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2020 Volume 36 Issue 8
2020, 36(8): 190503
doi: 10.3866/PKU.WHXB201905039
Abstract:
As an excellent photoelectric material, metal halide perovskites have been rapidly developed in the photovoltaic field. The power conversion efficiency of solar cells based on perovskite materials now exceeds 24%, which is close to the conversion efficiency of silicon-based solar cells. However, organic-inorganic hybrid perovskite materials are sensitive to light, oxygen, and moisture, particularly when combined in the ambient environment, limiting their commercial application in perovskite devices due to their poor environmental stability. Therefore, a comprehensive understanding of the degradation mechanism is the key for development of an effective method to inhibit the degradation of perovskite materials. Herein, the photo-induced degradation process of CH3NH3PbI3 films in air was studied by conventional optical and structural characterization methods, including ultraviolet-visible (UV-Vis) absorption spectroscopy, X-ray diffraction (XRD) and advanced transmission electron microscopy (TEM) equipped with a probe spherical aberration corrector. The CH3NH3PbI3 films were first decomposed into hexagonal PbI2 and amorphous phase, and subsequently oxidized to the amorphous phase under the combined effects of light and oxygen. The molecular formula of the amorphous phase was further confirmed as PbI2−2xOx (0.4 < x < 0.6) via X-ray energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS). Further analysis showed that the film degradation is mainly related to superoxide (O2•−) formed by combination of oxygen molecules and photoelectrons in the perovskite film. The organic part of the CH3NH3PbI3 is oxidized by O2•− and CH3NH3PbI3 is decomposed to form volatile products, such as CH3NH2 and I2, then degraded into PbI2, and oxidized to form the amorphous PbI2−2xOx. Therefore, during the initial degradation of film under light soaking in air, the degradation sites are mainly located at the interface between CH3NH3PbI3 and air. Many pores were observed on the film surface due to the large loss of volatile decomposition products during the initial degradation. The films then converted to a honeycomb hollow morphology due to the continuous consumption of material under light soaking, reducing the mass of the film as well. Finally, the entire film was oxidized to form an amorphous structure. Herein, for the first time, we report that the formation of amorphous oxides is accompanied by the degradation of perovskite film. This study presents a new understanding of the photo-induced degradation mechanism of perovskite films in air and provides novel theoretical guidance to promote the long-term stability of perovskite solar cells. ![]()
As an excellent photoelectric material, metal halide perovskites have been rapidly developed in the photovoltaic field. The power conversion efficiency of solar cells based on perovskite materials now exceeds 24%, which is close to the conversion efficiency of silicon-based solar cells. However, organic-inorganic hybrid perovskite materials are sensitive to light, oxygen, and moisture, particularly when combined in the ambient environment, limiting their commercial application in perovskite devices due to their poor environmental stability. Therefore, a comprehensive understanding of the degradation mechanism is the key for development of an effective method to inhibit the degradation of perovskite materials. Herein, the photo-induced degradation process of CH3NH3PbI3 films in air was studied by conventional optical and structural characterization methods, including ultraviolet-visible (UV-Vis) absorption spectroscopy, X-ray diffraction (XRD) and advanced transmission electron microscopy (TEM) equipped with a probe spherical aberration corrector. The CH3NH3PbI3 films were first decomposed into hexagonal PbI2 and amorphous phase, and subsequently oxidized to the amorphous phase under the combined effects of light and oxygen. The molecular formula of the amorphous phase was further confirmed as PbI2−2xOx (0.4 < x < 0.6) via X-ray energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS). Further analysis showed that the film degradation is mainly related to superoxide (O2•−) formed by combination of oxygen molecules and photoelectrons in the perovskite film. The organic part of the CH3NH3PbI3 is oxidized by O2•− and CH3NH3PbI3 is decomposed to form volatile products, such as CH3NH2 and I2, then degraded into PbI2, and oxidized to form the amorphous PbI2−2xOx. Therefore, during the initial degradation of film under light soaking in air, the degradation sites are mainly located at the interface between CH3NH3PbI3 and air. Many pores were observed on the film surface due to the large loss of volatile decomposition products during the initial degradation. The films then converted to a honeycomb hollow morphology due to the continuous consumption of material under light soaking, reducing the mass of the film as well. Finally, the entire film was oxidized to form an amorphous structure. Herein, for the first time, we report that the formation of amorphous oxides is accompanied by the degradation of perovskite film. This study presents a new understanding of the photo-induced degradation mechanism of perovskite films in air and provides novel theoretical guidance to promote the long-term stability of perovskite solar cells.
