And stronger adsorption was beneficial for combustion of chlorobenzene. Furthermore, 10%MnCo(6:1)/MCM-41 was not deactivated during a continuous reaction for 1000 h at 260℃ and displayed good resistance to water and benzene, which indicated that the catalyst could be used in a wide range of applications.
2018, 39(4): 857-866
doi: 10.1016/S1872-2067(17)62919-X
Abstract:
A series of both unsupported and coal-supported iron-oxygen compounds with gradual changes in microstructure were synthesized by a precipitation-oxidation process at 20 to 70℃. The relationship between the microstructures and catalytic activities of these precursors during direct coal liquefaction was studied. The results show that the microstructure could be controlled through adjusting the synthesis temperature during the precipitation-oxidation procedure, and that compounds synthesized at lower temperatures exhibit higher catalytic activity. As a result of their higher proportions of γ-FeOOH or α-FeOOH crystalline phases, the unsupported iron-oxygen compounds synthesized at 20-30℃, which also had high specific surface areas and moisture levels, generate oil yields 4.5%-4.6% higher than those obtained with precursors synthesized at 70℃. It was also determined that higher oil yields were obtained when the catalytically-active phase formed by the precursors during liquefaction (pyrrhotite, Fe1-xS) had smaller crystallites. Feed coal added as a carrier was found to efficiently disperse the active precursors, which in turn significantly improved the catalytic activity during coal liquefaction.
A series of both unsupported and coal-supported iron-oxygen compounds with gradual changes in microstructure were synthesized by a precipitation-oxidation process at 20 to 70℃. The relationship between the microstructures and catalytic activities of these precursors during direct coal liquefaction was studied. The results show that the microstructure could be controlled through adjusting the synthesis temperature during the precipitation-oxidation procedure, and that compounds synthesized at lower temperatures exhibit higher catalytic activity. As a result of their higher proportions of γ-FeOOH or α-FeOOH crystalline phases, the unsupported iron-oxygen compounds synthesized at 20-30℃, which also had high specific surface areas and moisture levels, generate oil yields 4.5%-4.6% higher than those obtained with precursors synthesized at 70℃. It was also determined that higher oil yields were obtained when the catalytically-active phase formed by the precursors during liquefaction (pyrrhotite, Fe1-xS) had smaller crystallites. Feed coal added as a carrier was found to efficiently disperse the active precursors, which in turn significantly improved the catalytic activity during coal liquefaction.
2018, 39(4): 867-875
doi: 10.1016/S1872-2067(17)62999-1
Abstract:
To investigate the role of oxygen defects on the photocatalytic activity of TiO2, the TiO2 nanocrystals with/without oxygen defects are successfully synthesized by the hydrothermal and sol-gel methods, respectively. The as-prepared TiO2 nanocrystals with defects are light blue and the absorption edge of light is towards the visible light region (~420 nm). Raman and X-ray photoelectron spectroscopy (XPS) measurements all confirm that the concentration of oxygen vacancies in the TiO2 synthesized by the sol-gel method is less than that synthesized through the hydrothermal route. The introduction of oxygen defects contributes to a new state in the band gap that narrows the band gap, which is the reason for the extension of light absorption into the visible light region. The photocurrent results confirm that this band-gap narrowing enhances the photocurrent response under simulated solar light irradiation. The TiO2 with oxygen defects shows a higher photocatalytic activity for decomposition of a methylene blue solution compared with that of the perfect TiO2 sample. The photocatalytic mechanism is discussed based on the density functional theory calculations and photoluminescence spectroscopy measurements.
To investigate the role of oxygen defects on the photocatalytic activity of TiO2, the TiO2 nanocrystals with/without oxygen defects are successfully synthesized by the hydrothermal and sol-gel methods, respectively. The as-prepared TiO2 nanocrystals with defects are light blue and the absorption edge of light is towards the visible light region (~420 nm). Raman and X-ray photoelectron spectroscopy (XPS) measurements all confirm that the concentration of oxygen vacancies in the TiO2 synthesized by the sol-gel method is less than that synthesized through the hydrothermal route. The introduction of oxygen defects contributes to a new state in the band gap that narrows the band gap, which is the reason for the extension of light absorption into the visible light region. The photocurrent results confirm that this band-gap narrowing enhances the photocurrent response under simulated solar light irradiation. The TiO2 with oxygen defects shows a higher photocatalytic activity for decomposition of a methylene blue solution compared with that of the perfect TiO2 sample. The photocatalytic mechanism is discussed based on the density functional theory calculations and photoluminescence spectroscopy measurements.