2020, 36(8): 190508
doi: 10.3866/PKU.WHXB201905083
Abstract:
Recently, MOF-derived metal oxides have been demonstrated as excellent semiconductor materials. Their derivatives can retain the high porosity and high specific surface area of the parent MOFs, effectively improving the adsorption capacity and mass transfer rate of reactive substances. Herein, UiO-67 was selected as the substrate due to its high hydrothermal stability and high specific surface area. Through the in situ growth of TiO2 nanoparticles, a series of double metal composite catalysts (ZrxTi/C) were produced after calcination at 400 ℃. The X-ray diffraction (XRD) patterns showed that the UiO-67 crystal structure of collapsed after calcination, forming a Zr-O-C/TiO2 heterojunction. Energy-dispersive spectrometry (EDS) mapping images showed that Ti, Zr, and O were evenly distributed throughout the materials without obvious aggregation. Compared to conventional inorganic semiconductor materials, ZrxTi/C heterojunction catalysts provided much higher BET surface area (317 m2·g-1) for the effective enrichment of contaminants on the catalyst surface. Tetracycline was selected as a representative antibiotic to study photodegradation performance under a 300 W Xenon lamp. Among the obtained catalysts, the Zr0.3Ti/C heterojunction catalyst exhibited the best photocatalytic efficiency, achieving 98% degradation within 30 min in a 10 mg·L-1 tetracycline solution. Fluorescence spectra, electrochemical impedance spectroscopy, and transient photocurrent responses showed that the Zr0.3Ti/C heterojunction catalyst exhibited the fastest charge-hole separation rate and a maximum photocurrent density of 8.75 μA·cm-2, which was 2.64 and 3.71 times those of UiO-67 and UiO-670.3/TiO2, respectively. The mechanism of tetracycline photodegradation was determined by UV-visible diffuse-reflectance absorption spectroscopy, Mott-Schottky plots, and electron spin resonance technology. A direct Z-scheme charge separation path was formed by the transfer and recombination of photoexcited e- in the conduction band (CB) of Zr-O-C with h+ in the valence band (VB) of TiO2, which effectively reduced the combination rate of e- and h+ in Zr-O-C and TiO2. The photodegradation rate constant of Zr0.3Ti/C was 16 and 3.7 times those of TiO2 and Zr-O-C, respectively, due to its large specific surface area and excellent tetracycline adsorption performance. Furthermore, the Zr-O-C/TiO2 heterostructure exhibiting suitable energy level matching and containing highly conductive carbon material improved the separation and migration of electron-hole pairs. Mechanistic studies revealed that the three types of radicals, superoxide radicals (O2•-), hydroxyl radicals (•OH), and a small amount of holes (h+) simultaneously promoted tetracycline photodegradation. After five recycling tests, Zr0.3Ti/C heterojunction catalyst maintained 91.2% removal efficiency for tetracycline, indicating good cycle stability. Combining the synergistic effects of adsorption and photodegradation, using bimetallic heterojunction composites with high specific surface area is promising for the photodegradation of environmental pollutants. ![]()
Recently, MOF-derived metal oxides have been demonstrated as excellent semiconductor materials. Their derivatives can retain the high porosity and high specific surface area of the parent MOFs, effectively improving the adsorption capacity and mass transfer rate of reactive substances. Herein, UiO-67 was selected as the substrate due to its high hydrothermal stability and high specific surface area. Through the in situ growth of TiO2 nanoparticles, a series of double metal composite catalysts (ZrxTi/C) were produced after calcination at 400 ℃. The X-ray diffraction (XRD) patterns showed that the UiO-67 crystal structure of collapsed after calcination, forming a Zr-O-C/TiO2 heterojunction. Energy-dispersive spectrometry (EDS) mapping images showed that Ti, Zr, and O were evenly distributed throughout the materials without obvious aggregation. Compared to conventional inorganic semiconductor materials, ZrxTi/C heterojunction catalysts provided much higher BET surface area (317 m2·g-1) for the effective enrichment of contaminants on the catalyst surface. Tetracycline was selected as a representative antibiotic to study photodegradation performance under a 300 W Xenon lamp. Among the obtained catalysts, the Zr0.3Ti/C heterojunction catalyst exhibited the best photocatalytic efficiency, achieving 98% degradation within 30 min in a 10 mg·L-1 tetracycline solution. Fluorescence spectra, electrochemical impedance spectroscopy, and transient photocurrent responses showed that the Zr0.3Ti/C heterojunction catalyst exhibited the fastest charge-hole separation rate and a maximum photocurrent density of 8.75 μA·cm-2, which was 2.64 and 3.71 times those of UiO-67 and UiO-670.3/TiO2, respectively. The mechanism of tetracycline photodegradation was determined by UV-visible diffuse-reflectance absorption spectroscopy, Mott-Schottky plots, and electron spin resonance technology. A direct Z-scheme charge separation path was formed by the transfer and recombination of photoexcited e- in the conduction band (CB) of Zr-O-C with h+ in the valence band (VB) of TiO2, which effectively reduced the combination rate of e- and h+ in Zr-O-C and TiO2. The photodegradation rate constant of Zr0.3Ti/C was 16 and 3.7 times those of TiO2 and Zr-O-C, respectively, due to its large specific surface area and excellent tetracycline adsorption performance. Furthermore, the Zr-O-C/TiO2 heterostructure exhibiting suitable energy level matching and containing highly conductive carbon material improved the separation and migration of electron-hole pairs. Mechanistic studies revealed that the three types of radicals, superoxide radicals (O2•-), hydroxyl radicals (•OH), and a small amount of holes (h+) simultaneously promoted tetracycline photodegradation. After five recycling tests, Zr0.3Ti/C heterojunction catalyst maintained 91.2% removal efficiency for tetracycline, indicating good cycle stability. Combining the synergistic effects of adsorption and photodegradation, using bimetallic heterojunction composites with high specific surface area is promising for the photodegradation of environmental pollutants.
2020, 36(8): 190602
doi: 10.3866/PKU.WHXB201906026
Abstract:
The electrocatalytic activity of commercial Pt/C for ethanol oxidation is relatively low, and the C―C bond is difficult to break. Thus, the complete oxidation process is not easy, and the fuel utilization efficiency becomes considerably reduced. Increasing the temperature can increase the reaction rate and enhance the mass transport; therefore, a temperature-controlled electrode was used during our in situ FTIRS (Fourier Transform Infrared Spectroscopy) investigation. The temperature sensor was placed at a certain distance from the surface of the electrode; thus, the surface temperature needed to be corrected. The temperature was calibrated using the "potentiometric" measurement method, which was because the potential-temperature coefficient of the redox couple is constant under certain conditions, and the electrode surface temperature was obtained by potential conversion at different temperatures during the experiment. The experimental results showed that the relationship between the heating temperature, Th, and the surface temperature, TS, was TS = 0.57Th + 7.71 (30 ℃ < Th ≤ 50 ℃) and TS = 0.62Th + 5.12 (50 ℃ < Th ≤ 80 ℃), and according to error analysis, the maximum error was 1 ℃. The temperature-controlled electrode was applied to investigate the electrooxidation of ethanol using both in situ FTIRS and cyclic voltammetry using a commercial Pt/C catalyst at different temperatures. Clearly, based on the CV curve for the oxidation of ethanol, with increasing temperature, the overall oxidation current increased, and the onset potential and peak potential both negatively shifted, indicating that thermal activation allows the oxidation reaction to proceed easier. Electrooxidation of ethanol showed two positive oxidation peaks, and the ratio of the first peak current to the second peak current was used to qualitatively evaluate the selectivity of CO2. Compared with at 25 ℃, the first peak current increased by 30% at 65 ℃, indicating that the high temperature was conducive to C―C bond cleavage. Comparing the in situ FTIRS recorded at 50 ℃, 35 ℃, and 25 ℃, we found that the onset potential of CO2 on the commercial Pt/C catalyst was lower by 200 mV, indicating that Pt/C can provide oxygen-containing species at lower potentials at high temperature; however, the onset potentials of CH3CHO and CH3COOH did not change with temperature. The CO2 selectivity was semi-quantitatively calculated by the area of CO2 compared with the area of CH3COOH from the FTIRS data. It was found that CO2 had the highest selectivity at high temperature and low potential, indicating that high temperature is conducive to complete ethanol oxidation during CO2 formation, possibly because both the ethanol bridge adsorption pattern and adsorbed OH (OHad) increased with temperature, enhancing subsequent COad and OHad oxidation reactions. The low selectivity of CO2 at the high potential was due to the adsorption of oxygen-containing species that occupied the surface-active site, blocking the adsorption of ethanol. ![]()
The electrocatalytic activity of commercial Pt/C for ethanol oxidation is relatively low, and the C―C bond is difficult to break. Thus, the complete oxidation process is not easy, and the fuel utilization efficiency becomes considerably reduced. Increasing the temperature can increase the reaction rate and enhance the mass transport; therefore, a temperature-controlled electrode was used during our in situ FTIRS (Fourier Transform Infrared Spectroscopy) investigation. The temperature sensor was placed at a certain distance from the surface of the electrode; thus, the surface temperature needed to be corrected. The temperature was calibrated using the "potentiometric" measurement method, which was because the potential-temperature coefficient of the redox couple is constant under certain conditions, and the electrode surface temperature was obtained by potential conversion at different temperatures during the experiment. The experimental results showed that the relationship between the heating temperature, Th, and the surface temperature, TS, was TS = 0.57Th + 7.71 (30 ℃ < Th ≤ 50 ℃) and TS = 0.62Th + 5.12 (50 ℃ < Th ≤ 80 ℃), and according to error analysis, the maximum error was 1 ℃. The temperature-controlled electrode was applied to investigate the electrooxidation of ethanol using both in situ FTIRS and cyclic voltammetry using a commercial Pt/C catalyst at different temperatures. Clearly, based on the CV curve for the oxidation of ethanol, with increasing temperature, the overall oxidation current increased, and the onset potential and peak potential both negatively shifted, indicating that thermal activation allows the oxidation reaction to proceed easier. Electrooxidation of ethanol showed two positive oxidation peaks, and the ratio of the first peak current to the second peak current was used to qualitatively evaluate the selectivity of CO2. Compared with at 25 ℃, the first peak current increased by 30% at 65 ℃, indicating that the high temperature was conducive to C―C bond cleavage. Comparing the in situ FTIRS recorded at 50 ℃, 35 ℃, and 25 ℃, we found that the onset potential of CO2 on the commercial Pt/C catalyst was lower by 200 mV, indicating that Pt/C can provide oxygen-containing species at lower potentials at high temperature; however, the onset potentials of CH3CHO and CH3COOH did not change with temperature. The CO2 selectivity was semi-quantitatively calculated by the area of CO2 compared with the area of CH3COOH from the FTIRS data. It was found that CO2 had the highest selectivity at high temperature and low potential, indicating that high temperature is conducive to complete ethanol oxidation during CO2 formation, possibly because both the ethanol bridge adsorption pattern and adsorbed OH (OHad) increased with temperature, enhancing subsequent COad and OHad oxidation reactions. The low selectivity of CO2 at the high potential was due to the adsorption of oxygen-containing species that occupied the surface-active site, blocking the adsorption of ethanol.
2020, 36(8): 191103
doi: 10.3866/PKU.WHXB201911035
Abstract:
Atomic-scale characterization of the atomic structure as well as molecular adsorption on an alloy surface plays a vital role in elucidating the catalytic mechanism of effective catalysts. Au-Cu alloy nanoparticles have important applications in catalyzing CO oxidation and CO2 reduction. However, the atomic-scale properties of Au-Cu alloy surfaces are rarely investigated. In particular, the physical and chemical properties of singly-dispersed doping atoms, either Au in Cu or vice versa, as well as their influence on the overall surface properties, have not been studied in detail. In response, we first prepared low-coverage bimetallic Au/Cu(111) and Cu/Au(111) films, which were then annealed at high temperature to realize single atomically-dispersed Au/Cu(111) and Cu/Au(111) surface alloys (SA). We characterized the surface structures and adsorption properties by low-temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS). For the SA-Au/Cu(111) system, we found that Au atoms can be incorporated in both the skin and subsurface layer of the Cu(111) substrate. These species can be readily distinguished from the topography contrast in STM. Moreover, STS measurements showed clear differences between the electronic states of doped Au atoms and the Cu host. In particular, we found that Au in the skin layer was strengthened while the subsurface Au showed weakened filled states at approximately −0.5 eV compared with the Cu(111) surface, which corresponds to the characteristic Shockley state of an Au surface. These altered electronic properties at the sites of doped atoms are also reflected by changes in the interactions with probe molecules. Adsorption experiments showed that Au atoms in the top surface prevented the binding of CO molecules, causing various adsorption vacancies in the CO adlayer. In contrast, the subsurface Au atoms had little influence on surface binding with CO molecules. For the SA-Cu/Au(111) system, we found that Cu atoms tend to aggregate into small clusters in the subsurface region of the Au(111) substrate. Only few Cu atoms can be stabilized at the elbow positions of the reconstructed top surface of Au(111). Adsorption experiments showed that only Cu atoms in the skin layer can adsorb CO molecules at liquid nitrogen temperature, while the subsurface Cu atoms cannot. On the other hand, the Au atoms around the doped Cu atoms do not seem to be influenced at all, possibly because of the weak effect of Cu. These experimental results provide details on the atomistic aspects of Au-Cu alloy surfaces, which can improve our understanding of the catalytic mechanism of Au-Cu alloy catalysts. ![]()
Atomic-scale characterization of the atomic structure as well as molecular adsorption on an alloy surface plays a vital role in elucidating the catalytic mechanism of effective catalysts. Au-Cu alloy nanoparticles have important applications in catalyzing CO oxidation and CO2 reduction. However, the atomic-scale properties of Au-Cu alloy surfaces are rarely investigated. In particular, the physical and chemical properties of singly-dispersed doping atoms, either Au in Cu or vice versa, as well as their influence on the overall surface properties, have not been studied in detail. In response, we first prepared low-coverage bimetallic Au/Cu(111) and Cu/Au(111) films, which were then annealed at high temperature to realize single atomically-dispersed Au/Cu(111) and Cu/Au(111) surface alloys (SA). We characterized the surface structures and adsorption properties by low-temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS). For the SA-Au/Cu(111) system, we found that Au atoms can be incorporated in both the skin and subsurface layer of the Cu(111) substrate. These species can be readily distinguished from the topography contrast in STM. Moreover, STS measurements showed clear differences between the electronic states of doped Au atoms and the Cu host. In particular, we found that Au in the skin layer was strengthened while the subsurface Au showed weakened filled states at approximately −0.5 eV compared with the Cu(111) surface, which corresponds to the characteristic Shockley state of an Au surface. These altered electronic properties at the sites of doped atoms are also reflected by changes in the interactions with probe molecules. Adsorption experiments showed that Au atoms in the top surface prevented the binding of CO molecules, causing various adsorption vacancies in the CO adlayer. In contrast, the subsurface Au atoms had little influence on surface binding with CO molecules. For the SA-Cu/Au(111) system, we found that Cu atoms tend to aggregate into small clusters in the subsurface region of the Au(111) substrate. Only few Cu atoms can be stabilized at the elbow positions of the reconstructed top surface of Au(111). Adsorption experiments showed that only Cu atoms in the skin layer can adsorb CO molecules at liquid nitrogen temperature, while the subsurface Cu atoms cannot. On the other hand, the Au atoms around the doped Cu atoms do not seem to be influenced at all, possibly because of the weak effect of Cu. These experimental results provide details on the atomistic aspects of Au-Cu alloy surfaces, which can improve our understanding of the catalytic mechanism of Au-Cu alloy catalysts.
2020, 36(8): 190504
doi: 10.3866/PKU.WHXB201905047
Abstract:
Volatile organic compounds (VOCs) are both harmful to human health and the environment; however, catalytic combustion offers a promising method for VOC purification because of its high efficiency without secondary pollution. Although manganese-based catalysts have been well studied for VOC catalytic oxidation, their catalytic activity at low temperature must be improved. Alkali metals as promoters have the potential to modulate the electronic and structural properties of the catalysts, improving their catalytic activity. Herein, a Ce0.65Zr0.35O2 support was prepared by co-precipitation and MnOx/Ce0.65Zr0.35O2 catalysts were obtained through the incipient-wetness impregnation method. The catalytic properties of K-modified MnOx/Ce0.65Zr0.35O2 for toluene oxidation with different molar ratios of K/Mn were investigated. In addition, the catalysts were characterized by XRD, UV/visible Raman, Hydrogen temperature program reduction (H2-TPR), Oxygen temperature programmed desorption (O2-TPD), X-ray photoelectron spectroscopy (XPS) and in situ diffuse reflectance FTIR spectroscopy (DRIFTS) experiments. The results showed that alkali metal doping with K significantly improved the catalytic activity. In particular, when the molar ratio of K/Mn was 0.2, the monolith catalyst Mn/Ce0.65Zr0.35O2-K-0.2 exhibited the best performance with the lowest complete conversion temperature T90 of 242 ℃ at a GHSV of 12000 h−1. The XRD results suggested that MnOx was uniformly distributed on the surface of the catalyst and that Mn4+ partially reduced to Mn3+ on the addition of K. The Raman spectrum demonstrated that with increasing K content, both the β- and α-MnO2 phases coexisted on the Mn/Ce0.65Zr0.35O2-K-0.2 catalyst, increasing the number of surface defect sites. The H2-TPR experiment results confirmed that Mn/Ce0.65Zr0.35O2-K-0.2 exhibited the lowest reduction temperature and good reducibility. From the O2-TPD experiments, it was clear that Mn/Ce0.65Zr0.35O2-K-0.2 contained the most surface adsorbed oxygen species and excellent lattice oxygen mobility, which benefitted the toluene oxidation activity. In addition, the XPS results suggested that the content of surface adsorbed oxygen species of the Mn/Ce0.65Zr0.35O2-K-0.2 catalyst was the highest among all the tested samples. In addition, toluene-TPSR in N2 as measured by in situ DRIFTs analysis demonstrated that available lattice oxygen was present in the Mn/Ce0.65Zr0.35O2-K-0.2 catalyst. Therefore, the Mn/Ce0.65Zr0.35O2-K-0.2 catalyst exhibited the best redox properties and oxygen mobility of the prepared samples and showed excellent activity toward toluene oxidation. Therefore, it was concluded that the addition of an appropriate amount of K improved the redox performance of the catalyst and increased the number of surface defect sites and mobility of the lattice oxygen of the catalyst as well as the concentration of the surface active oxygen species, thereby significantly improving catalytic ability. ![]()
Volatile organic compounds (VOCs) are both harmful to human health and the environment; however, catalytic combustion offers a promising method for VOC purification because of its high efficiency without secondary pollution. Although manganese-based catalysts have been well studied for VOC catalytic oxidation, their catalytic activity at low temperature must be improved. Alkali metals as promoters have the potential to modulate the electronic and structural properties of the catalysts, improving their catalytic activity. Herein, a Ce0.65Zr0.35O2 support was prepared by co-precipitation and MnOx/Ce0.65Zr0.35O2 catalysts were obtained through the incipient-wetness impregnation method. The catalytic properties of K-modified MnOx/Ce0.65Zr0.35O2 for toluene oxidation with different molar ratios of K/Mn were investigated. In addition, the catalysts were characterized by XRD, UV/visible Raman, Hydrogen temperature program reduction (H2-TPR), Oxygen temperature programmed desorption (O2-TPD), X-ray photoelectron spectroscopy (XPS) and in situ diffuse reflectance FTIR spectroscopy (DRIFTS) experiments. The results showed that alkali metal doping with K significantly improved the catalytic activity. In particular, when the molar ratio of K/Mn was 0.2, the monolith catalyst Mn/Ce0.65Zr0.35O2-K-0.2 exhibited the best performance with the lowest complete conversion temperature T90 of 242 ℃ at a GHSV of 12000 h−1. The XRD results suggested that MnOx was uniformly distributed on the surface of the catalyst and that Mn4+ partially reduced to Mn3+ on the addition of K. The Raman spectrum demonstrated that with increasing K content, both the β- and α-MnO2 phases coexisted on the Mn/Ce0.65Zr0.35O2-K-0.2 catalyst, increasing the number of surface defect sites. The H2-TPR experiment results confirmed that Mn/Ce0.65Zr0.35O2-K-0.2 exhibited the lowest reduction temperature and good reducibility. From the O2-TPD experiments, it was clear that Mn/Ce0.65Zr0.35O2-K-0.2 contained the most surface adsorbed oxygen species and excellent lattice oxygen mobility, which benefitted the toluene oxidation activity. In addition, the XPS results suggested that the content of surface adsorbed oxygen species of the Mn/Ce0.65Zr0.35O2-K-0.2 catalyst was the highest among all the tested samples. In addition, toluene-TPSR in N2 as measured by in situ DRIFTs analysis demonstrated that available lattice oxygen was present in the Mn/Ce0.65Zr0.35O2-K-0.2 catalyst. Therefore, the Mn/Ce0.65Zr0.35O2-K-0.2 catalyst exhibited the best redox properties and oxygen mobility of the prepared samples and showed excellent activity toward toluene oxidation. Therefore, it was concluded that the addition of an appropriate amount of K improved the redox performance of the catalyst and increased the number of surface defect sites and mobility of the lattice oxygen of the catalyst as well as the concentration of the surface active oxygen species, thereby significantly improving catalytic ability.
2020, 36(8): 190705
doi: 10.3866/PKU.WHXB201907057
Abstract:
Customizing core-shell nanostructures is considered to be an efficient approach to improve the catalytic activity of metal nanoparticles. Various physiochemical and green methods have been developed for the synthesis of core-shell structures. In this study, a novel liquid-phase hydrogen reduction method was employed to form core-shell Pt@Au nanoparticles with intimate contact between the Pt and Au particles, without the use of any protective or structure-directing agents. The Pt@Au core-shell nanoparticles were prepared by depositing Au metal onto the Pt core; AuCl4− was reduced to Au(0) by H2 in the presence of Pt nanoparticles. The obtained Pt@Au core-shell structured nanoparticles were characterized by transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), high-resolution TEM, fast Fourier transform, powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and H2-temperature programmed reduction (H2-TPR) analyses. The EDX mapping results for the nanoparticles, as obtained from their scanning transmission electron microscopy images in the high-angle annular dark-field mode, revealed a Pt core with Au particles grown on its surface. Fourier transform measurements were carried out on the high-resolution structure to characterize the Pt@Au nanoparticles. The lattice plane at the center of the nanoparticles corresponded to Pt, while the edge of the particles corresponded to Au. With an increase in the Au content, the intensity of the peak corresponding to Pt in the FTIR spectrum decreased slowly, indicating that the Pt nanoparticles were surrounded by Au nanoparticles, and thus confirming the core-shell structure of the nanoparticles. The XRD results showed that the peak corresponding to Pt shifted gradually toward the Au peak with an increase in the Au content, indicating that the Au particles grew on the Pt seeds; this trend was consistent with the FTIR results. Hence, it can be stated that the Pt@Au core-shell structure was successfully prepared using the liquid-phase hydrogen reduction method. The catalytic activity of the nanoparticles for the oxidation of toluene was evaluated using a fixed-bed reactor under atmospheric pressure. The XPS and H2-TPR results showed that the Pt1@Au1/Al2O3 catalyst had the best toluene oxidation activity owing to its lowest reduction temperature, lowest Au 4d & 4f and Pt 4d & 4f binding energies, and highest Au0/Auδ+ and Pt0/Pt2+ proportions. The Pt1@Au2Al2O3 catalyst showed high stability under dry and humid conditions. The good catalytic performance and high selectivity of Pt@Au/Al2O3 for toluene oxidation could be attributed to the high concentration of adsorbed oxygen species, good low-temperature reducibility, and strong interaction. ![]()
Customizing core-shell nanostructures is considered to be an efficient approach to improve the catalytic activity of metal nanoparticles. Various physiochemical and green methods have been developed for the synthesis of core-shell structures. In this study, a novel liquid-phase hydrogen reduction method was employed to form core-shell Pt@Au nanoparticles with intimate contact between the Pt and Au particles, without the use of any protective or structure-directing agents. The Pt@Au core-shell nanoparticles were prepared by depositing Au metal onto the Pt core; AuCl4− was reduced to Au(0) by H2 in the presence of Pt nanoparticles. The obtained Pt@Au core-shell structured nanoparticles were characterized by transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), high-resolution TEM, fast Fourier transform, powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and H2-temperature programmed reduction (H2-TPR) analyses. The EDX mapping results for the nanoparticles, as obtained from their scanning transmission electron microscopy images in the high-angle annular dark-field mode, revealed a Pt core with Au particles grown on its surface. Fourier transform measurements were carried out on the high-resolution structure to characterize the Pt@Au nanoparticles. The lattice plane at the center of the nanoparticles corresponded to Pt, while the edge of the particles corresponded to Au. With an increase in the Au content, the intensity of the peak corresponding to Pt in the FTIR spectrum decreased slowly, indicating that the Pt nanoparticles were surrounded by Au nanoparticles, and thus confirming the core-shell structure of the nanoparticles. The XRD results showed that the peak corresponding to Pt shifted gradually toward the Au peak with an increase in the Au content, indicating that the Au particles grew on the Pt seeds; this trend was consistent with the FTIR results. Hence, it can be stated that the Pt@Au core-shell structure was successfully prepared using the liquid-phase hydrogen reduction method. The catalytic activity of the nanoparticles for the oxidation of toluene was evaluated using a fixed-bed reactor under atmospheric pressure. The XPS and H2-TPR results showed that the Pt1@Au1/Al2O3 catalyst had the best toluene oxidation activity owing to its lowest reduction temperature, lowest Au 4d & 4f and Pt 4d & 4f binding energies, and highest Au0/Auδ+ and Pt0/Pt2+ proportions. The Pt1@Au2Al2O3 catalyst showed high stability under dry and humid conditions. The good catalytic performance and high selectivity of Pt@Au/Al2O3 for toluene oxidation could be attributed to the high concentration of adsorbed oxygen species, good low-temperature reducibility, and strong interaction.
2020, 36(8): 191204
doi: 10.3866/PKU.WHXB201912045
Abstract:
2020, 36(8): 200101
doi: 10.3866/PKU.WHXB202001013
Abstract:
2020, 36(8): 200101
doi: 10.3866/PKU.WHXB202001014
Abstract:
2020, 36(8): 200102
doi: 10.3866/PKU.WHXB202001023
Abstract:
2020, 36(8): 200102
doi: 10.3866/PKU.WHXB202001026
Abstract:
2020, 36(8): 200102
doi: 10.3866/PKU.WHXB202001028
Abstract:
2020, 36(8): 200103
doi: 10.3866/PKU.WHXB202001035
Abstract:
2020, 36(8): 200103
doi: 10.3866/PKU.WHXB202001036
Abstract:
2020, 36(8): 200201
doi: 10.3866/PKU.WHXB202002014
Abstract:
2020, 36(8): 200301
doi: 10.3866/PKU.WHXB202003010
Abstract:
2020, 36(8): 191204
doi: 10.3866/PKU.WHXB201912046
Abstract:
2020, 36(8): 200303
doi: 10.3866/PKU.WHXB202003034
Abstract:
2020, 36(8): 200305
doi: 10.3866/PKU.WHXB202003051
Abstract:
Project proposal review is the core of science foundation management. This article systematically overviews existing key issues in panel committee meetings in final reviews of scientific research grants, which are becoming an increasing concern in the science community. Moreover, it reassesses the importance and functionality of panel committee meetings, and accordingly provides suggestions and measures based on the "verification-rectification-and-selection" principle for further installation and improvement of the panel committee meeting mechanism.
Project proposal review is the core of science foundation management. This article systematically overviews existing key issues in panel committee meetings in final reviews of scientific research grants, which are becoming an increasing concern in the science community. Moreover, it reassesses the importance and functionality of panel committee meetings, and accordingly provides suggestions and measures based on the "verification-rectification-and-selection" principle for further installation and improvement of the panel committee meeting mechanism.
2020, 36(8): 200400
doi: 10.3866/PKU.WHXB202004002
Abstract:
The National Natural Science Foundation of China (NSFC) is a vital government agency supporting basic research and people to create knowledge and meet major national needs, where a rigorous and objective merit-based peer review mechanism is the key to funding the most promising research proposals. This invited comment overviews some recent attempts aimed at bettering the academic evaluation environment at the Department of Chemical Science in 2019, through measures such as grouped panel committee meetings, standardized panel committee meeting procedures, and review process refinement to improve the project review at panel committee meeting levels.
The National Natural Science Foundation of China (NSFC) is a vital government agency supporting basic research and people to create knowledge and meet major national needs, where a rigorous and objective merit-based peer review mechanism is the key to funding the most promising research proposals. This invited comment overviews some recent attempts aimed at bettering the academic evaluation environment at the Department of Chemical Science in 2019, through measures such as grouped panel committee meetings, standardized panel committee meeting procedures, and review process refinement to improve the project review at panel committee meeting levels.
2020, 36(8): 200404
doi: 10.3866/PKU.WHXB202004048
Abstract:
Peer review plays a crucial role in the fair and efficient allocation of high-quality research grants. This invited comment analyzes the fuzzy criteria and a possible bias in the peer review process, and explores the use of a potential approach based on fuzzy sets for tackling this challenge.
Peer review plays a crucial role in the fair and efficient allocation of high-quality research grants. This invited comment analyzes the fuzzy criteria and a possible bias in the peer review process, and explores the use of a potential approach based on fuzzy sets for tackling this challenge.
2020, 36(8): 200403
doi: 10.3866/PKU.WHXB202004033
Abstract:
